Distinct pulsatile prolactin secretory patterns during the estrous cycle: possible encoding for diverse physiological responses.

Abstract

PRL levels during the estrous cycle have been reported to be low, with the notable exception of the afternoon proestrous surge. The present study was designed to evaluate the pulsatile pattern of PRL secretion during the low secretory phases of the cycle. Animals were bled at 3-min intervals for 150 min during proestrus morning (AM), estrus AM and evening (PM), metestrus AM, and diestrus AM and PM. Using the algorithm Detect, pulse frequency, duration, interval, peak and trough values, area under the pulses, and mean PRL levels were calculated. PRL secretion was pulsatile during all stages of the estrous cycle, although the pattern varied considerably among the different cycle stages. Pulse frequency was highest during estrus and lowest during diestrus. All quantitative pulse parameters (peak, trough, amplitude, and area under pulse) were also higher during estrus and lowest in diestrus. Analysis of area under the pulse using frequency distribution indicated that at least two subpopulations of pulses, i.e. small mass and large mass pulses, were observed in certain stages of the cycle. Small mass pulses were present at all stages of the cycle; while their frequency remained unchanged, changes in other parameters were observed at different stages which did not always correlate with changes in mean PRL levels. Big mass pulses, on the other hand, presented a clear change in pulse frequency with values rising progressively from diestrus PM through proestrus to peak at estrus. Between estrus PM and metestrus AM these big mass PRL pulses essentially disappeared. In contrast to the marked changes in frequency, big mass PRL pulses were remarkably homogeneous in other quantitative characteristics. The results indicate that distinct changes in PRL pulsatility patterns occur during the estrous cycle; these changes are related to both the pattern and the quality of PRL pulses. Based on the observations of a companion study (28) and other data, we suggest that the genesis of the big mass and small mass PRL pulses involves dopaminergic and nondopaminergic mechanisms, respectively. The timing and selectivity of the changes in PRL pulsatile patterns suggest that those patterns may encode different signals for expression of the diverse actions of PRL on reproductive tissues.