The immature brain is prone to seizures but the underlying mechanisms are poorly understood. We explored the hypothesis that increased seizure susceptibility during early development is due to the excitatory action of GABA. Using noninvasive extracellular field potential and cell-attached recordings in CA3 of Sprague-Dawley rat hippocampal slices, we compared the developmental alterations in three parameters: excitatory actions of GABA, presence of the immature pattern of giant depolarizing potentials (GDPs) and severity of epileptiform activity generated by high potassium. The GABA(A) receptor agonist isoguvacine increased firing of CA3 pyramidal cells in neonatal slices while inhibiting activity in adults. A switch in the GABA(A) signalling from excitation to inhibition occurred at postnatal day (P) 13.5 +/- 0.4. Field GDPs were present in the form of spontaneous population bursts until P12.7 +/- 0.3. High potassium (8.5 mm) induced seizure-like events (SLEs) in 35% of slices at P7-16 (peak at P11.3 +/- 0.4), but only interictal activity before and after that age. The GABA(A) receptor antagonist bicuculline reduced the frequency or completely blocked SLEs and induced interictal clonic-like activity accompanied by a reduction in the frequency but an increase in the amplitude of the population spikes. In slices with interictal activity, bicuculline typically caused a large amplitude interictal clonic-like activity at all ages; in slices from P5-16 rats it was often preceded by one SLE at the beginning of bicuculline application. These results suggest that, in the immature hippocampus, GABA exerts dual (both excitatory and inhibitory) actions and that the excitatory component in the action of GABA may contribute to increased excitability during early development.

Activation of the G-protein-coupled receptor GPR54 by kisspeptins during normal puberty promotes the central release of gonadotropinreleasing 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.

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.

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.fertility ͉ gonadotropins ͉ HPG axis ͉ obesity ͉ starvation N utritional status and availability of energy stores exert a profound impact on reproductive function (1-3). Reproduction is an expensive energy-consuming process, and thus it is important that puberty, pregnancy, and lactation occur only when metabolic fuel is available (4). Availability of metabolic fuel is conveyed to the brain by peripherally generated signals, such as leptin, insulin, and ghrelin, as well as by centrally released peptides such as neuropeptide Y, melanocortins, and melanin-concentrating hormone (MCH). One or more of these signals can directly or indirectly link energy balance with reproduction at one or more levels of the hypothalamic-pituitary-gonadal (HPG) axis. Thus, insulin and leptin may indirectly influence GnRH neurons (2, 5-7); leptin could regulate the HPG axis via kisspeptin-containing hypothalamic neurons (8, 9). Kisspeptin and its receptor are critical for reproduction (10-12); both kisspeptin and kisspeptin receptor knockout mice fail to enter puberty, and humans with loss of function mutations in the kisspeptin receptor exhibit hypogonadotropic hypogonadism and are infertile (13-15). Mechanisti...

Recent studies indicate that the histaminergic system, which is critical for wakefulness, also influences learning and memory by interacting with cholinergic systems in the brain. Histamine-containing neurones of the tuberomammillary nucleus densely innervate the cholinergic and GABAergic nucleus of the medial septum/diagonal band of Broca (MSDB) which projects to the hippocampus and sustains hippocampal theta rhythm and associated learning and memory functions. Here we demonstrate that histamine, acting via H 1 and/or H 2 receptor subtypes, utilizes direct and indirect mechanisms to excite septohippocampal GABA-type neurones in a reversible, reproducible and concentration-dependent manner. The indirect mechanism involves local ACh release, is potentiated by acetylcholinesterase inhibitors and blocked by atropine methylbromide and 4-DAMP mustard, an M 3 muscarinic receptor selective antagonist. This indirect effect, presumably, results from a direct histamine-induced activation of septohippocampal cholinergic neurones and a subsequent indirect activation of the septohippocampal GABAergic neurones. In double-immunolabelling studies, histamine fibres were found in the vicinity of both septohippocampal cholinergic and GABAergic cell types. These findings have significance for Alzheimer's disease and other neurodegenerative disorders involving a loss of septohippocampal cholinergic neurones as such a loss would also obtund histamine effects on septohippocampal cholinergic and GABAergic functions and further compromise hippocampal arousal and associated cognitive functions.

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.

No abstract

Neurological and psychiatric disorders are frequently associated with disruption of various cognitive functions, but development of effective drug treatments for these conditions has proven challenging. One of the main obstacles is the poor predictive validity of our preclinical animal models. In the present study the effects of the γ-secretase inhibitor semagacestat was evaluated in preclinical in vivo electrophysiological models. Recently disclosed Phase III findings on semagacestat indicated that Alzheimer’s disease (AD) patients on this drug showed significantly worsened cognitive function compared to those treated with placebo. Since previous studies have shown that drugs impairing cognitive function (including scopolamine, NMDA (N-methyl-D-aspartate) receptor antagonists, and nociceptin receptor agonists) disrupt or decrease power of elicited theta oscillation in the hippocampus, we tested the effects of acute and sub-chronic administration of semagacestat in this assay. Field potentials were recorded across the hippocampal formation with NeuroNexus multi-site silicon probes in urethane anesthetized male C57BL/6 mice; hippocampal CA1 theta oscillation was elicited by electrical stimulation of the brainstem nucleus pontis oralis. Sub-chronic administration of semagacestat twice daily over 12 days at a dose known to reduce beta-amyloid peptide (Aβ) level [100 mg/kg, p.o. (per oral)] diminished power of elicited hippocampal theta oscillation. Acute, subcutaneous administration of semagacestat (100 mg/kg) produced a similar effect on hippocampal activity. We propose that the disruptive effect of semagacestat on hippocampal function could be one of the contributing mechanisms to its worsening of cognition in patients with AD. As it has been expected, both acute and sub-chronic administrations of semagacestat significantly decreased Aβ40 and Aβ42 levels but the current findings do not reveal the mode of action of semagacestat in disrupting hippocampal oscillignificantly reduced braination.