Fig 1.
Walking-induced increases in gamma power, firing rates and spike-gamma LFP correlations.
A, Histological reconstruction showing locations (red dots) of the recording electrodes in the prelimbic mPFC and MD thalamus. B, Photograph shows the rotating circular treadmill. The paddle in the back ensured that the rat continued walking. C, Representative wavelet-based scalograms show time-frequency plots of normalized LFP spectral power during epochs with the treadmill turned ON or OFF (white bar). Warmer colors indicate higher power. D, Representative FFT-based spectrogram shows time-frequency coherence between the mPFC and MD thalamus. E, Average LFP power spectra ± SEM for epochs with treadmill ON (black) or OFF (grey) on day 1 of recording (n = 12 rats). Insets show representative traces for each structure. F, Bar graphs shows mean total gamma LFP power with the treadmill ON and OFF from day 1 of recording (n = 12 rats). The gamma frequency range was determined using the 15 Hz surrounding the significant gamma peak (see Methods) between 40 and 70 Hz in each treadmill walking epoch. G, Representative pyramidal neuron (PYR, black) and interneuron (IN, grey) waveforms, raw signals and spike trains from the mPFC and MD thalamus during treadmill walking. H, Firing rates in the mPFC of PYR neurons (n = 118 neurons from 12 rats), and from MD thalamus neurons (n = 62 neurons from 12 rats). I, Mean ratios between peak-to-trough amplitudes of the original spike-triggered waveform average (STWA) and the mean of 20 shuffled STWAs for LFPs filtered from 40–70 Hz recorded from a neighboring wire (see Methods). A ratio of 1 (dashed horizontal line) indicates no difference between shuffled and unshuffled values. Values are reported as mean ± SEM. *: p < 0.05, **: p < 0.01, ***: p < 0.001, bootstrap tests.
Table 1.
Time course of treatments.
Fig 2.
Ketamine-induced increases in gamma LFP power and spiking activity during treadmill walking.
A, Representative wavelet-based scalograms show the time-frequency plots of LFP spectral power before ketamine (baseline) and 13–17 minutes after 10 mg/kg ketamine s.c. (Ket 15 min). B, Representative FFT-based spectrogram shows time-frequency coherence between the mPFC and MD thalamus before ketamine and after ketamine. C, Averaged LFP power spectra for epochs before ketamine (black), and after ketamine (green). Insets show representative traces for each structure. D, Bar graph shows mean total gamma LFP power (peak ± 7 Hz) from the mPFC (n = 10 rats) and the MD thalamus (n = 6 rats) during treadmill walking at baseline (black), 15 minutes after saline injection (white), and 15 minutes (green) or 75 minutes (green with diagonal stripes) after 10 mg/kg ketamine injection. E, Representative examples of gamma band-pass filtered LFP (40–70 Hz) in the mPFC and MD thalamus. F, Representative STWAs of a mPFC pyramidal neuron (D) and MD thalamus neuron (E) with LFPs filtered from 40–70 Hz 15 minutes before and after ketamine administration. G, I, Mean firing rates of pyramidal neurons in the mPFC (G, n = 45 neurons from 6 rats) and the MD thalamus (I, n = 37 neurons from 6 rats). H, J, Bar graphs show spike-gamma LFP correlations (filtered from 40–70 Hz) for putative mPFC pyramidal neurons (H) and MD thalamus neurons (J). Values are reported as mean ± SEM. Stars over a single bar indicate this value differs significantly from the three other conditions. *: p < 0.05, **: p < 0.01, ***: p < 0.001, paired bootstrap tests with Bonferroni correction.
Fig 3.
Effects of dopamine D4R agonist and antagonist on gamma power in the absence of ketamine.
Percent change in total gamma power (peak between 40 and 70 Hz ± 7 Hz, see Methods) in the mPFC (left) and MD thalamus (right) of rats injected (at arrows) with either saline (black curve), or the D4R agonist A-412997 (blue curve) at different doses (0.3 mg/kg at 1st arrow; 1.2 mg/kg at 2nd arrow; 1.5 mg/kg at 3rd arrow). The total dose was 3 mg/kg A-412997. The D4R antagonist L-745,870 (5 mg/kg, red arrow, n = 6 rats) was injected 90 min following the first injection of either saline or A-412997. *: p < 0.05, ***: p < 0.001, significant difference from vehicle injections, repeated measures one-way ANOVA with post-hoc paired bootstrap tests with Bonferroni correction #: p < 0.05, paired bootstrap test comparing percent change in power 15 minutes after 3 mg/kg A-412997 and 35 minutes after 5 mg/kg L-745,870.
Fig 4.
Effects of dopamine D4R agonist and antagonist on ketamine-induced gamma power.
A, D, Averaged LFP power spectra in the mPFC (A) and MD thalamus (D) at baseline (black) and 15 minutes after 10 mg/kg ketamine (green) with saline injection 30 minutes prior to ketamine administration (n = 6 rats). B, E, Average percent change in the mPFC (B) and MD thalamus (E) in the LFP power spectra, relative to baseline walking, 15 minutes after rats were injected with 10 mg/kg ketamine. The black bars indicate the gamma frequency range of each individual rat. The red trace indicates pretreatment with the antagonist (5 mg/kg L-745,870), the blue trace indicates the pretreatment with the agonist (3 mg/kg A-412997), and the black trace indicates the pretreatment with saline. All pretreatments were administered 30 minutes prior to ketamine, as shown in (C) and (F). Black traces are from the ketamine line spectra shown in (A) and (D) respectively. C, F, Changes in low gamma (peak between 40 and 70 Hz ± 7 Hz) power from the frequency ranges shown by the black bars in (B) and (E) in the mPFC (C) and MD thalamus (F), taken from 4 minute intervals 10–20 minutes apart, so that data was always taken with the treadmill ON. The pre-treatment injection (black arrow) of saline (black trace), D4R agonist (blue trace) or D4R antagonist (red trace), was administered 30 minutes before the injection of 10 mg/kg ketamine (green arrow). Blue stars indicate a significant change from saline pretreatment with the agonist (n = 6 rats). Data is presented as percent change from the rat’s baseline walking power on the same recording day. Values are reported as percentage ± SEM. **: p < 0.01, ***: p < 0.001, two-way repeated measures ANOVA followed by paired bootstrap tests with Bonferroni correction.
Fig 5.
Effects of dopamine D4R agonist and antagonist on ketamine-induced changes in spiking activity.
A-B, Average firing rates of putative mPFC pyramidal neurons (A) and MD thalamus neurons (B) at baseline (black), 15 minutes after the pretreatment injection (solid color), and 15 minutes after the subsequent additional administration of 10 mg/kg ketamine (striped). Bars in white, in red or in blue correspond to pretreatments with either saline, antagonist 5 mg/kg L-745,870 s.c., or agonist 3 mg/kg A-412997 s.c., respectively (saline: mPFC: n = 45 neurons, MD thalamus: n = 37 neurons; L-745,870: mPFC: n = 30 neurons, MD thalamus: n = 31 neurons; A-412997: mPFC: n = 38 neurons, MD thalamus: n = 31 neurons). C-D, Bar graphs show spike-gamma LFP correlations (filtered from 40–70 Hz) for mPFC pyramidal neurons (C) and MD thalamus neurons (D). There were no significant differences between pretreatment groups in the effect of ketamine on firing rates or spike-LFP correlations (p > 0.05, one-way ANOVAs). All data collected from treadmill walking epochs. Values are reported as mean ± SEM; neurons from 6 rats. *:p < 0.05, **:p < 0.01, ***:p < 0.001, paired bootstrap tests compare baseline walk and the drug treatments.