Nuclear benzodiazepine binding: possible interaction with thyroid hormone receptors

Alterations in brain-derived neurotrophic factor in the mouse hippocampus following acute but not repeated benzodiazepine treatment

Benzodiazepines (BZs) are safe drugs for treating anxiety, sleep, and seizure disorders, but their use also results in unwanted effects including memory impairment, abuse, and dependence. The present study aimed to reveal the molecular mechanisms that may contribute to the effects of BZs in the hippocampus (HIP), an area involved in drug-related plasticity, by investigating the regulation of immediate early genes following BZ administration. Previous studies have demonstrated that both brain derived neurotrophic factor (BDNF) and c-Fos contribute to memory- and abuse-related processes that occur within the HIP, and their expression is altered in response to BZ exposure. In the current study, mice received acute or repeated administration of BZs and HIP tissue was analyzed for alterations in BDNF and c-Fos expression. Although no significant changes in BDNF or c-Fos were observed in response to twice-daily intraperitoneal (i.p.) injections of diazepam (10 mg/kg + 5 mg/kg) or zolpidem (ZP; 2.5 mg/kg + 2.5 mg/kg), acute i.p. administration of both triazolam (0.03 mg/kg) and ZP (1.0 mg/kg) decreased BDNF protein levels within the HIP relative to vehicle, without any effect on c-Fos. ZP specifically reduced exon IV-containing BDNF transcripts with a concomitant increase in the association of methyl-CpG binding protein 2 (MeCP2) with BDNF promoter IV, suggesting that MeCP2 activity at this promoter may represent a ZP-specific mechanism for reducing BDNF expression. ZP also increased the association of phosphorylated cAMP response element binding protein (pCREB) with BDNF promoter I. Future work should examine the interaction between ZP and DNA as the cause for altered gene expression in the HIP, given that BZs can enter the nucleus and intercalate into DNA directly.

Diazepam affects the nuclear thyroid hormone receptor density and their expression levels in adult rat brain

Thyroid hormones (THs) are involved in the occurrence of anxiety and affective disorders; however, the effects following an anxiolytic benzodiazepine treatment, such as diazepam administration, on the mechanism of action of thyroid hormones has not yet been investigated. The effect of diazepam on the in vitro nuclear T3 binding, on the relative expression of the TH receptors (TRs) and on the synaptosomal TH availability were examined in adult rat cerebral hemispheres 24 h after a single intraperitoneal dose (5 mg/kg BW) of this tranquillizer. Although, diazepam did not affect the availability of TH either in blood circulation or in the synaptosomal fraction, it decreased (33%) the nuclear T3 maximal binding density (B(max)). No differences were observed in the equilibrium dissociation constant (K(d)). The TRalpha2 variant (non-T3-binding) mRNA levels were increased by 33%, whereas no changes in the relative expression of the T3-binding isoforms of TRs (TRalpha1, TRbeta1) were observed. This study shows that a single intraperitoneal injection of diazepam affects within 24 h, the density of the nuclear TRs and their expression pattern. The latest effect occurs in an isoform-specific manner involving specifically the TRalpha2 mRNA levels in adult rat brain.

Interaction between [3H]flunitrazepam and [3H]GABA binding in the cerebellum of reeler mice

It has been shown that in the cerebellum of reeler mutant mice GABA levels and GABA uptake increase while GABA binding decreases. This study shows that in the cerebellum of these mutants there is also an increase of benzodiazepine receptors. This increase is observed in cerebellar homogenates, in nuclei and in membranes. The increase in the density of central (i.e. clonazepam displacable) benzodiazepine receptors is primarily reflected in binding sites located in the GABA-receptor complex. In comparison to wild-type, GABA-modulin extracted from reeler cerebellum inhibits with a greater potency [3H]GABA binding. The increase in the central-type of benzodiazepine binding and its interaction with GABA binding, observed in cerebellar membranes, is interpreted as a functional response to the decrease in GABA binding and may reflect benzodiazepine receptor condensation and/or changes of subunit composition of the GABA/benzodiazepine receptor complex. The enhanced activity of reeler GABA-modulin reflects a functional response to the increased GABA levels in reeler cerebellum. The increase of the peripheral-type (i.e. PK 11195 displacable) of benzodiazepine receptors is probably due to metabolic changes that may accompany reeler cerebellar mutation. Differences in nuclear benzodiazepine binding between reeler and wild-type mice add a physiological importance to the nuclear binding of this drug.

Altered peripheral benzodiazepine receptor binding in cardiac and liver tissues from thyroidectomized rats

We measured the density and affinity of the peripheral benzodiazepine receptor (PBR) ligand, [3H]4'-Cl-diazepam, in cardiac ventricular and liver homogenates from thyroidectomized (TX) Holtzman adult male rats, and compared these data to sham-operated controls. When data from hypothyroid tissues were compared to those of controls, the densities of PBRs were decreased in cardiac ventricles but not in liver tissue. This reduction in cardiac PBR density is opposite to what has been reported for ventricular calcium channel density in hypothyroidism. PBR affinity for the ligand was increased in both the liver and ventricular homogenates from the hypothyroid tissues, but this difference was not seen in membranes prepared from the liver homogenates. Although 4'-Cl-diazepam affinity is reported to vary between tissues from different species, this is the first report of an in vivo hormone treatment induced change in the benzodiazepine type PBR affinity. Liver tissues from both groups failed to show any interaction when radiolabeled [3H]4'-Cl-diazepam was tested against competing concentrations of thyroid hormone analogs.