Catecholamine neurotransmission as a drug target

This chapter is relevant to Section M2 (ii) of the 2017 CICM Primary Syllabus, which expects the exam candidates to "outline the mechanisms by which drugs may affect neurotransmission and noradrenaline effect at the sympathetic nerve terminal". This was interrogated indirectly in Question 23 from the second paper of 2011, which asked for the "production, release, and fate of noradrenaline at the sympathetic nerve terminal", using similar language but asking for something fairly specific and not entirely related. Still, as the subjects are closely associated, it felt good to unite them in the same chapter, as the production release and fate of noradrenaline are all serious drug targets.

Nagatsu & Stjärnet (1997) do a
Biosynthesis of catecholamines is discussed elsewhere, and 

Mechanism of catecholamine exocytosis and reuptake

The majority of the events discussed here occur directly at the synapse and most of the molecular machinery required for mass manufacture and manipulation of catecholamines is found there. For example, tyrosine hydroxylase, the rate limiting enzyme of catecholamine synthesis, is made locally at the nerve terminal (but the mRNA required for this is transported there down the axon). The catecholamines that are created in the cytosol are then transported to the vesicles, stored there until release, and reclaimed back into the vesicles once their synaptic time is over. The following  diagram remains here to illustrate some of these mechanisms,  though it is very old, dating back to a time when the author's younger punk self was rebelling against every Tufteain principle of visual data representation (though in all fairness this is not the worst example of that).

Noradrenaline does hang around in the cytoplasm unescorted, but generally speaking you need to keep your catecholamines in vesicles.  The VMAT-2 protein (vesicular amine transporter) concentrates the catecholamines in the vesicles. It is promiscuous: i.e. it has equal affinity for dopamine, adrenaline, noradrenaline, serotonin and so forth.  VMAT-2 gets about 90% of the catecholamines into vesicles; the rest float about in the cytoplasm and get metabolized by mitochondrial MAO. 

 The low  pH inside the vesicles causes the catecholamines to become trapped; in the acidic environment they exist in their ionised water-soluble form, and thus cannot diffuse out.  These vesicles act as storage for all kinds of monoamines, and the VMAT-2 doesn’t care what it pumps. If dopamine enters the vesicles, it gets converted to noradrenaline by dopamine beta-hydroxylase (dβH)

Catecholamine exocytosis is a process mediated by a bunch of proteins which have little clinical relevance outside of being the targets for the botulinum toxin and the tetanus toxin. These proteins are SNAP-25, syntaxin and synaptobrevin. Botulinum and tetanus toxins are more famous for this very same effect but at the neuromuscular junction.  Nobody quite knows precisely what mechanisms trigger the exocytosis of catecholamines at the nerve terminals, but it looks like it is something to do with calcium influx through voltage-gated N-type calcium channels.  They open when the action potential reaches the synapse. The calcium influx then activates the vesicle fusion proteins, and exocytosis results. 

Presynaptic receptors play a role in the modulation of catecholamine release. Activation of presynaptic α2 receptors inhibits the release of catecholamines, and activation of presynaptic β-2 receptors enhances the release of catecholamines, making the system capable of activating and deactivating itself via a positive and negative feedback loop. An additional external control mechanism is the adenosine neurotransmitter system which, by activation of presynaptic A-1 adenosine receptors, inhibits the release of catecholamines. The common pathway shared by both networks is the synthesis of cAMP: if you increase cAMP (a β-2  effect), you enhance the release of catecholamines. 

The NET and DAT proteins (NorEpinephrine Transporter and DopAmine Transporter) remove catecholamine from the synapse.  These make attractive drug targets, and clever ligands can either block these transport proteins, or even cause their internalisation and reversal, turning them into exocytosis pumps.

Drug targets in catecholamine neurotransmission

Again, a gigantic diagram says a thousand words.

The sites where catecholamine transport exocytosis and reuptake can be interfered with by drugs are numerous. Very briefly:

• Tyrosine metabolism: α-Methyltyrosine blocks the conversion of tyrosine into L-DOPA, which is a rate-limiting step. This is one way of treating phaeochromocytoma.
• L-Dopa Metabolism: α-Methyldopa blocks  the conversion of L-DOPA into dopamine, and its active metabolite α-Methylnorepinephrine  is an alpha-2 receptor agonist, which is functionally similar to clonidine.
• Interference with storage: Reserpine blocks VMAT-2, and can thus result in the depletion of catecholamines from all your nerve endings
• Interference with the mechanisms of exocytosis: Botulinum and tetanus toxins proteolyse the fusion proteins, synaptobrevin specifically,  and thus prevent release of  catecholamines and acetylcholine.
• Inhibition of exocytosis: Clonidine and dexmedetomidine act here to inhibit release of catecholamines. Ditto α-Methylnorepinephrine as mentioned above.
• Stimulation of exocytosis Xanthines like caffeine act as adenosine antagonists; they inhibit the inhibition. The net result is an enhanced release of catecholamines.
• Interference with synaptic concentration: Indirect sympathomimetics act here by displacing catecholamines out of the synapse and into the extracellular fluid.
• Inhibition of reuptake: cocaine and the tricyclic antidepressants act at the NAT and DAT channels to decrease reuptake of noradrenaline, thus increasing its synaptic dwell-time.
• Reversal of reuptake channels: Amphetamines also cause DAT and NET dysfunction, by causing internalization of the DAT protein, and by causing it to malfunction and actually leak dopamine back into the synapse