An object of the present invention is to provide a remedy for dysfunction of central monoamine pathway, a method for screening a PTP.zeta. inhibitor or activator, which is useful as a remedy for gastric ulcer caused by Helicobacter pylori or pleiotrophin which is a heparin-binding secretory protein, and a non-human model animal being hyposensitive to a stimulant drug, VacA which is a toxin of Helicobacter pylori, or pleiotrophin by utilizing the physiological function of PTP.zeta.. After administering a subject material to PTP.zeta. knockout mice and wild-type mice, PTP.zeta. activity in the PTP.zeta. knockout mice and the wild-type mice is compared and evaluated to screen a PTP.zeta. inhibitor or activator. Examples of the comparison and the evaluation of the PTP.zeta. activity include the comparison and the evaluation of the function of central monoamine pathway such as changes in the level of central monoamine metabolism, sensitivity to a stimulant drug, the presence of dysfunction of mesolimbic dopamine pathway, level of acclimation to new circumstances, or stress-responsiveness, and the comparison and the evaluation of the level of binding to VacA, a toxin of Helicobacter pylori, or pleiotrophin.