3 Introduction: Serotonergic signaling is essential for a breadth of processes ranging from homeostasis to executive function and emotional regulation. In mammals, serotonin is thought to play a modulatory role in almost every physiological function. Dysfunction in serotonergic signaling system is associated with anxiety, depression, schizophrenia, migraines, autism, Parkinson's disease and Alzheimer's disease. Due to its importance, the serotonergic system is the target of many pharmaceuticals, such as the antidepressant drug families of monoaminoxidase inhibitors (MAOIs), selective serotonin reuptake inhibitors (SSRIs), and tricyclic antidepressants (TCAs). There are 7 types of serotonin receptors, and a number of subtypes. This diversity is a component of many diseases and provides important therapeutic potential for drugs targeted to individual serotonin receptor subtypes. For example, subtype-selective drugs can treat symptoms of migraine, anxiety, or psychosis, as well as induce hallucinations. In fundamental research, studies are underway to determine the effect of stimulating a serotonin receptor subtype on brain dynamics, to determine the origin of the brain's flexibility to produce different states. Overall, extensive research effort is expended on attempting to understand the role of serotonergic signaling systems in health and disease. However, there is a lack of tools to enable researchers to tease apart the signaling processes in these exquisitely interconnected networks. There are many challenges to solving this problem due to the nature of the central nervous system, one of the most notable is the challenge of recreating the unique kinetics of the chemical synapse, where a bolus of neurotransmitter is rapidly released then quickly cleared by active reuptake machinery. Mimicking this system requires a tool that can provide an incredibly rapid rise in signal, followed by an equally rapid fall. One strategy to produce a rapid rise in a bio-orthogonally controlled signal is via photocaged compounds. Photocages are photoremovable protecting groups that greatly reduce a compound's affinity for its receptor. Removal of this protecting group with the appropriate wavelength and intensity of light can result in the delivery of the active compound in less than a microsecond. A number of compounds important for neuroscience have been caged including serotonin (NPEC-5HT, BHQ-O-5HT, [Ru(bpy)2(PMe3)(5HT)] 2+ ), dopamine (BBHCM-DA, CNB-DA, CNV-DA, NPEC-DA, RuBi-DA), and subtype selective dopamine receptor D2/D3 antagonists dechloroeticlopride and sulpiride. Thus, although dopamine and serotonin have been caged multiple times (including commercial products), there is almost no work caging subtype selective modulators.