Comparison of the antinociceptive effects by conditioning stimulation of different amygdaloid nuclei in the rat.

Abstract

We have shown that conditioning stimulation of the amygdala has an inhibitory effect on nociceptive neurons of the medullary dorsal horn of the rat. The amygdala is considered to be a complex of anatomically different units. The aim of this study was to compare the inhibitory effects of different nucleus within the amygdala. The animals were anesthetized with N₂O-O₂ and 0.5% halothane and were immobilized with pancuronium bromide. A peripheral test stimulus (a single rectangular pulse of 2.0 msec in duration) was applied to the receptive field of nociceptive neurons, and ipsilateral amygdaloid conditioning stimuli to the recording site were trains of 33 pulses (100-300 μA) delivered at 330 Hz. Effective sites for inhibition were widely distributed throughout the amygdala except for the lateral nucleus. This finding is in contrast with our previous data that inhibitory sites were concentrated into the central nucleus in the cat. There were no significant differences in the mean inhibitory effects by the different amygdaloid nuclei, and the inhibitory effect was between 56.6 and 62.4% of the control (n=32). The present results suggest that there are marked species differences with regard to the antinociceptive mechanisms within the amygdala and support the anatomical observations that the efferent fibers of the amygdala originate mostly from the central nucleus that is projection focus from the other amygdaloid nuclei in the rat. [J Physiol Sci. 2006;56 Suppl:S180]