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.