Effect of histamine and acetylcholine on hypophysial stalk plasma dopamine and peripheral plasma prolactin levels

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.

Effect of selective blockade of catecholaminergic alpha and beta receptors on histamine-induced release of corticotropin and prolactin

We investigated the role of adrenergic receptors in histamine (HA)-induced release of corticotropin (ACTH) and prolactin (PRL) in conscious male rats. Specific alpha- or beta-receptor antagonists were administered intracerebroventricularly in doses of 1 mmol at time -20 min, and HA (270 nmol), the H1 receptor agonist 2-thiazolylethylamine (2-TEA; 2,180 nmol) or the H2 receptor agonist 4-methylHA (4-MeHA; 790 nmol) were administered intracerebroventricularly at -15 min. The animals were decapitated at 0 min, and plasma was analyzed for ACTH and PRL. Administration of HA and the histaminergic agonists stimulated ACTH secretion equally, while only HA and the H2 receptor agonist stimulated PRL secretion. Pretreatment with the adrenergic receptor antagonists had no effect on the ACTH response to the histaminergic compounds. In contrast, the PRL response to HA or 4-MeHA was inhibited or prevented by the alpha-receptor antagonists phenoxybenzamine and phentolamine, the alpha1-receptor antagonist prazocin, the beta-receptor antagonist propranolol and the beta1-receptor antagonist atenolol, whereas the alpha2-receptor antagonist yohimbine or the beta2-receptor antagonist ICI-118-551 had no effect. The study indicates that histaminergic neurons interact with the catecholaminergic neuronal system in regulation of PRL secretion, and that this interaction is dependent upon activation of alpha1- and beta1-receptors. In contrast, histaminergic neurons stimulate ACTH secretion independently of adrenergic receptor activation.

Effects of various neuroactive substances on single-unit activities of hypothalamic arcuate neurons in brain slices

Extracellular single-unit activities of 288 dorsomedial/ventrolateral hypothalamic arcuate (ARC) neurons were studied electrophysiologically in brain slices in vitro. We tested the effects of several neuroactive substances plus some of their analogs in this study. Among them, baclofen, a GABAb-receptor agonist, inhibited 95.6% of ARC neurons tested, although GABA itself was much less effective (23.8%). About half of baclofen's effect was blocked by phaclofen, a GABAB antagonist. Serotonin and dopamine also exhibited mostly inhibitory effects on responsive ARC neurons (80 and 78.4%, respectively), although more than half ARC neurons tested (53.3% of 169) were not responsive to dopamine. Neither ketanserin, a 5-HT2, nor domperidone, a D2 receptor antagonist, had much effect on blocking the actions of 5-HT or DA. Histamine and carbachol excited 67.4% and 52.2% of ARC neurons tested, respectively. Moreover, their effects were completely blocked by pyrilamine, a H1, and atropine, a muscarinic receptor antagonist, respectively. Ranitidine, a H2 receptor antagonist, however, was less effective. Norepinephrine had about equal number of excitation (33.3%) and inhibition (38.5%) on ARC neurons. Cholecystokinin-octapeptide sulphate (CCK-8S), a neuropeptide tested exerted potent excitatory effects on ARC neurons (62.8% of 137). In summary, ARC neurons in a more localized region aiming at the tuberoinfundibular dopaminergic neurons were selected in this study. Their responses to many agents and the implications on the regulation of prolactin secretion were discussed.

Evidence that histamine-stimulated prolactin secretion is not mediated by an inhibition of tuberoinfundibular dopaminergic neurons

The effects of histamine on prolactin secretion and the activity of tuberoinfundibular dopaminergic (DA) neurons were examined in male rats. Tuberoinfundibular DA neuronal activity was estimated in situ by measuring the metabolism [concentration of 3,4-dihydroxyphenylacetic acid (DOPAC)] and synthesis [accumulation of 3,4-dihydroxyphenylalanine (DOPA) after administration of a decarboxylase inhibitor] of dopamine in the median eminence. Intracerebroventricular (icv) injection of histamine produced a dose- and time-dependent increase in plasma prolactin levels but had no effect on DOPA accumulation or DOPAC concentrations in the median eminence. These results indicate that the stimulation of prolactin secretion following icv histamine is not mediated by an inhibition of tuberoinfundibular DA neurons.

Neuroendocrine functions of histamine

The neurotransmitter histamine (HA) participates in the neuroendocrine regulation of pituitary hormone secretion and in the regulation of some peripheral hormones. In general, HA has a stimulatory but indirect effect on the release of these hormones by activation of postsynaptic receptors in the hypothalamic region. The release of the pro-opiomelanocortin-derived peptides ACTH, beta-endorphin (beta-END), and alpha-melanocyte-stimulating hormone (alpha-MSH) occurs by stimulation of H1- and H2-receptors and seems to be mediated via release of corticotropin-releasing hormone and vasopressin from the hypothalamus. The HA-induced release of prolactin (PRL) involves H2-receptors in some hypothalamic areas and H1-receptors in other areas. The release of PRL occurs by histaminergic inhibition of tuberoinfundibular dopaminergic neurons and by stimulation of serotoninergic and vasopressinergic neurons. Histaminergic neurons seem to participate in the mediation of the stress-induced release of ACTH, beta-END, alpha-MSH, and PRL. The neurohypophysial hormones vasopressin and oxytocin are stimulated by HA, and a physiological role of HA in the control of vasopressin secretion is likely. HA stimulates the release of peripheral catecholamines and renin. The stress-induced increase in plasma catecholamines and plasma renin activity (PRA) seems also to involve central histaminergic neurons. The effect of HA and stress on peripheral catecholamines is mediated via H1- and H2-receptors, while that on PRA is mediated via H2-receptors.

Stimulation and inhibition of prolactin release from rat pituitary lactotrophs by the cholinomimetic carbachol in vitro. Influence of hormonal environment and intercellular contacts

The prolactin (PRL) response of perifused rat pituitary tissue to the cholinomimetic agent carbachol (CARB) was studied under various in vitro conditions. Perifusion of freshly removed hemipituitaries from 14-day-old rats with CARB did not affect basal PRL release. When established in organ culture for 3 days in a serum-free chemically defined medium, there was a significant increase of PRL release in response to CARB. This PRL releasing activity of CARB depended on the hormonal environment of the culture medium: supplementation of the culture medium with triiodothyronine (T3) and the glucocorticoid dexamethasone (DEX) completely reversed the PRL releasing activity of CARB into an inhibition of PRL release. In dispersed pituitary cells from immature rats, cultured as three-dimensional reaggregates, a similar reciprocal responsiveness to CARB existed which was also determined by T3 and DEX. This reciprocal responsiveness to CARB was preserved in adult female rats but was shifted to a more prominent inhibition in adult male rats. Tumoral PRL secreting GH3 cells, cultured as aggregates, always responded in an inhibitory way, irrespective of the hormonal environment. The expression of the reciprocal responses, in particular of the inhibitory pathway to CARB was dependent on close cellular contacts, as the inhibitory response of normal and tumoral pituitary cells, cultured as isolated cells on Cytodex beads, was completely absent. The stimulatory response of normal lactotrophs, cultured as isolated cells was, although attenuated, still preserved. The present data suggest that there exists a reciprocal responsiveness of normal lactotrophs to cholinomimetics depending on the hormonal environment and close cellular associations. In contrast, only inhibitory PRL responses occur in GH3 tumoral lactotrophs, which are not dependent on thyroid and glucocorticoid hormones.

Effects of brain histamine depletion on stimulated prolactin release in rats

We examined the effects of an irreversible inhibitor of brain histamine (HA) synthesis, alpha-fluoromethyl-histidine (alpha-FMH), on prolactin (PRL) release induced by an opiate agonist (morphine, M) or by a serotonergic agonist (MK212). alpha-FMH was administered intracerebroventricularly (i.c.v., 200 micrograms/rat) into freely moving rats with indwelling catheters in the carotid. M (6 mg/kg, intracarotid, i.a.) was administered simultaneously with or 3 h after alpha-FMH. MK212 (2.5 mg/kg, i.a.) was administered 3 h after alpha-FMH. Blood samples for assay for PRL were drawn at 0, 10, 20, 40 min after M or MK212 administration. alpha-FMH (3 h before) significantly reduced the PRL-releasing effect of M and MK212 but did not modify PRL release by M when administered simultaneously. The present results showing that the facilitatory actions of the opiate and serotonergic systems on PRL are impaired when brain HA synthesis is reduced, suggest that there is an HA-dependent step in opiate and serotonergic control of PRL.

Multifactorial control of pituitary hormone secretion: the "wheels" of the brain

An attempt is made to deal with the complexity of the nerve fibers in the median eminence. Visual aids are presented in the shape of "wheels" that depict a dynamic interplay of neurochemicals which result in the release of hormones from the anterior pituitary gland. The multiplicity of neurochemicals in the median eminence is perceived to be responsible for the integrated control of pituitary hormone releasing factors.

Involvement of brain histamine in basal and stress-induced release of prolactin in the rat

We investigated whether inhibition of brain histamine (HA) synthesis by alpha-fluoromethylhistidine (alpha-FMH) can influence basal or stimulated prolactin (PRL) release in male rats. alpha-FMH was administered either into the carotid (i.a., 20 and 100 mg/kg) or intracerebroventricularly (i.c.v., 200 micrograms/rat) into freely moving rats with indwelling catheters. Plasma PRL levels were measured 90, 120, 180 min later. Both i.a. and i.c.v. administration of alpha-FMH significantly inhibited basal PRL secretion at 120 and 180 min. When PRL secretion was stimulated by exposing rats to restraint stress, alpha-FMH administered 3 h before the stress (20 and 100 mg/kg, i.a.; 200 micrograms/rat, i.c.v.) was able to prevent the PRL surges at 10 and 20 min after stress. These results suggest that endogenous brain HA has a facilitatory role in the control of PRL secretion in rats.

Supersensitive endocrine response to physostigmine in dopamine-depleted rats: a model of depression?

Depressed patients exhibit an abnormal "supersensitive" increase in the plasma concentration of several pituitary hormones following intravenous injection of the acetyl cholinesterase inhibitor physostigmine (PHY). In the present study, we examined the effects of PHY treatments on the plasma concentrations of prolactin (PRL) and adrenocorticotrophic hormone (ACTH) in the rat. Physostigmine (0-0.6 mg/kg, s.c.) produced a dose-dependent increase in PRL and ACTH immunoreactivity in unoperated animals. Neurotoxin-induced depletion of brain dopamine (DA) or norepinephrine (NE) did not significantly alter baseline plasma PRL or ACTH values. Following depletion of brain DA, but not NE, animals exhibited a "supersensitive" increase in plasma ACTH values, which was evidenced by a sixfold left shift in the dose-response properties of PHY. These results suggest that there are intriguing parallels between the abnormal endocrine response to PHY demonstrated by depressed patients and that demonstrated by rats following depletion of central nervous system (CNS) DA levels.

Histamine-induced prolactin release and activity of tuberoinfundibular dopaminergic neurons in male rats

The role of tuberoinfundibular dopamine (DA) in the histamine-induced rise of plasma prolactin was studied in male rats. Right lateral ventricle (icv) histamine injection (30 micrograms/rat) caused a significant rise of plasma prolactin at 15 and 30 min; at 60 min values returned to basal levels. Histamine does not modified steady-state DA concentrations. For turnover evaluation, histamine was icv injected immediately or 30 min after the tyrosine hydroxylase inhibitor alpha-methyltyrosine (250 mg/kg i.p.) and DA concentration in the median eminence was measured at 0, 1 and 2 hours after the inhibitor injection. Rate constants of DA decline and DA synthesis rates were found similar in both controls (cerebrospinal fluid) and histamine-injected rats. These results indicate that the stimulatory action of histamine on prolactin release might not be associated to DA. It would be due mainly to its action on prolactin-releasing factors and to a minor extent, to changes in the dopaminergic system.

Effects of putative neurotransmitters on release of prolactin from pituitary glands of the domestic fowl co-incubated with hypothalamic tissue

Pituitary glands and hypothalami from broiler fowl heads were incubated alone or together with histamine, gamma-aminobutyric acid (GABA) or acetylcholine (ACh) as well as with catecholamines or neurotransmitter antagonists. Histamine and ACh stimulated, whereas GABA inhibited, the hypothalamus-induced release of prolactin, responses blocked by their specific antagonists. The dopamine antagonist pimozide, but not adrenergic (both alpha and beta), serotoninergic or cholinergic antagonists, prevented the actions of histamine and GABA. None of the antagonists except the cholinergic blocker, atropine, affected ACh-induced release of prolactin. Neither histamine nor ACh prevented inhibition of prolactin release by dopamine or stimulation of prolactin release by noradrenaline. GABA did not affect the response to noradrenaline. Furthermore, histamine, GABA and ACh had no effects on thyrotrophin releasing hormone-stimulated release of prolactin directly at the pituitary level. These results suggest that histamine and GABA affect prolactin release from chicken pituitaries in vivo by modifying the activity of the dopaminergic system. Acetylcholine may stimulate the secretion of prolactin releasing factor from the hypothalamus.

Histamine and the hypothalamus

The chemical tools that could be used to examine the function of histamine in the brain are considered together with the evidence linking histamine specifically with the hypothalamus. The distribution of histamine and the enzymes responsible for its synthesis and metabolism is consistent with there being both mast cells and histaminergic nerve terminals within the hypothalamus. Iontophoresis, mepyramine binding and histamine-stimulated adenylate cyclase studies suggest that both histamine H1- and H2- receptors are present in the hypothalamus. In addition, intracerebroventricularly injected histamine receptor agonists and antagonists affect many functions associated with the hypothalamus such as cardiovascular control, food intake, body temperature control, and pituitary hormones whose release is mediated via the hypothalamus, such as corticotropin, growth hormone, thyroid stimulating hormone, prolactin, gonadotropins and vasopressin. However, only in the case of thyroliberin release, prolactin release, body fluid control and blood pressure control is there evidence yet that such effects are mediated via histamine receptors actually in the hypothalamus. The effects of enzyme inhibitors suggest endogenous histamine may be involved in the physiological control of thyroid stimulating hormone, growth hormone and blood pressure, and the effects of receptor antagonists support a role for endogenous histamine in prolactin control. Otherwise, there is little evidence for a physiological role for endogenous, as against exogenous, histamine whether it be from histaminergic terminals or mast cells. In addition, few studies have tried to distinguish possible effects on presynaptic receptors, postsynaptic receptors, hypothalamic blood vessels or the hypophyseal portal blood vessels. It is concluded that although there is good evidence now linking histamine and the hypothalamus more specific studies are required, for instance using microinjection or in vitro techniques and the more specific chemical tools now available, to enable a clearer understanding of the physiological role of histamine in the hypothalamus.