Nitric oxide synthase in rat visual cortex: an immunohistochemical study

nNOS-induced tyrosine nitration of TRKB impairs BDNF signaling and restrains neuronal plasticity

Nitric oxide (NO) has been long recognized as an important modulator of neural plasticity, but characterization of the molecular mechanisms involved - specially the guanylyl cyclase-independent ones - has been challenging. There is evidence that NO could modify BDNF-TRKB signaling, a key mediator of neuronal plasticity. However, the mechanism underlying the interplay of NO and TRKB remains unclear. Here we show that NO induces nitration of the tyrosine 816 in the TRKB receptor in vivo and in vitro, and that post-translational modification inhibits TRKB phosphorylation and binding of phospholipase Cγ1 (PLCγ1) to this same tyrosine residue. Additionally, nitration triggers clathrin-dependent endocytosis of TRKB through the adaptor protein AP-2 and ubiquitination, thereby increasing translocation of TRKB away from the neuronal surface and directing it towards lysosomal degradation. Accordingly, inhibition of nitric oxide increases TRKB phosphorylation and TRKB-dependent neurite branching in neuronal cultures. In vivo, chronic inhibition of neuronal nitric oxide synthase (nNOS) dramatically reduced TRKB nitration and facilitated TRKB signaling in the visual cortex, and promoted a shift in ocular dominance upon monocular deprivation - an indicator of increased plasticity. Altogether, our data describe and characterize a new molecular brake on plasticity, namely nitration of TRKB receptors.

nNOS-expressing interneurons control basal and behaviorally evoked arterial dilation in somatosensory cortex of mice

Cortical neural activity is coupled to local arterial diameter and blood flow. However, which neurons control the dynamics of cerebral arteries is not well understood. We dissected the cellular mechanisms controlling the basal diameter and evoked dilation in cortical arteries in awake, head-fixed mice. Locomotion drove robust arterial dilation, increases in gamma band power in the local field potential (LFP), and increases calcium signals in pyramidal and neuronal nitric oxide synthase (nNOS)-expressing neurons. Chemogenetic or pharmocological modulation of overall neural activity up or down caused corresponding increases or decreases in basal arterial diameter. Modulation of pyramidal neuron activity alone had little effect on basal or evoked arterial dilation, despite pronounced changes in the LFP. Modulation of the activity of nNOS-expressing neurons drove changes in the basal and evoked arterial diameter without corresponding changes in population neural activity.

In the Piriform Cortex, the Primary Impetus for Information Encoding through Synaptic Plasticity Is Provided by Descending Rather than Ascending Olfactory Inputs

Information encoding by means of persistent changes in synaptic strength supports long-term information storage and memory in structures such as the hippocampus. In the piriform cortex (PC), that engages in the processing of associative memory, only short-term synaptic plasticity has been described to date, both in vitro and in anesthetized rodents in vivo. Whether the PC maintains changes in synaptic strength for longer periods of time is unknown: Such a property would indicate that it can serve as a repository for long-term memories. Here, we report that in freely behaving animals, frequency-dependent synaptic plasticity does not occur in the anterior PC (aPC) following patterned stimulation of the olfactory bulb (OB). Naris closure changed action potential properties of aPC neurons and enabled expression of long-term potentiation (LTP) by OB stimulation, indicating that an intrinsic ability to express synaptic plasticity is present. Odor discrimination and categorization in the aPC is supported by descending inputs from the orbitofrontal cortex (OFC). Here, OFC stimulation resulted in LTP (>4 h), suggesting that this structure plays an important role in promoting information encoding through synaptic plasticity in the aPC. These persistent changes in synaptic strength are likely to comprise a means through which long-term memories are encoded and/or retained in the PC.

Nitric oxide and the cardiovascular system

Nitric oxide (NO) generated by endothelial cells to relax vascular smooth muscle is one of the most intensely studied molecules in the past 25 years. Much of what is known about NO regulation of NO is based on blockade of its generation and analysis of changes in vascular regulation. This approach has been useful to demonstrate the importance of NO in large scale forms of regulation but provides less information on the nuances of NO regulation. However, there is a growing body of studies on multiple types of in vivo measurement of NO in normal and pathological conditions. This discussion will focus on in vivo studies and how they are reshaping the understanding of NO's role in vascular resistance regulation and the pathologies of hypertension and diabetes mellitus. The role of microelectrode measurements in the measurement of [NO] will be considered because much of the controversy about what NO does and at what concentration depends upon the measurement methodology. For those studies where the technology has been tested and found to be well founded, the concept evolving is that the stresses imposed on the vasculature in the form of flow-mediated stimulation, chemicals within the tissue, and oxygen tension can cause rapid and large changes in the NO concentration to affect vascular regulation. All these functions are compromised in both animal and human forms of hypertension and diabetes mellitus due to altered regulation of endothelial cells and formation of oxidants that both damage endothelial cells and change the regulation of endothelial nitric oxide synthase.

Synergistic neuroprotection by epicatechin and quercetin: Activation of convergent mitochondrial signaling pathways

In view of evidence that increased consumption of epicatechin (E) and quercetin (Q) may reduce the risk of stroke, we have measured the effects of combining E and Q on mitochondrial function and neuronal survival following oxygen-glucose deprivation (OGD). Relative to mouse cortical neuron cultures pretreated (24h) with either E or Q (0.1-10μM), E+Q synergistically attenuated OGD-induced neuronal cell death. E, Q and E+Q (0.3μM) increased spare respiratory capacity but only E+Q (0.3μM) preserved this crucial parameter of neuronal mitochondrial function after OGD. These improvements were accompanied by corresponding increases in cyclic AMP response element binding protein (CREB) phosphorylation and the expression of CREB-target genes that promote neuronal survival (Bcl-2) and mitochondrial biogenesis (PGC-1α). Consistent with these findings, E+Q (0.1 and 1.0μM) elevated mitochondrial gene expression (MT-ND2 and MT-ATP6) to a greater extent than E or Q after OGD. Q (0.3-3.0μM), but not E (3.0μM), elevated cytosolic calcium (Ca(2+)) spikes and the mitochondrial membrane potential. Conversely, E and E+Q (0.1 and 0.3μM), but not Q (0.1 and 0.3μM), activated protein kinase B (Akt). Nitric oxide synthase (NOS) inhibition with L-N(G)-nitroarginine methyl ester (1.0μM) blocked neuroprotection by E (0.3μM) or Q (1.0μM). Oral administration of E+Q (75mg/kg; once daily for 5days) reduced hypoxic-ischemic brain injury. These findings suggest E and Q activate Akt- and Ca(2+)-mediated signaling pathways that converge on NOS and CREB resulting in synergistic improvements in neuronal mitochondrial performance which confer profound protection against ischemic injury.

Homeostatic sleep pressure is the primary factor for activation of cortical nNOS/NK1 neurons

Cortical interneurons, immunoreactive for neuronal nitric oxide synthase (nNOS) and the receptor NK1, express the functional activity marker Fos selectively during sleep. NREM sleep 'pressure' is hypothesized to accumulate during waking and to dissipate during sleep. We reported previously that the proportion of Fos(+) cortical nNOS/NK1 neurons is correlated with established electrophysiological markers of sleep pressure. As these markers covary with the amount of NREM sleep, it remained unclear whether cortical nNOS/NK1 neurons are activated to the same degree throughout NREM sleep or whether the extent of their activation is related to the sleep pressure that accrued during the prior waking period. To distinguish between these possibilities, we used hypnotic medications to control the amount of NREM sleep in rats while we varied prior wake duration and the resultant sleep pressure. Drug administration was preceded by 6 h of sleep deprivation (SD) ('high sleep pressure') or undisturbed conditions ('low sleep pressure'). We find that the proportion of Fos(+) cortical nNOS/NK1 neurons was minimal when sleep pressure was low, irrespective of the amount of time spent in NREM sleep. In contrast, a large proportion of cortical nNOS/NK1 neurons was Fos(+) when an equivalent amount of sleep was preceded by SD. We conclude that, although sleep is necessary for cortical nNOS/NK1 neuron activation, the proportion of cells activated is dependent upon prior wake duration.

Perivascular iron deposits are associated with protein nitration in cerebral experimental autoimmune encephalomyelitis

Nitration of proteins, which is thought to be mediated by peroxynitrite, is a mechanism of tissue damage in multiple sclerosis (MS). However, protein nitration can also be catalyzed by iron, heme or heme-associated molecules independent of peroxynitrite. Since microhemorrhages and perivascular iron deposits are present in the CNS of MS patients, we sought to determine if iron is associated with protein nitration. A cerebral model of experimental autoimmune encephalomyelitis (cEAE) was utilized since this model has been shown to have perivascular iron deposits similar to those present in MS. Histochemical staining for iron was used together with immunohistochemistry for nitrotyrosine, eNOS, or iNOS on cerebral sections. Leakage of the blood-brain barrier (BBB) was studied by albumin immunohistochemistry. Iron deposits were colocalized with nitrotyrosine staining around vessels in cEAE mice while control animals revealed minimal staining. This finding supports the likelihood that nitrotyrosine formation was catalyzed by iron or iron containing molecules. Examples of iron deposits were also observed in association with eNOS and iNOS, which could be one source of substrates for this reaction. Extravasation of albumin was present in cEAE mice, but not in control animals. Extravasated albumin may act to limit tissue injury by binding iron and/or heme as well as being a target of nitration, but the protection is incomplete. In summary, iron-catalyzed nitration of proteins is a likely mechanism of tissue damage in MS.

Cortical nNOS neurons co-express the NK1 receptor and are depolarized by Substance P in multiple mammalian species

We have previously demonstrated that Type I neuronal nitric oxide synthase (nNOS)-expressing neurons are sleep-active in the cortex of mice, rats, and hamsters. These neurons are known to be GABAergic, to express Neuropeptide Y (NPY) and, in rats, to co-express the Substance P (SP) receptor NK1, suggesting a possible role for SP in sleep/wake regulation. To evaluate the degree of co-expression of nNOS and NK1 in the cortex among mammals, we used double immunofluorescence for nNOS and NK1 and determined the anatomical distribution in mouse, rat, and squirrel monkey cortex. Type I nNOS neurons co-expressed NK1 in all three species although the anatomical distribution within the cortex was species-specific. We then performed in vitro patch clamp recordings in cortical neurons in mouse and rat slices using the SP conjugate tetramethylrhodamine-SP (TMR-SP) to identify NK1-expressing cells and evaluated the effects of SP on these neurons. Bath application of SP (0.03–1 μM) resulted in a sustained increase in firing rate of these neurons; depolarization persisted in the presence of tetrodotoxin. These results suggest a conserved role for SP in the regulation of cortical sleep-active neurons in mammals.

Electrochemical nanosensor for real-time direct imaging of nitric oxide in living brain

As gaseous nitric oxide (NO), a critical and multifaceted biomarker, diffuses easily once released, identifying the precise sources of NO release is a challenge. This study developed a new technique for real-time in vivo direct NO imaging by coupling an amperometric NO nanosensor with scanning electrochemical microscopy. This technique provides three-dimensional information of the NO releasing sites in an intact living mouse brain with high sensitivity and spatial resolution. Immunohistochemical analysis was carried out to confirm the anatomical reliability of the acquired electrochemical NO image. The real-time NO imaging results were well matched with the corresponding immunohistochemical analysis of neuronal NO synthase immunoreactive (nNOS-IR) cells, i.e., NO releasing sites in a living brain. The imaged NO local concentrations were confirmed to be closely related to the location in depth, the size of the nNOS-IR cell, and the intensity of nNOS immunoreactivity. This paper demonstrates the first direct electrochemical NO imaging of a living brain.

Cerebral microvascular nNOS responds to lowered oxygen tension through a bumetanide-sensitive cotransporter and sodium-calcium exchanger

Na(+) cotransporters have a substantial role in neuronal damage during brain hypoxia. We proposed these cotransporters have beneficial roles in oxygen-sensing mechanisms that increase periarteriolar nitric oxide (NO) concentration ([NO]) during mild to moderate oxygen deprivation. Our prior studies have shown that cerebral neuronal NO synthase (nNOS) is essential for [NO] responses to decreased oxygen tension and that endothelial NO synthase (eNOS) is of little consequence. In this study, we explored the mechanisms of three specific cotransporters known to play a role in the hypoxic state: KB-R7943 for blockade of the Na(+)/Ca(2+) exchanger, bumetanide for the Na(+)-K(+)-2Cl(-) cotransporter, and amiloride for Na(+)/H(+) cotransporters. In vivo measurements of arteriolar diameter and [NO] at normal and locally reduced oxygen tension in the rat parietal cortex provided the functional analysis. As previously found for intestinal arterioles, bumetanide-sensitive cotransporters are primarily responsible for sensing reduced oxygen because the increased [NO] and dilation were suppressed. The Na(+)/Ca(2+) exchanger facilitated increased NO formation because blockade also suppressed [NO] and dilatory responses to decreased oxygen. Amiloride-sensitive Na(+)/H(+) cotransporters did not significantly contribute to the microvascular regulation. To confirm that nNOS rather than eNOS was primarily responsible for NO generation, eNOS was suppressed with the fusion protein cavtratin for the caveolae domain of eNOS. Although the resting [NO] decreased and arterioles constricted as eNOS was suppressed, most of the increased NO and dilatory response to oxygen were preserved because nNOS was functional. Therefore, nNOS activation secondary to Na(+)-K(+)-2Cl(-) cotransporter and Na(+)/Ca(2+) exchanger functions are key to cerebral vascular oxygen responses.

Cerebral microvascular dilation during hypotension and decreased oxygen tension: a role for nNOS

Endothelial (eNOS) and neuronal nitric oxide synthase (nNOS) are implicated as important contributors to cerebral vascular regulation through nitric oxide (NO). However, direct in vivo measurements of NO in the brain have not been used to dissect their relative roles, particularly as related to oxygenation of brain tissue. We found that, in vivo, rat cerebral arterioles had increased NO concentration ([NO]) and diameter at reduced periarteriolar oxygen tension (Po(2)) when either bath oxygen tension or arterial pressure was decreased. Using these protocols with highly selective blockade of nNOS, we tested the hypothesis that brain tissue nNOS could donate NO to the arterioles at rest and during periods of reduced perivascular oxygen tension, such as during hypotension or reduced local availability of oxygen. The decline in periarteriolar Po(2) by bath manipulation increased [NO] and vessel diameter comparable with responses at similarly decreased Po(2) during hypotension. To determine whether the nNOS provided much of the vascular wall NO, nNOS was locally suppressed with the highly selective inhibitor N-(4S)-(4-amino-5-[aminoethyl]aminopentyl)-N'-nitroguanidine. After blockade, resting [NO], Po(2), and diameters decreased, and the increase in [NO] during reduced Po(2) or hypotension was completely absent. However, flow-mediated dilation during occlusion of a collateral arteriole did remain intact after nNOS blockade and the vessel wall [NO] increased to approximately 80% of normal. Therefore, nNOS predominantly increased NO during decreased periarteriolar oxygen tension, such as that during hypotension, but eNOS was the dominant source of NO for flow shear mechanisms.

Expression of nitric oxide synthase isoforms is reduced in late-gestation ovine fetal brainstem

Fetal baroreflex responsiveness increases in late gestation. An important modulator of baroreflex activity is the generation of nitric oxide in the brainstem nuclei that integrate afferent and efferent reflex activity. The present study was designed to test the hypothesis that nitric oxide synthase (NOS) isoforms are expressed in the fetal brainstem and that the expression of one or more of these enzymes is reduced in late gestation. Brainstem tissue was rapidly collected from fetal sheep of known gestational ages (80, 100, 120, 130, 145 days gestation and 1 day and 1 wk postnatal). Neuronal (nNOS), inducible (iNOS), and endothelial (eNOS) mRNA was measured using real-time PCR methodology specific for ovine NOS isoforms. The three enzymes were measured at the protein level using Western blot methodology. In tissue prepared for histology separately, the cellular pattern of immunostaining was identified in medullae from late-gestation fetal sheep. Fetal brainstem contained mRNA and protein of all three NOS isoforms, with nNOS the most abundant, followed by iNOS and eNOS, respectively. nNOS and iNOS mRNA abundances were highest at 80 days' gestation, with statistically significant decreases in abundance in more mature fetuses and postnatal animals. nNOS and eNOS protein abundance also decreased as a function of developmental age. nNOS and eNOS were expressed in neurons, iNOS was expressed in glia, and eNOS was expressed in vascular endothelial cells. We conclude that all three isoforms of NOS are constitutively expressed within the fetal brainstem, and the expression of all three forms is reduced with advancing gestation. We speculate that the reduced expression of NOS in this brain region plays a role in the increased fetal baroreflex activity in late gestation.

The distribution of neural nitric oxide synthase-positive cerebrospinal fluid-contacting neurons in the third ventricular wall of male rats and coexistence with vasopressin or oxytocin

The detailed distribution of neural nitric oxide synthase (nNOS)-positive cerebrospinal fluid-contacting neurons (CSF-CN) was studied in the wall of the third ventricle of rats by anti-nNOS immunohistochemistry. The coexistence of nNOS and 8-arginine vasopressin (AVP) or oxytocin (OT) was also investigated in the CSF-CN using double labeling immunohistochemistry. The results demonstrated a widespread occurrence of nNOS-CSF-CN throughout the wall of the hypothalamic third ventricle. The vast majority of nNOS-CSF-CN cell bodies were of magnocellular type, commonly classified as oval, fusiform, multipolar, and inverted pear shape. These cell bodies were located in the ependyma, the subependyma, or the parenchyma, and their processes inserted in the ependymal layer or directly contacted with the CSF space. Electron microscopy demonstrated many nNOS-immunoreactive somas, dendrites, and/or axons that were situated at the subependyma, the ependyma, or the supraependyma. Generally, the distribution of OT-CSF-CN in the third ventricular wall was similar to the nNOS-CSF-CN and the ratio of NOS/OT co-expression was approximately 88%. In comparison, the distribution of AVP-CSF-CN was mainly restricted to the rostral part of the third ventricle and the ratio of nNOS/AVP co-expression was only about 6%. The widespread presence of nNOS-CSF-CN-expressing OT in the third ventricular region suggests that NO is an important messenger in the CSF-hypothalamo-hypophyseal neuroendocrine regulation that may in part act in concert with OT.