Influence of restraint on plasma prolactin and corticosterone in female rats

The effect of restraint on plasma prolactin and corticosterone concentrations was investigated in chronically catheterized, ovariectomized (OVX) or ovariectomized, oestrogen-treated (OVX-PEP) rats. Restraint was induced by tying the hind legs together. In OVX rats, prolactin levels were unchanged following restraint, either during the morning (10.00 h) or afternoon (14.00 h). Prolactin levels increased in OVX-PEP animals when restraint was initiated in the morning; when restraint was initiated in the afternoon the prolactin response depended upon the level of prolactin before restraint. If levels were low (pre-surge) the response to restraint was similar to that observed in the morning; if prolactin levels were high (surge) the response to restraint was reversed and the prolactin level declined. The morning prolactin response to restraint was significantly inhibited and the afternoon surge was retarded in adrenalectomized OVX-PEP (OVX-PEP-ADX) rats; however, in OVX-PEP animals maintained on 0-9% NaCl drinking solution, the morning prolactin response to restraint was also blunted, although the afternoon surge was normal. In OVX-PEP-ADX animals injected with either vehicle alone, or 2 or 4 mg corticosterone for 4 days, and sampled on the morning of day 5, the prolactin response to restraint was absent. Furthermore, when OVX-PEP animals were injected daily with either vehicle or 4 mg corticosterone/day, they showed no increase in prolactin in response to restraint when values were compared with those of uninjected animals. Corticosterone levels after restraint were higher than initial values in all of the above experimental conditions.