Miller, Steven M.

In mammalian peripheral sympathetic ganglia GABA acts presynaptically to facilitate cholinergic transmission and postsynaptically to depolarize membrane potential. The GABA effect on parasympathetic pancreatic ganglia is unknown. We aimed to determine the effect of locally applied GABA on cat pancreatic ganglion neurons. Ganglia with attached nerve trunks were isolated from cat pancreata. Conventional intracellular recording techniques were used to record electrical responses from ganglion neurons. GABA pressure microejection depolarized membrane potential with an amplitude of 17.4 ± 0.7 mV. Electrically evoked fast excitatory postsynaptic potentials were significantly inhibited (5.4 ± 0.3 to 2.9 ± 0.2 mV) after GABA application. GABA-evoked depolarizations were mimicked by the GABAAreceptor agonist muscimol and abolished by the GABAAreceptor antagonist bicuculline and the Cl−channel blocker picrotoxin. GABA was taken up and stored in ganglia during preincubation with 1 mM GABA; β-aminobutyric acid application after GABA loading significantly ( P < 0.05) increased depolarizing response to GABA (15.6 ± 1.0 vs. 7.8 ± 0.8 mV without GABA preincubation). Immunolabeling with antibodies to GABA, glial cell fibrillary acidic protein, protein gene product 9.5, and glutamic acid decarboxylase (GAD) immunoreactivity showed that GABA was present in glial cells, but not in neurons, and that glial cells did not contain GAD, whereas islet cells did. The data suggest that endogenous GABA released from ganglionic glial cells acts on pancreatic ganglion neurons through GABAAreceptors.

Effects of nitric oxide (NO) generated from sodium nitroprusside (SNP) on neurons of mouse superior mesenteric ganglion (SMG) were studied in vitro using intracellular recording techniques. SNP solutions caused a membrane hyperpolarization in the majority (64%) of the neurons tested or a hyperpolarization followed by a depolarization in 8% of the neurons tested. The SNP-induced hyperpolarization persisted in a low-Ca2+ high-Mg2+ solution, indicating a direct effect of NO on the postsynaptic membrane. The hyperpolarizing effect of SNP was reduced or abolished by oxyhemoglobin. Electrical stimulation of the colonic nerves evoked a late slow excitatory postsynaptic potential (late sEPSP) in a population of neurons in normal Krebs solution. The amplitude of the late sEPSP was significantly enhanced in the presence of NG-nitroL-arginine, a NO synthase inhibitor. The results, particularly those observed with the NO synthase inhibitor, suggest that endogenous NO was released in the mouse SMG by repetitive nerve stimulation and that it modulated slow synaptic transmission, presumably by a direct action on ganglionic neurons.

Carbon monoxide (CO) has been postulated to be a messenger in the gastrointestinal tract. The aims of this study were to determine the distribution of heme oxygenase (HO), the source for endogenous CO in the canine jejunum, and to determine the effects of CO on jejunal circular smooth muscle cells. HO-2 isoform was present in a population of myenteric and submucosal neuronal cell bodies, in nerve fibers innervating the muscle layers, and in smooth muscle cells. HO-1 isozyme was not detected in the canine jejunum. Exogenous CO increased whole cell current by 285 ± 86%, hyperpolarized the membrane potential by 8.5 ± 2.9 mV, and increased guanosine 3′,5′-cyclic monophosphate (cGMP) levels in smooth muscle cells. 8-Bromo- cGMP also increased the whole cell current. The data suggest that endogenous activity of HO-2 may be a source of CO in the canine jejunum and that exogenously applied CO can modulate intestinal smooth muscle electrical activity. It is therefore reasonable to suggest a role for endogenously produced CO as a messenger in the canine jejunum.

Electrical activity and synaptic responses were recorded intracellularly in 415 neurons of the mouse superior mesenteric ganglion (SMG) attached to a segment of distal colon in vitro. Eighty-seven percent of neurons tested received ongoing nicotinic cholinergic fast excitatory postsynaptic potentials (fEPSPs). Colonic distension caused an initial transient followed by a sustained, slowly adapting increase in fEPSP activity. Application of hexamethonium only to the colon reduced, but did not completely abolish, distension-evoked responses, suggesting direct projection of some distension-sensitive fibers. Ongoing fEPSPs were abolished when nerve trunks connecting the SMG to the colon were transected or blocked with tetrodotoxin applied to the colon. Intracellular labeling with horseradish peroxidase or lucifer yellow revealed that about 90% of neurons receiving colonic synaptic input had a caudally projecting axon; about 60% that did not receive colonic input had a rostrally projecting axon. The latter neurons were found only in the cephalad ganglion region. These results show that mouse SMG neurons receive colonic mechanosensory afferent synaptic input and thus may participate in sympathetic intestinal reflexes.

Abstract CD38/cyclic adenosine diphosphate ribose (cADPR) signaling plays an important role in the regulation of intracellular calcium responses to agonists in a variety of cells, including airway smooth muscle (ASM) cells. The present study was aimed at determining the effect of interleukin (IL)-13, a cytokine implicated in the pathogenesis of asthma, on CD38/cADPR signaling and to ascertain the contribution of CD38/cADPR signaling to IL-13–induced airway hyperresponsiveness. Human ASM cells maintained in culture were exposed to 50 ng/ml IL-13 for 22 h and levels of CD38 expression and intracellular calcium responses to agonists were measured. Treatment of human ASM cells with IL-13 resulted in increased CD38 expression as determined by real-time polymerase chain reaction, Western blot analysis, and indirect immunofluorescence. Increased CD38 expression was reflected as increased ADP-ribosyl cyclase activity in the ASM cell membranes. The net intracellular calcium responses to bradykinin, thrombin, and histamine were significantly (P ≤ 0.05) higher in cells treated with IL-13 compared with controls. Furthermore, 8-bromo-cADPR, a cADPR antagonist, attenuated IL-13–induced augmented intracellular calcium responses to agonists in human ASM cells. These findings indicate that the CD38/cADPR–dependent pathway has a major role in IL-13–induced modulation of calcium signaling in human ASM.

Carbon monoxide (CO) is proposed as a physiological messenger. CO activates cGMP and has a direct effect on potassium channels. Both actions of CO lead to hyperpolarization of a cell's resting membrane potential, suggesting that CO may function as a hyperpolarizing factor, although direct evidence is still lacking. Here we take advantage of the known membrane potential gradient that exists in the muscle layers of the gastrointestinal tract to determine whether CO is an endogenous hyperpolarizing factor. We find that heme oxygenase-2-null mice have depolarized smooth muscle cells and that the membrane potential gradient in the gut is abolished. Exogenous CO hyperpolarizes the membrane potential. Regions of the canine gastrointestinal tract that are more hyperpolarized generate more CO and have higher heme oxygenase activity than more depolarized regions. Our results suggest that CO is a critical hyperpolarizing factor required for the maintenance of intestinal smooth muscle membrane potential and gradient.

Nitric oxide (NO) and carbon monoxide (CO) seem to be neurotransmitters in the brain. The colocalization of their respective biosynthetic enzymes, neuronal NO synthase (nNOS) and heme oxygenase-2 (HO2), in enteric neurons and altered intestinal function in mice with genomic deletion of the enzymes ( nNOS Δ/Δ and HO2 Δ/Δ ) suggest neurotransmitter roles for NO and CO in the enteric nervous system. We now establish that NO and CO are both neurotransmitters that interact as cotransmitters. Small intestinal smooth muscle cells from nNOS Δ/Δ and HO2 Δ/Δ mice are depolarized, with apparent additive effects in the double knockouts ( HO2 Δ/Δ / nNOS Δ/Δ ). Muscle relaxation and inhibitory neurotransmission are reduced in the mutant mice. In HO2 Δ/Δ preparations, responses to electrical field stimulation are nearly abolished despite persistent nNOS expression, whereas exogenous CO restores normal responses, indicating that the NO system does not function in the absence of CO generation.