Desipramine antidepressant action

It has been known for over 25 years that many of the tricyclic antidepressants (TCAs), e.g. imipramine and amitriptyline, are potent inhibitors of both norepinephrine and 5-HT reuptake. Some tricyclic antidepressants, e.g. desipramine, inhibit the uptake of norepinephrine much more potently than the uptake of 5-HT. Thus, it was unclear for some time whether the inhibition of 5-HT uptake played any role in the antidepressant action of those TCAs that possessed this pharmacological property. Recently, however, effective antidepressants such as fluoxetine, paroxetine and sertraline have been marketed and these SSRIs are much more potent inhibitors of the uptake of 5-HT than that of norepinephrine (Fig. 13-8). Thus, selective inhibition of the uptake of either norepinephrine or 5-HT can result in an antidepressant effect (Ch. 55). 

Delgado, P. L., Miller, H. L., Salomon, R. M., Licinio, J., Krystal, J. H., Moreno, F. A., Heninger, G. R., Chamey, D. S. 1999, Tryptophan-depletion challenge in depressed patients treated with desipramine or fluoxetine implications for the role of serotonin in the mechanism of antidepressant action, Biol.Psychiatry, vol. 46, no. 2, pp. 212-220. 

Stahl, S. M. 1984, Regulation of neurotransmitter receptors by desipramine and other antidepressant drugs the neurotransmitter receptor hypothesis of antidepressant action, J.Clin.Psychiatry, vol. 45, no. 10, Pt 2, pp. 37-45. 

Lofepramine differs from imipramine by the attachment of a p-chlorophenacyl moiety to the N-aminopropyl side chain (Fig. 21.17). This change confers enhanced lipophilicity and the potential of more rapid distribution into the CNS with greater in vitro affinity and selectivity for NET. Its mechanism of antidepressant action is attributed to its rapid metabolism to the secondary amine metabolite, desipramine, which selectively inhibits the neuronal uptake of NE (66). 

The original monoamine hypothesis of depression states that depressions are associated with a deficiency of catecholamines, particularly norepinephrine, at functionally important adrenergic receptor sites in the brain. Elation conversely may be associated with an excess of such amines. The hypothesis was articulated in 1966 only after the mechanism of action of the tricyclic antidepressant desipramine and of the psychostimulants... 

Cocaine and desipramine inhibit the reuptake of monoamine neurotransmitters whereas amphetamine, which is a phenylalkylamine - similar in structure to the catecholamines, see Fig. 4 - competes for uptake and more importantly, evokes efflux of the monoamine neurotransmitters. All of them exert antidepressant effects. Cocaine and amphetamine are addictive whereas tricyclic antidepressants and their modern successors are not. The corollaty of the addictive properties is interference with DAT activity. Blockade of DAT by cocaine or efflux elicited by amphetamine produces a psychostimulant effect despite the different mechanisms even the experienced individual can hardly discern their actions. Because of the risk associated with inhibiting DAT mediated dopamine clearance the antidepressant effects of psychostimulants has not been exploited. 

All TCAs are either secondary- or tertiary-amines of a dibenzazepine nucleus (Fig. 20.3), and they all inhibit neuronal reuptake of noradrenaline and/or 5-HT but are much less potent as dopamine reuptake blockers. A common claim is that secondary amines (e.g. desipramine) are preferential inhibitors of noradrenaline uptake whereas the tertiary derivatives (e.g. imipramine, doxepin and amitryptyline) preferentially inhibit 5-HT uptake. However, when Richelson and Pfenning (1984) actually compared the effects of a wide range of antidepressants on the synaptosomal uptake of [ H]monoamines in vitro, and compared their A s, instead of merely ranking /C50S collected from different studies, they found that tertiary- and secondary-substituted compounds were equi-potent inhibitors of [ H]noradrenaline uptake. Moreover, all the TCAs turned out to be more potent inhibitors of [ H]noradrenaline than of [ H]5-HT uptake. Tertiary amines are even less convincing inhibitors of 5-HT reuptake in vivo, because any such action is diminished by their metabolism to secondary amines (e.g. imipramine to desipramine amitriptyline to nortriptyline). Only clomipramine retains any appreciable 5-HT uptake blocking activity in vivo with (an unimpressive) five-fold selectivity for 5-HT versus noradrenaline. 

Since that time, many other tricyclic antidepressants have been studied and put into use. They are all structurally related to imipramine. The active metabolite of imipramine is desipramine. This means that imipramine breaks down into desipramine in the body, and the resulting desipramine actually improves mood. Because their structures are so similar, scientists assume that they have a similar action in the body. 

Charney DS, Nelson JC Delusional and nondelusional unipolar depression further evidence for distinct subtypes. Am J Psychiatry 138 328-333, 1981 Charney DS, Menekes DB, Heninger GR Receptor sensitivity and the mechanism of action of antidepressant treatment. Arch Gen Psychiatry 38 1160-1180, 1981 Charney DS, Price LH, Heninger GR Desipramine-yohimbine combination treatment for refractory depression. Arch Gen Psychiatry 43 1155-1161, 1986 Charney DS, Goodman WK, Price LH, et al Serotonin function in OGD a comparison of the effects of tryptophan and mGPP in patients and healthy subjects. Arch Gen Psychiatry 45 177-185, 1988... 

Few data are available to guide this decision beyond clinical experience. There is a modest amount of evidence that nonresponders to TCAs, principally desipramine or imipramine, alone may respond to a SSRI alone and vice versa (136). No compelling evidence exists showing that nonresponders to one SSRI as a result of a lack of efficacy will respond to a second trial with another SSRI. There is limited confidence in the results of studies that have been done switching nonresponders from one SSRI to another for two reasons. First, virtually all have been open label and, second, most were conducted by the manufacturer of the second SSRI. Until there is more substantive evidence that switching from one SSRI to another is worthwhile, it may be more prudent to switch to a class of antidepressants with a different putative mechanism of action. 

This antidepressant can interact with other drugs via its two mechanisms of action serotonin and NE uptake inhibition. The former action means that the same pharmacodynamic interactions will occur with venlafaxine as with SSRIs, including the serotonin syndrome. At higher doses, venlafaxine is also prone to the same pharmacodynamic interactions as NSRIs such as secondary amine TCAs like desipramine and with newer NSRIs such reboxetine. Thus, the combination of high-dose venlafaxine plus an MAOl could produce a hypertensive crisis as well as the serotonin syndrome. 

The actions of amoxapine and maprotiline resemble those of TCAs such as desipramine. Both are potent NET inhibitors and less potent SERT inhibitors. In addition, both possess anticholinergic properties. Unlike the TCAs or other antidepressants, amoxapine is a moderate inhibitor of the postsynaptic D2 receptor. As such, amoxapine possesses some antipsychotic properties. 

It has been known since the mid-1980s that clomipramine, a potent but nonse-lective serotonin reuptake inhibitor, is effective in reducing OCD symptoms. Since then, numerous studies have confirmed the superiority of clomipramine over placebo in OCD patients, whereas other antidepressant medications with less potent inhibitory effects on serotonin reuptake (e.g., nortripytline, desipramine) seem to be ineffective in OCD. Demonstration of the anti-OCD actions of all five SSRIs, namely, fluoxetine, sertraline, paroxetine, fluvoxamine, and citalopram, also supports the hypothesis that the antiobsessional effects of these various pharmacologic agents is due to their potent serotonergic reuptake blocking activity. 

CNS and cardiovascular actions of d,l-methylphenidate could theoretically be enhanced by combination with agents that block norepinephrine reuptake, such as the tricyclic antidepressants desipramine or protriptyline, venlafaxine, duloxetine, atomoxetine, milnacipran, and reboxetine... 

Noradrenaline is transported by uptake systems that have been extensively studied. On release of noradrenaline from sympathetic nerve varicosities in the peripheral nervous system, it is subject to two uptake systems. Uptake 1 (UJ is a reuptake process where the noradrenaline is recovered by the nerve via a process that has a high affinity but relatively low maximum rate, whereas a second process, uptake 2 (Uj), clears noradrenaline from the tissues into extraneuronal sites by a low affinity, but fast, process (which is inhibited by GLUCOCORTICOIDS, phenoxybenzamine and normetanephrine). The first - the neuronal system - has been studied in detail, and is essentially the same process as used for dopamine and 5-hydroxytryptamine in the CNS. The U transport protein has now been cloned, and is one of a famiiy of transporter proteins which act as co-transporters for Na, Cl and the amine, driven by the ATP-generated electrochemical gradient for Na . This Ui noradrenaline reuptake process is inhibited by cocaine and amphetamine (thus accounting for some of their actions, particularly within the CNS), phenoxybenzamine and the extensive class of tricyclic and related compounds that are used as ANTIDEPRESSANTS (e.g. desipramine). 

HT, antagonist with antidepressant activity (123) it binds with high affinity (if < 10 nM) at 5-HT, receptors (124). Hence, trazodone (24), nefazodone (16), and mianserin (27) represent atypical antidepressants that have in common a high affinity for S-HTgA receptors and S-HTg antagonist action. It might be noted that certain tricyclic antidepressants also bind at 5-HT, receptors imipramine (11), desipramine (6), nortriptyline (17), and maprotiline (13), for example, bind with submicromolar Ki values (124) (Table 8.6). 

Evidence accumulated in 1978 for a catecholamine receptor supersensitivity theory of depression. 8 The therapeutic action of antidepressants may be due to delayed post-synaptic changes in receptor sensitivity, rather than to acute events like uptake. Various drugs, including TCA, mianserin, viloxazine and iprindol, as well as electroconvulsive therapy (ECT), but not selective 5-HT uptake inhibitors, caused central alpha-adrenoceptor subsensitivity in rats as measured by noradrenaline (NA)-associated adenylate cyclase or by receptor binding. In vivo, the effects were associated with chronic but not acute treatment, paralleling the clinical effects. MAOI may cause similar effects on chronic but not acute treatment. , 24-27 Brain NA turnover in rats was decreased by chronic desipramine and other TCA, but unaffected by iprindol and increased by mianserin.3,28... 

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