Analgesia induced by brief or more prolonged stress differs in its dependency on naloxone, 5-hydroxytryptamine and previous testing of analgesia.

Analgesia was determined in rats by a standard procedure in which the time taken to withdraw the tail from water at 51 degrees C was noted. The analgesic responses resulting from footshock applied for 30 sec and 30 min showed marked differences in pharmacological profile and in the dependence on experimental conditions. Thus, analgesia induced by footshock applied for 30 sec was enhanced when 5-HT in the brain was decreased by injection of p-chlorophenylalanine (PCPA), was unaffected by naloxone and was dependent on pre-exposure to the test-procedure for analgesia. In contrast analgesia induced by footshock applied for 30 min was attenuated by injection of PCPA and naloxone but was not dependent on pre-exposure to the analgesia test. It is suggested that the analgesia induced by footshock applied for 30 sec and 30 min reflects distinct mechanisms and that, in the former case, the animal behaves as if the shock occurred as a consequence of its motor response to the noxious heat applied during the pre-shock determination of analgesia.