Arcuate nucleus lesions reduce opioid stress-induced analgesia (SIA) and enhance non-opioid SIA in rats

When rats were tested more than two weeks following surgery, lesions of the medial basal hypothalamus centered on the arcuate nucleus enhanced a form of foot-shock stress-induced analgesia (SIA) (Expt. 1) that was not blocked by injections of the opiate receptor blocker, naltrexone (6 mg/kg; Expt. 3). These arcuate nucleus lesions reduced the SIA produced by the same stressor when similar rats were tested 3–4 days following surgery (Expt.2). Finally, when similar rats were tested more than 2 weeks following surgery, these lesions reduced a different form of SIA that was blocked by naltrexone (Expt. 4). There were no effects of the lesions or naltrexone on baseline pain reactivity in any of the experiments. We suggest that arcuate nucleus lesions disrupt a system important for the elaboration of opiate-mediated SIA (Expt. 4), perhaps by damaging the brain's sb-endorphin system. In response to damage to this opioid analgesic system, we hypothesize that the damaged brain initiates time-dependent compensatory changes (Expts. 1 and 2) in an undamaged non-opioid analgesic system, resulting in enhanced non-opiate-mediated SIA (Expts. 1 and 3).