Melanin-concentrating hormone directly inhibits GnRH neurons and blocks kisspeptin activation, linking energy balance to reproduction.

A link between energy balance and reproduction is critical for the survival of all species. Energy-consuming reproductive processes need to be aborted in the face of a negative energy balance, yet knowledge of the pathways mediating this link remains limited. Fasting and food restriction that inhibit fertility also upregulate the hypothalamic melanin-concentrating hormone (MCH) system that promotes feeding and decreases energy expenditure; MCH knockout mice are lean and have a higher metabolism but remain fertile. MCH also modulates sleep, drug abuse behavior, and mood, and MCH receptor antagonists are currently being developed as antiobesity and antidepressant drugs. Despite the clinical implications of MCH, the direct postsynaptic effects of MCH have never been reported in CNS neurons. Using patch-clamp recordings in brain slices from multiple lines of transgenic GFP mice, we demonstrate a strong inhibitory effect of MCH on an exclusive population of septal vGluT2-GnRH neurons that is activated by the puberty-triggering and preovulatory luteinizing hormone surge-mediating peptide, kisspeptin. MCH has no effect on kisspeptin-insensitive GnRH, vGluT2, cholinergic, or GABAergic neurons located within the same nucleus. The inhibitory effects of MCH are reproducible and nondesensitizing and are mediated via a direct postsynaptic Ba(2+)-sensitive K(+) channel mechanism involving the MCHR1 receptor. MCH immunoreactive fibers are in close proximity to vGluT2-GFP and GnRH-GFP neurons. Importantly, MCH blocks the excitatory effect of kisspeptin on vGluT2-GnRH neurons. Considering the role of MCH in regulating energy balance and of GnRH and kisspeptin in triggering puberty and maintaining fertility, MCH may provide a critical link between energy balance and reproduction directly at the level of the kisspeptin-activated vGluT2-GnRH neuron.

Gonadotropin inhibitory hormone inhibits basal forebrain vGluT2-gonadotropin-releasing hormone neurons via a direct postsynaptic mechanism.

The novel hypothalamic peptides avian gonadotropin inhibitory hormone (GnIH) and its mammalian analogue RFRP-3, are emerging as key negative regulators of reproductive functions across species. GnIH/RFRP-3 reduces gonadotropin release and may play an inhibitory role in ovulation and seasonal reproduction, actions opposite to that of the puberty-promoting kisspeptins. GnIH directly inhibits gonadotropin release from the anterior pituitary in birds. GnIH/RFRP-3-immunoreactive fibres also abut the preoptic-septal gonadotropin-releasing hormone (GnRH) neurons, suggesting an additional site of action that has not been studied at the cellular level. Using anatomical labelling and electrophysiological recordings in septal brain slices from GnRH-GFP, vGluT2-GFP and GAD67-GFP mice, we report inhibitory actions of GnIH/RFRP-3 on kisspeptin-activated vGluT2 (vesicular glutamate transporter 2)-GnRH neurons as well as on kisspeptin-insensitive GnRH neurons, but not on cholinergic or GABAergic neurons (n = 531). GnIH and RFRP-3 produced a strikingly similar non-desensitizing hyperpolarization following brief 15 s applications (GnIH: 9.3 +/- 1.9 mV; RFRP-3: 9.0 +/- 0.9 mV) with IC(50) values of 34 and 37 nm, respectively. The inhibitory effect was mediated via a direct postsynaptic Ba(2+)-sensitive K(+) current mechanism and could prevent or interrupt kisspeptin-induced activation of vGluT2-GnRH neurons. GnIH-immunoreactive fibres were in apparent contact with vGluT2-GFP neurons. Thus, GnIH/RFRP-3 could reduce GnRH and glutamate release in target brain regions and in the median eminence via a direct inhibition of vGluT2-GnRH neurons. This in turn could suppress gonadotropin release, influence reproductive development and alter sex behaviour.

Excitatory effects of the puberty-initiating peptide kisspeptin and group I metabotropic glutamate receptor agonists differentiate two distinct subpopulations of gonadotropin-releasing hormone neurons.

Activation of the G-protein-coupled receptor GPR54 by kisspeptins during normal puberty promotes the central release of gonadotropin-releasing hormone (GnRH) that, in turn, leads to reproductive maturation. In humans and mice, a loss of function mutations of GPR54 prevents the onset of puberty and leads to hypogonadotropic hypogonadism and infertility. Using electrophysiological, morphological, molecular, and retrograde-labeling techniques in brain slices prepared from vGluT2-GFP and GnRH-GFP mice, we demonstrate the existence of two physiologically distinct subpopulations of GnRH neurons. The first subpopulation is comprised of septal GnRH neurons that colocalize vesicular glutamate transporter 2 and green fluorescent protein and is insensitive to metabotropic glutamate receptor agonists, but is exquisitely sensitive to kisspeptin which closes potassium channels to dramatically initiate a long-lasting activation in neurons from prepubertal and postpubertal mice of both sexes. A second subpopulation is insensitive to kisspeptin but is uniquely activated by group I metabotropic glutamate receptor agonists. These two physiologically distinct classes of GnRH cells may subserve different functions in the central control of reproduction and fertility.

Neurokinins robustly activate the majority of septohippocampal cholinergic neurons.

In the brain, tachykinins acting via the three cloned neurokinin (NK) receptors are implicated in stress-related affective disorders. Hemokinin-1 is a novel tachykinin that reportedly prefers NK1 to NK2 or NK3 receptors. Although NK1 and NK3 receptors are abundantly expressed in the brain, NK2-receptor-mediated electrophysiological effects have rarely been described as NK2 receptors are expressed only in a few brain regions such as the nucleus of the medial septum/diagonal band. Medial septal/diagonal band neurons that control hippocampal mnemonic functions also colocalize NK1 and NK3 receptors. Functionally, intraseptal activation of all three NK receptors increases hippocampal acetylcholine release and NK2 receptors have specifically been implicated in stress-induced hippocampal acetylcholine release. Electrophysiological studies on the effects of NKs on septohippocampal cholinergic neurons are lacking and electrophysiological effects of hemokinin-1 have thus far not been reported in brain neurons. In the present study we examined the electrophysiological and pharmacological effects of multiple NKs on fluorescently tagged septohippocampal cholinergic neurons using whole-cell patch-clamp recordings in a rat brain slice preparation. We demonstrate that a vast majority of septohippocampal cholinergic cells are activated by NK1, NK2 and NK3 receptor agonists as well as by hemokinin-1 via direct post-synaptic mechanisms. Pharmacologically, hemokinin-1 recruits not only NK1 but also NK2 and NK3 receptors to activate septohippocampal cholinergic neurons that are the primary source of acetylcholine for the hippocampus.

Opposing role of synaptic and extrasynaptic NMDA receptors in regulation of the extracellular signal-regulated kinases (ERK) activity in cultured rat hippocampal neurons.

The extracellular signal-regulated kinases (ERK) signalling cascade is a key pathway that mediates the NMDA receptor (NMDAR)-dependent neuronal plasticity and survival. However, it is not clear yet how NMDARs regulate ERK activity. Stimulation of the NMDARs induces a complex modification of ERK that includes both ERK activation and inactivation and depends on particular experimental conditions. Here we show that there exists a differential restriction in the regulation of ERK activity that depends on the pool of NMDAR that was activated. The synaptic pool of NMDARs activates ERK whereas the extrasynaptic pool does not; on the contrary, it triggers a signalling pathway that results in the inactivation of ERK. As a result, simultaneous activation of both extrasynaptic and synaptic NMDAR using bath application of NMDA or glutamate (a typical protocol explored in the majority of studies) produced ERK activation that depended on the concentration of agonists and was always significantly weaker than those mediated by synaptic NMDARs. Since the activation of the extrasynaptic NMDA is attributed mainly to global release of glutamate occurring at pathological conditions including hypoxic/ischaemic insults, traumas and epileptic brain damage, the reported differential regulation of ERK cascade by NMDARs provides a unique mechanism for an early identification of the physiological and/or pathophysiological consequences of NMDAR activation. The negative regulation of the ERK activity might be one of the first signalling events determining brain injury and constitutes a putative target of new pharmacological applications.