BEHAVIORAL AND HORMONAL EFFECTS OF RESTRAINT STRESS AND FORMALIN TESTIN MALE AND FEMALE RATS

The formalin test was used to measure the analgesia induced by restrai nt in male and female rats. Animals were restrained for 30 min or left undisturbed in their cage and then (1) killed immediately to collect blood for hormonal determinations; or (2) subcutaneously injected with formalin in the hind paw (or sham-injected), introduced to an open fi eld for recording of behaviour, and killed at the end of this procedur e. In both experiments, corticosterone was found to be higher in femal es. In Experiment 1, the ability of restraint to be stressful was conf irmed by the increase in corticosterone in both sexes and by the decre ase of testosterone in males. In Experiment 2, restraint-treatment ind uced a reduction in licking and flexing that was limited to the second phase. The reduction occurred in different periods and to a different degree in the two sexes; it was greater in females. Spontaneous behav iours showed sex differences in restraint-treated but not in formalin- treated animals. The results show that the hormonal effects observed a fter restraint are not present after the formalin test and that the ma rked analgesia observed with phasic painful stimuli does not occur wit h a longer-lasting one such as that induced by formalin, after which o nly partial and short-lasting effects were observed, (C) 1998 Elsevier Science Inc.