STATE-DEPENDENT PHENOMENA IN CAT MASSETER MOTONEURONS

In the present study we explored the mechanisms of carbachol-induced m uscle atonia in the alpha-chloralose-anesthetized animal. We compared our findings to those that have been previously obtained in unanesthet ized cars during muscle atonia occurring during natural active sleep. Accordingly, in cats anesthetized with alpha-chloralose, intracellular records were obtained from masseter motoneurons before and after carb achol-induced motor. atonia. Following the induction of atonia, tile m embrane potential activity was dominated by high-frequency, discrete, hyperpolarizing potentials. These hyperpolarizing potentials were reve rsed in polarity by the intracellular injection of chloride ions and a bolished by the application of strychnine. These findings indicate tha t they were inhibitory postsynaptic potentials (IPSPs) mediated by gly cine. These IPSPs appeared exclusively during muscle atonia. In additi on, masseter motoneurons were significantly hyperpolarized and their r heobase increased. There was a decrease in input resistance and membra ne time constant. In the alpha-chloralose-anesthetized preparation, st imulation of the nucleus pontis oralis (NPO) induced IPSPs in masseter motoneurons following, but never prior to, the pontine injection of c arbachol. Thus, this is the first demonstration that 'reticular respon se-reversal' may be elicited in an anesthetized preparation. Another s tate-dependent phenomenon of active sleep, the occurrence of IPSPs in motoneurons that are temporally correlated with ponto-geniculo-occipit al (PGO) waves, was also observed in this preparation only after carba chol administration. Based an the data in this report, we conclude tha t the inhibitory system that mediates atonia during the state of activ e sleep can be activated ill an animal that is anesthetized with alpha -chloralose. Specifically, the neuronal groups that generate spontaneo us IPSPs, those that mediate the phenomenon of reticular response-reve rsal, and those involved in the generation of PGO waves are capable of being activated and remain functional during alpha-chloralose-anesthe sia.