Apomorphine hypothermia in the rat: Central sites and mechanisms

Abstract

Central injections of apomorphine (10, 20 or 40 μg) into the lateral ventricle or the rostral hypothalamus produced a dose-dependent hypothermie response in the rat. Systemic injections of apomorphine caused a fall in temperature which could be antagonized by pretreatment with pimozide (5 and 20 μg) injected into the rostral hypothalamus, but was not altered when pimozide (20 μg) was given in the lateral ventricle. Tolerance developed to the hypothermie effects of systemic or rostral hypothalamic injections of apomorphine when the same dose was repeated 24 or 48 hr later. However, no tolerance was observed upon repeated administration into the lateral ventricle. Behavioral thermoregulation studies showed that injection of apomorphine into the rostral hypothalamus caused the animals to stay under a heat lamp for a significantly shorter time than saline controls, indicating that the hypothermie effect of apomorphine is due to a downward shift in the thermoregulatory set point. It is suggested that apomorphine may be inducing a hypothermie effect via two different types of dopamine receptors at two different sites: (1) in the rostral hypothalamus, where receptors exhibit tolerance development and where the temperature change is due to a downward shift in the set point; and (2) in areas surrounding the lateral ventricle, where the receptors do not manifest tolerance. A relay from the lateral ventricular sites to the rostral hypothalamus is also a possibility though this cannot be determined from the present data.