INDUCTION OF LOW-AFFINITY GABA(A) RECEPTORS BY THE GABA-AGONIST THIP (4,5,6,7-TETRAHYDROISOXAZOLO[5,4-C]PYRIDIN-3-OL) IN CULTURED RAT CEREBELLAR GRANULE CELLS IS PREVENTED BY INHIBITION OF POLYAMINE BIOSYNTHESIS

GABA(A) agonist-induced formation of low-affinity GABA(A) receptors in cultured cerebellar granule cells was studied in the presence or abse nce of alpha-difluoromethylornithine (DFMO), a blocker of polyamine fo rmation, High- and low-affinity GABA(A) receptors were monitored by Sc atchard analysis of [H-3]GABA binding to membranes from cells cultured for either 4 or 10 days in the presence or absence of the GABA agonis t 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP), Cultures grow n for 4 days were exposed to THIP and DFMO for an additional period of 6 hr (acute exposure), whereas cultures grown for 10 days were expose d to the same agents during the entire culture period (chronic exposur e), Regardless of the culture period or drug exposure protocol, contro l cells expressed only a high-affinity (K-D 7 nM) binding site for GAB A, whereas the cultures treated with THIP for either 6 hr or 10 days e xhibited an additional low-affinity binding site (K-D similar to 500 n M), Chronic exposure to DFMO prevented the THIP induction of low-affin ity GABA(A) receptors, whereas acute exposure to DFMO had no effect on the ability of THIP to induce low-affinity GABA(A) receptors, Measure ments of the intracellular polyamine concentration demonstrated a slig ht decrease in the putrescine level in the granule cells exposed to DF MO or THIP + DFMO for 6 hr, In contrast, granule cells chronically (10 days) exposed to DFMO or THIP + DFMO were depleted of putrescine and spermidine, Hence, the ability of THIP to induce low-affinity GABA(A) receptors was prevented by the simultaneous depletion of the cellular content of putrescine and spermidine, whereas inhibition of ornithine decarboxylase and of putrescine formation was not sufficient to preven t THIP-induced receptor formation. (C) 1994 Wiley-Liss, Inc.