Chronic pentobarbital administration alters ?-aminobutyric acidA receptor ?6-subunit mRNA levels and diazepam-insensitive [3H]Ro15-4513 binding

Abstract

In order to study the chronic effects of pentobarbital, a positive GABAA receptor modulator, on the inverse agonist binding of the benzodiazepine site, binding of [3H]Ro15‐4513 and levels of GABAA receptor α6‐subunit mRNA were investigated in the brains of pentobarbital‐tolerant/dependent animals, using receptor autoradiography and in situ hybridization histochemistry in consecutive brain sections. Pentobarbital was administered to rats either 60 mg/kg, i.p., once, for acute treatment, or 300 μg/10μl/h i.c.v. continuously for 6 days via osmotic minipumps to render rats tolerant to pentobarbital. Rats assigned to the dependent group were sacrificed 24 h after discontinuance of pentobarbital infusion, while those assigned to the tolerant group were sacrificed at the end of infusion. The α6 subunit mRNA was increased in the tolerant group only. Diazepam‐insensitive [3H]Ro15‐4513 binding was increased in the cerebellar granule layer of pentobarbital‐tolerant and ‐dependent rats. No alterations in these parameters were observed in acutely treated animals. These data suggest that chronic pentobarbital treatment induced expression of α6‐subunit mRNA. This was in contrast to α1‐ and γ2‐subunit mRNA, which in tolerant animals are unchanged, but for which withdrawal triggers a surge in levels. Because the α6‐subunit is a major component of the diazepam‐insensitive [3H]Ro15‐4513 binding site, the increased diazepam‐insensitive [3H]Ro15‐4513 binding implied de novo synthesis of the receptor subunit protein. © 1996 Wiley‐Liss, Inc.

Identification of Structures within GABAAReceptor α Subunits That Regulate the Agonist Action of Pentobarbital

“…The unique pharmacological properties associated with the ␣6 subunit along with its restricted expression pattern in the adult brain (Laurie et al, 1992) make it a potentially useful target for selective modulation. Long-term administration of pentobarbital has been shown to increase the expression of the ␣6 subunit (Ito et al, 1996;Raol et al, 2005), which may be a result of the higher activity level of barbiturates at these receptors. The ␣6 subunits contribute to both postsynaptic and extrasynaptic receptor populations in cerebellar granule cells and play an important role in both phasic and tonic neurotransmission in this region (Brickley et al, 1996).…”

Regulation of GABAAReceptor Subunit Expression by Pharmacological Agents

“…During pentobarbital withdrawal (1 day), ␣1 mRNA was up-regulated in the neocortex and cerebellar granule and Purkinje cells (Table 13). The expression of cerebellar ␣6 mRNA was up-regulated by long-term treatment with pentobarbital (Ito et al, 1996a) (Table 12), and during withdrawal (1 day), the expression returned to control levels (Ito et al, 1996). Long-term treatment with pentobarbital drastically up-regulated ␤1 mRNA 600% over basal expression in the CA1 and CA2 regions of the hippocampus, although the expression was not affected in other hippocampal areas (Yin and Lee, 1998).…”

“…One-day pentobarbital withdrawal increased the [ 3 H]flunitrazepam B max values in the frontal cortex, cerebellum and striatum (Tseng et al, 1993b;Miyaoka et al, 1994 (Tseng et al, 1993b). Consistent with increased ␣6 mRNA expression in the study of Ito et al (1996a) …”

Tolerance to allopregnanolone with focus on the GABA‐A receptor

“…Barbiturates act on GABA‐A receptors through three distinct mechanisms, resulting in positive allosteric modulation, direct activation and inhibition. These effects are observed at different concentrations and are differentially affected by the subunit composition of GABA‐A receptors (Yu and Ho, 1990; Ito et al ., 1996a,b; Drafts and Fisher, 2006). However, possible changes in GABA‐A receptor subunit composition during tolerance to barbiturates have not yet been studied.…”