Effect of pilocarpine on the formalin-induced orofacial pain in rats

Abstract

In this study, the effects of subcutaneous (SC) injection of pilocarpine (a cholinomimetic agent) and atropine (a muscarinic receptors antagonist) were investigated on a tonic model of orofacial pain in rats. The contribution of the endogenous analgesic opioid system was assessed using naloxone (an opioid receptors antagonist). Tonic orofacial pain was induced by SC injection of a diluted formalin solution (1%, 50 μL) in the right upper lip, and the time spent face rubbing was measured in five min blocks for 1 h. Formalin induced a biphasic (first phase: 0-5 min and second phase: 15-35 min) pain response. Pilocarpine significantly (P < 0.05) suppressed both phases of orofacial pain. Atropine did not have any effect and naloxone non-significantly increased the intensity of pain when used alone. In the pre-injection examinations, atropine prevented, but naloxone did not reverse the antinociceptive effect of pilocarpine. The results indicated that SC injection of formalin in the orofacial region induced a marked biphasic pain. Pilocarpine via muscarinic cholinergic receptors produced antinociceptive effect in the orofacial formalin-induced pain. The endogenous opioid analgesic system may not have a role in pilocarpine-induced antinociception.

Keywords: Atropine; Naloxone; Orofacial pain; Pilocarpine; Rat.

Fig. 1

The orofacial pain response induced by SC injection of normal saline and formalin in the upper lip in rats. Each point represents the mean ± SEM. (n = 6). * indicates significant difference compared with normal saline and other 5-min blocks (P < 0.05).

Fig. 2

Effect of pilocarpine on the first (A) and second (B) phases of formalin-induced orofacial pain. Each column represents the mean ± SEM. (n = 6). * indicates significant difference compared with normal saline treated group (P < 0.05), ^† indicates significant difference compared with pilocarpine (0.125, 0.25, 0.5, 1, and 2 mg kg^-1) treated groups

Fig. 3

Effects of atropine and naloxone on the pilocarpine-induced antinociception in the first (A) and second (B) phases of orofacial formalin test. Each column represents the mean ± SEM. (n = 6). * indicates significant difference compared with other treated groups (P < 0.05).