Effect of nitric oxide on hyperpolarization-activated current in substantia gelatinosa neurons of rats

Retrograde response in axotomized motoneurons: nitric oxide as a key player in triggering reversion toward a dedifferentiated phenotype

The adult brain retains a considerable capacity to functionally reorganize its circuits, which mainly relies on the prevalence of three basic processes that confer plastic potential: synaptic plasticity, plastic changes in intrinsic excitability and, in certain central nervous system (CNS) regions, also neurogenesis. Experimental models of peripheral nerve injury have provided a useful paradigm for studying injury-induced mechanisms of central plasticity. In particular, axotomy of somatic motoneurons triggers a robust retrograde reaction in the CNS, characterized by the expression of plastic changes affecting motoneurons, their synaptic inputs and surrounding glia. Axotomized motoneurons undergo a reprograming of their gene expression and biosynthetic machineries which produce cell components required for axonal regrowth and lead them to resume a functionally dedifferentiated phenotype characterized by the removal of afferent synaptic contacts, atrophy of dendritic arbors and an enhanced somato-dendritic excitability. Although experimental research has provided valuable clues to unravel many basic aspects of this central response, we are still lacking detailed information on the cellular/molecular mechanisms underlying its expression. It becomes clear, however, that the state-switch must be orchestrated by motoneuron-derived signals produced under the direction of the re-activated growth program. Our group has identified the highly reactive gas nitric oxide (NO) as one of these signals, by providing robust evidence for its key role to induce synapse elimination and increases in intrinsic excitability following motor axon damage. We have elucidated operational principles of the NO-triggered downstream transduction pathways mediating each of these changes. Our findings further demonstrate that de novo NO synthesis is not only "necessary" but also "sufficient" to promote the expression of at least some of the features that reflect reversion toward a dedifferentiated state in axotomized adult motoneurons.

The treatment of Alzheimer's disease using Chinese medicinal plants: from disease models to potential clinical applications

ETHNOPHARMACOLOGICAL RELEVANCE

Alzheimer's disease (AD) is characterized by the sustained higher nervous disorders of the activities and functions of the brain. Due to its heavy burden on society and the patients' families, it is urgent to review the treatments for AD to provide basic data for further research and new drug development. Among these treatments, Chinese Material Medica (CMM) has been traditionally clinical used in China to treat AD for a long time with obvious efficacy. With the further research reports of CMM, new therapeutic materials may be recovered from troves of CMM. However, So far, little or no review work has been reported to conclude anti-AD drugs from CMM in literature. Therefore, a systematic introduction of CMM anti-AD research progress is of great importance and necessity. This paper strives to systematically describe the progress of CMM in the treatment of AD, and lays a basis data for anti-AD drug development from CMM, and provides the essential theoretical support for the further development and utilization of CMM resources through a more comprehensive research of the variety of databases regarding CMM anti-AD effects reports.

MATERIAL AND METHODS

Literature survey was performed via electronic search (SciFinder®, Pubmed®, Google Scholar and Web of Science) on papers and patents and by systematic research in ethnopharmacological literature at various university libraries.

RESULTS

This review mainly introduces the current research on the Chinese Material Medica (CMM) theoretical research on Alzheimer's disease (AD), anti-AD active constituent of CMM, anti-AD effects on AD models, anti-AD mechanism of CMM, and anti-AD effect of CMM formula.

CONCLUSION

Scholars around the world have made studies on the anti-AD molecular mechanism of CMM from different pathways, and have made substantial progress. The progress not only enriched the anti-AD theory of CMM, but also provided clinical practical significance and development prospects in using CMM to treat AD. Western pure drugs cannot replace the advantages of CMM in the anti-AD aspect. Therefore, in the near future, the development of CMM anti-AD drugs with a more clearly role and practical data will be a major trend in the field of AD drug development, and it will promote the use of CMM.

Dual effect of exogenous nitric oxide on neuronal excitability in rat substantia gelatinosa neurons

Nitric oxide (NO) is an important signaling molecule involved in nociceptive transmission. It can induce analgesic and hyperalgesic effects in the central nervous system. In this study, patch-clamp recording was used to investigate the effect of NO on neuronal excitability in substantia gelatinosa (SG) neurons of the spinal cord. Different concentrations of sodium nitroprusside (SNP; NO donor) induced a dual effect on the excitability of neuronal membrane: 1 mM of SNP evoked membrane hyperpolarization and an outward current, whereas 10 µM induced depolarization of the membrane and an inward current. These effects were prevented by hemoglobin and 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt (c-PTIO) (NO scavengers), phenyl N-tert-butylnitrone (PBN; nonspecific reactive oxygen species scavenger), and through inhibition of soluble guanylyl cyclase (sGC). Pretreatment with n-ethylmaleimide (NEM; thiol-alkylating agent) also decreased effects of both 1 mM and 10 µM SNP, suggesting that these responses were mediated by direct S-nitrosylation. Charybdotoxin (CTX) and tetraethylammonium (TEA) (large-conductance Ca²⁺-activated K⁺ channel blockers) and glybenclamide (ATP-sensitive K⁺ channel blocker) decreased SNP-induced hyperpolarization. La³⁺ (nonspecific cation channel blocker), but not Cs⁺ (hyperpolarization-activated K⁺ channel blocker), blocked SNP-induced membrane depolarization. In conclusion, NO dually affects neuronal excitability in a concentration-dependent manner via modification of various K⁺ channels.

Spinal hyperpolarization-activated cyclic nucleotide-gated cation channels at primary afferent terminals contribute to chronic pain

Hyperpolarization-activated cyclic nucleotide-gated cation channels (HCN channels) have large influences upon neuronal excitability. However, the participation of spinal HCN channels in chronic pain states, where pathological conditions are related to altered neuronal excitability, has not been clarified. Intraperitoneally (i.p.) or intrathecally (i.t.) administered ZD7288, a selective blocker of Ih channels, reduced thermal and mechanical hypersensitivity in mice under neuropathic conditions induced by the partial ligation of the sciatic nerve, while no analgesic effect was observed in naïve animals. Moreover, in the mouse formalin test, ZD7288 (i.p. and i.t.) reduced the licking/biting behavior observed during the second phase without affecting the first phase. To further explore the pain-modulatory action of spinal HCN channels, whole-cell patch clamp recordings were made from the visually identified substantia gelatinosa neurons in adult mouse spinal cord slices with an attached dorsal root, and A-fiber- and/or C-fiber-mediated monosynaptic excitatory postsynaptic currents (EPSCs) were evoked by electrical stimulation of the L4 or L5 dorsal root using a suction electrode. Bath-applied ZD7288 reduced A-fiber- and C-fiber-mediated monosynaptic EPSCs more preferentially in slices prepared from mice after peripheral nerve injury. In addition, ZD7288 reduced the frequency of miniature EPSCs without affecting their amplitude in cells receiving monosynaptic afferent inputs, indicating that it inhibits EPSCs via presynaptic mechanisms. The present behavioral and electrophysiological data suggest that spinal HCN channels, most likely at the primary afferent terminals, contribute to the maintenance of chronic pain.

Hyperpolarization-activated ion channels as targets for nitric oxide signalling in deep cerebellar nuclei

Most biological effects of nitric oxide (NO) in the brain are mediated by guanylyl cyclase-coupled NO receptors, whose activation results in increased intracellular cGMP levels. Apart from protein kinase activation little is known about subsequent cGMP signal transduction. In optic nerve axons, hyperpolarization-activated cyclic nucleotide-modulated cation (HCN) channels, which bind cGMP or cAMP directly, were recently suggested to be a target. The aim here was to test this possibility more directly. Neurones of the rat deep cerebellar nuclei were selected for this purpose, their suitability being attested by immunocytochemistry showing that the principal neurones expressed guanylyl cyclase protein and that NO synthase-containing fibres were abundant in the neuropil. Using whole-cell voltage-clamp recording, HCN channels in the neurones were activated in response to isoprenaline and exogenous cAMP but only occasionally did they respond to NO, although exogenous cGMP was routinely effective. With the less invasive sharp microelectrode recording technique, however, exogenous NO modulated the channels reproducibly, as measured by the size of the HCN channel-mediated voltage sag following hyperpolarization. Moreover, NO also blunted the subsequent rebound depolarizing potentials, consistent with it increasing the hyperpolarization-activated current. Optimizing the whole-cell solution to improve the functioning of NO-activated guanylyl cyclase failed to restore NO sensitivity. Minimizing cellular dialysis by using the perforated-patch technique, however, was successful. The results provide evidence that HCN channels are potential downstream mediators of NO signalling in deep cerebellar nuclei neurones and suggest that the more general importance of this transduction pathway may have been overlooked previously because of unsuitable recording methods.

The nitric oxide/cyclic guanosine monophosphate pathway modulates the inspiratory-related activity of hypoglossal motoneurons in the adult rat

Motoneurons integrate interneuronal activity into commands for skeletal muscle contraction and relaxation to perform motor actions. Hypoglossal motoneurons (HMNs) are involved in essential motor functions such as breathing, mastication, swallowing and phonation. We have investigated the role of the gaseous molecule nitric oxide (NO) in the regulation of the inspiratory-related activity of HMNs in order to further understand how neural activity is transformed into motor activity. In adult rats, we observed nitrergic fibers and bouton-like structures in close proximity to motoneurons, which normally lack the molecular machinery to synthesize NO. In addition, immunohistochemistry studies demonstrated that perfusion of animals with a NO donor resulted in an increase in the levels of cyclic guanosine monophosphate (cGMP) in motoneurons, which express the soluble guanylyl cyclase (sGC) in the hypoglossal nucleus. Modulators of the NO/cGMP pathway were micro-iontophoretically applied while performing single-unit extracellular recordings in the adult decerebrated rat. Application of a NO synthase inhibitor or a sGC inhibitor induced a statistically significant reduction in the inspiratory-related activity of HMNs. However, excitatory effects were observed by ejection of a NO donor or a cell-permeable analogue of cGMP. In slice preparations, application to the bath of a NO donor evoked membrane depolarization and a decrease in rheobase, which were prevented by co-addition to the bath of a sGC inhibitor. These effects were not prevented by reduction of the spontaneous synaptic activity. We conclude that NO from afferent fibers anterogradely modulates the inspiratory-related activity of HMNs by a cGMP-dependent mechanism in physiological conditions.

NO/cGMP-dependent modulation of synaptic transmission

Nitric oxide (NO) is a multifunctional messenger in the CNS that can signal both in antero- and retrograde directions across synapses. Many effects of NO are mediated through its canonical receptor, the soluble guanylyl cyclase, and the second messenger cyclic guanosine-3',5'-monophosphate (cGMP). An increase of cGMP can also arise independently of NO via activation of membrane-bound particulate guanylyl cyclases by natriuretic peptides. The classical targets of cGMP are cGMP-dependent protein kinases (cGKs), cyclic nucleotide hydrolysing phosphodiesterases, and cyclic nucleotide-gated (CNG) cation channels. The NO/cGMP/cGK signalling cascade has been linked to the modulation of transmitter release and synaptic plasticity by numerous pharmacological and genetic studies. This review focuses on the role of NO as a retrograde messenger in long-term potentiation of transmitter release in the hippocampus. Presynaptic mechanisms of NO/cGMP/cGK signalling will be discussed with recently identified potential downstream components such as CaMKII, the vasodilator-stimulated phosphoprotein, and regulators of G protein signalling. NO has further been suggested to increase transmitter release through presynaptic clustering of a-synuclein. Alternative modes of NO/cGMP signalling resulting in inhibition of transmitter release and long-term depression of synaptic activity will also be addressed, as well as anterograde NO signalling in the cerebellum. Finally, emerging evidence for cGMP signalling through CNG channels and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels will be discussed.

Bee venom injection produces a peripheral anti-inflammatory effect by activation of a nitric oxide-dependent spinocoeruleus pathway

Our recent data, obtained using a zymosan-induced inflammatory air pouch model in mice, have demonstrated that subcutaneous bee venom (BV) injection into the hind limb selectively activates the contralateral brain stem locus coeruleus (LC) and then via a descending noradrenergic pathway and subsequent adrenal medullary catecholamine release induces a potent anti-inflammatory effect. While the efferent limb of this BV-induced neuroimmune anti-inflammatory pathway is well documented, the afferent limb of this pathway is poorly understood. In particular the spinal mechanisms involved with BV activation of the LC are currently unknown. Spinal nitric oxide (NO) and its synthase (NOS) have been shown to play an important role in the transmission and amplification of neuronal information from the spinal cord to the brain stem. In the present study we evaluated whether spinal NO plays a role in BV-induced LC activation, since we have previously shown that LC activation underlies this 'BV-induced anti-inflammatory effect' (BVAI) using the mouse air pouch model. Intrathecal (i.t.) pretreatment with l-nitro arginine methyl ester (l-NAME, non-selective NOS inhibitor), hemoglobin (NO scavenger) or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, soluble guanylate cyclase inhibitor) abolished BVAI on zymosan-induced leukocyte migration into the air pouch. Moreover, i.t. injection of l-N-iminoethyl-lysine (l-NIL, inducible NOS inhibitor), but not 7-nitroindazole (7-NI, neuronal NOS inhibitor), also inhibited BVAI. BV injection significantly increased both the number of Fos immunoreactive neurons and tyrosine hydroxylase-Fos double labeling neurons in the contralateral LC in zymosan-induced inflamed mice. Importantly this increase in Fos expression in the LC was also completely inhibited by i.t. injection of l-NIL, but not by i.t. injection of 7-NI. Collectively these results indicate that spinal NO generated from inducible NOS is involved in the BV-induced LC activation that underlies BVAI.

Signaling from blood vessels to CNS axons through nitric oxide

Brain function is usually perceived as being performed by neurons with the support of glial cells, the network of blood vessels situated nearby serving simply to provide nutrient and to dispose of metabolic waste. Revising this view, we find from experiments on a rodent central white matter tract (the optic nerve) in vitro that microvascular endothelial cells signal persistently to axons using nitric oxide (NO) derived from the endothelial NO synthase (eNOS). The endogenous NO acts to stimulate guanylyl cyclase-coupled NO receptors in the axons, leading to a raised cGMP level which then causes membrane depolarization, apparently by directly engaging hyperpolarization-activated cyclic nucleotide-gated ion channels. The tonic depolarization and associated endogenous NO-dependent cGMP generation was absent in optic nerves from mice lacking eNOS, although such nerves responded to exogenous NO, with raised cGMP generation in the axons and associated depolarization. In addition to the tonic activity, exposure of optic nerves to bradykinin, a classical stimulator of eNOS in endothelial cells, elicited reversible NO- and cGMP-dependent depolarization through activation of bradykinin B2 receptors, to which eNOS is physically complexed. No contribution of other NO synthase isoforms to either the action of bradykinin or the continuous ambient NO level could be detected. The results suggest that microvascular endothelial cells participate in signal processing in the brain and can do so by generating both tonic and phasic NO signals.