Acute and chronic effects of corticosterone on 5-HT_1A receptor-mediated autoinhibition in the rat dorsal raphe nucleus

Abstract

Corticosteroid modulation of 5-HT_1A receptor function may contribute to the aetiology of affective disorders. To examine this modulation, the effects of acute and chronic corticosterone administration on 5-HT_1A autoreceptor function were investigated using in vitro electrophysiology in the rat dorsal raphe nucleus (DRN). The magnitude and time course of the inhibitory response to a submaximal dose of 5-HT was not affected by acute application of either corticosterone (30–200 nM) or dexamethasone (100 nM) in vitro, when tested either in slices from control rats or rats adrenalectomised two weeks prior to recording. For chronic treatment, rats were supplied with drinking water containing corticosterone (50 μg/ml) or ethanol vehicle (0.5%) for 25–31 days. The autoinhibitory response to 5-HT was significantly attenuated in the corticosterone-treated group; vehicle EC₅₀=48±8 μM vs. corticosterone EC₅₀=121±20 μM. Furthermore a subpopulation of 5-HT neurones from corticosterone-treated animals exhibited marked insensitivity to 5-HT. In situ hybridisation histochemistry showed that corticosterone did not affect the expression of mRNA encoding the 5-HT_1A receptor or either the type 1 and type 3 subunits of the G-protein linked inwardly rectifying K⁺ (GIRK) channel. However, GIRK2 subunit mRNA expression was significantly reduced. Thus, 5-HT_1A autoreceptor function in the DRN is attenuated following chronic, but not acute, exposure to elevated corticosterone levels, and this effect may involve changes to the receptor–effector coupling mechanism.

Introduction

Two common features of depressive illness are chronically elevated levels of adrenal corticosteroids (hypercortisolaemia) and dysfunction of serotonin neurotransmission. The hypothalamic–pituitary–adrenal (HPA) axis is dysregulated in a large proportion of depressed patients, leading to increased secretion of cortisol, particularly in the diurnal trough of the rhythm (Wong et al., 2000). It is thought that this hypercortisolaemia may contribute to the aetiology of the mood disorder, since Cushing’s patients who have increased adrenal activity frequently exhibit depressive symptoms which resolve upon correction of the endocrine dysfunction (Sonino and Fava, 2001). Serotonergic neurotransmission, particularly through the 5-hydroxytryptamine 1A (5-HT_1A) receptor, is also impaired in depressed patients (Bhagwagar et al., 2002). There is considerable evidence indicating that the HPA axis and the brain serotonin (5-HT) system interact, and some authors have argued that a primary deficit in one of these systems may lead to a functional impairment in the other (e.g. Dinan, 1994, McAllister-Williams et al., 1998).

The 5-HT_1A receptor subtype has been strongly implicated in the regulation of mood. The receptor is abundantly expressed in limbic areas, such as the hippocampus and hypothalamus (Verge et al., 1986), where it is post-synaptic to 5-HT neurones, as well as in the midbrain dorsal raphe nucleus (DRN) where it is expressed at somatodendritic sites (Sotelo et al., 1990, Riad et al., 2000). These somatodendritic 5-HT_1A receptors function as autoreceptors to negatively regulate the discharge of DRN 5-HT neurones (Sprouse and Aghajanian, 1987, Blier and de Montigny, 1990) and thus play an important role in constraining 5-HT release in forebrain regions.

Considerable evidence from basic and clinical studies indicates that corticosteroids modulate the post-synaptic 5-HT_1A receptor function. It has been demonstrated that acute application of low concentrations of corticosterone attenuates 5-HT_1A receptor function in the CA1 subfield of the rat hippocampus, while higher concentrations enhance hippocampal 5-HT_1A receptor function (Joëls et al., 1991, Joëls and de Kloet, 1992). Interestingly, long-term exposure to elevated plasma corticosterone levels was reported to attenuate 5-HT_1A receptor function in the hippocampus (Karten et al., 1999), the opposite to what is found in the acute situation. Clinical studies in healthy volunteers have also demonstrated that corticosteroids are capable of modulating 5-HT_1A receptor function. For example, the hypothermic response to the 5-HT_1A agonist buspirone is attenuated by repeated cortisol administration (Young et al., 1994).

Whilst the ability of corticosteroids to modulate post-synaptic 5-HT_1A receptor function has been well established, less evidence is available regarding the effects of corticosteroids on somatodendritic 5-HT_1A autoreceptor function. One in vitro electrophysiological study demonstrated that the corticosterone attenuated dorsal raphe 5-HT_1A receptor functions when applied acutely to rat brain slices (Laaris et al., 1995). The same authors subsequently showed that exposure to unpredictable stress, in combination with social isolation, desensitised 5-HT_1A autoreceptors in the DRN (Laaris et al., 1999). Since this effect was blocked in adrenalectomised animals, the authors proposed that elevated corticosterone levels during stress exposure caused the observed reduction in receptor function. However, no study has yet addressed the effect of chronic corticosterone directly.

The current studies measured 5-HT_1A autoreceptor function by in vitro electrophysiology of the DRN to investigate, firstly, whether acute corticosterone exposure would attenuate 5-HT_1A autoreceptor function and the time course of any such effect and, secondly, whether chronic exposure to elevated plasma corticosterone levels would alter 5-HT_1A autoreceptor function. For this, we have administered corticosterone through the drinking water in order to elevate levels across the diurnal cycle as they are in conditions of chronic stress. In addition, in situ hybridisation histochemistry was used to determine the impact of chronic corticosterone administration on mRNA expression for the 5-HT_1A receptor and the G-protein-coupled inwardly rectifying K⁺ channel (GIRK) subunits 1–3. These subunits form part of the receptor–effector complex as the 5-HT_1A autoreceptor in the DRN is linked to GIRK channels (Andrade et al., 1986, Penington et al., 1993, Williams et al., 1988).