The immediate-shock deficit and postshock analgesia: Implications for the relationship between the analgesic CR and UR

Rats received a 3-sec, 1-mA footshock either immediately or 3 min after placement in a chamber. Postshock pain sensitivity was assessed with the formalin test. The animals that received the 3-min delay between placement and shock showed an analgesic response compared with noshock controls. The immediate-shock animals did not. Thus the immediate-shock deficit, previously reported for freezing and defecation, also occurs for analgesia. This suggests that shock levels sufficient to condition analgesia are not necessarily sufficient to produce analgesia as an unconditional response. As with freezing, there is a dissociation between conditional and unconditional responses in the fear-conditioning system. Increasing immediate-shock levels to 6 sec, 2 mA produced a transient unconditional analgesia. For analgesia, a conditional response is more readily produced than an unconditional response.