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1. Extracellular recordings were obtained from 94 warm-sensitive, 6 cold-sensitive and 117 temperature-insensitive neurones in slices of the hypothalamic medial preoptic area of rats, to determine the effect of the GABAA agonist muscimol, the GABAA antagonist bicuculline, the GABAB agonist baclofen and the GABAB antagonist phaclofen on tonic activity and temperature sensitivity.2. Muscimol and baclofen dose-dependently inhibited the tonic activity of 69% (36/52) and 97 % (36/37) of the hypothalamic neurones, respectively, regardless of their type of thermosensitivity. In contrast, the GABAA antagonist bicuculline increased the tonic activity of the majority of neurones (58/83), while the GABAB antagonist phaclofen increased neuronal activity only in the high dose of 100 AIM. 3. The temperature sensitivity of hypothalamic neurones was only changed by ligands of GABAB receptors, and this effect was restricted to warm-sensitive neurones. The temperature coefficient (TC) was significantly increased by the GABAB agonist baclofen (ATC = 0-69 + 0-11 imp s-0C-1, P< 0.01, n = 18). In contrast, the GABAB antagonist phaclofen (10 uM) decreased the temperature sensitivity (ATC = -0-67 + 0 09 imp s-' 0C-, P < 0.01, n = 10) in doses which did not affect tonic activity. 4. The increase in temperature sensitivity due to the GABAB agonist baclofen was significantly enhanced by co-perfusion of the GABAA antagonist bicuculline, indicating an interaction of GABAA and GABAB receptor-mediated mechanisms with regard to neuronal thermosensitivity.5. The results suggest that neurones in the medial preoptic area are subject to GABA-mediated tonic inhibition resulting in modulation of tonic activity and temperature sensitivity of warm-sensitive neurones possibly involved in the control of body temperature. The data support the hypothesis that the hypo-or hyperthermic action of an endogenous substance is related to its effect on the thermosensitivity rather than on tonic activity of hypothalamic neurones.