Differential effects of ammonia on the benzodiazepine modulatory site on the GABA-A receptor complex of human brain.
Ammonia is a key factor in the pathogenesis of encephalopathies associated with liver failure. A direct effect of ammonia on GABAergic neurotransmission was proposed as a mechanism that may explain its neurotoxic effect on the basis of electrophysiological and biochemical studies performed in animal models of liver failure. In the present study, we investigated using a radiometric assay the effect of ammonia on the binding of GABA-A receptor ligands to membranes from normal human brains. Ammonium tartrate significantly decreased the maximal binding of [3H]flunitrazepam to well-washed frontal cortical membranes (366+/-63 fmol/mg protein in absence of ammonia versus 294.1+/-51 fmol/mg protein in presence of 2 mM ammonia; p<0.05). The efficacy of the effects of ammonia was within the millimolar range (IC50=4.8 mM). This effect was not seen in cerebellum or hippocampus. Ammonia exposure decreased the maximal binding of [3H]flumazenil (284.9+/-24.2 fmol/mg protein in absence of ammonia versus 146.4+/-15.6 fmol/mg protein in presence of 2 mM ammonia; p<0.01). This effect was seen with a greater potency (Imax=32.4%) and a lower IC50 (0.1 mM). Inhibition of [3H]flumazenil binding was significant in all brain regions. The apparent ammonia-induced decrease of [3H]flunitrazepam and [3H]flumazenil binding was due to a decrease in the binding affinities of these ligands for the benzodiazepine site. In contrast, ammonium tartrate exposure did not cause significant changes to the binding of [3H]muscimol in any brain region. These findings demonstrate that ammonia interacts negatively with components of the benzodiazepine-associated site at the GABA-A receptor complex in human brain in contrast to previous reports in the rat, and thus, does not support the notion that ammonia directly activates the GABA-A receptor complex resulting in increased GABAergic neurotransmission in human hepatic encephalopathy. These findings also suggest that positron emission tomography studies in cirrhotic patients using [11C]flumazenil may be underestimating GABA-A receptor sites depending upon the degree of hyperammonemia of the patient.
Ahboucha
Samir
S
Neuroscience Research Unit, CHUM (Campus Saint-Luc), 1058 St-Denis, Montreal, Que., Canada H2X 3J4.
Araqi
Fadwa
F
Layrargues
Gilles Pomier
GP
Butterworth
Roger F
RF
D000641:Ammonia / Q000378:metabolism* / Q000494:pharmacology; D001665:Binding Sites / Q000187:drug effects / Q000502:physiology; D001667:Binding, Competitive / Q000187:drug effects / Q000502:physiology; D001921:Brain / Q000378:metabolism* / Q000503:physiopathology; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D015536:Down-Regulation / Q000187:drug effects / Q000502:physiology; D005442:Flumazenil / Q000378:metabolism; D005445:Flunitrazepam / Q000378:metabolism; D018755:GABA Agonists / Q000378:metabolism; D018757:GABA Modulators / Q000378:metabolism; D006501:Hepatic Encephalopathy / Q000378:metabolism* / Q000503:physiopathology; D006801:Humans; D022124:Hyperammonemia / Q000378:metabolism* / Q000503:physiopathology; D008024:Ligands; D009118:Muscimol / Q000378:metabolism; D011869:Radioligand Assay; D011963:Receptors, GABA-A / Q000187:drug effects / Q000378:metabolism*; D013347:Subcellular Fractions; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology; D013644:Tartrates / Q000494:pharmacology
D018755:GABA Agonists; D018757:GABA Modulators; D008024:Ligands; D011963:Receptors, GABA-A; D013644:Tartrates; D009118:Muscimol; D005442:Flumazenil; D005445:Flunitrazepam; D000641:Ammonia; C029768:tartaric acid