Elsevier

Neuroscience

Volume 99, Issue 2, 11 September 2000, Pages 229-232
Neuroscience

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

https://doi.org/10.1016/S0306-4522(00)00182-2Get rights and content

Abstract

In previous studies, we observed a transient increase in dendritic spine frequency in the molecular layer of the dentate gyrus at 6 h following passive avoidance training [O’Malley A., O’Connell C. and Regan C. M. (1998) Neuroscience 87, 607–613]. To determine if a similar change is associated with spatial forms of learning, we have estimated time-dependent modulations of spine number in the dentate gyrus of the adult rat following water maze training. All animals exhibited significant reductions in the latency to locate 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, respectively) 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 sections 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-training time (1.32±0.18 spines/μm3) as it was not observed in paired controls exposed to the water maze for a similar swim-time (0.66±0.11 spines/μm3). The increase was transient as spine number returned to control levels at the 72 h post-training time.

These spine frequency changes are proposed to reflect increased synapse turnover rate and concomitant change in connectivity pattern in the processing of information for long-term storage.

Section snippets

Training

The spatial learning task employed has been described in detail previously.18 Briefly, the water maze apparatus consisted of a large circular pool (1 m diameter, 80 cm high, temperature 26±1°C) with a platform (11 cm diameter) submerged 1.5 cm below the water surface. Both the pool and the platform were constructed of black polyvinyl plastic and offered no intra-maze cues to guide escape behaviour. The experimental room contained several extra-maze visual cues. During training the platform was

Results

The water maze task was acquired by all animals as the swim latency times became significantly reduced between the first and the fifth trial (median and interquartile ranges of 60, 8 versus 8, 3 s, respectively; P<0.05, Mann–Whitney U-test; Table 1). The passive control animals explored the water maze for a time matched to their trained counterparts but in the absence of the platform. The improved performance of the task in the trained animals persisted in the recall trial at 6 h post-training

Discussion

The experiments reported here imply that among the sequence of physiological processes initiated following acquisition of a spatial task is modulation of spine number in the mid-molecular layer of the dentate gyrus. Sensory information arrives at the hippocampus via the perforant path entorhinal cortical efferents, 80% of which terminate in the dentate molecular layer as axospinous synapses.5., 13., 27. Thus, the observed change in spine number most probably relates to the elaboration of

Acknowledgments

This work was supported by Enterprise Ireland.

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