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.