Effects of GABAB ligands alone and in combination with paroxetine on hippocampal BDNF gene expression

Brain-derived neurotrophic factor (BDNF) has been suggested as a target for antidepressant treatment and chronic antidepressant drug administration shows a 'biphasic effect' on BDNF mRNA in rat hippocampus (transient decrease followed by an increase). In comparison, following acute administration only, an inhibitory action on BDNF gene expression is detected. The present study aimed to understand the mechanism behind the acute inhibitory action on BDNF gene expression by investigating the possible involvement of γ-aminobutyric acid (GABA) receptors in mediating this effect. Rats were injected with either saline, the GABA_A selective compound 4,5,6,7-tetrahydroisoxazolo[5,4- c]pyridin-3-ol (THIP), the benzodiazepine flunitrazepam or the GABA_B selective compound baclofen. BDNF mRNA levels were measured 4 h later using in-situ hybridization. Baclofen (10 mg/kg, i.p.), but not THIP (10 mg/kg, i.p.) or flunitrazepam (10 mg/kg, i.p.), administration resulted in significant inhibition of BDNF mRNA expression in the cornu ammonis 3 and dentate gyrus but not in the cornu ammonis 1 region of the hippocampus. The inhibitory effect of baclofen on hippocampal BDNF mRNA expression was significantly attenuated by pre-treatment the selective GABA_B antagonists, CGP 46381 and CGP 55845 (10 mg/kg, i.p.). The inhibitory action by the selective serotonin re-uptake inhibitor (SSRI) paroxetine on hippocampal BDNF mRNA was also attenuated by CGP 46381. Our findings suggest a role for GABA_B, but not GABA_A, receptor-mediated mechanisms in the inhibitory regulation of basal hippocampal BDNF gene expression. Our results indicate that GABA _B receptor activation may play a role in the antidepressant drug-induced inhibition of BDNF gene expression in the hippocampus.