Patricia L. Brooks

Glycinergic and GABAA-Mediated Inhibition of Somatic Motoneurons Does Not Mediate Rapid Eye Movement Sleep Motor Atonia

A hallmark of rapid eye movement (REM) sleep is a potent suppression of postural muscle tone. Motor control in REM sleep is unique because it is characterized by flurries of intermittent muscle twitches that punctuate muscle atonia. Because somatic motoneurons are bombarded by strychnine-sensitive IPSPs during REM sleep, it is assumed that glycinergic inhibition underlies REM atonia. However, it has never been determined whether glycinergic inhibition of motoneurons is indeed responsible for triggering the loss of postural muscle tone during REM sleep. Therefore, we used reverse microdialysis, electrophysiology, and pharmacological and histological methods to determine whether glycinergic and/or GABAA-mediated neurotransmission at the trigeminal motor pool mediates masseter muscle atonia during REM sleep in rats. By antagonizing glycine and GABAA receptors on trigeminal motoneurons, we unmasked a tonic glycinergic/GABAergic drive at the trigeminal motor pool during waking and non-rapid eye movement (NREM) sleep. Blockade of this drive potently increased masseter muscle tone during both waking and NREM sleep. This glycinergic/GABAergic drive was immediately switched-off and converted into a phasic glycinergic drive during REM sleep. Blockade of this phasic drive potently provoked muscle twitch activity in REM sleep; however, it did not prevent or reverse REM atonia. Muscle atonia in REM even persisted when glycine and GABAA receptors were simultaneously antagonized and trigeminal motoneurons were directly activated by glutamatergic excitation, indicating that a powerful, yet unidentified, inhibitory mechanism overrides motoneuron excitation during REM sleep. Our data refute the prevailing hypothesis that REM atonia is caused by glycinergic inhibition. The inhibitory mechanism mediating REM atonia therefore requires reevaluation.

The prion hypothesis in Parkinson's disease: Braak to the future

Parkinson’s disease (PD) is a progressive neurodegenerative disorder typified by the presence of intraneuronal inclusions containing aggregated alpha synuclein (αsyn). The progression of parkinsonian pathology and clinical phenotype has been broadly demonstrated to follow a specific pattern, most notably described by Braak and colleagues. In more recent times it has been hypothesized that αsyn itself may be a critical factor in mediating transmission of disease pathology from one brain area to another. Here we investigate the growing body of evidence demonstrating the ability of αsyn to spread transcellularly and induce pathological aggregation affecting neurons by permissive templating and provide a critical analysis of some irregularities in the hypothesis that the progression of PD pathology may be mediated by such a prion-like process. Finally we discuss some key questions that remain unanswered which are vital to determining the potential contribution of a prion-like process to the pathogenesis of PD.

Identification of the Transmitter and Receptor Mechanisms Responsible for REM Sleep Paralysis

During REM sleep the CNS is intensely active, but the skeletal motor system is paradoxically forced into a state of muscle paralysis. The mechanisms that trigger REM sleep paralysis are a matter of intense debate. Two competing theories argue that it is caused by either active inhibition or reduced excitation of somatic motoneuron activity. Here, we identify the transmitter and receptor mechanisms that function to silence skeletal muscles during REM sleep. We used behavioral, electrophysiological, receptor pharmacology and neuroanatomical approaches to determine how trigeminal motoneurons and masseter muscles are switched off during REM sleep in rats. We show that a powerful GABA and glycine drive triggers REM paralysis by switching off motoneuron activity. This drive inhibits motoneurons by targeting both metabotropic GABAB and ionotropic GABAA/glycine receptors. REM paralysis is only reversed when motoneurons are cut off from GABAB, GABAA and glycine receptor-mediated inhibition. Neither metabotropic nor ionotropic receptor mechanisms alone are sufficient for generating REM paralysis. These results demonstrate that multiple receptor mechanisms trigger REM sleep paralysis. Breakdown in normal REM inhibition may underlie common sleep motor pathologies such as REM sleep behavior disorder.

Impaired GABA and Glycine Transmission Triggers Cardinal Features of Rapid Eye Movement Sleep Behavior Disorder in Mice

Rapid eye movement (REM) sleep behavior disorder (RBD) is a neurological disease characterized by loss of normal REM motor inhibition and subsequent dream enactment. RBD is clinically relevant because it predicts neurodegenerative disease onset (e.g., Parkinsons disease) and is clinically problematic because it disrupts sleep and results in patient injuries and hospitalization. Even though the cause of RBD is unknown, multiple lines of evidence indicate that abnormal inhibitory transmission underlies the disorder. Here, we show that transgenic mice with deficient glycine and GABA transmission have a behavioral, motor, and sleep phenotype that recapitulates the cardinal features of RBD. Specifically, we show that mice with impaired glycine and GABAA receptor function exhibit REM motor behaviors, non-REM muscle twitches, sleep disruption, and EEG slowing—the defining disease features. Importantly, the RBD phenotype is rescued by drugs (e.g., clonazepam and melatonin) that are routinely used to treat human disease symptoms. Our findings are the first to identify a potential mechanism for RBD—we show that deficits in glycine- and GABAA-mediated inhibition trigger the full spectrum of RBD symptoms. We propose that these mice are a useful resource for investigating in vivo disease mechanisms and developing potential therapeutics for RBD.

Genetic association of CR1 with Alzheimer's disease: A tentative disease mechanism

CR1 is a novel Alzheimers disease (AD) gene identified by genome-wide association studies (GWAS). Recently, we showed that AD risk could be explained by an 18-kilobase insertion responsible for the complement component (3b/4b) receptor 1 (CR1)-S isoform. We investigated the relevance of the CR1 isoforms to AD in a Canadian dataset. Also, we genotyped rs4844610 tagging the GWAS-significant CR1 single nucleotide polymorphisms. Individuals with F/S genotype had a 1.8 times increased risk for AD compared with F/F genotype (p-adjusted = 0.003), while rs4844610 was only marginally significant (p-adjusted = 0.024). The analyses of brain samples demonstrated that the CR1-S isoform is expressed at lower protein levels than CR1-F (p < 0.0001) hence likely associated with increased complement activation. Intriguingly, our neuropathological results show that the pattern of CR1 expression in neurons is different between the F/F and F/S genotypes (filiform vs. vesicular-like profiles). Furthermore, double labeling studies supported a differential distribution of CR1 in neurons (endoplasmic reticulum intermediate compartment vs. lysosomes). These observations indicate that the CR1-S and CR1-F isoforms could be processed in different ways in neurons. In conclusion, our results support that the CR1-S isoform explains the GWAS signals and open a novel prospect for the investigation of CR1-related disease mechanisms.

GABAergic and Glycinergic Control of Upper Airway Motoneurons in Rapid Eye Movement Sleep

The aim of this study was to determine if GABA(B) receptors play a role in suppressing upper airway muscle tone in rapid eye movement (REM) sleep. The results reported herein indicate that GABA(B) receptors, acting in concert with GABA(A) and glycine receptors, play a role in mediating REM sleep atonia.