Hemorrhage- and Restraint-induced Analgesia in Male and Female Conscious Rats

It is well known that stress induces analgesia. This study was designed to demonstrate the stress-induced analgesia by employing hemorrhage and restraint and to investigate its mechanism and sex difference. The degree of pain was assessed by measuring the magnitude of jaw opening reflex produced by a noxious electrical stimulation in the dental pulp and by measuring the latency to withdraw the tail from a heat ray. Restraint showed an antinociceptive response. A significant increase in pain threshold on bleeding was shown and the increase was larger in male group than in female group. The tail flick latency (TFL) on bleeding after AVP antagonist injection into the ventricle was decreased and the decrease was greater in male rats than in female rats. Castration resulted in a significant reduction of TFL. This effect was reversed by treatment with sex hormones. TFL was decreased during hemorrhage in castrated rats. This response was opposite to that in non-castrated rats. TFL was further decreased during hemorrhage after infusion of AVP antagonist, and there was a significant sex difference. These results suggest that both restraint and hemorrhage produce an antinociception and that, in hemorrhage-induced analgesia, AVP and sex hormones may play an important role and male rats show a greater analgesic response.