Elsevier

Brain Research

Volume 850, Issues 1–2, 11 December 1999, Pages 136-143
Brain Research

Research report
Psychological stress increased corticotropin-releasing hormone mRNA and content in the central nucleus of the amygdala but not in the hypothalamic paraventricular nucleus in the rat

https://doi.org/10.1016/S0006-8993(99)02114-9Get rights and content

Abstract

The central administration of corticotropin-releasing hormone (CRH) to experimental animals sets into motion a coordinated series of physiological and behavioral events that promote survival during threatening situation. A large body of evidence suggest that CRH in the central nucleus of the amygdala (CEA) induces fear-related behaviors and is essential to fear conditioning; however, evidence of CRH-mediated activation of the amygdala under physiological situation is still limited. We report here a study of the impact of a psychological stressor on hypothalamic and amygdala CRH systems in the rat. Non-footshocked rats placed in a floored compartment surrounded by footshocked rats were defined as the psychological stress group. Rats were exposed to psychological stress for 15 min, and then sacrificed 1.5 and 3 h after cessation of stress. We found that our psychological stressor induced an increase in both CRH mRNA levels, as assessed by in situ hybridization histochemistry, and CRH content, as assessed by micropunch RIA, in the CEA. Exposure to the psychological stressor also caused a significant increase in CRH mRNA levels with a trend for an increase in CRH content in the dorsolateral subdivision of the bed nucleus of the stria terminalis (BNST) which is anatomically associated with the CEA. In contrast, psychological stress induced a small, but significant increase in type-1 CRH receptor (CRHR-1) mRNA in the hypothalamic paraventricular nucleus (PVN), while it failed to elevate either PVN CRH mRNA levels or content, CRH content in the median eminence (ME), or levels of plasma ACTH or corticosterone (CORT). Thus, in the context of a psychological stressor, the activation of the amygdala CRH system can occur without robust activation of the hypothalamic CRH system. In the light of previous data that the psychological stress-induced loss of sleep was reversed by the central administration of a CRH antagonist, these data suggest that CRH in the CEA may contribute to the psychological stress-evoked fear-related behavior such as hyperarousal. These data also indicate that in response to a psychological stressor, the amygdala CRH system is much more sensitive than is the CRH system emanating from the PVN.

Introduction

Corticotropin-releasing hormone (CRH) is a 41 amino acid neuropeptide that acts in discrete but widespread areas of the brain to coordinate physiological and behavioral responses to threatening situations 3, 31, 44. CRH pathways emanating from the paraventricular nucleus of the hypothalamus (PVN) activate the pituitary–adrenal axis and sympathetic nervous system while inhibiting so-called vegetative functions whose inhibition is adaptive during stress (e.g., feeding, sexual behavior, growth, reproduction) 6, 9. On the other hand, a growing body of evidence suggests that activation of CRH receptors in the central nucleus of the amygdala (CEA) and/or CRH pathways emanating from the CEA play an important role in the fear-related behaviors. Electrical lesions of the CEA, but not of the PVN, abolished many of the behavioral effects of centrally administered CRH such as conditioned startle [24], whereas chemical lesion of the CEA blocked fear-potentiated startle [23]. The direct injection of CRH antagonist into the CEA also diminished the stress-evoked freezing [41]or reduced emotionality in socially defeated rats [13].

Although both hypothalamic and amygdala-derived CRH are thought to interact in the activation of brainstem arousal centers such as the locus coeruleus (LC) [10], they do not necessarily respond in a similar fashion to the same stimuli. We and others have previously reported that while sustained glucocorticoid administration inhibits the hypothalamic CRH system, it causes a subtle but significant increase in the levels of CRH mRNA in the amygdala 26, 40, 43. On the other hand, although restraint or footshock stress which gives both physical and psychological threats to the rats have been shown to activate the hypothalamic CRH system 43, 44, evidence for the activation of amygdala CRH system under these mixed stressors is still limited. Thus, while two other groups demonstrated activation of the amygdala CRH system following mild restraint stress 15, 19, 35, we have shown that immobilization stress that increases PVN CRH mRNA fails to increase CRH mRNA levels in the CEA [32].

In contrast, whether the PVN and amygdala CRH systems are differentially responsive to psychological stressors has not yet been determined. Given the fact that the amygdala CRH system seems to play a preferential role in fear-related behaviors, we sought to test whether the amygdala CRH system was more sensitive than the hypothalamic CRH system to a psychological stressor. To test the impact of well characterized psychological stressor on the hypothalamic CRH system, we assessed CRH mRNA levels and content in the PVN, CRH content in the median eminence (ME), and the secretion of plasma ACTH and corticosterone (CORT). In the amygdala, we measured CRH mRNA and content in the CEA, as well as the bed nucleus of the stria terminalis (BNST), thought to represent extended amygdala. CRH content in the LC was also measured, because it has been suggested that the LC is one of the important target areas of CRH neurons emanating from the CEA 21, 42. In addition, as an index of tissue-specific effects of CRH, we measured mRNA levels of type-1 CRH receptor (CRHR-1) in the PVN, amygdala and BNST 16, 28.

Section snippets

Materials and methods

Male Wistar rats, weighing 250–280 g, were individually housed under conditions of controlled temperature and illumination (0800–2000 h) and were allowed ad libitum access to food and water.

Plasma CORT and ACTH responses to psychological stress

As shown in Table 1, plasma CORT and ACTH were unaltered at both 1.5 and 3 h following psychological stress. This is consistent with a previous report [38]demonstrating that psychological stress identical in the present study did not evoke robust pituitary–adrenocortical responses.

CRH mRNA changes in the PVN, CEA and BNST

Consistent with plasma ACTH and CORT responses, CRH mRNA in the PVN did not show any significant changes following psychological stress (Fig. 1Fig. 2). In contrast, CRH mRNA in the CEA significantly increased at both

Discussion

We report here that CRH mRNA and content are both increased in the CEA following the psychological stress generated in the communication box. Psychological stress also increased CRH mRNA in the dorsolateral subdivision of the BNST, whereas CRH content in that region showed a tendency towards an increase. In contrast, we found that psychological stress failed to elevate any components of the hypothalamic–pituitary–adrenocortical (HPA) axis (i.e., CRH mRNA and content in the PVN, CRH content in

Acknowledgements

We are extremely grateful to Dr. K. Mayo and Dr. W. Vale for providing the ribonucleotide probes for CRH and CRHR-1. We also wish to thank Drs. K. Asaba and M. Nishiyama for plasma CORT measurement, and Miss M. Nakatsukasa for her technical assistance.

References (45)

  • S. Makino et al.

    Differential regulation of type-1 and type-2α corticotropin-releasing hormone receptor mRNA in the hypothalamic paraventricular nucleus of the rat

    Mol. Brain Res.

    (1997)
  • T. Shibasaki et al.

    Psychological stress increases arousal through brain corticotropin-releasing hormone without significant increase in adrenocorticotropin and catecholamine secretion

    Brain Res.

    (1993)
  • G. Skofitsch et al.

    Distribution of corticotropin-releasing factor-like immunoreactivity in the rat brain by immunohistochemistry and radioimmunoassay: comparison and characterization of ovine and rat/human CRF antisera

    Peptides

    (1985)
  • A.H. Swiergiel et al.

    Attenuation of stress-induced behavior by antagonism of corticotropin-releasing factor receptors in the central amygdala in the rat

    Brain Res.

    (1993)
  • A.G. Watts

    The impact of physiological stimuli on the expression of corticotropin-releasing hormone (CRH) and other neuropeptide genes

    Front. Neuroendocrinol.

    (1996)
  • M.H. Whitnall

    Regulation of the hypothalamic corticotropin-releasing hormone neurosecretory system

    Prog. Neurobiol.

    (1993)
  • P.C. Wynn et al.

    Brain and pituitary receptors for corticotropin-releasing factor: localization and differential regulation after adrenalectomy

    Peptides

    (1984)
  • D.S. Albeck et al.

    Chronic social stress alters levels of corticotropin-releasing factor and arginine vasopressin mRNA in rat brain

    J. Neurosci.

    (1997)
  • M.R. Brown, L.A. Fisher, Regulation of the autonomic nervous system by corticotropin-releasing factor, in: E.B. De...
  • P.D. Butler et al.

    Corticotropin-releasing factor produces fear-enhancing and behavioral activating effects following infusion into the locus coeruleus

    J. Neurosci.

    (1990)
  • P.B. Chappell et al.

    Alterations in corticotropin-releasing factor-like immunoreactivity in discrete rat brain regions after acute and chronic stress

    J. Neurosci.

    (1986)
  • G.P. Chrousos et al.

    The concepts of stress and stress system disorders: overview of physical and behavioral homeostasis

    J. Am. Med. Assoc.

    (1992)
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