LOCALIZATION, PHYSIOLOGY, AND MODULATION OF A MOLLUSKAN DOPAMINERGIC SYNAPSE

We investigated the location, physiology, and modulation of an identif ied synapse from the central nervous system (CNS) of the mollusk Lymna ea stagnalis. Specifically, the excitatory synapse from interneuron ri ght pedal dorsal one (RPeD1) to neurons visceral dorsal two and three (VD2/3) was examined. The gross and fine morphology of these neurons w as determined by staining with Lucifer yellow or sulforhodamine. In pr eparations where RPeD1 was stained with Lucifer yellow and VD2/3 with sulforhodamine, the axon collaterals occupied similar regions, suggest ing that these neurons make physical contact in the CNS. Digital confo cal microscopy of these preparations revealed that presynaptic varicos ities made apparent contact (synapses) with smooth postsynaptic axon c ollaterals. The number of putative synapses per preparation was about five to 10. Regarding physiology, the synaptic latency was moderately rapid at 24.1 +/- 5.2 ms. Previous work indicated that RPeD1 uses dopa mine as a neurotransmitter. The RPeD1 --> VD2/3 excitatory postsynapti c potential (EPSP) and the VD2/3 bath-applied dopamine (100 mu M) resp onse displayed a similar decrease in input resistance and a similar pr edicted reversal potential (-31 vs. -26 mV), indicating that the synap se and exogenous dopamine activate the same conductance. Finally, bath -applied serotonin (10 mu M) rapidly and reversibly depressed the RPeD 1 --> VD2/3 synapse but did not affect the VD2/3 bath-applied dopamine (100 mu M) response, suggesting a presynaptic locus of action for ser otonin. The effect of serotonin was not associated with any changes to the pre-or postsynaptic membrane potential and input resistance, or t he presynaptic action potential half-width. The RPeD1 --> VD2/3 synaps e provides an opportunity to examine the anatomy and physiology of tra nsmission, and is amenable to the study of neuromodulation. (C) 1997 J ohn Wiley & Sons, Inc. J Neurobiol.