Vesicles catecholamine storage

Geffen LB, Livett BG, Rush RA (1969) Immunohistochemical localization of protein components of catecholamine storage vesicles. J Physiol (London) 204 593-605. 

Reserpine is a Rauwolfia alkaloid that has been used for centuries to treat insanity, insomnia and hypertension in humans. Reserpine inhibits normal sympathetic activity in both the CNS and the peripheral nervous system by binding to catecholamine storage vesicles, causing catecholamines to leak into the synapse so that they are not available for release when the presynaptic neuron is stimulated. This prevents the normal magnesium and ATP-dependent storage of catecholamines and 5-hydroxytryptamine in nerve cells, the result being catecholamine (norepinephrine (noradrenaline)) depletion. This results in the inhibition of normal sympathetic activity. 

O Connor DT, Frigon RP. Chromogranin A, the major catecholamine storage vesicle protein. J Biol Chem. 1984 259 3237-3247. 

An important aspect of the preparation and isolation of subcellular particles from brain regions is the criteria by which purity is assessed. Electron microscopy of the various subcellular fractions can provide among the best pieces of evidence for the presence in the preparation of the organelles or subcellular fragments of interest. However, a number of biochemical markers (usually enzymes) that have been established to be present in certain fractions can also be assayed to demonstrate the enrichment of the organelle of interest. For instance, acetylcholinesterase is a common marker for synap-tosomes dopamine-P-hydroxylase is a marker for catecholamine storage vesicles within the synaptosome and cytochrome c oxidase is a marker for mitochondria. Most of the enzymatic markers can be assayed routinely. 

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... 

Catecholamines are concentrated in storage vesicles that are present at high density within nerve terminals 213... 

Ordinarily, low concentrations of catecholamines are free in the cytosol, where they may be metabolized by enzymes including monoamine oxidase (MAO). Thus, conversion of tyrosine to l-DOPA and l-DOPA to dopamine occurs in the cytosol dopamine then is taken up into the storage vesicles. In norepinephrine-containing neurons, the final P-hydroxylation occurs within the vesicles. In the adrenal gland, norepinephrine is N-methylated by PNMT in the cytoplasm. Epinephrine is then transported back into chromaffin granules for storage. 

Octopamine (4.41), which carries a p-hydroxyl group, is taken up even more readily into storage vesicles and is, in turn, released when the neuron fires. As an adrenergic agonist, octopamine is, however, only about one-tenth as active as NE therefore, it acts as a very weak neurotransmitter. Compounds such as this behave like neurotransmitters of low potency, and are called false transmitters. On the other hand, octopamine may be a true transmitter in some invertebrates, with receptors that cannot be occupied either by other catecholamines or by serotonin. 

The storage and release of DA can be modified irreversibly by reserpine (3.1), just as in vesicles containing other catecholamines and serotonin. Dopamine release can be blocked specifically by y-hydroxybutyrate (4.78) or its precursor, butyrolactone, which can cross the blood-brain barrier. High doses of amphetamines do deplete the storage vesicles, but this is not their principal mode of action. Apparently, amantadine (4.79), an antiviral drug that is likewise beneficial in parkinsonism (and also perhaps to relieve fatigue in multiple sclerosis), may also act by releasing DA. 

The effect of released norepinephrine wanes quickly, because -90% is transported back into the axoplasm by a specific transport mechanism (norepinephrine transporter, NAT) and then into storage vesicles by the vesicular transporter (neuronal reuptake). The NAT can be inhibited by tricyclic antidepressants and cocaine. Moreover, norepinephrine is taken up by transporters into the effector cells (extraneuronal monoamine transporter, EMT). Part of the norepinephrine undergoing reuptake is enzymatically inactivated to normetanephrine via catecholamine O-methyltransferase (COMT, present in the cytoplasm of postjunctional cells) and to dihydroxymandelic acid via monoamine oxidase (MAO, present in mitochondria of nerve cells and postjunctional cells). 

Reserpine is an alkaloid from plants of the genus Rauwolfia, used in medicine since ancient times in southern Asia, particularly for insanity more recently, reserpine was extensively used in psychiatry but is now obsolete. Reserpine depletes adrenergic nerves of noradrenaline primarily by blocking amine storage within vesicles present in the nerve ending, so reducing stores of releasable transmitter. Its antihypertensive action is due chiefly to peripheral action, but it enters the CNS and depletes catecholamine stores there too this explains the sedation, depression and parkinsonian (extrapyramidal) side effects that can accompany its use. The effects on catecholamine storage persist for days to weeks after it is withdrawn. 

In addition to terminating the actions of released monoamines, the plasma membrane monoamine transporters present at neuronal locations function in sequence with vesicular monoamme transporters to recycle catecholamines for rerelease (Figure 29-3). Thus most of the norepinephrine released and recaptured by sympathetic nerves is sequestered back into storage vesicles, thereby substantially reducing the requirements for synthesis of new transmitter. 

In addition to synthesis of new transmitter, NE stores are also replenished by transport ofNE previously released to the extracellular fluid by the combined actions of a NE transporter (NET, or uptake 1) that terminates the synaptic actions of released NE and returns NE to the neuronal cytosol, and VMAT-2, the vesicular monoamine transporter, that refills the storage vesicles from the cytosolic pool ofNE ("see below). In the removal ofNE from the synaptic cleft, uptake by the NET is more important than extraneuronal uptake (ENT, uptake 2). The sympathetic nerves as a whole remove -87% of released NE via NET compared with 5% by extraneuronal ENT and 8% via diffusion to the circulation. By contrast, clearance of circulating catecholamines is primarily by nonneuronal mechanisms, with liver and kidney accounting for >60% of the clearance. Because VMAT-2 has a much higher affinity for NE than does the metabolic enzyme, monoamine oxidase, over 70% of recaptured NE is sequestered into storage vesicles. 

Adenosine and uridine di- and triphosphates have roles as extracellular signaling molecules. ATP is a component of the adrenergic storage vesicle and is released with catecholamines. Intracellular nucleotides may also reach the cell surface by other means and extracellular adenosine can result from cellular release or extracellular production from adenine nucleotides. Extracellular nucleotides and adenosine act on a family ofpurinergic receptors that is divided into two classes,... 

Fig. 48.5. Transport of catecholamines into storage vesicles. This is a secondary active transport based on the generation of a proton...

Catecholamine vesicle pump Storage vesicle transporter that pumps amine from cytoplasm into vesicle... 

Philippu A. Matthaei H. Transport and storage of catecholamines in vesicles. In Trendelenburg U. Weiner N. eds. Catecholamines I. New York Springer. 1988 1-42. 

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