Persistent α1-adrenergic receptor function in the nucleus locus coeruleus causes hyperexcitability in AD/HD model rats.

Spontaneously hypertensive rats (SHR) are widely used as a model of attention deficit hyperactivity disorder (ADHD) as their ADHD-like behaviors are restored by methylphenidate. However, a postnatal neural development in SHR is unknown. We performed whole cell patch clamp recordings from locus coeruleus (LC) neurons in neonatal [postnatal day (P) 3-5], juvenile (P21-28), and adult (P 49-56) SHR and age-matched Wistar rats to evaluate α1- and α2-adrenergic receptor (ARs) activities at each developmental period. LC neurons in neonatal Wistar rats and SHR showed no difference in resting membrane potential and spontaneous firing rate, while juvenile and adult SHR LC neurons showed depolarized resting membrane potential and faster spontaneous firing rate than in Wistar rats. Blockade of α1-AR activity by prazosin hyperpolarized the membrane and abolished spontaneous firings in all developmental periods in SHR LC neurons, but not in juvenile and adult Wistar rats. α1-AR stimulation by phenylephrine evoked an inward current in juvenile LC neurons in SHR, but not in juvenile Wistar rats. This phenylephrine-induced inward current was abolished by nonselective cation channel blockers. By contrast, α2-AR stimulation-induced outward currents in the presence of an α1-AR antagonist were equivalent in SHR and Wistar LC neurons. These data suggest that Wistar LC neurons lose α1-AR function during development, whereas α1-ARs remain functional in SHR LC neurons. Thus persistent intrinsic activity of α1-ARs may be a neural mechanism contributing to developmental disorders in juvenile SHRs.

Function and expression pattern of TRPM8 in bladder afferent neurons associated with bladder outlet obstruction in rats.

We investigated the function and expression pattern of the transient receptor potential melastatin-8 (TRPM8) in urinary bladder afferent neurons from control and bladder outlet obstruction (BOO) rats. BOO was produced and, after six weeks, the effects of intravesical infusion of menthol, the agonist of TRPM8, were investigated using unanesthetized cystometry. The intravesical infusion of menthol produced an increase in the micturition pressure in both sham surgery and BOO rats. In BOO rats, increased basal and threshold pressure and a decreased micturition interval were observed. Next, the population of TRPM8-positive and the co-expression proportion of TRPM8 with neurochemical markers (NF200 or TRPV1) in the bladder afferent neurons were each compared between the control and BOO rats using retrograde tracing and immunohistochemistry. The population of TRPM8-immunoreactive bladder afferent neurons was larger in BOO rats (3.28±0.43%) than in the control rats (1.33±0.18%). However, there were no statistical differences between the control and BOO rats in the co-expression proportion of neither TRPM8-NF200 (84.1±4.3% vs 79.7±2.7%, p=0.41) nor TRPM8-TRPV1 (33.3±3.6% vs 40.8±2.6%, p=0.08) in the bladder afferent neurons. The present results suggest that the neuronal input through TRPM8-positive bladder afferent neurons are augmented after BOO, however, the neurochemical phenotype of the up-regulated TRPM8-positive bladder afferent neurons is not changed after BOO.

Methylphenidate enhances inhibitory synaptic transmission by increasing the content of norepinephrine in the locus coeruleus of juvenile rats.

The present study examined the effect of methylphenidate (MPH), a psychostimulant, on nor-adrenergic transmission in the locus coeruleus (LC) of juvenile rats. Intracellular recordings showed that MPH (>3 µM) produced a hyperpolarizing response associated with a decrease in the rate of spontaneously firing action potentials. MPH (1 µM) enhanced the amplitude of the inhibitory postsynaptic potential (IPSP) mediated by norepinephrine (NE), but did not change the excitatory postsynaptic potential (EPSP) mediated by excitatory amino acids. Whole-cell patch-clamp recordings showed that MPH (0.3-30 µM) produced an outward current (I(MPH)) and enhanced the inhibitory postsynaptic current (IPSC) in neurons of the juvenile rat LC. MPH (30 µM) enhanced the NE-induced outward current (I(NE)). Bath-application of yohimbine (1 µM) produced an inward current and blocked the MPH-induced enhancement of the IPSC. Yohimbine (1 µM) depressed not only the I(NE) but also the I(MPH) in juvenile rat LC neurons. The current-voltage relationship of the I(MPH) showed inward rectification and reversed polarity at -91.1±4.3 mV (n=5). Ba(2+) (100 µM) blocked the I(MPH), indicating that the I(MPH) is mediated by Ba(2+)-sensitive inward rectifier K(+) current. These results suggest that MPH enhances inhibitory synaptic transmission by increasing the concentration of NE at noradrenergic synapses in juvenile rat LC neurons.

Expression of the TRPM8-immunoreactivity in dorsal root ganglion neurons innervating the rat urinary bladder.

The neurochemical phenotypes of the transient receptor potential melastatin-8 (TRPM8)-immunoreactive afferent neurons innervating the rat urinary bladder were examined by using a highly sensitive tyramide signal amplification method, combined with wheat-germ agglutinin-horseradish peroxidase (WGA-HRP) retrograde tracing. TRPM8-immunoreactivity was detected in a small proportion of the WGA-HRP-labeled bladder afferent neurons in the dorsal root ganglia of the Th13-L1 (1.14%) and the L6-S1 (1.27%), and these neurons were small in size (<600 microm(2)). The 82.6+/-3.8% of the TRPM8-immunoreactive bladder afferent neurons and 80.9+/-1.5% of the total population of the TRPM8-immunoreactive afferent neurons in the observed dorsal root ganglia expressed NF200. On the other hand, the proportions of the co-expression of TRPM8 and nociceptive markers such as calcitonin gene-related peptide (CGRP), transient receptor potential vanilloid-1 (TRPV1), and isolectin B4 (IB4) in the bladder afferent neurons (81.5+/-5.2% for CGRP, 36.1+/-4.0% for TRPV1, and 15.8+/-5.5% for IB4) were higher in comparison to those in the total population of the TRPM8-immunoreactive afferent neurons (21.9+/-2.4% for CGRP, 16.6+/-1.7% for TRPV1, and 5.4+/-0.5% for IB4), although no significant difference existed for IB4. Our results suggest that the TRPM8-expressing bladder afferents should be classified as Adelta-fibers and C-fibers, while some of these afferents may be involved in nociceptive sensations.

[Effects of milnacipran on neuronal excitability and synaptic transmission in neurons of the rat locus coeruleus].

Effects of milnacipran (MIL), a selective serotonin and noradrenaline (NA) reuptake inhibitor, on the neuronal excitability and synaptic transmission in the rat locus coeruleus (LC) were examined by intracellular and whole-cell patch-clamp recording techniques. We compared MIL and methylphenidate (MPH), a selective NA and dopamine reuptake inhibitor, as a therapeutic agent for attention deficit/hyperactivity disorder. Application of MPH (1-100 microM) and MIL (1-100 microM) to artificial cerebrospinal fluid (ACSF) produced a hyperpolarizing response in LC neurons in a concentration-dependent manner. Spontaneous firing of LC neurons was blocked during the hyperpolarization. The MIL-induced hyperpolarization was blocked by yohimbine (1 microM), an antagonist for alpha-adrenoceptors. These results suggest that the MIL-induced hyperpolarization is mediated by NA via alpha2-adrenoceptors in LC neurons. Under the whole-cell patch-clamp condition, prolonged application of MIL produced an outward current which lasted as long as MIL existed in the ACSF. The outward current induced by NA was enhanced by MIL in LC neurons. MIL enhanced the amplitude and duration of the inhibitory postsynaptic potential, while it depressed the excitatory postsynaptic potential. The results indicated that both MIL and MPH showed almost the same effects on neuronal activity and synaptic transmission in the rat LC. These results suggest that MIL increases the concentration of NA at synaptic clefts by inhibiting the NA reuptake system in the rat LC.

Mechanical unloading improves intracellular Ca2+ regulation in rats with doxorubicin-induced cardiomyopathy.

OBJECTIVES: We sought to assess whether mechanical unloading has beneficial effects on cardiomyocytes from doxorubicin-induced cardiomyopathy in rats. BACKGROUND: Mechanical unloading by a left ventricular assist device (LVAD) improves the cardiac function of terminal heart failure in humans. However, previous animal studies have failed to demonstrate beneficial effects of mechanical unloading in the myocardium. METHODS: The effects of mechanical unloading by heterotopic abdominal heart transplantation were evaluated in the myocardium from doxorubicin-treated rats by analyzing the intracellular free calcium level ([Ca(2+)](i)) and the levels of intracellular Ca(2+)-regulatory proteins. RESULTS: In doxorubicin-treated rats, the duration of cell shortening and [Ca(2+)](i) transients in cardiomyocytes was prolonged (432 +/- 28.2% of control in 50% relaxation time; 184 +/- 10.5% of control in [Ca(2+)](i) 50% decay time). Such prolonged time courses significantly recovered after mechanical unloading (114 +/- 10.4% of control in 50% relaxation time; 114 +/- 5.8% of control in 50% decay time). These effects were accompanied by an increase in sarcoplasmic reticulum Ca(2+) ATPase (SERCA2a) protein levels (0.97 +/- 0.05 in unloaded hearts vs. 0.41+/- 0.09 in non-unloaded hearts). The levels of other intracellular Ca(2+)-regulatory proteins (phospholamban and ryanodine receptor) were not altered after mechanical unloading in doxorubicin-treated hearts. These parameters in unloaded hearts without doxorubicin treatment were similar to normal hearts. CONCLUSIONS: Mechanical unloading increases functional sarcoplasmic reticulum Ca(2+) ATPase and improves [Ca(2+)](i) handling and contractility in rats with doxorubicin-induced cardiomyopathy. These beneficial effects of mechanical unloading were not observed in normal hearts.

Effects of milnacipran on the inhibitory postsynaptic potential in neurons of the rat locus coeruleus.

Effects of milnacipran (MIL), a serotonin and noradrenaline reuptake inhibitor (SNRI), on synaptic transmission were examined in the rat locus coeruleus (LC). Bath-application of MIL produced a hyperpolarization associated with a decrease in input resistance of LC neurons. The MIL-induced hyperpolarization reversed polarity near the equilibrium potential of K+. The MIL-induced hyperpolarization was blocked by yohimbine (1 microM). Clonidine, but not serotonin (5-hydroxytryptamine; 5-HT), produced a hyperpolarizing potential in LC neurons. The MIL-induced hyperpolarization reversed polarity at -114 +/- 3 mV (n=4). MIL (0.1-10 microM) depressed the amplitude of the excitatory postsynaptic potential (EPSP), while it enhanced the amplitude and duration of the inhibitory postsynaptic potential (IPSP). These results suggest that MIL hyperpolarizes LC neurons and enhances the IPSP by increasing endogenous noradrenaline (NA) concentration at synapses in LC neurons.

Protective role of heparin/heparan sulfate on oxalate-induced changes in cell morphology and intracellular Ca2+.

Alterations in intracellular Ca2+ ([Ca2+]i) are generally associated with cellular distress. Oxalate-induced cell injury of the renal epithelium plays an important role in promoting CaOx nephrolithiasis. However, the degree of change in intracellular free calcium ions in renal epithelial cells during oxalate exposure remains unclear. The aim of this study is to determine whether acute short-term exposure to oxalate produces morphological changes in the cells, induces a change in cytosolic Ca2+ levels in renal tubular epithelial cells and whether the application of extracellular glycosaminoglycans (GAGs) prevents these changes. Cultured Mardin-Darby canine kidney cells were exposed to oxalate, and changes in cytosolic Ca2+ were determined under various conditions. The effect of heparin and heparan sulfate (HS) during oxalate exposure was examined. The change in the GAG contents of the culture medium was also determined. Transmission electron microscopy (TEM) was performed for morphological analysis. The degree of change in cytosolic Ca2+ strongly correlated with oxalate concentration. Cytosolic Ca2+ levels decreased in parallel with an increase in the concentration of oxalate. However, this decrease was strongly inhibited by pretreatment with heparin or HS. TEM revealed cytoplasmic vacuolization, the appearance of flocculent material and mitochondrial damage after oxalate exposure. On the other hand, pretreatment with heparin or HS completely blocked these morphological changes. The present data suggest that acute exposure to a high concentration of oxalate challenges the renal cells, diminishes their viability and induces changes in cytosolic Ca2+ levels. Heparin and HS, which are known as potent inhibitors of CaOx crystallization, may also prevent oxalate-induced cell changes by stabilizing the cytosolic Ca2+ level.

Effects of methylphenidate on the membrane potential and current in neurons of the rat locus coeruleus.

Effects of methylphenidate (MPH), a therapeutic agent used in children presenting the attention deficit hyperactivity disorder (ADHD), on the membrane potential and current in neurons of the rat locus coeruleus (LC) were examined using intracellular and whole cell patch-clamp recording techniques. Application of MPH (30 microM) to artificial cerebrospinal fluid (ACSF) produced a hyperpolarizing response with amplitude of 12 +/- 1 mV (n = 29). Spontaneous firing of LC neurons was blocked during the MPH-induced hyperpolarization. Superfusion of LC neurons with ACSF containing 0 mM Ca(2+) and 11 mM Mg(2+) (Ca(2+)-free ACSF) produced a depolarizing response associated with an increase in spontaneous firing of the action potential. The MPH-induced hyperpolarization was blocked in Ca(2+)-free ACSF. Yohimbine (1 microM) and prazosin (10 microM), antagonists for alpha(2) and alpha(2B/2C) receptors, respectively, blocked the MPH-induced hyperpolarization in LC neurons. Tetrodotoxin (TTX, 1 microM) produced a partial depression of the MPH-induced hyperpolarization in LC neurons. Under the whole cell patch-clamp condition, MPH (30-300 microM) produced an outward current (I(MPH)) with amplitude of 110 +/- 6 pA (n = 17) in LC neurons. The I(MPH) was blocked by Co(2+) (1 mM). During prolonged application of MPH (300 microM for 45 min), the hyperpolarization gradually decreased in the amplitude and eventually disappeared, possibly because of depression of norepinephrine (NE) release from noradrenergic nerve terminals. At a low concentration (1 microM), MPH produced no outward current but consistently enhanced the outward current induced by NE. These results suggest that the MPH-induced response is mediated by NE via alpha(2B/2C)-adrenoceptors in LC neurons. I(MPH) was associated with an increase in the membrane conductance of LC neurons. The I(MPH) reversed its polarity at -102 +/- 6 mV (n = 8) in the ACSF. The reversal potential of I(MPH) was changed by 54 mV per decade change in the external K(+) concentration. Current-voltage relationship showed that the I(MPH) exhibited inward rectification. Ba(2+) (100 microM) suppressed the amplitude and the inward rectification of the I(MPH.) These results suggest that the I(MPH) is produced by activation of inward rectifier K(+) channels in LC neurons.

Effects of methylphenidate on the inhibitory postsynaptic potential in rat locus coeruleus neurons.

Effects of methylphenidate (MPH), an agent used clinically for the treatment of children presenting the attention-deficit/hyperactivity disorder (AD/HD), on synaptic transmission in the rat locus coeruleus (LC) were examined by intracellular recording methods. Bath-application of MPH (30 nM-3 microM) increased the amplitude of the inhibitory postsynaptic potential (IPSP), while it did not change the amplitude of the excitatory postsynaptic potential (EPSP). MPH increased the time-to-peak and the half-decay time of the IPSP in LC neurons. MPH increased the amplitude of spontaneous IPSP: individual spontaneous IPSPs merged one into the other so as to produce regular, long-lasting waves of hyperpolarization. Clonidine (10 nM), a selective agonist for alpha 2-adrenoceptors, depressed the IPSP without affecting the EPSP in LC neurons. The results suggest that MPH enhances inhibitory synaptic transmission in the rat LC by depressing the norepinephrine (NE) re-uptake system.