Immunohistochemistry of small intensely fluorescent (SIF) cells and of SIF cell-associated nerve fibers in the rat superior cervical ganglion

Identification of glucocorticoid receptors as potential modulators of parasympathetic and sympathetic neurons within rat intracardiac ganglia

Background

Emerging evidences indicate that glucocorticoid receptors (GR) play a regulatory role in cardiac function, particularly with regard to the autonomic nervous system. Therefore, this study aimed to demonstrate the expression and the precise anatomical location of GR in relation to the parasympathetic and sympathetic innervations of the heart.

Methods

The present study used tissue samples from rat heart atria to perform conventional reverse-transcriptase polymerase chain reaction (RT-PCR), Western blot, and double immunofluorescence confocal analysis of GR with the neuronal markers vesicular acetylcholine transporter (VAChT), tyrosine hydroxylase (TH), calcitonin gene-related peptide (CGRP) as well as the mineralocorticoid receptor (MR).

Results

Double immunofluorescence labeling revealed that GRs were co-expressed with VAChT in parasympathetic principal neuronal somata and nerve terminals innervating atrium. Also, GR colocalized with the sympathetic neuronal marker TH in a cluster of small intensely fluorescent (SIF) cells, on intracardiac nerve terminals and in the atrial myocardium. GR immunoreactivity was scarcely identified on CGRP-immunoreactive sensory nerve terminals. Approximately 20% of GR immunoreactive neuronal somata co-localized with MR. Finally, conventional RT-PCR and Western blot confirmed the presence of GR and MR in rat heart atria.

Conclusion

This study provides evidence for the existence of GR predominantly on cardiac parasympathetic neurons and TH-immunoreactive SIF cells suggesting a functional role of cardiac GR on cardiovascular function by modulation of the cardiac autonomic nervous system.

Identification of Mineralocorticoid Receptors, Aldosterone, and Its Processing Enzyme CYP11B2 on Parasympathetic and Sympathetic Neurons in Rat Intracardiac Ganglia

Recent interest has focused on the mineralocorticoid receptor (MR) and its impact on the myocardium and the performance of the heart. However, there is a lack of evidence about MR expression and its endogenous ligand aldosterone synthesis with specific regard to the intrinsic cardiac nervous system. Therefore, we looked for evidence of MR and aldosterone in sympathetic and parasympathetic neurons of intracardiac ganglia. Tissue samples from rat heart atria were subjected to conventional reverse-transcriptase polymerase chain reaction (PCR), Western blot, and double immunofluorescence confocal analysis of MR, corticosterone-inactivating enzyme 11β-hydroxysteroid-dehydrogenase-2 (11β-HSD2), aldosterone, and its processing enzyme CYP11B2 together with the neuronal markers vesicular acetylcholine transporter (VAChT) and tyrosine hydroxylase (TH). Our results demonstrated MR, 11β-HSD2, and CYP11B2 specific mRNA and protein bands in rat heart atria. Double immunofluorescence labeling revealed coexpression of MR immunoreactivity with VAChT in large diameter parasympathetic principal neurons. In addition, MR immunoreactivity was identified in TH-immunoreactive small intensely fluorescent (SIF) cells and in nearby VAChT- and TH-immunoreactive nerve terminals. Interestingly, the aldosterone and its synthesizing enzyme CYP11B2 and 11β-HSD2 colocalized in MR– immunoreactive neurons of intracardiac ganglia. Overall, this study provides first evidence for the existence of not only local expression of MR, but also of 11β-HSD2 and aldosterone with its processing enzyme CYP11B2 in the neurons of the cardiac autonomic nervous system, suggesting a possible modulatory role of the mineralocorticoid system on the endogenous neuronal activity on heart performance.

Developmental expression of δ-opioid receptors during maturation of the parasympathetic, sympathetic, and sensory innervations of the neonatal heart: early targets for opioid regulation of autonomic control

Evidence is accumulating regarding the local opioid regulation of heart function. However, the exact anatomical location of δ-opioid receptors (DORs) and expression during maturation of the autonomic and sensory innervations of the neonatal heart is unknown. Therefore, we aimed to characterize target sites for opioids in neonatal rat heart intracardiac ganglia at postnatal day (P)1, P7 and adulthood (P56-P84). Rat heart atria were subjected to reverse-transcriptase polymerase chain reaction, Western blot, radioligand binding, and immunofluorescence confocal analysis of DORs with the neuronal markers vesicular acetylcholine transporter (VAChT), tyrosine hydroxylase (TH), calcitonin gene-related peptide (CGRP), and substance P (SP). Our results demonstrated DOR mRNA, protein, and binding sites that gradually increased from P1 toward adulthood. Immunofluorescence confocal microscopy showed DOR co-localized with VAChT in large-diameter principal neurons, TH-immunoreactive (IR) small intensely fluorescent (SIF) catecholaminergic cells, and CGRP- or SP-IR afferent nerve terminals arborizing within intracardiac ganglia and atrial myocardium. Co-expression of DOR with VAChT-IR neurons was observed from the first day of birth (P1). In contrast, DORs on TH-IR SIF cells or CGRP-IR fibers were not observed in intracardiac ganglia of P1, but rather in P7 rats. The density of nerve fibers in atrial myocardium co-expressing DORs with different neuronal markers increased from neonatal age toward adulthood. In summary, the enhanced DOR expression parallel to the maturation of cardiac parasympathetic, sympathetic, and sensory innervation of the heart suggests that the cardiac opioid system is an important regulator of neonatal and adult heart function through the autonomic nervous system.

Identification of mu- and kappa-opioid receptors as potential targets to regulate parasympathetic, sympathetic, and sensory neurons within rat intracardiac ganglia

Recent interest has been focused on the opioid regulation of heart performance; however, specific allocation of opioid receptors to the parasympathetic, sympathetic, and sensory innervations of the heart is scarce. Therefore, the present study aimed to characterize such specific target sites for opioids in intracardiac ganglia, which act as a complex network for the integration of the heart's neuronal in- and output. Tissue samples from rat heart atria were subjected to RT-PCR, Western blot, radioligand-binding, and double immunofluorescence confocal analysis of mu (M)- and kappa (K)-opioid receptors (ORs) with the neuronal markers vesicular acetylcholine transporter (VAChT), tyrosine hydroxylase (TH), calcitonin gene-related peptide (CGRP), and substance P (SP). Our results demonstrated MOR- and KOR-specific mRNA, receptor protein, and selective membrane ligand binding. By using immunofluorescence confocal microscopy, MOR and KOR immunoreactivity were colocalized with VAChT in large-diameter parasympathetic principal neurons, with TH-immunoreactive small intensely fluorescent (SIF) cells, and on nearby TH-IR varicose terminals. In addition, MOR and KOR immunoreactivity were identified on CGRP- and SP-IR sensory neurons throughout intracardiac ganglia and atrial myocardium. Our findings show that MOR and KOR are expressed as mRNA and translated into specific receptor proteins on cardiac parasympathetic, sympathetic, and sensory neurons as potential binding sites for opioids. Thus, they may well play a role within the complex network for the integration of the heart's neuronal in- and output.

Fos expression in rat celiac ganglion: an index of the activation of postganglionic sympathetic nerves

To develop an index of the activation of abdominal sympathetic nerves, we used Fos immunostaining of the celiac ganglion (CG) taken from rats receiving nicotine, preganglionic nerve stimulation, or glucopenic agents. Subcutaneous nicotine injection moderately increased Fos expression in the principal ganglionic cells of the CG (17 +/- 4 Fos+ per mm(2), approximately 12% of all principal CG cells), whereas subcutaneous saline had no effect (0 +/- 0 Fos+ per mm(2); n = 7; P < 0.01). Greater Fos expression was obtained by applying nicotine topically to the CG (71 +/- 8 Fos+ per mm(2); 52% of all principal CG cells, n = 5; P < 0.01 vs. topical saline, n = 4) and by preganglionic nerve stimulation (126 +/- 9 Fos+ per mm(2); 94% of all principal CG cells, n = 11; P < 0.01 vs. nerve isolation, n = 7). Moderate Fos expression was also observed in the CG after intraperitoneal 2-deoxy-D-glucose (2DG) injection (21 +/- 2 Fos+ per mm(2); 16% of all principal CG cells, n = 5; P < 0.01 vs. saline ip) or insulin injection (16 +/- 2 Fos+ per mm(2); 12% of all principal CG cells, n = 6; P < 0.01 vs. saline ip). Furthermore, Fos expression induced by 2DG was dose and time dependent. These data demonstrate significant Fos expression in the CG in response to chemical, electrical, and reflexive stimulation. Thus Fos expression in the CG may be a useful index to describe various levels of activation of its postganglionic sympathetic neurons.

Ultrastructural localisation of NADPH-d/nNOS expression in the superior cervical ganglion of the hamster

This study examined NADPH-d and nNOS expression in the SCG of hamsters. By light microscopy, numerous NADPH-d/NOS positive processes were widely distributed in the ganglion. Ultrastructurally, the NADPH-d reaction product was associated with the membranous organelles of neuronal soma, dendrites, myelinated fibres, small granular cells, and axon profiles bearing agranular vesicles. The NOS immunoreaction product, on the other hand, was localised in the cytoplasm of principal neurons and dendrites. Some of the NADPH-d/NOS labelled processes formed junctional contacts including synapses or zonulae adherentia. Compared with the neurons, the nonneuronal cells in the ganglion, namely, macrophages, satellite cells and endothelial cells were labelled by NADPH-d but devoid of nNOS immunoreaction product. The results suggest that the NADPH-d/NOS positive fibres in the SCG originate not only from the projecting fibres of the lateral horns of thoracic spinal cord, but also from the principal neurons and small granular cells; some may represent visceral afferent fibres. Electron microscopic morphometry has shown that about 67% of the principal neurons contain NADPH-d reaction product, and that the majority were small to medium sized neurons based on cross-sectional areas in image analysis. On the basis of the present morphological study, it is concluded NO is produced by some local neurons and possibly some nonneuronal cells in the SCG as well as some fibres of extrinsic origin. In this connection, NO may serve either as a neurotransmitter or neuromodulator.

Secretoneurin: a functional neuropeptide in health and disease

Chromogranins belong to an evolutionarily conserved family of proteins that serve as neuropeptide pro-proteins, besides having other functions. The secretogranin-II-derived peptide secretoneurin is a 33-amino-acid polypeptide generated by proteolytic cleavage at paired dibasic sequences that exerts its effect by binding to specific receptors. Secretoneurin receptors have been kinetically and functionally characterized indicating that they are G-protein linked. Localization of secretoneurin and functional studies have helped to elucidate roles for secretoneurin, ranging from effects in the central nervous system to the modulation of the inflammatory response in the periphery. It has been shown that secretoneurin possesses biologic activities such as stimulation of dopamine release from striatal neurons and activation of monocyte migration, suggesting that the peptide may modulate both neurotransmission and inflammatory response. With an array of actions as diverse as that seen with other sensory neuropeptides, there is scope for numerous studies and therapeutic possibilities.

Plasticity in the phenotypic expression of catecholamines and vasoactive intestinal peptide in adult rat superior cervical and stellate ganglia after long-term hypoxia in vivo

Sympathetic ganglia in the adult rat contain various populations of nerve cells which demonstrate plasticity with respect to their transmitter phenotype. The plasticity of the neuronal cell bodies and of the small intensely fluorescent cells in the superior cervical and stellate ganglia in response to hypoxia in vivo (10% O2 for seven days) was assessed by studying the expression of catecholamines and vasoactive intestinal peptide. The levels of norepinephrine, dopamine, 3,4-dihydroxyphenylacetic acid and vasoactive intestinal peptide immunoreactivity were determined. In addition, the density of the immunohistochemical staining of cells for tyrosine hydroxylase and vasoactive intestinal peptide was evaluated. In the intact superior cervical ganglion, hypoxia increased the dopamine level as well as the density of small intensely fluorescent cells immunolabelled for tyrosine hydroxylase and vasoactive intestinal peptide. In the axotomized ganglion, hypoxia elicited a twofold rise in the level of the vasoactive intestinal peptide as well as enhancing the density of neuronal cell bodies immunostained for this peptide. Thus, the effect of hypoxia on the expression of vasoactive intestinal peptide expression in neurons was dependent on neural interactions. In the intact stellate ganglion, hypoxia alone induced a 1.5-fold increase in the density of neuronal cell bodies immunostained for vasoactive intestinal peptide. Thus, ganglia-specific factors appeared to play a role in determining changes in neuronal phenotype in response to hypoxia. The present study provides evidence for the involvement of dopamine and vasoactive intestinal peptide in ganglionic responses to long-term hypoxia as well as for differential responses by the two ganglionic cell populations, i.e. neuronal cell bodies and small intensely fluorescent cells. Changes in the expression of the vasoactive intestinal peptide during long-term hypoxia may be of energetic, trophic and/or synaptic significance. Hypoxia may be considered to be a vasoactive intestinal peptide-inducing factor in sympathetic ganglia.

Secretoneurin-like immunoreactivity in rat sympathetic, enteric and sensory ganglia

Distribution of secretoneurin-like immunoreactivity (SN-LI) was studied in the rat sympathetic ganglia/adrenal gland, enteric and sensory ganglia by immunohistochemical methods. SN-LI nerve fibers formed basket-like terminals surrounding many of the postganglionic neurons of the superior cervical, stellate, paravertebral chain ganglia, coeliac/superior mesenteric and inferior mesenteric ganglia. Postganglionic neurons of the superior cervical and other sympathetic ganglia exhibited low-to-moderate levels of SN-LI. In all these sympathetic ganglia, clusters of small diameter (< 10 microm) cells, which may correspond to the small intensely fluorescent (SIF) cells, were found to be intensely labeled. Surgical sectioning or ligation of the cervical sympathetic trunk for 7-10 days resulted in a nearly total loss of SN-LI fibers in the superior cervical ganglia, whereas immunoreactivity in the postganglionic neurons and small diameter cells remained essentially unchanged. In the thoracolumbar and sacral segments of the spinal cord, SN-LI nerve fibers were detected in the superficial layers of the dorsal horn as well as in the intermediolateral cell column (ILp). Occasionally, SN-LI somata were noted in the ILp. SN-LI nerve fibers formed a delicate plexus underneath the capsule of the adrenal gland, some of which traversed the adrenal cortex and reached the adrenal medulla. While heavily invested with SN-LI nerve terminals, chromaffin cells seemed to express a low level of SN-LI. In the enteric plexus, varicose SN-LI nerve fibers and terminals formed a pericellular network around many myenteric and submucous ganglion cells; the ganglionic neurons were lightly to moderately labeled. A population of ganglion cells in the dorsal root, nodose and trigeminal ganglia exhibited moderate-to-strong SN-LI. The detection of SN-LI in nerve fibers and somata of various sympathetic ganglia, enteric plexus and adrenal medulla and in somata of the sensory ganglia implies an extensive involvement of this peptide in sympathetic, enteric and sensory signal processing.

Coexistence of calcitonin gene-related peptide and NADPH-diaphorase in the canine superior cervical ganglion

By means of double staining technique of NADPH-diaphorase (NADPH-d) histochemistry and calcitonin gene-related peptide (CGRP) immunohistochemistry, we investigated the coexistence of NADPH-d reactivity and CGRP immunoreactivity in the canine superior cervical ganglion (SCG). Most of NADPH-d reactivity and CGRP immunoreactivity were coexisted in the principal postganglionic neurons. These neurons were distributed throughout the ganglion without specific localization. The present findings suggest the intimate role of CGRP and nitric oxide in postganglionic neurons of the canine SCG.

Muscarinic transmission decreases the number of SIF cells demonstrating catecholamine histofluorescence in rat superior cervical ganglia

Preganglionic electrical stimulation of the cervical sympathetic trunk to the rat superior cervical ganglia produced a mean reduction in the number of visible small intensely fluorescent (SIF) cells demonstrating catecholamine histofluorescence to 32% of the unstimulated contralateral control. The reduction in the number of catecholamine-positive SIF cells required the presence of specific blockers of catecholamine uptake and synthesis and was dependent on normal synaptic transmission. No change in the number of catecholamine-positive SIF cells was observed when ganglionic transmission occurred in solutions containing both hexamethonium and atropine or with atropine alone (97% of the unstimulated control). Furthermore, preganglionic stimulation in the presence of high magnesium/low calcium solutions, which effectively blocked synaptic transmission, prevented the stimulation-induced decrease in the number of catecholamine-positive SIF cells. Prolonged antidromic stimulation of the internal carotid nerve only reduced the number of catecholamine-positive SIF cells to 75% of the unstimulated contralateral control. These results suggest that preganglionic synaptic impulses can induce the release of catecholamines from SIF cells via muscarinic receptor activation. Furthermore, the necessity for pharmacological intervention of uptake and synthesis blockers of catecholamines in order to detect the synaptically-induced reduction in the number of catecholamine-positive SIF cells, suggests that synaptic transmission also modulates the synthesis of catecholamines in SIF cells within the rat superior cervical ganglia.

Localization and axotomy-induced regulation of the peptide secretoneurin in the rat superior cervical ganglion

This study demonstrates the localization and regulation of a novel neuropeptide of 33 amino acids, secretoneurin (SN), in the rat superior cervical ganglion. Gel filtration chromatography of ganglion proteins followed by a specific radioimmunoassay revealed that SN is the predominant cleavage product of secretogranin II, a member of the chromogranin/secretogranin protein family, in adult ganglia. SN was detected within the majority of nerve endings surrounding postganglionic neurons that were identified by the presence of synaptophysin and, in part, colocalized leu-encephalin. Applying immuno-electronmicroscopy, SN was localized to large dense core vesicles of neuronal and small intensely fluorescent (SIF) cells. In situ hybridization revealed the presence of secretogranin II mRNA in postganglionic neurons and, to a lesser extent, in SIF cells. One week after transection of the postganglionic branches SN levels were not significantly altered; however, a decrease of secretogranin II mRNA was observed in postganglionic neurons but not in SIF cells. After decentralization of the ganglion, SN-immunoreactive nerve terminals disappeared and intraganglionic SN levels were reduced by 70%, indicating the preganglionic origin of SN-positive nerve fibres and varicosities. Secretogranin II mRNA was slightly reduced under this condition. Combined axotomy and decentralization further diminished intraganglionic secretogranin II mRNA, although peptide levels increased significantly above control values under these conditions. Double-labelling immunofluorescence with antibodies against the somatodendritic marker microtubule-associated protein 2 (MAP2) revealed that the increase in SN immunoreactivity was due to an accumulation of SN in axonal processes of postganglionic neurons. SN immunoreactivity was also detected in dissociated neonatal superior cervical ganglion cultures and increased significantly upon treatment with nerve growth factor, the survival and differentiation factor of sympathetic neurons during perinatal development. Co-culture with non-neuronal cells or addition of leukaemia inhibitory factor, a cytokine known to stimulate synthesis of various peptides after nerve transection, did not influence SN immunoreactivity. Therefore, since no fixed relationship between SN and any of the known neuropeptides or neurotransmitters expressed in sympathetic neurons was observed, the expression of this novel peptide appears to be independently regulated.

Localization, regulation and functions of neurotransmitters and neuromodulators in cervical sympathetic ganglia

Cervical sympathetic ganglia represent a suitable model for studying the establishment and plasticity of neurochemical organization in the nervous system since sympathetic postganglionic neurons: (1) express several neuromediators, i.e., short acting transmitters, neuropeptide modulators and radicals, in different combinations; (2) receive synaptic input from a limited number of morphologically and neurochemically well-defined neuron populations in the central and peripheral nervous systems (anterograde influence on phenotype); (3) can be classified morphologically and neurochemically by the target they innervate (retrograde influence on phenotype); (4) regenerate readily, making it possible to study changes in neuromediator content after axonal lesion and their possible influence on peripheral nerve regeneration; (5) can be maintained in vitro in order to investigate effects of soluble factors as well as of membrane bound molecules on neuromediator expression; and (6) are easily accessible. Acetylcholine and noradrenaline, as well as neuropeptides and the recently discovered radical, nitric oxide, are discussed with respect to their localization and possible functions in the mammalian superior cervical and cervicothoracic (stellate) paravertebral ganglia. Furthermore, mechanisms regulating transmitter synthesis in sympathetic neurons in vivo and in vitro, such as soluble factors, cell contact or electrical activity, are summarized, since modulation of transmitter synthesis, release and metabolism plays a key role in the neuronal response to environmental influences.

Secretogranin II: molecular properties, regulation of biosynthesis and processing to the neuropeptide secretoneurin

Secretogranin II is an acidic secretory protein in large dense core vesicles of endocrine, neuroendocrine and neuronal tissues. It comprises, together with chromogranins A and B, the class of proteins collectively called chromogranins. In this review the physico-chemical properties, genomic organization, tissue distribution, synthesis regulation, ontogeny and physiological function of this protein are discussed. Secretogranin II gained interest recently for mainly three reasons: (1) secretogranin II is an excellent marker for the regulated secretory pathway due to its simple and specific metabolic labeling by inorganic sulfate; (2) secretogranin II occurs in a variety of neoplasms arising from endocrine and neuroendocrine cells and was shown to be a useful histological tumor marker for these cells; (3) secretogranin II is the precursor of the recently discovered neuropeptide secretoneurin which induces dopamine release in the striatum of the rat brain.