Postsynaptic spiking determines anti-Hebbian LTD in visual cortex basket cells

Christina You Chien Chou ^1,2 Wouter J Droogers ¹ Txomin Lalanne ¹ Eric Fineberg ¹ Tal Klimenko ¹ Hannah Owens ¹ P. Jesper Sjöström ^1*

• ¹ Research Institute, McGill University Health Center, Montreal, Canada
• ² Integrated Program in Neuroscience, McGill University, Montreal, Canada

Long-term plasticity at pyramidal cell to basket cell (PC→BC) synapses is important for the functioning of cortical microcircuits. It is well known that at neocortical PC→PC synapses, dendritic calcium (Ca 2+ ) dynamics signal coincident pre-and postsynaptic spiking which in turn triggers longterm potentiation (LTP). However, the link between dendritic Ca 2+ dynamics and long-term plasticity at PC→BC synapses of primary visual cortex (V1) is not as well known. Here, we explored if PC→BC synaptic plasticity in developing V1 is sensitive to postsynaptic spiking. Two-photon (2P) Ca 2+ imaging revealed that action potentials (APs) in dendrites of V1 layer-5 (L5) BCs backpropagated decrementally but actively to the location of PC→BC putative synaptic contacts. Pairing excitatory inputs with postsynaptic APs elicited dendritic Ca 2+ supralinearities for pre-beforepostsynaptic but not post-before-presynaptic temporal ordering, suggesting that APs could impact synaptic plasticity. In agreement, extracellular stimulation as well as high-throughput 2P optogenetic mapping of plasticity both revealed that pre-before-postsynaptic but not post-before-presynaptic pairing resulted in anti-Hebbian long-term depression (LTD). Our results demonstrate that V1 BC dendritic Ca 2+ nonlinearities and synaptic plasticity at PC→BC connections are both sensitive to somatic spiking.