A new approach to pathogenetic study of
hepatic encephalopathy was recently undertaken in order to identify the neurological alterations of the brain which characterize the
coma. In this study attention was firstly addressed to a correct and objective evaluation of the
comatose state in rats with
fulminant hepatic failure induced by
galactosamine. For this purpose visual evoked potentials were utilized since this electrophysiological test proved reliable and sensitive on the basis of an extensive pharmacological study. Two different stages of
coma were identified in the rat and they were named mild and severe. Receptor binding studies performed on brain membranes of these rats show in the mild stage an increased number of low and high affinity
GABA receptors and a decreased affinity of
dopamine receptors. The severe stage is characterized by the persistence of only high affinity
GABA receptors and a reduced number of
dopamine receptors. This imbalance between inhibitory and excitatory receptor systems may explain the generalized central nervous system depression which characterizes the
hepatic encephalopathy while the increased number of
benzodiazepine receptors found in both stages of
coma may account for the brain supersensitivity to
sedative administration of patients with
liver disease and for the
sedative-induced episodes of
coma. These receptor alterations may be attributed to a disuse and/or a partial degeneration of nerve terminals due to peripheral
neurotoxins (i.e.,
ammonia,
mercaptans,
short chain fatty acids) and the decrease of
glutamate decarboxylase activity and of
zinc levels in brain tissues seems to be respectively a direct and an indirect demonstration of this phenomenon. Bearing in mind the supersensitivity of the
GABA-benzodiazepine receptor system and their reciprocal interaction, a
benzodiazepine antagonist was administered to rats in mild stage of
encephalopathy. Electrophysiological and
benzodiazepine binding studies demonstrated that this treatment can temporarily counteract some of the neurological disturbances of the earlier stage of
coma and act as
antidote of the
sedative-induced episodes of
coma.