Effects of intra-hippocampal microinjection of vitamin B on the orofacial pain and memory impairments induced by scopolamine and orofacial pain in rats.

In the present study, we investigated the effects of microinjection of vitamin B into the hippocampus on the orofacial pain and memory impairments induced by scopolamine and orofacial pain. In ketamine-xylazine anesthetized rats, the right and left sides of the dorsal hippocampus (CA1) were implanted with two guide cannulas. Orofacial pain was induced by subcutaneous injection of formalin (1.5%, 50μl) into the right vibrissa pad, and the durations of face rubbing were recorded at 3-min blocks for 45min. Morris water maze (MWM) was used for evaluation of learning and memory. Finally, locomotor activity was assessed using an open-field test. Vitamin B attenuated both phases of formalin-induced orofacial pain. Prior administration of naloxone and naloxonazine, but not naltrindole and nor-binaltorphimine, prevented this effect. Vitamin B and physostigmine decreased latency time as well as traveled distance in Morris water maze. In addition, these chemicals improved scopolamine-induced memory impairment. The memory impairment induced by orofacial pain was improved by vitamin B and physostigmine used alone. Naloxone prevented, whereas physostigmine enhanced the memory improving effect of vitamin B in the pain-induced memory impairment. All the above-mentioned chemicals did not alter locomotor activity. The results of the present study showed that at the level of the dorsal hippocampus, vitamin B modulated orofacial pain through a mu-opioid receptor mechanism. In addition, vitamin B contributed to hippocampal cholinergic system in processing of memory. Moreover, cholinergic and opioid systems may be involved in improving effect of vitamin B on pain-induced memory impairment.