Serotonergic Modulation of Patterned Motor Output in Lymnaea Stagnalis

Rhythmic feeding motor output from the buccal ganglia of Lymnaea stagnalis was evoked by tonic depolarization of the pattern-initiating interneurone SO in the isolated central nervous system. Perfusion with 10~moll~' serotonin (5-HT) led to a reduction in frequency of the SO-driven rhythm, and in some cases rhythmic activity was completely blocked. The frequency reduction was predominantly due to an increase in duration of the 'inactive' phase of the rhythm. In a number of preparations, the normal buccal rhythm was replaced by an 'atypical' pattern of bursting in buccal motoneurones in the presence of 5-HT. This was characterized by the absence of one phase (N2) of interneuronal activity in the feeding pattern generator. Stimulation of the serotonergic giant cerebral interneurones (CGCs), to increase the mean spike frequency from 10 to 2-5 Hz, mimicked some of the effects of 5-HT perfusion. However, the timing of onset of CGC stimulation in relation to depolarization of SO was critical; prolonged activation of a CGC led to an apparent decrease in its effectiveness in suppressing the buccal rhythm.