Localization, physiology, and modulation of a molluskan dopaminergic synapse

We investigated the location, physiology, and modulation of an identified synapse from the central nervous system (CNS) of the mollusk Lymnaea stagnalis. Specifically, the excitatory synapse from interneuron right pedal dorsal one (RPeD1) to neurons visceral dorsal two and three (VD2/3) was examined. The gross and fine morphology of these neurons was determined by staining with Lucifer yellow or sulforhodamine. In preparations where RPeD1 was stained with Lucifer yellow and VD2/3 with sulforhodamine, the axon collaterals occupied similar regions, suggesting that these neurons make physical contact in the CNS. Digital confocal microscopy of these preparations revealed that presynaptic varicosities made apparent contact (synapses) with smooth postsynaptic axon collaterals. 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 dopamine as a neurotransmitter. The RPeD1 --> VD2/3 excitatory postsynaptic potential (EPSP) and the VD2/3 bath-applied dopamine (100-microM) response displayed a similar decrease in input resistance and a similar predicted reversal potential (-31 vs. -26 mV), indicating that the synapse and exogenous dopamine activate the same conductance. Finally, bath-applied serotonin (10 microM) rapidly and reversibly depressed the RPeD1 --> VD2/3 synapse but did not affect the VD2/3 bath-applied dopamine (100-microM) response, suggesting a presynaptic locus of action for serotonin. The effect of serotonin was not associated with any changes to the pre- or postsynaptic membrane potential and input resistance, or the presynaptic action potential half-width. The RPeD1 --> VD2/3 synapse provides an opportunity to examine the anatomy and physiology of transmission, and is amenable to the study of neuromodulation.