Transient spine density increases in the mid-molecular layer of hippocampal dentate gyrus accompany consolidation of a spatial learning task in the rodent

In previous studies, we observed a transient increase in dendritic spine fr equency in the molecular layer of the dentate gyrus at 6 h following passiv e avoidance training [O'Malley A., O'Connell C. and Regan C. M. (1998) Neur oscience 87, 607-613]. To determine if a similar change is associated with spatial forms of learning, we have estimated rime-dependent modulations of spine number in the dentate gyrus of the adult rat following water maze tra ining. All animals exhibited significant reductions in the latency to locat e the platform over the five training sessions of the single trial (median and interquartile ranges of 60, 8 versus 8, 3 s for trials 1 and 5, respect ively) and this improved performance was retained just prior to killing at the 6 h post-training time. The unbiased dissector stereological procedure was used to estimate spine number in serial pairs of ultrathin coronal sect ions obtained at a point 3.3 mm caudal of Bregma. This analysis revealed a significant learning associated increase in spine number at the 6 h post-tr aining time (1.32 +/- 0.18 spines/mu m(3)) as it was not observed in paired controls exposed to the water maze for a similar swim-time (0.66 +/- 0.11 spines/mu m(3)). The increase was transient as spine number returned to con trol levels at the 72 h post-training time. These spine frequency changes are proposed to reflect increased synapse tur nover rate and concomitant change in connectivity pattern in the processing of information for long-term storage. (C) 2000 IBRO. Published by Elsevier Science Ltd. All rights reserved.