Neonatal Clomipramine Treatment of Syrian Hamsters: Effect on the Circadian System

The circadian behavior of male Syrian hamsters injected with the serotonin/norepinephrine reuptake inhibitor clomipramine (15 mg/kg from postnatal days 8 to 21) was examined. Clomipramine treatment significantly augmented mean activity values of wheel running rhythm, as well as delayed its acrophase. After a 6-h phase advance of the light-dark cycle, reentrainment of clomipramine-treated hamsters took significantly longer than controls. Clomipramine-treated hamsters exhibited a shorter circadian period than controls in constant light conditions, but no differences were found in constant darkness. Light pulses applied at late subjective night to clomipramine-treated hamsters caused significantly reduced phase advances as compared to controls, while no differences were found in phase delay magnitudes when light pulses were applied during early subjective night. Administration of the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) at circadian time 8 significantly advanced the onset of activity to a greater extent in clomipramine-treated hamsters than in controls. The results indicate that neonatal clomipramine treatment of hamsters causes long-lasting changes in the circadian system, by increasing activity levels and by partially inhibiting light-evoked responses. An enhancement of a non-photic, serotonergic-induced response was also unveiled.