Climbing Fiber-Mediated Spillover Transmission to Interneurons Is Regulated by EAAT4

Neurotransmitter spillover is a form of communication not readily predicted by anatomic structure. In the cerebellum, glutamate spillover from climbing fibers recruits molecular layer interneurons in the absence of conventional synaptic connections. Spillover-mediated signaling is typically limited by transporters that bind and reuptake glutamate. Here, we show that patterned expression of the excitatory amino acid transporter 4 (EAAT4) in Purkinje cells regulates glutamate spillover to molecular layer interneurons. Using male and female Aldolase C-Venus knock-in mice to visualize zebrin microzones, we find larger climbing fiber-evoked spillover EPSCs in regions with low levels of EAAT4 compared with regions with high EAAT4. This difference is not explained by presynaptic glutamate release properties or postsynaptic receptor density but rather by differences in the glutamate concentration reaching receptors on interneurons. Inhibiting glutamate transport normalizes the differences between microzones, suggesting that heterogeneity in EAAT4 expression is a primary determinant of differential spillover. These results show that neuronal glutamate transporters limit extrasynaptic transmission in a non-cell-autonomous manner and provide new insight into the functional specialization of cerebellar microzones.

Automated summary

Neurotransmitter spillover, a form of communication not easily predicted by anatomy, is regulated by the patterned expression of the excitatory amino acid transporter 4 in the cerebellum. Through experiments using Aldolase C-Venus knock-in mice, researchers found that heterogeneity in EAAT4 expression is the primary determinant of differential spillover between neuronal microzones. This study highlights how neuronal glutamate transporters limit extrasynaptic transmission in a non-cell-autonomous manner and provides new insights into the functional specialization of cerebellar microzones.