Activation of GABA receptors in the hypothalamus stimulates secretion of growth hormone and prolactin

Mice with gene alterations in the GH and IGF family

Much of our understanding of GH's action stems from animal models and the generation and characterization of genetically altered or modified mice. Manipulation of genes in the GH/IGF1 family in animals started in 1982 when the first GH transgenic mice were produced. Since then, multiple laboratories have altered mouse DNA to globally disrupt Gh, Ghr, and other genes upstream or downstream of GH or its receptor. The ability to stay current with the various genetically manipulated mouse lines within the realm of GH/IGF1 research has been daunting. As such, this review attempts to consolidate and summarize the literature related to the initial characterization of many of the known gene-manipulated mice relating to the actions of GH, PRL and IGF1. We have organized the mouse lines by modifications made to constituents of the GH/IGF1 family either upstream or downstream of GHR or to the GHR itself. Available data on the effect of altered gene expression on growth, GH/IGF1 levels, body composition, reproduction, diabetes, metabolism, cancer, and aging are summarized. For the ease of finding this information, key words are highlighted in bold throughout the main text for each mouse line and this information is summarized in Tables 1, 2, 3 and 4. Most importantly, the collective data derived from and reported for these mice have enhanced our understanding of GH action.

Gamma-Aminobutyric Acid (GABA) Promotes Growth in Zebrafish Larvae by Inducing IGF-1 Expression via GABAA and GABAB Receptors

Insulin-like growth factor-1 (IGF-1) primarily increases the release of gamma-aminobutyric acid (GABA) in neurons; moreover, it is responsible for the promotion of longitudinal growth in children and adolescents. Therefore, in this study, we investigated whether exogenous GABA supplementation activates IGF-mediated growth performance. Zebrafish larvae treated with GABA at three days post fertilization (dpf) showed a significant increase in the total body length from 6 to 12 dpf through upregulation of growth-stimulating genes, including IGF-1, growth hormone-1 (GH-1), growth hormone receptor-1 (GHR-1), and cholecystokinin A (CCKA). In particular, at 9 dpf, GABA increased total body length from 3.60 ± 0.02 to 3.79 ± 0.03, 3.89 ± 0.02, and 3.92 ± 0.04 mm at concentrations of 6.25, 12.5, and 25 mM, and the effect of GABA at 25 mM was comparable to 4 mM β-glycerophosphate (GP)-treated larvae (3.98 ± 0.02 mm). Additionally, the highest concentration of GABA (50 mM) -induced death in 50% zebrafish larvae at 12 dpf. GABA also enhanced IGF-1 expression and secretion in preosteoblast MC3T3-E1 cells, concomitant with high levels of the IGF-1 receptor gene (IGF-1R). In zebrafish larvae, the GABA-induced growth rate was remarkably decreased in the presence of an IGF-1R inhibitor, picropodophyllin (PPP), which indicates that GABA-induced IGF-1 enhances growth rate via IGF-1R. Furthermore, we investigated the effect of GABA receptors on growth performance along with IGF-1 activation. Inhibitors of GABA_A and GABA_B receptors, namely bicuculline and CGP 46381, respectively, considerably inhibited GABA-induced growth rate in zebrafish larvae accompanied by a marked decrease in the expression of growth-stimulating genes, including IGF-1, GH-1, GHR-1, and CCKA, but not with an inhibitor of GABA_C receptor, TPMPA. Additionally, IGF-1 and IGF-1R expression was impaired in bicuculline and CGP 46381-treated MC3T3-E1 cells, but not in the cells treated with TPMPA. Furthermore, treatment with bicuculline and CGP 46381 significantly downregulated GABA-induced IGF-1 release in MC3T3-E1 cells. These data indicate that GABA stimulates IGF-1 release via GABA_A and GABA_B receptors and leads to growth promotion performance via IGF-1R.

Role of dietary gamma-aminobutyric acid in broiler chickens raised under high stocking density

The present study was conducted to evaluate the effects of dietary gamma-aminobutyric acid (GABA) in broiler chickens raised in high stocking density (HSD) on performance and physiological responses. A total of 900 male broiler chicks (Ross 308) at 1 d old were assigned in a 2 × 2 factorial arrangement to 4 treatments (10 replicates per treatment) with stocking density, 7.5 birds/m² (low stocking density; LSD) or 15 birds/m² (HSD), and dietary GABA, 0 or 100 mg/kg. Chickens raised in HSD exhibited a decrease in body weight gain in all phases (P < 0.05) and feed intake in starter and whole phases (P < 0.01), and an increase in feed conversion ratio in the finisher phase (P < 0.01) compared with LSD-raised chickens. However, dietary GABA did not affect growth performance nor interacted with stocking density on production variables. The HSD vs. LSD increased relative liver weight on d 35 whereas dietary GABA increased relative liver weight and decreased relative bursa weight on d 21. Both stocking density and dietary GABA affected yield and quality of breast and leg muscles. Dietary GABA increased (P < 0.05) width of tibia on d 35 and interacted (P = 0.054) with stocking density on breaking stocking density on d 35. The HSD vs. LSD group lowered (P < 0.05) feather coverage scores. Significant interaction between stocking density and GABA on surface temperature of shank on d 21 was noted (P = 0.024). Dietary GABA exhibited an opposite effect on the concentrations of cecal short-chain fatty acids depending on stocking density leading to a moderate to significant interaction. Stocking density decreased alpha-1-acid glycoprotein whereas dietary GABA decreased heterophil-to-lymphocyte ratio and corticosterone in blood or serum samples. Serum biochemical parameters were altered by stocking density or dietary GABA. It is concluded that dietary GABA alleviated stress indices including corticosterone and heterophil-to-lymphocyte ratio, but failed to reverse stocking density-induced growth depression.

Propofol Requirement in Patients with Growth Hormone-Secreting Pituitary Tumors Undergoing Transsphenoidal Surgery

Growth hormone (GH) secretion is regulated by various hormones or neurotransmitters, including gamma-aminobutyric acid. The aim of this study was to determine the propofol requirement in patients with GH-secreting pituitary tumors undergoing transsphenoidal surgery. General anesthesia was induced in 60 patients with GH-secreting tumors (GH group, n = 30) or nonfunctioning pituitary tumors (NF group, n = 30) using an effect-site target-controlled intravenous propofol infusion. The effect-site concentrations were recorded at both a loss of consciousness and a bispectral index (BIS) of 40, along with the effect-site concentration after extubation, during emergence from the anesthesia. The effect-site concentration of propofol was higher in the GH group than in the NF group at a loss of consciousness and a BIS of 40 (4.09 ± 0.81 vs. 3.58 ± 0.67, p = 0.009 and 6.23 ± 1.29 vs. 5.50 ± 1.13, p = 0.025, respectively) and immediately after extubation (1.60 ± 0.27 vs. 1.40 ± 0.41, p = 0.046). The total doses of propofol and remifentanil during anesthesia were comparable between the groups (127.56 ± 29.25 vs. 108.64 ± 43.16 µg/kg/min, p = 0.052 and 6.67 ± 2.89 vs. 7.05 ± 1.96 µg/kg/h, p = 0.550, respectively). The propofol requirement for the induction of a loss of consciousness and the achievement of a BIS of 40 is increased during the induction of general anesthesia in patients with GH-secreting tumors.

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.

Ethanol administration dampens the prolactin response to psychosocial stress exposure in sons of alcohol-dependent fathers

Genetic predisposition and exposure to alcohol and stress increase the risk for alcoholism, possibly by forming a threefold interaction. This is suggested by various aspects of alcohol-induced stress response dampening in offspring of alcoholics. We tested whether such an interaction is also revealed by prolactin secretion, which is predominantly controlled by hypothalamic dopamine. Plasma prolactin was measured during four experimental days in 26 young males with a paternal history of alcoholism (PHA) and in 22 family history negative (FHN) controls. A public speaking stress paradigm was applied on the first 2 days, and a non-stress acoustic startle experiment on the others. Before the tests, subjects drank alcohol (0.6 g/kg) or placebo in a randomized, double-blind crossover design. During placebo experiments, prolactin levels significantly increased after stress, but not after startle, and did not differ between risk groups. Alcohol administration significantly increased prolactin before stress and during startle in both groups, did not alter stress-induced prolactin stimulation in FHN, but significantly attenuated the prolactin stress response in PHA subjects. The alcohol effects on prolactin, cortisol, and adrenocorticotropin stress response were positively interrelated with each other. These data confirm that alcohol specifically dampens the stress response in PHA but not FHN subjects. Since prolactin responses to stress alone and alcohol alone were normal in PHA, we conclude that this genetic effect is not related to altered physiology of the hypothalamic dopaminergic system, but to risk-group specific alcohol effects on hierarchically higher brain areas controlling the stress response in general.

The tuberal lateral hypothalamus is a major target for GABAA--but not GABAB-mediated control of food intake

The lateral hypothalamus (LH) is a site of integration for control mechanisms of feeding behavior as it has extensive reciprocal connections with multiple intrahypothalamic and extrahypothalamic brain areas. Evidence suggests that blockade of ionotropric gamma-aminobutyric acid (GABA) receptors in the LH elicits eating in satiated rats. To determine whether this GABA(A) receptor antagonist effect is specific to the LH, the antagonist picrotoxin was injected into one of six nearby sites and food intake was measured. Picrotoxin at 133 pmol elicited eating in the LH, but not in surrounding sites (thalamus, lateral preoptic area, ventral tegmental area, dorsomedial hypothalamus, and entopeduncular nucleus). More specifically, picrotoxin injected into the tuberal LH (tLH) elicited eating, but was ineffective when injected into the anterior or posterior LH. We also investigated whether GABA(B) receptors in the LH participated in the control of food intake and found that neither blockade nor activation of these receptors under multiple conditions changed food intake. Collectively, our findings suggest that GABA(A) but not GABA(B) receptors in the tLH act to suppress feeding behavior.

GABA and glutamate in mating-activated cells in the preoptic area and medial amygdala of male gerbils

The posterodorsal medial amygdala (MeApd), the posterodorsal preoptic nucleus (PdPN), and the medial cell group of the sexually dimorphic preoptic area (mSDA) contain cells that are activated specifically at ejaculation as assessed by Fos expression. The mSDA also expresses Fos early in the mating context. Because little is known about the neurotransmitters of these activated cells, the possibility that they use gamma-aminobutyric acid (GABA) or glutamate was assessed. Putative glutamatergic cells were visualized with immunocytochemistry (ICC) for glutamate and its neuron-specific transporter. Their distributions were compared with those of GABAergic cells visualized with ICC for the 67-kDa form of glutamic acid decarboxylase (GAD(67)) and in situ hybridization for GAD(67) messenger RNA (mRNA). Colocalization of Fos and GAD(67) mRNA in recently mated males indicated that half of the activated cells in the PdPN, mSDA, and lateral MeApd are GABAergic. Colocalization of Fos and glutamate suggested that a quarter of the activated mSDA and lateral MeApd cells are glutamatergic. The PdPN does not appear to have glutamatergic cells. In the lateral MeApd, the percentage of activated cells that are GABAergic (45%) matches the percentage that project to the principal part of the bed nucleus of the stria terminalis (BST; 43%), and the percentage likely to be glutamatergic (27%) matches the percentage projecting to the mSDA (27%). The latter could help to trigger ejaculation. The distribution of GABAergic and putative glutamatergic cells in the caudal preoptic area, caudal BST, and medial amygdala of male gerbils is also described.

A preliminary study of baclofen-induced growth hormone release in generalised social phobia

OBJECTIVE

The authors sought to examine central GABA(B) receptor responses in patients with generalised social phobia using the growth hormone (GH) response to baclofen.

METHOD

Baclofen 20 mg was administered to 15 patients and 15 healthy matched controls and plasma growth hormone was monitored over a 3 h period.

RESULTS

The GH response in patients was significantly reduced in comparison to that of controls when calculated as both area under the curve (AUC) and delta (Delta) response.

CONCLUSIONS

The results suggest an abnormality of central GABA(B) receptor function in this disorder.

Benzodiazepines and anterior pituitary function

Benzodiazepines (BDZ) are one of the most prescribed classes of drugs because of their marked anxiolytic, anticonvulsant, muscle relaxant and hypnotic effects. The pharmacological actions of BDZ depend on the activation of 2 specific receptors. The central BDZ receptor, present in several areas of the central nervous system (CNS), is a component of the GABA-A receptor, the activation of which increases GABAergic neurotransmission and is followed by remarkable neuroendocrine effects. The peripheral benzodiazepine receptors (PBR), structurally and functionally different from the GABA-A receptor, have been shown in peripheral tissues but also in the CNS, in both neurones and glial cells, and in the pituitary gland. BDZ receptors bind to a family of natural peptides called endozepines, firstly isolated from neurons and glial cells in the brain and then in several peripheral tissues as well. Endozepines modulate several central and peripheral biological activities, including some neuroendocrine functions and synthetic BDZ are likely to mimic them, at least partially. BZD, especially alprazolam (AL), possess a clear inhibitory influence on the activity of the HPA axis in both animals and humans. This effect seems to be mediated at the hypothalamic and/or suprahypothalamic level via suppression of CRH. The strong negative influence of AL on hypothalamicpituitary-adrenal (HPA) axis agrees with its peculiar efficacy in the treatment of panic disorders and depression. BZD have also been shown to increase GH secretion via mechanisms mediated at the hypothalamic or supra-hypothalamic level, though a pituitary action cannot be ruled out. Besides the impact on HPA and somatotrope function, BDZ also significantly affect the secretion of other pituitary hormones, such as gonadotropins and PRL, probably acting through GABAergic mediation in the hypothalamus and/or in the pituitary gland. In all, BDZ are likely to represent a useful tool to investigate GABAergic activity and clarify its role in the neuroendocrine control of anterior pituitary function; their usefulness probably overrides what had been supposed before.

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.

Neuroendocrine control of growth hormone secretion

The secretion of growth hormone (GH) is regulated through a complex neuroendocrine control system, especially by the functional interplay of two hypothalamic hypophysiotropic hormones, GH-releasing hormone (GHRH) and somatostatin (SS), exerting stimulatory and inhibitory influences, respectively, on the somatotrope. The two hypothalamic neurohormones are subject to modulation by a host of neurotransmitters, especially the noradrenergic and cholinergic ones and other hypothalamic neuropeptides, and are the final mediators of metabolic, endocrine, neural, and immune influences for the secretion of GH. Since the identification of the GHRH peptide, recombinant DNA procedures have been used to characterize the corresponding cDNA and to clone GHRH receptor isoforms in rodent and human pituitaries. Parallel to research into the effects of SS and its analogs on endocrine and exocrine secretions, investigations into their mechanism of action have led to the discovery of five separate SS receptor genes encoding a family of G protein-coupled SS receptors, which are widely expressed in the pituitary, brain, and the periphery, and to the synthesis of analogs with subtype specificity. Better understanding of the function of GHRH, SS, and their receptors and, hence, of neural regulation of GH secretion in health and disease has been achieved with the discovery of a new class of fairly specific, orally active, small peptides and their congeners, the GH-releasing peptides, acting on specific, ubiquitous seven-transmembrane domain receptors, whose natural ligands are not yet known.

GABAergic function in detoxified heroin addicts: relationship to anxiety disorders

The function of the GABAergic system was examined in 20 subjects with heroin dependence and abuse, 2 months after detoxification, and in 10 healthy volunteers, by measuring the growth hormone (GH) response to a challenge with the GABA B receptor agonist baclofen. Ten heroin addicts had comorbid anxiety disorder (Group A), while the other ten had heroin addiction uncomplicated by Axis I and II psychopathologies (Group B). GH responses to baclofen stimulation of Group A patients were significantly blunted, while those of Group B subjects did not differ from responses of healthy volunteers. Our data show that the function of the GABAergic system is impaired only in heroin addicts with comorbid anxiety disorders (anxious cluster), suggesting that the GABA system is not persistently influenced by prolonged exposure to opioid receptor stimulation.

Thyrotropin-releasing hormone: inhibitory function on growth hormone through both somatostatin and growth hormone-releasing factor neurons

Double labelling immunohistochemistry using antibodies to thyrotropin releasing hormone (TRH) and somatostatin (SS) was undertaken in the anterior hypothalamus in 6 rats. Light microscopic quantitation revealed that 94.5% of SS immunopositive perikarya in the preoptic anterior hypothalamic area (PO/AHA) and 97.5% in the paraventricular nucleus appeared to be contacted by one or more TRH immunopositive terminals. In the chronically cannulated unanaesthetised male rat, unilateral microinjections of a range of doses of TRH were made in the PO/AHA, where SS neurons are located, or in the medial basal hypothalamus, where growth hormone (GH)-releasing factor (GRF) neurons are located. Transient reductions in GH plasma levels occurred only after injections of the highest (10 nmol) dose of TRH in both sites. The function of TRH inputs to both somatostatin and GRF neurons appears to be inhibitory for GH. The physiological conditions in which these inputs function remain to be defined.

Androgen receptor function during undernutrition

Hypothalamo-pituitary inhibition of reproductive function during undernutrition is well known, however, the physiological mechanisms leading to suppression of gonadotrophin secretion are not clear. A variety of studies have indicated that testicular negative feedback on LH secretion is enhanced during food restriction. To evaluate directly the suppression by endogenous androgens on hypothalamic GnRH pulse generator activity during food restriction and examine the mechanism underlying the increased testicular steroidal feedback, we examined (1) circulating bioactive LH (bLH) levels in response to selective cerebral androgen blockade by intraventricular administration of an androgen receptor antagonist (hydroxyflutamide, SCH 16423) and (2) the binding capacity and affinity of androgen receptors in medio-basal hypothalamus, pituitary and prostate during undernutrition of intact mature male rats. Hydroxyflutamide (20 micrograms in 10 microliters vehicle), but not vehicle alone, markedly increased bLH levels in both food restricted and ad-lib fed rats. However, the faster (geometric mean 11.4 vs 27.7 min) and greater (47.2 vs 21.9 ng/ml) increase in bLH level in food restricted compared with ad-lib fed controls demonstrates an enhanced sensitivity to blockade of androgenic negative feedback during undernutrition. Food restriction increased androgen receptor binding capacity in pituitary (3.36 vs 0.77 fmol/mg protein) but not in medio-basal hypothalamus or prostate while binding affinity was unchanged by undernutrition in all 3 tissues. These studies reveal that undernutrition both enhances tonic, androgen receptor-mediated feedback suppression of GnRH secretion and increases in pituitary (but not hypothalamic) androgen receptor numbers to cause inhibition of LH secretion.

Effects of GABA and L-glutamate on the gastric acid secretion and gastric defensive mechanisms in rat lateral hypothalamus

The effects of administration of an inhibitory GABAergic or excitatory glutaminergic neurotransmitter into the lateral hypothalamic area (LHA) on gastric acids, an aggressive mechanism, and transepithelial potential difference (PD) and mucosal blood flow (MBF), defensive mechanisms, were examined in anesthetized rats, since lesions of LHA in these animals cause gastric mucosal damage and electrical stimulation stimulates gastric acids and antral contractions. Microinfusion of the inhibitory neurotransmitter, muscimol (GABA agonist) resulted in an increase in gastric acid secretion and in PD and MBF. The GABA antagonists picrotoxin and bicuculline methiodide, in contrast, decreased these three factors. The excitatory neurotransmitter L-glutamate induced only an increase of MBF. Thus, the GABAergic system in LHA stimulates the gastric functions, both defensive and aggressive mechanisms, while the glutaminergic system increases only a portion of the defensive system. The results suggest that there is a significant interaction between LHA and stomach functions.

Alpha 2 and beta adrenoceptors in the mediobasal hypothalamus and alpha 2 adrenoceptors in the preoptic-anterior hypothalamus stimulate prolactin secretion in the conscious male rat

Plasma prolactin concentrations were measured in unanaesthetized male rats before and after stereotaxic microinjection of adrenergic agents into the mediobasal and preoptic-anterior hypothalamus. In the mediobasal hypothalamus injection of the alpha 2 agonist clonidine produced a dose-dependent increase in prolactin secretion over the dose range 0.1 to 10 nmoles, the stimulation due to 1 nmole being blocked by idazoxan (alpha 2 antagonist). Stimulation of prolactin release was also caused by isoprenaline (beta agonist) and was significantly reduced by the beta antagonist propranolol. The beta 2 agonist salbutamol was also effective in stimulating prolactin secretion. However, the adrenergic agonists, noradrenaline (mixed alpha and beta), phenylephrine (alpha 1) and tyramine (sympathomimetic) failed to affect prolactin secretion. In the preoptic-anterior hypothalamus clonidine caused a dose-dependent increase in prolactin secretion over the dose range 0.001 to 10 nmoles, the stimulation due to 0.1 nmole being abolished by idazoxan. While prolactin levels were significantly elevated by noradrenaline and tyramine, phenylephrine was ineffective. We conclude that the activation of alpha 2 and beta 2 adrenoceptors in the mediobasal hypothalamus and of alpha 2 adrenoceptors in the preoptic-anterior hypothalamus, on or near prolactin-regulating neurons, results in increased prolactin secretion. An alpha 1 inhibitory action in the mediobasal hypothalamus has however not been ruled out. Adrenergic inputs in the preoptic-anterior hypothalamus appear to exert a predominant facilitatory effect on prolactin secretion.

Endogenous catecholamines modulate growth hormone release in the conscious rat during hypoglycaemia but not in the basal state

To investigate the role of endogenous catecholamines and 5-hydroxytryptamine in the control of growth hormone (GH) secretion, secretory profiles of GH and prolactin were measured in conscious, male rats following intravenous administration of either 1) alpha 2 antagonist idazoxan 2 mg/kg, a dose that blocked alpha 2 agonist induced GH rise, 2) alpha 1 antagonist prazosin 1 mg/kg, 3) non-specific beta-blocker propranolol 1.5 mg/kg, a dose that prevented beta 2 agonist (salbutamol) induced inhibition, 4) serotonin antagonist cyproheptadine 0.5 mg/kg, a dose that inhibited serotonin agonist quipazine induced GH rise, or 5) control. No drug altered mean GH or prolactin levels and pulsatile GH release persisted. Unilateral injections of prazosin, propranolol and idazoxan were made into the medial basal hypothalamus and preoptic-anterior hypothalamic area and of cyproheptadine into the medial basal hypothalamus, all with no effect on short-term GH release. GH and prolactin secretory profiles were measured after giving rats 6 units/kg intravenous insulin. Blood glucose levels fell to less than 50% basal. Hypoglycaemia caused a non-significant 30% fall in mean 2 h GH. Intravenous idazoxan, prazosin, propranolol and cyproheptadine (doses as in first study) did not modify the blood glucose fall, but idazoxan produced a significant reduction of mean GH compared to insulin alone (4 +/- 1.1 ng/ml SEM, idazoxan/insulin versus 16 +/- 5.6 ng/ml, saline/insulin). The lack of an effect of alpha- and beta-blockers on normal, pulsatile GH release is against a role for endogenous catecholamines in controlling this release.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Activation of mu opioid receptors in the pre-optic-anterior hypothalamus releases prolactin in the conscious male rat

Microinjection of opioid agonists into the pre-optic-anterior hypothalamus (PO/AHA) was used to determine the identity of the opioid receptor subtype(s) involved in the stimulation of prolactin release. The mu agonist DAGO [(D-Ala2, NMe-Phe4, Gly-o15)-enkephalin] was the only opioid agonist to show dose-dependent release of prolactin, the lowest significant dose being 0.001 nmoles. Neither the specific delta agonist DPDPE [(D-Pen2, D-Pen5)-enkephalin] nor the specific kappa agonist U50,488H [(trans-3,4-dichloro-N-methyl-N-(2-(1-pyrrodinyl)-cyclohexyl)-benz ene acetamide] showed dose-dependent increase of prolactin secretion, or indeed any significant increase in prolactin secretion in the dose range 0.01-1 nmoles and 0.01-10 nmoles respectively. We suggest that mu (and not kappa or delta) opioid receptors in the PO/AHA are involved in the opioid stimulated release of prolactin in the conscious male rat.

Intrahypothalamic Mu-, not Delta- or Kappa-Opioid Receptor Activation Causes Growth Hormone Secretion

Abstract The possible effects of opioid receptor agonists on growth hormone (GH)-releasing factor or somatostatin neurons were examined by measuring the effects of localized intracerebral injections of mu-, delta- and kappa-selective agonists on GH secretion. Serial GH concentrations were measured in plasma in unanaesthetized male rats chronically prepared with venous and intracerebral cannulae, before and after treatment with bilateral intracerebral injections of opioid agonists in the preoptic anterior hypothalamic area and medial basal hypothalamus. In the medial basal hypothalamus, injections of the mu-agonist DAGO (Tyr-D-Ala-Gly-(Me)Phe-Gly-ol) caused dose-responsive increases in GH, the maximally effective dose being 0.001 nmoles. Injection of 10,000-fold higher doses of the delta-agonist DPDPE ([D-Pen, D-Pen]enkephalin) and the kappa-agonist U50,488H were also effective in stimulating GH secretion. In the preoptic anterior hypothalamic area, DAGO caused dose-responsive increases in GH, the maximally effective dose being 0.01 nmoles. U50.488H was ineffective at 1,000-fold higher doses while DPDPE was effective at 100- to 1,000-fold higher doses. We conclude that hypothalamic mu-opioid receptor activation on or near somatostatin or GH-releasing factor neurons causes GH secretion. Opioids capable of acting on other opioid receptors may also stimulate GH secretion, though only at doses that seem likely to affect mu-receptors.

GABA: a dominant neurotransmitter in the hypothalamus

To study the organization and distribution of the inhibitory amino acid neurotransmitter GABA in the medial hypothalamus, we used a postembedding immunocytochemical approach with colloidal gold. Quantitative analysis showed that half (49%) of all synapsing boutons studied were immunoreactive for GABA, based on immunogold staining of the suprachiasmatic, arcuate, supraoptic, and paraventricular nuclei. This was corroborated with pre-embedding peroxidase immunostaining with antisera against glutamate decarboxylase, the GABA synthetic enzyme. These data suggest that GABA is the numerically dominant neurotransmitter in the hypothalamus, and emphasize the importance of inhibitory circuits in the hypothalamus. Serial ultrathin sections were used to reconstruct GABA immunoreactive boutons and axons in three dimensions. With this type of analysis we found less morphological heterogeneity between GABA immunoreactive boutons than with single ultrathin sections. Single sections sometimes showed boutons containing only small clear vesicles, and other with both clear vesicles and small dense core vesicles. However, with serial sections through individual boutons, dense core vesicles were consistently found at the periphery of the pre-synaptic GABA immunoreactive boutons, suggesting probable co-localization of GABA with unidentified peptides in most if not all boutons throughout the hypothalamus. A positive correlation was found between the density of small clear vesicles and the intensity of immunostaining with colloidal gold particles. GABA immunoreactive axons generally made symmetrical type synaptic specializations, although a small percentage made strongly asymmetrical synaptic specializations. Vesicles in GABA immunoreactive boutons were slightly smaller than those in non-reactive boutons. Synaptic efficacy is related to the position of the synapse on the post-synaptic neuron. While the majority of GABA immunoreactive axons made synaptic contact with dendrites, the distribution of GABA immunoreactive synapses on somata and dendrites was the same as would be expected from a random distribution of all boutons. No preferential innervation of cell bodies by GABA immunoreactive terminals was found. Serial ultrathin sections showed that a GABA immunoreactive axon would sometimes make repeated synaptic contacts with a single postsynaptic neuron, indicating a high degree of direct control by the presynaptic GABAergic cell. Other immunoreactive axons made synaptic contact with a number of adjacent dendrites and cells, suggesting a role for GABA in synchronizing the activity of hypothalamic neurons. Based on the density of immunogold particles per unit area, varying concentrations of immunoreactive GABA were found in different presynaptic boutons in the hypothalamus.

Activation of hypothalamic gamma-aminobutyric Acid receptors resets the pendulum of the growth hormone clock

Abstract Plasma levels of growth hormone in male rats exhibit an ultradian rhythm having a periodicity of 3 to 4 h, bursts of growth hormone being separated by troughs when growth hormone secretion ceases. To determine if neurons in the vicinity of somatostatin neurons in the preoptic/anterior hypothalamic area participate in the generation of the rhythm, we temporarily inhibited this region with injections of the gamma-aminobutyric acid agonist, muscimol, in unanaesthetized rats previously prepared with a venous catheter and stereotaxically implanted intracerebral guide tubes. The injection resulted in an immediate burst of growth hormone release, followed by a trough period and another growth hormone burst, 3.4 +/- 0.1 h later. The induction of a trough and synchronization of growth hormone bursts was not reproduced by a burst of intravenously injected exogenous growth hormone at least as large as the first burst initiated by muscimol. These findings indicate that, in the short term, the ultradian rhythm is independent of growth hormone feedback and provide the first evidence that structures in the anterior hypothalamus receiving a gamma-aminobutyric acid input are an important component of the neural generator of the ultradian rhythm of growth hormone.

Chronic antidepressant drug treatment does not affect GH response to baclofen in depressed subjects

The growth hormone (GH) response to baclofen, a specific GABAB agonist, was tested in 8 male depressed patients before and after chronic treatment with amitriptyline (100 mg/day). No difference was seen in plasma GH response before and 28 days after amitriptyline treatment, suggesting that chronic antidepressant drug treatment does not increase hypothalamic GABAB receptor sites in humans. These data suggest that further studies need to support the hypothesis of a GABA involvement in the mechanism of action of antidepressant drugs.

Activation of opioid receptors in the mediobasal hypothalamus stimulates prolactin secretion in the conscious rat

Abstract In an attempt to localize the opioid receptor(s) (mu, delta and kappa) involved in opioid-stimulated prolactin release in the conscious male rat, opioid agonists were microinjected into the mediobasal hypothalamus and prolactin levels measured before and after injection. The specific mu agonist, DAGO ((D-Ala(2), NMe-Phe(4), Gly-ol(5))-enkephalin) was the most effective in eliciting prolactin release, the smallest effective dose being 0.01 nmoles. The specific delta agonist, DPDPE ((D-Pen(2), D-Pen(5))-enkephalin) had no significant effect even at the highest dose of 10 nmoles. The specific kappa agonist, U50,488H ((trans-3,4-dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzene acetamide) was effective at the doses 1.0 and 10 nmoles. We conclude that mu and kappa opioid receptors in the mediobasal hypothalamus are involved in opioid-stimulated prolactin release and that delta receptors are not.

Peripheral administration of picrotoxin and bicuculline stimulates in vivo somatostatin release from rat median eminence

The gamma-Aminobutyric acid-A (GABAA) antagonist picrotoxin and bicuculline were administered to male rats to determine their effects on somatostatin (SRIF) release, measured in unanesthetized animals stereotaxically implanted with push-pull cannula in the median eminence (ME). I.p. injection (3 mg/kg) of picrotoxin (n = 5) or bicuculline (n = 6) significantly increased (35.4 +/- 10.8 vs 13.7 +/- 4.3 pg/15 min; P less than 0.03 and 38 +/- 3.5 vs 14 +/- 1.8 pg/15 min; P less than 0.001, respectively) SRIF release from the ME compared to baseline levels measured in the same animals. In contrast, with local perfusion of picrotoxin, (10(-4) to 10(-6) M) SRIF release from the ME was not affected. These data suggest a physiological endogenous inhibitory tone of GABA on SRIF release.

Intrahypothalamic injection of somatostatin, not GRF, stimulates prolactin secretion

The distribution of somatostatin and growth hormone releasing factor (GRF) fibres in the hypothalamus suggests that they may be involved in physiological functions in addition to growth hormone control. GRF or somatostatin were stereotaxically injected into anterior or basal hypothalamic regions of unanesthetized male rats and effects on plasma prolactin measured. Somatostatin caused a small, significant, dose-responsive stimulation of prolactin secretion when injected in both hypothalamic regions, while GRF was without effect. Somatostatin may therefore have a minor intrahypothalamic role in regulating prolactin.

Stimulation of pituitary hormone secretion by neurotransmitter amino acids in humans

The effects of several neurotransmitter amino acids on pituitary hormone secretion were examined in normal humans. Oral administration of 10 g of glutamic acid stimulated the secretion of prolactin (PRL) and cortisol to approximately twice baseline values, with no effect on GH, TSH or LH. Aspartic acid (10 g), taurine (5 g), and cysteine (5 or 10 g) had no consistent effect on any hormone measured, although the lack of effect of aspartic acid may relate to the modest increments in serum concentration achieved. Glutamic acid may be an important modulator of PRL and ACTH secretion in humans.

Comparison of the effects of intraventricular taurine, GABA and homotaurine on serum prolactin levels in male rats

The effects of taurine (2-aminoethanesulphonic acid), gamma-aminobutyric acid (GABA) and homotaurine (3-aminopropanesulphonic acid), a structural analogue of both taurine and GABA, on serum prolactin (PRL) levels were compared in conscious, unrestrained male rats. Taurine, injected into the lateral brain ventricles at doses of 6 and 10 mumol per rat, elevated serum PRL level by 52% (P less than 0.01) and 90% (P less than 0.001), respectively. GABA elevated serum PRL level (41%, P less than 0.05) only at the lowest dose (1 mumol) tested. Homotaurine was the most effective compound, eliciting increases of 353% and 449% (P less than 0.001) at 6 and 10 mumol per rat, respectively. The rank order of the three amino acids in elevating serum PRL level bears some similarity to their known rank order of potency in altering cerebral dopamine metabolism.

Biochemical and functional aspects on the control of prolactin release by the hypothalamo-pituitary GABAergic system

A growing body of biochemical, immunohistochemical, and autoradiographic evidence indicates the presence of two different GABAergic systems in the mediobasal hypothalamus: one intrinsic, the tuberoinfundibular GABAergic system, and the other extrinsic, whose cell bodies are located outside the mediobasal hypothalamus and which projects to this area and establishes synaptic contacts with aminergic and peptidergic neurons involved in endocrine function. This particular anatomical configuration provides a rational basis to explain the dual action of GABA (inhibitory and stimulatory) on prolactin release. Different studies aimed at identifying the precise role of GABA on prolactin function have demonstrated that this system can be modulated, at the pre- and/or post-synaptic level, by different experimental maneuvers in which prolactin secretion is physiologically and pharmacologically altered. GABA mainly appears to be involved in feedback mechanisms preventing an exaggerated prolactin output during specific physiological situations. The ability of clinically tested, direct GABAmimetic compounds to lower prolactin secretion in the rat points towards a clinical usefulness of these drugs in particular spontaneous or induced neuroendocrine disorders. However, the possibility of a widespread use of this type of compounds is hampered by the lack of potent, specific and non-toxic GABA agonists suitable for clinical purposes.

Interaction between GABAergic neurotransmission and rat hypothalamic corticotropin-releasing hormone secretion in vitro

Corticotropin-releasing hormone (CRH) has been considered a major coordinator of the overall physical and behavioral response to stress. Moreover, prolonged hypersecretion of CRH has been implicated in the pathogenesis of disorders characterized by anxiety and/or depression. Drugs acting through the gamma-aminobutyric acid/benzodiazepine (GABA/BZD) receptor system have anxiolytic and/or antidepressant properties whereas benzodiazepine inverse agonists cause anxiety and stimulate the pituitary-adrenal axis in vivo. To examine the involvement of the GABA/BZD system in the regulation of hypothalamic CRH secretion, we studied the effects of various agonists and antagonists of GABAA and GABAB receptors using a sensitive rat hypothalamic organ culture with radioimmunoassayable CRH (IR-rCRH) as endpoint. The GABAA and GABAB receptor agonist GABA inhibited serotonin (5-HT)-induced IR-rCRH secretion from 10(-9) to 10(-6) M, but failed to do so at 10(-5) M. The GABAA receptor agonist muscimol was a weak inhibitor of 5-HT-induced IR-rCRH secretion, being effective only at the concentration of 10(-6) M. In contrast, the specific GABAB receptor agonist baclofen was able to inhibit 5-HT-induced IR-rCRH secretion from 10(-7) to 10(-5) M. The rank of potency was thus, GABA much greater than baclofen greater than muscimol. Bicuculline, a GABAA receptor antagonist, partially reversed the inhibitory effects of GABA. Diazepam, a classic benzodiazepine which interacts with the benzodiazepine-site of the GABAA receptor complex, inhibited 5-HT-induced IR-rCRH secretion from 3.3 X 10(-9) to 10(-5) M, an effect that could be reversed by the BZD inactive ligand Ro15-1788.(ABSTRACT TRUNCATED AT 250 WORDS)

Baclofen-induced growth hormone secretion is blunted in chronic schizophrenics: neuroendocrine evidence for a GABA disturbance in schizophrenia

To substantiate a previously reported disturbance of gamma-aminobutyric acid (GABA) in chronic schizophrenia, plasma growth hormone (GH) response to a direct GABA agonist (baclofen, 10 mg) was assessed in 12 unmedicated chronic schizophrenic males and 10 sex- and age-matched healthy controls. Baclofen and placebo were administered orally, in a double-blind design, and blood samples were collected before and 30, 60, 90, 120, 180, and 240 min after drug or placebo administration. Baclofen induced a clear-cut rise in plasma GH levels over baseline values, but the GH increase observed in the patients was significantly smaller than that in controls. These results support the idea that GABA mechanisms may be impaired in chronic schizophrenia.

GABAergic innervation of somatostatin-containing neurosecretory cells of the anterior periventricular hypothalamic area: a light and electron microscopy double immunolabelling study

Double immunolabelling on semithin sections revealed glutamate decarboxylase immunopositive dots surrounding somatostatin-containing cell sections in the rat periventricular hypothalamic area. Up to 12 appositions were observed per cell section with an average number of 2-3 and a unimodal distribution. At the electron microscopical level pre-embedding staining of glutamate decarboxylase showed that most immunoreactive elements consisted of immunolabelled axonal endings. Most of these glutamate decarboxylase immunopositive boutons were found within the neuropil where they frequently made synapses on unidentified dendrites. Some of them were apposed to somatostatin-containing cell bodies that were identified according to the presence of immunolabelled granules using combined immunogold post-embedding staining. In many instances glutamate decarboxylase immunoreactive endings were also found to be involved in synaptic contact with somatostatin-labelled perikarya, or neuronal processes. These contacts provide the morphological basis for a direct GABAergic control of the somatostatin-containing cells regulating the secretion of growth hormone.

Activation of serotonin receptors in the medial basal hypothalamus stimulates growth hormone secretion in the unanesthetized rat

In conscious rats, serotonin microinjected into the basal hypothalamus caused secretion of GH maximal within 10-25 min. The effects of serotonin on GH were blocked by the non-selective serotonin receptor blocker, metergoline 2.5 mg/kg, but not by the serotonin type 2 receptor blocker, ketanserine 0.2 mg/kg. Injections of serotonin in the preoptic/anterior hypothalamic area were without effect. It is concluded that activation of serotonin receptors, probably type I, on or near GH releasing factor neurons in the arcuate nucleus causes secretion of GH and that serotonin has no direct effect on or near somatostatin neurons in the preoptic anterior hypothalamic area.

Adrenoceptors in the preoptic-anterior hypothalamic area stimulate secretion of prolactin but not growth hormone in the male rat

Plasma growth hormone (GH) and prolactin concentrations were measured by radioimmunoassay in unanesthetized male rats after stereotaxic microinjection of adrenergic agents and 6-hydroxydopamine into the preoptic-anterior hypothalamic area (PO/AHA). Norepinephrine, epinephrine, isoprenaline and clonidine failed to stimulate GH, moreover, 16 nanomoles norepinephrine produced a decrease. However, these agents stimulated prolactin secretion and the mixed alpha antagonist phentolamine, administered systemically, inhibited the stimulatory action of epinephrine on prolactin secretion. GH and prolactin secretory patterns were not affected by 6-hydroxydopamine disruption of catecholamine terminals in the PO/AHA. GH responses to adrenergic agonists and the failure of 6-hydroxydopamine to affect GH secretory patterns indicate that PO/AHA norepinephrine afferents do not facilitate GH secretion. Taken in conjunction with previous studies, the results suggest that there must be an extra-hypothalamic site at which norepinephrine is stimulatory for GH. Prolactin responses suggest that alpha adrenoceptors in the PO/AHA may participate in prolactin secretion.