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 absence of alpha-difluoromethylornithine (DFMO), a blocker of polyamine formation, High- and low-affinity GABA(A) receptors were monitored by Scatchard 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 agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP), Cultures grown 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 exposed to the same agents during the entire culture period (chronic exposure), Regardless of the culture period or drug exposure protocol, control cells expressed only a high-affinity (K-D 7 nM) binding site for GABA, whereas the cultures treated with THIP for either 6 hr or 10 days exhibited an additional low-affinity binding site (K-D similar to 500 nM), Chronic exposure to DFMO prevented the THIP induction of low-affinity GABA(A) receptors, whereas acute exposure to DFMO had no effect on the ability of THIP to induce low-affinity GABA(A) receptors, Measurements of the intracellular polyamine concentration demonstrated a slight decrease in the putrescine level in the granule cells exposed to DFMO 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 prevent THIP-induced receptor formation. (C) 1994 Wiley-Liss, Inc.