BALTIMORE—When GABAergic neurons in the ventrolateral preoptic (VLPO) area were identified about 10 years ago, a long search for the key neuronal players in the transition from wakefulness to sleep appeared to be completed. However, if current studies are confirmed, room is being made for GABAergic median preoptic nucleus (MnPN) neurons, which appear to have a dominant role for the transition from arousal to a state of sleep.
“Our data suggest that MnPN and VLPO neurons play somewhat different but complementary roles in sleep regulation,” reported Ronald S. Szymusiak, PhD, Adjunct Professor in the Departments of Medicine and Neurobiology at the David Geffen School of Medicine at the University of California, Los Angeles. Summarizing a series of studies that he and others have conducted, Dr. Szymusiak now believes that “MnPN is more important for regulating transitions from wake to sleep, while VLPO is more important for sleep maintenance and continuity.” Dr. Szymusiak addressed the 22nd Annual Meeting of the Associated Professional Sleep Societies.
From Arousal to Sleep
The obstacle in the quest to identify which neurons were involved in bringing disparate parts of the CNS from a state of arousal to one of sleep was isolating cells with the potential of acting across several systems. After several hypotheses led to dead ends, the identification of VLPO neurons in the mid-1990s seemed to solve several theoretic problems. Not only do these neurons become active right before sleep and then discharge through the period of sleep, which is a pattern exactly reciprocal to that associated with neurons linked to arousal, but VLPO neurons were found to project to the brain stem and the posterior hypothalamus, accounting for action on multiple arousal systems.
“After the VLPO neurons were identified and shown to be active during sleep, the confirmatory studies made them synonymous with sleep regulation in the brain,” said Dr. Szymusiak. “However, we have since become aware of a second nucleus in the preoptic area as a site of sleep regulation. We believe that the MnPN neurons, which also project to multiple arousal systems, are also sleep activators.”
The MnPN neurons share numerous features with VLPO neurons that are consistent with a role in sleep regulation, according to Dr. Szymusiak, who is also a Research Career Scientist at the VA Greater Los Angeles Healthcare System. He noted that neurons in the MnPN are well situated to project to multiple arousal systems. A series of studies attempting to evaluate expression of the c-Fos protein, which is activated during the transition from wakefulness to sleep, found that activation of this gene was on the same order of prevalence in the MnPN as in the VLPO area.
The difference between MnPN and VLPO neurons in c-Fos expression in experimental conditions has provided much of the basis for separating their roles in sleep activity. Describing several of these experiments, Dr. Szymusiak said that if animal models are deprived of sleep, there is significantly higher c-Fos expression in MnPN neurons relative to VLPO neurons. Although c-Fos expression also increases in VLPO neurons, the greatest surge of expression in these occurs when animals are finally permitted to sleep.
“This suggests that the MnPN neurons play a more important role in increasing pressure to sleep,” Dr. Szymusiak commented. The conception that these cells play a more important role than VLPO neurons in changing the brain state from one of wakefulness to one of sleep is now consistent with a variety of experiments.
“We believe there is a coordinated temporal pattern of discharge between MnPN and VLPO neurons such that after sustained wakefulness, the MnPN neurons prime the pump so that there is a rapid activation of the VLPO system to produce short latency consolidated sleep,” Dr. Szymusiak explained. He described a functional coupling of MnPN and VLPO neurons in which GABA produced by the MnPN neurons projects to the VLPO neurons to activate sleep.
When neuron activation is evaluated in the context of EEG monitoring of wakefulness to sleep in animals, distinct patterns of discharge can be observed between MnPN and VLPO neurons, according to Dr. Szymusiak. In particular, the lead time for discharge of the MnPN neurons is 20 seconds or more before the actual onset of sleep. Moreover, the maximal discharge correlates with the time of sleep onset. In contrast, while the VLPO neurons begin to discharge before sleep onset, they do so at much shorter intervals, and the discharge rate in the beginning is relatively slow, increasing as sleep deepens.
Sleep Patterns and Mechanisms
One of the difficulties in sleep research has been that like patterns of sleep, the patterns of purported sleep mechanisms are not very homogeneous. For example, due to the relative, rather than absolute, discharge rates for both MnPN and VLPO neurons, calculations are expressed in percentages. Although there is a correlation between c-Fos expression, GABA production, and sleep efficiency, this must be addressed in relative terms. Other factors that may contribute to heterogeneity are now being evaluated for their contribution to sleep onset and maintenance. For example, growth hormone appears to be involved in stimulation and perhaps coupling of the activation of MnPN and VLPO neurons.