subunits arranged about a fivefold symmetry axis to form a central ion conducting pore. To date, three GABAA-ρ receptor subunits have been identified in humans: ρ1 () ρ2 () ρ3 () The above three subunits coassemble either to form functional homo-pentamers (ρ15, ρ25, ρ35) or hetero-pentamers (ρ1mρ2n, ρ2mρ3n where m + n = 5). There is also evidence that ρ1 subunits can form hetero-pentameric complexes with GABAA receptor γ2 subunits. Pharmacology There are several pharmacological differences that distinguish GABAA-ρ from GABAA and GABAB receptors. For example, GABAA-ρ receptors are: selectively activated by (+)-CAMP [(+)-cis-2-aminomethylcyclopropane-carboxylic acid] and blocked by TPMPA [(1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid]; not sensitive to the GABAB agonist baclofen nor the GABAA receptor antagonist bicuculline; not modulated by many GABAA receptor modulators such as barbiturates and benzodiazepines, but are modulated selectively by certain neuroactive steroids. Selective Ligands Agonists CACA CAMP GABOB Muscimol Antagonists Mixed GABAA-ρ / GABAB antagonists ZAPA ((Z)-3-[(Aminoiminomethyl)thio]prop-2-enoic acid) SKF-97541 (3-Aminopropyl(methyl)phosphinic acid) CGP-36742 (3-aminopropyl-n-butyl-phosphinic acid) Selective GABAA-ρ antagonists TPMPA (±)-cis-(3-Aminocyclopentyl)butylphosphinic acid (S)-(4-Aminocyclopent-1-enyl)butylphosphinic acid N2O Genetics In humans, GABAA-ρ receptor subunits ρ1 and ρ2 are encoded by the and genes which are found on chromosome 6 whereas the gene for ρ3 is found on chromosome 3. Mutations in the ρ1 or ρ2 genes may be responsible for some cases of autosomal recessive retinitis pigmentosa. References Transmembrane receptors GABA