Sleep-controlling neurons are sensitive and vulnerable to multiple forms of α-synuclein: implications for the early appearance of sleeping disorders in α-synucleinopathies.

Parkinson's disease, Multiple System Atrophy, and Lewy Body Dementia are incurable diseases called α-synucleinopathies as they are mechanistically linked to the protein, α-synuclein (α-syn). α-syn exists in different structural forms which have been linked to clinical disease distinctions. However, sleeping disorders (SDs) are common in the prodromal phase of all three α-synucleinopathies, which suggests that sleep-controlling neurons are affected by multiple forms of α-syn. To determine whether a structure-independent neuronal impact of α-syn exists, we compared and contrasted the cellular effect of three different α-syn forms on neurotransmitter-defined cells of two sleep-controlling nuclei located in the brainstem: the laterodorsal tegmental nucleus and the pedunculopontine tegmental nucleus. We utilized size exclusion chromatography, fluorescence spectroscopy, circular dichroism spectroscopy and transmission electron microscopy to precisely characterize ​​timepoints in the α-syn aggregation process with three different dominating forms of this protein (monomeric, oligomeric and fibril) and we conducted an in-depth investigation of the underlying neuronal mechanism behind cellular effects of the different forms of the protein using electrophysiology, multiple-cell calcium imaging, single-cell calcium imaging and live-location tracking with fluorescently-tagged α-syn. Interestingly, α-syn altered membrane currents, enhanced firing, increased intracellular calcium and facilitated cell death in a structure-independent manner in sleep-controlling nuclei, and postsynaptic actions involved a G-protein-mediated mechanism. These data are novel as the sleep-controlling nuclei are the first brain regions reported to be affected by α-syn in this structure-independent manner. These regions may represent highly important targets for future neuroprotective therapy to modify or delay disease progression in α-synucleinopathies.

α-Synuclein Responses in the Laterodorsal Tegmentum, the Pedunculopontine Tegmentum, and the Substantia Nigra: Implications for Early Appearance of Sleep Disorders in Parkinson's Disease.

BACKGROUND: Parkinson's disease (PD) is a neurodegenerative disorder associated with insoluble pathological aggregates of the protein α-synuclein. While PD is diagnosed by motor symptoms putatively due to aggregated α-synuclein-mediated damage to substantia nigra (SN) neurons, up to a decade before motor symptom appearance, patients exhibit sleep disorders (SDs). Therefore, we hypothesized that α-synuclein, which can be present in monomeric, fibril, and other forms, has deleterious cellular actions on sleep-control nuclei. OBJECTIVE: We investigated whether native monomer and fibril forms of α-synuclein have effects on neuronal function, calcium dynamics, and cell-death-induction in two sleep-controlling nuclei: the laterodorsal tegmentum (LDT), and the pedunculopontine tegmentum (PPT), as well as the motor-controlling SN. METHODS: Size exclusion chromatography, Thioflavin T fluorescence assays, and circular dichroism spectroscopy were used to isolate structurally defined forms of recombinant, human α-synuclein. Neuronal and viability effects of characterized monomeric and fibril forms of α-synuclein were determined on LDT, PPT, and SN neurons using electrophysiology, calcium imaging, and neurotoxicity assays. RESULTS: In LDT and PPT neurons, both forms of α-synuclein induced excitation and increased calcium, and the monomeric form heightened putatively excitotoxic neuronal death, whereas, in the SN, we saw inhibition, decreased intracellular calcium, and monomeric α-synuclein was not associated with heightened cell death. CONCLUSION: Nucleus-specific differential effects suggest mechanistic underpinnings of SDs' prodromal appearance in PD. While speculative, we hypothesize that the monomeric form of α-synuclein compromises functionality of sleep-control neurons, leading to the presence of SDs decades prior to motor dysfunction.

Comparison of bNOS and chat immunohistochemistry in the laterodorsal tegmentum (LDT) and the pedunculopontine tegmentum (PPT) of the mouse from brain slices prepared for electrophysiology.

BACKGROUND: Identification of cell phenotype from brain slices upon which in vitro electrophysiological recordings have been performed often relies on conducting post hoc immunohistochemistry on tissue that necessarily has not been ideally prepared for immunohistochemical procedures. In such studies, antibody labeling against neuronal nitric oxide synthase (bNOS) has been used to identify cholinergic neurons of the laterodorsal tegmental nucleus (LDT) and the pedunculopontine tegmental nuclei (PPT), two brainstem nuclei importantly involved in arousal. However, a widespread perception maintains that antibody staining for enzymes involved in synthesis or transport, of acetylcholine would be a more definitive marker and hence, preferable. NEW METHOD: Colocalization of bNOS and CHAT in the LDT/PPT, and presence of parvalbumin (PV), was examined in non-ideally prepared mouse brain slices using currently available antibodies. RESULTS: Using fluorescent-based immunohistochemistry in LDT/PPT slices prepared for in vitro recordings, a near 100% colocalization of bNOS and CHAT was observed. COMPARISON WITH EXISTING METHOD: We confirm in the mouse, findings of near 100% colocalization of bNOS and CHAT in the LDT/PPT, and we expand upon data from rat studies using optimally prepared tissue, that for dendritic visualization, bNOS staining exceeded the quality of CHAT staining for visualization of a higher degree of detail of fine processes. PV is not highly present in the mouse LDT/PPT. CONCLUSION: CHAT and bNOS are equally useful target proteins for immunofluorescent identification of cholinergic LDT/PPT cells in mouse brain slices prepared for in vitro recordings, however, antibody targeting of bNOS allows for a superior appreciation of structural detail.

Pharmacological evidence of functional inhibitory metabotrophic glutamate receptors on mouse arousal-related cholinergic laterodorsal tegmental neurons.

Cholinergic neurons of the pontine laterodorsal tegmentum (LDT) are importantly involved in neurobiological mechanisms governing states of arousal such as sleep and wakefulness as well as other appetitive behaviors, such as drug-seeking. Accordingly, mechanisms controlling their excitability are important to elucidate if we are to understand how these LDT neurons generate arousal states. Glutamate mediates the vast majority of excitatory synaptic transmission in the vertebrate CNS and while presence of glutamate input in the LDT has been shown and ionotropic responses to glutamate have been reported in the LDT, characterization of metabotropic responses is lacking. Therefore, electrophysiological responses and changes in levels of intracellular Ca(2+) in mouse cholinergic LDT neurons following application of specific mGluR agonists and antagonists were examined. Unexpectedly, both the mGluR(5)specific agonist, CHPG, and the group II mGluR (mGlu(2/3)) agonist, LY379268 (LY), induced a TTX-insensitive outward current/hyperpolarization. Both outward currents were significantly reduced by the mGluR antagonist MCPG and the CHPG-induced current was blocked by the specific mGluR(5) antagonist MTEP. Concurrent Ca(2+)imaging revealed that while CHPG actions did include release of Ca(2+) from CPA/thapsigargin-sensitive intracellular stores, actions of LY did not. Both CHPG- and LY-induced outward currents were mediated by a TEA-sensitive potassium conductance. The large-conductance, Ca(2+)-dependent potassium (BK) channel blocker, iberiotoxin, attenuated CHPG actions. Consistent with actions on the BK conductance, CHPG enhanced the amplitude of the fast component of the after hyperpolarizing potential, concurrent with a reduction in the firing rate. We conclude that stimulation of mGluR(5) and group II (mGluR(2/3)) elicits postsynaptically-mediated outward currents/hyperpolarizations in cholinergic LDT neurons. Effects of glutamatergic input would be, thus, expected not only to be excitation via stimulation of ionotropic glutamate receptors and mGluR(1), but also inhibition via actions at mGluR(5) and mGluR(2/3) on these neurons. As these two processes counteract each other, these surprising findings necessitate revision of predictions regarding the net level of excitation generated by glutamate input to cholinergic LDT cells and, by extension, the functional outcome of glutamate transmission on processes which these neurons regulate. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Differential actions of orexin receptors in brainstem cholinergic and monoaminergic neurons revealed by receptor knockouts: implications for orexinergic signaling in arousal and narcolepsy.

Orexin neuropeptides influence multiple homeostatic functions and play an essential role in the expression of normal sleep-wake behavior. While their two known receptors (OX1 and OX2) are targets for novel pharmacotherapeutics, the actions mediated by each receptor remain largely unexplored. Using brain slices from mice constitutively lacking either receptor, we used whole-cell and Ca(2+) imaging methods to delineate the cellular actions of each receptor within cholinergic [laterodorsal tegmental nucleus (LDT)] and monoaminergic [dorsal raphe (DR) and locus coeruleus (LC)] brainstem nuclei-where orexins promote arousal and suppress REM sleep. In slices from OX(-/-) 2 mice, orexin-A (300 nM) elicited wild-type responses in LDT, DR, and LC neurons consisting of a depolarizing current and augmented voltage-dependent Ca(2+) transients. In slices from OX(-/-) 1 mice, the depolarizing current was absent in LDT and LC neurons and was attenuated in DR neurons, although Ca(2+)-transients were still augmented. Since orexin-A produced neither of these actions in slices lacking both receptors, our findings suggest that orexin-mediated depolarization is mediated by both receptors in DR, but is exclusively mediated by OX1 in LDT and LC neurons, even though OX2 is present and OX2 mRNA appears elevated in brainstems from OX(-/-) 1 mice. Considering published behavioral data, these findings support a model in which orexin-mediated excitation of mesopontine cholinergic and monoaminergic neurons contributes little to stabilizing spontaneous waking and sleep bouts, but functions in context-dependent arousal and helps restrict muscle atonia to REM sleep. The augmented Ca(2+) transients produced by both receptors appeared mediated by influx via L-type Ca(2+) channels, which is often linked to transcriptional signaling. This could provide an adaptive signal to compensate for receptor loss or prolonged antagonism and may contribute to the reduced severity of narcolepsy in single receptor knockout mice.

Rethinking the fear circuit: the central nucleus of the amygdala is required for the acquisition, consolidation, and expression of Pavlovian fear conditioning.

In the standard model of pavlovian fear learning, sensory input from neutral and aversive stimuli converge in the lateral nucleus of the amygdala (LA), in which alterations in synaptic transmission encode the association. During fear expression, the LA is thought to engage the central nucleus of the amygdala (CE), which serves as the principal output nucleus for the expression of conditioned fear responses. In the present study, we reexamined the roles of LA and CE. Specifically, we asked whether CE, like LA, might also be involved in fear learning and memory consolidation. Using functional inactivation methods, we first show that CE is involved not only in the expression but also the acquisition of fear conditioning. Next, we show that inhibition of protein synthesis in CE after training impairs fear memory consolidation. These findings indicate that CE is not only involved in fear expression but, like LA, is also involved in the learning and consolidation of pavlovian fear conditioning.

Disparate cholinergic currents in rat principal trigeminal sensory nucleus neurons mediated by M1 and M2 receptors: a possible mechanism for selective gating of afferent sensory neurotransmission.

Neurons situated in the principal sensory trigeminal nucleus (PSTN) convey orofacial sensory inputs to thalamic relay regions and higher brain centres, and the excitability of these ascending tract cells is modulated across sleep/wakefulness states and during pain conditions. Moreover, acetylcholine release changes profoundly across sleep/wakefulness states and ascending sensory neurotransmission is altered by cholinergic agonists. An intriguing possibility is, therefore, that cholinergic mechanisms mediate such state-dependent modulation of PSTN tract neurons. We tested the hypotheses that cholinergic agonists can modulate PSTN cell excitability and that such effects are mediated by muscarinic receptor subtypes, using patch-clamp methods in rat and mouse. In all examined cells, carbachol elicited an electrophysiological response that was independent of action potential generation as it persisted in the presence of tetrodotoxin. Responses were of three types: depolarization, hyperpolarization or a biphasic response consisting of hyperpolarization followed by depolarization. In voltage-clamp mode, carbachol evoked corresponding inward, outward or biphasic currents. Moreover, immunostaining for the vesicle-associated choline transporter showed cholinergic innervation of the PSTN. Using muscarinic receptor antagonists, we found that carbachol-elicited PSTN neuron hyperpolarization was mediated by M2 receptors and depolarization, in large part, by M1 receptors. These data suggest that acetylcholine acting on M1 and M2 receptors may contribute to selective excitability enhancement or depression in individual, rostrally projecting sensory neurons. Such selective gating effects via cholinergic input may play a functional role in modulation of ascending sensory transmission, including across behavioral states typified by distinct cholinergic tone, e.g. sleep/wakefulness arousal levels or neuropathic pain conditions.

Urotensin II modulates rapid eye movement sleep through activation of brainstem cholinergic neurons.

Urotensin II (UII) is a cyclic neuropeptide with strong vasoconstrictive activity in the peripheral vasculature. UII receptor mRNA is also expressed in the CNS, in particular in cholinergic neurons located in the mesopontine tegmental area, including the pedunculopontine tegmental (PPT) and lateral dorsal tegmental nuclei. This distribution suggests that the UII system is involved in functions regulated by acetylcholine, such as the sleep-wake cycle. Here, we tested the hypothesis that UII influences cholinergic PPT neuron activity and alters rapid eye movement (REM) sleep patterns in rats. Local administration of UII into the PPT nucleus increases REM sleep without inducing changes in the cortical blood flow. Intracerebroventricular injection of UII enhances both REM sleep and wakefulness and reduces slow-wave sleep 2. Intracerebroventricular, but not local, administration of UII increases cortical blood flow. Moreover, whole-cell recordings from rat-brain slices show that UII selectively excites cholinergic PPT neurons via an inward current and membrane depolarization that were accompanied by membrane conductance decreases. This effect does not depend on action potential generation or fast synaptic transmission because it persisted in the presence of TTX and antagonists of ionotropic glutamate, GABA, and glycine receptors. Collectively, these results suggest that UII plays a role in the regulation of REM sleep independently of its cerebrovascular actions by directly activating cholinergic brainstem neurons.

Activity changes of the cat paraventricular hypothalamus during stressor exposure.

Dorso-medial paraventricular hypothalamus (PVH) activity was assessed by light scattering procedures in freely behaving cats during auditory stressor exposure. Acoustic noise (> 95dB) raised plasma ACTH concentrations, somatic muscle tonus, respiratory frequency and cardiac rates; PVH activity peaked 0.8s following stimulation, and then markedly declined below baseline to a trough at 9.7s. Hypothalamic responses were not uniformly distributed across the recorded PVH field. Activity changes emerged from subregions within the visualized area, and were widespread at the overall activity zenith and nadir. Isolated pixels appeared opposite in activity pattern to overall changes. We suggest that transient activity increases represent initial PVH neural stress responses, and that subsequent profound declines result from neural inhibitory feedback.