Hypothalamic neuronal responses to iontophoretic application of morphine in rats

Effects of thermal stimulation of the scrotum and of iontophoretic application of morphine on the activity of neurones in the preoptic anterior hypothalamic area were observed in 30 urethane-anaesthetised rats. The proportions of cold-responsive, warm-responsive and thermally unresponsive units were 20.7, 28.3 and 51%, respectively, of the total number of neurones tested. Iontophoretic application of morphine to neurones in the preoptic anterior hypothalamic area in rats resulted in excitation of the majority of cold-responsive cells and inhibition of the majority of warm-responsive cells tested. However, most of the thermally unresponsive cells were unaffected by application of morphine. These results provide a neuronal basis for the hypothesis that morphine, when administered directly into the hypothalamus, facilitates heat production and inhibits heat loss, which leads to hyperthermia.