Venlafaxine: discrepancy between in vivo 5-HT and NE reuptake blockade and affinity for reuptake sites.
Using an in vivo electrophysiological paradigm, venlafaxine and paroxetine displayed similar potency for suppressing the firing activity of dorsal raphe 5-HT neurons (ED50: 233 and 211 microg/kg i.v., respectively), while venlafaxine was three times less potent than desipramine (ED50: 727 and 241 microg/kg i.v., respectively) to suppress the firing activity of locus coeruleus NE neurons. The selective 5-HT1A receptor antagonist WAY 100635 (100 microg/kg, i.v.) reversed the suppressant effect of venlafaxine and paroxetine on the firing activity of 5-HT neurons and the alpha2-adrenoceptor antagonist piperoxane (1 mg/kg, i.v.) reversed those of venlafaxine and desipramine on the firing activity of NE neurons. The ED50 of venlafaxine on the firing activity of 5-HT neurons was not altered (ED50: 264 microg/kg) in noradrenergic-lesioned rats, while the suppressant effect of venlafaxine on the firing activity of NE neurons was greater in serotonergic-lesioned rats (ED50: 285 microg/kg). Taken together, these results suggest that, in vivo, venlafaxine blocks both reuptake processes, its potency to block the 5-HT reuptake process being greater than that for NE. Since the affinities of venlafaxine for the 5-HT and NE reuptake carriers are not in keeping with its potencies for suppressing the firing activity of 5-HT and NE neurons, the suppressant effect of venlafaxine on the firing activity of 5-HT and NE neurons observed in vivo may not be mediated solely by its action on the [3H]cyanoimipramine and [3H]nisoxetine binding sites. In an attempt to unravel the mechanism responsible for this peculiarity, in vitro superfusion experiments were carried out in rat brain slices to assess a putative monoamine releasing property for venlafaxine. (+/-)Fenfluramine and tyramine substantially increased the spontaneous outflow of [3H]5-HT and [3H]NE, respectively, while venlafaxine was devoid of such releasing properties.