Title of Invention: LAT1 transporters expressed in blood brain barrier cells Abstract: LAT1 is consistently expressed at high levels in brain microvessel endothelial cells. Disclosed herein are assays for determining whether a test material/molecule is a substrate for, and/or is actively transported by, the LAT1 transporter, and therefore a candidate substrate for crossing the blood brain barrier. The assays are useful in screening for therapeutic, cytotoxic or imaging compounds used in the treatment or diagnosis of neurological diseases. Detailed Description: This application claims the benefit of U.S. Provisional Application No. 60/540,754, filed Jan. 30, 2004, the disclosure of which is incorporated by reference herein in its entirety. TECHNICAL FIELD The disclosures herein relate to assays and methods of using the same for screening compounds and/or chemical moieties for their ability to be actively transported across the blood brain barrier. BACKGROUND The capillaries that supply blood to the tissues of the brain constitute the blood brain barrier (Goldstein et al. (1986) Scientific American 255:74-83; Pardridge, W. M. (1986) Endocrin. Rev. 7:314-330). The endothelial cells which form the brain capillaries are different from those found in other tissues in the body. Brain capillary endothelial cells are joined together by tight intercellular junctions which form a continuous wall against the passive diffusion of molecules from the blood to the brain and other parts of the central nervous system (CNS). These cells are also different in that they have few pinocytic vesicles which in other tissues allow somewhat unselective transport across the capillary wall. Also lacking are continuous gaps or channels running between the cells which would allow unrestricted passage. The blood-brain barrier functions to ensure that the environment of the brain is constantly controlled. The levels of various substances in the blood, such as hormones, amino acids and ions, undergo frequent small fluctuations which can be brought about by activities such as eating and exercise (Goldstein et al., cited supra). If the brain was not protected by the blood brain barrier from these variations in serum composition, the result could be uncontrolled neural activity. The isolation of the brain from the bloodstream is not complete. If this were the case, the brain would be unable to function properly due to a lack of nutrients and because of the need to exchange chemicals with the rest of the body. The presence of specific transport systems within the capillary endothelial cells assures that the brain receives, in a controlled manner, all of the compounds required for normal growth and function. In many instances, these transport systems consist of membrane-associated proteins which selectively bind and certain molecules across the barrier membranes. These transporter proteins are known as solute carrier transporters. The problem posed by the blood-brain barrier is that, in the process of protecting the brain, it excludes many potentially useful therapeutic agents. Presently, only substances which are sufficiently lipophilic can penetrate the blood-brain barrier (Goldstein et al., cited supra; Pardridge, W. M., cited supra). Some drugs can be modified to make them more lipophilic and thereby increase their ability to cross the blood brain barrier. However, each modification must be tested individually on each drug and the modification can alter the activity of the drug. Because the blood brain barrier is composed of brain microvessel endothelial cells, these cells have been isolated and cultured for use in in vitro model systems for studying the blood brain barrier (Bowman et. al, Brain microvessel endothelial cells in tissue culture: A model for study of blood-brain barrier permeability, Ann. Neurol. 14, 396-402 (1983); Audus and Borchardt, Characterization of an in vitro blood-brain barrier model system for studying drug transport and metabolism, Pharm, Res. 3, 81-87 (1986)). In vitro model systems of the blood brain barrier have been successfully derived from bovine, canine, human, murine, porcine, and rat cells, and have similar permeability properties due to similarity of the physiological characteristics of the blood brain barrier among mammals (Cserr et al., Blood-brain interfaces in vertebrates: a comparative approach, Am. J. Physiol. 246, R277-R288 (1984); Audus et al., The use of cultured epithelial and endothelial cells for drug transport and metabolism studies, Pharm. Res. 7, 435-451 (1990)). In these models, the cultured endothelial cells retain the characteristics of brain endothelial cells in vivo, such as morphology, specific blood brain barrier enzyme markers, and tight intercellular junctions. The cells can also be used for the study of passive diffusion, carrier mediated transport, and metabolism to specific factors affecting the blood brain barrier permeability. However, passaging of brain microvessel endothelial cells results in loss of specific endothelial and blood brain barrier markers as well as tight intercellular junctions (Brightman and Neuwelt (ed.), Implications of the blood-brain barrier and its manipulation, Vol. 1, Plenum Medical, New York, pp. 53-83 (1989)). Currently, primary cultures of brain microvessel endothelial cells are the principal tool for in vitro prediction of blood brain barrier permeability. Isolated and cultured primary brain cells developed previously have exhibited different properties primarily due to considerable variability in the starting material. For example, with respect to transcellular transport, rigorous comparison of data between different laboratories has been very difficult (Pardridge et al., Comparison of in vitro and in vivo models of drug transcytosis through the blood-brain barrier, J. Pharmacol. Exp. Thera. 253, 884-891 (1990); Masereeuw et al., In vitro and in vivo transport of zidovudine (AZT) across the blood-brain barrier and the effect of transport inhibitors. Pharm. Res., 11, 324-330 (1994)). Passaging primary cells can affect the differentiation of cells and lead to the selection of the most rapidly proliferating clones. Furthermore, the expression of some marker enzymes such as gamma-glutamyl transpeptidase as well as tight junctional complexity has been shown to decrease with time in culture and passage number (Meresse et. al., Bovine brain endothelial cells express tight junctions and monoamine oxidase activity in long-term culture, J. Neuorchem. 53, 1363-1371 (1989)). Some transporter substrates have been demonstrated to accumulate in the brain (see U.S. Pat. No. 6,489,302). Thus, it is apparent that the presently available clones of immortalized brain microvessel endothelial cell cultures suffer from individual drawbacks in terms of phenotype expression and homogeneic maintenance of that expression. This leads to difficulties with respect to accuracy and reproducibility in studies utilizing brain microvessel endothelial cells to model passage of chemical compounds and moieties, e.g., potential therapeutic compounds and/or drug moieties, across the blood brain barrier. SUMMARY Disclosed herein are methods of screening agents, conjugates or conjugate moieties for the ability to enter the CNS by crossing the blood brain barrier in order to treat or diagnose conditions within the CNS. These methods entail providing a cell expressing an LAT1 transporter, the transporter being situated in the plasma membrane of the cell. The cell is contacted with an agent, conjugate or conjugate moiety. Whether the agent, conjugate or conjugate moiety passes through the plasma membrane via the LAT1 transporter is determined. If the method comprises contacting the cell with an agent, the agent is a neuropharmaceutical agent or an imaging component. If the method comprises contacting the cell with a conjugate, the conjugate comprises an agent that is a neuropharmaceutical agent or an imaging component. If the method comprises contacting the cells with a conjugate moiety, the method further comprises linking the conjugate moiety to an agent that is a neuropharmaceutical agent or an imaging component. In some methods, the agent is other than a chemotherapeutic agent, an imaging agent or other agent used for treating or diagnosing cancer. In some methods, if the agent is a cytotoxic agent or an imaging component, the method further comprises administering the agent, conjugate, or conjugate moiety to a peripheral tissue of an animal and measuring the amount of agent, conjugate, or conjugate moiety that passes through the blood brain barrier into the brain of the animal. In other methods, if the agent is a cytotoxic agent or an imaging component, the method further comprises contacting the agent to one side of a polarized monolayer of brain microvessel endothelial cells; and determining whether the agent is transported across the polarized monolayer. In some methods, the cell endogenously expresses a LAT1 transporter. In other methods a nucleic acid molecule encoding a LAT1 transporter has been transfected or injected into the cell. In some methods the cell is a brain microvessel endothelial cell. In other methods the cell is an oocyte. In other methods the cell is a human embryonic kidney (HEK) cell. In other methods the cell is a Madin Darby canine kidney cell (MDCK). In still other methods, the cell is constructed to conditionally express the transporter. In some methods the agent, conjugate or conjugate moiety comprises an L-amino acids with bulky (>2 atoms), uncharged, and hydrophobic or aromatic side-chains; a similarly structured D-amino acid; or a gamma amino acid such as gabapentin; or a cyclic alpha amino acid such as bicyclohexane amino acid. In some methods the agent, conjugate or conjugate moiety is administered to an undiseased animal and any toxic effects are determined. In some methods the neuropharmaceutical agent is a cytotoxic neuropharmaceutical agent selected from the group consisting of platinum, nitrosourea, a phosphoramide group that is selectively cytotoxic to brain tumor cells, nitroimidizole, and nitrogen mustard. Disclosed herein are methods of screening agents, conjugates or conjugate moieties for the ability to enter the CNS by crossing the blood brain barrier wherein a cell used for testing is a brain microvessel endothelial cell that is one of a plurality of brain microvessel endothelial cells forming a polarized monolayer. An agent, conjugate or conjugate moiety is contacted to one side of the polarized monolayer and whether the agent, conjugate or conjugate moiety is transported into the brain microvessel endothelial cells or to the opposite side of the polarized monolayer is determined. Some methods further comprise administering the agent, conjugate, or conjugate moiety to a peripheral tissue of an animal and measuring the amount of agent, conjugate, or conjugate moiety that passes through the blood brain barrier into the brain of the animal. Disclosed herein are methods of screening an agent, conjugate or conjugate moiety for neuropharmacological activity useful for treating neurological disorders. In these methods, one determines whether the agent, conjugate or conjugate moiety is transported through a LAT1 transporter. One then administers the agent, conjugate or conjugate moiety to a test animal and determines whether the agent, conjugate or conjugate moiety is actively transported across the blood brain barrier by measuring agent, conjugate or conjugate moiety concentrations found in the CNS of the animal. For those agents, conjugates or conjugate moieties that are transported in sufficient quantities, the agents, conjugates or conjugate moieties can be further tested in animals suffering from a particular neurological disorder to determine whether the agents, conjugates or conjugate moieties have the requisite therapeutic neuropharmacological activity for treating such neurological disorder. Also disclosed herein are methods for in vitro screening of agents, conjugates or conjugate moieties for improved retention in the CNS. In these methods, one determines the substrate properties of a compound on both uptake transporters and efflux transporters. An agent, conjugate or conjugate moiety is first tested for activity on the LAT1 transporter. The agent, conjugate or conjugate moiety is then tested for substrate activity on an efflux transporter, such as P Glycoprotein (PgP). Those agents, conjugates or conjugate moieties active on both the efflux transporter and LAT1 are then modified and tested for a reduction of efflux substrate activity and retested for retention of activity on the LAT1 transporter. This iterative process produces an agent, conjugate or conjugate moiety with an increased ratio of substrate activities in the uptake and efflux systems, and improved retention of pharmacological levels of the modified agent, conjugate or conjugate moiety in the CNS. Disclosed herein are methods of screening an agent, conjugate or conjugate moiety for capacity to be transported into the brain, comprising determining whether the agent, conjugate or conjugate moiety specifically binds to a LAT1 transporter, contacting the agent to one side of a polarized monolayer of cells, and determining whether the agent is actively transported across the polarized monolayer. In some methods the specific binding is determined by contacting a cell expressing the LAT1 transporter, the transporter being situated in the plasma membrane of the cell, with a substrate of the LAT1 transporter, and determining whether the agent inhibits transport of the substrate across the polarized monolayer. Disclosed herein are pharmaceutical compositions comprising a therapeutic neuropharmaceutical agent, a cytotoxic neuropharmaceutical agent or an imaging component linked to a conjugate moiety to form a conjugate in which the conjugate moiety has a higher Vmax for the LAT1 transporter than the therapeutic neuropharmaceutical agent, cytotoxic neuropharmaceutical agent or imaging component alone. Some pharmaceutical compositions have at least 5 times the Vmax for LAT1 than the neuropharmaceutical agent or the imaging component alone. In some pharmaceutical compositions the conjugate has a Vmax for LAT1 that is at least 5% of the Vmax for LAT1 of a compound selected from the group comprising tryptophan, leucine, methionine, phenylalanine, bicyclohexane amino acid, L-dopa, gabapentin, and baclofen. In some pharmaceutical compositions the conjugate has a lower Vmax for an efflux transporter than the neuropharmaceutical agent or the imaging component alone. In some pharmaceutical compositions, the agent is not a cytotoxic agent, an imaging agent or other agent used for treating or diagnosing cancer. Disclosed herein are methods of formulating a therapeutic neuropharmaceutical agent, a cytotoxic neuropharmaceutical agent or an imaging component. These methods entail linking the therapeutic neuropharmaceutical agent, the cytotoxic neuropharmaceutical agent or the imaging component to a conjugate moiety to form a conjugate, wherein the conjugate moiety has a greater Vmax for a LAT1 transporter than the component alone. The conjugate is formulated with a pharmaceutical carrier as a pharmaceutical composition. In some methods, the agent is not a cytotoxic agent, an imaging agent or other agent used for treating or diagnosing cancer. Disclosed herein are methods of delivering a therapeutic neuropharmaceutical agent, a cytotoxic neuropharmaceutical agent or an imaging component. The methods involve administering to a patient a pharmaceutical composition comprising a therapeutic neuropharmaceutical agent, a cytotoxic neuropharmaceutical agent or an imaging component linked to a conjugate moiety to form a conjugate, wherein the conjugate has a higher Vmax for a LAT1 transporter than the therapeutic neuropharmaceutical agent, cytotoxic neuropharmaceutical agent or imaging component alone, whereby the conjugate passes through brain microvessel endothelial cells which make up the blood brain barrier, via the LAT1 transporter, into the CNS of the patient. Also disclosed herein are methods of delivering a conjugate, comprising administering to a patient a pharmaceutical composition comprising a neuropharmaceutical agent or imaging component linked to a conjugate moiety to form the conjugate, wherein the conjugate has a higher Vmax for a LAT1 transporter than the neuropharmaceutical agent or imaging component alone. In some methods the Vmax of the conjugate is at least two-fold higher than that of the neuropharmaceutical agent or imaging component alone. In some methods the neuropharmaceutical agent is a cytotoxic neuropharmaceutical selected from the group consisting of platinum, nitrosourea, a phosphoramide group selectively cytotoxic to brain tumor cells, nitroimidizole, and nitrogen mustard. In some methods, the agent is not a cytotoxic agent, an imaging agent or other agent used for treating or diagnosing cancer. Disclosed herein are methods of treating neurological disorders. These methods entail administering to a patient an effective amount of an agent that is transported by LAT1, wherein the agent is a conjugate comprising a therapeutic neuropharmaceutical agent, a cytotoxic neuropharmaceutical agent or an imaging component linked to a conjugate moiety. Disclosed herein are methods of screening an agent for decreased side effects in the central nervous system (CNS), comprising providing an agent having a pharmacological activity, wherein the pharmacological activity is useful for treating a disease present in a tissue other than the CNS, and the pharmacological activity results in undesired side effects in the CNS if the agent enters the CNS, modifying the agent, providing a cell expressing at least one efflux transporter protein that transports substrates out of the CNS, contacting the cell with the modified agent, and determining whether the modified agent is transported by the at least one efflux transporter protein with a higher Vmax than the agent, a higher Vmax indicating that the modification increases the capacity of the modified agent relative to the agent to be transported out of the CNS, thereby decreasing undesired side effects in the CNS. BRIEF DESCRIPTION OF THE FIGURES FIG. 1 shows the structures of known substrates of the LAT1 transporter. FIG. 2 shows uptake of 14C-gabapentin into oocytes injected or not injected with a LAT1 expression vector. FIG. 3 shows uptake of gabapentin and L-dopa into oocytes injected with LAT1 and 4F2HC cRNA. FIG. 4 shows competition for uptake of 14C-gabapentin into KB cells by BCH and gabapentin. FIG. 5 shows uptake of different concentrations of gabapentin into KB cells and that gabapentin uptake is inhibited by BCH. FIG. 6 shows measurement of LAT1 transport by an exchange assay. FIG. 7 shows an efflux transporter ATPase activity assay using membrane preparations containing the PgP efflux transporter and the PgP substrate verapamil. FIG. 8 shows an efflux transporter competition assay using the reporter molecule calcein-AM and the PgP substrate verapamil. FIG. 9 shows a direct efflux transport assay using a polarized monolayer of MDCK cells transfected with a tetracycline-inducible PgP expression construct. FIG. 10 shows (A) competition assays using HEK-TREx-LAT1-4F2hc cells with 3H-gabapentin as a substrate and gabapentin, phenylalanine and L-Dopa as competitors, and (B) a summary of the results in each assay including the assay variability (% error). FIG. 11 shows (A) exchange assays using HEK-TREx-LAT1-4F2hc cells with 3H-gabapentin as a labeled substrate and gabapentin, phenylalanine and L-Dopa as substrates, and (B) a summary of the results in each assay including the assay variability (% error). FIG. 12 shows direct uptake assays using HEK-TREx-LAT1-4F2hc cells with gabepentin, phenylalanine and L-Dopa as substrates. (A) Dose-response (left) and specific uptake (right) of gabapentin into cells induced (+ TET) or uninduced (no TET) to express hLAT1. (B) Dose-response (left) and specific uptake (right) of phenylalanine into cells induced (+ TET) or uninduced (no TET) to express hLAT1. (C) Dose-response (left) and specific uptake (right) of L-Dopa into cells induced (+ TET) or uninduced (no TET) to express hLAT1. Specific uptake was determined by subtracting the values obtained in cells not induced to express hLAT1 from those in the induced cells and graphed vs. the test concentration of each substrate. (D) A summary of the results in each assay including the assay variability (% error). FIG. 13 shows uptake of different concentrations of α-methyl-Dopa into HEK-TREx-LAT1-4F2hc cells induced (+ TET) or uninduced (no TET) to express hLAT1. DEFINITIONS “Transport by passive diffusion” refers to transport of an agent that is not mediated by a specific transporter protein. An agent that is substantially incapable of passive diffusion has a permeability across a standard cell monolayer (e.g., Caco-2 or MDCK cells or an artificial bilayer (PAMPA)) of less than 5×10−6 cm/sec, and usually less than 1×10−6 cm/sec in the absence of an efflux mechanism. A “substrate” of a transporter protein is a compound whose uptake into or passage through the plasma membrane of a cell is facilitated at least in part by a transporter protein. The term “ligand” of a transporter protein includes compounds that bind to the transporter protein. Some ligands are transported and are thereby also substrates. Some ligands inhibit or antagonize transport of a substrate by the transporter protein. Some ligands bind in a manner non-competitive with substrates and modulate the transport of substrates by the transporter protein. The term “neuropharmaceutical agent” is used to describe a compound that has or may have a pharmacological activity in the treatment or prophylaxis of a neurological disorder. Neuropharmaceutical agents include compounds that are known drugs, compounds for which pharmacological activity has been identified but which are undergoing further therapeutic evaluation, and compounds that are members of collections and libraries that are to be screened for a pharmacological activity. The neuropharmaceutical agent can be a compound having a therapeutic, prophylactic or cytotoxic effect on a neurological disease including any condition which affects biological functioning of the central nervous system. Examples of neurological diseases include cancer (e.g., brain tumors), Acquired Immune Deficiency Syndrome (AIDS), stroke, epilepsy, Parkinson's disease, multiple sclerosis, neurodegenerative disease, trauma, depression, Alzheimer's disease, migraine, pain, or a seizure disorder. Classes of neuropharmaceutical agents include proteins, antibiotics, adrenergic agents, anticonvulsants, small molecules, nucleotide analogs, chemotherapeutic agents, anti-trauma agents, peptides and other classes of agents used in treatment or prophylaxis of a neurological disorder. Examples of proteins include CD4 (including soluble portions thereof), growth factors (e.g., nerve growth factor and interferon), dopamine decarboxylase and tricosanthin. Examples of antibiotics include amphotericin B, gentamycin sulfate, and pyrimethamine. Examples of adrenergic agents (including blockers) include dopamine and atenolol. Examples of chemotherapeutic agents include adriamycin, methotrexate, cyclophosphamide, etoposide, and carboplatin. An example of an anticonvulsant which can be used is valproate and an anti-trauma agent which can be used is superoxide dismutase. Examples of peptides are somatostatin analogues and enkephalinase inhibitors. Nucleotide analogs which can be used include azido thymidine (hereinafter AZT), dideoxy Inosine (ddI) and dideoxy cytodine (ddc). The term “agent” is used to describe a compound that has or may have a pharmacological activity. Agents include compounds that are known drugs, compounds for which pharmacological activity has been identified but which are undergoing further therapeutic evaluation, and compounds that are members of collections and libraries that are to be screened for a pharmacological activity. A “pharmacological” activity means that an agent exhibits an activity in a screening system that indicates that the agent is or may be useful in the prophylaxis or treatment of a disease. The screening system can be in vitro, cellular, animal or human. Agents can be described as having pharmacological activity notwithstanding that further testing may be required to establish actual prophylactic or therapeutic utility in treatment of a disease. An agent is “orally active” if it can exert a pharmacological activity when administered via an oral route. A “peripheral tissue” means a tissue other than the CNS. A “conjugate” refers to a compound comprising a neuropharmaceutical agent or imaging component and a chemical moiety bound thereto, which moiety by itself or in combination with the neuropharmaceutical agent or imaging component renders the conjugate a substrate for active transport, for example rendering the conjugate to be a substrate for a transporter protein. The chemical moiety may or may not be subject to cleavage from the neuropharmaceutical agent or imaging component upon uptake and metabolism of the conjugate in the patient's body. In other words, the moiety may be cleavably bound to the neuropharmaceutical agent or imaging component or non-cleavably bound to the neuropharmaceutical agent or imaging component. The bond can be a direct (i.e., covalent) bond or the bond can be through a linker. In cases where the bond/linker is cleavable by metabolic processes, the neuropharmaceutical agent or imaging component, or a further metabolite of the neuropharmaceutical agent or imaging component, is the therapeutic or imaging entity. In cases where the bond/linker is not cleavable by metabolic processes, the conjugate itself is the therapeutic or imaging entity. Most typically, the conjugate comprises a prodrug having a metabolically cleavable moiety, where the conjugate itself does not have pharmacological activity but the component to which the moiety is cleavably bound does have pharmacological activity. Typically, the moiety facilitates therapeutic use of the neuropharmaceutical agent or imaging component by promoting uptake of the conjugate via a transporter. Thus, for example, a conjugate comprising a neuropharmaceutical agent and a conjugate moiety may have a Vmax for a transporter that is at least 2, 5, 10, 20, 50 or 100-fold higher than that of the neuropharmaceutical agent or imaging component alone. A conjugate moiety can itself be a substrate for a transporter or can become a substrate when linked to the neuropharmaceutical agent or imaging component. Examples of preferred conjugate moieties are tryptophan, leucine, methionine, phenylalanine, Bicyclohexane amino acid, L-dopa, gabapentin, and baclofen. Thus, a conjugate formed from a neuropharmaceutical agent or imaging component and a conjugate moiety can have higher CNS uptake activity than either the neuropharmaceutical agent, the imaging component, or the conjugate moiety alone. A “neuropharmacological” activity means that a neuropharmaceutical agent exhibits an activity in a screening system that indicates that the neuropharmaceutical agent is or may be useful in the prophylaxis or treatment of a neurological disease. The screening system can be in vitro, cellular, animal or human. Neuropharmaceutical agents can be described as having neuropharmacological activity notwithstanding that further testing may be required to establish actual prophylactic or therapeutic utility in treatment of a disease. Vmax and Km of a compound for a transporter are defined in accordance with convention. Vmax is the number of molecules of compound transported per second at saturating concentration of the compound. Km is the concentration of the compound at which the compound is transported at half of Vmax. When the goal is to transport an agent, conjugate or conjugate moiety into the CNS, a high Vmax for an influx transporter such as LAT1 is generally desirable. Likewise for the same goal, a low value of Km is typically desirable for transport of a compound present at low blood concentrations. In some cases a high value of Km is acceptable for the transport of compounds present at high concentrations in the blood. For these reasons, the intrinsic capacity of a compound to be transported by a particular transporter is usually expressed as the ratio Vmax of the compound/Vmax of a reference compound known to be a substrate for the transporter. Vmax is affected both by the intrinsic turnover rate of a transporter (molecules/transporter protein) and transporter density in the plasma membrane, which depends on expression level. In certain instances, the goal is to avoid transport into the CNS. In these instances, low Vmax for all influx transporters and a high Vmax for all efflux transporters expressed in the blood brain barrier is desirable. “EC50”, or “effective concentration 50”, is a measurement of the substrate concentration that results in a turnover rate 50% of the maximal turnover rate for the substrate (0.5 Vmax). A plasma membrane containing a monolayer of cells in physical contact with each other and having different sets of proteins embedded in the plasma membranes facing either side of the monolayer is described as being “polarized”. For example, brain microvessel endothelial cells in the blood brain barrier have a luminal side facing capillaries and exposed to blood, and an abluminal side facing cells of the central nervous system and exposed to cerebrospinal fluid. The luminal plasma membrane contains a different set of transmembrane and membrane-associated components than the abluminal plasma membrane of the same cell. Brain microvessel endothelial cells in culture can also be polarized, where the cells form a monolayer in culture that has a luminal and abluminal side. MDCK cells, when grown on filter membranes in transwell dishes, form a polarized monolayer in which one side of the monolayer is the apical side and the other is the basolateral side. “Sustained release” refers to release of a therapeutic or prophylactic amount of a drug or an active metabolite thereof over a period of time that is longer than a conventional formulation of the drug. For oral formulations, the term “sustained release” typically means release of the drug within the GI tract lumen over a period of from about 2 to about 30 hours, more typically over a period of about 4 to about 24 hours. Sustained release formulations achieve therapeutically effective concentrations of the drug in the systemic blood circulation over a prolonged period of time relative to that achieved by oral administration of a conventional formulation of the drug. “Delayed release” refers to release of the drug or an active metabolite thereof into the gastrointestinal lumen after a delay time period, typically a delay of about 1 to about 12 hours, relative to that achieved by oral administration of a conventional formulation of the drug. The phrase “specifically binds” when referring to a substrate or ligand of a LAT1 transporter refers to a specific interaction between a substrate or ligand and the LAT1 transporter in which the substrate or ligand binds preferentially with a LAT1 transporter and does not bind in a significant amount to most or any other proteins present in a biological sample. A substrate or ligand that specifically binds to a LAT1 transporter often has an association constant of 10-103 M−1, 105 M−1, 106 M−1 or 107 M−1, preferably 108 M−1 to 109 M−l or higher. However, some substrates or ligands of LAT1 transporters have much lower affinities and yet the binding can still be shown to be specific. Substrates of LAT1 can specifically bind to LAT1 and other proteins such as efflux transporters without specifically binding to other proteins. “Papp”, or “apparent permeability”, is a value that reflects the permeability of a test compound through a cell layer such as a polarized monolayer. The equation for determining Papp is as follows: P app = V · ⅆ C A · C 0 · ⅆ t ⁢ ( cm ⁢ / ⁢ sec ) where, V=volume of receiving chamber (in cm3, i.e., ml); dC/dt=steady state rate of appearance of applied compound in receiving chamber after primary lag time (in μM/sec); C0=concentration of compound in the donor chamber (in μM) A=area of the cell layer (in cm2) “Allelic variants” at the DNA level are the result of genetic variation between individuals of the same species. Some allelic variants at the DNA level that cause substitution, deletion or insertion of amino acids in proteins encoded by the DNA result in corresponding allelic variation at the protein level. “Cognate forms” of a gene refers to variation between structurally and functionally related genes between species. For example, the human gene showing the greatest sequence identity and closest functional relationship to a mouse gene is the human cognate form of the mouse gene. For sequence comparison, typically one sequence acts as a reference sequence, to which test sequences are compared. When using a sequence comparison algorithm, test and reference sequences are input into a computer, subsequence coordinates are designated, if necessary, and sequence algorithm program parameters are designated. The sequence comparison algorithm then calculates the percent sequence identity for the test sequence(s) relative to the reference sequence, based on the designated program parameters. Optimal alignment of sequences for comparison can be conducted, e.g., by the local homology algorithm of Smith & Waterman, Adv. Appl. Math. 2:482 (1981), by the homology alignment algorithm of Needleman & Wunsch, J. Mol. Biol. 48:443 (1970), by the search for similarity method of Pearson & Lipman, Proc. Nat'l. Acad. Sci. USA 85:2444 (1988), by computerized implementations of these algorithms (GAP, BESTFIT, FASTA, and TFASTA in the Wisconsin Genetics Software Package, Genetics Computer Group, 575 Science Dr., Madison, Wis.), or by visual inspection (see generally Ausubel et al., supra). Another example of an algorithm that is suitable for determining percent sequence identity and sequence similarity is the BLAST algorithm, which is described in Altschul et al., J. Mol. Biol. 215:403-410 (1990). Software for performing BLAST analyses is publicly available through the National Center for Biotechnology Information web site. This algorithm involves first identifying high scoring sequence pairs (HSPs) by identifying short words of length W in the query sequence, which either match or satisfy some positive-valued threshold score T when aligned with a word of the same length in a database sequence. T is referred to as the neighborhood word score threshold (Altschul et al., supra.). These initial neighborhood word hits act as seeds for initiating searches to find longer HSPs containing them. The word hits are then extended in both directions along each sequence for as far as the cumulative alignment score can be increased. Cumulative scores are calculated using, for nucleotide sequences, the parameters M (reward score for a pair of matching residues; always >0) and N (penalty score for mismatching residues; always <0). For amino acid sequences, a scoring matrix is used to calculate the cumulative score. Extension of the word hits in each direction are halted when: the cumulative alignment score falls off by the quantity X from its maximum achieved value; the cumulative score goes to zero or below, due to the accumulation of one or more negative-scoring residue alignments; or the end of either sequence is reached. For identifying whether a nucleic acid or polypeptide is within the scope hereof, the default parameters of the BLAST programs are suitable. The BLASTN program (for nucleotide sequences) uses as defaults a word length (W) of 11, an expectation (E) of 10, M=5, N=−4, and a comparison of both strands. For amino acid sequences, the BLASTP program uses as defaults a word length (W) of 3, an expectation (E) of 10, and the BLOSUM62 scoring matrix. The TBLASTN program (using protein sequence for nucleotide sequence) uses as defaults a word length (W) of 3, an expectation (E) of 10, and a BLOSUM 62 scoring matrix. (see Henikoff & Henikoff, Proc. Natl. Acad. Sci. USA 89:10915 (1989)). In addition to calculating percent sequence identity, the BLAST algorithm also performs a statistical analysis of the similarity between two sequences (see, e.g., Karlin & Altschul, Proc. Nat'l. Acad. Sci. USA 90:5873-5787 (1993)). One measure of similarity provided by the BLAST algorithm is the smallest sum probability (P(N)), which provides an indication of the probability by which a match between two nucleotide or amino acid sequences would occur by chance. For example, a nucleic acid is considered similar to a reference sequence if the smallest sum probability in a comparison of the test nucleic acid to the reference nucleic acid is less than about 0.1, more preferably less than about 0.01, and most preferably less than about 0.001. DETAILED DESCRIPTION I. General LAT1 is shown herein to be expressed at high levels in brain microvessel endothelial cells. This finding can be used to generate or isolate conjugates and agents having neuropharmacological or imaging activity useful for treatment, prophylaxis or diagnosis of neurological diseases. The invention provides methods of identifying agents, conjugates or conjugate moieties that are substrates for LAT1. For therapeutic purposes, agents or conjugates having inherent neuropharmacologic activity can be screened to determine whether they are substrates for LAT1. Alternatively, a conjugate moiety lacking such activity can be screened, and linked to a neuropharmacologic agent after screening. Agents or conjugates that both have neuropharmacologic activity and are substrates for LAT1 are preferentially transported into the CNS via LAT1 transporters after administration to a patient. Such an agent or conjugate by itself or in combination with another agent is effective in treatment or prophylaxis of a neurological disease. An analogous approach is used for imaging features of the brain. Agents and conjugates that have an imaging component and are substrates for LAT1 are preferentially transported into the CNS via LAT1 transporters. The imaging component is then detected by various methods such as detecting radioactive decay of the imaging component. The agents and conjugates can be used to image brain tumors overexpressing the LAT1 transporter. Optionally, the agents or conjugates have inherent affinity for, or are provided with a conjugate moiety that confers affinity for, a particular antigen or cell type within the brain. For example, the agents or conjugates can be provided with a targeting moiety to Aβ to allow imaging of plaques in Alzheimer's patients. II. LAT1 transporter The family of amino acid transporter/permeases (AAPs) contains at least 13 members in humans (SLC7A1-13). AAP transporters have 12 putative transmembrane domains, with both the amino and carboxy termini located on the cytoplasmic side. A sub-family of AAPs (referred to as LAT1 and LAT2) is specialized for the transport of large aromatic neutral amino acids. LAT1 and LAT2 transport a variety of neutral amino acids (leucine, valine, isoleucine, phenylalanine, tryptophan and methionine) and amino acid analogs (L-dopa, gabapentin, and bicyclohexane amino acid (BCH)). Both LAT1 and LAT2 co-assemble with the glycoprotein 4F2HC. LAT1 and LAT2 transporters are obligate exchange transporters. Thus, for each amino acid transported across the plasma membrane into the cell, another amino acid is effluxed out of the cell. It is now shown that LAT1 is highly expressed in brain microvessel endothelial cells. LAT1 is expressed at a level nearly 10-fold higher than other LAT family transporters with similar substrate specificity. It is desirable to generate agents, conjugates, and conjugate moieties for transport into the CNS that have activity for LAT1 due to this high expression level. The GenBank accession number for human LAT1 is NM—003983 (SEQ ID NO:1). The GenBank accession number for human 4F2HC is AB018010 (SEQ ID NO:2). Unless otherwise apparent from the context, reference to a transporter includes the amino acid sequence described in or encoded by the GenBank reference numbers NM-003983 and AB018010, and, allelic, cognate and induced variants and fragments thereof retaining essentially the same transporter activity. Usually such variants show at least 90% sequence identity to the exemplary Genbank nucleic acid or amino acid sequence. III. Methods of Screening to Identify LAT1 Substrates Agents known or suspected to have a neuropharmaceutical activity or to comprise an imaging component can be screened directly for their capacity to act as substrates of LAT1. Alternatively, conjugate moieties can be screened as substrates, and the conjugate moieties are then linked to a neuropharmaceutical agent or imaging component. In such methods, the conjugate moieties can optionally be linked to a neuropharmaceutical agent or imaging component, or other molecule during the screening process. If another molecule is used in place of a neuropharmaceutical agent or imaging component, the molecule can be chosen to resemble the structure of a neuropharmaceutical agent or imaging component ultimately intended to be linked to the conjugate moiety for neuropharmaceutical use. Alternatively, a conjugate moiety can be screened for a substrate activity alone and linked to a neuropharmaceutical agent or imaging component after screening. LAT1 proteins preferably transport L-amino acids with bulky (>2 atoms), uncharged and hydrophobic or aromatic side-chains. LAT1 also transports similar D-amino acids with lower affinity than L-amino acids. Additionally, LAT1 transports several gamma amino acids such as gabapentin and cyclic alpha amino acids such as bicyclohexane amino acid. The structures of each compound listed in Table 1 are depicted in FIG. 1. TABLE 1 Reported Affinity for SUBSTRATES LAT1 Tryptophan 0.35 mM Leucine 0.22 mM Methionine 0.25 mM Phenylalanine 0.025 mM Bicyclohexane amino acid 0.18 mM L-dopa 0.05 mM Gabapentin 0.15 mM Baclofen 0.25 mM Tryptophan, leucine, methionine, phenylalanine, bicyclohexane amino acid, L-dopa, α-methyl dopa, gabapentin, and baclofen are examples of LAT1 substrates that are candidates for conjugation to therapeutic neuropharmaceutical agents, cytotoxic neuropharmaceutical agents and imaging components. In some screening methods, the cells are transfected with DNA encoding the LAT1 transporter. HEK (human embryonic kidney) and CHO (Chinese hamster ovary) cells, for example, are suitable for transfection. Oocytes can be injected with LAT1 cRNA to express LAT1 transporter. In some methods, the only transporter expressed by the cells is the LAT1 transporter. In other methods, cells express LAT1 in combination with other transporters. In still other methods, agents, conjugate moieties or conjugates are screened on different cells expressing different transporters. Agents, conjugate moieties or conjugates can be screened either for specificity for the LAT1 transporter or for transport into cells endogenously expressing a plurality of transporters. In some methods, the results of a screening method (e.g., a competition uptake, exchange or direct uptake assay) using a cell expressing the LAT1 transporter can be compared with the results of a control cell(s) lacking the LAT1 transporter or in the presence of a specific inhibitor of the LAT1 transporter. In some methods, cells endogenously expressing the LAT1 transporter are used. Brain microvessel endothelial cells, for example, endogenously express the LAT1 transporter, as demonstrated in Example 1. Agents, conjugate moieties or conjugates can be screened for transport into cultured brain microvessel endothelial cells. Passaging cultures of brain microvessel endothelial cells typically causes the cells to lose differentiation characteristics such as the ability to form tight junctions. The propensity of passaged cells to lose differentiation characteristics can be avoided through the use of brain microvessel endothelial cells that are transformed with an SV40 large T antigen. See Terasaki et al., Drug Discovery Today 8:944-954 (2003). Inducible expression of the SV40 large T antigen allows cells to divide when the antigen is expressed and differentiate when the antigen is not expressed. Brain microvessel endothelial cells can be isolated from animals transgenic for the SV40 large T antigen, which can be expressed in a temperature-sensitive fashion. The cells are stimulated to divide by being cultured at the temperature at which the antigen is expressed. Once the cells have formed a monolayer, they are placed at a temperature at which the antigen is not expressed, causing the cells to stop dividing and differentiate. Differentiation results in the formation of tight junctions and the polarization of the plasma membranes. Monolayers of polarized cells are tested for the ability to transport agents, conjugates or conjugate moieties. In other methods, the ability to transport agents, conjugates or conjugate moieties is measured by administering the agent, conjugate, or conjugate moiety to a peripheral tissue of an animal. The amount of agent, conjugate, or conjugate moiety that passes through the blood brain barrier into the brain of the animal is measured either in the cerebral spinal fluid (CSF) or in whole brain tissue following brain perfusion with solution to remove compound from the brain vasculature. Generally, the brain penetration of the drug is reported as the ratio of the unbound compound in brain tissue or CSF to the unbound compound in the blood. In some methods, the measurement can be made on a specimen from the animal or in situ. In some methods, the ability of an agent, conjugate or conjugate moiety to specifically bind to a LAT1 transporter is tested. A known substrate of the LAT1 transporter and the agent, conjugate or conjugate moiety are added to cells expressing the LAT1 transporter. The amount or rate of transport of the substrate in the presence of the agent, conjugate or conjugate moiety is compared to the amount or rate of transport of the agent, conjugate or conjugate moiety in the absence of the test compound. If the amount or rate of transport of the substrate is decreased by the presence of the agent, conjugate or conjugate moiety, the agent, conjugate or conjugate moiety binds the LAT1 transporter. Agents, conjugates or conjugate moieties that bind the LAT1 transporter can be further analyzed to determine if they are transported by the LAT1 transporter or only adhere to the exterior of the transporter. Agents, conjugates or conjugate moieties that are transported by the LAT1 transporter can be further tested to determine if they are transported from one side of a monolayer of polarized cells to the other side, such as a monolayer of brain microvessel endothelial cells. Agents and conjugates having neuropharmaceutical activity and that that are transported by the LAT1 transporter can be used to form pharmaceutical compositions. Conjugate moieties that are transported by the LAT1 transporter can be linked to a therapeutic or cytotoxic neuropharmaceutical agent or an imaging component. Transport of a compound into a cell can be detected by detecting a signal from within a cell from any of a variety of reporters. The reporter can be as simple as a label such as a fluorophore, a chromophore, or a radioisotope. Confocal imaging can also be used to detect internalization of a label as it provides sufficient spatial resolution to distinguish between fluorescence on a cell surface and fluorescence within a cell; alternatively, confocal imaging can be used to track the movement of compounds over time. In another approach, transport of a compound is detected using a reporter that is a substrate for an enzyme expressed within a cell. Once the compound is transported into the cell, the substrate is metabolized by the enzyme and generates an optical signal that can be detected. Light emission can be monitored by commercial PMT-based instruments or by CCD-based imaging systems. In addition, assay methods utilizing liquid chromatography-mass spectroscopy (LC-MS-MS) detection of the transported compounds or electrophysiological signals indicative of transport activity are also employed. Mass spectroscopy is a powerful tool because it allows detection of very low concentrations of almost any compound, especially molecules for which a radiolabeled version is not available. It can also be used to distinguish substrates from nontransported ligands. These same detection methods can be used to determine if a compound is transported from one side of a monolayer of polarized cells to the other side by administering the compound to one side of the monolayer and sampling the media on the other side of the monolayer after a predetermined period of time. In some methods, multiple agents, conjugates or conjugate moieties are screened simultaneously and the identity of each agent, conjugate or conjugate moiety is tracked using tags linked to the agents, conjugates or conjugate moieties. In some methods, a preliminary step is performed to determine binding of an agent, conjugate or conjugate moiety to a transporter. Although not all agents, conjugates or conjugate moieties that bind to a transporter are substrates of the transporter, observation of binding is an indication that allows one to reduce the number of candidates from an initial repertoire. In some methods, the transport rate of an agent, conjugate or conjugate moiety is tested in comparison with the transport rate of a reference substrate for that transporter. For example, gabapentin, a natural substrate of LAT1, can be used as a reference. The comparison can be performed in separate parallel assays in which an agent, conjugate or conjugate moiety under test and the reference substrate are compared for uptake on separate samples of the same cells. Alternatively, the comparison can be performed in a competition format in which an agent, conjugate or conjugate moiety under test and the reference substrate are applied to the same cells. Typically, the agent, conjugate or conjugate moiety and the reference substrate are differentially labeled in such assays. In comparative assays, the Vmax of an agent, conjugate or conjugate moiety tested can be compared with that of a reference substrate. If an agent, conjugate moiety or conjugate has a Vmax of at least 1%, 5%, 10%, 20%, and most preferably at least 50% of the reference substrate for the LAT1 transporter, then the agent, conjugate moiety or conjugate is also a substrate for the LAT1 transporter. If transport of the agent, conjugate moiety or conjugate into the CNS is desired, a higher Vmax of the agent, conjugate moiety or conjugate relative to that of the reference substrate is preferred. Therefore, agents, conjugate moieties or conjugates having Vmax's of at least 1%, 5%, 10%, 20%, 50%, 100%, 150% or 200% (i.e., two-fold) of the Vmax of a reference substrate (e.g., gabapentin) for the transporter are screened in some methods. The components to which conjugate moieties are linked can by themselves show little or no detectable substrate activity for the transporter (e.g., Vmax relative to that of a reference substrate of less than 0.1% or 1%). Preferred agents, conjugates or conjugate moieties have a Vmax for LAT1 that is at least 5% of the Vmax for LAT1 of gabapentin. Preferred conjugates comprising a neuropharmaceutical agent or imaging component linked to a conjugate moiety preferably have a greater Vmax for LAT1 than the neuropharmaceutical agent or imaging component alone. Having determined that an agent, conjugate or conjugate moiety is a substrate for LAT 1, a further screen can be performed to determine its therapeutic activity in treatment or prophylaxis of a disease, or its cytotoxic activity against brain tumor cells. Usually the disease is neurological (i.e., the pathology occurs in the CNS). Alternatively, the diseased tissue is non-CNS tissue but is responsive to treatment by an agent that exerts a pharmacological effect on the CNS that in turn causes an effect on the diseased non-CNS tissue, such as an effect caused by the release of hormones from the CNS. Diseases of this type are also considered to be diseases of the CNS unless otherwise apparent from context. If the agent, conjugate or conjugate moiety does not have inherent therapeutic or cytotoxic activity, it is first linked to another chemical component having such therapeutic or cytotoxic properties. The agent, conjugate or conjugate moiety is then contacted with cells expressing LAT1. The contacting can be performed either on a population of cells in vitro, or the brain microvessel endothelial cells of a test animal via administration of the agent, conjugate or conjugate moiety to a test animal. The therapeutic or cytotoxic activity of the agent, conjugate or conjugate moiety is then determined from established protocols for that particular disease. Optionally, the effect of the agent, conjugate or conjugate moiety can be compared with a placebo. A further screen can be performed to determine toxicity of the agent, conjugate, or conjugate moiety to normal cells. The agent, conjugate or conjugate moiety is administered to a laboratory animal that is preferably in an undiseased state. Various tissues of the animal, such as liver, kidney, heart and brain are then examined for signs of pathology. Cells in the animal can
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