The invention relates to fragments of a mammalian nervous system protein, agrin, and to their use as a screening and therapeutic agents in controlling neural activity associated with the function of the Na.sup.+/K.sup.+-ATPase pump's function in neurons. Na.sup.+/K+-ATPases are the most important active transporters in animal cells, required for maintaining the electrochemical gradient responsible for resting membrane potential and function of other transport proteins. Accordingly, the current invention demonstrates agrin's ability to modulate activity of the .alpha.3Na.sup.+/K.sup.+-ATPase and suggests a direct role in controlling activity-dependent processes in neurons and other excitable cells, including cardiac muscle fibers, providing a molecular framework for identifying treatments for a variety of disorders characterized by dysregulation of cellular excitability such as epilepsy, nervous tissue trauma and coronary diseases.