Alteraciones genéticas en la producción, secreción y acción de las gonadotrofinas / Genetic disorders of gonadotropin synthesis, release and function

Pulsatile secretion of Gonadotropin-Releasing Hormone (GnRH) by the hipothalamus and its action on the pituitary gland is a complex process involving many pre and post natal events. For example, migration of GnRH neurons from the olfactory placode, GnRH release and signalling, normal anterior pituitary development and function are all needed to allow GnRH to stimulate pulsaltile pituitary secretion of follicle-stimulating hormone (FSH) and liteinizing hormone (LH). Hypogonadotropic hypogonadism can be the result of absent or inadequate GnRH secretion or action. Abnormalities in gonadotropin hormone release and function can arise from mutations in a variety of genes implicated in hypogonadotropic hypogonadism is continually growing. A given genotype at a single locus cannot reliably predict the phenotypic manifestations in any given member of affected families. Thus, the identification and characterization of these mutations are providing important information about the reproductive axis in humans and may result in improved treatment and counselling for patients with infertility.

Análogos do GnRH: bases moleculares e aplicações em reprodução assistida / GnRH analogues: molecular basis and applications in assisted reproduction

O decapeptídeo GnRH é o iniciador central da cascata hormonal reprodutiva. É gerado em neurônios hipotalâmicos a partir de um polipeptídeo precursor por processamento enzimático e liberado de maneira pulsátil na circulação portal para estimular a biossíntese e secreção dos hormônios luteinizante (LH) e folículo-estimulante (FSH) pela hipófise. Baixas doses de GnRH aplicadas de maneira pulsátil equivalem à liberação fisiológica portal e restauram a fertilidade em diferentes condições de anovulação. Por outro lado, altas doses de GnRH causam desensibilização do gonadotrofo e inibem a liberação hipofisária com conseqüente inibição da função ovariana. Pequenas modificações na molécula original do GnRH originam substâncias conhecidas como análogos do GnRH, que podem ser agonistas e antagonistas. Os agonistas provocam uma liberação inicial de gonadotrofinas (flare-up) seguida de desensibilização do gonadotrofo (down regulation). Já os antagonistas agem diretamente sobre o gonadotrofo, através de inibição competitiva dos receptores do GnRH. Esse fenômeno de quiescência hipofisária provocada pelos análogos do GnRH tem extensivas aplicações clínicas e o conhecimento sobre a fisiologia do GnRH é importante para aplicação mais racional destes análogos. Este artigo visa a descrever os mecanismos de ação destes análogos do GnRH, bem como revisar as diferentes indicações clínicas de seu uso em reprodução assistida

GnRH neurones population in the diencephalon of the coypu myocastor coypus

El objetivo del presente estudio fue investigar la distribución y morfología de las neuronas que sintetizan y almacenan el factor de liberación de gonadotrofinas (GnRH), en el diencéfalo del coipo (Myocastor coypus), roedor sudmericano del suborden histricomorpha. Para tales fines los encéfalos de tres coipos machos, adultos, fueron fijados por perfusión intra-arterial, utilizando una solución de paraformaldehído y ácido pícrico. Los bloques del hipotálamo fueron separados del resto del encéfalo y seccionados en micrótomo de congelación, obteniendo láminas coronales de 40 µm de espesor. Las secciones fueron sometidas a un proceso de recuperación antigénica por ultrasonido, para desenmascarar los antígenos ocultos, luego las secciones fueron procesadas utilizando una técnica de inmunohistoquímica para evidenciar las neuronas GnRH, usando un anticuerpo monoclonal (LRH 13). Un grupo de secciones seriadas fue coloreada con violeta de cresilo (técnica de NissI), mientras que las otras secciones fueron coloreadas usando el método de Kuver Barrera (azul luxol rápido y violeta de cresilo), para observar los núcleos y tractos nerviosos del hipotálamo, con el microscopio óptico. Los análisis morfométrico y cuantitativo de los cuerpos neuronales fue realizado usando un analizador de imágenes. Las neuronas GnRH inmunoreactivas observadas eran bipolares y alargadas. El número total de cuerpos neuronales para esta especie fue estimado en 1072 ñ 27. Todas las células fueron localizadas en el hipotálamo rostral, principalmente en el área preóptica. Pocas neuronas fueron observadas en el núcleo de la estría terminal y en el núcleo medial preóptico hipotalámico. Las fibras inmunorreactivas fueron visibles en la capa externa de la eminencia media. De acuerdo a los datos obtenidos, se concluye que la distribución de las neuronas GnRH en el coipo coinciden con la de otros roedores, tales como la rata

Interaction between NO and oxytocin: influence on LHRH release

Nitric oxide synthase (NOS)-containing neurons have been localized in various parts of the CNS. These neurons occur in the hypothalamus, mostly in the paraventricular and supraoptic nuclei and their axons project to the neural lobe of the pituitary gland. We have found that nitric oxide (NO) controls luteinizing hormone-releasing hormone (LHRH) release from the hypothalamus acting as a signal transducer in norepinephrine (NE)-induced LHRH release. LHRH not only releases LH from the pituitary but also induces sexual behavior.On the other hand, it is known that oxytocin also stimulates mating behavior and there is some evidence that oxytocin can increase NE release. Therefore, it occurred to us that oxytocin may also stimulate LHRH releave via NE and NO. To test this hypothesis, we incubated medial basal hypothalamic (MBH) explants from adult male rats in vitro. Following a preincubation period of 30 min, MBH fragments were incubated in Krebs-Ringer bicarbonate buffer in the presence of various concentrations of oxytocin. Oxytocin relesed LHRH at concentrations ranging from 0.1 nM to 1muM with a maximal stimulatory effect (P<0.001) at 0.1 muM, but with no stimulatory effect at 10 muM. That these effects were mediated by NO was shown by the fact that incubation of the tissues with NG-monomethyl-L-arginine (NMMA), a competitive inhibitor of NOS, blocked the stimulatory effects. Furthermore, the release of LHRH by oxytocin was also blocked by prazocin, an alpha1-adrenergic receptor antagonist, indicating that NE mediated this effect. Oxytocin at the same concentrations also increased the activity of NOS (P<0.01) as measured by the conversion of [14C]arginine to citrulline, which is produced in equimolar amounts with NO by the action of NOS. The release of LHRH induced by oxytocin was also accompanied by a significant (P<0.02) increase in the release of prostaglandin E2 (PGE2), a mediator of LHRH release that is released by NO. On the other hand, incubation of neural lobes with vaious concentrations of sodium nitroprusside (NP) (300 or 600 muM), a releaser of NO, revealed that NO acts to suppres (P<0.01) the release of oxytoxin. Therefore, our results indicate that oxytocin releases LHRH by stimulating NOS via NE, resulting in an increased release of NO, which increases PGE2 release that in turn induces LHRH release. Furthermore, the released NO can act back on oxytocinergic terminals to suppress the release of oxytocin in an ultrashort-loop negative feedback.

Maintenance of hypothalamic GnRH release during lactation in the rat: a push-pull perfusion study

The activity of the gonadotropin-releasing hormone (GnRH) pulse generator during lactation was assessed by direct determination of GnRH levels impinging upon the pituitary gland. Sprague-Dawley rats were implanted on day 15 of pregnancy with a push-pull perfusion cannula directed to the anterior pituitary. All implanted animals showed normal parturition, maternal behavior and lactation. Push-pull perfusions were performed in 15 rats suckling 11.0 +/- 0.8 pups (range 4-15) on day 7-20 of lactation and repeated on diestrous 1 after weaning in some of the same animals. GnRH content of the samples was assayed by RIA. GnRH pulses were clearly detected during lactation. Mean GnRH secretion rate was 1.9 +/- 0.3 pg/10 min (chi +/- SE, range between 0.5 and 3 pg/10 min) and interpulse interval was 37.5 +/- 1.7 min (range between 27 and 50 min). There was a significant decrease of about 19 percent in the interpulse interval after weaning. There was no significant difference in GnRH pulse amplitude nor in GnRH secretion rate between lactation and diestrous. These results demonstrate that nursing does not suppress the GnRH pulse generator in the rat

Neuroendocrine mechanisms of lactational infertility in women

The current knowledge on the mechanisms of lactational infertility, discussed during a symposium of investigators in this subject, is reviewed. Three periods of lactation are examined: the first weeks postpartum, the period of extended lactational amenorrhea and the recovery of ovarian function. In the first postpartum weeks the inhibition of ovarian function is accounted by diminished pituitary response to GnRH, since exogenous GnRH fails to elicit a LH increase. Suckling can extend the period of ovarian inhibition for weeks, months or years, although it does not fully suppress pulsatile secretion of LH beyond the first weeks. Extended lactational amenorrhea is associated with low LH plasma levels, a great PRL increase in response to suckling, low basal E2 levels and a suppression of estrogen positive feedback. Decreased immunoreactive LH levels may result from partial suppression of the LH pulse generator and a smaller mass of GnRH released in each burst. The role of neurotransmitters, PRL and ovarian factors is discussed. After the recovery of ovulatory cycles suckling still has a residual infertility effect, associated to inadequate luteal function. The sources of variation among women and populations were recognized. Areas in which research is needed to improve the understanding of the mechanisms that sustain lactational amenorrhea are suggested

Efeito da dopamina sobre a degradaçäo do hormônio liberador das gonadotrofinas em condiçöes de alteraçäo dos hormônios esteroídes: estudo em sinaptossomos hipotalâmicos da rata / Effect of dopamine on the degradation of gonadotrophin releasing hormone, under steroid hormones changes conditions: study in hypothalamic synaptosomes of rats

A regiäo médio-basal do hipotálamo é rica em terminais nervosos que secretam peptídeos hipofisiotróficos. O estudo in vitro dessa regiäo tem trazido inúmeras contribuiçöes para o conhecimento do controle endócrino do organismo. O procedimento mais simples é a utilizaçäo de fragmentos ou fatias hipotalâmicas porém um método que trouxe muita informaçäo para o estudo das secreçöes hipotalâmicas é a utilizaçäo de sinaptossomos, terminais nervosos isolados. O hormônio liberador das gonadotrofinas (LHRH) está largamente associado com a fraçäo sinaptossômica de homogeneizados hipotalâmicos. Neste trabalho fatores que modulam a liberaçäo desse peptídeo de suspensöes incubadas de sinaptossomos do hipotálamo da rata foram estudados, verificando-se que a dopamina em concentraçöes elevadas ativa a degraduaçäo do LHRH em animais previamente injetados com estradiol ou mistura de estradiol e progesterona. Em ratas ovariectomizadas este efeito só foi verificado quando os animais foram injetados com estradiol ou com estradiol seguido de progesterona, näo ocorrendo nas ratas que receberam o veículo ou progesterona sem estradiol. Sugere-se que essa degradaçäo do LHRH na presença da dopamina, que é dependente dos hormônios esteróides, seja um dos mecanismos fisiológicos de controle das gonadotrofinas hipofisárias