Estrogen modulation of hypothalamic neurons: Activation of multiple signaling pathways and gene expression changes

Hypothalamic target neurons of estrogen include neurosecretory neurons such as gonadotropin-releasing hormone (GnRH) and dopamine neurons, and local circuitry neurons such as proopiomelanocortin (POMC) and γ-aminobutyric acid (GABA) neurons. These and other hypothalamic neurons are involved in regulating numerous homeostatic functions including reproduction, thermoregulation, stress responses, feeding and motivated behaviors. Using a combination of techniques to examine the molecular mechanisms leading to physiological changes induced by estrogen, we find that both rapid effects and transcriptional changes alter excitability of hypothalamic neurons. We have identified membrane-initiated, rapid signaling pathways through which 17β-estradiol (E₂) alters synaptic responses in these neurons using whole-cell patch recording in hypothalamic slices from ovariectomized female guinea pigs. E₂ rapidly uncouples μ-opioid and GABA _B receptors from G protein-gated inwardly rectifying K⁺ (GIRK) channels in POMC and dopamine neurons as manifested by a reduction in the potency of μ-opioid and GABA_B receptor agonists to activate these channels. Inhibitors of phospholipase C, protein kinase C and protein kinase A block the actions of E₂, indicative that the E₂ receptor is G protein-coupled to activation of this cascade. Taking advantage of an animal model we developed to investigate estrogen's feedback actions on secretion of gonadotropin-releasing hormone (GnRH), we studied the transcriptional changes induced by estrogen using suppression subtractive hybridization (SSH) and microarray analysis. Many of the observed mRNA expression changes include transcripts encoding proteins critical for neurotransmitter release and receptor dynamics. Some of these include gec-1, PI3-kinase p55γ, rab11a GTPase, synaptobrevin2, synaptogyrin, taxilin, Ca²⁺-dependent activator protein for secretion (CAPS) and a number of proteins containing pleckstrin homology domains - domains that are involved in plasma membrane targeting of their host protein. In situ hybridization and quantitative film autoradiography analysis on selected transcripts show differential distribution and expression in hypothalamic nuclei. Furthermore, single-cell PCR analysis reveals these genes to be expressed in neurons such as POMC (and GnRH). Whether these expression changes are mediated by the classical or membrane estrogen receptors has yet to be delineated. More detailed investigations of transcript spatial localization within neurons and their temporal expression, i.e., within minutes or hours, will provide more insight regarding how estrogen alters neuronal excitability and synaptic efficacy that ultimately lead to changes in complex behavior.