Endogenous opiate analgesia induced by tonic immobility in guinea pigs

A function of the endogenous analgesic system is to prevent recuperative be haviors generated by tissue damage, thus preventing the emission of species -specific defensive behaviors. Activation of intrinsic nociception is funda mental for the maintenance of the behavioral strategy adopted, Tonic immobi lity (TI) is an inborn defensive behavior characterized by a temporary stat e of profound and reversible motor inhibition elicited by some forms of phy sical restraint. We studied the effect of TI behavior on nociception produc ed by the formalin and hot-plate tests in guinea pigs, The induction of TI produced a significant decrease in the number of flinches (18 +/- 6 and 2 /- 1 in phases 1 and 2) and lickings (6 +/- 2 and 1 +/- 1 in phases 1 and 2 ) in the formalin test when compared with control (75 +/- 13 and 22 +/- 6 f linches in phases 1 and 2; 28 +/- 7 and 17 +/- 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 ind uction of TI (0.67 +/- 0.1 and 0.53 +/- 0.13, respectively) when compared w ith control (-0.10 +/- 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), suggesti ng that the hypoalgesia observed after induction of TI behavior, as evaluat ed by the algesimetric formalin and hot-plate tests. is due to activation o f endogenous analgesic mechanisms involving opioid synapses.