GABAB receptors in the rabbit uterus may mediate contractile responses.

The effects of gamma-aminobutyric acid (GABA) and related compounds on the contractility of isolated rabbit uterus were examined. GABA and baclofen (10(-6)-10(-4) M) stimulated the spontaneous motility in a dose-dependent manner and showed cross-desensitization. The effect of baclofen was stereoselective for the (-)-enantiomer. Muscimol was ineffective, while bicuculline evoked marked contractions. Contractions elicited by submaximal electrical stimulation could be further increased by GABA and baclofen. The effects of GABA and baclofen on spontaneous contractility could not be antagonized by atropine, phentolamine, or tetrodotoxin. These findings indicate: (1) the presence of GABAB receptors in the rabbit uterus; (2) their involvement in the modulation of uterine contractility; (3) the extraneuronal location of uterine GABAB receptors which are thus most probably on smooth muscle cells; and (4) a possible role of inhibitory GABAA receptors in the modulation of spontaneous movements of the uterus.