Stimulation of lateral hypothalamic glutamate and acetylcholine efflux by nicotine: implications for mechanisms of nicotine-induced activation of orexin neurons

Relationships and Interactions between Ionotropic Glutamate Receptors and Nicotinic Receptors in the CNS

Although ionotropic glutamate receptors and nicotinic receptors for acetylcholine (ACh) have usually been studied separately, they are often co-localized and functionally inter-dependent. The objective of this review is to survey the evidence for interactions between the two receptor families and the mechanisms underlying them. These include the mutual regulation of subunit expression, which change the NMDA:AMPA response balance, and the existence of multi-functional receptor complexes which make it difficult to distinguish between individual receptor sites, especially in vivo. This is followed by analysis of the functional relationships between the receptors from work on transmitter release, cellular electrophysiology and aspects of behavior where these can contribute to understanding receptor interactions. It is clear that nicotinic receptors (nAChRs) on axonal terminals directly regulate the release of glutamate and other neurotransmitters, α7-nAChRs generally promoting release. Hence, α7-nAChR responses will be prevented not only by a nicotinic antagonist, but also by compounds blocking the indirectly activated glutamate receptors. This accounts for the apparent anticholinergic activity of some glutamate antagonists, including the endogenous antagonist kynurenic acid. The activation of presynaptic nAChRs is by the ambient levels of ACh released from pre-terminal synapses, varicosities and glial cells, acting as a 'volume neurotransmitter' on synaptic and extrasynaptic sites. In addition, ACh and glutamate are released as CNS co-transmitters, including 'cholinergic' synapses onto spinal Renshaw cells. It is concluded that ACh should be viewed primarily as a modulator of glutamatergic neurotransmission by regulating the release of glutamate presynaptically, and the location, subunit composition, subtype balance and sensitivity of glutamate receptors, and not primarily as a classical fast neurotransmitter. These conclusions and caveats should aid clarification of the sites of action of glutamate and nicotinic receptor ligands in the search for new centrally-acting drugs.

Nicotinic Acetylcholine Receptor Signaling in the Hypothalamus: Mechanisms Related to Nicotine's Effects on Food Intake

Despite health risks associated with smoking, up to 20% of the US population persist in this behavior; many smoke to control body weight or appetite, and fear of post-cessation weight gain can motivate continued smoking. Nicotine and tobacco use is associated with lower body weight, and cessation yields an average weight gain of about 4 kg, which is thought to reflect a return to the body weight of a typical nonsmoker. Nicotine replacement therapies can delay this weight gain but do not prevent it altogether, and the underlying mechanism for how nicotine is able to reduce weight is not fully understood. In rodent models, nicotine reduces weight gain, reduces food consumption, and alters energy expenditure, but these effects vary with duration and route of nicotine administration. Nicotine, acting through nicotinic acetylcholine receptors (nAChRs), increases the firing rate of both orexigenic agouti-related peptide and anorexigenic proopiomelanocortin neurons in the arcuate nucleus of the hypothalamus (ARC). Manipulation of nAChR subunit expression within the ARC can block the ability of nicotine and the nicotinic agonist cytisine from decreasing food intake; however, it is unknown exactly how this reduces food intake. This review summarizes the clinical and preclinical work on nicotine, food intake, and weight gain, then explores the feeding circuitry of the ARC and how it is regulated by nicotine. Finally, we propose a novel hypothesis for how nicotine acts on this hypothalamic circuit to reduce food intake. Implications: This review provides a comprehensive and updated summary of the clinical and preclinical work examining nicotine and food intake, as well as a summary of recent work examining feeding circuits of the hypothalamus. Synthesis of these two topics has led to new understanding of how nAChR signaling regulates food intake circuits in the hypothalamus.

T-type calcium channel enhancer SAK3 promotes dopamine and serotonin releases in the hippocampus in naive and amyloid precursor protein knock-in mice

T-type calcium channels in the brain mediate the pathophysiology of epilepsy, pain, and sleep. Recently, we developed a novel therapeutic candidate, SAK3 (ethyl 8'-methyl-2',4-dioxo-2-(piperidin-1-yl)-2'H-spiro[cyclopentane-1,3'-imidazo[1,2-a] pyridine]-2-ene-3-carboxylate), for Alzheimer's disease (AD). The cognitive improvement by SAK3 is closely associated with enhanced acetylcholine (ACh) release in the hippocampus. Since monoamines such as dopamine (DA), noradrenaline (NA), and serotonin (5-HT) are also involved in hippocampus-dependent learning and psychomotor behaviors in mice, we investigated the effects of SAK3 on these monoamine releases in the mouse brain. Oral administration of SAK3 (0.5 mg/kg, p.o.) significantly promoted DA and 5-HT releases in the naive mouse hippocampal CA1 region but not in the medial prefrontal cortex (mPFC), while SAK3 did not affect NA release in either brain region. The T-type calcium channel-specific inhibitor, NNC 55-0396 (1 μM) significantly antagonized SAK3-enhanced DA and 5-HT releases in the hippocampus. Interestingly, the α7 nicotinic ACh receptor (nAChR) antagonist, methyllycaconitine (1 nM) significantly inhibited DA release, and the α4 nAChR antagonist, dihydro-β-erythroidine (100 μM) significantly blocked both DA and 5-HT releases following SAK3 (0.5 mg/kg, p.o.) administration in the hippocampus. SAK3 did not alter basal monoamine contents both in the mPFC and hippocampus. SAK3 (0.5 mg/kg, p.o.) administration also significantly elevated DA and 5-HT releases in the hippocampal CA1 region of amyloid-precursor protein (APP)NL-GF knock-in (KI) mice. Moreover, hippocampal DA and 5-HT contents were significantly decreased in APPNL-GF KI mice. Taken together, our data suggest that SAK3 promotes monoamine DA and 5-HT releases by enhancing the T-type calcium channel and nAChR in the mouse hippocampus.

Nicotinic Cholinergic System in the Hypothalamus Modulates the Activity of the Hypothalamic Neuropeptides During the Stress Response

BACKGROUND

The hypothalamus harbors high levels of cholinergic neurons and axon terminals. Nicotinic acetylcholine receptors, which play an important role in cholinergic neurotransmission, are expressed abundantly in the hypothalamus. Accumulating evidence reveals a regulatory role for nicotine in the regulation of the stress responses. The present review will discuss the hypothalamic neuropeptides and their interaction with the nicotinic cholinergic system. The anatomical distribution of the cholinergic neurons, axon terminals and nicotinic receptors in discrete hypothalamic nuclei will be described. The effect of nicotinic cholinergic neurotransmission and nicotine exposure on hypothalamic-pituitaryadrenal (HPA) axis regulation at the hypothalamic level will be analyzed in view of the different neuropeptides involved.

METHODS

Published research related to nicotinic cholinergic regulation of the HPA axis activity at the hypothalamic level is reviewed.

RESULTS

The nicotinic cholinergic system is one of the major modulators of the HPA axis activity. There is substantial evidence supporting the regulation of hypothalamic neuropeptides by nicotinic acetylcholine receptors. However, most of the studies showing the nicotinic regulation of hypothalamic neuropeptides have employed systemic administration of nicotine. Additionally, we know little about the nicotinic receptor distribution on neuropeptide-synthesizing neurons in the hypothalamus and the physiological responses they trigger in these neurons.

CONCLUSION

Disturbed functioning of the HPA axis and hypothalamic neuropeptides results in pathologies such as depression, anxiety disorders and obesity, which are common and significant health problems. A better understanding of the nicotinic regulation of hypothalamic neuropeptides will aid in drug development and provide means to cope with these diseases. Considering that nicotine is also an abused substance, a better understanding of the role of the nicotinic cholinergic system on the HPA axis will aid in developing improved therapeutic strategies for smoking cessation.

Neuroanatomical Relationships between Orexin/Hypocretin-Containing Neurons/Nerve Fibers and Nicotine-Induced c-Fos-Activated Cells of the Reward-Addiction Neurocircuitry

Orexin/hypocretin-containing neurons in lateral hypothalamus (LH) are implicated in the neurobiology of nicotine addiction. However, the neuroanatomical relationships between orexin-neurons/nerve fibers and nicotine-activated cells within the reward-addiction neurocircuitry is not known. In the present study in mice, we first used c-Fos immunohistochemistry to identify CNS cells stimulated by an acute single injection of nicotine (NIC, 2 mg/kg, IP). Sequential double-labelling was then performed to identify the location of orexin-containing neurons and nerve fibers with respect to NIC-induced c-Fos activated cells and/or tyrosine hydroxylase (TH) immunoreactive (IR) cells of the mesocorticolimbic reward-addiction pathways. Orexin-IR nerve fibers and terminals were detected at multiple sites of the NIC reward-addiction circuitry in close apposition to, and intermingled with, NIC-induced c-Fos-IR cells of locus coeruleus (LC), ventral tegmental area (VTA), nucleus accumbens (Acb), LH and paraventricular thalamic nucleus (PVT). Double-labelling of orexin with TH showed frequent contact between orexin-IR nerve fibers and noradrenergic cells of LC. However, there was infrequent contact between the orexinergic fibers and the TH-expressing dopaminergic cells of VTA, dorsal raphe nucleus (DR), posterior hypothalamus (DA11), arcuate hypothalamic nucleus (DA12) and periventricular areas (DA14). The close anatomical contact between orexinergic nerve fibers and NIC-activated cells at multiple sites of the reward-addiction pathways suggests that orexinergic projections from LH are likely to be involved in modulating activity of the neurons that are directly impacted by acute administration of nicotine.

The dual orexin receptor antagonist TCS1102 does not affect reinstatement of nicotine-seeking

The orexin/hypocretin system is important for appetitive motivation towards multiple drugs of abuse, including nicotine. Both OX1 and OX2 receptors individually have been shown to influence nicotine self-administration and reinstatement. Due to the increasing clinical use of dual orexin receptor antagonists in the treatment of disorders such as insomnia, we examined whether a dual orexin receptor antagonist may also be effective in reducing nicotine seeking. We tested the effect of intracerebroventricular (i.c.v.) administration of the potent and selective dual orexin receptor antagonist TCS1102 on orexin-A-induced food self-administration, nicotine self-administration and reinstatement of nicotine-seeking in rats. Our results show that 30 μg of TCS1102 i.c.v. abolishes orexin-A-induced increases in food self-administration but does not reduce nicotine self-administration. Neither i.c.v. 10 μg nor 30 μg of TCS1102 reduced compound reinstatement after short-term (15 days) self-administration nicotine, but 30 μg transiently reduced cue/nicotine compound reinstatement after chronic self-administration (29 days). These results indicate that TCS1102 has no substantial effect on motivation for nicotine seeking following chronic self-administration and no effect after shorter periods of intake. Orexin receptor antagonists may therefore have little clinical utility against nicotine addiction.

Synaptic signalling and its interface with neuropathologies: snapshots from the past, present and future

This 'Past to Future' Review as part of the 60th anniversary year of the Journal of Neurochemistry focuses on synaptic transmission and associated signalling, and seeks to identify seminal progress in neurochemistry over the last 10 years which has advanced our understanding of neuronal communication in brain. The approach adopted analyses neurotransmitters on a case by case basis (i.e. amino acids, monoamines, acetylcholine, neuropeptides, ATP/purines and gasotransmitters) to highlight novel findings that have changed the way we view each type of transmitter, to explore commonalities and interactions, and to note how new insights have changed the way we view the biology of degenerative, psychiatric and behavioural conditions. Across all transmitter systems there was remarkable growth in the identification of targets likely to provide therapeutic benefit and which undoubtedly was driven by the elucidation of circuit function and new vistas of synaptic signalling. There has been an increasing trend to relate signalling to disease, notably for Alzheimer's and Parkinson's disease and related conditions, and which has occurred for each transmitter family. Forebrain circuitry and tonic excitatory control have been the centre of great attention yielding novel findings that will impact upon cognitive, emotional and addictive behaviours. Other impressive insights focus on gasotransmitters integrating activity as volume transmitters. Exciting developments in how serotonin, cholinergic, l-glutamate, galanin and adenosine receptors and their associated signalling can be beneficially targeted should underpin the development of new therapies. Clearly integrated, multifaceted neurochemistry has changed the way we view synaptic signalling and its relevance to pathobiology. Highlighted are important advances in synaptic signalling over the last decade in the Journal of Neurochemistry. Across all transmitter systems elucidation of circuit function, and notably molecular insights, have underpinned remarkable growth in the identification of targets likely to provide therapeutic benefit in neuropathologies. Another commonality was wide interest in forebrain circuitry and its tonic excitatory control. Increasingly observations relate to signalling in disease and behavioural conditions. This article is part of the 60th Anniversary special issue.

The novel α7 nicotinic acetylcholine receptor agonist EVP-6124 enhances dopamine, acetylcholine, and glutamate efflux in rat cortex and nucleus accumbens

BACKGROUND

Alpha7 and α4β2 nicotinic acetylcholine receptor (nAChR) agonists have been shown to improve cognition in various animal models of cognitive impairment and are of interest as treatments for schizophrenia, Alzheimer's disease, and other cognitive disorders. Increased release of dopamine (DA), acetylcholine (ACh), glutamate (Glu), and γ-aminobutyric acid (GABA) in cerebral cortex, hippocampus, and nucleus accumbens (NAC) has been suggested to contribute to their beneficial effects on cognition.

RESULTS

Using in vivo microdialysis, we found that EVP-6124 [(R)-7-chloro-N-quinuclidin-3-yl)benzo[b]thiophene-2-carboxamide], a high-affinity α7 nAChR partial agonist, at 0.1 mg/kg, s.c., increased DA efflux in the medial prefrontal cortex (mPFC) and NAC. EVP-6124, 0.1 and 0.3 mg/kg, also increased efflux of ACh in the mPFC but not in the NAC. Similarly, EVP-6124, 0.1 mg/kg, but not 0.03 and 0.3 mg/kg, significantly increased mPFC Glu efflux. Thus, EVP-6124 produced an inverted U-shaped curve for DA and Glu release, as previously reported for other α7 nAChR agonists. The three doses of EVP-6124 did not produce a significant effect on GABA efflux in either region. Pretreatment with the selective α7 nAChR antagonist, methyllycaconitine (MLA, 1.0 mg/kg), significantly blocked cortical DA and Glu efflux induced by EVP-6124 (0.1 mg/kg), suggesting that the effects of EVP-6124 on these neurotransmitters were due to α7 nAChR agonism. MLA only partially blocked the effects of EVP-6124 on ACh efflux in the mPFC.

CONCLUSION

These results suggest increased cortical DA, ACh, and Glu release, which may contribute to the ability of the α7 nAChR agonist, EVP-6124, to treat cognitive impairment and possibly other dimensions of psychopathology.

Role of lateral hypothalamus in two aspects of attention in associative learning

Orexin (hypocretin) and melanin-concentrating hormone (MCH) neurons are unique to the lateral hypothalamic (LH) region, but project throughout the brain. These cell groups have been implicated in a variety of functions, including reward learning, responses to stimulants, and the modulation of attention, arousal and the sleep/wakefulness cycle. Here, we examined roles for LH in two aspects of attention in associative learning shown previously to depend on intact function in major targets of orexin and MCH neurons. In experiments 1 and 2, unilateral orexin-saporin lesions of LH impaired the acquisition of conditioned orienting responses (ORs) and bilaterally suppressed FOS expression in the amygdala central nucleus (CeA) normally observed in response to food cues that provoke conditioned ORs. Those cues also induced greater FOS expression than control cues in LH orexin neurons, but not in MCH neurons. In experiment 3, unilateral orexin-saporin lesions of LH eliminated the cue associability enhancements normally produced by the surprising omission of an expected event. The magnitude of that impairment was positively correlated with the amount of LH damage and with the loss of orexin neurons in particular, but not with the loss of MCH neurons. We suggest that the effects of the LH orexin-saporin lesions were mediated by their effect on information processing in the CeA, known to be critical to both behavioral phenomena examined here. The results imply close relations between LH motivational amplification functions and attention, and may inform our understanding of disorders in which motivational and attentional impairments co-occur.

Stimulation of nicotine reward and central cholinergic activity in Sprague-Dawley rats exposed perinatally to a fat-rich diet

RATIONALE

While clinical studies show maternal consumption of palatable fat-rich diets during pregnancy to negatively impact the children's behaviors and increase their vulnerability to drug abuse, the precise behavioral and neurochemical mechanisms mediating these phenomena have yet to be examined.

OBJECTIVE

The study examined in rats whether gestational exposure to a high-fat diet (HFD) can increase the offspring's propensity to use nicotine and whether disturbances in central nicotinic cholinergic signaling accompany this behavioral effect.

METHODS

Rat offspring exposed perinatally to a HFD or chow diet were characterized in terms of their nicotine self-administration behavior in a series of operant response experiments and the activity of acetylcholinesterase (AChE) and density of nicotinic ACh receptors (nAChRs) in different brain areas.

RESULT

Perinatal HFD compared to chow exposure increased nicotine-self administration behavior during fixed ratio and dose-response testing and caused an increase in breakpoint using progressive ratio testing, while nicotine seeking in response to nicotine prime-induced reinstatement was reduced. This behavioral change induced by the HFD was associated with a significant reduction in activity of AChE in the midbrain, hypothalamus, and striatum and increased density of β2-nAChRs in the ventral tegmental area and substantia nigra and of α7-nAChRs in the lateral and ventromedial hypothalamus.

CONCLUSIONS

Perinatal exposure to a HFD increases the vulnerability of the offspring to excessive nicotine use by enhancing its reward potential, and these behavioral changes are accompanied by a stimulation of nicotinic cholinergic signaling in mesostriatal and hypothalamic brain areas important for reinforcement and consummatory behavior.

Mu-opioid stimulation in rat prefrontal cortex engages hypothalamic orexin/hypocretin-containing neurons, and reveals dissociable roles of nucleus accumbens and hypothalamus in cortically driven feeding

Mu-opioid receptor (μOR) stimulation within ventral medial prefrontal cortex (vmPFC) induces feeding and hyperactivity, resulting possibly from recruitment of glutamate signaling in multiple vmPFC projection targets. We tested this hypothesis by analyzing Fos expression in vmPFC terminal fields after intra-vmPFC μOR stimulation, and by examining of the impact of glutamate receptor blockade in two feeding-related targets of vmPFC, the lateral-perifornical hypothalamic area (LH-PeF) and nucleus accumbens shell (Acb shell), upon behavioral effects elicited by intra-vmPFC μOR stimulation in rats. Intra-vmPFC infusion of the μOR agonist, DAMGO, provoked Fos expression in the dorsomedial sector of tuberal hypothalamus (including the perifornical area) and increased the percentage of Fos-expressing hypocretin/orexin-immunoreactive neurons in these zones. NMDA receptor blockade in the LH-PeF nearly eliminated intra-vmPFC DAMGO-induced food intake without altering DAMGO-induced hyperactivity. In contrast, blocking AMPA-type glutamate receptors within the Acb shell (the feeding-relevant subtype in this structure) antagonized intra-vmPFC DAMGO-induced hyperlocomotion but enhanced food intake. Intra-vmPFC DAMGO also elevated the breakpoint for sucrose-reinforced progressive-ratio responding; this effect was significantly enhanced by concomitant AMPA blockade in the Acb shell. Conversely, intra-Acb shell AMPA stimulation reduced breakpoint and increased nonspecific responding on the inactive lever. These data indicate intra-vmPFC μOR signaling jointly modulates appetitive motivation and generalized motoric activation through functionally dissociable vmPFC projection targets. These findings may shed light on the circuitry underlying disorganized appetitive responses in psychopathology; e.g., binge eating and opiate or alcohol abuse, disorders in which μORs and aberrant cortical activation have been implicated.

Intravenous prenatal nicotine exposure increases orexin expression in the lateral hypothalamus and orexin innervation of the ventral tegmental area in adult male rats

BACKGROUND

Approximately 18% of pregnant women continue to smoke tobacco cigarettes throughout pregnancy. Offspring exposed to tobacco smoke in utero exhibit a higher incidence of drug use in later stages of development relative to non-exposed children. Animal models indicate that prenatal nicotine (PN) exposure alone alters the development of the mesocorticolimbic dopamine (DA) system, which, in part, organizes motivated behavior and reward. The orexin/hypocretin neuropeptide system, which originates in the lateral hypothalamus (LH), projects to key areas of the mesocorticolimbic DA pathway. Previous research suggests that orexin exerts a major influence on motivation and reward.

METHODS

The present experiments determined if intravenous (IV) PN exposure alters (1) the expression of orexin neurons and melanin-concentrating hormone (MCH; positive control) in the LH; and (2) orexin projections from the LH onto DA neurons in the ventral tegmental area (VTA). Dams were injected with IV nicotine (0.05 mg/kg/injection) or saline 3×/day during gestational days 8-21. Tissues from adult male offspring (∼130 days) were examined using immunohistochemistry.

RESULTS

Relative to controls, offspring of IV PN exposure showed (1) increased numbers of orexin neurons in the LH, and no changes in the expression of MCH; and (2) increased orexin appositions on DA cells in the VTA.

CONCLUSION

The findings indicate that the influence of PN exposure is enduring, and suggests that the PN-induced modification of orexin expression on mesolimbic circuitry may contribute to the reported changes in motivated behaviors related to food and drug reward observed in offspring prenatally exposed to nicotine.

Different levels in orexin concentrations and risk factors associated with higher orexin levels: comparison between detoxified opiate and methamphetamine addicts in 5 Chinese cities

This study sought to explore the degree of orexin levels in Chinese opiate and methamphetamine addicts and the differences between them. The cross-sectional study was conducted among detoxified drug addicts from Mandatory Detoxification Center (MDC) in five Chinese cities. Orexin levels were assayed with radioimmunoassay (RIA). Mann-Whitney U test and Kruskal-Wallis test were used to detect differences across groups, and logistic regression was used to explore the association between orexin levels and characteristics of demographic and drug abuse. Between November 2009 and January 2011, 285 opiates addicts, 112 methamphetamine addicts, and 79 healthy controls were enrolled. At drug withdrawal period, both opiate and methamphetamine addicts had lower median orexin levels than controls, and median orexin levels in opiate addicts were higher than those in methamphetamine addicts (all above P < 0.05). Adjusted odds of the above median concentration of orexin were higher for injection than "chasing the dragon" (AOR = 3.1, 95% CI = 1.2-7.9). No significant factors associated with orexin levels of methamphetamine addicts were found. Development of intervention method on orexin system by different administration routes especially for injected opiate addicts at detoxification phase may be significant and was welcome.

Effects of changes in energy homeostasis and exposure of noxious insults on the expression of orexin (hypocretin) and its receptors in the brain

This review summarizes data regarding the brain expression of the orexin (hypocretin) system including: prepro-orexin (PPO), orexin A (OxA), orexin B (OxB) and the two orexin receptors 1 and 2 (OxR1, OxR2). Clinical data is limited to OxA and OxB in cerebral spinal fluid and serum/plasma, thus necessitating the development of animal models to undertake mechanistic studies. We focus on changes in animal models that were either exposed to a regime of altered sleep, metabolic energy homeostasis, exposed to drugs and noxious insults. Many more expressional studies are available for PPO, OxA and OxB levels, compared to studies of the receptors. Interestingly, the direction and pattern of change for PPO, OxA and OxB is inconsistent amongst studies, whereas for the receptors, there tends to be increased expression for both OxR1 and OxR2 after alterations in energy homeostasis, and an increased expression after noxious insults or exposure to some drugs. The clinical implications of these results from animal models are discussed in light of the findings from human studies, and future research directions are suggested to fill knowledge gaps with regard to the orexin system, particularly during early brain development.

Molecules and circuits involved in nicotine addiction: The many faces of smoking

Tobacco smoking in humans is one of the most persistent and widespread addictions and is driven by nicotine in tobacco smoke. Over the last several decades, understanding of the molecular and cellular basis for nicotine addiction has increased tremendously as a result of pharmacological, molecular genetic, electrophysiological and behavioral studies of nicotine reinforcement. Studies of the biological basis for nicotine reinforcement has helped in the design of new treatments for smoking cessation such as varenicline; however, smokers report that they smoke for many reasons, including the ability to control symptoms of anxiety and depression or the desire to control appetite. Further, developmental exposure to tobacco smoke increases the likelihood of adult smoking. Here we review what is known about the molecular and circuit basis for a number of behaviors related to tobacco smoking. Leveraging the knowledge from studies of different behaviors mediated by nicotine receptors in multiple brain circuits could provide points of convergence that will inform future therapeutic development for smoking cessation. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Orexin receptors in the developing piglet hypothalamus, and effects of nicotine and intermittent hypercapnic hypoxia exposures

Orexin and its receptors (OxR1 and OxR2) play a significant role in arousal and sleep regulation. Using developing piglets, we aimed to determine the effects of nicotine and Intermittent Hypercapnic Hypoxia (IHH), alone or in combination, on orexin receptor expression in the hypothalamus. Four piglet groups were studied: control (n=14), nicotine (n=14), IHH (n=10) and nic+IHH (n=14). Applying immunohistochemistry for OxR1 and OxR2 expression, eight nuclei/areas of the hypothalamus: dorsal medial nucleus (DMN), arcuate nucleus (ARC), perifornical area (PFA), paraventricular nucleus (PVN), lateral hypothalamic area (LHA), ventral medial nucleus (VMN), supraoptic nucleus, retrochiasmatic part (SONr) and tuberal mammillary nucleus (TMN), were studied. Compared to controls, OxR1 and OxR2 were increased due to exposures, however this was region dependent. Nicotine increased OxR1 in the DMN (P<0.001) and SONr (P=0.036), and OxR2 in the DMN (P<0.001), VMN (P=0.014) and the TMN (P=0.026). IHH increased OxR1 in the DMN, PVN, VMN and SONr (P<0.01 for all), and OxR2 in DMN (P<0.001), PFA (P=0.001), PVN (P=0.004), VMN (P=0.041) and the TMN (P<0.001). The nic+IHH exposure increased OxR1 expression in all nuclei (TMN excluded) however, the changes were not significantly different from IHH alone. For OxR2, the increased expression after nic+IHH was significant compared to IHH in the DMN, ARC and SONr. These results show that nicotine increases orexin receptor expression in a region dependent manner. IHH induced increases were specific to arousal and stress related regions and nic+IHH results suggest that for OxR1, nicotine has no additive effect whereas for OxR2 it does, and is region dependent.

Acetylcholine as a neuromodulator: cholinergic signaling shapes nervous system function and behavior

Acetylcholine in the brain alters neuronal excitability, influences synaptic transmission, induces synaptic plasticity, and coordinates firing of groups of neurons. As a result, it changes the state of neuronal networks throughout the brain and modifies their response to internal and external inputs: the classical role of a neuromodulator. Here, we identify actions of cholinergic signaling on cellular and synaptic properties of neurons in several brain areas and discuss consequences of this signaling on behaviors related to drug abuse, attention, food intake, and affect. The diverse effects of acetylcholine depend on site of release, receptor subtypes, and target neuronal population; however, a common theme is that acetylcholine potentiates behaviors that are adaptive to environmental stimuli and decreases responses to ongoing stimuli that do not require immediate action. The ability of acetylcholine to coordinate the response of neuronal networks in many brain areas makes cholinergic modulation an essential mechanism underlying complex behaviors.

Nicotinic regulation of energy homeostasis

INTRODUCTION

The ability of nicotine, the primary psychoactive substance in tobacco smoke, to regulate appetite and body weight is one of the factors cited by smokers that prevents them from quitting and is the primary reason for smoking initiation in teenage girls. The regulation of feeding and metabolism by nicotine is complex, and recent studies have begun to identify nicotinic acetylcholine receptor (nAChR) subtypes and circuits or cell types involved in this regulation.

DISCUSSION

We will briefly describe the primary anatomical and functional features of the input, output, and central integration structures of the neuroendocrine systems that regulate energy homeostasis. Then, we will describe the nAChR subtypes expressed in these structures in mammals to identify the possible molecular targets for nicotine. Finally, we will review the effects of nicotine and its withdrawal on feeding and energy metabolism and attribute them to potential central and peripheral cellular targets.

Involvement of cholinergic mechanisms in the behavioral effects of dietary fat consumption

Clinical reports suggest a positive association between fat consumption and the incidence of hyperactivity, impulsivity and cognitive abnormalities. To investigate possible mechanisms underlying these disturbances under short-term conditions, we examined in Sprague-Dawley rats the influence of 7-day consumption of a high-fat diet (HFD) compared to chow on anxiety, novelty-seeking and exploratory behaviors and also on acetylcholine (ACh) neurotransmission that may mediate these behaviors. The HFD consumption, which elevated circulating fatty acids but produced no change in caloric intake or body weight, stimulated novelty-seeking and exploration in an open field, while reducing anxiety in an elevated plus maze. Using the Ellman assay to measure ACh esterase (AChE) activity that breaks down ACh, the second experiment showed HFD consumption to significantly reduce AChE activity in the frontal cortex, hypothalamus and midbrain. With measurements of [¹²⁵I]-epibatidine or [¹²⁵I]-bungarotoxin binding to nicotinic ACh receptors (nAChRs) containing β2 or α7 subunits, respectively, the results also showed HFD consumption to increase both β2-nAChR binding in the medial prefrontal cortex and substantia nigra and α7-nAChR binding in the lateral and ventromedial hypothalamus. When treated with an acute dose of the nicotinic antagonist, mecamylamine (0.5 mg/kg, sc), the HFD animals responded with significantly reduced exploratory and novelty-seeking behaviors, whereas the chow-consuming rats exhibited no response. These findings suggest that the exploratory and novelty-seeking behaviors induced by dietary fat may be mediated by enhanced nicotinic cholinergic activity, which is accompanied by increased density of β2-nAChRs in cortical and midbrain regions associated with impulsivity and locomotor activity and of α7-nAChRs in hypothalamic regions associated with arousal and energy balance.

Feeding behavior as seen through the prism of brain microdialysis

The knowledge of feeding behavior mechanisms gained through brain microdialysis is reviewed. Most of the chemical changes so far reported concern to the limbic system in rodents. A picture showing increases and decreases of extracellular neurotransmitters correlating to different aspects of feeding behavior is gradually emerging. Depending on the region, the same neurotransmitter may signal opposite aspects of feeding. Dopamine (DA) in the nucleus accumbens (NAC) correlates with food reward, stimulus saliency, and goal directed hyperlocomotion but in the ventromedial hypothalamus DA correlates with satiety and hypolocomotion. The findings accumulated in the last 25 years suggest that the control of a particular function relies on the interaction of several neurotransmitters rather than on a single neurotransmitter. The poor sensitivity of most analytical techniques hinders time and spatial resolution of microdialysis. Therefore, neurochemical correlates of short lasting behaviors are hard to figure out. As new and more sensitive analytical techniques are applied, new neurochemical correlates of feeding show up. Sometimes the proper analytical techniques are simply not available. As a consequence, critical signals such as neuropeptides are not yet completely placed in the puzzle. Despite such limitations, brain microdialysis has yielded a great deal of knowledge on the neurochemical basis of feeding.