Hyperglycemic effect of prolactin in anterior pituitary transplanted rats

The role of prolactin as a diabetogenic hormone was based on studies describing the ability of administered prolactin to raise the level of serum glucose. In the present study, female rats were made hyperprolactinemic by the transplantation of two anterior pituitary glands under the renal capsule, obtained from littermate donors. Chronic hyperprolactinemia increased blood glucose and impaired the glycolytic pathway of hepatic carbohydrate metabolism. However, basal serum insulin values were not different from those of controls. These data suggest that chronic hyperprolactinemia has an hyperglycemic effect and has a significant influence on glucose handling by the liver, through reduced glycolysis.     

Intrahypothalamic pituitary grafts elevate prolactin in the cerebrospinal fluid and attenuate prolactin release following ether stress

Anterior pituitary (AP) tissue grafted into the hypothalamus of female rats inhibits the luteotrophic prolactin (PRL) secretion which normally follows mating. Dopamine blockade has been shown to overcome this inhibition, suggesting that the grafts suppress PRL release from the in situ pituitary by the action of graft PRL increasing dopamine activity in the hypothalamus. To examine whether PRL levels in the cerebrospinal fluid (CSF) were elevated by the AP grafts, CSF samples were taken from 5 control rats and 10 rats bearing intrahypothalamic AP grafts. Mean PRL concentrations in the CSF of the control rats were 3.0 +/- 0.8 ng/ml. The grafted rats had significantly higher concentrations of PRL in their CSF, averaging 23.2 +/- 4.2 ng/ml (P less than 0.005). Plasma PRL concentrations were similar in the control and grafted rats. PRL release in response to 5 min of ether stress was examined in 8 control and 11 grafted rats. In control animals, PRL rose from 4.2 +/- 1.5 to 44.7 +/- 9.0 ng/ml following exposure to ether, but the response was significantly attenuated in the grafted rats, peaking at 9.3 +/- 1.4 ng/ml (P less than 0.001). This inhibition of response due to the grafts was evident within 1 week of graft placement. The results confirm that the presence of intrahypothalamic AP grafts led to the accumulation of supranormal PRL concentrations in the CSF. This elevated PRL suppressed pituitary PRL release in response to ether stress, probably by an autoregulatory feedback activation of the inhibitory tuberoinfundibular dopaminergic neurons in the hypothalamus.     

On the mechanism by which dopamine inhibits prolactin release in the anterior pituitary

An $\text{in}$$\text{vitro}$ perfusion system was used to assess the effects of chloride channel blockers, dopamine (DA) receptor agonists and antagonists, and GABA receptor agonists and antagonists on prolactin release from the mouse anterior pituitary. Dopamine and muscimol inhibited prolactin release ($\text{IC}{}_{50}{}^1\text{= 6 × 10}{}^{\mathrm{?8}}\text{M and}\text{10}{}^{\mathrm{?5}}\text{M}$ respectively). The GABA receptor antagonist bicuculline blocked the inhibition of prolactin release by muscimol but not dopamine. The dopamine receptor antagonist chlorpromazine blocked the dopamine- but not muscimol-induced inhibition of prolactin release. Haloperidol, however, reversed both the muscimol and dopamine induced inhibition of prolactin release. Furthermore, the chloride channel blocker picrotoxinin blocked the inhibition of prolactin release elicited by both dopamine and muscimol. These later results suggest that the anterior pituitary dopamine receptor which mediates the inhibition of prolactin release may be coupled to a picrotoxinin sensitive chloride ionophore and that haloperidol may affect the function of both DA and GABA receptors in the anterior pituitary.     

Various proteinase inhibitors decrease prolactin and growth hormone release by anterior pituitary cells

Proteinase inhibitors were tested for their ability to inhibit prolactin (PRL) and growth hormone (GH) release by cultured anterior pituitary cells of the rat. Inhibitors of microbial origin (chymostatin, elastatinal, leupeptin) had either no or a moderate effect on hormone release while some tripeptide aldehydes, especially those with lysine at their C terminus, inhibited markedly PRL and to a lesser extent GH release. Boc-DPhe-Phe-lysinal was the most effective on lactotrophs inhibiting PRL release more than 50% at 10(-4) M. The site(s) of action of tripeptide aldehydes remain to be elucidated.     

Stimulation of prolactin secretion from turkey anterior pituitary cells in culture

Prolactin (PRL) secretion by monolayer cultures of turkey anterior pituitary cells was significantly increased (up to 44-fold) by vasoactive intestinal peptide (VIP), arginine vasotocin (AVT), and by an extract of turkey hypothalami (HE). Several other neuropeptides (including thyrotropin-releasing hormone) and neurotransmitters were ineffective in influencing PRL secretion at doses up to 10(-6) M. The dynamic PRL response to HE and VIP was studied using superfused pituitary cells attached to microcarrier beads. HE, administered in 30-min pulses, resulted in a significant, dose-related increase in PRL secretion from a basal secretion rate of 2.32 ng/min/10(7) cells to a peak secretion rate of 127.13 ng/min/10(7) cells at the highest dose of HE tested (1 mg tissue-equivalent weight/ml). VIP significantly increased PRL secretion at all doses studied (from 10(-10) to 10(-6) M), with 10(-8) M VIP producing a response similar to that observed with 1 mg/ml HE. A highly significant (P less than 0.001) linear relationship was demonstrated between the log-dose of VIP administered and peak PRL secretion rate. These studies suggest that VIP, but not TRH, may be a physiological stimulus for PRL release in the turkey.     

Inhibition of prolactin release from anterior pituitary lactotrophs in culture by sulfur-containing analogs of dopamine

The effects of permanently charged and uncharged analogs of dopamine were examined for their ability to inhibit basal prolactin release from primary cultures of rat pituitary lactotrophs. The charged quaternary trimethyldopamine and the charged dimethylsulfonium analogs were active (IC50's were 4.3 and 31 microM, respectively) while the permanently uncharged monoethylsulfide was devoid of significant activity. Dopamine and dimethyldopamine, which are able to exist in both charged and uncharged forms, are more potent (IC50's were 36 and 44 nM, respectively) but all compounds were capable of approaching the same maximum degree of prolactin release inhibition. Haloperidol, a dopamine receptor antagonist, was able to block the actions of each of the agonists. The data suggest that (a) dopamine agonists do not have to be in the uncharged form in order to activate the dopamine receptor that regulates prolactin release, (b) the uncharged monomethylsulfide analog of dopamine is incapable of activating the dopamine receptor, and (c) the nitrogen on the side chain of dopamine can be replaced by another atom and still retain prolactin release inhibiting activity.     

The mechanism of action of cyproheptadine on prolactin release by cultured anterior pituitary cells

The role of a direct effect of serotonin (5-HT) on PRL secretion at the pituitary level is uncertain. The present study investigated the mechanism of action of the serotonin receptor-blocking agent cyproheptadine on PRL release by normal cultured rat anterior pituitary cells. Cyproheptadine (10 nM-10 microM) and its metabolite desmethylcyproheptadine (a compound which has lost its affinity for serotonin receptors) directly inhibited PRL release, while serotonin, investigated over a wide concentration range, did not reverse this inhibition. The cyproheptadine-mediated inhibition of PRL-release could be completely prevented by 50 microM verapamil. Cyproheptadine strongly inhibited TRH-induced PRL release in the absence, but not in the presence of verapamil. Our studies suggest that cyproheptadine inhibits PRL release at the pituitary level by a blockade of calcium influx at the cell membrane, without affecting the movement of Ca2+ between intracellular compartments.     

Localization of microfilaments in prolactin cells of the rat anterior pituitary gland

Summary Prolactin cells from anterior pituitary glands of normal non-lactating female rats, and lactating animals, some of which were separated from their pups for 48 hours, were examined ultrastructurally for the presence of microfilaments. Microfilaments were found in specific intracellular locations in all cells examined. They were in association with the nuclear envelope, the Golgi complex, the endoplasmic reticulum, small vesicles of the endoplasmic reticulum, and secretory granules. The possible role of microfilaments in the movement of intracellular organelles is considered.This investigation was supported by the National Institutes of Health grants AM 12583 and TW 02023.The authors wish to express their gratitude to Mr. M. G. Williams and Miss Pauline Cisneros for their excellent technical assistance.     

The effect of estradiol on output of adrenocorticotropin and prolactin by fetal sheep anterior pituitary cells

We examined the hypothesis that estradiol (E2) would affect fetal anterior pituitary corticotroph and lactotroph function in vitro, and that any effects would be influenced by gestational age. Anterior pituitary cells from fetal sheep at day 129 (n = 4) and at day 139 (n = 5) of gestation were cultured. After 96 h in culture, cells were treated for 18 h with E2 concentrations ranging from 0 to 1000 nM, in the presence or absence of 100 nM of corticotropin-releasing hormone (CRH), cortisol, arginine vasopressin (AVP), or CRH and cortisol, to examine their effects on corticotroph function. Cells were also treated with bromocriptine or increasing concentrations of E2 to study their effects on lactotroph function. Immunoreactive (ir) adrenocorticotropin (ACTH) and prolactin in the culture medium were measured by radioimmunoassay. Levels of cellular pro-opiomelanocortin (POMC) mRNA and prolactin mRNA were determined by in situ hybridization. Immunohistochemistry was used to determine the percentage of cells that were immunopositive for ACTH (corticotrophs) or prolactin (lactotrophs). ACTH output was stimulated by CRH treatment at day 139 but not at day 129 of gestation, and cortisol attenuated this response. ACTH output by cells cultured with 10 nM E2 and 100 nM CRH, at 139 days of gestation, was greater than with CRH alone (p < 0.05). E2 did not affect basal ACTH output or ACTH output with any other treatment or levels of POMC mRNA. Prolactin output was not affected by E2 treatment. Bromocriptine significantly decreased prolactin output but not levels of prolactin mRNA. We conclude that E2 may affect CRH-stimulated fetal sheep pituitary corticotroph function late in gestation, but only within a narrow, physiological range of concentration.     

Naltrexone partially inhibits the estrogen-induced increase in prolactin secretion and anterior pituitary weight

The effects of naltrexone, a specific opiate antagonist, on stimulation by estradiol benzoate (EB) of prolactin (PRL) release and anterior pituitary (AP) weight, were studied in gonadectomized female and male Sprague-Dawley rats. One week after castration, rats were injected for 10 days once daily with 2 μg EB alone, or together with twice daily injections of 2 mg naltrexone/kg body weight (BW). Blood was collected for radioimmunoassay of PRL by orbital sinus puncture on days 0 and 6, and by decapitation on day 11, at which time the AP was quickly removed, weighed and assayed for PRL.Serum PRL concentrations and AP weights were significantly increased by EB administration. These effects of EB were partially but significantly inhibited by naltrexone. These results suggest that endegenous opiates may be involved in the estrogen-induced rise in serum PRL and increase in pituatary weight.     

Effect of photoperiod variation on testes and accessory sex organs in the male blind mole rat Spalax ehrenbergi

The blind mole rat is a seasonally breeding fossorial rodent that is perceptionally blind. This study examines the effect of photoperiod on the morphology and histology of the male mole rat reproductive system, three groups of male mole rats were maintained in the laboratory under short day (SD) conditions (9L: 15D); long day (LD) conditions (15L:9D); and constant darkness (CD), and compared to animals trapped in the field (FL). It was found that the field animals revealed higher testes and prostate gland weights, higher prostate tubuli volume (v*) and lower testes tubuli volume (v*) compared to the other three groups. Distribution of the tubuli in the testes (Vv) was low in the FL group compared to the SD and LD groups but still higher than in the CD group. Distribution of lumen in the testes (Vv) was higher in the CD group in comparison to the other three groups. Distribution of interstitial tissue in the testes (Vv) was higher in the FL group than in the other three groups. Electrolytes and elements secreted from the prostate gland did not differ among the four groups. In the FL group distribution of the tubuli (Vv) in the prostate gland was low and lumen ratio (Vv) was high compared to the other three groups. Distribution of connective tissue in the prostate gland did not differ among all four groups. Testosterone levels and total sperm count was highest in the FL group. Sperm production was noted in all groups; however spermatid and spermatozoa cell production was higher in the FL group. This study shows that photoperiod could be important in initiating timing in the breeding season but that certain other conditions which are absent in the laboratory appear to be responsible for successful breeding in the field.     

Stimulation of spontaneous and dopamine-inhibited prolactin release from anterior pituitary reaggregate cell cultures by angiotensin peptides

In superfused anterior pituitary reaggregate cell cultures angiotensin II (AII) stimulated both spontaneous and dopamine-inhibited prolactin (PRL) release from subnanomolar concentrations. Angiotensin I (AI) and angiotensin III (AIII) also stimulated PRL release. The magnitude and rate of response to AI was equal to or only slightly lower than that to AII. However, the angiotensin converting enzyme (ACE) inhibitors captopril and teprotide (1 microM) completely abolished the PRL response to 0.1 nM AI and strongly reduced that to 1 nM AI. The intrinsic activity of AIII was lower than that of AII but could be enhanced by adding 2 microM of the aminopeptidase inhibitor amastatin to the superfusion medium. After withdrawal of AIII, PRL secretion rate rapidly returned to baseline levels, whereas after withdrawal of AI or AII, secretion fell to a level remaining significantly higher than basal release. The present findings indicate that stimulation of PRL release by AI is weak unless it is converted into AII by ACE and that aminopeptidase may be important in determining the magnitude and termination of the PRL response. Furthermore, the active peptides induce a different pattern of response.     

The effect of pituitary homografts on the accessory sex organs and serum hormonal levels with or without androgen in mature male rats

The effect of pituitary homografts on the accessory sex organs and hormonal levels were studied in Wistar mature male rats. Grafted rats were further divided into four experiments: rats were bled once daily via a jugular vein cannula for seven days to investigate when serum prolactin began to rise after transplantation. rats were decapitated on the seventh day after transplantation to test whether 7 days were long enough to show the effect of pituitary grafts on the weight of prostate and seminal vesicles. rats were orchiectomized or orchiectomized and adrenalectomized on the seventh day after transplantation and then decapitated 4 weeks later to test a long term action of pituitary grafts and hormonal levels on the accessory sex organs without androgen. Rats grafted with several pieces of muscle were used as controls in each experiment. The initial rise in serum prolactin level was observed on the fourth day after pituitary transplantation, and then a higher serum prolactin level was maintained thereafter. Despite the higher prolactin level in the pituitary-grafted rat than in the control, no significant differences from the control in the weight of prostates and seminal vesicles and adrenal gland and the concentrations of serum luteinizing hormone (LH) and follicular stimulating hormone (FSH) were measured. This result showed that the weight of accessory sex organs was not affected by a higher serum prolactin within seven days.(ABSTRACT TRUNCATED AT 250 WORDS)