By using an EEG biofeedback-induced sleep restriction methodology, reliable and quantifiable levels of sleep restriction were induced in rats that could be behaviourally indexed by a variant of the psychomotor vigilance test. The administration of exemplar pro-vigilant compounds, at doses with broadly comparable effects on EEG sleep-wake parameters, demonstrated that it was possible to determine differences in the capacity for task engagement induced by different agents. mGlu5 PAMs were identified as a novel pharmacological class of pro-vigilant drugs that induce a functionally distinct form of capacity for task engagement compared to the existing standards modafinil, amphetamine and caffeine.

The study of sleep restriction in rats has been fraught with methodological issues, which can generate confounds related to stress (Coenen and van Luijtelaar 1985; Rechtschaffen et al. 2002; Wurts and Edgar 2000) or require additional groups to control for activity/fatigue (e.g. Christie et al. 2008). In the present study, the EEG biofeedback-induced cage rotation protocol employed the minimum amount of movement necessary to wake the animal via vestibular reflex upon detection of an epoch of sleep, minimising both stress and movement confounds as best as possible. This work also benefited from quantification of EEG parameters throughout, allowing exact definition of the sleep-wake state of animals during both restriction and recovery periods. To date, very few behavioural pharmacological studies in sleep restricted rats have collected such information to guide interpretation.

When an 11 h sleep restriction was applied prior to the performance of a SRLT, rats very consistently lost around 6 h of NREM sleep and 40 min of REM sleep. This magnitude of sleep loss was sufficient to cause behavioural impairment on the SRLT task where the most marked effect was a large increase in errors of omission, accompanied by a smaller decrease in premature response rate and lengthening of RT. In humans, such effects on PVT performance can be observed after experimental total sleep deprivation or chronic sleep fragmentation (Van Dongen et al. 2003), but also as a consequence of clinically presented EDS (Czeisler et al. 2005; Dinges and Weaver 2003). A noticeable qualitative difference was evident between rat SRLT and human PVT with regard to sleep restriction effects on the RT parameter itself. These are very reliably detected in human studies, with performance lapses (i.e. RTs > 500 ms) being a defining hallmark of sleep restriction (Lim and Dinges 2008). However, during rat SRLT performance, the effects of sleep restriction on RT appeared to be relatively smaller. Future work should determine whether such discrepancies are related to motivational factors (i.e. animals responding for food versus humans adhering to verbal instruction), attentional load and/or arousal level differences.

Little preclinical work has actually studied the effect of sleep restriction on attentional processes in rodents, but the present results do display broad consistencies with the small, extant literature. For instance, it has been shown in rats that 24 h sleep deprivation (constant wheel turning method) followed by a PVT-like task (Christie et al. 2008) and 10 h total sleep deprivation (gentle handling method) followed by 5-choice serial RT testing (Cordova et al. 2006) impaired task performance. In both cases, response latencies and omissions/lapses increased significantly as a result of increased sleepiness, although the number of premature responses did not change. The induction of a change in premature response rate by sleep restriction may therefore depend on subtleties of assay protocols employed.

The most important overall finding from the present study was that different pro-vigilant pharmacologies had differential effects on the ability to restore rat SRLT performance despite all having significant effects on sleep/wake as assessed by EEG parameters. Of the four compounds tested, amphetamine (1 mg/kg) was the only drug to have marked negative effect on performance capacity of sleep-restricted animals, such that they completely disengaged from the task. Omissions increased drastically, and when animals did complete trials they occurred with very long response latencies. This profile is suggestive that this dose of amphetamine, while wake-promoting in EEG measures, results in the expression of stimulant hyperactivity in sleep-restricted rats that competes with task engagement. This represents a potentially important disconnect to existing clinical data, whereby most human studies report positive effects of standard 10 or 20 mg amphetamine doses on attentional and other cognitive tasks following sleep deprivation (see review by Minzenberg and Carter 2008). By comparison, caffeine (12 mg/kg), modafinil (300 mg/kg) and the mGlu5 PAM LSN2814617 (3 mg/kg) all had beneficial effects on SRLT performance in sleep-restricted rats. All three compounds significantly decreased omissions and increased the number of trials completed, allowing sleep-restricted animals to engage in the task more effectively. Caffeine and modafinil also displayed a trend-level tendency towards normalizing response latencies, although both compounds also had concomitant negative effects on premature response rates. Increases in premature responses were especially dramatic following modafinil administration, almost trebling in rate during the test session immediately following sleep restriction. Again, this finding is somewhat at odds with clinical data, which describes modafinil to be well tolerated with largely positive effects on human performance capacity and response inhibition parameters (Minzenberg and Carter 2008). Preclinical modafinil effects have been much more mixed, however, with reports of positive effects on stop signal (Eagle et al. 2007) and 5-choice serial RT (5CSRT) performance (Morgan et al. 2007) in normal animals, but negative effects on accuracy and impulse control in 5CSRT in normal (Waters et al. 2005) and REM sleep deprived (disc over water method) animals (Liu et al. 2011) in other studies. Several variables, including strain and age of animals, dose of modafinil, assay designs and protocol variants, may need to be carefully considered here to understand this mixture of effects.

The mGlu5 PAM LSN2814617 was found to have a remarkably powerful effect on EEG-defined wakefulness, considerably larger and more enduring than the effects observed for the other agents tested. Previous work on mGlu5 pharmacology has suggested that potentiation at this receptor can promote EEG-defined wakefulness in normal animals (Ahnaou et al. 2015; Gilmour et al. 2013; Gregory et al. 2013). Ahnaou and colleagues found that a 3 mg/kg dose of LSN2814617 enhanced slow alpha band oscillatory activity (defined as 8–11 Hz activity in their study) and reduced functional network connectivity, which the authors suggested may have been indicative of potential for impairment of cognition following dosing. In the present study, a 3 mg/kg LSN2814617 dose also increased power across an 8–11 Hz frequency band (data not presented), but by extending assessment of LSN2814617 to the measurement of functional capacity in a sleep-restricted state, the question related to the potential for cognitive impairment could also be addressed. LSN2814617 actually presented with the ability to restore functional capacity in sleep-restricted rats, displaying similar positive effects as modafinil and caffeine on trials completed and omission measures, but lacking significant effect on premature responding. Little is known at present as to why this pharmacology has such a marked effect on vigilance, although it may relate to the ability of mGlu5 receptor activation to directly and indirectly promote excitatory transmission via the NMDA receptor (Bird and Lawrence 2009). In a broader sense, the hypothesis that promotion of excitatory glutamatergic neurotransmission could be a potential mechanism of pro-vigilant effects is also substantiated by work on AMPAkine pharmacology. In this regard, the AMPA receptor PAM CX717 can restore performance of a delayed match-to-sample task in sleep deprived (gentle handling) nonhuman primates (Porrino et al. 2005), whilst also normalizing sleep deprivation-induced decreases in NMDA-mediated intracellular calcium release in the hippocampus (Hampson et al. 2009). Unfortunately, these effects have not yet translated into robust responses in clinical trials (Boyle et al. 2012; Wesensten et al. 2007), possibly due to lack of a direct measure of target engagement to inform human dosing regimens. Beyond glutamate, interactions of the mGlu5 receptor with other mechanisms postulated to play a role in sleep-wake regulation may also be important, for example Homer 1a (Ango et al. 2001; Ango et al. 2002; Maret et al. 2007) or adenosinergic transmission (Bachmann et al. 2012; Bodenmann et al. 2012; Gallopin et al. 2005; Okada et al. 2003). Finally, a recent PET imaging study in humans using the selective radioligand 11C-ABP688 has demonstrated that mGlu5 receptor availability is increased in several brain regions after one night of sleep deprivation (Hefti et al. 2013), which suggests that dynamic changes in mGlu5 receptor expression may play a fundamental role in sleep/wake homeostasis.

It is clear that a homeostatic process tracks the loss of sleep as a “sleep pressure” that will result in a dose-dependent compensation proportional to the debt accrued (Daan et al. 1984; Dijk et al. 1990). All treatments compared in the present study resulted in immediate attenuation of the compensatory sleep response elicited by 11 h of sleep restriction. However, the proportion of sleep pressure immediately relieved following pro-vigilant treatment was later recovered to a degree during the subsequent recording period, suggesting that requirement for homeostatic sleep was not completely alleviated by any compound. The most rapid and complete compensatory sleep response was observed for d-amphetamine, whilst both caffeine and modafinil produced equivalent proportions of NREM sleep recovery relative to the additional wakefulness gained. The greatest differential effect was observed for LSN2814617, where animals appeared to only recover 22% of lost NREM sleep versus 70% of REM sleep. This finding is interesting, as it is more typical for NREM sleep recovery to precede or at least occur concomitantly with REM sleep recovery (Berger and Oswald 1962; Borbely and Neuhaus 1979). Future work should consider how the relatively distinct effects of LSN2814617 on NREM versus REM compensatory sleep responses and performance capacity may be related.

In conclusion, the present study demonstrated that a PVT-like SRLT can be used in rats to detect behavioural impairments caused by a quantified loss of sleep. An mGlu5 PAM molecule was shown to produce marked wakefulness and an improvement in functional capacity of sleep-restricted animals, qualitatively distinct from that of amphetamine, caffeine and modafinil. The methodology and novel pharmacological effects described may offer utility for future work directed at understanding the translational correspondence of pro-vigilant drug effects between species.