Interaction of La3+ with GABAA receptors in rat cerebrocortical membranes as detected with [35S]t-butylbicyclophosphorothionate binding.

Lanthanides at micromolar concentrations stimulated binding of [35S]t-butylbicyclophosphorothionate (TBPS), a highly sensitive marker for allosteric modulation of the chloride channel of GABAA receptors, to purified rat cerebrocortical synaptosomal membranes in the absence of GABA. This trivalent cation effect appeared to reflect a specific and direct interaction with GABAA receptors, and could not be mimicked by divalent metal ions including Ca2+, Cd2+, Co2+, Sr2+, Mg2+ and Mn2+. On the other hand, Zn2+, a known allosteric regulator of GABAA receptors, inhibited TBPS binding, but its presence could not prevent stimulation of TBPS binding by La3+. We further examined the lanthanide binding site by monitoring La3(+)-induced changes in TBPS binding. La3+ reduced the Kd for TBPS without affecting Bmax. Stimulation of TBPS by La3+ was not additive with the increased TBPS binding induced by other agonists, including GABA (at nanomolar concentrations), neurosteroids or benzodiazepines (in the absence of GABA). GABA at micromolar concentrations inhibited TBPS binding and its inhibitory effect was potentiated by neurosteroids and benzodiazepines, but not by La3+. Thus, GABA, in the presence of La3+, inhibited TBPS binding in a monophasic manner with a Hill coefficient of approximately 2 and an IC50 of 6 microM. In the absence of La3+, the dose-response profile for GABA became biphasic with an appearance of a stimulatory phase at concentrations of GABA lower than 1 microM. Analysis of the initial stimulation yielded a Hill coefficient of 1 and an EC50 of 88 nM.(ABSTRACT TRUNCATED AT 250 WORDS)