Inhibitory effects of NPY on ganglionic transmission in myenteric neurones of the guinea-pig descending colon

Untangling Peripheral Sympathetic Neurocircuits

The sympathetic nervous system plays a critical role in regulating many autonomic functions, including cardiac rhythm. The postganglionic neurons in the sympathetic chain ganglia are essential components that relay sympathetic signals to target tissues and disruption of their activity leads to poor health outcomes. Despite this importance, the neurocircuitry within sympathetic ganglia is poorly understood. Canonically, postganglionic sympathetic neurons are thought to simply be activated by monosynaptic inputs from preganglionic cholinergic neurons of the intermediolateral cell columns of the spinal cord. Early electrophysiological studies of sympathetic ganglia where the peripheral nerve trunks were electrically stimulated identified excitatory cholinergic synaptic events in addition to retrograde action potentials, leading some to speculate that excitatory collateral projections are present. However, this seemed unlikely since sympathetic postganglionic neurons were known to synthesize and release norepinephrine and expression of dual neurochemical phenotypes had not been well recognized. In vitro studies clearly established the capacity of cultured sympathetic neurons to express and release acetylcholine and norepinephrine throughout development and even in pathophysiological conditions. Given this insight, we believe that the canonical view of ganglionic transmission needs to be reevaluated and may provide a mechanistic understanding of autonomic imbalance in disease. Further studies likely will require genetic models manipulating neurochemical phenotypes within sympathetic ganglia to resolve the function of cholinergic collateral projections between postganglionic neurons. In this perspective article, we will discuss the evidence for collateral projections in sympathetic ganglia, determine if current laboratory techniques could address these questions, and discuss potential obstacles and caveats.

Microbiota-modulated CART+ enteric neurons autonomously regulate blood glucose

The gut microbiota affects tissue physiology, metabolism, and function of both the immune and nervous systems. We found that intrinsic enteric-associated neurons (iEANs) in mice are functionally adapted to the intestinal segment they occupy; ileal and colonic neurons are more responsive to microbial colonization than duodenal neurons. Specifically, a microbially responsive subset of viscerofugal CART+ neurons, enriched in the ileum and colon, modulated feeding and glucose metabolism. These CART+ neurons send axons to the prevertebral ganglia and are polysynaptically connected to the liver and pancreas. Microbiota depletion led to NLRP6- and caspase 11-dependent loss of CART+ neurons and impaired glucose regulation. Hence, iEAN subsets appear to be capable of regulating blood glucose levels independently from the central nervous system.

Impact of Intestinal Peptides on the Enteric Nervous System: Novel Approaches to Control Glucose Metabolism and Food Intake

The gut is one of the most important sources of bioactive peptides in the body. In addition to their direct actions in the brain and/or peripheral tissues, the intestinal peptides can also have an impact on enteric nervous neurons. By modifying the endogenousproduction of these peptides, one may expect modify the "local" physiology such as glucose absorption, but also could have a "global" action via the gut-brain axis. Due to the various origins of gut peptides (i.e., nutrients, intestinal wall, gut microbiota) and the heterogeneity of enteric neurons population, the potential physiological parameters control by the interaction between the two partners are multiple. In this review, we will exclusively focus on the role of enteric nervous system as a potential target of gut peptides to control glucose metabolism and food intake. Potential therapeutic strategies based on per os administration of gut peptides to treat type 2 diabetes will be described.

Endogenous peptide YY and neuropeptide Y inhibit colonic ion transport, contractility and transit differentially via Y₁ and Y₂ receptors

BACKGROUND AND PURPOSE

Peptide YY (PYY) and neuropeptide Y (NPY) activate Y receptors, targets under consideration as treatments for diarrhoea and other intestinal disorders. We investigated the gastrointestinal consequences of selective PYY or NPY ablation on mucosal ion transport, smooth muscle activity and transit using wild-type, single and double peptide knockout mice, comparing mucosal responses with those from human colon.

EXPERIMENTAL APPROACH

Mucosae were pretreated with a Y₁ (BIBO3304) or Y₂ (BIIE0246) receptor antagonist and changes in short-circuit current recorded. Colonic transit and colonic migrating motor complexes (CMMCs) were assessed in vitro and upper gastrointestinal and colonic transit measured in vivo.

KEY RESULTS

Y receptor antagonists revealed tonic Y₁ and Y₂ receptor-mediated antisecretory effects in human and wild-type mouse colon mucosae. In both, Y₁ tone was epithelial while Y₂ tone was neuronal. Y₁ tone was reduced 90% in PYY⁻/⁻ mucosa but unchanged in NPY⁻/⁻ tissue. Y₂ tone was partially reduced in NPY⁻/⁻ or PYY⁻/⁻ mucosae and abolished in tetrodotoxin-pretreated PYY⁻/⁻ tissue. Y₁ and Y₂ tone were absent in NPYPYY⁻/⁻ tissue. Colonic transit was inhibited by Y₁ blockade and increased by Y₂ antagonism indicating tonic Y₁ excitation and Y₂ inhibition respectively. Upper GI transit was increased in PYY⁻/⁻ mice only. Y₂ blockade reduced CMMC frequency in isolated mouse colon.

CONCLUSIONS AND IMPLICATIONS

Endogenous PYY and NPY induced significant mucosal antisecretory tone mediated by Y₁ and Y₂ receptors, via similar mechanisms in human and mouse colon mucosa. Both peptides contributed to tonic Y₂-receptor-mediated inhibition of colonic transit in vitro but only PYY attenuated upper GI transit.

Peripheral peptide YY inhibits propulsive colonic motor function through Y2 receptor in conscious mice

Peptide YY (PYY) antisecretory effect on intestinal epithelia is well established, whereas less is known about its actions to influence colonic motility in conscious animals. We characterized changes in basal function and stimulated colonic motor function induced by PYY-related peptides in conscious mice. PYY(3-36), PYY, and neuropeptide Y (NPY) (8 nmol/kg) injected intraperitoneally inhibited fecal pellet output (FPO) per hour during novel environment stress by 90%, 63%, and 57%, respectively, whereas the Y(1)-preferring agonists, [Pro(34)]PYY and [Leu(31),Pro(34)]NPY, had no effect. Corticotrophin-releasing factor 2 receptor antagonist did not alter PYY(3-36) inhibitory action. PYY and PYY(3-36) significantly reduced restraint-stimulated defecation, and PYY(3-36) inhibited high-amplitude distal colonic contractions in restrained conscious mice for 1 h, by intraluminal pressure with the use of a microtransducer. PYY suppression of intraperitoneal 5-hydroxytryptophan induced FPO and diarrhea was blocked by the Y(2) antagonist, BIIE0246, injected intraperitoneally and mimicked by PYY(3-36), but not [Leu(31),Pro(34)]NPY. PYY(3-36) also inhibited bethanechol-stimulated FPO and diarrhea. PYY(3-36) inhibited basal FPO during nocturnal feeding period and light phase in fasted/refed mice for 2-3 h, whereas the reduction of food intake lasted for only 1 h. PYY(3-36) delayed gastric emptying after fasting-refeeding by 48% and distal colonic transit time by 104%, whereas [Leu(31),Pro(34)]NPY had no effect. In the proximal and distal colon, higher Y(2) mRNA expression was detected in the mucosa than in muscle layers, and Y(2) immunoreactivity was located in nerve terminals around myenteric neurons. These data established that PYY/PYY(3-36) potently inhibits basal and stress/serotonin/cholinergic-stimulated propulsive colonic motor function in conscious mice, likely via Y(2) receptors.

Motilin inhibits ganglionic transmission in the myenteric plexus of the guinea-pig ileum

Motilin is a key factor in triggering interdigestive migrating contractions. Our preceding study demonstrated that motilin caused membrane depolarizations in a minority of S and AH neurons in the myenteric plexus of the guinea-pig ileum after 18 h-fasting period; motilin depolarizations were small and seldom triggered action potentials. Then, the present study was undertaken to examine possible electrophysiological actions of motilin on the ganglionic transmission in the myenteric plexus. Intracellular recordings with sharp glass microelectrodes were made from myenteric S neurons having fast excitatory postsynaptic potentials (EPSPs), evoked by focal electrical stimulation. Motilin inhibited the fast EPSPs in amplitude, associated either with or without membrane depolarizations. Results obtained with the paired stimulus method suggested that the site for motilin-induced inhibition of fast EPSPs might be presynaptic. Furthermore, motilin did not decrease postsynaptic sensitivity to ACh, a main neurotransmitter mediating the fast EPSPs. Therefore, it is concluded that motilin might inhibit presynaptically ganglionic transmission in the myenteric plexus of the guinea-pig ileum.

Physiological role of neuropeptide Y in the regulation of the ascending phase of the peristaltic reflex

The physiological role of neuropeptide Y (NPY) and of specific NPY receptors in regulating the intestinal peristaltic reflex was examined in three-compartment flat-sheet preparations of rat colon. Graded muscle stretch or mucosal stimulation applied to the central compartment inhibited NPY release in the orad compartment where ascending contraction was measured. NPY and the Y1-receptor agonist [Leu31, Pro34]NPY inhibited, whereas the selective Y1-receptor antagonist BIBP 3226 augmented ascending contraction and substance P (SP) release in the orad compartment induced by muscle stretch or mucosal stimulation. Neither agonist nor antagonist had any effect on descending relaxation or VIP release in the caudad compartment. The Y2-receptor agonist NPY13-36 and antagonist BIIE 0246 had no effect on peptide release or mechanical response. The results indicate that suppression of a tonic inhibitory influence of NPY neurons on excitatory neurotransmitter release contributes substantially to the orad contractile phase of the peristaltic reflex. The effect of NPY on neurotransmitter release is mediated by Y1 receptors.

Tea catechin, (-)-epigallocatechin gallate, facilitates cholinergic ganglion transmission in the myenteric plexus of the guinea-pig small intestine

Intracellular recordings were made from myenteric S neurons of the guinea-pig ileum. One of the major tea catechins, (-)-epigallocatechin gallate (EGCG at concentrations from 1 to 20 microM), was applied by superfusion to examine its effect on cholinergic ganglion transmission in the myenteric plexus. Fast excitatory postsynaptic potentials (EPSPs) evoked by electrical stimulation to ganglia and/or internodal fiber tracts were augmented in amplitude by EGCG in about 60% of tested neurons without changing the postsynaptic sensitivity to acetylcholine (ACh) applied by ionophoresis. Furthermore, the amplitude-ratio of paired fast EPSPs was increased by EGCG. These results indicated that the site at which EGCG augmented the fast EPSPs was presynaptic. It is concluded that EGCG can facilitate the cholinergic ganglion transmission possibly by increasing the amount of ACh released and, together with its previously described depolarizing action on myenteric neurons, may modulate the activity of the myenteric plexus of the guinea-pig ileum.