Dopaminergic transmission in Lymnaea: Physiology, pharmacology, and plasticity

Synaptic transmission was investigated in the central nervous system (CNS) of the freshwater snail, Lymnaea stagnalis. The presynaptic neuron chosen was neuron Right Pedal Dorsal one (RPeD1), a respiratory interneuron which makes chemical connections with multiple postsynaptic neurons.; To examine sites of contact, RPeD1 and one of its postsynaptic cells, Visceral Dorsal two and three (VD2/3) were labelled with Lucifer Yellow and sulforhodamine. Digital confocal microscopy revealed sites of putative contact, presumably synapses.; Regarding physiology, the excitatory RPeD1 {dollar}to{dollar} VD2/3 synapse had a latency of {dollar}sim{dollar}24 ms and its excitatory postsynaptic potential (EPSP) reversed at {dollar}{lcub}sim{rcub}{lcub}-{rcub}25{dollar} mV. The inhibitory synapse from RPeD1 to another postsynaptic cell, Visceral Dorsal four (VD4), had a latency of {dollar}sim{dollar}40 ms, and its inhibitory postsynaptic potential reversed at {dollar}{lcub}sim{rcub}{lcub}-{rcub}90{dollar} mV.; Previous work indicated that RPeD1 contains dopamine. Appropriately, bath or pressure application of dopamine and general dopamine agonists to its postsynaptic cells mimicked the effects of RPeD1 stimulation. The reversal potentials of the dopamine response in VD2/3 and VD4 were similar to their evoked synaptic potentials. The VD2/3 dopamine response proved to be G-protein dependent. The D-2 dopamine receptor antagonist, ({dollar}pm{dollar}) sulpiride, blocked both synaptic transmission from RPeD1 to its postsynaptic cells, and the effects of bath- and pressure-applied dopamine.; The neuromodulatory effects of serotonin, a potential neuromodulator were tested. Serotonin rapidly and reversibly depressed the RPeD1 {dollar}to{dollar} VD2/3 EPSP, but did not alter the response of VD2/3 to dopamine, suggesting a presynaptic action.; Regarding variability, the signs of the RPeD1 {dollar}to{dollar} VD2/3 and RPeD1 {dollar}leftrightarrow{dollar} VD4 synapses were variable. The RPeD1 {dollar}to{dollar} VD2/3 synapse was either excitatory or not connected, while the RPeD1 {dollar}to{dollar} VD4 synapses was inhibitory, biphasic, or not connected. Furthermore, the reciprocal synapse from VD4 to RPeD1 was also variable, being inhibitory, biphasic, depolarizing, excitatory, or not connected. The possibility that synapses change sign was tested by placing the CNS in 24 hour organ culture. All synapses changed sign; furthermore, when cultured in brain-conditioned medium, the VD4 {dollar}to{dollar} RPeD1 synapse changed significantly more than in defined medium. The efficacy of the RPeD1 {dollar}to{dollar} VD2/3 synapse was enhanced following organ culture in conditioned medium, and this effect was mimicked by murine nerve growth factor.; This thesis has defined a dopaminergic system and revealed novel aspects of synaptic variability and plasticity.