Cellular Localization of Acid-Sensing Ion Channel 1 in Rat Nucleus Tractus Solitarii.

By determining its cellular localization in the nucleus tractus solitarii (NTS), we sought anatomical support for a putative physiological role for acid-sensing ion channel Type 1 (ASIC1) in chemosensitivity. Further, we sought to determine the effect of a lesion that produces gliosis in the area. In rats, we studied ASIC1 expression in control tissue with that in tissue with gliosis, which is associated with acidosis, after saporin lesions. We hypothesized that saporin would increase ASIC1 expression in areas of gliosis. Using fluorescent immunohistochemistry and confocal microscopy, we found that cells and processes containing ASIC1-immunoreactivity (IR) were present in the NTS, the dorsal motor nucleus of vagus, and the area postrema. In control tissue, ASIC1-IR predominantly colocalized with IR for the astrocyte marker, glial fibrillary acidic protein (GFAP), or the microglial marker, integrin αM (OX42). The subpostremal NTS was the only NTS region where neurons, identified by protein gene product 9.5 (PGP9.5), contained ASIC1-IR. ASIC1-IR increased significantly (157 ± 8.6% of control, p < 0.001) in the NTS seven days after microinjection of saporin. As we reported previously, GFAP-IR was decreased in the center of the saporin injection site, but GFAP-IR was increased in the surrounding areas where OX42-IR, indicative of activated microglia, was also increased. The over-expressed ASIC1-IR colocalized with GFAP-IR and OX42-IR in those reactive astrocytes and microglia. Our results support the hypothesis that ASIC1 would be increased in activated microglia and in reactive astrocytes after injection of saporin into the NTS.

Reduced responses to glutamate receptor agonists follow loss of astrocytes and astroglial glutamate markers in the nucleus tractus solitarii.

Saporin (SAP) or SAP conjugates injected into the nucleus tractus solitarii (NTS) of rats kill astrocytes. When injected in its unconjugated form, SAP produces no demonstrable loss of or damage to local neurons. However bilateral injections of SAP significantly attenuate responses to activation of baroreceptor reflexes that are mediated by transmission of signals through glutamate receptors in the NTS We tested the hypothesis that SAP would reduce cardiovascular responses to activation of NTS glutamate receptors despite its recognized ability to spare local neurons while killing local astrocytes. In animals treated with SAP and SAP conjugates or, as a control, with the toxin 6-hydroxydopamine (6-OHDA), we sought to determine if dose-related changes of arterial pressure (AP) or heart rate (HR) in response to injection into NTS of N-methyl-d-aspartate (NMDA) or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) were attenuated. Also we quantified changes in immunoreactivity (IR) for EAAT2, EAAC1, and VGluT2 in NTS after SAP and SAP conjugates. Our earlier studies showed that IR for NMDA and AMPA receptors was not changed after injection of SAP We found that EAAT2 and EAAC1, both found in astrocytes, were reduced by SAP or its conjugates but not by 6-OHDA In contrast, VGluT2-IR was increased by SAP or conjugates but not by 6-OHDA AP and HR responses to NMDA and AMPA were attenuated after SAP and SAP conjugate injection but not after 6-OHDA Results of this study are consistent with others that have shown interactions between astroglia and neurons in synaptic transmission mediated by glutamate receptor activation in the NTS.

Acid-sensing ion channel 1 and nitric oxide synthase are in adjacent layers in the wall of rat and human cerebral arteries.

Extracellular acidification activates a family of proteins known as acid-sensing ion channels (ASICs). One ASIC subtype, ASIC type 1 (ASIC1), may play an important role in synaptic plasticity, memory, fear conditioning and ischemic brain injury. ASIC1 is found primarily in neurons, but one report showed its expression in isolated mouse cerebrovascular cells. In this study, we sought to determine if ASIC1 is present in intact rat and human major cerebral arteries. A potential physiological significance of such a finding is suggested by studies showing that nitric oxide (NO), which acts as a powerful vasodilator, may modulate proton-gated currents in cultured cells expressing ASIC1s. Because both constitutive NO synthesizing enzymes, neuronal nitric oxide synthase (nNOS) and endothelial NOS (eNOS), are expressed in cerebral arteries we also studied the anatomical relationship between ASIC1 and nNOS or eNOS in both rat and human cerebral arteries. Western blot analysis demonstrated ASIC1 in cerebral arteries from both species. Immunofluorescent histochemistry and confocal microscopy also showed that ASIC1-immunoreactivity (IR), colocalized with the smooth muscle marker alpha-smooth muscle actin (SMA), was present in the anterior cerebral artery (ACA), middle cerebral artery (MCA), posterior cerebral artery (PCA) and basilar artery (BA) of rat and human. Expression of ASIC1 in cerebral arteries is consistent with a role for ASIC1 in modulating cerebrovascular tone both in rat and human. Potential interactions between smooth muscle ASIC1 and nNOS or eNOS were supported by the presence of nNOS-IR in the neighboring adventitial layer and the presence of nNOS-IR and eNOS-IR in the adjacent endothelial layer of the cerebral arteries.

Astrocytes in the rat nucleus tractus solitarii are critical for cardiovascular reflex control.

We have shown that an antibody to dopamine-ß-hydroxylase conjugated with saporin (anti-DBH-SAP) damages catecholamine neurons in the nucleus tractus solitarii (NTS) of rat, attenuates arterial baroreflexes, and leads to lability of arterial blood pressure, damage to cardiac myocytes, and, in some animals, sudden death. However, others have shown that injection of 6-hydroxydopamine (6-OHDA), a toxin devoid of saporin, also damaged NTS catecholamine neurons but did not lead to these cardiovascular changes. We found similar cardiovascular changes after injecting a different SAP conjugate to target NTS neurons with neurokinin (NK1) receptors. Because ribosome-inactivating proteins may be toxic to glia, we hypothesized that SAP, a ribosome-inactivating protein, might target glia whose loss could account for physiological changes. We tested this hypothesis by assessing effects on select neurons and on glia in the NTS after exposure to SAP, targeted SAP conjugates, or 6-OHDA. SAP and all SAP conjugates led to loss of immunoreactivity for glial fibrillary acidic protein, a marker for astrocytes, in the NTS while 6-OHDA did not. As reported previously, anti-DBH-SAP selectively killed noradrenergic neurons in the NTS while SAP conjugated to stabilized substance P (SSP-SAP) selectively killed neurons with NK1 receptors. In contrast, SAP produced no demonstrable neuronal damage. All injections led to activation of microglia in the NTS; however, only SAP and its conjugates attenuated cardiovascular reflexes while also producing lability of arterial pressure, damage to cardiac myocytes, and in some animals, sudden death. Thus, NTS astrocytes may play a role in mediating cardiovascular reflex transmission through the NTS.

Sudden death following selective neuronal lesions in the rat nucleus tractus solitarii.

In efforts to assess baroreflex and cardiovascular responses in rats in which substance P (SP) or catecholamine transmission had been eliminated we studied animals after bilateral injections into the nucleus tractus solitarii (NTS) of SP or stabilized SP (SSP) conjugated to saporin (SP-SAP or SSP-SAP respectively) or SAP conjugated to an antibody to dopamine-ß-hydroxylase (anti-DBH-SAP). We found that SP- and SSP-SAP eliminated NTS neurons that expressed the SP neurokinin-1 receptor (NK1R) while anti-DBH-SAP eliminated NTS neurons expressing tyrosine hydroxylase (TH) and DBH. The toxins were selective. Thus SP- or SSP-SAP did not eliminate TH/DBH neurons and anti-DBH-SAP did not eliminate NK1R neurons in the NTS. Each toxin, however, led to chronic lability of arterial blood pressure, diminished baroreflex function, cardiac ventricular irritability, coagulation necrosis of cardiac myocytes and, in some animals, sudden death associated with asystole. However, when TH/DBH neurons were targeted and eliminated by injection of 6-hydroxydopamine (6-OHDA), none of the cardiovascular or cardiac changes occurred. The studies reviewed here reveal that selective lesions of the NTS lead to altered baroreflex control and to cardiac changes that may lead to sudden death. Though the findings could support a role for SP or catecholamines in baroreflex transmission neither is proven in that NK1R colocalizes with glutamate receptors. Thus neurons with both are lost when treated with SP- or SSP-SAP. In addition, loss of catecholamine neurons after treatment with 6-OHDA does not affect cardiovascular control. Thus, the effect of the toxins may depend on an action of SAP independent of the effects of the SAP conjugates on targeted neuronal types.

Views of translational research from a somewhat translational scientist.

This review arose from a talk entitled "Identifying Targets" and given by the author at EB2011 at the invitation of the American Federation for Medical Research (AFMR). The presentation was part of the American Federation for Medical Research workshop entitled "Keys for Translation: Science and Strategy" and focused on identifying clinically relevant targets as a result of observations made during basic scientific studies. The review emphasizes that targets do not have to be the aim that drives basic discovery, but communication between the basic scientist and clinical investigators may aid recognition of such targets and their translation to clinical applications. Using one line of investigator-initiated research from his own laboratory as an example, the author emphasizes that basic discovery must be hypothesis driven and allowed to follow its logical sequence. Finding treatments, while always an aim of biomedical research, may arise as a result of basic studies that were not originally aimed at a target of translational research.

Decreased expression of neuronal nitric oxide synthase in the nucleus tractus solitarii inhibits sympathetically mediated baroreflex responses in rat.

Despite numerous studies it remains controversial whether nitric oxide (NO·) synthesized by neuronal NOS (nNOS) plays an excitatory or inhibitory role in transmission of baroreflex signals in the nucleus tractus solitarii (NTS). In the current studies we sought to test the hypothesis that nNOS is involved in excitation of baroreflex pathways in NTS while excluding pharmacological interventions in assessing the influence of nNOS. We therefore developed, validated and utilized a short hairpin RNA (shRNA) to reduce expression of nNOS in the NTS of rats whose baroreflex activity was then studied. We demonstrate downregulation of nNOS through transduction with adeno-associated virus type 2 (AAV2) carrying shRNA for nNOS. When injected bilaterally into NTS AAV2nNOSshRNA significantly reduced reflex tachycardic responses to acute hypotension while not affecting reflex bradycardic responses to acute increases of arterial pressure. Control animals treated with intravenous propranolol to block sympathetically mediated chronotropic responses manifested the same baroreflex responses as animals that had been treated with AAV2nNOSshRNA. Neither AAV2 eGFP nor AAV2nNOScDNA affected baroreflex responses. Blocking cardiac vagal influences with atropine similarly reduced baroreflex-mediated bradycardic responses to increases in arterial pressure both in control animals and in those treated with AAV2nNOSshRNA. We conclude that NO· synthesized by nNOS in the NTS is integral to excitation of baroreflex pathways involved in reflex tachycardia, a largely sympathetically mediated response, but not reflex bradycardia, a largely parasympathetically mediated response. We suggest that, at the basal state, nNOS is maximally engaged. Thus, its upregulation does not augment the baroreflex.

Sudden death and myocardial lesions after damage to catecholamine neurons of the nucleus tractus solitarii in rat.

Lesions that remove neurons expressing neurokinin-1 (NK1) receptors from the nucleus tractus solitarii (NTS) without removing catecholaminergic neurons lead to loss of baroreflexes, labile arterial pressure, myocardial lesions, and sudden death. Because destruction of NTS catecholaminergic neurons expressing tyrosine hydroxylase (TH) may also cause lability of arterial pressure and loss of baroreflexes, we sought to test the hypothesis that cardiac lesions associated with lability are not dependent on damage to neurons with NK1 receptors but would also occur when TH neurons in NTS are targeted. To rid the NTS of TH neurons we microinjected anti-dopamine ß-hydroxylase conjugated to saporin (anti-DBH-SAP, 42 ng/200 nl) into the NTS. After injection of the toxin unilaterally, immunofluorescent staining confirmed that anti-DBH-SAP decreased the number of neurons and fibers that contain TH and DBH in the injected side of the NTS while sparing neuronal elements expressing NK1 receptors. Bilateral injections in eight rats led to significant lability of arterial pressure. For example, on day 8 standard deviation of mean arterial pressure was 16.8 ± 2.5 mmHg when compared with a standard deviation of 7.83 ± 0.33 mmHg in six rats in which phosphate buffered saline (PBS) had been injected bilaterally. Two rats died suddenly at 5 and 8 days after anti-DBH-SAP injection. Seven-treated animals demonstrated microscopic myocardial necrosis as reported in animals with lesions of NTS neurons expressing NK1 receptors. Thus, cardiac and cardiovascular effects of lesions directed toward catecholamine neurons of the NTS are similar to those following damage directed toward NK1 receptor-containing neurons.

Collateral damage and compensatory changes after injection of a toxin targeting neurons with the neurokinin-1 receptor in the nucleus tractus solitarii of rat.

Injection into the nucleus tractus solitarii (NTS) of toxins that target substance P (SP) receptors ablates neurons that express neurokinin-1 (NK1) receptors, attenuates baroreflexes, and results in increased lability of arterial pressure. We and others have shown that the toxin leads to loss of neurons containing SP receptors and loss of GABAergic neurons in the NTS; but given that neither type neuron is thought to be integral to baroreflex transmission in NTS, mechanisms responsible for the cardiovascular changes remained unclear. Because NK1 receptors colocalize with N-methyl-d-aspartate (NMDA) receptors and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in NTS and because glutamate transmission may be integral to baroreflex transmission in the NTS we hypothesized that the toxic lesions may interrupt mechanisms for glutamate transmission. Interruption of those mechanisms could be responsible for the cardiovascular effects. We tested the hypothesis by performing fluorescent immunohistochemistry, confocal microscopy and image analysis after injecting stabilized SP-SAP (SSP-SAP) unilaterally into the NTS. We assessed changes in immunoreactivity (IR) of NMDA receptor subunit 1 (NMDAR1), AMPA receptor subunit 2 (GluR2), and 3 types of vesicular glutamate transporters (VGluT) as well as IR of gamma-aminobutyric acid receptors type b (GABAb), neuronal nitric oxide synthase (nNOS), tyrosine hydroxylase (TH), and protein gene product 9.5 (PGP 9.5), a neuronal marker, in the NTS. When compared to that of the same section of the un-injected NTS, IR decreased significantly in the injected side for NMDAR1 (p<0.01), GluR2 (p<0.01), VGluT3 (p<0.01), GABAb (p<0.001), and PGP9.5 (p<0.001). In contrast, IR for VGluT1 (p<0.001), VGluT2 (p<0.001), nNOS (p<0.001), and TH (p<0.001) increased significantly. We conclude that pathologic effects following ablation of neurons with NK1 receptors in NTS may result from interruption of neurotransmission through other neurochemical systems associated with NK1 receptors-containing neurons.

Targeting neurons of rat nucleus tractus solitarii with the gene transfer vector adeno-associated virus type 2 to up-regulate neuronal nitric oxide synthase.

Adeno-associated virus (AAV) has distinct advantages over other viral vectors in delivering genes of interest to the brain. AAV mainly transfects neurons, produces no toxicity or inflammatory responses, and yields long-term transgene expression. In this study, we first tested the hypothesis that AAV serotype 2 (AAV2) selectively transfects neurons but not glial cells in the nucleus tractus solitarii (NTS) by examining expression of the reporter gene, enhanced green fluorescent protein (eGFP), in the rat NTS after unilateral microinjection of AAV2eGFP into NTS. Expression of eGFP was observed in 1-2 cells in the NTS 1 day after injection. The number of transduced cells and the intensity of eGFP fluorescence increased from day 1 to day 28 and decreased on day 60. The majority (92.9 ± 7.0%) of eGFP expressing NTS cells contained immunoreactivity for the neuronal marker, protein gene product 9.5, but not that for the glial marker, glial fibrillary acidic protein. We observed eGFP expressing neurons and fibers in the nodose ganglia (NG) both ipsilateral and contralateral to the injection. In addition, eGFP expressing fibers were present in both ipsilateral and contralateral nucleus ambiguus (NA), caudal ventrolateral medulla (CVLM) and rostral ventrolateral medulla (RVLM). Having established that AAV2 was able to transduce a gene into NTS neurons, we constructed AAV2 vectors that contained cDNA for neuronal nitric oxide synthase (nNOS) and examined nNOS expression in the rat NTS after injection of this vector into the area. Results from RT-PCR, Western analysis, and immunofluorescent histochemistry indicated that nNOS expression was elevated in rat NTS that had been injected with AAV2nNOS vectors. Therefore, we conclude that AAV2 is an effective viral vector in chronically transducing NTS neurons and that AAV2nNOS can be used as a specific gene transfer tool to study the role of nNOS in CNS neurons.

Nitroxidergic innervation of human cerebral arteries.

A dense network of nerves containing neuronal nitric oxide synthase is present in cerebral vessels from experimental animals. The nerves may regulate cerebrovascular tone, protect the brain from stroke, and contribute to cluster headaches in humans; but studies in humans have shown only modest nitroxidergic innervation of cerebral vessels. We tested the hypothesis that nerve fibers containing neuronal nitric oxide synthase richly innervate human cerebral arteries. We used immunohistochemical techniques at post mortem and found dense neuronal nitric oxide synthase nerve staining in human cerebral vessel walls consistent with participation of nitroxidergic fibers in human physiological and pathophysiological processes.

Parasympathetic tonic dilatory influences on cerebral vessels.

Parasympathetic nerves from the pterygopalatine ganglia may participate in development of cluster headaches, in vascular responses to hypertension and in modulation of damage due to stroke. Stimulation of the nerves elicits cerebral vasodilatation, but it is not known if the nerves tonically influence cerebrovascular tone. We hypothesized that parasympathetics provide a tonic vasodilator influence and tested that hypothesis by measuring cerebral blood flow in anesthetized rats before and after removal of a pterygopalatine ganglion. Ganglion removal led to reduced cerebral blood flow without changing blood pressure. Thus, parasympathetic nerves provide tonic vasodilatory input to cerebral blood vessels.

Cardiac damage after lesions of the nucleus tractus solitarii.

Humans with central lesions that augment sympathetic nerve activity are predisposed to cardiac arrhythmias, myocardial lesions, and sudden death. Previously, we showed that selectively killing neurons with neurokinin-1 receptors in the nucleus tractus solitarii (NTS) of rats attenuated the baroreflex and, in some animals, led to sudden unexplained death within approximately 2 wk. Interruption of arterial baroreflexes is known to increase sympathetic activity. Here we tested the hypothesis that lesions in the NTS lead to fatal cardiac arrhythmias and myocardial lesions. We studied electrocardiograms, echocardiograms, blood pressure, and heart rate in 14 adult male rats after bilateral microinjection into the NTS of stabilized substance P conjugated to the toxin saporin and compared the variables in five sham control rats and in five animals with toxin injected outside the NTS. Only injection of toxin into the NTS led to increased lability of arterial blood pressure, a sign of baroreflex interruption. Two animals treated with toxin died suddenly. All animals engaged in normal activity until, in two, rapid development of asystole and death over 6-8 min. Cardiac function when examined by echocardiography was normal, but pathologic examination of the heart revealed diffuse microscopic areas of acute coagulation necrosis in the myocardium in five animals, focal subacute necrosis in two animals, and both changes in one animal. This study supports the hypothesis that NTS lesions interrupting the baroreflex may induce cardiac arrhythmias and myocardial changes similar to those seen in humans with central lesions and may lead to sudden cardiac death.

Parasympathetic stimulation elicits cerebral vasodilatation in rat.

Forebrain arteries receive nitroxidergic input from parasympathetic ganglionic fibers that arise from the pterygopalatine ganglia. Previous studies have shown that ganglionic stimulation in some species led to cerebral vasodilatation while interruption of those fibers interfered with vasodilatation seen during acute hypertension. Because the ganglionic fibers are quite delicate and are easily damaged when the ganglia are approached with published techniques we sought to develop a method that allowed clear exposure of the ganglia and permitted demonstration of cerebral vasodilatation with electrical stimulation of the ganglia in the rat. We had found that an orbital approach during which the eye was retracted for visualization of the ganglion precluded eliciting vasodilatation with ganglionic stimulation. In the current study approaching the ganglion through an incision over the zygomatic arch provided clear exposure of the ganglion and stimulation of the ganglion with that approach led to vasodilatation.

Neuronal nitric oxide mediates cerebral vasodilatation during acute hypertension.

Parasympathetic nerves from the pterygopalatine ganglia provide nitroxidergic innervation to forebrain cerebral blood vessels. Disruption of that innervation attenuates cerebral vasodilatation seen during acute hypertension as does systemic administration of a non-selective nitric oxide synthase (NOS) inhibitor. Although such studies suggest that nitric oxide (NO) released from parasympathetic nerves participates in vasodilatation of cerebral vessels during hypertension, that hypothesis has not been tested with selective local inhibition of neuronal NOS (nNOS). We tested that hypothesis through these studies performed in anesthetized rats instrumented for continuous measurement of blood pressure, heart rate and pial arterial diameter through a cranial window. We sought to determine if the nNOS inhibitor propyl-L-arginine delivered directly to the outer surface of a pial artery would (1) attenuate changes in pial arterial diameter during acute hypertension and (2) block nNOS-mediated dilator effects of N-methyl-D-aspartate (NMDA) delivered into the window but (3) not block vasodilatation elicited by acetylcholine (ACh) and mediated by endothelial NOS dilator. Without the nNOS inhibitor arterial diameter abruptly increased 70+/-15% when mean arterial pressure (MAP) reached 183+/-3 mm Hg while with nNOS inhibition diameter increased only 13+/-10% (p<0.05) even when MAP reached 191+/-4 mm Hg (p>0.05). The nNOS inhibitor significantly attenuated vasodilatation induced by NMDA but not ACh delivered into the window. Thus, local nNOS inhibition attenuates breakthrough from autoregulation during hypertension as does complete interruption of the parasympathetic innervation of cerebral vessels. These findings further support the hypothesis that NO released from parasympathetic fibers contributes to cerebral vasodilatation during acute hypertension.

NMDA receptors in nucleus tractus solitarii are linked to soluble guanylate cyclase.

We sought to test the hypothesis that cardiovascular responses to activation of ionotropic, but not metabotropic, glutamate receptors in the nucleus tractus solitarii (NTS) depend on soluble guanylate cyclase (sGC) and that inhibition of sGC would attenuate baroreflex responses to changes in arterial pressure. In adult male Sprague-Dawley rats anesthetized with chloralose, the ionotropic receptor agonists N-methyl-d-aspartate (NMDA) and dl-alpha-amino-3-hydroxy-5-methylisoxazole-propionic acid (AMPA) and the metabotropic receptor agonist trans-dl-amino-1,3-cyclopentane-dicarboxylic acid (ACPD) were microinjected into the NTS before and after microinjection of sGC inhibitors at the same site. Inhibition of sGC produced significant dose-dependent attenuation of cardiovascular responses to NMDA but did not alter responses produced by injection of AMPA or ACPD. Bilateral inhibition of sGC did not alter arterial pressure, nor did it attenuate baroreflex responses to pharmacologically induced changes in arterial pressure. This study links sGC with NMDA, but not AMPA or metabotropic, receptors in cardiovascular signal transduction through NTS.

Transmission of arterial baroreflex signals depends on neuronal nitric oxide synthase.

Because inhibition of neuronal nitric oxide synthase in the nucleus tractus solitarii blocks cardiovascular responses to activation of local glutamate receptors, and because glutamate is a neurotransmitter of baroreceptor afferent nerves, we sought to test the hypothesis that neuronal nitric oxide synthase inhibition would block baroreflex transmission and cause hypertension. We determined reflex heart rate responses to intravenous phenylephrine and sodium nitroprusside in 5 anesthetized rats before and after bilateral microinjection (100 nL) of the neuronal nitric oxide synthase inhibitor AR-R 17477 (7.5 nmol) into the nucleus tractus solitarii. The inhibitor significantly increased mean arterial pressure without affecting heart rate, and it significantly reduced the gain of the baroreflex. After administration of the inhibitor, reflex responses of heart rate to changes in mean arterial pressure were always less than those responses to the same, or less, change in mean arterial pressure in the same animal without administration of the inhibitor. Microinjection of saline (100 nL) bilaterally into the nucleus tractus solitarii did not lead to hypertension or change baroreflex responses. These data support the hypothesis and suggest that neuronal nitric oxide synthase is critical to transmission of baroreflex signals through the nucleus tractus solitarii.