Antidepressants preferentially enhance habituation to novelty in the olfactory bulbectomized rat

The mechanisms whereby antidepressant drugs exert their therapeutic effects remain unknown. Responses to stressful stimuli are currently thought to contribute to the onset and course of affective disorders. It has been postulated that antidepressants might act by ameliorating response patterns to challenging life events, such as processes of reactivity and/or habituation.

Using the olfactory bulbectomy (OBX) rat model, this study examined the effects of various antidepressants on measures of reactivity and habituation in behavioral tests assessing responses to novel stimuli.

Sham-operated and OBX rats received 21 daily injections of fluoxetine (10 mg/kg), amitriptyline (10 mg/kg), desipramine (10 mg/kg), buspirone (3 mg/kg), or vehicle. Forty-eight hours after the last injection, animals were tested in the open field, elevated plus maze, and startle apparatus. For each test, time series data were collected and fit with exponential random effects models, in which estimated parameters assessed behavioral reactivity and habituation.

Relative to sham controls, OBX rats displayed increased total locomotor activity in the open field and exhibited increased open arm behavior in the elevated plus maze. Through comparison with zinc sulfate-treated anosmic controls, these OBX-induced increases were attributed to both an augmentation of initial reactivity due to anosmia and an attenuation of the average rate of habituation. Chronic antidepressant treatment did not reduce the anosmia-related initial reactivity levels of OBX rats to that of sham controls. Rather, the antidepressants evoked their restorative effects by increasing the rate of habituation.

These findings suggest that antidepressants restore normal responding by permitting more effective adaptation to novel stimuli.