Effects of morphine on: spontaneous, dorsal raphe, spinal tract of trigeminal nucleus, medial lemniscus and reticular lateral magnocellular evoked responses of hypothalamic units, in naive and morphine physically dependent rats.

The spontaneous activity and the inputs to the medial basal hypothalamus (MBH) following dorsal raphe (DR), spinal tract of the trigeminal nerve (SpV), medial lemniscus (ML), reticular lateral magnocellular nucleus ( RLM ) and acoustic (Ac) stimulation and the effects of morphine and the opioid antagonist, naloxone, on these inputs, were investigated in morphine-naive and morphine-dependent animals. The observations were obtained in freely behaving animals previously implanted with permanent electrodes. The spontaneous activity of MBH neurons exhibits heterogenic spontaneous firing rates. This spontaneous activity is affected by acute and chronic morphine treatment. The MBH neuronal population exhibits neurophysiological patterns of tolerance of morphine dependence and withdrawal. The central input exerts a marked influence on MBH neurons in both naive and morphine-dependent animals. These inputs are modified by morphine challenge dose in both preparations, i.e., in morphine-naive and morphine-dependent animals, and are reversed by naloxone. The DR and Ac inputs affect the MBH neuronal activity differently from that observed following SpV, ML and RLM stimuli. The effects of morphine and naloxone on the DR and Ac input in morphine-naive and morphine-dependent animals differ from those observed following SpV, ML and RLM inputs. The MBH neurons exhibited a high percentage of convergence to Ac, DR, SpV, ML and RLM stimulation.