Effect of chronic fluoxetine and WAY-100635 treatment on serotonergic neurotransmission in the frontal cortex

Clinical augmentation strategies have shown that some improvement in antidepressant efficacy can be achieved by combining the β-adrenergic/serotonin (5-HT)1A/1B receptor antagonist (±)pindolol with a selective serotonin reuptake inhibitor (SSRI). This has lead to the hypothesis that a combination of a 5-HT_1A receptor antagonist with an SSRI will lead to a faster onset of antidepressant action. Although there is a significant accumulation of acute preclinical data supporting this rationale, until recently, there have been no investigations examining the chronic effects of combining an SSRI with a 5-HT_1A receptor antagonist. Here, we determined the chronic effects of fluoxetine (10 mg/kg o.d.), administered in combination with the selective 5-HT_1A receptor antagonist WAY-100635 (1 mg/kg b.i.d.), on serotonergic neurotransmission in the frontal cortex using in-vivo microdialysis. Following chronic administration of fluoxetine ± WAY-100635, functional changes in serotonergic neurotransmission, as well as 5-HT_1A autoreceptors, were assessed by administering fluoxetine or (±) 8-hydroxy-2-(di-n-propylamino)tetralin [(±) 8-OH-DPAT] 24 h after the last chronic dose. Chronic administration of WAY-100635 alone produced no detectable change in the functional status of the 5-HT_1A receptor. However, fluoxetine alone produced a time-dependent adaptation in serotonergic transmission such that fluoxetine (acutely administered on day 15) was able to produce a two fold increase in extracellular 5-HT levels but the decrease in response to 8-OH-DPAT was completely attenuated. These data indicate that the fluoxetine-induced adaptation was mediated by desensitization of the 5-HT_1A receptor. WAY-100635 given chronically in combination with fluoxetine blocked the SSRI-induced desensitization of the 5-HT_1A receptor. Furthermore, chronic treatment with this combination produced no tolerance in terms of its ability to acutely increase forebrain 5-HT levels. These data suggest that augmentation of an SSRI by combined pharmacotherapy with a 5-HT_1A antagonist would be effective upon prolonged exposure.