The concentration of glycerol-3-phosphate dehydrogenase (GPDH; sn-glycerol-3-phosphate:NAD(+) 2-oxidoreductase, EC 1.1.1.8) had previously been determined to be regulated by glucocorticoids in rat brain cells in vivo and in cell culture. We now demonstrate that concanavalin A (Con A) can inhibit the induction of GPDH in dose-dependent manner in C6 rat glioma cells and in primary cultures of rat brain oligodendrocytes. Con A is not cytotoxic, because its effect can be prevented or reversed by alpha-methyl mannoside. The inhibition specifically prevents the appearance of new molecules of GPDH, although Con A does not significantly inhibit protein synthesis in these cells, nor does it affect the activity of another soluble enzyme, lactate dehydrogenase. The ability to block enzyme induction is not limited to Con A, because other lectins also inhibit induction, with Ricinus communis agglutinin 60 being the most potent (50% inhibition of induction at 0.0083 muM) and wheat germ agglutinin being the least potent (50% inhibition of induction at 1.2 muM). The molecular mechanism by which Con A inhibits GPDH induction appears to be by the "down regulation" of the cytoplasmic glucocorticoid receptors, because exposure to Con A results in the loss of more than 90% of the receptor activity. Con A does not inhibit the receptor assay and no direct interaction between the receptor and Con A could be demonstrated. This down regulation is not tumor cell specific and appears to be a general phenomenon, because it occurs in normal oligodendrocytes and even in normal astrocytes (a cell type in which the gene for GPDH is not expressed). The down regulation of glucocorticoid receptors in normal brain cells suggests two important corollaries. First, it demonstrates the existence of a rate-limiting step controlling the glucocorticoid-dependent gene expression in brain cells and possibly represents a regulatory site common to all glucocorticoid target cells. Second, it suggests that the response to glucocorticoids of oligodendrocytes and astrocytes can be regulated in vivo by cell surface contact with endogenous lectins, neighboring cells, or both.