Role of Endogenous Dopamine in Regulation of Anterior Pituitary Hormone Secretion During Early Postpartum and Various Stages of the Estrous Cycle in Holstein Cows
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The role of endogenous dopamine, utilizing a dopamine antagonist (fluphenazine; FLU), in modulation of gonadotropin, growth hormone (GH) and prolactin (PRL) secretion during the early postpartum period and various stages of the estrous cycle was investigated in Holstein cows. Experiment 1 was conducted in anovulatory early postpartum cows. Fluphenazine caused a decrease (P < .05) in mean serum LH concentration and LH pulse frequency. Likewise, FLU caused a (P < .05) decrease in mean GH concentration. These results suggest that endogenous dopamine, at least in part, is responsible for regulation of LH and GH secretion in anovulatory Holstein cows. Experiment 2 was conducted in cyclic lactating Holstein cows during the mid-luteal phase of the estrous cycle. Mean serum LH and FSH concentrations, pulse frequencies, and peak amplitudes did not change in response to FLU. FLU did not affect mean serum GH concentration. These results suggest that a dopamine-mediated mechanism for modulation of gonadotropin and GH secretion is absent or perhaps overridden by high progesterone concentration during the luteal phase of the estrous cycle in lactating dairy cows. Experiment 3 was conducted during the early follicular phase of the estrous cycle in Holstein cows. During the follicular phase, FLU caused a decrease (P < .05) in mean serum LH concentration and LH pulse frequency. However, FLU had no effect on mean serum FSH concentration or pulse frequency. Further, FLU increased (P < .05) GH concentrations during the follicular phase. Experiment 4 was conducted during the early metestrus phase of the estrous cycle. During the metestrus phase, FLU tended to decrease (P < .1) mean LH concentration and suppressed (P < .05) LH pulse frequency but had no effect on FSH secretion. Fluphenazine caused a transient increase (P < .05) in mean serum GH concentration. The results of the third and fourth experiments suggest that, during the early follicular and metestrus phases of the estrous cycle, when progesterone concentration is low, modulation of LH and GH secretion, at least in part, is modulated by endogenous dopamine. However, a dopamine mediated mechanism for FSH secretion is absent during both phases of the estrous cycle in lactating Holstein cows. In all experiments FLU increased (P < .01) PRL secretion indicating that endogenous dopamine suppresses PRL secretion in cattle regardless of ovarian status. It is concluded that: 1) endogenous dopamine plays a stimulatory role in LH secretion during the anovulatory postpartum period and during the estrous cycle only when serum progesterone is low. 2) FLU decreased GH secretion in anovulatory postpartum Holstein cows but it increased GH secretion during the follicular and metestrus phases of the estrous cycle. However FLU had no effect on GH secretion during the luteal phase of the estrous cycle. Thus it appears that, modulation of GH secretion is dependent upon reproductive status and ovarian hormones secretion.
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