RECOVERY OF SPATIAL PERFORMANCE IN THE MORRIS WATER MAZE FOLLOWING BILATERAL TRANSECTION OF THE FIMBRIA FORNIX IN RATS/

The present study investigated whether spatial performance in the Morr is water maze (MWM) recovers after bilateral transection of the fimbri a/fornix (FF) in rats, whether such recovery results from restored or residual spatial cognitive capacity, and what contribution, if any, pr e-operative training makes to such recovery. Following surgery, rats w ere administered extensive training to a constant submerged platform l ocation with frequent probe tests to assess performance strategies. Fo llowing the attainment of asymptotic performance levels, rats were tes ted for acquisition of a second platform location. FF lesions were fou nd to produce a severe impairment both in pre-operatively trained rats (a retention or retrieval deficit) and in naive rats (an acquisition deficit) as shown by the use of indirect routes to the platform on sub merged platform trials and an absence of localized searching in the pl atform's area on probe trials. However, with further training, perform ance recovered in both groups, such that they eventually used direct e scape routes to the submerged platform and showed highly localized sea rching in its area on probe trials. When tested for acquisition of a s econd platform location, a substantial deficit reappeared, but was aga in overcome with additional training. Pre-operative training was found to attenuate the initial post-operative deficit and speed recovery of performance but did not affect asymptotic performance levels nor acqu isition of the second platform location. These data show that, though spatial cognition as assessed in the MWM is impaired by FF lesions, sp atial performance eventually recovers. Moreover, pre-operative trainin g, though of some initial post-operative benefit, is not essential for this recovery. The deficit shown in acquisition of the second platfor m location argues against recovery of spatial cognition and suggests t hat the basis of recovered performance is residual spatial cognitive c apacity. Several limitations of this residual capacity are apparent: ( i) rate of acquisition of spatial information is reduced; (ii) utiliza tion of spatial information stored pre-operatively is restricted; and (iii) translation of spatial information into navigational behaviour i s less efficient. The neural bases of this residual system are specula ted to include spared intra-hippocampal storage mechanisms and/or mech anisms involved in extra-hippocampal long-term memory consolidation wh ile the neural bases of the FF's contribution to spatial information s torage in the intact brain are speculated to involve theta synchroniza tion of hippocampal activity and the induction and expression of hippo campal long-term potentiation. (C) 1998 Elsevier Science B.V.