Abstract
1. 1. Intracellular recordings were made from Retzius cells in the segmental ganglia of the leech, Hirudo medicinalis, and the leech, Haemopsis sp. 5-Hydroxytryptamine (5-HT) hyperpolarized the membrane potential of these cells. 2. 2. The inhibitory effect of 5-HT was due to an increase in chloride permeability of the cell membrane. No evidence was found for a potassium component in this response. The internal chloride concentration was calculated to be 8·4 mM which gave a chloride equilibrium potential of — 70 mV. 3. 3. Part of the membrane potential was due to an electrogenic pump but 5-HT did not act by stimulating this system. 4. 4. By the use of a specific 5-HT antagonist, mianserin, and by desensitization experiments it was concluded that 5-HT and strychnine (which also had an inhibitory action on these cells) acted via different receptors. 5. 5. It is concluded that 5-HT acted directly on the Retzius cell membrane to increase chloride conductance.
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