Involvement of dopamine receptors within the dorsal hippocampus in suppression of the formalin-induced orofacial pain

It is widely established that the dopaminergic system has profound effects on pain modulation in different regions of the brain including the hippocampus, the salient area for brain functions. The orofacial region is one of the most densely innervated (by the trigeminal nerves) areas of the body susceptible to acute and chronic pains. In this study, we tried to examine the effects of dopamine receptors located in the dorsal hippocampus (CA1) region upon the modulation of orofacial pain induced by the formalin test. To induce orofacial pain in male Wistar rats, 50 μl of 1% formalin was subcutaneously injected into the upper lip. In control and experimental groups, two guide cannulae were stereotaxically implanted in the CA1, and SKF-38393 (0.25, 0.5, 1 and 2 μg/0.5 μl saline) as a D1-like receptor agonist, SCH-23390 (1 μg/0.5 μl saline) as a D1-like receptor antagonist, Quinpirole (0.5, 1, 2 and 4 μg/0.5 μl saline) as a D2-like receptor agonist and Sulpiride(3 μg/0.5 μl DMSO) as a D2-like receptor antagonist or vehicles were microinjected. For induction of orofacial pain, 50 μl of 1% formalin was subcutaneously injected into the left side of the upper lip. Results indicated that SKF-38393 at the dose of 1 and 2 μg significantly reduced pain during the first and second phases of observed pain while SCH-23390 reversed such analgesic effect. Moreover, there is a significant difference between groups in which animals received 2 and 4 μg quinpirole or vehicle in the first phase (early phase) of pain. The three high doses of this compound (1, 2 and 4 μg) appeared to have an analgesic effect during the second (late) phase. Furthermore, Sulpiride could potentially reverse the observed analgesic effects already induced by an agonist. Current findings suggest that the dorsal hippocampal dopamine receptors exert an analgesic effect during the orofacial pain test.