Histamine-induced rise in core temperature of chloral-anaesthetized rats: mediation by H2-receptors located in the preopticus area of hypothalamus. 
Intracerebroventricular (i.c.v.)  administration of histamine in chloral anaesthetized rats exposed to an ambient temperature of 22 C elicited a rise in their colonic temperature associated with a shivering.  This effect was shared by the H2 receptor agonists dimaprit and impromidine.  Impromidine is, in this respect, a partial agonist with an ED50 much lower than histamine.  The histamine-induced rise in core temperature was antagonized by cimetidine administered either centrally (in doses of 25-40 micrograms, i.c.v.)  or peripherally (large doses greater than or equal to 50 mg/kg i.p.)  This constitutes an indication for the crossing of the blood-brain barrier by cimetidine.  The H2 histamine receptors involved in this effect seem to be located mainly in the preopticus medialis nucleus (p.o.m.n.)  of the hypothalamus since bilateral microinjections of histamine (5 ng) into this nucleus induced the effect, whereas cimetidine injected into the p.o.m.n.,  antagonised the relative hyperthermia elicited by an intracerebroventricular administration of histamine.