Somatostatin binding reduced by ammonium acetate in the rat hippocampus can be reversed by treatment with N-carbamyl-L-glutamate plus L-arginine

Glutamine synthetase in brain: effect of ammonia

Glutamine synthetase (GS) in brain is located mainly in astrocytes. One of the primary roles of astrocytes is to protect neurons against excitotoxicity by taking up excess ammonia and glutamate and converting it into glutamine via the enzyme GS. Changes in GS expression may reflect changes in astroglial function, which can affect neuronal functions. Hyperammonemia is an important factor responsible of hepatic encephalopathy (HE) and causes astroglial swelling. Hyperammonemia can be experimentally induced and an adaptive astroglial response to high levels of ammonia and glutamate seems to occur in long-term studies. In hyperammonemic states, astroglial cells can experience morphological changes that may alter different astrocyte functions, such as protein synthesis or neurotransmitters uptake. One of the observed changes is the increase in the GS expression in astrocytes located in glutamatergic areas. The induction of GS expression in these specific areas would balance the increased ammonia and glutamate uptake and protect against neuronal degeneration, whereas, decrease of GS expression in non-glutamatergic areas could disrupt the neuron-glial metabolic interactions as a consequence of hyperammonemia. Induction of GS has been described in astrocytes in response to the action of glutamate on active glutamate receptors. The over-stimulation of glutamate receptors may also favour nitric oxide (NO) formation by activation of NO synthase (NOS), and NO has been implicated in the pathogenesis of several CNS diseases. Hyperammonemia could induce the formation of inducible NOS in astroglial cells, with the consequent NO formation, deactivation of GS and dawn-regulation of glutamate uptake. However, in glutamatergic areas, the distribution of both glial glutamate receptors and glial glutamate transporters parallels the GS location, suggesting a functional coupling between glutamate uptake and degradation by glutamate transporters and GS to attenuate brain injury in these areas. In hyperammonemia, the astroglial cells located in proximity to blood-vessels in glutamatergic areas show increased GS protein content in their perivascular processes. Since ammonia freely crosses the blood-brain barrier (BBB) and astrocytes are responsible for maintaining the BBB, the presence of GS in the perivascular processes could produce a rapid glutamine synthesis to be released into blood. It could, therefore, prevent the entry of high amounts of ammonia from circulation to attenuate neurotoxicity. The changes in the distribution of this critical enzyme suggests that the glutamate-glutamine cycle may be differentially impaired in hyperammonemic states.

The benzodiazepine antagonist CGS 8216 prevents hyperammonemia-induced somatostatin receptor reduction in the brain

Previous results from our group showed that hyperammonemia decreases the number of somatostatin (SS) receptors and that benzodiazepine receptors might regulate the number of SS receptors in rat brain. These findings together with the supersensitivity of benzodiazepine receptors in the hyperammonemic rat brain suggest that benzodiazepine receptors might mediate the effect of hyperammonemia on SS receptors. To assess this hypothesis we tested whether 2-phenylpyrazolo[3,4-c]-quinolin-3(5H)-one (CGS 8216), a benzodiazepine antagonist, prevented the effect of ammonium acetate on rat brain SS receptors. Administration of ammonium acetate (5 mmol/kg, i.p.) for 7 days did not affect the levels of somatostatin-like immunoreactivity but decreased the number of SS receptors in synaptosomes from the frontoparietal cortex and hippocampus without affecting their apparent affinity. This decrease could be blocked by the concomitant administration of CGS 8216 (10 mg/kg, i.p.). The benzodiazepine antagonist alone had no observable effect on the somatostatinergic system. These results suggested that the effect of hyperammonemia on SS receptors could be mediated, at least in part, through the benzodiazepine receptors.

Effect of ammonium acetate on the somatostatinergic system in the rat frontoparietal cortex

Short (90 min)-, mid (5 days)-, and long-term (15 days) ammonium acetate (5 mmol/kg IP) administration decreased the number of specific [125I][Tyr11]somatostatin receptors in synaptosomes from the frontoparietal cortex without changing the affinity constant. Administration of ammonium acetate did no affect the levels of somatostatin-like immunoreactivity in the frontoparietal cortex. The administration of a single dose of N-carbamyl-L-glutamate (1 mmol/kg) plus L-arginine (1 mmol/kg) 1 h before the last administration of ammonium acetate totally blocked the inhibitory effects of the latter on somatostatin receptor number in the frontoparietal cortex synaptosomes. N-Carbamyl-L-glutamate plus L-arginine alone had no observable effect on the somatostatinergic system. The decrease in the number of somatostatin receptors induced by ammonium acetate might reflect decreased target cell sensitivity to somatostatin, a phenomenon that could contribute to the depressed neuronal excitability induced by ammonia in the rat frontoparietal cortex.