DEVELOPMENT OF BENZODIAZEPINE AND PICROTOXIN (TERT-BUTYLBICYCLOPHOSPHOROTHIONATE) BINDING-SITES IN RAT CEREBELLAR GRANULE CELLS IN CULTURE

The specific bindings of [3H]flunitrazepam ([3H]FLU), [3H]CGS 8216, and t‐[35S]butylbicyclophosphorothionate ([35S]TBPS) to sites on rat cerebellar granule cells all increase from 4 to 15 days in culture, although their time courses differ. Specific [3H]FLU binding doubles, [3H]CGS 8216 binding triples, and [35S]TBPS binding increases about fourfold from 4 to 15 days in culture. Displacement studies, using the type I‐selective ligand CL 218,872, indicate that at 4 days the [3H]FLU binding sites are almost entirely “type II,'’judging from an IC50 value near 300 nM and a pseudo‐Hill number near 1. By 10 days, approximately equal numbers of type I and type II binding sites are present in the cultured cells, and this ratio remains constant thereafter (12 and 15 days). At days 10–15, both the IC50 value for CL 218,872 (near 100 nM) and the pseudo‐Hill number (near 0.7) remain constant and are significantly different from the values at culture day 4. The development of specific [35S]TBPS binding parallels that of [3H]CGS 8216 binding more closely than the development of [3H]FLU binding. The [3H]CGS 8216/[3H]FLU ratio increased by a factor of 1.6 from day 4 to day 15 (p < 0.001). Taken together, our data suggest the existence of several γ‐aminobutyric acidA (GA‐BAA) receptor subunits, the relative proportions of which change during development. The presence of the GABA‐mimetic 4,5,6,7‐tetrahydroisoxazolo[5,4c]pyridine‐3‐ol (THIP) in the culture medium had no apparent effect on any of the binding sites studied, although THIP was shown previously to induce low‐affinity GABA binding sites. Copyright © 1990, Wiley Blackwell. All rights reserved