GABA receptor-mediated modification of reticulo-ruminal myoelectrical activity in sheep.

Abstract

The involvement of central and peripheral GABA receptors in the control of the forestomach periodic motor activity was examined in five conscious ewes at rest, chronically fitted with electrodes implanted in the reticular and ruminal wall and a cannula placed in a cerebral lateral ventricle. Intravenous (IV) administration of the GABAA receptor agonist muscimol (5-50 micrograms/kg) did not affect reticulo-ruminal myoelectrical activity, while its intracerebroventricular (ICV) injection (0.05-0.10 micrograms/kg) dose-dependently increased the frequency of reticular and ruminal motor cycles and provoked rumination in 11 trials out of 15. Forestomach responses to ICV muscimol were abolished by a previous ICV treatment of the animals with the GABAA receptor antagonist bicuculline (0.5-4 micrograms/kg), which did not in itself influence reticular or ruminal motility. The GABAB receptor agonist baclofen, given either IV (300-800 micrograms/kg) or ICV (1-3 micrograms/kg), inhibited in a dose-dependent manner both reticular and ruminal activity, with the effects of its ICV administration appearing earlier and lasting longer. The GABAB receptor blocker baclofen given ICV alone (50-300 micrograms/kg) was ineffective, but such a treatment before IV or ICV administration of baclofen in part antagonized the inhibition of the forestomach myoelectrical cyclic activity. These pharmacological data suggest a possible excitatory role of strictly central GABAA and an inhibitory one of mainly central GABAB receptors in the regulation of reticulo-ruminal extrinsic motility in sheep. In this regulation, however, a probable peripheral component of inhibitory GABAB receptors should not be excluded.