Unit activity of amygdala and hippocampal neurons: effects of morphine and benzodiazepines

The effects of morphine sulfate and two benzodiazepine derivatives, chlordiazepoxide HCl and diazepam, were evaluated upon single unit activity of the amygdaloid nuclear complex and hippocampal formation in immobilized cats. All surgical procedures were performed under halothane anesthesia and all wound margins were infiltrated with Lidocaine after halothane withdrawal. Single unit activity was recorded extracellularly with platinum-iridium microelectrodes. Chlordiazepoxide HCl, 10.0-20.0 mg/kg i.v., or diazepam, 0.05-0.20 mg/kg i.v., suppressed spontaneous firing rates of the amygdala and the hippocampal neurons. In contrast, the spontaneous firing rates of neurons in these limbic structures were augmented by morphine sulfate, 0.50-2.00 mg/kg i.v. The morphine-induced augmentation of hippocampal neuronal activity was effectively antagonized by naloxone, 0.10-0.20 mg/kg i.v. However, naloxone, 0.20-0.40 mg/kg i.v., only partially suppressed the morphine induced augmentation of amygdala neuronal activity. In a dose-dependent fashion, chlordiazepoxide and diazepam administration prevented or antagonized morphine-induced augmentation of amygdala and hippocampal neuronal activity. Our results suggest that, in the cat, the amygdala and hippocampus may play an important role for morphine-induced behavioral responses. Moreover, our data imply that these two limbic structures may be the sites of tranquillizing actions of diazepam and chlordiazepoxide.