A possible dual regulation of prolactin release by the serotoninergic system in rats at pro-oestrus and during late pregnancy: role of ovarian hormones

Impact of ovarian hormones on the modulation of the serotonin transporter by fluvoxamine

Most preclinical studies examining the mechanism(s) of action of antidepressants are carried out using male animals. Blockade of serotonin transporter (SERT) function by selective serotonin reuptake inhibitors (SSRIs) is the initial event that triggers a not completely understood process that results in clinical improvement in depression. To investigate whether there are differences in the ability of SSRIs to inhibit the SERT between male and female rats at different phases of the estrous cycle, clearance of locally applied serotonin (5-HT) was measured by in vivo chronoamperometry. Local application of the SSRI, fluvoxamine, directly into the CA3 area of hippocampus increased significantly 5-HT clearance time parameters in male rats and female rats in estrus or diestrus, but not in proestrus. The contribution of ovarian steroids to this result was investigated in ovariectomized (OVX) rats treated with estradiol benzoate (EB) and/or progesterone (P). In OVX-control rats, fluvoxamine increased clearance time parameters, whereas EB and/or P treatment blocked this effect, consistent with what was seen in female rats in proestrus. This effect was gender-specific, since treatment of castrated rats with EB/P had no effect on the ability of fluvoxamine to slow 5-HT clearance. The time course of hormonal effects showed that 1-60 min after local application of 17-beta-estradiol (E(2)) into the CA3 region of OVX rats, fluvoxamine had no effect on clearance time of 5-HT. E(2)-BSA mimicked E(2)'s effects at 10 min but not at 60 min. Pretreatment with estrogen receptor antagonists blocked the effects of E(2). The finding that acutely both estradiol and progesterone can inhibit the ability of an SSRI to slow the clearance of 5-HT, may have important implications for the use of SSRIs in women.

Prolactin secretory surge during estrus coincides with increased dopamine activity in the hypothalamus and preoptic area and is not altered by ovariectomy on proestrus

Prolactin (PRL) secretory surges have been reported on the afternoons of both proestrus and estrous in cycling rats. As neuroendocrine regulation of estrous PRL surge is poorly understood, the present study aimed to investigate the involvement of hypothalamic dopamine and serotonin as well as of plasma ovarian steroids in this hormonal surge generation. For that, we determined the concentrations of dopamine, serotonin and their respective metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindole-3-acetic acid (5-HIAA) in the mediobasal hypothalamus (MBH) and medial preoptic area (MPOA) throughout the day of estrus and correlated them with plasma PRL levels. In a second study, we evaluated the effect of ovariectomy on the morning of proestrus on PRL surges of both proestrus and estrus. Dopamine turnover, as determined by DOPAC/dopamine ratio, increased in both the MBH and MPOA coinciding with the afternoon PRL surge on estrus. In contrast, both the concentration and turnover (5-HIAA/serotonin) of serotonin within these areas were unaltered during estrus. In addition, ovariectomy reduced plasma estradiol and progesterone levels but did not alter the PRL surges on proestrus and estrus. Considering that dopamine is the main inhibitor of PRL release and that PRL auto-regulates its secretion through a short-loop feedback mechanism, our present results suggest that PRL may suppress its own secretion during the estrus surge through the activation of the dopaminergic neurons in the MBH and MPOA. In addition, the PRL surge on estrus seems do not depend on either the activity of hypothalamic serotonin or the increased secretion of ovarian steroids on proestrus.

Hormonal profile and reproductive performance in lactation deficient (OFA hr/hr) and normal (Sprague–Dawley) female rats

Lactation deficiency may have important consequences on infant health, particularly in populations of low socioeconomic status. The OFAhr/hr(OFA) strain of rats, derived from Sprague–Dawley (SD) rats, has deficient lactation and is a good model of lactation failure. We examined the reproductive performance and hormonal profiles in OFA and SD strains to determine the cause(s) of the lactation failure of the OFA strain. We measured hormonal (PRL, GH, gonadotropins, oxytocin, and progesterone) levels by RIA in cycling, pregnant, and lactating rats and in response to suckling. Dopaminergic metabolism was assessed by determination of mediobasal hypothalamic dopamine and dihydroxyphenylacetic acid (DOPAC) concentrations by HPLC and tyrosine hydroxylase expression by immunocytochemistry and western blot. OFA rats have normal fertility but 50% of the litters die of malnutrition on early lactation; only 6% of the mothers show normal lactation. The OFA rats showed lower circulating PRL during lactation, increased hypothalamic dopamine and DOPAC, and impaired milk ejection with decreased PRL and oxytocin response to suckling. Before parturition, PRL release and lactogenesis were normal, but dopaminergic metabolism was altered, suggesting activation of the dopaminergic system in OFA but not in SD rats. The number of arcuate and periventricular neurons expressing tyrosine hydroxylase was higher in SD rats, but hypothalamic expression of TH was higher in OFA rats at the end of pregnancy and early lactation. These results suggest that the OFA rats have impaired PRL release linked with an augmented dopaminergic tone which could be partially responsible for the lactational failure.

Neurotransmitters involved in the opioid regulation of prolactin secretion at the end of pregnancy in rats

Using a pharmacological approach, we explored potential mechanisms for the regulation of prolactin secretion by opioid peptides at the end of pregnancy in rats. On day 19 of pregnancy, intracereboventricular administration of the mu-opioid receptor agonist (D-Ala2, NMe-Phe4, Gly-ol5)-enkephalin (DAMGO) or beta-endorphin (beta-END) induced a dose-related increase in serum prolactin levels 30 min later. Pretreatment with the opioid antagonist naloxone abolished the increase induced by DAMGO injection. At lower doses, DAMGO and beta-END did not modify the 3,4-dihydroxyphenylacetic acid/dopamine ratio, but at higher doses, the mu-agonists evoked a significant increase of the dopaminergic activity as compared with saline control. The time course of the effects of beta-END (2.5 microg/rat) showed a higher increase in serum prolactin levels at 15 min than at 30 min after treatment. The 3,4-dihydroxyphenylacetic acid/dopamine ratio increased 15 min after beta-END administration and was even higher 30 min later. Neither the selective kappa-agonist U50,488H nor the selective delta-agonist (D-Pen2, D-Pen5)- enkephalin were able to modify the serum prolactin levels at the doses studied. To evaluate potential neurotransmitters involved in the regulation of prolactin secretion at the end of pregnancy, we combined the administration of serotoninergic or GABAergic antagonists with the opioid agonist DAMGO. The serotonin 5-HT2 receptor antagonist ketanserin increased the serum prolactin levels and potentiated the effect of DAMGO. The intracerebroventricular administration of SR-95531 did not modify the serum prolactin concentration under basal conditions, but partially prevented the increase induced by DAMGO injection. The intracerebroventricular administration of the GABA(B) receptor antagonist phaclofen had no effect on the serum prolactin levels either in naive or DAMGO-treated rats. The present results support the proposal that activation of mu-opioid receptors stimulates prolactin secretion at the end of pregnancy. Although the exact mechanisms by which the opioid system modulates prolactin secretion at the end of pregnancy are unclear, these results suggest an interaction of the opioidergic system with serotoninergic and GABAergic systems, without ruling out a direct or indirect action on dopaminergic neurons. In conclusion, the opioid system may regulate prolactin secretion at the end of pregnancy through either stimulatory (present results) or inhibitory actions previously described.

Regulation of serotonin re-uptake transporter mRNA expression by ovarian steroids in rhesus macaques

It has been widely hypothesized that the ovarian steroids, estrogen (E) and progesterone (P), act on serotonin neurons to modulate mood and increase prolactin secretion in women. However, information is needed on the molecular consequences of ovarian hormone action in serotonin neurons. This study examined the effect of E and P on the expression of mRNA for the serotonin re-uptake transporter (SERT) in monkeys using in situ hybridization and a 253 bp human SERT cRNA probe. Monkeys (n=5 animals/group) were ovariectomized and hysterectomized (spayed) and then untreated (control), or treated, with E for 28 days (E treated) or treated with E for 28 days and supplemented with P for the last 14 days of the E regimen (E+P treated). Densitometric analysis of autoradiographs with gray-level thresholding was performed at five levels of the dorsal and median raphe. The number of pixels exceeding background in defined areas was obtained (pixel number). The average pixel number for spayed, E- and E+P-treated groups was 22 280+/-3517, 15 227+/-1714, and 14 827+/-2042, respectively, in the combined dorsal and median raphe. In the E- and E+P-treated groups compared to the control group, there was a 32% and 33% decrease in SERT mRNA signal represented by pixel number (ANOVA, P<0.05). Hence, E- and E+P-treated groups were significantly less than the control group, but they were not different from one another. Also, there were significantly fewer SERT mRNA-positive cells in the dorsal raphe of E- and E+P-treated groups (ANOVA, P<0.001). Therefore E, with or without P, reduces SERT mRNA expression. These results suggest that the ability of P to increase prolactin secretion in E-primed monkeys does not involve an action at the level of SERT gene transcription. Hence, the mechanism by which the CNS transduces the action of P on prolactin secretion remains to be elucidated. However, these data suggest that one action of E replacement therapy in postmenopausal women may be to decrease expression of the SERT gene.

Ovarian steroid regulation of tryptophan hydroxylase mRNA expression in rhesus macaques

Progesterone (P) stimulates prolactin secretion through an unknown neural mechanism in estrogen (E)-primed female monkeys. Serotonin is a stimulatory neurotransmitter in prolactin regulation, and this laboratory has shown previously that E induces progestin receptors (PR) in serotonin neurons. Therefore, we questioned whether E and/or E+P increased serotonin neural function. The expression of mRNA for tryptophan hydroxylase (TPH) was examined in ovariectomized (spayed) control, E-treated (28 d), and E+P-treated monkeys (14 d E and 14 d E+P) using in situ hybridization and a 249 bp TPH cRNA probe generated with RT-PCR (n = 5 animals/group). Densitometric analysis of film autoradiographs revealed a ninefold increase in TPH mRNA in E-treated macaques compared to spayed animals (p < 0.05). With supplemental P treatment, TPH mRNA signal was increased fivefold over spayed animals (p < 0.05), but was not significantly different compared to E-treated animals. These results were verified by grain counts from photographic emulsion-coated slides. There were significantly higher single-cell levels of TPH mRNA in serotonergic neurons of the dorsal raphe in E- and E+P-treated groups (p < 0.05). These data indicate that E induces TPH gene expression in nonhuman primates and that the addition of P has little additive effect on TPH gene expression. Thus, the action of P on prolactin secretion is probably not mediated at the level of TPH gene transcription. However, because P increases raphe serotonin content in E-primed rodents, the possibility remains that P may have other actions on post-translational processing or enzyme activity.

Central administration of serotonin decreases tyrosine hydroxylase catalytic activity and messenger ribonucleic Acid signal levels in the hypothalamus of female rats

We investigated the effect of central serotonin (5-hydroxytryptamine, 5-HT) administration on hypothalamic tuberoinfundibular dopamine neurons and related changes in neuronal activity to circulating prolactin (PRL) levels. Ovariectomized rats were treated with either vehicle or 5-HT through a lateral ventricular cannula in one of two dose paradigms: 1) a bolus of 20 μg, with tissues taken at 30 min, or 2) the same bolus immediately followed by 20 μg/30 min via a syringe pump for 120 min, and tissues taken at 120 min. Blood samples were taken throughout experiments and plasma PRL determined by radioimmunoassay. Under both paradigms, NSD 1015, a dihydroxyphenylalanine (DOPA) decarboxylase inhibitor (25 mg/kg intraarterially) was injected 10 min before decapitation and brain excision followed by stalk-median eminence dissection. The rate of DOPA accumulation, determined by measuring DOPA levels in the stalk-median eminence by high-performance liquid chromatography with electrochemical detection was used as a measure of tyrosine hydroxylase (TH) catalytic activity. Stalk-median eminence DOPA accumulation in control rats was 29.9 ± 4.2 and 28.8 ± 4,4 ng/mg protein (30 and 120 min experiments, respectively). DOPA accumulation in 5-HT-treated rats was significantly reduced (P<0.05) after 30 min to 17.8 ± 1.2 ng/mg protein, but it was similar (21.7 ± 3.9) to controls after 120 min of 5-HT infusion. 5-HT levels in the stalk-median eminence of rats treated with 5-HT were 13- to 17-fold greater than controls (16.9 to 18.5 ng/mg protein). Plasma PRL levels in both groups increased 10-fold after 5-HT treatment with a peak at 5 min, returning to baseline by 120 min. TH mRNA levels were determined by in situ hybridization in a second group of rats which were treated with the 20μg bolus and subsequent 120 min infusion of 5-HT. TH mRNA signal levels in the arcuate nucleus of control rats averaged 144 ± 21 grains/cell. After treatment with 5-HT, TH mRNA levels in the arcuate nucleus were significantly lower (P<0.0001) with 69±14 grains/cell. In a third group of rats, the effects of the 30 min 5-HT treatment on TH catalytic activity and circulating PRL levels was challenged with two 5-HT(2) receptor antagonists, LY53857 (5 mg/kg intraperitoneally) or ketanserin (10 mg/kg intraperitoneally). Neither the 5-HT-induced decrease in TH catalytic activity nor the increase in PRL was altered by pretreatment (120 min) with 5-HT(2) antagonists. These data suggest that central 5-HT is capable of decreasing TH activity and TH mRNA levels in the tuberoinfundibular dopamine neurons and that the decrease in dopaminergic neuronal activity may contribute to the 5-HT-induced PRL rise. The changes in TH catalytic activity and PRL after intracerebroventricular administration of 5-HT do not appear to be mediated by 5-HT(2) receptors.

Neurohormonal Factors Involved in the Control of the Nocturnal Prolactin Surge that Precedes Parturition in the Rat

A nocturnal surge of prolactin (PRL) occurs in the dark period preceding parturition in the rat. The roles of oxytocin, vasoactive intestinal peptide (VIP), serotonin and the opioids in controlling the antepartum PRL surge were investigated by examining PRL secretion over the last 2 days of pregnancy in the presence of antagonists to these neurohormonal factors. Serial blood samples were collected from unanesthetized, freely moving rats via indwelling jugular cannulae, and plasma PRL was measured by radioimmunoassay. In control rats PRL levels rose in a nocturnal surge peaking at 223 ± 34 ng/ml (n = 6) at 0500 h on day 21 of pregnancy, the day of parturition. Intra-arterial infusion of the oxytocin antagonist desGly-NH(2) d(CH(2) )(5) [Tyr(Me)(2) , Thr(4) ]-OVT at a dose sufficient to completely block milk ejection (10 μg/h) had no effect on this PRL surge. Infusion of the VIP antagonist [4Cl-D-Phe(6) ,Leu(17) ]-VIP at 2 μg/h from 2200 h on day 20 until 0500 h on day 21 significantly attenuated the antepartum PRL surge, reducing the peak to 76 ± 28 ng/ml at 0500 h on day 21 (n = 6; P<0.001). Naloxone, the opiate receptor antagonist, inhibited the antepartum PRL surge in a dose-dependent manner. Infused at 2 mg/h naloxone partially reduced the magnitude of the PRL surge, which peaked at 128 ± 24 ng/ml at 0300 h on day 21 (n = 4; P<0.05), while at 10 mg/h naloxone totally abolished the PRL surge (n = 6; P<0.001). Injection of the serotonin synthesis inhibitor ρ-chlorophenylalanine (250 mg/kg, sc at 1700 h on days 19 and 20 of pregnancy) increased the magnitude of the antepartum PRL surge to a peak of 327 ± 48 ng/ml at 0500 h on day 21 (n = 5), compared with 244 ± 24 ng/ml at the same time in vehicle-injected controls (P<0.05; n = 5). The results demonstrate that the antepartum PRL surge is stimulated by an opioid mechanism, and also by VIP. Oxytocin and serotonin have no role in stimulating PRL secretion during late pregnancy.

A selective serotonin reuptake inhibitor, fluoxetine hydrochloride, modulates the pulsatile release of prolactin in postmenopausal women

We investigated serotoninergic regulation of prolactin release in estrogen-withdrawn postmenopausal women by using a serotonin reuptake inhibitor. Subjects underwent frequent blood sampling after placebo (basal) or fluoxetine administration. Mean 24-hour serum prolactin concentrations increased significantly in response to fluoxetine. Objective pulse analysis revealed no change in prolactin pulse frequency with serotoninergic stimulation, but maximal serum prolactin peak heights increased significantly. Multiple-parameter deconvolution disclosed no change in prolactin half-life, but a significant increase in the total mass of prolactin secreted per 24 hours during fluoxetine administration. Cosinor analysis of the prolactin time series showed a significant increase in the circadian amplitude and mean without any change in the time of maximal concentration during treatment with fluoxetine. We conclude that short-term activation of the serotoninergic system in the absence of substantial estradiol and opiatergic tone significantly increases the secretion of prolactin in postmenopausal women.