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In spite of intensive research, the problem of treating antidepressant-resistant depressive patients has not yet been solved. Our previous studies demonstrated that joint administration of a tricyclic antidepressant drug, imipramine (IMI) with the uncompetitive antagonist of NMDA receptors, amantadine (AMA), produced stronger "antidepressant" effect in the forced swimming test (Porsolt's test) than the treatment with either drug alone given. Since it has been suggested that dopamine receptors, among others, may play a role in anti-immobility effect of IMI, in the present study we examined the effect of AMA (10 mg/kg) and IMI (5 and 10 mg/kg) given separately or jointly, as a single dose or repeatedly (twice daily for 14 days) on the dopamine D2 and D3 receptors in the rat brain, using receptor autoradiography. Following repeated administration of AMA alone or given in combination with IMI (5 mg/kg), the binding of [3H]quinpirole (dopamine D2/D3 receptors agonist) was increased, and similar changes were observed at the level of mRNA encoding dopamine D2 receptors. We used [3H]7-OH-DPAT to selectively label the dopamine D3 receptors. This experiment has shown that AMA given repeatedly did not induce statistically significant changes in the D3 receptor binding, while IMI at both used doses, increased the [3H]7-OH-DPAT binding, and this effect was still observed after repeated joint administration of AMA with both doses of IMI. However, using both radioligands, we did not observe any synergistic or even additive effects in the binding studies after joint administration of AMA and IMI. Nevertheless, we can conclude that repeated administration of AMA, given together with IMI, induces the up-regulation of dopamine D2 and D3 receptors in the rat brain, and this effect may explain their synergistic action observed in the behavioral studies involving dopaminergic transmission.
The purpose of the present study was to evaluate the effects of acute and repeated treatment with two antidepressant drugs (ADs)of opposite pharmacological profile,i.e. tianpetine (TIA,serotonin reuptake enhancer)and fluoxetine (FLU,serotonin reuptake inhibitor)on the levels of Met-Enkephalin,(Met-Enk,a member of opioid peptide family, which has been suggested to play a role in the mechanism of action ADs)as well as on mRNA coding for proenkephalin (mRNA PENK)in various regions of the rat brain, pituitary,adrenal glands and plasma.Male Wistar rats were treated acutely or repeatedly (10 mg/kg p.o.,twice daily for 14 days)with TIA or FLU.Tissue for biochemical experiments was taken 2 h after last dose of appropriate drug.The levels of Met-Enk were estimated by radioimmunoassay,mRNA PENK was measured using in situ hybridization.From the results obtained in the present study it may be concluded that repeated administration of TIA or FLU induced similar changes in the levels of Met- Enk in the rat hippocampus,striatum,hypothalamus and neurointermediate lobe of pituitary.Such an effect is interesting,especially if one takes into account the differences in pharmacological profile between these two antidepressant drugs.It may be suggested that serotonin level might not be crucial for inducing the alterations in the content of Met-Enk.Since we did not observe any changes in the levels of PENK mRNA in the studied rat brain regions after repeated administration of TIA or FLU,it seems that the observed changes in the levels of Met-Enk do not result from effects of these antidepressants on biosynthesis of PENK,but rather from alterations in the peptide release.Another interesting finding of the present study was that in the anterior lobe of pituitary,adrenal glands and plasma,repeated administration of TIA induced alterations in the contents of Met-Enk,while repeated administration of FLU remained without any effect.It is tempting to speculate that such a differentiation between the effects of these two antidepressants might be linked to the well known feature of TIA (but not FLU)which has been shown to reduce both basal and stress-evoked activity of the hypothalamic-pituitary-adrenal (HPA)axis.
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