Extraneuronal catecholamine in the iris of the rat: a consequence of nonsynaptic neurotransmission?

Nonsynaptic diffusion neurotransmission (NDN) in the brain

The synapse has dominated the conceptual model of neurotransmission; other mechanisms, such as neuromodulation, have been considered to support and complement synaptic transmission. In this commentary, the conceptual framework considers synaptic transmission as one of several mechanisms of neurotransmission. One of these is nonsynaptic diffusion neurotransmission (NDN), which includes both the diffusion of neurotransmitters and other neuroactive substances through the extracellular fluid to reach extrasynaptic receptors, and the diffusion of substances such as nitric oxide through both the extracellular fluid and cellular membranes to act within the cell. The possible roles of NDN in mass, sustained functions such as mood, sleep and brain "tone", as well as in various other functions, such as in long term potentiation, at the retinal, lateral geniculate nucleus and visual cortex levels of the visual system, in recovery from brain damage and in neuropharmacology, are explored.

Protein patterns in human vestibular ganglion cells and hair cells, with functional interpretations

Cytoskeletal organization was analysed in the vestibular ganglion cells and in the sensory epithelia of all five vestibular organs of the adult human temporal bone. A pancellular rigidity of the upper surfaces of the vestibular organs, evidenced by large quantities of intermediate filaments and actin, seems of importance for the mechanoelectrical transduction by opposing the motion of the cupulae and statoconial layers. Immunostaining for the calcium-binding protein synaptophysin in the sensory hair bundles and in the area close to the cuticular plates--the first of its kind to be demonstrated in human vestibular organs--indicates the presence of calcium-dependent ionic channels. The hypothesis is presented that the calyx might be involved in a short-loop feed back control of type I hair cells, i.e. of the mechanoelectrical transduction itself. Subpopulations of vestibular ganglion cells were identified by the staining pattern of cytoskeletal proteins, but not by ordinary ultrastructural analysis.

Exocytosis from large dense cored vesicles outside the active synaptic zones of terminals within the trigeminal subnucleus caudalis: a possible mechanism for neuropeptide release

It has been hypothesized that chemical interactions between neurons in the central nervous system can occur in the absence of well defined synaptic complexes, but morphological correlates have been difficult to find. The present study demonstrates exocytotic release from large (70-130 nm) dense cored vesicles at structurally nonspecialized areas along the plasmalemma of structurally different categories of terminals and occasionally from dendrites and axons within the neuropil of the trigeminal subnucleus caudalis. In rats, the marginal (lamina I) and substantia gelatinosa (lamina II) layers contain the central terminals of primary afferent fibers from the infraorbital nerve that supply the skin and whiskers (vibrissae). Different types of interneurons are also present and may modify the input being relayed to higher centers. While exocytotic profiles were present in control animals, they increased significantly (P less than 0.01) on the ipsilateral side 1-24 h after a unilateral skin lesion in the vibrissae area. A second increase (P less than 0.001) occurred 14-15 days after the lesion. Virtually all examples of large vesicle exocytosis were observed at structurally nonspecialized sites while those at the active synaptic zones involved small clear vesicles. Substance P-like immunofluorescence, present in controls and on the ipsilateral side during the first 6 days, subsequently declined until 4 weeks after surgery when some recovery was noted. The increase in large vesicle exocytosis and the decrease in substance P are interpreted to reflect functional adjustments of different neurons in response to the lesion. The exocytosis involving large dense cored vesicles may serve to deliver transmitters and/or neuropeptide modulators to appropriate receptors in a wider area than release into a specialized synaptic cleft would allow.

Microspectrofluorometry in biogenic amine research

Microspectrofluorometry has played an important part in the development and understanding of fluorescence techniques for the demonstration of biogenic amines. This paper reviews briefly the techniques currently available for the characterization and localization of biogenic amines, and considers the basic design considerations applicable to the construction of a microspectrofluorometer, with special emphasis on the automated correction of spectral data. Spectral data for the commonly occurring biogenic amines dopamine, noradrenaline and 5-hydroxytryptamine have been summarized and the criteria applicable to the spectral analysis of the individual amines and their mixtures discussed. Discrepancies between spectral data derived from tissues and model systems are reviewed and emphasis is given to the importance of biochemical studies of amines prior to definitive microspectrofluorometric analysis. The conceptual basis of quantitative microspectrofluorometry is reviewed with a particular emphasis on the limitations of such studies. The analysis of terminal fields by scanning microspectrofluorometry and its application to the study of extracellular amine distribution is discussed.

Quantification of formaldehyde induced fluorescence and its application in neurobiology

The methods used for measurements of fluorescence intensity in histochemical preparation will be described. Under certain conditions, the FIF intensity of biological objects shows a linear relationship with the local monoamine concentration. Microfluorometry enables determination of changes in monoamine concentrations in cell bodies, terminal axons and extraneuronal structures. In certain neuronal systems, FIF intensity of cell bodies appears to be correlated with neuronal activity. FIF intensity measurements of terminal axons can provide information on turnover of monoamine in structures as small as individual varicosities. Finally, the significance of extraneuronal FIF intensity in relationship to neurotransmission will be discussed. This paper aims to show that determinations of FIF provide information on the functional activity of neuronal systems.

Image analysis of catecholamine fluorescence

Image analysis methods are being developed for measuring the density and distribution of perivascular noradrenergic nerves and their varicosities, demonstrated by fluorescence histochemistry. Image analysis is recommended in comparison with other methods for the quantitation of fluorescence, because image resolution is greater and larger areas of tissues can be scanned and measured more rapidly. Applications of image analysis have provided new and valuable information regarding sympathetic nerves: for example, we have shown rapid growth of perivascular nerves during perinatal development in the rabbit, and loss of nerves in different vessels in postnatal life and in old age; regional variations of nerve density and varicosity numbers in the central ear artery of the rabbit have also been demonstrated.