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.