The effects of neurally active amino acids on prolactin secretion

The Glutamate-Glutamine (GABA) Cycle: Importance of Late Postnatal Development and Potential Reciprocal Interactions between Biosynthesis and Degradation

The gold standard for studies of glutamate-glutamine (GABA) cycling and its connections to brain biosynthesis from glucose of glutamate and GABA and their subsequent metabolism are the elegant in vivo studies by (13)C magnetic resonance spectroscopy (NMR), showing the large fluxes in the cycle. However, simpler experiments in intact brain tissue (e.g., immunohistochemistry), brain slices, cultured brain cells, and mitochondria have also made important contributions to the understanding of details, mechanisms, and functional consequences of glutamate/GABA biosynthesis and degradation. The purpose of this review is to attempt to integrate evidence from different sources regarding (i) the enzyme(s) responsible for the initial conversion of α-ketoglutarate to glutamate; (ii) the possibility that especially glutamate oxidation is essentially confined to astrocytes; and (iii) the ontogenetically very late onset and maturation of glutamine-glutamate (GABA) cycle function. Pathway models based on the functional importance of aspartate for glutamate synthesis suggest the possibility of interacting pathways for biosynthesis and degradation of glutamate and GABA and the use of transamination as the default mechanism for initiation of glutamate oxidation. The late development and maturation are related to the late cortical gliogenesis and convert brain cortical function from being purely neuronal to becoming neuronal-astrocytic. This conversion is associated with huge increases in energy demand and production, and the character of potentially incurred gains of function are discussed. These may include alterations in learning mechanisms, in mice indicated by lack of pairing of odor learning with aversive stimuli in newborn animals but the development of such an association 10-12 days later. The possibility is suggested that analogous maturational changes may contribute to differences in the way learning is accomplished in the newborn human brain and during later development.

GABAergic mediation of stress-induced secretion of corticosterone and oxytocin, but not prolactin, by the hypothalamic paraventricular nucleus

The hypothalamus-pituitary-adrenal axis (HPA) participates in mediating the response to stressful stimuli. Within the HPA, neurons in the medial parvocellular region of paraventricular nucleus (PVN) of the hypothalamus integrate excitatory and inhibitory signals triggering secretion of corticotropin-releasing hormone (CRH), the main secretagogue of adrenocorticotropic hormone (ACTH). Stressful situations alter CRH secretion as well as other hormones, including prolactin and oxytocin. Most inputs to the PVN are of local origin, half of which are GABAergic neurons, and both GABA-A and GABA-B receptors are present in the PVN. The objective of the present study was to investigate the role of GABA-A and GABA-B receptors in the PVN's control of stress-induced corticosterone, oxytocin and prolactin secretion. Rats were microinjected with saline or different doses (0.5, 5 and 50 pmol) of GABA-A (bicuculine) or GABA-B (phaclofen) antagonists in the PVN. Ten minutes later, they were subjected to a stressor (ether inhalation) and blood samples were collected 30 min before and 10, 30, 60, 90 and 120 min after the stressful stimulus to measure hormone levels by radioimmunoassay. Our results indicate that GABA acts in the PVN to inhibit stress-induced corticosterone secretion via both its receptor subtypes, especially GABA-B. In contrast, GABA in the PVN stimulates oxytocin secretion through GABA-B receptors and does not alter prolactin secretion.

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.

Ionotropic glutamate-receptor gene expression in hypothalamus: localization of AMPA, kainate, and NMDA receptor RNA with in situ hybridization

In situ hybridization and Northern blots were used to study the ionotropic subtypes of the glutamate receptor in the rat hypothalamus. Widespread expression of AMPA, kainate, and NMDA receptor RNA was found in the hypothalamus with the transcripts the same size and number as found in other regions of the brain. Most of the glutamate-receptor subunits studied were expressed in greater amounts in hippocampus than in hypothalamus; GluR5, on the other hand, showed a greater expression in hypothalamus than in hippocampus. On the basis of Northern blot analysis, all regions of the brain examined, including hypothalamus, cerebral cortex, cerebellum, olfactory bulb, and hippocampus, expressed all eight of the subunits studied. Each subunit showed different relative expressions in the different regions. In the hypothalamus, GluR1 and GluR2 were among the most widely expressed of the non-NMDA ionotropic receptors. Other AMPA-preferring receptors, GluR3 and -R4, were also found, but to a lesser extent. Scattered cells expressed the kainate-preferring receptors GluR5, -R6, and -R7. The NMDA receptor NMDAR1 was detected throughout the hypothalamus. In many regions of the hypothalamus, only scattered cells showed detectable expression of the glutamate-receptor mRNA as detected by autoradiographic silver grains over neurons; unlabeled cells were mixed among labeled cells. Every region of the hypothalamus had several different glutamate receptors. The expression of many different types of ionotropic glutamate receptors throughout the hypothalamus suggests that multiple modes of ion channel regulation by glutamate probably operate here and provides further support for the importance of the excitatory transmitter glutamate in hypothalamic regulation.

Effects of intraventricular taurine, homotaurine and GABA on serum prolactin and thyrotropin levels in female and in male rats

Serum prolactin and thyrotropin levels of conscious, unrestrained male and female rats were compared after intracerebroventricular (i.c.v.) administration of taurine, gamma-aminobutyric acid (GABA) and homotaurine. The amino acids studied had no clear effect on serum basal thyrotropin levels in male or female rats. All amino acids elevated serum prolactin levels in female rats at the dose of 5 mumol/rat; homotaurine by about 18-fold, taurine and GABA by 3-fold. Only homotaurine elevated serum prolactin of male rats at this dose, but its effect was less pronounced (p < 0.01) in male than in female rats. Although homotaurine was clearly more potent than the two other amino acids, at the dose of 10 mumol/rat taurine and GABA also elevated serum prolactin in male rats. These findings show that there are gender-related differences in the responses of serum prolactin levels to homotaurine, taurine and GABA in rats. The tuberoinfundibular dopaminergic pathway, which exerts tonic inhibitory influence on prolactin secretion, is sexually differentiated. Hence the gender-related differences in the effects of the amino acids on prolactin secretion suggest that they might inhibit dopamine release from the median eminence. In case of homotaurine, the gender effect was most pronounced. The less clear dependence of GABA's effect on the gender is in accordance with the suggestions that GABA influences the secretion of serum prolactin by more than one mechanism.

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.

The arcuate nucleus: a site for gamma-aminobutyric acid regulation of prolactin secretion

The effects on prolactin (Prl) secretion following microinfusion of muscimol or N-methyl-aspartate (NMA) into the medial basal hypothalamus (MBH) of male rats was determined. A highly significant increase in Prl occurred following microinfusion of muscimol (500 pmol) into the arcuate nucleus. Microinfusion of muscimol into nearby sites, such as the ventromedial nucleus and dorsal medial nucleus, was not effective in altering Prl secretion. Similarly, microinfusions of artificial cerebrospinal fluid were not effective. NMA (50 pmol), a dose found to affect hormone secretion in other CNS areas, did not alter Prl secretion when infused into the MBH. The present study was able to discriminate between CNS areas by using small volumes (250 nl) and small quantities of the amino acid agonists. This data indicates that GABAergic systems in the arcuate nucleus control Prl release from the pituitary, and that one possible mechanism is via the tuberoinfundibular dopamine system.

Gamma aminobutyric acid mediates ventromedial hypothalamic mechanisms controlling the execution of lordotic responses in the female rat

To study the participation of gamma-aminobutyric acid (GABA) in the control of sexual behavior in the female rat, the effect of GABA and picrotoxin injections in the ventromedial hypothalamic nucleus (VMN) upon lordosis frequency and multiunit spike activity (MUSA) was determined. Infusion of 100 micrograms GABA into conscious rats reduced lordotic responsiveness within 15 min after injection; with a similar time course, the same dose markedly reduced MUSA in urethane anesthetized rats. Forty-five min after injection lordotic responsiveness recuperated to preinjection levels; at this time MUSA showed a rebound increase in neuron firing frequency. The possible relation between ventromedial hypothalamic neuronal activity and capacity for lordotic responses was further tested with injections of a local anesthetic: 1 microliter of 2% Xylocaine infused into the VMN produced similar results, suppressing MUSA and lordotic responsiveness for ca. 45 min beginning immediately after injection. Microinjections of GABA antagonist picrotoxin had the opposite effects: 0.1 microgram increased MUSA and lordotic responsiveness at 5 and 45 min; however at 20 min, when MUSA was at its highest, lordosis frequency was not elevated. Injections of solvent had no consistent effects on either measure. Two conclusions many be tentatively drawn from these data: (a) the VMN is the origin of a neural signal which exerts a moment-to-moment gating control on the execution of lordosis, and (b) the generation and/or the output of this signal is under the control of a GABAergic hypothalamic mechanism which normally exerts an inhibitory effect on the display of lordotic responses.

Glutamate decarboxylase-immunoreactive boutons in synaptic contacts with hypothalamic dopaminergic cells: a light and electron microscopy study combining immunocytochemistry and radioautography

Double post-embedding immunolabeling of both tyrosine hydroxylase and glutamate decarboxylase on 1-micron semi-thin sections allowed the visualization of numerous endings that use gamma-aminobutyrate as a transmitter apposed to dopaminergic cell bodies in the periventricular-arcuate hypothalamic complex. Up to fifteen glutamate decarboxylase-positive contacts per tyrosine hydroxylase-positive cell profile could be observed. In some favourable planes of section glutamate decarboxylase-positive endings were also seen in close apposition to proximal dopaminergic dendrites. About 250 tyrosine hydroxylase-positive cell profiles, whose diameter approached the maximum diameter of the dopaminergic cells, were surveyed. An average of 7.4 glutamate decarboxylase-positive contacts were counted on these profiles. From these figures it was estimated that a dopaminergic cell body was contacted on average by 75-175 terminals that use gamma-aminobutyrate as a transmitter. At the electron-microscopic level, the nature of these contacts was investigated by a method combining radioautographic detection of cell bodies having taken up tritiated dopamine and pre-embedding immunostaining of glutamate decarboxylase containing endings. Glutamate decarboxylase-positive axon terminals were seen apposed to somatic and dendritic elements. On some favorable planes of section, they were found to be engaged in morphologically defined synaptic complexes of the symmetrical or asymmetrical type. A number of the postsynaptic perikarya were labelled by tritiated dopamine and, in agreement with the light microscopic observations, they were frequently seen in contact with more than one immunopositive ending. The present findings provide a morphological substratum for a direct gamma-aminobutyrate control of the tuberoinfundibular dopaminergic neurons. Such a control could account more particularly for the central, stimulatory effects of gamma-aminobutyrate on prolactin secretion.

Glutamate in hypothalamic and limbic structures of diestrous, proestrous, ovariectomized and ovariectomized estrogen-treated rats

Concentrations of glutamic acid were determined in the nucleus accumbens (ACB), medial preoptic area (MPO), anterior and posterior mediobasal hypothalamus (AMBH, PMBH) and mediocortical amygdala (AMY) of diestrous (D), proestrous (P), ovariectomized (OVX) and OVX rats treated with estradiol-benzoate (EB) 12 h and 24 h before decapitation. Significant changes of glutamate concentrations were found in the ACV and PMBH. Glutamate is reduced in OVX when compared to D, P and OVX-EB rats. No significant changes of glutamate concentrations could be detected in the MPO, AMBH, and AMY.

The central actions of glycine and strychnine on prolactin and LH secretion

Unanesthetized diestrous female rats were tested for the prolactin response following an intraventricular injection of several doses of the neuroinhibitory amino acid glycine, and the antagonist strychnine. Glycine in doses of 1.0 or 0.1 mumoles increased pituitary prolactin release, significantly elevating plasma hormone levels. Direct pituitary effects for glycine were not observed. Strychnine, a glycine antagonist, was effective in blocking the prolactin release caused by glycine in doses as low as 5 nmoles. Intraventricular glycine administration did not alter pituitary LH release significantly. These studies suggest a central stimulatory role for the neuroinhibitory amino acid glycine in provoking prolactin secretion, and that this effect is strychnine sensitive.

Light- and electron-microscopic immunocytochemistry of glutamic acid decarboxylase (GAD) in the basal hypothalamus: morphological evidence for neuroendocrine gamma-aminobutyrate (GABA)

GABAergic cells and axon terminals were localized in the basal hypothalamus of differnet species (rat, mouse and cat), by means of an immunocytochemical approach using a specific and well-characterized antiserum to the GABA biosynthetic enzyme, glutamate decarboxylase. Light-microscopic visualization was performed with an indirect immunofluorescence method and electron-microscopic observations were made on material with pre-embedding staining and use of the peroxidase-antiperoxidase procedure. At the light-microscopic level, a dense immunofluorescent plexus was observed over both the medial and lateral parts of the external layer of the median eminence. The labelling extended from the rostal part of the median eminence up to the pituitary stalk. Over the subependymal and internal layers only a few immunoreactive dots were visible, except around the blood vessels where they appeared more concentrated. Immunoreactive varicosities could be found following the outlines of the capillary loops and lining tanycyte processes, especially in the median eminance midportion. At the electron-microscopic level, the immunolabelling was exclusively found over neuronal profiles in the median eminence. The latter represented a small fraction of the total number of varicosities visible on the same section. Labelled profiles typically contained numerous small clear synaptic vesicles and only a few or no dense-core vesicles. In the subependymal and internal layers, rare labelled endings were found close to ependymal cells or among transversally cut fibers, respectively. In the palisadic zone, elongated positive boutons were visible intermingled with bundles of unlabelled axons and glial or ependymal processes. In the neurohemal contact zone, immunoreactive endings were observed among unlabelled neurosecretory endings in close vicinity to fenestrated capillary perivascular space. Small moderately intense immunofluorescent varicosities were observed all over the hypothalamus. The density of the glutamate decarboxylase-positive network was higher than in most diencephalic regions. Intraventricular or topical injection of colchicine allowed the visualization of small lightly immunoreactive cells in the diffusion area of colchicine. In the arcuate nucleus labelled axonal endings containing small pleomorphic synaptic vesicles and sometimes a few dense-core vesicles were observed at the electron-microscopic level. Typical synaptic junctions were commonly found between positive endings and unlabelled perikarya, or more frequently, unlabelled dendrites. These findings show that glutamate decarboxylase-containing endings are localized ed in several strategic sites for potential GABAergic neuroendocrine regulations. The GABAergic endings found among neurosecretory endings in the neurohemal contact zone may provide the morphological support for the release of gamma-aminobutyrate into the portal blood flow as an hypothalamic hypophysiotropic hormone.

Role of GABA in the control of thyrotropin secretion in the rat

Various doses of GABA from 0.25 to 5 mumol injected into the third ventricle decrease serum TSH rapidly. The same effect was observed with GABOB (10 mumol), the hydroxylated form of GABA. The inhibitory effect of both of these drugs was prevented by picrotoxin injection (1 microgram). Peripheral injection of GABAergic drugs such as GABOB or AOAA also decreased serum TSH. In vitro, the addition of GABA (from 6.7 . 10(-6) to 6.7 . 10(-4) M) to the incubation medium of hemi-anterior pituitary did not modify the liberation of TSH. To test the physiological role of GABA in the regulation of thyrotropin function the circadian TSH rhythm was used as a model. Both GABAergic inhibitors, picrotoxin (10 microgram/kg b.w.) as well as semicarbazide (150 mg/kg b.w.), induced an increase of the low basal nocturnal level of TSH (centered on the 02.00 h time point) without altering the diurnal peak of TSH. We conclude that GABA has an inhibitory effect on central thyrotropin control via an inhibition of TRH release from the hypothalamus and might be, at least partly, responsible for the low nocturnal levels of serum TSH observed during the circadian physiological rhythm.

GABA-related enzymes in the hypothalamus of rats treated with estradiol

The possibility that the activity of GABA-related enzymes in the hypothalamus, glutamic acid decarboxylase (GAD) and GABA-transaminase (GABA-T), may be modified by circulating prolactin levels was investigated in castrated female and male rats. Estradiol benzoate was chosen to induce an increase, while bromergocriptine was used to obtain a decrease of serum prolactin levels. Acute treatment with estradiol benzoate brought about 3 h later, a significant increase of serum prolactin, and a slight, non-significant increase of GAD activity in the hypothalamus. Chronic treatment with estradiol benzoate for 12 days induced significant increases in serum prolactin and GAD activity in the hypothalamus. Bromergocriptine significantly blocked this increase of GAD activity. Bromergocriptine-treated rats had significant increases of GABA-T activity in the hypothalamus. The results of this investigation support the possibility of the existence of a relationship between GABA-related enzymes in the hypothalamus and circulating prolactin levels.

Enhanced prolactin release by injection of glycine in the medial preoptic area (mPOA) of the rat

Injection of glycine into the medial preoptic area (mPOA) in doses of 12.5, 25 and 50 micrograms produced a marked increase in prolactin release in freely-moving male rats. Neither taurine nor beta-alanine had effect on prolactin. These amino acids were unable to modify the resting plasma LH levels. The present studies show that the mPOA is a target site for the prolactin releasing action of glycine. Furthermore, they suggest a participation of this amino acid in the mPOA mechanism for control of prolactin secretion in the rat.

GABAergic control of anterior pituitary hormone secretion

Anatomical and biochemical studies have identified a hypothalamic tubero-infundibular GABAergic system, which plays a functional role on anterior pituitary hormone secretion. Experimental and clinical evidence support the presence of a dual component in the action of GABA; one mediated via the central nervous system and the other exerted directly at the anterior pituitary level. The two sites of action may be responsible for the excitatory and inhibitory effects of GABA on pituitary hormone and especially prolactin secretion. The future characterization of this system will provide a better understanding of the involvement of GABA in the physiology of anterior pituitary hormone secretion and will contribute to the development of new pharmacological agents for the therapy of neuroendocrine disorders.

Involvement of GABA in the feedback action of estradiol on gonadotropin and prolactin release: hypothalamic GABA and catecholamine turnover rates

The concentrations and turnover rates of norepinephrine (NE), dopamine (DA) and gamma-aminobutyric acid (GABA) were measured in discrete brain areas of diestrous (D), proestrous (P), ovariectomized (OVX) and ovariectomized rats treated with estradiol-benzoate (EB) 12 or 24 h before decapitation. The turnover of NE in the medial preoptic area (MPO) correlates well with plasma LH levels under all endocrine conditions showing high NE turnover in P and OVX and low NE turnover in D and OVX-EB animals. The DA turnover shows no hormone-dependent changes in the MPO. In those animals where estrogens exert no (OVX) or a negative feedback action (D, OVX-EB) on LH the GABA turnover correlates inversely with LH and preoptic NE turnover showing low GABA turnover values in OVX and high values in D and OVX-EB. For P animals the inverse correlation cannot be confirmed. It is concluded that GABA mediates the negative feedback action of estrogens to LH-RH perikarya located in the MPO. GABA might act by presynaptic inhibition of NE axon terminals. This hypothesis is supported by morphological findings which indicate that axon terminals in the MPO are in close contact without separating glial lamellae. In the anterior mediobasal hypothalamus (AMBH) NE turnover correlates best with serum prolactin levels being high in P and OVX animals 24 h after EB treatment. The DA turnover is increased in OVX rats 24 h after EB. It is not yet clear if this increase might be a consequence of the elevated prolactin levels. GABA turnover in the AMBH shows no significant changes. GABA concentrations and turnover rates were also determined in the mediocortical amygdala where estrogen receptors have been reported and in the nucleus accumbens. No significant changes could be observed in these regions.

Effects of the neuroexcitatory amino acids aspartate and glutamate on LH secretion

The effects of infusing aspartate or glutamate into the third ventricle of unanesthetized female animals was examined. LH levels were significantly enhanced within 5-15 min following aspartate (0.5 and 0.2 micromoles), separated by one hour and twenty-four hours, promoted consecutive increases in plasma LH. Plasma FSH levels, however, were not altered in any of the animals examined. This study further supports the findings that neuroexcitatory amino acids promote LH secretion.

Na+ independent binding of [3H] muscimol to a membrane fraction of the brains of normal and dwarf mice

Although their body weights were decreased by about 77% and their brain weights by about 30%, high-affinity [3H] muscimol binding to a cerebral membrane fraction was not altered in hereditary pituitary dwarf mice. Marked changes in the level of pituitary growth-associated hormones do not appear to be associated with a change in cerebral GABA-receptors.

Sensitivity of identified medial hypothalamic neurons to GABA, glycine and related amino acids; influence of bicuculline, picrotoxin and strychnine on synaptic inhibition

Medial hypothalamic neurons in pentobarbital anesthetized rats were identified by location and response to electrical stimulation of the amygdala, medial preoptic area, midbrain periaqueductal gray and median eminence. Cells were then examined for their sensitivity to microiontophoretic applications of GABA, glycine and 3 related amino acids, i.e. beta-guanidinopropionic acid, delta-aminovaleric acid and beta-alanine. Application of all agents decreased the spontaneous and glutamate or aspartate evoked activity of the majority of neurons in all identified categories. The majority of neurons were more sensitive to beta-guanidinopropionate, delta aminovalerate and GABA than to glycine and beta-alanine. Bicuculline and picrotoxin produced a selective and reversible antagonism of depressions evoked by GABA and GABA-like amino acids whereas strychnine produced a selective and reversible antagonism of depressions evoked by glycine and beta-alanine. Bicuculline and picrotoxin, but not strychnine, were observed to diminish synaptic inhibition evoked by electrical stimulation of several sites when these agents were administered by microiontophoresis and by i.v. injections in convulsive doses. These observations suggest that many medial hypothalamic neurons have both GABA and glycine receptors but that GABA may have the more important role as a neurotransmitter common to afferent or recurrent inhibitory hypothalamic pathways.

Stimulation of LH and FSH secretion following intraventicular injection of cysteic acid but not taurine

Taurine and related neurally active metabolites, were tested for their ability to influence basal secretion of luteinizing hormone (LH) and follicle stimulating hormone (FSH). The animal model used was the unanesthetized, unrestrained adult rat with an indwelling catheter in the jugular vein and a cannula implanted in the right lateral cerebroventricle or anterior pituitary. The proposed inhibitory neurotransmitter traurine, and its metabolic precursor, hypotaurine, did not affect the secretion of LH or FSH following infusion of 0.2 or 2.0 mumol intraventricularly or into the pituitary. In contrast, intraventricular injection of cysteic acid, a neurally excitant amino acid, in doses of 2.0 and 0.2 mumol promoted pituitary secretion of LH in both male and female rats. FSH secretion was also increased slightly by cysteic acid (2.0 mumol). These studies provide additional evidence that excitatory amino acids have a stimulatory role in the release of pituitary gonadotropins.

Neuroleptic-induced prolactin rise: influence of pharmacological alterations of different neurotransmitter system

Oral administration of 2 neuroleptic drugs, haloperidol and LR511 induced in male rats a marked, dose-dependent and sustained rise of plasma prolactin.

Autoradiography of GABA in the rat hypothalamic median eminence

Light microscopy autoradiographs of the rat hypothalamic median eminence were prepared after injection of high specific activity tritiated GABA and GABA structural analogs. Following intracardiac injection of labeled GABA with short (15 min) survival time, a dense accumulation of silver grains was observed over the external layer of the median eminence. The silver grains appeared much less numerous and randomly scattered over the internal layer. No conspicuously labeled cells could be detected in the median eminence. A similar pattern of labeling was observed after 10 min in vitro incubation of the median eminence with a low concentration (2.5 x 10(-7) M) of labeled GABA. Clusters of silver grains were also visible over the external layer following intraventricular injection of labeled GABA. In this latter case, however, other sites of labeling were revealed over the internal and ependymal layers. The dense labeling over the external layer with tritiated GABA was partially reduced by a simultaneous intracardiac injection of a 50-fold excess of non-radioactive cis-aminocyclohexane carboxylic acid--a reported preferential substrate for GABA neuronal uptake--but it was not displaced by a 2000-fold excess of non-radioactive beta-alanine--a reported specific substrate for GABA glial uptake. Intracardiac injection of triated beta-alanine led to a faint and even labeling over the entire median eminence with no preferential accumulation of silver grains over the various layers. Following intraventricular injection of labeled beta-alanine the tanycytes and their processes as well as numerous glial cells appeared heavily labeled. These results suggested that there exist cell elements in the external layer of the hypothalamic median eminence which are capable of accumulating exogenous GABA according to its neuronal uptake characteristics. Although the exact nature of these cells is not readily apparent at this stage of our investigations, these findings led us to speculate that there might be a subpopulation of GABAergic nerve endings in the vicinity of the primary plexus capillaries.

Sodium dependence of GABA transport in rat hypothalamic synaptosomes

The sodium dependence of gamma-aminobutyric acid (GABA) uptake has been studied in rat hypothalamic synaptosomes, and the results compared to previous studies in cortical synaptosomes. Initial velocity of GABA uptake was measured as a function of both GABA and sodium concentration, and these data were fitted to the rate equation for each of several plausible models. The minimal best-fit model was found to be identical to that for cortical synaptosomes. However, the constants which quantitate the model were found to differ for hypothalamus and cortex. As a result, uptake at any combination of [Na] and [G] in hypothalamic synaptosomes is approximately double that in cortical synaptosomes. The rate equation for the minimal best-fit model was utilized to define and compute certain parameters which are useful in comparing transport mechanisms (Vmax, Va, Kt, Jm, and k Na). In all cases, differences were found between hypothalamus and cortex.

The effect of parenteral glutamate treatment on the localization of neurotransmitters in the mediobasal hypothalamus

The localization of cholinergic, aminergic and amino acid-ergic neurones in the mediobasal hypothalamus has been studied in normal rat brain and in brains where neurones in nucleus arcuatus were destroyed by repeated administration of 2 mg/g body weight monosodium glutamate to newborn animals. In normal animals acetylcholinesterase staining, choline acetyltransferase and aromatic L-amino acid decarboxylase were concentrated in the median eminence and the arcuate nucleus. Glutamate decarboxylase was concentrated at the boundary between the ventromedial and the arcuate nuclei, with lower activity in the arcuate nucleus and very low activity in the median eminence. Nucleus arcuatus contained an intermediate level of high affinity glutamate uptake. In the lesioned animals, there were significant decreases in choline acetyltransferase, acetylcholinesterase staining and glutamate decarboxylase in the median eminence, whereas choline acetyltransferase activity and acetylcholinesterase staining, but not glutamate decarboxylase activity, were decreased in nucleus arcuatus. Aromatic L-amino acid decarboxylase was unchanged in all regions studied. The high affinity uptakes of glutamate, dopamine and noradrenaline, and the endogenous amino acid levels were also unchanged in the treated animals. The results indicate the existence of acetylcholine- and GABA-containing elements in the tuberoinfundibular tract. They further indicate that the dopamine cells in the arcuate nucleus are less sensitive to the toxic effect of glutamate than other cell types, possibly because they contain less glutamate receptors.