Networking of glucagon-like peptide-1 axons with GnRH neurons in the basal forebrain of male mice revealed by 3DISCO-based immunocytochemistry and optogenetics

GLP-1 modulated the firing activity of nigral dopaminergic neurons in both normal and parkinsonian mice

The spontaneous firing activity of nigral dopaminergic neurons is associated with some important roles including modulation of dopamine release, expression of tyrosine hydroxylase (TH), as well as neuronal survival. The decreased neuroactivity of nigral dopaminergic neurons has been revealed in Parkinson's disease. Central glucagon-like peptide-1 (GLP-1) functions as a neurotransmitter or neuromodulator to exert multiple brain functions. Although morphological studies revealed the expression of GLP-1 receptors (GLP-1Rs) in the substantia nigra pars compacta, the possible modulation of GLP-1 on spontaneous firing activity of nigral dopaminergic neurons is unknown. The present extracellular in vivo single unit recordings revealed that GLP-1R agonist exendin-4 significantly increased the spontaneous firing rate and decreased the firing regularity of partial nigral dopaminergic neurons of adult male C57BL/6 mice. Blockade of GLP-1Rs by exendin (9-39) decreased the firing rate of nigral dopaminergic neurons suggesting the involvement of endogenous GLP-1 in the modulation of firing activity. Furthermore, the PKA and the transient receptor potential canonical (TRPC) 4/5 channels are involved in activation of GLP-1Rs-induced excitatory effects of nigral dopaminergic neurons. Under parkinsonian state, both the exogenous and endogenous GLP-1 could still induce excitatory effects on the surviving nigral dopaminergic neurons. As the mild excitatory stimuli exert neuroprotective effects on nigral dopaminergic neurons, the present GLP-1-induced excitatory effects may partially contribute to its antiparkinsonian effects.

Cholinergic Control of GnRH Neuron Physiology and Luteinizing Hormone Secretion in Male Mice: Involvement of ACh/GABA Cotransmission

Gonadotropin-releasing hormone (GnRH)-synthesizing neurons orchestrate reproduction centrally. Early studies have proposed the contribution of acetylcholine (ACh) to hypothalamic control of reproduction, although the causal mechanisms have not been clarified. Here, we report that in vivo pharmacogenetic activation of the cholinergic system increased the secretion of luteinizing hormone (LH) in orchidectomized mice. 3DISCO immunocytochemistry and electron microscopy revealed the innervation of GnRH neurons by cholinergic axons. Retrograde viral labeling initiated from GnRH-Cre neurons identified the medial septum and the diagonal band of Broca as exclusive sites of origin for cholinergic afferents of GnRH neurons. In acute brain slices, ACh and carbachol evoked a biphasic effect on the firing rate in GnRH neurons, first increasing and then diminishing it. In the presence of tetrodotoxin, carbachol induced an inward current, followed by a decline in the frequency of miniature postsynaptic currents (mPSCs), indicating a direct influence on GnRH cells. RT-PCR and whole-cell patch-clamp studies revealed that GnRH neurons expressed both nicotinic (α4β2, α3β4, and α7) and muscarinic (M1-M5) AChRs. The nicotinic AChRs contributed to the nicotine-elicited inward current and the rise in firing rate. Muscarine via M1 and M3 receptors increased, while via M2 and M4 reduced the frequency of both mPSCs and firing. Optogenetic activation of channelrhodopsin-2-tagged cholinergic axons modified GnRH neuronal activity and evoked cotransmission of ACh and GABA from a subpopulation of boutons. These findings confirm that the central cholinergic system regulates GnRH neurons and activates the pituitary-gonadal axis via ACh and ACh/GABA neurotransmissions in male mice.

The GLP-1 agonist, exendin-4, stimulates LH secretion in female sheep

Our previous studies showed that microinjection into the median eminence of the sheep of glucagon-like peptide- 1 (GLP-1) or its receptor agonist exendin-4 stimulates luteinising hormone (LH) secretion, but it is unknown whether the same effect may be obtained by systemic administration of the same. The present study measured the response in terms of plasma LH concentrations to intravenous (iv) infusion of exendin-4. A preliminary study showed that infusion of 2 mg exendin-4 into ewes produced a greater LH response in the follicular phase of the oestrous cycle than the luteal phase. Accordingly, the main study monitored plasma LH levels in response to either 0.5 mg or 2 mg exendin-4 or vehicle (normal saline) delivered by jugular infusion for 1 h in the follicular phase of the oestrous cycle. Blood samples were collected at 10 min intervals before, during and after infusion. Both doses of exendin-4 increased mean plasma LH concentrations and increased LH peripheral pulse amplitude. There was no effect on inter-pulse interval or timing of the preovulatory LH surge. These doses of exendin-4 did not alter plasma insulin or glucose concentrations. Quantitative PCR of the gastrointestinal tract samples from a population of ewes confirmed the expression of the preproglucagon gene (GCG). Expression increased aborally and was greatest in the rectum. It is concluded that endogenous GLP-1, most likely derived from the hindgut, may act systemically to stimulate LH secretion. The present data suggest that this effect may be obtained with levels of agonist that are lower than those functioning as an incretin.

Central and peripheral mechanisms involved in the control of GnRH neuronal function by metabolic factors

Gonadotropin-releasing hormone (GnRH) neurons are the final output pathway for the brain control of reproduction. The activity of this neuronal population, mainly located at the preoptic area of the hypothalamus, is controlled by a plethora of metabolic signals. However, it has been documented that most of these signal impact on GnRH neurons through indirect neuronal circuits, Kiss1, proopiomelanocortin, and neuropeptide Y/agouti-related peptide neurons being some of the most prominent mediators. In this context, compelling evidence has been gathered in recent years on the role of a large range of neuropeptides and energy sensors in the regulation of GnRH neuronal activity through both direct and indirect mechanisms. The present review summarizes some of the most prominent recent advances in our understanding of the peripheral factors and central mechanisms involved in the metabolic control of GnRH neurons.

Recent advances in emerging PCOS therapies

Polycystic ovary syndrome is a prevalent endocrinopathy involving androgen excess, and anovulatory infertility. The disorder is also associated with many comorbidities such as obesity and hyperinsulinemia, and an increased risk of cardiovascular complications. Reproductive, endocrine, and metabolic symptoms are highly variable, with heterogenous phenotypes adding complexity to clinical management of symptoms. This review highlights recent findings regarding emerging therapies for treating polycystic ovary syndrome, including i) pharmacological agents to target androgen excess, ii) modulation of kisspeptin signalling to target central neuroendocrine dysregulation, and iii) novel insulin sensitisers to combat peripheral metabolic dysfunction.

Kisspeptin neuron electrophysiology: Intrinsic properties, hormonal modulation, and regulation of homeostatic circuits

The obligatory role of kisspeptin (KISS1) and its receptor (KISS1R) in regulating the hypothalamic-pituitary-gonadal axis, puberty and fertility was uncovered in 2003. In the few years that followed, an impressive body of work undertaken in many species established that neurons producing kisspeptin orchestrate gonadotropin-releasing hormone (GnRH) neuron activity and subsequent GnRH and gonadotropin hormone secretory patterns, through kisspeptin-KISS1R signaling, and mediate many aspects of gonadal steroid hormone feedback regulation of GnRH neurons. Here, we review knowledge accrued over the past decade, mainly in genetically modified mouse models, of the electrophysiological properties of kisspeptin neurons and their regulation by hormonal feedback. We also discuss recent progress in our understanding of the role of these cells within neuronal circuits that control GnRH neuron activity and GnRH secretion, energy balance and, potentially, other homeostatic and reproductive functions.