Nerve growth factor (NGF) and glial cell line-derived neurotrophic factor (GDNF) regulate substance P release in adult spinal sensory neurons

NGF increases expression and content of substance P in developing and mature spinal sensory neurons. The role this neurotrophin plays in peptide release, however, is less clear. Accordingly, we examined substance P release from cultures of mature rat sensory neurons, which do not require NGF for survival. Neurons grown without NGF have a low but detectable basal release, which increases with depolarization by KCl (50 mM) but never achieves statistical significance. In contrast, basal release is 3 times higher from neurons that have been cultured in the presence of NGF, and KCl depolarization triples the amount of SP released. Stimulation with capsaicin (10^–7 M) yields similar results. Residual peptide remaining after capsaicin stimulation is refractory to release for up to 24 h. Bradykinin does not induce SP secretion from mature neurons nor does it potentiate the action of capsaicin. GDNF, which also increases SP content, mimics NGF. Addition of NGF to the bath during release does not directly induce SP secretion, nor does it alter the effects of KCl, capsaicin, or bradykinin. It appears therefore that NGF increases SP release indirectly by increasing intracellular stores.