Evidence for Electrical Coupling in the SubCoeruleus (SubC) Nucleus

NMDA Enhances and Glutamate Attenuates Synchrony of Spontaneous Phase-Locked Locus Coeruleus Network Rhythm in Newborn Rat Brain Slices

Locus coeruleus (LC) neurons are controlled by glutamatergic inputs. Here, we studied in brain slices of neonatal rats NMDA and glutamate effects on phase-locked LC neuron spiking at ~1 Hz summating to ~0.2 s-lasting bell-shaped local field potential (LFP). NMDA: 10 μM accelerated LFP 1.7-fold, whereas 25 and 50 μM, respectively, increased its rate 3.2- and 4.6-fold while merging discrete events into 43 and 56% shorter oscillations. After 4–6 min, LFP oscillations stopped every 6 s for 1 s, resulting in ‘oscillation trains’. A dose of 32 μM depolarized neurons by 8.4 mV to cause 7.2-fold accelerated spiking at reduced jitter and enhanced synchrony with the LFP, as evident from cross-correlation. Glutamate: 25–50 μM made rhythm more irregular and the LFP pattern could transform into 2.7-fold longer-lasting multipeak discharge. In 100 μM, LFP amplitude and duration declined. In 25–50 μM, neurons depolarized by 5 mV to cause 3.7-fold acceleration of spiking that was less synchronized with LFP. Both agents: evoked ‘post-agonist depression’ of LFP that correlated with the amplitude and kinetics of V_m hyperpolarization. The findings show that accelerated spiking during NMDA and glutamate is associated with enhanced or attenuated LC synchrony, respectively, causing distinct LFP pattern transformations. Shaping of LC population discharge dynamics by ionotropic glutamate receptors potentially fine-tunes its influence on brain functions.

Spatiotemporal characterization of cellular tau pathology in the human locus coeruleus-pericoerulear complex by three-dimensional imaging

Tau pathology of the noradrenergic locus coeruleus (LC) is a hallmark of several age-related neurodegenerative disorders, including Alzheimer's disease. However, a comprehensive neuropathological examination of the LC is difficult due to its small size and rod-like shape. To investigate the LC cytoarchitecture and tau cytoskeletal pathology in relation to possible propagation patterns of disease-associated tau in an unprecedented large-scale three-dimensional view, we utilized volume immunostaining and optical clearing technology combined with light sheet fluorescence microscopy. We examined AT8⁺ pathological tau in the LC/pericoerulear region of 20 brains from Braak neurofibrillary tangle (NFT) stage 0-6. We demonstrate an intriguing morphological complexity and heterogeneity of AT8⁺ cellular structures in the LC, representing various intracellular stages of NFT maturation and their diverse transition forms. We describe novel morphologies of neuronal tau pathology such as AT8⁺ cells with fine filamentous somatic protrusions or with disintegrating soma. We show that gradual dendritic atrophy is the first morphological sign of the degeneration of tangle-bearing neurons, even preceding axonal lesions. Interestingly, irrespective of the Braak NFT stage, tau pathology is more advanced in the dorsal LC that preferentially projects to vulnerable forebrain regions in Alzheimer's disease, like the hippocampus or neocortical areas, compared to the ventral LC projecting to the cerebellum and medulla. Moreover, already in the precortical Braak 0 stage, 3D analysis reveals clustering tendency and dendro-dendritic close appositions of AT8⁺ LC neurons, AT8⁺ long axons of NFT-bearing cells that join the ascending dorsal noradrenergic bundle after leaving the LC, as well as AT8⁺ processes of NFT-bearing LC neurons that target the 4th ventricle wall. Our study suggests that the unique cytoarchitecture, comprised of a densely packed and dendritically extensively interconnected neuronal network with long projections, makes the human LC to be an ideal anatomical template for early accumulation and trans-neuronal spreading of hyperphosphorylated tau.

Orexin signaling modulates synchronized excitation in the sublaterodorsal tegmental nucleus to stabilize REM sleep

The relationship between orexin/hypocretin and rapid eye movement (REM) sleep remains elusive. Here, we find that a proportion of orexin neurons project to the sublaterodorsal tegmental nucleus (SLD) and exhibit REM sleep-related activation. In SLD, orexin directly excites orexin receptor-positive neurons (occupying ~3/4 of total-population) and increases gap junction conductance among neurons. Their interaction spreads the orexin-elicited partial-excitation to activate SLD network globally. Besides, the activated SLD network exhibits increased probability of synchronized firings. This synchronized excitation promotes the correspondence between SLD and its downstream target to enhance SLD output. Using optogenetics and fiber-photometry, we consequently find that orexin-enhanced SLD output prolongs REM sleep episodes through consolidating brain state activation/muscle tone inhibition. After chemogenetic silencing of SLD orexin signaling, a ~17% reduction of REM sleep amounts and disruptions of REM sleep muscle atonia are observed. These findings reveal a stabilization role of orexin in REM sleep.

Orexin signaling is provided by diffusely distributed fibers and involved in different brain circuits that orchestrate sleep and wakefulness states. Here, the authors show that a proportion of orexin neurons project to the sublaterodorsal tegmental nucleus and exhibit rapid eye movement (REM) sleep-related actions.

Neuroepigenetics of arousal: Gamma oscillations in the pedunculopontine nucleus

Four major discoveries on the function of the pedunculopontine nucleus (PPN) have significantly advanced our understanding of the role of arousal in neurodegenerative disorders. The first was the finding that stimulation of the PPN-induced controlled locomotion on a treadmill in decerebrate animals, the second was the revelation of electrical coupling in the PPN and other arousal and sleep-wake control regions, the third was the determination of intrinsic gamma band oscillations in PPN neurons, and the last was the discovery of gene transcription resulting from the manifestation of gamma activity in the PPN. These discoveries have led to novel therapies such as PPN deep brain stimulation (DBS) for Parkinson's disease (PD), identified the mechanism of action of the stimulant modafinil, determined the presence of separate mechanisms underlying gamma activity during waking versus REM sleep, and revealed the presence of gene transcription during the manifestation of gamma band oscillations. These discoveries set the stage for additional major advances in the treatment of a number of disorders.

Trigeminal, Visceral and Vestibular Inputs May Improve Cognitive Functions by Acting through the Locus Coeruleus and the Ascending Reticular Activating System: A New Hypothesis

It is known that sensory signals sustain the background discharge of the ascending reticular activating system (ARAS) which includes the noradrenergic locus coeruleus (LC) neurons and controls the level of attention and alertness. Moreover, LC neurons influence brain metabolic activity, gene expression and brain inflammatory processes. As a consequence of the sensory control of ARAS/LC, stimulation of a sensory channel may potential influence neuronal activity and trophic state all over the brain, supporting cognitive functions and exerting a neuroprotective action. On the other hand, an imbalance of the same input on the two sides may lead to an asymmetric hemispheric excitability, leading to an impairment in cognitive functions. Among the inputs that may drive LC neurons and ARAS, those arising from the trigeminal region, from visceral organs and, possibly, from the vestibular system seem to be particularly relevant in regulating their activity. The trigeminal, visceral and vestibular control of ARAS/LC activity may explain why these input signals: (1) affect sensorimotor and cognitive functions which are not directly related to their specific informational content; and (2) are effective in relieving the symptoms of some brain pathologies, thus prompting peripheral activation of these input systems as a complementary approach for the treatment of cognitive impairments and neurodegenerative disorders.

Implications of gamma band activity in the pedunculopontine nucleus

The fact that the pedunculopontine nucleus (PPN) is part of the reticular activating system places it in a unique position to modulate sensory input and fight-or-flight responses. Arousing stimuli simultaneously activate ascending projections of the PPN to the intralaminar thalamus to trigger cortical high-frequency activity and arousal, as well as descending projections to reticulospinal systems to alter posture and locomotion. As such, the PPN has become a target for deep brain stimulation for the treatment of Parkinson's disease, modulating gait, posture, and higher functions. This article describes the latest discoveries on PPN physiology and the role of the PPN in a number of disorders. It has now been determined that high-frequency activity during waking and REM sleep is controlled by two different intracellular pathways and two calcium channels in PPN cells. Moreover, there are three different PPN cell types that have one or both calcium channels and may be active during waking only, REM sleep only, or both. Based on the new discoveries, novel mechanisms are proposed for insomnia as a waking disorder. In addition, neuronal calcium sensor protein-1 (NCS-1), which is over expressed in schizophrenia and bipolar disorder, may be responsible for the dysregulation in gamma band activity in at least some patients with these diseases. Recent results suggest that NCS-1 modulates PPN gamma band activity and that lithium acts to reduce the effects of over expressed NCS-1, accounting for its effectiveness in bipolar disorder.

Pedunculopontine Gamma Band Activity and Development

This review highlights the most important discovery in the reticular activating system in the last 10 years, the manifestation of gamma band activity in cells of the reticular activating system (RAS), especially in the pedunculopontine nucleus, which is in charge of waking and rapid eye movement (REM) sleep. The identification of different cell groups manifesting P/Q-type Ca(2+) channels that control waking vs. those that manifest N-type channels that control REM sleep provides novel avenues for the differential control of waking vs. REM sleep. Recent discoveries on the development of this system can help explain the developmental decrease in REM sleep and the basic rest-activity cycle.

Pedunculopontine arousal system physiology-Implications for schizophrenia

Schizophrenia is characterized by major sleep/wake disturbances including increased vigilance and arousal, decreased slow wave sleep, and increased REM sleep drive. Other arousal-related symptoms include sensory gating deficits as exemplified by decreased habituation of the blink reflex. There is also dysregulation of gamma band activity, suggestive of disturbances in a host of arousal-related mechanisms. This review examines the role of the reticular activating system, especially the pedunculopontine nucleus, in the symptoms of the disease. Recent discoveries on the physiology of the pedunculopontine nucleus help explain many of these disorders of arousal in, and point to novel therapeutic avenues for, schizophrenia.

Pedunculopontine Nucleus Gamma Band Activity-Preconscious Awareness, Waking, and REM Sleep

The pedunculopontine nucleus (PPN) is a major component of the reticular activating system (RAS) that regulates waking and REM sleep, states of high-frequency EEG activity. Recently, we described the presence of high threshold, voltage-dependent N- and P/Q-type calcium channels in RAS nuclei that subserve gamma band oscillations in the mesopontine PPN, intralaminar parafascicular nucleus (Pf), and pontine subcoeruleus nucleus dorsalis (SubCD). Cortical gamma band activity participates in sensory perception, problem solving, and memory. Rather than participating in the temporal binding of sensory events as in the cortex, gamma band activity in the RAS may participate in the processes of preconscious awareness, and provide the essential stream of information for the formulation of many of our actions. That is, the RAS may play an early permissive role in volition. Our latest results suggest that (1) the manifestation of gamma band activity during waking may employ a separate intracellular pathway compared to that during REM sleep, (2) neuronal calcium sensor (NCS-1) protein, which is over expressed in schizophrenia and bipolar disorder, modulates gamma band oscillations in the PPN in a concentration-dependent manner, (3) leptin, which undergoes resistance in obesity resulting in sleep dysregulation, decreases sodium currents in PPN neurons, accounting for its normal attenuation of waking, and (4) following our discovery of electrical coupling in the RAS, we hypothesize that there are cell clusters within the PPN that may act in concert. These results provide novel information on the mechanisms controlling high-frequency activity related to waking and REM sleep by elements of the RAS.

Gamma band activity in the RAS-intracellular mechanisms

Gamma band activity participates in sensory perception, problem solving, and memory. This review considers recent evidence showing that cells in the reticular activating system (RAS) exhibit gamma band activity, and describes the intrinsic membrane properties behind such manifestation. Specifically, we discuss how cells in the mesopontine pedunculopontine nucleus, intralaminar parafascicular nucleus, and pontine SubCoeruleus nucleus dorsalis all fire in the gamma band range when maximally activated, but no higher. The mechanisms involve high-threshold, voltage-dependent P/Q-type calcium channels, or sodium-dependent subthreshold oscillations. Rather than participating in the temporal binding of sensory events as in the cortex, gamma band activity in the RAS may participate in the processes of preconscious awareness and provide the essential stream of information for the formulation of many of our actions. We address three necessary next steps resulting from these discoveries: an intracellular mechanism responsible for maintaining gamma band activity based on persistent G-protein activation, separate intracellular pathways that differentiate between gamma band activity during waking versus during REM sleep, and an intracellular mechanism responsible for the dysregulation in gamma band activity in schizophrenia. These findings open several promising research avenues that have not been thoroughly explored. What are the effects of sleep or REM sleep deprivation on these RAS mechanisms? Are these mechanisms involved in memory processing during waking and/or during REM sleep? Does gamma band processing differ during waking versus REM sleep after sleep or REM sleep deprivation?

The serendipity case of the pedunculopontine nucleus low-frequency brain stimulation: chasing a gait response, finding sleep, and cognition improvement

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) is an efficacious therapy for Parkinson’s disease (PD) but its effects on non-motor facets may be detrimental. The low-frequency stimulation (LFS) of the pedunculopontine nucleus (PPN or the nucleus tegmenti pedunculopontini – PPTg-) opened new perspectives. In our hands, PPTg-LFS revealed a modest influence on gait but increased sleep quality and degree of attentiveness. At odds with potential adverse events following STN-DBS, executive functions, under PPTg-ON, ameliorated. A recent study comparing both targets found that only PPTg-LFS improved night-time sleep and daytime sleepiness. Chances are that different neurosurgical groups influence either the PPN sub-portion identified as pars dissipata (more interconnected with GPi/STN) or the caudal PPN region known as pars compacta, preferentially targeting intralaminar and associative nucleus of the thalamus. Yet, the wide electrical field delivered affects a plethora of en passant circuits, and a fine distinction on the specific pathways involved is elusive. This review explores our angle of vision, by which PPTg-LFS activates cholinergic and glutamatergic ascending fibers, influencing non-motor behaviors.

Gap junction channels and hemichannels in the CNS: regulation by signaling molecules

Coordinated interaction among cells is critical to develop the extremely complex and dynamic tasks performed by the central nervous system (CNS). Cell synchronization is in part mediated by connexins and pannexins; two different protein families that form gap junction channels and hemichannels. Whereas gap junction channels connect the cytoplasm of contacting cells and coordinate electric and metabolic activities, hemichannels communicate intra- and extra-cellular compartments and serve as diffusional pathways for ions and small molecules. Cells in the CNS depend on paracrine/autocrine communication via several extracellular signaling molecules, such as, cytokines, growth factors, transmitters and free radical species to sense changes in microenvironment as well as to adapt to them. These signaling molecules modulate crucial processes of the CNS, including, cellular migration and differentiation, synaptic transmission and plasticity, glial activation, cell viability and microvascular blood flow. Gap junction channels and hemichannels are affected by different signaling transduction pathways triggered by these paracrine/autocrine signaling molecules. Most of the modulatory effects induced by these signaling molecules are specific to the cell type and the connexin and pannexin subtype expressed in different brain areas. In this review, we summarized and discussed most of the relevant and recently published information on the effects of signaling molecules on connexin or pannexin based channels and their possible relevance in CNS physiology and pathology. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

Coherence and frequency in the reticular activating system (RAS)

This review considers recent evidence showing that cells in the reticular activating system (RAS) exhibit (1) electrical coupling mainly in GABAergic cells, and (2) gamma band activity in virtually all of the cells. Specifically, cells in the mesopontine pedunculopontine nucleus (PPN), intralaminar parafascicular nucleus (Pf), and pontine dorsal subcoeruleus nucleus dorsalis (SubCD) (1) show electrical coupling, and (2) all fire in the beta/gamma band range when maximally activated, but no higher. The mechanism behind electrical coupling is important because the stimulant modafinil was shown to increase electrical coupling. We also provide recent findings demonstrating that all cells in the PPN and Pf have high threshold, voltage-dependent P/Q-type calcium channels that are essential to gamma band activity. On the other hand, all SubCD, and some PPN, cells manifested sodium-dependent subthreshold oscillations. A novel mechanism for sleep-wake control based on transmitter interactions, electrical coupling, and gamma band activity is described. We speculate that continuous sensory input will modulate coupling and induce gamma band activity in the RAS that could participate in the processes of preconscious awareness, and provide the essential stream of information for the formulation of many of our actions.

Control of sleep and wakefulness

This review summarizes the brain mechanisms controlling sleep and wakefulness. Wakefulness promoting systems cause low-voltage, fast activity in the electroencephalogram (EEG). Multiple interacting neurotransmitter systems in the brain stem, hypothalamus, and basal forebrain converge onto common effector systems in the thalamus and cortex. Sleep results from the inhibition of wake-promoting systems by homeostatic sleep factors such as adenosine and nitric oxide and GABAergic neurons in the preoptic area of the hypothalamus, resulting in large-amplitude, slow EEG oscillations. Local, activity-dependent factors modulate the amplitude and frequency of cortical slow oscillations. Non-rapid-eye-movement (NREM) sleep results in conservation of brain energy and facilitates memory consolidation through the modulation of synaptic weights. Rapid-eye-movement (REM) sleep results from the interaction of brain stem cholinergic, aminergic, and GABAergic neurons which control the activity of glutamatergic reticular formation neurons leading to REM sleep phenomena such as muscle atonia, REMs, dreaming, and cortical activation. Strong activation of limbic regions during REM sleep suggests a role in regulation of emotion. Genetic studies suggest that brain mechanisms controlling waking and NREM sleep are strongly conserved throughout evolution, underscoring their enormous importance for brain function. Sleep disruption interferes with the normal restorative functions of NREM and REM sleep, resulting in disruptions of breathing and cardiovascular function, changes in emotional reactivity, and cognitive impairments in attention, memory, and decision making.

Developmental changes in glutamatergic fast synaptic neurotransmission in the dorsal subcoeruleus nucleus

STUDY OBJECTIVES

The dorsal subcoeruleus nucleus (SubCD) is involved in the generation of rapid eye movement sleep (REM), a state distinguished by high-frequency EEG activity, muscle atonia, and ponto-geniculo-occipital (PGO) waves. Activation of the SubCD by injection of the glutamate (GLU) receptor agonist kainic acid (KA) produced a REM sleep-like state with muscle atonia. We tested the hypothesis that developmental changes in the GLU excitability of SubCD neurons could underlie the developmental decrease in REM sleep that occurs in the rat from postnatal days 10-30.

DESIGN

Sagittal sections containing the SubCD were cut using 9-15 day old rat pups. Whole-cell patch clamp recordings were performed on SubCD neurons and responses were measured following electrical stimulation or bath application of the GLU receptor agonists N-methyl-D-aspartic acid (NMDA) or KA.

MEASUREMENTS AND RESULTS

Pharmacological or electrical stimulation increased non-cholinergic excitatory postsynaptic currents (EPSCs) in SubCD neurons, which were blocked by GLU receptor antagonists. Although no developmental changes were observed in the relative contribution of AMPA/KA and NMDA receptors to the responses, there was a developmental decrease in the half-width duration of both evoked and miniature EPSCs. Bath application of NMDA or KA revealed a developmental decrease in the direct response of SubCD neurons to these agonists.

CONCLUSIONS

The SubCD receives glutamatergic input, which may be involved in activation of SubCD neurons during REM sleep. A developmental decrease in the glutamatergic excitability of these neurons could underlie the developmental decrease in REM sleep observed in humans and rodents.

Gamma band activity in the reticular activating system

This review considers recent evidence showing that cells in three regions of the reticular activating system (RAS) exhibit gamma band activity, and describes the mechanisms behind such manifestation. Specifically, we discuss how cells in the mesopontine pedunculopontine nucleus (PPN), intralaminar parafascicular nucleus (Pf), and pontine subcoeruleus nucleus dorsalis (SubCD) all fire in the beta/gamma band range when maximally activated, but no higher. The mechanisms behind this ceiling effect have been recently elucidated. We describe recent findings showing that every cell in the PPN have high-threshold, voltage-dependent P/Q-type calcium channels that are essential, while N-type calcium channels are permissive, to gamma band activity. Every cell in the Pf also showed that P/Q-type and N-type calcium channels are responsible for this activity. On the other hand, every SubCD cell exhibited sodium-dependent subthreshold oscillations. A novel mechanism for sleep–wake control based on well-known transmitter interactions, electrical coupling, and gamma band activity is described. The data presented here on inherent gamma band activity demonstrates the global nature of sleep–wake oscillation that is orchestrated by brainstem–thalamic mechanism, and questions the undue importance given to the hypothalamus for regulation of sleep–wakefulness. The discovery of gamma band activity in the RAS follows recent reports of such activity in other subcortical regions like the hippocampus and cerebellum. We hypothesize that, rather than participating in the temporal binding of sensory events as seen in the cortex, gamma band activity manifested in the RAS may help stabilize coherence related to arousal, providing a stable activation state during waking and paradoxical sleep. Most of our thoughts and actions are driven by pre-conscious processes. We speculate that continuous sensory input will induce gamma band activity in the RAS that could participate in the processes of pre-conscious awareness, and provide the essential stream of information for the formulation of many of our actions.

Sleep deprivation and sleep recovery modifies connexin36 and connexin43 protein levels in rat brain

Gap junctional communication is mainly mediated by connexin36 and connexin43 in neurons and astrocytes, respectively. It has been suggested that connexin36 allows electrical coupling between neurons whereas connexin43 participates in several process including release of ATP. It was recently reported that blockage of gap junctional communication mediated by connexin36 can disrupt the sleep architecture of the rat. However, there is no experimental approach about effects of sleep deprivation on connexins expression. Therefore, we examined in adult male Wistar rats whether protein levels of connexin36 and connexin43 change in pons, hypothalamus, and frontal cortex after 24 h of total sleep deprivation and 4 h of sleep recovery. Western blot revealed that total sleep deprivation significantly decreases the levels of connexin36 in the hypothalamus and this decrease maintains after sleep recovery. Meanwhile, connexin43 is not altered by total sleep deprivation but interestingly the sleep recovery period induces an increase of this connexin. These results suggest that electrical coupling between hypothalamic neurons could be altered by sleep deprivation and that sleep recovery drives changes in connexin43 expression probably as a mechanism related to ATP release and energy regulation during sleep.

The pedunculopontine tegmental nucleus: from basic neuroscience to neurosurgical applications: arousal from slices to humans: implications for DBS

One element of the reticular activating system (RAS) is the pedunculopontine nucleus (PPN), which projects to the thalamus to trigger thalamocortical rhythms and the brainstem to modulate muscle tone and locomotion. The PPN is a posterior midbrain site known to induce locomotion in decerebrate animals when activated at 40-60 Hz, and has become a target for DBS in disorders involving gait deficits. We developed a research program using brainstem slices containing the PPN to study the cellular and molecular organization of this region. We showed that PPN neurons preferentially fire at gamma band frequency (30-60 Hz) when maximally activated, accounting for the effects of electrical stimulation. In addition, we developed the P13 midlatency auditory evoked potential, which is generated by PPN outputs, in freely moving rats. This allows the study of PPN cellular and molecular mechanisms in the whole animal. We also study the P50 midlatency auditory evoked potential, which is the human equivalent of the rodent P13 potential, allowing us to study PPN-related processes detected in vitro, confirmed in the whole animal, and tested in humans. Previous findings on the P50 potential in PD suggest that PPN output in this disorder is overactive. This translational research program led to the discovery of a novel mechanism of sleep-wake control based on electrical coupling, pointing the way to a number of new clinical applications in the development of novel stimulants (e.g., modafinil) and anesthetics. In addition, it provides methods for monitoring therapeutic efficacy of DBS in humans and animal models.

Cholinergic and glutamatergic agonists induce gamma frequency activity in dorsal subcoeruleus nucleus neurons

The dorsal subcoeruleus nucleus (SubCD) is involved in generating two signs of rapid eye movement (REM) sleep: muscle atonia and ponto-geniculo-occipital (PGO) waves. We tested the hypothesis that single cell and/or population responses of SubCD neurons are capable of generating gamma frequency activity in response to intracellular stimulation or receptor agonist activation. Whole cell patch clamp recordings (immersion chamber) and population responses (interface chamber) were conducted on 9- to 20-day-old rat brain stem slices. All SubCD neurons (n = 103) fired at gamma frequency when subjected to depolarizing steps. Two statistically distinct populations of neurons were observed, which were distinguished by their high (>80 Hz, n = 24) versus low (35-80 Hz, n = 16) initial firing frequencies. Both cell types exhibited subthreshold oscillations in the gamma range (n = 43), which may underlie the gamma band firing properties of these neurons. The subthreshold oscillations were blocked by the sodium channel blockers tetrodotoxin (TTX, n = 21) extracellularly and N-(2,6-dimethylphenylcarbamoylmethyl)triethylammonium bromide (QX-314) intracellularly (n = 5), indicating they were sodium channel dependent. Gamma frequency subthreshold oscillations were observed in response to the nonspecific cholinergic receptor agonist carbachol (CAR, n = 11, d = 1.08) and the glutamate receptor agonists N-methyl-d-aspartic acid (NMDA, n = 12, d = 1.09) and kainic acid (KA, n = 13, d = 0.96), indicating that cholinergic and glutamatergic inputs may be involved in the activation of these subthreshold currents. Gamma band activity also was observed in population responses following application of CAR (n = 4, P < 0.05), NMDA (n = 4, P < 0.05) and KA (n = 4, P < 0.05). Voltage-sensitive, sodium channel-dependent gamma band activity appears to be a part of the intrinsic membrane properties of SubCD neurons.

Chapter 11--novel mechanism for hyperreflexia and spasticity

We established that hyperreflexia is delayed after spinal transection in the adult rat and that passive exercise could normalize low frequency-dependent depression of the H-reflex. We were also able to show that such passive exercise will normalize hyperreflexia in patients with spinal cord injury (SCI). Recent results demonstrate that spinal transection results in changes in the neuronal gap junction protein connexin 36 below the level of the lesion. Moreover, a drug known to increase electrical coupling was found to normalize hyperreflexia in the absence of passive exercise, suggesting that changes in electrical coupling may be involved in hyperreflexia. We also present results showing that a measure of spasticity, the stretch reflex, is rendered abnormal by transection and normalized by the same drug. These data suggest that electrical coupling may be dysregulated in SCI, leading to some of the symptoms observed. A novel therapy for hyperreflexia and spasticity may require modulation of electrical coupling.

L-Dopa effect on frequency-dependent depression of the H-reflex in adult rats with complete spinal cord transection

This study investigated whether l-dopa (DOPA), locomotor-like passive exercise (Ex) using a motorized bicycle exercise trainer (MBET), or their combination in adult rats with complete spinal cord transection (Tx) preserves and restores low frequency-dependent depression (FDD) of the H-reflex. Adult Sprague-Dawley rats (n=56) transected at T8-9 had one of five treatments beginning 7 days after transection: Tx (transection only), Tx+Ex, Tx+DOPA, Tx+Ex+DOPA, and control (Ctl, no treatment) groups. After 30 days of treatment, FDD of the H-reflex was tested. Stimulation of the tibial nerve at 0.2, 1, 5, and 10Hz evoked an H-reflex that was recorded from plantar muscles of the hind paw. No significant differences were found at the stimulation rate of 1Hz. However, at 5Hz, FDD of the H-reflex in the Tx+Ex, Tx+DOPA and Ctl groups was significantly different from the Tx group (p<0.01). At 10Hz, all of the treatment groups were significantly different from the Tx group (p<0.01). No significant difference was identified between the Ctl and any of the treatment groups. These results suggest that DOPA significantly preserved and restored FDD after transection as effectively as exercise alone or exercise in combination with DOPA. Thus, there was no additive benefit when DOPA was combined with exercise.

Oocyte triplet pairing for electrophysiological investigation of gap junctional coupling

Gap junctions formed by expressing connexin subunits in Xenopus oocytes provide a valuable tool for revealing the gating properties of intercellular gap junctions in electrically coupled cells. We describe a new method that consists of simultaneous triple recordings from 3 apposed oocytes expressing exogenous connexins. The advantages of this method are that in one single experiment, 1 oocyte serves as control while a pair of oocytes, which have been manipulated differently, may be tested for different gap junctional properties. Moreover, we can study simultaneously the gap junctional coupling of 3 different pairs of oocytes in the same preparation. If the experiment consists of testing the effect of a single drug, this approach will reduce the time required, as background coupling in control pairs of oocytes does not need to be measured separately as with the conventional 2 oocyte pairing. The triplet approach also increases confidence that any changes seen in junctional communication are due to the experimental treatment and not variation in the preparation of oocytes or execution of the experiment. In this study, we show the example of testing the gap junctional properties among 3 oocytes, 2 of which are expressing rat connexin36.

Gamma band unit activity and population responses in the pedunculopontine nucleus

The pedunculopontine nucleus (PPN) is involved in the activated states of waking and paradoxical sleep, forming part of the reticular activating system (RAS). The studies described tested the hypothesis that single unit and/or population responses of PPN neurons are capable of generating gamma band frequency activity. Whole cell patch clamp recordings (immersion chamber) and population responses (interface chamber) were conducted on 9- to 20-day-old rat brain stem slices. Regardless of cell type (I, II, or III) or type of response to the nonselective cholinergic receptor agonist carbachol (excitation, inhibition, biphasic), almost all PPN neurons fired at gamma band frequency, but no higher, when subjected to depolarizing steps (50 +/- 2 Hz, mean +/- SE). Nonaccommodating neurons fired at 18-100 Hz throughout depolarizing steps, while most accommodating neurons exhibited gamma band frequency of action potentials followed by gamma band membrane oscillations. These oscillations were blocked by the sodium channel blocker tetrodotoxin (TTX), suggesting that at least some are mediated by sodium currents. Population responses in the PPN showed that carbachol induced peaks of activation in the theta and gamma range, while glutamatergic receptor agonists induced overall increases in activity at theta and gamma frequencies, although in differing patterns. Gamma band activity appears to be a part of the intrinsic membrane properties of PPN neurons, and the population as a whole generates different patterns of gamma band activity under the influence of specific transmitters. Given sufficient excitation, the PPN may impart gamma band activation on its targets.

Enhancement of Synchronization Between Hippocampal and Amygdala Theta Waves Associated With Pontine Wave Density

Theta waves in the amygdala are known to be synchronized with theta waves in the hippocampus. Synchronization between amygdala and hippocampal theta waves is considered important for neuronal communication between these regions during the memory-retrieval process. These theta waves are also observed during rapid eye movement (REM) sleep. However, few studies have examined the mechanisms and functions of theta waves during REM sleep. This study examined correlations between the dynamics of hippocampal and amygdala theta waves and pontine (P) waves in the subcoeruleus region, which activates many brain areas including the hippocampus and amygdala, during REM sleep in rats. We confirmed that the frequency of hippocampal theta waves increased in association with P wave density, as shown in our previous study. The frequency of amygdala theta waves also increased with in associated with P wave density. In addition, we confirmed synchronization between hippocampal and amygdala theta waves during REM sleep in terms of the cross-correlation function and found that this synchronization was enhanced in association with increased P wave density. We further studied theta wave synchronization associated with P wave density by lesioning the pontine subcoeruleus region. This lesion not only decreased hippocampal and amygdala theta frequency, but also degraded theta wave synchronization. These results indicate that P waves enhance synchronization between regional theta waves. Because hippocampal and amygdala theta waves and P waves are known to be involved in learning and memory processes, these results may help clarify these functions during REM sleep.

Therapeutic approaches for spinal cord injury induced spasticity

Spasticity is evident in both humans and animals following spinal cord injury (SCI) and can contribute to significant functional limitation and disruption in quality of life of patients with this disorder. This mini-review describes a number of preclinical and clinical studies that promise to improve outcomes for, especially in terms of spasticity and hyper-reflexia, patients with SCI. A gold standard for the quantification of spasticity has proved elusive, but the combination of H-reflex frequency dependent depression and a novel stretch reflex (SR) windup protocol have the potential to provide new insights. As the pathophysiology of hyper-reflexia and spasticity continue to be investigated, the documented onset in the animal model of SCI provides critical time points for further study into these complex mechanisms. The positive effects of a passive exercise protocol and several potential pharmacological interventions are reviewed as well as a novel potential mechanism of action. Further work is needed to determine additional mechanisms that are involved in SCI, and how to optimize multiple therapies to overcome some of the deficits induced by SCI.

Cholinergic modulation of GABAergic and glutamatergic transmission in the dorsal subcoeruleus: mechanisms for REM sleep control

STUDY OBJECTIVES

Dorsal subcoeruleus (SubCD) neurons are thought to promote PGO waves and to be modulated by cholinergic afferents during REM sleep. We examined the differential effect of the cholinergic agonist carbachol (CAR) on excitatory and inhibitory postsynaptic currents (PSCs), and investigated the effects of CAR on SubCD neurons during the developmental decrease in REM sleep.

DESIGN

Whole-cell patch clamp recordings were conducted on brainstem slices of 7- to 20-day-old rats.

MEASUREMENTS AND RESULTS

CAR acted directly on 50% of SubCD neurons by inducing an inward current, via both nicotinic and muscarinic M1 receptors. CAR induced a potassium mediated outward current via activation of M2 muscarinic receptors in 43% of SubCD cells. Evoked stimulation established the presence of NMDA, AMPA, GABA, and glycinergic PSCs in the SubCD. CAR was found to decrease the amplitude of evoked EPSCs in 31 of 34 SubCD cells, but decreased the amplitude of evoked IPSCs in only 1 of 13 SubCD cells tested. Spontaneous EPSCs were decreased by CAR in 55% of cells recorded, while spontaneous IPSCs were increased in 27% of SubCD cells. These findings indicate that CAR exerts a predominantly inhibitory role on fast synaptic glutamatergic activity and a predominantly excitatory role on fast synaptic GABAergic/glycinergic activity in the SubCD.

CONCLUSION

We hypothesize that during REM sleep, cholinergic "REM-on" neurons that project to the SubCD induce an excitation of inhibitory interneurons and inhibition of excitatory events leading to the production of coordinated activity in SubCD projection neurons. The coordination of these projection neurons may be essential for the production of REM sleep signs such as PGO waves.

Identification of cholinergic and non-cholinergic neurons in the pons expressing phosphorylated cyclic adenosine monophosphate response element-binding protein as a function of rapid eye movement sleep

Recent studies have shown that in the pedunculopontine tegmental nucleus (PPT), increased neuronal activity and kainate receptor-mediated activation of intracellular protein kinase A (PKA) are important physiological and molecular steps for the generation of rapid eye movement (REM) sleep. In the present study performed on rats, phosphorylated cyclic AMP response element-binding protein (pCREB) immunostaining was used as a marker for increased intracellular PKA activation and as a reflection of increased neuronal activity. To identify whether activated cells were either cholinergic or noncholinergic, the PPT and laterodorsal tegmental nucleus (LDT) cells were immunostained for choline acetyltransferase (ChAT) in combination with pCREB or c-Fos. The results demonstrated that during high rapid eye movement sleep (HR, approximately 27%), significantly higher numbers of cells expressed pCREB and c-Fos in the PPT, of which 95% of pCREB-expressing cells were ChAT-positive. With HR, the numbers of pCREB-positive cells were also significantly higher in the medial pontine reticular formation (mPRF), pontine reticular nucleus oral (PnO), and dorsal subcoeruleus nucleus (SubCD) but very few in the locus coeruleus (LC) and dorsal raphe nucleus (DRN). Conversely, with low rapid eye movement sleep (LR, approximately 2%), the numbers of pCREB expressing cells were very few in the PPT, mPRF, PnO, and SubCD but significantly higher in the LC and DRN. The results of regression analyses revealed significant positive relationships between the total percentages of REM sleep and numbers of ChAT+/pCREB+ (Rsqr=0.98) cells in the PPT and pCREB+ cells in the mPRF (Rsqr=0.88), PnO (Rsqr=0.87), and SubCD (Rsqr=0.84); whereas significantly negative relationships were associated with the pCREB+ cells in the LC (Rsqr=0.70) and DRN (Rsqr=0.60). These results provide evidence supporting the hypothesis that during REM sleep, the PPT cholinergic neurons are active, whereas the LC and DRN neurons are inactive. More importantly, the regression analysis indicated that pCREB activation in approximately 98% of PPT cholinergic neurons, was caused by REM sleep. Moreover the results indicate that during REM sleep, PPT intracellular PKA activation and a transcriptional cascade involving pCREB occur exclusively in the cholinergic neurons.

Modafinil normalized hyperreflexia after spinal transection in adult rats

STUDY DESIGN

Hyperreflexia occurs after spinal cord injury and can be assessed by measuring low frequency-dependent depression of the H-reflex in the anesthetized animal.

OBJECTIVE

To determine the effects of Modafinil (MOD), given orally, following a complete SCI compared with animals receiving MBET and transected untreated animals and examine if changes exist in Connexin 36 (Cx-36) protein levels in the lumbar enlargement of animals for the groups described.

SETTING

Center for Translational Neuroscience, Little Rock, AR, USA.

METHODS

Adult female rats underwent complete transection (Tx) at T10 level. H-reflex testing was performed 30 days following Tx in one group, and after initiation of treatment with MOD in another group, and after MBET training in the third group. The Lumbar enlargement tissue was harvested and western blots were performed after immunoprecipitation techniques to compare Cx-36 protein levels.

RESULTS

Statistically significant decreases in low frequency-dependent depression of the H-reflex were observed in animals that received MOD and those that were treated with MBET compared with the Tx, untreated group. Statistically significant changes in Cx-36 protein levels were not observed in animals treated with MOD compared with Tx, untreated animals.

CONCLUSION

Normalization of the loss of low frequency -dependent depression of the H-reflex was demonstrated in the group receiving MOD and the group receiving MBET compared with the Tx, untreated group. Further work is needed to examine if Cx-36 protein changes occur in specific subregions of the spinal cord.

Modafinil increases arousal determined by P13 potential amplitude: an effect blocked by gap junction antagonists

STUDY OBJECTIVES

We recorded the effects of administration of the stimulant modafinil on the amplitude of the sleep state-dependent auditory P13 evoked potential in freely moving rats, a measure of arousal thought to be generated by the cholinergic arm of the reticular activating system, specifically the pedunculopontine nucleus (PPN).

DESIGN

Groups of rats were implanted for recording auditory evoked responses and the effects on P13 potential amplitude of intracranial injections into the PPN of neuroactive agents determined.

MEASUREMENTS AND RESULTS

The effects of intracranial injections into the PPN of modafinil showed that P13 potential amplitude increased in a dose-dependent manner at doses of 100, 200, and 300 microM. The effect was blocked by pretreatment with either of the gap junction antagonists carbenoxolone (300 microM) or mefloquine (25 microM), which by themselves slightly decreased P13 potential amplitude.

CONCLUSIONS

These results suggest that modafinil increases arousal levels as determined by the amplitude of the P13 potential, an effect blocked by gap junction antagonists, suggesting that one mechanism by which modafinil increases arousal may be by increasing electrical coupling.

The onset of hyperreflexia in the rat following complete spinal cord transection

STUDY DESIGN

Hyperreflexia occurs after spinal cord injury (SCI) and can be assessed by measuring low frequency-dependent depression of the H-reflex. Previous studies showed the time course for the onset of hyperreflexia to occur between 6-28 days in the contusion model of SCI.

OBJECTIVE

To determine the time course of the onset of hyperreflexia in the transection model of SCI and examine changes in Connexin-36 (Cx-36) protein levels in the lumbar enlargement of animals.

SETTING

Spinal Cord Injury Mobilization Program of the Center for Translational Neuroscience, the research arm of the Jackson T. Stephens Neuroscience Institute, Little Rock, AR, USA.

METHODS

Adult female rats underwent transection at T10 level. Low frequency-dependent depression of the H-reflex was tested at 7, 14 and 30 days post-transection. Lumbar enlargement tissue was harvested following reflex testing and western blots were performed after immunoprecipitation to compare Cx-36 protein levels.

RESULTS

Significant decreases in low frequency-dependent depression of the H-reflex were observed in animals tested 14 and 30 days post-transection compared with control animals, but it was not different from control animals at 7 days. Significant decreases in Cx-36 protein levels were observed in animals 7 days post-transection compared with controls.

CONCLUSION

Rats transition to a state of hyperreflexia between 7 and 14 days post-transection. Cx-36 protein levels decreased at 7 days post-transection and gradually returned to control levels by 30 days post-transection. These data suggest there may be a relationship between changes in neuronal gap junction protein levels and the delayed onset of hyperreflexia.

The effects of passive exercise therapy initiated prior to or after the development of hyperreflexia following spinal transection

Hyperreflexia develops after spinal cord injury (SCI) in the human and in the spinal cord transected animal, and can be measured by the loss of low frequency-dependent depression of the H-reflex. Previous studies demonstrated normalization of low frequency-dependent depression of the H-reflex using passive exercise when initiated prior to the development of hyperreflexia. We examined the effects of passive exercise prior to compared to after the development of hyperreflexia in the transected rat. Adult female rats underwent complete transection (Tx) at T10. Frequency-dependence of the H-reflex was tested following passive exercise for 30 days, initiated prior to hyperreflexia in one group compared to initiation after hyperreflexia became established, and compared to intact and untreated Tx groups. An additional Tx group completed 60 days of exercise initiated after hyperreflexia was established. Lumbar enlargement tissue was harvested for western blot to compare Connexin-36 protein levels in control vs Tx animals vs Tx animals that were passively exercised. No differences in whole tissue were evident, although regional differences may still be present in Connexin-36 levels. Statistically significant decreases in low frequency-dependent depression of the H-reflex were observed following 30 days of exercise initiated prior to the onset of hyperreflexia, and also after 60 days of exercise when initiated after hyperreflexia had been established, compared with Tx only animals. We concluded that modulation of spinal circuitry by passive exercise took place when initiated before and after the onset of hyperreflexia, but different durations of exercise were required.

The developmental decrease in REM sleep: the role of transmitters and electrical coupling

STUDY OBJECTIVES

This mini-review considers certain factors related to the developmental decrease in rapid eye movement (REM) sleep, which occurs in favor of additional waking time, and its relationship to developmental factors that may influence its potential role in brain development.

DESIGN

Specifically, we discuss some of the theories proposed for the occurrence of REM sleep and agree with the classic notion that REM sleep is, at the least, a mechanism that may play a role in the maturation of thalamocortical pathways. The developmental decrease in REM sleep occurs gradually from birth until close to puberty in the human, and in other mammals it is brief and coincides with eye and ear opening and the beginning of massive exogenous activation. Therefore, the purported role for REM sleep may change to involve a number of other functions with age.

MEASUREMENTS AND RESULTS

We describe recent findings showing that morphologic and physiologic properties as well as cholinergic, gamma amino-butyric acid, kainic acid, n-methyl-d-aspartic acid, noradrenergic, and serotonergic synaptic inputs to mesopontine cholinergic neurons, as well as the degree of electrical coupling between mostly noncholinergic mesopontine neurons and levels of the neuronal gap-junction protein connexin 36, change dramatically during this critical period in development. A novel mechanism for sleep-wake control based on well-known transmitter interactions, as well as electrical coupling, is described.

CONCLUSION

We hypothesize that a dysregulation of this process could result in life-long disturbances in arousal and REM sleep drive, leading to hypervigilance or hypovigilance such as that observed in a number of disorders that have a mostly postpubertal age of onset.

Neuronal connexin expression in the cochlear nucleus of big brown bats

We present immunohistochemical data describing the presence and distribution of connexins, structural component of gap junctions, in the cochlear nuclei of adult big brown bats (Eptesicus fuscus). Echolocating big brown bats show microsecond scale echo-delay sensitivity that requires accurate synchronization of neuronal responses to the timing of echoes. Midbrain and auditory cortical neuronal response timing is similar to that observed in other non-echolocating mammals, suggesting that lower auditory processing nuclei may have specialized mechanisms for obtaining the required temporal hyperacuity. Our data shows that connexin 36, a gap junction protein specific to neurons, is most densely expressed in the bat's cochlear nuclear complex, the medullary region that receives and processes first-order afferents from the auditory nerve. Cx36 expression is absent in the cochlear nucleus of normal mice, which have high-frequency hearing sensitivity similar to big brown bats. Glial connexins, Cx26 and Cx43, expressed in astrocytes and several inner ear structures, are also found in the bat cochlear nucleus complex, associated with major fiber tracts in and around the cochlear nuclei. The extensive presence of neuronally-associated Cx36 in brainstem auditory structures of adult bats suggests a possible role for gap junctions in mediating echo-delay hyperacuity.

Electrical coupling: novel mechanism for sleep-wake control

STUDY OBJECTIVES

Recent evidence suggests that certain anesthetic agents decrease electrical coupling, whereas the stimulant modafinil appears to increase electrical coupling. We investigated the potential role of electrical coupling in 2 reticular activating system sites, the subcoeruleus nucleus and in the pedunculopontine nucleus, which has been implicated in the modulation of arousal via ascending cholinergic activation of intralaminar thalamus and descending activation of the subcoeruleus nucleus to generate some of the signs of rapid eye movement sleep.

DESIGN

We used 6- to 30-day-old rat pups to obtain brainstem slices to perform whole-cell patch-clamp recordings.

MEASUREMENTS AND RESULTS

Recordings from single cells revealed the presence of spikelets, manifestations of action potentials in coupled cells, and of dye coupling of neurons in the pedunculopontine nucleus. Recordings in pairs of pedunculopontine nucleus and subcoeruleus nucleus neurons revealed that some of these were electrically coupled with coupling coefficients of approximately 2%. After blockade of fast synaptic transmission, the cholinergic agonist carbachol was found to induce rhythmic activity in pedunculopontine nucleus and subcoeruleus nucleus neurons, an effect eliminated by the gap junction blockers carbenoxolone or mefloquine. The stimulant modafinil was found to decrease resistance in neurons in the pedunculopontine nucleus and subcoeruleus nucleus after fast synaptic blockade, indicating that the effect may be due to increased coupling.

CONCLUSIONS

The finding of electrical coupling in specific reticular activating system cell groups supports the concept that this underlying process behind specific neurotransmitter interactions modulates ensemble activity across cell populations to promote changes in sleep-wake state.

Modafinil enhances thalamocortical activity by increasing neuronal electrotonic coupling

Modafinil (Provigil, Modiodal), an antinarcoleptic and mood-enhancing drug, is shown here to sharpen thalamocortical activity and to increase electrical coupling between cortical interneurons and between nerve cells in the inferior olivary nucleus. After irreversible pharmacological block of connexin permeability (i.e., by using either 18beta-glycyrrhetinic derivatives or mefloquine), modafinil restored electrotonic coupling within 30 min. It was further established that this restoration is implemented through a Ca(2+)/calmodulin protein kinase II-dependent step.