Research reportMetyrapone pretreatment prevents the behavioral and neurochemical sequelae induced by stress
Introduction
Evidence from clinical studies suggests a good correlation between the degree of anxiety in humans and plasma concentrations of cortisol 4, 46. Increases in circulating cortisol, whether endogenously produced by stress, pathological states such as Cushing's disease, depression or other psychiatric disorders, or, as effected through exogenous administration, are often associated with a state of anxiety characterized by a sense of apprehension and fear 14, 25, 53.
Circulating corticosteroids enter the brain, where they bind to intracellular mineralocorticoid and glucocorticoid receptors (MR and GR). The MR and GR are both localized in high densities in certain brain structures involved in emotional behavior such as septum, amygdala, hippocampus and cerebral cortex 9, 24, 52. Adrenal glucocorticoids appear to have a dual and frequently opposite action on the behavioral expression of fear and anxiety. For instance, some report that corticosterone (CS) administration may cause anxiolytic-like effects, whereas adrenalectomy may produce anxiogenic-like effects, which can be reversed by CS replacement therapy 13, 54. In contrast, fear-motivated immobility can be abolished by adrenalectomy and restored after CS administration [6]. Moreover, recent reports have shown either anxiolytic or anxiogenic-like effects following treatment with MR and/or GR antagonists in various animal models of anxiety, meanwhile, a recent review argues for an anxiolytic effect of the hormone 22, 23, 41.
The mechanism(s) by which CS regulates the expression of anxiety is not yet known. The GABAergic system may be one potential mediator of this modulation since CS affects the activity of GABAergic neurons. Additionally, it is well-established that GABA is critically involved in the effect of anxiolytic drugs and, consequently, in anxiety response [5]. Indeed, glucocorticoids have been shown to modulate the density of GABA 27, 28, TBPS [29]and benzodiazepine binding sites [28], as well as GABA-mediated chloride flux in vitro [38]. In addition, adrenalectomy-induced changes in GABA [21], muscimol [27], and flunitrazepan [11]binding in selective brain regions have been reported. Furthermore, adrenalectomy reverses the effect of acute stress on GABA activated chloride influx and modifies the alteration of benzodiazepine binding sites induced by prior stress 34, 47, 55, 56.
Taken together, all these results suggest a possible involvement of adrenal steroids in the modulation of, at least some, behavioral and neurochemical changes associated with the exposure to an aversive event.
In accordance with previous reports [1], we have recently demonstrated that a previous exposure to a brief restraint session induced an anxiogenic-like behavior assessed in the elevated plus-maze (EPM), where fear is behaviorally expressed by rats' passive avoidance of novel open arms 17, 39, 44. This behavioral response was paralleled by a clear reduction of cortical chloride uptake following GABA stimulation [31]. Considering that exposure to such a brief restraint induced a robust increase in CS secretion [31], the present study was conducted to examine the potential involvement of CS in the behavioral sequelae and in the functional changes of the GABAA receptor complex induced by this brief aversive experience (15-min restraint session).
Accordingly, we addressed the following issues: (a) the effect(s) of metyrapone, a 11-β-hydroxylase inhibitor that blocks CS synthesis, administered before restraint, on the subsequent behavioral performance in the EPM and on the activity of the supramolecular GABAA complex, in pooled cortical tissue in vitro by means of chloride uptake assessment following GABA stimulation; (b) the effect of metyrapone on the time course of changes in serum CS concentrations, determined in individual rats, before and after exposure to the restraint session and also before and after the subsequent exposure to the EPM; (c) whether exogenous corticosteroid administration, either CS or the synthetic corticosteroid dexamethasone (DEXA), could mimic the behavioral alteration in the EPM observed following the exposure to the restraint session.
Section snippets
Animals
Male Wistar rats from our breeding stock, weighing 280–320 g were used. Animals were housed in groups of 6 per cage with a constant light–dark cycle (lights off from 1900 to 0700 h). Temperature (22°C) and humidity (60%) were also kept constant. Animals were allowed at least 1 week of habituation to the animal room. Experiments were performed between 1100 and 1400 h, with the experimenter unaware of the treatment condition.
Restraint
Rats were transferred to an experimental room where the stressor was
Experiment 1
This experiment was devised to evaluate the effect of metyrapone on behavioral performance in the EPM in animals with or without the previous restraint experience. Rats were handled daily for 1 week prior to testing, namely, removed, weighed, and returned to their home cages. On day eight, subjects were randomly divided into four groups. The four groups comprised a 2×2 factorial design. Rats were given either metyrapone (n=15) or vehicle (n=14) 3 h before restraint; a similar treatment was
Experiment 1: effect of metyrapone on the behavioral responses to restraint
As was described in a previous report [31], the exposure to a 15 min restraint session induced a decrease in the percentage of time spent on open arms (treatment F1,50=8.62, p<0.005) as compared with unrestrained animals.
Metyrapone treatment significantly modified the decrease in the percentage of time spent on open arms induced by restraint (treatment×stress interaction, F1,50=5.89, p<0.019) (Fig. 1). Metyrapone- and vehicle-unrestrained animals did not significantly differ in percentage of
Discussion
As previously reported [31], the exposure to as little as 15 min of restraint resulted in significant behavioral and neurochemical changes when stressed animals were subsequently tested in the EPM 24 h later. These changes were manifested as a reduced open arm activity, and a decreased efficacy and potency of GABA to stimulate Cl− flux in cerebral cortical membranes.
The present study shows that the behavioral consequences induced by a brief restraint, and the associated changes in GABAA
Acknowledgements
This research was supported by grants from CONICOR, SECYT-UNC and CONICET (Argentina). The authors are grateful to Elsa Pereyra and Estela Salde for their technical assistance and to Cristina Aráoz for generously providing us with horse serum.
References (57)
- et al.
Corticosterone is involved in foot-shock-induced inactivity in rats
Physiol Behav.
(1996) - et al.
GABAergic and dopaminergic transmission in the rat cerebral cortex: effect of stress, anxiolytic and anxiogenic drugs
Pharmacol. Ther.
(1990) - et al.
Ethopharmacological analysis of rat behavior on the elevated plus-maze
Pharmacol. Biochem. Behav.
(1994) - et al.
Benzodiazepine receptors in rat brain are altered by adrenalectomy
Brain Res.
(1986) - et al.
Presynaptic effects of glucocorticoids on dopaminergic and cholinergic synaptosomes
Life Sci.
(1987) - et al.
Role of 5-HT in stress, anxiety and depression
Pharm. Biochem. Behav.
(1996) - et al.
An assessment of the elevated X-maze for studying anxiety-modulating drugs
J. Pharmacol. Toxicol. Methods
(1993) - et al.
Long term control of neuronal excitability by corticosteroid hormones
J. Steroid Biochem. Mol. Biol.
(1995) - et al.
The influence of ACTH and corticosterone on GABA receptor binding in rat brain
Brain Res.
(1982) - et al.
Anxiolytic-like effects of selective mineralocorticoid and glucocorticoid antagonists on fear-enhanced behavior in the elevated plus-maze
Psychoneuroendocrinology
(1995)