Changes of GABA_A receptor binding and subunit mRNA level in rat brain by infusion of NOS inhibitor

In the present study, we have investigated the effects of prolonged inhibition of nitric oxide synthase (NOS) by infusion of NOS inhibitor, L-nitroarginine, to examine the pentobarbital-induced sleep, modulation of GABA_A receptor binding, and GABA_A receptor subunit mRNA level in rat brain. Pre-treatment with L-nitroarginine 30 min before pentobarbital treatment (60 mg/kg, i.p.) significantly increased the duration of sleep in rats. However, the duration of pentobarbital-induced sleep was shortened by the prolonged infusion of L-nitroarginine into ventricle. We have investigated the effect of NOS inhibitor on GABA_A receptor binding characteristics in discrete areas of brain regions by using autoradiographic and in situ hybridization techniques. Rats were infused with L-nitroarginine (10, 100 pmol/10 μl/h, i.c.v.) for 7 days, through pre-implanted cannula by osmotic minipumps. The levels of [³H]muscimol and [³H]flunitrazepam binding were markedly elevated in almost all of brain regions including cortex, caudate putamen, thalamus, hippocampus, and cerebellum. However, there was no change in the level of [³⁵S]TBPS binding. The levels of β2-subunit were elevated in the cortex, brainstem, and cerebellar granule layers. By contrast, the levels of β3-subunit were significantly decreased in the cortex, hippocampus, and cerebellar granule layers in L-nitroarginine-infused rats. Following L-nitroarginine treatment, the levels of α6- and δ-subunits which were strictly localized to the cerebellum, were not changed in the cerebellar granule layer. These results show that the prolonged inhibition of NOS by L-nitroarginine-infusion markedly elevates [³H]muscimol and [³H]flunitrazepam binding throughout the brain, and alters GABA_A receptor subunit mRNA levels in different directions. Chronic inhibition of NO generation has differential effects on the various expressions of GABA_A receptor subunits. These suggest the involvement of different regulatory mechanisms for the NO-induced expression of GABA_A receptor.