Neuromodulatory effects of acetylcholine and serotonin on the sensitivity of leech mechanoreceptors

Abstract

1. Each segmental ganglion of the leech Hirudo medicinatis contains 6 touch (T) cells, 4 pressure (P) cells and 4 nociceptive (N) cells. The receptive terminals of these cells innervate the skin in discrete areas. These cells are known to have extrasynaptic receptors. 2. We tested the effect of transmitter substances present in leech CNS on the sensitivity of T and P cells to mechanical stimuli. Substances tested included octopamine, FMRFamide, proctolin, substance P, glutamate, GABA, acetylcholine and serotonin. 3. Only acetylcholine and serotonin had consistent effects. Serotonin (1 × 10 −3 M) increased the number of action potentials of T cells elicited by a standard stimulus. Serotonin (1 × 10 −4 M) and acetylcholine (1 × 10 −3 M) increased the number and frequency of action potentials in P cells.