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Primary cultures of neonatal rat cortical neurons and astrocytes synthesize and secrete nerve growth factor (NGF). Co-culturing neurons with astrocytes decreased NGF secretion in the co-cultures. The inhibition of co-culture NGF secretion was partially reversible upon selectively decreasing the number of neurons by glutamate treatment. Interleukin-1 beta (IL-1) stimulated NGF secretion from astrocytes, and the magnitude of this secretion was decreased in the co-cultures. Thus, co-culture with neurons decreases astroglial cell secretion of NGF and down-regulates astroglial responsiveness to IL-1.

Neonatal rat cortical astrocytes in primary culture synthesize and secrete nerve growth factor (NGF). Interleukin-1 beta(IL-1) and basic fibroblast growth factor (bFGF) treatment of astrocytes increased NGF mRNA content by about 2-fold. The effect of these two factors was specific, because other growth factors, such as tumor necrosis factor-alpha, insulin-like growth factor-1, and epidermal growth factor, failed to change NGF mRNA content. The concentrations of IL-1 and bFGF causing half-maximal stimulation were 1 unit/ml and 1 ng/ml, respectively. The increase in NGF mRNA elicited by IL-1 and bFGF was maximal at 3 hr of incubation. In the presence of IL-1 this increase persisted for 36 hr, whereas in the presence of bFGF the initial increase in NGF mRNA was followed by a decrease to 50% of control levels after 24 hr of incubation. Readdition of bFGF after 24 hr of treatment gave a similar increase in NGF mRNA content, suggesting that the decrease at 24 hr was not due to receptor desensitization. The effect of IL-1 was reversible, because removal of IL-1 after 3 hr of incubation resulted in a decrease of NGF mRNA content to control levels by 6 hr, whereas a readdition of IL-1 at this time led to a 2-3-fold increase in NGF mRNA content after an additional 3 hr of treatment. This second increase in NGF mRNA was also maintained for several hours. The combined treatment of astrocytes with maximally effective doses of IL-1 and bFGF produced an additive increase in NGF mRNA content, suggesting that different mechanisms are operative. Treatment of astrocytes with cycloheximide increased (about 6-fold) NGF mRNA content, and this content failed to increase further with IL-1 or bFGF treatment. Experiments using actinomycin D indicated that IL-1 increased the stability of the NGF mRNA. bFGF treatment failed to change this parameter. Thus, IL-1 increases NGF mRNA content in astrocytes, at least in part, by stabilizing mRNA, whereas bFGF does not affect mRNA stability but may act at the level of NGF gene transcription.

Primary cultures of neonatal rat cortical astrocytes contain low cellular levels (about 2 pg/mg of protein) of nerve growth factor (NGF), but secrete significant amounts of NGF into the culture medium (about 540 pg of NGF/mg of cell protein/38-h incubation). Incubation of astrocytes with interleukin-1 (IL-1) increased the cellular content of NGF and the amount secreted by about threefold. In comparison, cerebellar astrocytes secreted significant amounts of NGF, and the secretion was also stimulated by IL-1. The stimulatory action of IL-1 on astrocytes prepared from cortex was dose- and time-dependent. Concentrations of IL-1 causing half-maximal and maximal stimulation of NGF secretion were 1 and 10 U/ml, respectively). Maximal NGF secretion induced by IL-1 (10 U/ml) was seen following 38 h of incubation. The basal secretion of NGF was reduced by about 50% under Ca2(+)-free conditions; however, the percent stimulation of NGF secretion by IL-1 was the same in the absence or presence of Ca2+. The stimulatory action of IL-1 was specific, because other glial growth factors and cytokines were almost ineffective in stimulating NGF secretion from cortical astroglial cells. IL-1 treatment also increased cellular NGF mRNA content twofold. The results indicate that IL-1 specifically triggers a cascade of events, independent of cell growth, which regulate NGF mRNA content and NGF secretion by astrocytes.