Analgesia induced by cold-water stress: Attenuation following hypophysectomy

In addition to the well-known activation of the pituitary-adrenal axis, acute exposure to severe stressors includes a temporary analgesia in rats. Thus, the present study investigates whether the pituitary was involved in the mediation of analgesia induced by severe cold-water swim (CWS) stress. Flinch-jump thresholds were measured 30 min following 3.5-min swims in water temperatures ranging from 2-35°C. Compared with untreated normal rats, hypophysectomized rats, receiving corticosterone and thyroxin, displayed significantly less CWS-induced analgesia, while similarly-supplemented normal rats exhibited significantly more CWS-induced analgesia. In a second experiment, operant liminal escape pain thresholds were determined following acute and chronic CWS. Whereas normal rats exhibited profound analgesia following the initial swims, the hypophysectomized rats never displayed any CWS-induced operant escape shifts. Stress-induced alterations in general activity levels and/or thermoregulation were shown to be unrelated to the diminished effectiveness of CWS to produce analgesia in hypophysectomized rats. These data imply that the pituitary is involved in the mediation of CWS-induced analgesia.