Intracerebroventricular Corticotropin-releasing Hormone Research Articles

Corticotropin-releasing hormone activates ERK1/2 MAPK in specific brain areas

Corticotropin-releasing hormone (CRH) coordinates hormonal and behavioral responses to stress. The mitogen-activated protein kinase extracellular signal-related kinase 1/2 (ERK1/2) mediates several functions in different forebrain structures and recently has been implicated in CRH signaling in cultured cells. To study in vivo CRH-mediated activation of central ERK1/2, we investigated the expression pattern of the phosphorylated ERK1/2(p-ERK1/2) in the mouse brain after intracerebroventricular CRH injections. As shown by immunohistochemistry and confocal microscopy analysis, CRH administration increased p-ERK1/2 levels specifically in the CA3 and CA1 hippocampal subfields and basolateral complex of the amygdala, both structures related to external environmental information processing and behavioral aspects of stress. Other regions such as hypothalamic nuclei and the central nucleus of the amygdala, also related to central CRH system but involved in the processing of the ascending visceral information and neuroendocrine-autonomic response to stress, did not show CRH-mediated ERK1/2 activation. To dissect the involvement of CRH receptor 1 (CRHR1) and CRHR2, we used conditional knockout mice in which Crhr1 is inactivated in the anterior forebrain and limbic structures. The conditional genetic ablation of Crhr1 inhibited the p-ERK1/2 increase, underlining the involvement of CRHR1 in the CRH-mediated activation. These findings underscore the fact that CRH activates p-ERK1/2 through CRHR1 only in selected brain regions, pointing to a specific role of this pathway in mediating behavioral adaptation to stress.

A stress-induced anxious state in male rats: Corticotropin-releasing hormone induces persistent changes in associative learning and startle reactivity

Exposure to intense inescapable stressors induces a persistent anxious state in rats. The anxious state is evident as increased sensory reactivity and enhanced associative learning. We examine whether similar neurobehavioral changes are observed after intracerebroventricular (ICV) administration of corticotropin releasing hormone (CRH). Two behaviors were observed: acoustic startle responses (ASRs) and acquisition of the classically conditioned eyeblink response. Male Sprague-Dawley rats were administered ICV CRH either in a single dose (1.0 microg/rat) or in three doses each separated by 30 min. Exaggerated ASRs were evident 2 hours after either CRH treatment; however, only the rats given three injections exhibited a persistently exaggerated ASR apparent 24 hours after CRH treatment. Rats administered three injections of CRH also exhibited faster acquisition of the eyeblink conditioned response beginning 24 hours after treatment. Yet, we did not find evidence for a persistent activation of the HPA-axis response; three CRH injections did not lead to elevated basal plasma corticosterone levels the following morning. Repeated treatment with CRH over a 1.5-hour period models some of the behavioral changes observed after exposure to intense inescapable stressors.

The Role of Corticotropin-Releasing Hormone in the Dorsal Raphe Nucleus in Mediating the Behavioral Consequences of Uncontrollable Stress

Inescapable shock (IS) produces subsequent interference with escape behavior and increased fear conditioning that has been linked to increased activity and release of serotonin (5-HT) from neurons within the caudal dorsal raphe nucleus (DRN) both at the time of IS and later behavioral testing. Extrahypothalamic corticotropin-releasing hormone (CRH) has been implicated in many stress-related phenomena and has recently been shown to increase DRN 5-HT activity in the same caudal DRN area at which IS increases 5-HT activity. The current set of studies therefore examined the role of CRH in mediating the behavioral sequelae of IS. Intra-DRN microinjection of the nonselective CRH receptor antagonist d-Phe CRH (12-41) blocked the IS-induced behavioral changes when administered before IS but not when administered before later behavioral testing. Furthermore, intra-DRN administration of CRH in the absence of IS dose-dependently mimicked the effects of IS and interfered with escape behavior and increased fear conditioning 24 hr later. This effect was specific to injection of CRH into the caudal DRN and was not produced by microinjection into the rostral DRN. Intracerebroventricular CRH produced escape deficits and potentiated fear conditioning 24 hr later at only much higher doses, further confirming the site specificity of the effects. The potential role of the caudal DRN in states of anxiety is discussed.

Role of the Septum in the Excitatory Effect of Corticotropin-Releasing Hormone on the Acoustic Startle Reflex

Intracerebroventricular administration of corticotropin-releasing hormone (CRH) elicits a constellation of behavioral, autonomic, and endocrinological changes typically observed in stress. One of the behavioral changes after intracerebroventricular CRH is a profound increase of startle amplitude (CRH-enhanced startle). The present study examined the role of the septum in CRH-enhanced startle. The septum has direct and indirect connections to the amygdala and inhibits the amygdala. Electrophysiological data show that CRH in the septum is inhibitory. Therefore, it has been hypothesized that intracerebroventricular CRH inhibits the septum, which in turn disinhibits the amygdala, resulting in a constellation of changes via activation of amygdala efferent targets. In testing this hypothesis, it was found that electrolytic lesions of the medial septum, but not the lateral septum, blocked CRH-enhanced startle. However, fiber-sparing chemical lesions of the medial septum did not block CRH-enhanced startle, suggesting that the blockade seen with the electrolytic lesions was caused by damage to fibers of passage. A major fiber bundle passing through the medial septum is the fornix, the primary efferent pathway for the hippocampus. Fimbria transection blocked CRH-enhanced startle almost completely, whereas the large electrolytic lesions of the dorsal hippocampus did not block CRH-enhanced startle. Taken together, these data suggest that perhaps the ventral hippocampus and its efferent target areas, which communicate via the fimbria, may be critically involved in CRH-enhanced startle.

Role of the Hippocampus, the Bed Nucleus of the Stria Terminalis, and the Amygdala in the Excitatory Effect of Corticotropin-Releasing Hormone on the Acoustic Startle Reflex

Previously, we demonstrated that transection of the fimbria/fornix blocked the excitatory effect of corticotropin-releasing hormone (CRH) on startle (CRH-enhanced startle), suggesting that the hippocampus and its efferent target areas that communicate via the fimbria may be critically involved in CRH-enhanced startle. The bed nucleus of the stria terminalis (BNST) receives direct projections from the ventral hippocampus via the fimbria/fornix. Therefore, the role of the ventral hippocampus, the BNST, and the amygdala in CRH-enhanced startle was investigated. NMDA lesions of the BNST completely blocked CRH-enhanced startle, whereas chemical lesions of the ventral hippocampus and the amygdala failed to block CRH-enhanced startle. However, the same amygdala-lesioned animals showed a complete blockade of fear-potentiated startle, a conditioned fear response sensitive to manipulations of the amygdala. In contrast, BNST-lesioned rats had normal fear-potentiated startle. This indicates a double dissociation between the BNST and the amygdala in two different paradigms that enhance startle amplitude. Microinfusions of CRH into the BNST, but not into the ventral hippocampus, mimicked intracerebroventricular CRH effects. Furthermore, infusion of a CRH antagonist into the BNST blocked CRH-enhanced startle in a dose-dependent manner. Control studies showed that this blockade did not result from either leakage of the antagonist into the ventricular system or a local anesthetic effect caused by infusion of the antagonist into the BNST. The present studies strongly suggest that CRH in the CSF can activate the BNST, which could lead to activation of brainstem and hypothalamic BNST target areas involved in anxiety and stress responses.

Glucocorticoid treatment increases the ability of CRH to induce seizures.

We examined whether glucocorticoids could enhance the ability of corticotropin-releasing hormone (CRH) to induce seizures. Rats were treated with systemic glucocorticoids (dexamethasone, 100 micrograms) or vehicle for either 3 days (chronic) or 2 h (acute) before intracerebroventricular CRH (3 or 10 micrograms) or saline injections and then monitored for 8 h following each injection. Our results suggest that chronic, but not acute, glucocorticoid pretreatment increases the likelihood of CRH-induced seizures.

Behavioral and cardiac responses after intracerebroventricular corticotropin-releasing hormone (CRH) administration: Role of adrenal cortical hormones

Intracerebroventricularly (icv) administered corticotropin-releasing hormone (CRH) produces a dose-dependent increase in heart rate in association with behavioral activation. The present study was designed to investigate whether these CRH-induced responses are dependent on adrenal function. The effects of adrenalectomy (ADX) and subsequent corticosterone replacement were studied. Administration icv of 300 ng of CRH failed to produce behavioral activation and tachycardia in ADX rats. Corticosterone replacement restored the CRH-induced behavioral response to preoperative levels, whereas the CRH-induced tachycardia was partially restored. This latter result may be related to the fact that the baseline heart rate of ADX animals appeared to be significantly higher than that of corticosterone-treated ADX animals. It is concluded that circulating adrenal corticosterone in ADX rats is involved in the expression of the behavioral and cardiac effect of central CRH.

Peripheral but not intracerebroventricular corticotropin-releasing hormone (CRH) produces antinociception which is not opioid mediated

Corticotropin-releasing hormone and endogenous opioid peptide systems are both activated during stress. An elevation of the pain threshold also occurs under conditions of stress. In the present study the effects of CRH on antinociception were examined. Rats were treated with CRH either centrally (i.c.v.) or peripherally (intracardially; i.c.) and their tail-flick latencies were measured. Central application of CRH (0–30 μg) was without effect on the analgesic test, while after peripheral application (0–32 μg) CRH produced a dose-dependent increase in tail-flick latencies. In a subsequent experiment we examined the possible involvement of endogenous opioids in the peripheral CRH-induced antinociceptive effects. To this end, two approaches were used: animals were either acutely treated with the opioid antagonist naloxone (3 or 6 mg/kg), or they were rendered tolerant to morphine, and then tested with CRH. In both cases, CRH effects on the tail-flick latencies were not modified. Our findings suggest that: (a) CRH may modulate pain sensitivity during stress; (b) opioids do not mediate this effect; and (c) brain CRH receptors are probably not involved in these processes.

Behavioral and physiologic effects of CRH administered to infant primates undergoing maternal separation

Infant rhesus monkeys briefly separated from their mothers emit frequent distress vocalizations and alter their activity levels. These behavioral alterations are accompanied by physiologic changes and activation of the hypothalamic-pituitary-adrenal axis. Because corticotropin-releasing hormone (CRH) is a major regulator of the pituitary-adrenal system and has been implicated as a mediator of other aspects of the stress response, we examined the effects of centrally and peripherally administered CRH on the infant monkey's separation response. Intracerebroventricular (ICV) doses less than 10 micrograms did not alter behavior; 10-micrograms doses inhibited behavior without affecting distress vocalizations. The behavioral inhibition did not appear to be due to nonspecific sedation and may be related to increased fearfulness. It was accompanied by increases in adrenocorticotropic hormone (ACTH) and cortisol and small but significant decreases in body temperature and mean arterial pressure. Within an individual animal, no relationship was seen between ICV CRH's effects on physiologic parameters and its effects on behavior. ICV CRH produced significant increases in plasma concentrations of CRH, suggesting the possibility that the effects of CRH were mediated through peripheral mechanisms. However, 10 micrograms of CRH administered intravenously (IV) had no effect on behavior, blood pressure, or body temperature, nor did it increase plasma concentrations of ACTH or cortisol beyond the expected separation-induced elevation. This is interesting because plasma levels of CRH after IV CRH were much greater than after ICV CRH. Our findings suggest that in infant rhesus monkeys undergoing maternal separation, brain CRH systems mediate behavioral inhibition and pituitary-adrenal activation.

ICV-CRH potently affects behavior without altering antinociceptive responding

To explore the hypothesized integrative function of corticotropin releasing hormone (CRH) in the stress response, stress-related behaviors including antinociception were studied in rats after either intracerebroventricular (ICV) or peripheral administration of CRH. The effects of low-dose (0.3 ωg) and high-dose (3.0 ωg) ICVCRH were compared to those of vehicle, employing a within- S design. The two doses yielded comparable behavioral changes suggestive of increased arousal and stress. These changes were characterized by significant increases in grooming, walking, burrowing, self-gnawing, and pica, and decreases in rearing and sleeping. None of these effects of ICVCRH were obtained with peripheral administration of the same doses. The hot-plate test of analgesia failed to show a significant effect of ICVCRH or peripherally administered CRH. A between- S experiment incorporating both the tail-flick and the hot-plate tests of analgesia compared ICVCRH (3.0 ωg) with vehicle. ICVCRH did not affect antinociceptive responding in either of these tests. In contrast, ICV morphine (10 ωg) yielded potent analgesia in both tests. Thus, with doses of ICVCRH yielding clear evidence of stress-related behavior, no evidence of analgesia was obtained. These findings question the possible role of central CRH systems in antinociceptive processes.