Behavioral specificity of non-genomic glucocorticoid effects in rats: Effects on risk assessment in the elevated plus-maze and the open-field
Introduction
When facing stressful or challenging situations, rats develop both an endocrinological and a behavioral response that aims at a successful coping with the situation. The acute endocrine response is characterized by a rapid increase in the secretion of different hormones, e.g., plasma glucocorticoids, noradrenaline, adrenaline, and vasopressin. Rapid changes in plasma glucocorticoid levels have been shown to correlate with a series of behaviors, such as the lordotic response in rats (Kubli-Garfias, 1990), perch hoping in sparrows (Breuner et al., 1998), attack latency in mice (Brain et al., 1971, Poole and Brain, 1974), and offensive behavior in rats (Mikics et al., 2004). However, the underlying mechanisms of acute glucocorticoid effects on behavior have not been well understood.
Several lines of evidence indicate that glucocorticoid hormones do not only exert their effects on the genome by mediating gene expression but also have rapid, non-genomic effects on neuronal membrane processes in the brain (Borski, 2000, Falkenstein et al., 2000, Makara and Haller, 2001, Wehling, 1997). This was demonstrated in models at the molecular, cellular, and behavioral level as well (Chen et al., 1991, Di et al., 2003, Mikics et al., 2004, Sze and Iqbal, 1994a, Sze and Iqbal, 1994b). In general, non-genomic glucocorticoid effects can be differentiated from genomic ones by criteria based on the particularities of the genomic mechanism. Thus, effects occurring within less than 15 min (the shortest known lag time for protein synthesis; Hallahan et al., 1973) and resistant to protein synthesis inhibition and/or blockade of mineralocorticoid (MR) and glucocorticoid (GR) receptors can be attributed to non-genomic mechanisms (Makara and Haller, 2001). However, most behavioral studies only focused on the rapidity of the effects. So far, there are only three behavioral studies in which the involvement of non-genomic mechanisms was proven. In the roughskin newt (Taricha granulosa), glucocorticoids rapidly inhibit male courtship behavior via non-genomic actions; moreover, a specific non-genomic glucocorticoid membrane receptor was described in this species (Orchinik et al., 1991, Rose and Moore, 1999). In rats, novelty-induced locomotion was rapidly enhanced by large doses of corticosterone, and this effect was resistant to both protein synthesis inhibition and MR and GR blockade (Sandi et al., 1996a, Sandi et al., 1996b). Recently, we demonstrated that glucocorticoids are able to activate the aggressive response of resident rats very rapidly via non-genomic mechanisms. The effect occurred within 2–7 min and was resistant to protein synthesis inhibition. Interestingly, glucocorticoid effects occurring 20–25 min after hormone administration were inhibited by protein synthesis blockade which suggests that the very rapid non-genomic effects were rapidly overtaken by genomic ones (Mikics et al., 2004).
Glucocorticoid synthesis increases rapidly in a variety of challenging situations; e.g., plasma levels increased within 3–5 min after exposure to the plus-maze, airpuff, and social defeat (Engelmann et al., 1996, File et al., 1994, Sgoifo et al., 1996). In addition, glucocorticoids were shown to rapidly affect a variety of behaviors (see above). These findings raise questions regarding the specificity and roles of glucocorticoids in controlling behavior. Glucocorticoids may act simply as arousing factors that promote behavioral activation. This assumption, however, is not supported by data on a certain degree of behavioral specificity; e.g., glucocorticoids increased offensive threats and dominant postures in a previous study but did not alter social behaviors (Mikics et al., 2004). We propose here that an acute increase in glucocorticoids affects coping responses. This assumption may explain both behavioral specificity in a certain situation, and context dependency (i.e., different effects in different situations). The aim of the present studies was to assess this hypothesis by studying the effects of glucocorticoids in the elevated plus-maze. This test appeared especially suitable for such a study as it assesses three different kinds of responses: locomotion (shown by total and/or closed arm entries), anxiety (shown by open arm exploration), and risk assessment (Pellow et al., 1985, Rodgers and Cole, 1993). The latter is generally viewed as a measure of anxiety, but factor analysis and pharmacological studies suggest that it is not controlled by, and has a different significance than, open arm exploration (Cole and Rodgers, 1994, Cruz et al., 1994). In an earlier study we have shown that plus-maze-induced increases in plasma glucocorticoids correlate significantly with risk assessment only, suggesting that the effects of this hormone are behavior specific in this test. In the present study we have also investigated rapid glucocorticoid effects in the open field, as this test is also able to measure all three locomotion (by distance covered), anxiety (by central area exploration), and risk assessment (by stretch attend posture) (Prut and Belzung, 2003). As glucocorticoid effects were essentially the same in the two tests, only one- the elevated plus-maze was assessed in detail.
Section snippets
Animals
The subjects were male Wistar rats (Charles River Laboratories, Hungary) weighing 350–400 g (at the age of 60–70 days). Animals were housed in groups of five or six per cage prior to experimentation in plastic cages measuring 59.5 × 38 × 22 cm (1354G, Eurostandard Type IV). They were kept under reversed day–night schedule (lights off at 10:00 h, lights on at 22:00 h) in a temperature (22 ± 2°C) and humidity (60 ± 10%) controlled environment with free access to laboratory rat food (Sniff
Experiment 1: Effects of acute inhibition of glucocorticoid synthesis on EPM behavior
Data are summarized in Fig. 1. Metyrapone treatment significantly decreased corticosterone levels measured immediately after testing (F(1,18) = 31.97; P < 0.0001). Metyrapone treatment decreased the frequency of total SAP (the sum of “protected” and unprotected” SAPs) significantly (H(1,20) = 7.48, P = 0.006) and there was a tendency towards an effect on the frequency of protected SAP (H(1,20) = 2.68, P = 0.01). Metyrapone did not affect “classical” locomotion or anxiety parameters such as the
Discussion
The acute inhibition of endogenous glucocorticoid production by metyrapone resulted in a decrease of risk assessment behavior (as characterized by stretched attend postures) on the elevated plus-maze. Corticosterone administered 22 or 2 min before testing restored behavioral patterns to levels seen in vehicle-injected animals. Conventional measures of anxiety and locomotion were not affected by metyrapone or corticosterone treatment in the elevated plus-maze and open-field tests. Thus,
Acknowledgment
This work was supported by OTKA Grant No. T046785.
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