Contributions of carotid bodies, retrotrapezoid nucleus neurons and preBötzinger complex astrocytes to the CO2-sensitive drive for breathing

Current models of respiratory CO2 chemosensitivity are centred around the function of a specific population of neurons residing in the medullary retrotrapezoid nucleus (RTN). However, there is significant evidence suggesting that chemosensitive neurons exist in other brainstem areas, including the rhythm-generating region of the medulla oblongata – the preBötzinger complex (preBötC). There is also evidence that astrocytes, non-neuronal brain cells, contribute to central CO2 chemosensitivity. In this study, we reevaluated the relative contributions of the RTN neurons, the preBötC astrocytes, and the carotid body chemoreceptors in mediating the respiratory responses to CO2 in experimental animals (adult laboratory rats). To block astroglial signalling via exocytotic release of transmitters, preBötC astrocytes were targeted to express the tetanus toxin light chain (TeLC). Bilateral expression of TeLC in preBötC astrocytes was associated with ∼20% and ∼30% reduction of the respiratory response to CO2 in conscious and anaesthetized animals, respectively. Carotid body denervation reduced the CO2 respiratory response by ∼25%. Bilateral inhibition of RTN neurons transduced to express Gi-coupled designer receptors exclusively activated by designer drug (DREADDGi) by application of clozapine-N-oxide reduced the CO2 response by ∼20% and ∼40% in conscious and anaesthetized rats, respectively. Combined blockade of astroglial signalling in the preBötC, inhibition of RTN neurons and carotid body denervation reduced the CO2-induced respiratory response by ∼70%. These data further support the hypothesis that the CO2-sensitive drive to breathe requires inputs from the peripheral chemoreceptors and several central chemoreceptor sites. At the preBötC level, astrocytes modulate the activity of the respiratory network in response to CO2, either by relaying chemosensory information (i.e. they act as CO2 sensors) or by enhancing the preBötC network excitability to chemosensory inputs

Excitatory Modulation of the preBötzinger Complex Inspiratory Rhythm Generating Network by Endogenous Hydrogen Sulfide

Hydrogen Sulfide (H2S) is one of three gasotransmitters that modulate excitability in the CNS. Global application of H2S donors or inhibitors of H2S synthesis to the respiratory network has suggested that inspiratory rhythm is modulated by exogenous and endogenous H2S. However, effects have been variable, which may reflect that the RTN/pFRG (retrotrapezoid nucleus, parafacial respiratory group) and the preBötzinger Complex (preBötC, critical for inspiratory rhythm generation) are differentially modulated by exogenous H2S. Importantly, site-specific modulation of respiratory nuclei by H2S means that targeted, rather than global, manipulation of respiratory nuclei is required to understand the role of H2S signaling in respiratory control. Thus, our aim was to test whether endogenous H2S, which is produced by cystathionine-β-synthase (CBS) in the CNS, acts specifically within the preBötC to modulate inspiratory activity under basal (in vitro/in vivo) and hypoxic conditions (in vivo). Inhibition of endogenous H2S production by bath application of the CBS inhibitor, aminooxyacetic acid (AOAA, 0.1–1.0 mM) to rhythmic brainstem spinal cord (BSSC) and medullary slice preparations from newborn rats, or local application of AOAA into the preBötC (slices only) caused a dose-dependent decrease in burst frequency. Unilateral injection of AOAA into the preBötC of anesthetized, paralyzed adult rats decreased basal inspiratory burst frequency, amplitude and ventilatory output. AOAA in vivo did not affect the initial hypoxia-induced (10% O2, 5 min) increase in ventilatory output, but enhanced the secondary hypoxic respiratory depression. These data suggest that the preBötC inspiratory network receives tonic excitatory modulation from the CBS-H2S system, and that endogenous H2S attenuates the secondary hypoxic respiratory depression

Noradrenergic Modulation of the Piriform Cortex: A Possible Avenue for Understanding Pre-Clinical Alzheimer’s Disease Pathogenesis

Olfactory dysfunction is one of the biomarkers for Alzheimer’s disease (AD) diagnosis and progression. Deficits with odor identification and discrimination are common symptoms of pre-clinical AD, preceding severe memory disorder observed in advanced stages. As a result, understanding mechanisms of olfactory impairment is a major focus in both human studies and animal models of AD. Pretangle tau, a precursor to tau tangles, is first observed in the locus coeruleus (LC). In a recent animal model, LC pretangle tau leads to LC fiber degeneration in the piriform cortex (PC), a cortical area associated with olfactory dysfunction in both human AD and rodent models. Here, we review the role of LC-sourced NE in modulation of PC activity and suggest mechanisms by which pretangle tau-mediated LC dysfunction may impact olfactory processing in preclinical stage of AD. Understanding mechanisms of early olfactory impairment in AD may provide a critical window for detection and intervention of disease progression

Tripartite signalling by NMDA receptors

Abstract N-methyl-d-aspartate receptors (NMDARs) are excitatory glutamatergic receptors that are fundamental for many neuronal processes, including synaptic plasticity. NMDARs are comprised of four subunits derived from heterogeneous subunit families, yielding a complex diversity in NMDAR form and function. The quadruply-liganded state of binding of two glutamate and two glycine molecules to the receptor drives channel gating, allowing for monovalent cation flux, Ca2+ entry and the initiation of Ca2+-dependent signalling. In addition to this ionotropic function, non-ionotropic signalling can be initiated through the exclusive binding of glycine or of glutamate to the NMDAR. This binding may trigger a transmembrane conformational change of the receptor, inducing intracellular protein-protein signalling between the cytoplasmic domain and secondary messengers. In this review, we outline signalling cascades that can be activated by NMDARs and propose that the receptor transduces signalling through three parallel streams: (i) signalling via both glycine and glutamate binding, (ii) signalling via glycine binding, and (iii) signalling via glutamate binding. This variety in signal transduction mechanisms and downstream signalling cascades complements the widespread prevalence and rich diversity of NMDAR activity throughout the central nervous system and in disease pathology

Modulation of L-type calcium channels in Alzheimer’s disease: A potential therapeutic target

Calcium plays a fundamental role in various signaling pathways and cellular processes in the human organism. In the nervous system, voltage-gated calcium channels such as L-type calcium channels (LTCCs) are critical elements in mediating neurotransmitter release, synaptic integration and plasticity. Dysfunction of LTCCs has been implicated in both aging and Alzheimer’s Disease (AD), constituting a key component of calcium hypothesis of AD. As such, LTCCs are a promising drug target in AD. However, due to their structural and functional complexity, the mechanisms by which LTCCs contribute to AD are still unclear. In this review, we briefly summarize the structure, function, and modulation of LTCCs that are the backbone for understanding pathological processes involving LTCCs. We suggest targeting molecular pathways up-regulating LTCCs in AD may be a more promising approach, given the diverse physiological functions of LTCCs and the ineffectiveness of LTCC blockers in clinical studies

Age-Dependent Contributions of NMDA Receptors and L-Type Calcium Channels to Long-Term Depression in the Piriform Cortex

In the hippocampus, the contributions of N-methyl-D-aspartate receptors (NMDARs) and L-type calcium channels (LTCCs) to neuronal transmission and synaptic plasticity change with aging, underlying calcium dysregulation and cognitive dysfunction. However, the relative contributions of NMDARs and LTCCs in other learning encoding structures during aging are not known. The piriform cortex (PC) plays a significant role in odor associative memories, and like the hippocampus, exhibits forms of long-term synaptic plasticity. Here, we investigated the expression and contribution of NMDARs and LTCCs in long-term depression (LTD) of the PC associational fiber pathway in three cohorts of Sprague Dawley rats: neonatal (1–2 weeks), young adult (2–3 months) and aged (20–25 months). Using a combination of slice electrophysiology, Western blotting, fluorescent immunohistochemistry and confocal imaging, we observed a shift from an NMDAR to LTCC mediation of LTD in aged rats, despite no difference in the amount of LTD expression. These changes in plasticity are related to age-dependent differential receptor expression in the PC. LTCC Cav1.2 expression relative to postsynaptic density protein 95 is increased in the associational pathway of the aged PC layer Ib. Enhanced LTCC contribution in synaptic depression in the PC may contribute to altered olfactory function and learning with aging

Hippocampal hyperphosphorylated tau-induced deficiency is rescued by L-type calcium channel blockade

Aging and Alzheimer's disease are associated with chronic elevations in neuronal calcium influx via L-type calcium channels. The hippocampus, a primary memory encoding structure in the brain, is more vulnerable to calcium dysregulation in Alzheimer's disease. Recent research has suggested a link between L-type calcium channels and tau hyperphosphorylation. However, the precise mechanism of L-type calcium channel-mediated tau toxicity is not understood. In this study, we seeded a human tau pseudophosphorylated at 14 amino acid sites in rat hippocampal cornu ammonis 1 region to mimic soluble pretangle tau. Impaired spatial learning was observed in human tau pseudophosphorylated at 14 amino acid sites-infused rats as early as 1-3 months and worsened at 9-10 months post-infusion. Rats infused with wild-type human tau exhibited milder behavioural deficiency only at 9-10 months post-infusion. No tangles or plaques were observed in all time points examined in both human tau pseudophosphorylated at 14 amino acid sites and human tau-infused brains. However, human tau pseudophosphorylated at 14 amino acid sites-infused hippocampus exhibited a higher amount of tau phosphorylation at S262 and S356 than the human tau-infused rats at 3 months post-infusion, paralleling the behavioural deficiency observed in human tau pseudophosphorylated at 14 amino acid sites-infused rats. Neuroinflammation indexed by increased Iba1 in the cornu ammonis 1 was observed in human tau pseudophosphorylated at 14 amino acid sites-infused rats at 1-3 but not 9 months post-infusion. Spatial learning deficiency in human tau pseudophosphorylated at 14 amino acid sites-infused rats at 1-3 months post-infusion was paralleled by decreased neuronal excitability, impaired NMDA receptor-dependent long-term potentiation and augmented L-type calcium channel-dependent long-term potentiation at the cornu ammonis 1 synapses. L-type calcium channel expression was elevated in the soma of the cornu ammonis 1 neurons in human tau pseudophosphorylated at 14 amino acid sites-infused rats. Chronic L-type calcium channel blockade with nimodipine injections for 6 weeks normalized neuronal excitability and synaptic plasticity and rescued spatial learning deficiency in human tau pseudophosphorylated at 14 amino acid sites-infused rats. The early onset of L-type calcium channel-mediated pretangle tau pathology and rectification by nimodipine in our model have significant implications for preclinical Alzheimer's disease prevention and intervention

Aging differentially affects LTCC function in hippocampal CA1 and piriform cortex pyramidal neurons.

Aging is associated with cognitive decline and memory loss in humans. In rats, aging-associated neuronal excitability changes and impairments in learning have been extensively studied in the hippocampus. Here, we investigated the roles of L-type calcium channels (LTCCs) in the rat piriform cortex (PC), in comparison with those of the hippocampus. We employed spatial and olfactory tasks that involve the hippocampus and PC. LTCC blocker nimodipine administration impaired spontaneous location recognition in adult rats (6-9 months). However, the same blocker rescued the spatial learning deficiency in aged rats (19-23 months). In an odor-associative learning task, infusions of nimodipine into either the PC or dorsal CA1 impaired the ability of adult rats to learn a positive odor association. Again, in contrast, nimodipine rescued odor associative learning in aged rats. Aged CA1 neurons had higher somatic expression of LTCC Cav1.2 subunits, exhibited larger afterhyperpolarization (AHP) and lower excitability compared with adult neurons. In contrast, PC neurons from aged rats showed higher excitability and no difference in AHP. Cav1.2 expression was similar in adult and aged PC somata, but relatively higher in PSD95- puncta in aged dendrites. Our data suggest unique features of aging-associated changes in LTCCs in the PC and hippocampus