Ophthalmate tripeptide is a signaling molecule that regulates striatal neurotransmission

Dopamine has long been regarded as the key neurotransmitter governing motor function through the direct and indirect striatal outflow pathways. However, the tripeptide ophthalmate (ophthalmic acid, OA), a glutathione analog, has recently emerged as another regulator of motor function. While the neural mechanisms by which OA regulates motor function remain unknown, we explored how OA behaves as a striatal neuromodulator. Using radiotracer-based uptake and release assays in mouse striatal tissue, combined with whole-cell electrophysiological recordings, we found that OA is taken up through a saturable, glutathione-competitive transport mechanism and is released in a Ca{superscript 2}-dependent manner upon depolarization, consistent with regulated exocytotic release. OA enhanced depolarization-evoked release of {gamma}-aminobutyric acid (GABA) but not glutamate, indicating selective modulation of distinct neurotransmitter systems. OA and dopamine reciprocally regulated one another: OA enhanced dopamine release, while D2 dopamine receptor activation suppressed OA release. Whole-cell recordings from medium spiny neurons (MSNs) showed that OA increased the amplitude of evoked AMPA receptor-mediated currents and shifted short-term plasticity from facilitation toward depression at excitatory synapses onto direct-pathway MSNs (dMSNs). These findings are consistent with an increase in presynaptic glutamate release probability, while sparing synapses onto indirect-pathway MSNs (iMSNs). Together, these findings establish OA as a striatal neuromodulator that interacts with other neurotransmitter systems and preferentially potentiates direct-pathway transmission. This novel discovery identifies a candidate mechanism within the basal ganglia circuitry that governs motor control, with major relevance to Parkinsons disease and other movement disorders.