This invention relates to the use of ADNF III polypeptides in the treatment of mental diseases or disorders, including schizophrenia. Embodiments of the invention provide methods for treating mental disorders, including schizophrenia, in a subject by administering a NAP, an 8-amino-acid peptide derived from Activity Dependent Neurotrophic Factor (ADNF III), in an amount sufficient to reduce or eliminate symptoms. The ADNF III polypeptides include polypeptides, analogs, subsequences, and D-amino acid versions (either wholly D-amino acid peptides or mixed D- and L-amino acid peptides), and combinations thereof which contain the active core sites and provide neuroprotective and anti-schizophrenic functions.