Conditional Gdi1 Deletion on Synaptic Function Figure 7. Short-term plasticity at Cortico-LA synapses is perturbed in absence of aGDI. (A) Iterative stimulations (20 Hz/4 sec) were applied at Cortico-LA synapses in Gdi1 WT and Gdi1-null mice. Scale bars: 50 (up) and 100 (bottom) pA and 250 msec. (B) The pronounced synaptic depression in Gdi1-null mice is well illustrated by the extraction of the last EPSCs of the train response. Scale bars: 50 (up) and 100 (bottom) pA and 10 msec. (C) Mean EPSC amplitude at a given stimulation for each genotype. (D) Cumulative plot based on the same data, allowing to visualize the deficit in refilling rate at Gdi1-null synapses (see material and methods section for further details). (E1) Ready releasable pool size (RRP) and refilling rate (E2) were calculated in Gdi1-null and Gdi1 WT preparations and expressed as % of initial EPSC size. Number of recorded cells is indicated. **p,0,01. doi:10.1371/journal.pone.0029763.g007 mimicked the constitutive mutation. Thus, these results show that the presence of pre-synaptic aGDI is essential in maintaining normal excitatory projections to the LA. Delayed absence of aGDI causes additional pre-synaptic deficits We next compared short-term plasticity at cortico-LA synapses in PND 100 Gdi1 WT, Gdi1-null and Gdi1flox/Y mice using trains of 80 stimulations 20 Hz in magnitude (Fig. 8G). Post-synaptic responses to high frequency trains were affected at synapses and were characterized by higher fatigue compared to WT synapses (Fig. 8G). However, analysis of cumulative EPSCs showed a major difference between both mutant mice: as seen in young animals (Fig. 7), a decrease in the refilling rate is present in not only PND 100 Gdi1-null mice but also Gdi1flox/Y PND 100 mice (Fig. 8I). More surprisingly, the RRP size of Gdi1flox/Y cortico-LA synapses was found to be significantly lower than in Gdi1 WT and Gdi1-null mice (Gdi1 WT: 22876312% of initial EPSC, Gdi1-null: 21816270%, Gdi1flox/Y: 12016239%, p.0.05 for both, Fig. 8H). This unexpected observation may indicate the existence of cellular mechanisms that correct the synaptic dysfunction when the mutation occurs during early development. Discussion We previously showed that Gdi1-null mice had a specific hippocampus-dependent short-term memory impairment and overfriendly behavior [5]. We suggested that the memory deficit might be caused by alterations in the SV pool and short-term synaptic plasticity involving alterations of different trafficking pathways that are controlled by several RAB GTPase proteins [6]. A major limitation for the study of aGDI function in the central nervous system (CNS) is that Gdi1 is ubiquitously expressed and that complete ablation of Gdi1 occurs in all brain regions and all brain cell types throughout development in the mice. In this study, we used the CRE/lox P recombination system to spatially and temporally restrict knockout of the Gdi1 gene to postmitotic neurons and glia of the anterior forebrain. Our results show that Gdi1flox/Y mice, in which Gdi1 is strongly reduced from PND 56 in the hippocampus, cortex and amygdala, were viable and fertile, and unlike the Gdi1-null mice, they did not present visible morphological or neuro-pathological phenotypes. The learning and memory and behavioral characterization of these mice showed no difference in emotion, explorative behavior, spatial memory or contextual fear conditioning. In contrast, Gdi1flox/Y mice suffered from a specific short-term memory deficit, as indicated by alterations in working memory in the radial maze PLoS ONE | www.plosone.org 8 January 2012 | Volume 7 | Issue 1 | e29763