Mechanism of transmitter release.

Publisher Summary This chapter discusses the presently available electron cytochemical methods for the analysis of biogenic monoamines, norepinephrine (NE), dopamine (DA), and serotonin (5-HT) and emphasizes on the state of knowledge of the cytochemical reactions involved in localizations. On the basis of a large number of experimental approaches, which combine various neurocytological and histochemical observations, the association of NE-storing nerve terminals in the peripheral sympathetic nervous system is well correlated with intra-axonal small granular vesicles (SGV). Large granular vesicles (LGV) occurring in these nerves seem to play only a minor role in the storage. A nonvesicular binding material may be ubiquitously present throughout the monoamine perikaryon and portions of the axons of these cells. However, the exact composition of the SGV granule and its qualitative and quantitative relation to the monoamine are believed to be present in the vesicle, which remains to be established.

The Fine Structural Localization of Biogenic Monoamines in Nervous Tissue

Previous studies with the Nauta technique have established that fibres which originate in two important areas — the hippocampus and the hypothalamus — converge upon the cells of the septal nuclear complex. The purpose of this study was to investigate the anatomical basis of how the septal cells could differentiate between fibres from the two sources. Differences in the mode of termination of these two systems have been studied quantitatively at the electron microscope level by using the orthograde degeneration of terminals after lesions of the fimbria and the medial forebrain bundle. In the medial septal nucleus, the hippocampal fibres account for 35% of the terminals, and in the lateral septal nucleus, 43% of the terminals on the same side and a further 13% on the opposite side. These terminals are at least 98% axodendritic and 91% of them contain predominantly clear synaptic vesicles of 500 A diameter. The hypothalamic fibres are the source of up to 19% of the axodendritic terminals in the medial septal nucleus, but considerably fewer in the lateral septal nucleus. In contrast to the hippocampal afferents, the hypothalamo-septal system has two characteristic features: firstly, the fibres give rise to up to 24% of the axosomatic terminals in the medial septal nucleus, and secondly, 63% of the terminals contain a population of vesicles with significantly higher proportions of dense centred vesicles of 800–1000 A diameter.

A comparison of the mode of termination of the hippocampal and hypothalamic afferents to the septal nuclei as revealed by electron microscopy of degeneration

Cytochemical methods for the localization of central catecholamine-containing synaptic terminals have been developed from an extensive foundation of biochemical work and from extrapolation of results on the peripheral sympathetic nervous system. Direct localization of catecholamines in central nerve terminals in some parts of the brain can now be obtained by fixation with permanganates. More broadly applicable, but less direct localizing methods depend upon selective accumulation of tritiated catecholamines for autoradiography or the accumulation of reactive catecholamine congeners which act as markers with conventional fixation. The pattern of acute degenerative changes which result after treatment with 6-hydroxydopamine can also be used to provide an indirect localization of the terminals which had stored catecholamines. When the results of each of the methods are combined, the present techniques indicate that catecholamine-containing terminals in the brain can be identified more confidently than any other system of neurotransmitter substances. Nevertheless, there is considerable need for future cytochemical innovation. Present electron cytochemical reactions can identify only a few nonenzymatic components of cells, most of’ which fall into the category of macromolecules such as deoxyribonucleic acid, glycogen, lipids and some partially characterized proteins. In neuropharmacologic research, special effort has been devoted to the development of methods for the cytochemical identification of small molecules believed to act as chemical synaptic transmitters. Such data are important because they provide powerful support for a transmitter role for the identified substances when related to specific defined synaptic terminals. The currently available methods for the localization of the catecholamines norepinephrine (NE) and dopamine (DA) have already been reviewed many times in the recent literature (9-11, 37, 46, 48, 49, 59). In this review, particular attention is placed on the status of the cytochemical methods involved in the localization of these substances in central synaptic terminals. The basic objectives are to evaluate the approaches to the direct cytochemical localization of the catecholamine molecules, for the identification of or

Ultrastructural identification of catecholamine-containing central synaptic terminals.

The pineal organ of the dogfish,Scyliorhinus canicula, is a long, thin, tubular structure consisting of an end-vesicle and a stalk The pineal parenchyma consists of receptor cells, glycogen-storing cells, supporting cells, cells containing dense granules of 1,500–3,000 A diameter, cytosome-rich cells, and ganglion cells The latter alledgedly give rise to the diffusely distributed pineal tract which runs to the posterior commissure A few pineal fibres diverge to the habenular commissure The receptor cells have well-developed outer segments with morphological features characteristic of the retinal cone Interaction between receptor cells and ganglion cells take place in neuropil-like areas Boutons are found which are believed to belong to the receptor cells because of the presence of occasional synaptic rods The few synapses observed always display synaptic vesicles both pre- and post-synaptically The functional significance of the reported morphological features is discussed with the aid of the pertinent literature and it is postulated that the pineal organ of the dogfish is a photosensitive organ

Light and electron microscopic studies on the pineal organ of the dogfish, Scyliorhinus canicula L.

The fine structure of ventromedial hypothalamic nucleus was studied in the rat with the electron microscope under circumstances of elevated brain monoamine level following treatment with a monoamine oxidase inhibitor Nialamid. The number of granular vesicles (size in diameter 450–1100 A) in synaptic terminals increased after Nialamid treatment significantly, while their size did not change; the number of agranular vesicles remained unchanged. The time courses of the increase of granular vesicles and elevation of brain noradrenaline content were approximately parallel. It is inferred that the granular vesicles of size 450 to 1100 A may possibly be the storage sites of noradrenaline.

Szabó D

Papers

Correlation between noradrenaline content of the brain and the number of granular vesicles in rat hypothalamus during Nialamid administration.