IDENTIFICATION OF A PUTATIVE MECHANOSENSORY NEURON IN LYMNAEA - CHARACTERIZATION OF ITS SYNAPTIC AND FUNCTIONAL CONNECTIONS WITH THE WHOLE-BODY WITHDRAWAL INTERNEURON

1. In this study, we identified a putative mechanosensory neuron in th e freshwater pond snail Lymnaea stagnalis. This sensory neuron, termed right parietal dorsal 3 (RPD3), mediates part of the whole-body withd rawal behavior via the activation of a withdrawal interneuron. 2. RPD3 is located in the central ring ganglia, where its soma is situated on the dorsal surface of the right parietal ganglion. Intracellular inje ction of the dye Lucifer yellow revealed that RPD3 has both central an d peripheral axonal projections. 3. In isolated-CNS preparations, RPD3 was quiescent. In semiintact preparations, however, a gentle/moderate mechanical touch (by a pair of blunt forceps) to the mantle cavity or columellar musculature elicited action potentials in RPD3 in the abse nce of prepotential activity. Furthermore, mechanical stimulus-induced action potentials in RPD3 persisted in the presence of zero Ca(2+)/hi gh Mg2+ and high Ca2+/high Mg2+ salines. Together, these data suggest that: RPD3 is mast likely to be a primary sensory neuron. 4. In both i solated-CNS and semi-intact preparations, intracellular depolarization of RPD3 excited the whole-body withdrawal interneuron right pedal dor sal 11 (RPeD11). This synaptic connection persisted in tile presence o f high Ca3+ and high Mg2+ saline, suggesting that it is likely to be m onosynaptic. Moreover, when stimulated electrically, the interneuron R PeD11 induced an hyperpolarizing response In RPD3. The possibility of this connection being monosynaptic was not tested, however, in the pre sent study. Together, these data demonstrate that RPD3 excites RPeD11, which in turn may inhibit RPD3 activity. 5. In the semi-intact prepar ation, a mechanical touch to the mantle edge excited RPD3, which in tu rn generated action potentials in RPeD11. Zero Ca2+ saline blocked thi s synaptic connection between RPD3 and RPeD11, suggesting that it is c hemical. 6. To demonstrate that RPD3 was sufficient to induce the with drawal response and that the withdrawal behavior was mediated indirect ly via RPeD11, we made simultaneous intracellular recordings from thes e two neurons while monitoring muscle contractions via a tension trans ducer. Intracellular depolarization of RPDS elicited action potentials in RPeD11, followed by the contraction of the columellar muscle. Simi lar stimulation of RPD3 failed to excite a simultaneously hyperpolariz ed RPeD11 and as a result, no contraction of the columellar muscle occ urred. Direct Intracellular depolarization of RPeD11. however, induced the contraction of the columellar muscle. These data suggest that RPD 3-induced withdrawal behavior is mediated in part via RPeD11.