Opioid nature of learned helplessness and stress induced analgesia observed without re-exposure to shock.
It has been shown that uncontrollable shocks that produce learned helplessness also produce long-term opioid analgesia if th animal is re-exposed to shock immediately before the test. The present study was conducted in order to investigate if this effect can be observed 24h after the uncontrollable shock treatment without re-exposure to shock, and if it is opioid mediated. Long-term analgesia was found in the absence of re-exposure to shock, and was prevented by an i.p. injection of naloxone (10mg/kg) administered 10min before the test. The learned helplessness effect produced by the same shock treatment was prevented by the administration of 10 and 20mg/kg of naloxone 10min before the shuttlebox test, but not by a lower naloxone dose (5mg/kg). These findings suggest that the shock re-exposure requirement proposed in previous studies is not crucial in determining the long-term analgesia, and that both the long-term analgesia and the learned helplessness effect produced by this shock treatment were opioid mediated.
Department of Pharmacology, Faculty of Medical Sciences, Universidade Estadual de Campinas (UNICAMP), Campinas, SP, Brazil and Department of Experimental Psychology, Institute of Psychology, Universidade de &Stilde;o Paulo (USP), &Stilde;o Paulo, Brazil.
M.H.L.
MH
Hunziker