Nerve growth factor (NGF) content in adult rat brain tissues is several-fold higher than generally reported and is largely associated with sedimentable fractions.

Initial studies had revealed that the bioactivity of nerve growth factor (NGF) in sonicates of adult rat hippocampal formation (HF) is several-fold greater in their pellet than their supernatant fractions. Such observations have prompted an analysis of NGF antigen (NGF-Ag) contents in pellets and supernatants from a variety of adult rat CNS tissues, both in the absence and the presence of exogenous beta-NGF. With HF tissues, NGF-Ag in the supernatants was comparable to most literature values, but pellet NGF-Ag was 3 to 5 times that amount. All other CNS tissue sonicates also revealed 3-6 fold higher NGF-Ag in their pellets than their supernatants, hence overall NGF-Ag contents were greatly in excess of reported ones. Presentation of mouse beta-NGF to a tissue, its sonicate, or its standard pellet resulted in a transfer to the final pellet of 30-50% of the added soluble NGF-Ag (and 30% of the added bioactivity). This percentage is much lower than that present in native pellets (80%), suggesting that the association of endogenous NGF with particulate matter may involve at least two compartments. Treatments of pellets with salt, alkaline pH, and/or the detergent Triton X-100 have revealed a third subset, namely additional pellet NGF-Ag that was not initially recognized by the antibody in our ELISA assay. The treatments also caused substantial release of NGF from pellet to supernatant. Further studies should clarify the nature of the association between NGF and the three subsets of pellet NGF and allow the investigation of the pellet molecules responsible for it.