The present experiments used urethane-anesthetized rats and single cell recording to study the electrophysiological properties of ventromedial hypothalamic (VMH) cells following different doses of morphine and naloxone, applied microiontophoretically. More than 45% of ventromedial hypothalamic units reacted in a dose-response fashion to local application of morphine. In the majority of the ventromedial hypothalamic neurons, naloxone failed to reverse the effects of morphine. Naloxone alone had effects on 37% of the ventromedial hypothalamic units. The ventromedial hypothalamic units exhibited different response patterns from those observed from other CNS sites in response to the microiontophoretic application of morphine and naloxone; this difference is discussed. The present neurophysiological findings support the existence of opiate target sites with multiple opiate receptors within the ventromedial hypothalamus.

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