Pathway-specific expression of PKC and PKA in sympathetic neurons

Generating diversity: Mechanisms regulating the differentiation of autonomic neuron phenotypes

Sympathetic and parasympathetic postganglionic neurons innervate a wide range of target tissues. The subpopulation of neurons innervating each target tissue can express unique combinations of neurotransmitters, neuropeptides, ion channels and receptors, which together comprise the chemical phenotype of the neurons. The target-specific chemical phenotype shown by autonomic postganglionic neurons arises during development. In this review, we examine the different mechanisms that generate such a diversity of neuronal phenotypes from the pool of apparently homogenous neural crest progenitor cells that form the sympathetic ganglia. There is evidence that the final chemical phenotype of autonomic postganglionic neurons is generated by both signals at the level of the cell body that trigger cell-autonomous programs, as well as signals from the target tissues they innervate.

Mechanisms of Angiotensin II–Mediated Decreases in Intraneuronal Ca 2+ in Calcium-Loaded Stellate Ganglion Neurons

Our laboratory has reported previously that angiotensin II, type-1 (AT 1 ) receptor stimulation in isolated stellate ganglion neurons decreases intraneuronal calcium concentration ([Ca 2+ ]i) acutely if baseline [Ca 2+ ]i is high and increases [Ca 2+ ]i if baseline [Ca 2+ ]i is low. Part of the angiotensin II (Ang II) effect in high Ca 2+ neurons is mediated through stimulation of Na + –Ca 2+ exchange. Current experiments were conducted to identify additional steps in the signaling pathways. In Ca 2+ -loaded neurons, Ang II–induced decreases in [Ca 2+ ]i were attenuated by phospholipase C inhibition (U73122) or nitric oxide (NO) synthase inhibition ( l -NMMA) and were mimicked by the cGMP analogue 8-Br-cGMP. Protein kinase C (PKC) inhibition (bisindolylmaleimide I or Go6976) and protein kinase G (PKG) inhibition (KT5823) partially blocked Ang II–mediated decreases in [Ca 2+ ]i, but complete blockade of Ang II effects was obtained with combined PKC and PKG inhibition. Modulation of inositol triphosphate (IP 3 )-inducible ER Ca 2+ release by [Ca 2+ ]i was investigated using furaptra, an ER-retaining dye. IP 3 -mediated ER Ca 2+ release in β-escin–permeabilized neurons was measured after clamping of [Ca 2+ ]i from 50 nM to 800 nM. Maximal ER Ca 2+ release was observed at ≈200 nM [Ca 2+ ]i, with noted blunting of release at higher [Ca 2+ ]i. Steady-state mRNA transcript and protein levels revealed that the principal IP 3 R isoform expressed was IP 3 R-II. These results suggest that Ca 2+ loading in stellate ganglion neurons promotes Ang II-mediated decreases in [Ca 2+ ]i via PKC and NO/cGMP/PKG pathways and inhibits IP 3 R-II–mediated ER Ca 2+ release.

Heterogeneous expression of SNAP-25 and synaptic vesicle proteins by central and peripheral inputs to sympathetic neurons

Neurons in prevertebral sympathetic ganglia receive convergent synaptic inputs from peripheral enteric neurons in addition to inputs from spinal preganglionic neurons. Although all inputs are functionally cholinergic, inputs from these two sources have distinctive neurochemical and functional profiles. We used multiple-labeling immunofluorescence, quantitative confocal microscopy, ultrastructural immunocytochemistry, and intracellular electrophysiologic recordings to examine whether populations of inputs to the guinea pig coeliac ganglion express different levels of synaptic proteins that could influence synaptic strength. Boutons of enteric intestinofugal inputs, identified by immunoreactivity to vasoactive intestinal peptide, showed considerable heterogeneity in their immunoreactivity to synaptosome-associated protein of 25 kDa (SNAP-25), synapsin, synaptophysin, choline acetyltransferase, and vesicular acetylcholine transporter. Mean levels of immunoreactivity to these proteins were significantly lower in terminals of intestinofugal inputs compared with terminals of spinal preganglionic inputs. Nevertheless, many boutons with undetectable levels of SNAP-25 immunoreactivity formed morphologically normal synapses with target neurons. Treatment with botulinum neurotoxin type A (20-50 nM for 2 hours in vitro) generated significant cleavage of SNAP-25 and produced similar dose- and time-dependent inhibitions of synaptic transmission from all classes of inputs, regardless of their mean level of SNAP-25 expression. The simplest interpretation of these results is that only synaptic boutons with detectable levels of SNAP-25 immunoreactivity contribute significantly to fast cholinergic transmission. Consequently, the low synaptic strength of intestinofugal inputs to final motor neurons in sympathetic pathways may be due in part to the low proportion of their boutons that express SNAP-25 and other synaptic proteins.

Synaptic density, convergence, and dendritic complexity of prevertebral sympathetic neurons

Prevertebral sympathetic ganglia contain a unique population of final motor neurons receiving convergent synaptic inputs not only from spinal preganglionic neurons, but also from peripheral intestinofugal neurons projecting from the gut. We used quantitative confocal and ultrastructural immunohistochemistry to determine how this increased synaptic convergence is accommodated by sympathetic final motor neurons in the celiac ganglion of guinea pigs. Terminals of intestinofugal neurons were identified by their immunoreactivity to vasoactive intestinal peptide. Stereologic analyses were based on transects and point counts at confocal and ultrastructural levels. The relative amount of dendritic neuropil in the medial regions of the ganglion was approximately 2.5 times greater than in the lateral regions of the ganglion, consistent with the 2 to 3 times difference in average dendritic field size of neurons in these regions. The total numbers of boutons and synaptic profiles showed significant positive correlations with the relative amount of neuropil in a region. However, the overall density of synaptic boutons was twice as high in the medial region of the ganglion compared with the lateral regions. Because the relative density of preganglionic synapses was similar in each region, this difference was due to the selective projection of intestinofugal inputs to neurons in the medial celiac ganglion, where they provided 45% of synaptic contacts. These results show that, compared with vasoconstrictor neurons, sympathetic neurons regulating gastrointestinal activity support a higher number of convergent inputs in two ways: in addition to having larger dendritic fields, they also have a twofold higher density of synapses.

Development of convergent synaptic inputs to subpopulations of autonomic neurons

Visceromotor neurons in mammalian prevertebral sympathetic ganglia receive convergent synaptic inputs from spinal preganglionic neurons and peripheral intestinofugal neurons projecting from the enteric plexuses. Vasomotor neurons in the same ganglia receive only preganglionic inputs. How this pathway-specific pattern of connectivity is established is unknown. We have used a combination of immunohistochemical, ultrastructural, and electrophysiological techniques to investigate the development of synaptic inputs onto visceromotor and vasomotor neurons in the celiac ganglion of guinea pigs. Functional synaptogenesis occurred primarily from early fetal (F30-F35) to midfetal (F36-F45) stages, after the neurochemical differentiation of vasomotor and visceromotor neurons but before establishment of their electrophysiological phenotypes. Intestinofugal inputs were detected only on presumptive visceromotor neurons located primarily in medial regions of the ganglion. The number of ultrastructurally identified synaptic profiles increased in parallel with functional synaptogenesis, especially in medial regions, where dendritic growth rates also were higher. However, the expression of immunoreactivity to choline acetyltransferase in the terminals of inputs was very low until late fetal stages, after functional transmission already had been established. These results show that peripheral intestinofugal neurons directly establish appropriate functional connections with their target visceromotor neurons simultaneously with the development of functional preganglionic inputs to both visceromotor and vasomotor neurons. It seems likely that synaptogenesis occurs independently of the neurochemical differentiation of the target neurons but is closely related to the pathway-specific dendritic development of those neurons.

Development of electrophysiological and morphological diversity in autonomic neurons

The generation of neuronal diversity requires the coordinated development of differential patterns of ion channel expression along with characteristic differences in dendritic geometry, but the relations between these phenotypic features are not well known. We have used a combination of intracellular recordings, morphological analysis of dye-filled neurons, and stereological analysis of immunohistochemically labeled sections to investigate the development of characteristic electrical and morphological properties of functionally distinct populations of sympathetic neurons that project from the celiac ganglion to the splanchnic vasculature or the gastrointestinal tract of guinea pigs. At early fetal stages, neurons were significantly more depolarized at rest compared with neurons at later stages, and they generally fired only a single action potential. By mid fetal stages, rapidly and slowly adapting neurons could be distinguished with a topographic distribution matching that found in adult ganglia. Most rapidly adapting neurons (phasic neurons) at this age had a long afterhyperpolarization (LAH) characteristic of mature vasomotor neurons and were preferentially located in the lateral poles of the ganglion, where most neurons contained neuropeptide Y. Most early and mid fetal neurons showed a weak M current, which was later expressed only by rapidly-adapting and LAH neurons. Two different A currents were present in a subset of early fetal neurons and may indicate neurons destined to develop a slowly adapting phenotype (tonic neurons). The size of neuronal cell bodies increased at a similar rate throughout development regardless of their electrical or neurochemical phenotype or their topographical location. In contrast, the rate of dendritic growth of neurons in medial regions of the ganglion was significantly higher than that of neurons in lateral regions. The apparent cell capacitance was highly correlated with the surface area of the soma but not the dendritic tree of the developing neurons. These results demonstrate that the well-defined functional populations of neurons in the celiac ganglion develop their characteristic electrophysiological and morphological properties during early fetal stages of development. This is after the neuronal populations can be recognized by their neurochemical and topographical characteristics but long before the neurons have finished growing. Our data provide strong circumstantial evidence that the development of the full phenotype of different functional classes of autonomic final motor neurons is a multi-step process likely to involve a regulated sequence of trophic interactions.

Differential Expression of Functionally Identified and Immunohistochemically Identified NK1 Receptors on Sympathetic Neurons

We have used multiple-labeling immunohistochemistry, intracellular dye-filling, and intracellular microelectrode recordings to characterize the distribution of tachykinin receptors and substance P boutons on subpopulations of neurons within the guinea pig celiac ganglion. Superfusion of substance P (SP, 1 μM for 1 min) depolarized 42% of tonic neurons and inhibited afterhyperpolarizations in 66% of long afterhyperpolarizing (LAH) neurons without significant desensitization. Twenty-one percent of tonic neurons and 24% of LAH neurons responded to the NK3 agonist senktide but did not respond to SP, indicating SP did not activate NK3 receptors at this concentration. All effects of SP were abolished by the selective NK1 receptor antagonist, SR140333, but not by the selective NK3 receptor antagonist, SR142801, suggesting that exogenous SP activated a receptor with NK1 pharmacology. No dye-filled LAH neuron and only 50% of tonic neurons responding to SP expressed NK1 receptor immunoreactivity (NK1-IR). All neurons responding to SP had SP immunoreactive fibers within one cell diameter, indicating good spatial matching between SP release sites and target neurons. These results indicate that SP may act via a receptor with NK1-like pharmacology that has a C terminus not recognized by antibodies to the intracellular domain of the conventional NK1 receptor. Inward currents evoked by SP acting on this NK1-like receptor or senktide acting through NK3 receptors had identical current-voltage relationships. In LAH neurons, both agonists suppressed I sAHP without reducing I AHP. Responses evoked by SP and senktide were resistant to PKC inhibitors, suggesting that the transduction mechanisms for the NK1-like receptor and the NK3 receptor may be similar.

Neurochemical differentiation of functionally distinct populations of autonomic neurons

The coeliac ganglion of guinea pigs displays a unique topographical arrangement of neurochemically and functionally distinct populations of sympathetic neurons. The authors used multiple-labeling immunohistochemistry to investigate the neurochemical differentiation of these neurons during embryonic and fetal development. Sympathoadrenal precursors, located on either side of the abdominal aorta, were intensely immunoreactive for tyrosine hydroxylase (TH-IR), neurofilament, and the human natural killer 1 antibody at midembryonic stages (Carnegie stages 16-19). During late embryonic stages (stages 20-23), a single bilobed ganglion had formed. At this time, neuropeptide Y immunoreactivity (NPY-IR) was widely expressed in sympathetic neurons (with moderate TH-IR) and chromaffin cells (with intense TH-IR). The onset of somatostatin (Som-IR) expression followed that of NPY-IR and was restricted to sympathetic neurons. However, at late embryonic stages, most TH-IR neurons with Som-IR also expressed NPY-IR (a combination of peptides not found in the mature coeliac ganglion). Between late embryonic stages and the end of the early fetal period, there was a significant increase in the proportion of neurons in lateral regions that had both NPY-IR and TH-IR. At the same time, there was an increase in the proportion of neurons in medial regions that had both Som-IR and TH-IR. Neurons expressing both Som-IR and TH-IR were rarely observed in lateral regions of the coeliac ganglion. Thus, a clear topography within the coeliac ganglion is established during late embryonic and early fetal stages of development and reflects that found in the mature animal by the end of the early fetal period.

Vagal-enteric interface: vagal activation-induced expression of c-Fos and p-CREB in neurons of the upper gastrointestinal tract and pancreas

Many gastrointestinal and pancreatic functions are under strong modulatory control by the brain via the vagus nerve. To start identifying location and neurochemical phenotype of the enteric neurons receiving functional vagal efferent input, we activated vagal preganglionic neurons either by electrical or chemical stimulation and examined the expression of phosphorylated CREB (c-AMP response element binding protein) and the immediate early gene c-Fos. There was no spontaneous expression of both markers in the pancreas and considerable spontaneous expression of p-CREB but not Fos in the upper GI-tract. Unilateral electrical vagal stimulation-induced p-CREB was found in 40% of neurons in the head of the pancreas. Fos expression was found in 70-90% of neurons in the esophagus and stomach, in 20-30% of myenteric plexus neurons and 5-15% in submucosal neurons of the proximal duodenum. Double-labeling experiments showed that a majority of pancreatic neurons and about 25-35% of neurons in the stomach and duodenum contain NADPH-diaphorase and that many of these receive functional vagal input. Other neurons that can be vagally activated contain gastrin-releasing peptide or calretinin. Chemical stimulation of the dorsal surface of the caudal brainstem with the stable TRH analog RX77368 resulted in selective activation of vagal efferents with expression of Fos in a small number of gastric myenteric plexus neurons. The results demonstrate the suitability of this method to investigate magnitude and local distribution of vagal input to the enteric nervous system as well as specificity of its neurochemically coded pathways. They represent the first step in the identification of function-specific units of parasympathetic vagal outflow.

Reflex patterns in preganglionic sympathetic neurons projecting to the superior cervical ganglion in the rat

Reflex patterns in preganglionic neurons projecting in the cervical sympathetic trunk (CST) were analyzed in response to stimulation of various afferent systems. We focused on the question whether these preganglionic neurons can be classified into functionally distinct subpopulations. Reflex responses were elicited by stimulation of trigeminal and spinal nociceptive, thermoreceptive as well as baroreceptor and chemoreceptor afferents. Multi- and single fiber preparations were studied in baroreceptor intact and sino-aortically denervated animals. Spontaneous activity of 36 preganglionic single neurons ranged from 0.2 to 3.5 imp/s (median= 1.11 imp/s). The degree of cardiac rhythmicity (CR) in the activity of sympathetic neurons was 69.5+/-13% (mean+/-S.D.; N=52; range=39-95%). Noxious stimulation of acral skin activated the majority (67%) of sympathetic preparations by 37+/-25% (N=35) above pre-stimulus activity; 15% were inhibited. In these neurons the response to noxious stimulation of acral skin was significantly correlated with the degree of CR (P<0.001, N=52) in that neurons showing the strongest excitation to noxious stimulation displayed the strongest CR. Noxious mechanical stimulation of body trunk skin (N=60) inhibited the majority (80%) of fiber preparations tested (by 34+/-18% of pre-stimulus activity, N=48); an activation was not observed. Cold stimulation of acral (N=9) and body trunk skin (N=42) activated most fiber preparations. Trigeminal stimulation evoked a uniform reflex activation of preganglionic neurons (+79+/-73% of pre-stimulus activity, N=32). Chemoreceptor stimulation by systemic hypercapnia elicited inhibitory (-31+/-19%, N=8) as well as excitatory (+59+/-5%, N=4) responses. These results show that preganglionic sympathetic neurons projecting to target organs in the head exhibit distinct reflex patterns to stimulation of various afferent systems; however, a clear classification into different functional subgroups did not emerge. Furthermore, reflex patterns showed a segmental organization to noxious cutaneous stimulation of acral parts and body trunk reflecting a differential central integration of spinal afferent input. Compared with the cat the reflex organization of sympathetic neurons projecting to the head seems to be less differentiated in the anesthetized rat.

Electrophysiological and morphological diversity of mouse sympathetic neurons

We have used multiple-labeling immunohistochemistry, intracellular dye-filling, and intracellular microelectrode recordings to characterize the morphological and electrical properties of sympathetic neurons in the superior cervical, thoracic, and celiac ganglia of mice. Neurochemical and morphological characteristics of neurons varied between ganglia. Thoracic sympathetic ganglia contained three main populations of neurons based on differential patterns of expression of immunoreactivity to tyrosine hydroxylase, neuropeptide Y (NPY) and vasoactive intestinal peptide (VIP). In the celiac ganglion, nearly all neurons contained immunoreactivity to both tyrosine hydroxylase and NPY. Both the overall size of the dendritic tree and the number of primary dendrites were greater in neurons from the thoracic and celiac ganglia compared with those from the superior cervical ganglion. The electrophysiological properties of sympathetic neurons depended more on their ganglion of origin rather than their probable targets. All neurons in the superior cervical ganglion had phasic firing properties and large afterhyperpolarizations (AHPs). In addition, 34% of these neurons displayed an afterdepolarization preceding the AHP. Superior cervical ganglion neurons had prominent I(M), I(A), and I(H) currents and a linear current-voltage relationship between -60 and -110 mV. Neurons from the thoracic ganglia had significantly smaller action potentials, AHPs, and apparent cell capacitance compared with superior cervical ganglion neurons, and only 18% showed an afterdepolarization. All neurons in superior cervical ganglia and most neurons in celiac ganglia received at least one strong preganglionic input. Nearly one-half the neurons in the celiac ganglion had tonic firing properties, and another 15% had firing properties intermediate between those of tonic and phasic neurons. Most celiac neurons showed significant inward rectification below -90 mV. They also expressed I(A), but with slower inactivation kinetics than that of superior cervical or thoracic neurons. Both phasic and tonic celiac ganglion neurons received synaptic inputs via the celiac nerves in addition to strong inputs via the splanchnic nerves. Multivariate statistical analysis revealed that the properties of the action potential, the AHP, and the apparent cell capacitance together were sufficient to correctly classify 80% of neurons according to their ganglion of origin. These results indicate that there is considerable heterogeneity in the morphological, neurochemical, and electrical properties of sympathetic neurons in mice. Although the morphological and neurochemical characteristics of the neurons are likely to be related to their peripheral projections, the expression of particular electrophysiological traits seems to be more closely related to the ganglia within which the neurons occur.