Interactions between heterotypic stressors and corticosterone reveal integrative mechanisms for controlling corticotropin-releasing hormone gene expression in the rat paraventricular nucleus

Abstract

Although the convergence of neural and humoral afferent information onto paraventricular neuroendocrine corticotropin-releasing hormone (CRH) neurons is a major determinant for adaptive stress responses, the underlying integrative mechanisms are poorly understood. To dissect the relative contributions made by neural afferents and corticosterone to these processes, we determined how the concurrent application of two heterotypic physiological stressors, chronic dehydration (produced by drinking hypertonic saline) and sustained hypovolemia (produced by subcutaneous injections of polyethylene glycol), is interpreted by the synthetic and secretory activity of CRH neurons using in situ hybridization and plasma ACTH measurements. These two stressors are encoded by relatively simple, distinct, and well defined sets of neural afferents to CRH neurons. Both increase plasma corticosterone, but they have opposing actions on CRH gene expression when applied separately. In the first experiment, we showed that chronic dehydration suppresses CRH gene transcription after hypovolemia, but not the preproenkephalin and c-fos mRNA responses or ACTH secretion. In the second, we showed that negative feedback actions of corticosterone do not suppress CRH gene activation after hypovolemia, but instead determine the prestress lower limit of a range within which the CRH gene then responds. Collectively, these data show that at least two processes are integrated to control how the CRH gene responds to multiple stimuli. First, the presence of corticosterone, which although permissive for appropriately activating the CRH gene during hypovolemia, does not mediate the suppressed gene response. Second, neural afferent-driven processes that encode dehydration play a central role in suppressing CRH activation.

Fig. 1.

The effect of drinking either water (W) or hypertonic saline for 3 d (HS) on the mean + SEM plasma concentrations of ACTH (A) and corticosterone (B) measured 5 hr after either subcutaneous vehicle (Veh) or PEG (P) injections. ns, Not significant.

Fig. 2.

The effect of drinking either water (W) or hypertonic saline for 3 d (HS) on the mean + SEM CRH mRNA (A) and CRH hnRNA (B) levels in the medial parvicellular part of the hypothalamic paraventricular nucleus measured 5 hr after either subcutaneous vehicle (Veh) or PEG (P) injections.

Fig. 3.

Photomicrographs of the hypothalamic paraventricular nucleus [approximately level 26 of Swanson (1998)] showing the effects of drinking either water or hypertonic saline for 3 d (HS) on the response 5 hr later to either subcutaneous vehicle (Veh) or PEG (P) injections of CRH mRNA, CRH hnRNA, ppENK mRNA, and c-fos mRNA as detected by in situ hybridization. The top row shows adjacent thionin-stained sections (Nissl). Eachcolumn shows adjacent sections from one animal chosen as representative of each treatment.

Fig. 4.

The effect of drinking either water (W) or hypertonic saline for 3 d (HS) on the mean + SEM level of preproenkephalin mRNA (A) and c-fos mRNA (B) in the medial parvicellular part of the hypothalamic paraventricular nucleus and c-fos mRNA in the posterior magnocellular part of the hypothalamic paraventricular nucleus (C) measured 5 hr after either subcutaneous vehicle (Veh) or PEG (P) injections.

Fig. 5.

Thymus weights (A) and CRH mRNA levels in the medial parvicellular part of the hypothalamic paraventricular nucleus (B) were significantly correlated to the log₁₀ plasma corticosterone concentration in ADX animals given a subcutaneous pellet containing various doses of corticosterone and injected with either 0.9% saline vehicle (○; dashed line) or 40% PEG (●; solid line). PEG injections did not effect either the slope or the Y intercept of the relationship between corticosterone and thymus weights but did significantly increase the Y intercept of the relationship between corticosterone and CRH mRNA levels. See Results for levels of statistical significance.