Selective modification by opiates of neuronal activity of the medial basal hypothalamus

Abstract

Multiunit activity was recorded from medial basal hypothalamic (MBH) neurons in freely behaving rats previously implanted with permanent Nichrome electrodes (62 μm in diameter). The effects of cumulative doses of morphine (0.5, 1.0, 5.0, 10.0, and 30.0 mg/kg, i.p.) and one dose of naloxone (1.0 mg/kg, i.p.) on spontaneous electrical discharges were studied. Dose-related responses were observed, i.e., each incremental dose of morphine elicited additional alterations of multiunits (25, 38, 59, 72, and 78% after 0.5, 1.0, 5.0, 10.0, and 30.0 mg/kg morphine, respectively). This procedure was utilized to identify and classify the MBH neurons with relation to the response patterns induced by morphine and its antagonist, naloxone. Four patterns (increase, decrease, biphasic changes in firing rates, or no effect) of response to morphine and three to naloxone (reversal, no effect, sensitivity to naloxone but not to morphine) were identified, i.e., a total of seven different patterns which may provide insights into neurophysiological actions relating to different functional properties of medial basal hypothalamic neurons. It was noticed that only the decreases of firing produced by morphine were blocked by naloxone. These observations also provide fundamental information of importance in studies of the physiological properties such as tolerance and dependence underlying chronic effects of opiates.