Corticosterone attenuates the antidepressant-like effects elicited by melatonin in the forced swim test in both male and female rats.

Melatonin has been demonstrated to increase activity in the forced swim test (FST), a putative model of antidepressant efficacy, indicating that it may possess antidepressant-like qualities. It has been suggested that corticosterone can interfere with the efficacy of antidepressants, an effect that has previously been demonstrated in the FST. This experiment examined the effects of melatonin and corticosterone, independently and in combination, on the behaviours of both male and female rats in the FST. Corticosterone, melatonin, combined vehicles or a combined melatonin/corticosterone regimen were administered for 20 days, after which the animals were observed in the FST. As seen in previous research, melatonin elicited an antidepressant-like effect in the FST by reducing immobile behaviour (P<.01) and increasing active behaviour (P<.01). Corticosterone was found to reduce activity (P<.01) and increase immobility (P<.01), as well as attenuate the anti-immobility effects of melatonin (P=.03). These findings suggest that while melatonin may possess antidepressant-like qualities, high levels of corticosterone seem capable of attenuating these effects.