Kisspeptin neurons in the mediobasal hypothalamus (MBH) are critical targets of ovarian estrogen feedback regulating mammalian fertility. To reveal molecular mechanisms underlying this signaling, we thoroughly characterized the estrogen-regulated transcriptome of kisspeptin cells from ovariectomized transgenic mice substituted with 17β-estradiol or vehicle. MBH kisspeptin neurons were harvested using laser-capture microdissection, pooled, and subjected to RNA sequencing. Estrogen treatment significantly (
p.adj
. < 0.05) up-regulated 1,190 and down-regulated 1,139 transcripts, including transcription factors, neuropeptides, ribosomal and mitochondrial proteins, ion channels, transporters, receptors, and regulatory RNAs. Reduced expression of the excitatory serotonin receptor-4 transcript (
Htr4
) diminished kisspeptin neuron responsiveness to serotonergic stimulation. Many estrogen-regulated transcripts have been implicated in puberty/fertility disorders. Patients (
n
= 337) with congenital hypogonadotropic hypogonadism (CHH) showed enrichment of rare variants in putative CHH-candidate genes (e.g.,
LRP1B
,
CACNA1G
,
FNDC3A
). Comprehensive characterization of the estrogen-dependent kisspeptin neuron transcriptome sheds light on the molecular mechanisms of ovary–brain communication and informs genetic research on human fertility disorders.
Based on the type-I cannabinoid receptor (CB1) content of hypophysiotropic axons and the involvement of tanycytes in the regulation of the hypothalamic-pituitary-thyroid (HPT) axis, we hypothesized that endocannabinoids are involved in the tanycyte-induced regulation of TRH release in the median eminence (ME). We demonstrated that CB1-immunoreactive TRH axons were associated to DAGLa-immunoreactive tanycyte processes in the external zone of ME and showed that endocannabinoids tonically inhibit the TRH release in this tissue. We showed that glutamate depolarizes the tanycytes, increases their intracellular Ca 2+ level and the 2-AG level of the ME via AMPA and kainite receptors and glutamate transport. Using optogenetics, we demonstrated that glutamate released from TRH neurons influences the tanycytes in the ME. In summary, tanycytes regulate TRH secretion in the ME via endocannabinoid release, whereas TRH axons regulate tanycytes by glutamate, suggesting the existence of a reciprocal microcircuit between tanycytes and TRH terminals that controls TRH release.
Glucagon-like peptide-1 (GLP-1) inhibits food intake and regulates glucose homeostasis. These actions are at least partly mediated by central GLP-1 receptor (GLP-1R). Little information is available, however, about the subcellular localization and the distribution of the GLP-1R protein in the rat brain. To determine the localization of GLP-1R protein in the rat brain, immunocytochemistry was performed at light and electron microscopic levels. The highest density of GLP-1R-immunoreactivity was observed in the circumventricular organs and regions in the vicinity of these areas like in the arcuate nucleus (ARC) and in the nucleus tractus solitarii (NTS). In addition, GLP-1R-immunreactive (IR) neuronal profiles were also observed in a number of telencephalic, diencephalic and brainstem areas and also in the cerebellum. Ultrastructural examination of GLP-1R-immunoreactivity in energy homeostasis related regions showed that GLP-1R immunoreactivity is associated with the membrane of perikarya and dendrites but GLP-1R can also be observed inside and on the surface of axon varicosities and axon terminals. In conclusion, in this study we provide a detailed map of the GLP-1R-IR structures in the CNS. Furthermore, we demonstrate that in addition to the perikaryonal and dendritic distribution, GLP-1R is also present in axonal profiles suggesting a presynaptic action of GLP-1. The very high concentration of GLP-1R-profiles in the circumventricular organs and in the ARC and NTS suggests that peripheral GLP-1 may influence brain functions via these brain areas.
GLP-1 exerts its anorexigenic effect at least partly via the POMC neurons of the arcuate nucleus (ARC). These neurons are known to express GLP-1 receptor (GLP-1R).
To determine whether in addition to its direct effect, GLP-1 also modulates, how neuronal inputs can regulate the POMC neurons by acting on presynaptic terminals, ultrastructural and electrophysiological studies were performed on tissues of adult male mice.
GLP-1R-immunoreactivity was associated with the cell membrane of POMC neurons and with axon terminals forming synapses on these cells. The GLP-1 analog Exendin 4 (Ex4) markedly increased the firing rate of all examined POMC neurons and depolarized these cells. These effects of Ex4 were prevented by intracellular administration of the G-protein blocker GDP-β-S.
Ex4 also influenced the miniature and evoked postsynaptic currents (PSCs) of POMC neurons. Ex4 increased the frequency of miniature excitatory PSCs and the amplitude of the evoked excitatory PSCs in half of the POMC neurons. Ex4 increased the frequency of miniature inhibitory PSCs and the amplitudes of the evoked inhibitory PSCs in one-third of neurons. These effects of Ex4 were not influenced by intracellular GDP-β-S, indicating that GLP-1-signaling directly stimulates a population of axon terminals innervating the POMC neurons. The different Ex4 responsiveness of their mPSCs indicates the heterogeneity of the POMC neurons of the ARC.
In summary, our data demonstrate that in addition to its direct excitatory effect on the POMC neurons, GLP-1-signaling also facilitates the presynaptic input of these cells by acting on presynaptically localized GLP-1R.
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