GABAERGIC INHIBITION OF HICCUP-LIKE REFLEX INDUCED BY ELECTRICAL-STIMULATION IN MEDULLA OF CATS

We hypothesize that the hiccup reflex is actively inhibited through GA BA(B), receptor within central connections of the hiccup reflex are. B ecause the hiccup-like reflex can be elicited by electrical stimulatio n to a limited area within the medullary reticular formation, the hicc up-evoking site (HES), electrical stimulation (50-100 mu A, three trai n pulses at 20 Hz) was delivered to HES by means of a metal electrode containing 1.0 mM baclofen, in anesthetized spontaneously breathing ca ts. The evoked response was characterized by a brief powerful increase in diaphragmatic activity and a temporal suppression of the posterior cricoarytenoid muscle, laryngeal dilator, which corresponded to the f ixed motor pattern of hiccup reflex. The hiccup-like response was rapi dly suppressed after microinjection of baclofen (0.1-0.5 nmol) into HE S, indicating that HES has GABA(B) receptors. In the other experiments , to histologically examine the inputs to the hiccup reflex are, uncon jugated cholera toxin subunit B (UCTB) was injected into HES. Followin g injections of UCTB, retrogradely labelled cells were found distribut ed in various areas of the lower brainstem. Among these areas, the nuc leus raphe magnus (RM) Is reported to have GABA-containing cells. It i s thus hypothesized that RM is most likely to be the source of the GAB Aergic inhibitory inputs to the hiccup reflex are. (C) 1998 Elsevier S cience Ireland Ltd. All rights reserved.