Neuromodulation of central inhibition on peripheral mechanosensory afferents

100 g reserpine, suspended in sunflower oil, was injected into the crop of six leeches (Hirudo medicinalis). Five leeches were injected with oil alone. The efficacy of delivering reserpine in this manner was assessed by measuring swimming bout duration. Four days after injection, swimming bouts were significantly longer in reserpine-treated leeches than in oil-treated or normal leeches. The amplitude of excitatory potentials elicited in velocity sensitive skin mechanoreceptors (T cells) by action potentials in mechanosensory cells which respond to sustained deformation of the skin (P cells) was no different in reserpine-treated leeches than in untreated leeches. The amplitude of inhibitory potentials in these T cells, produced by interneurons excited by P cells, was significantly larger in reserpine-treated leeches. Depletion of monoamines enhances inhibition in T cells. This is consistent with the established roles played by monoamines in promoting leech swimming and feeding behaviour.