Alterations of cortical inhibition have been proposed to play a crucial role in modulation of cortical activity. While optogenetic manipulation of different functional groups of interneurons has become an important tool to study the roles of different cells in sensory processing, the timing of light relative to sensory stimulation becomes a confounding factor, and the pattern of supra-threshold activation of inhibitory neurons may not be physiologically accurate. Here, we circumvent these limitations by using stable step-function opsin (SSFO), which can be rendered continuously active or inactive with short pulses of light. We expressed SSFO in parvalbumin-positive (PV+) interneurons in the primary auditory cortex of mice, and recorded both local field potentials (LFP) and spiking responses to tone pips of varying frequency in awake animals.
Prolonged low-level activation of PV+ cells profoundly changed the dynamics of spike responses in several and diverse ways. While spontaneous activity and sustained responses to the tones were mostly suppressed during PV+ activation, onset and offset responses were either enhanced or reduced less than spontaneous responses, thus increasing signal-to-noise ratio. More consistently, we observed that best frequencies of offset responses shifted towards lower frequency by as much as an octave when SSFO was turned on. This shift was much less pronounced in sustained and onset responses. Additionally, PV+ activation resulted in finer tuning of offset responses in the majority of recorded cells. Intriguingly, tuning broadened for a small fraction of units whose spontaneous activity increased during activation. Possibly, theses units were PV+ cells directly activated by SSFO.
Analysis of LFP data confirmed the contrary effect of PV+ activation on transient and sustained responses. Activation of SSFO caused a decrease of the power in the high-γ range (50-150Hz) during spontaneous and sustained tone-response phases. Both onset and offset responses were boosted compared to control, and offset responses increased more than onset response. Generally, SSFO activation increased power in the low-frequency range of the LFP (<50Hz) and decreased power in the high-frequency range (50-150Hz, high-γ).