Previous studies have established that pulsatile PRL secretion is characterized in both male and female rats by at least two classes of secretory episodes, i.e., big and small mass pulses. This manner of secretion appears to both provide a physiological means for encoding trophic signals to peripheral organs and to be exquisitely regulated. Earlier studies in which a complete dopaminergic blockade was induced, suggested that big mass PRL pulses could be due to sporadic interruptions in the prevailing inhibitory dopaminergic tone exerted by the hypothalamus. If this hypothesis is correct, then either by abolishing the dopaminergic tone or maintaining a continuous dopaminergic input, big mass pulses should disappear. This hypothesis has recently been demonstrated in experiments using dopaminergic agonists and antagonists, since the incidence of big mass PRL pulses is dramatically reduced by both treatments. Under a continuous dopaminergic input PRL secretion is pulsatile, indicating that other hypothalamic factor(s) different from dopamine is involved in the generation of pulsatile PRL secretion. After using frequency distribution analysis, big mass PRL pulses were absent, thus, under these conditions small mass pulses were the only class of pulses contributing to the pulsatile nature of PRL secretion. Interestingly, a complete blockade of the dopaminergic input did not abolish the pulsatile nature of PRL secretion. Moreover, the dopaminergic blockade showed the same qualitative phenomenon, i.e., big mass PRL pulses were also absent when the animals were treated with a dopaminergic antagonist. These observations demonstrate that sporadic interruptions in the dopaminergic tone regulate the appearance of a specific class of PRL secretory events and suggest that other factor(s) (PRF) are able to drive pulsatile PRL secretion even under a strong dopaminergic status. Studies combining passive immunoneutralization paradigms and blockade or enhancement of the dopaminergic tone will provide information on the Interactions of peptidergic and aminergic signals which result, after being integrated at the pituitary level, in pulsatile PRL secretion.
