The role of hypothalamic amines in controlling prolactin surge in the pseudopregnant rat (author's transl)

Abstract

In order to elucidate the role of hypothalamic amines in controlling the diurnal (D) and nocturnal (N) surges of prolactin (PRL) induced by cervical stimulation, chronological relationships between dopamine (DA), norepinephrine (NE) and serotonin concentrations in the hypothalamus and PRL concentrations in the anterior pituitary and plasma were studied in pseudopregnant (PSP) rats. In addition, the effects of diethyldithiocarbamic acid (DDC), an inhibitor of dopamine-β-hydroxylase, and DL-p-chlorophenylalanine (PCPA), an inhibitor of tryptophan hydroxylase, on two PRL surges were also investigated. Rats were made pseudopregnant by cervical stimulation performed at 10.00 h on estrus (Day 0). The concentrations of NE, DA and serotonin in the hypothalamus and PRL in the anterior pituitary and plasma were determined every two hours between 15.00 h on diestrus 1 and 13.00 h on diestrus 2 of the estrous cycle and during the corresponding periods of PSP. D- and N-surges of PRL in the PSP rats were clearly observed at 19.00 h on Day 1 and at 07.00 h on Day 2 of PSP, respectively. During the diestrus 1 and 2 stages of the estrous cycle, PRL concentrations in the anterior pituitary were consistently low and unvarying. In contrast to this, a marked increase in the concentration of PRL in the anterior pituitary was observed during the corresponding period of PSP, just after the D- and N-surges of PRL.Hypothalamic NE concentrations increased during the D-surge of PRL in normal cyclic rats as well as in PSP rats, while in PSP rats, an additional increase in hypothalamic NE concentrations was observed during the initial stage of N-surge of PRL. Serotonin concentrations in the hypothalamus were significantly higher in PSP rats than in normal cyclic rats during the N-surge of PRL. DA concentrations in the hypothalamus significantly decreased during the D-surge of PRL in PSP rats, whereas those of PSP rats as well as normal cyclic rats markedly increased during the N-surge of PRL. These results suggest a possibility that these hypothalamic amines may have an important role on the occurrence of the D- and N-surges of PRL in PSP rats. DDC injected intraperitoneally (500mg/kg) at 12.00 or 21.00 h on Day 1 of PSP produced a dramatic decrease in NE concentrations in the hypothalamus. Under these conditions, initiation of both D- and N-surges of PRL was significantly suppressed, whereas the magnitude and peak time of each surge were the same as those of normal PSP rats. PCPA injected intraperitoneally (250mg/kg) at 12.00 h on estrus of PSP also produced a dramatic decrease in serotonin concentrations in the hypothalamus, but the N-surge of PRL was only partially suppressed. PRL concentrations in the anterior pituitary were normal in these DDC- or PCPA-treated animals during the time period observed.These results suggest that hypothalamic NE and serotonin are not essential for the occurrence of D- and N-surges of PRL, but they have a facilitatory effect on hypothalamic neural mechanisms inducing PRL release in response to the cervical sitimulation.