Phagocytosis of aggrecan-positive perineuronal nets surrounding motor neurons by reactive microglia expressing MMP-9 in TDP-43Q331K ALS model mice.

Perineuronal nets (PNNs) are extracellular matrix structures that surround excitable neurons and their proximal dendrites. PNNs play an important role in neuroprotection against oxidative stress. Oxidative stress within motor neurons can act as a trigger for neuronal death, and this has been implicated in motor neuron degeneration in amyotrophic lateral sclerosis (ALS). We therefore characterised PNNs around alpha motor neurons and the possible contributing cellular factors in the mutant TDP-43Q331K transgenic mouse, a slow onset ALS mouse model. PNNs around alpha motor neurons showed significant loss at mid-stage disease in TDP-43Q331K mice compared to wild type strain control mice. PNN loss coincided with an increased expression of matrix metallopeptidase-9 (MMP-9), an endopeptidase known to cleave PNNs, within the ventral horn. During mid-stage disease, increased numbers of microglia and astrocytes expressing MMP-9 were present in the ventral horn of TDP-43Q331K mice. In addition, TDP-43Q331K mice showed increased levels of aggrecan, a PNN component, in the ventral horn by microglia and astrocytes during this period. Elevated aggrecan levels within glia were accompanied by an increase in fractalkine expression, a chemotaxic protein responsible for the recruitment of microglia, in alpha motor neurons of onset and mid-stage TDP-43Q331K mice. Following PNN loss, alpha motor neurons in mid-stage TDP-43Q331K mice showed increased 3-nitrotyrosine expression, an indicator of protein oxidation. Together, our observations along with previous PNN research provide suggests a possible model whereby microglia and astrocytes expressing MMP-9 degrade PNNs surrounding alpha motor neurons in the TDP-43Q331K mouse. This loss of nets may expose alpha-motor neurons to oxidative damage leading to degeneration of the alpha motor neurons in the TDP-43Q331K ALS mouse model.

Perineuronal nets are phagocytosed by MMP-9 expressing microglia and astrocytes in the SOD1G93A ALS mouse model.

AIMS: Perineuronal nets (PNNs) are an extracellular matrix structure that encases excitable neurons. PNNs play a role in neuroprotection against oxidative stress. Oxidative stress within motor neurons can trigger neuronal death, which has been implicated in amyotrophic lateral sclerosis (ALS). We investigated the spatio-temporal timeline of PNN breakdown and the contributing cellular factors in the SOD1G93A strain, a fast-onset ALS mouse model. METHODS: This was conducted at the presymptomatic (P30), onset (P70), mid-stage (P130), and end-stage disease (P150) using immunofluorescent microscopy, as this characterisation has not been conducted in the SOD1G93A strain. RESULTS: We observed a significant breakdown of PNNs around α-motor neurons in the ventral horn of onset and mid-stage disease SOD1G93A mice compared with wild-type controls. This was observed with increased numbers of microglia expressing matrix metallopeptidase-9 (MMP-9), an endopeptidase that degrades PNNs. Microglia also engulfed PNN components in the SOD1G93A mouse. Further increases in microglia and astrocyte number, MMP-9 expression, and engulfment of PNN components by glia were observed in mid-stage SOD1G93A mice. This was observed with increased expression of fractalkine, a signal for microglia engulfment, within α-motor neurons of SOD1G93A mice. Following PNN breakdown, α-motor neurons of onset and mid-stage SOD1G93A mice showed increased expression of 3-nitrotyrosine, a marker for protein oxidation, which could render them vulnerable to death. CONCLUSIONS: Our observations suggest that increased numbers of MMP-9 expressing glia and their subsequent engulfment of PNNs around α-motor neurons render these neurons sensitive to oxidative damage and eventual death in the SOD1G93A ALS model mouse.

Investigating the Role of GABA in Neural Development and Disease Using Mice Lacking GAD67 or VGAT Genes.

Normal development and function of the central nervous system involves a balance between excitatory and inhibitory neurotransmission. Activity of both excitatory and inhibitory neurons is modulated by inhibitory signalling of the GABAergic and glycinergic systems. Mechanisms that regulate formation, maturation, refinement, and maintenance of inhibitory synapses are established in early life. Deviations from ideal excitatory and inhibitory balance, such as down-regulated inhibition, are linked with many neurological diseases, including epilepsy, schizophrenia, anxiety, and autism spectrum disorders. In the mammalian forebrain, GABA is the primary inhibitory neurotransmitter, binding to GABA receptors, opening chloride channels and hyperpolarizing the cell. We review the involvement of down-regulated inhibitory signalling in neurological disorders, possible mechanisms for disease progression, and targets for therapeutic intervention. We conclude that transgenic models of disrupted inhibitory signalling-in GAD67+/- and VGAT-/- mice-are useful for investigating the effects of down-regulated inhibitory signalling in a range of neurological diseases.

Capsaicin causes robust reduction in glycinergic transmission to rat hypoglossal motor neurons via a TRPV1-independent mechanism.

The effect of capsaicin on glycinergic synaptic transmission to juvenile rat hypoglossal motor neurons in acute brainstem slices was evaluated in the presence of TTX. Capsaicin caused a robust decrease in miniature IPSC frequency, amplitude, and half-width, showing that this effect is independent of action potential generation. In the presence of capsazepine, a classic TRPV1 antagonist, capsaicin was still able to reduce spontaneous inhibitory postsynaptic current (IPSC) amplitude and frequency. We further investigated whether the effect of capsaicin on glycinergic transmission to hypoglossal motor neurons is pre- or postsynaptic in nature by recording pairs of evoked IPSCs. Interestingly, capsaicin also reduced evoked IPSC amplitude without affecting paired-pulse ratio, indicating a postsynaptic mechanism of action. Significant reduction was also observed in evoked IPSC half-width, rise time, and decay tau. We also show that capsaicin does not have any effect on either transient (It) or sustained (Is) potassium currents. Finally, we also show that the hyperpolarization-activated cationic current (Ih) also remains unchanged after capsaicin application. NEW & NOTEWORTHY Capsaicin reduces the amplitude of quantal and evoked glycinergic inhibitory neurotransmission to brainstem motor neurons without altering activity-dependent transmitter release. This effect of capsaicin is not due to activation of TRPV1 receptors, as it is not blocked by capsazepine, a TRPV1 receptor antagonist. Capsaicin does not alter voltage-dependent potassium current or the hyperpolarization-activated cationic current in brainstem motor neurons.

A rat model of ataxia-telangiectasia: evidence for a neurodegenerative phenotype.

Ataxia-telangiectasia (A-T), an autosomal recessive disease caused by mutations in the ATM gene is characterised by cerebellar atrophy and progressive neurodegeneration which has been poorly recapitulated in Atm mutant mice. Consequently, pathways leading to neurodegeneration in A-T are poorly understood. We describe here the generation of an Atm knockout rat model that does not display cerebellar atrophy but instead paralysis and spinal cord atrophy, reminiscent of that seen in older patients and milder forms of the disorder. Loss of Atm in neurons and glia leads to accumulation of cytosolic DNA, increased cytokine production and constitutive activation of microglia consistent with a neuroinflammatory phenotype. Rats lacking ATM had significant loss of motor neurons and microgliosis in the spinal cord, consistent with onset of paralysis. Since short term treatment with steroids has been shown to improve the neurological signs in A-T patients we determined if that was also the case for Atm-deficient rats. Betamethasone treatment extended the lifespan of Atm knockout rats, prevented microglial activation and significantly decreased neuroinflammatory changes and motor neuron loss. These results point to unrepaired damage to DNA leading to significant levels of cytosolic DNA in Atm-deficient neurons and microglia and as a consequence activation of the cGAS-STING pathway and cytokine production. This in turn would increase the inflammatory microenvironment leading to dysfunction and death of neurons. Thus the rat model represents a suitable one for studying neurodegeneration in A-T and adds support for the use of anti-inflammatory drugs for the treatment of neurodegeneration in A-T patients.

Glycinergic Neurotransmission: A Potent Regulator of Embryonic Motor Neuron Dendritic Morphology and Synaptic Plasticity.

Emerging evidence suggests that central synaptic inputs onto motor neurons (MNs) play an important role in developmental regulation of the final number of MNs and their muscle innervation for a particular motor pool. Here, we describe the effect of genetic deletion of glycinergic neurotransmission on single MN structure and on functional excitatory and inhibitory inputs to MNs. We measured synaptic currents in E18.5 hypoglossal MNs from brain slices using whole-cell patch-clamp recording, followed by dye-filling these same cells with Neurobiotin, to define their morphology by high-resolution confocal imaging and 3D reconstruction. We show that hypoglossal MNs of mice lacking gephyrin display increased dendritic arbor length and branching, increased spiny processes, decreased inhibitory neurotransmission, and increased excitatory neurotransmission. These findings suggest that central glycinergic synaptic activity plays a vital role in regulating MN morphology and glutamatergic central synaptic inputs during late embryonic development. SIGNIFICANCE STATEMENT: MNs within the brainstem and spinal cord are responsible for integrating a diverse array of synaptic inputs into discrete contractions of skeletal muscle to achieve coordinated behaviors, such as breathing, vocalization, and locomotion. The last trimester in utero is critical in neuromotor development, as this is when central and peripheral synaptic connections are made onto and from MNs. At this time-point, using transgenic mice with negligible glycinergic postsynaptic responses, we show that this deficiency leads to abnormally high excitatory neurotransmission and alters the dendritic architecture responsible for coherently integrating these inputs. This study compliments the emerging concept that neurodevelopmental disorders (including autism, epilepsy, and amyotrophic lateral sclerosis) are underpinned by synaptic dysfunction and therefore will be useful to neuroscientists and neurologists alike.

The antihypertensive drug pindolol attenuates long-term but not short-term binge-like ethanol consumption in mice.

Alcohol dependence is a debilitating disorder with current therapies displaying limited efficacy and/or compliance. Consequently, there is a critical need for improved pharmacotherapeutic strategies to manage alcohol use disorders (AUDs). Previous studies have shown that the development of alcohol dependence involves repeated cycles of binge-like ethanol intake and abstinence. Therefore, we used a model of binge-ethanol consumption (drinking-in-the-dark) in mice to test the effects of compounds known to modify the activity of neurotransmitters implicated in alcohol addiction. From this, we have identified the FDA-approved antihypertensive drug pindolol, as a potential candidate for the management of AUDs. We show that the efficacy of pindolol to reduce ethanol consumption is enhanced following long-term (12 weeks) binge-ethanol intake, compared with short-term (4 weeks) intake. Furthermore, pindolol had no effect on locomotor activity or consumption of the natural reward sucrose. Because pindolol acts as a dual beta-adrenergic antagonist and 5-HT1A/1B partial agonist, we examined its effect on spontaneous synaptic activity in the basolateral amygdala (BLA), a brain region densely innervated by serotonin and norepinephrine-containing fibres. Pindolol increased spontaneous excitatory post-synaptic current frequency of BLA principal neurons from long-term ethanol-consuming mice but not naïve mice. Additionally, this effect was blocked by the 5-HT1A/1B receptor antagonist methiothepin, suggesting that altered serotonergic activity in the BLA may contribute to the efficacy of pindolol to reduce ethanol intake following long-term exposure. Although further mechanistic investigations are required, this study demonstrates the potential of pindolol as a new treatment option for AUDs that can be fast-tracked into human clinical studies.

Motor cortex layer V pyramidal neurons exhibit dendritic regression, spine loss, and increased synaptic excitation in the presymptomatic hSOD1(G93A) mouse model of amyotrophic lateral sclerosis.

Motor cortex layer V pyramidal neurons (LVPNs) regulate voluntary control of motor output and selectively degenerate (along with lower motor neurons) in amyotrophic lateral sclerosis. Using dye-filling and whole-cell patch clamping in brain slices, together with high-resolution spinning disk confocal z-stack mosaics, we characterized the earliest presymptomatic cortical LVPN morphologic and electrophysiological perturbations in hSOD1(G93A) (SOD1) mice to date. Apical dendritic regression occurred from postnatal day (P) 28, dendritic spine loss from P21, and increased EPSC frequency from P21 in SOD1 LVPNs. These findings demonstrate extensive early changes in motor cortex of the SOD1 mouse model, which thus recapitulates clinically relevant cortical pathophysiology more faithfully than previously thought.

Emerging Roles of Filopodia and Dendritic Spines in Motoneuron Plasticity during Development and Disease.

Motoneurons develop extensive dendritic trees for receiving excitatory and inhibitory synaptic inputs to perform a variety of complex motor tasks. At birth, the somatodendritic domains of mouse hypoglossal and lumbar motoneurons have dense filopodia and spines. Consistent with Vaughn's synaptotropic hypothesis, we propose a developmental unified-hybrid model implicating filopodia in motoneuron spinogenesis/synaptogenesis and dendritic growth and branching critical for circuit formation and synaptic plasticity at embryonic/prenatal/neonatal period. Filopodia density decreases and spine density initially increases until postnatal day 15 (P15) and then decreases by P30. Spine distribution shifts towards the distal dendrites, and spines become shorter (stubby), coinciding with decreases in frequency and increases in amplitude of excitatory postsynaptic currents with maturation. In transgenic mice, either overexpressing the mutated human Cu/Zn-superoxide dismutase (hSOD1(G93A)) gene or deficient in GABAergic/glycinergic synaptic transmission (gephyrin, GAD-67, or VGAT gene knockout), hypoglossal motoneurons develop excitatory glutamatergic synaptic hyperactivity. Functional synaptic hyperactivity is associated with increased dendritic growth, branching, and increased spine and filopodia density, involving actin-based cytoskeletal and structural remodelling. Energy-dependent ionic pumps that maintain intracellular sodium/calcium homeostasis are chronically challenged by activity and selectively overwhelmed by hyperactivity which eventually causes sustained membrane depolarization leading to excitotoxicity, activating microglia to phagocytose degenerating neurons under neuropathological conditions.

Structural and functional characterization of dendritic arbors and GABAergic synaptic inputs on interneurons and principal cells in the rat basolateral amygdala.

The basolateral amygdala (BLA) is a complex brain region associated with processing emotional states, such as fear, anxiety, and stress. Some aspects of these emotional states are driven by the network activity of synaptic connections, derived from both local circuitry and projections to the BLA from other regions. Although the synaptic physiology and general morphological characteristics are known for many individual cell types within the BLA, the combination of morphological, electrophysiological, and distribution of neurochemical GABAergic synapses in a three-dimensional neuronal arbor has not been reported for single neurons from this region. The aim of this study was to assess differences in morphological characteristics of BLA principal cells and interneurons, quantify the distribution of GABAergic neurochemical synapses within the entire neuronal arbor of each cell type, and determine whether GABAergic synaptic density correlates with electrophysiological recordings of inhibitory postsynaptic currents. We show that BLA principal neurons form complex dendritic arborizations, with proximal dendrites having fewer spines but higher densities of neurochemical GABAergic synapses compared with distal dendrites. Furthermore, we found that BLA interneurons exhibited reduced dendritic arbor lengths and spine densities but had significantly higher densities of putative GABAergic synapses compared with principal cells, which was correlated with an increased frequency of spontaneous inhibitory postsynaptic currents. The quantification of GABAergic connectivity, in combination with morphological and electrophysiological measurements of the BLA cell types, is the first step toward a greater understanding of how fear and stress lead to changes in morphology, local connectivity, and/or synaptic reorganization of the BLA.

Timeline of hypoglossal motor neuron death and intrinsic tongue muscle denervation in high-copy number SOD1G93A mice.

In amyotrophic lateral sclerosis (ALS) postmortem tissue and the SOD1 mouse model at mid-disease, death of hypoglossal motor neurons (XII MNs) is evident. These XII MNs innervate the intrinsic and extrinsic tongue muscles, and despite their importance in many oral and lingual motor behaviours that are affected by ALS (e.g., swallowing, speech, and respiratory functions), little is known about the timing and extent of tongue muscle denervation. Here in the well-characterised SOD1G93A (high-copy) mouse model, we evaluated XII MN numbers and intrinsic tongue muscle innervation using standard histopathological approaches, which included stereological evaluation of Nissl-stained brainstem, and the presynaptic and postsynaptic evaluation of neuromuscular junctions (NMJs), using synapsin, neurofilament, and α-bungarotoxin immunolabelling, at presymptomatic, onset, mid-disease, and endstage timepoints. We found that reduction in XII MN size at onset preceded reduced XII MN survival, while the denervation of tongue muscle did not appear until the endstage. Our study suggests that denervation-induced weakness may not be the most pertinent feature of orolingual deficits in ALS. Efforts to preserve oral and respiratory functions of XII MNs are incredibly important if we are to influence patient outcomes.

Pre- and postsynaptic mechanisms underlying inhibition of hypoglossal motor neuron excitability by riluzole.

Riluzole is the sole treatment for amyotrophic lateral sclerosis (ALS), but its therapeutically relevant actions on motor neurons are not well defined. Whole cell patch-clamp recordings were made from hypoglossal motor neurons (HMs, n = 25) in brain stem slices from 10- to 23-day-old rats anesthetized with pentobarbital sodium to investigate the hypothesis that riluzole inhibits HMs by multiple mechanisms. Riluzole (20 µM) hyperpolarized HMs by decreasing an inward current, inhibited voltage-gated persistent Na(+) and Ca(2+) currents activated by slow voltage ramps, and negatively shifted activation of the hyperpolarization-activated cationic current (IH). Repetitive firing of HMs was strongly inhibited by riluzole, which also increased action potential threshold voltage and rheobase and decreased amplitude and maximum rise slope but did not alter the maximal afterhyperpolarization amplitude or decay time constant. HM rheobase was inversely correlated with persistent Na(+) current density. Glutamatergic synaptic transmission was inhibited by riluzole by both pre- and postsynaptic effects. Riluzole decreased activity-dependent glutamate release, as shown by decreased amplitude of evoked and spontaneous excitatory postsynaptic currents (EPSCs), decreased paired-pulse ratio, and decreased spontaneous, but not miniature, EPSC frequency. However, riluzole also decreased miniature EPSC amplitude and the inward current evoked by local application of glutamate onto HMs, suggesting a reduction of postsynaptic glutamate receptor sensitivity. Riluzole thus has a marked inhibitory effect on HM activity by membrane hyperpolarization, decreasing firing and inhibiting glutamatergic excitation by both pre- and postsynaptic mechanisms. These results broaden the range of mechanisms controlling motor neuron inhibition by riluzole and are relevant to researchers and clinicians interested in understanding ALS pathogenesis and treatment.

Complement drives circuit modulation in the adult brain.

Once widely considered an immune-privileged organ, the brain is now known to be intimately intertwined with immune-system activation. In particular, the complement system, an enzymatic cascade conferring innate immunity, has crucial functions for several neurodevelopmental and neuromigratory mechanisms. Recent advances have demonstrated the neurological importance of complement activation in the adult brain, whereby phagocytosis of weakened synapses biologically encodes "forgetting" of information through complement activation. Neurophysiologically, complement factors can also influence the brain's computational processes, increasing neuronal calcium influx and neurotransmitter release and altering synaptic strength. The complement system's effects on synaptic connectivity can also be observed in many pathological conditions including epilepsy, schizophrenia, and viral-induced cognitive deficits, where perturbations of complement-stimulated synaptic remodelling lead to severe dysfunction. In this review we provide an overview of current knowledge for complement in neurodevelopment, and examine recent evidence highlighting a critical physiological role of complement in the plasticity of the adult brain. This is especially relevant due to the explosion of complement-targeted therapeutics in clinical trials to treat neurological disorders.

Cholinergic basal forebrain degeneration due to sleep-disordered breathing exacerbates pathology in a mouse model of Alzheimer's disease.

Although epidemiological studies indicate that sleep-disordered breathing (SDB) such as obstructive sleep apnea is a strong risk factor for the development of Alzheimer's disease (AD), the mechanisms of the risk remain unclear. Here we developed a method of modeling SDB in mice that replicates key features of the human condition: altered breathing during sleep, sleep disruption, moderate hypoxemia, and cognitive impairment. When we induced SDB in a familial AD model, the mice displayed exacerbation of cognitive impairment and the pathological features of AD, including increased levels of amyloid-beta and inflammatory markers, as well as selective degeneration of cholinergic basal forebrain neurons. These pathological features were not induced by chronic hypoxia or sleep disruption alone. Our results also revealed that the cholinergic neurodegeneration was mediated by the accumulation of nuclear hypoxia inducible factor 1 alpha. Furthermore, restoring blood oxygen levels during sleep to prevent hypoxia prevented the pathological changes induced by the SDB. These findings suggest a signaling mechanism whereby SDB induces cholinergic basal forebrain degeneration.

Functional divergence of the two Elongator subcomplexes during neurodevelopment.

The highly conserved Elongator complex is a translational regulator that plays a critical role in neurodevelopment, neurological diseases, and brain tumors. Numerous clinically relevant variants have been reported in the catalytic Elp123 subcomplex, while no missense mutations in the accessory subcomplex Elp456 have been described. Here, we identify ELP4 and ELP6 variants in patients with developmental delay, epilepsy, intellectual disability, and motor dysfunction. We determine the structures of human and murine Elp456 subcomplexes and locate the mutated residues. We show that patient-derived mutations in Elp456 affect the tRNA modification activity of Elongator in vitro as well as in human and murine cells. Modeling the pathogenic variants in mice recapitulates the clinical features of the patients and reveals neuropathology that differs from the one caused by previously characterized Elp123 mutations. Our study demonstrates a direct correlation between Elp4 and Elp6 mutations, reduced Elongator activity, and neurological defects. Foremost, our data indicate previously unrecognized differences of the Elp123 and Elp456 subcomplexes for individual tRNA species, in different cell types and in different key steps during the neurodevelopment of higher organisms.

Penetratin peptide potentiates endogenous calcium-activated chloride currents in Xenopus oocytes.

Calcium-activated chloride currents (CaCCs) are required for epithelial electrolyte and fluid secretion, fertilization, sensory transduction and excitability of neurons and smooth muscle. Defolliculated Xenopus oocytes express a robust CaCC formed by a heterologous group of proteins including transmembrane protein 16A (TMEM16A) and bestrophins. Penetratin, a 17-amino acid peptide, potentiated endogenous oocyte CaCCs by ~50-fold at 10 µM, recorded using a two-electrode voltage clamp. CaCC potentiation was rapid and dose-dependent (EC50=3.2 µM). Penetratin-potentiated currents reversed at -18 mV and were dependent on the extracellular divalent cations present, showing positive regulation by Ca2+ and Mg2+ but effective block by Zn2+ (IC50=5.9 µM). Extracellular Cd2+, Cu2+ and Ba2+ resulted in bimodal responses: CaCC inhibition at low but potentiation at high concentrations. Intracellular BAPTA injection, which prevents activation of CaCCs, and the Cl- channel blockers niflumic acid and DIDS significantly reduced potentiation. In contrast, the K+ channel blockers Cs+, TEA, tertiapin-Q and halothane had no significant effect. This pharmacological profile is consistent with penetratin potentiation of zinc-sensitive CaCCs that are activated by influx of extracellular Ca2+. These findings may stimulate basic research on CaCCs in native cells and may lead to development of novel therapeutics targeting disorders caused by insufficient chloride secretion.

Neurophysiological Mechanisms Underlying Cortical Hyper-Excitability in Amyotrophic Lateral Sclerosis: A Review.

Amyotrophic lateral sclerosis (ALS) is a progressive neuromotor disease characterized by the loss of upper and lower motor neurons (MNs), resulting in muscle paralysis and death. Early cortical hyper-excitability is a common pathological process observed clinically and in animal disease models. Although the mechanisms that underlie cortical hyper-excitability are not completely understood, the molecular and cellular mechanisms that cause enhanced neuronal intrinsic excitability and changes in excitatory and inhibitory synaptic activity are starting to emerge. Here, we review the evidence for an anterograde glutamatergic excitotoxic process, leading to cortical hyper-excitability via intrinsic cellular and synaptic mechanisms and for the role of interneurons in establishing disinhibition in clinical and experimental settings. Understanding the mechanisms that lead to these complex pathological processes will likely produce key insights towards developing novel therapeutic strategies to rescue upper MNs, thus alleviating the impact of this fatal disease.

Reduced Inhibitory Inputs On Basolateral Amygdala Principal Neurons Following Long-Term Alcohol Consumption.

Recent studies have shown that manipulating basolateral amygdala (BLA) activity can affect alcohol consumption, particularly following chronic and/or long-term intake. Although the mechanisms underlying these effects remain unclear, the BLA is highly sensitive to emotional stimuli including stress and anxiety. Negative emotional states facilitate alcohol craving and relapse in patients with alcohol use disorders. Consequently, the aim of this study was to determine the effect of long-term (10 weeks) alcohol drinking on synaptic activity in BLA principal neurons. We utilized an intermittent drinking paradigm in rats, which facilitated escalating, binge-like alcohol intake over the 10 week drinking period. We then recorded spontaneous excitatory and inhibitory postsynaptic currents of BLA principal neurons from long-term alcohol drinking rats and aged-matched water drinking controls. Excitatory postsynaptic current properties from long-term alcohol drinking rats were unchanged compared to those from age-matched water drinking controls. Conversely, we observed significant reductions of inhibitory postsynaptic current amplitude and frequency in long-term ethanol drinking rats compared to age-matched water drinking controls. These results highlight substantive decreases in basal inhibitory synaptic activity of BLA principal neurons following long-term alcohol consumption. A loss of inhibitory control in the BLA could explain the high incidence of compulsive drinking and stress- or anxiety-induced relapse in patients with alcohol use disorders.

Size-dependent dendritic maladaptations of hypoglossal motor neurons in SOD1G93A mice.

The total motor neuron (MN) somato-dendritic surface area is correlated with motor unit type. MNs with smaller surface areas innervate slow (S) and fast fatigue-resistant (FR) motor units, while MNs with larger surface areas innervate fast fatigue-intermediate (FInt) and fast fatigable (FF) motor units. Differences in MN surface area (equivalent to membrane capacitance) underpin the intrinsic excitability of MNs and are consistent with the orderly recruitment of motor units (S > FR > FInt > FF) via the Size Principle. In amyotrophic lateral sclerosis (ALS), large MNs controlling FInt and FF motor units exhibit earlier denervation and death, compared to smaller and more resilient MNs of type S and FR motor units that are spared until late in ALS. Abnormal dendritic morphologies in MNs precede neuronal death in human ALS and in rodent models. We employed Golgi-Cox methods to investigate somal size-dependent changes in the dendritic morphology of hypoglossal MNs in wildtype and SOD1G93A mice (a model of ALS), at postnatal (P) day ~30 (pre-symptomatic), ~P60 (onset), and ~P120 (mid-disease) stages. In wildtype hypoglossal MNs, increased MN somal size correlated with increased dendritic length and spines in a linear fashion. By contrast, in SOD1G93A mice, significant deviations from this linear correlation were restricted to the larger vulnerable MNs at pre-symptomatic (maladaptive) and mid-disease (degenerative) stages. These findings are consistent with excitability changes observed in ALS patients and in rodent models. Our results suggest that intrinsic or synaptic increases in MN excitability are likely to contribute to ALS pathogenesis, not compensate for it.

Size-Dependent Vulnerability of Lumbar Motor Neuron Dendritic Degeneration in SOD1G93A Mice.

The motor neuron (MN) soma surface area is correlated with motor unit type. Larger MNs innervate fast fatigue-intermediate (FInt) or fast-fatiguable (FF) muscle fibers in type FInt and FF motor units, respectively. Smaller MNs innervate slow-twitch fatigue-resistant (S) or fast fatigue-resistant (FR) muscle fibers in type S and FR motor units, respectively. In amyotrophic lateral sclerosis (ALS), FInt and FF motor units are more vulnerable, with denervation and MN death occurring for these units before the more resilient S and FR units. Abnormal MN dendritic arbors have been observed in ALS in humans and rodent models. We used a Golgi-Cox impregnation protocol to examine soma size-dependent changes in the dendritic morphology of lumbar MNs in SOD1G93A mice, a model of ALS, at pre-symptomatic, onset and mid-disease stages. In wildtype control mice, the relationship between MN soma surface area and dendritic length or dendritic spine number was highly linear (i.e., increased MN soma size correlated with increased dendritic length and spines). By contrast, in SOD1G93A mice, this linear relationship was lost and dendritic length reduction and spine loss were observed in larger MNs, from pre-symptomatic stages onward. These changes correlated with the neuromotor symptoms of ALS in rodent models. At presymptomatic ages, changes were restricted to the larger MNs, likely to comprise vulnerable FInt and FF motor units. Our results suggest morphological changes of MN dendrites and dendritic spines are likely to contribute ALS pathogenesis, not compensate for it. Anat Rec, 303:1455-1471, 2020. © 2019 American Association for Anatomy.