Ventromedial hypothalamus is highly sensitive to prostaglandin E2 for producing fever in rabbits

1. The febrile responses induced by intraventricular or intrapreoptic (bilateral) injections of prostaglandin E2 (PGE2) were investigated in the same group of rabbits. Both injections produced dose-dependent fever over a range of 100-2000 ng. However the magnitude of febrile responses induced by ventricular injections was significantly greater than those by intrapreoptic injections. This indicates that there exist regions more sensitive to PGE2 than the preoptic region for producing fever. 2. To explore the regions sensitive to PGE2, the effects of microinjection (1 microliter) of PGE2 (50 and 100 ng) on the rectal temperature were extensively examined in the forty regions of the brain stem. The results showed that the preoptic and anterior hypothalamic region, and the ventromedial hypothalamic region are highly sensitive to PGE2 for producing fever. 3. The febrile responses to PGE2 (50-1000 ng) microinjected into the preoptic region were compared with those induced by injection in the ventromedial hypothalamic region. Fever induced by injection in the ventromedial hypothalamic region was significantly greater than that by injection into the preoptic region. 4. Fever induced by PGE2 injected into the ventromedial hypothalamic region was due to increased heat production in the cold environment (10 degrees C), while in 24 degrees C environment heat losses were reduced without significant changes in heat production. 5. The present results show that the ventromedial hypothalamic region is the most sensitive region to PGE2 for producing fever.