Changes in electrophysiological properties of lumbar motoneurons in the à- chloralose anesthetized CAT during carbachol-induced motor inhibition. Ming-Chu Xi, Rong-Huan Liu, Jack Yamuy, Francisco R. Morales, and Michael H. Chase. Department of Physiology and the Brain Research Institute, UCLA School of Medicine, Los Angeles, CA 90024.
APStracts 4:0070N, 1997.
ABSTRACT
The present study was undertaken a) to examine the neuronal mechanisms responsible for the inhibition of spinal cord motoneurons which occurs in à- chloralose anesthetized cats following the microinjection of carbachol into the nucleus pontis oralis (NPO), and b) to determine if the inhibitory mechanisms are the same as those which are responsible for the postsynaptic inhibition of motoneurons that is present during naturally-occurring active sleep. Accordingly, the basic electrophysiological properties of lumbar motoneurons were examined, utilizing intracellular recording techniques, in cats anesthetized with à-chloralose and compared with those present during naturally-occurring active sleep. The intrapontine administration of carbachol resulted in a sustained reduction in the amplitude of the spinal cord Ia- monosynaptic reflex. Discrete large amplitude inhibitory postsynaptic potentials (IPSPs), that are only present during the state of active sleep in the chronic cat (Chase and Morales 1990), were also observed in high-gain recordings from lumbar motoneurons following the injection of carbachol. During carbachol-induced motor inhibition, lumbar motoneurons exhibited a statistically significant decrease in input resistance, membrane time constant and a reduction in the amplitude of the action potential's afterhyperpolarization (AHP). In addition, there was a statistically significant increase in rheobase and in the delay between the initial segment (IS) and soma dendritic (SD) portions of the action potential (IS-SD delay). There was a significant increase in the mean motoneuron resting membrane potential (i.e., hyperpolarization). The preceding changes in the electrophysiological properties of motoneurons, as well as the development of discrete IPSPs, indicate that lumbar motoneurons are postsynaptically inhibited following the intrapontine administration of carbachol in cats which are anesthetized with à-chloralose. These changes in the electrophysiological properties of lumbar motoneurons were found to be comparable to those that take place during the atonia of active (REM) sleep in chronic cats (Morales and Chase 1981; Soja et al. 1991). The present results support the conclusion that the neural system which is responsible for motor inhibition during naturally-occurring active sleep can also be activated in à-chloralose anesthetized cats following the injection of carbachol into the NPO. 

Received 23 August 1996; accepted in final form 6 March 1997.
APS Manuscript Number J683-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 3 April 1997