Effects of corticotropin-releasing factor on neuronal activity in the serotonergic dorsal raphe nucleus

Endogenous Opioids: The Downside of Opposing Stress

Our dynamic environment regularly exposes us to potentially life-threatening challenges or stressors. To answer these challenges and maintain homeostasis, the stress response, an innate coordinated engagement of central and peripheral neural systems is initiated. Although essential for survival, the inappropriate initiation of the stress response or its continuation after the stressor is terminated has pathological consequences that have been linked to diverse neuropsychiatric and medical diseases. Substantial individual variability exists in the pathological consequences of stressors. A theme of this Special Issue is that elucidating the basis of individual differences in resilience or its flipside, vulnerability, will greatly advance our ability to prevent and treat stress-related diseases. This can be approached by studying individual differences in "pro-stress" mediators such as corticosteroids or the hypothalamic orchestrator of the stress response, corticotropin-releasing factor. More recently, the recognition of endogenous neuromodulators with "anti-stress" activity that have opposing actions or that restrain stress-response systems suggests additional bases for individual differences in stress pathology. These "anti-stress" neuromodulators offer alternative strategies for manipulating the stress response and its pathological consequences. This review uses the major brain norepinephrine system as a model stress-response system to demonstrate how co-regulation by opposing pro-stress (corticotropin-releasing factor) and anti-stress (enkephalin) neuromodulators must be fine-tuned to produce an adaptive response to stress. The clinical consequences of tipping this fine-tuned balance in the direction of either the pro- or anti-stress systems are emphasized. Finally, that each system provides multiple points at which individual differences could confer stress vulnerability or resilience is discussed.

Protracted effects of chronic stress on serotonin-dependent thermoregulation

Chronic stress is known to affect serotonin (5HT) neurotransmission in the brain and to alter body temperature. The body temperature is controlled in part, by the medial preoptic area (mPOA) of the hypothalamus. To investigate the effect of chronic stress on 5HT and how it affects body temperature regulation, we examined whether exposure to a chronic unpredictable stress (CUS) paradigm produces long-term alterations in thermoregulatory function of the mPOA through decreased 5HT neurotransmission. Adult male Sprague-Dawley rats underwent 21 d of CUS. Four days after the last stress exposure, basal body temperature in the home cage and body temperature in a cold room maintained at 10 °C were recorded. The CUS rats had significantly higher subcutaneous basal body temperature at 13:00 h compared to unstressed (NoStress) rats. Whereas the NoStress rats were able to significantly elevate body temperature from basal levels at 30 and 60 min of exposure to the cold room, the CUS rats showed a hypothermic response to the cold. Treatment during CUS with metyrapone, a corticosterone synthesis inhibitor, blocked stress-induced decrease in body temperature in response to the cold challenge. CUS also decreased 5HT transporter protein immunoreactivity in the mPOA and 5HT2A/C agonist injection into the mPOA after CUS exposure caused stressed rats to exhibit a sensitized hyperthermic response to cold. These results indicate that the CUS induced changes to the 5HTergic system alter mPOA function in thermoregulation. These findings help us to explain the mechanisms underlying chronic stress-induced disorders such as chronic fatigue syndrome wherein long lasting thermoregulatory deficits are observed.

Prevention of alcohol-heightened aggression by CRF-R1 antagonists in mice: critical role for DRN-PFC serotonin pathway

Alcohol can escalate aggressive behavior in a significant subgroup of rodents, humans, and nonhuman primates. The present study investigated whether blockade of corticotropin-releasing factor receptor type 1 (CRF-R1) could prevent the emergence of alcohol-heightened aggression in mice. The serotonin (5-HT) pathway from the dorsal raphe nucleus (DRN) to the medial prefrontal cortex (mPFC) by CRF-R1 was investigated as a possible target for the prevention of alcohol-heightened aggressive behavior. Male CFW mice that reliably exhibited aggressive behaviors after consuming 1 g/kg of alcohol received systemic or intra-DRN administration of CRF-R1 antagonists, CP-154,526 or MTIP, before a confrontation with a male conspecific. Blockade of DRN CRF-R1 receptors with both antagonists significantly reduced only alcohol-heightened aggression, whereas systemic administration reduced both alcohol-heightened and species-typical aggression. Next, a 5-HT1A agonist, 8-OH-DPAT, was coadministered with CP-154,526 into the DRN to temporarily disrupt 5-HT activity. This manipulation abolished the antiaggressive effects of intra-DRN CP-154,526. In the mPFC, in vivo microdialysis revealed that extracellular 5-HT levels were increased in mice that consumed alcohol and were then injected with CP-154,526, both systemically or intra-DRN. Neither alcohol nor CP-154,526 alone affected 5-HT release in the mPFC. The present results suggest the DRN as a critical site for CRF-R1 to modulate alcohol-heightened aggression via action on the serotonergic DRN-PFC pathway.

The Deakin/Graeff hypothesis: focus on serotonergic inhibition of panic

The Deakin/Graeff hypothesis proposes that different subpopulations of serotonergic neurons through topographically organized projections to forebrain and brainstem structures modulate the response to acute and chronic stressors, and that dysfunction of these neurons increases vulnerability to affective and anxiety disorders, including panic disorder. We outline evidence supporting the existence of a serotonergic system originally discussed by Deakin/Graeff that is implicated in the inhibition of panic-like behavioral and physiological responses. Evidence supporting this panic inhibition system comes from the following observations: (1) serotonergic neurons located in the 'ventrolateral dorsal raphe nucleus' (DRVL) as well as the ventrolateral periaqueductal gray (VLPAG) inhibit dorsal periaqueductal gray-elicited panic-like responses; (2) chronic, but not acute, antidepressant treatment potentiates serotonin's panicolytic effect; (3) contextual fear activates a central nucleus of the amygdala-DRVL/VLPAG circuit implicated in mediating freezing and inhibiting panic-like escape behaviors; (4) DRVL/VLPAG serotonergic neurons are central chemoreceptors and modulate the behavioral and cardiorespiratory response to panicogenic agents such as sodium lactate and CO2. Implications of the panic inhibition system are discussed.

Functional antagonism between nociceptin/orphanin FQ and corticotropin-releasing factor in rat anxiety-related behaviors: involvement of the serotonergic system

Nociceptin/orphanin FQ (N/OFQ) acts as an anxiolytic-like agent in the rat and behaves as a functional antagonist of corticotropin-releasing factor (CRF) due to its ability to oppose CRF biological actions. In response to stress, CRF triggers changes in neurotransmitter systems including serotonin (5-HT). The role of 5-HT1A receptor in anxiety has been supported by preclinical and clinical studies. The present study investigated the possible functional antagonism between N/OFQ (1nmol/rat) and CRF (0.2nmol/rat) in anxiety-related conditions in rats, using elevated plus maze and defensive burying tests, in order to confirm previous literature results. Moreover, possible changes in the serotonergic system were studied in areas rich of serotonergic neurons: frontal cortex and pons. In both tests N/OFQ showed anxiolytic-like effects while CRF displayed anxiogenic-like effects. N/OFQ before CRF treatment counteracted the anxiogenic-like effects evoked by CRF. In frontal cortex, N/OFQ significantly decreased 5-HT levels but did not modify the hydroxyindoleacetic acid (5-HIAA) ones; CRF modified neither 5-HT nor 5-HIAA content but counteracted changes induced by N/OFQ alone. In pons, N/OFQ induced no change in serotonergic activity while CRF significantly decreased 5-HT levels and increased 5-HIAA content. The two peptides' combination reinstated serotonergic parameters to controls. In frontal cortex, N/OFQ increased the 5HT1A receptor density but reduced its affinity, while CRF alone did not induce any change. In pons, CRF decreased 5HT1ABmax and KD whereas N/OFQ was ineffective. All biochemical modifications were reverted by N/OFQ plus CRF treatment. The present study confirms that N/OFQ counteracts CRF anxiogenic-like effects in the behavioral tests evaluated. These effects may involve central serotonergic mechanisms since N/OFQ plus CRF induces a reversion of serotonergic changes provoked by single peptide. Our data support the hypothesis that N/OFQ may behave as functional CRF antagonist, this action being of interest for the treatment of anxiety disorders.

Localisation and stress-induced plasticity of GABAA receptor subunits within the cellular networks of the mouse dorsal raphe nucleus

The dorsal raphe nucleus (DRN) provides the major source of serotonin to the central nervous system (CNS) and modulates diverse neural functions including mood. Furthermore, DRN cellular networks are engaged in the stress-response at the CNS level allowing for adaptive behavioural responses, whilst stress-induced dysregulation of DRN and serotonin release is implicated in psychiatric disorders. Therefore, identifying the molecules regulating DRN activity is fundamental to understand DRN function in health and disease. GABAA receptors (GABAARs) allow for brain region, cell type and subcellular domain-specific GABA-mediated inhibitory currents and are thus key regulators of neuronal activity. Yet, the GABAAR subtypes expressed within the neurochemically diverse cell types of the mouse DRN are poorly described. In this study, immunohistochemistry and confocal microscopy revealed that all serotonergic neurons expressed immunoreactivity for the GABAAR alpha2 and 3 subunits, although the respective signals were co-localised to varying degrees with inhibitory synaptic marker proteins. Only a topographically located sub-population of serotonergic neurons exhibited GABAAR alpha1 subunit immunoreactivity. However, all GABAergic as well as non-GABAergic, non-serotonergic neurons within the DRN expressed GABAAR alpha1 subunit immunoreactivity. Intriguingly, immunoreactivity for the GABAAR gamma2 subunit was enriched on GABAergic rather than serotonergic neurons. Finally, repeated restraint stress increased the expression of the GABAAR alpha3 subunit at the mRNA and protein level. The study demonstrates the identity and location of distinct GABAAR subunits within the cellular networks of the mouse DRN and that stress impacts on the expression levels of particular subunits at the gene and protein level.

Computational modeling of spike generation in serotonergic neurons of the dorsal raphe nucleus

Serotonergic neurons of the dorsal raphe nucleus, with their extensive innervation of limbic and higher brain regions and interactions with the endocrine system have important modulatory or regulatory effects on many cognitive, emotional and physiological processes. They have been strongly implicated in responses to stress and in the occurrence of major depressive disorder and other psychiatric disorders. In order to quantify some of these effects, detailed mathematical models of the activity of such cells are required which describe their complex neurochemistry and neurophysiology. We consider here a single-compartment model of these neurons which is capable of describing many of the known features of spike generation, particularly the slow rhythmic pacemaking activity often observed in these cells in a variety of species. Included in the model are 11 kinds of ion channels: a fast sodium current INa, a delayed rectifier potassium current IKDR, a transient potassium current IA, a slow non-inactivating potassium current IM, a low-threshold calcium current IT, two high threshold calcium currents IL and IN, small and large conductance potassium currents ISK and IBK, a hyperpolarization-activated cation current IH and a leak current ILeak. In Sections 3-8, each current type is considered in detail and parameters estimated from voltage clamp data where possible. Three kinds of model are considered for the BK current and two for the leak current. Intracellular calcium ion concentration Cai is an additional component and calcium dynamics along with buffering and pumping is discussed in Section 9. The remainder of the article contains descriptions of computed solutions which reveal both spontaneous and driven spiking with several parameter sets. Attention is focused on the properties usually associated with these neurons, particularly long duration of action potential, steep upslope on the leading edge of spikes, pacemaker-like spiking, long-lasting afterhyperpolarization and the ramp-like return to threshold after a spike. In some cases the membrane potential trajectories display doublets or have humps or notches as have been reported in some experimental studies. The computed time courses of IA and IT during the interspike interval support the generally held view of a competition between them in influencing the frequency of spiking. Spontaneous activity was facilitated by the presence of IH which has been found in these neurons by some investigators. For reasonable sets of parameters spike frequencies between about 0.6Hz and 1.2Hz are obtained, but frequencies as high as 6Hz could be obtained with special parameter choices. Topics investigated and compared with experiment include shoulders, notches, anodal break phenomena, the effects of noradrenergic input, frequency versus current curves, depolarization block, effects of cell size and the effects of IM. The inhibitory effects of activating 5-HT1A autoreceptors are also investigated. There is a considerable discussion of in vitro versus in vivo firing behavior, with focus on the roles of noradrenergic input, corticotropin-releasing factor and orexinergic inputs. Location of cells within the nucleus is probably a major factor, along with the state of the animal.

Individual differences in the neurobiology of social stress: implications for depression-cardiovascular disease comorbidity

Stress initiates a cascade of complex neural and peripheral changes that promote healthy adaption to stress, but when unabated, leads to pathology. Fascinating individual differences arise in the ability to cope with a stressor, rendering an individual more or less likely to develop stress-induced pathologies such as depression, anxiety, and cardiovascular disease. In this review we evaluate recent findings that investigate the neural underpinnings of adopting a passive or active coping response during social defeat stress. Because passive coping is associated with vulnerability to stress-related pathologies and active coping confers resiliency, understanding neurobiological adaptations associated with these diverse coping strategies may reveal biomarkers or targets impacting stress susceptibility. The co-occurrence of stress-induced depression and cardiovascular disease is becoming increasingly clear. Therefore this review focuses on the central mechanisms capable of contributing to psychopathology and cardiovascular disease such as corticotropin releasing factor, neuropeptide Y, monoamines, cytokines and oxidative stress. The impetus for this review is to highlight neurobiological systems that warrant further evaluation for their contribution to the pathophysiology of depression-cardiovascular disease comorbidity.

Relaxin-3/RXFP3 networks: an emerging target for the treatment of depression and other neuropsychiatric diseases?

Animal and clinical studies of gene-environment interactions have helped elucidate the mechanisms involved in the pathophysiology of several mental illnesses including anxiety, depression, and schizophrenia; and have led to the discovery of improved treatments. The study of neuropeptides and their receptors is a parallel frontier of neuropsychopharmacology research and has revealed the involvement of several peptide systems in mental illnesses and identified novel targets for their treatment. Relaxin-3 is a newly discovered neuropeptide that binds, and activates the G-protein coupled receptor, RXFP3. Existing anatomical and functional evidence suggests relaxin-3 is an arousal transmitter which is highly responsive to environmental stimuli, particularly neurogenic stressors, and in turn modulates behavioral responses to these stressors and alters key neural processes, including hippocampal theta rhythm and associated learning and memory. Here, we review published experimental data on relaxin-3/RXFP3 systems in rodents, and attempt to highlight aspects that are relevant and/or potentially translatable to the etiology and treatment of major depression and anxiety. Evidence pertinent to autism spectrum and metabolism/eating disorders, or related psychiatric conditions, is also discussed. We also nominate some key experimental studies required to better establish the therapeutic potential of this intriguing neuromodulatory signaling system, including an examination of the impact of RXFP3 agonists and antagonists on the overall activity of distinct or common neural substrates and circuitry that are identified as dysfunctional in these debilitating brain diseases.

Increased anxiety in corticotropin-releasing factor type 2 receptor-null mice requires recent acute stress exposure and is associated with dysregulated serotonergic activity in limbic brain areas

Background

Corticotropin-releasing factor type 2 receptors (CRFR2) are suggested to facilitate successful recovery from stress to maintain mental health. They are abundant in the midbrain raphe nuclei, where they regulate serotonergic neuronal activity and have been demonstrated to mediate behavioural consequences of stress. Here, we describe behavioural and serotonergic responses consistent with maladaptive recovery from stressful challenge in CRFR2-null mice.

Results

CRFR2-null mice showed similar anxiety levels to control mice before and immediately after acute restraint stress, and also after cessation of chronic stress. However, they showed increased anxiety by 24 hours after restraint, whether or not they had been chronically stressed.

Serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) contents were quantified and the level of 5-HIAA in the caudal dorsal raphe nucleus (DRN) was increased under basal conditions in CRFR2-null mice, indicating increased 5-HT turnover. Twenty-four hours following restraint, 5-HIAA was decreased only in CRFR2-null mice, suggesting that they had not fully recovered from the challenge. In efferent limbic structures, CRFR2-null mice showed lower levels of basal 5-HT in the lateral septum and subiculum, and again showed a differential response to restraint stress from controls.

Local cerebral glucose utilization (LCMRglu) revealed decreased neuronal activity in the DRN of CRFR2-null mice under basal conditions. Following 5-HT receptor agonist challenge, LCMRglu responses indicated that 5-HT_1A receptor responses in the DRN were attenuated in CRFR2-null mice. However, postsynaptic 5-HT receptor responses in forebrain regions were intact.

Conclusions

These results suggest that CRFR2 are required for proper functionality of 5-HT_1A receptors in the raphe nuclei, and are key to successful recovery from stress. This disrupted serotonergic function in CRFR2-null mice likely contributes to their stress-sensitive phenotype. The 5-HT content in lateral septum and subiculum was notably altered. These areas are important for anxiety, and are also implicated in reward and the pathophysiology of addiction. The role of CRFR2 in stress-related psychopathologies deserves further consideration.

Role of GABAA receptors in dorsal raphe nucleus in stress-induced reinstatement of morphine-conditioned place preference in rats

RATIONALE

The serotonin (5-hydroxytryptamine, 5-HT) system plays an important role in stress-related psychiatric disorders and substance abuse. Our data indicate that stress inhibits the dorsal raphe nucleus (DRN)-5-HT system via stimulation of GABA synaptic activity by the stress neurohormone corticotropin-releasing factor and, more recently, that morphine history sensitizes DRN-5-HT neurons to GABAergic inhibitory effects of stress.

OBJECTIVES

We tested the hypothesis that DRN GABAA receptors contribute to stress-induced reinstatement of morphine-conditioned place preference (CPP).

METHODS

First, we tested if activation of GABAA receptors in the DRN would reinstate morphine CPP. Second, we tested if blockade of GABAA receptors in the DRN would attenuate swim stress-induced reinstatement of morphine CPP. CPP was induced by morphine (5 mg/kg) in a 4-day conditioning phase followed by a conditioning test. Upon acquiring conditioning criteria, subjects underwent 4 days of extinction training followed by an extinction test. Upon acquiring extinction criteria, animals underwent a reinstatement test. For the first experiment, the GABAA receptor agonist muscimol (50 ng) or vehicle was injected into the DRN prior to the reinstatement test. For the second experiment, the GABAA receptor antagonist bicuculline (75 ng) or vehicle was injected into the DRN prior to a forced swim stress, and then, animals were tested for reinstatement of CPP.

RESULTS

Intraraphe injection of muscimol reinstated morphine CPP, while intraraphe injection of bicuculline attenuated swim stress-induced reinstatement.

CONCLUSIONS

These data provide evidence that GABAA receptor-mediated inhibition of the serotonergic DRN contributes to stress-induced reinstatement of morphine CPP.

Corticotropin-releasing factor-related peptides, serotonergic systems, and emotional behavior

Corticotropin-releasing factor (CRF) is a 41-amino acid neuropeptide that is involved in stress-related physiology and behavior, including control of the hypothalamic-pituitary-adrenal (HPA) axis. Members of the CRF family of neuropeptides, including urocortin 1 (UCN 1), UCN 2, and UCN 3, bind to the G protein-coupled receptors, CRF type 1 (CRF₁) and CRF₂ receptors. In addition, CRF binding protein (CRFBP) binds both CRF and UCN 1 and can modulate their activities. There are multiple mechanisms through which CRF-related peptides may influence emotional behavior, one of which is through altering the activity of brainstem neuromodulatory systems, including serotonergic systems. CRF and CRF-related peptides act within the dorsal raphe nucleus (DR), the major source for serotonin (5-HT) in the brain, to alter the neuronal activity of specific subsets of serotonergic neurons and to influence stress-related behavior. CRF-containing axonal fibers innervate the DR in a topographically organized manner, which may contribute to the ability of CRF to alter the activity of specific subsets of serotonergic neurons. CRF and CRF-related peptides can either increase or decrease serotonergic neuronal firing rates and serotonin release, depending on their concentrations and on the specific CRF receptor subtype(s) involved. This review aims to describe the interactions between CRF-related peptides and serotonergic systems, the consequences for stress-related behavior, and implications for vulnerability to anxiety and affective disorders.

Opiate exposure and withdrawal dynamically regulate mRNA expression in the serotonergic dorsal raphe nucleus

Previous results from our lab suggest that hypofunctioning of the serotonergic (5-HT) dorsal raphe nucleus (DRN) is involved in stress-induced opiate reinstatement. To further investigate the effects of morphine dependence and withdrawal on the 5-HT DRN system, we measured gene expression at the level of mRNA in the DRN during a model of morphine dependence, withdrawal and post withdrawal stress exposure in rats. Morphine pellets were implanted for 72h and then either removed or animals were injected with naloxone to produce spontaneous or precipitated withdrawal, respectively. Animals exposed to these conditions exhibited withdrawal symptoms including weight loss, wet dog shakes and jumping behavior. Gene expression for brain-derived neurotrophic factor (BDNF), tyrosine kinase receptor B (TrkB), corticotrophin releasing-factor (CRF)-R1, CRF-R2, alpha 1 subunit of the GABAA receptor (GABAA-α1), μ-opioid receptor (MOR), 5-HT1A receptor, tryptophan hydroxylase2 (TPH2) and the 5-HT transporter was then measured using quantitative real-time polymerase chain reaction at multiple time-points across the model of morphine exposure, withdrawal and post withdrawal stress. Expression levels of BDNF, TrkB and CRF-R1 mRNA were decreased during both morphine exposure and following 7days of withdrawal. CRF-R2 mRNA expression was elevated after 7days of withdrawal. 5-HT1A receptor mRNA expression was decreased following 3h of morphine exposure, while TPH2 mRNA expression was decreased after 7days of withdrawal with swim stress. There were no changes in the expression of GABAA-α1, MOR or 5-HT transporter mRNA. Collectively these results suggest that alterations in neurotrophin support, CRF-dependent stress signaling, 5-HT synthesis and release may underlie 5-HT DRN hypofunction that can potentially lead to stress-induced opiate relapse.

The role of the dorsal raphé nucleus in reward-seeking behavior

Pharmacological experiments have shown that the modulation of brain serotonin levels has a strong impact on value-based decision making. Anatomical and physiological evidence also revealed that the dorsal raphé nucleus (DRN), a major source of serotonin, and the dopamine system receive common inputs from brain regions associated with appetitive and aversive information processing. The serotonin and dopamine systems also have reciprocal functional influences on each other. However, the specific mechanism by which serotonin affects value-based decision making is not clear. To understand the information carried by the DRN for reward-seeking behavior, we measured single neuron activity in the primate DRN during the performance of saccade tasks to obtain different amounts of a reward. We found that DRN neuronal activity was characterized by tonic modulation that was altered by the expected and received reward value. Consistent reward-dependent modulation across different task periods suggested that DRN activity kept track of the reward value throughout a trial. The DRN was also characterized by modulation of its activity in the opposite direction by different neuronal subgroups, one firing strongly for the prediction and receipt of large rewards, with the other firing strongly for small rewards. Conversely, putative dopamine neurons showed positive phasic responses to reward-indicating cues and the receipt of an unexpected reward amount, which supports the reward prediction error signal hypothesis of dopamine. I suggest that the tonic reward monitoring signal of the DRN, possibly together with its interaction with the dopamine system, reports a continuous level of motivation throughout the performance of a task. Such a signal may provide "reward context" information to the targets of DRN projections, where it may be integrated further with incoming motivationally salient information.

Cellular adaptations of dorsal raphe serotonin neurons associated with the development of active coping in response to social stress

BACKGROUND

Social stress is a risk factor for affective disorders for certain vulnerable individuals. Stress and depression are linked in part through regulation of the dorsal raphe (DR)-serotonin (5-HT) system by the stress-related neuropeptide, corticotropin-releasing factor (CRF). We used a rat social stress model that shows individual differences in coping strategies to determine whether differences in CRF-5-HT interactions underlie individual differences in the vulnerability to social stress.

METHODS

Rats were exposed to the resident-intruder model of social stress for 5 days. In vivo single-unit recordings assessed DR-5-HT neuronal responses to CRF and immunoelectron microscopy assessed CRF1 and CRF2 cellular localization 24 hours after the last stress.

RESULTS

Rats responded to social stress passively, assuming defeat with short latencies (48%), or actively, with proactive behaviors and longer defeat latencies (LL, 52%). Whereas CRF (30 ng, intra-DR) inhibited 5-HT neuronal activity of control and SL rats, it activated 5-HT neurons of LL rats, an effect that was CRF2-mediated. Consistent with this, social stress promoted CRF1 internalization together with CRF2 recruitment to the plasma membrane of DR neurons selectively in LL rats.

CONCLUSIONS

These data suggest that a proactive coping strategy toward social stress is associated with a redistribution of CRF1 and CRF2 in DR-5-HT neurons that primes the system to be activated by subsequent stress. The lack of this adaptation in passive coping rats may contribute to their depressive-like phenotype. These studies provide a cellular mechanism for individual differences in stress responses and consequences.

Coordinate regulation of noradrenergic and serotonergic brain regions by amygdalar neurons

Based on the importance of the locus coeruleus-norepinephrine (LC-NE) system and the dorsal raphe nucleus-serotonergic (DRN-5-HT) system in stress-related pathologies, additional understanding of brain regions coordinating their activity is of particular interest. One such candidate is the amygdalar complex, and specifically, the central nucleus (CeA), which has been implicated in emotional arousal and is known to send monosynaptic afferent projections to both these regions. Our present data using dual retrograde tract tracing is the first to demonstrate a population of amygdalar neurons that project in a collateralized manner to the LC and DRN, indicating that amygdalar neurons are positioned to coordinately regulate the LC and DRN, and links these brain regions by virtue of a common set of afferents. Further, we have also characterized the phenotype of a population of these collaterally projecting neurons from the amygdala as containing corticotropin releasing factor or dynorphin, two peptides heavily implicated in the stress response. Understanding the co-regulatory influences of this limbic region on 5HT and NE regions may help fill a gap in our knowledge regarding neural circuits impacting these systems and their adaptations in stress.

Role of corticotropin-releasing factor in the median raphe nucleus in yohimbine-induced reinstatement of alcohol seeking in rats

The pharmacological stressor yohimbine increases ongoing alcohol self-administration and reinstates alcohol seeking in rats. This effect is attenuated by systemic injections of a corticotropin-releasing factor (CRF) antagonist. The brain sites involved in CRF's role in yohimbine-induced alcohol taking and seeking are unknown. We report that injections of the CRF receptor antagonist d-Phe CRF into the median raphe nucleus (MRN) attenuated yohimbine-induced reinstatement of alcohol seeking but had no effect on yohimbine-induced increases in alcohol intake during ongoing self-administration. Results indicate an important role of MRN CRF receptors in yohimbine-induced reinstatement of alcohol seeking but not yohimbine-induced increases in alcohol intake.

Biophysical properties and computational modeling of calcium spikes in serotonergic neurons of the dorsal raphe nucleus

Serotonergic neurons of the dorsal raphe nuclei, with their extensive innervation of nearly the whole brain have important modulatory effects on many cognitive and physiological processes. They play important roles in clinical depression and other psychiatric disorders. In order to quantify the effects of serotonergic transmission on target cells it is desirable to construct computational models and to this end these it is necessary to have details of the biophysical and spike properties of the serotonergic neurons. Here several basic properties are reviewed with data from several studies since the 1960s to the present. The quantities included are input resistance, resting membrane potential, membrane time constant, firing rate, spike duration, spike and afterhyperpolarization (AHP) amplitude, spike threshold, cell capacitance, soma and somadendritic areas. The action potentials of these cells are normally triggered by a combination of sodium and calcium currents which may result in autonomous pacemaker activity. We here analyse the mechanisms of high-threshold calcium spikes which have been demonstrated in these cells the presence of TTX (tetrodotoxin). The parameters for calcium dynamics required to give calcium spikes are quite different from those for regular spiking which suggests the involvement of restricted parts of the soma-dendritic surface as has been found, for example, in hippocampal neurons.

Effects of fluoxetine on CRF and CRF1 expression in rats exposed to the learned helplessness paradigm

RATIONALE

Stress is a common antecedent reported by people suffering major depression. In these patients, extrahypothalamic brain areas, like the hippocampus and basolateral amygdala (BLA), have been found to be affected. The BLA synthesizes CRF, a mediator of the stress response, and projects to hippocampus. The main hippocampal target for this peptide is the CRF subtype 1 receptor (CRF1). Evidence points to a relationship between dysregulation of CRF/CRF1 extrahypothalamic signaling and depression.

OBJECTIVE

Because selective serotonin reuptake inhibitors (SSRIs) are the first-line pharmacological treatment for depression, we investigated the effect of chronic treatment with the SSRI fluoxetine on long-term changes in CRF/CRF1 signaling in animals showing a depressive-like behavior.

METHODS

Male Wistar rats were exposed to the learned helplessness paradigm (LH). After evaluation of behavioral impairment, the animals were treated with fluoxetine (10 mg/kg i.p.) or saline for 21 days. We measured BLA CRF expression with RT-PCR and CRF1 expression in CA3 and the dentate gyrus of the hippocampus with in situ hybridization. We also studied the activation of one of CRF1's major intracellular signaling targets, the extracellular signal-related kinases 1 and 2 (ERK1/2) in CA3.

RESULTS

In saline-treated LH animals, CRF expression in the BLA increased, while hippocampal CRF1 expression and ERK1/2 activation decreased. Treatment with fluoxetine reversed the changes in CRF and CRF1 expressions, but not in ERK1/2 activation.

CONCLUSION

In animals exposed to the learned helplessness paradigm, there are long-term changes in CRF and CRF1 expression that are restored with a behaviorally effective antidepressant treatment.

Alcohol in excess: CRF₁ receptors in the rat and mouse VTA and DRN

RATIONALE

Manipulation of the stress neuropeptide corticotropin-releasing factor (CRF), specifically central antagonism of the type 1 receptors (CRF-R1), effectively reduces alcoholic-like ethanol drinking in rodents. Escalated consumption is largely controlled by neurocircuitry that is important for reward and affect, such as the ventral tegmental area (VTA) and the dorsal raphé nucleus (DRN).

OBJECTIVE

The current studies investigated the role of CRF-R1 within the VTA and DRN and their relation to escalated ethanol drinking in two species. An additional goal was to explore whether high alcohol-drinking individuals would be more affected by CRF-R1 antagonism than low alcohol-drinking individuals.

METHODS

With a two-bottle choice drinking procedure, adult male C57BL/6J mice and Long-Evans rats were given 24-h access to 20 % ethanol and water on an intermittent schedule. Rats and mice were implanted with cannulae targeting the VTA or DRN. Doses of the CRF-R1 antagonist CP-154,526 (butyl-[2,4,6-trimethylphenyl)-7H-pyrrolo[2,3-d]pyrimidin-4-yl]ethylamine)) were microinfused to modulate drinking of ethanol and water over the course of 24 h.

RESULTS

In both mice and rats, intra-VTA CP-154,526 selectively decreased ethanol intake, while identical doses (0.3 and 0.6 μg) infused intra-DRN reduced both ethanol and water drinking. Long-Evans rats displayed a range of individual differences for ethanol preference, and CP-154,526 suppressed ethanol drinking in the high-preferring animals regardless of brain site manipulation.

CONCLUSIONS

The current findings confirm previous studies that blockade of CRF-R1 efficaciously reduces escalated drinking while also suggesting that the effects of intermittent access on alcohol consumption may require CRF interaction with dopamine in the VTA.

Corticotropin-releasing factor (CRF)-induced disruption of attention in rats is blocked by the κ-opioid receptor antagonist JDTic

Stress often disrupts behavior and can lead to psychiatric illness. Considerable evidence suggests that corticotropin-releasing factor (CRF) plays an important role in regulating the effects of stress. CRF administration produces stress-like effects in humans and laboratory animals, and CRF levels are elevated in individuals with stress-related illness. Recent work indicates that κ-opioid receptor (KOR) antagonists can block CRF effects, raising the possibility that at least some of the effects of stress are mediated via KORs. Here we examined the effects of CRF on performance in the 5-choice serial reaction time task (5CSRTT), a test used to quantify attention in rodents, as well as functional interactions between CRF and KORs. Male Sprague-Dawley rats were trained in the 5CSRTT and then each was implanted with an intracerebroventricular (ICV) cannula. After recovery and restabilization of performance, they received a single intraperitoneal (IP) injection of vehicle or JDTic (10 mg/kg), a KOR antagonist with long-lasting (>14 days) effects. In subsequent sessions, rats received ICV infusions of CRF (0.25-1.0 μg) or vehicle and were tested 60 min later. CRF dose-dependently disrupted performance as reflected by decreases in correct responding, increases in omission errors, increases in latencies to respond correctly, and increases in time to complete the session. JDTic attenuated each of these CRF-induced deficits while having no effects on its own. The persistent ability of JDTic to disrupt KOR function was confirmed using the tail immersion assay. These findings indicate that KOR antagonists can prevent acute stress-related effects that degrade performance in tasks requiring attention.

Differential lipopolysaccharide-induced immune alterations in the hippocampus of two mouse strains: effects of stress

Immunological activation may result in the development of depressive-like symptoms in a large percentage of patients treated with cytokine-based therapies. The mechanisms underlying susceptibility to cytokine-induced depression are currently unknown; however activation of the tryptophan catabolising enzyme indoleamine 2,3-dioxygenase (IDO) is associated with the induction of cytokine-induced depression. Peripheral administration of lipopolysaccharide (LPS) is one of the most commonly used immunological challenges in animal models of cytokine-induced depression. Inbred mouse strains are useful tools in the investigation of the neurobiology of psychiatric illnesses. In this study we hypothesised that two strains which differ in stress susceptibility, namely the BALB/c and C57BL/6J mice, would respond differentially to LPS and swim-stress in cytokine profile, corticosterone concentrations and mRNA expression of genes coding for the tryptophan metabolising enzymes, IDO1, IDO2, Tph1 and Tph2. The stress-sensitive BALB/c strain exhibited increased depressive-like behaviour and enhanced corticosterone concentrations in response to LPS. Furthermore, swim-stress attenuated the LPS-induced corticosterone response in BALB/c mice only. LPS significantly increased plasma interleukin (IL)-1β and tumour necrosis factor α (TNFα) concentrations to a greater extent in BALB/c mice. The LPS-induced increase in IL-1β mRNA expression was significantly attenuated by swim-stress in the hippocampus of C57BL/6J but not in BALB/c mice. TNFα mRNA expression was significantly increased in BALB/c mice only; this increase was attenuated by swim-stress. Tph1 mRNA expression was upregulated in the brainstem of C57BL/6J mice post-LPS and following the combination of swim-stress and LPS in BALB/c mice. In the hippocampus Tph1 and Tph2 mRNA expression was increased in C57BL/6J but not BALB/c mice in response to LPS challenge and swim-stress. Conversely, IDO2 but not IDO1 mRNA expression was significantly altered following swim-stress and LPS, particularly in the hippocampus of BALB/c mice. These data indicate altered central mRNA expression of tryptophan metabolising enzymes and immune activation in BALB/c mice compared to the normo-sensitive C57BL/6J strain.

Brain-Gut Interactions in IBS

Irritable bowel syndrome (IBS) is a common gastrointestinal disorder with an estimated prevalence of 10–20%. Current understanding of the pathophysiology of IBS is incomplete due to the lack of a clearly identified pathological abnormality and due to the lack of reliable biomarkers. Possible mechanisms believed to contribute to IBS development and IBS like symptoms include physical stressors, such as infection or inflammation, psychological, and environmental factors, like anxiety, depression, and significant negative life events. Some of these mechanisms may involve the brain-gut axis (BGA). In this article we review the current knowledge on the possible involvement of the BGA in IBS and discuss new directions for potential future therapies of IBS.

Morphine history sensitizes postsynaptic GABA receptors on dorsal raphe serotonin neurons in a stress-induced relapse model in rats

The serotonin (5-hydroxytryptamine, 5-HT) system plays an important role in stress-related psychiatric disorders and substance abuse. Previous work has shown that the dorsal raphe nucleus (DR)-5-HT system is inhibited by swim stress via stimulation of GABA synaptic activity by the stress neurohormone corticotropin-releasing factor (CRF). Additionally, the DR 5-HT system is regulated by opioids. The present study tests the hypothesis that the DR 5-HT system regulates stress-induced opioid relapse. In the first experiment, electrophysiological recordings of GABA synaptic activity in 5-HT DR neurons were conducted in brain slices from Sprague-Dawley rats that were exposed to swim stress-induced reinstatement of previously extinguished morphine conditioned place preference (CPP). Behavioral data indicate that swim stress triggers reinstatement of morphine CPP. Electrophysiology data indicate that 5-HT neurons in the morphine-conditioned group exposed to stress had increased amplitude of inhibitory postsynaptic currents (IPSCs), which would indicate greater postsynaptic GABA receptor density and/or sensitivity, compared to saline controls exposed to stress. In the second experiment, rats were exposed to either morphine or saline CPP and extinction, and then 5-HT DR neurons from both groups were examined for sensitivity to CRF in vitro. CRF induced a greater inward current in 5-HT neurons from morphine-conditioned subjects compared to saline-conditioned subjects. These data indicate that morphine history sensitizes 5-HT DR neurons to the GABAergic inhibitory effects of stress as well as to some of the effects of CRF. These mechanisms may sensitize subjects with a morphine history to the dysphoric effects of stressors and ultimately confer an enhanced vulnerability to stress-induced opioid relapse.

Effect of lamotrigine and carbamazepine on corticotropin-releasing factor-associated serotonergic transmission in rat dorsal raphe nucleus

Corticotropin-releasing factor (CRF) and serotonin are important transmitters of the pathophysiology of mood disorder. To clarify the mechanisms of action of lamotrigine (LTG) and carbamazepine (CBZ), we determined their effects on serotonin release associated with CRF in rat dorsal raphe nucleus (DRN) and median prefrontal cortex (mPFC) using dual-probe microdialysis. Neither perfusion with CRF1 nor CRF2 antagonists into DRN-affected serotonin release in DRN and mPFC. Perfusion of 10 μM CRF into DRN increased serotonin release in both regions, whereas 0.1 μM CRF decreased and had no effect on serotonin release in DRN and mPFC, respectively. Pre-perfusion with CRF1 antagonist into DRN inhibited 0.1 μM CRF-induced serotonin reduction, whereas pre-perfusion with CRF2 antagonist in DRN inhibited 10 μM CRF-induced serotonin elevation, without affecting 0.1 μM CRF-induced serotonin reduction. LTG perfusion concentration dependently decreased serotonin releases in DRN and mPFC. Therapeutic and supratherapeutic concentrations of CBZ increased and decreased serotonin releases in both regions, respectively. Pre-perfusion with sub-therapeutic concentration LTG inhibited CRF1-induced serotonin reduction without affecting CRF2-induced serotonin release, whereas pre-perfusion with therapeutic concentration of LTG inhibited both CRF1- and CRF2-actions. In contrast, both therapeutic and supratherapeutic concentrations of CBZ inhibited CRF2-induced serotonin release without affecting CRF1-induced serotonin reduction. Neither LTG nor CBZ affected the CRF-induced cAMP production in cells over-expressing CRF1 and CRF2 receptors. This study demonstrated that inhibition of CRF2-receptor-mediated serotonergic transmission is a mechanism shared by LTG and CBZ, two clinically related compounds, whereas LTG but not CBZ inhibits CRF1-receptor-mediated serotonergic transmission. Therefore, these mechanisms may contribute to the clinical actions of these agents.

The effect of citalopram on midbrain CRF receptors 1 and 2 in a primate model of stress-induced amenorrhea

We have demonstrated marked differences in the neurobiology of the serotonin system between stress-sensitive (SS) and stress-resilient (SR) cynomolgus macaques characterized in a model of stress-induced amenorrhea, also called functional hypothalamic amenorrhea (FHA). Dysfunction of the serotonin system in SS monkeys suggested that administration of a selective serotonin reuptake inhibitor (SSRI) might correct FHA. This study examines the effect of escitalopram (CIT) administration to SS and SR monkeys on corticotrophin-releasing factor (CRF) receptor 1 (CRF-R1) and CRF receptor 2 (CRF-R2) gene expression in the serotonin cell body region of the midbrain dorsal raphe. CRF-R1 was not significantly different between groups. There was a significant effect of treatment and a significant interaction between treatment and stress sensitivity on the average CRF-R2-positive pixel area (P < .004 and P < .006, respectively) and on the average number of CRF-R2-positive cells (P < .023 and P < .025, respectively). CIT significantly increased CRF-R2-positive pixel area and cell number in the SS group (pixel area P < .001; cell number P < .01; Bonferoni) but not in the SR group. In summary, CIT administration tended to decrease CRF-R1, but the small animal number precluded significance. CIT administration significantly increased CRF-R2 only in SS animals. These data suggest that the administration of CIT reduces anxiogenic components and increases anxiolytic components of the CRF system in the midbrain serotonin network, which in turn leads to improved ovarian function. Moreover, these data raise the possibility that SSRIs may be effective in the treatment of stress-induced infertility.

Extended access cocaine self-administration differentially activates dorsal raphe and amygdala corticotropin-releasing factor systems in rats

Cocaine-induced neuroadaptation of stress-related circuitry and increased access to cocaine each putatively contribute to the transition from cocaine use to cocaine dependence. The present study tested the hypothesis that rats receiving extended versus brief daily access to cocaine would exhibit regional differences in levels of the stress-regulatory neuropeptide corticotropin-releasing factor (CRF). A secondary goal was to explore how CRF levels change in relation to the time since cocaine self-administration. Male Wistar rats acquired operant self-administration of cocaine and were assigned to receive daily long access (6 hours/day, LgA, n=20) or short access (1 hour/day, ShA, n=18) to intravenous cocaine self-administration (fixed ratio 1, ~0.50 mg/kg/infusion). After at least 3 weeks, tissue CRF immunoreactivity was measured at one of three timepoints: pre-session, post-session or 3 hours post-session. LgA, but not ShA, rats showed increased total session and first-hour cocaine intake. CRF immunoreactivity increased within the dorsal raphe (DR) and basolateral, but not central, nucleus of the amygdala (BLA, CeA) of ShA rats from pre-session to 3 hours post-session. In LgA rats, CRF immunoreactivity increased from pre-session to 3 hours post-session within the CeA and DR but tended to decrease in the BLA. LgA rats showed higher CRF levels than ShA rats in the DR and, pre-session, in the BLA. Thus, voluntary cocaine intake engages stress-regulatory CRF systems of the DR and amygdala. Increased availability of cocaine promotes greater tissue CRF levels in these extrahypothalamic brain regions, changes associated here with a model of cocaine dependence.

Corticotropin-releasing factor antagonist reduces activation of noradrenalin and serotonin neurons in the locus coeruleus and dorsal raphe in the arousal response accompanied by yawning behavior in rats

We previously reported that intracerebroventricular (icv) administration of corticotropin-releasing factor (CRF) antagonist attenuates the arousal response during yawning behavior in rats. However, the CRF-related pathway involved in the arousal response during yawning is still unclear. In the present study, we assessed the involvement of the CRF-containing pathway from the hypothalamic paraventricular nucleus (PVN) to the locus coeruleus (LC) and the dorsal raphe nucleus (DRN) in the arousal response during frequent spontaneous yawning, which was induced by several microinjections of l-glutamate into the PVN in anesthetized rats, using c-Fos immunohistochemistry. The PVN stimulation showed significant increases in activation of PVN CRF neurons, LC noradrenalin (NA) neurons and DRN serotonin (5-HT) neurons as well as arousal response during yawning. But icv administration of a CRF receptor antagonist, α-helical CRF (9-41), significantly inhibited the activation of both LC NA neurons and DRN 5-HT neurons except the activation of CRF neurons in the PVN, and significantly suppressed the arousal response during yawning. These results suggest that the CRF-containing pathway from PVN CRF neurons to LC NA neurons and DRN 5-HT neurons can be involved in the arousal response during yawning behavior.

Corticotropin-releasing factor in the norepinephrine nucleus, locus coeruleus, facilitates behavioral flexibility

Corticotropin-releasing factor (CRF), the stress-related neuropeptide, acts as a neurotransmitter in the brain norepinephrine nucleus, locus coeruleus (LC), to activate this system during stress. CRF shifts the mode of LC discharge from a phasic to a high tonic state that is thought to promote behavioral flexibility. To investigate this, the effects of CRF administered either intracerebroventricularly (30-300 ng, i.c.v.) or directly into the LC (intra-LC; 2-20 ng) were examined in a rat model of attentional set shifting. CRF differentially affected components of the task depending on dose and route of administration. Intracerebroventricular CRF impaired intradimensional set shifting, reversal learning, and extradimensional set shifting (EDS) at different doses. In contrast, intra-LC CRF did not impair any aspect of the task. The highest dose of CRF (20 ng) facilitated reversal learning and the lowest dose (2 ng) improved EDS. The dose-response relationship for CRF on EDS performance resembled an inverted U-shaped curve with the highest dose having no effect. Intra-LC CRF also elicited c-fos expression in prefrontal cortical neurons with an inverted U-shaped dose-response relationship. The number of c-fos profiles was positively correlated with EDS performance. Given that CRF excites LC neurons, the ability of intra-LC CRF to activate prefrontal cortical neurons and facilitate EDS is consistent with findings implicating LC-norepinephrine projections to medial prefrontal cortex in this process. Importantly, the results suggest that CRF release in the LC during stress facilitates shifting of attention between diverse stimuli in a dynamic environment so that the organism can adapt an optimal strategy for coping with the challenge.

Decreased plasma ghrelin contributes to anorexia following novelty stress

We hypothesized that anorexia induced by novelty stress caused by exposure to a novel environment may be due to activation of corticotropin-releasing factor (CRF) and subsequently mediated by decreasing peripheral ghrelin concentration via serotonin (5-HT) and melanocortin-4 receptors (MC4R). Each mouse was transferred from group-housed cages to individual cages to establish the novelty stress. We observed the effect of changes in feeding behavior in a novel environment using the method of transferring group-housed mice to individual cages. We investigated the effect of an intracerebroventricular injection of antagonists/agonists of CRF1/2 receptors (CRF1/2Rs), 5-HT(1B)/(2C) receptors (5-HT(1B)/(2C)R), and MC4R to clarify the role of each receptor on the decrease in food intake. Plasma ghrelin levels were also measured. The novelty stress caused a reduction in food intake that was abolished by administering a CRF1R antagonist. Three hours after the novelty stress, appetite reduction was associated with reduced levels of neuropeptide Y/agouti-related peptide mRNA, increased levels of proopiomelanocortin mRNA in the hypothalamus, and a decrease in plasma ghrelin level. Administering a CRF1R antagonist, a 5-HT(1B)/(2C)R antagonist, an MC4R antagonist, exogenous ghrelin, and an enhancer of ghrelin secretion, rikkunshito, resolved the reduction in food intake 3 h after the novelty stress by enhancing circulating ghrelin concentrations. We showed that anorexia during a novelty stress is a process in which CRF1R is activated at the early stage of appetite loss and is subsequently activated by a 5-HT(1B)/(2C)R and MC4R stimulus, leading to decreased peripheral ghrelin concentrations.

Collateralized dorsal raphe nucleus projections: a mechanism for the integration of diverse functions during stress

The midbrain dorsal raphe nucleus (DR) is the origin of the central serotonin (5-HT) system, a key neurotransmitter system that has been implicated in the expression of normal behaviors and in diverse psychiatric disorders, particularly affective disorders such as depression and anxiety. One link between the DR-5-HT system and affective disorders is exposure to stressors. Stress is a major risk factor for affective disorders, and stressors alter activity of DR neurons in an anatomically specific manner. Stress-induced changes in DR neuronal activity are transmitted to targets of the DR via ascending serotonergic projections, many of which collateralize to innervate multiple brain regions. Indeed, the collateralization of DR efferents allows for the coordination of diverse components of the stress response. This review will summarize our current understanding of the organization of the ascending DR system and its collateral projections. Using the neuropeptide corticotropin-releasing factor (CRF) system as an example of a stress-related initiator of DR activity, we will discuss how topographic specificity of afferent regulation of ascending DR circuits serves to coordinate activity in functionally diverse target regions under appropriate conditions.

Effects of citalopram on serotonin and CRF systems in the midbrain of primates with differences in stress sensitivity

This chapter reviews the neurobiological effects of stress sensitivity and s-citalpram (CIT) treatment observed in our nonhuman primate model of functional hypothalamic amenorrhea (FHA). This type of infertility, also known as stress-induced amenorrhea, is exhibited by cynomolgus macaques. In small populations, some individuals are stress-sensitive (SS) and others are highly stress-resilient (HSR). The SS macaques have suboptimal secretion of estrogen and progesterone during normal menstrual cycles. SS monkeys also have decreased serotonin gene expression and increased CRF expression compared to HSR monkeys. Recently, we found that CIT treatment improved ovarian steroid secretion in SS monkeys, but had no effect in HSR monkeys. Examination of the serotonin system revealed that SS monkeys had significantly lower Fev (fifth Ewing variant, rodent Pet1), TPH2 (tryptophan hydroxylase 2), 5HT1A autoreceptor and SERT (serotonin reuptake transporter) expression in the dorsal raphe than SR monkeys. However, CIT did not alter the expression of either Fev, TPH2, SERT or 5HT1A mRNAs. In contrast, SS monkeys tended to have a higher density of CRF fiber innervation of the dorsal raphe than HSR monkeys, and CIT significantly decreased the CRF fiber density in SS animals. In addition, CIT increased CRF-R2 gene expression in the dorsal raphe. We speculate that in a 15-week time frame, the therapeutic effect of S-citalopram may be achieved through a mechanism involving extracellular serotonin inhibition of CRF and stimulation of CRF-R2, rather than alteration of serotonin-related gene expression.

Persistent escalation of alcohol drinking in C57BL/6J mice with intermittent access to 20% ethanol

BACKGROUND

Intermittent access (IA) to drugs of abuse, as opposed to continuous access, is hypothesized to induce a kindling-type transition from moderate to escalated use, leading to dependence. Intermittent 24-hour cycles of ethanol access and deprivation can generate high levels of voluntary ethanol drinking in rats.

METHODS

The current study uses C57BL/6J mice (B6) in an IA to 20% ethanol protocol to escalate ethanol drinking levels. Adult male and female B6 mice were given IA to 20% ethanol on alternating days of the week with water available ad libitum. Ethanol consumption during the initial 2 hours of access was compared with a short-term, limited access "binge" drinking procedure, similar to drinking-in-the-dark (DID). B6 mice were also assessed for ethanol dependence with handling-induced convulsion, a reliable measure of withdrawal severity.

RESULTS

After 3 weeks, male mice given IA to ethanol achieved high stable levels of ethanol drinking in excess of 20 g/kg/24 h, reaching above 100 mg/dl blood ethanol concentrations, and showed a significantly higher ethanol preference than mice given continuous access to ethanol. Also, mice given IA drank about twice as much as DID mice in the initial 2-hour access period. B6 mice that underwent the IA protocol for longer periods of time displayed more severe signs of alcohol withdrawal. Additionally, female B6 mice were given IA to ethanol and drank significantly more than males (ca. 30 g/kg/24 h).

DISCUSSION

The IA method in B6 mice is advantageous because it induces escalated, voluntary, and preferential per os ethanol intake, behavior that may mimic a cardinal feature of human alcohol dependence, though the exact nature and site of ethanol acting in the brain and blood as a result of IA has yet to be determined.

Alterations in the central CRF system of two different rat models of comorbid depression and functional gastrointestinal disorders

Clinical evidence suggests comorbidity between depression and irritable bowel syndrome (IBS). Early-life stress and genetic predisposition are key factors in the pathophysiology of both IBS and depression. Thus, neonatal maternal separation (MS), and the Wistar-Kyoto (WKY) rat, a genetically stress-sensitive rat strain, are two animal models of depression that display increased visceral hypersensitivity and alterations in the hypothalamic-pituitary-adrenal axis. Corticotrophin-releasing factor (CRF) is the primary peptide regulating this axis, acting through two receptors: CRF1 and CRF2. The central CRF system is also a key regulator in the stress response. However, there is a paucity of studies investigating alterations in the central CRF system of adult MS or WKY animals. Using in-situ hybridization we demonstrate that CRF mRNA is increased in the paraventricular nucleus (PVN) of WKY rats and the dorsal raphé nucleus (DRN) of MS animals, compared to Sprague-Dawley and non-separated controls, respectively. Additionally, CRF1 mRNA was higher in the PVN, amygdala and DRN of both animal models, along with high levels of CRF1 mRNA in the hippocampus of WKY animals compared to control animals. Finally, CRF2 mRNA was lower in the DRN of MS and WKY rats compared to control animals, and in the hippocampus and amygdala of MS rats. These results show that the central CRF system is altered in both animal models. Such alterations may affect HPA axis regulation, contribute to behavioural changes associated with stress-related disorders, and alter the affective component of visceral pain modulation, which is enhanced in IBS patients.

Functional organization of the dorsal raphe efferent system with special consideration of nitrergic cell groups

The serotonin (5HT) system of the brain is involved in many CNS functions including sensory perception, stress responses and psychological disorders such as anxiety and depression. Of the nine 5HT nuclei located in the mammalian brain, the dorsal raphe nucleus (DRN) has the most extensive forebrain connectivity and is implicated in the manifestation of stress-related psychological disturbances. Initial investigations of DRN efferent connections failed to acknowledge the rostrocaudal and mediolateral organization of the nucleus or its neurochemical heterogeneity. More recent studies have focused on the non-5HT contingent of DRN cells and have revealed an intrinsic intranuclear organization of the DRN which has specific implications for sensory signal processing and stress responses. Of particular interest are spatially segregated subsets of nitric oxide producing neurons that are activated by stressors and that have unique efferent projection fields. In this regard, both the midline and lateral wing subregions of the DRN have emerged as prominent loci for future investigation of nitric oxide function and modulation of sensory- and stressor-related signals in the DRN and coinciding terminal fields.

Topographical distribution of corticotropin-releasing factor type 2 receptor-like immunoreactivity in the rat dorsal raphe nucleus: co-localization with tryptophan hydroxylase

Corticotropin-releasing factor (CRF) and CRF-related neuropeptides are involved in the regulation of stress-related physiology and behavior. Members of the CRF family of neuropeptides bind to two known receptors, the CRF type 1 (CRF₁) receptor, and the CRF type 2 (CRF₂) receptor. Although the distribution of CRF₂ receptor mRNA expression has been extensively studied, the distribution of CRF₂ receptor protein has not been characterized. An area of the brain known to contain high levels of CRF₂ receptor mRNA expression and CRF₂ receptor binding is the dorsal raphe nucleus (DR). In the present study we investigated in detail the distribution of CRF₂ receptor immunoreactivity throughout the rostrocaudal extent of the DR. CRF₂ receptor-immunoreactive perikarya were observed throughout the DR, with the highest number and density in the mid-rostrocaudal DR. Dual immunofluorescence revealed that CRF₂ receptor immunoreactivity was frequently co-localized with tryptophan hydroxylase, a marker of serotonergic neurons. This study provides evidence that CRF₂ receptor protein is expressed in the DR, and that CRF₂ receptors are expressed in topographically organized subpopulations of cells in the DR, including serotonergic neurons. Furthermore, these data are consistent with the hypothesis that CRF₂ receptors play an important role in the regulation of stress-related physiology and behavior through actions on serotonergic and non-serotonergic neurons within the DR.

Overlapping neurobiology of learned helplessness and conditioned defeat: implications for PTSD and mood disorders

Exposure to traumatic events can increase the risk for major depressive disorder (MDD) as well as posttraumatic stress disorder (PTSD), and pharmacological treatments for these disorders often involve the modulation of serotonergic (5-HT) systems. Several behavioral paradigms in rodents produce changes in behavior that resemble symptoms of MDD and these behavioral changes are sensitive to antidepressant treatments. Here we review two animal models in which MDD-like behavioral changes are elicited by exposure to an acute traumatic event during adulthood, learned helplessness (LH) and conditioned defeat. In LH, exposure of rats to inescapable, but not escapable, tailshock produces a constellation of behavioral changes that include deficits in fight/flight responding and enhanced anxiety-like behavior. In conditioned defeat, exposure of Syrian hamsters to a social defeat by a more aggressive animal leads to a loss of territorial aggression and an increase in submissive and defensive behaviors in subsequent encounters with non-aggressive conspecifics. Investigations into the neural substrates that control LH and conditioned defeat revealed that increased 5-HT activity in the dorsal raphe nucleus (DRN) is critical for both models. Other key brain regions that regulate the acquisition and/or expression of behavior in these two paradigms include the basolateral amygdala (BLA), central nucleus of the amygdala (CeA) and bed nucleus of the stria terminalis (BNST). In this review, we compare and contrast the role of each of these neural structures in mediating LH and conditioned defeat, and discuss the relevance of these data in developing a better understanding of the mechanisms underlying trauma-related depression. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Stress-hyperresponsive WKY rats demonstrate depressed dorsal raphe neuronal excitability and dysregulated CRF-mediated responses

Major depression is a debilitating psychiatric disease that may be precipitated by a dysregulation of stress neurocircuitry caused by chronic or severe stress exposure. Moreover, hyperresponsivity to stressors correlates with depressed mood and may contribute to the etiology of major depression. The serotonergic dorsal raphe nucleus (DRN) is an important site in the neurocircuitry underlying behavioral responses to stressors, and is tightly regulated, in part, by a combination of intrinsic cell properties, autoinhibition, and GABAergic synaptic transmission. The stress-related neurotransmitter corticotropin-releasing factor (CRF) modulates DRN neuronal excitability and subsequent 5-HT release in the forebrain. Wistar Kyoto (WKY) rats exhibit exaggerated behavioral responses to stressors, that is, stress hyperresponsivity, and are considered an animal model of depression. To better understand the neurobiological basis of the stress hyperresponsivity, we used a combination of mRNA analysis and whole-cell electrophysiological techniques to measure differences in intrinsic activity and receptor response, in 5-HT- and non-5-HT-containing neurons of the DRN in WKY rats compared with Sprague-Dawley controls. In the WKY rat, there was a decrease in the neuronal excitability of 5-HT neurons coupled with decreased TPH2 production. Additionally, we found that CRF did not increase GABAergic activity in 5-HT neurons as is normally seen in 5-HT neurons of Sprague-Dawley controls. The CRF modulation of 5-HT DRN neurotransmission at the single-cell level is selectively disrupted in the WKY animal model of depression and may be one of the cellular correlates underlying depression.

Brain serotonin receptors and transporters: initiation vs. termination of escalated aggression

RATIONALE

Recent findings have shown a complexly regulated 5-HT system as it is linked to different kinds of aggression.

OBJECTIVE

We focus on (1) phasic and tonic changes of 5-HT and (2) state and trait of aggression, and emphasize the different receptor subtypes, their role in specific brain regions, feed-back regulation and modulation by other amines, acids and peptides.

RESULTS

New pharmacological tools differentiate the first three 5-HT receptor families and their modulation by GABA, glutamate and CRF. Activation of 5-HT(1A), 5-HT(1B) and 5-HT(2A/2C) receptors in mesocorticolimbic areas, reduce species-typical and other aggressive behaviors. In contrast, agonists at 5-HT(1A) and 5-HT(1B) receptors in the medial prefrontal cortex or septal area can increase aggressive behavior under specific conditions. Activation of serotonin transporters reduce mainly pathological aggression. Genetic analyses of aggressive individuals have identified several molecules that affect the 5-HT system directly (e.g., Tph2, 5-HT(1B), 5-HT transporter, Pet1, MAOA) or indirectly (e.g., Neuropeptide Y, αCaMKII, NOS, BDNF). Dysfunction in genes for MAOA escalates pathological aggression in rodents and humans, particularly in interaction with specific experiences.

CONCLUSIONS

Feedback to autoreceptors of the 5-HT(1) family and modulation via heteroreceptors are important in the expression of aggressive behavior. Tonic increase of the 5-HT(2) family expression may cause escalated aggression, whereas the phasic increase of 5-HT(2) receptors inhibits aggressive behaviors. Polymorphisms in the genes of 5-HT transporters or rate-limiting synthetic and metabolic enzymes of 5-HT modulate aggression, often requiring interaction with the rearing environment.

Investigation of a central nucleus of the amygdala/dorsal raphe nucleus serotonergic circuit implicated in fear-potentiated startle

Serotonergic systems are thought to play an important role in control of motor activity and emotional states. We used a fear-potentiated startle paradigm to investigate the effects of a motor-eliciting stimulus in the presence or absence of induction of an acute fear state on serotonergic neurons in the dorsal raphe nucleus (DR) and cells in subdivisions of the central amygdaloid nucleus (CE), a structure that plays an important role in fear responses, using induction of the protein product of the immediate-early gene, c-Fos. In Experiment 1 we investigated the effects of fear conditioning training, by training rats to associate a light cue (conditioned stimulus, CS; 1000 lx, 2 s) with foot shock (0.5 s, 0.5 mA) in a single session. In Experiment 2 rats were given two training sessions identical to Experiment 1 on days 1 and 2, then tested in one of four conditions on day 3: (1) placement in the training context without exposure to either the CS or acoustic startle (AS), (2) exposure to 10 trials of the 2 s CS, (3) exposure to 40 110 dB AS trials, or (4) exposure to 40 110 dB AS trials with 10 of the trials preceded by and co-terminating with the CS. All treatments were conducted during a 20 min session. Fear conditioning training, by itself, increased c-Fos expression in multiple subdivisions of the CE and throughout the DR. In contrast, fear-potentiated startle selectively increased c-Fos expression in the medial subdivision of the CE and in serotonergic neurons in the dorsal part of the dorsal raphe nucleus (DRD). These data are consistent with previous studies demonstrating that fear-related stimuli selectively activate DRD serotonergic neurons. Further studies of this mesolimbocortical serotonergic system could have important implications for understanding mechanisms underlying vulnerability to stress-related psychiatric disorders, including anxiety and affective disorders.

Stress and tension-type headache mechanisms

Stress is widely demonstrated as a contributing factor in tension-type headache (TTH). The mechanisms underlying this remain unclear at present. Recent research indicates the importance of central pain processes in tension-type headache (TTH) pathophysiology. Concurrently, research with animals and healthy humans has begun to elucidate the relationship between stress and pain processing in the central nervous system, including central pain processes putatively dysfunctional in TTH. Combined, these two fields of research present new insights and hypotheses into possible mechanisms by which stress may contribute to TTH. To date, however, there has been no comprehensive review of this literature. The present paper provides such a review, which may be valuable in facilitating a broader understanding of the central mechanisms by which stress may contribute to TTH.

Behavioral and pharmacogenetics of aggressive behavior

Serotonin (5-HT) has long been considered as a key transmitter in the neurocircuitry controlling aggression. Impaired regulation of each subtype of 5-HT receptor, 5-HT transporter, synthetic and metabolic enzymes has been linked particularly to impulsive aggression. The current summary focuses mostly on recent findings from pharmacological and genetic studies. The pharmacological treatments and genetic manipulations or polymorphisms of aspecific target (e.g., 5-HT1A receptor) can often result in inconsistent results on aggression, due to "phasic" effects of pharmacological agents versus "trait"-like effects of genetic manipulations. Also, the local administration of a drug using the intracranial microinjection technique has shown that activation of specific subtypes of 5-HT receptors (5-HT1A and 5-HT1B) in mesocorticolimbic areas can reduce species-typical and other aggressive behaviors, but the same receptors in the medial prefrontal cortex or septal area promote escalated forms of aggression. Thus, there are receptor populations in specific brain regions that preferentially modulate specific types of aggression. Genetic studies have shown important gene-environment interactions; it is likely that the polymorphisms in the genes of 5-HT transporters or rate-limiting synthetic and metabolic enzymes of 5-HT (e.g., MAOA) determine the vulnerability to adverse environmental factors that escalate aggression. We also discuss the interaction between the 5-HT system and other systems. Modulation of 5-HT neurons in the dorsalraphe nucleus by GABA, glutamate and CRF profoundly regulate aggressive behaviors. Also, interactions of the 5-HT system with other neuropeptides(arginine vasopressin, oxytocin, neuropeptide Y, opioid) have emerged as important neurobiological determinants of aggression. Studies of aggression in genetically modified mice identified several molecules that affect the 5-HT system directly (e.g., Tph2, 5-HT1B, 5-HT transporter, Pet1, MAOA) or indirectly[e.g., BDNF, neuronal nitric oxide (nNOS), aCaMKII, Neuropeptide Y].The future agenda delineates specific receptor subpopulations for GABA, glutamate and neuropeptides as they modulate the canonical aminergic neurotransmitters in brainstem, limbic and cortical regions with the ultimate outcome of attenuating or escalating aggressive behavior.

Chemical neuroanatomy of the dorsal raphe nucleus and adjacent structures of the mouse brain

Serotonin neurons play a major role in many normal and pathological brain functions. In the rat these neurons have a varying number of cotransmitters, including neuropeptides. Here we studied, with histochemical techniques, the relation between serotonin, some other small-molecule transmitters, and a number of neuropeptides in the dorsal raphe nucleus (DRN) and the adjacent ventral periaqueductal gray (vPAG) of mouse, an important question being to establish possible differences from rat. Even if similarly distributed, the serotonin neurons in mouse lacked the extensive coexpression of nitric oxide synthase and galanin seen in rat. Although partly overlapping in the vPAG, no evidence was obtained for the coexistence of serotonin with dopamine, substance P, cholecystokinin, enkephalin, somatostatin, neurotensin, dynorphin, thyrotropin-releasing hormone, or corticotropin-releasing hormone. However, some serotonin neurons expressed the gamma-aminobutyric acid (GABA)-synthesizing enzyme glutamic acid decarboxylase (GAD). Work in other laboratories suggests that, as in rat, serotonin neurons in the mouse midline DRN express the vesicular glutamate transporter 3, presumably releasing glutamate. Our study also shows that many of the neuropeptides studied (substance P, galanin, neurotensin, dynorphin, and corticotropin-releasing factor) are present in nerve terminal networks of varying densities close to the serotonin neurons, and therefore may directly or indirectly influence these cells. The apparently low numbers of coexisting messengers in mouse serotonin neurons, compared to rat, indicate considerable species differences with regard to the chemical neuronatomy of the DRN. Thus, extrapolation of DRN physiology, and possibly pathology, from rat to mouse, and even human, should be made with caution.

Swim stress differentially blocks CRF receptor mediated responses in dorsal raphe nucleus

Modulation of the serotonergic (5-HT) neurotransmitter system arising from the dorsal raphe nucleus (DR) is thought to support the behavioral effects of swim stress, i.e., immobility. In vivo pharmacological and anatomical studies suggest that corticotropin-releasing factor (CRF) and γ-aminobutyric acid (GABA) synaptic transmission closely interact to set the response of the DR to swim stress. To investigate the cellular basis of these physiological mechanisms the effects of ovine CRF (oCRF) on GABA(A)-dependent miniature inhibitory postsynaptic currents (mIPSCs) in 5-HT and non-5-HT DR neurons in acute mesencephalic slices obtained from rats either naïve or 24h after a 15 min swim stress session were tested. In this study, the effect of swim stress alone was to decrease the holding current, i.e., hyperpolarize the neuron, and to increase the amplitude and charge of mIPSCs recorded from non-5-HT neurons. Ovine CRF (10 nM) induced an increase in mIPSC frequency in 5-HT neurons recorded from naïve rats, an effect that was suppressed by swim stress. The inward current elicited by oCRF in both 5-HT and non-5-HT neurons was also blocked by swim stress. Ovine CRF increased mIPSCs amplitude and charge in both 5-HT and non-5-HT neurons, but this effect was not modified by swim stress. In concert with our previous findings that swim stress decreased input resistance, action potential threshold and action potential duration and increased glutamatergic synaptic activity the overall primary effect of swim stress is to increase the excitability of 5-HT neurons. These data provide a mechanism at the cellular level for the immobility induced by swim stress and identifies critical components of the raphe circuitry responsible for the altered output of 5-HT neurons induced by swim stress.

Localized expression of urocortin genes in the developing zebrafish brain

The corticotropin-releasing hormone (CRH) family consists of four paralogous genes, CRH and urocortins (UCNs) 1, 2, and 3. In a previous study, we analyzed CRH in the teleost model organism zebrafish and its transcript distribution in the embryonic brain. Here, we describe full-length cDNAs encoding urotensin 1 (UTS1), the teleost UCN1 ortholog, and UCN3 of zebrafish. Major expression sites of uts1 in adult zebrafish are the caudal neurosecretory system and brain. By using RT-PCR analysis, we show that uts1 mRNA is also present in ovary, maternally contributed to the embryo, and expressed throughout embryonic development. Expression of ucn3 mRNA was detected in a range of adult tissues and during developmental stages from 24 hours post fertilization onward. Analysis of spatial transcript distributions by whole-mount in situ hybridization revealed limited forebrain expression of uts1 and ucn3 during early development. Small numbers of uts1-synthesizing neurons were found in subpallium, hypothalamus, and posterior diencephalon, whereas ucn3-positive cells were restricted to telencephalon and retina. The brainstem was the main site of uts1 and ucn3 synthesis in the embryonic brain. uts1 Expression was confined to the midbrain tegmentum; distinct hindbrain cell groups, including locus coeruleus and Mauthner neurons; and the spinal cord. ucn3 Expression was localized to the optic tectum, serotonergic raphe, and distinct rhombomeric cell clusters. The prominent expression of uts1 and ucn3 in brainstem is consistent with proposed roles of CRH-related peptides in stress-induced modulation of locomotor activity through monoaminergic brainstem neuromodulatory systems.

Blocking corticotropin-releasing factor-2 receptors, but not corticotropin-releasing factor-1 receptors or glucocorticoid feedback, disrupts the development of conditioned defeat

Several neuroendocrine signals of the hypothalamic-pituitary-adrenal (HPA) axis are released following exposure to stressful events. It has long been proposed that the signals in this cascade each act to modify ongoing and future behavior. In this study we investigated whether blocking glucocorticoid synthesis, corticotropin-releasing factor (CRF)-1 receptors, or CRF-2 receptors during social defeat would alter subsequent behavioral responses. We used a conditioned defeat model in Syrian hamsters in which social defeat results in a dramatic shift from territorial aggression to increased submissive and defensive behavior in future social encounters. We found that intracerebroventricular administration of anti-sauvagine-30, a CRF-2 receptor antagonist, prior to social defeat training reduced the acquisition of conditioned defeat. In contrast, the acquisition of conditioned defeat was not altered by the CRF-1 receptor antagonist CP-154,526 or the glucocorticoid synthesis inhibitor metyrapone. Our results suggest that CRF, and perhaps related neuropeptides such as urocortins, act at CRF-2 receptors to promote the development of defeat-induced changes in social behavior, whereas signaling at CRF-1 and glucocorticoid receptors plays a negligible role in this process.

Chronic amphetamine treatment enhances corticotropin-releasing factor-induced serotonin release in the amygdala

Amphetamine use is associated with dysphoric states, including heightened anxiety, that emerge within 24h of withdrawal from the drug. Corticotropin-releasing factor increases serotonin release in the central nucleus of the amygdala, and this neurochemical circuitry may play a role in mediating fear and anxiety states. We have previously shown that chronic amphetamine treatment increases corticotropin-releasing factor receptor type-2 levels in the serotonergic dorsal raphe nucleus of the rat. Therefore, we hypothesized that chronic amphetamine treatment would enhance the amygdalar serotonergic response to corticotropin-releasing factor infused into the dorsal raphe nucleus. Male rats were injected once-daily with d-amphetamine (2.5mg/kg i.p., or saline) for two weeks. Serotonin release within the central nucleus of the amygdala in response to intra-raphe infusion of corticotropin-releasing factor (100 ng) was measured 24h after the last treatment in urethane-anesthetized (1.8 mg/kg, i.p.) rats using in vivo microdialysis. Rats pretreated with amphetamine showed significantly enhanced serotonin release in the central nucleus of the amygdala in response to corticotropin-releasing factor infusion when compared to saline pretreated rats. Furthermore, this enhanced response was blocked by the corticotropin-releasing factor type-2 receptor antagonist antisauvagine-30 (2 microg) infused into the dorsal raphe nucleus. These results suggest increased sensitivity to corticotropin-releasing factor as mediated by type-2 receptors following chronic amphetamine treatment, which may underlie dysphoric states observed during amphetamine withdrawal.