Effects of combined medial septal area, fimbria–fornix and entorhinal cortex tetrodotoxin inactivations on passive avoidance response consolidation in the rat

Abstract

On the basis of previous experimental evidence, it has been concluded that the entorhinal cortex (EC), the fimbria–fornix (FF) complex and medial septal area (MSA) do not take part in the consolidation phase of passive avoidance response (PAR) memorization. On the other hand, a mnemonic role during consolidation of at least two of these structures has been argued, based on several considerations. In order to ascertain whether the EC and FF are still involved in PAR memorization during consolidation, the coupled fully reversible functional tetrodotoxin (TTX) inactivation of MSA, FF and EC was performed in rats having undergone a PAR training. In Experiment 1 MSA, FF and EC were inactivated pair-wise (FF and EC always bilaterally). Permanently cannulated animals were injected stereotaxically with TTX (5 ng in 0.5 μl saline) or saline (0.5 μl) immediately following PAR acquisition. It was shown that combined FF–EC inactivation induced PAR retention impairment, whereas FF–MSA and EC–MSA inactivation was not followed by amnesic effects. Having obtained a positive result, in Experiment 2 the combined FF–EC inactivation was performed at different post-acquisition delays (0.25 h, 1.5 h, 6 h), so as to assess the duration of their involvement in PAR consolidation. It was shown that only the coupled inactivation performed at the shortest post-acquisition delay was followed by amnesic effects. Thus EC and FF play a definite role during early consolidation. The results are discussed in relation to EC, FF, MSA, and hippocampal involvement in PAR memorization, as reported in previous studies, and to their connectivity.