Nigel P. Pedersen

Sleep state switching.

We take for granted the ability to fall asleep or to snap out of sleep into wakefulness, but these changes in behavioral state require specific switching mechanisms in the brain that allow well-defined state transitions. In this review, we examine the basic circuitry underlying the regulation of sleep and wakefulness and discuss a theoretical framework wherein the interactions between reciprocal neuronal circuits enable relatively rapid and complete state transitions. We also review how homeostatic, circadian, and allostatic drives help regulate sleep state switching and discuss how breakdown of the switching mechanism may contribute to sleep disorders such as narcolepsy.

Reassessment of the structural basis of the ascending arousal system

The “ascending reticular activating system” theory proposed that neurons in the upper brainstem reticular formation projected to forebrain targets that promoted wakefulness. More recent formulations have emphasized that most neurons at the pontomesencephalic junction that participate in these pathways are actually in monoaminergic and cholinergic cell groups. However, cell‐specific lesions of these cell groups have never been able to reproduce the deep coma seen after acute paramedian midbrain lesions that transect ascending axons at the caudal midbrain level. To determine whether the cortical afferents from the thalamus or the basal forebrain were more important in maintaining arousal, we first placed large cell‐body‐specific lesions in these targets. Surprisingly, extensive thalamic lesions had little effect on electroencephalographic (EEG) or behavioral measures of wakefulness or on c‐Fos expression by cortical neurons during wakefulness. In contrast, animals with large basal forebrain lesions were behaviorally unresponsive and had a monotonous sub‐1‐Hz EEG, and little cortical c‐Fos expression during continuous gentle handling. We then retrogradely labeled inputs to the basal forebrain from the upper brainstem, and found a substantial input from glutamatergic neurons in the parabrachial nucleus and adjacent precoeruleus area. Cell‐specific lesions of the parabrachial‐precoeruleus complex produced behavioral unresponsiveness, a monotonous sub‐1‐Hz cortical EEG, and loss of cortical c‐Fos expression during gentle handling. These experiments indicate that in rats the reticulo‐thalamo‐cortical pathway may play a very limited role in behavioral or electrocortical arousal, whereas the projection from the parabrachial nucleus and precoeruleus region, relayed by the basal forebrain to the cerebral cortex, may be critical for this process. J. Comp. Neurol. 519:933–956, 2011.

Locus Ceruleus and Anterior Cingulate Cortex Sustain Wakefulness in a Novel Environment

Locus ceruleus (LC) neuronal activity is correlated with the waking state, yet LC lesions produce only minor alterations in daily wakefulness. Here, we report that sustained elevations in neurobehavioral and EEG arousal in response to exposure to an environment with novel stimuli, including social interaction, are prevented by selective chemical lesions of the LC in rats. Similar results are seen when the anterior cingulate cortex (ACC), which receives especially dense LC innervation, is selectively denervated of LC input or is ablated by the cell-specific neurotoxin ibotenic acid. Anterograde tracing combined with tyrosine hydroxylase immunohistochemistry demonstrates ACC terminals in apposition with the distal dendrites of LC neurons. Our data implicate the ACC as both a source of input to the LC as well as one of its targets and suggests that the two structures engage in a dialog that may provide a critical neurobiological substrate for sustained attention.

Basal forebrain control of wakefulness and cortical rhythms

Wakefulness, along with fast cortical rhythms and associated cognition, depend on the basal forebrain (BF). BF cholinergic cell loss in dementia and the sedative effect of anti-cholinergic drugs have long implicated these neurons as important for cognition and wakefulness. The BF also contains intermingled inhibitory GABAergic and excitatory glutamatergic cell groups whose exact neurobiological roles are unclear. Here we show that genetically targeted chemogenetic activation of BF cholinergic or glutamatergic neurons in behaving mice produced significant effects on state consolidation and/or the electroencephalogram but had no effect on total wake. Similar activation of BF GABAergic neurons produced sustained wakefulness and high-frequency cortical rhythms, whereas chemogenetic inhibition increased sleep. Our findings reveal a major contribution of BF GABAergic neurons to wakefulness and the fast cortical rhythms associated with cognition. These findings may be clinically applicable to manipulations aimed at increasing forebrain activation in dementia and the minimally conscious state.

Glutamatergic Signaling from the Parabrachial Nucleus Plays a Critical Role in Hypercapnic Arousal

The mechanisms of arousal from apneas during sleep in patients suffering from obstructive sleep apnea are not well understood. However, we know that respiratory chemosensory pathways converge on the parabrachial nucleus (PB), which sends glutamatergic projections to a variety of forebrain structures critical to arousal, including the basal forebrain, lateral hypothalamus, midline thalamus, and cerebral cortex. We tested the role of glutamatergic signaling in this pathway by developing an animal model for repetitive CO2 arousals (RCAs) and investigating the effect of deleting the gene for the vesicular glutamate transporter 2 (Vglut2) from neurons in the PB. We used mice with lox P sequences flanking exon2 of the Vglut2 gene, in which adeno-associated viral vectors containing genes encoding Cre recombinase and green fluorescent protein were microinjected into the PB to permanently and selectively disrupt Vglut2 expression while labeling the affected neurons. We recorded sleep in these mice and then investigated the arousals during RCA. Vglut2 deletions that included the external lateral and lateral crescent subdivisions of the lateral PB more than doubled the latency to arousal and resulted in failure to arouse by 30 s in >30% of trials. By contrast, deletions that involved the medial PB subdivision had minimal effects on arousal during hypercapnia but instead increased non-rapid eye movement (NREM) sleep by ∼43% during the dark period, and increased delta power in the EEG during NREM sleep by ∼50%. Our results suggest that glutamatergic neurons in the lateral PB are necessary for arousals from sleep in response to CO2, while medial PB glutamatergic neurons play an important role in promoting spontaneous waking.

Brainstem circuitry regulating phasic activation of trigeminal motoneurons during REM sleep.

Background Rapid eye movement sleep (REMS) is characterized by activation of the cortical and hippocampal electroencephalogram (EEG) and atonia of non-respiratory muscles with superimposed phasic activity or twitching, particularly of cranial muscles such as those of the eye, tongue, face and jaw. While phasic activity is a characteristic feature of REMS, the neural substrates driving this activity remain unresolved. Here we investigated the neural circuits underlying masseter (jaw) phasic activity during REMS. The trigeminal motor nucleus (Mo5), which controls masseter motor function, receives glutamatergic inputs mainly from the parvocellular reticular formation (PCRt), but also from the adjacent paramedian reticular area (PMnR). On the other hand, the Mo5 and PCRt do not receive direct input from the sublaterodorsal (SLD) nucleus, a brainstem region critical for REMS atonia of postural muscles. We hypothesized that the PCRt-PMnR, but not the SLD, regulates masseter phasic activity during REMS. Methodology/Principal Findings To test our hypothesis, we measured masseter electromyogram (EMG), neck muscle EMG, electrooculogram (EOG) and EEG in rats with cell-body specific lesions of the SLD, PMnR, and PCRt. Bilateral lesions of the PMnR and rostral PCRt (rPCRt), but not the caudal PCRt or SLD, reduced and eliminated REMS phasic activity of the masseter, respectively. Lesions of the PMnR and rPCRt did not, however, alter the neck EMG or EOG. To determine if rPCRt neurons use glutamate to control masseter phasic movements, we selectively blocked glutamate release by rPCRt neurons using a Cre-lox mouse system. Genetic disruption of glutamate neurotransmission by rPCRt neurons blocked masseter phasic activity during REMS. Conclusions/Significance These results indicate that (1) premotor glutamatergic neurons in the medullary rPCRt and PMnR are involved in generating phasic activity in the masseter muscles, but not phasic eye movements, during REMS; and (2) separate brainstem neural circuits control postural and cranial muscle phasic activity during REMS.

Supramammillary glutamate neurons are a key node of the arousal system

Basic and clinical observations suggest that the caudal hypothalamus comprises a key node of the ascending arousal system, but the cell types underlying this are not fully understood. Here we report that glutamate-releasing neurons of the supramammillary region (SuMvglut2) produce sustained behavioral and EEG arousal when chemogenetically activated. This effect is nearly abolished following selective genetic disruption of glutamate release from SuMvglut2 neurons. Inhibition of SuMvglut2 neurons decreases and fragments wake, also suppressing theta and gamma frequency EEG activity. SuMvglut2 neurons include a subpopulation containing both glutamate and GABA (SuMvgat/vglut2) and another also expressing nitric oxide synthase (SuMNos1/Vglut2). Activation of SuMvgat/vglut2 neurons produces minimal wake and optogenetic stimulation of SuMvgat/vglut2 terminals elicits monosynaptic release of both glutamate and GABA onto dentate granule cells. Activation of SuMNos1/Vglut2 neurons potently drives wakefulness, whereas inhibition reduces REM sleep theta activity. These results identify SuMvglut2 neurons as a key node of the wake−sleep regulatory system.Supramammillary nucleus (SuM) neurons have been studied in the context of REM sleep but their possible role in mediating wakefulness is not known. Here the authors elucidate the distinct functional contributions of three subpopulations in the SuM on electrographical and behavioral arousal in mice using genetically targeted approaches.

In the flicker of an eye

Dreams were tentatively associated with twitching of the eyelids and somatic muscles during sleep in 1868 by Griesinger (Finger, 1994). However, it was not until 1953 that Aserinsky & Kleitman (1953) identified a discrete sleep stage they termed rapid eye movement (REM) sleep characterized by recurrent episodes of non-respiratory muscle atonia and desynchronized EEG, coinciding with enriched dream content. Superimposed upon these tonic REM features, they observed phasic phenomena including increased heart rate, irregular and shallower respiration, brief muscle twitches, and REMs hallmark, bursts of rapid eye movements. Although there is some evidence for the correlation of dream content and eye movements, REMs are maintained or increased after destruction or removal of the cerebral cortex, and continue in animals after midbrain transection (Jouvet, 1962), arguing against them being caused by cortical activity. Remarkably, given the role of REMs in defining this interesting behavioural state, the study of the REMs themselves has been largely phenomenological. In standard polysomnography, the electro-oculogram (EOG) is used for the measurement of eye movement, helping to easily identify REM sleep. This method measures the alteration in charge between two electrodes due to the movement of the large polarized retina with the globe of the eye. The EOG is not, however, suitable to determine slow eye movements or absolute eye position accurately. As a result, earlier studies focused mainly on the very rapid and predominantly conjugate horizontal eye movements that characterize REMs. An early study employing the monocular magnetic search coil technique maintained a similar focus (Vanni-Mercier et al. 1994). Magnetic search coil techniques, with scleral annuli were introduced in 1963 by Robinson and gained popularity in the 1980s. They are currently the technique of choice for experimental studies in which accurate absolute measurement of eye position in all three axes is required. In the present issue of The Journal of Physiology, Marquez-Ruiz & Escudero (2008a,b) use binocular search coils to describe accurately REMs in cats, including, for the first time, measurement of tonic and vergence movements, associated changes in abducens activity, and the relationship of these phenomena to ponto-geniculooccipital waves. Although REMs are stereotypical and occur reliably as a marker of REM sleep, the oculomotor control during REM sleep remains poorly understood. The abducens nucleus is responsible for abduction of the globe and conjugate adduction via a projection to the contra-lateral oculomotor nucleus. It receives input from a variety of structures (Maciewicz et al. 1977), including the vestibular nuclei and dorsal paragigantocellular region of the rostral medulla, both of which are REM active. Vestibular inputs to the oculomotor nuclei play an important role in disynaptic vestibuloocular reflexes. However, the inputs from the region immediately caudal to the abducens nucleus are less well understood. In non-human primates, this dorsal paragigantocellular region is currently characterized as containing inhibitory burst neurons (Strassman et al. 1986), and in rats, as containing REM-on neurons (Goutagny et al. 2008). Saccadic eye movements consist of two components. The first, a rapid and large extra-ocular muscle contraction, begins a rapid rotation of the globe. Subsequently, a step function then maintains eye position. Present models describe excitatory burst neurons of the paramedian pontine reticular formation and vestibular nuclei, and neurons of the nucleus prepositus hypoglossi, respectively, as responsible for these two components of horizontal saccades (Leigh & Zee, 2006). During REM sleep, however, eye movements consist of only the first burst component, without a subsequent step function. In a series of papers in the mid-1960s, Pompieano and Morrison (see Pompeiano & Morrison, 1965) describe the loss of REMs and other phasic phenomena in cats following electrolytic lesions of the inferior and medial vestibular nuclei. The lesions were somewhat variable as the technique was difficult to control and destroyed fibres of passage. The premotor region mediating the burst thus remains poorly characterized, but represents an opportunity to identify phasic REM mechanisms, and may assist in differentiating their functional anatomy from the tonic REM circuitry. The identification of the source of phasic activity in REM sleep is of clinical importance, as phasic autonomic activation during REM sleep increases the risk for myocardial ischaemia or arrhythmia. Marquez-Ruiz and Escudero here comprehensively characterize two important components of REMs: a slow change in tonic eye position between wakefulness, non-REM and REM sleep, upon which is superimposed phasic REMs during REM sleep. The tonic change in eye position is from an outward and divergent position, to a downward and convergent position during REM. This novel finding leads us to ask what differential tonic changes occur in oculomotor nuclei during the transitions into non-REM and REM sleep. It may be, for example, that abducens motorneurons are inhibited to a relatively greater extent than those of the oculomotor and trochlear nuclei during REM, as would be consistent with Marquez-Ruiz and Escuderos description of abducens nucleus inhibition during REM. These papers by Marquez-Ruiz and Escudero provide important details about the precise nature of REMs and abducens unit activity that may constitute an important alternative approach to studying oculomotor control, and further our understanding of the mechanisms of both phasic and tonic REM sleep phenomena. Phasic REM features are loosely coordinated, and it is likely that they originate in a brain structure one or two synapses from the abducens nucleus. These articles and the utilized techniques will allow future studies that narrow candidate regions.

Stereotactic laser amygdalohippocampotomy for mesial temporal lobe epilepsy

To evaluate the outcomes 1 year and longer following stereotactic laser amygdalohippocampotomy for mesial temporal lobe epilepsy in a large series of patients treated over a 5‐year period since introduction of this novel technique.

Neuromodulation Using Optogenetics and Related Technologies

Abstract Optogenetics is defined as the use of light-based technologies that are genetically targeted to certain cellular groups or proteins. In practice, optogenetics most often refers to the use of genetically modified, microbial rhodopsins (pigmented light-sensitive membrane proteins) to manipulate the excitability of genetically targeted neurons. Given this high degree of temporal control over targeted neurons, with a well-understood mechanism of action, these techniques have resulted in a substantial contribution to basic neuroscience, as well as generating significant clinical translational interest, considering the nonspecificity of electrical stimulation. While optogenetics is presently an investigational technology, these tools have been used with translational promise in nonhuman primates. This chapter provides an overview of the development, mechanisms, diversity, and translatability of this technique. Other related genetically targeted techniques will also be described.