Kinetics of tert-[³⁵S]butylbicyclophosphorothionate binding in the cerebral cortex of newborn and adult rats: Effects of GABA and receptor desensitization

The effects of GABA on the kinetics of tert-[³⁵S]butylbicyclophosphorothionate ([³⁵S]TBPS) binding to the convulsant site of GABA_A receptors were studied in membrane suspensions from the cerebral cortex of newborn (1-day-old) and adult (90-day-old) rats. TBPS dissociation was biphasic in neonates and adults, indicating that more than one interconvertible state of [³⁵S] TBPS binding sites may be present in the cerebral cortex. In the absence of GABA, the fast (T_1/2, 11 min) and slow (T_1/2, 77 min) components of TBPS dissociation in newborn rats were approximately fourfold slower than in adults. The acceleration of the dissociation rates caused by 2 μM GABA, however, was more robust in neonates than in adults (six- to ninefold vs. twofold increase, respectively). Moreover, the dissociation rates of TBPS in membranes preincubated with 2 μM GABA (dissociation started by adding 40 μM picrotoxin) were two- to fourfold slower than in membranes preincubated without GABA (dissociation started by adding 40 μM picrotoxin plus 2 μM GABA). Taken together, these results suggest that (1) the closed state of GABAA receptors is associated with a more effective steric barrier for the binding of TBPS in neonates compared with adults, (2) GABA produces a larger acceleration of the binding kinetics of TBPS in neonates than in adults, and (3) long incubations with GABA may cause receptor desensitization, which in turn slows down the dissociation rates of TBPS.