A central inhibitory action of 5‐hydroxytryptamine in the leech

Previous studies indicated that 5‐HT reduced the number of spontaneous excitatory junctional potentials (ejp's) that occurred in leech body wall muscle cells. The present study confirms these findings and shows that the ejp's arise from impulses in motoneuron L. This study further shows that 5‐HT acts by hyperpolarizing and reducing the membrane resistance of neuron L, thus inhibiting the motoneuron and reducing the frequency of spontaneous ejp's on body wall muscle cells. These effects of 5‐HT are not seen when the ganglion is bathed in a high magnesium solution, a finding that suggests that 5‐HT does not act directly on the membrane of motoneuron L. This study demonstrates that 5‐HT can have a central inhibitory effect on body wall muscle contractions. Previous studies provide evidence that 5‐HT may act as a direct neuromuscular inhibitory transmitter and may also take part in peripheral presynaptic inhibition. Thus, if further studies confirm these suggestions, the well‐known inhibitory effect of 5‐HT on leech body wall muscle is a more complex process than was previously thought.