Suppression of AMPA receptor exocytosis contributes to hippocampal LTD

Abstract

The decrease in number of AMPA-type glutamate receptor (AMPAR) at excitatory synapses causes long-term depression (LTD), a cellular basis of learning and memory. The number of postsynaptic AMPAR is regulated by the balance of exo- and endocytosis, and enhanced endocytosis of AMPAR has been suggested to underlie the LTD expression. However, it remains unclear how endo- and exocytosis of AMPAR change during LTD. In this study, we addressed this question by analyzing exocytosis and endocytosis of AMPAR by imaging super-ecliptic pHlorin (SEP)-tagged AMPAR around postsynaptic structure formed directly on the glass surface in the hippocampal culture prepared from rat embryos of both sex. Contrary to a prevailing view on the LTD expression by endocytosis enhancement, The LTD induction by NMDA application only transiently enhanced endocytosis of SEP-tagged GluA1 subunits of AMPAR, which was counteracted by simultaneous augmentation of exocytosis. As a result, soon after the start of the LTD induction (∼1 min), the surface AMPAR did not markedly decrease. Thereafter, the surface GluA1-SEP gradually decreased (2∼5 min) and kept at a low level until the end of observation (> 30 min). Surprisingly, this gradual and sustained decrease of surface AMPAR was accompanied not by the enhanced endocytic events of GluA1, but by the suppression of exocytosis. Altogether, our data highlights an unprecedented mechanism for the LTD expression by attenuation of exocytosis of AMPAR, but not by enhanced endocytosis, together with a reduction of postsynaptic AMPAR scaffolding protein PSD95.

SIGNIFICANCE STATEMENT

It has been generally assumed that long-term depression (LTD) is expressed by enhancement of AMPAR endocytosis. Previous studies reported that endocytosis-related protein was involved in LTD and that significant amount of cell-surface AMPAR moved into intracellular compartments during LTD. Here, we report changes of cell-surface amount of AMPAR, and where and when individual exo- and endocytosis occurred during LTD. Cell-surface AMPAR gradually decreased in synchrony with suppression of exocytosis but not with enhancement of endocytosis. These results suggest the decrease of cell-surface AMPAR amount during LTD was caused not by enhancement of endocytosis but rather by suppression of exocytosis, which revises current understanding of the expression mechanism of LTD.