Jeffrey M. Palmer

Presynaptic inhibition produced by histamine at nicotinic synapses in enteric ganglia

Intracellular methods were used to record fast excitatory postsynaptic potentials in myenteric neurons of the guinea-pig small intestine in vitro. The excitatory postsynaptic potentials were suppressed by hexamethonium, mimicked by acetylcholine and assumed to be mediated by nicotinic cholinergic receptors. Application of histamine either by addition to the superfusion solution or by focal application from fine-tipped pipettes reversibly reduced the amplitude or abolished the excitatory postsynaptic potentials. Postsynaptic responses to focal application of acetylcholine by pressure ejection from micropipettes were either unaffected or were potentiated by histamine. Failure of histamine to affect antidromic action potentials excluded a local anesthetic action on the presynaptic fibers. Neither 2-methylhistamine nor dimaprit, which are selective H1 and H2 agonists respectively, suppressed the excitatory postsynaptic potentials when applied in concentrations nearly one hundred times greater than the ED50 for histamine. The selective H1 and H2 antagonists, pyrilamine and cimetidine did not suppress the inhibitory action of histamine when applied separately or in combination. Based on these results, the presynaptic receptors involved in this inhibitory mechanism appeared to be of a pharmacologically atypical histamine receptor subtype. The putative histamine agonist, N,alpha-methylhistamine, which has been reported to have high stereoselectivity and activity for a receptor subtype classified as H3, potently reduced or abolished the excitatory postsynaptic potentials. The ED50 for N,alpha-methylhistamine was 8.8 nM compared to an ED50 of 220 nM for histamine. Burimamide, a histamine antagonist with higher activity at putative H3 receptors than H2 receptors, effectively reversed the inhibitory action of histamine on the excitatory postsynaptic potentials.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcitonin gene-related peptide excites myenteric neurons

Intracellular methods were used to record electrical behavior of myenteric neurons in guinea-pig ileum in vitro. Calcitonin gene-related peptide (CGRP; 1 nM to 1 microM) and calcitonin (1-100 microM) were applied by addition to the superfusion solution of longitudinal muscle-myenteric plexus preparations. Both peptides were applied also by pressure ejection from fine-tipped micropipettes. CGRP, applied by either method, evoked a long-lasting depolarization of the cell membranes that was dose-dependent (ED50 = 50 nM) and was associated with an increase in the input resistance, suppression of post-spike hyperpolarizing potentials and enhanced excitability in all neurons that were tested. The enhanced excitability was reflected by a significant increase in the number of action potentials evoked by intracellular injection of constant current depolarizing pulses. Enhanced excitability also was apparent as a train of spikes that appeared at the crests of the CGRP-induced depolarization. The excitatory action of CGRP simulated slow synaptic excitation. Application of calcitonin did not evoke any changes in electrical behavior of myenteric neurons. The results are consistent with a neurotransmitter or neuromodulator role for CGRP in the enteric nervous system and suggest that it may participate in local neurohumoral regulation of gastrointestinal effector systems.

Neuroimmune communication in the submucous plexus of guinea pig colon after infection with Trichinella spiralis

BACKGROUND/AIMSnEnteric neuroimmune communication in gastrointestinal hypersensitivity responses includes antigen detection by mast cells and release of chemical messages to the enteric nervous system. The aim of this study was to analyze the electrical and synaptic behavior of neurons in the colonic submucous plexus during exposure to Trichinella spiralis antigen in animals infected earlier with the parasite.nnnMETHODSnMicroelectrodes were used to record in submucous neurons of guinea pig distal colon during application of Trichinella antigen.nnnRESULTSnNeurons in sensitized animals were more excitable than in controls. Hyperexcitability was seen as a greater probability of spontaneous action potential discharge and repetitive firing to depolarizing current or exposure to acetylcholine. Application of histaminergic antagonists reversed the augmented excitability, suggesting endogenously released histamine as a responsible factor. Antigenic exposure increased neuronal excitability and suppressed nicotinic transmission at fast cholinergic synapses only in sensitized animals. Effects on excitability, but not presynaptic inhibitory effects, were blocked by cimetidine.nnnCONCLUSIONSnSignaling between mucosal mast cells and the enteric nervous system is involved in colonic anaphylactic responses to sensitizing antigens. Histamine is a paracrine signal in the communication pathway.

GLP-2 Receptor Agonism Ameliorates Inflammation and Gastrointestinal Stasis in Murine Postoperative Ileus

Glucagon-like peptide 2 (GLP-2) is a pleiotropic intestinotrophic hormone that we hypothesized could lessen gastrointestinal inflammation associated with postoperative ileus (POI). To test this idea, the prophylactic timing and dose of a long-acting variant of human GLP-2 linked to the Fc portion of murine immunoglobulin G (IgG) (GLP-2/IgG) was optimized in a murine model of POI. Surgically treated mice received a single dose of GLP-2/IgG, IgG isotype control, or phosphate-buffered saline 1 to 48 h before small bowel surgical manipulation. The distribution of orally fed fluorescein isothiocyanate-dextran and histological analyses of myeloperoxidase-positive immune cells were determined 24 and 48 h postoperatively. TaqMan quantitative polymerase chain reaction was used to determine early changes in mRNA expression in the muscularis or mucosa. In normal mice, prolonged exposure to GLP-2 increased upper gastrointestinal (GI) transit and mucosal weight. When administered 1 or 3 h before surgery, GLP-2/IgG reduced the leukocyte infiltrate 24 and 48 h postoperatively and improved GI transit 48 h postoperatively. Surgical manipulation rapidly increased gene expression of proinflammatory cytokines and enzymes for kinetically active mediators in the mucosa and muscularis. GLP-2/IgG2a affected the expression of genes associated with mucosal inflammation and barrier function. We conclude that prophylactic treatment with a long-acting GLP-2 agonist ameliorates inflammation and improves intestinal dysmotility associated with surgical manipulation of the bowel. The action of GLP-2 is consistent with a lessening of inflammation, leading to a more rapid recovery.

Neuropharmacology of the muscarinic antagonist telenzepine in myenteric ganglia of the guinea-pig small intestine

Intracellular recording methods were used to investigate the actions of the putative M1 muscarinic receptor antagonist telenzepine on the electrical and synaptic behavior of myenteric neurons. Telenzepine had no effect on resting membrane potential, input resistance, excitability and antidromic potentials in both AH/type 2 and S/type 1 neurons, when applied in concentrations of 0.1-2000 nM, although higher concentrations (10-100 microM) did have a significant non-specific effect on the postsynaptic membrane. Micromolar concentrations of telenpzepine (1-2 microM) had no effect on excitatory responses to substance P, vasoactive intestinal peptide, the nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium or the nicotinic action of acetylcholine. Nicotinic fast excitatory postsynaptic potentials were also unaffected by 2 microM telenzepine. In contrast, at submicromolar concentrations (100 nM), telenzepine abolished responses to either muscarine or the muscarinic component of the acetylcholine response. The excitatory effect of muscarine at postsynaptic M1 receptors was dose dependently inhibited by telenzepine (0.1-1000 nM) at concentrations which had no effect on the electrical properties of the cells. This effect was slowly reversible, usually requiring more than 60 min for significant recovery. The threshold dose of telenzepine as an antagonist of the muscarinic depolarization in AH/type 2 neurons was in the range of 0.1-1 nM. The IC50 concentration of telenzepine needed to abolish the response was 8.5 nM. A small proportion of stimulus-evoked slow excitatory postsynaptic potentials in both AH/type 2 and S/type 1 cells were abolished by 1 microM telenzepine, while the majority of them remained unaffected, indicating that some slow excitatory postsynaptic potentials are mediated by the muscarinic action of released acetylcholine.(ABSTRACT TRUNCATED AT 250 WORDS)