Occurrence of morphine tolerance and dependence in the nucleus paragigantocellularis neurons.

The occurrence of morphine tolerance and dependence in the nucleus paragigantocellularis neurons was investigated. The spontaneous activity was recorded from the nucleus paragigantocellularis neurons of urethane-anesthetized rats, using single unit recording. Morphine microinjected (20 mg/ml, 120-200 nl) into the nucleus paragigantocellularis of control rats had both excitatory and inhibitory effects. These effects were reversed by microinjection of naloxone, revealing the possible involvement of mu receptors. Morphine microinjected into morphine-dependent rats failed to change the spontaneous activity of the nucleus paragigantocellularis neurons that accounts for the occurrence of tolerance to morphine in these neurons. Microinjection of naloxone (25 mg/ml, 120-200 nl) in control rats had no effect on the spontaneous firing rate of the nucleus paragigantocellularis neurons but in morphine-dependent rats, either alone or after morphine microinjection, naloxone increased neuronal activity significantly, indicating the occurrence of dependence on morphine in the nucleus paragigantocellularis neurons. These data show that the nucleus paragigantocellularis neurons may play a role in physical dependence on morphine. This conclusion is consistent with the finding, that activation of the nucleus paragigantocellularis by electrical stimulation in morphine-naive rats can elicit behaviors similar to those observed during naloxone-precipitated morphine withdrawal.