Chronic administration of 13-cis-retinoic acid does not alter the number of serotoninergic neurons in the mouse raphe nuclei

The synthetic retinoid 13-cis-retinoic acid (13-cis-RA), prescribed for the treatment of severe nodular acne, has been linked to an increased incidence of depression. Chronic treatment studies in rodents have shown that 13-cis-RA induces an increase in depression-related behaviours and a functional uncoupling of the hippocampus and dorsal raphe nucleus (DRN). Changes in the number of serotoninergic neurons in the DRN have been reported in depressed human patients. Given that retinoids have apoptotic effects, we hypothesised that a decrease in the number of serotoninergic neurons in the DRN or median raphe nucleus (MRN) would lead to decreased serotoninergic tone and in turn to the behavioural changes seen with 13-cis-RA administration. Here, we used immunolabelling and unbiased stereological methods to estimate the number of serotonin (5-hydroxytryptamine, 5-HT) neurons in the MRN and DRN of vehicle control and 13-cis-RA-treated adult mice. In the MRN, the number of 5-HT immunolabelled cells was 1815 ± 194 in control, compared with 1954 ±111 in 13–cis-RA treated tissues. The number of 5-HT immunolabelled cells was much higher in the DRN, with 7148 ± 377 cells in the control, compared with 7578 ± 424 in the 13-cis-RA treated group. Further analysis of the DRN revealed that there were no changes in the number of 5-HT neurons within distinct subregions of the DRN. Similarly, changes in the density of serotoninergic neurons or in the volume of the MRN or DRN were not observed in 13-cis-RA treated animals. These data show that apoptotic actions of 13-cis-RA do not occur in vivo at drug concentrations that induce changes in depression-related behaviour and functional uncoupling of the DRN and hippocampus. The potential pro-depressant behavioural and molecular effects associated with chronic administration of 13-cis-RA may result from changes in serotoninergic activity rather than changes in the number of serotoninergic neurons