Effect of repeated treatment with tianeptine and fluoxetine on central dopamine D₂/D₃ receptors

Abstract

Tianeptine (TIA) is an antidepressant drug that has been shown to decrease extracellular serotonin level and reveals no affinity for neurotransmitter receptors. The present study was aimed at determining whether repeated TIA treatment induced any adaptive changes in the central dopamine D₂/D₃ system (behavioural and biochemical) similar to those reported earlier for tricyclic antidepressants. Experiments were carried out on male Wistar rats. TIA was administered at a dose of 5 and 10 mg/kg once or repeatedly (twice daily for 14 days). Fluoxetine (FLU), used as a reference compound, was also administered at a dose of 10 mg/kg. The results obtained showed that TIA or FLU administered repeatedly increased the hyperlocomotion induced by D-amphetamine and 7-hydroxy-dipropylaminotetralin (7-OH-DPAT). Biochemical study revealed a decrease in the [³H]7-OH-DPAT binding sites after acute and repeated treatment with TIA or FLU in the islands of Calleja minor, as well as in the shell part of nucleus accumbens septi. On the other hand, both TIA and FLU administered repeatedly increased the binding of [³H]quinpirole (a D₂/D₃ receptor agonist) in the nucleus caudatus as well as in the core part of the nucleus accumbens septi. Similar effects have been observed when dopamine D₂/D₃ receptors were visualized with the use of [³H]raclopride, a dopamine D₂/D₃ receptor antagonist. However, TIA and FLU induced a decrease in the level of mRNA encoding for dopamine D₂ receptors, not only after repeated but also after acute treatment. These results indicate that repeated TIA and FLU administration induces adaptive changes in the dopaminergic D₂/D₃ system and especially enhances the functional responsiveness of dopamine D₂ and D₃ receptors. However, the question of whether this increased responsiveness is important for clinical antidepressant efficacy remains open.