Postnatal development and kinetics of [3H]gaboxadol binding in rat brain: In vitro homogenate binding and quantitative autoradiography

The postnatal development of the binding of the GABAA receptor agonist [3H]gaboxadol in rat brain was investigated. Using brain tissue from rats obtained at postnatal days 1, 10, 25, and > 25 (adult), the binding of [3H]gaboxadol and the benzodiazepine [3H]flunitrazepam to GABAA receptors was compared in homogenate binding assays and quantitative receptor autoradiography. Kinetic and equilibrium data obtained in homogenate binding studies revealed two different [3H]gaboxadol affinities. A kinetically derived KD of 3.7 nM in adult cerebellum, calculated from the association and dissociation rate constants kon (1.45 × 108 M− 1 min− 1) and koff (0.54 min− 1) was contrasted by an equilibrium KD of 38.6 nM, obtained by homologous competition experiments. Quantitative analysis of autoradiographic data revealed an increase in specific [3H]gaboxadol binding sites during brain development, which resembles the anatomical and temporal pattern of the postnatal expression of the extrasynaptic δ subunit of GABAA receptors. In conclusion, by the radioligand binding data obtained on native tissue, binding of gaboxadol to GABAA receptors located outside the synaptic junctions could be postulated.