Volume transmission

The reuptake process does not capture all of the released catecholamine. Diffusion away from the nerve terminal to distant sites can occur and has been termed volume transmission [23]. Volume transmission allows the stimulation of extrasynaptic receptors, which has been described for dopamine [24] and norepinephrine [25]. Brain regions differ in their capacity for catecholamine reuptake thus, whereas extracellular dopamine concentrations are dominated by release in the cerebral cortex, in the striatum dopamine concentrations are dominated by reuptake [26]. These regional differences in extracellular dopamine kinetics correlate with levels of dopamine transporter [27]. 

Agnati,L. F.,Zoli,M.,Stromberg,I. and Fuxe,K. Intercellular communication in the brain wiring versus volume transmission. Neuroscience 69 711-726,1995. 

The primary catabolic pathway for 5-HT is oxidative deamination by the enzyme monoamine oxidase 237 In addition to classical synaptic transmission, 5-HT may relay information by volume transmission or paracrine neurotransmission 238 5 -HT may be involved in a wide variety of behaviors by setting the tone of brain activity in relationship to the state of behavioral arousal/activity 238... 

Del Arco A, Segovia G, Fuxe K, Mora F. 2003. Changes in dialysate concentrations of glutamate and GABA in the brain An index of volume transmission mediated actions J Neurochem 85(1) 23-33. 

Fuxe K, Agnati LF (eds) (1991) Volume transmission in the brain novel mechanisms for neural transmission. Raven Press, New York... 

A fourth modification of the classical model involves nitric oxide (NO), a new form of nonsynaptic interneuronal communication, or volume transmission. Nitric Oxide inhibits the uptake of dopamine, norepinephrine (Lonart and Johnson, 1994), and serotonin (Asano et ah, 1997) into neurons and is closely linked to glutamate-mediated neurotransmission. Nitric Oxide synthase is switched on only by glutamatergic receptors and appears to enhance the strength of glutamatergic input to monoaminergic neurons without requiring direct synaptic contact (Kiss and Vizi, 2001). 

From a communications viewpoint the CCN is multiply noded (each osteo-cyte is a node) and multiply connected (Figure 4). Each osteocytic process is a connection between two osteocytes, and each osteocyte is multiply connected to a number of osteocytes that are near neighbors. Cell-to-cell communication is considered first below, then some speculative considerations of the ability of the CCN to compute as well as signal are described. It is useful to note the possibility that bone cells, like neurons, may communicate intercellular information by volume transmission, a process that does not require direct cytological contact, but rather utilizes charges in the environment [59, 117, 181]. 

Volume Transmission in the Brain, Editors J. Fuxe and L.F. Agnati Raven Press,... 

Only the first type of neurotransmitter release mediates the fast point-to-point synaptic transmission process at classical synapses (sometimes referred to as wiring transmission). All of the other types of neurotransmitter release effect one or another form of volume transmission whereby the neurotransmitter signal acts diffusely over more prolonged time periods (Agnati et al., 1995). Of these volume transmitter pathways, the time constants and volumes involved differ considerably. For example, diffusible neurotransmitters such as nitric oxide act relatively briefly in a localized manner, whereas at least some neuropeptides act on the whole brain, and can additionally act outside of it (i.e., function as hormones). There is an overlap between wiring and volume neurotransmission in that all classical neurotransmitters act as wiring transmitters via ionotropic receptors, and also act as volume transmitters via G-protein-coupled receptors. Moreover, neuromodulators in turn feed back onto classical synaptic transmission. 

Quantitatively, synaptic transmission is the dominant form of communication between neurons. A single look at an electron micrograph reveals that synapses with their appendant organelles, especially synaptic vesicles, are abundant in brain, whereas LDCVs are only observed occasionally (Figure 2). However, this does not mean that synaptic transmission is more important than the volume transmission pathways. The two principally different signaling pathways play distinct roles in information processing by the brain, and both are essential for brain function. 

Bjelke B, Strdmberg I, O Connor WT, Andbjer B, Agnati LF, Fuxe K (1994) Evidence for volume transmission in the dopamine denervated neostriatum of the rat after a unilateral nigral 6-OHDA microinjection. Studies with systemic D-amphetamine treatment. Brain Res 662 11-24. 

Tao L, Nicholson C. Diffusion of albumins in rat cortical slices and relevance to volume transmission. Neuroscience 1996 75 839-47. 

The dynamics of the release and uptake of dopamine into brain extracellular space are currently under intense investigation[20-22]. Dopamine is a well-known extrasynaptic messenger that functions via volume transmission, escaping from the synaptic cleft to bind to extrasynaptic receptors and transporters. High sensitivity, chemical selectivity, and fast temporal resolution are all desirable characteristics in detecting neurotransmitters in vivo. In practice, it is difficult to achieve all of these with one method. 

Bjelke, B., et al.. Long distance pathways of diffusion for dextran along fibre bundles in brain. Relevance for volume transmission. NeuroReport, 1995, 6, 1005-1009. 

Tao, L. and C. Nicholson, Diffusion of Albumins in Rat Cortical Slices and Relevance to Volume Transmission. New York New York University Medical Center, 1996. 

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