Distinct Spiking Patterns of Excitatory and Inhibitory Neurons and LFP Oscillations in Prefrontal Cortex During Sensory Discrimination

Abstract

Prefrontal cortex (PFC) are broadly linked to various aspects of behavior. During sensory discrimination, PFC neurons can encode a range of task related information, including the identity of sensory stimuli and related behavioral outcome. However, it remains largely unclear how different neuron subtypes and local field potential (LFP) oscillation features in the mouse PFC are modulated during sensory discrimination. To understand how excitatory and inhibitory PFC neurons are selectively engaged during sensory discrimination and how their activity relates to LFP oscillations, we used tetrode recordings to probe well-isolated individual neurons, and LFP oscillations, in mice performing a three-choice auditory discrimination task. We found that a majority of PFC neurons, 78% of the 711 recorded individual neurons, exhibited sensory discrimination related responses that are context and task dependent. Using spike waveforms, we classified these responsive neurons into putative excitatory neurons with broad waveforms or putative inhibitory neurons with narrow waveforms, and found that both neuron subtypes were transiently modulated, with individual neurons' responses peaking throughout the entire duration of the trial. While the number of responsive excitatory neurons remain largely constant throughout the trial, an increasing fraction of inhibitory neurons were gradually recruited as the trial progressed. Further examination of the coherence between individual neurons and LFPs revealed that inhibitory neurons exhibit higher spike-field coherence with LFP oscillations than excitatory neurons during all aspects of the trial and across multiple frequency bands. Together, our results demonstrate that PFC excitatory neurons are continuously engaged during sensory discrimination, whereas PFC inhibitory neurons are increasingly recruited as the trial progresses and preferentially coordinated with LFP oscillations. These results demonstrate increasing involvement of inhibitory neurons in shaping the overall PFC dynamics toward the completion of the sensory discrimination task.

Keywords: LFP oscillations; auditory discrimination; rodent prefrontal cortex; single unit activity; spike field coherence.

FIGURE 1

Three-choices auditory discrimination task and putative neuron classification. (A) During the auditory discrimination task, mice self-initiated each trial by triggering a motion detector at the “initiation” location (left, “Task”). Upon trial-start, one of the three auditory stimuli was initiated. On the other end of the arena, there were three reward locations, each paired with a specific auditory stimulus. Mice were given 5 s to reach one of the three reward locations. If mice reached the correctly paired reward location, a reward was provided. If mice failed to reach the correct reward location, they experienced a 5 s timeout period, with a bright light illuminating the arena. Auditory stimuli were played throughout the entire trial, until mice reached one of the reward locations or trigged the timeout period. During “No Task” blocks, both initiation and reward locations were covered with a different floor. The same auditory stimuli were present for 1.5–3 s, followed by a 1.5–3 s intertrial interval, while the mice were freely moving in the arena. (B) Behavioral performance during the first three recording sessions (left) and the last three recording sessions (right). All mice had correct rates above random chance of 33% (N = 6 mice). (C) The trial duration, the time from trial-initiation when mice self-initiate a trail at the initiation location until trail-completion when they reach the reward location, during the first three recording sessions (left) and the last three recording sessions (right). Each gray line represents mean ± standard deviation of an individual mouse. (D) Representative waveforms of excitatory and inhibitory neurons recorded with tetrodes. (E) Binomial distribution of the width of spike waveforms, with a clear separation at 0.4 ms between the two peaks, which was used as a threshold to identify putative excitatory (blue) and putative inhibitory (red) neurons.

FIGURE 2

Excitatory and inhibitory neuron spiking during a three-choice auditory discrimination behavioral task. (A) A representative excitatory neuron with elevated firing rates at trial start. Raster plots of spike activities from all trials were aligned to the trial start during the auditory discrimination “Task” block (A1 top) and during the “No Task” block (A1 bottom). Trials were sorted by trial durations from the shortest to the longest. Each blue or red dot represents a spike, and the gray and black dots indicate trial start and trial end, respectively. The average firing rates of the same neuron across all trials during the auditory discrimination tasks (dark line, “Task”) and during “No Task” (light line), aligned to the trial start (A2 top), and to the trial end (A2 bottom). Normalized firing rates throughout the entire trial duration of the “Task” block (A3 dark line) and of the “No Task” block (A3 light line). (B) Similar to (A), but from a representative excitatory neuron with elevated firing rate in the middle of the task. (C) Similar to (A), but from a representative inhibitory neuron with elevated firing rate at the trial end.

FIGURE 3

Context and stage-dependent modulation of spiking activity. (A) Normalized population firing rates during the discrimination task (A1) and during the “No Task” condition (A2, sorted in the same order as in A1. The 130 neurons recorded only in auditory discrimination task without corresponding “No Task” block were filled with dark blue). Neurons were grouped by type (excitatory and inhibitory) and sorted based on the timing of their peak firing rates. For each neuron, the firing rate was normalized by its z-score over the −20 to 120% stage. (B) Distribution of excitatory (blue bars) and inhibitory neurons (red bars), based on the timing of their peak firing rate during the task (p < 0.01, χ²-test).

FIGURE 4

PFC neurons discriminate different auditory cues. (A) A representative neuron with increased response to the presentation of one auditory cue (25 click/s), but not to the other two (10 kHz sine wave and 100 click/s). Left: spike raster plot for trials presented with different auditory cues (blue: 10 kHz sine wave; red: 25 click/s; green: 100 click/s), and sorted by trial durations. Right: normalized firing rate during each auditory stimulus across trials. (B) Another example neuron responded to two auditory cues (10 kHz sine wave and 100 click/s), but not to the third (25 click/s). (C) Discriminatory ability of each neuron presented as discrimination scores, defined as one minus the p-values calculated with one-way ANOVA between the firing rates to the presentation of the three auditory cues at different trial stages. Neurons were grouped as excitatory (top) and inhibitory (bottom), and sorted by the total duration when they were discriminative. Discriminative responses of individual PFC neurons only occurred during the discrimination task (C1), but not during “No Task” condition (C2, the neurons are sorted in the same order as C1). (D) The percentage of excitatory (blue) and inhibitory (red) neurons showing sound discrimination throughout trial stages.

FIGURE 5

LFP oscillations and selective coherence with excitatory vs. inhibitory neurons during auditory discrimination. (A) Average LFP power spectrum during the auditory discrimination task from one example recording session, aligned at trial-start (left) and trial-end (right). (B) Average LFP power spectrum during “No Task” block from the same recording session as (A). (C) Normalized LFP oscillation powers at theta (C1, 5–8 Hz), beta (C2, 15–30 Hz), and gamma frequencies (C3, 30–50 Hz), aligned at trial-start (left), and at trial end (right), during auditory discrimination (black line). LFP powers were normalized as z-scores to the 2 s window analyzed. Shaded areas indicate standard error (N = 5 animals). (D) Spike-field coherences of excitatory neurons (blue, mean ± standard error of mean) and inhibitory neurons (red, mean ± standard error of mean), aligned at trial-start (left), and at trial-end (right), during the discrimination task, at theta (D1), beta (D2), and gamma frequencies (D3), aligned to trial start (left) or to trial end (right). (Excitatory = 331 neurons, inhibitory = 91 neurons, two tailed, unpaired, t-test, *p < 0.05).

FIGURE 6

LFP oscillation power diverged between correct and incorrect trials. (A) Spectrogram of the correct trials from one representative recording session, aligned to trial-start (left) and to trial-end (right). (B) Spectrogram of the incorrect trials from the same recording session as (A), aligned to trial-start (left) and to trial-end (right). (C,E) Normalized population LFP oscillation powers of correct (black line) and incorrect (gray line) trials, aligned to trial-start (C) and to trial end (E). (D,F) The difference of LFP power, defined as the averaged z-scores of 500 ms windows before minus after at trial start (D), at trial end (F). (N = 5, paired t-test, *p < 0.05).