CHARACTERIZATION OF PITUITARY MEDIATION OF STRESS-INDUCED ANTINOCICEPTION IN RATS

Antinociception, induced by continuous cold-water swims (CCWS) and cer tain parameters of inescapable foot shock, is reduced in hypophysectom ized rats receiving supplements of corticosterone and 1-thyroxine. To assess which lobe of the pituitary gland is involved in this effect, t he first experiment compared the effects of total hypophysectomy and p osterior lobectomy in supplemented rats upon CCWS antinociception on t he tail-flick and jump tests and upon continuous inescapable foot shoc k antinociception on the tail-flick test. Total hypophysectomy, but no t posterior lobectomy, significantly reduced CCWS antinociception on b oth tests in supplemented rats relative to sham surgery. Both total an d posterior hypophysectomy either reduced or potentiated foot shock an tinociception as functions of shock intensity or duration of exposure in supplemented rats. To assess whether hormonal supplementation is ne cessary for the observed effects, the second experiment examined CCWS antinociception in sham-operated and hypophysectomized rats that recei ved either no hormonal supplements or corticosterone andor 1-thyroxine . These regimens failed to alter CCWS antinociception in sham-operated rats. Treatment of hypophysectomized rats with corticosterone and 1-t hyroxine either separately or together significantly reduced CCWS anti nociception. In contrast, if hypophysectomized rats did not receive su pplements, CCWS antiociception was significantly potentiated relative to sham-operated controls. These effects could not be attributed to tr eatment-induced changes in either body weight or CCWS hypothermia. The se data suggest that the anterior lobe of the pituitary gland and adre nal cortex are involved in the mediation and/or maintenance of CCWS an tinociception.