SUMMARY Rodents begin to use bilaterally coordinated, rhythmic sweeping of their vibrissae (“whisking”) for environmental exploration around two weeks after birth. Whether and how vibrissal control circuitry changes after birth is unknown, and relevant premotor circuitry remains poorly characterized. Using a modified rabies virus transsynaptic tracing strategy, we labeled neurons synapsing directly onto vibrissa facial motor neurons (vFMNs). Sources of potential excitatory, inhibitory, and modulatory vFMN premotor neurons, and differences between the premotor circuitry for vFMNs innervating intrinsic versus extrinsic vibrissal muscles, were systematically characterized. The emergence of whisking is accompanied by the addition of “new” sets of bilateral excitatory inputs to vFMNs from neurons in the lateral paragigantocellularis (LPGi). Furthermore, descending axons from the motor cortex directly innervate LPGi premotor neurons. Thus, neural modules well suited to facilitate the bilateral coordination and cortical control of whisking are added to premotor circuitry in parallel with the emergence of this exploratory behavior.

SummarySensory information is encoded within the brain in distributed spatiotemporal patterns of neuronal activity. Understanding how these patterns influence behavior requires a method to measure and to bidirectionally perturb with high spatial resolution the activity of the multiple neuronal cell types engaged in sensory processing. Here, we combined two-photon holography to stimulate neurons expressing blue light-sensitive opsins (ChR2 and GtACR2) with two-photon imaging of the red-shifted indicator jRCaMP1a in the mouse neocortex in vivo. We demonstrate efficient control of neural excitability across cell types and layers with holographic stimulation and improved spatial resolution by opsin somatic targeting. Moreover, we performed simultaneous two-photon imaging of jRCaMP1a and bidirectional two-photon manipulation of cellular activity with negligible effect of the imaging beam on opsin excitation. This all-optical approach represents a powerful tool to causally dissect how activity patterns in specified ensembles of neurons determine brain function and animal behavior.

We describe refinements in optogenetic methods for circuit mapping that enable measurements of functional synaptic connectivity with single-neuron resolution. By expanding a two-photon beam in the imaging plane using the temporal focusing method and restricting channelrhodopsin to the soma and proximal dendrites, we are able to reliably evoke action potentials in individual neurons, verify spike generation with GCaMP6s, and determine the presence or absence of synaptic connections with patch-clamp electrophysiological recording.DOI: http://dx.doi.org/10.7554/eLife.14193.001

Brain-derived neurotrophic factor (BDNF) has been postulated to be a key signaling molecule in regulating synaptic strength and overall circuit activity. In this context, we have found that BDNF dramatically increases the frequency of spontaneously initiated action potentials in hippocampal neurons in dissociated culture. Using analysis of unitary synaptic transmission and immunocytochemical methods, we determined that chronic treatment with BDNF potentiates both excitatory and inhibitory transmission, but that it does so via different mechanisms. BDNF strengthens excitation primarily by augmenting the amplitude of AMPA receptor-mediated miniature EPSCs (mEPSCs) but enhances inhibition by increasing the frequency of mIPSC and increasing the size of GABAergic synaptic terminals. In contrast to observations in other systems, BDNF-mediated increases in AMPA-receptor mediated mEPSC amplitudes did not require activity, because blocking action potentials with tetrodotoxin for the entire duration of BDNF treatment had no effect on the magnitude of this enhancement. These forms of synaptic regulations appear to be a selective action of BDNF because intrinsic excitability, synapse number, and neuronal survival are not affected in these cultures. Thus, although BDNF induces a net increase in overall circuit activity, this results from potentiation of both excitatory and inhibitory synaptic drive through distinct and selective physiological mechanisms.