The effects on thermoregulatory mechanisms produced by intracerebroventricular injections of histamine in the sheep

Abstract 1. 1. The effects on thermoregulatory mechanisms of intracerebroventricular (ICV) injections of histamine at doses between 5and 1000 nmol.kg −1 have been studied in sheep at different ambient temperatures ( T a ). 2. 2. In doses up to 800 nmol.kg −1 ICV histamine increased respiratory frequency (RF) and peripheral vasomotor tone (PVMT) at T a 20°C. These two effects were prevented by, respectively, methysergide and atropine. 3. 3. At higher dose levels or at higher T a ICV histamine decreased RF and PVMT. 4. 4. ICV histamie only rarely increased heat production. 5. 5. It is suggested that histamine may be excitatory at some synapses and inhibitory at others in the hypothalamic thermoregulatory centre.