Dendritic inhibitory synapses are most efficient in modulating excitatory inputs localized
on the same dendrite, but it is unknown whether their location is random or regulated.
Here, we show that the formation of inhibitory synapses can be directed by excitatory
synaptic activity on the same dendrite. We stimulated dendritic spines close to a
GABAergic axon crossing by pairing two-photon glutamate uncaging with postsynaptic
depolarization in CA1 pyramidal cells. We found that repeated spine stimulation promoted
growth of a GABAergic bouton onto the same dendrite. The dendritic feedback signal
required postsynaptic activation of DAGL, which produces the endocannabinoid 2-AG,
and was mediated by CB1 receptors. We could also induce inhibitory bouton growth by
local, brief applications of 2-AG. Our findings reveal a dendritic signaling mechanism
to trigger growth of an inhibitory bouton at dendritic locations with strong excitatory
synaptic activity, and this mechanism may serve to ensure inhibitory control over
clustered excitatory inputs.
