Previous results from our laboratory have indicated that small intravenous doses of the alpha 2-adrenergic agonist clonidine increase serotonin (5-HT) neurotransmission by attenuating the release of endogenous norepinephrine (NE), as a result of the activation of alpha 2-adrenergic autoreceptor on NE neurons, and that high doses of clonidine decrease 5-HT neurotransmission by directly activating alpha 2-adrenergic heteroreceptors on 5-HT terminals. The aim of the present study was to assess whether antidepressant treatments that increase the synaptic concentration of NE or 5-HT alter the ability of clonidine to modulate 5-HT neurotransmission through these two alpha 2-adrenoceptors. Rats were treated for 3 weeks with 0.75 mg/kg per day of befloxatone (a reversible inhibitor of monoamine oxidase A), 10 mg/kg per day of nisoxetine (a selective NE reuptake inhibitor), 10 mg/kg per day of paroxetine (a selective 5-HT reuptake inhibitor) or saline using subcutaneous osmotic minipumps (removed 48 hours before the experiment). No significant change in the effect of the small dose of clonidine (10 micrograms/kg, i.v.) was found following the befloxatone, the nisoxetine, or the paroxetine treatments. The reduction of 5-HT neurotransmission by the high dose of clonidine (400 micrograms/kg, i.v.) was no longer present in rats treated with nisoxetine or befloxatone, but was unaltered in those treated with paroxetine. Furthermore, in rats pretreated with the NE neurotoxin 6-hydroxydopamine, a long-term treatment with befloxatone failed to alter the reducing effect of the high dose of clonidine but abolished the reducing effect of the low dose of clonidine. These results suggest that antidepressant drugs that increase NE synaptic concentration induce a desensitization of alpha 2-heteroreceptor on 5-HT terminals.