Effects of morphine on single unit activity of the amygdala in cats

Single neuronal activity has been recorded extra-cellularly from the nucleus amygdaloideus centralis (pars lateralis) (Acl), the nucleus amygdaloideus centralis (pars medialis) (Acm), the nucleus amygdaloideus basalis (pars magnocellularis) (Abm), the nucleus amygdaloideus lateralis (Al), and the nucleus amygdaloideus basalis (pars parvocellularis) (Abp). The majority of the Acl, Acm, and Abm neurons were excited by nociceptive stimulation such as pinching the skin with serrated forceps and/or intraarterial injection of bradykinin. The nociceptive neurons were also driven by non-nociceptive stimulation such as tapping of deep tissues and bending hairs with an air-puff. Their receptive fields were large. After the intravenous administration of morphine, all nociceptive neurons became unresponsive to nociceptive stimuli, although they were driven by non-nociceptive stimuli. Intravenous naloxone antagonized the antinociceptive action of morphine. This suggests that morphine has selective and inhibitory effects on impulse transmission to these nociceptive neurons, and the amygdala, especially the Acl, Acm, and Abm, plays an important role in central nociceptive processing.