||It is known that the density of peripheral benzodiazepine receptors (PBR) increases after brain damage. Astrocytes are among the cell types where PBR ligand binding has been detected and may be involved in the response to neuronal injury and regeneration. Consistent with the hypothesis, the apparent density of PER sites in astrocytes is increased by both cytokines and neurotoxins. However, microglia, the resident macrophages which represent 5-15% of glial cell populations have not been evaluated for the presence of the PER. In the present study, we report the presence of [H-3]Ro5-4864 binding in microglial cells. In particular, we used BV-2 cells, an immortalized cell line of murine microglial cells. High affinity binding of [H-3]Ro5-4864 to a single site was detected in membranes prepared from BV-2 cells (K-D = 4.4 nM, B-max = 3,800 fmoles/mg protein). Various ligands for the PER displaced [H-3]Ro5-4864 binding with the following rank order of potencies: PK11195 = Ro5-4864 > FGIN-1-27 > triazolam = diazepam > beta-pro-pyl-beta-carboline-3-carboxylate = clonazepam > lorazepam = flurazepam >> chloridazepoxide = clorazepate. Subcellular fractionationstudies indicate that the majority of the Ro5-4864 binding sites is in the mitochondrial fraction. The remainder is found in nonmitochondrial cell fractions. The [H-3]Ro5-4864 binding observed on intact cells had characteristics similar to those found on membranes. The presence of a high density of PBRs in these cells establish the basis for additional investigations into their possible functional role, if any, in the microglial response to neuronal injury. (C) 1996 Wiley-Liss, Inc.