Effects of arousal- and feeding-related neuropeptides on dopaminergic and GABAergic neurons in the ventral tegmental area of the rat

Knocking down the transcript of protein kinase C-lambda modulates hypothalamic glutathione peroxidase, melanocortin receptor and neuropeptide Y gene expression in amphetamine-treated rats

It has been reported that neuropeptide Y (NPY) contributes to the behavioral response of amphetamine (AMPH), a psychostimulant. The present study examined whether protein kinase C (PKC)-λ signaling was involved in this action. Moreover, possible roles of glutathione peroxidase (GP) and melanocortin receptor 4 (MC4R) were also examined. Rats were given AMPH daily for 4 days. Hypothalamic NPY, PKCλ, GP and MC4R were determined and compared. Pretreatment with α-methyl-para-tyrosine could block AMPH-induced anorexia, revealing that endogenous catecholamine was involved in regulating AMPH anorexia. PKCλ, GP and MC4R were increased with maximal response on Day 2 during AMPH treatment, which were concomitant with the decreases in NPY. cAMP response element binding protein (CREB) DNA binding activity was increased during AMPH treatment, revealing the involvement of CREB-dependent gene transcription. An interruption of cerebral PKCλ transcript could partly block AMPH-induced anorexia and partly reverse NPY, MC4R and GP mRNA levels to normal. These results suggest that PKCλ participates in regulating AMPH-induced anorexia via a modulation of hypothalamic NPY gene expression and that increases of GP and MC4R may contribute to this modulation. Our results provided molecular evidence for the regulation of AMPH-induced behavioral response.

Drugs of abuse and stress impair LTP at inhibitory synapses in the ventral tegmental area

Synaptic plasticity in the ventral tegmental area (VTA) is modulated by drugs of abuse and stress and is hypothesized to contribute to specific aspects of addiction. Both excitatory and inhibitory synapses on dopamine neurons in the VTA are capable of undergoing long-term changes in synaptic strength. While the strengthening or weakening of excitatory synapses in the VTA has been widely examined, the role of inhibitory synaptic plasticity in brain reward circuitry is less established. Here, we investigated the effects of drugs of abuse, as well as acute stress, on long-term potentiation of GABAergic synapses onto VTA dopamine neurons (LTP(GABA)). Morphine (10 mg/kg i.p.) reduced the ability of inhibitory synapses in midbrain slices to express LTP(GABA) both at 2 and 24 h after drug exposure but not after 5 days. Cocaine (15 mg/kg i.p.) impaired LTP(GABA) 24 h after exposure, but not at 2 h. Nicotine (0.5 mg/kg i.p.) impaired LTP(GABA) 2 h after exposure, but not after 24 h. Furthermore, LTP(GABA) was completely blocked 24 h following brief exposure to a stressful stimulus, a forced swim task. Our data suggest that drugs of abuse and stress trigger a common modification to inhibitory plasticity, synergizing with their collective effect at excitatory synapses. Together, the net effect of addictive substances or stress is expected to increase excitability of VTA dopamine neurons, potentially contributing to the early stages of addiction.

Central dopaminergic circuitry controlling food intake and reward: implications for the regulation of obesity

Prevalence of obesity in the general population has increased in the past 15 years from 15% to 35%. With increasing obesity, the coincident medical and social consequences are becoming more alarming. Control over food intake is crucial for the maintenance of body weight and represents an important target for the treatment of obesity. Central nervous system mechanisms responsible for control of food intake have evolved to sense the nutrient and energy levels in the organism and to coordinate appropriate responses to adjust energy intake and expenditure. This homeostatic system is crucial for maintenance of stable body weight over long periods of time of uneven energy availability. However, not only the caloric and nutritional value of food but also hedonic and emotional aspects of feeding affect food intake. In modern society, the increased availability of highly palatable and rewarding (fat, sweet) food can significantly affect homeostatic balance, resulting in dysregulated food intake. This review will focus on the role of hypothalamic and mesolimbic/mesocortical dopaminergic (DA) circuitry in coding homeostatic and hedonic signals for the regulation of food intake and maintenance of caloric balance. The interaction of dopamine with peripheral and central indices of nutritional status (e.g., leptin, ghrelin, neuropeptide Y), and the susceptibility of the dopamine system to prenatal insults will be discussed. Additionally, the importance of alterations in dopamine signaling that occur coincidently with obesity will be addressed.

Dopamine-based reward circuitry responsivity, genetics, and overeating

Data suggest that low levels of dopamine D2 receptors and attenuated responsivity of dopamine-target regions to food intake is associated with increased eating and elevated weight. There is also growing (although mixed) evidence that genotypes that appear to lead to reduced dopamine signaling (e.g., DRD2, DRD4, and DAT) and certain appetite-related hormones and peptides (e.g., ghrelin, orexin A, leptin) moderate the relation between dopamine signaling, overeating, and obesity. This chapter reviews findings from studies that have investigated the relation between dopamine functioning and food intake and how certain genotypes and appetite-related hormones and peptides affect this relation.

Co-localization of TRHR1 and LepRb receptors on neurons in the hindbrain of the rat

We have reported a highly cooperative interaction between leptin and thyrotropin releasing hormone (TRH) in the hindbrain to generate thermogenic responses (Hermann et al., 2006) (Rogers et al., 2009). Identifying the locus in the hindbrain where leptin and TRH act synergistically to increase thermogenesis will be necessary before we can determine the mechanism(s) by which this interaction occurs. Here, we performed heat-induced epitope recovery techniques and in situ hybridization to determine if neurons or afferent fibers in the hindbrain possess both TRH type 1 receptor and long-form leptin receptor [TRHR1; LepRb, respectively]. LepRb receptors were highly expressed in the solitary nucleus [NST], dorsal motor nucleus of the vagus [DMN] and catecholaminergic neurons of the ventrolateral medulla [VLM]. All neurons that contained LepRb also contained TRHR1. Fibers in the NST and the raphe pallidus [RP] and obscurrus [RO] that possess LepRb receptors were phenotypically identified as glutamatergic type 2 fibers (vglut2). Fibers in the NST and RP that possess TRHR1 receptors were phenotypically identified as serotonergic [i.e., immunopositive for the serotonin transporter; SERT]. Co-localization of LepRb and TRHR1 was not observed on individual fibers in the hindbrain but these two fiber types co-mingle in these nuclei. These anatomical arrangements may provide a basis for the synergy between leptin and TRH to increase thermogenesis.

Low cerebrospinal fluid and plasma orexin-A (hypocretin-1) concentrations in combat-related posttraumatic stress disorder

The hypothalamic neuropeptide, orexin-A has a number of regulatory effects in humans and pre-clinical evidence suggests a link to neuroendocrine systems known to be pathophysiologically related to posttraumatic stress disorder (PTSD). However, there are no reports of central nervous system (CNS) or peripheral orexin-A concentrations in patients with PTSD, or any anxiety disorder. Cerebrospinal fluid (CSF) and plasma levels of orexin-A were serially determined in patients with PTSD and healthy comparison subjects to characterize the relationships between orexin-A (in the CNS and peripheral circulation) and central indices of monoaminergic neurotransmission and to determine the degree to which CNS orexin-A concentrations reflect those in the circulating blood. CSF and plasma samples were obtained serially over a 6-h period in 10 male combat veterans with chronic PTSD and 10 healthy male subjects through an indwelling subarachnoid catheter. Orexin-A concentrations were determined in plasma and CSF and CSF levels of the serotonin metabolite, 5-hydroxyindolacetic acid (5-HIAA), and the dopamine metabolite, homovanillic acid (HVA), were determined over the sampling period. CSF and plasma orexin-A concentrations were significantly lower in the patients with PTSD as compared with healthy comparison subjects at all time points. In addition, CSF orexin-A concentrations strongly and negatively correlated with PTSD severity as measured by the Clinician-Administered PTSD Scale (CAPS) in patients with PTSD. Peripheral and CNS concentrations of orexin-A were correlated in the healthy comparison subjects and peripheral orexin-A also correlated with CNS serotonergic tone. These findings suggest low central and peripheral orexin-A activity in patients with chronic PTSD are related to symptom severity and raise the possibility that orexin-A is part of the pathophysiological mechanisms of combat-related PTSD.

Functional NPY variation as a factor in stress resilience and alcohol consumption in rhesus macaques

CONTEXT

Neuropeptide Y (NPY) counters stress and is involved in neuroadaptations that drive escalated alcohol drinking in rodents. In humans, low NPY expression predicts amygdala response and emotional reactivity. Genetic variation that affects the NPY system could moderate stress resilience and susceptibility to alcohol dependence.

OBJECTIVE

To determine whether functional NPY variation influences behavioral adaptation to stress and alcohol consumption in a nonhuman primate model of early adversity (peer rearing).

DESIGN

We sequenced the rhesus macaque NPY locus (rhNPY) and performed in silico analysis to identify functional variants. We performed gel shift assays using nuclear extract from testes, brain, and hypothalamus. Levels of NPY in cerebrospinal fluid were measured by radioimmunoassay, and messenger RNA levels were assessed in the amygdala using real-time polymerase chain reaction. Animals were exposed to repeated social separation stress and tested for individual differences in alcohol consumption. Animals were genotyped for -1002 T > G, and the data were analyzed using analysis of variance.

SETTING

National Institutes of Health Animal Center. Subjects Ninety-six rhesus macaques. Main Outcome Measure Behavior arousal during social separation stress and ethanol consumption.

RESULTS

The G allele altered binding of regulatory proteins in all nuclear extracts tested, and -1002 T > G resulted in lower levels of NPY expression in the amygdala. Macaques exposed to adversity had lower cerebrospinal fluid NPY levels and exhibited higher levels of arousal during stress, but only as a function of the G allele. We also found that stress-exposed G allele carriers consumed more alcohol and exhibited an escalation in intake over cycles of alcohol availability and deprivation.

CONCLUSIONS

Our results suggest a role for NPY promoter variation in the susceptibility to alcohol use disorders and point to NPY as a candidate for examining gene x environment interactions in humans.

Substance P inhibits GABAB receptor signalling in the ventral tegmental area

Substance P (SP) and its receptors are involved in anxiety-related behaviours and regulate the intake of drugs of abuse and alcohol. Within the midbrain ventral tegmental area (VTA), a region that is clearly involved in the control of these behaviours, SP is released by stress and has been shown to trigger relapse. SP activates neurokinin (NK) receptors, which excites midbrain dopamine (DA) neurons and leads to increased DA in target regions. In this study, we have investigated the mechanisms underlying SP actions in the VTA, specifically investigating interactions between SP and GABA(B) receptors. We show that in VTA neurons, NK receptor activation closes an inwardly rectifying potassium channel, and moreover inhibits GABA(B) receptor-mediated transmission through an interaction that depends upon phospholipase C (PLC), intracellular calcium and protein kinase C (PKC).

Does the difference between physically active and couch potato lie in the dopamine system?

Obesity and other inactivity related diseases are increasing at an alarming rate especially in Western societies. Because of this, it is important to understand the regulating mechanisms involved in physical activity behavior. Much research has been done in regard to the psychological determinants of physical activity behavior; however, little is known about the underlying genetic and biological factors that may contribute to regulation of this complex trait. It is true that a significant portion of any trait is regulated by genetic and biological factors. In the case of voluntary physical activity behavior, these regulating mechanisms appear to be concentrated in the central nervous system. In particular, the dopamine system has been shown to regulate motor movement, as well as motivation and reward behavior. The pattern of regulation of voluntary physical activity by the dopamine system is yet to be fully elucidated. This review will summarize what is known about the dopamine system and regulation of physical activity, and will present a hypothesis of how this signaling pathway is mechanistically involved in regulating voluntary physical activity behavior. Future research in this area will aid in developing personalized strategies to prevent inactivity related diseases.

Energy expenditure and aging

The study of energy expenditure (EE) has deep roots in understanding aging and lifespan in all species. In humans, total EE decreases substantially in advanced age resulting from parallel changes in resting metabolic rate (RMR) and activity EE. For RMR, this reduction appears to be due to a reduction in organ mass and specific metabolic rates of individual tissues. However, these anatomical changes explain very little regarding the decline in activity EE, which is governed by both genetic and environmental sources. The biological control centers for activity EE are closely coupled with body mass fluctuations and seem to originate in the brain. Several candidate neuromodulators may be involved in the age-related reduction of activity EE that include: orexin, agouti-related proteins and dopaminergic pathways. Unfortunately, the existing body of research has primarily focused on how neuromodulators influence weight gain and only a few studies have been performed in aging models. Recent evidence suggests that activity EE has an important role in dictating lifespan and thus places emphasis on future research to uncover the underlying biological mechanisms. The study of EE continues to unlock clues to aging.

Ethanol action on dopaminergic neurons in the ventral tegmental area: interaction with intrinsic ion channels and neurotransmitter inputs

The dopaminergic system originating in the midbrain ventral tegmental area (VTA) has been extensively studied over the past decades as a critical neural substrate involved in the development of alcoholism and addiction to other drugs of abuse. Accumulating evidence indicates that ethanol modulates the functional output of this system by directly affecting the firing activity of VTA dopamine neurons, whereas withdrawal from chronic ethanol exposure leads to a reduction in the functional output of these neurons. This chapter will provide an update on the mechanistic investigations of the acute ethanol action on dopamine neuron activity and the neuroadaptations/plasticities in the VTA produced by previous ethanol experience.

Neuronal pathways linking substance P to drug addiction and stress

Accumulating evidence suggests that the neuropeptide substance P (SP) and its principal receptor neurokinin 1 (NK1) play a specific role in the behavioral response to opioids and stress that may help to initiate and maintain addictive behavior. In animal models, the NK1 receptor is required for opioids to produce their rewarding and motivational effects. SP neurotransmission is also implicated in the behavioral response to stress and in the process of drug sensitization, potentially contributing to vulnerability to addiction or relapse. However, SP neurotransmission only plays a minor role in opioid-mediated antinociception and the development of opioid tolerance. Moreover, the effects of SP on addiction-related behavior are selective for opioids and evidence supporting a role in the response to cocaine or psychostimulants is less compelling. This review will summarize the effects of SP neurotransmission on opioid-dependent behaviors and correlate them with potential contributing neural pathways. Specifically, SP neurotransmission within components of the basal forebrain particularly the nucleus accumbens and ventral pallidum as well as actions within the ascending serotonin system will be emphasized. In addition, cellular- or network-level interactions between opioids and SP signaling that may underlie the specificity of their relationship will be reviewed.

Convergent actions of orexin/hypocretin and CRF on dopamine neurons: Emerging players in addiction

The ventral tegmental area (VTA) is a brain region centrally involved in the development and expression of a variety of behaviors associated with drug use. Orexin/hypocretin (ox/hcrt) and corticotropin releasing factor (CRF) are both peptides released into the VTA, and an increasing number of studies show that both ox/hcrt and CRF in the VTA exert powerful modulatory effects on a variety of behaviors produced by drugs of abuse. Importantly, at a cellular and synaptic level, there is strong evidence supporting the hypothesis that these neuropeptides exert similar effects onto N-methyl-d-aspartate receptors (NMDARs) expressed in VTA neurons, suggesting a common molecular target in the actions of these peptides on this circuitry and ultimately on behaviors associated with drug use. In this review, we discuss emerging evidence that ox/hcrt and CRF directly target dopamine neurons by modulating excitatory synaptic activity, and that interaction of ox/hcrt and CRF may play a central role in addictive behaviors. Understanding these interactions in more detail may suggest new therapeutic targets in the treatment of relapse to substance abuse.

Deletion of agouti-related protein blunts ethanol self-administration and binge-like drinking in mice

The melanocortin (MC) system is composed of peptides that are cleaved from the polypeptide precursor proopiomelanocortin (POMC). Recent pharmacological and genetic evidence suggests that melanocortin receptor (MCR) signaling modulates neurobiological responses to ethanol and ethanol intake. Agouti-related protein (AgRP) is synthesized by neurons in the arcuate nucleus of the hypothalamus and is a natural antagonist of MCRs. Because central administration of the functionally active AgRP fragment AgRP-(83-132) increases ethanol intake by C57BL/6 J mice, we determined if mutant mice lacking normal production of AgRP (AgRP(-/-)) and maintained on a C57BL/6 J genetic background would show reduced self-administration of ethanol relative to littermate wild-type (AgRP(+/+)) mice. AgRP(-/-) mice showed reduced 8% (v/v) ethanol-reinforced lever-pressing behavior relative to AgRP(+/+) mice in daily 2-h sessions, but normal sucrose-, saccharin- and water-reinforced lever-pressing. Similarly, AgRP(-/-) mice showed reduced consumption of 8% ethanol in a two-bottle limited access test (2 h/day), although this effect was largely sex-dependent. Using drinking-in-the-dark (DID) procedures, AgRP(-/-) mice showed blunted binge-like drinking of 20% (v/v) ethanol which was associated with lower blood ethanol levels (85 mg/dl) relative to AgRP(+/+) mice (133 mg/dl) after 4 h of intake. AgRP(-/-) mice showed normal ethanol metabolism and did not show altered sensitivity to the sedative effects of ethanol. These observations with genetically altered mice are consistent with previous pharmacological data and suggest that endogenous AgRP signaling modulates the reinforcing properties of ethanol and binge-like ethanol drinking.

Hypocretin mechanisms in nicotine addiction: evidence and speculation

BACKGROUND

The hypocretin/orexin system has been implicated in arousal mechanisms, sleep, and sleep disorders, including narcolepsy, and more recently in drug addiction. Theoretically, hypocretin (hcrt) mechanisms appear to be potential substrates for nicotine addiction: arousal and attentional mechanisms influence use and withdrawal symptoms, and hcrt systems overlap anatomically with a number of brain regions associated with nicotine addiction.

OBJECTIVE

This review summarizes the studies that have examined hcrt mechanisms in the effects of nicotine and describes hcrt innervation of, and effects in, several brain regions implicated in nicotine addiction. The review speculates on the possible mechanisms by which hcrt may contribute to nicotine addiction in these regions, with the objective of encouraging research in this area.

RESULTS

In a small literature, both experimenter-administered and self-administered nicotine have been shown to elicit or depend on hcrt signaling. However, although untested in experimental designs, there is compelling evidence that hcrt mechanisms in the ventral tegmental area, the pontine region, thalamocortical circuits, the prefrontal cortex, and the amygdala could have a broad influence on nicotine addiction.

CONCLUSIONS

Evidence reviewed leads to the conclusion that hcrt mechanisms could mediate several dimensions of nicotine addiction, including a multi-faceted regulation of mesocorticolimbic dopaminergic function, but beyond dopaminergic mechanisms, hcrt could influence nicotine use and relapse during abstinence through broadly based arousal/attentional effects. These speculative ideas need to be examined experimentally; the potential gains are a more thorough understanding of the pathophysiology of nicotine addiction, and the discovery of novel targets for the development of pharmacotherapeutics.

Orexin-1 receptor antagonism decreases ethanol consumption and preference selectively in high-ethanol--preferring Sprague--Dawley rats

Work from our laboratory has shown that orexin (ORX; or hypocretin) neurons in the lateral hypothalamus are involved in preference for morphine, cocaine, and food. Other groups have demonstrated a connection between the ORX system and ethanol-related behaviors. Here, we extended those results to investigate, in outbred Sprague-Dawley rats, the relationship between ethanol preference and the ORX system. In Experiment 1, rats were trained to drink 10% ethanol using the intermittent access (IA) technique. In Experiment 2, different groups of rats were trained to drink 10% ethanol using either IA or the sucrose-fade (SF) technique. Following ethanol-drinking acquisition, ethanol preference was assessed using two-bottle-choice tests. The rats were then tested for changes in preference with additional two-bottle-choice tests following administration of the orexin-1 receptor antagonist SB-334867 (SB; 30 mg/kg, intraperitoneally). Differences in ethanol preference were observed across individuals, with a significantly higher ethanol preference observed in rats trained to drink using IA compared with SF. In both Experiments 1 and 2, SB reduced ethanol preference selectively in rats with high ethanol preference. These results demonstrate a strong, causal relationship between the ORX system and ethanol preference in outbred rats. These findings provide additional evidence that the ORX system provides opportunities to develop novel treatments for alcohol abuse.

CRF enhancement of GIRK channel-mediated transmission in dopamine neurons

Dopamine neurons in the ventral midbrain contribute to learning and memory of natural and drug-related rewards. Corticotropin-releasing factor (CRF), a stress-related peptide, is thought to be involved in aspects of relapse following drug withdrawal, but the cellular actions are poorly understood. This study investigates the action of CRF on G-protein-linked inhibitory postsynaptic currents (IPSCs) mediated by GIRK (Kir3) channels in dopamine neurons. CRF enhanced the amplitude and slowed the kinetics of IPSCs following activation of D2-dopamine and GABA(B) receptors. This action was postsynaptic and dependent on the CRF(1) receptor. The enhancement induced by CRF was attenuated by repeated in vivo exposures to psychostimulants or restraint stress. The results indicate that CRF influences dopamine- and GABA-mediated inhibition in the midbrain, suggesting implications for the chronic actions of psychostimulants and stress on dopamine-mediated behaviors.

Chronic cocaine enhances corticotropin-releasing factor-dependent potentiation of excitatory transmission in ventral tegmental area dopamine neurons

Current concepts suggest that stress-induced release of neuromodulators such as corticotropin-releasing factor (CRF) can drive drug-dependent behaviors. Although previous drug exposure can enhance behavioral and neurochemical responses to stress, it is unclear how such drug exposure alters the CRF modulation of excitatory synapses onto ventral tegmental area (VTA) dopamine neurons, a key locus of drug- and stress-induced neuroadaptation. Here, we demonstrate that, after repeated cocaine exposure, the magnitude and duration of the CRF-induced potentiation of NMDA receptor (NMDAR)-mediated neurotransmission was significantly increased compared with naive and saline-treated mice. Furthermore, CRF enhanced AMPA receptor (AMPAR)-mediated transmission only in mice that were exposed to cocaine. Increased frequency of AMPAR-mediated spontaneous miniature EPSCs and the intracellular blockade of CRF potentiation of AMPAR-mediated transmission suggest both presynaptic and postsynaptic effects of CRF. Importantly, pharmacological experiments revealed that CRF receptor 1 and protein kinase A pathways were newly recruited after repeated cocaine for the enhancement of CRF-induced NMDAR potentiation and the appearance of AMPAR potentiation. Thus, enhanced CRF-induced potentiation of excitatory synaptic transmission onto VTA dopamine neurons after cocaine preexposure is likely to produce an abnormal increase in dopamine release during stressful events and could augment activation of addictive behaviors in response to stress.

Responses of limbic and extrapyramidal substance P systems to nicotine treatment

RATIONALE

Neuropeptides are linked to the psychopathology of stimulants of abuse, principally through dopamine mechanisms. Substance P (SP) is one of these neuropeptides and is associated with both limbic and extrapyramidal dopaminergic pathways and likely contributes to the pharmacology of these stimulants. The effects of nicotine on these dopamine systems have also been extensively studied; however, its effects on the associated SP pathways have received little attention.

OBJECTIVES

In the present study, we elucidated the effects of nicotine treatment on limbic and extrapyramidal SP systems by measuring changes in associated SP tissue concentrations.

MATERIALS AND METHODS

Male Sprague-Dawley rats received (+/-)nicotine 4.0 mg/kg/day (0.8 mg/kg, intraperitoneally; five injections at 2-h intervals) in the presence or absence of selective dopamine D1 and D2 receptor antagonists or a nonselective nicotinic acetylcholine receptor antagonist.

RESULTS

The nicotine treatment significantly but temporarily decreased substance P-like immunoreactivity (SPLI) content in the ventral tegmental area (VTA) and substantia nigra 12-18 h after drug exposure. The nicotine-mediated changes in SPLI were selectively blocked by pretreatment with mecamylamine as well as a dopamine D1, D2, or both receptor antagonists. Other brain areas that also selectively demonstrated nicotine-related declines in SPLI content included prefrontal cortex, the nucleus accumbens shell, and the very posterior caudate.

CONCLUSIONS

These findings indicate that some limbic and basal ganglia SP systems are significantly affected by exposure to nicotine through processes mediated by nicotinic and dopaminergic receptors, suggesting a role for SP pathways in nicotine's limbic and extrapyramidal effects.

Effect of Inhibition of the Central Nucleus of the Amygdala and Drug Experience on the Regions Underlying Footshock-Induced Reinstatement of Morphine Seeking

This study assessed the effect of inhibition of the central nucleus of the amygdala (CeA) and drug experience on brain regions underlying footshock-induced reinstatement of morphine-seeking behaviour in rats. The difference in time spent in two chambers of a place-preference apparatus was used to measure morphine-conditioned place preference. Fos was measured as a marker of neuronal activation in the ventral bed nucleus of the stria terminalis (BNSTv) and ventral tegmental area (VTA). Footshock was found to enhance Fos expression in the BNSTv regardless of drug experience. In the VTA, morphine and footshock had an interactive effect on the increase in Fos expression. Inhibition of the CeA decreased Fos expression in the BNSTv regardless of drug experience, whereas in the VTA this effect only occurred in morphine-treated rats. These results suggest that drug experience has no differential effect on the BNSTv however morphine produces footshock sensitization in the VTA. CeA inhibition modulates the footshock-induced activity of these regions of the brain and attenuates reinstatement of drug seeking behaviour.

CRF1 receptor activation increases the response of neurons in the basolateral nucleus of the amygdala to afferent stimulation

The basolateral nucleus (BLA) of the amygdala contributes to the consolidation of memories for emotional or stressful events. The nucleus contains a high density of CRF1 receptors that are activated by corticotropin-releasing factor (CRF). Modulation of the excitability of neurons in the BLA by CRF may regulate the immediate response to stressful events and the formation of associated memories. In the present study, CRF was found to increase the amplitude of field potentials recorded in the BLA following excitatory afferent stimulation, in vitro. The increase was mediated by CRF1 receptors, since it could be blocked by the selective, non-peptide antagonists, NBI30775 and NBI35583, but not by the CRF2-selective antagonist, astressin 2B. Furthermore, the CRF2-selective agonist, urocortin II had no effect on field potential amplitude. The increase induced by CRF was long-lasting, could not be reversed by subsequent administration of NBI35583, and required the activation of protein kinase C. This effect of CRF in the BLA may be important for increasing the salience of aversive stimuli under stressful conditions, and for enhancing the consolidation of associated memories. The results provide further justification for studying the efficacy of selective antagonists of the CRF1 receptor to reduce memory formation linked to emotional or traumatic events, and suggest that these compounds might be useful as prophylactic treatments for stress-related illnesses such as post-traumatic stress disorder.

Corticotropin-releasing factor binding protein within the ventral tegmental area is expressed in a subset of dopaminergic neurons

Corticotropin-releasing factor (CRF) and related peptides play a role in mediating neuronal effects of stress. These peptides mediate stress responses by their interactions with the CRF receptors and the CRF-binding protein (CRF-BP). Because the CRF-BP is implicated in neurotransmission within the ventral tegmental area (VTA), we investigated whether the CRF-BP is expressed in VTA neurons. By in situ hybridization, we detected cellular expression of CRF-BP mRNA in the VTA; no such expression was seen in neighboring substantia nigra pars compacta (SNC) or substantia nigra pars reticulata. By double in situ hybridization, we determined that VTA neurons with CRF-BP mRNA coexpressed transcripts encoding either tyrosine hydroxylase [TH; a marker for dopamine (DA) neurons] or glutamic acid decarboxylase [GAD; synthesizing enzyme of gamma-aminobutyric acid (GABA)]. Neurons with CRF-BP mRNA represented 25% of the total population of TH-expressing neurons and 28% of the total population of GAD-expressing neurons, indicating that discrete subpopulations of dopaminergic and GABAergic neurons are present in the VTA. Within the total population of neurons containing CRF-BP mRNA, 70% coexpressed TH mRNA and only 27% coexpressed GAD mRNA. As far as we are aware, we provide the first anatomical evidence that a molecule, CRF-BP, is encoded by DAergic neurons of the VTA but not by those of the SNC. We propose, based on the observation that the majority of VTA neurons expressing CRF-BP mRNA are DAergic, that in the VTA interactions of CRF-BP with CRF, or with CRF-related peptides, are likely to be mediated predominantly by DAergic neurons.

CRF facilitates calcium release from intracellular stores in midbrain dopamine neurons

Changes in cytosolic calcium are crucial for numerous processes including neuronal plasticity. This study investigates the regulation of cytosolic calcium by corticotropin-releasing factor (CRF) in midbrain dopamine neurons. The results demonstrate that CRF stimulates the release of intracellular calcium from stores through activation of adenylyl cyclase and PKA. Imaging and photolysis experiments showed that the calcium originated from dendrites and required both functional IP3 and ryanodine receptor channels. The elevation in cytosolic calcium potentiated calcium-sensitive potassium channels (sK) activated by action potentials and metabotropic Gq-coupled receptors for glutamate and acetylcholine. This increase in cytosolic calcium activated by postsynaptic Gs-coupled CRF receptors may represent a fundamental mechanism by which stress peptides and hormones can shape Gq-coupled receptor-mediated regulation of neuronal excitability and synaptic plasticity in dopamine neurons.

Synapses between corticotropin-releasing factor-containing axon terminals and dopaminergic neurons in the ventral tegmental area are predominantly glutamatergic

Interactions between stress and the mesocorticolimbic dopamine (DA) system have been suggested from behavioral and electrophysiological studies. Because corticotropin-releasing factor (CRF) plays a role in stress responses, we investigated possible interactions between neurons containing CRF and those producing DA in the ventral tegmental area (VTA). We first investigated the cellular distribution of CRF in the VTA by immunolabeling VTA sections with anti-CRF antibodies and analyzing these sections by electron microscopy. We found CRF immunoreactivity present mostly in axon terminals establishing either symmetric or asymmetric synapses with VTA dendrites. We established that nearly all CRF asymmetric synapses are glutamatergic, insofar as the CRF-immunolabeled axon terminals in these synapses coexpressed the vesicular glutamate transporter 2, and that the majority of CRF symmetric synapses are GABAergic, insofar as the CRF-immunolabeled axon terminals in these synapses coexpressed glutamic acid decarboxylase, findings that are of functional importance. We then looked for synaptic interactions between CRF- and DA-containing neurons, by using antibodies against CRF and tyrosine hydroxylase (TH; a marker for DA neurons). We found that most synapses between CRF-immunoreactive axon terminals and TH neurons are asymmetric (in the majority likely to be glutamatergic) and suggest that glutamatergic neurons containing CRF may be part of the neuronal circuitry that mediates stress responses involving the mesocorticolimbic DA system. The presence of CRF synapses in the VTA offers a mechanism for interactions between the stress-associated neuropeptide CRF and the mesocorticolimbic DA system.

Corticotropin-releasing factor increases mouse ventral tegmental area dopamine neuron firing through a protein kinase C-dependent enhancement of Ih

Stress induces the release of the peptide corticotropin-releasing factor (CRF) into the ventral tegmental area (VTA), and also increases dopamine levels in brain regions receiving dense VTA input. Therefore, stress may activate the mesolimbic dopamine system in part through the actions of CRF in the VTA. Here, we explored the mechanism by which CRF affects VTA dopamine neuron firing. Using patch-clamp recordings from brain slices we first determined that the presence of I(h) is an excellent predictor of dopamine content in mice. We next showed that CRF dose-dependently increased VTA dopamine neuron firing, which was prevented by antagonism of the CRF receptor-1 (CRF-R1), and was mimicked by CRF-R1 agonists. Inhibition of the phospholipase C (PLC)-protein kinase C (PKC) signalling pathway, but not the cAMP-protein kinase A (PKA) signalling pathway, prevented the increase in dopamine neuron firing by CRF. Furthermore, the effect of CRF on VTA dopamine neurons was not attenuated by blockade of I(A), I(K(Ca)) or I(Kir), but was completely eliminated by inhibition of I(h). Although cAMP-dependent modulation of I(h) through changes in the voltage dependence of activation is well established, we surprisingly found that CRF, through a PKC-dependent mechanism, enhanced I(h) independent of changes in the voltage dependence of activation. Thus, our results demonstrated that CRF acted on the CRF-R1 to stimulate the PLC-PKC signalling pathway, which in turn enhanced I(h) to increase VTA dopamine neuron firing. These findings provide a cellular mechanism of the interaction between CRF and dopamine, which can be involved in promoting the avoidance of threatening stimuli, the pursuit of appetitive behaviours, as well as various psychiatric conditions.

Stressor- and corticotropin releasing factor-induced reinstatement and active stress-related behavioral responses are augmented following long-access cocaine self-administration by rats

RATIONALE

Stressful events during periods of drug abstinence likely contribute to relapse in cocaine-dependent individuals. Excessive cocaine use may increase susceptibility to stressor-induced relapse through alterations in brain corticotropin-releasing factor (CRF) responsiveness.

OBJECTIVES

This study examined stressor- and CRF-induced cocaine seeking and other stress-related behaviors in rats with different histories of cocaine self-administration (SA).

MATERIALS AND METHODS

Rats self-administered cocaine under short-access (ShA; 2 h daily) or long-access (LgA; 6 h daily) conditions for 14 days or were provided access to saline and were tested for reinstatement by a stressor (electric footshock), cocaine or an icv injection of CRF and for behavioral responsiveness on the elevated plus maze, in a novel environment and in the light-dark box after a 14- to 17-day extinction/withdrawal period.

RESULTS

LgA rats showed escalating patterns of cocaine SA and were more susceptible to reinstatement by cocaine, EFS, or icv CRF than ShA rats. Overall, cocaine SA increased activity in the center field of a novel environment, on the open arms of the elevated plus maze, and in the light compartment of a light-dark box. In most cases, the effects of cocaine SA were dependent on the pattern/amount of cocaine intake with statistically significant differences from saline self-administering controls only observed in LgA rats.

CONCLUSIONS

When examined after several weeks of extinction/withdrawal, cocaine SA promotes a more active pattern of behavior during times of stress that is associated with a heightened susceptibility to stressor-induced cocaine-seeking behavior and may be the consequence of augmented CRF regulation of addiction-related neurocircuitry.

Neuropeptidergic mediators of spontaneous physical activity and non-exercise activity thermogenesis

Lean individuals have high levels of spontaneous physical activity (SPA) and the energy expenditure derived from that activity, termed non-exercise activity thermogenesis or NEAT, appears to protect them from obesity. Conversely, obesity in different human populations is characterized by low levels of SPA and NEAT. Like in humans, elevated SPA in rats appears to protect against obesity: obesity-resistant rats have significantly greater SPA and NEAT than obesity-prone rats. We review the literature on brain mechanisms important in mediating SPA and NEAT. The focus is on neuropeptides, including cholecystokinin, corticotropin-releasing hormone (also known as corticotropin-releasing factor), neuromedin U, neuropeptide Y, leptin, agouti-related protein, orexin-A (also known as hypocretin-1), and ghrelin. We also review information regarding interactions between these neuropeptides and dopamine, a neurotransmitter important in mediating motor function. Finally, we present evidence that elevated signaling of pathways mediating SPA and NEAT may protect against weight gain and obesity.

Orexin axons in the rat ventral tegmental area synapse infrequently onto dopamine and gamma-aminobutyric acid neurons

Cells in the ventral tegmental area (VTA) facilitate motivated behaviors, and the activity of VTA neurons is regulated by dense projections from the lateral hypothalamic area (LHA). Orexin (Orx) neurons in the lateral and perifornical hypothalamus play important roles in arousal, feeding, and energy metabolism. Orx cells contribute substantially to the LHA projection to the rat midbrain. However, the morphological features of Orx fibers in the VTA and whether they synapse onto dopamine (DA) or gamma-aminobutyric acid (GABA) neurons have not yet been investigated. We utilized immunoperoxidase and immunogold-silver staining to examine the morphological features and synaptic incidence of Orx-labeled axons in the VTA. We then combined immunoperoxidase labeling for Orx with immunogold-silver labeling for GABA or for tyrosine hydroxylase (TH) in DA neurons. Electron microscopic analysis revealed that most Orx-labeled axons in the VTA were passing fibers. The less common Orx varicosities were occasionally apposed to TH- or GABA-labeled dendrites without synapsing. Only a small proportion of Orx-positive axons synapsed onto dendrites or soma. The synapses included both asymmetric and symmetric types and targeted TH- and GABA-labeled profiles with equal frequency. These findings suggest that most Orx fibers in the VTA are axons passing to caudal brainstem structures. However, Orx does mediate some direct synaptic influence on VTA DA and GABA neurons. Additional nonsynaptic effects are suggested by the presence of numerous dense-cored vesicles. These studies have important implications for understanding the mechanisms whereby Orx can alter behavior through regulating VTA DA and GABA cell activity.

Systemic ghrelin sensitizes cocaine-induced hyperlocomotion in rats

The feeding-relevant pathway by which food restriction (FR) augments cocaine action is unknown. Systemic administration of the 28-amino acid acylated peptide ghrelin (1-10 nmol) increases food intake in rats and circulating levels of rat ghrelin are up-regulated by FR. The present experiment examined the impact of repeated administration of ghrelin or vehicle on the subsequent capacity of cocaine to enhance locomotion in rats. Male Sprague-Dawley rats were pretreated daily for seven days with 0, 5 or 10 nmol rat ghrelin (i.p.) in the home cage. On the 8th day, rats were transported to a testing room, placed in a locomotion chamber for 15 min, and then injected (i.p.) with 0, 7.5, or 15 mg/kg cocaine hydrochloride. Locomotor activity was monitored over a 45 min post-cocaine period. Pretreatment with 5 or 10 nmol ghrelin alone did not significantly increase basal locomotion relative to that of the 0 nmol ghrelin group. Rats pretreated with 5 nmol or 10 nmol ghrelin showed an enhanced locomotor response after treatment with 15 mg/kg cocaine relative to rats treated with 0 nmol ghrelin. These results indicate that acute injection of ghrelin, at a feeding-relevant dose, can augment the acute effects of cocaine on locomotion in rats.

Is dopamine a physiologically relevant mediator of feeding behavior?

The hypothalamus integrates various hormonal and neuronal signals to regulate appetite and metabolism and thereby serves a homeostatic purpose in the regulation of body weight. Additional neural circuits that are superimposed on this system have the potential to override the homeostatic signals, resulting in either gluttony or anorexia at the extremes. Midbrain dopamine neurons have long been implicated in mediating reward behavior and the motivational aspects of feeding behavior. Recent results reveal that hormones implicated in regulating the homeostatic system also impinge directly on dopamine neurons; for example, leptin and insulin directly inhibit dopamine neurons, whereas ghrelin activates them. Here, I discuss the predictions and implications of these new findings as they relate to dopamine signaling and the physiology of appetite control.