Effect of five-consecutive-day exposure to an anxiogenic stressor on sleep-wake activity in rats.

Repeated exposure to an anxiogenic stressor (AS) is a known environmental factor for the development of depression, yet the progression of sleep-wake (S-W) changes associated with the onset of AS-induced depression (ASID) is not completely understood. Thus, the aim of this study was to identify these progressive S-W changes by developing ASID in rats, via repeated exposure to an AS, and compare this ASID-associated sleep phenotype with the sleep phenotype of human depression. To achieve this aim, rats were first recorded for a 6 h period of baseline S-W activity without AS. Then, rats were subjected to 5 days of AS [Day 1: inescapable foot-shock; 5 trials of 3 s foot-shocks (1.0 mA) at 3 min intervals; Days 3-5: 15 trials of 5 s foot-shocks at 45 s intervals]. S-W activity was recorded for 6 h immediately after each AS treatment session. Two days later rats were again recorded for 6 h of S-W activity, but with no exposure to the AS (NASD). Compared to the baseline day: Day 1 of AS (ASD-1) increased wakefulness, slow-wave sleep (SWS) latency, and rapid-eye movement (REM) sleep latency, but decreased the total amount of REM sleep; ASD-2 animals remained awake throughout the 6 h S-W recording period; ASD-3, ASD-4, and ASD-5 (ASDs-3-5) decreased wakefulness, SWS latency, and REM sleep latency, but increased the total amount of REM sleep. Interestingly, these results reveal that initial exposure to the AS versus later, repeated exposure to the AS produced opposing S-W changes. On NASD, animals exhibited baseline-like S-W activity, except slightly less REM sleep. These results suggest that repeated AS produces a sleep phenotype that resembles the sleep phenotype of depression in humans, but consistent re-exposure to the AS is required. These results are promising because the methodological simplicity and reversibility of the ASID-associated S-W phenotype could be more advantageous than other animal models for studying the pathophysiological mechanisms that underlie the expression of ASID.

Fear extinction memory consolidation requires potentiation of pontine-wave activity during REM sleep.

Sleep plays an important role in memory consolidation within multiple memory systems including contextual fear extinction memory, but little is known about the mechanisms that underlie this process. Here, we show that fear extinction training in rats, which extinguished conditioned fear, increased both slow-wave sleep and rapid-eye movement (REM) sleep. Surprisingly, 24 h later, during memory testing, only 57% of the fear-extinguished animals retained fear extinction memory. We found that these animals exhibited an increase in phasic pontine-wave (P-wave) activity during post-training REM sleep, which was absent in the 43% of animals that failed to retain fear extinction memory. The results of this study provide evidence that brainstem activation, specifically potentiation of phasic P-wave activity, during post-training REM sleep is critical for consolidation of fear extinction memory. The results of this study also suggest that, contrary to the popular hypothesis of sleep and memory, increased sleep after training alone does not guarantee consolidation and/or retention of fear extinction memory. Rather, the potentiation of specific sleep-dependent physiological events may be a more accurate predictor for successful consolidation of fear extinction memory. Identification of this unique mechanism will significantly improve our present understanding of the cellular and molecular mechanisms that underlie the sleep-dependent regulation of emotional memory. Additionally, this discovery may also initiate development of a new, more targeted treatment method for clinical disorders of fear and anxiety in humans that is more efficacious than existing methods such as exposure therapy that incorporate only fear extinction.

REM Sleep Regulating Mechanisms in the Cholinergic Cell Compartment of the Brainstem.

Rapid eye movement (REM) sleep is a highly evolved yet paradoxical behavioral state (highly activated brain in a paralyzed body) in mammalian species. Since the discovery of REM sleep and its physiological distinction from other sleep states1, a vast number of studies in neurosciences have been dedicated toward understanding the mechanisms and functions of this behavioral state. Collectively, studies have shown that each of the physiological events that characterize the behavioral state of REM sleep is executed by distinct cell groups located in the brainstem. These cell groups are discrete components of a widely distributed network, rather than a single REM sleep center. The final activity within each of these executive cell groups is controlled by the ratio of cholinergic neurotransmission emanating from the pedunculopontine tegmentum (PPT) to aminergic neurotransmission emanating from the locus coeruleus (LC) and raphe nucleus (RN). In this review, we summarize the most recent findings on the cellular and molecular mechanisms in the PPT cholinergic cell compartment that underlie the regulation of REM sleep. This up-to-date review should allow clinicians and researchers to better understand the effects of drugs and neurologic disease on REM sleep.

Calcium/calmodulin kinase II in the pedunculopontine tegmental nucleus modulates the initiation and maintenance of wakefulness.

The pedunculopontine tegmentum nucleus (PPT) is critically involved in the regulation of wakefulness (W) and rapid eye movement (REM) sleep, but our understanding of the mechanisms of this regulation remains incomplete. The present study was designed to determine the role of PPT intracellular calcium/calmodulin kinase (CaMKII) signaling in the regulation of W and sleep. To achieve this aim, three different concentrations (0.5, 1.0, and 2.0 nmol) of the CaMKII activation inhibitor, KN-93, were microinjected bilaterally (100 nl/site) into the PPT of freely moving rats, and the effects on W, slow-wave sleep (SWS), REM sleep, and levels of phosphorylated CaMKII (pCaMKII) expression in the PPT were quantified. These effects, which were concentration-dependent and affected wake-sleep variables for 3 h, resulted in decreased W, due to reductions in the number and duration of W episodes; increased SWS and REM sleep, due to increases in episode duration; and decreased levels of pCaMKII expression in the PPT. Regression analyses revealed that PPT levels of pCaMKII were positively related with the total percentage of time spent in W (R(2) = 0.864; n = 28 rats; p < 0.001) and negatively related with the total percentage of time spent in sleep (R(2) = 0.863; p < 0.001). These data provide the first direct evidence that activation of intracellular CaMKII signaling in the PPT promotes W and suppresses sleep. These findings are relevant for designing a drug that could treat excessive sleepiness by promoting alertness.