Effect of corticotropin releasing factor receptor 1 antagonist on extracellular norepinephrine, dopamine and serotonin in hippocampus and prefrontal cortex of rats in vivo

Functional Interplay of Type-2 Corticotrophin Releasing Factor and Dopamine Receptors in the Basolateral Amygdala-Medial Prefrontal Cortex Circuitry

BACKGROUND

Basolateral amygdala (BLA) excitatory projections to medial prefrontal cortex (PFC) play a key role controlling stress behavior, pain, and fear. Indeed, stressful events block synaptic plasticity at the BLA-PFC circuit. The stress responses involve the action of corticotrophin releasing factor (CRF) through type 1 and type 2 CRF receptors (CRF1 and CRF2). Interestingly, it has been described that dopamine receptor 1 (D1R) and CRF peptide have a modulatory role of BLA-PFC transmission. However, the participation of CRF1 and CRF2 receptors in BLA-PFC synaptic transmission still is unclear.

METHODS

We used in vivo microdialysis to determine dopamine and glutamate (GLU) extracellular levels in PFC after BLA stimulation. Immunofluorescence anatomical studies in rat PFC synaptosomes devoid of postsynaptic elements were performed to determine the presence of D1R and CRF2 receptors in synaptical nerve endings.

RESULTS

Here, we provide direct evidence of the opposite role that CRF receptors exert over dopamine extracellular levels in the PFC. We also show that D1R colocalizes with CRF2 receptors in PFC nerve terminals. Intra-PFC infusion of antisauvagine-30, a CRF2 receptor antagonist, increased PFC GLU extracellular levels induced by BLA activation. Interestingly, the increase in GLU release observed in the presence of antisauvagine-30 was significantly reduced by incubation with SCH23390, a D1R antagonist.

CONCLUSION

PFC CRF2 receptor unmasks D1R effect over glutamatergic transmission of the BLA-PFC circuit. Overall, CRF2 receptor emerges as a new modulator of BLA to PFC glutamatergic transmission, thus playing a potential role in emotional disorders.

Changes in hypothalamic neurotransmitter and prostanoid levels in response to NMDA, CRF, and GLP-1 stimulation

Determination of neuroactive substances, such as neurotransmitters and prostanoids, in the extracellular space of the living brain is a very important technique in neuroscience. The hypothalamic paraventricular nucleus (PVN) is one of the most important autonomic control centers in the brain. Recently, we demonstrated that thromboxane (Tx) A2 in the PVN may play an important role in adrenomedullary outflow evoked by N-methyl-D-aspartate (NMDA), corticotrophin-releasing factor (CRF), and glucagon-like peptide-1 (GLP-1) stimulation using microdialysis sampling and liquid chromatography-ion trap tandem mass spectrometry (LC-ITMS(n)). In the present study, we investigated whether centrally administered NMDA, CRF, and GLP-1 can release five neurotransmitters, acetylcholine (ACh), histamine, glutamate (Glu), γ-aminobutyric acid (GABA), and serotonin (5-HT), along with six prostanoids, TxB2, prostaglandin (PG) E2, PGD2, 15-deoxy-∆(12,14) (15d)-PGJ2, 6-keto-PGF1α, and PGF2α in rat PVN microdialysates after optimization of LC-ITMS(n) conditions . All stimulations increased the levels of 5-HT, TxB2, PGE2, and PGF2α (except for 5-HT stimulated with GLP-1). Only NMDA increased the levels of ACh, Glu, and GABA. Treatment with dizocilpine maleate (MK-801), a noncompetitive NMDA receptor antagonist, attenuated the NMDA-induced increase in the levels of neuroactive substances except for Glu. This is the first study to use LC-ITMS(n) to analyze neurotransmitters and prostanoids in the same microdialysates from rat PVN.

CRF1 receptor antagonists do not reverse pharmacological disruption of prepulse inhibition in rodents

RATIONALE

As enhanced corticotropin-releasing factor (CRF) transmission is associated with induction of sensorimotor gating deficits, CRF₁ receptor antagonists may reverse disrupted prepulse inhibition (PPI), an operational measure of sensorimotor gating.

OBJECTIVES

To determine the effects of CRF₁ receptor antagonists in pharmacological models of disrupted PPI and to determine if long-term elevated central CRF levels alter sensitivity towards PPI disrupting drugs.

METHODS

CP154,526 (10-40 mg/kg), SSR125543 (3-30 mg/kg) and DMP695 (40 mg/kg) were tested on PPI disruption provoked by D-amphetamine (2.5, 3 mg/kg), ketamine (5, 30 mg/kg) and MK801 (0.2, 0.5 mg/kg) in Wistar rats, C57Bl/6J and CD1 mice, and on spontaneously low PPI in Iffa Credo rats and DBA/2J mice. PPI-disrupting effects of D-amphetamine (2.5-5 mg/kg) and MK801 (0.3-1 mg/kg) were examined in CRF-overexpressing (CRFtg) mice, which display PPI deficits. Finally, we determined the influence of CP154,526 on D-amphetamine-induced dopamine outflow in nucleus accumbens and prefrontal cortex of CRFtg mice using in vivo microdialysis.

RESULTS

No CRF₁-antagonists improved PPI deficits in any test. CRFtg mice showed blunted PPI disruption in response to MK801, but not D-amphetamine. Further, D-amphetamine-induced dopamine release was less pronounced in CRFtg versus wild-type mice, a response normalized by pretreatment with CP154,526.

CONCLUSION

The inability of CRF₁ receptor antagonists to block pharmacological disruption of sensorimotor gating suggests that the involvement of CRF₁ receptors in the modulation of dopaminergic and glutamatergic neurotransmission relevant for sensory gating is limited. Furthermore, the alterations observed in CRFtg mice support the notion that long-term elevated central CRF levels induce changes in these neurotransmitter systems.

Contrasting effects of pretraining, posttraining, and pretesting infusions of corticotropin-releasing factor into the lateral amygdala: attenuation of fear memory formation but facilitation of its expression

BACKGROUND

The lateral nucleus of the amygdala (LA) is a crucial part of the neural circuitry underlying the formation and storage of memories established through fear conditioning. To investigate corticotropin-releasing factor (CRF) contributions to fear memory in LA, the present experiments tested the effects of intra-LA infusions on the formation and expression of memory after Pavlovian fear conditioning.

METHODS

In experiment 1, CRF was infused bilaterally into LA of rats 1 hour before fear conditioning training. Two days later, rats were tested for conditioned stimulus (CS)-elicited freezing behavior in a distinct context. In experiment 2, rats were infused with CRF in LA immediately after auditory fear conditioning and then tested 2 days later. In experiment 3, rats were fear conditioned and then 2 days later infused with CRF in LA 1 hour before fear memory testing to assess effects on the expression of fear memory. Finally, we repeated the pretraining and pretesting experiments with the central nucleus of the amygdala infusions.

RESULTS

Rats given either pretraining or posttraining CRF infusions in LA showed dose-dependent suppression of CS-elicited freezing in the fear memory test session. In contrast, rats given pretesting CRF showed facilitation of CS-elicited freezing. Corticotropin-releasing factor infusions into the central nucleus of the amygdala had no effect when given before-training or testing.

CONCLUSIONS

Corticotropin-releasing factor infusions into LA impair the consolidation of memory for fear conditioning but enhance the expression of pre-established fear memories. These findings may have important implications for understanding mechanisms underlying contributions of CRF to fear-related disorders.

Tyrosine depletion lowers in vivo DOPA synthesis in ventral hippocampus

In vivo dopamine synthesis in the medial prefrontal cortex of the rat is sensitive to the availability of tyrosine. Whether other limbic cortical dopamine terminal regions are similarly tyrosine-dependent is not known. In this study we examined the effects of tyrosine depletion on dopamine synthesis and catecholamine levels in the ventral hippocampus. A tyrosine- and phenylalanine-free neutral amino acid mixture was used to lower brain tyrosine levels in rats undergoing in vivo microdialysis. In one group, NSD-1015 was included in perfusate to permit measurement of DOPA levels. In a second group, NSD-1015 was not included in perfusate so that catecholamine levels could be assayed. Tyrosine depletion significantly lowered DOPA levels in the NSD-1015 treated group and lowered DOPAC but not dopamine or noradrenaline levels in the group not exposed to NSD-1015. We conclude that while catecholamine synthesis in the ventral hippocampus declines when tyrosine availability is lowered, under basal conditions, compensatory mechanisms are able to maintain stable extracellular catecholamine levels.

Corticotropin-releasing factor and noradrenergic signalling exert reciprocal control over startle reactivity

Corticotropin-releasing factor (CRF) and norepinephrine (NE) levels are altered in post-traumatic stress disorder and may be related to symptoms of hyperarousal, including exaggerated startle, in these patients. In animals, activation of both systems modulates anxiety behaviours including startle plasticity; however, it is unknown if they exert their actions orthogonally or dependently. We tested the hypothesis that NE receptor activation is required for CRF effects on startle and that CRF1 receptor activation is required for NE effects on startle. The study examined the effects of: (1) α2 agonist clonidine (0.18 mg/kg i.p.), α1 antagonist prazosin (0.8 mg/kg), and β1/2 antagonist propranolol (0.8, 8.0 mg/kg) pretreatment on ovine-CRF (oCRF)- (0.6 nmol) induced increases in startle reactivity and disruption of prepulse inhibition (PPI); (2) α2 antagonist atipamezole (1-30 mg/kg) and α1 agonist cirazoline (0.025-1.0 mg/kg) treatment on startle; (3) CRF1 antagonist (antalarmin, 14 mg/kg) pretreatment on atipamezole- (10.0 mg/kg) induced increases in startle. oCRF robustly increased startle and reduced PPI. Pretreatment with clonidine or prazosin, but not propranolol, blocked oCRF-induced increases in startle but had no effect on oCRF-induced disruptions in PPI. Atipamezole treatment increased startle, which was partially attenuated by CRF1 antagonist pretreatment. Cirazoline treatment did not increase startle. These findings suggest that CRF modulation of startle, but not PPI, requires activation of α1 adrenergic receptors, while CRF1 activation also contributes to NE modulation of startle. These data support a bi-directional model of CRF-NE modulation of stress responses and suggest that both systems must be activated to induce stress effects on startle reactivity.

Roles of two subtypes of corticotrophin-releasing factor receptor in the corticostriatal long-term potentiation under cocaine withdrawal condition

The roles of two subtypes of corticotrophin-releasing factor (CRF) receptor in corticostriatal synaptic plasticity under cocaine withdrawal condition were examined in this study. Neither the resting membrane potential and input resistance of striatal neurons nor the long-term potentiation (LTP) of corticostriatal slices were affected by cocaine withdrawal. CRF dose-dependently enhanced in vitro corticostriatal LTP in rats from both cocaine-withdrawal and saline-control groups. Yet, the enhancement of corticostriatal LTP by CRF (20, 40, 80 nM) was significantly greater in the cocaine-withdrawal group than in the control group. CRF(1)-selective antagonist (NBI 27914, 100 nM) attenuated the CRF-induced enhancement of corticostriatal LTP in both groups, whereas the CRF(2)-selective antagonist (astression2B, 100 nM) attenuated the enhanced corticostriatal LTP only in the cocaine-withdrawal group. Importantly, urocortin2 (a CRF(2)-selective agonist, 40 nM) selectively increased corticostriatal LTP in the cocaine-withdrawal group, but not in the saline controls. The urocortin2-induced enhancement of LTP was totally blocked by astression2B (100 nM). These results suggest that the CRF system modulate neuroadaptive changes in the corticostriatal circuit during cocaine withdrawal, and the CRF(2) in this area mediate an important mechanism that contributes to the relapse of cocaine addiction.

Urocortin-1 and -2 double-deficient mice show robust anxiolytic phenotype and modified serotonergic activity in anxiety circuits

The urocortin (Ucn) family of neuropeptides is suggested to be involved in homeostatic coping mechanisms of the central stress response through the activation of corticotropin-releasing factor receptor type 2 (CRFR2). The neuropeptides, Ucn1 and Ucn2, serve as endogenous ligands for the CRFR2, which is highly expressed by the dorsal raphe serotonergic neurons and is suggested to be involved in regulating major component of the central stress response. Here, we describe genetically modified mice in which both Ucn1 and Ucn2 are developmentally deleted. The double knockout mice showed a robust anxiolytic phenotype and altered hypothalamic-pituitary-adrenal axis activity compared with wild-type mice. The significant reduction in anxiety-like behavior observed in these mice was further enhanced after exposure to acute stress, and was correlated with the levels of serotonin and 5-hydroxyindoleacetic acid measured in brain regions associated with anxiety circuits. Thus, we propose that the Ucn/CRFR2 serotonergic system has an important role in regulating homeostatic equilibrium under challenge conditions.

Chemical modification of class II G protein-coupled receptor ligands: frontiers in the development of peptide analogs as neuroendocrine pharmacological therapies

Recent research and clinical data have begun to demonstrate the huge potential therapeutic importance of ligands that modulate the activity of the secretin-like, Class II, G protein-coupled receptors (GPCRs). Ligands that can modulate the activity of these Class II GPCRs may have important clinical roles in the treatment of a wide variety of conditions such as osteoporosis, diabetes, amyotrophic lateral sclerosis and autism spectrum disorders. While these receptors present important new therapeutic targets, the large glycoprotein nature of their cognate ligands poses many problems with respect to therapeutic peptidergic drug design. These native peptides often exhibit poor bioavailability, metabolic instability, poor receptor selectivity and resultant low potencies in vivo. Recently, increased attention has been paid to the structural modification of these peptides to enhance their therapeutic efficacy. Successful modification strategies have included d-amino acid substitutions, selective truncation, and fatty acid acylation of the peptide. Through these and other processes, these novel peptide ligand analogs can demonstrate enhanced receptor subtype selectivity, directed signal transduction pathway activation, resistance to proteolytic degradation, and improved systemic bioavailability. In the future, it is likely, through additional modification strategies such as addition of circulation-stabilizing transferrin moieties, that the therapeutic pharmacopeia of drugs targeted towards Class II secretin-like receptors may rival that of the Class I rhodopsin-like receptors that currently provide the majority of clinically used GPCR-based therapeutics. Currently, Class II-based drugs include synthesized analogs of vasoactive intestinal peptide for type 2 diabetes or parathyroid hormone for osteoporosis.

Stress, alcohol and drug interaction: an update of human research

A challenging question that continues unanswered in the field of addiction is why some individuals are more vulnerable to substance use disorders than others. Numerous risk factors for alcohol and other drugs of abuse, including exposure to various forms of stress, have been identified in clinical studies. However, the neurobiological mechanisms that underlie this relationship remain unclear. Critical neurotransmitters, hormones and neurobiological sites have been recognized, which may provide the substrates that convey individual differences in vulnerability to addiction. With the advent of more sophisticated measures of brain function in humans, such as functional imaging technology, the mechanisms and neural pathways involved in the interactions between drugs of abuse, the mesocorticolimbic dopamine system and stress systems are beginning to be characterized. This review provides a neuroadaptive perspective regarding the role of the hormonal and brain stress systems in drug addiction with a focus on the changes that occur during the transition from occasional drug use to drug dependence. We also review factors that contribute to different levels of hormonal/brain stress activation, which has implications for understanding individual vulnerability to drug dependence. Ultimately, these efforts may improve our chances of designing treatment strategies that target addiction at the core of the disorder.

Differential blockade of CRF-evoked behaviors by depletion of norepinephrine and serotonin in rats

RATIONALE

Central administration of corticotropin-releasing factor (CRF) elicits a specific pattern of behavioral responses resembling a stress-like state and is anxiogenic in rodent models of anxiety.

OBJECTIVES

Specific behaviors evoked by the administration of CRF were measured. The roles of CRF receptor subtypes and that of serotonergic and noradrenergic systems in mediating these responses were studied.

MATERIALS AND METHODS

Burying, grooming, and head shakes were quantified in rats following intracerebroventricular administration of CRF and urocortin II and after pretreatment with antagonists. The role of forebrain norepinephrine in the behavioral responses to CRF (0.3 microg) was examined following pretreatment with the neurotoxin DSP-4 and that of serotonin after depletion using systemic administration of para-chlorophenylalanine (p-CPA).

RESULTS

CRF at 0.3 and 3.0 microg caused robust increases in burying, grooming, and head shakes, but urocortin II was ineffective. Pretreatment with either antalarmin or propranolol significantly attenuated the CRF-evoked behaviors. Destruction of forebrain norepinephrine pathways blocked spontaneous burying behavior elicited by CRF and conditioned burying directed towards an electrified shock probe. In contrast, depletion of 5-HT selectively attenuated CRF-evoked grooming.

CONCLUSIONS

Overt behavioral responses produced by CRF, burying, grooming, and head shakes appeared to be mediated through the CRF(1) receptor. Spontaneous burying behavior evoked by CRF or conditioned burying directed towards a shock probe was disrupted by lesion of the dorsal noradrenergic bundle and may represent anxiety-like behavior caused by CRF activation of the locus ceruleus. In contrast, CRF-evoked increases in grooming were dependent on serotonin.

Corticotropin-releasing factor in the basolateral amygdala enhances memory consolidation via an interaction with the beta-adrenoceptor-cAMP pathway: dependence on glucocorticoid receptor activation

Extensive evidence indicates that stress hormone effects on the consolidation of emotionally influenced memory involve noradrenergic activation of the basolateral complex of the amygdala (BLA). The present experiments examined whether corticotropin-releasing factor (CRF) modulates memory consolidation via an interaction with the beta-adrenoceptor-cAMP system in the BLA. In a first experiment, male Sprague Dawley rats received bilateral infusions of the CRF-binding protein ligand inhibitor CRF(6-33) into the BLA either alone or together with the CRF receptor antagonist alpha-helical CRF(9-41) immediately after inhibitory avoidance training. CRF(6-33) induced dose-dependent enhancement of 48 h retention latencies, which was blocked by coadministration of alpha-helical CRF(9-41), suggesting that CRF(6-33) enhances memory consolidation by displacing CRF from its binding protein, thereby increasing "free" endogenous CRF concentrations. In a second experiment, intra-BLA infusions of atenolol (beta-adrenoceptor antagonist) and Rp-cAMPS (cAMP inhibitor), but not prazosin (alpha(1)-adrenoceptor antagonist), blocked CRF(6-33)-induced retention enhancement. In a third experiment, the CRF receptor antagonist alpha-helical CRF(9-41) administered into the BLA immediately after training attenuated the dose-response effects of concurrent intra-BLA infusions of clenbuterol (beta-adrenoceptor agonist). In contrast, alpha-helical CRF(9-41) did not alter retention enhancement induced by posttraining intra-BLA infusions of either cirazoline (alpha(1)-adrenoceptor agonist) or 8-br-cAMP (cAMP analog). These findings suggest that CRF facilitates the memory-modulatory effects of noradrenergic stimulation in the BLA via an interaction with the beta-adrenoceptor-cAMP cascade, at a locus between the membrane-bound beta-adrenoceptor and the intracellular cAMP formation site. Moreover, consistent with evidence that glucocorticoids enhance memory consolidation via a similar interaction with the beta-adrenoceptor-cAMP cascade, a last experiment found that the CRF and glucocorticoid systems within the BLA interact in influencing beta-adrenoceptor-cAMP effects on memory consolidation.

Moderate perinatal arsenic exposure alters neuroendocrine markers associated with depression and increases depressive-like behaviors in adult mouse offspring

Arsenic is one of the most common heavy metal contaminants found in the environment, particularly in water. We examined the impact of perinatal exposure to relatively low levels of arsenic (50 parts per billion, ppb) on neuroendocrine markers associated with depression and depressive-like behaviors in affected adult C57BL/6J mouse offspring. Whereas most biomedical research on arsenic has focused on its carcinogenic potential, a few studies suggest that arsenic can adversely affect brain development and neural function. Compared to controls, offspring exposed to 50 parts per billion arsenic during the perinatal period had significantly elevated serum corticosterone levels, reduced whole hippocampal CRFR 1 protein level and elevated dorsal hippocampal serotonin 5HT 1A receptor binding and receptor-effector coupling. 5HT 1A receptor binding and receptor-effector coupling were not different in the ventral hippocampal formation, entorhinal or parietal cortices, or inferior colliculus. Perinatal arsenic exposure also significantly increased learned helplessness and measures of immobility in a forced swim task. Taken together, these results suggest that perinatal arsenic exposure may disrupt the regulatory interactions between the hypothalamic-pituitary-adrenal axis and the serotonergic system in the dorsal hippocampal formation in a manner that predisposes affected offspring to depressive-like behavior. These results are the first to demonstrate that relatively low levels of arsenic exposure during development can have long-lasting adverse effects on behavior and neurobiological markers associated with these behavioral changes.

Discriminative stimulus properties of the selective norepinephrine reuptake inhibitor, reboxetine, in rats: a characterization with alpha/beta-adrenoceptor subtype selective ligands, antidepressants, and antagonists at neuropeptide receptors

Although little information is available concerning discriminative stimulus (DS) properties of antidepressants, rats can be trained to recognize the selective norepinephrine (NE) reuptake inhibitor, reboxetine (2.5 mg/kg i.p.). By analogy to reboxetine (effective dose50, 1.1), 'full' (80%) substitution dose50 was obtained with the NE reuptake inhibitors, nisoxetine (4.9), nomifensine (0.5) and BW1555,U88 (1.0). Full substitution was also attained with the NE/serotonin (5-HT) reuptake inhibitors, S33005 (0.3), venlafaxine (4.8) and duloxetine (26.8), and the tricyclics, imipramine (2.5) and clomipramine (2.9). In contrast, the 5-HT reuptake inhibitors, citalopram, sertraline and paroxetine (all >2.5), and the 5-HT reuptake inhibitors/5-HT2 receptor antagonists, nefazodone and trazodone (both >10.0), did not substitute for reboxetine. The 'atypical' antidepressants, mirtazapine (>10.0) and mianserin (>2.5), similarly failed to substitute. DS properties of reboxetine were dose-dependently blocked by the alpha1-adrenoceptor (AR) antagonists, prazosin (inhibitory dose50, 0.3) and WB4101 (0.5), but resistant to the alpha2-AR antagonists, atipamezole (>0.63), idazoxan (>2.5) and RX821,002 (>0.08), and to the beta1-AR and beta2-AR antagonists, betaxolol (>2.5) and ICI118,551 (>10.0). Interestingly, the neurokinin-1 receptor antagonist, GR205,171, stereospecifically substituted for reboxetine (1.1) compared to its less active isomer, GR226,206 (>10.0). The corticotrophin-releasing factor-1 antagonists, DMP695 (>40), CP154,526 (>10.0) and SN003 (>40.0), and the melanin-concentrating hormone-1 antagonist, SNAP-7941 (>40.0), failed to substitute for reboxetine. In conclusion, DS properties of reboxetine are mimicked by antidepressants recognizing NE transporters, and require functionally intact alpha1-ARs for their expression. The neurokinin-1 antagonist, GR205,171, mimics the interoceptive properties of reboxetine, possibly reflecting its elevation of extracellular levels of NE in corticolimbic structures.

The CRH1 antagonist CP154,526 failed to alter ischemia-induced neurodegeneration and spatial memory deficits in rats but inhibited behavioral activity in the novel open field

Corticotropin-releasing hormone (CRH) has been implicated in ischemia-induced neurotoxicity, due in part to excitatory effects at the hippocampus, and the demonstrated neuroprotective effects of centrally administered, non-specific CRH antagonists. However, a number of issues remain to be clarified from these studies, including the relative contribution of CRH receptor subtypes, and the efficacy of these compounds to alter ischemia-induced behavioral impairments. In the current study, a highly selective, systemically administered CRH1 antagonist (CP154,526) failed to reverse global ischemia-induced cell death in hippocampal CA1 neurons or spatial memory impairments as assessed in the radial arm maze. Similarly, central administration of alpha-helical CRH failed to confer protection against ischemic damage. Interestingly, CRH1 antagonism reversed ischemia-induced hyperactivity in a novel open field, suggesting that modulation of this behavior is independent of effects on hippocampal CA1 cell loss. Failure of the current study to demonstrate neuroprotective effects of either the selective or non-selective CRH antagonists tested challenges the proposed neurotoxic role of CRH in global ischemia. These findings are discussed in relationship to recent findings reconsidering the participation of CRH in excitotoxic-mediated cellular damage.

Interactions between corticotropin-releasing hormone and serotonin: implications for the aetiology and treatment of anxiety disorders

The amount of evidence for a role of aberrant serotoninergic neurotransmission in the aetiology of anxiety disorders, such as generalised anxiety and panic disorder, has been increasing steadily during the past several years. Although the picture is far from complete yet--partly due to the large number of serotonin (5-HT) receptors and the often-disparate effects of receptor agonists and antagonists in animal models of anxiety--SSRIs and the 5-HT1A agonist buspirone have now earned their place in the treatment of anxiety disorders. However, these drugs show--as they do in depressed patients--a delayed onset of improvement. Therefore, new therapeutical strategies are being explored. Corticotropin-releasing hormone (CRH), which plays a key role in the autonomic, neuroendocrine and behavioural responses to stress, is a strong anxiogenic neuropeptide and a promising candidate for therapeutical intervention in anxiety disorders. The neuroanatomical localisation of CRH, its congeners (the urocortins) and their receptors within the serotoninergic raphé nuclei suggests that interactions between the CRH system and 5-HT may play a role in fear and anxiety. In this chapter, I will discuss studies from my own and other laboratories showing that CRH and the urocortins influence several aspects of serotoninergic neurotransmission, including the firing rate of 5-HT neurones and the release and synthesis of this monoamine. Moreover, the interactions between CRH and 5-HT during psychologically stressful challenges will be discussed. Finally, I will review data showing that long-term alterations in the CRH system lead to aberrant functioning of serotoninergic neurotransmission under basal and/or stressful conditions. From this growing set of data the picture is emerging that the CRH system exerts a vast modulatory influence on 5-HT neurotransmission. An aberrant cross-talk between CRH and 5-HT may be of crucial importance in the neurobiology of anxiety disorders and represents, therefore, a promising goal for therapeutical intervention in these psychiatric diseases.

Chronic corticotropin-releasing factor type 1 receptor antagonism with antalarmin regulates the dopaminergic system of Fawn-Hooded rats

Corticotropin-releasing factor is a neuropeptide associated with the integration of physiological and behavioural responses to stress and also in the modulation of affective state and drug reward. The selective, centrally acting corticotropin-releasing factor type 1 receptor antagonist, antalarmin, is a potent anxiolytic and reduces volitional ethanol consumption in Fawn-Hooded rats. The efficacy of antalarmin to reduce ethanol consumption increased with time, suggestive of adaptation to reinforcement processes and goal-directed behaviour. The aim of the present study was to examine the effects of chronic antalarmin treatment on reward-related regions of Fawn-Hooded rat brain. Bi-daily antalarmin treatment (20 mg/kg, i.p.) for 10 days increased tyrosine hydroxylase messenger RNA expression throughout the ventral mesencephalon. Following chronic antalarmin the density of dopaminergic terminals within the basal ganglia and amygdaloid complex were reduced, as was dopamine transporter binding within the striatum. Receptor autoradiography indicated an up-regulation of dopamine D2, but no change in D1, binding in striatum, and Golgi-Cox analysis of striatal medium spiny neurones indicated that chronic antalarmin treatment increased spine density. Thus, chronic antalarmin treatment modulates dopaminergic pathways and implies that chronic treatment with drugs of this class may ultimately alter postsynaptic signaling mechanisms within the basal ganglia.

Time-dependent changes in CRH concentrations and release in discrete brain regions following global ischemia: effects of MK-801 pretreatment

The excitatory actions of corticotropin-releasing hormone (CRH) in the brain and the neuroprotective effects of CRH antagonists in models of ischemia suggest a role for this peptide in the cascade of events leading to cellular damage. The present study aimed to characterize endogenous activation of CRH in discrete brain regions following global ischemia. Time-dependent changes in CRH concentrations were assessed in 10 brain regions including hippocampal, parahippocampal, and hypothalamic regions as well as the amygdala and the frontal cortex at three post-ischemic intervals: 4, 24, and 72 h (Experiment 1). The impact of pretreatment with a neuroprotective dose of the NMDA antagonist (5R,10S)-(+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801; hydrogen maleate) on 24-h ischemia-induced CRH concentrations in the 10 brain regions was also determined (Experiment 2). In vivo microdialysis was used to assess dynamic fluctuations in CRH release at the dorsal hippocampus (CA1 pyramidal layer) and central nucleus of the amygdala (CeA; Experiment 3). Our findings revealed a rapid elevation of CRH concentrations at the piriform cortex (Pir) and hypothalamic nuclei following global ischemia. This was followed by decreased CRH concentrations at the amygdala, the frontal cortex (FC), the CA3, and the hypothalamus 24-h post-ischemia. MK-801 reversed the decreases in the hypothalamic nuclei but not in the other brain regions. Seventy-two hours post-ischemia, CRH levels returned to control values in all regions except the dentate gyrus (DG) where elevated CRH levels were observed. In vivo, a significant increase in CRH release in response to global ischemia was found at the CeA with no alterations at the CA1. These findings support brain region-specific ischemia-induced CRH alterations and suggest that CRH actions to mediate neuronal damage at the hippocampal CA1 layer may be indirect.

CRF and CRF receptors: role in stress responsivity and other behaviors

Since corticotropin-releasing factor (CRF) was first characterized, a growing family of ligands and receptors has evolved. The mammalian family members include CRF, urocortinI (UcnI), UcnII, and UcnIII, along with two receptors, CRFR1 and CRFR2, and a CRF binding protein. These family members differ in their tissue distribution and pharmacology. Studies have provided evidence supporting an important role of this family in regulation of the endocrine and behavioral responses to stress. Although CRF appears to play a stimulatory role in stress responsivity through activation of CRFR1, specific actions of UcnII and UcnIII on CRFR2 may be important for dampening stress sensitivity. As the only ligand with high affinity for both receptors, UcnI's role may be promiscuous. Regulation of the relative contribution of the two CRF receptors to brain CRF pathways may be essential in coordinating physiological responses to stress. The development of disorders related to heightened stress sensitivity and dysregulation of stress-coping mechanisms appears to involve regulatory mechanisms of CRF family members.

Altered serotonergic neurotransmission but normal hypothalamic-pituitary-adrenocortical axis activity in mice chronically treated with the corticotropin-releasing hormone receptor type 1 antagonist NBI 30775

Antagonists of the corticotropin-releasing hormone receptor type 1 (CRH-R1) are regarded as promising tools for the treatment of stress-related psychiatric disorders. Owing to the intricate relationship between CRH and serotonin (5-HT), we studied the effects of chronic oral treatment of C57Bl6/N mice with the CRH-R1 antagonist NBI 30775 (formerly known as R121919) on hippocampal serotonergic neurotransmission during basal (on 15th day of treatment) and stress (forced swimming; on 16th day of treatment) conditions by in vivo microdialysis. Given the important role of CRH in the regulation of hypothalamic-pituitary-adrenocortical (HPA) axis activity and behavior, the effects of NBI 30775 on dialysate-free corticosterone levels, and on home cage and forced swimming-related behavior were also assessed. Chronic administration of NBI 30775 (18.4+/-0.9 mg/kg/day) did not result in alterations in food consumption and body weight. NBI 30775 caused complex changes in hippocampal serotonergic neurotransmission. Whereas no effects on the diurnal rhythms of 5-HT and its metabolite 5-hydroxyindoleacetic acid were found, the responses of the neurotransmitter and its metabolite to 10 min of forced swim stress were reduced and prolonged, respectively. NBI 30775 did not change free corticosterone levels over the diurnal rhythm. Moreover, NBI 30775-treated mice showed a similar forced swim stress-induced increase in corticosterone as observed in the control group. No effects of NBI 30775 on home cage, and swim stress-related active behaviors (climbing, swimming) and immobility were found. Thus, whereas chronic antagonism of CRH-R1 did not compromise HPA axis performance and behavior, distinct changes in serotonergic neurotransmission developed. Owing to the important role of 5-HT in the pathophysiology of mood and anxiety disorders, the latter observation may contribute to the therapeutical efficacy of CRH-R1 antagonists in these illnesses.

Anxiogenic-like effect of corticotropin-releasing factor receptor 2 antisense oligonucleotides infused into rat brain

Corticotropin-releasing factor (CRF) is widely distributed in the brain and coordinates behavioural responses to stress. Its receptor subtypes, CRF-R1 and CRF-R2, are expressed in the brain. For this study, we tested the effect of a continuous infusion of CRF-R2 antisense oligonucleotides into the lateral ventricle on anxiety-related behaviours in rats. Our results indicate that CRF-R2 antisense oligonucleotides produced an anxiogenic-like effect in elevated plus maze, black and white box and conditioned fear stress in rats. No significant effect on general locomotor activity was seen. These results indicate that inhibition of CRF-R2 induces an increase in anxiety-related behaviours suggesting an anxiogenic-like effect.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

The CRF1 receptor antagonist, DMP695, abolishes activation of locus coeruleus noradrenergic neurones by CRF in anesthetized rats

Corticotropin-releasing factor (CRF)(1) receptors have been implicated in the excitatory influence of CRF upon noradrenergic perikarya of the locus coeruleus. This study thus characterized the influence of the novel CRF(1) receptor antagonist, DMP695 (N-(2-chloro-4,6-dimethylphenyl)-1-[1-methoxymethyl-(2-methoxyethyl]-6-methyl-1H-1,2,3-triazolo[4,5-c]pyridin-4-amine mesylate), upon the electrical activity of noradrenergic perikarya in the locus coeruleus of anesthetized rats. Intracerebroventricular injection of CRF dose-dependently (0.05-4.0 microg) enhanced the firing rate of noradrenergic cell bodies and transformed their firing pattern into a burst mode. This action was dose-dependently abolished by i.v. administration of DMP695 (0.125-2.0 mg/kg i.v.), which did not itself modify the electrical activity of noradrenergic neurones. These data demonstrate antagonist properties of DMP695 at central CRF(1) receptors excitatory to ascending noradrenergic neurones, an action which may contribute to its distinctive profile of anxiolytic properties.

The pharmacology of CP-154,526, a non-peptide antagonist of the CRH1 receptor: a review

Since CRH has been shown to mediate stress-induced physiological and behavioral changes, it has been hypothesized that CRH receptor antagonists may have therapeutic potential in disorders that involve excessive CRH activity. CP-154,526 and its close analog antalarmin are potent, brain-penetrable, selective nonpeptide CRH1 receptor antagonists that were discovered in an effort to develop compounds with efficacy in CNS disorders precipitated by stress. Since its discovery many investigators have used CP-154,526 as a tool to study the pharmacology of CRH and its receptors and to evaluate its therapeutic potential in a variety of CNS and peripheral disorders. Systemically-administered CP-154,526 has been demonstrated to antagonize CRH- and stress-induced neuroendocrine, neurochemical, electrophysiological, and behavioral effects. These findings support the hypothesis that CRH1 receptor antagonists may have therapeutic utility in a number of neuropsychiatric disorders. CP-154,526, as well as other CRH1 receptor antagonists that have since been discovered, have also shown activity in several preclinical models of anxiety, depression, and substance abuse, while having little effect on locomotor activity and motor function. Although these effects are on occasion inconsistent among different laboratories, clinical evaluation of CRH1 antagonists appears justified on the basis of these and clinical data implicating the involvement of CRH in several CNS disorders. The effects of CRH1 antagonists on cognition, neurodegeneration, inflammation, and the gastrointestinal system have not been as extensively characterized and additional studies will be necessary to evaluate their therapeutic potential in these areas.

Evidence that acute serotonergic activation potentiates the locomotor-stimulating effects of corticotropin-releasing hormone in juvenile chinook salmon (Oncorhynchus tshawytscha)

The present study investigated whether the serotonergic system is involved in mediating the behavioral effects of corticotropin-releasing hormone (CRH) in juvenile spring chinook salmon, Oncorhynchus tshawytscha. An intracerebroventricular (ICV) injection of CRH induced hyperactivity. The effect of CRH was potentiated in a dose-dependent manner by the concurrent administration of the serotonin (5-HT) selective reuptake inhibitor fluoxetine. However, administration of fluoxetine alone had no effect on locomotor activity, suggesting that the locomotor-stimulating effect of CRH is mediated by the activation of the serotonergic system. Conversely, ICV injections of the 5-HT(1A) receptor antagonist NAN-190 attenuated the effect of CRH on locomotor activity when given in combination with CRH but had no effect when administered alone. These results provide the first evidence to support the hypothesis that the effect of CRH on locomotor activity in teleosts is mediated by activating the serotonergic system.

Forced swim stress activates rat hippocampal serotonergic neurotransmission involving a corticotropin-releasing hormone receptor-dependent mechanism

Serotonin is important for adequate coping with stress. Aberrant serotonin function is implicated in the aetiology of major depression and anxiety disorders. Dysregulation of the hypothalamic-pituitary-adrenocortical axis, involving elevated corticotropin-releasing hormone (CRH) activity, also plays a role in these stress-related illnesses. Here we studied the effects of stress on hippocampal serotonin and the role of the CRH system using in vivo microdialysis. First, rats were subjected to a forced swim stress, resulting in a dramatic increase in hippocampal serotonin (1500% of baseline), which was associated with the occurrence of diving behaviour. The diving-associated increase in serotonin depended on activation of CRH receptors, as it was antagonized by intracerebroventricular pretreatment with D-Phe-CRH12-41. Secondly, the effects of intracerebroventricular administration of CRH and urocortin (0.03-1.0 microg) were studied. Both CRH and urocortin caused a dose-dependent rise in hippocampal serotonin (maximally 350% of baseline) and 5-hydroxyindoleacetic acid levels, suggesting the involvement of CRH receptor type 1. Because the effects of urocortin were prolonged, CRH receptor type 2 could play a role in a later phase of the neurotransmitter response. Experiments using adrenalectomized rats showed that CRH-induced serotonin changes were adrenally independent. These data suggest that the raphe-hippocampal serotonin system is able to mount, CRH receptor-dependent, responses to specific stressful situations that surpass the usually observed maximal increases of about 300% of baseline during stress and enhanced vigilance.