Elsevier

Brain Research

Volume 861, Issue 2, 14 April 2000, Pages 288-295
Brain Research

Research report
Corticosterone delivery to the amygdala increases corticotropin-releasing factor mRNA in the central amygdaloid nucleus and anxiety-like behavior

https://doi.org/10.1016/S0006-8993(00)02019-9Get rights and content

Abstract

The present study examined the effects of stereotaxic delivery of corticosterone to the amygdala on anxiety-like behavior and corticotropin-releasing factor (CRF) mRNA level in the central nucleus of the amygdala (CeA). Micropellets (30 μg) of crystalline corticosterone or cholesterol (control) were implanted bilaterally at the dorsal margin of the CeA in Wistar rats. Seven days post-implantation, anxiety-like behavior was accessed using an elevated plus-maze. CRF mRNA level in the CeA was determined by in situ hybridization 4 h after being tested on the elevated plus-maze. Corticosterone implants increased indices of anxiety on the elevated plus-maze and produced a concomitant increase in both basal level of CRF mRNA per neuron and the number of neurons with CRF hybridization signal in the CeA. The plus-maze increased CRF mRNA levels in the CeA of cholesterol implanted rats to the elevated basal levels observed in corticosterone treated animals. Exposure to the plus-maze did not increase CRF mRNA level in the CeA of corticosterone implanted rats beyond elevated basal levels. Taken together, these findings support the involvement of the amygdala in anxiety-like behaviors in response to chronically elevated corticosterone and suggests that elevated glucocorticoids may increase anxiety by inducing CRF expression in the CeA.

Introduction

Numerous studies highlight the amygdala as a limbic structure central to the expression of fear and anxiety 11, 28. Electrical stimulation of the amygdala produces feelings of fear or anxiety in humans and elicits a constellation of behaviors consistent with a fear response in cats 8, 15, 22. In rats, lesions of the amygdala reduce anxiety-like behaviors and attenuate neuroendocrine and autonomic responses to a variety of stressors 13, 27. Lesions of the central amygdaloid nucleus (CeA), a major efferent pathway from the amygdala, reproduce many of the effects observed with complete amygdala lesions implicating this nucleus in the generation of fear and anxiety 4, 13, 27.

The CeA is a major extrahypothalamic site of corticotropin-releasing factor (CRF) expression and recent studies support a role for CRF in mediating fear and anxiety 18, 19, 20. Intracerebroventricular (i.c.v.) administration of CRF elicits anxiety-like behaviors while infusion of CRF antagonists into the CeA reduce such behaviors, suggesting that CRF neurons in the CeA are involved in behaviors interpreted as anxiety 21, 41, 48. Levels of both CRF mRNA and peptide are increased in the CeA in response to restraint stress and during periods of heightened anxiety associated with alcohol or cannabinoid withdrawal 12, 24, 25, 40. Increased basal levels of CRF mRNA in the CeA of Fawn-hooded rats has also been proposed as a possible mechanism for the predisposition of this strain to exhibit a high degree of anxiety-related behaviors [1].

In addition to CRF, the hypothalamo–pituitary–adrenal axis may be involved in the modulation of anxiety. Sustained elevation of plasma corticosterone increases indices of anxiety while inhibition of corticosterone synthesis reduces anxiety induced by a behavioral stressor 7, 9, 30, 43. Chronically elevated corticosterone also increases CRF mRNA in the CeA 33, 34, 47, 50. Further, glucocorticoid receptors are expressed in CRF neurons in the CeA, providing a mechanism for direct glucocorticoid regulation of CRF expression in this nucleus [23]. Upregulation of CRF in the CeA may contribute to anxiety-like behaviors observed with chronically elevated corticosterone 45, 46.

The purpose of this study was to determine the effects of directly elevating corticosterone in the amygdala on anxiety and CRF mRNA levels in the CeA. We hypothesized that stereotaxic administration of corticosterone to the amygdala would increase CRF mRNA in the CeA with a concomitant increase in anxiety-like behavior on the elevated plus-maze.

Section snippets

Animals

All protocols were approved by the University of Oklahoma Health Sciences Center Institutional Animal Care and Use Committee. Male Wistar rats (Charles River Laboratories, Wilmington, MA) weighing 350–450 g on the day of the elevated plus-maze test were maintained on 12:12 light–dark cycle (lights off 1800 to 0600 h) with food and water available ad libitum. Rats were housed in groups of three prior to stereotaxic surgery and single-housed thereafter.

Experimental design

Upon arrival, rats were habituated to the

Statistical analysis

Data are presented as mean±standard error of the mean (S.E.M.). Independent T-test were used to test for significant differences in behaviors on the elevated plus-maze. A two (cholesterol, corticosterone) by two (basal, elevated plus-maze) analysis of variance (ANOVA) was performed on in situ hybridization data followed with Bonferroni adjusted pairwise comparisons where appropriate.

Effect of chronically elevated corticosterone in the amygdala on anxiety-like behaviors and locomotor activity

Corticosterone implants reduced open arm exploration on the elevated plus-maze, indicating increased anxiety. Number of open arm entries, time exploring open arms, percent open arm entries, percent time in the open arms, time scanning, and number of open arm end explorations were reduced by corticosterone implants (p<0.03, Table 1). While corticosterone implants reduced exploration of the open arms, overall exploratory activity on the elevated plus-maze was similar for both groups, indicating

Discussion

The current study is the first demonstration that direct administration of corticosterone to the amygdala increases CRF mRNA in the CeA in conjunction with an increase in anxiety-like behavior on the elevated plus-maze. These results extend previous studies showing that chronic systemic administration of corticosterone increased CRF expression in the CeA 33, 34, 47, 50or increased behaviors interpreted as anxiety 7, 9, 30. We also found that CRF mRNA increased in the CeA in response to 15 min

Acknowledgements

This research was supported by a Graduate Student Association Dissertation Research Minigrant to J.D.S. We thank Mei Wu for scoring the elevated plus-maze sessions.

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