Analgesic effect of electrical stimulation and microinjection of L-glutamate into the paragigantocellularis nucleus on phasic and tonic pain

  The paragigantocellularis (PGi) nucleus constitutes a large portion of the ventral pontomedullary reticular formation. Neurons within the PGi have been implicated in a variety of functions including cardiovascular regulation, respiratory control, pain and analgesia. Investigators have demonstrated that electrical stimulation and microinjection of L-glutamate into the PGi produce antinociception in phasic pain. In this study, we measured the analgesic effects of electrical stimulation and glutamate microinjection into the PGi using the formalin test and the tail-flick method in rats. A bipolar stimulation electrode and a guide cannula were stereotaxically inserted into the right PGi of individual rats under brief ketamine and xylazine anesthesia. One week after the implantation, a monophasic square wave (500 µs, 400 µA, 50 Hz) was delivered into the PGi for 20 seconds via the implanted electrode. Glutamate (0.5 µl, 44 nM/rat) was injected through the guide cannula. One minute after electrical stimulation or glutamate injection, the tail-flick and formalin (50 ml, 2.5%) tests were performed. The results showed that electrical stimulation and glutamate injection into the PGi caused marked antinociception in both the tail-flick and the early phase of the formalin test compared to the control rats. We conclude that electrical stimulation and glutamate injection into the PGi cause marked antinociception in phasic pain and moderate antinociception in tonic pain. These findings suggest that the PGi is also involved in alleviation of tonic pain.