Neurotrophins have long been thought important for the survival and differentiation of neurons during development and for their maintenance during adulthood. Recent work has demonstrated that neurotrophins may also play a role in certain forms of neuronal plasticity. The experiments proposed here seek to understand the action of brain derived neurotrophic factor (BDNF), and possibly other neurotrophins, in an activity-dependent homeostatic process that we have observed in cortical cultures. When deprived of activity for an extended period, these cultures respond by increasing their excitatory synaptic strengths; when subject to enhanced activity for an extended period, they respond by decreasing those strengths. Activity levels appear to regulate synaptic strengths in order to keep firing rates within certain bounds--to prevent networks from saturating or falling silent. There is some reason to believe that this homeostatic process is mediated by BDNF, possibly through a calcium-dependent mechanism. To explore this issue, fura-2 imaging will be used to examine how BDNF application affects resting calcium levels in cultured visual cortical neurons. Whole cell patch recordings will determine whether incubation in BDNF abolishes the effects of activity deprivation on synaptic plasticity, and, if so, whether BDNF~s action depends upon intracellular calcium. Immunohistochemistry will be used to discover wether BDNF's action on a different homeostatic process--reduced GABA expression in activity-deprived cultures--is calcium dependent. Most of these experiments will be repeated to determine if other neutrophins have similar effects.