c-Fos- and JunB-immunoreactivities in the enteric nervous system of the guinea-pig ileum

Cannabinoid CB2 receptors in the enteric nervous system modulate gastrointestinal contractility in lipopolysaccharide-treated rats

Enhanced intestinal transit due to lipopolysaccharide (LPS) is reversed by cannabinoid (CB)2 receptor agonists in vivo, but the site and mechanism of action are unknown. We have tested the hypothesis that CB2 receptors are expressed in the enteric nervous system and are activated in pathophysiological conditions. Tissues from either saline- or LPS-treated (2 h; 65 microg/kg ip) rats were processed for RT-PCR, Western blotting, and immunohistochemistry or were mounted in organ baths where electrical field stimulation was applied in the presence or absence of CB receptor agonists. Whereas the CB2 receptor agonist JWH133 did not affect the electrically evoked twitch response of the ileum under basal conditions, in the LPS-treated tissues JWH133 was able to reduce the enhanced contractile response in a concentration-dependent manner. Rat ileum expressed CB2 receptor mRNA and protein under physiological conditions, and this expression was not affected by LPS treatment. In the myenteric plexus, CB2 receptors were expressed on the majority of neurons, although not on those expressing nitric oxide synthase. LPS did not alter the distribution of CB2 receptor expression in the myenteric plexus. In vivo LPS treatment significantly increased Fos expression in both enteric glia and neurons. This enhanced expression was significantly attenuated by JWH133, whose action was reversed by the CB2 receptor antagonist AM630. Taking these facts together, we conclude that activation of CB2 receptors in the enteric nervous system of the gastrointestinal tract dampens endotoxin-induced enhanced intestinal contractility.

Cholecystokinin activates specific enteric neurons in the rat small intestine

Cholecystokinin (CCK) is a peptide hormone released from the I-cells of the upper small intestine. CCK evokes a variety of physiological responses, such as stimulation of pancreatic secretion, reduction of food intake and inhibition of gastric emptying. Previously, we reported that CCK activates enteric neurons in the rat. However the specific subpopulations of enteric neurons activated by CCK have not been identified. In the work reported here, we utilized immunohistochemical detection of nuclear Fos, a marker for neuronal activation, and selected phenotypic markers to identify some of the neuronal subpopulations activated by CCK. The phenotypic markers that we examined were: nitric oxide synthase (NOS), neurokinin-1 receptor (NK-1R), calbindin (Cal), Calretinin (Calr), and neurofilament-M (NF-M). We found that in the myenteric plexus of the rat duodenum and jejunum, CCK activated NOS immunoreactive neurons. In the submucosal plexus of duodenum and jejunum, CCK activated Cal, Calr and NF-M immunoreactive neurons. CCK failed to activate NK-1R immunoreactive neurons in either plexus. Our results indicate that CCK activates distinct enteric neurons in the rat upper small intestine. Furthermore the fact that NOS immunoreactive neurons were activated suggests that CCK modulates the activity of inhibitory motor neurons in the myenteric plexus. Expression of Fos immunoreactivity in Calr and Cal immunoreactive neurons is consistent with a role for CCK in modulation of intrinsic sensory and/or secretomotor neuronal activity in the submucosal plexus.

Capsaicin-sensitive extrinsic afferents are involved in acid-induced activation of distinct myenteric neurons in the rat stomach

Challenge of the rat gastric mucosa with 0.5 mol L(-1) HCl activates nitrergic neurons in the myenteric plexus as visualized by c-Fos immunohistochemistry. In the present study, we characterized the activated neurons more extensively by their chemical coding and investigated whether a neural pathway that involves capsaicin-sensitive extrinsic afferents and/or cholinergic neurons transmitting via nicotinic receptors contributes to the activation of myenteric neurons. In multiple labelling experiments, c-Fos was examined for co-localization with nitric oxide synthase (NOS), vasoactive intestinal peptide (VIP), neuropeptide Y (NPY), enkephalin (ENK), gastrin-releasing peptide (GRP), substance P (SP), calbindin D-28k (CALB) and neurofilament 145 (NF 145). All c-Fos-positive neurons were immunoreactive for NOS, VIP, NPY and NF 145, but not for SP, ENK, GRP and CALB. Nerve fibres co-expressing NOS, VIP and NPY were predominantly found in the external muscle layer and in the muscularis mucosae but rarely in the mucosa. Pre-treatment with capsaicin or hexamethonium or a combination of both pre-treatments reduced HCl-induced c-Fos expression by 54, 66 and 63%, respectively. Acid challenge of the stomach, therefore, leads to activation of presumably inhibitory motor neurons responsible for muscle relaxation. Activation of these neurons is partly mediated by capsaicin-sensitive afferents and involves ganglionic transmission via nicotinic receptors.

Expression of immediate early genes in sensory ganglia

C-Jun and c-Fos belong to the family of immediate early genes. Apart from their role as transcription factors, a basal expression was shown for them in central nervous system tissues. The expression of c-Jun and c-Fos in sensory ganglia of guinea pig, rat and murine sensory ganglia was examined under normal, unstimulated conditions by quantitative double-immunohistochemistry. 4.6 +/- 2.8% of neuron-specific protein gene-product 9.5 -positive cells in nodose ganglia, 51.6 +/- 2.1% in jugular ganglia, 46.4 +/- 3.0% in trigeminal ganglia and 42.5 +/- 1.3% of cervical dorsal root ganglia neurons were positive for c-Jun in the guinea pig (less than 1% for c-Fos). In rat and mouse, less than 1% of the sensory neurons exhibited c-Jun and c-Fos-immunoreactivity. The high basal expression of c-Jun in guinea pig sensory neurons suggests that in this species the presence of c-Jun does not only depend on specific stimulation and is not exclusively associated with neuronal plasticity of gene expression and functional changes.

Intracisternal TRH analog induces Fos expression in gastric myenteric neurons and glia in conscious rats

Activation of gastric myenteric cells by intracisternal injection of the stable thyrotropin-releasing hormone (TRH) analog RX-77368, at a dose inducing near maximal vagal cholinergic stimulation of gastric functions, was investigated in conscious rats. Fos immunoreactivity was assessed in gastric longitudinal muscle-myenteric plexus whole mount preparations 90 min after intracisternal injection. Fos-immunoreactive cells were rare in controls (~1 cell/ganglion), whereas intracisternal RX-77368 (50 ng) increased the number to 24.8 +/- 1.8 and 26.8 +/- 2.2 cells/ganglion in the corpus and antrum, respectively. Hexamethonium (20 mg/kg sc) prevented Fos expression by 90%, whereas atropine (2 mg/kg sc) had no effect. The neuronal marker protein gene product 9.5 and the glial markers S-100 and glial fibrillary acidic proteins showed that RX-77368 induced Fos in both myenteric neurons and glia. Vesicular ACh transporter and calretinin were detected around the activated myenteric neurons. These results indicated that central vagal efferent stimulation by intracisternal RX-77368 activates gastric myenteric neurons as well as glial cells mainly through nicotinic ACh receptors in conscious rats.

Consequences of intestinal inflammation on the enteric nervous system: neuronal activation induced by inflammatory mediators

The ENS is responsible for the regulation and control of all gastrointestinal functions. Because of this critical role, and probably as a consequence of its remarkable plasticity, the ENS is often relatively well preserved in conditions where the architecture of the intestine is seriously disrupted, such as in IBD. There are structural and functional changes in the enteric innervation in animal models of experimental intestinal inflammation and in IBD. These include both up and down regulation of transmitter expression and the induction of new genes in enteric neurons. Using Fos expression as a surrogate marker of neuronal activation it is now well established that enteric neurons (and also enteric glia) respond to inflammation. Whether this "activation" is limited to a short-term functional response, such as increased neuronal excitability, or reflects a long-term change in some aspect of the neuronal phenotype (or both) has yet to be firmly established, but it appears that enteric neurons are highly plastic in their response to inflammation.

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.

Functional vagal input to gastric myenteric plexus as assessed by vagal stimulation-induced Fos expression

Immunohistochemical detection of c-Fos expression was used to identify gastric myenteric plexus neurons that receive excitatory input from vagal efferent neurons activated by electrical stimulation of the cervical vagi in anesthetized rats. Vagal stimulation-induced Fos expression increased with higher pulse frequency, so that with 16 Hz (rectangular pulses of 1 mA/0.5 ms for 30 min) approximately 30% and with 48 Hz 90% of all neurons near the lesser curvature were Fos positive. In sham-stimulated rats there was no Fos expression. The percentage of Fos-activated neurons was only slightly smaller (85% with 48 Hz) near the greater curvature. Prior atropine administration (1 mg/kg ip) had little effect on vagal stimulation-induced Fos expression, and in unilaterally stimulated rats there was no Fos expression on the contralateral (noninnervated) side of the stomach, ruling out mediation by gastric motility or secretory responses. However, polysynaptic recruitment of third- and higher-order neurons cannot be ruled out completely. These results support the idea that, at least in the stomach, functional excitatory innervation of myenteric plexus neurons by the efferent vagus is profuse and widespread, refuting the notion of only a few vagal "command neurons."

Prolonged exposure to cigarette smoke blocks the neurotoxicity induced by kainic acid in rats

We examined the effects of cigarette smoke (CS) on three parameters associated with kainic acid (KA)-induced neurotoxicity: seizure activity, cell loss in the hippocampus, and increased Fos-related antigen (FRA) expression. Animals were exposed to the main stream of CS from 15 Kentucky 2R1F research cigarettes containing 28.6 mg tar and 1.74 mg nicotine per cigarette, for 10 min a day, 6 days per week, for 4 weeks, using an automatic smoking machine. KA administration (10 mg/kg, i.p.) produced robust behavioral convulsions lasting 4-5 h. Pre-exposure to CS significantly reduced the seizures, mortality, and severe loss of cells in regions CA1 and CA3 of the hippocampus after KA administration. Consistently, pre-exposure to CS significantly attenuated the KA-induced increased FRA immunoreactivity in the hippocampus. In contrast, pretreatment with central nicotinic antagonist, mecamylamine (2 or 10 mg/kg, i.p.) blocked the neuroprotective effects mediated by CS in a dose-dependent manner. These results indicate that CS exposure provides neuroprotection against the KA insult via nicotinic receptor activation.

Plasticity in the enteric nervous system

Enteric ganglia can maintain integrated functions, such as the peristaltic reflex, in the absence of input from the central nervous system, which has a modulatory role. Several clinical and experimental observations suggest that homeostatic control of gut function in a changing environment may be achieved through adaptive changes occurring in the enteric ganglia. A distinctive feature of enteric ganglia, which may be crucial during the development of adaptive responses, is the vicinity of the final effector cells, which are an important source of mediators regulating cell growth. The aim of this review is to focus on the possible mechanisms underlying neuronal plasticity in the enteric nervous system and to consider approaches to the study of plasticity in this model. These include investigations of neuronal connectivity during development, adaptive mechanisms that maintain function after suppression of a specific neural input, and the possible occurrence of activity-dependent modifications of synaptic efficacy, which are thought to be important in storage of information in the brain. One of the applied aspects of the study of plasticity in the enteric nervous system is that knowledge of the underlying mechanisms may eventually enable us to develop strategies to correct neuronal alterations described in several diseases.

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.

C-Fos in enteric nerves after extrinsic denervation of guinea pig ileum

BACKGROUND

Gastrointestinal function is controlled partly by an interaction between extrinsic (sympathetic, parasympathetic, sensory) and intrinsic (enteric) nerves. However, normal gut function occurs in the absence of extrinsic innervation as enteric nerves adapt to the loss of extrinsic nerves from the gut wall. Expression of the proto-oncogene product, c-Fos, is a signal for activity-dependent changes in gene expression and immunocytochemical detection of c-Fos is used as a marker for changes in neuronal activity. The purpose of this study was to determine if enteric neurons in guinea pig ileum respond to loss of extrinsic innervation by expressing c-Fos protein.

MATERIALS AND METHODS

Fos protein was localized using immunohistochemical methods and an antiserum raised against synthetic Fos. Segments of ileum were extrinsically denervated by crushing the mesenteric nerves in anesthetized animals or by treating animals with 6-hydroxydopamine (6-OH-DA) or capsaicin to destroy sympathetic and extrinsic sensory nerves, respectively.

RESULTS

One week after surgical extrinsic denervation of loops of ileum, 12 +/- 1 nuclei/submucosal ganglion and 114 +/- 6 nuclei/myenteric ganglion contained Fos immunoreactivity (ir). These values were greater (P < 0.05) than those from unoperated segments from the same animals (4 +/- 1 Fos-ir nuclei/submucosal ganglion and 13 +/- 4 Fos-ir nuclei/myenteric ganglion) or from sham-operated segments. Significantly more nuclei contained Fos-ir at 4, 7, 10, and 24 weeks after denervation. Finally, capsaicin or 6-OH-DA treatment increased the number of Fos-ir nuclei in enteric ganglia.

CONCLUSIONS

These data suggest that Fos expression may be part of the adaptation of enteric nerves to extrinsic denervation.

Functional alterations in jejunal myenteric neurons during inflammation in nematode-infected guinea pigs

Intracellular recordings of jejunal myenteric neurons with an afterspike hyperpolarization (AH) from Trichinella spiralis-infected animals showed enhanced excitability on days 3, 6, and 10 postinfection (PI) compared with uninfected animals. Lower membrane potential, increased membrane input resistance, decreased threshold for action potential discharge, decreased AH amplitude and duration, and increased fast excitatory postsynaptic potential amplitude and duration were characteristic of neuronal recordings from infected animals. Concurrent with electrophysiological changes during T. spiralis infection, increased cytochrome oxidase activity, a marker of neuronal metabolic activity, and the expression of nuclear c-Fos immunoreactivity, an indicator of transcriptional-translational activity, were also observed in myenteric ganglion cells. Double-labeling for calbindin-immunoreactive myenteric neurons revealed that approximately 50% of these neurons also expressed increased c-Fos immunoreactivity during T. spiralis infection. Myeloperoxidase activity was significantly higher in the jejunum of T. spiralis-infected guinea pigs on days 3, 6, and 10 PI vs. uninfected counterparts. The expression of c-Fos in calbindin-immunoreactive neurons together with enhanced neuronal electrical and metabolic activity during nematode-induced intestinal inflammation suggests the onset of excitation-transcription coupled changes in enteric neural microcircuits.

Microwave staining of intestinal whole-mount preparations with Cuprolinic blue

Encouraged by the knowledge that microwaves have a beneficial effect on immunohistochemical reactions, the present study aimed to find out whether microwaves could improve the Cuprolinic Blue staining of enteric neurons as well as the actual method that has been developed for gastrointestinal whole-mount preparations. In addition to incorporating a microwave application in the method described by Holst and Powley (1995), some other modifications were made: two incubations before incubation in the staining solution and free-floating incubations. In the whole-mount preparations, most, if not all, enteric neurons were stained by Cuprolinic Blue. These neurons appeared as blue-green cells with non-reacting nuclei and neuronal processes. At higher magnification, the cytoplasm was characterized by a fine blue-green granulation, and the nucleolus in the nucleus appeared as a blue iridescent structure. Non-specific staining occurred in fibrocytes and epithelial cells but, because of their location and appearance, they could easily be distinguished from neurons. The modified incubations and the incorporation of a microwave application into the conventional Cuprolinic Blue staining method turn the method into an easy-to-use one that seems to visualize most, if not all, enteric neurons in whole-mount preparations of the pig jejunum.

Prostaglandin E2 activation of VIP secretomotor neurons in the guinea pig ileum

The effect of prostaglandin E2 (PGE2) on the activity-related expression of the proto-oncogene c-fos in specific populations of enteric neurons was investigated. Segments of guinea-pig ileum were incubated in vitro in the presence or absence of PGE2, and whole mounts of the myenteric and submucosal plexus were prepared for immunocytochemical localization of Fos, VIP and NPY. Control tissues exhibited a low number of Fos-immunoreactive (Fos-IR) neurons (7 +/- 2% of total). Incubation of the tissues with 10-1000 nM PGE2 for 30 min caused a concentration-dependent increase in Fos-IR submucosal neurons (maximum at 100 nM; 39 +/- 6%), which was not inhibited by TTX. PGE2 did not evoke an increase in Fos-IR myenteric neurons. In double labeling experiments, Fos colocalized exclusively with VIP in the submucosal plexus, and not with NPY. Exposure of stripped segments of guinea pig ileum in Ussing chambers to 100 nM PGE2 evoked an increase in short circuit current (20 +/- 7 microA/cm2), of which the initial rapid phase could be abolished by TTX, and not by atropine and hexamethonium. It is concluded that PGE2 can activate VIP non-cholinergic secretomotor neurons.

Regional differences in the number of neurons in the myenteric plexus of the guinea pig small intestine and colon: an evaluation of markers used to count neurons

BACKGROUND

Subsets of myenteric neurons have been identified. To determine the proportional representation of neurons in each, it is necessary to relate the number of neurons in the subset to that of the complete set. Prior estimates of total numbers of neurons, obtained with many different markers, have varied widely.

METHODS

Markers were compared for counting myenteric neurons in dissected laminar preparations of guinea pig duodenum, jejunum-ileum, and colon; the effect of stretching preparations on these counts was also determined. Markers included the visualization of single-stranded nucleic acid with cuprolinic blue and the immunocytochemical demonstration of neuron specific enolase (NSE), PGP9.5, S-100, and the constitutive expression of a Fos related antigen (FRA).

RESULTS

Neurons could not be counted accurately by demonstrating NSE, PGP9.5, or S-100. The number of neurons detected by demonstrating FRA was consistently less than that determined with cuprolinic blue (approximately 65%). Cuprolinic blue-derived estimates of neuron numbers were higher than most reported in the literature, but comparable to those recently obtained with "a nerve cell body" antiserum. Ganglionic area was found to be stretch independent. The rank order of neurons/cm2 and ganglionic area/ unit resting length was colon > duodenum >> jejunum-ileum; more neurons were found in the myenteric plexus of the colon (7.3 x 10(6)) than in that of the entire small intestine (6.5 x 10(6)).

CONCLUSIONS

Prior studies that have obtained denominators for estimating the proportions of myenteric neuronal subsets with markers that do not reveal the entire population should be re-evaluated. The guinea pig colon contains a surprisingly large number of neurons, the physiological significance of which must be determined.