Driving fast-spiking cells induces gamma rhythm and controls sensory responses

Abstract

Cortical gamma oscillations (20-80 Hz) predict increases in focused attention, and failure in gamma regulation is a hallmark of neurological and psychiatric disease. Current theory predicts that gamma oscillations are generated by synchronous activity of fast-spiking inhibitory interneurons, with the resulting rhythmic inhibition producing neural ensemble synchrony by generating a narrow window for effective excitation. We causally tested these hypotheses in barrel cortex in vivo by targeting optogenetic manipulation selectively to fast-spiking interneurons. Here we show that light-driven activation of fast-spiking interneurons at varied frequencies (8-200 Hz) selectively amplifies gamma oscillations. In contrast, pyramidal neuron activation amplifies only lower frequency oscillations, a cell-type-specific double dissociation. We found that the timing of a sensory input relative to a gamma cycle determined the amplitude and precision of evoked responses. Our data directly support the fast-spiking-gamma hypothesis and provide the first causal evidence that distinct network activity states can be induced in vivo by cell-type-specific activation.

Figure 1. AAV DIO ChR2–mCherry gives Cre-dependent and cell-type-specific expression of light-activated channels in vivo

a, AAV DIO ChR2–mCherry with Cre-dependent expression of ChR2 produced cell-type-specific targeting of light-activated channels. In the presence of Cre, ChR2–mCherry is inverted into the sense direction and expressed from the EF-1α (EEF1A1) promoter. ITR, inverted terminal repeat; pA, poly(A); WPRE, woodchuck hepatatis B virus post-transcriptional element. b, ChR2–mCherry was robustly expressed in PV⁺ interneurons in barrel cortex of adult PV–Cre mice. c, A corresponding injection in αCamKII–Cre mice resulted in exclusive labelling of excitatory neurons. d, e, ChR2–mCherry expression in PV–Cre mice was confined to cells expressing PV. e, PV⁺ cells with ChR2–mCherry expression and typical FS interneuron morphology. f, g, ChR2–mCherry expression in αCamKII–Cre mice is confined to neurons immuno-negative for PV. g, ChR2–mCherry-expressing cells with typical pyramidal neuron morphology. Scale bars: b, c, 100 μm; d–g, 25 μm.

Figure 2. Light-evoked activity in FS-PV⁺ inhibitory interneurons suppresses sensory processing in nearby excitatory neurons

a, Light-activated RS and FS cells recorded in layers 2/3 and 4 of barrel cortex in PV–Cre and αCamKII–Cre mice, respectively, formed two discrete overall populations based on waveform properties. b, Intracellular in vivo recording of an RS cell in a PV–Cre animal. A 1-ms pulse of blue light at low power evoked an IPSP with a sharp onset. c, The latency to light-activated FS spikes (filled circles) agreed well with the onset latency of the resulting IPSPs (open circles). The IPSP time to peak decreased with increasing power (low power: 46 mW mm⁻²; high power: 68 mW mm⁻²). d, Sustained activation of FS inhibitory interneurons eliminated sensory responses in nearby RS neurons. A layer 2/3 FS cell was reliably activated by a 10-ms light pulse (blue line; left panel). An RS cell recorded on the same tetrode responded to vibrissa deflection (red bar; centre panel). Activation of inhibitory activity simultaneously with vibrissa deflection eliminated the RS sensory response (right panel). e, Mean RS vibrissa response decreased significantly in the presence of increased FS cell activity. **P < 0.01; error bars, mean ± s.e.m.

Figure 3. FS inhibitory interneurons generate gamma oscillations in the local cortical network

a, In response to 40-Hz light pulses (blue bars), this FS cell fired reliably at 25-ms intervals, giving an instantaneous firing frequency of 40 Hz (inset). b, Average spike probability per light-pulse cycle in light-activated FS and RS cells in the PV–Cre and αCamKII–Cre mice, respectively (RS, n = 17, open circles; FS, n = 22, filled circles). c, Example of the increase in power at ~40 Hz in the LFP caused by activation of FS cells by light pulses at 40 Hz. d, Mean power ratio in each frequency band in response to light activation of FS (filled circles) and RS (open circles) cells at those frequencies. e, f, Comparison of the effect of activating FS and RS cells at 8 and 40 Hz on relative LFP power in those frequency bands. Black bars, relative power in the baseline LFP; blue bars, relative power in the presence of light pulses. g, Average spike probability of FS cells per light pulse cycle in response to three levels of light intensity. h, Mean power ratios from LFP recordings at the light intensity levels shown in g. i, The trace shows spontaneously occurring gamma activity in the LFP. Brief activation of FS cells (blue asterisk) prolonged the duration of the ongoing gamma cycle and consequently shifted the phase of the following cycles. The duration of the cycle during which the light stimulus was given (Light) was significantly longer than the preceding (Pre) or the following (Post) cycle. **P < 0.01; error bars, mean ± s.e.m.

Figure 4. Gamma oscillations gate sensory responses of excitatory neurons

a, In each trial, FS-PV⁺ inhibitory interneurons were activated at 40 Hz and a single vibrissa deflection (whisker stimulus, WS) was presented at one of five phases. b, Baseline response of one layer 4 RS cell to single vibrissa deflections, shown in units of spikes per trial. c, Responses of the same cell when the whisker was deflected at each of five temporal phases relative to the induced gamma oscillation. d, Average spikes evoked per trial under each condition. Dotted line indicates baseline responses. e, Timing of the RS spike response, measured as median spike latency. f, Spike precision of the RS responses. g, Schematic model of the gating of sensory responses by gamma oscillations. IPSP and LFP examples are averaged data traces. *P < 0.05, **P < 0.01; error bars, mean ± s.e.m.