Noradrenaline storage

Studies of release of noradrenaline from sympathetic neurons provided the first convincing evidence that impulse (Ca +)-dependent release of any transmitter depended on vesicular exocytosis. Landmark studies carried out in the 1960s, using the perfused cat spleen preparation, showed that stimulation of the splenic nerve not only led to the detection of noradrenaline in the effluent perfusate but the vesicular enzyme, DpH, was also present. As mentioned above, this enzyme is found only within the noradrenaline storage vesicles and so its appearance along with noradrenaline indicated that both these factors were released from the vesicles. By contrast, there was no sign in the perfusate of any lactate dehydrogenase, an enzyme that is found only in the cell cytosol. The processes by which neuronal excitation increases transmitter release were described in Chapter 4. 

Although some studies show that noradrenaline inhibits neuronal firing it is generally considered to increase behavioural activity and arousal. This impression is borne out to the extent that CNS stimulants, like amphetamine, increase release of noradrenaline and produce behavioural and EEG arousal, while reserpine, which reduces noradrenaline storage and hence release, causes psychomotor retardation. It is also supported by... 

The transporters for 5HT, noradrenaline and dopamine, biogenic monoamines, are genetically related, exist as single isoforms and are expressed on the surface of nerve cells, which use monoamines as (or convert them into) their cognate neurotransmitter. The single-isoform monoamine transporters fulfil all three fundamental functions (reuptake, limiting synaptic transmission, and control of the extracellular neurotransmitter concentration). Inactivation of DAT, NET, or SERT results in an increased extracellular lifetime and level of monoamine neurotransmitter, but decreased intracellular storage and evoked release (Fig. 3). 

There are numerous transmitter substances. They include the amino acids glutamate, GABA and glycine acetylcholine the monoamines dopamine, noradrenaline and serotonin the neuropeptides ATP and NO. Many neurones use not a single transmitter but two or even more, a phenomenon called cotransmission. Chemical synaptic transmission hence is diversified. The basic steps, however, are similar across all neurones, irrespective of their transmitter, with the exception of NO transmitter production and vesicular storage transmitter release postsynaptic receptor activation and transmitter inactivation. Figure 1 shows an overview. Nitrergic transmission, i.e. transmission by NO, differs from transmission by other transmitters and is not covered in this essay. 

The effects of drugs on the synthesis, storage, release and destruction of noradrenaline, summarised in Fig. 8.4, are discussed in the following sections. 

The pathway for synthesis of the catecholamines dopamine, noradrenaline and adrenaline, illustrated in Fig. 8.5, was first proposed by Hermann Blaschko in 1939 but was not confirmed until 30 years later. The amino acid /-tyrosine is the primary substrate for this pathway and its hydroxylation, by tyrosine hydroxylase (TH), to /-dihydroxyphenylalanine (/-DOPA) is followed by decarboxylation to form dopamine. These two steps take place in the cytoplasm of catecholaminereleasing neurons. Dopamine is then transported into the storage vesicles where the vesicle-bound enzyme, dopamine-p-hydroxylase (DpH), converts it to noradrenaline (see also Fig. 8.4). It is possible that /-phenylalanine can act as an alternative substrate for the pathway, being converted first to m-tyrosine and then to /-DOPA. TH can bring about both these reactions but the extent to which this happens in vivo is uncertain. In all catecholamine-releasing neurons, transmitter synthesis in the terminals greatly exceeds that in the cell bodies or axons and so it can be inferred... 

After reuptake into the cytosol, some noradrenaline may be taken up into the storage vesicles by the vesicular transporter and stored in the vesicles for subsequent release (see above). However, it is thought that the majority is broken down within the cytosol of the nerve terminal by monoamine oxidase (MAO ECl.4.3.4). A second degradative enzyme, catechol-O-methyl transferase (COMT EC2.1.1.6), is found mostly in nonneuronal tissues, such as smooth muscle, endothelial cells or glia. The metabolic pathway for noradrenaline follows a complex sequence of alternatives because the metabolic product of each of these enzymes can act as a substrate for the other (Fig 8.8). This could enable one of these enzymes to compensate for a deficiency in the other to some extent. 

In other respects the storage of 5-HT resembles that of noradrenaline with its uptake by vesicles resting on energy-dependent, vesicular monoamine transporters (VMATs) (see Chapter 8). Functional disruption of this transporter, either through competitive inhibition (e.g. by methylenedioxymethamphetamine (MDMA, Ecstasy )) or dissipation... 

Reserpine inhibits the synaptic vesicular storage of the monoamines dopamine, serotonin and noradrenaline. As a result they leak out into the cytoplasm where they are inactivated by monoamine oxidase this causes their long-lasting depletion. The resulting low levels of dopamine underlie the antipsychotic actions of reserpine (Chapter 11), whereas the reduced noradrenaline levels underlie its antihypertensive actions. Finally, the resulting low levels of serotonin and noradrenaline mean that reserpine also induces depression. These severe side effects mean that reserpine is no longer used clinically as a treatment for schizophrenia (Chapter 11). 

The noradrenaline normally contained in the storage granules can be partly or completely replaced by structurally related sympathomimetic amines, either by injection of the amine itself, or of suitable precursors such as a-methyl-DOPA or a-methyl-w-tyrosine. These amines can be depleted from the heart by guanethidine in the same way as the noradrenaline which they had replaced. a-Methylnoradrenaline [337] and metaraminol [338] are depleted less readily than noradrenaline from rabbit or rat hearts, whereas dopamine, octopamine and w-octopamine are depleted more readily than noradrenaline [339]. The more rapid depletion of these last three compounds was attributed to weaker binding in the storage granules [339], but could equally well be due to their greater susceptibility to destruction by monoamine oxidase, since both a-methyl-noradrenaline and metaraminol are resistant to attack by monoamine oxidase. 

Since the main clinical use for antisympathotonics is in the treatment of essential hypertension, such drugs will be discussed in Chapter 20 in more detail. The alkaloid reserpine from Rauwolfia serpentina was the first drug used clinically to reduce sympathetic tone. Reserpine reduce the ability of storage and release of various transmitters (adrenaline, noradrenaline, serotonine and dopamine) by an irreversible destruction of the axonal vesicle membranes. The duration of the reserpine effect is actually determined by the de novo synthesis of these structure. Beside various central side effects like sedation, depression, lassitude and nightmares the pattern of unwanted effects of reserpine is determined by the shift of the autonomic balance towards the parasympathetic branch myosis, congested nostrils, an altered saliva production, increased gastric acid production, bardycardia and diarrhea. As a consequence of the inhibition of central dopamine release, reserpine infrequently shows Parkinson-like disturbances of the extrapyramidal system. 

The question as to whether all of the possible iWmethylation compounds of noradrenaline are indeed commonly present in storage places in the body such as the adrenal medulla, or the carotid body, or other ganglia or peripheral synapses will probably not be answered soon, for the intensity of the pharmacological activity of iV -dimethylnoradrenaline and noradrenaline trimethylammonium ion is not great as reported by Stehle, Melville, and Oldham (14) who unfortunately do not give any of the chemical details regarding the identity or purity of the preparations they used. 

In D. dendriticum, a number of types of presumptive aminergic synapses have been described (277) (Fig. 2.10). Evidence that the dense-core granules in this and other species may contain storage granules of a monoamine such as noradrenaline ( = norepinephrine), dopamine or 5-HT is based on microspectrofluorometric analysis, histochemical reactions and biochemical analysis (206, 277, 295-297). 

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