GAD65 deficient mice are susceptible to ethanol-induced impairment of motor coordination and facilitation of cerebellar neuronal firing

γ-aminobutyric acid (GABA) is a major inhibitory neurotransmitter and its concentrations in the brain could be associated with EtOH-induced impairment of motor coordination. GABA is synthesized by two isoforms of glutamate decarboxylase (GAD): GAD65 and GAD67. Mice deficient in GAD65 (GAD65-KO) can grow up to adulthood, and show that GABA concentration in their adult brains was 50-75% that of wild-type C57BL/6 mice (WT). Although a previous study showed that there was no difference in recovery from the motor-incoordination effect of acute intraperitoneally administered injections of 2.0 g/kg EtOH between WT and GAD65-KO, the sensitivity of GAD65-KO to acute EtOH-induced ataxia has not been fully understood. Here, we sought to determine whether motor coordination and spontaneous firing of cerebellar Purkinje cells (PCs) in GAD65-KO are more sensitive to the effect of EtOH than in WT. Motor performance in WT and GAD65-KO was examined by rotarod and open-field tests following acute administration of EtOH at lower-doses, 0.8, 1.2 and 1.6 g/kg. In a rotarod test, there was no significant difference between WT and GAD65-KO in terms of baseline motor coordination. However, only the KO mice showed a significant decrease in rotarod performance of 1.2 g/kg EtOH. In the open-field test, GAD65-KO showed a significant increase in locomotor activity after 1.2 and 1.6 g/kg EtOH injections, but not WT. In in vitro studies of cerebellar slices, the firing rate of PCs was increased by 50 mM EtOH in GAD65-KO compared with WT, whereas no difference was observed in the effect of EtOH at more than 100 mM between the genotypes. Taken together, GAD65-KO are more susceptible to the effect of acute EtOH exposure on motor coordination and PC firing than WT. This different sensitivity could be attributed to the basal low GABA concentration in the brain of GAD65-KO.

Benzodiazepine-dependent stabilization of GABA(A) receptors at synapses

GABA(A) receptors constitutively enter and exit synapses by lateral diffusion in the plane of the neuronal membrane. They are trapped at synapses through their interactions with gephyrin, the main scaffolding protein at inhibitory post-synaptic densities. Previous work has shown that the synaptic accumulation and diffusion dynamics of GABA(A)Rs are controlled via excitatory synaptic activity. However, it remains unknown whether GABA(A)R activity can itself impact the surface trafficking of the receptors. Here we report the effects of GABA(A)R agonists, antagonists and allosteric modulators on the receptor's surface dynamics. Using immunocytochemistry and single particle tracking experiments on mouse hippocampal neurons, we show that the agonist muscimol decreases GABA(A)R and gephyrin levels at synapses and accelerates the receptor's lateral diffusion within 30–120 min of treatment. In contrast, the GABA(A)R antagonist gabazine increased GABA(A)R amounts and slowed down GABA(A)R diffusion at synapses. The response to GABA(A)R activation or inhibition appears to be an adaptative regulation of GABAergic synapses. Surprisingly, the positive allosteric modulator diazepam abolished the regulation induced by muscimol, and this effect was observed on α1, α2, α5 and γ2 GABA(A)R subunits. Altogether these results indicate that diazepam stabilizes synaptic GABA(A)Rs and thus prevents the agonist-induced regulation of GABA(A)R levels at synapses. This occurred independently of neuronal activity and intracellular calcium and involved GABA(A)R–gephyrin interactions, suggesting that the changes in GABA(A)R diffusion depend on conformational changes of the receptor. Our study provides a new molecular mechanism involved in the adaptative response to changes in GABA(A)R activity and benzodiazepine treatments.

Identification of CD44 as a cell surface marker for Müller glia precursor cells

In the retina, both neurons and glia differentiate from a common progenitor population. CD44 cell surface antigen is a hyaluronic acid receptor expressed on mature Müller glial cells. We found that in the developing mouse retina, expression of CD44 was transiently observed at or around birth in a subpopulation of c-kit-positive retinal progenitor cells. During in vitro culture, purified CD44/c-kit-positive retinal progenitor cells exclusively differentiated into Müller glial cells and not into neurons, suggesting that CD44 marks a subpopulation of retinal progenitor cells that are fated to become glia. Over-expression of CD44 inhibited the extension of processes by Müller glial cells and neurons. Notch signaling is known to be involved in the specification of retinal progenitors into a glial fate. Activation of Notch signaling increased the number of CD44-positive cells, and treatment with the Notch signal inhibitor, DAPT, at early, but not later, stages of retinal development abolished both CD44-positive cells and Müller glial cells. Together, CD44 was identified as an early cell surface marker of the Müller glia lineage, and Notch signalling was involved in commitment of retinal progenitor cells to CD44 positive Müller glial precursor cells.

Ablation of NMDA receptors enhances the excitability of hippocampal CA3 neurons

Synchronized discharges in the hippocampal CA3 recurrent network are supposed to underlie network oscillations, memory formation and seizure generation. In the hippocampal CA3 network, NMDA receptors are abundant at the recurrent synapses but scarce at the mossy fiber synapses. We generated mutant mice in which NMDA receptors were abolished in hippocampal CA3 pyramidal neurons by postnatal day 14. The histological and cytological organizations of the hippocampal CA3 region were indistinguishable between control and mutant mice. We found that mutant mice lacking NMDA receptors selectively in CA3 pyramidal neurons became more susceptible to kainate-induced seizures. Consistently, mutant mice showed characteristic large EEG spikes associated with multiple unit activities (MUA), suggesting enhanced synchronous firing of CA3 neurons. The electrophysiological balance between fast excitatory and inhibitory synaptic transmission was comparable between control and mutant pyramidal neurons in the hippocampal CA3 region, while the NMDA receptor-slow AHP coupling was diminished in the mutant neurons. In the adult brain, inducible ablation of NMDA receptors in the hippocampal CA3 region by the viral expression vector for Cre recombinase also induced similar large EEG spikes. Furthermore, pharmacological blockade of CA3 NMDA receptors enhanced the susceptibility to kainate-induced seizures. These results raise an intriguing possibility that hippocampal CA3 NMDA receptors may suppress the excitability of the recurrent network as a whole in vivo by restricting synchronous firing of CA3 neurons.

Modulation of synaptic plasticity by physiological activation of M1 muscarinic acetylcholine receptors in the mouse hippocampus

The muscarinic acetylcholine receptor (mAChR) has been considered one of the neurotransmitter receptors regulating hippocampal synaptic plasticity, which likely plays a critical role in learning and memory. In previous studies, however, muscarinic agonists were used at relatively high concentrations, and the subtype selectivity of muscarinic antagonists was not satisfactory. Thus, it remains to be answered whether physiological levels of ACh are involved in the regulation of synaptic plasticity and which mAChR subtypes are responsible for such effects. We found in this study that a low concentration (50 nM) of carbachol enhanced long-term potentiation (LTP) of excitatory synaptic transmission in mouse hippocampal slices. Notably, this enhancing effect was abolished in M1 mAChR knock-out (KO) but not in M3 mAChR KO mice, although LTP itself was intact in both mutant mice. Furthermore, we found that repetitive stimulation in the stratum oriens, which presumably triggered the release of endogenous ACh from cholinergic terminals, could enhance LTP in wild-type mice but not in M1 mAChR KO mice. These results suggest that physiologically released ACh from cholinergic fibers modulates hippocampal synaptic plasticity through the postsynaptic M1 mAChR activation.

Upregulation of the pro-apoptotic BH3-only peptide harakiri in spinal neurons of amyotrophic lateral sclerosis patients

DNA fragmentation and activation of caspase-1, implicating involvement of apoptosis, have been reported in the spinal cord of amyotrophic lateral sclerosis (ALS) patients and transgenic mouse models of ALS. Because BH3-only members of the Bcl-2 family have pro-apoptotic activity, we examined the expression of the BH3-only peptide harakiri (Hrk) in the spinal cord of ALS patients. In situ expression of Hrk mRNA and immunoreactivity against the Hrk peptide were verified in the spinal neurons. In the immunoblot analysis, upregulated Hrk protein migrated at 16 kDa. Heterodimerization of Hrk with Bcl-2 was detected by immunoprecipitation, which suggests the competition of Hrk and anti-apoptotic Bcl-2. These findings suggest that Hrk plays a role in apoptotic events in ALS pathogenesis.

Cyclic AMP regulates the calcium transients released from IP(3)-sensitive stores by activation of rat kappa-opioid receptors expressed in CHO cells

We analyzed intracellular Ca(2+)and cAMP levels in Chinese hamster ovary cells expressing a cloned rat kappa opioid receptor (CHO-kappa cells). Although expression of kappa(kappa)-opioid receptors was confirmed with a fluorescent dynorphin analog in almost all CHO-kappa cells, the kappa-specific agonists, U50488H or U69593, induced a Ca(2+) transient only in 35% of the cells. The Ca(2+) response occurred in all-or-none fashion and the half-maximal dosage of U50488H (812.1nM) was higher than that (3.2nM) to inhibit forskolin-stimulated cAMP. The kappa-receptors coupled to G(i/o)proteins since pertussis toxin significantly reduced the U50488H actions on intracellular Ca(2+) and cAMP. The Ca(2+) transient originates from IP(3)-sensitive internal stores since the Ca(2+) response was blocked by a PLC inhibitor (U73122) or by thapsigargin depletion of internal stores while removal of extracellular Ca(2+) had no effect. Interestingly, application of dibutyryl cAMP (+ 56.2%) or 8-bromo-cAMP (+ 174.7%) significantly increased the occurrence of U50488H-induced Ca(2+) mobilization while protein kinase A (PKA) inhibitors, Rp-cAMP (-32.3%) or myr-psi PKA (-73.9%) significantly reduced the response. Therefore, it was concluded that cAMP and PKA activity can regulate the Ca(2+) mobilization. These results suggest that the kappa receptor-linked cAMP cascade regulates the occurrence of kappa-opioid-mediated Ca(2+) mobilization.

A novel function of synapsin II in neurotransmitter release

Although synapsin has been localized to presynaptic structures, its function remains poorly understood. In the present study, we investigated the presynaptic function of synapsin II using a synaptic vesicle recycling process using synapsin-II-overexpressing NG108-15 cells. Western blot analysis with antibodies for synaptic-vesicle-associated protein indicated that the number of synaptic vesicles was approximately doubled in synapsin II transfectants as reported previously. In differentiated synapsin-II-overexpressing and control cells, the application of high potassium induced strong intracellular calcium elevation along neurites and varicosities after differentiation and a weak calcium rise in the cell bodies. The uptake and release of the fluorescent dye FM1-43 revealed that synaptic vesicle recycling in synapsin-II-transfected cells occurred with the same kinetics in the cell body and neuritic varicosities. Furthermore, the area labeled with FM1-43 fluorescence in the synapsin-II-transfected cells was approximately twice as much as in control cells after stimulation, and ATP released after synaptic vesicle fusion with the plasma membrane in synapsin-II-expressing cells was significantly elevated relative to controls. The number of synaptic vesicles paralleled the amount of transmitter released from the cells leading to the conclusion that the number of releasable synaptic vesicles were increased by synapsin II transfection into NG108-15 cells, suggesting that synapsin II may have a role in the regulation of synaptic vesicle number in presynapse-like structures in NG108-15 cells.

Distal sensory axonopathy after sarin intoxication

A 51-year-old man inhaled sarin during a terrorist attack on the Tokyo subway system and died 15 months later. Neuropathologic examination revealed marked nerve fiber decrease in the sural nerve, moderate nerve fiber loss in the sciatic nerve, and unremarkable dorsal root ganglia, dorsal roots, and posterior column of the spinal cord. This pathology is consistent with dying-back degeneration of the peripheral nervous system and could represent a late sequela of sarin intoxication.

[Diameter decrement of abdominal aorta and redistribution of systemic blood flow in non-ambulant Duchenne muscular dystrophy patients]

Cardiomyopathy is a major life-threatening complication in Duchenne muscular dystrophy (DMD). Cardiac output is always low in advanced stage patients. Impaired muscles presumably require extremely low circulation, while blood flow to brain and internal organs is probably maintained. Therefore, the redistribution of systemic circulation must be another important factor which determines whether or not the cardiomyopathy results in fatal heart failure. Although great efforts have been focused on the cardiac function, this problem has seldom attracted attentions. Assuming that diameter of blood vessels reflects blood flow, we measured the diameter of abdominal aorta on CT at four levels: 1) beginning of the abdominal aorta, 2) before and 3) after the branching of renal arteries, 4) before bifurcation of the common iliac arteries. Six bed-ridden DMD patients were compared with 10 ambulant patients suffering from other neurological and non-neurological diseases. We obtained following results. First, the diameter of abdominal aorta is significantly smaller in DMD patients. Second, the decrement is most prominent between before and after the branching of the renal arteries. These results clearly showed predilection of renointestinal circulation at the sacrifice of lower extremities. This narrowing of distal aorta was also observed in Kugelberg-Welander disease and understood as a sequelae of longstanding abolition of lower extremity functions.

[Tracheal deformity observed in tracheostomized Duchenne muscular dystrophy patients who can speak with appropriately inflated cuff of tracheal tube]

Respiratory failure occurring during late adolescence is a major cause of death in Duchenne muscular dystrophy (DMD). A number of respiratory assistance techniques including tracheostomy and positive pressure mechanical ventilation have been utilized not only to prolong DMD patients' life but also to improve quality of their life. It is surprising that most of the tracheostomized DMD patients can speak under positive pressure ventilatory assistance. Preserved functions of glosso-pharyngo-laryngeal muscles in these patients presumably make it possible to control the air leak around tracheal tube, which does not necessitate tight inflation of the tube cuff. Recently we noticed that some patients are still able to speak even when the cuff is fully inflated. In order to understand the mechanism of this phenomenon, we visualized the topographical relationship between trachea and the tube cuff using computed tomography by filling the cuff with aqueous contrast medium. We obtained following findings: 1) Trachea is flat and deviated from normal midline position, and 2) the tube cuff is located eccentrically in trachea resulting in compression of one side of the inner wall and leaving open space on the other side. We conclude that the tracheal malposition and deformity underlie this strange phenomenon and tight inflation of the cuff to prevent air leak may result in unfavorable complications in DMD patients.