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Mitsuru Sanada

Publications and source records attributed to Mitsuru Sanada.

9 recordsLinked to original sources
PubMed2006

Cholinergic modulation of periaqueductal grey neurons: does it contribute to epileptogenesis after organophosphorus nerve agent intoxication?

Previous work has shown that a single focal microinjection of the unselective cholinergic agonist, carbachol, into the periaqueductal grey (PAG) of the midbrain is sufficient to induce forebrain seizures in rats. In order to determine the cholinergic mechanisms underlying epileptogenesis at the cellular and network level of the PAG, we performed whole-cell recordings from rat PAG neurons in vitro and examined how the activation of muscarinic and nicotinic receptors modulates cellular excitability and synaptic responses. Stimulation of muscarinic receptors produced either a pirenzepine-sensitive depolarization (40% of PAG neurons), or a gallamine-sensitive hyperpolarization (20%), suggesting the involvement of M1 and M2 receptors, respectively. In the remaining neurons (40%), no change was observed. Voltage-clamp recordings showed that muscarinic depolarization resulted from the inhibition of a resting K(+) current, in part accompanied by simultaneous activation of a presumed non-selective cation current. Muscarinic hyperpolarization was caused by the activation of a G protein-coupled, inwardly rectifying K(+) current. Stimulation of muscarinic receptors enhanced the frequency of spontaneous inhibitory postsynaptic currents (IPSCs), but strongly suppressed evoked IPSCs. In addition, nicotine almost doubled the frequency of miniature IPSCs. Based on our findings and the network properties of the PAG, we advance a scenario in which excessive stimulation of cholinergic receptors would substantially contribute to generalized seizures after organophosphorus nerve agent poisoning.

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PubMed2006

Effect of protein kinase Cbeta inhibitor on Ca2+ homeostasis in diabetic sensory neurons.

To elucidate the direct effect of selective protein kinase Cbeta inhibitor LY333531 on diabetic sensory neurons, we examined intracellular Ca(2+) concentration in isolated rat dorsal root ganglion neurons using the fluorescent Ca(2+) indicator fura-2. The duration of calcium transients induced by high (50 mM) extracellular K in small diabetic dorsal root ganglion neurons was significantly prolonged compared with that in control neurons. This prolonged intracellular Ca concentration elevation in diabetic neurons was normalized rapidly and reversibly by LY333531 in a dose-dependent manner, and the effect of LY333531 was completely abolished by pretreating the neurons with mitochondrial calcium uniporter inhibitor, Ruthenium 360. These results suggest that LY333531 has an ameliorating effect on calcium homeostasis of diabetic sensory neurons via mitochondrial calcium buffering.

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PubMed2004

Cytosolic Ca2+ under high glucose with suppressed Na+/K+ pump activity in rat sensory neurons.

Cytosolic Ca2+ concentration ([Ca2+]i) was measured in isolated rat dorsal root ganglion (DRG) neurons using the fluorescent Ca2+ indicator fura-2. Exposure to high (50 mM) extracellular K+ evoked a robust increase in [Ca2+]i, which was almost totally abolished by concomitant presence of nisoldipine (10 microM) and omega-conotoxin GVIA (10 microM). Whereas either high (30 mM) D-glucose alone or ouabain (100 microM) alone did not appreciably affect the high K+-induced [Ca2+]i elevation, neurons pretreated with high D-glucose together with ouabain exhibited a significantly larger [Ca2+]i response to high K+ stimulation, which was almost completely inhibited by nisoldipine and omega-conotoxin GVIA. These results suggest that a combination of high glucose and suppressed Na+/K+ pump activity potentiates the [Ca2+]i elevation stimulated by activation of the voltage-gated Ca2+ channels in rat DRG neurons.

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PubMed2003

Amelioration of retarded neurite outgrowth of dorsal root ganglion neurons by overexpression of PKCdelta in diabetic rats.

To examine which isoform of protein kinase C (PKC) may be associated with impaired nerve regeneration in diabetes, we compared neurite outgrowth of isolated dorsal root ganglion (DRG) neurons in streptozocin (STZ)-induced diabetic and control rats. Neurite outgrowth was significantly retarded in diabetic neurons. Rottlerin, a PKCdelta specific inhibitor, significantly retracted neurite outgrowth whereas Gö6976, an inhibitor specific for classical PKCs, had no effect, suggesting a significant role of PKCdelta in neurite outgrowth of DRG neurons. The expression of phosphorylated PKCdelta, but not total PKCdelta, in DRGs was decreased in diabetic rats. When this reduced expression was restored by overexpressing the PKCdelta in isolated DRG neurons, retardation of neurite outgrowth was significantly reversed in diabetic rats. These results suggest that a decrease in phosphorylated PKCdelta is at least in part responsible for impaired neurite outgrowth in diabetes, and that PKCdelta plays a significant role in the pathogenesis of diabetic neuropathy. This observation provides a useful clue for the treatment of diabetic neuropathy.

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PubMed2003

Diabetic neuropathy and nerve regeneration.

Diabetic neuropathy is the most common peripheral neuropathy in western countries. Although every effort has been made to clarify the pathogenic mechanism of diabetic neuropathy, thereby devising its ideal therapeutic drugs, neither convinced hypotheses nor unequivocally effective drugs have been established. In view of the pathologic basis for the treatment of diabetic neuropathy, it is important to enhance nerve regeneration as well as prevent nerve degeneration. Nerve regeneration or sprouting in diabetes may occur not only in the nerve trunk but also in the dermis and around dorsal root ganglion neurons, thereby being implicated in the generation of pain sensation. Thus, inadequate nerve regeneration unequivocally contributes to the pathophysiologic mechanism of diabetic neuropathy. In this context, the research on nerve regeneration in diabetes should be more accelerated. Indeed, nerve regenerative capacity has been shown to be decreased in diabetic patients as well as in diabetic animals. Disturbed nerve regeneration in diabetes has been ascribed at least in part to all or some of decreased levels of neurotrophic factors, decreased expression of their receptors, altered cellular signal pathways and/or abnormal expression of cell adhesion molecules, although the mechanisms of their changes remain almost unclear. In addition to their steady-state changes in diabetes, nerve injury induces injury-specific changes in individual neurotrophic factors, their receptors and their intracellular signal pathways, which are closely linked with altered neuronal function, varying from neuronal survival and neurite extension/nerve regeneration to apoptosis. Although it is essential to clarify those changes for understanding the mechanism of disturbed nerve regeneration in diabetes, very few data are now available. Rationally accepted replacement therapy with neurotrophic factors has not provided any success in treating diabetic neuropathy. Aside from adverse effects of those factors, more rigorous consideration for their delivery system may be needed for any possible success. Although conventional therapeutic drugs like aldose reductase (AR) inhibitors and vasodilators have been shown to enhance nerve regeneration, their efficacy should be strictly evaluated with respect to nerve regenerative capacity. For this purpose, especially clinically, skin biopsy, by which cutaneous nerve pathology including nerve regeneration can be morphometrically evaluated, might be a safe and useful examination.

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PubMed2003

Properties of the Na+/K+ pump current in small neurons from adult rat dorsal root ganglia.

1 The present investigation was undertaken to characterize the Na(+)/K(+) pump current in small (<or=25 microm in soma diameter) dorsal root ganglion (DRG) neurons isolated from lumbar L4-6 segments of adult rats. 2. The Na(+)/K(+) pump current was identified as an ouabain-sensitive current during square voltage steps to membrane potentials between +40 and -120 mV, using the whole-cell patch-clamp technique in which Ca(2+) and K(+) channel currents and Na(+)/Ca(2+) exchange currents were minimized. The Na(+)/K(+) pump current was practically time-independent over the entire voltage range examined and exhibited a voltage-dependence; its current - voltage (I-V) relationship displayed a positive slope at potentials between -120 and 0 mV but nearly plateau levels at positive membrane potentials. 3. The concentration-dependent block of Na(+)/K(+) pump current (activated by 30 mM pipette Na(+)) by ouabain at concentrations between 0.1 microM and 5 mM was biphasic and was well described using a two-binding site model with dissociation constants for high- and low-affinity binding sites of 0.20 and 140.1 microM, respectively. The relative amplitude of the Na(+)/K(+) pump current produced by low- and high-affinity sites (probably alpha1beta1 and alpha3beta1 isozymes, respectively) was estimated to be 13 : 1 in the presence of 30 mM Na(+) in the pipette solution. 4. Additionally, the activation of Na(+)/K(+) pump current by pipette Na(+) at concentrations ranging from 5 to 100 mM also exhibited a biphasic concentration dependence which can be reasonably well fitted by assuming the existence of two isozymes having high and low affinities for Na(+) (6.7 and 67.6 mM, respectively). 5. Thus, the present investigation provides functional evidence to suggest that the Na(+)/K(+) ATPase comprises two functionally distinct isozymes as expected for alpha1beta1 and alpha3beta1 in rat small DRG neurons.

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