Peripheral nerve pathways to neurons in the guinea pig inferior mesenteric ganglion determined electrophysiologically after chronic nerve section

Peripheral synaptic pathways to neurons in the guinea pig inferior mesenteric ganglion (IMG) were studied. Nerve trunks innervating neurons in the ganglion were surgically sectioned and intracellular electrical responses to nerve stimulation were measured 6-8 days after surgery. In all animals ganglia were decentralized by removal of the lumbar sympathetic chain ganglia L2 through L4 and in addition two peripheral nerves were sectioned leaving the ganglion innervated by only one peripheral nerve. Fast and slow excitatory postsynaptic potential (EPSP) were evoked with electrical stimulation of each of the nerve trunks and with distension of the colon. The thresholds to evoke fast EPSPs and the amplitude of slow EPSPs were compared for each nerve trunk among the different surgical groups including sham-operated controls and completely denervated ganglia. Both fast and slow EPSPs could be evoked electrically from each intact peripheral nerve trunk after the other three nerve trunks had been sectioned, which demonstrates that nerve fibers with cell bodies in the regions innervated by the peripheral nerves make functional synaptic connections with neurons in the inferior mesenteric ganglion. In general, nerve sections increased the threshold for evoking fast EPSPs and decreased the amplitude of electrically-evoked slow EPSPs compared to control ganglia. Synaptic potentials could also be evoked with stimulation of cut nerve trunks, demonstrating that branches of nerve fibers from peripheral nerves enter other nerve trunks. The hypogastric nerve was unique in that branches of axons eliciting fast but not slow synaptic potentials in the ganglion entered this nerve trunk. Distension-induced fast and slow EPSPs were present only if the lumbar colonic nerve was intact and they were not altered by section of the other nerve trunks. In contrast, the slow EPSPs evoked with electrical stimulation of the lumbar colonic nerve were significantly smaller when at least one other nerve trunk was sectioned suggesting that the axon branches from other nerve trunks which enter the lumbar colonic nerve are not activated by distension. These studies demonstrate that neurons eliciting either fast or slow synaptic potentials with cell bodies in regions innervated by the peripheral nerve trunks make functional synaptic connections with neurons of the inferior mesenteric ganglion. The results also suggest that the majority of mechanosensory neurons mediating excitatory synaptic responses to colon distension are neurons with a peripheral cell body.     

Serotonin receptor subtypes that depolarize guinea pig inferior mesenteric ganglion neurons

Our previous studies indicated that serotonin (5-HT) depolarized a majority of guinea pig inferior mesenteric ganglion (IMG) neurons and may be another transmitter for the noncholinergic late slow excitatory postsynaptic potential (ls-EPSP) in the IMG. However, the subtypes of 5-HT receptor mediating these responses have not yet been identified. Using intracellular recording, we examined the effect of 5-HT receptor antagonists with specificity to various 5-HT receptor subtypes on the 5-HT-mediated depolarization and ls-EPSP in IMG neurons in vitro. Cyproheptadine, a 5-HT(1/2) receptor antagonist, reversibly inhibited the slow, but not the fast, depolarization and ls-EPSP in the 5-HT-sensitive neurons. Both mianserin and spiperone, 5-HT(2) and 5-HT(1A) receptor antagonists, did not significantly alter either the fast or slow depolarizing responses or the ls-EPSP. The 5-HT(3) receptor antagonist MDL 72222 (Bemesetron) completely inhibited the fast depolarization with little diminution of the slow depolarization and ls-EPSP. Superfusion of putative 5-HT(1P) receptor antagonist, BRL 24924 (Renzapride), reversibly attenuated both the depolarization and ls-EPSP. However, 5-HT-insensitive neurons with ls-EPSP were found to be insensitive to both cyproheptadine and BRL 24924. In most 5-HT-sensitive neurons, the 5-HT(3) receptor agonist, 2-methyl-5-HT, and the selective 5-HT(1P) agonist, MCPP or 5-OHIP, evoked a fast and a slow depolarization in 55.6 and 71.4% of the neurons, respectively, without a significant effect on the membrane potential in 85.7 and 100% of the 5-HT-insensitive neurons. In 5-HT-sensitive neurons, MDL 72222 reversibly abolished the fast depolarization induced by 2-methyl-5-HT; BRL 24924 significantly inhibited the slow depolarization induced by MCPP or 5-OHIP, but not by SP. Prolonged superfusion of 5-HT-sensitive neurons with MCPP abolished the evoked ls-EPSP without inhibition of action potential. These results suggest that the fast and slow depolarizations in these neurons are mediated by 5-HT(3) and 5-HT(1P) receptor subtypes, respectively. The latter may also mediate the ls-EPSP in 5-HT-sensitive neurons.     

Substance P-mediated membrane currents in voltage-clamped guinea pig inferior mesenteric ganglion cells

Responses to substance P (SP) and to hypogastric nerve stimulation were recorded from voltage-clamped guinea pig inferior mesenteric ganglion (IMG) neurons, and compared with those to muscarine. Muscarine produced a voltage-dependent inward current accompanied by a reduced input conductance and inhibition of IM a time- and voltage-dependent K+-current (Brown and Adams: Nature 283:673-676, 1980). SP also produced an inward current, accompanied by a fall in input conductance (20 out of 31 cells) or a rise in input conductance (7 out of 31 cells). The fall in input conductance was not accompanied by an inhibition of M-current (unlike frog ganglia: Adams et al.: British Journal of Pharmacology 79:330-333, 1983) or an inhibition of the inward rectifier current (unlike globus pallidus neurons: Stanfield et al.: Nature 315:498-501, 1985). Repetitive hypogastric nerve stimulation (10-20 Hz, 2-10 s) produced a slow inward postsynaptic current lasting 1-3 min, with decreases or increases of input conductance matching those produced by SP. The postsynaptic current did not show a consistent or reproducible change in amplitude on varying the holding potential between -90 and -25 mV. It is concluded that SP and hypogastric stimulation produce complex and variable changes in ionic conductance in IMG neurons.     

The role of serotonin in non-cholinergic excitatory transmission in the guinea pig inferior mesenteric ganglion.

The non-cholinergic late slow excitatory postsynaptic potential (ls-EPSP) of the guinea pig inferior mesenteric ganglion (IMG) was previously believed to be mediated by substance P (SP) or several other neuropeptides. Yet, the pharmacological evidence presented here indicates that serotonin (5-HT) may be another transmitter for the ls-EPSP in the guinea pig IMG. Repetitive stimulation of the presynaptic nerves elicited ls-EPSP in about half of the IMG neurons. Application of 5-HT or SP caused, in a portion of the IMG neurons, a slow depolarization similar to ls-EPSP. Fifty-six out of 88 (63.6%) neurons with ls-EPSP and 13 out of 35 (37.1%) neurons with ls-EPSP were sensitive to 5-HT and SP, respectively. Superfusion of the ganglia with 5-HT markedly suppressed the ls-EPSP evoked in 5-HT sensitive neurons. Similarly, exogenously applied SP attenuated the ls-EPSP of SP-sensitive neurons. However, prolonged superfusion of 5-HT or SP had no effect on the ls-EPSP elicited in 5-HT or SP-insensitive neurons, respectively. Furthermore, the ls-EPSPs elicited in 5-HT-sensitive neurons as well as the 5-HT-induced depolarization were reversibly suppressed by cyproheptadine, a 5-HT antagonist, and enhanced by fluoxetine, a 5-HT reuptake inhibitor. In contrast, the ls-EPSP of 5-HT insensitive neurons and SP-induced depolarization were not appreciably changed by those two drugs. Pretreatment with p-chlorophenylalanine, a 5-HT biosynthesis inhibitor, did not change the general electrophysiological characteristics of the neurons and did not suppress nicotinic neurotransmission, but markedly reduced the occurrence rate of ls-EPSP from 53.8% to 15.1% (P less than 0.005). Collectively, our results indicate that, besides SP, 5-HT may be involved in mediating the ls-EPSP in a subpopulation of neurons in the guinea pig IMG. The type of transmitter mediating ls-EPSP is apparently not limited to 5-HT and SP, as about 30% of the neurons with ls-EPSP were found to be insensitive to both 5-HT and SP and prolonged superfusion with both did not affect appreciably the ls-EPSP elicited in these neurons.     

Immediate-early gene expression in the inferior mesenteric ganglion and colonic myenteric plexus of the guinea pig.

Activation of neurons in the inferior mesenteric ganglion (IMG) was assessed using c-fos, JunB, and c-Jun expression in the guinea pig IMG and colonic myenteric plexus during mechanosensory stimulation and acute colitis in normal and capsaicin-treated animals. Intracolonic saline or 2% acetic acid was administered, and mechanosensory stimulation was performed by passage of a small (0.5 cm) balloon either 4 or 24 hr later. Lower doses of capsaicin or vehicle were used to activate primary afferent fibers during balloon passage. c-Jun did not respond to any of the stimuli in the study. c-fos and JunB were absent from the IMG and myenteric plexus of untreated and saline-treated animals. Acetic acid induced acute colitis by 4 hr, which persisted for 24 hr, but c-fos was found only in enteric glia in the myenteric plexus and was absent from the IMG. Balloon passage induced c-fos and JunB in only a small subset of IMG neurons and no myenteric neurons. However, balloon passage induced c-fos and JunB in IMG neurons (notably those containing somatostatin) and the myenteric plexus of acetic acid-treated animals. After capsaicin treatment, c-fos and JunB induction by balloon passage was inhibited in the IMG, but there was enhanced c-fos expression in the myenteric plexus. c-fos and JunB induction by balloon stimulation was also mimicked by acute activation of capsaicin-sensitive nerves. These data suggest that colitis enhances reflex activity of the IMG by a mechanism that involves activation of both primary afferent fibers and the myenteric plexus.     

Separate origins for the dynorphin and enkephalin immunoreactive fibers in the inferior mesenteric ganglion of the guinea pig

By different denervation procedures the origin of dynorphin-(1-17) and enkephalin immunoreactive fibers in the guinea pig inferior mesenteric ganglion was investigated. It was found that the dynorphin-(1-17)-positive fibers reached the ganglion predominantly via the colonic nerves and to a lesser extent via the hypogastric and intermesenteric nerves whereas the enkephalin-positive fibers reached the ganglion via the lumbar splanchnic nerves. These findings show that the dynorphin-(1-17) and enkephalin systems are separate in this ganglion.     

An ultrastructural study of dynorphin-immunoreactive nerve fibers and terminals in the celiac-superior mesenteric ganglion of the guinea pig

Dynorphin-immunoreactive nerve fibers and terminals were identified in the celiac-superior mesenteric ganglion of the guinea pig at the ultrastructural level with the peroxidase-antiperoxidase technique. The immunostained material was localized in the large dense core vesicles of the terminals but was also present diffusely in the axoplasm. The terminals formed numerous axodendritic and a few axosomatic contacts, interpreted as synapses, with the principal ganglion cells. These findings suggest that dynorphin plays a role as a neurotransmitter or neuromodulator in the ganglion and, taken together with earlier findings, indicate an involvement of dynorphin neurons in the intestino-intestinal inhibitory reflex.     

Intracellular labeling of auditory nerve fibers in guinea pig: central and peripheral projections

Auditory-nerve fibers (ANFs) in the cat have been subdivided according to spontaneous rate (SR), with high-SR fibers showing the lowest thresholds. Cochlear terminals of the three SR groups differ in caliber and synaptic position around the inner hair cell (Liberman [1982b] Science 216:1239–1241); central terminals differ in degree of branching and in which subregions of the cochlear nucleus (CN) are targeted (Liberman [1991] J. Comp. Neurol. 313:240–258). The present study investigates whether these SR-based differences in ANF connections are unique to the cat. Thirty ANFs from 15 guinea pigs were intracellularly labeled after measuring characteristic frequency, threshold, and SR. Labeled cochlear projections showed significant SR-based differences in axonal caliber, with low- and medium-SR fibers 20–40% thinner than those of high-SR fibers for both peripheral and central (modiolar) axons. Spatial segregation in the inner hair cell area could not be assessed; however, the peripheral axons in the osseous spiral lamina showed the same SR-based organization reported for the cat (Kawase and Liberman [1992] J. Comp. Neurol. 319:312–318). Labeled central projections also showed significant SR-based differences. Low- and medium-SR fibers: 1) were more highly branched, 2) sent significantly more terminals to the small-cell cap region of the CN, and 3) produced endbulb terminals (on spherical cells) that were significantly more complex than high-SR fibers. All of these SR-based trends for both central and peripheral projections are analogous to those reported in the cat, and, thus, may represent a fundamental organizational principle of the mammalian ear. J. Comp. Neurol. 381:188-202, 1997. © 1997 Wiley-Liss, Inc.     

Calbindin-immunoreactive nerve terminals in the guinea pig coeliac ganglion originate from colonic nerve cells

Previous work has shown that calbindin-immunoreactive (calbindin-IR) nerve terminals are numerous in guinea pig prevertebral ganglia. A high proportion of those colonic nerve cells that project to the inferior mesenteric ganglia are calbindin-IR, but none of the neurons that project from the small intestine to the coeliac ganglion are immunoreactive for calbindin. The present work was designed to determine the source of the calbindin-IR fibres and the pathways by which they reach the coeliac ganglion. Sections through the major nerve trunks that connect with the coeliac ganglion revealed numerous calbindin-IR fibres in the inferior coeliac nerves and in the intermesenteric nerves, while there were very few fibres in the splanchnic or superior coeliac nerves. When all peripheral nerve connections to a lobe of the coeliac ganglion were cut, all calbindin-IR terminals degenerated. Cutting the ileo-caeco-colic nerves caused a substantial reduction in the density of nerve fibres in the coeliac ganglion, whereas no significant reduction could be detected when the intermesenteric nerves were cut. However, lesion of both the ileo-caeco-colic and intermesenteric nerves caused all the calbindin-IR nerve fibres in the coeliac ganglion to degenerate. It is concluded that most or all of the calbindin-reactive nerve terminals in the coeliac ganglion originate from the large intestine and that most reach the ganglion via the ileo-caeco-colic nerves. Thus many colonic intestinofugal neurons, supplying both the coeliac and inferior mesenteric ganglia, are immunoreactive for calbindin, whereas small intestinal intestinofugal neurons are not immunoreactive for this protein.     

The three-dimensional structure of neurons in the guinea pig inferior mesenteric and pelvic hypogastric ganglia

The three-dimensional (3-D) morphology of sympathetic inferior mesenteric ganglion (IMG) neurons and sympathetic-parasympathetic pelvic hypogastric ganglion (PHG) neurons was studied using confocal laser scanning microscopy. Cell bodies of IMG neurons were disc-shaped and were arranged orderly in layers. The dendritic arbor of individual neurons was confined to a plane with a thickness that did not exceed the thickness of the parent cell body. The actual dendritic surface area (71,400 micron 2) and volume (81,500 micron 3) of the IMG neurons were up to 100-fold larger than previously reported for similar sympathetic neurons using data of 2-D measurements and estimations of the third dimension. PHG neurons had a much smaller dendritic surface area (4100 micron 2) and volume (2400 micron 3) compared to IMG neurons. The ratio dendritic/somal surface area for individual IMG and PHG neurons ranged from 5:1 to 14:1 and from 0.1:1 to 6:1, respectively. The total dendritic path-length was 8-42 times greater for IMG than for PHG neurons. Neurons in the IMG were either stellate with radiating dendrites or bipolar-shaped with dendrites emerging from the two poles of the cell body. Neurons in the PHG were of two morphological types. One type (nearly 2/3 of all the imaged PHG neurons) had two to seven relatively long dendrites and an axon; the other type had only one to three short unbranched dendrites and an axon. The spatial organization of neurons within the ganglia and the structural features of individual neurons are likely to have important implications regarding connectivity patterns between neurons within the ganglion as well as on how information is processed by the ganglion.     

Afterspike-hyperpolarization of neurons in the inferior mesenteric ganglion in guinea-pig

Intracellular recordings were made from neurons (n = 121) in the inferior mesenteric ganglion (IMG) in guinea-pig. The afterspike hyperpolarization (ASH) following a single action potential was studied in IMG cells which received an excitatory, cholinergic innervation from mechanosensory nerves in the gastrointestinal tract. The amplitude of ASH was dependent on the membrane potential of IMG cells and the concentration of K+ in the bathing solution. The reversal potential of ASH (-80- -90 mV, in normal Krebs solution) appeared to follow the equilibrium potential for K+, as [K+]o was changed, suggesting that ASH was the product of K+-efflux. Further evidence suggested that a major component of the K+-efflux was dependent on the concentration of Ca2+ in the bathing medium. Elevation and reduction of [Ca2+]o increased and decreased, respectively, the amplitude and duration of ASH. In the presence of tetrodotoxin, depolarizing current pulses elicited spike-like events which (1) were dependent on [Ca2+]o and the degree of depolarization by current-clamp and (2) were followed by afterhyperpolarizations that were also dependent on [Ca2+]o and degree of depolarization by current-clamp. In the combined presence of tetrodotoxin and tetraethylammonium, depolarizing current pulses elicited prolonged action potentials (up to 100 ms in duration) followed by prolonged ASH (up to 3 s in duration). Spike-like events, prolonged action potentials and their afterhyperpolarizations were reduced in amplitude and duration when the calcium-channel blocking ion, Co2+, or blocking drug, verapamil, was present in the bathing medium. In normal Krebs solution, the ASH of action potentials produced by nerve stimulation was reduced but not abolished in the presence of Co2+. These results suggested that Ca2+ entered IMG cells during depolarization and activated the K+-conductance mechanisms responsible for the ASH. However, an initial component of the ASH may have involved other voltage-dependent K+-currents known to be activated during the excitation of sympathetic neurons. The amplitude and duration of ASH differed during non-synaptic and synaptic excitation of IMG cells, and differed when action potentials resulted from fast and slow EPSPs. In addition, the amplitude and duration of ASH were altered by noradrenaline, by the cholinomimetic, carbachol, and by 3 neuropeptides present in the IMG, namely leucine-enkephalin, substance P and vasoactive intestinal polypeptide.(ABSTRACT TRUNCATED AT 400 WORDS)     

Demonstration of preganglionic fibers in the inferior mesenteric ganglion, pelvic ganglia and related nerves of the guinea pig by anterogradely transported wheat germ agglutinin-horseradish peroxidase conjugate

Preganglionic sympathetic neurons in the guinea pig were labeled by injections of wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) into the L2 and L3 spinal cord segments. After anterograde transport of the tracer the following areas were examined for the presence of HRP-labeled fibers: the inferior mesenteric ganglion (IMG), the pelvic ganglia, the hypogastric and colonic nerves. In the ganglia labeling appeared predominantly as clusters of varicose-like profiles suggestive of being axon terminals. Particularly in the pelvic ganglia, these profiles appeared to surround the contours of some of the ganglion cell bodies. Numerous HRP-positive fibers were present in the hypogastric nerves, but only occasional fibers were observed in the colonic nerves. The pattern of labeling differed markedly from that described previously after anterograde transport of WGA-HRP in sensory fibers of the IMG, hypogastric and colonic nerves. Furthermore, the results from this and the previous study show that anterograde tracing with WGA-HRP can be a useful means for analyzing the structural organization of various fiber inputs to autonomic ganglia.     

The effect of optic nerve section on form deprivation myopia in the guinea pig

Myopia is induced when a growing eye wears a diffuser that deprives it of detailed spatial vision (form deprivation, FD). In chickens with optic nerve section (ONS), FD myopia still occurs, suggesting that the signals underlying myopia reside within the eye. As avian eyes differ from mammals, we asked whether local mechanisms also underlie FD myopia in a mammalian model. Young guinea pigs underwent either sham surgery followed by FD (SHAM + FD, n = 7); or ONS followed by FD (ONS + FD, n = 7); or ONS without FD (ONS, n = 9). FD was initiated 3 days after surgery with a diffuser that was worn on the surgically treated eye for 14 days. Animals with ONS + FD developed −8.9 D of relative myopia and elongated by 135 μm more than in their untreated eyes after 2 weeks of FD. These changes were significantly greater than those in SHAM + FD animals (−5.5 D and 40 μm of elongation after 14 days of FD), and reflected exaggerated elongation of the posterior vitreous chamber. The myopia reversed when FD was discontinued, despite ONS, but eyes did not recover back to normal (30 days after surgery, ONS + FD eyes still retained −3 D of relative myopia when SHAM+FD animals had returned to normal). No long-term residual myopia was present after ONS alone, ruling out a surgical artifact. Although the gross mechanism signaling myopic ocular growth and its recovery in the young mammalian eye does not require an intact optic nerve, its fine-tuning is disrupted by ONS.     

Primary sensory afferents in the inferior mesenteric ganglion and related nerves of the guinea pig. An experimental study with anterogradely transported wheat germ agglutinin-horseradish peroxidase conjugate

Peripheral visceral afferents in the guinea pig were labeled by injections of wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) into the L2 and L3 dorsal root ganglia bilaterally. After anterograde transport of the tracer the following areas were examined for the presence of HRP-labeled fibers: the inferior mesenteric ganglion (IMG), the inferior mesenteric artery (IMA) with surrounding tissue, the hypogastric nerves, parts of the descending and sigmoid colon as well as the urinary bladder. Large numbers of heavily labeled fibers were found in the IMG, in the colonic nerves around the IMA and in the hypogastric nerves. In the IMG, profiles suggestive of being labeled axon terminals were observed. Labeled fibers were observed in the muscle layers of the colon and in the bladder wall. The results show that anterograde tracing with WGA-HRP can be used successfully in analyzing the morphology and structural organization of visceral afferents in the periphery.     

Effects of mesenteric nerve stimulation on the electrical activity of myenteric neurons in the guinea pig ileum

Effects of mesenteric nerve stimulation on the electrical activity of 28 myenteric neurons were investigated in the myenteric flaps innervated with mesenteric nerves. Mesenteric nerve stimulation evoked slow excitatory postsynaptic potentials (EPSPs), whose amplitude and duration were 24.5 +/- 5.5 mV and 374.6 +/- 58.9 s in 7 AH/Type 2 neurons, respectively. Such slow EPSPs mimic the slow depolarizing action induced by exogenous substance P. It is, therefore, likely that slow EPSPs might be in part mediated by the release of substance P.     

Neuronal projections to the guinea pig stellate ganglion investigated by retrograde tracing

Previous electrophysiological studies have revealed a peripheral sensory input to the stellate ganglion which does not originate from the dorsal root ganglia. The present retrograde tracing study aimed at evaluating whether the parent cell bodies are located in the periphery, i.e. in mediastinal ganglia. Following injection of Fast blue or wheat germ agglutinin-horseradish peroxidase into the right stellate ganglion of the guinea pig, retrogradely labelled cell bodies were observed in the intermediolateral and intercalated nuclei of the spinal cord as well as in dorsal root ganglia at segmental levels C8 to T6. In another case, the stellate ganglion was resected and replaced by a sponge soaked with 10 μl of Fast blue. Labelling of preganglionic and sensory neurons parallelled that obtained by tracer injections. In neither case, however, were retrogradely labelled neurons found within or around the thoracic viscera (thymus, trachea, bronchi, esophagus, heart, great vessels of upper mediastinum) when these were cut serially en bloc. Controls performed by injection of Fast blue into the inferior mesenteric ganglion and investigation of the distal colon showed that our experimental protocol was able to visualize a peripheral projection towards a sympathetic ganglion — in this case from myenteric ganglia to the inferior mesenteric ganglion. We conclude that, in contrast to the circuitry connecting prevertebral sympathetic ganglia with the gut, the neuronal cell bodies providing peripheral sensory input from thoracic viscera to the right stellate ganglion most likely are not located within the mediastinal ganglia. Instead, they may reside within the stellate ganglion itself.     

Topography of NPY-, somatostatin-, and VIP-immunoreactive, neuronal subpopulations in the guinea pig celiac-superior mesenteric ganglion and their projection to the pylorus

The topography of the peptidergic neuronal subpopulations in the guinea pig celiac-superior mesenteric ganglion was studied analyzing the distribution of immunoreactivity to neuropeptide Y (NPY), somatostatin (SOM), and vasoactive intestinal polypeptide (VIP)/polypeptide HI (PHI). For comparison, the ganglion was also studied using antisera against the 2 catecholamine-synthesizing enzymes tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH). Approximately 65% of the neuronal cell bodies contained NPY-like immunoreactivity (NPY-LI), whereas 25% of the principal ganglion cells contained SOM-like immunoreactivity (SOM-LI). Though occasional cells were found to contain both NPY-LI and SOM-LI, these peptides had a complementary distribution in the ganglion, with NPY cells in the celiac poles and SOM cells in the superior mesenteric pole. The vast majority of both the NPY- and SOM-positive cells also contained TH-like immunoreactivity (TH-LI), confirming their catecholaminergic, presumably noradrenergic, nature. Some noradrenergic neurons seemed to lack NPY- and SOM-LI. Small numbers of VIP/PHI-containing cell bodies were found in areas where the NPY-immunoreactive neurons predominated. Many of the VIP/PHI-positive cells contained NPY-LI and occasionally also TH-LI. The immunohistochemical markers were also observed in fibers. Thus, a comparatively weak NPY-LI was seen in smooth fibers, probably representing axons and axon bundles. SOM-LI was seen in a similar type of fiber but also in more strongly fluorescent fibers with a varicose appearance. The latter fibers were observed only in the SOM-dominated part of the ganglion, often surrounding the ganglion cells. Varicose fibers with a similar distribution containing DBH-like immunoreactivity (DBH-LI) were also seen. In addition, DBH- and TH-LI were seen in smooth axonlike processes. VIP-positive fibers exhibited a very dense fiber network, almost exclusively related to the SOM cell-dominated part of the ganglion. The projection of the postganglionic sympathetic neurons was studied with special reference to the pylorus using a combination of retrograde axonal tracing and indirect immunofluorescence techniques. Seventy-two hours after injection of the fluorescent tracer Fast Blue into the pyloric sphincter, labeled neurons were found in the ganglion. By comparing the Fast Blue-labeled cells with the immunoreactive cell bodies, neurons containing both dye and NPY- or SOM-LI were observed. In elution-restaining experiments, it was established that the majority of these cells were also immunoreactive to TH, indicating that they produce noradrenaline.(ABSTRACT TRUNCATED AT 400 WORDS)