Cytosolic neurotransmitters

Fig. 1 General model of the release of vesicular neurotransmitter stores in response to cellular depolarization and the reuptake of the neurotransmitters by the monoamine transporters. Cytosolic neurotransmitters are taken into vesicles by VMAT and stored until the cell becomes depolarized, causing these vesicular stores to fuse with the plasma membrane and release the neurotransmitters into the synaptic cleft. Neurotransmitters in the synaptic cleft are available to bind pre- or postsynaptic receptors. Termination of signaling occurs when the neurotransmitters are taken back into the presynaptic cell by the monoamine transporters...

Together with dopamine, adrenaline and noradrenaline belong to the endogenous catecholamines that are synthesized from the precursor amino acid tyrosine (Fig. 1). In the first biosynthetic step, tyrosine hydroxylase generates l-DOPA which is further converted to dopamine by the aromatic L-amino acid decarboxylase ( Dopa decarboxylase). Dopamine is transported from the cytosol into synaptic vesicles by a vesicular monoamine transporter. In sympathetic nerves, vesicular dopamine (3-hydroxylase generates the neurotransmitter noradrenaline. In chromaffin cells of the adrenal medulla, approximately 80% of the noradrenaline is further converted into adrenaline by the enzyme phenylethanolamine-A-methyltransferase. 

NO is a gaseous neurotransmitter implicated in signaling in the central and peripheral nervous system as well as in the immune system and the vasculature. NO is formed from L-arginine by nitric oxide synthase (NOS). There are three isoforms of NOS. All isoforms require NADPH as a cofactor, use L-arginine as a substrate, and are inhibited by Nw-nitro-L-arginine methyl ester (L-NAME). The three isoforms are separate gene products. One isoform of NOS is a cytosolic, calcium/calmodulin-independent, inducible enzyme (iNOS). It is found in macrophages, neutrophils, vascular smooth muscle, and endothelia. The iNOS... 

Adenosine is not a classical neurotransmitter because it is not stored in neuronal synaptic granules or released in quanta. It is generally thought of as a neuromodulator that gains access to the extracellular space in part from the breakdown of extracellular adenine nucleotides and in part by translocation from the cytoplasm of cells by nucleoside transport proteins, particularly in stressed or ischemic tissues (Fig. 17-2C). Extracellular adenosine is rapidly removed in part by reuptake into cells and conversion to AMP by adenosine kinase and in part by degradation to inosine by adenosine deaminases. Adenosine deaminase is mainly cytosolic but it also occurs as a cell surface ectoenzyme. 

Release is another area of difference conventional neurotransmitters are secreted from small synaptic vesicles (SSVs) after cytosolic [Ca2+] transiently reaches concentrations of 50-100 pmol/1, while peptides are released from LDCVs at lower concentrations of cytosolic [Ca2+] (Fig. 18-3). Conventional neurotransmitter release is thought to occur very close to the site of Ca2+ entry (see Chs 9,10), while neuropeptides are typically released at a distance from the site of Ca2+ entry (Fig. 18-4). Furthermore the Ca2+ that stimulates exocytosis from LDCVs may come from either internal stores or the extracellular fluid. Thus, the location of LDCVs relative to the site of Ca2+ influx has a very crucial impact on the intensity of stimulation necessary for secretion to occur. 

Tyrosine phosphorylation plays an important role in synaptic transmission and plasticity. Evidence for this role is that modulators of PTKs and PTPs have been shown to be intimately involved in these synaptic functions. Among the various modulators of PTKs, neuro-trophins have been extensively studied in this regard and will be our focus in the following discussion (for details of growth factors, see Ch. 27). BDNF and NT-3 have been shown to potentiate both the spontaneous miniature synaptic response and evoked synaptic transmission in Xenopus nerve-muscle cocultures. Neurotrophins have also been reported to augment excitatory synaptic transmission in central synapses. These effects of neurotrophins in the neuromuscular and central synapses are dependent on tyrosine kinase activities since they are inhibited by a tyrosine kinase inhibitor, K-252a. Many effects of neurotrophins on synaptic functions have been attributed to the enhancement of neurotransmitter release BDNF-induced increase in neurotransmitter release is a result of induced elevation in presynaptic cytosolic calcium. Accordingly, a presynaptic calcium-depen-dent phenomenon - paired pulse facilitation - is impaired in mice deficient in BDNF. 

The exact mechanisms by which BDNF enhances the induction of LTP remain obscure. Nevertheless, both pre-and postsynaptic mechanisms appear to be possible. As mentioned above, BDNF elevates presynaptic cytosolic calcium level and thus increases vesicular neurotransmitter release. When a postsynaptic neuron is injected with a Trk tyrosine kinase inhibitor (K252a), BDNF-augmented LTP is curtailed this suggests that a postsynaptic mechanism is adopted by BDNF in the manifestation of LTP. It has been postulated that neurotrophins may act as... 

Receptor-effector mechanisms include (1) enzymes with catalytic activities, (2) ion channels that gate the transmembrane flux of ions (ionotropic receptors), (3) G protein-coupled receptors that activate intracellular messengers (metabotropic receptors), and (4) cytosolic receptors that regulate gene transcription. Cytosolic receptors are a specific mechanism of many steroid and thyroid hormones. The ionotropic and metabotropic receptors are discussed in relevance to specific neurotransmitters in chapter 2. 

The three best-known examples of biochemical oscillations were found during the decade 1965-1975 [40,41]. These include the peroxidase reaction, glycolytic oscillations in yeast and muscle, and the pulsatile release of cAMP signals in Dictyostelium amoebae (see Section V). Another decade passed before the development of Ca " " fluorescent probes led to the discovery of oscillations in intracellular Ca +. Oscillations in cytosolic Ca " " have since been found in a variety of cells where they can arise spontaneously, or after stimulation by hormones or neurotransmitters. Their period can range from seconds to minutes, depending on the cell type [56]. The oscillations are often accompanied by propagation of intracellular or intercellular Ca " " waves. The importance of Ca + oscillations and waves stems from the major role played by this ion in the control of many key cellular processes—for example, gene expression or neurotransmitter secretion. 

A process similar to endocytosis occurs in the reverse direction when it is known as exocytosis (Figure 5.11). Membrane-bound vesicles in the cytosol fuse with the plasma membrane and release their contents to the outside of the cell. Both endocytosis and exocytosis are manifestations of the widespread phenomenon of vesicular transport, which not only ferries materials in and out of cells but also between organelles, e.g. from the endoplasmic reticulum to the Golgi and then to the lysosomes or to the plasma membrane for secretion (Chapter 1). Many hormones are also secreted in this way, as are neurotransmitters from one nerve into a synaptic junction that joins two nerves (Chapters 12 and 14). 

The arrival of the action potential at the presynaptic terminal opens voltage-dependent Ca ion channels in the plasma membrane so that the Ca ions enter the cytosol down their concentration gradient. This results in activation of a Ca -binding cytosolic or a membrane protein. This facilitates movement of the vesicles to the membrane and formation of a fusion pore through which the neurotransmitter is discharged into the synaptic cleft (i.e. exocytosis). This occurs within about 0.1 ms of the arrival of the depolarisation (Figure 14.8). The process of exocytosis lasts for only a short time, since the Csl ion concentration in the cytosol is rapidly lowered due to the ion extrusion from the cell (Appendix 14.3). 

Schematic illustration of the interrelationships between glutamate and NO in synaptic function in the cetebellum. The presynaptic nerve terminal synthesizes, stores, and releases glutamate (G) as the neurotransmitter by exocytosis as illustrated. The glutamate diffu.ses across the synaptic cleft and interacts with postsynaptic NMDA recepti>rs ( ) that are coupled to calcium (Ca ) channels. Ca influx occurs and the free intracellular Ca complexes with calmtxlulin and activates NO synthase. NADPH is also required hir conversion, and the products of the reaction are NO plus L-citrulline. NO diffuses out of the piistsynaptic cell to interact with nearby target cells, one of which is the presynaptic neuron that released the glutamate in the first place. NO stimulates cytosolic guanylate cyclase and cyclic GMP (cGMP) formation presynaptically, hut the consequence of this pre.synaptic modification is unknown.

The postsynaptic nerve ending, which is usually the tip of an axonal dendrite, has its own set of proteins, which varies to some extent with the nature of the neurotransmitter. In excitatory cells the plasma membrane of the postsynaptic neuron is thickened to — 30—40 ran to form the "postsynaptic density," a disc-like structure of clustered receptors of two types, which extends 30 ran into the cytosol.593 594 Only single receptor channels are indicated in Fig. 30-20, but many receptors are present in the clusters594 595 as are other specialized proteins. One of these, designated... 

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