2790 J. Garthwaite constantly diffuse in and out of the NO-generating compartment. Consequently, an efﬁcient extracellular scavenger will inevitably deplete both intracellular and extracellular NO. Generally speaking, however, NO does appear to signal to the opposite synaptic partner, as in the examples illustrated (Fig. 1), but both pre- and postsynaptic actions are feasible (irrespective of the site of generation), depending on location of the receptors. The actions of NO on either structure, despite employing the same transduction mechanism (cGMP), obey no general rules and, even when a similar effect is observed, the underlying mechanisms may be different. In one of the sample synapses (Fig. 1B), the hyperpolarizing postsynaptic potential observed on stimulation of nitrergic nerves was ascribed to the activation of background K+ channels, a major contributor being a member of the two-pore-domain K+ channel family, TREK-1, with the effect of NO being transduced through serine phosphorylation by PKG (reviewed in Sanders & Koh, 2006). TREK-1 channels are widely expressed in the brain, often in association with GABAergic neurones (Fink et al., 1996; Hervieu et al., 2001) and it will be interesting to know whether NO–cGMP operates through these channels elsewhere. The EPSP in the snail neurones (Fig. 1A) appears to be caused by closure of K+ channels, although this was not explicitly examined (Park et al., 1998). A similar NO-mediated EPSP has been recorded in an Aplysia neurone modulating the feeding circuit where it was shown that, through cGMP, synaptically released NO inhibited background K+ channels (Jacklet & Tieman, 2004). Together, the two examples encapsulate observations made at many different CNS synapses. A large literature describes various reversible effects of NO and ⁄ or cGMP on neuronal excitability and ⁄ or synaptic transmission, the effects being generally classiﬁable as excitatory or inhibitory. It is useful to give a few examples, selected on the basis of evidence that endogenous NO engages the same mechanism. This is a particularly important criterion here, bearing in mind that exogenous NO (or its method of delivery) may produce unphysiological effects and that too much cGMP may also be unphysiological, leading, for example, to inadvertent activation of cAMP-dependent protein kinases (Sausbier et al., 2000). In some cases, endogenous activity is promoted by administration of the NOS substrate l-arginine, and it is relevant that the l-arginine concentration in the cerebrospinal ﬂuid is 20 lm (Martens-Lobenhoffer et al., 2007) but is normally omitted from solutions used to incubate in vitro preparations, possibly resulting in a suppression of NO-mediated transmission in some experiments. It should be cautioned, however, that l-arginine may have effects unrelated to NOS activity (Hentall, 1995; Rivadulla et al., 1997). Frequently missing are measurements of NO itself but, unfortunately, there are no reliable methods yet for directly measuring endogenously generated NO in tissues with the necessary sensitivity or spatial and temporal precision (reviewed in Keynes & Garthwaite, 2004; Wang et al., 2006a). Nevertheless, useful data at a more gross level have been obtained from certain electrode designs (e.g. Shibuki & Kimura, 1997) but many of those that have been used suffer from suspect speciﬁcity and inadequate sensitivity. Postsynaptic actions Earlier in vivo experiments on the lateral geniculate and ventrobasal nuclei of the thalamus showed that NO had a marked facilitatory effect on neuronal responses to natural stimuli (light and whisker stimula- tion, respectively), as well as on the excitatory effect of locally administered glutamatergic agonists (Do et al., 1994; Cudeiro et al., 1996; Shaw et al., 1999). A major source of the NO in these nuclei is in afferent cholinergic ﬁbres, suggesting that presynaptically derived NO is at work whereas the downstream enhancement of excitation was probably postsynaptic, putatively through direct engagement by cGMP of HCN channels that are prominent in the thalamus (Shaw et al., 1999). A similar general enhancing effect of NO was seen in the visual cortex, where inhibition of NO synthesis depressed responses to exogenous agonists and to visual stimuli (Cudeiro et al., 1997; Kara & Friedlander, 1999) and in the somatosensory cortex, where NO participated in the ‘wake-up’ electrical activity observed following stimulation of the basal forebrain cholinergic afferents (Marino & Cudeiro, 2003). In the striatum in vivo too, NO (probably derived from local interneurones that are rich in nNOS) modulated excitation of medium spiny neurones in such a way as to enhance EPSPs (West & Grace, 2004). Its effect here was associated with membrane depolarization, perhaps brought about by cGMP suppressing K+ conductances. Beyond these pioneering in vivo studies, NO–cGMP has also been found to have depolarizing actions on several different types of central neurone in vitro, including a population of paraventricular neurones (Bains & Ferguson, 1997), striatal cholinergic neurones (Centonze et al., 2001), trigeminal motoneurones (Abudara et al., 2002) and optic nerve axons (Garthwaite et al., 2006). HCN channels were implicated as the transducers in the latter two instances. In a pituitary cell line, endogenous NO elicited a cGMP-dependent inhibition of K+ channels, an effect of which would also be membrane depolar- ization, although cytosolic Ca2+ oscillations were actually measured in the experiments (Secondo et al., 2006). Alternatively, CNG channel activation following NO-evoked cGMP accumulation may produce excitatory postsynaptic responses in central neurones, as was shown ﬁrst in a population of retinal ganglion cells (Ahmad et al., 1994; Kawa & Sterling, 2002) and, more recently, in medial vestibular nucleus neurones (Podda et al., 2008), but the engagement of this pathway by endogenous NO has not yet been demonstrated. Postsynaptic inhibitory effects of NO–cGMP on neuronal ﬁring, or hyperpolarisations, have also been reported (Schmid & Pehl, 1996; Rauch et al., 1997; Xu et al., 1998; Riediger et al., 2006; Sardo et al., 2006). Potential mechanisms include the activation of various classes of K+ channel (Han et al., 2006; Kang et al., 2007; Mironov & Langohr, 2007; Chai & Lin, 2008) but, again, there is little evidence yet for endogenous NO acting in this way (but see Cudeiro et al., 1997, for an in vivo example). Presynaptic actions An altogether different response to NO is exempliﬁed by results from the paraventricular nucleus, which is important in autonomic and endocrine homeostasis. NMDA or angiotensin was found to cause a barrage of GABAergic inhibitory postsynaptic potentials (IPSPs) in the magnocellular neurones of this region that was mediated, at least in part, by endogenous NO exciting GABAergic neurones (Bains & Ferguson, 1997; Latchford & Ferguson, 2003). These results were extended to spinally projecting neurones within this nucleus, whose ﬁring was regulated by NO modulating the GABAergic tone (Li et al., 2002, 2003). In such neurones, studies of miniature GABAergic IPSPs (recorded in the presence of tetrodotoxin to prevent circuit-based activity) strongly suggested that NO was acting on presynaptic terminals to increase the probability of vesicular GABA release (Li et al., 2002), a mechanism that has been attributed to NO acting through cGMP and PKG (Li et al., 2004). Presynaptic voltage-gated K+ channels, speciﬁcally subtypes Kv1.1 and 1.2 but perhaps others as well, have recently been implicated (Yang et al., 2007). Where studied in the accessible calyx of Held nerve terminals in the brainstem, Kv1 ª The Author (2008). Journal Compilation ª Federation of European Neuroscience Societies and Blackwell Publishing Ltd European Journal of Neuroscience, 27, 2783–2802