Interactions of galanin and opioids in nociceptive modulation in the arcuate nucleus of hypothalamus in rats

The fact that galanin, β-endorphin and their receptors are present in the arcuate nucleus of hypothalamus (ARC), coupled with our previous observation that both β-endorphin and galanin play antinociceptive roles in pain modulation in the ARC, made it of interest to study their interactions. The hindpaw withdrawal latency (HWL) in response to noxious thermal and mechanical stimulation was assessed by the hot-plate test and the Randall Selitto Test. We showed that the antinociceptive effect induced by intra-ARC injection of galanin was dose-dependently attenuated by the following intra-ARC injection of naloxone. Furthermore, intra-ARC administration of the selective μ-opioid receptor antagonist β-funaltrexamine (β-FNA) attenuated the increased HWL induced by intra-ARC injection of galanin in a dose-dependent manner, while the δ-opioid receptor antagonist naltrindole or the κ-opioid receptor antagonist nor-binaltorphimine (nor-BNI) did not. Moreover, intra-ARC injection of a galanin receptor antagonist galantide attenuated intraperitoneal morphine-induced increases in HWLs. These results demonstrate that the antinociceptive effect of galanin was related to the opioid system, especially μ-opioid receptor was involved in, and that systemic morphine induced antinociception involves galanin in the ARC.