Subunit- and brain region-specific reduction of GABAA receptor subunit mRNAs during chronic treatment of rats with diazepam

Abstract

The mRNA levels for several GABAA receptor subunits were measured by Northern blot analysis. Rats were treated for 3 wk by continuous release of diazepam (DZP) from subcutaneous reservoirs, and then sacrificed immediately or 48 h after removing the reservoirs. Poly(A)+ RNAs, isolated from cerebral cortex, cerebellum, and hippocampus, were hybridized with oligonucleotide probes for GABAA receptor subunits and a cDNA probe for beta-actin. Subunit mRNAs were expressed relative to the corresponding beta-actin mRNA. DZP treatment decreased the alpha 1 subunit mRNA level 40% in hippocampus, but it was not changed in cortex or cerebellum. The alpha 5 subunit mRNA level was decreased in cerebral cortex (28%) and hippocampus (15%). The gamma 2 subunit mRNA level was decreased (40%) only in cortex. DZP treatment did not affect alpha 2, alpha 3, alpha 4, beta 2, or beta 3 subunit mRNA levels. Decreases in mRNA levels had reversed within 48 h after stopping chronic treatment. Acute DZP did not change alpha 1, alpha 5, or gamma 2 subunit mRNA levels. The decreases in GABAA receptor subunit mRNA levels were specific to subunit and brain region. These results, coupled with those after chronic flurazepam treatment, also indicated that the effects on GABAA receptor subunit mRNA levels are specific to the benzodiazepine (BZ) used for chronic treatment.

Regulation of GABAAReceptor Subunit Expression by Pharmacological Agents

“…Long-term treatment with abecarnil, imidazenil, triazolam and zolpidem in vivo had no effect on cortical ␣5 mRNA expression (Holt et al, 1996(Holt et al, , 1997aImpagnatiello et al, 1996;Ramsey-Williams and Carter., 1996). Hippocampal ␣5 mRNA was slightly down-regulated by long-term treatment with diazepam in the study by Wu et al (1994) but not in that of Impagnatiello et al (1996). Long-term treatment with imidazenil and triazolam did not affect hippocampal ␣5 mRNA expression RamseyWilliams and Carter, 1996).…”

“…No effect of long-term treatment with flurazepam treatment was found on cortical ␥2 polypeptide (Chen et al, 1999). Long-term diazepam treatment increased ␥2 mRNA expression in the cerebellum in one (Holt et al, 1999) of three studies , whereas no effect of long-term treatment with (Kang and Miller 1991;Wu et al, 1994;Impagnatiello et al, 1996;Tietz et al, 1999). In accordance, long-term treatment with flurazepam treatment had no effect on ␥2 polypeptide in the cerebellum or hippocampus (Chen et al, 1999).…”

“…␥2 Subunit. Long-term treatment of rats with abecarnil, diazepam, and lorazepam down-regulated ␥2 mRNA expression in the cerebral cortex (Kang and Miller, 1991;Primus and Gallager, 1992;Wu et al, 1994;Holt et al, 1996). A reduction in ␥2 mRNA was seen in the frontoparietal motor cortex by diazepam but not by imidazenil treatment Longone et al, 1996).…”

Differential Regulation of GABAA Receptor Gene Expression by Ethanol in the Rat Hippocampus Versus Cerebral Cortex

“…Whereas chronic ethanol exposure did not alter GABAA receptor a 1 subunit peptide levels in hippocampus, other GABAergic drugs have been shown to regulate GABAA receptor a1 subunit peptide expression in the hippocampus. For example, chronic administration of both diazepam and flurazepam has been shown to decrease levels of hippocampal GABAA receptor a1 subunit mRNA (Tietz et al, 1993;Wu et al, 1994;Impagnatiello et al, 1996). Hence, chronic activation of distinct GABAA isoreceptors by different GABAA receptor modulators might lead to different effects on subunit expression.…”

Requirement of α5-GABAAReceptors for the Development of Tolerance to the Sedative Action of Diazepam in Mice

“…Uncoupling of the allosteric interaction between the benzodiazepine binding site and the GABA site, probably linked to GABA A receptor internalization, has been proposed as a correlate of diazepam tolerance (Hutchinson et al, 1996;Itier et al, 1996;Primus et al, 1996;Ali and Olsen, 2001;Costa et al, 2001). Furthermore, subtle changes in the expression of GABA A receptor subunits were described notably in the cerebral cortex and hippocampus (Wu et al, 1994;Impagnatiello et al, 1996;Pesold et al, 1997;Arnot et al, 2001). In particular, a selective decrease in the expression of genes encoding the ␣ 1 -and ␥ 2 -subunits in dendrites and spines of cortical pyramidal cells has been associated with tolerance to the anticonvulsant action of diazepam (Costa et al, 2002).…”