Changes of GABA A receptor binding and subunit mRNA level in rat brain by infusion of subtoxic dose of MK-801

Abstract

In the present study, we have investigated the effects of prolonged inhibition of NMDA receptor by infusion of subtoxic dose of MK-801 to examine the modulation of GABA A receptor binding and GABA A receptor subunit mRNA level in rat brain. It has been reported that NMDA-selective glutamate receptor stimulation alters GABA A receptor pharmacology in cerebellar granule neurons in vitro by altering the levels of selective subunit. However, we have investigated the effect of NMDA antagonist, MK-801, on GABA A receptor binding characteristics in discrete brain regions by using autoradiographic and in situ hybridization techniques. The GABA A receptor bindings were analyzed by quantitative autoradiography using [ 3H]muscimol, [ 3H]flunitrazepam, and [ 35S]TBPS in rat brain slices. Rats were infused with MK-801 (1 pmol/10 μl per h, i.c.v.) for 7 days, through pre-implanted cannula by osmotic minipumps (Alzet, model 2ML). The levels of [ 3H]muscimol binding were highly elevated in almost all of brain regions including cortex, caudate putamen, thalamus, hippocampus, and cerebellum. However, the [ 3H]flunitrazepam binding and [ 35S]TBPS binding were increased only in specific regions; the former level was increased in parts of the cortex, thalamus, and hippocampus, while the latter binding sites were only slightly elevated in parts of thalamus. The levels of β2-subunit were elevated in the frontal cortex, thalamus, hippocampus, brainstem, and cerebellar granule layers while the levels of β3-subunit were significantly decreased in the cortex, hippocampus, and cerebellar granule layers in MK-801-infused rats. The levels of α6- and δ-subunits, which are highly localized in the cerebellum, were increased in the cerebellar granule layer after MK-801 treatment. These results show that the prolonged suppression of NMDA receptor function by MK-801-infusion strongly elevates [ 3H]muscimol binding throughout the brain, increases regional [ 3H]flunitrazepam and [ 35S]TBPS binding, and alters GABA A receptor subunit mRNA levels in different directions. The chronic MK-801 treatment has differential effect on various GABA A receptor subunits, which suggests involvement of differential regulatory mechanisms in interaction of NMDA receptor with the GABA receptors.