Lack of effect of repeated administration of milnacipran, a double noradrenaline and serotonin reuptake inhibitor, on the beta-adrenoceptor-linked adenylate cyclase system in the rat cerebral cortex

Serotonin and Norepinephrine Reuptake Inhibitors

This chapter covers antidepressants that fall into the class of serotonin (5-HT) and norepinephrine (NE) reuptake inhibitors. That is, they bind to the 5-HT and NE transporters with varying levels of potency and binding affinity ratios. Unlike the selective serotonin (5-HT) reuptake inhibitors (SSRIs), most of these antidepressants have an ascending rather than a flat dose-response curve. The chapter provides a brief review of the chemistry, pharmacology, metabolism, safety and adverse effects, clinical use, and therapeutic indications of each antidepressant. Venlafaxine, a phenylethylamine, is a relatively weak 5-HT and weaker NE uptake inhibitor with a 30-fold difference in binding of the two transporters. Therefore, the drug has a clear dose progression, with low doses predominantly binding to the 5-HT transporter and more binding of the NE transporter as the dose ascends. Venlafaxine is metabolized to the active metabolite O-desmethylvenlafaxine (ODV; desvenlafaxine) by CYP2D6, and it therefore is subject to significant inter-individual variation in blood levels and response dependent on variations in CYP2D6 metabolism. The half-life of venlafaxine is short at about 5 h, with the ODV metabolite being 12 h. Both parent compound and metabolite have low protein binding and neither inhibit CYP enzymes. Therefore, both venlafaxine and desvenlafaxine are potential options if drug-drug interactions are a concern, although venlafaxine may be subject to drug-drug interactions with CYP2D6 inhibitors. At low doses, the adverse effect profile is similar to an SSRI with nausea, diarrhea, fatigue or somnolence, and sexual side effects, while venlafaxine at higher doses can produce mild increases in blood pressure, diaphoresis, tachycardia, tremors, and anxiety. A disadvantage of venlafaxine relative to the SSRIs is the potential for dose-dependent blood pressure elevation, most likely due to the NE reuptake inhibition caused by higher doses; however, this adverse effect is infrequently observed at doses below 225 mg per day. Venlafaxine also has a number of potential advantages over the SSRIs, including an ascending dose-antidepressant response curve, with possibly greater overall efficacy at higher doses. Venlafaxine is approved for MDD as well as generalized anxiety disorder, social anxiety disorder, and panic disorder. Desvenlafaxine is the primary metabolite of venlafaxine, and it is also a relatively low-potency 5-HT and NE uptake inhibitor. Like venlafaxine it has a favorable drug-drug interaction profile. It is subject to CYP3A4 metabolism, and it is therefore vulnerable to enzyme inhibition or induction. However, the primary metabolic pathway is direct conjugation. It is approved in the narrow dose range of 50-100 mg per day. Duloxetine is a more potent 5-HT and NE reuptake inhibitor with a more balanced profile of binding at about 10:1 for 5HT and NE transporter binding. It is also a moderate inhibitor of CYP2D6, so that modest dose reductions and careful monitoring will be needed when prescribing duloxetine in combination with drugs that are preferentially metabolized by CYP2D6. The most common side effects identified in clinical trials are nausea, dry mouth, dizziness, constipation, insomnia, asthenia, and hypertension, consistent with its mechanisms of action. Clinical trials to date have demonstrated rates of response and remission in patients with major depression that are comparable to other marketed antidepressants reviewed in this book. In addition to approval for MDD, duloxetine is approved for diabetic peripheral neuropathic pain, fibromyalgia, and musculoskeletal pain. Milnacipran is marketed as an antidepressant in some countries, but not in the USA. It is approved in the USA and some other countries as a treatment for fibromyalgia. It has few pharmacokinetic and pharmacodynamic interactions with other drugs. Milnacipran has a half-life of about 10 h and therefore needs to be administered twice per day. It is metabolized by CYP3A4, but the major pathway for clearance is direct conjugation and renal elimination. As with other drugs in this class, dysuria is a common, troublesome, and dose-dependent adverse effect (occurring in up to 7% of patients). High-dose milnacipran has been reported to cause blood pressure and pulse elevations. Levomilnacipran is the levorotary enantiomer of milnacipran, and it is pharmacologically very similar to the racemic compound, although the side effects may be milder within the approved dosing range. As with other NE uptake inhibitors, it may increase blood pressure and pulse, although it appears to do so less than some other medications. All medications in the class can cause serotonin syndrome when combined with MAOIs.

Evidence for a differential opioidergic involvement in the analgesic effect of antidepressants: prediction for efficacy in animal models of neuropathic pain?

BACKGROUND AND PURPOSE

Antidepressants are one of the recommended treatments for neuropathic pain. However, their analgesic action remains unpredictable, and there are no selection criteria for clinical use. Better knowledge of their mechanism of action could help highlight differences underlying their unequal efficacy.

EXPERIMENTAL APPROACH

We compared the activity of a tricyclic antidepressant (clomipramine) with selective 5-HT and noradrenaline reuptake inhibitors (milnacipran and duloxetine) in streptozocin-induced diabetic and chronic constriction nerve injury-induced neuropathic rats, after repeated injections. We looked for an opioidergic mechanism in their action.

KEY RESULTS

Abolition of mechanical hyperalgesia was observed in mononeuropathic rats after five injections of clomipramine (5 mg·kg(-1) , s.c.) and milnacipran (10 or 20 mg·kg(-1) , i.p.) and in diabetic rats after clomipramine. An additional antinociceptive effect was obtained with five injections of duloxetine (3 mg·kg(-1) , i.p.) in both models and milnacipran (10 mg·kg(-1) , i.p.) in diabetic rats. These effects were observed with plasma antidepressant concentrations similar to those found in patients treated for neuropathic pain. Naloxone (1 mg·kg(-1) , i.v.) only suppressed the anti-hyperalgesic effects of clomipramine in both models of pain and of milnacipran in the traumatic model.

CONCLUSIONS AND IMPLICATIONS

The opioid system appears to be involved in the mechanism of action of antidepressants that only have an anti-hyperalgesic effect but not in those that have a stronger (i.e. antinociceptive) effect. These differences between the antidepressants occurred whatever the aetiology of the neuropathy and, if confirmed in clinical trials, could be used to decide which antidepressant is administered to a patient with neuropathic pain.

Repeated administration of milnacipran induces rapid desensitization of somatodendritic 5-HT1A autoreceptors but not postsynaptic 5-HT1A receptors

The effects of the repeated administration of milnacipran, a serotonin (5-HT)-noradrenaline reuptake inhibitor (SNRI), on the functional status of somatodendritic 5-HT1A receptors, and postsynaptic 5-HT1A receptors were explored using electrophysiological approaches in rats. In-vitro electrophysiological recordings in the dorsal raphe nucleus showed that 5-HT inhibited the firing of serotonergic neurones, and the selective 5-HT1A receptor antagonist, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane carboxamide (WAY 100635), reversed the inhibitory effect of 5-HT. The potency of 5-HT to inhibit the firing of serotonergic neurones was slightly attenuated after 3 days of treatment with milnacipran (30 mg/kg, p.o., twice daily), and significantly attenuated after 7 or 14 days treatment at the same dose. The tricyclic antidepressant, imipramine, did not significantly modify the inhibitory effect of 5-HT. After 7 days treatment at 30 mg/kg, p.o., once daily, milnacipran reduced the potency of 5-HT to inhibit the firing of serotonergic neurones, whereas the selective serotonin reuptake inhibitors, fluvoxamine and fluoxetine (60 and 30 mg/kg, p.o., once daily, respectively), did not modify it under these conditions. Treatment with milnacipran (30 mg/kg, p.o., twice daily) for 14 days did not change the inhibition of the CA1 field potential in rat hippocampal slices by 5-HT. These data suggest that somatodendritic 5-HT1A receptors, but not postsynaptic 5-HT1A receptors, rapidly desensitize in response to the repeated administration of milnacipran.

Cellular mechanisms in the vulnerability to depression and response to antidepressants

As a testable heuristic, the concept of stress response and adaptation is highly appealing, and the support for the concept is strong. This explanatory model of depression may account for hitherto apparently discordant facts--contradictory symptoms, antidepressant drugs that act on differing systems, facilitation of antidepressant response by augmentation, and response to psychotherapy and pharmacotherapy. This article has focused narrowly on specific cellular elements of the stress-adaptational mechanisms, including the AC-PKA and PLC-PKC transductional cascades, together with specific response elements, such as the HPA axis, BDNF, and NMDA receptors; however, other important mechanisms, including specific receptor subtypes (e.g., 5-HT1A and NE alpha 2), transmitter systems (e.g., acetylcholine and depamine), and hormones (e.g., thyroid and growth hormones and prolactin), which may be important, have not been discussed. As the complex interactions of these systems gradually yield to investigation, not only will new treatments be developed, but better matching of treatment to patient may become an achievable goal.

Milnacipran: an antidepressant with dual selectivity of action on noradrenaline and serotonin uptake

Milnacipran is a new antidepressant which possesses potent and doubly selective action in that it inhibits both the re-uptake of serotonin and noradrenaline without any effect on other neurotransmitter systems. The almost equipotent inhibition of serotonin and noradrenaline by milnacipran is functionally reflected in the several-fold and long-lasting increase of the levels of these monoamines in the brain and in antidepressant-like effects in animals. In man, milnacipran distinguishes itself from many other antidepressants by its simple pharmacokinetics. It shows linear dose-concentration relationship over a dose range of 25-200 mg/day. It is rapidly and extensively absorbed and almost completely eliminated after 12 h (t1/2 approx. 8 h). Steady-state plasma levels are reached within 32-48 h after twice daily oral administration. Milnacipran is highly bioavailable (>85 per cent) and its metabolism does not involve the cytochrome P450 enzyme system. In clinical studies, milnacipran showed antidepressant efficacy similar to that of TCAs and SSRIs and superior to that of placebo. At the optimum dose of 100 mg/day, after 4-8 weeks of treatment, 60-64 per cent of in- or out-patients with major depression improve (>/=50 per cent reduction of HAMD and MADRS score) and about 32-39 per cent of them achieve full remission (HAMD score</=7). Milnacipran has proved to be a very safe drug with a benign adverse event profile clearly superior to that of TCAs and, to a certain extent, that of SSRIs. Only about 10 per cent of patients experience side-effects and only dysuria occurred more frequently (2 per cent) with milnacipran than with TCAs or SSRIs. Milnacipran appears therefore to be an antidepressant with a very favourable benefit/risk ratio. Copyright 2000 John Wiley & Sons, Ltd.

In vivo cyclic AMP responses in rat brain are not modified by chronic electroconvulsive shock

In vivo microdialysis was used to determine the effects of chronic electroconvulsive shock (ECS), given daily for 10 days, on the cyclic AMP (cAMP) responses to noradrenaline (NA) and to forskolin in the cortex of freely-moving conscious rats. Microdialysis probes were inserted on the final day of ECS administration, and cAMP responses measured the following day. Chronic ECS did not modify the responses to either NA or forskolin. These results differ markedly from those previously obtained with incubated slices of rat brain. It is concluded that care is needed in interpreting results obtained by either ex vivo or in vitro methods.