1. Mediators of non-adrenergic non-cholinergic inhibitory neurotransmission in porcine jejunum
N M Matsuda, S M Miller, L Sha, G Farrugia, J H Szurszewski Neurogastroenterol Motil. 2004 Oct;16(5):605-12. doi: 10.1111/j.1365-2982.2004.00574.x.
The purpose of this study was to determine the non-adrenergic non-cholinergic inhibitory neurotransmitter in pig jejunum. Intracellular electrical activity was recorded from circular smooth muscle cells. Inhibitory junction potentials (IJPs) evoked by electrical field stimulation were inhibited by tetrodotoxin (1 micromol L(-1)), omega-conotoxin GVIA (0.1 micromol L(-1)) tetrodotoxin, apamin (1 micromol L(-1)), 1-[6-((17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-1H-pyrrole-2,5-dione (U-73122; 10 micromol L(-1)) but not by N omega-nitro-l-arginine (l-NNA; 100 micromol L(-1)), haemoglobin (10 micromol L(-1)), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 10 micromol L(-1)) or 9-(tetrahydro-2-furyl)adenine (SQ-22536; 10 micromol L(-1)). S-nitroso-N-acetylpenicillamine (SNAP) hyperpolarized the membrane potential. This was inhibited by ODQ (3 micromol L(-1)) and charybdotoxin (0.1 micromol L(-1)). Adenosine-5-triphosphate (ATP; 100 micromol L(-1)) and 2-methylthio ATP (2-MeS-ATP; 100 micromol L(-1)) did not hyperpolarize the membrane potential and 6-N-N-diethyl-beta- gamma -dibromomethylene-d-adenosine-5'-triphosphate (ARL67156; 100 micromol L(-1)) did not modify IJPs. Carbon monoxide (CO; 10%) and tricarbonyl dichlororuthenium dimer ([Ru(CO3Cl2)]2; 100 micromol L(-1)) hyperpolarized the membrane potential however zinc, copper and tin protoporphyrin IX (100 micromol L(-1)) did not alter IJPs. Vasoactive intestinal peptide (VIP) hyperpolarized the membrane potential but 4-Cl-d-Phe6-Leu17-VIP (1 micromol L(-1)) did not modify IJPs. Pituitary adenylate cyclase activating peptide (PACAP)38 (0.5 micromol L(-1)) hyperpolarized the membrane potential. This was inhibited by apamin (1 micromol L(-1)) but not by tetrodotoxin (1 micromol L(-1)). Pituitary adenylate cyclase activating peptide6-38 (1 micromol L(-1)) inhibited IJPs. These data suggest that inhibitory neurotransmission in pig jejunum is mediated partly by PACAP.
2. Inhibitors of synaptosomal gamma-hydroxybutyrate transport
S J McCormick, G Tunnicliff Pharmacology. 1998 Sep;57(3):124-31. doi: 10.1159/000028233.
Synaptosomes prepared from mouse brain possess a Na+-dependent transport system for gamma-hydroxybutyrate displaying saturation kinetics, the transport constant (Kt) for which was calculated as 31 +/- 9 micromol/l. Several gamma-hydroxybutyrate and gamma-aminobutyric acid (GABA) structural analogues were tested as potential inhibitors of gamma-hydroxybutyrate transport. The most effective inhibitor was harmaline (Ki = 94 +/- 21 micromol/l), a known competitive inhibitor of Na+ binding to certain transport proteins. 2-Hydroxycinnamic acid, 3-(2-furyl)acrylic acid and citrazinic acid also inhibited transport and were competitive with respect to gamma-hydroxybutyrate. The least effective gamma-hydroxybutyrate analogues were 3-hydroxypropane sulfonic acid (Ki = 4.1 +/- 0.8 mmol/l) 3,5-dihydroxybenzoic acid (Ki = 6.1 +/- 2. 8 mmol/l) and 3-hydroxybenzoic acid (Ki = 6.9 +/- 3.3 mmol/l), although 2-hydroxypropane sulfonic acid and kynurenic acid had no measurable effects. Four inhibitors of GABA transport - nipecotic acid, guvacine, ketamine and beta-alanine and GABA itself, were without effect on gamma-hydroxybutyrate transport. These results show that certain drugs that structurally resemble gamma-hydroxybutyrate have the capacity to compete with gamma-hydroxybutyrate at its recognition site on the transporter. By examining the structure of such inhibitors, we can learn more about the properties of the substrate binding site on the carrier protein. Moreover, the absence of inhibition by GABA uptake inhibitors shows that gamma-hydroxybutyrate transport is a separate entity from GABA transport.
