Psychopharmacology
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Randomized Controlled Trial Clinical Trial
Comparison of the effects of venlafaxine, paroxetine and desipramine on the pupillary light reflex in man.
The time-course of the pupillary light reflex response is determined by the successive activation of the parasympathetic and sympathetic innervations of the iris, the latency and the amplitude reflecting parasympathetic and the recovery time mainly sympathetic activity. ⋯ The increase in resting pupil diameter could be indicative of parasympathetic inhibition and/or sympathetic activation. The shortening of the recovery time of the light reflex response is consistent with sympathetic potentiation resulting from noradrenaline uptake blockade in the iris. The prolongation of the latency and decrease of the amplitude of the light reflex response are indicative of a parasympatholytic effect of venlafaxine. However, as venlafaxine has negligible affinity for muscarinic cholinoceptors, this effect cannot be attributed to the blockade of cholinoceptors in the iris. A possible explanation for this finding is that it reflects a central rather than a peripheral effect of the drug: the blockade of noradrenaline uptake in the brain could lead to the potentiation of the noradrenergic inhibition of central parasympathetic (Edinger-Westphal) neurones. These results demonstrate the ability of therapeutically relevant single doses of venlafaxine to potentiate noradrenergic responses in man, consistent with the blockade of noradrenaline uptake.