Induction of neuronal nitric oxide synthase by sympathetic denervation is mediated via alpha 2-adrenoceptors in the jejunal myenteric plexus

Priapism caused by partial deficiency of tetrahydrobiopterin through hypofunction of the sympathetic neurons in sepiapterin reductase gene-disrupted mice

6R-L-erythro-5,6,7,8-tetrahydrobiopterin (BH4) is an essential cofactor for aromatic L-amino acid hydroxylases, including tyrosine hydroxylase (TH), alkylglycerol monooxygenase, and three types of nitric oxide (NO) synthases (NOS). Sepiapterin reductase (SPR) catalyzes the third step of BH4 biosynthesis. SPR gene-disrupted (Spr-/- ) mice exhibit a dystonic posture, low body weight, hyperphenylalaninemia, and unstable hypertension with endothelial dysfunction. In this study, we found that Spr-/- mice suffered from a high incidence of severe priapism. Their erections persisted for months. The biopterin, BH4, and norepinephrine contents, and TH protein levels in the penile tissue of Spr-/- mice without and with priapism were significantly reduced compared to those of Spr+/+ mice. In contrast, their neural NOS (nNOS) protein levels were increased, and the cyclic guanosine monophosphate (cGMP) levels were remarkably elevated in the penises of Spr-/- mice with priapism. The symptoms were relieved by repeated administration of BH4. The biopterin, BH4, and norepinephrine contents were increased in penile homogenates from BH4-supplemented Spr-/- mice, and the TH protein levels tended to increase, and their nitrite plus nitrate levels were significantly lower than those of vehicle-treated Spr-/- mice and were approximately the same as vehicle- and BH4-supplemented Spr+/+ mice. Thus, we deduced that the priapism of Spr-/- mice is primarily caused by hypofunction of the sympathetic neurons due to cofactor depletion and the loss of TH protein and, further, dysregulation of the NO/cGMP signaling pathway, which would be caused by disinhibition of nNOS-containing neurons and/or abnormal catabolism of cyclic nucleotides is suggested.

Calcium-sensing receptor (CaSR) agonist R568 inhibits small intestinal motility of mice through neural and non-neural mechanisms

Gastrointestinal motility (GI) disorder causes symptoms such as dyspepsia, abdominal distention, and constipation and severely affects quality of life. The calcium (Ca²⁺)-sensing receptor (CaSR) expressed in the digestive tract can be activated by amino acids and participates in GI motility regulation. This study is designed to explore the effect and underlying mechanism of CaSR agonist R568 on the small intestine motility of mice in vivo and ex vivo. R568 was given to male C57BL/6 mice by gavage or incubated with isolated jejunum and ileum segments to observe its effects on GI motility and the involved neurons, neurotransmitters and hormones were detected by fluorescence immunohistochemistry and enzyme-linked immunosorbent assays. The in vivo results showed that the intestinal propulsive rate reduced in response to oral intake of R568. R568 treatment increased the numbers of nitric oxide synthase-positive neurons and nitric oxide release but decreased the choline acetyl transferase-positive neurons and acetylcholine release in the myenteric plexuses. R568 increased the secretion of cholecystokinin in the intestinal tissues and serum but had no effect on the secretion of glucagon like peptide-1. Ex vivo results showed that R568 inhibited the contractility of intestinal strips from the jejunum and ileum. Nitric oxide synthase (NOS) inhibitor L-nitroarginine methyl ester (L-NAME), M-receptor antagonist-atropine, and tetrodotoxin (TTX) failed to block the effect of R568. CaSR co-expressed with interstitial cells of Cajal (ICCs) in the myenteric plexus suggests the possibility that ICCs mediated the effect of R568. Our findings demonstrate that CaSR activation inhibited intestinal motility, and both the enteric nervous system and non-neural mechanism are involved in this process.

Long-term dietary L-arginine supplementation increases endothelial nitric oxide synthase and vasoactive intestinal peptide immunoexpression in rat small intestine

BACKGROUND AND AIMS

Nitric oxide (NO) and vasoactive intestinal polypeptide (VIP) are important intestinal neurotransmitters that coexist in the gut enteric nervous system and play an important role in intestinal physiology (e.g., absorption, motility, fluid secretion and smooth muscle relaxation). It is also known that cold exposure alters several aspects of gastrointestinal physiology and induces hyperphagia to meet increased metabolic demands, but there are no data regarding NO and VIP involvement in intestinal response during acclimation to cold. The objective of this study was to determine the influence of long-term L-arginine supplementation on the expression of the three isoforms of nitric oxide synthase (NOS) and VIP in small intestine of rats acclimated to room temperature or cold.

METHODS

Animals (six per group) acclimated to room temperature (22 ± 1 °C) and cold (4 ± 1 °C), respectively, were treated with 2.25% L-arginine, a substrate for NOSs, or with 0.01% N(ω)-nitro-L-arginine methyl ester, an inhibitor of NOSs, for 45 days. The topographical distribution of VIP and NOSs expression in small intestine was studied by immunohistochemistry, and ImageJ software was used for semiquantitative densitometric analysis of their immunoexpression.

RESULTS

Long-term dietary L-arginine supplementation increases VIP and NOSs immunoexpression at room temperature while at cold increases the endothelial NOS, inducible NOS and VIP but decrease neuronal NOS in rat small intestine.

CONCLUSION

Our results demonstrate that long-term dietary L-arginine supplementation modulates NOSs and VIP immunoexpression in rat small intestine with respect to ambient temperature, pointing out the eNOS as a predominant NOS isoform with an immunoexpression pattern similar to VIP.

Expression and function of intestinal hexose transporters after small intestinal denervation

BACKGROUND

The role of neural regulation in expression and function of intestinal hexose transporters is unknown. The aim of this study is to determine the role of intestinal innervation in gene expression and function of the membrane hexose transporters, SGLT1, GLUT2, and GLUT5 in the enterocyte. We hypothesize that denervation of the small intestine decreases expression of hexose transporters, which leads to decreased glucose absorption.

METHODS

Six groups of Lewis rats were studied (n = 6 each) as follows: control, 1 week after sham laparotomy, 1 and 8 weeks after syngeneic (no immune rejection) orthotopic small-bowel transplantation (SBT) (SBT1 and SBT8) to induce complete extrinsic denervation, and 1 and 8 weeks after selective disruption of intrinsic neural continuity to jejunoileum by gut transection and reanastomosis (T/A1 and T/A8). All tissue was harvested between 8 AM and 10 AM. In duodenum, jejunum, and ileum, mucosal messenger RNA (mRNA) levels were quantitated by real-time polymerase chain reaction (PCR), protein by Western blotting, and transporter-mediated glucose absorption using the everted sleeve technique.

RESULTS

Across the 6 groups, the relative gene expression of hexose transporter mRNA and protein levels were unchanged, and no difference in transporter-mediated glucose uptake was evident in any region. The glucose transporter affinity (K(m)) and functional transporter levels (V(max)) calculated for duodenum and jejunum showed no difference among the 6 groups.

CONCLUSION

Baseline regulation of hexose transporter function is not mediated tonically by intrinsic or extrinsic neural continuity to the jejunoileum.

Role of selective alpha and beta adrenergic receptor mechanisms in rat jejunal longitudinal muscle contractility

Gut motility is modulated by adrenergic mechanisms. The aim of our study was to examine mechanisms of selective adrenergic receptors in rat jejunum. Spontaneous contractile activity of longitudinal muscle strips from rat jejunum was measured in 5-ml tissue chambers. Dose-responses (six doses, 10(-7) -3 x 10(-5)M) to norepinephrine (NE, nonspecific), phenylephrine (PH, alpha1), clonidine (C, alpha2), prenalterol (PR, beta1), ritodrine (RI, beta2), and ZD7714 (ZD, beta3) were evaluated with and without tetrodotoxin (TTX, nerve blocker). NE(3 x 10(-5)M) inhibited 74 +/- 5% (mean +/- SEM) of spontaneous activity. This was the maximum effect. The same dose of RI(beta2), PH(alpha1), or ZD(beta(3)) resulted in an inhibition of only 56 +/- 5, 43 +/- 4, 33 +/- 6, respectively. The calculated concentration to induce 50% inhibition (EC50) of ZD(beta3) was similar to NE, whereas higher concentrations of PH(alpha1) or RI(beta2) were required. C(alpha2) and PR(beta1) had no effect. TTX changed exclusively the EC50 of RI from 4.4 +/- 0.2 to 2.7 +/- 0.8% (p < 0.04). Contractility was inhibited by NE (nonspecific). PH(alpha1), RI(beta2), and ZD(beta3) mimic the effect of NE. TTX reduced the inhibition by RI. Our results suggest that muscular alpha1, beta2, and beta3 receptor mechanisms mediate adrenergic inhibition of contractility in rat jejunum. beta2 mechanisms seem to involve also neural pathways.

Activity-dependent regulation of tyrosine hydroxylase expression in the enteric nervous system

The regulation of neuromediator expression by neuronal activity in the enteric nervous system (ENS) is currently unknown. Using primary cultures of ENS derived from rat embryonic intestine, we have characterized the regulation of tyrosine hydroxylase (TH), a key enzyme involved in the synthesis of dopamine. Depolarization induced either by 40 mm KCl, veratridine or by electrical field stimulation produced a robust and significant increase in the proportion of TH immunoreactive (TH-IR) neurons (total neuronal population was identified with PGP9.5 or Hu) compared to control. This increase in the proportion of TH-IR neurons was significantly reduced by the sodium channel blocker tetrodotoxin (0.5 microm), demonstrating that neuronal activity was critically involved in the effects of these depolarizing stimuli. KCl also increased the proportion of VIP-IR but not nNOS-IR enteric neurons. The KCl-induced increase in TH expression was partly reduced in the presence of the nicotinic receptor antagonist hexamethonium (100 microm), of noradrenaline (1 microm) and of the alpha(2)-adrenoreceptor agonist clonidine (1 microm). Combining pharmacological and calcium imaging studies, we have further shown that L-type calcium channels were involved in the increase of TH expression induced by KCl. Finally, using specific inhibitors, we have shown that both protein kinases A and C as well as the extracellular signal-regulated kinases were required for the increase in the proportion of TH-IR neurons induced by KCl. These results are the first demonstration that TH phenotype of enteric neurons can be regulated by neuronal activity. They could also set the basis for the study of the pathways and mechanisms involved in the neurochemical plasticity observed both during ENS development and in inflammatory enteric neuropathies.

Restraint stress alters the duodenal expression of genes important for lipid metabolism in rat

Stress, such as trauma and injury, is known to cause transcriptional changes in various tissues; however, there is little information on tissue-specific gene expression in response to stress. Here, we have examined duodenal gene expression in rats subjected to whole-body immobilization in order to elucidate the mechanism underlying the stress response in the duodenum--one of the tissues that is most sensitive to external stress. DNA microarray analysis revealed that the immobilization for 2 weeks caused great changes in gene expression in the rat duodenum: 165 genes exhibited more than a two-fold change in expression level (103 up-regulated; 62 down-regulated). In addition, functional classification of these genes showed that immobilization preferentially stimulated the expression of genes related to lipid metabolism, including genes encoding mitochondrial HMG-CoA synthase, a key enzyme in ketogenesis; solute carrier 27A2, a fatty acid transporter; and dienoyl CoA reductase, a key enzyme in beta-oxidation. To elucidate the factors mediating these immobilization-induced changes, we treated rats and small intestinal IEC-6 cells with dexamethasone and hydrogen peroxide. In both rats and IEC-6 cells, treatment with dexamethasone induced changes in gene expression that mimicked the immobilization-mediated increase in expression of the mitochondrial HMG-CoA synthase and dienoyl CoA reductase transcripts, suggesting that stress-induced synthesis of glucocorticoid hormones mediates, at least in part, the stress response in the duodenum. These results suggest that immobilization may alter lipid metabolism in the small intestine by modifying the expression of specific genes through which the small intestine may seek to protect itself from stress-induced damage.

Distribution of adrenergic receptors in the enteric nervous system of the guinea pig, mouse, and rat

Adrenergic receptors in the enteric nervous system (ENS) are important in control of the gastrointestinal tract. Here we describe the distribution of adrenergic receptors in the ENS of the ileum and colon of the guinea pig, rat, and mouse by using single- and double-labelling immunohistochemistry. In the myenteric plexus (MP) of the rat and mouse, alpha2a-adrenergic receptors (alpha2a-AR) were widely distributed on neurons and enteric glial cells. alpha2a-AR mainly colocalized with calretinin in the MP, whereas submucosal alpha2a-AR neurons colocalized with vasoactive intestinal polypeptide (VIP), neuropeptide Y, and calretinin in both species. In the guinea pig ileum, we observed widespread alpha2a-AR immunoreactivity on nerve fibers in the MP and on VIP neurons in the submucosal plexus (SMP). We observed extensive beta1-adrenergic receptor (beta1-AR) expression on neurons and nerve fibers in both the MP and the SMP of all species. Similarly, the beta2-adrenergic receptor (beta2-AR) was expressed on neurons and nerve fibers in the SMP of all species, as well as in the MP of the mouse. In the MP, beta1- and beta2-AR immunoreactivity was localized to several neuronal populations, including calretinin and nitrergic neurons. In the SMP of the guinea pig, beta1- and beta2-AR mainly colocalized with VIP, whereas, in the rat and mouse, beta1- and beta2-AR were distributed among the VIP and calretinin populations. Adrenergic receptors were widely localized on specific neuronal populations in all species studied. The role of glial alpha2a-AR is unknown. These results suggest that sympathetic innervation of the ENS is directed toward both enteric neurons and enteric glia.

Gastrointestinal Function Regulation by Nitrergic Efferent Nerves

Gastrointestinal (GI) smooth muscle responses to stimulation of the nonadrenergic noncholinergic inhibitory nerves have been suggested to be mediated by polypeptides, ATP, or another unidentified neurotransmitter. The discovery of nitric-oxide (NO) synthase inhibitors greatly contributed to our understanding of mechanisms involved in these responses, leading to the novel hypothesis that NO, an inorganic, gaseous molecule, acts as an inhibitory neurotransmitter. The nerves whose transmitter function depends on the NO release are called "nitrergic", and such nerves are recognized to play major roles in the control of smooth muscle tone and motility and of fluid secretion in the GI tract. Endothelium-derived relaxing factor, discovered by Furchgott and Zawadzki, has been identified to be NO that is biosynthesized from l-arginine by the constitutive NO synthase in endothelial cells and neurons. NO as a mediator or transmitter activates soluble guanylyl cyclase and produces cyclic GMP in smooth muscle cells, resulting in relaxation of the vasculature. On the other hand, NO-induced GI smooth muscle relaxation is mediated, not only by cyclic GMP directly or indirectly via hyperpolarization, but also by cyclic GMP-independent mechanisms. Numerous cotransmitters and cross talk of autonomic efferent nerves make the neural control of GI functions complicated. However, the findingsrelated to the nitrergic innervation may provide us a new way of understanding GI tract physiology and pathophysiology and might result in the development of new therapies of GI diseases. This review article covers the discovery of nitrergic nerves, their functional roles, and pathological implications in the GI tract.