CRF receptor antagonist attenuates stress-induced noradrenaline release in the medial prefrontal cortex of rats

Noradrenergic stimulation of α1 adrenoceptors in the medial prefrontal cortex mediates acute stress-induced facilitation of seizures in mice

Stress is one of the critical facilitators for seizure induction in patients with epilepsy. However, the neural mechanisms underlying this facilitation remain poorly understood. Here, we investigated whether noradrenaline (NA) transmission enhanced by stress exposure facilitates the induction of medial prefrontal cortex (mPFC)-originated seizures. In mPFC slices, whole-cell current-clamp recordings revealed that bath application of picrotoxin induced sporadic epileptiform activities (EAs), which consisted of depolarization with bursts of action potentials in layer 5 pyramidal cells. Addition of NA dramatically shortened the latency and increased the number of EAs. Simultaneous whole-cell and field potential recordings revealed that the EAs are synchronous in the mPFC local circuit. Terazosin, but not atipamezole or timolol, inhibited EA facilitation, indicating the involvement of α₁ adrenoceptors. Intra-mPFC picrotoxin infusion induced seizures in mice in vivo. Addition of NA substantially shortened the seizure latency, while co-infusion of terazosin into the mPFC inhibited the effect of NA. Finally, acute restraint stress shortened the latency of intra-mPFC picrotoxin infusion-induced seizures, whereas prior infusion of terazosin reversed this stress-induced shortening of seizure latency. Our findings suggest that stress facilitates the induction of mPFC-originated seizures via NA stimulation of α₁ adrenoceptors.

The Antidepressant Effect of L-Tyrosine-Loaded Nanoparticles: Behavioral Aspects

BACKGROUND

Depression has been linked to disruption in the cerebral levels of specific neurotransmitters. L-tyrosine is a precursor of more than one of the neurotransmitters affected by depression. Even though setbacks of monoamines precursors include high doses and low efficiency, many studies have suggested using L-tyrosine as antidepressant.

PURPOSE

The purpose of this study was to explore the possible antidepressant effect of L-tyrosine loaded in a nanoparticle-designed formula, using behavioral tests in acute and chronic mild stress (CMS) models of depression in rats.

METHODS

Animals from both models received L-tyrosine-loaded nanoparticles (5 or 10 mg/kg), L-tyrosine solution (10 mg/kg), fluoxetine (10 mg/kg) or placebo daily for 21 days. Rats from the acute stress model of depression were subjected to open field and forced swim tests (FSTs). For the CMS model, sucrose preference test was carried out. Additionally, 3 profiles of the nanoparticles formula were tested in vitro. High dissolution rate and entrapment efficiency were obtained from the in vitro tests. Moreover, L-tyrosine-loaded nanoparticles 10 mg/kg and fluoxetine 10 mg/kg significantly decreased the immobility time in the FST, concomitant with restoration of the basal levels of locomotor activity, distance travelled and rearing counts. Also, an increase of the sucrose consumption was recorded in the sucrose preference test after treatment with L-tyrosine-loaded nanoparticles 10 mg/kg and fluoxetine 10 mg/kg.

RESULTS

The positive results after treatment with L-tyrosine-loaded nanoparticles, through behavioral tests, are probably attributed to restorating the basal levels of the cerebral noradrenaline.

CONCLUSION

The effects of L-tyrosine administration on the cerebral levels of tyrosine hydroxylase and corticotropin-releasing factor should be further investigated.

Experiential and genetic contributions to depressive- and anxiety-like disorders: clinical and experimental studies

Stressful events have been implicated in the precipitation of depression and anxiety. These disorders may evolve owing to one or more of an array of neuronal changes that occur in several brain regions. It seems likely that these stressor-provoked neurochemical alterations are moderated by genetic determinants, as well as by a constellation of experiential and environmental factors. Indeed, animal studies have shown that vulnerability to depressive-like behaviors involve mechanisms similar to those associated with human depression (e.g., altered serotonin, corticotropin releasing hormone and their receptors, growth factors), and that the effects of stressors are influenced by previous stressor experiences, particularly those encountered early in life. These stressor effects might reflect sensitization of neuronal functioning, phenotypic changes of processes that lead to neurochemical release or receptor sensitivity, or epigenetic processes that modify expression of specific genes associated with stressor reactivity. It is suggested that depression is a life-long disorder, which even after effective treatment, has a high rate of re-occurrence owing to sensitized processes or epigenetic factors that promote persistent alterations of gene expression.

A centrally acting, anxiolytic angiotensin II AT1 receptor antagonist prevents the isolation stress-induced decrease in cortical CRF1 receptor and benzodiazepine binding

Long-term pretreatment with an angiotensin II AT1 antagonist blocks angiotensin II effects in brain and peripheral organs and abolishes the sympathoadrenal and hypothalamic-pituitary-adrenal responses to isolation stress. We determined whether AT1 receptors were also important for the stress response of higher regulatory centers. We studied angiotensin II and corticotropin-releasing factor (CRF) receptors and benzodiazepine binding sites in brains of Wistar Hannover rats. Animals were pretreated for 13 days with vehicle or a central and peripheral AT1 antagonist (candesartan, 0.5 mg/kg/day) via osmotic minipumps followed by 24 h of isolation in metabolic cages, or kept grouped throughout the study (grouped controls). In another study, we determined the influence of a similar treatment with candesartan on performance in an elevated plus-maze. AT1 receptor blockade prevented the isolation-induced increase in brain AT1 receptors and decrease in AT2 binding in the locus coeruleus. AT1 receptor antagonism also prevented the increase in tyrosine hydroxylase mRNA in the locus coeruleus. Pretreatment with the AT1 receptor antagonist completely prevented the decrease in cortical CRF1 receptor and benzodiazepine binding produced by isolation stress. In addition, pretreatment with candesartan increased the time spent in and the number of entries to open arms of the elevated plus-maze, measure of decreased anxiety. Our results implicate a modulation of upstream neurotransmission processes regulating cortical CRF1 receptors and the GABA(A) complex as molecular mechanisms responsible for the anti-anxiety effect of centrally acting AT1 receptor antagonists. We propose that AT1 receptor antagonists can be considered as compounds with possible therapeutic anti-stress and anti-anxiety properties.

Comparison of the effects of diazepam, the CRF1 antagonist CP-154,526 and the group II mGlu receptor agonist LY379268 on stress-evoked extracellular norepinephrine levels

The present study used an elevated platform procedure to investigate the effects of diazepam, a CRF1 antagonist CP-154,526 and a group II mGlu2/3 receptor agonist LY379268 on stress-evoked increase in extracellular norepinephrine (NE). Pretreatment with either diazepam (1 mg/kg, i.p.), CP-154,526 (20 mg/kg, i.p.) or LY379268 (1, 3 and 10 mg/kg, p.o.) significantly reduced platform stress-evoked NE. Interestingly, at the highest dose tested (10 mg/kg) LY379268 caused a marked increase in baseline NE levels. We tested whether this effect would diminish after repeated dosing. In contrast to acute administration, a challenge injection of LY379268 after repeated dosing (10 mg/kg x days) did not alter basal NE. Importantly, although less effective, LY379268 still significantly reduced stress-evoked NE. We further show that this increase in basal NE may involve mGlu2/3 receptor regulation of the GABAergic system. To this end, administration of the GABAB agonist, baclofen (4 mg/kg, i.p.), 2 h after dosing with LY379268, reversed the increase in baseline NE. These data suggest that, like diazepam and CP-154,526, group II mGlu2/3 receptor agonists can attenuate stress-evoked increase in extracellular NE in the rat prefrontal cortex. In addition they reveal a 'stress-like' increase in NE after high doses of LY379268 which may reflect mGlu3 receptor modulation of GABAergic transmission.

Functional magnetic resonance mapping of intracerebroventricular infusion of a neuroactive peptide in the anaesthetised rat

Pharmacological magnetic resonance imaging (phMRI) methods map the cerebral haemodynamic response to challenge with psychotropic agents as a surrogate for drug-induced changes in brain activity. However, many neuroactive compounds present low blood-brain barrier penetration and thus systemic administration may result in insufficient brain concentration. Intracerebroventricular (ICV) administration has been long used as an effective way of bypassing the blood-brain barrier in studies with non-brain-penetrant compounds, such as neuropeptides. In order to extend the range of pharmacological substances accessible to phMRI, we have developed methods to map relative cerebral blood volume (rCBV) changes induced by in situ ICV administration of neuroactive agents in the anaesthetised rat. We have applied this method to study for the first time the phMRI response to central administration of a neuropeptide, the metabolically stable and potent NK1 receptor agonist GR-73632. ICV administration of 4.2 pmol of GR-73632 produced a rapid onset and sustained rCBV increase in several brain structures, such as the amygdala, the caudate putamen and the cortex. These results demonstrate the feasibility of phMRI as a tool to study the functional correlates of brain activity induced by central administration of neuroactive agents.

Role of noradrenergic projections to the bed nucleus of the stria terminalis in the regulation of the hypothalamic-pituitary-adrenal axis

The bed nucleus of the stria terminalis (BNST) plays an important role in the regulation of the hypothalamic-pituitary-adrenal (HPA) axis during stress and it is a major extrahypothalamic relay to the paraventricular nucleus of the hypothalamus (PVN) from the amygdala and the hippocampus. In this review, we discuss the anatomical, neurochemical and behavioral evidence that substantiate a role for noradrenergic terminals of the anterior BNST in the regulation of the HPA axis. We propose the hypothesis that BNST noradrenaline (NA) participates in the regulation of the hippocampal inhibitory influence on the HPA axis activation. The observation that NA exerts a tonic inhibitory effect upon glutamatergic transmission in the anterior BNST supports this hypothesis. We also discuss the known mechanisms involved in the regulation of BNST NA extracellular levels and the possible interactions between NA and corticotropin-releasing hormone (CRH), and of CRH with glutamate (GLU) in the regulation of the HPA axis activity exerted by the BNST. The evidence discussed in the present review situates the BNST as a key extrahypothalamic center that relays and integrates limbic and autonomic information related to stress responses suggesting that dysregulation in the functioning of the BNST may underlie the pathophysiology of stress-related psychiatric disorders.

Pavlovian autoshaping procedures increase plasma corticosterone and levels of norepinephrine and serotonin in prefrontal cortex in rats

Pavlovian autoshaping procedures provide for pairings of a small object conditioned stimulus (CS) with a rewarding substance unconditioned stimulus (US), resulting in the acquisition of complex sequences of CS-directed skeletal-motor responses or autoshaping conditioned responses (CRs). Autoshaping procedures induce higher post-session levels of corticosterone than in controls receiving CS and US randomly, and the enhanced post-session corticosterone levels have been attributed to the appetitive or arousal-inducing effects of autoshaping procedures. Enhanced corticosterone release can be induced by aversive stimulation or stressful situations, where it is often accompanied by higher levels of norepinephrine (NE) and serotonin (5-HT) in prefrontal cortex (PFC) but not in striatum (ST). Effects of autoshaping procedures on post-session corticosterone levels, NE contents in PFC, and 5-HT contents in PFC and ST were investigated in male Long-Evans rats. Post-session blood samples revealed higher corticosterone levels in the CS-US Paired group (n = 46) than in the CS-US Random control group (n = 21), and brain samples revealed higher levels of PFC NE and 5-HT in CS-US Paired group. Striatal 5-HT levels were unaltered by the autoshaping procedures. Autoshaping procedures provide for appetitive stimulation and induce an arousal-like state, as well as simultaneous stress-like changes in plasma corticosterone and monoamine levels in PFC. Autoshaping, therefore, may be useful for the study of endocrine and central processes associated with appetitive conditions.

Differential involvement of amygdaloid CRH system(s) in the salience and valence of the stimuli

Anxiety is a heterogeneous term encompassing not only state or trait characteristics but also a wide range of pathologies such as generalized anxiety disorders, phobias, panic and obsessive-compulsive disorders, acute stress disorder, and posttraumatic stress disorder. Given that diverse forms of anxiety exist, numerous animal models have been developed, which are considered to be useful in identifying mechanisms underlying anxiety states. Examples of such animal models include paradigms that assess the behavioral response to neurogenic (or painful stimuli) or psychogenic stressors or to cues that had previously been associated with painful stimuli. The present report presents data regarding the impact of stressors on corticotropin-releasing hormone (CRH), and relates these to changes in anxiety-like states. Specifically, we demonstrate that (1) psychogenic stressors influence the in vivo release of CRH at the central nucleus of the amygdala (CeA); (2) although CRH changes within the CeA are exquisitely sensitive to stressors, they are also elicited by positive stimuli; and (3) while treatment with diazepam attenuates behavioral signs of anxiety, the CRH release associated with a stressor is unaffected by the treatment. The position is offered that although release of CRH within the CeA is increased under stressful conditions, it is not a necessary condition for the consequent behavioral expression of anxiety-like reactions, at least not in minimally threatening situations. We suggest that the CRH responses at the CeA may be involved in a preparatory capacity and, as such, may accompany a range of emotionally significant stimuli, be they appetitive or aversive.

Corticotropin-releasing hormone receptor blockade fails to alter stress-evoked catecholamine release in prefrontal cortex of control or chronically stressed rats

Although it is well documented that stress can increase the activity of central dopamine and norepinephrine neurons, little is known about the role of other neurotransmitters in modulating this response. Previous studies have implicated corticotropin-releasing hormone in modulating stress-evoked changes in the activity of locus coeruleus neurons. The present study examines whether corticotropin-releasing hormone contributes to stress-evoked increases in extracellular norepinephrine and dopamine in rat medial prefrontal cortex, as monitored by in vivo microdialysis. As noted previously, 30 min of tail-shock increased extracellular levels of norepinephrine and dopamine in the medial prefrontal cortex of naïve rats, and this was enhanced in rats previously exposed to chronic cold ( approximately 5 degrees C for 2-3 weeks). Previous intraventricular administration of a corticotropin-releasing hormone antagonist (D-Phe-corticotropin-releasing hormone; 3 and 9 microg) did not alter the tail-shock evoked in increase in extracellular levels of norepinephrine and dopamine in either naïve or chronically cold-exposed rats. Intraventricular administration of 3 microg of D-Phe-corticotropin-releasing hormone attenuated the increase in extracellular norepinephrine induced by co-administration of 3 microg of corticotropin-releasing hormone, confirming the efficacy of this compound. Results of the present study suggest that endogenous corticotropin-releasing hormone does not play a role in modulating the release of norepinephrine and dopamine occurring in response to acute tail-shock or the expression of a potentiated response to tail-shock in rats exposed chronically to cold.

Central alpha-adrenergic receptors and corticotropin releasing factor mediate hemodynamic responses to acute cold stress

Behavioral stress is likely to contribute to the development of hypertension in susceptible individuals. We reported that hemodynamic response patterns to acute startle vary and that those patterns predict the predisposition of rats to sustained stress-induced elevations in arterial pressure. Since considerable evidence suggests that central catecholamines and corticotropin releasing factor (CRF) contribute to the regulation of arterial pressure and the development of hypertension, we investigated the role of central alpha-adrenergic receptors and CRF in mediating different hemodynamic response patterns to acute cold water stress in conscious rats. Rats were instrumented for arterial pressure, heart rate and cardiac output determination and for intracerebroventricular (icv) administration of selective antagonists. After acclimation to a water tight cage, ice water (1 cm deep) was rapidly added then drained 1 min later. Although the early startle response to cold water stress elicited a pressor response in all rats, the hemodynamic response pattern varied between rats. Vascular responders (n=19) had an initial considerable increase in systemic vascular resistance and a decrease in cardiac output. In contrast, mixed responders (n=11) had a smaller increase in vascular resistance and an increase in cardiac output. Pretreatment with phentolamine (30 microgram/5 microliter, icv, n=8), prazosin (10 microgram/5 microliter, icv, n=12) or alpha-helical CRF(9-41) (10 microgram/5 microliter, icv, n=9) prevented the decrease in cardiac output elicited by acute cold water stress in vascular responders without affecting mixed responders. Yohimbine (3 microgram/5 microliter, icv, n=8) pretreatment did not alter hemodynamic responses. Therefore, we conclude that central alpha(1)-adrenoceptors and CRF mediate the specific hemodynamic response patterns to acute startle and may be responsible for the predisposition to develop hypertension in vascular responders.

Depression as a spreading adjustment disorder of monoaminergic neurons: a case for primary implication of the locus coeruleus

A model for the pathophysiology of depression is discussed in the context of other existing theories. The classic monoamine theory of depression suggests that a deficit in monoamine neurotransmitters in the synaptic cleft is the primary cause of depression. More recent elaborations of the classic theory also implicitly include this postulate, other theories of depression frequently prefer to depart from the monoamine-based model altogether. We suggest that the primary defect emerges in the regulation of firing rates in brainstem monoaminergic neurons, which brings about a decrease in the tonic release of neurotransmitters in their projection areas, an increase in postsynaptic sensitivity, and concomitantly, exaggerated responses to acute increases in the presynaptic firing rate and transmitter release. It is proposed that the initial defect involves, in particular, the noradrenergic innervation from the locus coeruleus (LC). Dysregulation of the LC projection activities may lead in turn to dysregulation of serotonergic and dopaminergic neurotransmission. Failure of the LC function could explain the basic impairments in the processing of novel information, intensive processing of irrational beliefs, and anxiety. Concomitant impairments in the serotonergic neurotransmission may contribute to the mood changes and reduction in the mesotelencephalic dopaminergic activity to loss of motivation, and anhedonia. Dysregulation of CRF and other neuropeptides such as neuropeptide Y, galanin and substance P may reinforce the LC dysfunction and thus further weaken the adaptivity to stressful stimuli.

An inhibitory effect of cytokine-induced neutrophil chemoattractant on corticotropin-releasing factor-induced increase in locomotor activity

To investigate whether cytokine-induced neutrophil chemoattractant (CINC) has an influence on corticotropin-releasing factor (CRF) in the central nervous system, the effects of intracerebroventricular (i.c.v.) injection of CINC on CRF-induced behavior were examined. Intracerebroventricular CRF injection produced an increase in locomotor activity, which was significantly reduced by i.c.v. injection of CINC. The intravenous injection of CINC did not alter CRF-induced locomotor hyperactivity. These results suggested that CINC has a functional antagonistic action on the response to CRF and may attenuate stress responses.

The effects of CRA 1000, a non-peptide antagonist of corticotropin-releasing factor receptor type 1, on adaptive behaviour in the rat

Intracerebrally administered CRF has been demonstrated to elicit several behavioural deficits in novel and potentially stressful experimental paradigms, and to promote activity in familiar situations. This study examined the effect of CRA 1000, a novel non-peptide antagonist of CRF(1)receptors, on rat behaviour in tests of anxiolytic and antidepressant activity and novelty-oriented behaviour. CRA 1000 (1.25-10 mg/kg) had no major effect in elevated plus-maze and social interaction tests. However, CRA 1000 (5 mg/kg) significantly reduced immobility in the forced swimming test, suggesting an antidepressant-like effect. In the exploration box test, CRA 1000 (1.25 mg/kg) had an anxiolytic effect on rat exploratory behaviour both in intact rats and after lesioning of the projections of locus coeruleus by DSP-4 (50 mg/kg) treatment. A higher dose of CRA 1000 (5 mg/kg) tended to have anxiolytic-like effects in DSP-4 pretreated rats, but in intact animals this dose prevented the increase in exploration which develops with repeated exposure to initially anxiety-provoking situations. Taken together, these experiments demonstrate that CRF1 receptor blockade by CRA 1000 has antidepressant-like effects, does not have a robust anti-anxiety effect in non-stressed animals, but does have anxiolytic-like effects in more complex tasks, which can be observed also after denervation of the locus coeruleus projections. However, large doses of CRF1 receptor antagonists may reduce motivation of exploratory behaviour in familiar environments.

Footshock-induced changes in brain catecholamines and indoleamines are not mediated by CRF or ACTH

Stressful treatments have long been associated with increased activity of brain catecholaminergic and serotonergic neurons. An intracerebroventricular (icv) injection of the corticotropin-releasing factor (CRF) also activates brain catecholaminergic neurons. Because brain CRF-containing neurons appear to be activated during stress, it is possible that CRF mediates the catecholaminergic activation. This hypothesis has been tested by assessing the responses in brain catecholamines and indoleamines to footshock in mice pretreated icv with a CRF receptor antagonist, and in mice lacking the gene for CRF (CRFko mice). Consistent with earlier results, icv administration of CRF increased catabolites of dopamine and norepinephrine, but failed to alter tryptophan concentrations or serotonin catabolism. A brief period of footshock increased plasma corticosterone and the concentrations of tryptophan and the catabolites of dopamine, norepinephrine and serotonin in several brain regions. Mice injected icv with 25 microg alpha-helical CRF(9-41) prior to footshock had neurochemical responses that were indistinguishable from controls injected with vehicle, while the increase in plasma corticosterone was slightly attenuated in some experiments. CRFko mice exhibited neurochemical responses to footshock that were indistinguishable from wild-type mice. However, whereas wild-type mice showed the expected increase in plasma corticosterone, there was no such increase in CRFko mice. Similarly, hypophysectomized mice also showed normal neurochemical responses to footshock, but no increase in plasma corticosterone. Hypophysectomy itself elevated brain tryptophan and catecholamine and serotonin metabolism. Treatment with ACTH icv or peripherally failed to induce any changes in cerebral catecholamines and indoleamines. These results suggest that CRF and its receptors, and ACTH and other pituitary hormones, are not involved in the catecholamine and serotonin responses to a brief period of footshock.

Clonidine blocks stress-induced reinstatement of heroin seeking in rats: an effect independent of locus coeruleus noradrenergic neurons

Using a reinstatement procedure, it has been shown that intermittent footshock stress reliably reinstates extinguished drug-taking behaviour in rats. Here we studied the role of noradrenaline (NE), one of the main brain neurotransmitters involved in responses to stress, in reinstatement of heroin seeking. We first determined the effect of clonidine, an alpha-2 adrenergic receptor agonist that decreases NE cell firing and release, on stress-induced reinstatement of heroin seeking. Rats were trained to self-administer heroin (0.1 mg/kg per infusion, IV, three 3-h sessions per day) for 9-10 days. Extinction sessions were given for up to 11 days during which saline was substituted for the drug. Tests for reinstatement were then conducted after exposure to intermittent footshock (5, 15 and 30 min, 0.5 mA). During testing, clonidine was injected systemically (10-40 microgram/kg, i.p.) or directly into the lateral or fourth ventricles (1-3 microram). Clonidine (1-2 microgram per site) or its charged analogue, 2-[2, 6-diethylphenylamino]-2-imidazole (ST-91, 0.5-1 microgram per site), was also injected bilaterally into the locus coeruleus (LC), the main noradrenergic cell group in the brain. Clonidine blocked stress-induced reinstatement of drug seeking when injected systemically or into the cerebral ventricles. In contrast, neither clonidine nor ST-91 consistently altered stress-induced reinstatement when injected into the locus coeruleus. We therefore studied the effect of lesions of the lateral tegmental NE neurons on stress-induced reinstatement. 6-Hydroxydopamine (6-OHDA) lesions performed after training for heroin self-administration had no effect on extinction of heroin-taking behaviour, but significantly attenuated reinstatement induced by intermittent footshock. These data suggest that: (i) clonidine prevents stress-induced relapse to heroin seeking by its action on neurons other than those of the locus coeruleus; and (ii) activation of the lateral tegmental NE neurons contributes to stress-induced reinstatement of heroin seeking.

Discrete local application of corticotropin-releasing factor increases locus coeruleus discharge and extracellular norepinephrine in rat hippocampus

The most prominent afferents impinging upon the noradrenergic neurons of the locus coeruleus (LC) utilize GABA and glutamate. However, peptide neurotransmitters such as galanin, neuropeptide Y, and corticotropin-releasing factor (CRF) have also been localized to LC afferents. The evidence for CRF modulation of LC activity was examined in the present studies. Specifically, the impact of local CRF administration on both LC-NE discharge characteristics and release of norepinephrine (NE) in hippocampus was determined. First, the ability of CRF microinfused into the LC area to increase NE efflux in the dorsal hippocampus was determined using in vivo microdialysis techniques in awake rats. CRF into the LC dose-dependently increased extracellular NE in the ipsilateral hippocampus. Second, a more detailed analysis was performed in halothane-anesthetized rats by characterizing the electrophysiological activity of LC-NE neurons in response to local application of CRF. Changes in the firing rate and pattern of single LC-NE neurons were measured while simultaneously monitoring the extracellular level of NE in hippocampus. A dose of 30 ng CRF applied directly into LC via pressure ejection elicited an 88% increase in the discharge rate of LC-NE neurons and increased the incidence of burst firing from 14% to 33%. This manipulation simultaneously increased extracellular NE in hippocampus by 63%. The CRF-induced increases in discharge rate of LC-NE neurons and extracellular NE efflux in hippocampus were prevented by prior i.c.v. administration of the CRF antagonist, d-PheCRF(12-41 )(3 microg / 3 microl). The present findings demonstrate that CRF applied directly into the LC increases both the activity of LC-NE neurons and the release of NE in an LC terminal region. The shift in activity of LC-NE neurons to more burst-like firing in response to CRF may provide a means for enhanced release of NE in LC projection fields. This is the first report to demonstrate a dose-dependent increase in extracellular NE levels evoked by intra-LC infusion of CRF in unanesthetized animals.

Food deprivation modulates chronic stress effects on object recognition in male rats: role of monoamines and amino acids

An object recognition task was used to determine if chronic restraint stress (6 h/day for 21 days) impairs non-spatial memory, since chronic restraint is known to impair spatial memory. In addition, food deprivation was tested as a possible modulating factor of any stress effect in this non-reward-dependent task. Following 3 weeks of daily restraint, subjects were tested for open field activity and object recognition (over different delay intervals) during one week in two separate experiments. Experiment 1 involved testing under low demand conditions (small arena) while experiment 2 involved testing under higher-demand conditions (large arena). Basal monoamine and amino acid levels (home cage) were assessed in experiment one and monoamine arousal levels (following a sample trial) were assessed in experiment two. We observed that chronic stress impaired object recognition when the delay was extended beyond 1 h, and that food deprivation could attenuate the degree of impairment. In addition, chronic stress was associated with increased norepinephrine levels in both the amygdala and hippocampus, and dopamine (HVA/DA, DOPAC/DA) in prefrontal cortex. These changes were not observed in stress subjects that were subsequently food deprived. Food deprived subjects had higher basal serotonin activity in prefrontal cortex and hippocampus as well as higher serum CORT levels. Results suggest that food deprivation may act as a novel stress, thereby increasing subjects' arousal and attention toward the objects, which aids stressed subjects, especially in low-demand conditions. Both restraint and food deprivation affected select limbic areas associated with memory functioning.

Physiological and neurochemical aspects of corticotropin-releasing factor actions in the brain: the role of the locus coeruleus

Corticotropin-releasing factor (CRF) is both a major regulator of the hypothalamo-pituitary-adrenal (HPA) axis and the activity of the autonomic nervous system. Besides, it exerts numerous effects on other physiological functions such as appetite control, motor and cognitive behavior and immune function. The basis for these effects is constituted by its distribution in hypothalamic and extra-hypothalamic brain areas, the latter being represented by limbic structures such as the central nucleus of the amygdala or by brain stem neurons such as the locus coeruleus (LC) or nucleus of the solitary tract (NTS). The effects of CRF are mediated through recently described CRF-receptor subtypes, whose molecular biology, biochemistry and pharmacological regulation are discussed in detail. In the second part of this review, we will focus on the physiology of CRF-systems in the brain, with a particular emphasis on cardiovascular regulation, respiration, appetite control and stress-related behavior. Finally, the role of the locus coeruleus in the control of CRF-mediated behavioral activities is discussed. The interaction of noradrenergic and CRF-neurons clearly implies that CRF appears to directly activate LC neurons in a stressful situation, thus ultimately coordinating the bodily response to a stressful stimulus.

KAT45, a noradrenergic human pheochromocytoma cell line producing corticotropin-releasing hormone

KAT45 cells were derived from a human pheochromocytoma, which also caused ectopic Cushing's syndrome, and developed into a cell line spontaneously after the continuous primary culture of the tumor cells. These human pheochromocytoma cells were compared with the extensively characterized PC12 rat pheochromocytoma cell line. KAT45 cells resembled PC12 cells in morphology, proliferation rate, response to cholinergic stimuli, and the development of dendrite-like projections after exposure to nerve growth factor. They produced norepinephrine and epinephrine in a ratio of 50:1, as opposed to production of dopamine by PC12 cells, in amounts 1 order of magnitude higher compared with PC12. Because of the ectopic Cushing's syndrome in our patient, her normal ACTH level, and the knowledge that PC12 cells and even normal rat chromaffin cells appear to produce CRH, we examined whether KAT45 cells also produced this neuropeptide. Indeed, KAT45 cells released authentic CRH and contained an apparently intact CRH transcript. Nicotine and KCl depolarization stimulated the secretion of CRH, whereas interleukin-1beta, glucocorticoids, and nerve growth factor stimulated its synthesis. In addition to the potential systemic effects of CRH, which in our patient produced ectopic Cushing's syndrome, CRH can exert paracrine effects within normal or tumoral adrenals. We used KAT45 cells as a model for the study of the local role of CRH. CRH affected several parameters of KAT45 cell metabolism, including their proliferation rate, synthesis of catecholamines, and production of POMC-derived peptides. KAT45 cells, in addition to the data they provided regarding the in vitro profile of a human CRH-producing pheochromocytoma, may prove to be a valuable auxiliary to the PC12 cell line.

Corticotropin-releasing factor in the caudal pontine reticular nucleus mediates the expression of fear-potentiated startle in the rat

The fear-potentiated startle paradigm is a valuable model for the investigation of the neuronal basis of fear. Previous studies have demonstrated that the neuropeptide corticotropin-releasing factor (CRF) plays an important role in fear-related processes, notably in the potentiation of the acoustic startle response. The present study investigated the role in fear-potentiated startle of CRF in the caudal pontine reticular nucleus, a brain nucleus that mediates the acoustic startle response. First, we showed that the central nucleus of the amygdala gives rise to a CRFergic projection to the caudal pontine reticular nucleus. In the second experiment, we iontophoretically applied CRF to caudal pontine reticular nucleus neurons and extracellularly recorded the activity of these neurons. CRF had a mainly excitatory effect on the tone-evoked activity of the neurons. In our third experiment, we injected the CRF antagonist alpha-helical CRF into the caudal pontine reticular nucleus of awake rats. Here, alpha-helical CRF dose-dependently blocked fear-potentiated startle, but had no effect on the baseline startle amplitude. The present results show that CRF-containing neurons which project from the central nucleus to the caudal pontine reticular nucleus are important for the enhancement of startle by fear, and further characterize the hypothetical neuronal circuitry underlying the expression of fear-potentiated startle.

CRF receptor antagonist attenuates immobilization stress-induced norepinephrine release in the prefrontal cortex in rats

Neuroanatomical, neurophysiological, and behavioral studies suggest that brain stem nucleus locus coeruleus (LC) plays an important role in stress response. The present study was designed to clarify, whether infusion of CRF antagonist, alpha hCRF, into LC could attenuate or block stress-induced changes in norepinephrine (NE) concentrations in microdialysates collected from the medial prefrontal cortex (PFM). Rats were implanted with a bilateral cannulae assembly aimed in the LC and a microdialysis probe (4 mm active membrane length) into the LC. Immobilization of animals significantly increased the concentration of NE in microdialysates from PFM to a maximum of 170.8 +/- 12.8% of the baseline ten minutes after the onset of stressor. Concentration of NE in dialysates remained significantly elevated for the next 40 min. Infusion of alpha hCRF into the LC significantly attenuated stress-induced increase in PFM NE concentration in samples collected at 10, 20, 30, and 50 min after the onset of immobilization. Infusion of alpha hCRF alone (no immobilization) did not change concentrations at any time during sample collection. These results are consistent with other studies and suggest that stress can facilitate NE release in the PFM through the activation of the CRF system in the brain.

Corticotropin-releasing factor receptor antagonist infused into the locus coeruleus attenuates immobilization stress-induced defensive withdrawal in rats

It has been proposed that corticotropin-releasing factor (CRF) released during stress in the region of the locus coeruleus (LC) induces changes in behavior that are typical indices of anxiety. The experiments tested the ability of a CRF antagonist, alpha hCRF9-41, to attenuate stress-induced defensive withdrawal in rats. 1 microgram of alpha hCRF in 300 nl was infused bilaterally in the LC of rats 10 min prior to 30 min immobilization. The apparatus consisted of a small chamber set on one side of a 1 m open field, into which the rat was placed to start the test. Restraint induced defensive withdrawal in rats familiar with the apparatus and significantly increased latency time to emerge from the chamber, total time and mean time spent in the chamber. Infusion of alpha hCRF into the LC prior to restraint significantly decreased total and mean time spent in the chamber comparing to stressed animals. These results are consistent with anatomical, electrophysiological and neurochemical evidence that CRF receptors located in, or close to, the LC region influence behaviors induced by stress.

Depression as a spreading neuronal adjustment disorder

The outlines of a theory of the pathophysiology of depression are presented. The classic monoamine theory of depression as well as its more recent elaborations suggests that a deficit in monoamine neurotransmitters in the synaptic cleft is the primary cause of depression. We suggest that the primary defect emerges in the regulation of firing rates in brainstem monoaminergic neurons, which brings about a decrease in the tonic release of neurotransmitters in their projection areas, an increase in postsynaptic sensitivity and, concomitantly, exaggerated responses to acute increases in presynaptic firing rate and transmitter release. We propose that the initial defect involves, in particular, the noradrenergic innervation from the locus coeruleus, which in turn leads to dysregulation of 5-HT-ergic and dopaminergic neurotransmission.

Reciprocal cross-desensitization of locus coeruleus electrophysiological responsivity to corticotropin-releasing factor and stress

While acutely administered corticotropin-releasing factor (CRF) and acute stress each activate neurons of the locus coeruleus (LC), desensitization to both develops with repeated treatment. The present experiments were designed to investigate whether cross-desensitization develops between CRF and stress. Because acute hemodynamic stress caused by intravenous infusion of sodium nitroprusside increases LC electrophysiological discharge rate via a CRF-dependent mechanism, it was hypothesized that repeated CRF administration would cause desensitization to the effect of this stressor on LC. For a complementary experiment, it was hypothesized that repeated stress, which presumably results in the repeated release of endogenous CRF, would result in desensitization to subsequent exogenous CRF. The results of the first experiment showed that repeated intracerebroventricular (i.c.v.) administration of CRF caused a significant attenuation of the sodium nitroprusside-induced increase in LC discharge rate seen in naive rats, although this pretreatment actually potentiated the decrease in blood pressure produced by sodium nitroprusside. In the second experiment, either one or eight sessions of white-noise stress attenuated the effect of CRF on LC activity 24 h after the last stress exposure, and this attenuation was more pronounced following eight sessions of stress than following one session. In a test of the specificity of this effect, stress-induced desensitization did not generalize to the LC electrophysiological response to clonidine (i.c.v.). One week following the last of eight sessions of stress, LC responsivity to CRF had recovered to control levels. These experiments demonstrate reciprocal cross-desensitization between CRF and stress using LC electrophysiological responsivity as an assay. This modifiability of the interaction between CRF and the LC may represent the operation of mechanisms mediating adaptive responding to stress.