Amnesic effects of preacquisition, postacquisition, or preretrieval tetrodotoxin administration into the medial septal area on rat's passive avoidance memorization.

By means of local administration of tetrodotoxin (TTX) fully reversible functional inactivation of the medial septal area (MSA) of the rat was obtained in order to define the role of this structure in the retention of a conditioned passive avoidance response (PAR). In permanently cannulated animals, TTX (5 ng in 0.5 microl saline) or saline (0.5 microl) was injected in the MSA. TTX or saline was administered to six different groups of rats, respectively 1 h before PAR acquisition, immediately after PAR acquisition, and 1 h before PAR retrieval, performed 48 h after the acquisition trial. It was shown that MSA preacquisition and preretrieval TTX injections were followed by significant PAR retention impairment, while postacquisition TTX administration had no effect on PAR retention. The results indicate a well-defined mnemonic role of MSA during the acquisition and retrieval periods but not during the consolidation of PAR engram. The experimental evidence is discussed in relation to other reports and to MSA connectivity with other subcortical structures.