MODULATION OF CONDUCTION BLOCK IN LEECH MECHANOSENSORY NEURONS

Conduction block is a mechanism of activity-dependent neuronal plastic ity, but little is known about its possible neuromodulation. Extensive activity in leech touch (T), pressure (P), and nociceptive (N) mechan osensory neurons results in conduction block of their minor receptive fields. We have examined whether the duration of conduction block coul d be modulated by the serotonergic Retzius neurons or by application o f serotonin (5-HT). Activation of one Retzius cell reduced the duratio n of conduction block in T and P cell posterior fields, but their ante rior fields and N cell fields were unaffected. Perfusion with 5-HT had stronger effects, reducing the duration of conduction block in T, P, and lateral N cells in the posterior fields and either reducing or mor e often enhancing the expression of conduction block in anterior field s. The effects of 5-HT on posterior fields were blocked by the nonspec ific 5-HT antagonist methysergide and were partly suppressed by the 5- HT2 antagonist ketanserin. To determine the site of 5-HT action, the c entral ganglion or peripheral skin was perfused independently. T and t o a greater extent P cells showed a preferential sensitivity to applic ation of 5-HT onto the central ganglion. Interestingly, medial N cells exhibited a progressive decrease in the duration of conduction block during repeated trials (''wind-up'') that was unaffected by 5-HT. We c onclude that secretion of 5-HT by the Retzius cells has a central modu latory effect on the duration of conduction block in T, P, and lateral N cells.