Neural nitric oxide transmission 2789 Fig. 2. Dynamics of synaptic NO. (A) Superimposed on an electron micrograph of an excitatory synapse (from Kennedy, 2000, with permission) are contours of NO concentrations (inner red ring, 1 nm; outer wine-coloured ring, 0.25 nm) predicted to be formed at steady-state by a 7 · 7 array of active NOS molecules located (approximately) in the postsynaptic density. Each NOS molecule was assumed to generate 20 NO molecules per second, based on the initial rates of NO formation by nNOS reported by Santolini et al. (2001) after correcting for temperature. Seven of the source molecules are in the plane of vision at the centre of the contours, the yellow colour signifying 2 nm NO. (B) NO proﬁles resulting from transient NOS activation at different distances from the sources, colour-coded to the positions of the contours in A. The waveform of the input NOS activity was chosen to resemble the time-course of a unitary NMDA receptor-mediated rise in postsynaptic Ca2+, and is superimposable on the output (NO concentration) curves, when scaled. (C) Transduction of an NO transient. The empirical kinetic scheme for NO receptor activation (Garthwaite, 2005) is shown at the top and the resulting activity of receptor-associated guanylyl cyclase activity (GC) and the increase in cGMP are depicted for NO arriving near the middle of the presynaptic terminal (purple contour in A) as a single pulse (upper panel) or during a sustained 2-s period of NOS activity (lower panel). Data are from C.N. Hall and J. Garthwaite (unpublished results). However, we do not have to rely merely on these theoretical calculations for the evidence. The value is consistent with vascular relaxation being maximal at NO concentrations near the base of the concentration–response curve for cGMP generation (Mullershausen et al., 2006), with cGMP-dependent phosphorylation being triggered by subnanomolar NO concentrations (Mo et al., 2004), and with data from NO receptor-knockout mice showing that NO can still relax blood vessels when 94% of the associated guanylyl cyclase activity has been eliminated (Mergia et al., 2006). All these experimental ﬁndings suggest that cells have a large receptor reserve, ensuring that low-amplitude NO signals are captured and transduced. As well as mediating point-to-point transmission at synapses, there may be situations in which neuronally derived NO performs as a ‘volume’ transmitter. There are two anatomical scenarios where volume transmission is most likely. One is where there is a plexus of NO-releasing nerve ﬁbres that have a density and degree of synchronous activity that will allow a regional ‘cloud’ of NO to be formed (Philippides et al., 2005). The other is when there is an anatomically appropriate segregation of NO sources and targets, as exempliﬁed in the insect brain by the so-called mushroom body, which is involved in associative learning. In the ‘stalk’ of the mushroom body, the NO sources were located in a sheath of nerve ﬁbres ( 30 lm thick) which enwrapped a 30-lm-diameter core of NOS- negative axons expressing the NO receptors; the design features, together with a theoretical analysis, suggested that the outer sleeve could act like a heating jacket, radiating NO to the inner core to generate a relatively uniform concentration within it (Ott et al., 2007). Volume transmission by NO has attractions for certain theories of learning in which alterations in synaptic strength are governed by the combination of the ambient NO concentration in the region and the coincident synaptic activity (Montague & Sejnowski, 1994). An alternative to neurones as the source of ambient NO in this setting might be the capillary endothelial cells (see below). Acute synaptic actions of NO A special property of NO compared with conventional neurotrans- mitters is its free diffusion through aqueous and lipid environments, so it is not possible to predict where it will act after being synthesised in either pre- or postsynaptic sites from the standpoint of diffusion alone (Fig. 2A). Indeed, a special advantage of a messenger such as NO would be that it provides a simultaneous signal to both pre- and postsynaptic elements, of probable importance in coordinating responses on the two sides of the synapse. A common assumption in the literature is that if an NO-mediated response is inhibited by a scavenger that remains extracellular, such as haemoglobin, NO must be acting intercellularly. This assumption is quite wrong. NO travels randomly and surprisingly quickly. With a tissue diffusion coefﬁcient of 848 lm2 ⁄ s (Liu et al., 2008) the average NO molecule travels 0.8 lm (twice the diameter of the postsynaptic density) in 100 ls (Lancaster, 1997) which means that, on biological time-scales, it will ª The Author (2008). Journal Compilation ª Federation of European Neuroscience Societies and Blackwell Publishing Ltd European Journal of Neuroscience, 27, 2783–2802