It is today well recognized that all pituitary hormones in particular, and hormones in general, are secreted in an intermittent, pulsatile manner. This special manner of secretion appears to be of paramount importance in encoding regulatory signals; i.e. stimulatory, inhibitory, which are integrated at the target organ level to obtain a determined target organ response to the signal. Recently, our group has provided new insights concerning the characterization and the regulation of pulsatile prolactin secretion. In fact, pulsatile prolactin secretion dramatically varies depending on the estrous cycle stage in rats, suggesting that different patterns convey distinct signals to target tissues. In an attempt to decode these putative signals, we evaluated the total amount of hormone released per pulse (area under the pulse), and with the aid of frequency distribution analysis, we observed that at least two different classes of pulses could be defined; big and small mass prolactin pulses. Interestingly, the maximum incidence of big mass prolactin pulses was observed during the afternoon of estrus, an estrous cycle stage in which prolactin secretion appears to protect the corpus luteum from the luteolytic actions of LH secreted during metestrus. This observation suggested to us that, in fact, prolactin actions on the corpus luteum are encoded in these big mass prolactin pulses. In addition, big mass prolactin pulses seem to be precisely regulated by sporadic interruptions in the dopaminergic inhibitory tone exerted by the hypothalamus in both male and female rats. Initial experiments, using D2 dopaminergic receptor antagonists, showed that the dopaminergic tone appears to be required to observe big mass prolactin pulses. Experiments in which the dopaminergic tone is either completely abolished or continuously present will demonstrate whether sporadic interruptions of the dopaminergic tone are involved in the genesis of big mass prolactin pulses.
