Summary Efficient retrograde access to projection neurons for the delivery of sensors and effectors constitutes an important and enabling capability for neural circuit dissection. Such an approach would also be useful for gene therapy, including the treatment of neurodegenerative disorders characterized by pathological spread through functionally connected and highly distributed networks. Viral vectors, in particular, are powerful gene delivery vehicles for the nervous system, but all available tools suffer from inefficient retrograde transport or limited clinical potential. To address this need, we applied in vivo directed evolution to engineer potent retrograde functionality into the capsid of adeno-associated virus (AAV) — a vector that has shown promise in neuroscience research and the clinic. A newly evolved variant, rAAV2-retro, permits robust retrograde access to projection neurons with efficiency comparable to classical synthetic retrograde tracers, and enables sufficient sensor/effector expression for functional circuit interrogation and in vivo genome editing in targeted neuronal populations.

Layer 4 neurons in primary sensory cortices receive direct sensory information from the external world. A general feature of these neurons is their selectivity to specific features of the sensory stimulation. Various theories try to explain the manner in which these neurons are driven by their incoming sensory information. In all of these theories neurons are regarded as simple elements summing small biased inputs to create tuned output through the axosomatic amplification mechanism. However, the possible role of active dendritic integration in further amplifying the sensory responses and sharpening the tuning curves of neurons is disregarded. Our findings show that dendrites of layer 4 spiny stellate neurons in the barrel cortex can generate local and global multi-branch N-methyl-D-aspartate (NMDA) spikes, which are the main regenerative events in these dendrites. In turn, these NMDA receptor (NMDAR) regenerative mechanisms can sum supralinearly the coactivated thalamocortical and corticocortical inputs. Using in vivo whole-cell recordings combined with an intracellular NMDAR blocker and membrane hyperpolarization, we show that dendritic NMDAR-dependent regenerative responses contribute substantially to the angular tuning of layer 4 neurons by preferentially amplifying the preferred angular directions over non-preferred angles. Taken together, these findings indicate that dendritic NMDAR regenerative amplification mechanisms contribute markedly to sensory responses and critically determine the tuning of cortical neurons.

Highlights d Populations of layer 2-3 pyramidal neurons in M1 report motor performance outcome d Success and failure activity is late, prolonged, and dissociated from kinematics and reward d At trial start, layer 5 pyramidal tract activity is affected by previous outcome d Post-movement activity in M1 is required for motor performance and learning