Origin of neuronal nitric oxide synthase (NOS)-immunoreactive fibers in guinea pig parasympathetic cardiac ganglia

Comparative specialization of intrinsic cardiac neurons in humans, mice, and pigs

Intrinsic cardiac neurons (ICNs) play a crucial role in the proper functioning of the heart; yet a paucity of data pertaining to human ICNs exists. We took a multidisciplinary approach to complete a detailed cellular comparison of the structure and function of ICNs from mice, pigs, and humans. Immunohistochemistry of whole and sectioned ganglia, transmission electron microscopy, intracellular microelectrode recording and dye filling for quantitative morphometry were used to define the neurophysiology, histochemistry, and ultrastructure of these cells across species. The densely packed, smaller ICNs of mouse lacked dendrites, formed axosomatic connections, and had high synaptic efficacy constituting an obligatory synapse. At Pig ICNs, a convergence of subthreshold cholinergic inputs onto extensive dendritic arbors supported greater summation and integration of synaptic input. Human ICNs were tonically firing, with synaptic stimulation evoking large suprathreshold excitatory postsynaptic potentials like mouse, and subthreshold potentials like pig. Ultrastructural examination of synaptic terminals revealed conserved architecture, yet small clear vesicles (SCVs) were larger in pigs and humans. The presence and localization of ganglionic neuropeptides was distinct, with abundant VIP observed in human but not pig or mouse ganglia, and little SP or CGRP in pig ganglia. Action potential waveforms were similar, but human ICNs had larger after-hyperpolarizations. Intrinsic excitability differed; 93% of human cells were tonic, all pig neurons were phasic, and both phasic and tonic phenotypes were observed in mouse. In combination, this publicly accessible, multimodal atlas of ICNs from mice, pigs, and humans identifies similarities and differences in the evolution of ICNs.

Anatomical evidence of non-parasympathetic cardiac nitrergic nerve fibres in rat

Neuronal nitric oxide synthase (nNOS)-derived nitric oxide (NO) plays a major role in the neural control of circulation and in many cardiovascular diseases. However, the exact mechanism of how NO regulates these processes is still not fully understood. This study was designed to determine the possible sources of nitrergic nerve fibres supplying the heart attempting to imply their role in the cardiac neural control. Sections of medulla oblongata, vagal nerve, its rootlets and nodose ganglia, vagal cardiac branches, Th₁ -Th₅ spinal cord segments, dorsal root ganglia of C₈ -Th₅ spinal nerves, and stellate ganglia from 28 Wistar rats were examined applying double immunohistochemical staining for nNOS combined with choline acetyltransferase (ChAT), peripherin, substance P, calcitonin gene-related peptide, tyrosine hydroxylase or myelin basic protein. Our findings show that the most abundant population of purely nNOS-immunoreactive (IR) neuronal somata (NS) was observed in the nodose ganglia (37.4 ± 1.3%). A high number of nitrergic NFs spread along the vagal nerve and entered its cardiac branches. All nitrergic neuronal somata (NS) in the nucleus ambiguus were simultaneously immunoreactive (IR) to ChAT and composed only a small subset of neurons (6%). In the dorsal nucleus of vagal nerve, biphenotypic nNOS-IR/ChAT-IR neurons composed 7.0 ± 1.0%, while small purely nNOS-IR neurons were scarce. Nitrergic NS were plentifully distributed within the nuclei of solitary tract. In the examined dorsal root and stellate ganglia, a few nitrergic NS were sporadically present. The majority of sympathetic NS in the intermediolateral nucleus were simultaneously immunoreactive for nNOS and ChAT. In conclusion, an abundant population of nitrergic NS in the nodose ganglion implies that neuronal NO is involved in afferent cardiac innervation. Nevertheless, nNOS-IR neurons identified within vagal nuclei may play a role in the transmission of preganglionic parasympathetic nerve impulses.

PACAP expression in explant cultured mouse major pelvic ganglia

The major pelvic ganglia (MPG) contain both parasympathetic and sympathetic postganglionic neurons and provide much of the autonomic innervation to urogenital organs and components of the lower bowel. Whereas many parasympathetic neurons were found to express vasoactive intestinal polypeptide (VIP), no MPG neurons exhibited immunoreactivity for pituitary adenylate cyclase-activating polypeptide (PACAP). However, in 3-day cultured MPGs, numerous PACAP-IR cells and nerve fibers were present, and transcript levels for PACAP increased significantly. In 3-day cultured MPGs, PACAP immunoreactivity was seen in cells that were also immunoreactive for VIP or neuronal nitric oxide synthase, but not tyrosine hydroxylase, indicating that PACAP expression occurred preferentially in MPG parasympathetic postganglionic neurons. Transcript levels for the VPAC2, but not VPAC1 or PAC1 receptor, also increased significantly following 3 days in culture. Transcript levels of activating transcription factor 3 (ATF-3), a marker of cellular injury, were increased 64-fold in 3-day explants, and ATF-3-IR nuclei were evident in both TH-IR and nNOS-IR neurons as well as in non-neuronal cells. In sum, these results demonstrate that, although only the parasympathetic neurons in explant cultured MPGs increase expression of PACAP, both sympathetic and parasympathetic postganglionic neurons in the cultured MPG whole-mount increase expression of ATF-3.

Localization of multiple neurotransmitters in surgically derived specimens of human atrial ganglia

Dysfunction of the intrinsic cardiac nervous system is implicated in the genesis of atrial and ventricular arrhythmias. While this system has been studied extensively in animal models, far less is known about the intrinsic cardiac nervous system of humans. This study was initiated to anatomically identify neurotransmitters associated with the right atrial ganglionated plexus (RAGP) of the human heart. Biopsies of epicardial fat containing a portion of the RAGP were collected from eight patients during cardiothoracic surgery and processed for immunofluorescent detection of specific neuronal markers. Colocalization of markers was evaluated by confocal microscopy. Most intrinsic cardiac neuronal somata displayed immunoreactivity for the cholinergic marker choline acetyltransferase and the nitrergic marker neuronal nitric oxide synthase. A subpopulation of intrinsic cardiac neurons also stained for noradrenergic markers. While most intrinsic cardiac neurons received cholinergic innervation evident as punctate immunostaining for the high affinity choline transporter, some lacked cholinergic inputs. Moreover, peptidergic, nitrergic, and noradrenergic nerves provided substantial innervation of intrinsic cardiac ganglia. These findings demonstrate that the human RAGP has a complex neurochemical anatomy, which includes the presence of a dual cholinergic/nitrergic phenotype for most of its neurons, the presence of noradrenergic markers in a subpopulation of neurons, and innervation by a host of neurochemically distinct nerves. The putative role of multiple neurotransmitters in controlling intrinsic cardiac neurons and mediating efferent signaling to the heart indicates the possibility of novel therapeutic targets for arrhythmia prevention.

Neurturin suppresses injury-induced neuronal activating transcription factor 3 expression in cultured guinea pig cardiac ganglia

Cultured guinea pig atrial whole mounts containing the intrinsic cardiac ganglia were used as an in vitro model to investigate the induction of the stress/injury marker activating transcription factor 3 (ATF-3). ATF-3 expression was quantified by using immunocytochemical labeling and real-time PCR. In freshly isolated ganglia, no neuronal or Schwann cell nuclei exhibited ATF-3 immunoreactivity. In 2-hour cultures, the induction of ATF-3 expression was evident in many Schwann cell nuclei, whereas no neuronal nuclei were ATF-3 immunoreactive. Beginning at 4 hours, the percentage of neurons with ATF-3-immunoreactive nuclei increased progressively, and, by 48 hours in culture, approximately 95% of the cardiac neurons had ATF-3-immunoreactive nuclei. Neurturin significantly suppressed ATF-3 expression in 48-hour-cultured neurons without effect on ATF-3 expression in Schwann cell nuclei. Neuturin also could reverse neuronal ATF-3 expression after its induction. The suppression of ATF-3 induction by neurturin was mediated by activation of the phosphatidylinositol 3-kinase and mitogen-activated protein kinase pathways. Glial-derived neurotrophic factor (GDNF) also suppressed neuronal ATF-3 induction during culture. However, culture in serum-free media, presence of nerve growth factor, or addition of pituitary adenylate cyclase-activating polypeptide had no effect on ATF-3 induction in the 48-hour-cultured cardiac neurons. By 4 hours in culture, there was a significant increase in ATF-3 transcript levels, and neurturin partially suppressed ATF-3 transcript levels in 48-hour cultures. It is proposed that the loss of target-derived neurturin is a potential mechanism stimulating injury-induced expression of ATF-3 in cardiac neurons.

Neurochemical diversity of afferent neurons that transduce sensory signals from dog ventricular myocardium

While much is known about the influence of ventricular afferent neurons on cardiovascular function in the dog, identification of the neurochemicals transmitting cardiac afferent signals to central neurons is lacking. Accordingly, we identified ventricular afferent neurons in canine dorsal root ganglia (DRG) and nodose ganglia by retrograde labeling after injecting horseradish peroxidase (HRP) into the anterior right and left ventricles. Primary antibodies from three host species were used in immunohistochemical experiments to simultaneously evaluate afferent somata for the presence of HRP and markers for two neurotransmitters. Only a small percentage (2%) of afferent somata were labeled with HRP. About half of the HRP-identified ventricular afferent neurons in T(3) DRG also stained for substance P (SP), calcitonin gene-related peptide (CGRP), or neuronal nitric oxide synthase (nNOS), either alone or with two markers colocalized. Ventricular afferent neurons and the general population of T(3) DRG neurons showed the same labeling profiles; CGRP (alone or colocalized with SP) being the most common (30-40% of ventricular afferent somata in T(3) DRG). About 30% of the ventricular afferent neurons in T(2) DRG displayed CGRP immunoreactivity and binding of the putative nociceptive marker IB(4). Ventricular afferent neurons of the nodose ganglia were distinct from those in the DRG by having smaller size and lacking immunoreactivity for SP, CGRP, and nNOS. These findings suggest that ventricular sensory information is transferred to the central nervous system by relatively small populations of vagal and spinal afferent neurons and that spinal afferents use a variety of neurotransmitters.

Autonomic control and innervation of the atrioventricular junctional pacemaker

BACKGROUND

The main physiologic function of the AV junction is control of timing between atrial and ventricular excitation. However, under pathologic conditions, the AV junction may become the pacemaker of the heart. Unlike the well-characterized sinoatrial node (SAN), autonomic control of the AV junctional pacemaker has not been studied.

OBJECTIVE

The purpose of this study was to characterize the autonomic control and innervation of the AV junctional pacemaker.

METHODS

The response of rabbit AV junctional pacemaker to autonomic stimulation was investigated using optical mapping, autonomic modulation via subthreshold stimulation (n = 12), and quantitative immunohistochemistry (n = 5), and the density of parasympathetic and sympathetic innervation in optically mapped preparations was quantified.

RESULTS

Subthreshold stimulation applied adjacent to the conduction system in the triangle of Koch autonomically modulates the junctional rate, and parasympathetic and sympathetic components can be separated with atropine and the beta-blocker nadolol. Subthreshold stimulation increased the rate maximally to 2.1 +/- 0.4 times when applied with atropine. Unlike the SAN pacemaker, which shifts significantly in response to autonomic stimulation, the AV junctional pacemaker remains stationary (most often in the inferior nodal extension), moving in only 5% of subthreshold stimulation trials. Staining with tyrosine hydroxylase and choline acetyltransferase revealed heterogeneous innervation within the AV junction.

CONCLUSION

AV junctional rhythm can be autonomically modulated with subthreshold stimulation to produce junctional rates of 145 +/- 16 bpm (cycle length 412 +/- 29 ms), similar to sinus rates in rabbit. Unlike the SAN, the anatomic location of the AV junctional pacemaker is stable during autonomic modulation.

NO Differentially Regulates Neurotransmission to Premotor Cardiac Vagal Neurons in the Nucleus Ambiguus

NO is involved in the neural control of heart rate, and NO synthase expressing neurons and terminals have been localized in the nucleus ambiguus where parasympathetic cardiac vagal preganglionic neurons are located; however, little is known about the mechanisms by which NO alters the activity of premotor cardiac vagal neurons. This study examines whether the NO donor sodium nitroprusside ([SNP] 100 μmol/L) and precursor, l -arginine (10 mmol/L), modulate excitatory and inhibitory synaptic neurotransmission to cardiac vagal preganglionic neurons. Glutamatergic, GABAergic, and glycinergic activity to cardiac vagal neurons was examined using whole-cell patch-clamp recordings in an in vitro brain slice preparation in rats. Both SNP, as well as l -arginine, increased the frequency of GABAergic neurotransmission to cardiac vagal preganglionic neurons but decreased the amplitude of GABAergic inhibitory postsynaptic currents. In contrast, both l -arginine and SNP inhibited the frequency of glutamatergic and glycinergic synaptic events in cardiac vagal preganglionic neurons. SNP and l -arginine also decreased glycinergic inhibitory postsynaptic current amplitude, and this response persisted in the presence of tetrodotoxin. Inclusion of the NO synthase inhibitor 7-nitroindazole (100 μmol/L) prevented the l -arginine–evoked responses. These results demonstrate that NO differentially regulates excitatory and inhibitory neurotransmission, facilitating GABAergic and diminishing glutamatergic and glycinergic neurotransmission to cardiac vagal neurons.

Regional cardiac ganglia projections in the guinea pig heart studied by postmortem DiI tracing

Our purpose was to identify and localize intrinsic cardiac ganglia innervating distinct regions of the heart using postmortem tracing of nerve projections with DiI, a method not previously used to study the intrinsic cardiac nervous system. We also investigated the possibility of collateral innervation of myocardium and intrinsic ganglia. In isolated paraformaldehyde-fixed guinea pig hearts, crystals of DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) were inserted into the posterior ventricular myocardium below the atrioventricular groove, the right atrium, or the left ventricular septum. Hearts were placed in the dark at 37 degrees C for 2-14 weeks to allow DiI diffusion within neuronal membranes. Labeled neurons were observed in intracardiac ganglia after at least 4 weeks of dye exposure. Labeling was restricted to the inferior-most ganglia (those near the atrioventricular groove) when DiI was inserted into the posterior ventricular myocardium and to ganglia near the sinus node after right atrial DiI placement. Application of DiI to the left ventricular septum resulted in neuron labeling in ganglia primarily in the interatrial septum near the atrioventricular node. After 8 weeks, DiI-labeled nerve fibers and varicosities were seen surrounding unlabeled neurons in some ganglia, suggesting that axons terminating in or passing through the DiI application site in posterior ventricular tissue had collateral branches innervating these ganglia. These results indicate that intrinsic innervation of major cardiac subdivisions is accomplished by regionally segregated cardiac ganglia. Also, tracing with DiI has provided evidence for collateral nerve projections that could be the substrate for novel intracardiac regulatory circuits.

Presence and co-localization of vasoactive intestinal polypeptide with neuronal nitric oxide synthase in cells and nerve fibers within guinea pig intrinsic cardiac ganglia and cardiac tissue

The presence of vasoactive intestinal polypeptide (VIP) has been analyzed in fibers and neurons within the guinea pig intrinsic cardiac ganglia and in fibers innervating cardiac tissues. In whole-mount preparations, VIP-immunoreactive (IR) fibers were present in about 70% of the cardiac ganglia. VIP was co-localized with neuronal nitric oxide synthase (nNOS) in fibers innervating the intrinsic ganglia but was not present in fibers immunoreactive for pituitary adenylate cyclase-activating polypeptide, choline acetyltransferase (ChAT), tyrosine hydroxylase, or substance P. A small number of the intrinsic ChAT-IR cardiac ganglia neurons (approximately 3%) exhibited VIP immunoreactivity. These few VIP-IR cardiac neurons also exhibited nNOS immunoreactivity. After explant culture for 72 h, the intraganglionic VIP-IR fibers degenerated, indicating that they were axons of neurons located outside the heart. In cardiac tissue sections, VIP-IR fibers were present primarily in the atria and in perivascular connective tissue, with the overall abundance being low. VIP-IR fibers were notably sparse in the sinus node and conducting system and generally absent in the ventricular myocardium. Virtually all VIP-IR fibers in tissue sections exhibited immunoreactivity to nNOS. A few VIP-IR fibers, primarily those located within the atrial myocardium, were immunoreactive for both nNOS and ChAT indicating they were derived from intrinsic cardiac neurons. We suggest that, in the guinea pig, the majority of intraganglionic and cardiac tissue VIP-IR fibers originate outside of the heart. These extrinsic VIP-IR fibers are also immunoreactive for nNOS and therefore most likely are a component of the afferent fibers derived from the vagal sensory ganglia.

Architecture and age-related analysis of the neuronal number of the guinea pig intrinsic cardiac nerve plexus

The aims of the present study have been to determine the architecture of the guinea pig intrinsic cardiac nerve plexus (ICNP) and to test whether or not the heart of this species undergoes decrease in neuronal number with aging. Nine young (3-4 weeks of age) and nine adult (18-24 months of age) animals were examined employing histochemistry for acetylcholinesterase to reveal the ICNP in total hearts. The number of intracardiac neurons in seven animals was assessed via counting of the nerve cells both on total hearts and in serial sections of the atrial walls. The intracardiac neurons from adult guinea pigs were amassed within 329 +/- 15 ganglia. The hearts of young animals contained significantly fewer ganglia, only 211 +/- 27. In adult guinea pigs approximately 60% of the intracardiac neurons were distributed within ganglia of not more than 20 neurons, but the ganglia of such size accumulated only 45% of the neurons in young animals. The total number of the intracardiac neurons estimated per guinea pig heart was 2321 +/- 215, and this number did not differ significantly between young and adult animals. The nerves entering the guinea pig heart were found both in the arterial and venous part of the heart hilum. The nerves from the arterial part of the heart hilum proceeded into the ventricles, but the nerves from the venous part of the hilum formed a nerve plexus of the cardiac hilum located on the heart base. Within the guinea pig epicardium, intrinsic nerves divided into six routes and proceeded to separate atrial, ventricular and septal regions. In conclusion, findings of this study contradict the age-related decrease of the neuronal number in the guinea pig heart and illustrate the remarkable similarity in the architecture of the intracardiac nerve plexuses between guinea pig and rat.

Co-expression pattern of neuronal nitric oxide synthase and two variants of choline acetyltransferase in myenteric neurons of porcine ileum

Cholinergic enteric neurons were demonstrated immunohistochemically so far by using antibodies staining the common choline acetyltransferase (cChAT) in neurons of the central nervous system. The results of staining in the enteric nervous system of various species were, however, not satisfactory. We describe here findings obtained with a newly raised antibody against a peripheral variant of choline acetyltransferase (pChAT) in myenteric neurons of the pig small intestine. Triple labelling for pChAT/cChAT/neuronal nitric oxide synthase (nNOS) revealed 19.7% of 1664 neurons (within 40 ganglia) to be immunoreactive exclusively for pChAT whereas 29.6% were positive for cChAT alone and 18.8% were reactive only for nNOS. Colocalization of pChAT and cChAT was found in 22.4%, of pChAT and nNOS in 8.1% and of cChAT and nNOS in 1.4%. All three markers were simultaneously found in only 1 of 1664 neurons. To investigate the presence and possible colocalization of the above markers within morphologically defined neuron types, triple labelling of cChAT or nNOS with pChAT and a neurofilament (NF) antibody pool was applied and the coexpression patterns of pChAT and cChAT as well as of pChAT and nNOS in 120 neurons of each type were recorded. All type I, II, IV and V neurons displayed immunoreactivity either for one or both cholinergic markers. These neuron types were considered to be cholinergic. All type VI neurons, a descending neuron population, were negative for cChAT but positive for nNOS. However, 95% were immunoreactive for both pChAT and nNOS. The physiological significance of the possible co-existence of acetylcholine and nitric oxide within type VI neurons remains to be clarified. It is concluded that the pChAT and cChAT antibodies used here recognize partly different populations of enteric neurons in the pig. Thus, for total immunohistochemical characterization of cholinergic enteric neurons both forms of choline acetyltransferase have to be considered.

Targeting neuronal nitric oxide synthase with gene transfer to modulate cardiac autonomic function

Microdomains of neuronal nitric oxide synthase (nNOS) are spatially localised within both autonomic neurons innervating the heart and post-junctional myocytes. This review examines the use of gene transfer to investigate the role of nNOS in cardiac autonomic control. Furthermore, it explores techniques that may be used to improve upon gene delivery to the cardiac autonomic nervous system, potentially allowing more specific delivery of genes to the target neurons/myocytes. This may involve modification of the tropism of the adenoviral vector, or the use of alternative viral and non-viral gene delivery mechanisms to minimise potential immune responses in the host. Here we show that adenoviral vectors provide an efficient method of gene delivery to cardiac-neural tissue. Functionally, adenovirus-nNOS can increase cardiac vagal responsiveness by facilitating cholinergic neurotransmission and decrease beta-adrenergic excitability. Whether gene transfer remains the preferred strategy for targeting cardiac autonomic impairment will depend on site-specific promoters eliciting sustained gene expression that results in restoration of physiological function.

Pituitary adenylate cyclase activating polypeptide (PACAP) decreases neuronal somatostatin immunoreactivity in cultured guinea-pig parasympathetic cardiac ganglia

Postganglionic parasympathetic neurons in guinea-pig cardiac ganglia exhibit choline acetyltransferase (ChAT)-immunoreactivity, and a large fraction (60%) of the ChAT-positive cardiac neurons co-express somatostatin-immunoreactivity. This co-expression remained when the cardiac ganglia explants were maintained in culture for 72 h (40% somatostatin-immunoreactive). The guinea-pig cardiac ganglia neurons express the high affinity pituitary adenylate cyclase activating polypeptide (PACAP)-selective PAC1 receptor, and treatment of the ganglia explants with 20 nM PACAP27 for 72 h to evaluate PACAP regulation of somatostatin expression revealed a dramatic 85% decrease in the number of somatostatin-IR neurons (6% somatostatin-IR neurons) compared with untreated control explant preparations. The decrease in percentage of somatostatin-IR neurons by PACAP27 was time- and concentration-dependent, and selective for PACAP27; PACAP38 and vasoactive intestinal polypeptide were less effective. PACAP6-38, a PACAP antagonist, eliminated the PACAP27-induced change in somatostatin positive neurons. The PACAP-mediated decrease in somatostatin-IR neurons was eliminated in calcium-deficient solutions and by the addition of nifedipine, indicating a requirement for calcium influx through L-type calcium channels. The addition of either the calmodulin inhibitor N-(4-aminobutyl)-1-naphthalenesulfonamide or the MEK inhibitor PD98059, also eliminated the PACAP27-induced decrease in somatostatin-IR cells. The PACAP27-mediated effect on somatostatin expression was not affected by inhibitors of protein kinase A or phospholipase C, but was reduced by the adenylyl cyclase inhibitor SQ22356, suggesting cAMP involvement. Semiquantitative and quantitative reverse transcription PCR prosomatostatin transcript measurements showed that cardiac ganglia prosomatostatin mRNA levels were not diminished by chronic PACAP27 exposure despite the dramatic decrement in somatostatin-expressing neurons. Neuronal peptide-IR content represents a balance between production and secretion. These results suggested that one of the primary effects of PACAP exposure may be enhanced levels of neuropeptide release that exceeded production levels, resulting in somatostatin depletion and a decrement in the number of identifiable somatostatin-expressing cardiac neurons.

Subcellular localization of neuronal nitric oxide synthase in the rat nucleus of the solitary tract in relation to vagal afferent inputs

In the nucleus of the solitary tract (NTS), nitric oxide (NO) modulates neuronal circuits controlling autonomic functions. A proposed source of this NO is via nitric oxide synthase (NOS) present in vagal afferent fibre terminals, which convey visceral afferent information to the NTS. Here, we first determined with electron microscopy that neuronal NOS (nNOS) is present in both presynaptic and postsynaptic structures in the NTS. To examine the relationship of nNOS to vagal afferent fibres the anterograde tracer biotinylated dextran amine was injected into the nodose ganglion and detected in brainstem sections using peroxidase-based methods. nNOS was subsequently visualised using a pre-embedding immunogold procedure. Ultrastructural examination revealed nNOS immunoreactivity in dendrites receiving vagal afferent input. However, although nNOS-immunoreactive terminals were frequently evident in the NTS, none were vagal afferent in origin. Dual immunofluorescence also confirmed lack of co-localisation. Nevertheless, nNOS immunoreactivity was observed in vagal afferent neurone cell bodies of the nodose ganglion. To determine if these labelled cells in the nodose ganglion were indeed vagal afferent neurones nodose ganglion sections were immunostained following application of cholera toxin B subunit to the heart. Whilst some cardiac-innervating neurones were also nNOS immunoreactive, nNOS was never detected in the central terminals of these neurones. These data show that nNOS is present in the NTS in both pre- and postsynaptic structures. However, these presynaptic structures are unlikely to be of vagal afferent origin. The lack of nNOS in vagal afferent terminals in the NTS, yet the presence in some vagal afferent cell bodies, suggests it is selectively targeted to specific regions of the same neurones.

The pharmacology of nitric oxide in the peripheral nervous system of blood vessels

Unanticipated, novel hypothesis on nitric oxide (NO) radical, an inorganic, labile, gaseous molecule, as a neurotransmitter first appeared in late 1989 and into the early 1990s, and solid evidences supporting this idea have been accumulated during the last decade of the 20th century. The discovery of nitrergic innervation of vascular smooth muscle has led to a new understanding of the neurogenic control of vascular function. Physiological roles of the nitrergic nerve in vascular smooth muscle include the dominant vasodilator control of cerebral and ocular arteries, the reciprocal regulation with the adrenergic vasoconstrictor nerve in other arteries and veins, and in the initiation and maintenance of penile erection in association with smooth muscle relaxation of the corpus cavernosum. The discovery of autonomic efferent nerves in which NO plays key roles as a neurotransmitter in blood vessels, the physiological roles of this nerve in the control of smooth muscle tone of the artery, vein, and corpus cavernosum, and pharmacological and pathological implications of neurogenic NO have been reviewed. This nerve is a postganglionic parasympathetic nerve. Mechanical responses to stimulation of the nerve, mainly mediated by NO, clearly differ from those to cholinergic nerve stimulation. The naming "nitrergic or nitroxidergic" is therefore proposed to avoid confusion of the term "cholinergic nerve", from which acetylcholine is released as a major neurotransmitter. By establishing functional roles of nitrergic, cholinergic, adrenergic, and other autonomic efferent nerves in the regulation of vascular tone and the interactions of these nerves in vivo, especially in humans, progress in the understanding of cardiovascular dysfunctions and the development of pharmacotherapeutic strategies would be expected in the future.

Innervation of guinea-pig stellate ganglia by nitric oxide synthase, cocaine- and amphetamine-regulated transcript protein- and pituitary adenylate cyclase activating polypeptide-immunoreactive fibers

The present study analyzed using immunohistochemical labeling the distribution and co-localization of nitric oxide synthase (NOS), cocaine- and amphetamine-regulated transcript peptide (CARTp) and pituitary adenylate cyclase activating polypeptide (PACAP) with choline acetyltransferase (ChAT)-immunoreactive fibers in the guinea-pig stellate ganglia. ChAT-immunoreactive fibers make pericellular baskets around virtually all stellate ganglia neurons. Pericellular baskets of NOS, CARTp and PACAP fibers were also present around numerous stellate ganglia neurons. Although all the NOS and PACAP fibers also exhibited ChAT immunoreactivity, only some of the CARTp fibers were ChAT-immunoreactive. No evidence of co-localization of NOS, PACAP and CARTp was obtained.These results indicate that NOS, PACAP and CARTp are present in distinct preganglionic axons innervating the guinea-pig stellate ganglia.

TRPC6 immunoreactivity is colocalized with neuronal nitric oxide synthase in extrinsic fibers innervating guinea pig intrinsic cardiac ganglia

Tachykinins depolarize guinea pig intracardiac neurons by activating nonselective cationic channels. Recently, members of the transient receptor potential family of membrane channels (TRPC) have been implicated in the generation of G protein-coupled receptor-activated nonselective cationic currents. We have investigated whether guinea pig cardiac neurons exhibit immunoreactivity to TRPC. Our results showed that nerve fibers within guinea pig intrinsic cardiac ganglia exhibited immunoreactivity to TRPC6. After culture of cardiac ganglia whole-mount explants for 72 hours, the TRPC6-IR fiber networks were absent. Therefore, the TRPC6-IR fibers were derived from sources extrinsic to the heart. A small percentage ( approximately 3%) of intracardiac neurons also exhibited TRPC6 immunoreactivity in control preparations, and the percentage of cells exhibiting TRPC6 immunoreactivity was not changed following explant culture for 72 hours. The few intrinsic TRPC6-IR neurons also exhibited nitric oxide synthase (NOS) immunoreactivity, indicating that they were nitrergic as well. We compared the immunohistochemical staining patterns of TRPC6-IR fibers with the staining patterns of a number of other neurotransmitters or neurotransmitter synthetic enzymes that mark specific extrinsic inputs to the intrinsic cardiac ganglia. The TRPC6-IR fibers were not immunoreactive for choline acetyltransferase, tyrosine hydroxylase, or substance P. However, the TRPC6-IR fibers exhibited immunoreactivity to neuronal NOS. Therefore, we propose that the TRPC6-IR fibers within the guinea pig intrinsic cardiac ganglia are vagal sensory fibers that also contain NOS. We found, in support of this conclusion, that TRPC6-IR cells were also present in sections of nodose ganglia.

Distribution of cocaine- and amphetamine-regulated transcript peptide in the guinea pig intrinsic cardiac nervous system and colocalization with neuropeptides or transmitter synthetic enzymes

This study was conducted to establish the presence of cocaine- and amphetamine-regulated transcript peptide (CARTp) immunoreactivity in neurons and fibers within guinea pig atrial whole-mount preparations containing the intrinsic cardiac ganglia. Many cardiac ganglia, but not all, in a given whole-mount preparation, were innervated by CARTp-immunoreactive (IR) fibers. Following explant culture of whole mounts for 72 hours, the CARTp-IR fiber networks were absent, but the number of CARTp-IR neurons was increased markedly. These observations suggested that the majority of the CARTp-IR fibers in the intracardiac ganglia were derived from sources extrinsic to the heart. In control whole-mount preparations, very few CARTp-positive neurons were present. The few intrinsic CARTp-IR neurons also exhibited choline acetyltransferase (ChAT) immunoreactivity, indicating that they make up a small subpopulation of cholinergic postganglionic neurons. Some CARTp-IR neurons also exhibited nitric oxide synthase (NOS) immunoreactivity, indicating that they were nitrergic as well. We compared the immunohistochemical staining patterns of CARTp-IR fibers with the staining patterns of a number of other neurotransmitters or neurotransmitter synthetic enzymes that mark specific extrinsic inputs. The CARTp-IR fibers were not immunoreactive for ChAT, tyrosine hydroxylase, calcitonin gene-related peptide, or substance P. However, virtually all CARTp-IR fibers exhibited immunoreactivity to neuronal NOS (a marker for nitric oxide-producing neurons). CARTp-IR cells and NOS-IR cells were present in the nodose ganglia. In addition, CARTp-IR neurons in the nodose also were stained positively for NADPH-diaphorase. Thus, we propose that most CARTp-IR fibers within the guinea pig intrinsic cardiac ganglia are vagal afferent fibers that also contain NOS.

The intrinsic origin of nitric oxide synthase immunoreactive nerve fibers in the right atrium of the guinea pig

We previously reported three kinds of nitric oxide synthase-immunoreactive (NOS-ir) axons in the guinea pig heart: the sparse fiber network covering the right atrium, the basket-like endings around intracardiac neurons, and the axons in the septal region. The sparse NOS-ir nerve fiber network in the right atrium remained after vagotomy and has been suggested to be originated from intrinsic cardiac ganglia. Using Chorera toxin B as a retrograde tracer, we determined a part of them were derived from cardiac ganglionic neurons located in the area near the vena cavae.