There remains a need for more efficacious antidepressant agents. Although two-thirds of patients will ultimately respond to anti-depressant treatment, one-third of patients respond to placebo, and remission is frequently sub-maximal (residual symptoms). In addition to post-treatment relapse, depressive symptoms can even recur in the course of long-term therapy (tachyphylaxis). Also, currently available antidepressants all elicit undesirable side-effects, and new agents should be divested of the distressing side-effects of both first and second-generation antidepressants.
Another serious drawback of all antidepressants is the requirement for long-term administration prior to maximal therapeutic efficacy. Although some patients show a partial response within 1–2 weeks, in general one must reckon with a delay of 3–6 weeks before full efficacy is attained. In general, this delay to onset of action is attributed to a spectrum of long-term adaptive changes. These include receptor desensitization, alterations in intracellular transduction cascades and gene expression, the induction of neurogenesis, and modifications in synaptic architecture and signaling.
Depression has been associated with impaired neurotransmission of serotonergic, noradrenergic, and dopaminergic pathways, although most pharmacologic treatment strategies directly enhance only 5HT and NE neurotransmission. In some patients with depression, DA-related disturbances improve upon treatment with antidepressants, it is presumed by acting on serotonergic or noradrenergic circuits, which then affect DA function. However, most antidepressant treatments do not directly enhance DA neurotransmission, which may contribute to residual symptoms, including impaired motivation, concentration, and pleasure.
Preclinical and clinical research indicates that drugs inhibiting the reuptake of all (3?) of these neurotransmitters can produce a more rapid onset of action and greater efficacy than traditional antidepressants.
DA may promote neurotrophic processes in the adult hippocampus, as 5-HT and NA do. It is thus possible that the stimulation of multiple signalling pathways resulting from the elevation of all three monoamines may account, in part, for an accelerated and/or greater antidepressant response.
Dense connections exist between monoaminergic neurons. Dopaminergic neurotransmission regulates the activity of 5-HT and NE in the dorsal raphe nucleus (DR) and locus coeruleus (LC), respectively. In turn, the ventral tegmental area (VTA) is sensitive to 5-HT and NE release.
In the case of SSRIs, the promiscuity among transporters means that there may be more than a single type of neurotransmitter to consider (e.g. 5-HT, DA, NE, etc.) as mediating the therapeutic actions of a given medication. MATs are able to transport monoamines other than their "native" neurotransmitter. It was advised to consider the role of the organic cation transporters (OCT) and the plasma membrane monoamine transporter (PMAT).
To examine the role of monoamine transporters in models of depression DAT, NET, and SERT knockout (KO) mice and wild-type littermates were studied in the forced swim test (FST), the tail suspension test, and for sucrose consumption. The effects of DAT KO in animal models of depression are larger than those produced by NET or SERT KO, and unlikely to be simply the result of the confounding effects of locomotor hyperactivity; thus, these data support reevaluation of the role that DAT expression could play in depression and the potential antidepressant effects of DAT blockade.
The SSRIs were intended to be highly selective at binding to their molecular targets. However it may be an oversimplification, or at least controversial in thinking that complex psychiatric (and neurological) diseases are easily solved by such a monotherapy. While it may be inferred that dysfunction of 5-HT circuits is likely to be a part of the problem, it is only one of many such neurotransmitters whose signaling can be affected by suitably designed medicines attempting to alter the course of the disease state.
Most common CNS disorders are highly polygenic in nature; that is, they are controlled by complex interactions between numerous gene products. As such, these conditions do not exhibit the single gene defect basis that is so attractive for the development of highly-specific drugs largely free of major undesirable side-effects ("the magic bullet"). Second, the exact nature of the interactions that occur between the numerous gene products typically involved in CNS disorders remain elusive, and the biological mechanisms underlying mental illnesses are poorly understood.
Clozapine and dimebon are examples of drugs used in the treatment of CNS disorders that have a superior efficacy precisely because of their "multifarious" broadspectrum mode of activity. Likewise, in cancer chemotherapeutics, it has been recognized that drugs active at more than one target have a higher probability of being efficacious.
In addition, the nonselective MAOIs and the TCA SNRIs are widely believed to have an efficacy that is superior to the SSRIs that are normally picked as the first-line choice of agents for/in the treatment of MDD and related disorders. The reason for this is based on the fact that SSRIs are safer than nonselective MAOIs and TCAs. This is both in terms of there being less mortality in the event of overdose, but also less risk in terms of dietary restrictions (in the case of the nonselective MAOIs), hepatotoxicity (MAOIs) or cardiotoxicity (TCAs).
'AIDS cocktails' is another example of where it is simply more advantageous to target multiple receptors than it is to find a magic bullet for any one single receptor.
In the case of epibatidine, it is clearly this drug's lack of selectivity that separates its analgesic actions from its potential to cause lethality. However, if we look at a different painkiller called Vioxx, despite being a highly selective agent, it was less safe to use than some of the earlier COX-2 inhibitors.
Read more about this topic: Serotonin–norepinephrine–dopamine Reuptake Inhibitor
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