THE EFFECTS OF SUBCUTANEOUS INJECTIONS OF GLUCOCORTICOIDS ON AMEBOID MICROGLIA IN POSTNATAL RATS

Subcutaneous injections of glucocorticoids into postnatal rats resulte d in a drastic reduction in the number of amoeboid microglial cells in the corpus callosum as shown by their labelling with the monoclonal a ntibodies of the OX-series, ED1, lectin and rhodamine isothiocynate (R hIc), In rats receiving 2 or 3 injections of glucocorticoids and kille d at the age of 4 or 7 days, between 40 to 60% of the callosal amoeboi d microglial cells were depleted when compared with the corresponding control animals, The cells that survived the glucocorticoid treatments became ramified, while those in the controls of the same age group re mained round or amoeboidic, In rats killed at 2 or 3 weeks of age, the microglia became extremely ramified with a concomitant diminution in their immunostaining, particularly in the glucocorticoid-injected rats . In rats receiving glucocorticoid injections along with RhIc, the RhI e-laden amoeboid microglia appeared round and amoeboidic and were inte nsely stained with OX-42, suggesting their activation and upregulation of complement type 3 receptors when compared with rats receiving only glucocorticoids, Compared with the control, cellular proliferation co ntinued in rats given glucocorticoid injection as indicated by the occ urrence of many bromodeoxyuridine-lakelled cells in the corpus callosu m at the age of 6 days, Ultrastructural studies confirmed the presence of mitotic cells identified as amoeboid microglia because of their la belling with isolectin, A striking ultrastructural feature in glucocor ticoids-injected rats was the wide occurrence of amoeboid microglial c ells that had ingested a variable number of lectin-labelled cells, It is concluded from this study that the drastic reduction of amoeboid mi croglia after glucocorticoid injections can be attributed to the suppr ession of their precursor cells, monocytes, Another possible explanati on is the acceleration of their degeneration process, probably greatly enhanced by glucocorticoids; the degenerating amoeboid microglia were readily eliminated by the surviving amoeboid microglial cells through endocytosis. Glucocorticoids also accelerated the maturation process of the persisting amoeboid microglia to become ramified in form.