Glycine at hypoglossal motor nucleus: genioglossus activity, CO(2) responses, and the additive effects of GABA.
There is evidence for glycine and GABA(A)-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 whether coapplication of glycine and muscimol (GABA(A)- 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 (control), glycine (0.0001-10 mM), or muscimol (0.1 microM). Increasing glycine at the HMN produced graded suppression of GG activity (P < 0.001), although the GG still responded to stimulation with 7% inspired CO(2) (P = 0.002). Strychnine (0.1 mM) reversed the glycine-mediated suppression of GG activity, whereas 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 behaviors such as rapid eye movement sleep.
Morrison
Janna L
JL
Departments of Medicine and Physiology, University of Toronto, Toronto, Ontario, Canada M5S 1A8.
Sood
Sandeep
S
Liu
Xia
X
Liu
Hattie
H
Park
Eileen
E
Nolan
Philip
P
Horner
Richard L
RL
D000818:Animals; D002245:Carbon Dioxide / Q000494:pharmacology; D004357:Drug Synergism; D004576:Electromyography; D005998:Glycine / Q000037:antagonists & inhibitors / Q000378:metabolism*; D018684:Glycine Agents / Q000494:pharmacology; D007002:Hypoglossal Nerve / Q000502:physiology*; D008297:Male; D008526:Medulla Oblongata / Q000502:physiology*; D009132:Muscles / Q000187:drug effects / Q000502:physiology; D051381:Rats; D017208:Rats, Wistar; D011963:Receptors, GABA-A / Q000378:metabolism; D013331:Strychnine / Q000494:pharmacology; D014059:Tongue / Q000187:drug effects / Q000502:physiology*; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
D018684:Glycine Agents; D011963:Receptors, GABA-A; D002245:Carbon Dioxide; D005680:gamma-Aminobutyric Acid; D013331:Strychnine; D005998:Glycine