SEROTONERGIC MODULATION OF SWIMMING SPEED IN THE PTEROPOD MOLLUSK CLIONE-LIMACINA .3. CEREBRAL NEURONS

Swim acceleration in Clione limacina can occur via central inputs to p attern generator interneurons and motor neurons and through peripheral inputs to the swim musculature. In the previous paper, peripheral mod ulation of the swim muscles,vas shown to increase wing contractility. In the present paper, central inputs are described that trigger an inc rease in swim frequency and an increase in motor neuron activity, In d issected preparations, spontaneous acceleration from slow to fast swim ming included an increase in the cycle frequency, a baseline depolariz ation in the swim interneurons and an increase in the intensity of mot oneuron firing, Similar effects could be elicited by bath application of 10(-5) mol l(-1) serotonin. Two clusters of cerebral serotonin-immu noreactive interneurons were found to produce acceleration of swimming accompanied by changes in neuronal activity, Posterior cluster neuron s triggered an increase in swim frequency, depolarization of the swim interneurons, an increase in general exciter motoneuron activity and a ctivation of type 12 interneurons and pedal peripheral modulatory neur ons, Cells from the anterior cerebral cluster also increased swim freq uency, increased activity in the swim motoneurons and activated type 1 2 interneurons, pedal peripheral modulatory neurons and the heart exci ter neuron. The time course of action of the anterior cluster neurons did not greatly outlast the duration of spike activity, while that of the posterior cluster neurons typically outlasted burst duration, It a ppears that the two discrete clusters of serotonin-immunoreactive neur ons have similar, but not identical, effects on swim neurons, raising the possibility that the two serotonergic cell. groups modulate the sa me target cells through different cellular mechanisms.