Gamma amino-butyric acid and the control of GnRH secretion in sheep

Abstract

The release of GnRH from nerve terminals in the median eminence into the portal vessels is influenced by factors in the internal and external environment of the animal. In the former category are the gonadal steroid hormones oestrogen and progesterone which alter the characteristics of GnRH secretion during the oestrous and seasonal cycles. These cannot exert their actions directly on the GnRH neurones as they do not possess hormone receptors. Therefore, some other steroid-sensitive neuronal system must relay this information to the GnRH neurones. Gamma amino-butyric acid (GABA) neurones are good candidates for this role as they contain steroid hormone receptors and synapse on GnRH neurones. Recent studies in ewes have sought to identify a role for GABA in mediating the actions of both oestrogen and progesterone on GnRH release. The technique of microdialysis was used to monitor GABA concentrations in areas containing GnRI-I cell bodies during the oestrogen-induced surge of GnRH and during progesterone negative feedback. Concentrations of this inhibitory neurotransmitter have been shown to fall in the former situation where GnRH release is being stimulated, but to be increased when progesterone is depressing GnRH release. GABA may also be important in mediating the seasonal switch in the negative feedback actions of oestradiol. During the anoestrous season, when oestradiol is a potent inhibitor of GnRH secretion, specific GABA receptor antagonists can stimulate neurohormone release, an action that is not observed in the breeding season when oestrogen is much less potent.

Does buck stimulation affect the reproductive performance of goats synchronized with Medroxyprogesterone Acetate (MAP) sponges?

“…P 4 levels was found non-significantly higher trend on day 7 in buck exposed compared to buck isolated groups respectively (p>0.05). P 4 prevents pulsatile release of the gonadotropin releasing hormone (GnRH) causing cessation of its production and secretion in ewe (36). The slight rise in P 4 concentration on day 7 has been influenced by MAP to impede LH secretion hence earlier production of endogenous P 4 in the observation period (37).…”

Ovarian Response and Fertility after Short-Term Progestagen/eCG Treatments Are Compromised in Nulliparous Sheep during Non-Breeding Season

“…Hence, during the luteal phase of the cycle, when progesterone concentrations are high, the frequency of the GnRH/LH pulses is low. The decrease in progesterone concentration after luteolysis allows the GnRH/LH pulse frequency to increase when stimulated by increased estrogen concentrations [30]. We hypothesize that the use of progesterone-based devices for a short time should overcome this lack of endogenous progesterone, and in a similar way as described for long-term treatments [31,32].…”

Respuesta Reproductiva en Ovejas De Pelo Sincronizadas Con Acetato De Medroxiprogesterona Impregnado en Esponjas Intravaginales No Comerciales

“…La concentración de progesterona fue superior a 1 ng/ml del día 0 al 6 en el T1, al 4 en el T2, al 7 en el T3 y al 5 en el T4 (Figura 2), lo cual representa el tiempo de vida del cuerpo lúteo, puesto que los progestágenos utilizados en los protocolos de sincronización no fueron detectados por el radioinmunoanálisis; sin embargo, las variables reproductivas muestran la efectividad de estos fármacos en la sincronización de estros. De los días 7 al 10, se observaron concentraciones de progesterona menores a 1 ng/ml (Figura 2), lo cual demostró la ausencia de cuerpos lúteos funcionales y durante este periodo los progestágenos exógenos liberados por las esponjas ejercieron un efecto análogo a la progesterona natural, e inhibieron la secreción pulsátil de GnRH/LH (Robinson, 1995;Arroyo-Ledezma et al, 2006), impidiendo la maduración folicular y la ovulación.…”