ROLE OF DORSAL HIPPOCAMPUS IN ACQUISITION, CONSOLIDATION AND RETRIEVAL OF RATS PASSIVE-AVOIDANCE RESPONSE - A TETRODOTOXIN FUNCTIONAL INACTIVATION STUDY

By means of local administration of tetrodotoxin (TTX) fully reversibl e functional inactivation of rat's dorsal hippocampus (DH) was obtaine d in order to define the role of this structure in the memorization of a conditioned passive avoidance response (PAR). In Experiment 1, on p ermanently cannulated animals, TTX (10 ng in 1.0 mu l saline) or salin e (1.0 mu l) was injected uni- or bilaterally in the DH, respectively 1 h before PAR acquisition, immediately after PAR acquisition, and 1 h before PAR retrieval, always performed 48 h after the acquisition tri al. It was shown that both pre-acquisition and pre-retrieval DH uni- o r bilateral blockades were followed by significant PAR retention impai rment, while in post-acquisition only the bilateral blockade determine d PAR retention impairment. In Experiment 2, on three different groups of rats, TTX (10 ng in 1 mu l saline) was bilaterally administered, u nder general ketamine anesthesia (100 mg/kg), into the DH at different post-acquisition delays (0.25, 1.5, 6 h). Retrieval testing, 48 h aft er treatment, showed that post-acquisition bilateral DH blockade cause d PAR impairment only when performed 0.25 or 1.5 h after acquisition. The results indicate a well defined mnemonic role of DH during the acq uisition, consolidation and retrieval of PAR engram. The experimental evidence is discussed in relation to other reports and to DH connectiv ity with the medial septal area and the amygdala.