Background & Aims : Brain histamine, through H3 receptor, has an important role in central perception of pain. The present study investigated the effects of intra-hippocampal microinjection of histamine and thioperamide (a histamine H3 antagonist) on tonic pain in orofacial region of rats.
Materials & Methods: In 72 male Wistar rats two guide cannulae were implanted into the dorsal hippocampus bilaterally. Orofacial region tonic pain was induced by subcutaneous injection of formalin (50 µl, %1) into the upper lip, and rubbing the injected site (face rubbing) was recorded in 3 min blocks for 45 min.
Results: Subcutaneous injection of formalin in the upper lip produced a marked biphasic (first phase: 0-3 min and second phase: 15-33 min) pain response. The intensity of pain in the first and second phases was significantly (P<0.05) suppressed with intra-hippocampal injection of histamine (1 and 2 µg) and thioperamide (2.5 and 5 µg). Pretreatment with thioperamide before histamine enhanced the antinociceptive effect of histamine.
Conclusion: The results showed that intra-hippocampal injection of histamine and thioperamide suppressed the first and the second phases of orofacial tonic pain. Blockade of histamine H3 receptor with thioperamide enhanced the antinociceptive effect of histamine.
Source: Urmia Med J 2011: 22(3): 276 ISSN: 1027-3727