Inhibitory effect of amygdaloid conditioning stimulation on nociceptive neurons in the rat medullary dorsal horn

We have shown that conditioning stimulation of the amygdaloid nucleus has an inhibitory effect on nociceptive responses of the somatosensory cortex. However, the site of inhibitory action in the nociceptive ascending system is not clear. The aim of this study was to determine whether amygdaloid conditioning stimulation exerts an influence on nociceptive neurons in the medullary dorsal horn of the rat. 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. The conditioning stimulation had almost no effect on the discharges of non-nociceptive neurons. In contrast, the conditioning stimulation markedly inhibited the activities in 32 of 51 nociceptive neurons (24/40 WDR neurons, 8/11 NS neurons). The amygdaloid inhibitory effect was 68.3±14.4% (n=32) at maximum and was not influenced by naloxone (2 mg/kg, i.v.). The present results suggest that the amygdaloid nucleus inhibits ascending nociceptive information at the 2nd-order neurons via a non-opioid system. The amygdaloid antinociceptive effect may provide a neurophysiological basis for the stress-induced analgesia (SIA) observed in the presence of stress and fear. [Jpn J Physiol 55 Suppl:S161 (2005)]