The posterior pituitary: regulation of anterior pituitary prolactin secretion

Revealing Sexual Dimorphism in Prolactin Regulation From Early Postnatal Development to Adulthood in Murine Models

Serum prolactin (PRL) levels exhibit a gradual rise both in male and female rats from birth to adulthood, with females consistently displaying higher levels compared to age-matched males. This pattern has traditionally been attributed to the development and maturation of endocrine and neuroendocrine networks responsible for regulating PRL synthesis and secretion. However, the effect of dopamine (DA), which acts as an inhibitory factor on lactotroph function, also increases from birth to puberty, particularly in females. Nonetheless, the secretion of PRL remains higher in females compared to males. On the other hand, the observed sex differences in serum PRL levels during early postnatal development cannot be attributed to the influence of estradiol (E2). While serum E2 levels gradually increase after birth, only after 45 days of life do the disparities in E2 levels between females and males become evident. These observations collectively suggest that neither the maturation of hypothalamic DA regulation nor the rise in E2 levels can account for the progressive and sustained elevation in serum PRL levels and the observed sexual dimorphism during postnatal development. This review highlights the importance of recent discoveries in animal models that shed light on inhibitory mechanisms in the control of PRL secretion within the pituitary gland itself, that is intrapituitary mechanisms, with a specific emphasis on the role of transforming growth factor β1 and activins in PRL secretion.

Seasonality of prolactin in birds and mammals

In most animals, annual rhythms in environmental cues and internal programs regulate seasonal physiology and behavior. Prolactin, an evolutionarily ancient hormone, serves as a molecular correlate of seasonal timing in most species. Prolactin is highly pleiotropic with a wide variety of well-documented physiological effects; in a seasonal context prolactin is known to regulate annual changes in pelage and molt. While short-term homeostatic variation of prolactin secretion is under the control of the hypothalamus, long-term seasonal rhythms of prolactin are programmed by endogenous timers that reside in the pituitary gland. The molecular basis of these rhythms is generally understood to be melatonin dependent in mammals. Prolactin rhythmicity persists for several years in many species, in the absence of hypothalamic signaling. Such evidence in mammals has supported the hypothesis that seasonal rhythms in prolactin derive from an endogenous timer within the pituitary gland that is entrained by external photoperiod. In this review, we describe the conserved nature of prolactin signaling in birds and mammals and highlight its role in regulating multiple diverse physiological systems. The review will cover the current understanding of the molecular control of prolactin seasonality and propose a mechanism by which long-term rhythms may be generated in amniotes.

Three lactation-related hormones: Regulation of hypothalamus-pituitary axis and function on lactation

The endocrine system plays a central role in many aspects of lactation, including mammogenesis (mammary gland development), lactogenesis (onset of lactation), and galactopoiesis (maintenance of milk secretion). Many hormones of the endocrine system directly or indirectly regulate lactation process. The secretion of prolactin (PRL), one of the most important lactation-related hormones, is inhibited by hypothalamus-pituitary dopaminergic system and stimulated by hypothalamus-pituitary oxytocinergic system. This hormone is essential in all stages of lactation. The growth hormone (GH) regulates metabolism and the distribution of nutrients between tissues mammary glands, and stimulates the production of IGF-I from the liver which binds to IGF-IR of mammary epithelial cells (MECs) to indirectly promote lactation. The synthesis and secretion of estrogen (E) are affected by the hypothalamus-pituitary axis. The hormone regulates duct morphogenesis and MECs proliferation. It also modulates the synthesis and secretion of PRL and GH, which together regulate the lactation in female animals. In this article, we reviewed the three main lactation-related hormones (PRL, GH, and E), summarize their regulation by the hypothalamus-pituitary axis and how they influence lactation.

Patterns of prolactin secretion

Prolactin is pleotropic in nature affecting multiple tissues throughout the body. As a consequence of the broad range of functions, regulation of anterior pituitary prolactin secretion is complex and atypical as compared to other pituitary hormones. Many studies have provided insight into the complex hypothalamic-pituitary networks controlling prolactin secretion patterns in different species using a range of techniques. Here, we review prolactin secretion in both males and females; and consider the different patterns of prolactin secretion across the reproductive cycle in representative female mammals with short versus long luteal phases and in seasonal breeders. Additionally, we highlight changes in the pattern of secretion during pregnancy and lactation, and discuss the wide range of adaptive functions that prolactin may have in these important physiological states.

GnRH Neurons Provide Direct Input to Hypothalamic Tyrosine Hydroxylase Immunoreactive Neurons Which Is Maintained During Lactation

Gonadotropin releasing hormone (GnRH) neurons provide neuronal input to the preoptic area (POA) and the arcuate nucleus (Arc), two regions involved critically in the regulation of neuroendocrine functions and associated behaviors. These areas contain tyrosine hydroxylase immunoreactive (TH-IR) neurons, which play location-specific roles in the neuroendocrine control of both the luteinizing hormone and prolactin secretion, as well as, sexually motivated behaviors. Concerning changes in the activity of GnRH neurons and the secretion pattern of GnRH seen under the influence of rising serum estrogen levels and during lactation, we tested the hypothesis that the functional state of GnRH neurons is mediated via direct synaptic connections to TH-IR neurons in the POA and Arc. In addition, we examined putative changes of these inputs in lactating mice and in mothers separated from their pups. Confocal microscopic and pre-embedding immunohistochemical studies on ovariectomized mice treated with 17β-estradiol (OVX+E2) provided evidence for direct appositions and asymmetric synapses between GnRH-IR fiber varicosities and TH-IR neurons in the POA and the Arc. As TH co-localizes with kisspeptin (KP) in the POA, confocal microscopic analysis was continued on sections additionally labeled for KP. The TH-IR neurons showed a lower level of co-labeling for KP in lactating mice compared to OVX+E2 mice (16.1 ± 5% vs. 57.8 ± 4.3%). Removing the pups for 24 h did not alter significantly the KP production in TH-IR neurons (17.3 ± 4.6%). The mean number of GnRH-IR varicosities on preoptic and arcuate TH cells did not differ in the three animal models investigated. This study shows evidence that GnRH neurons provide direct synaptic inputs to POA and Arc dopaminergic neurons. The scale of anatomical connectivity with these target cells was unaltered during lactation indicating a maintained GnRH input, inspite of the altered hormonal condition.

Hypothalamus as an endocrine organ

The endocrine hypothalamus constitutes those cells which project to the median eminence and secrete neurohormones into the hypophysial portal blood to act on cells of the anterior pituitary gland. The entire endocrine system is controlled by these peptides. In turn, the hypothalamic neuroendocrine cells are regulated by feedback signals from the endocrine glands and other circulating factors. The neuroendocrine cells are found in specific regions of the hypothalamus and are regulated by afferents from higher brain centers. Integrated function is clearly complex and the networks between and amongst the neuroendocrine cells allows fine control to achieve homeostasis. The entry of hormones and other factors into the brain, either via the cerebrospinal fluid or through fenestrated capillaries (in the basal hypothalamus) is important because it influences the extent to which feedback regulation may be imposed. Recent evidence of the passage of factors from the pars tuberalis and the median eminence casts a new layer in our understanding of neuroendocrine regulation. The function of neuroendocrine cells and the means by which pulsatile secretion is achieved is best understood for the close relationship between gonadotropin releasing hormone and luteinizing hormone, which is reviewed in detail. The secretion of other neurohormones is less rigid, so the relationship between hypothalamic secretion and the relevant pituitary hormones is more complex.

60 YEARS OF NEUROENDOCRINOLOGY: The hypothalamo-prolactin axis

The hypothalamic control of prolactin secretion is different from other anterior pituitary hormones, in that it is predominantly inhibitory, by means of dopamine from the tuberoinfundibular dopamine neurons. In addition, prolactin does not have an endocrine target tissue, and therefore lacks the classical feedback pathway to regulate its secretion. Instead, it is regulated by short loop feedback, whereby prolactin itself acts in the brain to stimulate production of dopamine and thereby inhibit its own secretion. Finally, despite its relatively simple name, prolactin has a broad range of functions in the body, in addition to its defining role in promoting lactation. As such, the hypothalamo-prolactin axis has many characteristics that are quite distinct from other hypothalamo-pituitary systems. This review will provide a brief overview of our current understanding of the neuroendocrine control of prolactin secretion, in particular focusing on the plasticity evident in this system, which keeps prolactin secretion at low levels most of the time, but enables extended periods of hyperprolactinemia when necessary for lactation. Key prolactin functions beyond milk production will be discussed, particularly focusing on the role of prolactin in inducing adaptive responses in multiple different systems to facilitate lactation, and the consequences if prolactin action is impaired. A feature of this pleiotropic activity is that functions that may be adaptive in the lactating state might be maladaptive if prolactin levels are elevated inappropriately. Overall, my goal is to give a flavour of both the history and current state of the field of prolactin neuroendocrinology, and identify some exciting new areas of research development.

TIDAL WAVES: Network mechanisms in the neuroendocrine control of prolactin release

Neuroendocrine tuberoinfundibular dopamine (TIDA) neurons tonically inhibit pituitary release of the hormone, prolactin. Through the powerful actions of prolactin in promoting lactation and maternal behaviour while suppressing sexual drive and fertility, TIDA neurons play a key role in reproduction. We summarize insights from recent in vitro studies into the membrane properties and network behaviour of TIDA neurons including the observations that TIDA neurons exhibit a robust oscillation that is synchronized between cells and depends on intact gap junction communication. Comparisons are made with phasic firing patterns in other neuronal populations. Modulators involved in the control of lactation - including serotonin, thyrotropin-releasing hormone and prolactin itself - have been shown to change the electrical behaviour of TIDA cells. We propose that TIDA discharge mode may play a central role in tuning the amount of dopamine delivered to the pituitary and hence circulating prolactin concentrations in different reproductive states and pathological conditions.

Transient Willis-Ekbom's disease (restless legs syndrome) during pregnancy may be caused by estradiol-mediated dopamine overmodulation

Willis-Ekbom's disease (WED), formerly called restless legs syndrome, is more common in pregnant than in non-pregnant women, implying that the physiological and biochemical changes during pregnancy influence its development. During pregnancy, many hormone levels undergo significant changes, and some hormones significantly increase in activity and can interfere with other hormones. For example, the steroid hormone estradiol interferes with the neuroendocrine hormone dopamine. During pregnancy, the activity of the thyroid axis is enhanced to meet the increased demand for thyroid hormones during this state. Dopamine is a neuroendocrine hormone that diminishes the levels of thyrotropin and consequently of thyroxine, and one of the roles of the dopaminergic system is to counteract the activity of thyroid hormones. When the activity of dopamine is not sufficient to modulate thyroid hormones, WED may occur. Robust evidence in the medical literature suggests that an imbalance between thyroid hormones and the dopaminergic system underpins WED pathophysiology. In this article, we present evidence that this imbalance may also mediate transient WED during pregnancy. It is possible that the main hormonal alteration responsible for transient WED of pregnancy is the excessive modulation of dopamine release in the pituitary stalk by estradiol. The reduced quantities of dopamine then cause decreased modulation of thyrotropin, leading to enhanced thyroid axis activity and subsequent WED symptoms. Iron deficiency may also be a predisposing factor for WED during pregnancy, as it can both diminish dopamine and increase thyroid hormone.

Dual effects of hyperprolactinemia on carrageenan-induced inflammatory paw edema in rats

OBJECTIVES

The effects of short-term 5-day and long-term 30-day hyperprolactinemia induced by domperidone (1.7 mg/kg/day, s.c.) or ectopic pituitary graft on the acute inflammatory response induced by carrageenan were evaluated in male rats. Both models of hyperprolactinemia effectively increased serum prolactin (PRL) levels.

METHODS

The volume in milliliters of inflammatory edema was measured by plethysmography 1, 2, 3, 4, 6, 8 and 24 h after carrageenan injection. The areas under the inflammatory time-response curves were compared. Additionally, the effects of hyperprolactinemia on body weight and serum corticosterone levels were evaluated.

RESULTS

In both domperidone-treated and pituitary graft-implanted animals, short-term 5-day hyperprolactinemia increased the inflammatory response, while long-term 30-day hyperprolactinemia had anti-inflammatory effects. Body weight was not affected by either short- or long-term hyperprolactinemia.

CONCLUSION

These results show that PRL has biphasic effects on the carrageenan-induced inflammatory response.

Colocalization of somatostatin receptor subtypes (SSTR1-5) with somatostatin, NADPH-diaphorase (NADPH-d), and tyrosine hydroxylase in the rat hypothalamus

The hypothalamus is a major site of somatostatin (SST) production and action. SST is synthesized in several hypothalamic nuclei and involved in a variety of functions. Using SST receptor (SSTR)-specific antibodies, we localized SSTR subtypes in the rat hypothalamus. In addition, we also demonstrated SSTRs colocalization with SST, NADPH-diaphorase (NADPH-d), and tyrosine hydroxylase (TH). SSTR1 is strongly localized in neurons in all major hypothalamic nuclei as well as in nerve fibers in the zona externa of the median eminence and the ependyma of the third ventricle. SSTR2 is also well expressed in most regions but with a relatively lower abundance in comparison to SSTR1. In contrast, SSTR3 is localized primarily in the paraventricular nucleus, dorsomedial hypothalamic nucleus, arcuate nucleus, and median eminence. SSTR4-like immunoreactivity is mainly confined to the arcuate nucleus, ventromedial hypothalamic nucleus, median eminence, and ependymal cells of third ventricle, with the rare SSTR4-positive neuron in the paraventricular nucleus. SSTR5 is the least expressed subtype occurring only in few cells in the inner layer of the median eminence. Overall, SSTR1 is the predominant subtype, followed by SSTR2, 4, 3, and 5. Combined immunofluorescence, immunocytochemistry, and histochemistry were used to demonstrate SSTRs colocalization with SST, TH, and NADPH-d. SSTRs colocalization with SST, TH, and NADPH-d displays in a region and receptor specificity. Colocalization of SST and NADPH-d with SSTRs in hypothalamic regions was similar, suggesting that SST and NADPH-d producing cells are same. In contrast, TH was selectively coexpressed with SSTRs in the hypothalamus in a receptor-specific manner. Taken together, these data suggest that SSTRs may interact with NADPH-d and TH to exert a physiological role in concert within the hypothalamus.

Neuroendocrine regulation of prolactin secretion during late pregnancy: easing the transition into lactation

Prolactin is an anterior pituitary hormone critical for maintaining pregnancy and lactation. Under normal conditions, prolactin secretion is tightly regulated by inhibitory dopaminergic neuronal systems within the mediobasal hypothalamus in a process known as short-loop negative feedback. This review focuses on neuroendocrine adaptations to prolactin negative feedback during late pregnancy. It is suggested that, in terms of prolactin regulation, late pregnancy is a transition period into lactation because many of the neuroendocrine adaptations promoting hyperprolactinemia in lactation develop during late pregnancy. As a consequence, the maternal brain is geared to provide unrestrained prolactin release critical for milk production, maternal care and thus survival of the offspring before parturition. The mechanisms responsible for these changes are discussed.

Dopamine as a prolactin (PRL) inhibitor

Dopamine is a small and relatively simple molecule that fulfills diverse functions. Within the brain, it acts as a classical neurotransmitter whose attenuation or overactivity can result in disorders such as Parkinson's disease and schizophrenia. Major advances in the cloning and characterization of biosynthetic enzymes, transporters, and receptors have increased our knowledge regarding the metabolism, release, reuptake, and mechanism of action of dopamine. Dopamine reaches the pituitary via hypophysial portal blood from several hypothalamic nerve tracts that are regulated by PRL itself, estrogens, and several neuropeptides and neurotransmitters. Dopamine binds to type-2 dopamine receptors that are functionally linked to membrane channels and G proteins and suppresses the high intrinsic secretory activity of the pituitary lactotrophs. In addition to inhibiting PRL release by controlling calcium fluxes, dopamine activates several interacting intracellular signaling pathways and suppresses PRL gene expression and lactotroph proliferation. Thus, PRL homeostasis should be viewed in the context of a fine balance between the action of dopamine as an inhibitor and the many hypothalamic, systemic, and local factors acting as stimulators, none of which has yet emerged as a primary PRL releasing factor. The generation of transgenic animals with overexpressed or mutated genes expanded our understanding of dopamine-PRL interactions and the physiological consequences of their perturbations. PRL release in humans, which differs in many respects from that in laboratory animals, is affected by several drugs used in clinical practice. Hyperprolactinemia is a major neuroendocrine-related cause of reproductive disturbances in both men and women. The treatment of hyperprolactinemia has greatly benefited from the generation of progressively more effective and selective dopaminergic drugs.

Systemic relaxin in pregnant pony mares grazed on endophyte-infected fescue: effects of fluphenazine treatment

Tall fescue is one of the most widely grown forage grasses for horses in the United States. However, it is frequently infected with the endophyte Neotyphodium coenophialum which produces ergot alkaloids that cause severe adverse effects in the pregnant mare. The objectives of this study were to determine the effects of fescue toxicosis and fluphenazine on circulating relaxin in pregnant pony mares and evaluate the usefulness of relaxin as a monitor of treatment efficacy. Twelve mares were maintained on endophyte-infected tall fescue pasture. Group TRT (n = 6), received 25 mg of fluphenazine decanoate (i.m.) on Day 320 of gestation while Group UTRT served as untreated controls. Daily blood samples were collected from Day 300 of gestation until Day 3 post partum and analyzed for plasma relaxin concentrations using a homologous equine radioimmunoassay. Mean gestation lengths were 330 +/- 0.7 and 336.5 +/- 3.2 days for TRT and UTRT mares, respectively (P = 0.07). Mean plasma relaxin concentrations in both groups of mares during the week before treatment (Day 313 to 319) were not different (UTRT, 53.4 +/- 11.3 ng/mL; TRT, 61.4 +/- 9.3 ng/mL). In the week after treatment (Day 320 to 326), mean plasma relaxin tended to be higher (P = 0.1) in TRT mares (66.7 +/- 6.2 ng/mL) when compared with UTRT mares (49.6 +/- 6.6 ng/mL), representing a 17.1 ng/mL difference in circulating relaxin between the two groups. Systemic relaxin during the last week before delivery (days relative to parturition) for UTRT and TRT mares was 45.7 +/- 6.7 and 64.7 +/- 6.4 ng/mL (P = 0.06), respectively. At Day -8 and Day -5 relative to parturition, systemic relaxin in TRT mares was significantly higher (P < 0.05) than in UTRT mares. Three of the six UTRT mares and one TRT mare showed clinical symptoms of fescue toxicosis. In the week before delivery, circulating relaxin in mares with problematic pregnancies (39.9 +/- 7.8 ng/mL) was significantly lower than concentrations measured in mares with normal pregnancies (63.4 +/- 5.4 ng/mL; P = 0.03). Clinical observations suggest that a one-time injection with fluphenazine improved pregnancy outcome by reducing the adverse effects of fescue toxicosis concomitant with a stabilization of plasma relaxin concentrations. These data support the hypothesis that systemic relaxin may be a useful biochemical means of monitoring placental function and treatment efficacy in the mare.

Synergetic effect of pimozide and thyrotropin releasing hormone on prolactin and thyrotropin release during the drying off of ewes

The effect of pimozide and/or TRH was investigated on plasma prolactin, thyrotropin, T(4) and T(3) and udder distension in 38 ewes during drying off by feed restriction. The effect of daily injections of 2mg pimozide (s.c.), combined or not with TRH stimulation (200µg, i.v.) on three different days of the drying off period was examined. Blood samples were taken twice daily in each group for 9 days, while blood sampling on the days of TRH injection was also performed at 0, 15, 30min, and 1, 2 and 4h post-injection. Plasma was assayed for PRL, TSH, T(4) and T(3) levels. Udder distension and mastitis incidence were recorded at the end of the drying off period. TRH and pimozide both resulted in elevated plasma PRL levels and acted in a synergetic way. Udder distension and the incidence of mastitis was only influenced by pimozide. The TSH as well as the T(3) response to TRH was increased in ewes under a continuous influence of pimozide and T(3) peaks following TRH injection occurred earlier than T(4) peaks. The higher effect of pimozide upon TRH stimulated PRL and TSH release at day 8 compared to days 0 and 3 indicates a progressive involvement of dopamine on the inhibition of PRL and the sensitivity of the thyrotrophs to TRH during drying off.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Immunoneutralization of prolactin prevents stimulatory feedback of prolactin on hypothalamic neuroendocrine dopaminergic neurons

We have found that exogenous prolactin (PRL) stimulates all three populations of hypothalamic neuroendocrine dopaminergic neurons. In this study, we investigated the effects of immunoneutralization of endogenous PRL on the activity of these neurons. Injection of 17beta-estradiol (E2) (20 microg subcutaneously) 10 d after ovariectomy induced a proestrus-like increase in PRL in peripheral plasma the following afternoon. At 1000 h the day after E2 injection, rats received either rabbit antirat PRL antiserum (PRL-AS) (200 microL) or normal rabbit serum (NRS, 200 microL, controls) intraperitoneally. Groups of rats were then decapitated every 2 h from 1100 h to 2100 h. Trunk blood was collected and serum extracted with protein A to remove the PRL-AS/PRL complex, and the remaining free PRL was measured by radioimmunoassay. Sites of neuroendocrine dopaminergic nerve terminals, the median eminence (ME), and intermediate and neural lobes of the pituitary gland were excised and stored for determination of dopamine (DA) and 3,4-dihydroxyphenyl acetic acid (DOPAC) concentrations by high-performance liquid chromatography electrochemical detection (EC). In addition, the anterior lobe of the pituitary gland, the locus of DA action, was collected. The concentration of PRL in NRS-treated animals increased by 1500 h, peaked by 1700 h, and returned to low levels by 2100 h. PRL-AS prevented the increase in PRL secretion in response to E2. The turnover of DA (DOPAC:DA ratio; an index of dopaminergic neuronal activity) in the ME of NRS-treated animals increased at 1500 h and rapidly returned to basal levels. Treatment with PRL-AS prevented the increase in DA turnover in the ME. DA turnover in the intermediate lobe increased coincident with the peak of PRL in serum of NRS-treated rats. PRL-AS administration prevented increased DA turnover in the intermediate lobe. The turnover of DA in the neural lobe increased by 1300 h and decreased steadily through 2100 h. However, administration of PRL-AS minimally suppressed the turnover of DA in the neural lobe. Moreover, administration of PRL-AS attenuated the rise of DA in the anterior lobe associated with the waning phase of the E2-induced PRL surge. These results clearly indicate that endogenous PRL regulates its own secretion by activating hypothalamic neuroendocrine dopaminergic neurons.

Mechanisms of age-related changes in gonadotropin-releasing hormone receptor messenger ribonucleic acid content in the anterior pituitary of male rats

To determine the mechanism(s) of age-related changes in gonadotropin release from pituitary gonadotrophs in male rats, we measured the number of GnRH (gonadotropin-releasing hormone) receptor containing cells and expression of GnRH receptor mRNA per cell in the anterior pituitary. An in situ hybridization procedure was performed using young (six months) and old (24-25 months) Wistar rats. An image analysis system was employed for the autoradiographic analysis. The number of pituitary cells increased during aging (approximately 45%, p < 0.01). On the other hand, the number of GnRH receptor mRNA-containing cells decreased (approximately 25%, p < 0.05). The percentage of these cells in old rats decreased to less than a half of that in young animals (p < 0.01). GnRH receptor mRNA per cell in old rats was only 7% lower than in young (p < 0.01). These results suggest that loss of pituitary gonadotroph GnRH receptors and response is primarily due to the loss of gonadotrophs, and that the death mechanism(s) are responsible for decreased stimulation of Gn release during aging.

A stress-sensitive chemokinergic neuronal pathway in the hypothalamo-pituitary system

Recently we found that cytokine-induced neutrophil chemoattractant influenced anterior pituitary hormone release in vitro. These observations prompted us to investigate the possibility of the existence of cytokine-induced neutrophil chemoattractant in the hypothalamus. Immunohistochemistry showed that cytokine-induced neutrophil chemoattractant-like immunoreactivity existed in the paraventricular hypothalamic nucleus, the supraoptic nucleus, both the internal and the external layers of the median eminence and the posterior pituitary. Since the paraventricular hypothalamic nucleus plays a pivotal role in response to stressful stimuli, we examined the effect of a single episode of immobilization stress on cytokine-induced neutrophil chemoattractant messenger RNA expression in the paraventricular hypothalamic nucleus. Immobilization stress induced strong hybridization signals of cytokine-induced neutrophil chemoattractant messenger RNA in the parvocellular and magnocellular subdivision of the paraventricular hypothalamic nucleus within 15 min, and cytokine-induced neutrophil chemoattractant-like immunostaining intensity in the posterior pituitary started to increase around the periphery of the posterior lobe at 30 min after stress and extended to the whole lobe at 1 h after stress. The increase in the serum cytokine-induced neutrophil chemoattractant in response to stress showed a kinetically biphasic pattern. A first phase occurred within 15 min which may be due to an immediate release of stored cytokine-induced neutrophil chemoattractant in the neurohypophysis, since hypophysectomy completely blocked this phase. A second phase may reflect the release of newly synthesized cytokine-induced neutrophil chemoattractant in the paraventricular hypothalamic nucleus and/or peripheral cytokine-induced neutrophil chemoattractant, since hypophysectomy could not reduce this phase. These data suggest that cytokine-induced neutrophil chemoattractant in the paraventricular hypothalamic nucleus was immediately synthesized in response to stress, and then released into the peripheral blood via the hypothalamo-neurohypophysial system, revealing the presence of a stress-sensitive chemokinergic neuronal pathway in the hypothalamo-pituitary system.

Dopamine D2 receptor mRNA in the pituitary during the oestrous cycle, pregnancy and lactation in the rat

We have used the technique of quantitative in situ hybridization in order to study the changes in the levels of expression of D2 receptor mRNA in the anterior pituitary gland of female rats at different stages of the reproductive cycle. Plasma prolactin levels in the same animals were determined by radioimmunoassay. Rats in the prooestrous, oestrous, dioestrous 1, dioestrous 2 phases of the oestrous cycle and in pregnant, lactating, ovariectomised and ovariectomised animals treated with diethylstilbestrol (DES) have been examined. Our results show that expression of D2 receptor mRNA in the anterior pituitary gland varies during the oestrous cycle, with the lowest expression measured during oestrus. Expression levels increased during dioestrus 1, reaching the highest values in dioestrus 2 and declining again in prooestrus. Expression of D2 receptor mRNA was reduced during pregnancy when compared to lactating animals. In ovariectomised animals, the level of D2 receptor mRNA was similar to that observed in intact animals during oestrus. Ovariectomised animals treated with DES showed dramatically increased prolactin levels, while D2 receptor mRNA remained low. Prolactin secretion might be controlled not only by variations in the release and plasma concentrations of dopamine itself, but also by modulation of D2 receptor expression in pituitary cells. Our results suggest that the variations in D2 receptor density in the anterior pituitary cells at different physiological states are, at least to some extent, regulated at the level of gene expression.

Denervation of the rat posterior pituitary gland: validation of a stereotaxic method

A stereotaxic surgical method was developed for interrupting the nerve fibres running through the rat pituitary stalk to the posterior pituitary gland without obliterating the hypothalamo-pituitary portal circulation. The pituitary stalk was compressed by the blunt tip of an L-shaped rotating knife. Successful operations produced mild diabetes insipidus, disappearance of arginine vasopressin from the neural lobe, accumulation of arginine vasopressin and neurosecretory material in the pituitary stalk and no infarction in the anterior lobe of the pituitary gland. In female rats, the oestrous cycle was only temporarily disturbed. Plasma prolactin and corticosterone levels were high during the first 24 h after the stalk compression but returned to normal baseline levels from the second day after the operation. One week after the operation plasma adrenocorticotropin and prolactin levels were in the control range while plasma alpha-melanocyte-stimulating hormone was elevated. Denervation of the posterior pituitary gland may help in studying the neural control of intermediate lobe function and the role of the neural lobe in various endocrine conditions, and may serve as a model for lesions of the pituitary stalk and formation of ectopic neurohypophysis in the human.

Topography of short portal vessels in the rat pituitary gland: a scanning electron-microscopic and morphometric study of corrosion cast replicas

We applied scanning electron microscopy combined with imaging and morphometric techniques to analyze the dorsal topography and morphology of short portal vessels linking the capillary beds of the pituitary neural and anterior lobes in adult male albino rats. The pituitary microvasculature was replicated by intracarotid injection of Batson's No. 17 compound producing plastic casts that were advantageous for comprehensive morphometric analyses using an imaging device. The analysis revealed the existence of two types of portal vessels having quantitatively different morphological properties. The bilateral venular plexus of 3-4 vessels located at the base of the infundibular stalk (each venule measuring 300 microns in length and 32 microns in diameter) appears to be the major part of the short portal system in the dorsum of the rat pituitary gland. Narrower capillary-like shunt vessels (6.8 microns in diameter), of about the same length as the venules, were situated throughout other subregions of the intermediate lobe cleft. The short portal vessels of both types made direct anastomoses with the capillary networks in the neural and anterior lobes. The neural lobe capillaries were twice as numerous (1324 per mm2), and only half as wide (6.2 microns), as the sinusoidal capillaries in the anterior lobe (density of 637 per mm2; diameter of 13.7 microns). The topographical position of the portal venular system suggests that the caudolateral subregions of the pituitary neural and anterior lobes have a functional relationship dependent on rapid interlobe transfer of neurohumoral factors such as hormones via the portal blood. This process appears to be supplemented throughout the rest of the cleft between the two lobes by a small number of capillary shunts that supply the epithelial cell lobules of the intermediate lobe in situ. The findings collectively indicate that this portal system provides a constant stream of neurohumoral information that is shared moment-by-moment between the pituitary neural and anterior lobes.

Osmotic stimuli attenuate vasoactive intestinal peptide gene expression in the rat anterior pituitary gland

Studies on vasoactive intestinal peptide (VIP) in the anterior pituitary gland have shown that it is synthesized locally, physiologically regulated, and may act as a paracrine/autocrine factor. We have now investigated the regulation of anterior pituitary VIP gene expression in rats during osmotic stimulation. Both salt-loading and dehydration resulted in a progressive and marked reduction in VIP mRNA levels as determined by Northern analysis, to 10% of control levels at 14 days of salt-loading. The 1.7 and 1.0 kb VIP RNA transcripts were equally affected. Since anterior pituitary VIP is partially localized in lactotrophs we also measured prolactin (PRL) mRNA levels. In contrast to VIP, PRL mRNA levels were increased during both osmotic paradigms, the mRNA levels being significantly raised after 5 days of salt-loading to 130% of controls. Further experiments, conducted to examine the mechanism by which VIP gene expression is down-regulated during osmotic stimulation, demonstrated that dopamine and angiotensin II do not appear to be involved. The results show dissociated regulation of VIP and PRL during osmotic stimulation and provide suggestive evidence of a role for anterior pituitary VIP in the animal's osmoregulatory responses. VIP may therefore be a paracrine factor with diverse functional roles.

The effect of genetic variability (degree of homozygosity) on serum levels of the anterior pituitary hormones prolactin, corticotropin, and growth hormone in rats

Male and female wild Norway rats (Rattus norvegicus Erxleben) and males and female albino outbred rats (Ipf:RIZ) were crossbred. The resulting animals (F1 hybrids) were the control, noninbred group (0% inbred). By systematic full-sib mating, two experimental groups (50 and 91% of inbred) were produced. Half of each group (both males and females) was exposed to physical stress (3 days of starvation and 3 hr of swimming). The other half of each group was anesthetized using ether to collect blood. The anterior pituitary hormone concentrations of prolactin (PRL), corticotropin (ACTH), and growth hormone (rGH) in blood serum were determined by the radioimmunoassay method. Significant relationships between the PRL, ACTH, and rGH concentrations in blood serum and the inbreeding coefficient were observed: A significant PRL content decrease in blood serum occurred (linear function) and the rGH and ACTH content diminished significantly rapidly (quadratic function). These changes were affected by an increase in homozygosity. Stress significantly influenced PRL, ACTH, and rGH concentrations as well. The sex of rats significantly determined PRL and ACTH content only. Hormone levels were also influenced by interactions between the factors studied (inbred level, sex, stress).

Modulation of pituitary gonadotropins and prolactin secretion by testosterone in vitro

Investigations were undertaken to study the differential modulation of LH, FSH and PRL secretion by testosterone (T) using whole pituitary (PI) or pituitary-hypothalamus coincubates (PHC) as in vitro constructs. PI and PHC from intact and castrated rats were incubated with or without T thrice, for 24 h each, (24 h x 3, total incubation period 72 h). The spent media was replenished every 24 h. At the end of 72 h, a few of the pituitary glands were challenged with 10 nM LHRH for 4 h. The spent media and pituitary glands were analyzed for LH, FSH and PRL using specific RIAs. Incubation of PI or PHC from intact rats with T stimulated the release of LH and FSH but inhibited the release of PRL. T had no effect on the intrapituitary contents of LH but inhibited intrapituitary contents of FSH and PRL, as compared to controls incubated without T. Castration increased intrapituitary contents of LH and FSH with concomitant decrease in PRL levels. Incubation of PI or PHC from castrated rats with T inhibited intrapituitary contents of LH to intact pituitary levels, while PRL levels were further reduced instead of being ameliorated. It is concluded that PI or PHC can be used as convenient in vitro models to monitor the effect of castration or of T modulation of pituitary and hypothalamus functions. T does not affect the synthesis of LH at the gonadotroph level but facilitates the regulation of intracellular LH and FSH levels. It is postulated that T inhibits the synthesis of FSH/PRL at the gonadotroph/lactotroph levels.

Effect of reserpine on the inhibition of prolactin released from different pituitary constructs in vitro by dopamine, bromocriptine and apomorphine

Hourly release of Prolactin by pituitary constructs 1 whole pituitary (PI), adenohypophysis (P-N) and pituitary-hypothalamus co-incubate (PHC) were compared. Adenohypophysis secreted significantly more prolactin than PI and PHC, while PHC secreted significantly less than PI. Co-incubation of (P-N) with posterior pituitary reduced the elevated secretion of prolactin. Addition of dopamine (10(-7) M), bromocriptine (10(-7) M) and apomorphine (5 x 10(-8) M) to these constructs did not affect the release of prolactin from PI but inhibited the same from (P-N) and PHC. Treatment with reserpine increased serum prolactin levels but intrapituitary prolactin contents were decreased. Hourly release of prolactin from pituitary constructs derived from reserpine-treated rats was significantly reduced as compared to ascorbic acid--treated controls. Inclusion of dopamine (10(-7) M), bromocriptine (10(-7) M) and apomorphine (5 x 10(-8) M) in these constructs inhibited prolactin secretion further. In vitro addition of perphenazine stimulated the release of prolactin by PHC but was without any effect on PI and (P-N). The data are interpreted to suggest that dopamine in posterior pituitary may be an important determinant of hypothalamic modulation of prolactin secretion.

Estradiol-induced prolactinomas: differential effects on dopamine in posterior pituitary and median eminence

Prolactin release is inhibited by dopamine and stimulated by estradiol. Dopamine is released from nerve terminals in the median eminence and posterior pituitary. Estradiol may act directly on the anterior pituitary or by modulating the two dopaminergic systems. Estradiol treatment induces the formation of prolactinomas in Fischer 334 rats. Therefore, this strain was chosen as the experimental model. The first objective was to determine whether estradiol differentially regulates the two dopaminergic systems. The second objective was to explore whether the anterior pituitary in estradiol-treated rats acquires the capability for de novo synthesis of dopamine. Rats were ovariectomized and implanted with estradiol capsules (OVEX + E2). Controls were untreated ovariectomized rats (OVEX). Three weeks thereafter, rats were killed. Anterior and posterior pituitaries and medial basal hypothalami (MBH) were removed and individually incubated for 60 min in Hank's balanced salt solution containing 10 microCi [3H-]tyrosine. The median eminence was then dissected from the MBH. Tissues were homogenized in perchloric acid and the supernatant fluids were extracted with alumina. Both endogenous and tritiated dopamine were simultaneously quantitated by HPLC. Prolactinoma formation in OVEX + E2 rats was confirmed by dramatic rise (50-fold) in plasma prolactin levels and marked enlargement (3-fold) of the anterior pituitary. Estradiol treatment caused a significant 60% reduction in both dopamine content and synthesis in the median eminence. In contrast, estradiol treatment affected neither dopamine content nor synthesis in the posterior pituitary. There was no evidence for de novo synthesis of dopamine in anterior pituitaries from either OVEX or OVEX + E2 rats. We conclude that the two dopaminergic systems which regulate prolactin secretion, exhibit a differential response to estradiol.

Presence of tyrosine-hydroxylase activity in anterior pituitary adenomas and ectopic anterior pituitaries in male rats

There is evidence for local regulatory effects of dopamine (DA) and norepinephrine (NE) on the release of prolactin (PRL) and other hormones from the anterior pituitaries transplanted to an ectopic site. In order to further explore these suspected regulatory mechanisms and to determine if they may exist also in pituitary tumors, we have examined the activity of tyrosine-hydroxylase (TH) in ectopic pituitaries and in hyperplastic pituitaries of estrogen-treated rats. Adult male rats with a pituitary graft in each kidney and sham-operated controls were examined 5 weeks after surgery. Rats implanted for 3 months with Silastic capsules containing diethylstilbestrol (DES) or with empty capsules were used 10 weeks after the capsules were removed. TH activity in ectopic anterior pituitaries of grafted rats was significantly higher than that measured in the eutopic anterior pituitaries of control animals. Similarly, TH activity was significantly higher in DES-induced pituitary tumors than in control pituitaries. These data support the possible existence of local catecholaminergic mechanism(s) that could be modulating PRL secretion from pituitary tumors and from ectopic pituitaries.

Prolactin secretion in posterior pituitary lobectomized rats: differential effects of 5-hydroxytryptophan and ether

The objective of this study was to determine whether the posterior pituitary mediates the prolactin (PRL) releasing activities of serotonin and ether. Ovariectomized (OVEX) rats were subjected to posterior pituitary lobectomy (LOBEX) or sham surgery (SHAM). Either 1 or 12 days after surgery, rats were injected i.v. with 20 mg/kg b. wt. of 5-hydroxytryptophan (5-HTP), which is a precursor of serotonin. A second group of rats was exposed to ether vapors for 10 min. Blood was collected from a jugular cannula before and after the treatments and analyzed for PRL. On either 1 or 12 days after surgery, injection of 5-HTP increased plasma PRL levels 5-10 fold in both LOBEX and SHAM rats. This was followed by a decline to preinjection levels within 60-90 min. LOBEX and SHAM male rats which were injected i.p. with 50 mg/kg b. wt. of 5-HTP, also showed marked and similar elevations of plasma PRL levels 12 days after surgery. Exposure of OVEX rats to ether elicited a 2-3 fold rise in plasma PRL levels only on day 1, but not on day 12, after LOBEX. The ether-induced rise in PRL was similar in SHAM rats tested on both days. These results indicate that the 5-HTP-induced rise in plasma PRL levels is independent of the posterior pituitary, regardless of the sex, the route of drug administration or the length of time after surgery. In contrast, the PRL response to ether stress is diminished within 12 days after LOBEX. The evidence that the PRL responses to 5-HTP and to ether might be mediated via different neuronal mechanisms is discussed.

Immunocytochemical study of the GABAergic innervation of the mouse pituitary by use of antibodies against gamma-aminobutyric acid (GABA)

The GABAergic innervation of the mouse pituitary, including the median eminence, was studied at light-microscopic and ultrastructural levels by use of a pre-embedding immunocytochemical technique with antibodies directed against GABA. In the median eminence, a high density of GABA-immunoreactive fibers was found in the external layer where the GABAergic varicosities were frequently observed surrounding the blood vessels of the primary capillary plexus. In the internal and subependymal layers, only few fibers were immunoreactive. The intense labeling of the external layer was observed in the entire rostro-caudal extent of the median eminence. In the pituitary proper, a dense network of GABA-immunoreactive fibers was revealed throughout the neural and intermediate lobes, entering via the hypophyseal stalk. The anterior and tuberal lobes were devoid of any immunoreactivity. The GABA-immunoreactive terminals were characterized in the median eminence, and in the intermediate and posterior lobes at the electron-microscopic level. They contained small clear vesicles, occasionally associated with dense-core vesicles or neurosecretory granules. In the intermediate lobe they were seen to be in contact with the glandular cells. In the posterior lobe and in the median eminence, GABA-immunoreactive terminals were frequently located in the vicinity of blood vessels. These results further support the concept of a role of GABA in the regulation of hypophyseal functions, via the portal blood for the anterior lobe, directly on the cells in the intermediate lobe, and via axo-axonic mechanisms in the median eminence and posterior lobe.

Evidence for dopamine receptor-mediated inhibition of prolactin cell function in the female Syrian hamster

Since the exact nature of the hypothalamic regulation of prolactin (PRL) cell activity in the photosensitive female Syrian hamster is unknown, the present investigation was designed to determine whether dopamine (DA), a physiological PRL-inhibitory hormone in the rat, inhibits the synthesis and release of female hamster PRL in vitro via a DA receptor-mediated mechanism. Anterior pituitary glands from long photoperiod-exposed adult female Syrian hamsters were incubated in the presence of increasing concentrations of DA (5 nM, 500nM and 50 microM) in Kreb's Ringer Bicarbonate medium for 2 hours following a preincubation period of 1 hour in medium not containing DA. While PRL synthesis, as measured by the incorporation of 3H-leucine into PRL, was unaffected by DA, the release of immunoreactive (RIA)-PRL into the medium was inhibited by 55% and 53% by 500 nM and 50 microM DA, respectively; however, these same concentrations of DA inhibited the release of 3H-PRL into the medium by only 25% and 23%, respectively. The DA receptor blocker, pimozide (PIM) was effective in blocking the PRL-inhibitory effects of DA (500 nM) on both RIA- and 3H-PRL release in vitro. These are the first data suggesting that DA directly inhibits PRL release from female hamster anterior pituitary glands via a DA receptor-mediated mechanism.

Vasopressin and oxytocin reduce plasma prolactin levels of conscious rats in basal and stress conditions. Study of the characteristics of the receptor involved

Subcutaneous administration of arginine vasopressin (AVP) to conscious rats induced a dose-dependent increase of plasma ACTH and beta-endorphin levels and decrease of plasma prolactin (PRL) levels 30 min later. AVP similarly reduced PRL increase induced by exposure to a novel environment stress. Oxytocin (OT) was also active but 5-fold less potent than AVP. The study of several analogs with specific agonistic and antagonistic activity on the oxytocic, vasopressor and antidiuretic receptors of OT and AVP suggests that the receptor involved in this effect does not fit into this classification.

Dopamine levels in the anterior pituitary gland monitored by in vivo electrochemistry

The present study was designed to assess the contributions of the long portal vessel and neurointermediate lobe routes of dopamine delivery to the anterior pituitary gland. Dopamine levels were monitored in the anterior pituitary of anesthetized diestrus 1 rats using in vivo differential pulse voltammetry at a carbon paste electrode. Measurements were taken during control periods, following neurointermediate lobectomy and hypophysial stalk section or after inhibition of catecholamine synthesis and dopamine infusion. Neurointermediate lobectomy resulted in a slight rise in plasma prolactin and a significant fall in voltammetric current. Subsequent stalk section elevated prolactin significantly and further reduced the voltammetric output. Inhibiting catecholamine synthesis significantly elevated plasma prolactin and reduced the voltammetric current. Stepped infusions of dopamine then suppressed plasma prolactin and elevated the voltammetric output in a dose related manner. The final values of both parameters were not significantly different from pre-inhibition levels. These data provide direct estimates of the relative contributions of various vascular routes to dopamine in the anterior pituitary and support the sufficiency of dopamine as the physiological inhibitor of prolactin secretion.

Organization of tyrosine-hydroxylase immunopositive neurons in the brain of the crested newt, Triturus cristatus carnifex

The localization of neurons, fibers, and terminals containing tyrosine hydroxylase (TH)-like immunoreactivity was studied in the brain of the crested newt by using an antiserum to rat phaeochromocytoma tyrosine hydroxylase. Immunoreactive cells and fibers were found in the spinal cord, the medulla oblongata (lateral periventricular areas), and the acousticolateral area. In the tegmentum mesencephali, two bilateral clusters of labelled cells were localized in the ventrolateral periventricular gray extending toward the caudal hypothalamus. In the hypothalamic tuberal lobes, the TH-like reactive neurons, frequently of CSF-contacting type, lined the dorsal wall of the lateral infundibular recesses. A thick network of TH-like reactive nerve fibers and terminals was observed in the perivascular zone of the median eminence and in the adenohypophysial pars intermedia. A number of labelled cell bodies were also found in the dorsal thalamus (pars intercalaris diencephali), the paraventricular organ, and the ventral wall of the preoptic recess. In the telencephalon, immunoreactive innervation was identified in the striatum, together with immunopositive cell bodies in the olfactory bulbs. The pattern of organization of TH-immunoreactive systems in the newt showed, except for some peculiarities (e.g., the labelled cell bodies in dorsal thalamus), close similarities to the arrangement typical of mammals.

Ontogenesis of immunoreactive tyrosine hydroxylase-containing neurons in rat hypothalamus

By employing anti-tyrosine hydroxylase (TH) serum, the ontogenesis of hypothalamic dopamine (DA) neurons was immunohistochemically examined with special attention to the medial basal hypothalamic area. DA neurons first appeared in the lateral hypothalamic walls on day 13.5 of gestation and in the anterior periventricular region and arcuate nucleus on day 15.5-16.5. In the arcuate nucleus, the appearance of the neurons was confined to the ventrolateral (VL) region, but extended to the periventricular region thereafter. About day 10 postnatally, the population of the arcuate DA neurons conjoins anterodorsally with the cell population in the anterior periventricular region. Concomitant with this, DA neurons in the VL region of the nucleus diminished in number and in stainability, becoming barely visible. Interestingly enough, the latter neurons reappeared after an anterolateral deafferentation of the medial basal hypothalamus. This did not occur in pregnant and lactating rats. Although most of the arcuate DA neurons were retarded by neonatal administration of monosodium glutamate, the immunoreactive fibers remained almost intact in the medial portion of the median eminence. It is concluded that in the periventricular-arcuate complex, DA neurons seem to play different roles relating with their ontogenetic heterogeneity.

The choice of a model for studying the hypothalamus-pituitary interactions in vitro

Comparative studies on the release of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin (Prl) by the whole pituitary, pituitary plus hypothalamus and pituitary-hypothalamus complex (PHC) were undertaken to choose an appropriate model for studying the hypothalamus-pituitary interactions in vitro and to relate the importance of the intact neural connections between pituitary and hypothalamus on hypothalamus-pituitary interactions. Also the effect of including dopamine (DA) at 1 X 10(-7) mol/1 in these different in vitro systems on the release of LH, FSH and Prl was investigated. The pituitary released increasing amounts of LH and FSH at 2, 4 and 6 h but the amount of Prl released remained unchanged. The rates of release of LH, FSH and Prl by the pituitary were different and were characteristic of each hormone. Co-incubation of pituitary with hypothalamus stimulated the release of LH and FSH but inhibited the release of Prl. Pituitary-hypothalamus complex behaved almost identical to behaved almost identical to pituitary plus hypothalamus system. Inclusion of 1 X 10(-7) M DA in the incubation medium stimulated the release of LH (80%) but inhibited the release of Prl (71%) by PHC. FSH was unaffected. DA had no significant effect on the release of LH, FSH and Prl by pituitary and pituitary plus hypothalamus systems. It is suggested that PHC is the system of choice for studying hypothalamus-pituitary interactions in vitro.

Differential effects of morphine on the rates of dopamine turnover in the neural and intermediate lobes of the rat pituitary gland

The acute administration of morphine to male rats decreased the rate of dopamine turnover in the median eminence and in the neural lobe of the pituitary, but was without effect in the intermediate lobe of the pituitary. Pretreatment with the opiate antagonist, naltrexone, reduced the effects of morphine. These results indicate that morphine, by acting on opiate receptors, inhibits the activity of tuberoinfundibular dopaminergic neurons that terminate in the median eminence and those tuberohypophysial dopaminergic neurons that terminate in the neural lobe of the pituitary.

Interaction between the posterior pituitary and LHRH in the control of LH secretion

We have recently reported that the posterior lobe of the pituitary differentially inhibits the secretion of prolactin (PRL) and luteinizing hormone (LH), but not follicle stimulating hormone (FSH) throughout the estrous cycle. Removal of the posterior pituitary (posterior pituitary lobectomy) results in elevations of plasma LH on all days of the cycle except on diestrus-day-2. In the present study we examined: whether the control of LH release involves an interaction between the posterior pituitary and hypothalamic luteinizing hormone-releasing hormone (LHRH), and whether the elevation of LH seen following posterior lobectomy is due to the removal of a posterior pituitary substance(s) which alters anterior pituitary sensitivity to LHRH. In order to block the action of hypothalamic LHRH, a potent LHRH inhibitory analog (50 micrograms) was injected SC two hours prior to removal of the posterior pituitary in estrous rats. Administration of the inhibitory analog completely eliminated the elevation of plasma LH seen following posterior lobectomy, but did not alter the posterior lobectomy-induced rise of plasma PRL, or plasma FSH concentrations. In order to test whether anterior pituitary sensitivity to LHRH is altered by posterior lobectomy, a moderate dose of LHRH (15 ng) was administered to both posterior lobectomized and sham lobectomized estrous rats. The time-course and magnitude of the LH response to LHRH was similar in both groups. The results are consistent with the hypothesis that LH secretion is controlled by an interaction between hypothalamic LHRH and the posterior lobe of the pituitary, but this interaction does not appear to involve lobectomy-induced changes in anterior pituitary responsiveness to LHRH.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of catecholamine neurons in the hypothalamus and preoptic region of mouse

The distribution and morphology of cells containing tyrosine hydroxylase (TH) were mapped by using the immunoperoxidase technique in the hypothalamus and preoptic area in two strains of mouse, CBA/J and BALB/cJ. On the basis of rostral-caudal contiguities between cell aggregates, hypothalamic preoptic neurons were subdivided into three arbitrary groups: (1) dorsal, (2) intermediate, and (3) ventral. New or more prominent collections of TH cells were observed, and in some regions, cells were more complexly organized than originally described. In the dorsal group, a rostral collection of small ovoid cells, previously not described, were located in the anterior preoptic nucleus (APN) of Loo ('31) and extended rostrally and ventrally into the preoptic periventricular gray. The next constituent occupied the paraventricular nucleus (PVN), and was composed of two classes of cells: (1) a small ovoid cell within anterior and medial parvocellular PVN in contiguity rostrally with a similar cell in APN and (2) a larger, angular cell within and adjacent to the lateral PVN in contiguity caudally with cells in the zona incerta (ZI). Further caudally, a larger and more pleomorphic collection of TH neurons was localized in the medial ZI, particularly at midtuberal levels. These cells were not scattered, as previously reported, but were differentiated into two clear-cut densities, a larger medial island and a more elongated lateral island. Cells of ZI, both large and small, extended caudally into the dorsal hypothalamic and subparafascicular nuclei and periventricular gray. In contrast to previous descriptions, no cells were seen in the nucleus reuniens. In the intermediate group, the most rostral constituent occupied the preoptic periventricular gray, extended as far as the lamina terminalis, and merged dorsocaudally with cells in APN. While the variably shaped cells of the hypothalamic periventricular gray (PVG) were still present in the retrochiasmatic region, a striking absence of these cells was noted at midtuberal levels between the dorsomedial and arcuate hypothalamic nuclei. At this level, a new group of small-round TH cells, resembling those of the arcuate nucleus, was identified in the dorsomedial hypothalamic nucleus (DMN). At caudal tuberal levels, similar neurons were found in the posterior hypothalamic nucleus (PH). These neurons overflowed medially into the PVG and caudoventrally into the arcuate nucleus. In the ventral group, the most rostral constituent, composed of both small and ovoid cells in the retrochiasmatic area, appeared to represent the rostral commissural portion of the arcuate nucleus (Arc).(ABSTRACT TRUNCATED AT 400 WORDS)

Increase in pituitary dopaminergic receptors after monosodium glutamate treatment

We investigated whether a decrease in arcuate nucleus dopamine (DA) levels resulting from neonatal treatment with monosodium glutamate (MSG) affects the anterior pituitary DA receptors in adult male rats. MSG treatment resulted in a significant reduction in medial basal hypothalamic (MBH) DA levels, no change in its norepinephrine (NE) and epinephrine (E) concentrations, and a marked increase in circulating prolactin (PRL). Scatchard analyses of DA binding characteristics to anterior pituitary membranes using [3H]spiperone revealed linear plots, suggesting a single class of high-affinity, low-capacity binding sites. The DA binding capacity was significantly higher in MSG-treated rats than in controls with no change in affinity. The data indicate that anterior pituitary DA receptors change in accordance with altered physiological conditions. The increase in the number of DA receptors following destruction of the arcuate nucleus is probably a direct effect of reduced DA levels reaching the anterior pituitary gland.

The organization of noradrenergic pathways from the brainstem to the paraventricular and supraoptic nuclei in the rat

Axonal transport and immunohistochemical methods have been used to clarify the organization of pathways from noradrenergic and adrenergic cell groups in the brainstem to the paraventricular (PVH) and supraoptic (SO) nuclei of the hypothalamus. First, the location of such cells was determined with a combined retrograde tracer-immunofluorescence method. The fluorescent tracer, True Blue, was injected into the PVH or the SO, and sections through the brainstem were stained with anti-(rat) DBH, a specific marker for noradrenergic and adrenergic neurons. It was found that, after injections in the PVH, doubly labeled neurons were confined almost exclusively to 3 cell groups, the A1 region of the ventral medulla, which contained a majority of such cells, the A2 region in the dorsal vagal complex, and the locus coeruleus (A6 region). After injections centered in the SO an even greater proportion of doubly labeled cells were found in the A1 region, although some were also found in the A2 and A6 regions. The topography of doubly labeled cells indicates that these projections arise primarily from noradrenergic neurons, although adrenergic cells in both the C1 and the C2 groups probably contribute as well. Because well over 80% of the retrogradely labeled cells in these three regions were also DBH-positive, we next placed injections of [3H]amino acids into each of them in different groups of animals, and traced the course and distribution of the ascending (presumably DBH-positive) projections to the PVH and SO in the resulting autoradiograms. Injections centered in the A1 region labeled a substantial projection to most parts of the parvocellular division of the PVH, and was most dense in the dorsal and medial parts. In addition, terminal fields were labeled on those parts of the magnocellular division of the PVH, and of the SO, in which vasopressinergic cell bodies are concentrated. Injections centered in the A2 region also labeled a projection to the parvocellular division of the PVH that was topographically similar, but less dense, than that from the A1 region. In contrast, [3H]amino acid injections centered in the locus coeruleus labeled a moderately dense projection to the PVH that was limited to the medialmost part of the parvocellular division. Neither the A2 nor the A6 cell groups project to the magnocellular parts of PVH, or to the SO. The autoradiographic material, and additional double-labeling experiments, were used to identify and to characterize projections that interconnect the A1, A2 and A6 regions, as well as possible projections from these cell groups to the spinal cord. These results may be summarized as follows: a substantial projection from the nucleus of the solitary tract to the A1 region was identified, but this pathway does not arise from catecholaminergic neurons in the A2 cell group. DBH-stained cells in the A1 region project back to the dorsal vagal complex, as well as quite massively to the locus coeruleus (A6 region)...

Involvement of the neurointermediate lobe of the pituitary gland in the secretion of prolactin and luteinizing hormone in the rat

The relationship between prolactin (PRL) secretion and the neurointermediate lobe (NIL) of the pituitary gland was investigated. Plasma PRL concentrations in rats bearing anterior pituitaries autografted with or without the NIL to the renal capsule were elevated to equal extents at 1 through 6 weeks after surgery (p greater than 0.10). PRL levels in ovariectomized rats in which the NIl had been removed surgically (NIL-X) or only visualized (NIL-C) were 3-7 ng/ml 4, 7, and 28 days after surgery (p greater than 0.10); however, they were slightly higher in NIL-X vs. NIL-C rats 14 days after surgery (p less than 0.05). Plasma luteinizing hormone (LH) concentrations in NIL-C rats increased by 36% from 2 to 4 weeks after surgery (p less than 0.5); this increase was not detected in NIL-X rats. PRL and LH surges were induced by estradiol implants in ovariectomized NIL-X and NIL-C rats; the profiles of the PRL surges were superimposable, although the magnitude of the LH surge was only 50% that in NIL-C rats (p less than 0.5). These results cast doubt on the importance of the NIL in the regulation of PRL secretion either via secreting hypophysiotropic hormones or via conducting anterior pituitary hormones directly to the median eminence. However, the NIL may have a physiologically important role in the regulation of LH secretion.