Prior stress attenuates the analgesic response but sensitizes the corticosterone and cortical dopamine responses to stress 10 days later.

This study demonstrates that pre-exposure to stress influences subsequent effects of stress on pain sensitivity (stress-induced analgesia) and on plasma corticosterone and brain catecholamine activity. Animals exposed to a 30 min shock session (S1 = 8, 5.0 s shocks) 10 days earlier showed a significant attenuation of shock-induced analgesia, as measured by increased latency of tail withdrawal from a hot water bath immediately after a 40 s, 1.6 mA footshock (S2). Animals exposed to shock 10 days before testing also exhibited a higher plasma corticosterone response to testing than did all other groups. Norepinephrine (NE) levels in the frontal cortex and dopamine (DA) and dihydroxyphenylacetic acid (DOPAC) levels in the frontal cortex and nucleus accumbens were not altered in any group. However, the DOPAC/DA ratio in the frontal cortex was increased by analgesia testing, and this increase was enhanced only by the combination of shock 10 days before testing and shock immediately before the test (S1 + S2). These results are consistent with previous reports from this laboratory which indicate that an animal's acute response to stress is strongly influenced by its past history of stress.