ABSTRACT
A function of the endogenous analgesic system is to prevent recuperative behaviors generated by tissue damage, thus preventing the emission of species-specific defensive behaviors. Activation of intrinsic nociception is fundamental for the maintenance of the behavioral strategy adopted. Tonic immobility (TI) is an inborn defensive behavior characterized by a temporary state of profound and reversible motor inhibition elicited by some forms of physical restraint. We studied the effect of TI behavior on nociception produced by the formalin and hot-plate tests in guinea pigs. The induction of TI produced a significant decrease in the number of flinches (18 + or - 6 and 2 + or - 1 in phases 1 and 2) and lickings (6 + or - 2 and 1 + or - 1 in phases 1 and 2) in the formalin test when compared with control (75 + or - 13 and 22 + or - 6 flinches in phases 1 and 2; 28 + or - 7 and 17 + or - 7 lickings in phases 1 and 2). In the hot-plate test our results also showed antinociceptive effects of TI, with an increase in the index of analgesia 30 and 45 min after the induction of TI (0.67 0.1 and 0.53 + or - 0.13, respectively) when compared with control (-0.10 + or - 0.08 at 30 min and -0.09 0.09 at 45 min). These effects were reversed by pretreatment with naloxone (1 mg/kg, ip), suggesting that the hypoalgesia observed after induction of TI behavior, as evaluated by the algesimetric formalin and hot-plate tests, is due to activation of endogenous analgesic mechanisms involving opioid synapses