Effect of selective and non-selective serotonin receptor activation on l-DOPA-induced therapeutic efficacy and dyskinesia in parkinsonian rats

•5-HTP prevents worsening of l-DOPA anti-parkinsonian efficacy induced by eltoprazine.•Co-administration of 5-HTP with eltoprazine shows additive effect on dampening of LID.•Eltoprazine and 5-HTP reduce LID without altering striatal SERT expression.

Selective activation of 5-HT1 receptors has been shown to produce near to full suppression of l-DOPA-induced dyskinesia (LID) in animal models of Parkinson's disease; however, a reduction of the therapeutic effect of l-DOPA has been reported in several studies. Conversely, we recently found that increasing the serotonergic tone with chronic administration of the serotonin precursor 5-hydroxy-tryptophan (5-HTP) can reduce LID in 6-OHDA-lesioned rats, without affecting l-DOPA efficacy.To directly compare the effects of selective versus non-selective serotonin receptor activation, here we first tested different acute doses of the 5-HT1A/1B receptor agonist eltoprazine and 5-HTP on LID in order to identify doses of the individual compounds showing similar anti-dyskinetic efficacy in l-DOPA-primed dyskinetic rats. About 50% reduction of LID was observed with 0.1 mg/kg and 24 mg/kg of eltoprazine and 5-HTP, respectively; we then compared the effect of the two drugs, individually and in combination, on l-DOPA-induced stepping test in l-DOPA-naïve parkinsonian animals and LID over three weeks of l-DOPA treatment. Results showed that eltoprazine induced significant worsening of l-DOPA-mediated performance in the stepping test, while 5-HTP did not. Interestingly, combination of 5-HTP with eltoprazine prevented the reduction in the forelimb use induced by eltoprazine. Moreover, 5-HTP and eltoprazine given individually showed similar efficacy also upon chronic treatment, and had additive effect in dampening the appearance of LID when given in combination. Finally, chronic administration of eltoprazine and/or 5-HTP did not affect striatal serotonin innervation, compared to l-DOPA alone, as measured by serotonin transporter expression.