Noradrenaline Amitriptyline

Antidepressants are used in the treatment of neuropathic pain and headache. They include the classic tricyclic compounds and are divided into nonselective nor-adrenaline/5-HT reuptake inhibitors (e.g., amitriptyline, imipramine, clomipramine, venlafaxine), preferential noradrenaline reuptake inhibitors (e.g., desipramine, nortriptyline) and selective 5-HT reuptake inhibitors (e.g., citalopram, paroxetine, fluoxetine). The reuptake block leads to a stimulation of endogenous monoaminer-gic pain inhibition in the spinal cord and brain. In addition, tricyclics have NMDA receptor antagonist, endogenous opioid enhancing, Na+ channel blocking, and K+ channel opening effects which can suppress peripheral and central sensitization. Block of cardiac ion channels by tricyclics can lead to life-threatening arrhythmias. The selective 5-HT transporter inhibitors have a different side effect profile and are safer in cases of overdose [3]. 

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. 

Many antidepressant drugs have pronounced effects on sleep. Several tricyclic compounds (amitriptyline and others) have sedative actions while others (imipramine and others) are less sedative or even stimulant. Monoamine oxidase inhibitors (MAOIs) have central stimulant effects and may cause insomnia. Specific serotonin reuptake inhibitors (SSRls) and combined serotonin, noradrenaline reuptake inhibitors (SNRIs) can also cause insomnia. 

This group includes compounds with actions on a range of neurotransmitter systems. Their antidepressant efficacy is mediated by reuptake inhibition of serotonin and noradrenaline, although side-effects such as sedation may also be useful. Their use in anxiety disorders is supported by a long history of clinical experience and a reasonable evidence base from controlled trials. Studies support the use of clomipramine (a potent serotonin reuptake inhibitor) in panic disorder and OCD (Lecrubier et al. 1997 Clomipramine Collaborative Study Group 1991), of imipramine in panic disorder and GAD (Cross-National Collaborative Panic Study 1992 Rickels et al. 1993), and of amitriptyline in PTSD (Davidson et al. 1993a). No controlled studies support the use of TCAs in social anxiety disorder. 

Another approach to correct neurotransmission is to inhibit the reuptake of the neurotransmitters into their presvnaptic endings. If the presynaptic reuptake mechanism of a neurotransmitter is blocked then more of the neurotransmitter will stay in the synaptic cleft and be functionally available. Many antidepressant drugs, called reuptake inhibitors , are thought to act via this mechanism. If selective for serotonin they are called selective serotonin reuptake inhibitors (SSRIs, Chapter 1), but if selective for both serotonin and noradrenaline they are called serotonin noradrenaline reuptake inhibitors (SNRIs). Most older antidepressants, such as the tricyclic compounds amitriptyline, imipramine and clomipramine, have little specificity for any of the neurotransmitters fluoxetine, paroxetine, citalopram and a few others are specific for serotonin venlafaxine is a representative of the SNRIs. A more recent mixed-uptake inhibitor is mirtazepine, and some similar compounds are about to be launched. 

Amitriptyline hydrochloride a tricyclic anti-depressant drug which inhibits serotonin and noradrenaline re-uptake. 

Amitriptyline is most frequently used, starting with 10 mg at night increasing to 75 mg. Nortriptyline is better tolerated by some patients. Their general action is to inhibit noradrenaline (norepinephrine) re-uptake by nerve terminals and benefit in neuropathic pain may follow enhanced activity in noradrenergic pain inhibitory paths in the spinal cord. 

A 61-year-old woman taking amitriptyline 25 mg/day underwent oophorectomy for ovarian cancer under combined general and epidural lumbar anesthesia. After the administration of the local anesthetic she developed hypotension refractory to high doses of ephedrine and dopaminergic drugs. Control was achieved with noradrenaline 200 pg. 

Tricyclic antidepressants were named after their characteristic chemical structure, although a number of the newer drugs in this group have a different chemical structure but still share similar pharmacological profiles. Tricyclic antidepressants inhibit the re-uptake of serotonin and noradrenaline into nerve endings. This leads to a build up of neurotransmitter at receptor sites in synapses. They also reduce the number of some receptors, which may contribute to the therapeutic action, but their complete mechanism of action is unknown. Examples of tricyclic antidepressants include imipramine and amitriptyline (for use of amitriptyline in neuropathic pain see Chapter 12). 

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