Serotonergic modulation of swimming speed in the pteropod mollusc Clione limacina. II. Peripheral modulatory neurons.

A symmetrical cluster of serotonin-immunoreactive neurons in the pedal ganglia of Clione limacina has been described morphologically and physiologically. At least five of the cluster neurons send axons to the ipsilateral wing that branch throughout the entire wing area. Activation of these cells did not produce a motor effect in non-swimming preparations, but did enhance contractility in swimming preparations. Activity in the pedal neurons did not produce detectable central effects as neither swim interneuron nor swim motor neuron activities were altered. Most notable was a lack of a change in swim frequency, a characteristic of swim acceleration. Activity in the pedal neurons did enhance the size of muscle junctional potentials and spike-like responses, but only in slow-twitch muscles. The peripheral modulatory effect was blocked by the serotonin antagonist mianserin.