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

Abstract

Abstract-Singleneuronal 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 (AI), 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 intra arterial 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 intra venous administration of morphine, all nociceptive neurons became un responsive 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.It has been shown that some of the amygdala neurons respond to sciatic nerve stimulation (1), but nociceptive stimuli were not used in this report.On the other hand, it has been shown that the amygdala of monkey brain contained the greatest amounts of opiate binding receptor and a morphine-like peptide, methionine enkephalin (2, 3). Moreover, Rodgers (4) reported that bilateral microinjection of morphine into the cortico-medial amygdala produced a dose-dependent increase in the aversive threshold. However, no experiments have yet been made which allows one to evaluate the effects of morphine on unit activities of the amygdala.An attempt is made in this paper to determine whether the amygdala neurons are driven by the peripheral nociceptive stimuli, and a further object of this work is to study the antinociceptive action of morphine on the neurons of the amygdala. MATERIALS AND METHODSExperiments were performed on 38 adult cats. The animals were initially anesthetized with sodium pentobarbital (Nembutal) at 35 mg/kg i.p. Tracheal and vascular can nulation were performed, all operative wounds were carefully closed with sutures,