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Articles in PresS, published online ahead of print August 2, 2002
J Appl Physiol, 10.1152/jap.00464.2002
Submitted on May 28, 2002
Accepted on July 22, 2002
1 Department of Medicine and Physiology, University of Toronto, Toronto, Ontario, Canada
2 Department of Human Anatomy and Physiology, Conway Institute, Dublin, Ireland
* To whom correspondence should be addressed. E-mail: richard.horner{at}utoronto.ca.
There is evidence for glycine and GABAA receptor mediated inhibition of hypoglossal motoneurons in-vitro. However, comparable studies have not been performed in-vivo and the interactions of such mechanisms with integrative reflex respiratory control have also not been determined. This study tests the hypotheses that glycine at the hypoglossal motor nucleus (HMN) will suppress genioglossus (GG) muscle activity, even in the presence of hypercapnic respiratory stimulation, and the effects of glycine will be blocked by strychnine. We also determined if co-application of glycine and muscimol (GABAA receptor agonist) to the HMN is additive in suppressing GG activity. Twenty-four urethane-anesthetized, tracheotomized and vagotomized rats were studied. Diaphragm and GG activities, the electroencephalogram and blood pressure were recorded. Microdialysis probes were implanted into the HMN for delivery of artificial cerebrospinal fluid (ACSF control), glycine (0.0001-10mM) or muscimol (0.1µM). Increasing glycine at the HMN produced graded suppression of GG activity (p<0.001) although the GG still responded to stimulation with 7% inspired CO2 (p=0.002). Strychnine (0.1mM) reversed the glycine-mediated suppression of GG activity while combined glycine and muscimol were additive in GG muscle suppression. It remains to be determined whether the recruitment of such glycine and GABA mechanisms explains the periods of major GG suppression in behaviours such as rapid-eye-movement sleep.
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