Effects of repeated fluoxetine on anxiety-related behaviours, central serotonergic systems, and the corticotropic axis in SHR and WKY rats
Introduction
The therapeutic benefit of selective serotonin reuptake inhibitors (SSRIs) in the therapy of depressive disorders is now well established (Lane et al., 1995). In addition, numerous human and animal data also indicate that SSRIs may affect anxiety-related behaviours, which is in keeping with the role of central serotonin (5-hydroxytryptamine, 5-HT) in the modulation of these behaviours (Griebel, 1995; Handley, 1995). In humans, SSRIs are evenly prescribed for panic disorders (Westenberg and den Boer, 1988), obsessive–compulsive disorders (Insel et al., 1985), social phobia (Westenberg, 1992), and anxious depression (Sheehan et al., 1992). Actually, the recognition of high comorbidity of depression with anxiety disorders (Sartorius et al., 1996) could provide a frame to understand why compounds such as SSRIs display antidepressant and anxiolytic properties on repeated administration, but anxiety, nervousness and agitation on subchronic administration or after overdose (Westenberg and den Boer, 1988).
In rodents, both acute and repeated treatment studies with SSRIs have yielded contradictory results, i.e. anxiety, anxiolysis or no effects (Griebel, 1995). Among the variables that underlie such a heterogeneity of results, the nature of the anxiety model (e.g. conditioned versus unconditioned), the dose and the route of administration of the SSRIs, the inclusion (or not) of a wash-out period, and even the species, are likely candidates. Recent studies have however shown that one additional source of experimental variability which should be taken into account is the genetic background of the animals (for a review: Ramos and Mormède, 1998). Thus, mouse (Trullas and Skolnick, 1993, Mathis et al., 1995, Clément et al., 1997) and rat (Pellow et al., 1985, Glowa and Hansen, 1994, Ramos et al., 1997) studies have shown that anxiety-related behaviours vary with the animal strain. This finding is in line with human data which indicate that a complex trait such as emotionality is partly, albeit significantly, driven by genetic factors (Plomin et al., 1994, Bouchard, 1994; Kendler, 1996). In this context, it is relevant to add that the allelic variation in the functional expression of the 5-HT transporter, the selective target of SSRIs, is associated with mood disorders, including anxiety-related traits (Lesch et al., 1996).
Taking into account the key role of genetics in the estimation of anxiety-related behaviours, the aim of our study was to analyse the effects of repeated fluoxetine, the most widely used SSRI, on anxiety-related behaviours in spontaneously hypertensive rats (SHR) and Wistar–Kyoto (WKY) rats. This choice was based on the reports that the WKY rats, from which SHR rats derive(Okamoto and Aoki, 1963), display low activity and high anxiety (as assessed in the open field and the elevated plus-maze tests), compared with SHR rats (McCarty, 1983, Gentsch et al., 1987, Söderpalm, 1989, Paré, 1989, Ramos et al., 1997). Of note is the observation that the genetic difference in locomotor activity between WKY and SHR rats is not linked to blood pressure differences, i.e. the phenotype upon which the two strains were selected (Whitehorn et al., 1983).
Because the initial target of SSRIs is the central serotonergic system, we also examined the effects of fluoxetine on direct (cell body and nerve terminal 5-HT transporters in the midbrain and the hippocampus, respectively) and indirect (5-HT metabolism, hippocampal 5-HT1A receptors, cortical 5-HT2A receptors) targets of SSRIs (Johnson, 1991, Mongeau et al., 1997) to test whether strain-related behavioural differences were associated with divergent neurochemical effects of fluoxetine. Lastly, the recent and promising findings indicating that the activity of the hypothalamo–pituitary–adrenal (HPA) axis (which is tightly linked to central serotonergic systems: Chaouloff, 1993, Chaouloff et al., 1995) is an additional key target of mood-improving drugs (including tricyclics and monoamine oxidase inhibitors; for a review: Holsboer and Barden, 1996), led us to include this neuroendocrine aspect in our study. Accordingly, basal and stress-induced corticosterone release, adrenal weights, and radioligand binding analyses of hippocampal mineralocorticoid (MR) and glucocorticoid (GR) receptors were included in the present study.
Section snippets
Animals and housing conditions
Male SHR and WKY rats (IFFA CREDO, Les Oncins, France), aged 4–5 weeks on arrival, were housed 3 per cage under constant temperature (22±1°C) and a 12 h light/dark cycle (lights on at 07:00 h), with free access to food and water.
All rats were used at least 2 weeks after their arrival. The experiments described herein were in conformity with the French legislation on animal welfare (O.J. number 87-848).
Experimental procedures
Rats from each strain were assigned to one of the following three group treatments (i) water,
Plasma fluoxetine and norfluoxetine concentrations in fluoxetine-pretreated SHR and WKY rats
As shown in Table 1, a 21 day fluoxetine treatment (5 or 10 mg/kg daily), followed by a 2 day wash-out period, increased blood norfluoxetine concentrations (F(2,28)=98.09, P<0.0001) in SHR and WKY rats. Although norfluoxetine concentrations tended to be higher in 10 mg/kg fluoxetine-treated WKY rats, compared with their respective SHR counterparts (Table 1), neither the strain influence nor the pretreatment×strain interaction reached significance. As far as the parent compound is concerned,
Discussion
The aim of the present study was to analyse some key psychoneuroendocrine effects of a repeated treatment with the SSRI, fluoxetine, in two rat strains, namely the SHR and WKY strains, that differ in their emotivity profiles. By means of the elevated plus-maze and open field tests, which allow (see below) a distinction between anxiety- and locomotion-related behaviours, it was observed that repeated fluoxetine administration was not endowed with anxiolytic properties. Actually, some behavioural
Acknowledgements
The authors wish to thank Y. Mellerin for taking care of the animals, and Conseil Régional d’Aquitaine for financial support.
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