Stress-induced analgesia evoked by intraperitoneal injection of hypertonic saline: evidence for its occurrence in vasopressin deficient rats.

Experiments were carried out to investigate whether vasopressin is involved in stress-induced analgesia. Intraperitoneal injection of hypertonic saline caused a significant and dose-related increase in the latency to the tail-flick response of the rat to noxious heat and was used as a stimulus for stress-induced analgesia. Neither the pituitary nor opioid peptides appeared to be involved, since the response occurred in hypophysectomized rats and was not reduced by the opiate antagonist naloxone. Furthermore hypertonic-saline analgesia was clearly potentiated in hypophysectomized rats in comparison to sham-operated controls. Hypertonic-saline analgesia was also observed in vasopressin-deficient (homozygous Brattleboro) rats similar in both magnitude and duration to that in normal rats of the same strain (Long Evans). It was concluded that vasopressin was not involved in stress-induced analgesia evoked by hypertonic saline.