The effect of desmopressin infusion into dorsal raphe nucleus on pain modulation and morphine analgesia in rats tail flick reflex

Recent neuroanatomical and behavioral evidence has indicated that vasopressin (VA) increases pain threshold. The dorsal raphe nucleus (DRN) is an important nucleus in pain modulation. Anatomical studies have shown that DRN receives vasopressinergic fibers originating in the hypothalamic paraventricular nucleus. The aim of the present study was to examine the effects of intra-DRN injection of desmopressin (DES) on pain threshold and morphine antinociception in tail flick reflex. DES (0.01 mg/ml) was injected through a single cannula aimed at the DRN for activation of VA receptors of DRN neurons; control group received saline. In experiment 1, rats received intra-DRN injection of DES or saline 10 min before tail flick reflex. Injection of DES into DRN decreased tail flick latency at 45 and 60 min (P