Neurotransmitters types

Fig. 3. Ca dependent (vesicular) release of multiple neurotransmitter types from rat hippocampal synaptosomes. To evoke release, the extracellular K concentration was raised to 30 mA/by adding KCl. The basis of the neurotransmitter type categories (Type I, and so forth) is described in Note 19. The measurements are expressed in log[fmol/mg protein]. See Section 3.4, and Note 21 for further discussion. GLU = glutamate, NA = noradrenaline, DA = dopamine, CCK = cholecystokinin, met-Enk = met-enkephalin.

The pioneer work on the connexion between constitution and activity in the phenylethylamine series of sympathomimetic drugs was carried out by Barger and Dale (1910). It has since become clear that direct action of the hormone and neurotransmitter type, is strongest in examples with hydroxy-groups in the 3-and 4-positions of the benzene ring, i.e. the catecholamines such as norepinephrine, epinephrine and dopamine. The indirect action of examples without these embellishments has been outlined in Sections 9.4.3 (p. 358) and 7.6.3 (p. 300). As recounted in Section 12.1 the D-catecholamines (Fig. 12.1) have much more biological activity than their L-enantiomers. 

Elucidation of the stmctural requirements for dmg interaction at the recognition site is by the study of stmcture—activity relationships (SAR), in which, according to a specific biologic response, the effects of systematic molecular modification of a parent dmg stmcture are determined. Such studies have permitted the classification of discrete classes of pharmacological receptors. For example, the neurotransmitter acetylcholine acts at both peripheral and central receptors which are of at least three distinct types. The effects of acetylcholine are mimicked in smooth and cardiac muscles and secretory... 

Glutamate is a small amino acid which constitutes the most important neurotransmitter at excitatory synapses in the mammalian brain. Glutamate can act on several different types of receptors including cation channels and G-protein-coupled receptors. 

Ubiquitous mitochondrial monoamine oxidase [monoamine oxygen oxidoreductase (deaminating) (flavin-containing) EC 1.4.3.4 MAO] exists in two forms, namely type A and type B [ monoamine oxidase (MAO) A and B]. They are responsible for oxidative deamination of primary, secondary, and tertiary amines, including neurotransmitters, adrenaline, noradrenaline, dopamine (DA), and serotonin and vasoactive amines, such as tyramine and phenylethylamine. Their nonselec-tive and selective inhibitors ( selective MAO-A and -B inhibitors) are employed for the treatment of depressive illness and Parkinson s disease (PD). 

Of the several classes of receptors for endogenous chemical signals [3], two are used as postsynaptic receptors in synaptic transmission ligand-gated ion channels (LGICs) and G protein-coupled receptors (GPCRs Fig. 1). Due to the large number of transmitters and the existence of several receptor types for almost all, postsynaptic receptor activation is the most diversified step of synaptic transmission. Table 1 shows selected neurotransmitter receptors. 

Synaptic vesicles are the organelles in axon terminals that store neurotransmitters and release them by exocytosis. There are two types, the large dense-core vesicles, diameter about 90 nm, that contain neuropeptides, and the small synaptic vesicles, diameter about 50nm, that contain non-peptide transmitters. About ten vesicles per synapse are docked to the plasma membrane and ready for release, the readily releasable pool . Many more vesicles per synapse are stored farther away from the plasma membrane, the resting pool . When needed, the latter vesicles may be recruited into the readily releasable pool. Neuronal depolarization and activation of voltage-sensitive Ca2+... 

Synaptic vesicles isolated from brain exhibit four distinct vesicular neurotransmitter transport activities one for monoamines, a second for acetylcholine, a third for the inhibitory neurotransmitters GABA and glycine, and a fourth for glutamate [1], Unlike Na+-dependent plasma membrane transporters, the vesicular activities couple to a proton electrochemical gradient (A. lh+) across the vesicle membrane generated by the vacuolar H+-ATPase ( vacuolar type proton translocating ATPase). Although all of the vesicular transport systems rely on ApH+, the relative dependence on the chemical and electrical components varies (Fig. 1). The... 

By far the most important amino acids are the a-amino acids, in which the —NH2 group is attached to the carbon atom next to the carboxyl group, as in glycine. However, other types of amino acids are common and play an important biological role. For instance, the y-amino acid NH2CH2CH2CH2COOH is the neurotransmitter GABA. 

Figure 1.8 Some basic neuronal systems. The three different brain areas shown (I, II and III) are hypothetical but could correspond to cortex, brainstem and cord while the neurons and pathways are intended to represent broad generalisations rather than recognisable tracts. A represents large neurons which have long axons that pass directly from one brain region to another, as in the cortico spinal or cortico striatal tracts. Such axons have a restricted influence often only synapsing on one or a few distal neurons. B are smaller inter or intrinsic neurons that have their cell bodies, axons and terminals in the same brain area. They can occur in any region and control (depress or sensitise) adjacent neurons. C are neurons that cluster in specific nuclei and although their axons can form distinct pathways their influence is a modulating one, often on numerous neurons rather than directly controlling activity, as with A . Each type of neuron and system uses neurotransmitters with properties that facilitate their role...

These criteria should be regarded as guidelines rather than rules. As guidelines they provide a reasonable scientific framework of the type of investigations that must be undertaken to establish the synaptic role of a substance. As rigid rules they could preclude the discovery of more than one type of neurotransmitter or one form of neurotransmission. Nevertheless, the criteria have been widely employed and often expanded to include other features which will be considered as subdivisions of the main criteria. 

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