Age-dependent decrease in the lethal threshold of pentylenetetrazole in mice

We previously reported that the minimal effective concentrations in plasma and brain for inducing the maximal seizure after intraperitoneal injection of pentylenetetrazole (PTZ) significantly increased with age in mice. We also observed that some old mice died without the maximal seizure during the observation period of 15 min. To examine how the lethal dose changes with age in the mouse, in the present study we employed a continuous intravenous infusion of PTZ that enabled us to control the dose, either stopping the infusion after the maximal seizure or continuing it until the lethal dose was reached. This study has demonstrated that the lethal threshold of PTZ in the oldest mice (27 months for males and 30 months for females) was significantly lower than that for the respective younger groups. We conclude that our observation in old mice of death occurring before the appearance of maximal seizure is due to a decrease in the lethal threshold combined with an increase in the threshold for maximal seizure.