INCREASE IN OMEGA-3 (PERIPHERAL TYPE BENZODIAZEPINE) BINDING-SITES IN THE RAT CORTEX AND STRIATUM AFTER LOCAL INJECTION OF INTERLEUKIN-1, TUMOR-NECROSIS-FACTOR-ALPHA AND LIPOPOLYSACCHARIDE

The possible involvement of lymphokines in the glial reaction/proliferation that follows brain injury has been investigated by measuring the density of omega-3 (peripheral type benzodiazepine) binding sites associated to glial cells and macrophages after local injection of lymphokines in the rat cerebral cortex and striatum. Omega-3 site densities were measured either by quantitative autoradiography in brain sections or by conventional binding in membrane using [H-3]PK 14105 or [H-3]PK 11195 as ligands. Intracortical or intrastriatal infusion of interleukin-1 (10 and 20 units) caused a marked increase in the density of omega-3 sites (+83% and +80%, respectively, when compared to saline-infused animals) around the injection site at 7 days postinjection. There was a good spatial correspondence between the autoradiographic distribution of omega-3 sites and the distribution of reactive astrocytes (as assessed by GFAP immunostaining) or acid phosphatase rich cells (phagocytes). Significant increases in omega-3 site densities were also observed in striatal homogenates 1 week after local administration of tumor necrosis factor-alpha (TNF-alpha). The maximal increase (+80%) was observed after the administration of 3 units, higher and lower doses resulting in smaller increases. Intrastriatal injection of E. coli lipopolysaccharide (LPS), a bacterial endotoxin known to stimulate interleukin-1 and TNF-alpha production by microglial cells in culture, also resulted in significant increases in omega-3 site densities in striatal homogenates (maximal increase, +170% 1 week after the injection of 200 ng). In contrast, no change in the density of two binding sites associated with neurons ([H-3]TCP which indexes NMDA receptor density and [H-3]Ro15 1788 which binds to central omega-1-2 (benzodiazepine) receptors) was observed after intrastriatal injection of interleukin-1, TNF-alpha or LPS. The intrastriatal injection of interleukin-2 (5-20 units) or of the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (FMLP) failed to alter omega-3 site or NMDA receptor densities in striatal homogenates. Based on the present results, we suggest that the increase in omega-3 site densities observed in experimental lesions and in human neuropathological states may be mediated by a sequential mechanism involving the activation of microglia or monocytes, the release of interleukin-1 and/or TNF-alpha and the promotion, by these cytokines, of the astroglial reaction.