Hippocampal noradrenergic responses to CRF injected into the locus coeruleus of unanesthetized rats

Corticotropin-releasing factor receptor 1 (CRF-R1) antagonists: Promising agents to prevent visceral hypersensitivity in irritable bowel syndrome

Corticotropin-releasing factor (CRF) is a 41-amino acid polypeptide that coordinates the endocrine system, autonomic nervous system, immune system, and physiological behavior. CRF is a signaling regulator in the neuro-endocrine-immune (NEI) network that mediates visceral hypersensitivity. Rodent models to simulate changes in intestinal motility similar to those reported in the irritable bowel syndrome (IBS), demonstrate that the CRF receptor 1 (CRF-R1) mediates intestinal hypersensitivity under many conditions. However, the translation of preclinical studies into clinical trials has not been successful possibly due to the lack of sufficient understanding of the multiple variants of CRF-R1 and CRF-R1 antagonists. Investigating the sites of action of central and peripheral CRF is critical for accelerating the translation from preclinical to clinical studies.

A role of noradrenergic receptors in anxiolytic-like effect of high CRF in the rat frontal cortex

Corticotropin releasing factor (CRF) is a neuropeptide widely distributed in the brain as a hormonal modulator and neurotransmitter. The best known behavioral function of CRF is activation of stress and anxiety via the hypothalamus and limbic structures but the role of CRF in the cortex is still poorly understood. Our previous studies have shown anxiolytic-like effects of high doses of CRF injected into the Fr2 frontal cortex and involvement of CRF1 receptors (R) in that effect. These results seemed to be controversial as most other studies suggested anxiogenic and not anxiolytic effects of CRF1R stimulation. Since stress is associated with adrenergic system, in the present study, we focused on participation of alpha1 and alpha2 or beta adrenergic receptors in the anxiolytic-like effect of CRF. Moreover, we verified whether these effects of CRF in the Fr2 were really connected with CRF1R. Male Wistar rats were bilaterally microinjected with CRF in a dose of 0.2 μg/1 μl/site or with the specific agonist of CRF1R, stressin 1 (0.2-0.0125 μg/1 μl/site) into the Fr2 area. The elevated plus maze (EPM) test was performed 30 min later to assess the anxiolysis. An involvement of noradrenergic receptors in the CRF induced anxiolytic-like effect in the Fr2 was studied by pretreatment with the alpha1 antagonist prazosin, alpha2 agonist clonidine, alpha2 antagonist RS 79948 or beta antagonist propranolol, 20-30 min before CRF. The influence on anxiety was assessed in the EPM test. The results show that anxiolytic behavior after CRF microinjection into the Fr2 area seems to be mainly connected with the CRF1R activation because a similar effect was observed after stressin 1 administration and it was blocked by CRF1R antagonist. The results observed after administration of noradrenergic ligands indicated that anxiolytic effects of CRF in the Fr2 engaged the alpha1 and alpha2 adrenergic receptors but not beta receptors.

Maternal social stress during late pregnancy affects hypothalamic-pituitary-adrenal function and brain neurotransmitter systems in pig offspring

Maternal stress in pregnant sows may induce long-lasting alterations in the behavior, physiology, and immunity of their offspring. The aim of the present study was to investigate the consequences of repeated social stress during late gestation on determinants of the hypothalamic-pituitary-adrenal axis and on hippocampal neurotransmitter profiles in pig offspring. All pregnant gilts were housed in pairs. Each Stress gilt was mixed with an unfamiliar gilt twice a week between days 77 and 105 of gestation (n=18). Control gilts were housed in stable pairs over the same period (n=18). Plasma cortisol and corticosteroid binding globulin (CBG) were measured in 1 male and 1 female per litter in a basal situation on postnatal days (PND) 4, 26, and 60 and in a stressful situation at PND 28 (2 d after weaning) and 62 (2 d after relocation to a new building). Prenatal stress had no effect on plasma cortisol, but it decreased CBG at PND 26. Brain and adrenals were collected from 1 female per litter after weaning or relocation at PND 28 and PND 62. Adrenals were additionally collected at PND 4. Glucocorticoid receptor binding in the hippocampus and hypothalamus was not affected by prenatal treatment. However, prenatal stress increased the expression of 11beta-hydroxysteroid dehydrogenase type 1 mRNA in the hippocampus after weaning (P<0.05) and after relocation (P=0.08). In addition, prenatally stressed piglets showed an increased 5-hydroxyindole-3-acetic acid to 5-hydroxytryptamine ratio in the hippocampus after weaning and increased hippocampal c-fos mRNA expression and noradrenaline concentration after relocation (P<0.05). Prenatal stress also increased the relative adrenal weight at PND 4 and the cell density in the cortex and the medulla at PND 28, whereas no difference was found for activities of catecholamine-synthesising enzymes in the medulla. Overall, our data indicate that repeated social stress during pregnancy has long-lasting consequences on hypothalamic-pituitary-adrenal axis and hippocampal neurotransmitter activity in the offspring of pigs.

Dose-dependent effects of the CRF(1) receptor antagonist R317573 on regional brain activity in healthy male subjects

BACKGROUND

Corticotropin-releasing factor receptor type 1 (CRF(1)) antagonists have been proposed as therapeutic agents in the treatment of mood and anxiety disorders although clinical evidence supporting their development and understanding of a dose-response relationship has been lacking.

METHODS

We tested two doses of the CRF(1) antagonist R317573 for effects on regional cerebral glucose metabolism (rCMglu) using [(18)F] fluoro-2-deoxy-D: -glucose (FDG) positron emission tomography (PET) following single-dose challenges in a double-blind, placebo-controlled, cross-over design, in 12 healthy male volunteers.

RESULTS

Single 30- and 200-mg doses of R317573 resulted in dose-related changes in rCMglu. Relative increases in rCMglu were observed in frontal cortical regions while relative decreases occurred in the putamen and right amygdala after both doses. Relative decreases occurred in cerebellum and right parahippocampal gyrus following the higher dose.

CONCLUSIONS

R317573 appears to produce acute dose-dependent changes in rCMglu. Effects occurred in regions that may be behaviorally relevant to mood and anxiety disorders. In some regions, these effects may be related to the receptor (target) density. Measuring acute effects on rCMglu with FDG-PET may offer a method for defining pharmacologically active doses for central nervous system targets for which selective radiotracers are lacking.

An associativity requirement for locus coeruleus-induced long-term potentiation in the dentate gyrus of the urethane-anesthetized rat

Norepinephrine has been hypothesized to provide a learning and memory signal. Norepinephrine long-term potentiation of perforant path input to the dentate gyrus of the hippocampus provides a model for norepinephrine initiated memory processes. However, in vitro, the pairing of perforant path stimulation and norepinephrine is not required for the occurrence of norepinephrine-dependent long-term potentiation. Since bath application of norepinephrine induces long-term changes in 2nd messenger signalling and differs in a number of ways from physiological norepinephrine release, the present study is an in vivo test of the associative requirement for the pairing of perforant path input with norepinephrine to induce long-term potentiation. Phasic activation of the locus coeruleus is provided by glutamate infusion into the locus coeruleus to initiate transient norepinephrine release in the hippocampus of urethane-anesthetized Sprague-Dawley rats. Perforant path stimulation (0.067 Hz) was given throughout the experiment in the paired condition. In the unpaired condition perforant path stimulation was interrupted 10 min prior to locus coeruleus activation and resumed 10 min after locus coeruleus activation. Locus coeruleus-induced long-term potentiation of both EPSP slope and population spike only occurred in the pairing condition. This result argues that, in vivo, temporal proximity of locus coeruleus-associated norepinephrine release and perforant path stimulation are required to induce long-term plasticity. The associativity requirement for locus coeruleus activation and perforant path stimulation in vivo is consistent with the hypothesis that norepinephrine can initiate circuit changes supporting learning and memory.

Noradrenergic modulation of the respiratory neural network

Noradrenergic dysregulation has been reported in human pathologies affecting the control of breathing, such as sudden infant death syndrome, congenital central hypoventilation syndrome and Rett syndrome. Noradrenergic neurons, located predominantly in pontine nuclei, are among the earliest to arise within the hindbrain and play an essential role in the maturation of the respiratory network. Noradrenergic neurons also play a major role in the modulation of the respiratory motor pattern from birth through adulthood. The critical importance of this signaling system in respiratory control is illustrated by the severe respiratory disturbances associated with gene mutations affecting noradrenergic neurons (Phox2 and Mecp2). Here, the role of catecholaminergic pontine nuclei in the control of breathing, the cellular effects of norepinephrine on the respiratory network and the pathological consequence to breathing of abnormalities in this signaling system will be discussed.

Spontaneous nicotine withdrawal potentiates the effects of stress in rats

Anxiety is a common symptom of nicotine withdrawal in humans, and may predict an inability to abstain from cigarette smoking. It is not clear if self-reports of anxiety during abstinence reflect increased baseline anxiety and/or increased responses to exogenous stressors. We hypothesized that nicotine withdrawal selectively exacerbates reactivity to aversive stimuli in rodents. Here, we investigated the effect of withdrawal from chronic nicotine administration (3.16 mg/kg per day base, delivered via subcutaneous osmotic minipumps) in the light-enhanced startle (LES) test in Wistar rats. In this procedure, baseline startle responding in the dark is compared to startle responding when the chamber is brightly lit. Bright illumination is aversive for rats and potentiates the startle response. Hence, this procedure allows comparisons of withdrawal effects on startle reactivity between relatively neutral and stressful contexts. We found that spontaneous nicotine withdrawal (24 h post-pump removal) did not influence baseline startle responding, but produced a selective increase in LES. Precipitated nicotine withdrawal through injections of one of two nicotinic acetylcholine receptor (nAChR) antagonists, dihydro-beta-erythroidine hydrobromide (DHbetaE: 0, 1.5, 3, or 6 mg/kg) or mecamylamine (0, 1, 2, or 4 mg/kg), did not influence baseline startle responding or LES. These results suggest that spontaneous nicotine withdrawal selectively potentiates responses to anxiogenic stimuli, but does not by itself produce a strong anxiogenic effect. These findings support the hypothesis that nicotine withdrawal exacerbates stress responding, and indicate LES may be a useful model to examine withdrawal effects on anxiety.

The role of corticotropin-releasing factor and noradrenaline in stress-related responses, and the inter-relationships between the two systems

Substantial evidence indicates that brain neurons containing and secreting noradrenaline and corticotropin-releasing factor (CRF) are activated during stress, and that physiological and behavioural responses observed during stress can be induced by exogenous administration of CRF and adrenoceptor agonists. This review focusses on the evidence for the involvement of these two factors in stress-related responses, and the inter-relationships between them. The possible abnormalities of these two systems in depressive illness are also discussed.

Effects of chlordiazepoxide on footshock- and corticotropin-releasing factor-induced increases in cortical and hypothalamic norepinephrine secretion in rats

Noradrenergic and corticotropin-releasing factor (CRF) neuronal systems within the brain have been implicated in stress and anxiety. Synaptic release of cerebral norepinephrine (NE) is increased during stress, and following intracerebral CRF administration. Benzodiazepines are commonly used anxiolytic drugs but information on their effects on the stress- and CRF-related release of NE is limited. We have used in vivo microdialysis to test the effects of the benzodiazepine, chlordiazepoxide (CDP) on the noradrenergic responses to footshock and intracerebroventricular CRF in the medial hypothalamus and the medial prefrontal cortex (PFM) of freely moving rats. Footshock (60 x 0.1-0.2 mA shocks in 20 min) significantly increased microdialysate concentrations of NE in the first sample collected after initiating the footshock. In the hypothalamus, microdialysate NE was augmented 64% above baseline. A second footshock session (100 min after the first footshock) increased microdialysate NE to 313% of the baseline. Thus the noradrenergic responses to footshock were enhanced by preceding footshocks. CRF (100 ng) administered into the locus coeruleus (LC) almost tripled microdialysate concentrations of NE in the PFM. CDP (5mg/kg, i.p.) had no statistically significant effects on the basal dialysate concentrations of NE, but it significantly attenuated both footshock- and CRF-induced increases in dialysate NE. CDP may exert a direct inhibitory effect on the noradrenergic neurons, alter the input to LC noradrenergic neurons, or alter the ability of CRF to activate the LC noradrenergic system.

Norepinephrine and the dentate gyrus

Norepinephrine's role in the dentate gyrus is assessed based on a review of what is known about its innervation and receptor patterns and its functional effects at both cellular and behavioral levels. The data support seven hypotheses: (1) Norepinephrine's functional actions are primarily mediated by beta adrenoceptors and include electrophysiological enhancement of responses to excitatory input and glycogenolytic metabolic support of excitatory synaptic activity. (2) At the cellular level, locus coeruleus burst release of norepinephrine transiently inhibits feedforward interneurons and either excites or inhibits subpopulations of feedback interneurons. Consistent with reduced feedforward inhibition, granule cell firing is transiently increased. Concomitant EEG effects include transient increases in theta power and decreases in beta and gamma power. (3) Norepinephrine selectively promotes the processing of medial perforant path spatial input. This effect is mediated both through short- and long-term potentiation of cell excitability and through delayed potentiation of synaptic input. A critical level of norepinephrine release is required for long-term effects to norepinephrine alone. Norepinephrine release switches early phase frequency-induced long-term potentiation of perforant path input to an enduring late phase form and can reinstate decayed long-term potentiation. Norepinephrine also promotes frequency-induced potentiation of granule cell output at the mossy fiber to CA3 connection. (4) Local increases in norepinephrine accompany glutamate release and release of other neurotransmitters providing a mechanism for norepinephrine enhancement effects independent of locus coeruleus firing. (5) Stimuli, such as novelty and reward and punishment, which activate locus coeruleus neurons, enhance responses to medial perforant path input and engage late phase frequency-induced long-term potentiation through beta adrenoceptor activation. (6) Behavioral studies are consistent with the mechanistic evidence for a norepinephrine role in promoting learning and memory and assisting retrieval. (7) The overall profile suggests lower levels of norepinephrine may facilitate pattern completion or memory retrieval while higher levels would recruit global remapping and promote long-term episodic memory.

Agonist-induced internalization of corticotropin-releasing factor receptors in noradrenergic neurons of the rat locus coeruleus

Corticotropin-releasing factor (CRF) acts within the locus coeruleus (LC), to modulate activity of the LC-norepinephrine (NE) system. Combining molecular and cellular approaches, we demonstrate CRF receptor (CRFr) mRNA expression in Sprague-Dawley rat LC and provide the first in vivo evidence for agonist-induced internalization of CRFr. CRFr mRNA was detected in LC micropunches by RT-PCR. In dual labelling immunofluorescence studies, tyrosine hydroxylase (TH) containing neurons exhibited CRFr labelling. At the ultrastructural level, immunogold-silver labelling for CRFr was localized to the plasma membrane of TH-immunoperoxidase labelled dendrites. CRF (100 ng) injection into the LC produced a robust neuronal activation that peaked 10-15 min after injection and was maintained for the duration of the recording. This was associated with CRFr internalization in LC neurons that was apparent at 5 and 30 min after injection. By 5 min after injection the ratio of cytoplasmic to total dendritic CRFr-labelling was 0.81 +/- 0.01 in rats injected with CRF and 0.59 +/- 0.02 in rats injected with artificial cerebrospinal fluid (ACSF; P < 0.0001). Enhanced internalization of CRFr was maintained at 30 min after CRF injection, with the ratio being 0.86 +/- 0.02 for CRF-injected cases and 0.57 +/- 0.03 for ACSF-injected cases (P < 0.0001). Internalized CRFr was associated with early endosomes, indicative of degradation or recycling. Agonist-induced CRFr internalization in LC neurons may underlie acute desensitization to CRF or stress. This process may be a pivotal target by which stressors or pharmacological agents regulate the sensitivity of the LC-NE system to CRF and subsequent stressors.

Distinctive stress effects on learning during puberty

Puberty is a time of significant change in preparation for adulthood. Here, we examined how stressful experience affects cognitive and related hormonal responses in male and female rats prior to, during and after puberty. Groups were exposed to an acute stressor of brief periodic tailshocks and tested 24 h later in an associative memory task of trace eyeblink conditioning. Exposure to the stressor did not alter conditioning in males or females prior to puberty but enhanced conditioning in both males and females during puberty. The enhancement occurred in pubescent females irrespective of the estrous cycle. In adulthood, sex differences in trace conditioning and the response to stress emerged: females outperformed males under unstressed conditions, but after stressor exposure, trace conditioning in females was impaired whereas that in males was enhanced. These differences were not related to changes in gross motor activity or other nonspecific measures of performance. The effects of acute stress on corticosterone, estradiol, progesterone, and testosterone were also measured. Stressor exposure increased the concentration of corticosterone in all age groups, although sex differences were only evident in adults. All reproductive hormones except estradiol increased with age in a predictable and sex dependent fashion and none were affected by stressor exposure. Estradiol decreased in male rats across age, and remained stable for female rats. Together, these data indicate that males and female respond similarly to learning opportunities and stressful experience before and during puberty; it is in adulthood that sex differences and the opposite responses to stress arise.

Corticotropin-releasing hormone directly activates noradrenergic neurons of the locus ceruleus recorded in vitro

The neuropeptide corticotropin-releasing hormone (CRH) activates locus ceruleus (LC) neurons, thereby increasing norepinephrine levels throughout the CNS. Despite anatomical and physiological evidence for CRH innervation of the LC, the mechanism of CRH-evoked activation of LC neurons is unknown. Moreover, given the apparent absence of mRNA for CRH receptors in LC neurons, the exact location of action of CRH within the cerulear region is debated. Using in vitro intracellular recordings from rat brainstem, we examined whether CRH exerts a direct effect on LC neurons and which ionic currents are likely affected by CRH. We demonstrate that CRH dose-dependently increases the firing rate of LC neurons through a direct (TTX- and cadmium-insensitive) mechanism by decreasing a potassium conductance. The CRH-evoked activation of LC neurons is, at least in part, mediated by CRH1 receptors and a cAMP-dependent second messenger system. These data provide additional support that CRH functions as an excitatory neurotransmitter in the LC and the hypothesis that dysfunction of the CRH peptidergic and noradrenergic systems observed in patients with mood and anxiety disorders are functionally related.

Rate-dependent behavioral effects of stimulation of central motoric alpha(1)-adrenoceptors: hypothesized relation to depolarization blockade

AIM

The purpose of this review is to clarify how central alpha(1)-adrenoceptors control behavioral activity under varying conditions of activity and stress.

METHOD

The literature is reviewed regarding the behavioral actions of alpha(1)-agonists and antagonists, and alpha(2)-agonists and antagonists under conditions of high and low baseline activity and stress.

RESULTS

It was found that alpha(1)-receptor stimulation of active behavior has a number of similarities to rate dependency including: (1) a dependence on low-active, low-stress conditions or on the prior depletion of endogenous brain catecholamines; (2) a nonmonotonic dose-response relationship with high doses producing a fall-off or actual depression of activity; (3) a failure to be blocked at high agonist doses by alpha(1)-antagonists; and (4) a facilitation by alpha(2)-adrenoceptor agonists which produce an opposing hyperpolarization.

DISCUSSION

To explain these findings, it is proposed that high levels of stimulation of central alpha(1)-receptors produce, in host neurons, a depolarization block that impedes nerve impulse generation and inhibits active behavior. This effect is assumed to be precluded or mitigated by low-active, low-stress conditions, depletion of brain catecholamines, and by hyperpolarizing alpha(2)-agonists, and to be reversed at high agonist doses by alpha(1)-antagonists.

CONCLUSION

Because brain alpha(1)-receptors are not only involved in motor activity but also in the mechanism of action of antidepressant and stimulant drugs, arousal, anxiety, stress and psychosis, a depolarization block from intense stimulation of these receptors could have broad psychopharmacological consequences and underlie rate dependency to a variety of stimulant drugs.

Brain Circuits Involved in Corticotropin-Releasing Factor-Norepinephrine Interactions during Stress

Corticotropin-releasing factor (CRF)- and norepinephrine (NE)-containing neurons in the brain are activated during stress, and both have been implicated in the behavioral responses. NE neurons in the brain stem can stimulate CRF neurons in the hypothalamic paraventricular nucleus (PVN) to activate the hypothalamic-pituitary-adrenocortical axis and may affect other CRF neurons. CRF-containing neurons in the PVN, the amygdala, and other brain areas project to the area of the locus coeruleus (LC), and CRF injected into the LC alters the electrophysiologic activity of LC-NE neurons. Neurochemical studies have indicated that CRF applied intracerebroventricularly or locally activates the LC-NE system, and microdialysis and chronoamperometric measurements indicate increased NE release in LC-NE terminal fields. However, chronoamperometric studies indicated a significant delay in the increase in NE release, suggesting that the CRF input to LC-NE neurons is indirect. The reciprocal interactions between cerebral NE and CRF systems have been proposed to create a "feed-forward" loop. It has been postulated that a sensitization of such a feed-forward loop may underlie clinical depression. However, in the majority of studies, repeated or chronic stress has been shown to decrease the behavioral and the neurochemical responsivity to acute stressors. Repeated stress also seems to decrease the responsivity of LC neurons to CRF. These results do not provide support for a feed-forward hypothesis. However, a few studies using certain tasks have indicated sensitization, and some other studies have suggested that the effect of CRF may be dose dependent. Further investigations are necessary to establish the validity or otherwise of the feed-forward hypothesis.