Cortical zeta-inhibitory peptide injection reduces local sleep need

Abstract

Local sleep need within cortical circuits exhibits extensive interregional variability and appears to increase following learning during preceding waking. Although the biological mechanisms responsible for generating sleep need are unclear, this local variability could arise as a consequence of wake-dependent synaptic plasticity. To test whether cortical synaptic strength is a proximate driver of sleep homeostasis, we developed a novel experimental approach to alter local sleep need. One hour prior to light onset, we injected zeta-inhibitory peptide (ZIP), a pharmacological antagonist of protein kinase Mζ, which can produce pronounced synaptic depotentiation, into the right motor cortex of freely behaving rats. When compared with saline control, ZIP selectively reduced slow-wave activity (SWA; the best electrophysiological marker of sleep need) within the injected motor cortex without affecting SWA in a distal cortical site. This local reduction in SWA was associated with a significant reduction in the slope and amplitude of individual slow waves. Local ZIP injection did not significantly alter the amount of time spent in each behavioral state, locomotor activity, or EEG/LFP power during waking or REM sleep. Thus, local ZIP injection selectively produced a local reduction in sleep need; synaptic strength, therefore, may play a causal role in generating local homeostatic sleep need within the cortex.

Keywords: local sleep; rat; sleep homeostasis; slow-wave activity; synaptic plasticity; zeta-inhibitory peptide.

Figure 1.

Local ZIP injection does not alter the sleep/wake cycle or locomotor activity. (A) Experimental timeline. (B) Typical electrophysiological recordings depict how neuronal activity (RMLFP/LPEEG) and muscle tone (EMG) change across behavioral state. Behavioral state was manually scored in 4 s epochs throughout the experiment. (C) Average time spent in each behavioral state, bout duration, and locomotor activity (across all states, or waking epochs only) did not significantly differ following saline and ZIP injections. RMS = root-mean squared. N = 8 (except EMG activity where N = 7).

Figure 2.

Average RMLFP power spectra under conditions of high- and low-sleep pressure (first and last 3 hr of the light period, respectively) following saline and ZIP injections. Complete statistical analyses of these data are presented in Table 1. Black circles depict significant differences within each frequency bin between saline and ZIP conditions (post hoc t-tests; uncorrected for multiple comparisons, p < 0.05).

Figure 3.

Local ZIP injection selectively reduces SWA and SWE at injection site. (A and B) Average hourly SWA (the summed EEG/LFP power between 0.5 and 4.0 Hz) during the light period following injection of saline or ZIP. SWA significantly decreases across the light period in both locations following both injections, whereas ZIP injection significantly reduces SWA only at the injection site (p < 0.05). A black circle indicates a significant hourly difference between ZIP and Saline (post hoc paired t-tests; p < 0.05). (C and D) Cumulative SWE during the light period following injection of saline or ZIP. ZIP injection significantly reduces SWE at the injection site only (p < 0.05). SWE is depicted for each channel as a percentage of the total SWE observed during the light period following saline injection. N = 8 (right motor); N = 7 (left parietal).

Figure 4.

Homeostatic sleep pressure and local ZIP injections affect characteristics of individual slow waves. (A and B) Average waveforms of individual slow waves in the motor and parietal cortices under high global sleep pressure (first 3 hr of light period; solid lines) and low global sleep pressure (last 3 hr of light period; dashed lines) following saline or ZIP injection. (C and D) Average amplitude, ascending/descending slope, and multipeak wave incidence in the motor and parietal cortices following saline or ZIP injection during high global sleep pressure (first 3 hr of light period), medium sleep pressure (hours 4.5–7.5 of light period), and low sleep pressure (last 3 hr of light period). Each characteristic is plotted as a percentage of its corresponding value observed under high sleep pressure following saline injections. ZIP injections significantly reduce slow-wave amplitude and ascending/descending slope (see Table 2 for complete stats for C/D). N = 8 (right motor); N = 7 (left parietal).