Acetylcholine detection by a modified HPLC-ED method improves the assessment of cholinergic function in the myenteric plexus of the guinea-pig colon

Acetylcholine Release from Urinary Bladder Smooth Muscles of Non-Insulin-Dependent Diabetic Rats

INTRODUCTION

To investigate the mechanism of voiding dysfunction in non-insulin-dependent diabetes mellitus, we attempted to measure the acetylcholine (ACh) release using an in vivo microdialysis technique and measuring the detrusor pressure after electrical field stimulation (EFS) of the pelvic nerve.

MATERIALS AND METHODS

Eight- and 32-week-old female Goto-Kakizaki (GK) rats (non-insulin-dependent diabetes mellitus model) and age-matched female Wistar rats (controls) were used in this study. The pelvic nerve was exposed on a bipolar platinum electrode to EFS, and a cannula was inserted into the bladder to measure the detrusor pressure. The microdialysis probe was inserted into the bladder wall and was connected to a microinfusion syringe pump. Dialysate was constantly perfused, collected in a microtube, and then injected into the ACh assay system. Histological examinations were performed by staining with hematoxylin and eosin and S-100 immunohistochemical staining in bladder preparations of both GK and control rats.

RESULTS

In 8-week-old rats, both detrusor pressures and amounts of ACh release of GK rats were not significantly different from those of control rats. In 32-week-old rats, both detrusor pressures and ACh releases were only significantly increased at 5 and 10 Hz of EFS. In the histological study, the number of nerve fibers or bundles of 32-week-old GK rats was significantly decreased as compared with control rats.

CONCLUSION

The present data suggest that the decrease in EFS-induced ACh release in GK rats, which may be caused by the decreased number of nerve fibers, may contribute to the decrease in bladder contractions.

Non-neuronal cholinergic system in human bladder urothelium

OBJECTIVES

To evaluate the non-neuronal cholinergic system in the isolated human bladder. Recently, several reports have demonstrated that non-nerve-evoked acetylcholine (ACh) release may contribute to various pathophysiologic conditions.

METHODS

Human bladders were obtained from 15 patients. Bladder strips with or without urothelium were suspended in an organ bath filled with Krebs-Henseleit solution. ACh release was measured using microdialysis and high-performance liquid chromatography. The contribution of urothelium to non-nerve-evoked ACh release was tested, and the effects of age and elevation of the resting tension (0 to 40 mN) on ACh release were also evaluated. Furthermore, choline acetyltransferase (ChAT) immunohistochemical staining of bladder was performed.

RESULTS

Immunohistochemistry showed marked ChAT-positive staining in the urothelium and suburothelial region. Tetrodotoxin-insensitive non-nerve-evoked ACh release occurred and was significantly greater in strips with than without urothelium. The non-nerve-evoked ACh release from strips with urothelium increased with age, and the positive correlation between age and release was significant. Stretching of the strips caused increases in non-nerve-evoked ACh release that was significantly greater in strips with than without urothelium. The stretch-induced release of non-nerve-evoked ACh increased with age.

CONCLUSIONS

These data have demonstrated that a non-neuronal cholinergic system is present in the human bladder and that the urothelium/suburothelium partially releases ACh. We found a significant age-related and stretch-induced increase in non-nerve-evoked ACh release. This suggests that the non-neuronal cholinergic system may contribute to the physiology and pathophysiology of human bladder function.

Involvement of glutamate receptors of the NMDA type in the modulation of acetylcholine and glutamate overflow from the guinea pig ileum during in vitro hypoxia and hypoglycaemia

The involvement of NMDA glutamate receptors in the effects of glucose/oxygen deprivation (in vitro ischaemia) on spontaneous endogenous acetylcholine and glutamate overflow from the guinea pig ileum was studied. Neurotransmitter overflow was measured by HPLC. Deprivation of glucose in the medium slightly reduced acetylcholine overflow, and did not significantly influence glutamate overflow. During oxygen deprivation and glucose/oxygen deprivation, acetylcholine overflow augmented with a biphasic modality: an early peak was followed by a long lasting increase, whereas glutamate overflow increased with a rapid and sustained modality. The effects of glucose/oxygen deprivation on both acetylcholine and glutamate overflow were abolished after reperfusion with normal oxygenated medium. Acetylcholine and glutamate overflow induced by glucose/oxygen deprivation were significantly reduced in the absence of external Ca(2+) as well as by the addition of the mitochondrial Na(+)-Ca(2+) exchanger blocker, CGP 37157, and of the endoplasmic reticulum Ca(2+)/ATPase blocker, thapsigargin. +/-AP5, an NMDA receptor antagonist, and 5,7-diCl-kynurenic acid, an antagonist of the glycine site associated to NMDA receptor, markedly depressed glucose/oxygen deprivation-induced acetylcholine and glutamate overflow as well. Our results suggest that in vitro simulated ischaemia evokes acetylcholine and glutamate overflow from the guinea pig ileum, which is partly linked to an increase in intracellular Ca(2+) concentration dependent on both Ca(2+) influx from the extracellular space and Ca(2+) mobilization from the endoplasmic reticulum and mitochondrial stores. During glucose/oxygen deprivation, ionotropic glutamate receptors of the NMDA type exert both a positive feedback modulation of glutamate output and contribute to increased acetylcholine overflow.

Involvement of protein kinase C in the adaptive changes of cholinergic neurons to sympathetic denervation in the guinea pig myenteric plexus

Supersensitivity to muscarinic, kappa- and mu-opioid agents modulating cholinergic neurons in the guinea pig colon develops after chronic sympathetic denervation. A possible role for protein kinase C (PKC) in contributing to development of these sensitivity changes was investigated. The PKC activator, phorbol-12-myristate-13-acetate (PMA), enhanced acetylcholine (ACh) overflow in preparations obtained from normal animals. The facilitatory effect of PMA was significantly reduced after prolonged exposure to the phorbol ester and by the PKC inhibitors, chelerythrine and calphostin C. Subsensitivity to the facilitatory effect of PMA developed after chronic sympathetic denervation. In this experimental condition, immunoblot analysis revealed reduced levels of PKC in myenteric plexus synaptosomes. The facilitatory effect of the muscarininc antagonist, scopolamine, on ACh overflow was significantly reduced by the phospolipase C (PLC) inhibitor, U73122, chelerythrine and calphostin C, both in normal and denervated animals. However, in both experimental groups, PLC antagonists and PKC antagonists did not affect the inhibitory effect of the muscarinic agonist, oxotremorine-M on ACh overflow. The inhibitory effects of U69593 (kappa-opioid receptor agonist) and DAMGO (mu-opioid receptor agonist) on ACh overflow significantly increased in the presence of U73122, chelerythrine and calphostin C in preparations obtained from normal animals, but not in those obtained from sympathetically denervated animals. These results indicate that activation of PKC enhances ACh release in the myenteric plexus of the guinea pig colon. At this level, chronic sympathetic denervation entails a reduced efficiency of the enzyme. In addition, PKC is involved in the inhibitory modulation of ACh release mediated by muscarinic-, kappa- and mu-opioid receptors, although with different modalities. Muscarinic receptors inhibit PKC activity, whereas kappa- and mu-opioid receptors increase PKC activity. Both the inhibitory and the facilitatory effect on PKC involve modulation of PLC activity. The possibility that the change in PKC activity represents one of the biochemical mechanisms at the basis of development of sensitivity changes to opioid and muscarinic agents after chronic sympathetic denervation is discussed.

Evidence for a glutamatergic modulation of the cholinergic function in the human enteric nervous system via NMDA receptors

Several reports suggest that enteric cholinergic neurons are subject to a tonic inhibitory modulation, whereas few studies are available concerning the role of facilitatory pathways. Glutamate, the main excitatory neurotransmitter in the central nervous system (CNS), has recently been described as an excitatory neurotransmitter also in the guinea-pig enteric nervous system (ENS). The present study aimed at investigating the presence of glutamatergic neurons in the ENS of the human colon. At this level, the presence of ionotropic glutamate receptors of the NMDA type, and their possible interaction with the enteric cholinergic function was also studied. In the human colon, L-glutamate and NMDA concentration dependently enhance spontaneous endogenous acetylcholine overflow in Mg2+-free buffer, both effects being significantly reduced by the antagonists, (+/-)-2-amino-5-phosphonopentanoic acid (+/- AP5) and 5,7-diCl-kynurenic acid. In the presence of Mg2+, the facilitatory effect of L-glutamate changes to inhibition, while the effect of NMDA is significantly reduced. In addition, morphological investigations reveal that glutamate- and NR1-immunoreactivities are present in enteric cholinergic neurons and glial cells in both myenteric and submucosal plexus. These findings suggest that, as described for the guinea-pig ileum, glutamatergic neurons are present in enteric plexuses of the human colon. Modulation of the cholinergic function can be accomplished through NMDA receptors.

Improved method for the routine determination of acetylcholine and choline in brain microdialysate using a horseradish peroxidase column as the immobilized enzyme reactor

A modified microbore high-performance liquid chromatography-immobilized enzyme reactor-electrochemical detection system for acetylcholine (ACh) and choline (Ch) was developed. The system used the horseradish peroxidase and a solution mediator ferrocene to convert the analyte into an oxidized ferrocene species which was detected electrochemically by reduction at 0 mV. There was an excellent linear relationship between the concentration of ACh/Ch and the peak height over the range of 1-5000 nmol/l. The limit of detection for ACh was 2 fmol/5 microl (S/N=3:1). Compared with the common method recommended by Bioanalytical System Inc. (BAS), this method exhibits a 200-fold improvement in the detection limit. The ACh and Ch levels in rat brain microdialysate were examined.

Sympathetic denervation-induced changes in G protein expression in enteric neurons of the guinea pig colon

Chronic sympathetic denervation entails subsensitivity to alpha(2)-adrenoceptor agonists and supersensitivity to kappa- and mu-opioid receptor agonists modulating cholinergic neurons in the guinea pig colon. A possible role for signal transduction G proteins in contributing to development of these sensitivity changes was investigated. Pertussis toxin (PTX), a blocker of the G(i/o)-type family of G proteins significantly reduced the inhibitory effects of UK14,304 (alpha(2)-adrenoceptor agonist), U69593 (kappa-opioid receptor agonist) and DAMGO (mu-opioid receptor agonist) on acetylcholine (ACh) overflow in preparations obtained from normal animals, but not in those obtained from sympathetically denervated animals. In this experimental condition, immunoblot analysis revealed reduced levels of G(alphao), G(alphai2), G(alphai3) and G(beta) in myenteric plexus synaptosomes. On reverse, synaptosomal levels of G(alphai1) and G(alphaz), a PTX-insensitive G-protein, increased after chronic ablation of the sympathetic pathways. These data suggest that changes in the function and expression of inhibitory G proteins coupled to alpha(2)-adrenoceptors, kappa- and mu-opioid receptors occur in the myenteric plexus of the guinea pig colon after chronic sympathetic denervation. The possibility that regulation of G proteins represents one of the biochemical mechanisms at the basis of the changes in sensitivity of enteric cholinergic neurons to alpha(2)-adrenoceptor, kappa- and mu-opioid receptor agonists is discussed.

Modulation of enteric cholinergic neurons by hetero- and autoreceptors: cooperation among inhibitory inputs

In the guinea-pig colon, acetylcholine (ACh) release from intrinsic cholinergic motor neurons is inhibited by adrenoceptors, opioid and muscarinic receptors. Chronic sympathetic denervation resulted in supersensitivity to the inhibitory effect of DAMGO (mu-opioid agonist) on ACh release and on the peristaltic reflex. After chronic treatment with naltrexone (NTX) supersensitivity to DAMGO and subsensitivity to UK14,304 (alpha2-adrenoceptor agonist) developed for both functional parameters. The facilitatory effect of scopolamine on ACh release remained unchanged after chronic NTX treatment, whereas it was potentiated after chronic sympathetic denervation. These data suggest the existence of a functional interaction between different inhibitory pathways modulating cholinergic motor neurons in the guinea-pig colon. Namely, chronic manipulation of an inhibitory pathway may entail adaptive sensitivity changes in another inhibitory pathway so that homeostasis can be maintained.