Sleep-waking discharge patterns of median preoptic nucleus neurons in rats

Abstract

Several lines of evidence show that the preoptic area (POA) of the hypothalamus is critically implicated in the regulation of sleep. Functionally heterogeneous cell groups with sleep-related discharge patterns are located both in the medial and lateral POA. Recently a cluster of neurons showing sleep-related c-Fos immunoreactivity was found in the median preoptic nucleus (MnPN). To determine the specificity of the state-related behaviour of MnPN neurons we have undertaken the first study of their discharge patterns across the sleep-waking cycle. Nearly 76 % of recorded cells exhibited elevated discharge rates during sleep. Sleep-related units showed several distinct types of activity changes across sleep stages. Two populations included cells displaying selective activation during either non-rapid eye movement (NREM) sleep (10 %) or REM sleep (8 %). Neurons belonging to the predominant population (58 %) exhibited activation during both phases of sleep compared to wakefulness. Most of these cells showed a gradual increase in their firing rates prior to sleep onset, elevated discharge during NREM sleep and a further increase during REM sleep. This specific sleep-waking discharge profile is opposite to that demonstrated by wake-promoting monoaminergic cell groups and was previously found in cells localized in the ventrolateral preoptic area (vlPOA). We hypothesize that these vlPOA and MnPN neuronal populations act as parts of a GABAergic/galaninergic sleep-promoting ('anti-waking') network which exercises inhibitory control over waking-promoting systems. MnPN neurons that progressively increase activity during sustained waking and decrease activity during sustained sleep states may be involved in homeostatic regulation of sleep.

Figure 1. Locations of different types of recorded neurons within the median preoptic nucleus and their distribution among subjects

A and B, rostral MnPN; C and D, caudal MnPN. Note that NREM sleep-related neurons (▿) were predominantly recorded in the rostral MnPN whereas the vast majority of NREM/REM-(♦) and REM sleep-related cells (▵) were found within the caudal portion of this structure. W/REM sleep-related (*) and state-indifferent (○) cells did not have predominant locations. E, distribution of different cell types among subjects. F, photomicrograph of a coronal section through the MnPN showing the microwire tracts. Arrow indicates the site of electrolytic lesion at the site of the most ventral recording. SI, state-indifferent neurons; CA, commissure anterior; MS, medial septum; och, optic chiasm; fx, fornix.

Figure 2. Classification of MnPN neurons based on the firing rate dynamics across the sleep-waking cycle

According to the results of cluster analysis, neurons were subdivided into 10 clusters (C1-10). The clusters with the same state-related discharge profile but with different firing rates were identified as belonging to the same taxonomic units. All cells were classified into three main groups: (1) sleep-related neurons exhibiting activation during sleep compared to W; (2) W/REM sleep-related cells displaying elevated firing rates during both W and REM sleep compared to NREM; (3) state-indifferent neurons. The group of sleep-related cells included three populations: NREM/REM-related neurons exhibiting activation during both phases of sleep compared to W, and NREM and REM sleep-related neurons showing selective activation during NREM and REM sleep, respectively. In all the clusters except that belonging to the state-indifferent group the changes of firing rates across the sleep-waking cycle were statistically significant (ANOVA, P < 0.001). Evaluation of inter-stage differences in discharge frequency for each cluster was done using the Tukey HSD post hoc test: *P < 0.05; **P < 0.01; ***P < 0.001.

Figure 3. The discharge pattern of an individual NREM/REM sleep-related MnPN neuron across the sleep-waking cycle

In this figure and in Figs 4–6 the hypnogram (top), the extracellularly recorded unit activity (unit), firing rate (rate), electroencephalogram (EEG) and neck electromyogram (EMG) are displayed. Note the increase in discharge rate in NREM sleep compared to W and in REM sleep compared to NREM.

Figure 4. The discharge pattern of NREM/REM sleep-related neurons exhibiting gradual increase in their firing rate prior to sleep onset

A and B, recordings showing the discharge of NREM/REM sleep-related neurons across the sleep-waking cycle. a and b, expanded tracing from the section shown by bars in A and B showing changes of neurons’ activity during W and on transition to NREM sleep. Note that during W, the neuron shown in panel A was silent during periods with locomotor activity (phasic bursts in EMG channel), then exhibited an increase of firing rate during periods with decreased muscle tone, and had a maximal firing rate during the 15 s period prior to NREM sleep onset. However, in general, the changes of cell activity across the sleep-waking cycle were not correlated with alterations in muscle tone level. The neuron shown in B exhibited a gradual increase in firing rate during a sustained period of W with no correlation with changes in muscle tone. Note that in panel A the histogram of discharge rate shows the mean firing rate within 5 s bins.

Figure 6. The discharge pattern of a neuron showing selective activation during NREM sleep across the sleep-waking cycle

Note that firing rate dramatically decreases during W and REM sleep compared to NREM. Discharge rate changes do not precede state transitions.

Figure 5. Discharge pattern of a NREM/REM sleep-related neuron, which gradually decreases its firing rate during individual NREM sleep episodes and their sequences and during REM sleep

A and B, 66 and 10 min continuous recordings showing the discharge of a NREM/REM sleep-related neuron across the sleep-waking cycle. C, regression function and correlation between the firing rate of the cell and NREM sleep episode number in the sequence of episodes. D, difference in the firing rate of the same cell in different NREM sleep quarters. Note that in A and B the histogram of discharge rate shows the mean firing rate within 5 s bins. *P < 0.05; ***P < 0.001, Tukey's HSD post hoc test.

Figure 7. The discharge pattern of a REM-related neuron during different stages of the sleep-waking cycle

W1, wakefulness with desynchronized frontoparietal EEG,; W2, wakefulness with theta rhythm in the EEG. Note the increased mean firing rate and intraburst frequency during REM sleep. In states with theta rhythm in the EEG (W2 and REM sleep) bursts are phase-locked to the surface-positive component of theta waves.