Non-invasive detection of narcolepsy type I phenotypical features and disease progression by continuous home-cage monitoring of activity in two mouse models: the HCRT-KO and DTA model

Narcolepsy type 1 (NT1) is a neurological disorder caused by disruption of hypocretin (HCRT; or orexin) neurotransmission leading to fragmented sleep/wake states, excessive daytime sleepiness, and cataplexy (abrupt muscle atonia during wakefulness). Electroencephalography and electromyography (EEG/EMG) monitoring is the gold standard to assess NT1 phenotypical features in both humans and mice. Here, we evaluated the digital ventilated home-cage (DVC®) activity system as an alternative to detect NT1 features in two NT1 mouse models: the genetic HCRT-knockout (-KO) model, and the inducible HCRT neuron-ablation hcrt-tTA;TetO-DTA (DTA) model, including both sexes. NT1 mice exhibited an altered dark phase activity profile and increased state transitions, compared to the wild-type (WT) phenotype. An inability to sustain activity periods >40 min represented a robust activity-based NT1 biomarker. These features were observable within the first weeks of HCRT neuron degeneration in DTA mice. We also created a nest-identification algorithm to differentiate between inactivity and activity, inside and outside the nest as a sleep and wake proxy, respectively, showing significant correlations with EEG/EMG-assessed sleep/wake behavior. Lastly, we tested the sensitivity of the activity system to detect behavioral changes in response to interventions such as repeated saline injection and chocolate. Surprisingly, daily consecutive saline injections significantly reduced activity and increased nest time of HCRT-WT mice. Chocolate increased total activity in all mice, and increased the frequency of short out-of-nest inactivity episodes in HCRT-KO mice. We conclude that the DVC® system provides a useful tool for non-invasive monitoring of NT1 phenotypical features, and has the potential to monitor drug effects in NT1 mice.

Sex-related differences within sleep-wake dynamics, cataplexy, and EEG fast-delta power in a narcolepsy mouse model

Narcolepsy type 1 (NT1) is a sleep-wake disorder caused by selective loss of hypocretin (HCRT, also called orexin) neurons. Although the prevalence of NT1 is equal in men and women, sex differences in NT1 symptomatology have been reported in humans and other species. Yet, most preclinical studies fail to include females, resulting in gender bias within translational drug development. We used hcrt-tTA;TetO DTA mice (NT1 mice) that lose their HCRT neurons upon dietary doxycycline removal to examine in detail the effect of sex on NT1 symptoms and sleep-wake characteristics. We recorded 24-h electroencephalography (EEG), electromyography (EMG), and video in adult male and female NT1 mice for behavioural state quantification. While conducting this study, we recognized another type of behavioural arrest different from cataplexy: shorter lasting and with high δ power. We termed these delta attacks and propose a set of criteria for quantifying these in future research. Our findings show that both sexes exhibit high behavioural state instability, which was markedly higher in females with more behavioural arrests interrupting the wake episodes. Females exhibited increased wake at the expense of sleep during the dark phase, and decreased rapid-eye-movement (REM) sleep during the 24-h day. During the dark phase, fast-δ (2.5-4 Hz) in non-rapid-eye-movement (NREM) sleep and θ (6-10 Hz) EEG spectral power in REM sleep were lower in females compared to males. We demonstrate that biologically driven sex-related differences exist in the symptomatology of NT1 mice which calls for including both sexes in future research.

Dissociable and Paradoxical Roles of Rat Medial and Lateral Orbitofrontal Cortex in Visual Serial Reversal Learning

Much evidence suggests that reversal learning is mediated by cortico-striatal circuitries with the orbitofrontal cortex (OFC) playing a prominent role. The OFC is a functionally heterogeneous region, but potential differential roles of lateral (lOFC) and medial (mOFC) portions in visual reversal learning have yet to be determined. We investigated the effects of pharmacological inactivation of mOFC and lOFC on a deterministic serial visual reversal learning task for rats. For reference, we also targeted other areas previously implicated in reversal learning: prelimbic (PrL) and infralimbic (IL) prefrontal cortex, and basolateral amygdala (BLA). Inactivating mOFC and lOFC produced opposite effects; lOFC impairing, and mOFC improving, performance in the early, perseverative phase specifically. Additionally, mOFC inactivation enhanced negative feedback sensitivity, while lOFC inactivation diminished feedback sensitivity in general. mOFC and lOFC inactivation also affected novel visual discrimination learning differently; lOFC inactivation paradoxically improved learning, and mOFC inactivation had no effect. We also observed dissociable roles of the OFC and the IL/PrL. Whereas the OFC inactivation affected only perseveration, IL/PrL inactivation improved learning overall. BLA inactivation did not affect perseveration, but improved the late phase of reversal learning. These results support opponent roles of the rodent mOFC and lOFC in deterministic visual reversal learning.

GABAA receptor β1 -subunit knock-out mice show increased delta power in NREM sleep and decreased theta power in REM sleep

γ-Aminobutyric acid (GABA) acting through heteropentameric GABA_A receptors plays a pivotal role in the sleep-promoting circuitry. Whereas the role of the different GABA_A receptor α-subunits in sleep regulation and in mediating the effect of benzodiazepines for treatment of insomnia is well-described, the β-subunits are less studied. Here we report the first study characterizing sleep in mice lacking the GABA_A receptor β₁ -subunit (β₁ ^-/- mice). We show that β₁ ^-/- mice have a distinct and abnormal sleep phenotype characterized by increased delta power in non-rapid eye movement (NREM) sleep and decreased theta activity in rapid eye movement (REM) sleep compared to β₁ ^+/+ mice, without any change in the overall sleep-wake architecture. From GABA_A receptor-specific autoradiography, it is further demonstrated that functional β₁ -subunit-containing GABA_A receptors display the highest binding levels in the hippocampus and frontal cortex. In conclusion, this study suggests that the GABA_A receptor β₁ -subunit does not play an important role in sleep initiation or maintenance but instead regulates the power spectrum and especially the expression of theta rhythm. This provides new knowledge on the complex role of GABA_A receptor subunits in sleep regulation. In addition, β₁ ^-/- mice could provide a useful mouse model for future studies of the physiological role of delta and theta rhythms during sleep.

Glutamatergic and Serotonergic Modulation of Rat Medial and Lateral Orbitofrontal Cortex in Visual Serial Reversal Learning

Adapting behavior to a dynamic environment requires both steadiness when the environment is stable and behavioral flexibility in response to changes. Much evidence suggests that cognitive flexibility, which can be operationalized in reversal learning tasks, is mediated by cortico-striatal circuitries, with the orbitofrontal cortex (OFC) playing a prominent role. The OFC is a functionally heterogeneous region, and we have previously reported differential roles of lateral (lOFC) and medial (mOFC) regions in a touchscreen serial visual reversal learning task for rats using pharmacological inactivation. Here, we investigated the effects of pharmacological overactivation of these regions using a glutamate transporter 1 (GLT-1) inhibitor, dihydrokainate (DHK), which increases extracellular glutamate by blocking its reuptake. We also tested the impact of antagonism of the serotonin 2A receptor (5-HT_2AR), which modulates glutamate action, in the mOFC and lOFC on the same task. Overactivation induced by DHK produced dissociable effects in the mOFC and lOFC, with more prominent effects in the mOFC, specifically improving performance in the early, perseveration phase. Intra-lOFC DHK increased the number of omitted responses without affecting errors. In contrast, blocking the 5-HT_2AR in the lOFC impaired reversal learning overall, while mOFC 5-HT_2AR blockade had no effect. These results further support dissociable roles of the rodent mOFC and lOFC in deterministic visual reversal learning and indicate that modulating glutamate transmission through blocking the GLT-1 and the 5-HT_2AR have different roles in these two structures.

This study further supports dissociable roles of specific orbitofrontal subregions, as well as glutamatergic and serotonergic transmission in these subregions, in cognitive flexibility. This knowledge will add to the understanding of specific neural mechanisms underlying inflexible behaviour across psychiatric disorders.