Some characteristics of muscle-conditioned media with excitatory and inhibitory influences on neurite outgrowth from early neural tube explants.

This study examined the growth of basal plate explants from 40-hr chick embryos containing the trigeminal motor nucleus in the presence of target muscle-conditioned medium (MCM) from day-4, day-10, and day-18 embryos. We had previously found that MCM derived from the 4-day target tissue enhanced neurite outgrowth from these explants, while target MCM from 10- and 18-day embryos inhibited it. For the present study, target MCM was treated with trypsin to assess the nature of the active fractions, or pre-incubated in polyornithine-coated dishes to determine the possible presence and relative contributions of substrate-binding vs. soluble components. Both trypsinization and pre-incubation abolished the outgrowth-enhancing potential of the 4-day MCM, indicating a protein or polypeptide substrate-binding active fraction, with no appreciable active soluble component. When the 10-day and 18-day target MCM were trypsinized, the inhibitory influence was reversed, and neurite outgrowth was enhanced. Similarly, when the MCM from these embryos was pre-incubated over polyornithine, the inhibitory influence was lost and was replaced by a stimulatory one. These results suggest that the 10- and 18-day MCM contain an active inhibitory fraction that is a protein or polypeptide, and which binds to a positively charged substrate. When this inhibitory fraction is inactivated or depleted, an excitatory soluble fraction is revealed. Such a dual nature in MCM has not been seen previously and may be expressed only when the media are assayed with very early neural tissue.