Antisera to vasoactive intestinal polypeptide inhibit basal prolactin release from dispersed anterior pituitary cells

Vasoactive intestinal polypeptide (VIP) has been identified in hypothalamic tissue, is secreted into hypophysial portal blood, and stimulates prolactin (PRL) release in vivo and in vitro. It has been proposed, therefore, that VIP is a physiologic PRL-releasing factor. In this study, we confirm that VIP stimulates PRL release from rat pituitary cells in vitro, and demonstrate that an anti-VIP antiserum blocks VIP-induced PRL secretion. Surprisingly, the anti-VIP antiserum inhibited basal PRL secretion from rat pituitary cells in 3 separate experiments. Data from these experiments were pooled, as the responses were similar, revealing basal PRL release of 10.7 +/- 1.3 ng rPRL/10(5) cells (X +/- SE), while anti-VIP antisera significantly inhibited release to 4.4 +/- 0.6 ng rPRL/10(5) cells (p less than 0.001). PRL release in incubates containing control non-immune sera did not differ from basal release, 8.1 ng rPRL/10(5) cells. A further control experiment was conducted wherein cells were incubated with an anti-ACTH antiserum, representing another hyperimmune serum, which had no effect on PRL secretion. These data suggest that VIP, in addition to its possible role as a hypothalamic-derived PRL-releasing factor, may play a role within the pituitary as a regulator of basal PRL secretion.     

Effect of passive immunization with antisera to vasoactive intestinal polypeptide and peptide histidine isoleucine amide on 5-hydroxy-L-tryptophan-induced prolactin release in rats

The present study was aimed to clarify, by use of the passive immunization method, the involvement of endogenous vasoactive intestinal polypeptide (VIP) and peptide histidine isoleucine amide (PHI)-like peptides in the stimulation of PRL-like immunoreactive material release induced by 5-hydroxy-L-tryptophan (5HTP), a serotonin precursor. We used conscious, freely moving male rats of the Wistar strain (BW, 250-300 g) chronically cannulated with atrial catheters. Anti-VIP serum (AVS) and anti-PHI serum (APS), each generated in rabbits against synthetic porcine VIP and natural porcine PHI, respectively, were highly potent [maximum binding capacity (Bmax): AVS, 55.5 nmol/ml; APS, 5.53 nmol/ml] and specific. Bolus injection of 5HTP (10 mg/kg BW) through the catheter caused a significant increase in plasma PRL (nanograms per ml) in rats pretreated with normal rabbit serum (NRS) [14.3 +/- (SE) 3.8----56.3 +/- 11.2], with AVS (12.3 +/- 3.5----48.5 +/- 6.2), with APS (10.5 +/- 3.9----43.5 +/- 8.8), and with AVS plus APS (9.0 +/- 1.4----28.5 +/- 2.7). The basal PRL concentrations did not differ significantly among these groups, whereas the PRL responses to 5HTP were significantly blunted in AVS plus APS-pretreated rats (P less than 0.05 vs. NRS). To eliminate the modification by dopaminergic control of 5HTP-induced PRL release, the next experiment was performed in rats repeatedly injected with sulpiride, a dopamine receptor antagonist (5 mg/kg BW), every 30 min. The first injection of sulpiride caused a prompt and marked increase in plasma PRL, followed by decreasing but still high levels of plasma PRL upon the subsequent injections of sulpiride every 30 min. The cumulative release area of PRL after pretreatment with AVS plus APS or APS alone was significantly lower than that after NRS (P less than 0.05). The same dose of 5HTP resulted in a significant further increase in plasma PRL exceeding the levels elevated by sulpiride injections in NRS-treated rats. Prior simultaneous administration of AVS and APS resulted in a complete suppression of 5HTP-induced PRL release, whereas pretreatment with either AVS or APS showed only a minimal effect. These results suggest that endogenous VIP and PHI-like peptides are PRL-releasing factors, involved at least in the mechanism of 5HTP-induced PRL release, in which the dopaminergic control may be also involved.     

Evidence that thyrotropin-releasing hormone is not a major prolactin-releasing factor during suckling in the rat

The purpose of this study was to investigate whether TRH could be an important PRL-releasing factor during suckling in the rat. Plasma PRL, TSH, beta-endorphin-like immunoreactivity, and GH responses in serial blood samples from unanesthetized suckled rats were determined. The resulting hormonal profile was compared with that obtained when TRH (500 ng/kg BW, iv) was injected at the onset of suckling. Suckling evoked a rise in plasma levels of PRL, beta-endorphin-like immunoreactivity, and GH, but not in TSH. In contrast, exogenous TRH caused a 9-fold increase in plasma TSH levels during suckling without further increasing the PRL response. Since plasma PRL responses are reportedly enhanced by previous suckling, we also determined plasma PRL and TSH levels when TRH (25 ng/rat, iv) was given 30 min after a brief suckling episode. TRH caused a 2.5-fold increase in plasma TSH, but did not significantly increase plasma PRL levels. Since suckling increases plasma PRL without increasing plasma TSH, and TRH increases TSH but not PRL levels, we conclude that TRH is not a major PRL-releasing factor during suckling.     

Vasoactive intestinal peptide is a hypothalamic prolactin-releasing neuropeptide in the turkey (Meleagris gallopavo)

The hypothesis that vasoactive intestinal peptide (VIP) functions as a hypothalamic prolactin (PRL)-releasing peptide in the turkey was tested by determining the effects of hypothalamic VIP immunoneutralization and pituitary VIP receptor blockade on hypothalamic extract (HE)-induced PRL secretion from dispersed anterior pituitaries. Incubation of cells with porcine VIP (pVIP; 0.5 or 10 nM) significantly stimulated PRL secretion. This effect was inhibited in a dose-related manner by 1-hr preincubation of pVIP with a VIP antisera (A/S; 1:500-1:50,000). Likewise, HE (0.3 equivalent)-stimulated PRL secretion was inhibited by preincubation with VIP A/S (P less than 0.0001). A 96-98% reduction in PRL secretion was obtained from cells cultured with HE, that was previously incubated with 1/500 dilution of antiserum. Pretreatment of pituitary cells for 15 min with [4Cl-D-Phe6,Leu17] VIP, a VIP receptor antagonist (10(-5) M), significantly depressed the PRL response to 0.5 nM VIP (9.9 +/- 0.5 micrograms/500,000 cells vs 4.9 +/- 0.1 micrograms/500,000 cells; 22.4 +/- 0.9 micrograms/500,000 cells vs 14.7 +/- 0.4 micrograms/500,000 cells) or 0.3 eq HE (8.8 +/- 0.6 micrograms/500,000 cells vs 5.2 +/- 0.2 micrograms/500,000 cells; 15.3 +/- 0.3 micrograms/500,000 cells vs 8.2 +/- 0.2 micrograms/500,000 cells). These results suggest that hypothalamic stimulation of PRL secretion appears to be mediated by receptors specific for VIP and that VIP is an endogenous hypothalamic PRL-releasing peptide in the turkey.     

Paraventricular nucleus mediates prolactin secretory responses to restraint stress, ether stress, and 5-hydroxy-L-tryptophan injection in the rat

The role of the paraventricular nucleus (PVN) in mediating acute stimulatory PRL responses was investigated in conscious male rats. Electrolytic lesions, verified histologically at autopsy, were stereotaxically made in the PVN region, and sham lesions were made in control rats. Blood was obtained through a chronically indwelling catheter in the right atrium. PVN-lesioned (PVL) rats showed significantly lower T3 levels 1 week after surgery (less than 34.4 ng/dl) compared with sham (mean +/- SEM, 91.2 +/- 5.0 ng/dl) and intact (86.8 +/- 2.0 ng/dl) animals, verifying a lesion in the PVN. T3 was restored to normal (95.6 +/- 1.8 ng/dl) by daily sc administration of T4 (10 micrograms/kg BW) for at least 4 days before the day of the experiments. Basal PRL levels in PVL rats did not differ significantly from those in control or sham-lesioned animals. In response to restraint stress, plasma PRL levels of PVL rats did not rise, in contrast to marked elevation in PRL in sham and intact rats [PRL levels (mean +/- SEM; nanograms per ml), basal to peak: PVL, 4.3 +/- 0.3 to 4.5 +/- 0.4; sham, 4.5 +/- 0.5 to 47.0 +/- 4.1; intact, 4.0 +/- 0.3 to 46.3 +/- 4.9]. PVL also resulted in the complete inhibition of PRL secretion induced by 30-min inhalation of ether (basal to peak: PVL, 3.3 +/- 0.3 to 4.5 +/- 0.2; sham, 5.7 +/- 0.8 to 19.9 +/- 0.9; intact, 3.3 +/- 0.4 to 27.9 +/- 4.0). The stimulatory effect on plasma PRL in sham and intact rats by one iv bolus injection of the serotonin precursor 5-hydroxy-L-tryptophan (5-HTP; 10 mg/kg BW) was completely abolished in PVL animals (basal to peak: PVL, 3.7 +/- 0.6 to 5.2 +/- 1.4; sham, 6.7 +/- 0.6 to 36.0 +/- 0.5; intact, 4.1 +/- 1.2 to 33.3 +/- 3.2). In contrast to the marked alteration in PRL regulation, PVL rats exhibited a typical ultradian rhythm of plasma GH secretion during a 6-h observation period and increased release of GH induced by iv injection of 5-HTP [GH (nanograms per ml), basal to peak; PVL, 4.5 +/- 0.6 to 21.0 +/- 4.9; sham, 3.7 +/- 0.3 to 18.4 +/- 4.4; intact, 2.9 +/- 0.1 to 17.8 +/- 3.5]. These findings indicate that PRL responses to stress and to serotonin act through the PVN, the site of origin of several putative PRL-releasing factors.(ABSTRACT TRUNCATED AT 400 WORDS)     

A prospective longitudinal study of the release of oxytocin and prolactin in response to infant suckling in long term lactation

We studied prospectively eight healthy postpartum breast-feeding women for 6 months during early, middle, and late lactation. Blood for measurement of oxytocin (OT), PRL, and arginine vasopressin was drawn before and every 3 min from each women during 15 min of infant suckling for two consecutive feedings during each stage of lactation. Basal plasma OT was not different in breast-feeding [0.7 +/- 0.1 (+/- SEM) microU/ml] and nonbreast-feeding women (0.8 +/- 0.2 microU/ml). OT increased significantly in response to infant suckling (P less than 0.00001) to 5.9 +/- 0.5 microU/ml and remained elevated throughout a feeding. OT was released during infant suckling in an episodic pattern in some, but not all, women; peak OT varied among women (5.0-23.3 microU/ml). There was no significant difference in the mean stimulated OT or the pattern of release comparing the first and second feedings of the same day. The mean OT (n = 8) released during 15 min of infant suckling was not significantly different in early (3.9 +/- 0.3 microU/ml), middle (4.5 +/- 0.3 microU/ml), and late (5.8 +/- 0.4 microU/ml) lactation. In the four women who breast fed exclusively, the mean stimulated OT was significantly higher (P less than 0.01) during late lactation (8.6 +/- 0.4 microU/ml) vs. early (4.6 +/- 0.4 microU/ml) or middle (6.1 +/- 0.4 microU/ml) lactation. In the other four women who provided formula supplements, OT did not change. Plasma arginine vasopressin did not increase in response to infant suckling. Plasma PRL increased in response to infant suckling, reaching a peak at 15 min. Mean basal PRL decreased progressively from weeks 1-24 postpartum. Mean peak PRL decreased significantly from early (162 +/- 29) to middle (130 +/- 15) to late (77 +/- 10) ng/ml lactation (P less than 0.05). OT release in response to infant suckling continues throughout the first 6 months postpartum in breast-feeding women, and the pattern is reproducible. The maximum release of OT is dependent upon continuous regular nipple stimulation. In contrast to basal and suckling-induced levels of PRL, which decreased with time postpartum, basal and suckling-induced OT release did not decrease from early to late lactation.     

Effects of calcium channel blockade with verapamil on the prolactin responses to TRH, L-dopa, and bromocriptine.

To determine the mechanisms by which calcium channel blockade with verapamil causes hyperprolactinemia, the authors investigated the effects of this blockade on the prolactin (PRL) responses to stimulation by thyrotropin releasing hormone (TRH) and inhibition by dopamine, using L-dopa and bromocriptine. Verapamil, given for 1 week at a dosage of 240 mg orally to eight healthy volunteers, induced a significant elevation of basal PRL levels (17.3 +/- 1.8 ng/ml to 30.9 +/- 4.3 ng/ml, p < 0.005). Verapamil also caused an increase in the PRL response to a TRH (100 micrograms). However, when the increased basal level was considered by calculating the area under the THR response curve and subtracting the basal values, this increase (1763.4 +/- 202.6 ng/ml.min to 2260.6 +/- 223.9 ng/ml.min) was not found to be statistically significant (p > 0.05). Verapamil had no effect on the basal or TRH-stimulated thyroid stimulating hormone levels. In these same volunteers, PRL levels decreased from 13.2 +/- 2.5 ng/ml to a nadir of 5.5 +/- 1.6 ng/ml in response to L-dopa. After 1 week of verapamil 240 mg, basal PRL levels were elevated to 21.5 +/- 3.1 ng/ml, then decreased to 8.2 +/- 1.8 ng/ml with L-dopa. The percentage decreased in PRL in response to L-dopa (60 +/- 5% versus 62 +/- 3%) were not significantly different (p > 0.05). Verapamil had no effect on the basal or L-dopa-stimulated growth hormone levels. Bromocriptine 2.5 mg given to five volunteers twice daily caused PRL levels to fall from 13.3 +/- 1.6 ng/ml to 5.0 +/- 0.9 ng/ml.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence that thyrotropin-releasing hormone and a hypothalamic prolactin-releasing factor may function in the release of prolactin in the lactating rat

We have compared the effectiveness of TRH and a rat hypothalamic PRL-releasing factor (PRF; previously incubated with rat serum to destroy TRH) in stimulating the release of PRL into the plasma of conscious lactating rats when injected before and after pituitary PRL had been depleted and transformed into releasable PRL by 10 min of suckling. TRH (1.25 microgramsss) and PRF [equivalent to 2.5 stalk median eminence (SME) fragments] each caused a small increase (38 and 30 ng, respectively) in the plasma PRL concentration within 10 min when injected into nondepleted mothers. The levels then fell quickly. Suckling, by comparison, caused a sustained 175 ng/ml increase above basal levels. Though PRL depletion occurred, as expected, as a result of suckling, there was no measurable depletion within the pituitaries of TRH- or PRF-injected rats. By contrast, the iv administration of TRH (doses ranging from 2-250 ng) and hypothalamic PRF (doses ranging from 0.2-1.0 SME equivalent) after depletion-transformation had been effected by 10 min of suckling resulted in a rapid and, in most instances, a sustained elevation in the plasma PRL concentration comparable to that seen after suckling. Dose-response relationships, though, were not clearly evident with either PRF or TRH. Neither saline, 1.25 microgram TRH previously incubated in serum, 50 mU oxytocin, 1 microgram dopamine, 25 microgram LHRH, nor an extract of cerebral cortex prepared in the same manner as hypothalamic TRH caused plasma PRL to rise after PRL depletion. We conclude that TRH and possibly a separate hypothalamic PRF have a stimulatory action upon the releasable, but not upon the depletion-transformation, phase of PRL secretion in the lactating rat.     

Posterior pituitary lobectomy abolishes the suckling-induced rise in prolactin (PRL): evidence for a PRL-releasing factor in the posterior pituitary

The aim of this study was to determine the role of the posterior pituitary in the regulation of PRL release during suckling. Lactating rats were subjected to posterior pituitary lobectomy (LOBEX) or sham surgery (SHAM) and separation from pups in the evening; experimental manipulations and blood collection were performed the next morning. In the first experiment rats were divided into three groups: SHAM, LOBEX, and LOBEX treated with a vasopressin analog, 1-desamino-8-D-arginine vasopressin and oxytocin. Plasma PRL levels in SHAM rats increased 20- to 25-fold upon introduction of pups and remained elevated for the duration of suckling. In contrast, basal plasma PRL levels in LOBEX rats were 3- to 4-fold higher than in SHAM but suckling failed to induce a further increase. Treatment of LOBEX rats with 1-desamino-8-D-arginine vasopressin and oxytocin reduced water consumption and allowed for milk ejection and milk intake by the pups but did not restore the suckling-induced rise in PRL. The second experiment tested the functional integrity of the hypothalamic dopamine (DA) and serotonergic systems after LOBEX and the ability of LOBEX-lactating rats to respond to PRL-releasing stimuli other than suckling. Injections of alpha-methyl-para tyrosine, an inhibitor of tyrosine hydroxylase, and 5-hydroxytryptophan, a precursor of serotonin, caused 20- to 30-fold rises in plasma PRL levels in both LOBEX and SHAM rats. Exposure to ether elicited a 3- to 4-fold rise in PRL which was higher in magnitude and of longer duration in LOBEX than in SHAM rats. Conclusions: Removal of the posterior pituitary from lactating rats results in an increase in basal PRL levels and a complete abolishment of the suckling-induced rise. Vasopressin and oxytocin restore water balance and milk ejection in the LOBEX rat but fail to affect PRL secretion. The LOBEX-lactating rat is not refractory to PRL-releasing stimuli other than suckling and its hypothalamic DA and serotonergic systems are functionally intact. In addition to DA, the posterior pituitary appears to contain a PRL-releasing factor(s) which mediates the suckling-induced rise in PRL.     

Plasma prolactin-releasing factor-like activity in the amenorrhea-galactorrhea syndrome.

An in vitro bioassay for plasma PRL-releasing factor-like activity has been developed. The method is a three-phase methanol extraction of plasma with extracts of 1.0 ml plasma adjusted to a final volume of 50 microliter. Single 50-microliter aliquots of extract were incubated in 1.0 ml Krebs-Ringer phosphate (KRP) buffer with one rat hemipituitary after a 1-h preincubation. Samples were obtained basally and 30 min after addition of the extract. During each set of incubations, a parallel series of hemipituitaries was incubated in KRP alone. The total nanograms of rat PRL released per mg pituitary tissue during the initial 30 min after preincubation was calculated for all studies. The mean quantity released in KRP alone was considered basal and was subtracted from values obtained during incubation with plasma extracts. The quantity remaining was considered PRL-releasing activity (PRA) of plasma, expressed as nanograms of rat PRL released per mg pituitary. The PRA in plasma from 13 patients with the amenorrhea-galactorrhea syndrome was 132 +/- 17 ng/mg pituitary (X +/- SE), which was significantly greater (P less than 0.001) than the PRA in plasma from eight matched controls [31 +/- 10 ng/mg pituitary (X +/- SE)]. The patients' individual PRL levels were elevated (range, 48-248 ng/ml), and when compared to the PRA in the samples, a highly significant (P less than 0.001) positive correlation evolved. These results indicate that a circulating PRL-releasing factor-like material present in normal plasma is higher in plasma from hyperprolactinemic patients in direct relationship to the PRL concentration. It is possible that this material is related to the pathogenesis of PRL-secreting pituitary disorders.     

Periovulatory plasma prolactin response to gonadotropin-releasing hormone: role of endogenous opiates

It has been previously shown that in normal women during the periovulatory period, prolactin (PRL) levels increase after the administration of nonspecific stimuli, such as growth hormone-releasing hormone or gonadotropin-releasing hormone (GnRH). In order to gain insight into the mechanism of this response, we have tested the effect of a naloxone infusion (1.6 mg/h) on the PRL response to GnRH in 5 normal females, aged 20-27 years, tested during the periovulatory period. Naloxone was administered starting 60 min before GnRH administration (100 micrograms as an i.v. bolus). Naloxone clearly blunted the PRL response (basal 10.7 +/- 1.7 ng/ml, peak 11.8 +/- 0.2 ng/ml at 30 min, versus: basal 9.0 +/- 0.5 ng/ml, peak 20.6 +/- 3.9 ng/ml at 45 min after GnRH alone; significance of difference between peaks: p less than 0.05). A secondary late increase of PRL levels was observed, reaching about 60% of basal levels at 120 min (16.8 +/- 2.6 ng/ml). These data indicate that periovulatory PRL dynamics are altered by naloxone administration and suggest a possible involvement of opioid peptides in the 'paradoxical' PRL response to GnRH in normal subjects.     

Ether releases large amounts of prolactin from rat pituitaries previously "depleted" by short term suckling.

The effect of ether inhalation upon plasma PRL levels was assessed in lactating rats whose pituitary stores of the hormone first had been depleted by a short term (10-min) period of suckling in comparison with those whose PRL stores had not been depleted by prior suckling. Ether, unlike suckling or handling-decapitation stress, did not deplete pituitary PRL and caused only a small increase (36 to 52 ng/ml) in the plasma concentration of the hormone. However, when ether was given 10, 60, or 120 min after the plasma concentration had subsided from a prior 10-min period of suckling, the plasma concentration of PRL in each instance rose within 5--10 min to 200--250 ng/ml and was sustained at that level for the remainder of the 30-min exposure period. The amplitude and profile of the plasma PRL concentration after ether inhalation were similar to those obtained when a second suckling, rather than ether, was administered after the short term suckling. These data support the hypothesis that releasable PRL derives from depleted PRL and also indicate that, once it is formed, releasable PRL remains available for discharge into the circulation for a relatively long period of time.     

Evidence for a synergistic effect of somatostatin on vasoactive intestinal polypeptide-induced prolactin release in the rat: comparison with its effect on thyrotropin (TSH)-releasing hormone-stimulated TSH release

The present experiments were carried out to clarify the role of endogenous somatostatin (SRIF) in the regulation of PRL and TSH release. The effects of electrical stimulation of the hypothalamic periventricular nucleus (PE) on vasoactive intestinal polypeptide (VIP)-induced PRL and TRH-stimulated TSH secretion were studied using pentobarbital-anesthetized male rats bearing indwelling cannulae in the right atria. The animals were implanted in the PE with bipolar concentric stimulating electrodes 1 week before the experiments began. The effects of a bolus injection or a continuous infusion of SRIF-14 (iv, 7.6 or 10 nmol/100 g BW, respectively) on the PRL or TSH release induced by VIP or TRH were also examined. Electrical stimulation of the PE significantly enhanced VIP-induced PRL release 19 min after the bolus injection of VIP (from 29.3 +/- 7.2 to 59.7 +/- 14.9 ng/ml, P less than 0.05). A bolus injection of SRIF had a similar effect and increased the PRL response to VIP (from 29.3 +/- 7.2 to 114.7 +/- 22.4 ng/ml, P less than 0.01). Continuous infusion of SRIF did not decrease the stimulatory effect of VIP on PRL release; on the contrary it significantly increased the PRL response to a first VIP injection (10 min after the onset of SRIF-14 infusion) over that observed after a second administration of VIP. Neither electrical stimulation of the PE nor the bolus SRIF-14 injection modified basal PRL secretion. Electrical stimulation of the PE slightly but significantly increased the TSH response to a bolus injection of TRH, but had no effect on the basal TSH release. In contrast, both the bolus injection and the continuous infusion of SRIF-14 significantly and persistently inhibited the TRH-stimulated TSH release. These results suggest that 1) SRIF does not inhibit VIP-induced PRL secretion in vivo but rather enhances it through some unknown mechanism; 2) SRIF inhibits TRH-stimulated TSH secretion.     

Suckling-induced changes of vasoactive intestinal peptide concentrations in hypothalamic areas implicated in the control of prolactin release

The present study was designed to investigate whether the vasoactive intestinal peptide (VIP) concentration in hypothalamic nuclei, dorsal raphe nucleus (DR) and pituitary lobes of lactating rats changes in physiological situations when prolactin (PRL) secretion is stimulated (suckling) or inhibited (pup separation). In addition VIP levels in blood plasma were determined in both situations. Acute suckling induced changes in VIP concentration only in the rostral part of the anterior hypothalamic (rAHN) and the paraventricular (PVN) nuclei of all the brain areas examined. VIP concentration in the rAHN increased at 5 min from 3.52 +/- 0.30 (mean +/- SEM) to 8.67 +/- 1.91 ng/mg protein (p less than 0.05) but fell to baseline values after 30 min suckling (p less than 0.05; 5 vs. 30 min). Although changes in VIP concentration in the suprachiasmatic nucleus (SCN) did not attain statistical significance, they followed the same trends as the changes of VIP in the rAHN. The opposite pattern of changes was observed in the PVN with a decrease in VIP concentration following 5 min suckling (p less than 0.01). At 30 min the VIP values showed a trend towards 0-min values. Pup removal did not affect VIP concentrations in the rAHN, PVN, SCN, median eminence, supraoptic nucleus and DR. VIP values were not detectable in the arcuate nucleus in any of the experimental situations examined. Lactation increased VIP concentration only in the rAHN and PVN when lactating rats with their pups were compared with virgin female diestrous rats. VIP concentration in the anterior lobe of the pituitary from lactating rats did not change with pup separation or suckling.(ABSTRACT TRUNCATED AT 250 WORDS)     

Growth hormone and prolactin responses to bolus and sustained infusions of GRH-1-40-OH in man

To determine whether GRH stimulates PRL secretion we studied the effects of iv bolus injections and prolonged infusions of GRH 1-40-OH on PRL and GH serum levels in normal volunteers. Eight patients with acromegaly, two of whom had elevated basal levels of PRL, were also tested with single bolus injections. Six normal subjects given 3.3 micrograms/kg bolus injections of GRH showed a mean increment of GH of 22.0 +/- 1.7 ng/ml (mean +/- SE). A small rise in PRL was noted in 5 of the 6 subjects (mean peak level of 6.4 +/- 1.9 ng/ml vs basal level of 3.3 +/- 0.4 ng/ml, p less than 0.05). During the continuous intusion of GRH (10 ng/kg/min), GH levels rose gradually from a mean baseline of 1.1 +/- 0.1 ng/ml to a mean peak of 30.0 +/- 7.2 ng/ml at about 2 h and then slowly declined to a nadir of 4.2 +/- 0.4 ng/ml at 330 min. PRL levels did not rise significantly during the infusion. To determine whether the decline in GH levels in the face of continued infusion was due to loss of GH responsiveness, a 3.3 micrograms/kg bolus of GRH was given during the nadir at 330 min; this GH increment was significantly less than that obtained by the GRH bolus injection without the infusion (12.9 +/- 3.5 ng/ml vs 22.0 +/- 1.7 ng/ml, p less than 0.05). The PRL response to the GRH bolus was the same during the infusion of GRH as before. In each of 8 acromegalic patients (including two who had initially elevated basal PRL levels) GRH led to an increase in both GH and PRL levels. PRL and GH levels spontaneously fluctuated in parallel in 4 acromegalic cases studied with repeated samples over 6 h during placebo administration. These experiments show that GRH has significant, though weak, PRF effect in normals and that it is more potent PRF in acromegalic patients. Furthermore, the effects on GH and PRL of a sustained infusion of GRH for 5 1/2 h are both qualitatively and quantitatively different. These results suggest that the GRH effect is exerted either on different pituitary receptors for GH and PRL regulation, or that the releasable pools of the two hormones have different sizes and/or turnover times.     

Does bromocriptine block thyrotropin-releasing hormone-induced prolactin release during pregnancy?

PRL responses to 200 microgram of iv TRH were measured in 16 healthy women with normal early pregnancy before and at the endo of bromocriptine treatment of 5.0--7.5 mg daily for 1--2 weeks. Before the start of bromocriptine, TRH caused a PRL elevation from 19.1 +/- 2.2 to 95.2 +/- 12.6 ng/ml (mean +/- SE) after 20 min, with a mean maximal PRL increment of 71.7 +/- 11.6 ng/ml. Bromocriptine suppressed basal plasma PRL level to 3.6 +/- 0.8 ng/ml (P less than 0.001). TRH then caused a PRL rise to 18.8 +/- 1.8 ng/ml at 20 min, with a mean maximal PRL increment of 15.7 +/- 1.8 ng/ml. The absolute PRL response was significantly smaller (P less than 0.001) during bromocriptine intake than before, whereas the mean percent increments in PRL levels after TRH administration were similar in the presence and absence of bromocriptine. Fifteen of these women were restudied with TRH stimulation 4--6 weeks after legal abortion, and the PRL responses to TRH were normal. When 7 of these women were once again treated with bromocriptine and retested with TRH, no absolute or relative PRL response to TRH emerged. These results release differs between the pregnant and nonpregnant states.     

Plasma levels of vasoactive intestinal polypeptide and oxytocin in response to suckling, electrical stimulation of the mammary nerve and oxytocin infusion in rats

The aim of the present study was to measure plasma levels of vasoactive intestinal polypeptide (VIP) and oxytocin following suckling, electrical stimulation of the mammary nerve and oxytocin infusion in lactating rats. Trunk blood was collected by decapitation after 5 and 20 min of suckling in conscious lactating rats. Repeated blood samples were drawn from the carotid artery in anesthetized rats, in connection with suckling, oxytocin infusion (0.22 nmol/l/kg/h) and electrical stimulation of the mammary nerve (5 V, 5 Hz, 2 ms). VIP and oxytocin levels were measured by radioimmunoassay. In conscious rats, VIP levels rose significantly from 18 +/- 5 to 102 +/- 30 pM after 5 min of suckling and to 123 +/- 25 pM after 20 min of suckling when milk ejection occurred. Oxytocin levels rose significantly from 90 +/- 24 to 269 +/- 45 pM during milk ejection. Suckling, oxytocin infusion and mammary nerve stimulation in anesthetized rats raised VIP levels significantly from 13 +/- 2, 18 +/- 5 and 10 +/- 2 to 43 +/- 8, 45 +/- 16 and 53 +/- 22 pM, respectively, whereas oxytocin levels rose from 111 +/- 34 to 294 +/- 66 pM after 20 min of suckling and to a peak value of 500 +/- 70 pM after oxytocin infusion. This study shows that VIP is elevated in plasma in lactating rats when the pups are suckling. The results showing that VIP levels rise following mammary nerve stimulation and oxytocin infusions indicate that both neurogenic and hormonal mechanisms can contribute to the regulation of VIP levels in plasma.     

Dopamine affects basal and augmented pituitary hormone secretion.

Although the role of the neurotransmitter, dopamine (DA), in the regulation of PRL has been well documented, controversy exists regarding its participation in the regulation of the other pituitary hormones. Consequently, we infused DA into six healthy male subjects (ages 19-32) and studied its effects on both basal pituitary hormone levels and augmented hormonal release induced by insulin hypoglycemia (ITT), TRH, and gonadotropin-releasing hormone (GnRH). DA alone produced a modest though significant increase in GH concentration from 2.2 +/- 0.5 to 11.9 +/- 3.7 ng/ml (P less than 0.05) by 60 min, but the peak incremental GH response to ITT was significantly inhibited by DA (43.5 +/- 5.0 vs. 16.3 +/- 3.3 ng/ml; P less than 0.01). PRL concentrations fell during the DA infusion (20.4 +/- 3.0 to 10.6 +/- 1.5 ng/ml; P less than 0.02) at 235 min, and the PRL responses to both ITT and TRH were completely abolished. Although the basal LH and FSH concentrations were unaffected by DA, the incremental LH response to GnRH was inhibited (45.5 +/- 10.6 to 24.4 +/- 5.4 mIU/ml; P less than 0.05), while the FSH response was unchanged. DA significantly reduced the basal TSH concentration from 3.9 +/- 0.2 to 2.5 +/- 0.2 micro U/ml (P less than 0.01) at 230 min and blunted the peak incremental TSH response to TRH (6.0 +/- 1.5 vs. 2.9 +/- 0.9 microU/ml; P less than 0.01). DA had no effect on basal cortisol levels, the cortisol response to ITT, basal plasma glucose, or the degree of hypoglycemia after ITT. Our data provide new evidence that DA has an inhibitory as well as a stimulatory role in the regulation of GH secretion in normal humans. It inhibits centrally as well as peripherally mediated PRL secretion and blunts the LH response to GnRH. In addition, DA lowers both basal and TRH-mediated TSH release, confirming the reports of other investigators.     

Galanin is a physiological regulator of spontaneous pulsatile secretion of growth hormone in the male rat.

To determine whether galanin (GAL), a 29-amino acid neuropeptide, plays a role in the physiological regulation of the pulsatile secretion of GH and PRL in the male rat, secretory patterns of both hormones were studied in freely moving animals after GAL passive immunoneutralization. Adult male Sprague-Dawley rats were equipped with iv and intracerebroventricular catheters. After 7 days, 3 microliters of a specific GAL antiserum (GAL-AS) or normal rabbit serum (NRS; controls) were infused in the third ventricle of 10 rats, 25 and 1 h before the animals were bled every 15 min for 6 h (1000-1600 h). Plasma GH and PRL concentrations were measured by RIA, and the hormonal secretory patterns were analyzed by the PULSAR program. Control rats, treated with NRS, displayed typical GH secretion, with pulses of high amplitude (167 +/- 27 ng/ml) and low frequency (2.4 +/- 0.2 pulses/6 h), separated by periods of low trough levels (3.8 +/- 0.6 ng/ml). Rats treated with GAL-AS had altered pulsatile GH secretion. Pulse height was markedly reduced (77 +/- 15 ng/ml; P less than 0.01 vs. controls), and peak frequency was higher (3.6 +/- 0.5 pulses/6 h; P less than 0.05), while GH baseline levels and integrated GH secretion over the 6-h sampling period remained unaltered. Injection of rat GH-releasing hormone (1 microgram/rat, iv) caused a similar GH stimulation in both groups of rats, as determined by the peak GH response at 5 min (368 +/- 112 vs. 342 +/- 81 ng/ml) or by the integrated GH response over 1 h (5.13 +/- 1.30 vs. 4.77 +/- 1.15 micrograms.min/ml in NRS- and GAL-AS-treated rats, respectively; P less than 0.05). In contrast to GH, pulsatile secretion of PRL was not affected by the GAL-AS treatment. These results indicate that GAL is a physiological regulator of spontaneous pulsatile secretion of GH, but not PRL, in the male rat. The influence of GAL on GH secretion appears to be exerted within the hypothalamus, mainly by a stimulation of GRF secretion. However, the changes in GH pulse frequency observed after GAL immunoneutralization suggest that GAL might also influence the somatostatin inhibitory tone.     

Vasoactive intestinal polypeptide and dopamine: effect on prolactin secretion in normal women and patients with microprolactinomas

In order to investigate the influence of dopamine (DA) in modulating prolactin (PRL) response to vasoactive intestinal polypeptide (VIP), VIP (75 micrograms i.v. over 12 min) was administered to normal women and patients with microprolactinomas during saline or DA (0.06 micrograms/kg BW/min) infusion. In 8 normal women VIP caused PRL to rise from 7.9 +/- 0.8 (mean +/- SE) to 33.4 +/- 17.1 ng/ml (p less than 0.01), the peak occurring at 15 min, while it did not elicit any significant modification in serum PRL levels in 18 patients with microprolactinomas. DA infusion lowered serum PRL by 67 and 58.2% at 120 min in normal women and in patients with microprolactinomas, respectively, and abolished VIP-induced PRL response in normal women without influencing PRL response in patients with microprolactinomas. PRL responsiveness to VIP was not restored by dopaminergic disinhibition (domperidone 10 mg i.v.) in 4 patients tested. Two previously unresponsive patients showed a VIP-induced PRL increase, superimposable on that recorded in normal women, after successful selective adenomectomy. These data suggest that DA, although able to suppress VIP-induced PRL response in normals, does not play any major role in causing unresponsiveness to VIP in prolactinomas. The lack of PRL responsiveness to VIP might be intrinsic to the adenoma or due to alterations of PRL secretion regulatory mechanisms other than DA secondary to the presence of the tumor, or to a depletion of the readily releasable pool of PRL.