EFFECT OF MIDBRAIN STIMULATIONS ON THERMOREGULATORY VASOMOTOR RESPONSES IN RATS

1. Efferent projections eliciting vasodilatation when the preoptic are a is warmed were investigated by monitoring tail vasomotor responses o f ketamine-anaesthetized rats when brain areas mere stimulated electri cally (0.2 mA, 200 mu s, 30 Hz) or with the excitatory amino acid D,L- homocysteic acid (1 mM, 0.3 mu l). 2. Both stimulations elicited vasod ilatation when applied within a region extending from the most caudal part of the lateral hypothalamus to the ventrolateral periaqueductal g rey mat ter (PAG) and the reticular formation ventrolateral to the PAG . 3. Vasodilatation elicited by preoptic warming was suppressed when e ither stimulation was applied within the rostral part of the ventral t egmental area (VTA). 4. Sustained vasodilatation was elicited by knife cuts caudal to the VTA, and vasodilatation elicited by preoptic warmi ng was suppressed by cuts either rostral to the VTA or in the region i ncluding the FAG and the reticular formation ventrolateral to it. 5. T hese results, together with the results of earlier physiological and h istological studies, suggest that warm-sensitive neurones in the preop tic area send excitatory signals to vasodilatative neurones in the cau dal part of the lateral hypothalamus, ventrolateral PAG and reticular formation, and send inhibitory signals to vasoconstrictive neurones in the rostral part of the VTA.