2026 Volume 13 Issue 4 Pages 161-170
To assess the toxicity profile of drugs in pre-clinical studies, it is important to dose the test article up to the maximum tolerable dose (MTD) or maximum feasible dose (MFD). The recent increase in novel modalities has necessitated a widened range of dosing routes, including intrahippocampal administration. However, information on the maximum injection volume and injection duration for intrahippocampal dosing in rats remains limited. Here, we investigated the MFD for intrahippocampal administration in rats by examining the maximum injection duration and injection speed under the maximum anesthesia time. Results showed that, even with warming, the maximum anesthesia duration was within 110 min; and excluding surgical procedures, the maximum possible infusion time was 60 min. In addition, when the infusion speed of intrahippocampal injection exceeded 200 nL/min, backflow from the hippocampus was observed. Thus, the maximum infusion speed was determined to be 100 nL/min. Under these conditions, no abnormalities were noted in general clinical observations, body weight or food consumption. Furthermore, on histopathological examination, recessed focus at the injection site, hemorrhage and inflammation in the submeningeal area, and focal microgliosis in the hippocampus were observed. These changes were considered attributable to the dosing procedure, however, and evaluation of the toxicity of the test article remained possible. Therefore, a single intrahippocampal injection (in the case of two sites per animal, 100 nL/min, 30-min injection time/site) to rats is considered appropriate in defining a MFD.
