Neuroendocrine, biogenic amine and behavioral responsiveness to a repeated foot-shock-induced analgesia (FSIA) stressor in Sprague-Dawley (CD) and Fischer-344 (CDF) rats.

The neurobehavioral responsiveness of two strains of rats, Fischer-344 (CDF) and Sprague-Dawley (CD), to a repeated foot-shock-induced analgesia (FSIA) stress was compared in this study. Rats were either restrained or freely moving during shock presentation (sham controls were exposed to the shock environment only). The foot-shock (15-s, 1.5-mA scrambled electric shock) was observed to induce analgesia in the CDF, but not the CD strain following acute presentation; analgesia was evaluated using time for tail-withdrawal from hot water (55 degrees C). Both strains exhibited an analgesic response when latency to tail withdrawal was evaluated just prior to daily FSIA presentations over 15 total sessions indicating that these rat strains were behaviorally conditioned to this repeated stressor. However, the levels of conditioned analgesic responses to foot-shock were: greater in the CDF and most evident when rats were restrained on the shock-grid while being administered the foot-shock. All rats were quickly sacrificed following the 15th conditioning session to determine the effects of this stressor on neurotransmitter and neuroendocrine function in both strains of rat. Experimental subjects were exposed to the shock grid but not shocked during this last session. The following was found: plasma corticosterone (CORT) and prolactin levels and adrenal CORT levels were significantly increased by repeated stress in the CDF strain; only plasma CORT levels were elevated in the CD rat; pituitary immunoreactive beta-endorphin levels were significantly higher (+46%) amongst all experimental groups in the CDF strain, but stress was not observed to alter peptide steady-state levels in either strain; dopamine (DA), 5-hydroxytryptamine and metabolites (5-hydroxyindoleacetic acid and dihydroxyphenylacetic acid) levels were generally higher in the hypothalamus and frontal cortex of the CDF rat but turnover rates (implied from metabolite/amine ratios) indicated that these systems were more sluggish in this rat strain; hypothalamic DA turnover was significantly attenuated by repeated FSIA + restraint in both strains, but the dynamics of this effect appeared to be different between rat strains; and frontal cortex 5-HT turnover was significantly elevated by repeated FSIA + restraint in only the CDF rat. This research indicates that the CDF rat is extremely sensitive to an acute FSIA stress and it is less able than the CD rat to adapt to repeated presentation of this stress.