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OBJECTIVE: The purpose of this study was to use functional magnetic resonance imaging (fMRI) to probe the neural circuitry associated with reactivity to negative and positive affective stimuli in patients with major depressive disorder before treatment and after 2 and 8 weeks of treatment with venlafaxine. Relations between baseline neural activation and response to treatment were also evaluated. METHOD: Patients with major depressive disorder (N=12) and healthy comparison subjects (N=5) were scanned on three occasions, during which trials of alternating blocks of affective and neutral pictorial visual stimuli were presented. Symptoms were evaluated at each testing occasion, and both groups completed self-report measures of mood. Statistical parametric mapping was used to examine the fMRI data with a focus on the group-by-time interactions. RESULTS: Patients showed a significant reduction in depressive symptoms with treatment. Group-by-time interactions in response to the negative versus neutral stimuli were found in the left insular cortex and the left anterior cingulate. At baseline, both groups showed bilateral activation in the visual cortices, lateral prefrontal cortex, and amygdala in response to the negative versus neutral stimuli, with patients showing greater activation in the visual cortex and less activation in the left lateral prefrontal cortex. Patients with greater relative anterior cingulate activation at baseline in response to the negative versus neutral stimuli showed the most robust treatment response. CONCLUSIONS: The findings underscore the importance of the neural circuitry activated by negative affect in depression and indicate that components of this circuitry can be changed within 2 weeks of treatment with antidepressant medication.
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CYP2D6 is involved in the O-demethylation metabolic pathway of venlafaxine in humans. In this study, we investigated whether this isozyme is stereoselective. Plasma samples from seven CYP2D6 extensive metabolizers (EMs) and five CYP2D6 poor metabolizers (PMs), collected during a period without and with coadministration of quinidine, were analysed. Subjects were administered venlafaxine hydrochloride 18.75 mg orally every 12 h for 48 h on two occasions (1 week apart); once alone and once during the concomitant administration of quinidine sulphate every 12 h. Blood and urine samples were collected under steady-state conditions over one dosing interval (12 h). The present results show that, although CYP2D6 catalyses the O-demethylation of both enantiomers of venlafaxine, it displays a marked stereoselectivity towards the (R)-enantiomer. The oral clearance of (R)-venlafaxine was found to be nine-fold higher in EMs compared to PMs [median (range) 173 (29-611) l/h versus 20 (16-24) l/h, P < 0.005], while it was two-fold higher for (S)-venlafaxine [73 (32-130) l/h versus 37 (21-44) l/h, P < 0.05]. In EMs, quinidine decreased (R)- and (S)-venlafaxine oral clearance by 12-fold ( 0.05) and four-fold ( 0.05), respectively. In contrast, quinidine did not have any effects on renal clearance of (R)-venlafaxine [4 (2-10) l/h for venlafaxine alone versus 5 (0.6-7) l/h for venlafaxine + quinidine] and of (S)-venlafaxine [4 (1-7) l/h for venlafaxine alone versus 3 (0.4-6) l/h for venlafaxine + quinidine]. The coadministration of quinidine to EMs resulted in an almost complete inhibition of the partial metabolic clearance of (R)-venlafaxine to O-demethylated metabolites [127 (10-493) l/h down to 1 (0.1-3) l/h, 0.05], while a seven-fold reduction was measured for (S)-venlafaxine [47 (14-94) l/h versus 7 (1-19) l/h, 0.05]. In PMs, coadministration of quinidine did not significantly change oral clearance and partial metabolic clearance of (R)- and (S)-venlafaxine to its various metabolites. In contrast, data obtained on the partial metabolic clearance of (R)- and (S)-venlafaxine to N-demethylated metabolites, a reaction which is mediated by CYP3A4, suggest a lack of stereoselectivity of this enzyme.
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Pain. 1996 Nov;68(1):151-5.
Venlafaxine hydrochloride (Effexor) relieves thermal hyperalgesia in rats with an experimental mononeuropathy.
Lang E, Hord AH, Denson D.
Department of Anesthesiology, Emory University School of Medicine, Atlanta, GA 30322, USA.
Venlafaxine hydrochloride (Effexor) is a structurally novel antidepressant that inhibits reuptake of 5-hydroxytryptamine and noradrenaline, but unlike the older antidepressants, has few side-effects. The objective of this study was to determine whether venlafaxine relieves thermal hyperalgesia in rats with neuropathic pain due to chronic constriction injury (CCI) of the sciatic nerve. Paw withdrawal latency (PWL) to heat was tested for each hind paw. A painful neuropathy was induced in 24 male Sprague-Dawley rats (Group 1) as described by Bennett and Xie. Rats randomly received either oral venlafaxine (22 mg/kg) or placebo via gavage feeding beginning the day after surgery. Postoperative PWL testing began 3 days after CCI (Time 0). A second group of 12 rats (Group 2) was used to confirm that venlafaxine reverses hyperalgesia in rats with a fully developed neuropathic lesion. These animals began to receive oral venlafaxine (22 mg/kg) starting on the third postoperative day, after the presence of thermal hyperalgesia was verified through PWL testing. Testing was continued for 5 days, during venlafaxine administration. A third group of 12 rats (Group 3) had activity measured before and after treatment with venlafaxine (22 mg/kg). Rats in the placebo group developed thermal hyperalgesia while those that received venlafaxine did not. Venlafaxine also appeared to have a mild non-specific analgesic effect that increased PWL in the sham limb. In Group 2, thermal hyperalgesia was present on day 3, but following treatment with venlafaxine, thermal hyperalgesia resolved. Activity measurements confirmed that venlafaxine was not sedating in this rat model.
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