Inhibition of nociceptive responses in the rat^'s medullary dorsal horn by glutamate microinjection into the amygdala

Abstract

We have shown that conditioning electrical stimulation of the amygdala has an inhibitory effect on nociceptive neurons of the medullary dorsal horn of the rat. The purpose of the present study is to determine whether the inhibitory effect depends on the activation of cell bodies or passing fibers by glutamate microinjection into 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. Thirteen nociceptive neurons recorded in the superficial layer of the trigeminal caudal nucleus and the medial reticular nucleus, and the nociceptive responses of these neurons were inhibited by the conditioning electrical stimulation in the central, basomedial and basolateral amygdaloid nuclei. Microinjection of 0.5 M monosodium glutamate (5 μl) into these amygdaloid nuclei induced inhibition of 5 WDR neurons and 2 NS neurons. Most potent inhibition occurred at 5 min after start of injection, and the recovery of inhibition to 50% took place around 25 min. These findings suggest that the excitation of cell bodies in the amygdala contribute to the inhibitory effects on the nociceptive neurons in the medullary dorsal horn. [J Physiol Sci. 2007;57 Suppl:S100]