The endpoint of nRT participation probability was progressively lower with increasing spindle length. In contrast, the participation probability
of TC cells displayed continuous increase during both long and short spindles until one to two cycles before spindle termination (from 35%–40% to 40%–45%). During natural sleep and under urethane anesthesia the spike per burst and probability trajectories were similar (Figure S6) confirming relatively intact spindle genesis under this anesthetic (for statistical analysis of the trajectories under both conditions see Figure S7). We conclude that check details spindles are characterized by a progressive decrease in the burst size of nRT neurons. TC cells show a steady increase in participation probability irrespective of spindle length with no change in burst size. In addition nRT but not TC cells display distinct activity trajectories during short and long spindles. The large difference in duration-specific nRT activity prompted us to investigate how the measured variables at the first cycle correlate with the duration of spindles. The probability of nRT participation in the first cycle was strongly correlated with spindle duration (r = −0.91; p < 0.001; Figure 7A), whereas same measure of TC cells displayed only weakly significant correlation
(r = 0.63; p = 0.047). In addition the number of spikes per burst in the first cycle also showed significant (-)-p-Bromotetramisole Oxalate correlation with spindle length in TC cells (Figure 7B). We also correlated the values of all variables between the first and last cycles. Only the probability of nRT participation selleck chemicals llc between the first and last cycle displayed significant correlation (r = 0.88, p < 0.001; Figure 7C). Similar pattern was observed under urethane anesthesia (Figure S8). These data show that the length of the spindle was correlated with the pattern of neuronal activity measured on the first cycle and in case of nRT cells the activity follows a fixed trajectory to a well-determined endpoint. These data allow two alternative scenarios
about the control of spindle length. First, spindle length might be causally determined by nRT activity on the first cycle alone. Alternatively, the correlation might occur because first-cycle nRT activity is under the control of the ongoing network state. To explore these possibilities we induced sleep spindles optogenetically (Halassa et al., 2011) in parvalbumin-channelrhodopsin (PV-ChR) (three animals, eight sessions) and vesicular-GABA transporter-channelrhodopsin (vGAT-ChR) mice (nine animals, 17 sessions). These strains express channelrhodopsin in both somata and axon terminals of nRT cells. Laser stimuli were delivered either to the nRT somata (n = 10), or to nRT axon terminals in VB (n = 15) with identical results.