LAUSR

Understanding how excitatory (E) and inhibitory (I) inputs are integrated by neurons requires monitoring their subthreshold behavior. We probed the subthreshold dynamics using optogenetic depolarizing pulses in hippocampal neuronal assemblies in freely moving mice. Excitability decreased during sharp-wave ripples coupled with increased I. In contrast to this “negative gain,” optogenetic probing showed increased within-field excitability in place cells by weakening I and unmasked stable place fields in initially non–place cells. Neuronal assemblies active during sharp-wave ripples in the home cage predicted spatial overlap and sequences of place fields of both place cells and unmasked preexisting place fields of non–place cells during track running. Thus, indirect probing of subthreshold dynamics in neuronal populations permits the disclosing of preexisting assemblies and modes of neuronal operations. Description Unmasking place fields in hippocampal CA1 A basic transformation process in the brain is the conversion of a neuron’s excitatory and inhibitory inputs to spikes. Experimentally examining the transformation process requires access to subthreshold membrane dynamics. To date, only intracellular recordings meet this requirement. Valero et al., using a new technique based on optogenetic stimulation to probe the excitability of neurons, examined the subthreshold activity dynamics of CA1 pyramidal neurons during sharp-wave ripples, theta oscillations, and place fields. During sharp-wave ripples, overall excitability shifted toward synaptic inhibition. However, during theta waves and in the center of place fields, excitability moved in the direction of synaptic excitation. This stimulation unmasked the place fields of nonplace cells, indicating that the proportion of place cells in CA1 is much higher than previously thought. —PRS Hippocampal assemblies and brain-state-dependent excitatory/inhibitory relationships are revealed using pulsed optogenetic stimulation.