Method for making selective lesions of the hippocampus in macaque monkeys using NMDA and a longitudinal surgical approach.

We describe a method for making selective lesions of the hippocampus in macaque monkeys, using a magnetic resonance imaging (MRI)-guided stereotaxic approach in which the excitotoxin N-methyl-D-aspartic acid (NMDA) is injected at intervals along a single needle track that extends longitudinally through the rostrocaudal extent of the hippocampus. Procedures were conducted on six rhesus monkeys (Macaca mulatta) and were assessed with either in vivo MRI (n = 3) or postmortem microscopic examination of the tissue after standard histological processing of the brains (n = 3). Based on our extensive experience with the standard stereotaxic procedure in which ibotenic acid (IBO) is injected via the dorsal approach, we report that the new method provides a viable and potentially advantageous alternative to the standard procedure. First, the longitudinal approach combined with N-methyl-D-aspartic acid (NMDA) injections increases the reliability and efficacy of hippocampal excitotoxic lesions, probably by limiting the leakage of injectant into the ventricles. Second, the present procedure led to more rapid postoperative recovery compared with that after the standard procedure. Third, because the new method requires fewer needle penetrations than the standard method, it most likely reduces the chances of infarction in extrahippocampal tissue. Finally, the new surgical approach may provide a mechanism for infusing agents into the hippocampus from a single cannula.