Induction of hyperlocomotion in mice exposed to a novel environment by inhibition of serotonin reuptake. A pharmacological characterization of diverse classes of antidepressant agents.

This study characterized the influence of acute administration of diverse classes of antidepressant agent upon the spontaneous locomotor activity (LA) of mice in a novel, open-field environment. The selective serotonin (5-HT) reuptake inhibitors (SSRIs), citalopram, fluoxetine, paroxetine, fluvoxamine, litoxetine and zimelidine, dose-dependently enhanced LA. Their actions were mimicked by the mixed 5-HT/noradrenaline (NA) reuptake inhibitors (SNRIs), venlafaxine, duloxetine and S33005. In contrast, clomipramine only slightly elevated LA and two further tricyclics, imipramine and amitriptyline, were inactive. Further, the selective NA vs. 5-HT reuptake inhibitors (NARIs), reboxetine, desipramine, maprotiline, nisoxetine and nortriptyline all failed to increase LA. The "atypical antidepressants," mianserin and mirtazapine, neither of which modify 5-HT reuptake, as well as the mixed SSRI/5-HT(2) antagonists, nefazodone and trazodone, also failed to increase LA. Doses of SSRI and SNRI which increased LA did not modify motor performance in the rotarod test. Further, they did not enhance LA in rats, suggesting that this response is characteristic of mice. Finally, upon prehabituation of mice to the activity chamber, the SSRI, citalopram, and the SNRI, venlafaxine, failed to increase LA. In conclusion, in mice exposed to a novel environment, inhibition of 5-HT reuptake by SSRIs and SNRIs enhances spontaneous LA in the absence of a generalized influence upon motor function. This response provides a simple parameter for characterization of SSRIs and SNRIs, and differentiates them from other classes of antidepressant agent. Although an influence upon arousal and/or anxiety is likely related to the increase in LA, the functional significance of this response requires additional elucidation.