Orexinergic and opioidergic receptors interaction in the dentate gyrus of the hippocampus on modulation of formalin-induced inflammatory pain responses in the rat.

Pain is a multidimensional experience involving sensory, emotional, and cognitive components. The hippocampal formation specifically the dentate gyrus (DG) plays a role in pain processing. Both the opioid and orexin systems independently modulate pain: Opioids affect nociception and emotional aspects, while orexins link pain to arousal, behavior, and autonomic responses. This study aimed to investigate the interaction between orexinergic and opioidergic receptors within the DG in modulating formalin-induced inflammatory pain in rats. One hundred eighteen adults male Wistar rats received intra-DG microinjections of orexin-A (0.5-2 nmol), the OX1R antagonist (SB334867; 3-100 nmol), the mu opioid receptor antagonist (Naloxone; 5-45 nmol) alone or in combination with morphine (25 nmol). All microinjections of SB334867 and naloxone or their vehicle (Saline/DMSO 12%) were done into the DG region 5 min before intra-DG microinjection of orexin-A or morphine or their vehicle. Microinjections were done in a volume of 0.5 μl. Formalin (2.5%, 50 μl) was then injected subcutaneously into the hind paw 5 min later to induce biphasic inflammatory pain, and nociceptive behaviors were recorded during early (first 5 min) and late (last 45 min) phases. Orexin-A administration (0.5-2 nmol) significantly reduced nociceptive behaviors in both phases, indicating potent analgesic effects. Pre-treatment with SB334867 (3-100 nmol) or naloxone (5-45 nmol) attenuated orexin -A-induced antinociception. Moreover, co-administration of SB334867 with morphine significantly inhibited morphine-induced antinociceptive responses, suggesting an interaction between orexinergic and opioidergic systems in the DG. Locomotor activity tests confirmed that drug administrations did not impair general motor function. These findings support the hypothesis that the orexinergic system modulates inflammatory pain partly through opioid receptor mechanisms within the hippocampal DG. Understanding this neural interaction provides novel insights into pain modulation and presents a promising target for developing more effective analgesic therapies with fewer side effects.