Evidence that TASK1 channels contribute to the background current in AH/type II neurons of the guinea-pig intestine.

Neurons that have AH (designation of neurons with a prominent and prolonged after hyperpolarizing potential that follows the action potential) electrophysiological characteristics and type II morphology (AH/type II neurons) are the first neurons in reflex circuits in the small intestine. Thus, the state of excitation of these neurons strongly influences the properties of enteric reflexes. The resting outward current in the type II neurons is reduced, causing depolarization and increased excitability, when protein kinase C (PKC) or synaptic inputs are activated, suggesting that regulation of background channels is an important determinant of the state of excitability of these neurons. However, the channels that carry the background current are not yet identified. We used intracellular microelectrodes to record from myenteric AH/type II neurons of the guinea-pig ileum, immunohistochemistry to localize channels and reverse transcriptase-polymerase chain reaction (RT-PCR) to characterize channel transcripts. The blockers of TASK1 channels, bupivacaine (1 mM) and methanandamide (10 muM), depolarized AH/type II neurons by 11.6 mV and 7.9 mV, respectively, and increased resting input resistance by about 30%. The reversal potential determined for the effect of bupivacaine was -92 mV, indicating that bupivacaine acts at K(+) channels, without significant action on other channel types that are open at rest. The membrane potential of type II neurons was depolarized by acidification to pH 6.4, but this depolarization was associated with decreased input resistance and was not reduced by bupivacaine. Thus an unidentified current that is activated by reduced pH masks effects on TASK channels. Slow excitatory post-synaptic potentials in the neurons were reduced in amplitude by methanandamide, suggesting that they are generated in part by closure of TASK1 channels. TASK1 immunoreactivity occurred in all type II neurons (determined by double labeling for IB4 and NeuN), but no type II neurons were immunoreactive for TASK2 or TASK3. These latter channels were localized to non-type II neurons. Transcripts for TASK1, TASK2, TASK3 and other two-pore-domain potassium channels were found in ganglion extracts. It is concluded that TASK1 channels contribute to the resting outward current in AH/type II neurons, and that neurotransmitters that evoke slow depolarizations in these neurons do so through the closure of resting K(+) channels that include TASK1 channels.

Damage to enteric neurons occurs in mice that develop fatty liver disease but not diabetes in response to a high-fat diet.

BACKGROUND: Disorders of gastrointestinal functions that are controlled by enteric neurons commonly accompany fatty liver disease. Established fatty liver disease is associated with diabetes, which itself induces enteric neuron damage. Here, we investigate the relationship between fatty liver disease and enteric neuropathy, in animals fed a high-fat, high-cholesterol diet in the absence of diabetes. METHODS: Mice were fed a high-fat, high-cholesterol diet (21% fat, 2% cholesterol) or normal chow for 33 weeks. Liver injury was assessed by hematoxylin and eosin, picrosirius red staining, and measurement of plasma alanine aminotransaminase (ALT). Quantitative immunohistochemistry was performed for different types of enteric neurons. KEY RESULTS: The mice developed steatosis, steatohepatitis, fibrosis, and a 10-fold increase in plasma ALT, indicative of liver disease. Oral glucose tolerance was unchanged. Loss and damage to enteric neurons occurred in the myenteric plexus of ileum, cecum, and colon. Total numbers of neurons were reduced by 15-30% and neurons expressing nitric oxide synthase were reduced by 20-40%. The RNA regulating protein, Hu, became more concentrated in the nuclei of enteric neurons after high-fat feeding, which is an indication of stress on the enteric nervous system. There was also disruption of the neuronal cytoskeletal protein, neurofilament medium. CONCLUSIONS & INFERENCES: Enteric neuron loss and damage occurs in animals with fatty liver disease in the absence of glucose intolerance. The enteric neuron damage may contribute to the gastrointestinal complications of fatty liver disease.

Investigation of the presence of ghrelin in the central nervous system of the rat and mouse.

Ghrelin and ghrelin receptor agonist have effects on central neurons in many locations, including the hypothalamus, caudal brain stem, and spinal cord. However, descriptions of the distributions of ghrelin-like immunoreactivity in the CNS in published work are inconsistent. We have used three well-characterized anti-ghrelin antibodies, an antibody to the unacylated form of ghrelin, and a ghrelin peptide assay in rats, mice, ghrelin knockout mice, and ghrelin receptor reporter mice to re-evaluate ghrelin presence in the rodent CNS. The stomach served as a positive control. All antibodies were effective in revealing gastric endocrine cells. However, no specific staining could be found in the brain or spinal cord. Concentrations of antibody 10 to 30 times those effective in the stomach bound to nerve cells in rat and mouse brain, but this binding was not reduced by absorbing concentrations of ghrelin peptide, or by use of ghrelin gene knockout mice. Concentrations of ghrelin-like peptide, detected by enzyme-linked immunosorbent assay in extracts of hypothalamus, were 1% of gastric concentrations. Ghrelin receptor-expressing neurons had no adjacent ghrelin immunoreactive terminals. It is concluded that there are insignificant amounts of authentic ghrelin in neurons in the mouse or rat CNS and that ghrelin receptor-expressing neurons do not receive synaptic inputs from ghrelin-immunoreactive nerve terminals in these species.

Identification of subunits of voltage-gated calcium channels and actions of pregabalin on intrinsic primary afferent neurons in the guinea-pig ileum.

BACKGROUND: The intrinsic primary afferent neurons (IPANs) in the intestine are the first neurons of intrinsic reflexes. Action potential currents of IPANs flow partly through calcium channels, which could feasibly be targeted by pregabalin. The aim was to determine whether pregabalin-sensitive α2δ1 subunits associate with calcium channels of IPANs and whether α2δ1 subunit ligands influence IPAN neuronal properties. METHODS: We used intracellular electrophysiological recording and in situ hybridisation to investigate calcium channel subunit expression in guinea-pig enteric neurons. KEY RESULTS: The α subunits of N (α1B) and R (α1E) type calcium channels, and the auxiliary α2δ1 subunit, were expressed by IPANs. This is the first discovery of the α2δ1 subunit in enteric neurons; we therefore investigated its functional role, by determining effects of the α2δ1 subunit ligand, pregabalin, that inhibits currents carried by channels incorporating this subunit. Pregabalin (10 µmol L(-1)) reduced the action potential duration. The effect was not increased with increase in concentration to 100 µmol L(-1). If N channels were first blocked by ω-conotoxin GVIA (0.5 µmol L(-1)), pregabalin had no effect on the residual inward calcium current. Reduction of the calcium current by pregabalin substantially inhibited the after-hyperpolarising potential (AHP) and increased neuron excitability. CONCLUSION & INFERENCES: Intrinsic primary afferent neurons express functional N (α1B) channel-forming subunits that are associated with α2δ1 modulatory subunits and are inhibited by pregabalin, plus functional R (α1E) channels that are not sensitive to binding of pregabalin to α2δ subunits. The positive effects of pregabalin in irritable bowel syndrome (IBS) patients might be partly mediated by its effect on enteric neurons.

Functional and in situ hybridization evidence that preganglionic sympathetic vasoconstrictor neurons express ghrelin receptors.

Agonists of ghrelin receptors can lower or elevate blood pressure, and it has been suggested that the increases in blood pressure are caused by actions at receptors in the spinal cord. However, this has not been adequately investigated, and the locations of neurons in the spinal cord that express ghrelin receptors, through which blood pressure increases may be exerted, are not known. We investigated the effects within the spinal cord of two non-peptide ghrelin receptor agonists, GSK894490 and CP464709, and two peptide receptor agonists, ghrelin and des-acyl ghrelin, and we used polymerase chain reaction (PCR) and in situ hybridization to examine ghrelin receptor expression. I.v. application of the non-peptide ghrelin receptor agonists caused biphasic changes in blood pressure, a brief drop followed by a blood pressure increase that lasted several minutes. The blood pressure rise, but not the fall, was antagonized by i.v. hexamethonium. Application of these agonists or ghrelin peptide directly to the spinal cord caused only a blood pressure increase. Des-acyl ghrelin had no significant action. The maximum pressor effects of agonists occurred with application at spinal cord levels T9 to T12. Neither i.v. nor spinal cord application of the agonists had significant effect on heart rate or the electrocardiogram. Ghrelin receptor gene expression was detected by PCR and in situ hybridization. In situ hybridization localized expression to neurons, including autonomic preganglionic neurons of the intermediolateral cell columns at all levels from T3 to S2. The numbers of ghrelin receptor expressing neurons in the intermediolateral cell columns were similar to the numbers of nitric oxide synthase positive neurons, but there was little overlap between these two populations. We conclude that activation of excitatory ghrelin receptors on sympathetic preganglionic neurons increases blood pressure, and that decreases in blood pressure caused by ghrelin agonists are mediated through receptors on blood vessels.