In the female rat, a complex interplay of both stimulatory and inhibitory hypothalamic factors controls the secretion of prolactin. Prolactin regulates a large number of physiological processes from immunity to stress. In the following review, we have chosen to focus on the control of prolactin secretion in the female rat in response to suckling, mating and ovarian steroids. In all three of these states, dopamine, released from neurones in the mediobasal hypothalamus, is a potent inhibitory signal regulating prolactin secretion. Early research has determined that the relief of dopaminergic tone is not enough to account for the full surge of prolactin secretion observed in response to the suckling stimulus, launching a search for possible prolactin-releasing factors. This research has since broadened to include searching for prolactin-releasing factors controlling prolactin secretion following mating or ovarian steroids. A great deal of literature has suggested that this prolactin-releasing factor may include oxytocin. Oxytocin receptors are present on lactotrophs. These oxytocin receptors respond to exogenous oxytocin and antagonism of endogenous oxytocin inhibits lactotroph activity. In addition, the pattern of oxytocin neuronal activity and oxytocin release correlate with the release of prolactin. Here we suggest that not only is oxytocin stimulating prolactin secretion, but we also hypothesize that prolactin secretion is controlled by a complex network of positive (oxytocin) and negative (dopamine) feedback loops. In the present review, we will discuss this literature and attempt to describe the circuitry we believe may be responsible for controlling prolactin secretion.

In female rats, estradiol (E(2)) and suckling induce prolactin (PRL) secretion. This involves inhibition of hypothalamic dopaminergic tone and stimulation by a PRL-releasing hormone, possibly oxytocin (OT). Infusing an OT antagonist (OTA) i.v., we evaluated the role of OT on suckling- and E(2)-induced PRL secretion. Three days after parturition at 0900 h, lactating dams were fitted with 24-h osmotic minipumps filled with saline or OTA. On d 5 of lactation, pups were separated from their dams for 6 h. Immediately or 20 min after the resumption of suckling, dam trunk blood was collected. Also, ovariectomized (OVX) rats were treated with E(2) (OVE) and OTA at 1000 h on d 1. Blood samples were obtained from 1300 to 2100 h on d 2 for PRL measurements. Additionally, OVX rats were evaluated on d 2 after receiving progesterone (P(4)). OTA blocked suckling and E(2)-induced release of PRL but not that induced by E(2)+P(4). Pups from treated dams failed to gain weight when allowed to nurse for 20 min on d 5 but gained more than 7 g when nursed on d 7 of lactation, indicating that the OTA was active 48 h later. Western blot analysis showed that E(2) treatment increased OT receptors in the anterior pituitary when compared with OVX animals. No further increase was observed in response to the P(4), suggesting that the enhancing effect of P(4) on E(2)-induced PRL release may act through mechanisms independent of OT. These data demonstrate the role of OT in the control of suckling and steroid-induced PRL secretion.

Poletini MO, McKee DT, Kennett JE, Doster J, Freeman ME. Knockdown of clock genes in the suprachiasmatic nucleus blocks prolactin surges and alters FRA expression in the locus coeruleus of female rats. Am J Physiol Endocrinol Metab 293: E1325-E1334, 2007. First published August 28, 2007; doi:10.1152/ajpendo.00341.2007.-The nature of the circadian signal from the suprachiasmatic nucleus (SCN) required for prolactin (PRL) surges is unknown. Because the SCN neuronal circadian rhythm is determined by a feedback loop of Period (Per) 1, Per2, and circadian locomotor output cycles kaput (Clock) gene expressions, we investigated the effect of SCN rhythmicity on PRL surges by disrupting this loop. Because lesion of the locus coeruleus (LC) abolishes PRL surges and these neurons receive SCN projections, we investigated the role of SCN rhythmicity in the LC neuronal circadian rhythm as a possible component of the circadian mechanism regulating PRL surges. Cycling rats on proestrous day and estradiol-treated ovariectomized rats received injections of antisense or random-sequence deoxyoligonucleotide cocktails for clock genes (Per1, Per2, and Clock) in the SCN, and blood samples were taken for PRL measurements. The percentage of tyrosine hydroxylase-positive neurons immunoreactive to Fos-related antigen (FRA) was determined in ovariectomized rats submitted to the cocktail injections and in a 12:12-h light:dark (LD) or constant dark (DD) environment. The antisense cocktail abolished both the proestrous and the estradiolinduced PRL surges observed in the afternoon and the increase of FRA expression in the LC neurons at Zeitgeber time 14 in LD and at circadian time 14 in DD. Because SCN afferents and efferents were probably preserved, the SCN rhythmicity is essential for the magnitude of daily PRL surges in female rats as well as for LC neuronal circadian rhythm. SCN neurons therefore determine PRL secretory surges, possibly by modulating LC circadian neuronal activity. norepinephrine; PERIOD1; PERIOD2 IN FEMALE RATS, PROLACTIN (PRL) secretion has been proposed to result from synchronization among a circadian neural signal from the suprachiasmatic nucleus (SCN), both inhibitory and stimulatory neurotransmitter actions on the pituitary gland, and modulating effects of ovarian steroids (19). As a consequence of this synchronization, female rats display cyclical increases of PRL secretion characterized by two surges: the preovulatory surge of proestrous afternoon (64) and the secondary surge of estrous afternoon (12,70).The preovulatory surge of PRL depends on the levels of plasma ovarian steroids, because the increase of estradiol titers induces (45) and the increase of progesterone amplifies (76) this surge. Estradiol treatment in ovariectomized (OVX) rats induces daily PRL surges that occur at roughly the same time of day as the proestrous surge (13). Shifting of the light phase results in a coincident shift of the proestrous surge (10), and the estradiol-induced PRL surges of OVX rats free run in constant light (52), but the m...

Background/Aims: Cervical stimulation induces a circadian rhythm of prolactin secretion and antiphase dopamine release. The suprachiasmatic nucleus (SCN) controls this rhythm, and we propose that it does so through clock gene expression within the SCN. Methods: To test this hypothesis, serial blood samples were taken from animals injected with an antisense deoxyoligonucleotide cocktail for clock genes (generated against the 5′ transcription start site and 3′ cap site of per1, per2, and clock mRNA) or with a random-sequence deoxyoligonucleotide in the SCN. To determine whether disruption of clock genes in the SCN compromises the neural mechanism controlling prolactin secretion, we sacrificed another group of rats (under the same treatments) at 12.00 or 17.00 h. Dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) were measured using HPLC/electrochemical detection in the median eminence as well as the intermediate and the neural lobe of the pituitary gland, and the DOPAC:dopamine ratio was used as an index of dopamine activity. Vasoactive intestinal polypeptide (VIP) content was determined in tissue punches of the SCN and paraventricular nucleus (PVN), an SCN efferent. Results: Treatment with clock gene antisense deoxyoligonucleotide cocktail abolished both the diurnal and nocturnal prolactin surges induced by cervical stimulation. This treatment abolished the antiphase relationship established by cervical stimulation between dopamine neuronal activity and prolactin secretion. Also, VIP content increased in the SCN and decreased in the PVN. Conclusion: These results suggest that the SCN clock determines the circadian rhythm of prolactin secretion in cervically stimulated rats by regulating dopamine neuronal activity and VIP inputs to the PVN.