Vol. 285, Issue 2, 404-412, May 1998

Electrophysiological Characterization of the Effect of Long-Term Duloxetine Administration on the Rat Serotonergic and Noradrenergic Systems¹

Lynne E. Rueter, Claude De Montigny and Pierre Blier

Neurobiological Psychiatry Unit, Department of Psychiatry,McGill University, Montreal, Quebec, Canada

Duloxetine is a dual serotonin (5-HT)/norepinephrine (NE) reuptake blocker with antidepressant potential. In the present in vivo electrophysiological study, the changes in the function of the rat 5-HT and NE systems after 2- and 21-day administration of duloxetine (20 mg/kg/day) were assessed in the dorsal hippocampus and the dorsal raphe nucleus (DRN). The firing rate of DRN neurons was decreased after 2 days of duloxetine, but returned to the control level after 21-day administration. This recovery of firing rate was presumably due to the desensitization of the DRN somatodendritic 5-HT_1A autoreceptors found after long-term duloxetine administration. Overall serotonergic tone was assessed by examining the ability of the 5-HT_1A antagonist WAY 100635 to alter hippocampal firing. WAY 100635 increased hippocampal firing rates in 21-day treated rats to a greater extent than in 2-day treated or control rats, suggesting that long-term administration induced an increase in endogenous levels of 5-HT in postsynaptic regions. This increase in 5-HT levels was accompanied by selective changes in the 5-HT and NE systems induced by long-term duloxetine administration, i.e., the desensitization of the alpha-2 adrenergic heteroreceptor on 5-HT terminals and the continued blockade of the 5-HT transporters. In contrast, the sensitivity of the alpha-2 adrenergic and terminal 5-HT_1B autoreceptors, as well as that of the postsynaptic 5-HT_1A receptor after 21-day treatment was unchanged. Therefore, this study demonstrates that duloxetine increases serotonergic tone in a limbic forebrain structure and may therefore be effective in the treatment of depression.