Body Temperature Elevation Per Se Induces the Late Phase Syndrome

In conscious guinea pigs, the rise in core temperature (Tc) induced by intraperitoneal heating was followed by a decrease to below its initial level when the heating was ended. The magnitude and duration of this decrease depended on the extent and duration of the hyperthermia. When a substantial Tc decrease occurred, it was preceded by the reversal of the hyperthermia-associated ear skin vasodilation to a vasoconstriction, despite high values of Tc and continued heating. “Hyperthermia-induced hypothermia” was accompanied by sleep-like behavior, enhanced lability of Tc, and decreased responsiveness of ear skin vasomotion to changes in Tc. These features resemble the “late phase syndrome”, i.e., the thermoregulatory changes characteristic of the late phase of fever.