Diffuse neuronal systems

Whereas membrane-delimited actions occur within microdomains in the membrane, second messenger-mediated effects can occur over considerable distances. Finally, an important consequence of the involvement of G proteins in receptor signaling is that, in contrast to the brief effect of ionotropic receptors, the effects of metabotropic receptor activation can last tens of seconds to minutes. Metabotropic receptor predominate in the diffuse neuronal systems in the CNS (see below). 

Most of the neuronal systems in the CNS can be divided into two broad categories hierarchical systems and nonspecific or diffuse neuronal systems. 

Again, keep in mind that the control attributed to each lobe mentioned above is not exclusive of the other lobes because of the integrated network of neurons connecting various portions of the brain. In addition there are other distinct bodies or diffuse neuronal systems which lie within the lobes just described. 

This compound is found in diffuse neuronal systems in the CNS, particularly in the raphe nuclei it appears to play a major role in the expression of mood, since many antidepressant drugs are thought to increase its functional activity. 

Catecholamines (dopamine, norepinephrine), opioid peptides, and serotonin act as neurotransmitters in nonspecific or diffuse neuronal systems. Glutamate is the primary excitatory transmitter in hierarchical neuronal systems. The answer is (B). 

Enolase is a glycolytic enzyme also known as phosphopyru-vate hydratase. Neuron-specific enolase (NSE) is the form of enolase found in neuronal tissue and in the cells of the diffuse neuroendocrine system and the amine precursor uptake, and decarboxylation (APUD) tissue. NSE is found in tumors associated with the neuroendocrine origin, including small cell lung cancer (SCLC), neuroblastoma, pheochromocytoma, carcinoid, medullary carcinoma of the thyroid, melanoma, and pancreatic endocrine tumors. 

C. Role of CNS Organization The CNS contains two types of neuronal systems hierarchical and diffuse. 

The APUD concept of a diffuse neuroendocrine system has been defined as follows by A.G.E.Pearse [in Centrally Acting Peptides, J. Hughes (ed.), MacMillan, 1978] The cells of the APUD series, producing peptides active as hormones or as neurotransmitters, are all derived from neuroendocrine-programed cells originating from the ectoblast. They constitute a third (endocrine or neuroendocrine) division of the nervous system whose cells act as third line effectors to support, modulate or amplify the action of neurons in the somatic and autonomic divisions, and possibly as tropins to both neuronal and non-neuronal cells . 

Wharton J, Polak JM, Cole GA, Marangos PJ, Pearse AGE. Neuron specific enolase as an immunohistochemical marker for diffuse neuroendocrine system in human fetal lung. J Histochem C hochem 1981 29 1359-1364. 

The results of a number of studies demonstrate that the gas nitric oxide (NO) plays a functional role in the central nervous system. This all originated with the discovery that the so-called endothelium-derived relaxing factor (EDRF), found in blood vessels, and thought to be a peptide, was in fact NO. The potential roles of this freely diffusible gas have subsequently been extended to many other tissues and organs but we will concentrate on the possible neuronal roles of what is obviously a novel mediator. There are also suggestions that the closely related carbon monoxide may also have a function in the central nervous system. 

Cholesterol transport and regulation in the central nervous system is distinct from that of peripheral tissues. Blood-borne cholesterol is excluded from the CNS by the blood-brain barrier. Neurons express a form of cytochrome P-450, 46A, that oxidizes cholesterol to 24(S)-hydroxycholesterol [11] and may oxidize it further to 24,25 and 24,27-dihydroxy products [12]. In other tissues hydroxylation of the alkyl side chain of cholesterol at C22 or C27 is known to produce products that diffuse out of cells into the plasma circulation. Although the rate of cholesterol turnover in mature brain is relatively low, 24-hydroxylation may be a principal efflux path to the liver because it is not further oxidized in the CNS [10]. 

The factors are secreted into the extracellular milieu where they diffuse and then act in a paracrine fashion on other cells (Fig. 27-1). Indeed, there is evidence that this type of paracrine support is necessary to sustain neurons as they extend their processes over long distances in the developing nervous system [2]. An analogous process, autocrine stimulation, occurs when a cell synthesizes and secretes a growth factor to which the cell itself is responsive. In this case, the cell provides its own trophic support. 

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