Effects of ghrelin on neuronal activity in the ventromedial nucleus of the hypothalamus in infantile rats: an in vitro study

Ghrelin Regulates Expression of the Transcription Factor Pax6 in Hypoxic Brain Progenitor Cells and Neurons

The nature of brain impairment after hypoxia is complex and recovery harnesses different mechanisms, including neuroprotection and neurogenesis. Experimental evidence suggests that hypoxia may trigger neurogenesis postnatally by influencing the expression of a variety of transcription factors. However, the existing data are controversial. As a proof-of-principle, we subjected cultured cerebral cortex neurons, cerebellar granule neurons and organotypic cerebral cortex slices from rat brains to hypoxia and treated these cultures with the hormone ghrelin, which is well-known for its neuroprotective functions. We found that hypoxia elevated the expression levels and stimulated nuclear translocation of ghrelin’s receptor GHSR1 in the cultured neurons and the acute organotypic slices, whereas ghrelin treatment reduced the receptor expression to normoxic levels. GHSR1 expression was also increased in cerebral cortex neurons of mice with induced experimental stroke. Additional quantitative analyses of immunostainings for neuronal proliferation and differentiation markers revealed that hypoxia stimulated the proliferation of neuronal progenitors, whereas ghrelin application during the phase of recovery from hypoxia counteracted these effects. At the mechanistic level, we provide a link between the described post-ischemic phenomena and the expression of the transcription factor Pax6, an important regulator of neural progenitor cell fate. In contrast to the neurogenic niches in the brain where hypoxia is known to increase Pax6 expression, the levels of the transcription factor in cultured hypoxic cerebral cortex cells were downregulated. Moreover, the application of ghrelin to hypoxic neurons normalised the expression levels of these factors. Our findings suggest that ghrelin stimulates neurogenic factors for the protection of neurons in a GHSR1-dependent manner in non-neurogenic brain areas such as the cerebral cortex after exposure to hypoxia.

Neural mechanisms of aggression across species

Aggression is a social behavior essential for securing resources and defending oneself and family. Thanks to its indispensable function in competition and thus survival, aggression exists widely across animal species, including humans. Classical works from Tinbergen and Lorenz concluded that instinctive behaviors including aggression are mediated by hardwired brain circuitries that specialize in processing certain sensory inputs to trigger stereotyped motor outputs. They further suggest that instinctive behaviors are influenced by an animal's internal state and past experiences. Following this conceptual framework, here we review our current understanding regarding the neural substrates underlying aggression generation, highlighting an evolutionarily conserved 'core aggression circuit' composed of four subcortical regions. We further discuss the neural mechanisms that support changes in aggression based on the animal's internal state. We aim to provide an overview of features of aggression and the relevant neural substrates across species, highlighting findings in rodents, primates and songbirds.

Effects of ghrelin on the electrical activities of substantia nigra dopaminergic neurons treated with MPP

Ghrelin, a 28 amino acid brain-gut peptide, has attracted increasing attention for its neuroprotective effect in Parkinson's disease (PD). In view of the pivotal role of excitability of dopaminergic neurons in substantia nigra pars compacta (SNc) in the function of nigrostriatal system, it is of great significance to elucidate the regulation of electrical activity of dopaminergic neurons by ghrelin, especially in PD pathogenesis. In this study, we tackled this issue by probing the effects of ghrelin on the electrophysiological properties of dopaminergic neurons in acute application of Methyl-4-phenylpyridinium (MPP⁺), a potent parkinsonizing agent in primates and rodents, with whole cell patch clamp technique. We first observed that MPP⁺ (10, 20 and 50 μM) inhibited the spontaneous firing activity of dopaminergic neurons with dose-dependent and time-dependent properties. MPP⁺ also hyperpolarized the membrane potential, inhibited the evoked firing activity and reduced the amplitude of the inward rectification characteristic (Sag) in dopaminergic neurons. Importantly, ghrelin (100 nM) could improve the above effects of MPP⁺ on the electrical activities of dopaminergic neurons. The potential mechanism of this phenomenon may be that ghrelin upregulated hyperpolarization-activated cyclic nucleotide-gated channel current (I_h) to antagonize the inhibition of MPP⁺ on I_h, thereby improving the electrical activities of dopaminergic neurons.

Effects of Feeding-Related Peptides on Neuronal Oscillation in the Ventromedial Hypothalamus

The ventromedial hypothalamus (VMH) plays an important role in feeding behavior, obesity, and thermoregulation. The VMH contains glucose-sensing neurons, the firing of which depends on the level of extracellular glucose and which are involved in maintaining the blood glucose level via the sympathetic nervous system. The VMH also expresses various receptors of the peptides related to feeding. However, it is not well-understood whether the action of feeding-related peptides mediates the activity of glucose-sensing neurons in the VMH. In the present study, we examined the effects of feeding-related peptides on the burst-generating property of the VMH. Superfusion with insulin, pituitary adenylate cyclase-activating polypeptide, corticotropin-releasing factor, and orexin increased the frequency of the VMH oscillation. In contrast, superfusion with leptin, cholecystokinin, cocaine- and amphetamine-regulated transcript, galanin, ghrelin, and neuropeptide Y decreased the frequency of the oscillation. Our findings indicated that the frequency changes of VMH oscillation in response to the application of feeding-related peptides showed a tendency similar to changes of sympathetic nerve activity in response to the application of these substances to the brain.

Ghrelin accelerates synapse formation and activity development in cultured cortical networks

BACKGROUND

While ghrelin was initially related to appetite stimulation and growth hormone secretion, it also has a neuroprotective effect in neurodegenerative diseases and regulates cognitive function. The cellular basis of those processes is related to synaptic efficacy and plasticity. Previous studies have shown that ghrelin not only stimulates synapse formation in cultured cortical neurons and hippocampal slices, but also alters some of the electrophysiological properties of neurons in the hypothalamus, amygdala and other subcortical areas. However, direct evidence for ghrelin's ability to modulate the activity in cortical neurons is not available yet. In this study, we investigated the effect of acylated ghrelin on the development of the activity level and activity patterns in cortical neurons, in relation to its effect on synaptogenesis. Additionally, we quantitatively evaluated the expression of the receptor for acylated ghrelin--growth hormone secretagogue receptor-1a (GHSR-1a) during development.

RESULTS

We performed electrophysiology and immunohistochemistry on dissociated cortical cultures from neonates, treated chronically with acylated ghrelin. On average 76±4.6% of the cortical neurons expressed GHSR-1a. Synapse density was found to be much higher in ghrelin treated cultures than in controls across all age groups (1, 2 or 3 weeks). In all cultures (control and ghrelin treated), network activity gradually increased until it reached a maximum after approximately 3 weeks, followed by a slight decrease towards a plateau. During early developmental stages (1-2 weeks), the activity was much higher in ghrelin treated cultures and consequently, they reached the plateau value almost a week earlier than controls.

CONCLUSIONS

Acylated ghrelin leads to earlier network formation and activation in cultured cortical neuronal networks, the latter being a possibly consequence of accelerated synaptogenesis.

The central nervous system sites mediating the orexigenic actions of ghrelin

The peptide hormone ghrelin is important for both homeostatic and hedonic eating behaviors, and its orexigenic actions occur mainly via binding to the only known ghrelin receptor, the growth hormone secretagogue receptor (GHSR). GHSRs are located in several distinct regions of the central nervous system. This review discusses those central nervous system sites that have been found to play critical roles in the orexigenic actions of ghrelin, including hypothalamic nuclei, the hippocampus, the amygdala, the caudal brain stem, and midbrain dopaminergic neurons. Hopefully, this review can be used as a stepping stone for the reader wanting to gain a clearer understanding of the central nervous system sites of direct ghrelin action on feeding behavior, and as inspiration for future studies to provide an even-more-detailed map of the neurocircuitry controlling eating and body weight.

Ghrelin increases GABAergic transmission and interacts with ethanol actions in the rat central nucleus of the amygdala

The neural circuitry that processes natural rewards converges with that engaged by addictive drugs. Because of this common neurocircuitry, drugs of abuse have been able to engage the hedonic mechanisms normally associated with the processing of natural rewards. Ghrelin is an orexigenic peptide that stimulates food intake by activating GHS-R1A receptors in the hypothalamus. However, ghrelin also activates GHS-R1A receptors on extrahypothalamic targets that mediate alcohol reward. The central nucleus of the amygdala (CeA) has a critical role in regulating ethanol consumption and the response to ethanol withdrawal. We previously demonstrated that rat CeA GABAergic transmission is enhanced by acute and chronic ethanol treatment. Here, we used quantitative RT-PCR (qRT-PCR) to detect Ghsr mRNA in the CeA and performed electrophysiological recordings to measure ghrelin effects on GABA transmission in this brain region. Furthermore, we examined whether acute or chronic ethanol treatment would alter these electrophysiological effects. Our qRT-PCR studies show the presence of Ghsr mRNA in the CeA. In naive animals, superfusion of ghrelin increased the amplitude of evoked inhibitory postsynaptic potentials (IPSPs) and the frequency of miniature inhibitory postsynaptic currents (mIPSCs). Coapplication of ethanol further increased the ghrelin-induced enhancement of IPSP amplitude, but to a lesser extent than ethanol alone. When applied alone, ethanol significantly increased IPSP amplitude, but this effect was attenuated by the application of ghrelin. In neurons from chronic ethanol-treated (CET) animals, the magnitude of ghrelin-induced increases in IPSP amplitude was not significantly different from that in naive animals, but the ethanol-induced increase in amplitude was abolished. Superfusion of the GHS-R1A antagonists D-Lys3-GHRP-6 and JMV 3002 decreased evoked IPSP and mIPSC frequency, revealing tonic ghrelin activity in the CeA. D-Lys3-GHRP-6 and JMV 3002 also blocked ghrelin-induced increases in GABAergic responses. Furthermore, D-Lys3-GHRP-6 did not affect ethanol-induced increases in IPSP amplitude. These studies implicate a potential role for the ghrelin system in regulating GABAergic transmission and a complex interaction with ethanol at CeA GABAergic synapses.

Food cues and ghrelin recruit the same neuronal circuitry

BACKGROUND

Cues that are associated with the availability of food are known to trigger food anticipatory activity (FAA). This activity is expressed as increased locomotor activity and enables an animal to prepare for maximal utilization of nutritional resources. Although the exact neural network that mediates FAA is still unknown, several studies have revealed that the medial hypothalamus is involved. Interestingly, this area is responsive to the anorexigenic hormone leptin and the orexigenic hormone ghrelin that have been shown to modulate FAA. However, how FAA is regulated by neuronal activity and how leptin and ghrelin modulate this activity is still poorly understood.

OBJECTIVE

We aimed to examine how the total neuronal population and individual neurons in the medial hypothalamus respond to cue-signaled food availability in awake, behaving rats. In addition, ghrelin and leptin were injected to investigate whether these hormones could have a modulatory role in the regulation of FAA.

DESIGN

Using in vivo electrophysiology, neuronal activity was recorded in the medial hypothalamus in freely moving rats kept on a random feeding schedule, in which a light cue signaled upcoming food delivery. Ghrelin and leptin were administered systemically following the behavioral paradigm.

RESULTS

The food-predictive cue induced FAA as well as a significant increase in neural activity on a population level. More importantly, a sub-population of medial hypothalamic neurons displayed highly correlated identical responses to both ghrelin and FAA, suggesting that these neurons are part of the network that regulates FAA.

CONCLUSION

This study reveals a role for ghrelin, but not leptin, signaling within medial hypothalamus in FAA on both a population level and in single cells, identifying a subset of neurons onto which cue information and ghrelin signaling converge, possibly to drive FAA.

Ghrelin postsynaptically depolarizes dorsal raphe neurons in rats in vitro

Ghrelin promotes growth hormone (GH) secretion and feeding. Recent studies further showed that ghrelin displayed a defending effect against the depressive-like symptoms and affected sleep in animals and humans. Serotonergic system is considered to be implicated in feeding, depression and other mood disorders, and sleep. The dorsal raphe nucleus (DRN) utilizes serotonin (5-HT) as its major neurotransmitter and expresses GH secretagogue receptors (GHS-Rs). Therefore, the present study was carried out to examine the electrophysiological effect of ghrelin on rat DRN neurons in vitro and determine the ionic mechanism involved. Whole-cell recording revealed that ghrelin depolarized DRN neurons dose-dependently in tetrodotoxin-containing artificial cerebrospinal fluid (TTX ACSF). Pretreatment with [D-Lys(3)]-GHRP-6, a selective antagonist for GHS-Rs, antagonized the ghrelin-induced depolarization. The depolarization was significantly reduced in a low-Na(+) TTX ACSF and in a high-K(+) TTX ACSF and was abolished in the combination of both ACSFs, suggesting that the ghrelin-induced depolarization is mediated by a dual ionic mechanism including an increase in nonselective cationic conductance and a decrease in K(+) conductance. The experiments on the reversal potential also supported an involvement of the dual ionic mechanism in the ghrelin-induced depolarization. On the basis of their electrophysiological and pharmacological properties, approximately 80% of DRN neurons were classified as putative 5-HT-containing neurons and ghrelin depolarized 75% of them. These results suggest that DRN neurons, especially 5-HT-containing neurons, might be involved in the neural mechanisms through which ghrelin participates in the development and/or regulation of feeding behavior, sleep-wake states and depressive-like symptoms.

Electrophysiological effect of ghrelin and somatostatin on rat hypothalamic arcuate neurons in vitro

Growth hormone (GH) secretion from the pituitary gland is partly regulated by GH releasing hormone (GHRH)-containing neurons located in the hypothalamic arcuate nucleus (ARC). GHRH-containing neurons express the GH secretagogue (GHS) receptor (GHS-R) and the somatostatin (SRIF) receptor. Recently, an endogenous ligand for the GHS-R named ghrelin was found. Therefore, it seems that both ghrelin and SRIF are involved in the hypothalamic regulation of GH release via GHRH-containing neurons in the ARC. In extracellular single unit recordings from in vitro hypothalamic slice preparations from rats, application of 100 nM ghrelin substantially excited ARC neurons (82.5%), whereas 1 microM SRIF substantially inhibited them (81.8%). The ghrelin-induced excitatory and SRIF-induced inhibitory effects on ARC neurons were dose-dependent and persisted during synaptic blockade using low-Ca(2+)/high-Mg(2+) solution. In addition, the effects were antagonized by [D-Lys(3)]-GHRP-6, a GHS-R antagonist, and CYN154806, a SRIF receptor subtype sst2 antagonist, respectively. When ghrelin and SRIF were sequentially applied to ARC neurons, 95.2% were excited by ghrelin and inhibited by SRIF. Similarly, 85.0% of ARC neuroendocrine cells that project to the median eminence were excited by ghrelin and inhibited by SRIF. These results indicate that ARC neuroendocrine cells projecting to the median eminence are dose-dependently, postsynaptically and oppositely regulated by ghrelin through GHS-R and SRIF via the SRIF sst2 receptor subtype. Our results also suggest that most of these ARC neuroendocrine cells are presumably GHRH-containing neurons and are involved in the cellular processes through which ghrelin and SRIF participate in the hypothalamic regulation of GH release.

Electrophysiological effects of neuropeptide S on rat ventromedial hypothalamic neurons in vitro

The newly identified neuropeptide S (NPS) is a ligand for a previously orphan G protein-coupled GPR 154 receptor, now named the NPS receptor (NPSR). Previous studies have shown that NPS induces hyperlocomotion, increases arousal and suppresses anxiety-like behaviors via NPSR. Although NPS also inhibits food intake, nothing is known about the neuronal mechanisms underlying this action. Anatomical studies show that NPSRs are expressed abundantly in the dorsomedial part of the ventromedial hypothalamic nucleus (VMH), a satiety center for food intake. Hence, we examined the electrophysiological effects of NPS on rat VMH neurons in vitro. NPS predominantly depolarized the VMH neurons, and the effects were postsynaptic and dose-dependent. Membrane resistance was significantly decreased during the depolarization, suggesting an opening of some ionic channels. The NPS-induced depolarization was significantly attenuated in Ca(2+)-free, NiCl(2)-containing and mibefradil-containing TTX ACSFs, but it did not disappear. The NPS-induced depolarization was also attenuated in low-Na(+) TTX ACSF, and completely abolished in Ca(2+)-free/low-Na(+) TTX ACSF. Pretreatment with 30 microM KB-R7943, an inhibitor of forward-mode Na(+)/Ca(2+) exchanger, did not have any significant effect on the NPS-induced depolarization in Ca(2+)-free TTX ACSF. These results suggest that NPS depolarizes VMH neurons via activations of R- and T-type Ca(2+) channels and nonselective cation channels, and that VMH neurons might be involved in the cellular process through which NPS participates in the regulation of food intake and energy homeostasis.

Ghrelin stimulation of growth hormone-releasing hormone neurons is direct in the arcuate nucleus

Background

Ghrelin targets the arcuate nucleus, from where growth hormone releasing hormone (GHRH) neurones trigger GH secretion. This hypothalamic nucleus also contains neuropeptide Y (NPY) neurons which play a master role in the effect of ghrelin on feeding. Interestingly, connections between NPY and GHRH neurons have been reported, leading to the hypothesis that the GH axis and the feeding circuits might be co-regulated by ghrelin.

Principal Findings

Here, we show that ghrelin stimulates the firing rate of identified GHRH neurons, in transgenic GHRH-GFP mice. This stimulation is prevented by growth hormone secretagogue receptor-1 antagonism as well as by U-73122, a phospholipase C inhibitor and by calcium channels blockers. The effect of ghrelin does not require synaptic transmission, as it is not antagonized by γ-aminobutyric acid, glutamate and NPY receptor antagonists. In addition, this hypothalamic effect of ghrelin is independent of somatostatin, the inhibitor of the GH axis, since it is also found in somatostatin knockout mice. Indeed, ghrelin does not modify synaptic currents of GHRH neurons. However, ghrelin exerts a strong and direct depolarizing effect on GHRH neurons, which supports their increased firing rate.

Conclusion

Thus, GHRH neurons are a specific target for ghrelin within the brain, and not activated secondary to altered activity in feeding circuits. These results support the view that ghrelin related therapeutic approaches could be directed separately towards GH deficiency or feeding disorders.

Relationship of arousal to circadian anticipatory behavior: ventromedial hypothalamus: one node in a hunger-arousal network

The mechanisms by which animals adapt to an ever-changing environment have long fascinated scientists. Different forces, conveying information regarding various aspects of the internal and external environment, interact with each other to modulate behavioral arousal. These forces can act in concert or, at times, in opposite directions. These signals eventually converge and are integrated to influence a common arousal pathway which, depending on all the information received from the environment, supports the activation of the most appropriate behavioral response. In this review we propose that the ventromedial hypothalamic nucleus (VMN) is part of the circuitry that controls food anticipation. It is the first nucleus activated when there is a change in the time of food availability, silencing of VMN ghrelin receptors decreases food-anticipatory activity (FAA) and, although lesions of the VMN do not abolish FAA, parts of the response are often altered. In proposing this model it is not our intention to exclude parallel, redundant and possibly interacting pathways that may ultimately communicate with, or work in concert with, the proposed network, but rather to describe the neuroanatomical requirements for this circuit and to illustrate how the VMN is strategically placed and connected to mediate this complex behavioral adaptation.

Electrophysiological effects of ghrelin on laterodorsal tegmental neurons in rats: an in vitro study

Ghrelin, a gut and brain peptide, is a potent stimulant for growth hormone (GH) secretion and feeding. Recent studies further show a critical role of ghrelin in the regulation of sleep-wakefulness. Laterodorsal tegmental nucleus (LDT), that regulates waking and rapid eye movement (REM) sleep, expresses GH secretagogue receptors (GHS-Rs). Thus, the present study was carried out to examine electrophysiological effects of ghrelin on LDT neurons using rat brainstem slices, and to determine the ionic mechanism involved. Whole cell recording revealed that ghrelin depolarizes LDT neurons dose-dependently in normal artificial cerebrospinal fluid (ACSF). The depolarization persisted in tetrodotoxin-containing ACSF (TTX ACSF), and is partially blocked by the application of [D-Lys3]-GHRP-6, a selective antagonist for GHS-Rs. Membrane resistance during the ghrelin-induced depolarization increased by about 18% than that before the depolarization. In addition, the ghrelin-induced depolarization was drastically reduced in high-K+ TTX ACSF with a K+ concentration of 13.25 mM. Reversal potentials obtained from I-V curves before and during the depolarization were about -83 mV, close to the equilibrium potential of the K+ channel. Most of the LDT neurons recorded were characterized by an A-current or both the A-current and a low threshold Ca2+ spike, and they were predominantly cholinergic. These results indicate that ghrelin depolarizes LDT neurons postsynaptically and dose-dependently via GHS-Rs, and that the ionic mechanisms underlying the ghrelin-induced depolarization include a decrease of K+ conductance. The results also suggest that LDT neurons are implicated in the cellular processes through which ghrelin participates in the regulation of sleep-wakefulness.

Human ghrelin ameliorates organ injury and improves survival after radiation injury combined with severe sepsis

In the terrorist radiation exposure scenario, radiation victims are likely to suffer from additional injuries such as sepsis. Our previous studies have shown that ghrelin is protective in sepsis. However, it remains unknown whether ghrelin ameliorates sepsis-induced organ injury and mortality after radiation exposure. The purpose of this study is to determine whether human ghrelin attenuates organ injury and improves survival in a rat model of radiation combined injury (RCI) and, if so, the potential mechanism responsible for the benefit. To study this, adult male rats were exposed to 5-Gy whole body irradiation followed by cecal ligation and puncture (CLP, a model of sepsis) 48 h thereafter. Human ghrelin (30 nmol/rat) or vehicle (saline) was infused intravenously via an osmotic minipump immediately after radiation exposure. Blood and tissue samples were collected at 20 h after RCI (68 h after irradiation or 20 h after CLP) for various measurements. To determine the longterm effect of human ghrelin after RCI, the gangrenous cecum was removed at 5 h after CLP and 10-d survival was recorded. In addition, vagotomy or sham vagotomy was performed in sham and RCI animals immediately prior to ghrelin administration, and various measurements were performed at 20 h after RCI. Our results showed that serum levels of ghrelin and its gene expression in the stomach were decreased markedly at 20 h after RCI. Administration of human ghrelin attenuated tissue injury markedly, reduced proinflammatory cytokine levels, decreased tissue myeloperoxidase activity, and improved survival after RCI. Furthermore, elevated plasma levels of norepinephrine (NE) after RCI were reduced significantly by ghrelin. However, vagotomy prevented ghrelin's beneficial effects after RCI. In conclusion, human ghrelin is beneficial in a rat model of RCI. The protective effect of human ghrelin appears to be attributed to re-balancing the dysregulated sympathetic/parasympathetic nervous systems.

Stomach ghrelin-secreting cells as food-entrainable circadian clocks

Increases in arousal and activity in anticipation of a meal, termed "food anticipatory activity" (FAA), depend on circadian food-entrainable oscillators (FEOs), whose locations and output signals have long been sought. It is known that ghrelin is secreted in anticipation of a regularly scheduled mealtime. We show here that ghrelin administration increases locomotor activity in nondeprived animals in the absence of food. In mice lacking ghrelin receptors, FAA is significantly reduced. Impressively, the cumulative rise of activity before food presentation closely approximates a Gaussian function (r = 0.99) for both wild-type and ghrelin receptor knockout animals, with the latter having a smaller amplitude. For both groups, once an animal begins its daily anticipatory bout, it keeps running until the usual time of food availability, indicating that ghrelin affects response threshold. Oxyntic cells coexpress ghrelin and the circadian clock proteins PER1 and PER2. The expression of PER1, PER2, and ghrelin is rhythmic in light-dark cycles and in constant darkness with ad libitum food and after 48 h of food deprivation. In behaviorally arrhythmic-clock mutant mice, unlike control animals, there is no evidence of a premeal decrease in oxyntic cell ghrelin. Rhythmic ghrelin and PER expression are synchronized to prior feeding, and not to photic schedules. We conclude that oxyntic gland cells of the stomach contain FEOs, which produce a timed ghrelin output signal that acts widely at both brain and peripheral sites. It is likely that other FEOs also produce humoral signals that modulate FAA.

Electrophysiological effects of ghrelin on pedunculopontine tegmental neurons in rats: An in vitro study

Ghrelin is a potent stimulant for growth hormone (GH) secretion and feeding. Recent studies further show a critical role of ghrelin in the regulation of sleep-wakefulness. Pedunculopontine tegmental nucleus (PPT), which regulates waking and rapid eye movement (REM) sleep, expresses GH secretagogue receptors (GHS-Rs). Thus, the present study was carried out to examine electrophysiological effects of ghrelin on PPT neurons using rat brainstem slices, and to determine the ionic mechanism involved. Whole cell recording revealed that ghrelin depolarizes PPT neurons dose-dependently in normal artificial cerebrospinal fluid (ACSF). The depolarization persisted in tetrodotoxin-containing ACSF, although action potentials did not occur. Application of [d-Lys(3)]-GHRP-6, a selective antagonist for GHS-Rs, almost blocked the ghrelin-induced depolarization. Furthermore, the ghrelin-induced depolarization was reduced in high K(+) ACSF or low Na(+) ACSF, and abolished in high K(+)-low Na(+) ACSF or in a combination of low Na(+) ACSF and recordings with Cs(+)-containing pipettes. An inhibitor of Na(+)/Ca(2+) exchanger had no effect on the depolarization. Most of the PPT neurons recorded were characterized by an A-current or both the A-current and a low threshold Ca(2+) spike, and they were predominantly cholinergic as revealed by nicotinamide adenine dinucleotide phosphate-diaphorase staining. These results suggest that ghrelin depolarizes PPT neurons postsynaptically and dose-dependently via GHS-Rs, and that the ionic mechanisms underlying the ghrelin-induced depolarization include a decrease of K(+) conductance and an increase of non-selective cationic conductance. The results also support the notion that ghrelin plays a role in the regulation of sleep-wakefulness.