The influence of oestrogen administration in vivo on in vitro prolactin release. Interaction between dopamine and thyrotrophin releasing hormone.

The influence of oestrogen administered to the ovariectomized rat on the interaction between dopamine (DA) and thyrotrophin releasing hormone (TRH) on the release of radioimmunoassayable (RIA) and [3H]leucine incorporated into prolactin ([3H]PRL) was examined in vitro. Dopamine had a more marked suppressing effect on newly synthetized PRL (80%), as determined [3H]PRL, than on total PRL (50%), as determined by RIA-PRL. The administration of 5 micrograms of oestradiolbenzoate (OeB) for 7 days resulted in blocking the suppressing effect of DA when RIA-PRL was measured but not when [3H]PRL was measured. The administration of 5 micrograms of OeB enabled TRH to partially override the suppressing effect of DA and the degree of response was more marked when RIA-PRL was measured than when [3H]PRL was measured. The administration of 50 micrograms of OeB for 3 days enabled TRH to override the DA blockade of prolactin release to levels comparable to that of the control when RIA-PRL was measured but had little to no effect on [3H]PRL. The results are discussed in relation to the two storage pools of PRL in the pituitary and the data suggest that DA acts predominantly to suppress the newly synthetized, rapidly releasable pool. Oestrogen acts to block DA action on the older more stable PRL pool. The ability of TRH to override the DA blockade of PRL release depends upon the presence of oestrogen; here TRH acts predominantly on the older more stable pool of PRL. Oestrogen's action on disrupting the DA suppression of PRL release appears to be related to the time of day the hormone is administered subsequent to when the pituitary is exposed to DA in vitro.