The role of GABA receptors in the control of the omasal myoelectrical activity in sheep.

Gamma-aminobutyric acid (GABA) receptor-mediated modification of omasal spiking activity (SA) was studied in six conscious ewes at rest, chronically fitted with electrodes implanted in the reticular and omasal walls and a cannula placed in the left cerebral lateral ventricle. Intracerebroventricular (0.1 microgram kg-1) but not intravenous (25 micrograms kg-1) administration of muscimol increased both the duration of omasal SA and the frequency of groups of spiking bursts (GSB) from the reticulum and oral omasum; the frequency of GSB from the aboral omasum decreased. These responses were antagonised by intracerebroventricular pretreatment with bicuculline (0.5 microgram kg-1). Both intravenous (500 micrograms kg-1) and intracerebroventricular (1 microgram kg-1) administration of baclofen inhibited reticular SA; in both the oral and aboral omasum, while the duration of SA increased, the frequency of GSB decreased. Reticuloomasal responses to intravenous or intracerebroventricular baclofen were greatly antagonised or abolished, respectively, by a previous intracerebroventricular injection of phaclofen (80 micrograms kg-1). The main conclusion which could be drawn from these results is that exclusively central GABAA and mainly central GABAB receptors mediate a prolongation of omasal body motility in sheep.