Effects of a serotonin agonist and antagonist on cortically induced rhythmical jaw movements in the anesthetized guinea pig

Abstract

The effects of systemic injections of a serotonin agonist (quipazine), and antagonist (methysergide), on rhythmical jaw movements (RJMs) in the ketamine anesthetized guinea pig were examined. It was observed that quipazine, (1) significantly elevated the electrical threshold for inducing RJMs by repetitive stimulation of the masticatory cortex and (2) reduced the amplitude of the digastric and masseter EMG activity during RJMs. In contrast, the excitability of the short latency pathway from masticatory cortex to digastric motoneurons, and the excitability of the digastric reflex evoked by electrical stimulation of the mucosa of the tongue were not significantly affected by quipazine administration. Methysergide administration had little effect on the threshold for evoking RJMs, and on the excitability of the digastric reflex and short latency pathway from cortex to digastric motoneurons. These data suggest that the central pattern generator (CPG) for RJMs is not critically dependent upon serotonin receptor activation since RJMs can be induced after methysergide administration. On the other hand, excitation of the CPG by repetitive cortical stimulation can be inhibited by serotonin receptor activation. These results are interpreted as supporting our previously described model of RJM production by repetitive cortical stimulation.