An investigation of the mechanisms responsible for acute fluoxetine‐induced anxiogenic‐like effects in mice

Abstract

Although selective 5‐hydroxytryptamine (5‐HT) reuptake inhibitors (SSRIs) are widely used in the chronic treatment of several anxiety disorders, increased anxiety has been observed in some patients at the beginning of treatment with these compounds. Similar increases in anxiety‐related behaviors have been observed in animal studies following a single injection with SSRIs. The mechanism underlying this effect is unclear. The aim of the present study was to investigate the effects of a variety of psychoactive compounds on the anxiogenic‐like activity of fluoxetine. The drugs used included the benzodiazepine diazepam, the 5‐HT 1A receptor partial agonist buspirone, the 5‐HT 1A receptor antagonists pindolol and WAY‐100635, the non‐selective 5‐HT 2 receptor antagonists methiothepin, mianserin and ritanserin, the non‐selective dopamine (DA) receptor antagonist haloperidol, the D 1 antagonist SCH23390, the selective D 2 antagonist raclopride, the D 2/3 agonist quinelorane, the cholecystokinin B (CCK B ) receptor antagonist LY 288513, and the corticotropin‐releasing factor 1 (CRF 1 ) receptor antagonist CP‐154,526. Experiments were performed in the free‐exploration test. This model is based on the strong neophobic reactions exhibited by BALB/c mice when confronted simultaneously with a familiar and a novel environment. When administered alone, diazepam (1 and 2 mg/kg), buspirone (1 mg/kg) and mianserin (0.3 mg/kg) produced anxiolytic‐like effects as they significantly increased exploratory activity of the novel compartment. In contrast, fluoxetine (20 mg/kg) almost completely suppressed exploration of the novel area. Diazepam reversed the anxiogenic‐like as well as the locomotor impairment induced by fluoxetine, while quinelorane blocked only the anxiogenic action of fluoxetine. None of the other compounds was able to counteract this effect. Taken together, these results suggest that dopaminergic mechanisms may underlie, at least in part, the behavioral effects of fluoxetine in the free‐exploration test, whereas 5‐HT 1A , 5‐HT 2 , CCK B and CRF 1 receptors may not be involved primarily in these effects.