Thermoregulation in the rat: deficits following 6-OHDA injections in the hypothalamus

Bilateral microinjections of 6-hydroxydopamine (6-OHDA) were made in a volume of 0.5--0.75 microliter through chronically implanted cannulae into anterior hypothalamic, preoptic loci. Sites were selected at which 1.0 to 12.5 microgram of norepinephrine (NE) had previously elicited a fall in the rat's body temperature. After 2.0 to 6.0 microgram of 6-OHDA were injected in the same volume at the same loci, a comparable hypothermia ensued. When the rats were exposed repeatedly for one-hour intervals to an environmental temperature of either 35.0 degrees C or 8.0 degrees C, they were unable to thermoregulate against the heat and their colonic temperature rose. In some experiments, the rats also failed to defend adequately against the cold ambient temperature, but mainly following the microinjection of the higher doses of 6-OHDA . The intakes of food and water were generally suppressed; this was accompanied by a transient decline in body weight. Overall, the severity, duration and direction of the thermoregulatory impairment depended upon the anatomical site of injection and the dose regimen of the neurotoxin employed. These results offer further evidence that an intact catecholaminergic pathway within the anterior hypothalamus is required for the rat's physiological control of heat loss in a warm environmental temperature.