Reversible lesion of the rat's orbitofrontal cortex interferes with hippocampus-dependent spatial memory

In this study, tetrodotoxin (TTX) inactivation was employed to evaluate the involvement of the rat's orbitofrontal cortex (OFC) in hippocampus-dependent spatial memory using Morris water maze (MWM) and place avoidance learning (PAL) tasks. In Experiment 1, rats trained in MWM task with two blocks of four trials per day for 3 consecutive days received bilateral injections of either TTX or saline into the OFC 60 min before each daily training session. The acquisition of spatial memory was evaluated 24 h after the last training day and it was shown an impairment by the TTX. In Experiment 2, bilateral intra-OFC injections of TTX or saline were made immediately after two blocks of four trials. Testing 24 h later, it was revealed that TTX also impairs spatial memory consolidation. In Experiments 3 and 4, rats were trained in a single 30-min session to avoid a 60degrees segment of the stable circular (80-cm diameter) arena, entering which was punished by a mild shock (PAL task) and retention was tested 24 h later in a 30-min extinction session. Bilateral injections of TTX or saline were made into the OFC 60 min before training or immediately after training. Again, TTX impaired the place avoidance retention when it was injected into the OFC either before (acquisition phase) or after (consolidation phase) training. These findings indicate that functional integrity of the OFC is necessary for both the acquisition and the consolidation of hippocampus-dependent spatial memory in rats. (C) 2003 Elsevier B.V. All rights reserved.