Thesis

Neuromodulation in the medicinal leech, Hirudo medicinalis

The primary goal of my thesis project is to classify and characterize mechanisms of neuromodulation in the central nervous system of the medicinal leech, Hirudo medicinafis. In the first set of experiments, N-cells were tested for the presence of autoreceptors to the putative peptide transmitter. There was no effect of A VP on N-cells (2-sample t, p>0.05). Although the statistical power of the test was low, the sample sizes required to detect a significant difference strongly suggest that N-cells do not possess autoreceptors. In a second set of experiments, longitudinal motor neurons (L-cells) were exposed to A VP and recorded from continuously. There was a significant hyperpolarizing effect of A VP on the L-cells (paired Hest=3.26, u=6, p=0.017).This effect was blocked in both High-magnesium and High-calcium salines, implying that the effect of A VP is indirect. This result suggests that the effect of A VP on L-cells is to relax certain defined behaviors, such as bending and swimming. As a means of studying neuromodulation in a sensory input-motor output system, Pressure cells (P-cells) were exposed to lO.oM pilocarpine, and electrical properties measured. There was a highly significant decrease in peak depolarization (paired t= 5.16, u=9, p=0.001) and a highly significant increase in depolarization duration (paired t= 5.16, u=9, p=0.006). This effect was further characterized by investigating the P-Anterior Pagoda synapse. There was no effect of pilocarpine on the P-AP synapse (t-test, p>0.05). These results suggest that AVP's affects are widespread, and provide evidence for other forms of simple learning in the leech.

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