Segregation of Kainate and AMPA Synapses

Kainate receptors have long been the poor stepchildren of the ionotropic glutamate receptor family. First, they were named for an agonist that also activates AMPA receptors. Then, even after molecular cloning established their unique composition, their small conductance and the lack of selective antagonists hampered study. However, their presynaptic and postsynaptic function is now well established in several brain regions, including layer IV of the barrel cortex. There, glutamatergic transmission abruptly switches from kainateto AMPA receptor-mediated transmission during the first postnatal week. This week, Bannister et al. report that this developmental switch is associated with total segregation of these receptors, indicating that individual synapses use either kainate or AMPA receptors, but not both. The authors used strontium to evoke single quantal events. Kainate receptor-mediated events had small quantal amplitude, slow kinetics, and large charge transfer, whereas AMPA-mediated transmission had larger quanta, fast kinetics, and small charge transfer. The slow kainate receptor kinetics could not be attributed to an extrasynaptic localization.