The Impact of the Nonpeptide Corticotropin-Releasing Hormone Antagonist Antalarmin on Behavioral and Endocrine Responses to Stress *

The nonpeptide CRH antagonist antalarmin has been shown to block both behavioral and endocrine responses to CRH. However, it’s potential activity in blunting behavioral and endocrine sequelae of stressor exposure has not been assessed. Because antagonism of central CRH by a-helical CRH attenuates conditioned fear responses, we sought to test antalarmin in this regard. In addition, it remains unclear as to whether this is a result of receptor blockade during conditioning or during testing. Thus, we explored whether CRH mediates the induction or expression of conditioned fear (freezing in a context previously associated with 2 footshocks; 1.0 mA, 5 sec each). Furthermore, because rats previously exposed to inescapable shock (IS; 100 shocks, 1.6 mA, 5 sec each), demonstrate enhanced fear conditioning, we investigated whether this effect would be blocked by antalarmin. Antalarmin (20 mg/kgz2 ml ip) impaired both the induction and expression of conditioned fear. In addition, antalarmin blocked the enhancement of fear conditioning produced by prior exposure to IS. Despite the marked behavioral effects observed in antalarmin-treated rats, antalarmin had no effect on IS-induced rises in ACTH or corticosterone. However, antalarmin did block the ACTH response produced by exposure to 2 footshocks. (Endocrinology 140: 79–86, 1999) C IS A 41-amino acid peptide initially identified as a hypothalamic factor responsible for stimulating ACTH from the anterior pituitary (1). Stressors induce the synthesis and release of CRH from cells of the paraventricular nucleus, into the portal blood, initiating the hypothalamopituitary-adrenal (HPA) response to stressors (2–4). CRH is also involved in the mediation of autonomic and behavioral sequelae of exposure to stressors. The intracerebroventricular (icv) administration of CRH produces autonomic activation (5) and many of the same behavioral (6–9), neurochemical (10), and electrophysiological (11) alterations that are produced by stressors. Furthermore, the icv administration of CRH antagonists, such as a-helical CRH9–41 and D-Phe CRH12–41, can blunt or block these stress-induced alterations in behavior and autonomic activity (12–15). Many of these effects can be obtained by infusing CRH or it’s antagonists into nonhypothalamic sites, such as the locus coeruleus and amygdala (16–19), and these effects persist in hypophysectomized and dexamethasone-treated subjects (12). These facts, together with the wide extrahypothalamic distribution of high-affinity CRH receptors (20, 21) and CRH-like immunoreactivity (22–24), suggest that CRH functions as a neurotransmitter, as well as a hormone, and that it mediates stress-related behavioral responses by action at extrahypothalamic sites (25, 26). The critical role of brain CRH in mediating stress-related phenomena (along with the possible importance of brain CRH) in a number of human disorders, such as depression, posttraumatic stress disorder, and bulimia (27–30), has motivated the recent development of a nonpeptide CRH receptor antagonist capable of readily crossing the blood-brain barrier. The compound, first synthesized by Chen (31), is a pyrrolopyrimidine and has been called CP-154,526 (32) and antalarmin (33). It has high affinity for the CRH receptor, with selectivity for the Type 1 receptor, CRHR-1. The compound, here to be called antalarmin, blocked or blunted the effects of CRH on adenylate cyclase in membranes from rat cortex and pituitary, plasma ACTH increases in response to iv CRH, locus coeruleus electrical activity in response to icv CRH, and the potentiation of the startle reflex by icv CRH (32). In addition, antalarmin blunted the increase in startle produced by a light that had been paired with footshock (32) and a carageenin-induced sc inflammation (33). The purpose of the present experiments was both to further explore the effects of this nonpeptide CRH antagonist and to investigate the role of CRH in several phenomena. One phenomenon was fear conditioning. The term fear conditioning refers to the fact that both discrete and contextual cues that are present during exposure to a stressor such as footshock come, themselves, to elicit behavioral and physiReceived April 14, 1998. Address all correspondence and requests for reprints to: Terrence Deak, Campus Box 345, Department of Psychology, University of Colorado, Boulder, Colorado 80309-0345. E-mail: tdeak@clipr.colorado. edu. * This research was supported by NIMH Grant MH-50479 and the Undergraduate Research Opportunities Program at the University of Colorado at Boulder. 0013-7227/99/$03.00/0 Vol. 140, No. 1 Endocrinology Printed in U.S.A. Copyright © 1999 by The Endocrine Society