Region-specific changes in CNS muscarinic acetylcholine receptors in a rat model of hyperammonemia.

Multiple neurotransmitter systems have been implicated in the etiology of cerebral dysfunction in acute and chronic hyperammonemic states. Involvement of the neurotransmitter systems of glutamate and gamma-aminobutyric acid has been reported, whereas not much information is available on the role of the cholinergic system in the etiology of hyperammonemic states. In the present investigation, muscarinic acetylcholine receptors (mAChR) were studied, using tritium-labelled quinuclidinyl benzilate ([3H]QNB), in rats administered ammonium acetate (AA), and the changes in the treated rats were compared with measurements in normal rats. The presence of two affinity (high and low) systems for [3H]QNB binding was observed in the cerebral cortex (CC), while a single affinity system was seen in the cerebellum (CE) and pons-medulla (PM). A decrease in the Bmax of both the high and low affinity systems for [3H]QNB binding was observed in the CC with no significant change in the Kd values in rats administered an acute dose of AA (25 mmol/kg of body weight). The Bmax values were unaltered in CE and PM, but a significant increase in the Kd value was observed in the CE. Studies of [3H]QNB binding in the presence of pirenzepine (a specific antagonist of M1 receptors) indicated the predominance of non-M1-type (M2, M3, M4, and M5) receptors (85-90% of the total specific binding) in the CE and PM, whereas in the CC, 60% was represented by non-M1 and 40% by M1 receptors. Reduction in [3H]QNB binding to M1 receptors was observed in the CC and PM of rats administered an acute dose of AA. Administration of lower doses of AA (2.5 mmol/kg of body weight) had no effect on CC and CE mAChRs, while an increase in non-M1 and a decrease in M1 receptors was observed in the PM. As the neurotransmitter receptors play a key role in signal transduction, the observed changes in receptor functions may be responsible for some of the behavioral changes reported in hyperammonemic states.

Ammonia-induced alterations in the activities of synaptosomal cholinesterases of rat brain under in vitro and in vivo conditions.

Effects of in vivo and in vitro pathophysiological concentrations of ammonium acetate were studied on the activities of acetyl and pseudocholinesterases in the synaptosomes isolated from cerebral cortex, cerebellum and brain stem of rat brain. Administration of subacute and acute doses of ammonium acetate elevated the activities of both the cholinesterases in synaptosomes of all the above three regions. A linear relationship (r = 0.98) was observed between brain-ammonia levels and magnitude of elevation in the activities of cerebral cholinesterases. Kinetic analysis revealed that this elevation was due to a change in the Vm but not in the Km of the enzymes. Incubation of synaptosomes isolated from normal animals with 1, 5 and 10 mM ammonium acetate marginally elevated the activities of these enzymes and had no effects on purified enzyme. It is suggested that the changes in the activities might be due to altered architecture of the membranes which exposes more number of catalytic sites.