GABAB receptor function and subunit expression in the rat spinal cord as indicators of stress and the antinociceptive response to antidepressants

Experiments were undertaken to examine whether once daily i.p. administration of either of two antidepressants used for the treatment of neuropathic pain, amitriptyline (10 mg/kg) and fluoxetine (5 mg/kg), to rats for 7 days modifies GABAB receptor function and subunit expression in the lumbar spinal cord. The results indicate that, as previously reported for desipramine, both amitriptyline and fluoxetine increase the pain threshold to a thermal stimulus, the expression of GABAB(1) subunits, and baclofen-stimulated [35S]GTPγS binding, a measure of GABAB receptor function. The effects of antidepressant administration on GABAB(1b) and GABAB(2) subunit expression in spinal cord are more variable than for GABAB(1a). It was also discovered that repeated daily exposure to a thermal stimulus or immobilization stress increases GABAB(1a) expression in the lumbar spinal cord, with no commensurate change in thermal pain threshold or GABAB receptor sensitivity. These results support a relationship between GABAB receptors and the action of antidepressants. The findings demonstrate that drug-induced increases in GABAB receptor function can occur independently of any change in GABAB receptor subunit expression and are consistent with the notion that GABAB receptor subunits have multiple functions, only one of which is dimerization to form GABAB receptors. The data also suggest that GABAB subunit gene expression may serve as a preclinical marker of antidepressant efficacy and of drug- or stress-induced modifications in central nervous system activity.