Effect of single or repeated estrogen administration on tuberoinfundibular GABA neurons and anterior pituitary GABA receptors: biochemical and functional studies

Adenohypophyseal and hypothalamic GABA B receptor subunits are downregulated by estradiol in adult female rats

Gamma-aminobutyric acid (GABA) participates in neuroendocrine regulation. Since steroid hormones have been shown to modulate the GABAergic system, here we evaluated the effect of chronic in vivo estradiol administration on GABA B receptor (GABA(B)R) expression. GABA(B1) and GABA(B2) subunits were analyzed by Western Blot and RT-PCR, in hypothalami and anterior pituitaries of adult female rats: a) treated for 1 week with estradiol-valerate (a single dose of 100 mug /kg: E1), b) implanted with a 10 mg pellet of estradiol-benzoate for 5 weeks (E5) or c) on proestrous (P), d) ovariectomized (OVX). Pituitary GABA(B)R levels were correlated to a biological effect: baclofen, a GABA(B)R agonist, action on intracellular calcium titers ([Ca(2+)](i)) in pituitary cells. E5 pituitaries showed a significant decrease in the expression of GABA(B1) and GABA(B2) mRNAs compared to P. The GABA(B1a) splice variant of GABA(B1) was always more abundant than GABA(B1b) in this tissue. Similar to the pituitary, hypothalamic GABA(B1) and GABA(B2) mRNAs decreased in E5; this was confirmed at the protein level. In the hypothalamus GABA(B1b) was the main variant expressed in P rats, and was the one significantly sensitive to estradiol-induced decrease, as determined by Western Blots. Castration did not modify GABA(B)R expression with regards to P in either tissue. In P pituitary cells baclofen induced a decrease in [Ca(2+)](i), in contrast this effect was lost in E5 cells. We conclude that chronic estradiol treatment negatively regulates the expression of the GABA(B)R subunits in the pituitary and the hypothalamus. This effect is coupled to a loss of baclofen action on intracellular calcium in pituitary cells.

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.

Functional sexual dimorphism of the nucleolar organizer regions in the tuberomamillary nucleus

The tuberomamillary nucleus (TM) is a cluster of magnocellular neurons that are located close to the mamillary recess or basal surface of the mamillary body (MBs) and is the only known source of histamine (HA). The nucleolar organizer regions (NORs) of the nervous cells of the subnuclei E1, E2 and E3 of the TM are quantified in this paper to discern the existence of possible sexual differences in the overall neuronal protein synthesis between male and female rats in two phases of the oestrous cycle (oestrous-dioestrous). We have used the argentic impregnation technique of the NORs (AgNORs) in both bilateral nuclei of the TM and found functional bilateral symmetry in its AgNORs. Furthermore, we demonstrate the existence of significant differences (P < 0.001) between sexes and between two phases of the oestrous cycle studied (oestrous-dioestrous) in the parameter of number of AgNORs per neuron. Hormonal manipulation of the TM in the rat during the two periods, postnatal or adult, shows the importance of the activational effect of the hormones on the activity of cellular protein synthesis.

Gonadal hormones as promoters of structural synaptic plasticity: cellular mechanisms

It is now obvious that the CNS is capable of undergoing a variety of plastic changes at all stages of development. Although the magnitude and distribution of these changes may be more dramatic in the immature animal, the adult brain retains a remarkable capacity for undergoing morphological and functional modifications. Throughout development, as well as in the postpubertal animal, gonadal steroids exert an important influence over the architecture of specific sex steroid-responsive areas, resulting in sexual dimorphisms at both morphological and physiological levels. We are only now beginning to gain insight into the mechanisms involved in gonadal steroid-induced synaptic changes. The number of synaptic inputs to specific neuronal populations is sexually dimorphic and this can be modulated by changes in the sex steroid environment. These modifications can be correlated with other morphological changes, such as glial cell activation, that are occurring simultaneously in the same anatomical area. Indeed, the close physical relationship between glial cells and neuronal synaptic contacts makes them an ideal candidate for participating in this process. Interestingly, not only can the morphology and immunoreactivity of glial cells be modulated by gonadal steroids, but a close negative correlation between the number of synapses and the amount of glial ensheathing of a neuron has been demonstrated, suggesting an active participation of these cells in this process. Glia have sex steroid receptors, are capable of producing and metabolizing steroids, and can produce other neuronal trophic factors in response to sex steroids. Hence, their role in gonadal steroid-induced synaptic plasticity is becoming more apparent. In addition, there is recent evidence that this process may involve certain cell surface molecules, such as the N-CAMs, since a specific isoform of this molecule, previously referred to as the embryonic form, is found in those areas of the brain which maintain the capacity to undergo synaptic remodelling. However, there is much work to be done in order to fully understand this phenomenon and before bringing it into a clinical setting in hopes of treating neurodegenerative diseases or injuries to the nervous system.

Estradiol induces plasticity of gabaergic synapses in the hypothalamus

The number of axosomatic synapses on arcuate neurons of the adult rat hypothalamus fluctuates following the sequence of increasing circulatory estradiol during the ovarian cycle. To determine whether estrogen is affecting GABAergic synaptic contacts we studied the number of GABA-immunoreactive axosomatic synapses in adult ovariectomized rats injected either with 17 beta estradiol (100 micrograms/100 g body weight) or with sesame oil vehicle. The number of immunoreactive axosomatic synapses was significantly reduced in estradiol-treated rats (77 +/- 8 vs 56 +/- 6 synapses per 1000 microns of perikaryal membrane in control and estradiol-treated rats, respectively) while the number of non-immunoreactive synapses was not significantly affected by the hormonal treatment (44 +/- 6 vs 35 +/- 5 synapses per 1000 microns of perikaryal membrane in control and estradiol-treated rats, respectively). Estradiol administration also resulted in a significant decrease in the percentage of perikaryal membrane covered by immunoreactive synapses. These results suggest that physiological levels of estradiol may induce a remodeling of GABAergic inhibitory inputs on arcuate neurons.

Effects of ovariectomy and steroid replacement on GABAA receptor binding in female rat brain

The specific binding of tritiated muscimol to gamma-aminobutyric acid (GABA) receptor sites was studied in distinct brain areas of female rats during different endocrine states. In diestrous rats with intact ovaries the highest receptor densities were found in the cortex (10.24 pmol/mg protein) and the lowest concentrations in the mediobasal hypothalamus (3.29 pmol/mg protein). Four weeks after removal of the ovaries, the number of binding sites was enhanced up to 2.4-fold in all brain areas investigated: the preoptic brain area, mediobasal hypothalamus, corticomedial amygdala, and cerebral cortex. The affinity of the binding sites remained unchanged. Substitution of estradiol and progesterone reduced the number of binding sites to values seen before ovariectomy. The induction of an afternoon surge of LH by estradiol that could be blocked by enhancing the GABAergic tone was accompanied by a distinct reduction in Bmax in the preoptic area in the morning. These results give evidence that ovarian hormones modulate GABAergic neurotransmission by regulation of GABAA receptor synthesis or degradation.

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.

Morphological evidence for a direct neuroendocrine GABAergic control of the anterior pituitary in teleosts

The anterior pituitary of teleosts is unique among vertebrates in receiving a direct innervation which represents the morphological support of the neuroendocrine control of pars distalis functions. The participation of GABAergic fibers in this innervation was studied by means of immunocytochemistry at the light and electron microscopic levels, using antibodies against GABA. Immunoreactive fibers, characterized by the presence of small clear and dense cored vesicles, were detected in all parts of the gland. Immunopositive terminals were found in close, sometimes synaptic-like, contact with most glandular cell types in the anterior lobe. The data strongly suggest that in teleosts, as in mammals, GABA is involved in the neuroendocrine control of anterior pituitary functions.

Effect of THIP and SL 76002, two clinically experimented GABA-mimetic compounds, on anterior pituitary GABA receptors and prolactin secretion in the rat

In the present study, the ability of three direct GABA agonists, muscimol, THIP and SL 76002 to displace 3H-GABA binding from anterior pituitary and medio-basal hypothalamus membranes was evaluated. Further, the effect of both THIP and SL 76002 on baseline prolactin levels or after stimulation of hormone release with haloperidol has been also studied. Either muscimol, THIP or SL 76002 have shown to posses 7-, 7- and 3-fold higher affinity, respectively, for the central nervous system than for the anterior pituitary 3H-GABA binding sites. Moreover, THIP and SL 76002 have demonstrated to be respectively, 25- and 1000- fold less potent than muscimol in inhibiting 3H- GABA binding at the level of the anterior pituitary and about 25- and 2700- fold less potent at the level of the medio-basal hypothalamus. Under basal conditions, either THIP or SL 76002 were ineffective to reduce prolactin release. However, after stimulation of prolactin secretion through blockade of the dopaminergic neurotransmission with haloperidol (0.1 mg/kg), both THIP (10 mg/kg) and SL 76002 (200 mg/kg) significantly counteracted the neuroleptic-induced prolactin rise with a potency which is in line with their ability to inhibit 3H-GABA binding in the anterior pituitary. The present results indicate that both compounds inhibit prolactin release under specific experimental situations probably through a GABAergic mechanism. In view of the endocrine effects of these GABA-mimetic compounds, the possibility arises for an application of these type of drugs in clinical neuroendocrinology.