The Gonadotropin surge: on the role of estrogen-induced synaptic plasticity and rapid, non-genomic regulatory mechanisms

Hypothalamic mechanisms maintain the reproductive cycle and fertility in all mammalian species. In females, the cyclic nature of the reproductive functions is based upon the responsiveness of the neuroendocrine hypothalamus to the fluctuating levels of estrogen (E2), and the responsiveness of the ovaries to the fluctuating levels of gonadotropins. This cyclic and reciprocal function is maintained by alternating negative- and positive feedbacks. Failure of the positive feedback results in anovulation and sterility. The regulatory effect of E2 on pituitary gonadotropin release is mediated by a complex neuronal system located in the mediobasal hypothalamus (MBH). It is well established that excitatory amino acid (EAA) neurotransmission is an essential component in the regulation of the gonadotropin-releasing hormone (GnRH) delivery system. However, the morphological interconnection of these systems is not fully understood. The first objective of the present study was to determine whether or not alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors – as indicators of aspartate/glutamatergic innervation – are present in the major neuronal populations, such as the neuropeptide-Y- (NPY), galanin- (GAL) and tyrosine-hydroxylase- (TH) containing neurons of the arcuate nucleus (AN) of the female rat. The results of our experiments suggest that an excitatory aspartate/glutamatergic input is implicated in the regulation of the examined neuropeptide containing AN neurons but not in that of TH-IR cells of the same area. Previous studies indicate that an E2-induced synaptic plasticity (EISP) is part of the mechanism through which E2 regulates the function of hypothalamic neurons. To better understand the mechanism of the EISP, we aimed to determine the identity of hypothalamic neurons that undergo EISP, and compare the findings with simultaneous changes in plasma E2- and LH-concentrations. Our results indicate that in non-human primates (Cercopithecus aethiops) E2 induces a characteristic pattern of changes in the number of axo-somatic synapses in the MBH that plays a major role in the regulation of the secretion/release of gonadotrop hormone-releasing hormone (GnRH). The E2-responsiveness of MBH neurons has been explained by their expression of estrogen receptors. However, some of the E2-dependent regulatory mechanisms of the basal forebrain are mainly associated with rapid and short-lived events that occur during the midcycle positive gonadotropin feedback. The rapid events that take place in MBH neurons occur within minutes after exposure to E2, and are mediated by signaling systems upstream of gene-activation. Rapid E2-induced effects on the activation of intracellular signaling systems have been observed in a variety of brain regions. We focused on identifying the activated form of the extracellularly regulated kinases 1 and 2 (ERK1/2) in the developing and adult rat cerebellum. To ensure the validity of the applied concentrations of E2 on neurons, we chose to test the rapid effects of E2 on the activation of the ERK-pathway in cerebellar cells, which are known to lack aromatase (estrogen-synthase) at any age, and characterized the time-, dose- and age dependency of these rapid effects, both in cultured cerebellar granule cells and in intact live cerebellum. Our in vitro and in vivo results provided evidence that E2 can rapidly activate neuronal ERK1/2 intracellular signaling. It is suggested that the activation of the ERK1/2-pathway within the MBH can also be modulated rapidly by E2, as it occurs in other brain areas