The present invention provides polypeptides (termed "egl-28 polypeptides") that modulate neurotransmitter- and psychostimulant-induced responses. The invention also provides methods of modulating such responses, as well as and anti-egl-28 antibodies. The invention encompasses related polynucleotides, vectors, host cells, production methods, and compositions. Moreover, the invention includes methods for prescreening or screening for test agents that modulate neurotransmitter and psychostimulant responses.