β-Endorphin: Central sites of analgesia, catalepsy and body temperature changes in rats

β-Endorphin was microinjected into rat brain in order to localize central sites associated with some of its pharmacologic effects: namely, analgesia (inhibition of the tail-flick response), catalepsy and changes in body temperature. Microinjections (1 μl) were made bilaterally under halothane anesthesia and the effects of β-endorphin were repeatedly assessed at 15- or 30-min intervals for 2 hr. β-Endorphin produced analgesia and catalepsy when it was injected at low doses (ED₅₀, 1.3 to 2.7 μg) into the medial preoptic area, nucleus accumbens, anterior hypothalamus and the periaqueductal gray-4th ventricular spaces. Brain areas of intermediate sensitivities (ED₅₀, 3.7 to 16 μg) were the medial thalamus, posterior hypothalamus and areas around the fasciculus retroflexus. The frontal cortex, striatum and lateral areas of the brain were relatively insensitive (ED₅₀ > 17 μg) to the effects of β-endorphin on analgesia and catalepsy. β-Endorphin had complex effects on body temperature. For example, when β-endorphin was injected into the nucleus accumbens or preoptic area, low doses (1.1 - 2.1 μg) produced hyperthermia; higher doses (8.5 μg) produced hypothermia. The brain regions in which low doses of β-endorphin elicit pharmacologic effects correspond to the anatomic areas in which the endogenous β-endorphin system is distributed. Similar correspondence to the endogenous enkephalin system was not obtained.