title: Endogenous opiate analgesia induced by tonic immobility in guinea pigs.
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 +/- 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 flinches 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 induction of TI (0.67 +/- 0.1 and 0.53 +/- 0.13, respectively) when compared with 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), 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.
authors.affiliation: Departamento de Fisiologia, Faculdade de Medicina de Ribeirão Preto, Universidade de São Paulo, Ribeirão Preto, SP, Brasil.
authors.forename: C R
authors.initials: CR
authors.lastname: Leite-Panissi
authors.forename: C L
authors.initials: CL
authors.lastname: Rodrigues
authors.forename: M R
authors.initials: MR
authors.lastname: Brentegani
authors.forename: L
authors.initials: L
authors.lastname: Menescal-De-Oliveira
mesh_terms: D000818:Animals; D001522:Behavior, Animal / Q000502:physiology*; D006168:Guinea Pigs; D007103:Immobilization / Q000502:physiology*; D008297:Male; D009270:Naloxone / Q000494:pharmacology*; D009292:Narcotic Antagonists / Q000494:pharmacology*; D018847:Opioid Peptides / Q000037:antagonists & inhibitors* / Q000378:metabolism
chemical_list: D009292:Narcotic Antagonists; D018847:Opioid Peptides; D009270:Naloxone