Cristina Giaroni

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.

Adrenergic mechanisms in the control of gastrointestinal motility: From basic science to clinical applications

Over the years, a vast literature has accumulated on the adrenergic mechanisms controlling gut motility, blood flow, and mucosal transport. The present review is intended as a survey of key information on the relevance of adrenergic mechanisms modulating gut motility and will provide an outline of our knowledge on the distribution and functional role of adrenoceptor subtypes mediating motor responses. alpha1-Adrenoceptors are located postsynaptically on smooth muscle cells and, to a lesser extent, on intrinsic neurons; alpha2-adrenoceptors may be present both pre- and postsynaptically, with presynaptic auto- and hetero-receptors playing an important role in the modulation of neurotransmitter release; beta-adrenoceptors are found mainly on smooth muscle cells. From a clinical standpoint, adrenoceptor agonists/antagonists have been investigated as potential motility inhibiting (antidiarrheal/antispasmodic) or prokinetic agents, although at present their field of application is limited to select patient groups.

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.

N-Methyl-di-aspartate receptors modulate neurotransmitter release and peristalsis in the guinea pig isolated colon ☆

To assess the role of NMDA receptors in modulating neurotransmitter release in the myenteric plexus, we studied the effects of L-glutamic acid and NMDA on endogenous acetylcholine and noradrenaline overflow (assayed by HPLC) from the guinea pig isolated distal colon. L-Glutamic acid and NMDA enhanced electrically evoked acetylcholine and noradrenaline overflow and these effects were reversed by selective NMDA receptor antagonists. The possible functional significance of these findings was studied by measuring the efficiency of the colonic peristaltic reflex in the presence of NMDA receptor agonists. NMDA inhibited propulsion velocity at all concentrations tested, this effect being antagonized by (+/-)-2-amino-5-phosphonopentanoic acid and virtually abolished in sympathetically denervated animals. In conclusion, the inhibitory effect of NMDA on peristalsis, being almost entirely dependent on the integrity of sympathetic pathways, could be, at least in part, due to NMDA-induced noradrenaline release.

Postnatal development of P2 receptors in the murine gastrointestinal tract

The actions of purine and pyrimidine compounds on isolated segments of the mouse intestine were investigated during postnatal development. The localization of P2Y(1), P2Y(2), P2Y(4), P2X(1,) P2X(2) and P2X(3) receptors were examined immunohistochemically, and levels of expression of P2Y(1), P2X(1) and P2X(2) were studied by Western immunoblot. From day 12 onwards, the order of potency for relaxation of longitudinal muscle of all regions was 2-MeSADP>or=alpha,beta-meATP>or=ATP=UTP=adenosine, suggesting P2Y(1) receptors. This was supported by the sensitivity of responses to 2-MeSADP to the selective antagonist MRS 2179 and P2Y(1) receptor immunoreactivity on longitudinal muscle and a subpopulation of myenteric neurons. A further alpha,beta-meATP-sensitive P2Y receptor subtype was also indicated. ATP and UTP were equipotent suggesting a P2Y(2) and/or P2Y(4) receptor. Adenosine relaxed the longitudinal muscle in all regions via P1 receptors. The efficacy of all agonists to induce relaxation of raised tone preparations increased with age, being comparable to adult by day 20, the weaning age. During postnatal development the contractile response of the ileum and colon was via P2Y(1) receptors, while the relaxant response mediated by P2Y(1) receptors gradually appeared along the mouse gastrointestinal tract, being detectable in the stomach from day 3 and in the duodenum from day 6. In the ileum and colon relaxant responses to 2-MeSADP were not detected until days 8 and 12, respectively. 2-MeSADP induced contractions on basal tone preparations from day 3, but decreased significantly at day 12 and disappeared by day 20. At day 8, contractions of colonic longitudinal muscle to ATP showed no desensitisation suggesting the involvement of P2X(2) receptors. Immunoreactivity to P2X(2) receptors only was observed on the longitudinal muscle of the colon and ileum from day 1 and on a subpopulation of myenteric neurons from day 3. These data suggest that P2Y(1) receptors undergo postnatal developmental changes in the mouse gut, with a shift from contraction to relaxation. Such changes occur 1 week before weaning and may contribute to the changes that take place in the gut when the food composition changes from maternal milk to solid food.

Calcium-channel blockers and gastrointestinal motility: Basic and clinical aspects

Several calcium-channel blockers currently in use for the treatment of cardiovascular disorders have recently been tested for their effects on gastrointestinal motility. The rationale for this approach centers on the concept that calcium-channel blockers are at least as potent in inhibiting intestinal smooth muscle as in relaxing vascular smooth muscle. This review will give an outline of the most recent findings on the role of calcium and calcium channels in smooth muscle and neuronal function in the digestive system. It will also consider the mechanisms by which calcium-channel blockers may affect gastrointestinal motility and assess potential clinical applications in gastroenterology. The main goal for researchers in this field will be the development of gut-selective agents, with no cardiovascular side effects.

Glutamate receptors of the AMPA type modulate neurotransmitter release and peristalsis in the guinea-pig isolated colon

To assess the role of AMPA and kainate receptors in modulating neurotransmitter release from the myenteric plexus, the effect of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and kainic acid on endogenous acetylcholine (ACh) and noradrenaline (NA) overflow from the guinea-pig isolated colon was studied. AMPA inhibited spontaneous ACh overflow and increased electrically-evoked NA overflow. Kainic acid did not influence both ACh and NA overflow. AMPA-mediated effects on ACh and NA overflow were significantly reduced by the AMPA/kainate antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, CNQX. The inhibitory effect of AMPA on ACh overflow could be due, at least in part, to the AMPA-induced NA overflow as it was greatly reduced after adrenoceptor blockade and virtually abolished in sympathetically-denervated animals. The possible functional significance of these findings was studied by measuring the efficiency of the peristaltic reflex in the presence of the different agonists. The efficiency of peristalsis was enhanced by AMPA, whereas it was not modified by kainic acid. In conclusion, AMPA receptors, but not kainate receptors, may play a role in the modulation of ACh and NA release and of peristalsis in the guinea-pig colon.

Tonic modulation of neurotransmitter release in the guinea-pig myenteric plexus: effect of μ and κ opioid receptor blockade and of chronic sympathetic denervation ☆

We have studied the effects of mu- and kappa-opioid receptor blockade on endogenous acetylcholine and noradrenaline overflow from the myenteric plexus of the guinea-pig isolated colon. Cyprodime (putative mu-selective antagonist) and nor-binaltorphimine (kappa-selective antagonist) had a concentration-dependent facilitatory effect on both acetylcholine and noradrenaline overflow. Moreover, in colonic specimens obtained from sympathetically denervated animals, the effect of opioid antagonists on acetylcholine overflow was significantly higher with respect to normal preparations. Evidence is thus given in favour of an involvement of mu- and kappa-opioid receptor pathways in the tonic modulation of neurotransmitter release at the colonic level. Enhanced sensitivity to the effect of mu and kappa antagonists after chronic sympathetic denervation is strongly suggestive for the existence of a functional link between opioid and adrenergic pathways in this model.

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.

Modulation of neurotransmitter release by opioid μ- and κ-receptors from adrenergic terminals in the myenteric plexus of the guinea-pig colon: effect of α2-autoreceptor blockade

We have studied the effect of [D-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin (DAMGO, opioid mu-receptor agonist) and ICI-204,448 (kappa-receptor agonist) on endogenous noradrenaline release in the guinea-pig isolated distal colon. DAMGO enhances noradrenaline over-flow and this effect is antagonized by naloxone (pIC50 = 10.27) and nor-binaltorphimine (pIC50 = 7.97), and concentration-dependently turned into inhibition by yohimbine. ICI-204,448 inhibits noradrenaline overflow and is antagonized by naloxone (pIC50 = 9.38) and nor-binaltorphimine (pIC50 = 10.48), but is not affected by yohimbine. Evidence is thus given that mu- and kappa-opioid receptors modulate noradrenaline release in the guinea-pig colon. Modifications by yohimbine of the effect of DAMGO indicate the existence of a functional relationship between mu-receptors and alpha(2)-autoreceptors in this model.