Excitatory amino acid response in isolated nucleus tractus solitarii neurons of the rat

Nitrous oxide directly inhibits action potential-dependent neurotransmission from single presynaptic boutons adhering to rat hippocampal CA3 neurons

We evaluated the effects of N2O on synaptic transmission using a preparation of mechanically dissociated rat hippocampal CA3 neurons that allowed assays of single bouton responses evoked from native functional nerve endings. We studied the effects of N2O on GABAA, glutamate, AMPA and NMDA receptor-mediated currents (IGABA, IGlu, IAMPA and INMDA) elicited by exogenous application of GABA, glutamate, (S)-AMPA, and NMDA and spontaneous, miniature, and evoked GABAergic inhibitory and glutamatergic excitatory postsynaptic current (sIPSC, mIPSC, eIPSC, sEPSC, mEPSC and eEPSC) in mechanically dissociated CA3 neurons. eIPSC and eEPSC were evoked by focal electrical stimulation of a single bouton. Administration of 70% N2O altered neither IGABA nor the frequency and amplitude of both sIPSCs and mIPSCs. In contrast, N2O decreased the amplitude of eIPSCs, while increasing failure rates (Rf) and paired-pulse ratios (PPR) in a concentration-dependent manner. On the other hand, N2O decreased IGlu, IAMPA and INMDA. Again N2O did not change the frequency and amplitude of either sEPSCs of mEPSCs. N2O also decreased amplitudes of eEPSCs with increased Rf and PPR. The decay phases of all synaptic responses were unchanged. The present results indicated that N2O inhibits the activation of AMPA/KA and NMDA receptors and also that N2O preferentially depress the action potential-dependent GABA and glutamate releases but had little effects on spontaneous and miniature releases.

The effects of volatile anesthetics on synaptic and extrasynaptic GABA-induced neurotransmission

Examination of volatile anesthetic actions at single synapses provides more direct information by reducing interference by surrounding tissue and extrasynaptic modulation. We examined how volatile anesthetics modulate GABA release by measuring spontaneous or miniature GABA-induced inhibitory postsynaptic currents (mIPSCs, sIPSCs) or by measuring action potential-evoked IPSCs (eIPSCs) at individual synapses. Halothane increased both the amplitude and frequency of sIPSCs. Isoflurane and enflurane increased mIPSC frequency while sevoflurane had no effect. These anesthetics did not alter mIPSC amplitudes. Halothane increased the amplitude of eIPSCs, with a decrease in failure rate (Rf) and paired-pulse ratio. In contrast, isoflurane and enflurane decreased the eIPSC amplitude and increased Rf, while sevoflurane decreased the eIPSC amplitude without affecting Rf. Volatile anesthetics did not change kinetics except for sevoflurane, suggesting that presynaptic mechanisms dominate changes in neurotransmission. Each anesthetic showed somewhat different GABA-induced response and these results suggest that GABA-induced synaptic transmission cannot have a uniformly common site of action as suggested for volatile anesthetics. In contrast, all volatile anesthetics concentration-dependently enhanced the GABA-induced extrasynaptic currents. Extrasynaptic receptors containing α4 and α5 subunits are reported to have high sensitivities to volatile anesthetics. Also, inhibition of GABA uptake by volatile anesthetics results in higher extracellular GABA concentration, which may lead to prolonged activation of extrasynaptic GABAA receptors. The extrasynaptic GABA-induced receptors may be major site of volatile anesthetic-induced neurotransmission. This article is part of a Special Issue entitled 'Extrasynaptic ionotropic receptors'.

Electrophysiological and chemical properties in subclassified acutely dissociated cells of rat trigeminal ganglion by current signatures

In the present study, we subclassified acutely dissociated trigeminal ganglion (TRG) cells of rats using a current signature method in whole cell patch-clamp recordings. Using modified criteria for cell classification for the dorsal root ganglion (DRG), TRG cells were subclassified into nine cell types: 1-5, 7-9, and 13. Types 1, 3, and 7 were in the small cell groups (15-24 μm); types 4, 5, and 8-13 were in the medium cell groups (25-38 μm); and type 2 was a mixed group of both cell sizes. Types 1-3, 5, and 7 showed high-input resistance and types 1, 2, and 7 showed more depolarized resting membrane potentials. Types 1, 2, and 5-13 expressed long-duration action potentials (APs), but types 3 and 4 expressed short-duration APs. Sensitivities to capsaicin, protons, and adenosine 5'-triphosphate (ATP) in TRG cell types largely corresponded to DRG cell types. However, different from the matched DRG types, half of TRG type 1 cells were capsaicin insensitive, showing desensitizing proton-induced currents, and types 5, 7, and 9 exhibited slow-desensitizing ATP-induced currents. Types 4, 5, and 8-13 had nicotine sensitivity, but the other cell types were insensitive. These results indicate that the "current signatures" classification is a useful means to separate TRG cells into internally homogeneous subpopulations that were distinct from other cell types. Furthermore, the data suggest some specific differences in the chemical responsiveness of some cell types between the TRG and DRG.

Cell subpopulations of nicotine-sensitive subfornical organ neurons in rat

The subfornical organ (SFO), which is related to drinking and cardiovascular regulation, is activated by central application of nicotine (NIC) and angiotensin II (ANG). However, NIC-induced drinking is much smaller than ANG-induced one although approximately 60% of SFO neurons are affected by both NIC and ANG. Therefore, some specific subpopulations of SFO neurons for NIC or ANG may be related to such different drinking responses. To clarify subpopulations of NIC-sensitive neurons, electrophysiological properties of SFO neurons with the application of NIC was investigated at whole-cell patch-clamp recordings. Based on our developed electrophysiological criteria of the recovery kinetics of tetraethylammonium-resistant transient outward K(+) currents, two sub-types (F- and S-type neurons) were distinguished. Twenty-nine dissociated SFO neurons were examined to determine whether they showed NIC-induced inward currents. Most F-type neurons (n=19/21) showed NIC sensitivity, but most S-type neurons (n=7/8) did not. Our previous study had demonstrated that half of the F-type and all of the S-type units showed ANG sensitivity. These suggests that almost all of the NIC-sensitive SFO neurons were electrophysiologically classified as the F-type, but not S-type, and this differs in part from angiotensin sensitivity. The different subpopulations for chemical sensitivities in the SFO may be related to different drinking responses.

Transient outward K+ currents in rat dissociated subfornical organ neurones and angiotensin II effects

Although angiotensin II inhibits transient outward K+ currents (I(A)s) in subfornical organ neurones, there is no evidence concerning which Kv channels are involved. We investigated I(A)-generating Kv channels in dissociated rat subfornical organ neurones, using molecular, electrophysiological and pharmacological techniques, and studied the effects of angiotensin II. Conventional RT-PCR showed the presence of mRNAs for channels of the Kv3.4, Kv1.4 and Kv4 families, which are capable of generating I(A)s. Tetraethylammonium at 1 mm, which blocks Kv3 channel-derived currents, and blood-depressing substance-I, a Kv3.4-specific blocker, at 2 microm suppressed the I(A)-like component of whole-cell outward currents in some neurones. 4-Aminopyridine at 5 mm inhibited I(A)s in the presence of tetraethylammonium at 1 mm. Cd2+ at 300 microm shifted the activation and inactivation curves of the 4-aminopyridine-sensitive and tetraethylammonium-resistant I(A)s positively. The tetraethylammonium-resistant I(A)s showed fast and slow components during the process of recovery from inactivation, but the slow component was not seen in all neurones. The time constant of the fast recovery component was less than 200 ms, while that of the slow recovery component was around 1 s. Using single-cell RT-PCR, mRNAs for Kv4.2 and Kv4.3L were detected frequently, but those for Kv1.4 and Kv3.4 were seen only rarely. Angiotensin II at 30 nm inhibited the fast recovery component of tetraethylammonium-resistant I(A)s in many neurones. These results suggest that the fast recovery component of the tetraethylammonium-resistant I(A) in subfornical organ neurones depends upon Kv4, and that it can be modulated by angiotensin II.

Orexins cause depolarization via nonselective cationic and K+ channels in isolated locus coeruleus neurons

The locus coeruleus (LC) contains noradrenergic neurons that are innervated by orexin (ORX)-like immunoreactive axons and express both orexin receptor-1 and -2. We studied effects of ORX-A and -B (ORX-A/B) on dissociated LC neurons by using whole-cell patch clamp techniques. In current-clamp mode, LC neurons were depolarized by application of ORX-A (10(-7) M) [53% of neurons tested; 9.0+/-0.2 mV (n=5)], or ORX-B (10(-7) M) [38% of neurons tested; 4.0+/-0.1 mV (n=5)]. Firing frequencies of action potentials increased during application [1.1+/-0.2 Hz (n=5) in ORX-A; 0.8+/-0.2 Hz (n=5) in ORX-B] and returned to the control level [0.2+/-0.1 Hz (n=5)] after removal. The ORX-A/B-induced depolarization was well maintained in the presence of TTX (3x10(-7) M), CNQX (10(-6) M) and AP5 (10(-5) M). In voltage-clamp mode, removal of external Na+ suppressed both ORX-A/B-induced currents and shifted their reversal potentials from approximately -45 mV to -60 mV. In addition, ORX-A/B inhibited sustained K+ currents. These results suggest that ORX-A/B increase the firing frequency of LC neurons through the depolarization probably produced by both augmentation of the nonselective cationic conductance and inhibition of the sustained K+ conductance.

Activation by N-acetyl-L-aspartate of acutely dissociated hippocampal neurons in rats via metabotropic glutamate receptors

PURPOSE

We previously reported that an increase in the N-acetyl-L-aspartate (NAA) level due to the lack of aspartoacylase gene was found in the brain of the tremor rat (tm/tm), which is a mutant with a causative gene named tm that shows epileptic seizures. Therefore, NAA is suggested to be one of the factors involved in the induction of epileptic seizures. Patch-clamp studies were performed to determine whether NAA produces an excitatory effect on acutely dissociated rat hippocampal neurons.

METHODS

Acutely dissociated hippocampal neurons were prepared from normal Wistar rats aged 3-4 weeks. NAA-induced currents were investigated by using the whole-cell voltage-clamp recording technique.

RESULTS

Application of NAA at concentrations of 100 nM to 1 mM through a U-tube for 2 s produced an inward current in a concentration-dependent manner at a holding potential of -60 mV. When the current-voltage relation was examined, the reversal potential of the NAA-induced current was found to be approximately 0 mV. The NAA-induced current was inhibited by bath application of the metabotropic glutamate receptor (mGluR) antagonist (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG) and by intracellular application of guanosine 5'-O-(2-thiodiphosphate) (GDP-betaS), a nonhydrolyzable GDP analogue. However, the NAA-induced current remained unaffected by glutamic acid diethyl ester, a non-N-methyl-D-aspartate (NMDA)-subtype ionotropic glutamate receptor antagonist, or the voltage-dependent ion channel blockers tetrodotoxin, CdCl2, and tetraethylammonium-chloride. Conversely, the mGluR agonist, trans-(1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid (ACPD) also induced an inward current, with a reversal potential of 0 mV. The ACPD-induced current also was inhibited by MCPG.

CONCLUSIONS

These results suggest that NAA acts on the G protein-coupled mGluRs to induce an inward current that results in excitation of the neurons, thereby contributing to the occurrence of epileptic seizures.

An electrophysiological study of muscarinic and nicotinic receptors of rat paratracheal ganglion neurons and their inhibition by Z-338

1. To study the mechanisms involved in the action of Z-338, a newly synthesized gastroprokinetic agent, experiments were performed with the paratracheal ganglion cells acutely dissociated from 2-week-old Wistar rats. The effects of Z-338 on both nicotinic and muscarinic responses of the ganglion cells were studied by nystatin perforated patch recording configuration under the current- and voltage-clamp conditions. 2. Acetylcholine (ACh) or nicotine, and muscarine or oxotremorine-M (OX-M) induced membrane depolarization with rapid and slow time courses respectively, followed by repetitive generation of action potentials in the ganglion cell. Corresponding to the membrane depolarization induced by cholinergic agents, ACh induced biphasic inward currents with rapid and slow time courses under the voltage-clamp condition. Nicotine and muscarine or OX-M evoked inward currents with rapid and slow time courses, respectively. The rapid and slow inward currents were accompanied by increase and decrease in the membrane conductance, respectively. In addition, OX-M dose-dependently suppressed the M-type K(+) current evoked in response to hyperpolarizing voltage-steps from V(H) of -25 mV to -50 mV, indicating that the activation of muscarinic acetylcholine receptors inhibits M-type K(+) current, thus inducing inward current in the ganglion cell. 3. Z-338 competitively suppressed the inward currents induced by OX-M through M(1) ACh receptor, and uncompetitively suppressed the currents induced by nicotine. 4. The inhibitory actions of Z-338 on the membrane depolarization and corresponding inward currents mediated by M(1)-muscarinic and neuronal nicotinic ACh receptors in the isolated ganglion cells were discussed in relation to the inhibitory actions on autoreceptors in the parasympathetic nerve terminals, which would explain the gastroprokinetic actions of Z-338.

Reversibility and cation selectivity of the K(+)-Cl(-) cotransport in rat central neurons

The reversibility and cation selectivity of the K(+)-Cl(-) cotransporter (KCC), which normally extrudes Cl(-) out of neurons, was investigated in dissociated lateral superior olive neurons of rats using the gramicidin perforated patch technique. Intracellular Cl(-) activity (alpha[Cl(-)](i)) was maintained well below electrochemical equilibrium as determined from the extracellular Cl(-) activity and the holding potential, where the pipette and external solutions contained 150 mM K(+) ([K(+)](pipette)) and 5 mM K(+) ([K(+)](o)), respectively. Extracellular application of 1 mM furosemide or elevated [K(+)](o) increased alpha[Cl(-)](i). When the pipette solution contained 150 mM Cs(+) ([Cs(+)](pipette)), alpha[Cl(-)](i) increased to a value higher than the passive alpha[Cl(-)](i). An increase of alpha[Cl(-)](i) with the [Cs(+)](pipette) was not due to the simple blockade of net KCC by the intracellular Cs(+) since alpha[Cl(-)](i), with the pipette solution containing 75 mM Cs(+) and 75 mM K(+), reached a value between those obtained using the [K(+)](pipette) and the [Cs(+)](pipette). The higher-than-passive alpha[Cl(-)](i) with the [Cs(+)](pipette) was reduced by 1 mM furosemide, but not by 20 microM bumetanide or Na(+)-free external solution, indicating that the accumulation of [Cl(-)](i) in the [Cs(+)](pipette) was mediated by a KCC operating in a reversed mode rather than by Na(+)-dependent, bumetanide-sensitive mechanisms. Replacement of K(+) in the pipette solution with either Li(+) or Na(+) mimicked the effect of Cs(+) on alpha[Cl(-)](i). On the other hand, Rb(+) mimicked K(+) in the pipette solution. These results indicate that K(+) and Rb(+), but not Cs(+), Li(+), or Na(+), can act as substrates of KCC in LSO neurons.

Preferential inhibition of L- and N-type calcium channels in the rat hippocampal neurons by cilnidipine

The effect of a dihydropyridine Ca2+ antagonist, cilnidipine, on voltage-dependent Ca2+ channels was studied in acutely dissociated rat CA1 pyramidal neurons using the nystatin-perforated patch recording configuration under voltage-clamp conditions. Cilnidipine had no effect on low-voltage-activated (LVA) Ca2+ channels at the low concentrations under 10(-6) M. On the other hand, cilnidipine inhibited the high-voltage-activated (HVA) Ca2+ current (I(Ca)) in a concentration-dependent manner and the inhibition curve showed a step-wise pattern; cilnidipine selectively reduced only L-type HVA I(Ca) at the low concentrations under 10(-7) and 10(-6) M cilnidipine blocked not only L- but also N-type HVA I(Ca). At the high concentration over 10(-6) M cilnidipine non-selectively blocked the T-type LVA and P/Q- and R-type HVA Ca2+ channels. This is the first report that cilnidipine at lower concentration of 10(-6) M blocks both L-and N-type HVA I(Ca) in the hippocampal neurons.

The prostaglandin E series modulates high-voltage-activated calcium channels probably through the EP3 receptor in rat paratracheal ganglia

The modulation of high-voltage-activated (HVA) Ca2+ channels by the prostaglandin E series (PGE1 and PGE2) was studied in the paratracheal ganglion cells. Prostaglandin E1, E2, STA2 (a stable analogue of thromboxane A2), 17-phenyl-trinor-PGE2 (an EP1-selective agonist) and sulprostone (an EP3-selective agonist) inhibited the HVA Ca2+ current (HVA ICa) dose-dependently, and the rank order of potency to inhibit HVA Ca2+ channels was sulprostone>PGE2, PGE1>STA2>>17-phenyl-trinor-PGE2. SC-51089 (10(-5) M), a selective EP1-receptor antagonist, showed no effect on the PGE1- or PGE2-induced inhibition of the HVA ICa, thereby indicating that PGE1- and PGE2-induced inhibition of the HVA Ca2+ channels is possibly mediated by the EP3 receptor. The PGE1-sensitive component of the current was markedly reduced in the presence of omega-conotoxin-GVIA (3x10(-6) M), but not with nifedipine (3x10(-6) M). PGE1 and PGE2 also inhibited the remaining ICa in a saturating concentration of nifedipine, omega-conotoxin-GVIA and omega-conotoxin-MVIIC, suggesting that R-type Ca2+ channels are involved. The inhibitory effect of PGE1 or sulprostone was prevented by pretreatment with pertussis toxin [islet activating protein (IAP)] or phorbol-12-myristate-13-acetate (PMA), and the protein kinase C (PKC) inhibitor chelerythrine blocked the action of PMA. It was concluded that PGE1 selectively reduces both N- and R-type Ca2+ currents by activating a G-protein probably through the EP3 receptor in paratracheal ganglion cells.

ATP-sensitive and Ca2+-activated K+ channel activities in the rat locus coeruleus neurons during metabolic inhibition

Locus coeruleus (LC) is the significant nucleus for consciousness and it is sensitive to metabolic inhibition. We investigated the effects of a metabolic inhibitor sodium cyanide (NaCN) on the rat dissociated LC neurons using nystatin-perforated patch recordings. Under voltage-clamp (VH=-40 mV), application of NaCN evoked outward currents composed of ATP-sensitive and Ca2+-dependent K+ channel currents (IKATP and IKCa2+). Onset of IKATP was faster than that of IKCa2+. Prolonged application of NaCN brought IKATP rundown but not IKCa2+ rundown. Okadaic acid prevented IKATP rundown, indicating that KATP channels are deactivated by dephosphorylation with protein phosphatase.

Functional properties of ionotropic glutamate receptor channels in rat sacral dorsal commissural neurons

Nystatin perforated patch and conventional whole-cell recording configurations were used to characterize the properties of ionotropic glutamate receptor (GluR) channels in neurons freshly dissociated from the rat sacral dorsal commissural nucleus (SDCN). L-Glutamate (Glu), N-methyl-D-aspartate (NMDA), quisqualate (QA), alpha-amino-3-hydroxy-5-methyl-4-isoxazoleprop ionate (AMPA) and kainate (KA) applied via a Y-tube produced inward currents at -44 mV which increased in a concentration-dependent manner; they desensitized when induced at higher concentrations except for the KA-induced current (IKA). (1S-3R)1-amino-cyclopentane-1,3-dicarboxylate (1S-3R-ACPD) evoked no response. The EC50 and Hill coefficient (nH) values of the GluR responses were 3.3 x 10(-5) M, 0.74 for Glu; 9.0 x 10(-5) M, 0.83 for NMDA; 6.4 x 10(-7) M, 1.30 for QA; 1.3 x 10(-4) M, 1.10 for AMPA and 9.6 x 10(-5) M, 1.30 for KA, respectively. The reversal potentials of the GluR responses were all near 0 mV. The 6-Cyano-7-nitroquinoxaline-2-3-dione (CNQX) and D-2-amino-5-phosphonovalerate (D-APV) suppressed the non-NMDA and NMDA responses in a concentration-dependent manner, respectively. Cyclothiazide strongly potentiated both KA- and AMPA-induced responses while concanavalin A potentiated both the responses to a much lesser degree. NS-102 produced no significant effect on either KA- or AMPA-activated currents, while GYKI 52466 reversibly blocked both the currents. The Ca2+ permeabilities (PCa/PCs) of the NMDA and AMPA receptor channels were 8.33 and 1.23, respectively. In addition, the current-voltage (I-V) relationship of IKA showed little rectification. There was a poor correlation between the Ca2+ permeability and the shape of the I-V curves of IKA. These results suggest that rat SDCN neurons possess NMDA and non-NMDA receptor channels, and express AMPA type receptors with unique properties (slow desensitization to AMPA, high Ca2+ permeability but lack of inward rectification). These ionotropic receptor channels may play important roles in mediating and regulating pelvic visceral information including nociception.

Regulation of intracellular chloride by cotransporters in developing lateral superior olive neurons

The regulatory mechanisms of intracellular Cl- concentration ([Cl-]i) were investigated in the lateral superior olive (LSO) neurons of various developmental stages by taking advantage of gramicidin perforated patch recording mode, which enables neuronal [Cl-]i measurement. Responses to glycine changed from depolarization to hyperpolarization during the second week after birth, resulting from [Cl-]i decrease. Furosemide equally altered the [Cl-]i of both immature and mature LSO neurons, indicating substantial contributions of furosemide-sensitive intracellular Cl- regulators; i.e., K+-Cl- cotransporter (KCC) and Na+-K+-Cl- cotransporter (NKCC), throughout this early development. Increase of extracellular K+ concentration and replacement of intracellular K+ with Cs+ resulted in [Cl-]i elevation at postnatal days 13-15 (P13-P15), but not at P0-P2, indicating that the mechanism of neuronal Cl- extrusion is sensitive to both furosemide and K+-gradient and poorly developed in immature LSO neurons. In addition, removal of extracellular Na+ decreased [Cl-]i at P0-P2, suggesting the existence of extracellular Na+-dependent and furosemide-sensitive Cl- accumulation in immature LSO neurons. These data show clearly that developmental changes of Cl- cotransporters alter [Cl-]i and are responsible for the switch from the neonatal Cl- efflux to the mature Cl- influx in LSO neurons. Such maturational changes in Cl- cotransporters might have the important functional roles for glycinergic and GABAergic synaptic transmission and the broader implications for LSO and auditory development.

Molecular cloning and functional characterization of a novel receptor-activated TRP Ca2+ channel from mouse brain

Characterization of mammalian homologues of Drosophila TRP proteins, which induce light-activated Ca2+ conductance in photoreceptors, has been an important clue to understand molecular mechanisms underlying receptor-activated Ca2+ influx in vertebrate cells. We have here isolated cDNA that encodes a novel TRP homologue, TRP5, predominantly expressed in the brain. Recombinant expression of the TRP5 cDNA in human embryonic kidney cells dramatically potentiated extracellular Ca2+-dependent rises of intracellular Ca2+ concentration ([Ca2+]i) evoked by ATP. These [Ca2+]i transients were inhibited by SK&F96365, a blocker of receptor-activated Ca2+ entry, and by La3+. Expression of the TRP5 cDNA, however, did not significantly affect [Ca2+]i transients induced by thapsigargin, an inhibitor of endoplasmic reticulum Ca2+-ATPases. ATP stimulation of TRP5-transfected cells pretreated with thapsigargin to deplete internal Ca2+ stores caused intact extracellular Ca2+-dependent [Ca2+]i transients, whereas ATP suppressed [Ca2+]i in thapsigargin-pretreated control cells. Furthermore, in ATP-stimulated, TRP5-expressing cells, there was no significant correlation between Ca2+ release from the internal Ca2+ store and influx of extracellular Ca2+. Whole-cell mode of patch-clamp recording from TRP5-expressing cells demonstrated that ATP application induced a large inward current in the presence of extracellular Ca2+. Omission of Ca2+ from intrapipette solution abolished the current in TRP5-expressing cells, whereas 10 nM intrapipette Ca2+ was sufficient to support TRP5 activity triggered by ATP receptor stimulation. Permeability ratios estimated from the zero-current potentials of this current were PCa:PNa:PCs = 14.3:1. 5:1. Our findings suggest that TRP5 directs the formation of a Ca2+-selective ion channel activated by receptor stimulation through a pathway that involves Ca2+ but not depletion of Ca2+ store in mammalian cells.

Acetylcholine modulation of high-voltage-activated calcium channels in the neurones acutely dissociated from rat paratracheal ganglia

1. The modulation of high-voltage-activated (HVA) Ca2+ channels by acetylcholine (ACh) was studied in the paratracheal ganglion cells acutely dissociated from 2-week-old Wistar rats by use of the nystatin perforated patch recording configuration under voltage-clamp conditions. 2. ACh inhibited the HVA Ca2+ currents in a concentration- and voltage-dependent manner. 3. The inhibition was mimicked by a muscarinic agonist, oxotremorine. Pirenzepine and methoctramine produced parallel shifts to the right in the ACh concentration-response curves. Schild analysis of the ACh concentration-ratios yield pA2 values for pirenzepine and methoctramine of 6.85 and 8.57, respectively, suggesting the involvement of an M2 receptor. 4. Nifedipine, omega-conotoxin-GVIA and omega-conotoxin-MVIIC reduced the HVA I(Ca) by 16.8, 59.2 and 6.3%, respectively. A current insensitive to all of these Ca2+ antagonists, namely 'R-type', was also observed. The results indicated the existence of L-, N-, P/Q-, and R-type Ca2+ channels. 5. The ACh-sensitive current component was markedly reduced in the presence of omega-conotoxin-GVIA, but not with both nifedipine and omega-conotoxin-MVIIC. ACh also inhibited the R-type HVA I(Ca) remaining in saturating concentrations of nifedipine, omega-conotoxin-GVIA and omega-conotoxin-MVIIC. 6. The inhibitory effect of ACh was prevented by pretreatment with pertussis toxin. 7. It was concluded that ACh selectively reduces both the N- and R-type Ca2+ channels, by activating pertussis toxin sensitive G-protein through the M2 muscarinic receptor in paratracheal ganglion cells.

alpha2-Adrenoceptor-mediated potassium currents in acutely dissociated rat locus coeruleus neurones

1. The noradrenaline (NA)-activated response was investigated in neurones acutely dissociated from the rat locus coeruleus (LC) using nystatin-perforated, conventional whole-cell and inside-out patch recording modes under current- and voltage-clamp conditions. 2. Under current-clamp conditions, NA hyperpolarized the LC neurones, abolishing the spontaneous action potentials. In voltage-clamp studies, NA induced an inwardly rectifying K+ current (INA) in a concentration-dependent manner with a half-maximum effective concentration of 2.2 x 10(-7) M. 3. INA was mimicked by the alpha2-agonist UK14304 but was inhibited by either the alpha2B/alpha2C antagonist ARC239 or the alpha1- and alpha2B/alpha2C antagonist prazosin, suggesting the contribution of alpha2B/alpha2C adrenoceptors. 4. INA was inhibited by the intracellular application of GDPbetaS but fully activated by intracellular perfusion of GTPgammaS. 5. In the inside-out recording mode, the application of GTP to the cytoplasmic side of the patch membrane markedly enhanced the open probability of the NA-activated single channels which represented the inwardly rectifying properties. 6. These results indicate that the activation of alpha2B/alpha2C adrenoceptors coupled with GTP-binding protein directly activates the inwardly rectifying K+ currents in rat LC neurones, thus resulting in a decrease in the spontaneous firing activities.

Ca2+-activated K+ currents in rat locus coeruleus neurons induced by experimental ischemia, anoxia, and hypoglycemia

Ca2+-activated K+ currents in rat locus coeruleus neurons induced by experimental ischemia, anoxia, and hypoglycemia. J. Neurophysiol. 78: 2674-2681, 1997. The effects of metabolic inhibition on membrane currents and N-methyl--aspartic acid (NMDA)-induced currents were investigated in dissociated rat locus coeruleus (LC) neurons by using the nystatin perforated patch recording mode under voltage-clamp conditions. Changes in the intracellular Ca2+ concentration ([Ca2+]i) during the metabolic inhibition were also investigated by using the microfluometry with a fluorescent probe, Indo-1. Removal of both the oxygen and glucose (experimental ischemia), deprivation of glucose (hypoglycemia), and a blockade of electron transport by sodium cyanide (NaCN) or a reduction of the mitochondrial membrane potential with carbonyl cyanide-p-trifluoromethoxyphenyl-hydrazone(FCCP) as experimental anoxia all induced a slowly developing outward current (IOUT) at a holding potential of -40 mV. The application of 10(-4) M NMDA induced a rapid transient peak and a successive steady state inward current and a transient outward current immediately after washout. All treatments related to metabolic inhibition increased the NMDA-induced outward current(INMDA-OUT) and prolonged the one-half recovery time of INMDA-OUT. The reversal potentials of both IOUT and INMDA-OUT were close to the K+ equilibrium potential (EK) of -82 mV. Either charybdotoxin or tolbutamide inhibited the IOUT and INMDA-OUT, suggesting the contribution of Ca2+-activated and ATP-sensitive K+ channels, even though the inhibitory effect of tolbutamide gradually diminished with time. Under the metabolic inhibition, the basal level of [Ca2+]i was increased and the one-half recovery time of the NMDA-induced increase in [Ca2+]i was prolonged. The IOUT induced by NaCN was inhibited by a continuous treatment of thapsigargin but not by ryanodine, indicating the involvement of inositol 1,4, 5-trisphosphate (IP3)-induced Ca2+ release (IICR) store. These findings suggest that energy deficiency causes Ca2+ release from the IICR store and activates continuous Ca2+-activated K+ channels and transient ATP-sensitive K+ channels in acutely dissociated rat LC neurons.

Dopamine-denervation enhances the trophic activity in striatum: evaluation by morphological and electrophysiological development in PC12D cells

To evaluate the possibility that dopamine (DA) denervation enhances the trophic activity in striatum, normal or DA-depleted striatal tissue extract (N- or L-extract, respectively) was obtained, and their trophic effects on PC12D cells were investigated from the viewpoints of differentiation using morphological and electrophysiological analyses. Treatment with N- or L-extract induced neurite outgrowth in a concentration-dependent manner, and induced the enlargement of cell size. These effects were stronger in L-extract than in N-extract. Cation currents were investigated in whole cell patch-clamp mode. Development of cation current started with delayed-rectifier type K+ current (IK) and transient type K+ current (IA), followed by Ca2+ current (ICa) and tetrodotoxin-sensitive Na+ current (INa). INa was expressed more frequently in L-extract treated cells than N-extract treated cells at D7-9. The larger IK amplitude in L-extract treatment at D7-9 seemed to be related to the expression of INa. Development of IA was similar at any stage for both treatments. ICa development started at D3-5 after treatments, and the amplitude and current density were similar in both treatments. ICa was strongly blocked by omega-conotoxin GVIA (3 microM), indicating that N-type channels were mainly expressed after treatments. The data suggests that L-extract has stronger effects to hasten the differentiation of PC12D cells than N-extract by promoting the neurite outgrowth, cell enlargement and expression of voltage-dependent cation channels, especially INa and IK.

The pharmacology of amino-acid responses in septal neurons

Septal cholinergic neurons are known to play an important role in cognitive processes including learning and memory through afferent innervation of the hippocampal formation and cerebral cortex. The septum contains not only cholinergic neurons but also various types of neurons including GABA (gamma-aminobutyric acid)-ergic neurons. Although synaptic transmission in the septum is mediated primarily by the activation of excitatory and inhibitory amino-acid receptors, it is possible that a distinct phenotype of neuron is endowed with a different type for each of the amino-acid receptors and thus they play different roles from each other, since it has been demonstrated within the septum that there is a regional distribution of various types of amino-acid receptor subunits, their expression as different combinations within a specific cell may produce receptor channels with disparate functional properties. As a first step towards knowing the various functions of septal cholinergic neurons, we characterized the functional properties of glutamate, GABA (type A; GABAA) and glycine receptor channels on cultured rat septal neurons which were histologically identified to be cholinergic. These were similar to those of receptor channels on other types of neurons, except for the actions of some neuromodulators. The septal N-methyl-D-aspartate receptor channel was distinct in being less sensitive to Mg2+ and in a voltage-dependent action of Zn2+. The septal GABAA receptor channel exhibited a lanthanide site whose activation resulted in a positive allosteric interaction with a binding site of pentobarbital. The septal glycine receptor channel was only positively modulated by Zn2+; this action of Zn2+ was not accompanied by an inhibitory effect. Our data suggest that the amino-acid receptors on septal cholinergic neurons may play a distinct role compared to other types of neurons; this difference depends on the actions of neuromodulators and metal cations. It would be interesting to compare these effects recorded in tissue culture to those observed with septal cholinergic neurons in slice preparations.

Pharmacologic characteristics of excitatory gamma-amino-butyric acid (GABA) receptors in a snail neuron

1. The pharmacologic characteristics of excitatory gamma-aminobutyric acid (GABA) receptors, termed muscimol II type GABA receptors, found in a giant neuron type, v-LCDN (ventral-left cerebral distinct neuron), of an African giant snail (Achatina fulica Férussac), were studied using the mammalian GABA receptor agonists, antagonists and synergists and GABA uptake inhibitor using the voltage clamp technique. 2. GABA and its agonists, ejected by brief pressure, produced an inward current (Iin) of the following order of potency: trans-t-aminocrotonic acid (TACA) > GABA > muscimol > isoguvacine > 5-aminopentanoic acid and cis-4-aminocrotonic acid (CACA). (+/-)-Baclofen and 3-aminopropylphosphonic acid (APPA) were ineffective. The Iin values produced by GABA, TACA, isoguvacine and CACA were stable for at least 60 min, whereas the Iin induced by muscimol was not. 3. According to the dose-response curves of GABA, TACA, isoguvacine and CACA, measured by the varied pressure duration method, the ED50 value of CACA was larger than those of the other compounds, and Emax of TACA was larger than that of GABA, whereas Emax values of isoguvacine and CACA were smaller. 4. The perfusion of beta-alanine, pentobarbital and 5-aminopentanoic acid inhibited the Iin induced by GABA, whereas (-)-bicuculline, pitrazepin, diazepam and 2-hydroxysaclofen had no effect. 5. From the effects of beta-alanine on the dose-response curves of GABA, measured by the varied pressure duration method, beta-alanine competitively inhibited the Iin caused by GABA. According to the effects of pentobarbital on the dose-response curves of GABA, this drug noncompetitively inhibited the Iin using the varied pressure duration method, and partly competitively and partly noncompetitively using the Y-tube method. The effects of 5-aminopentanoic acid on the dose-response curves of GABA indicated that this drug noncompetitively inhibited the Iin using the varied pressure duration method, and partly noncompetitively and partly uncompetitively using the Y-tube method. 6. The pharmacologic features of the Achatina muscimol II type GABA receptors were similar to those of mammalian GABAC (GABAp1) receptors, except for the effects of pentobarbital.

Electrophysiological and pharmacological characteristics of ionotropic glutamate receptors in medial vestibular nucleus neurons: a whole cell patch clamp study in acutely dissociated neurons

A patch clamp study was performed to determine which subtype of ionotropic glutamate receptors is involved in the glutamate-induced excitation of the medial vestibular nucleus (MVN) neurons. Whole cell recording was performed on MVN neurons that were acutely dissociated by enzymatic and mechanical treatments. Application of glutamate at a concentration of 100 microM produced a current with a reversal potential of approximately 0 mV. The glutamate-induced current was completely blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM), a non-N-methyl-D-aspartate (NMDA)-receptor antagonist. Application of alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) and kainic acid (KA), non-NMDA-receptor agonists, at concentrations of 30 and 100 microM produced a concentration-dependent depolarization concomitantly with an increase in firing rates during current clamp recording. During voltage clamp recording, glutamate, AMPA and KA elicited a concentration-dependent current with an equilibrium potential of approximately 0 mV. To clarify whether NMDA receptors are present in MVN neurons, the effects of glycine on the glutamate- and NMDA-induced current were examined. Two types of NMDA receptor-mediated current (types 1 and 2) were obtained in terms of the difference in sensitivity to both magnesium ion and MK-801, which act on the NMDA-receptor channel. In the type 1 neurons, the NMDA-induced current was not apparently blocked by magnesium ion or MK-801, although a larger current was obtained in the absence of magnesium ion. In the type 2 neurons, marked blockade of the NMDA-induced current was seen in the presence of magnesium ion and MK-801, as previously reported in other neurons of the central nervous system. These findings indicate the presence of both non-NMDA and NMDA receptors, which are involved in primary afferent transmission, in the MVN neuron, and two distinct types of NMDA receptors.

Amino acid substitutions in the rat Na+, K(+)-ATPase alpha 2-subunit alter the cation regulation of pump current expressed in HeLa cells

1. To study the functional role of negatively charged amino acids (E327 and D925) located in the transmembrane region of the rat alpha 2-isoform of the Na+, K(+)-ATPase (rat alpha 2*) in ion transport, the effects of mutations on external K+ dependence and internal Na+ dependence of pump currents were assessed by the patch-clamp technique in combination with a system for rapid solution changes. 2. Amino acid residues were replaced by glutamine (E327Q) or leucine (D925L) and were introduced into rat alpha 2* cDNA which encodes a ouabain-resistant isoform. These mutant enzymes were stably expressed in HeLa cells. The endogenous ouabain-sensitive HeLa cell Na+, K(+)-ATPase activity was selectively inhibited by 1 microM ouabain present in both the growing media and the assay solution. 3. External K(+)- and internal Na(+)-dependent pump activation was observed in all cells expressing rat alpha 2*, E327Q or D925L; however, the apparent affinities were significantly reduced by the mutations. 4. In E327Q, the activation of pump current was slightly slower than for rat alpha 2*, whereas the deactivation rate was faster. In contrast, D925L produced pump current having dramatically slower activation and deactivation kinetics. 5. These results indicate that these negatively charged amino acids (E327 and D925) are important in cation-induced conformational changes of the protein, which are intermediate steps in the pump mechanism.

Tissue extract from dopamine-depleted striatum enhances differentiation of cultured striatal type-1 astrocytes

The effects of tissue extract from dopamine (DA)-depleted striatum (lesion extract, L-ext) on morphological and electrophysiological natures of cultured striatal astrocytes were investigated. L-ext treatment suppressed the proliferation of type-1 astrocytes. They became fibrous in a concentration-dependent manner. These changes were not observed in type-2 astrocytes. By whole cell patch-clamp recording, two kinetically and pharmacologically distinct voltage-activated potassium currents, A current and delayed rectifier, were identified. L-ext treatment enhanced both currents in type-1 astrocytes, but only A current in type-2. Data suggest that in tissue extract from DA-depleted striatum, there are increased trophic activities that promote the differentiation of type-1 astrocytes.

Modulation of pump function by mutations in the first transmembrane region of Na(+)-K(+)-ATPase alpha 1-subunit

The Cys in the first transmembrane region of the Na(+)-K(+)-adenosinetriphosphatase (ATPase) alpha 1-subunit has been shown to be a critical determinant of cardiac glycoside binding. To study the role of this Cys on ion transport activity, we measured pump currents in HeLa cells expressing wild-type or mutant alpha 1-subunit cDNAs. The endogenous ouabainsensitive Na(+)-K(+)-ATPase was selectively inhibited by growing the cells in 0.1 microM ouabain. A Cys-to-Tyr substituted mutant exhibited decreased sensitivity to digitoxin but not digoxin compared with wild type. The decreased affinity for digitoxin was due to a faster dissociation rate. In contrast, the Cys-to-Ala substitution did not significantly alter the sensitivity to digitoxin or digoxin. Both wild-type and mutant cells displayed marked external K(+)-dependent pump currents; however, the affinity for K+ was reduced by the mutations. The decrease in K+ affinity was due to a slower association rate. The results show that the Cys that interacts with cardiac glycosides also participates in the sensitivity of the pump to external K+.

Time-dependent rundown of GABA response in mammalian cns neuron during experimental anoxia

Gamma-Aminobirtyric acid (GABA) is one of the major neurotransmitters in the mammalian central nervous system (CNS). The activation of post-synaptic GABAA receptor-chloride channel complex is thought to underlie inhibitory postsynaptic potentials ubiquitously in various CNS regions. GABAA receptors are modulated by convulsant, hypnotic-anticonvulsant, anxiolytic and anxiogenic agents and endogenous agents such as nurosteroids and intracellular calcium, ATP, and cyclic AMP. The function of GABAA receptor in CNS neuron is also affected by some pathophysiological processes, e.g., anoxia. For example, it is currently believed that delayed neuronal death after brain ischemia results from excessive cell excitability and/or loss of inhibition. In the present study, we investigated how the GABA-gated chloride current is affected by anoxic conditions. All experiments were carried out on neurons freshly dissociated from rat CNS by the use of both conventional and nystatin perforated patch recording configurations. The GABA response showed a considerable rundown with time in anoxic condition. The rundown was prevented by adding either ouabain or SPAI-I (Na+-K+ ATPase inhibitor-I), suggesting that the experimental anoxia reduced GABA response by decreasing intracellular ATP synthesis. This result was also confirmed by finding that the direct decrease of intracellular ATP concentration using a conventional whole-cell patch recording mode inhibited the GABA-gated chloride response in mammalian CNS neurons.

Pharmacological studies on mechanisms involved in Ca2+ oscillations in rat megakaryocytes

Extracellular application of ATP evoked the oscillatory K+ currents (IKCa) reflecting oscillation in cytoplasmic Ca2+ concentration ([Ca2-]i) of megakaryocyte isolated from rat bone marrow. We have reported that the [Ca2+], oscillation was regulated by intracellular Ca(2+)-pumping activity (Uneyama H.C. Uneyama and N. Akaike, 1993, J. Biol. Chem. 268, 168). Here we found that the Ca2+ pump of the megakaryocyte could be divided into at least two classes according to the sensitivity to phosphorylation-modulating drugs. The effects of protein kinase C and cyclic AMP-dependent protein kinase are complementary, and the effect of Ca2+/calmodulin is independent of the above two kinases. In addition, this is the first report concerning the physiological regulation of Ca(2+)-ATPase in living cells.

Block of P-type Ca2+ channels in freshly dissociated rat cerebellar Purkinje neurons by diltiazem and verapamil

We investigated the effects of organic Ca2+ channel blockers, diltiazem and verapamil, on the high voltage-activated P-type Ca2+ channels in freshly isolated rat Purkinje neurons. Both diltiazem and verapamil blocked P-type Ca2+ channel current without any change in the current-voltage relation. The block was concentration-dependent. In the presence of these agents, the inactivation curve was shifted to hyperpolarizing potentials. The characteristics of block of P-type Ca2+ channels by diltiazem and verapamil are similar to that of L-type Ca2+ channels. These results indicate that both benzothiazepine and phenylalkylamine react with P-type Ca2+ channels and suggest that some structural features common to which operate in both L-type and P-type Ca2+ channels may be involved in drug binding to these channels.

Norepinephrine modulates high voltage-activated calcium channels in freshly dissociated rat nucleus tractus solitarii neurons

The effects of norepinephrine on the low- and high-voltage-activated calcium channels in the neurons acutely dissociated from the nucleus tractus solitarius of 2- to 3-week-old rats were investigated in the nystatin perforated patch recording configuration under voltage-clamp conditions. The norepinephrine had no effect on the low voltage-activated calcium channel but inhibited the high voltage-activated calcium channel in a concentration-, time- and voltage-dependent manner. The norepinephrine slowed the activation phase of the high voltage-activated calcium channel current and the maximum inhibition was 30% of the total current amplitude measured 10 ms after the current activation. The inhibitory effect was eliminated by applying larger depolarizing prepulses. The pretreatment with pertussis toxin completely blocked the norepinephrine effect on high-voltage activated calcium channels, suggesting the contribution of pertussis toxin-sensitive Gi/Go-proteins to the norepinephrine-induced inhibition. Yohimbine but not prazosin nor propranolol antagonized the norepinephrine-induced inhibition, suggesting the involvement of alpha 2-adrenoceptor in norepinephrine-induced inhibition of the high voltage-activated calcium channels. omega-Conotoxin-GVIA, omega-agatoxin-IVA, nicardipine and omega-conotoxin-MVIIC blocked the high voltage-activated calcium channel current by 26, 9, 36 and 11% of the total current respectively, suggesting the existence of N-, P-, L- and Q-type calcium channels in the nucleus tractus solitarii neurons. The current being insensitive to these calcium channel antagonists, termed R-type calcium channel current, also existed. This residual R-type calcium channel was completely blocked by adding 200 microM CD2+. The norepinephrine significantly inhibited N- and P-type calcium channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Stable expression and coupling of cardiac L-type Ca2+ channels with beta 1-adrenoceptors

A number of neurotransmitters modulate cardiac dihydropyridine-sensitive L-type Ca2+ channels through several homologous G protein-coupled receptors. Previous studies that have examined receptor-Ca2+ channel interactions have suffered because of the coexpression of various receptor subtypes in native cells. To study the functional coupling of a particular receptor subtype to these channels, rabbit cardiac Ca2+ channel alpha 1 and skeletal beta and alpha 2/delta subunits were stably expressed in baby hamster kidney cells. In this stable cell line, Ca2+ channels remained at high levels (> 1000 fmol/mg protein, or 2700 channels per cell) over extended times. The expressed recombinant Ca2+ channels displayed the voltage dependence of activation and inactivation, unitary conductance, and pharmacology characteristic of native cardiac L-type Ca2+ channels. Subsequent coexpression of the beta 1-adrenoceptors (150 to 300 fmol/mg protein) with the Ca2+ channels resulted in cell responsiveness to the extracellular application of isoproterenol. These results indicate that heterogeneous expression in mammalian cells provides a useful system for studying both biophysical analysis of Ca2+ channel properties and receptor-coupled regulatory processes.

Ionic properties of IK,n in outer hair cells of guinea pig cochlea

The ionic properties of voltage-dependent K+ current activated at the resting membrane potential (IK,n) of outer hair cells (OHC) isolated from the guinea pig cochlea were studied using a patch-clamp technique in a whole-cell recording mode. The reversal potential of IK,n indicated a high selectivity for K+, and the relative permeability ratios for various monovalent cations were K+:Rb+:NH4+ = 1:1.21:0.13. Decrease in extracellular Cl- inhibited the IK,n. IK,n was blocked by Cs+ and Ba2+, although the inhibitory manner of Cs+ and Ba2+ were voltage-dependent and voltage-independent, respectively. By the use of puff-application method, the local application of Ba2+ to basolateral surface of OHC shifted the holding current level in an inward direction, whereas the application to apex and hair showed little change. Indicating that the IK,n channels preferentially locate at the basolateral region of cell membrane.

Actions of a new Ca2+ channel antagonist, CD832, on two types of Ca2+ channels in smooth muscle

The purpose of this study was to determine the characteristics of blockade of L- and T-type Ca2+ channels by a new Ca2+ channel antagonist, 3-(2-nicotinoylamino)ethyl 5-(3-nitrooxypropyl) 2,6-dimethyl-4-(3-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylate hydrochloride (CD832), in comparison with those of nifedipine, in rat aortic smooth muscle cells in primary culture. In a whole-cell configuration under voltage-clamp, CD832 and nifedipine dose dependently blocked both L- and T-type Ca2+ currents (L- and T-type ICa) without shifting the axis of the current-voltage relations. The 50% inhibition concentration (IC50) of CD832 was 310 nM for L-type and 1.1 microM for T-type ICa. The IC50 of nifedipine was 36 nM for L-type and 2.7 microM for T-type ICa. CD832 blocked both types of ICa from the first step pulses after drug application, shifted their steady-state inactivation curves to the left, and did not significantly accelerate the current decay. The effect of CD832 reached its maximum within 6 min and was hardly washed out, while that of nifedipine reached a maximum within 30 s and could be washed out within 3 min. These results suggest that (1) CD832 blocks L-type ICa less potently than does nifedipine, (2) CD832 blocks T-type ICa more potently than does nifedipine, (3) CD832 blocks not only the resting state but, more preferentially, the inactivated state of L- and T-type Ca2+ channels, and (4) CD832 has a slow and long-acting activity in blocking both types of ICa as compared with nifedipine.

Suramin and reactive blue 2 are antagonists for a newly identified purinoceptor on rat megakaryocyte

1. The effects of purinoceptor antagonists on ATP-induced oscillatory K(+)-currents in rat isolated megakaryocytes were investigated. 2. Both reactive blue-2 (RB-2), a selective antagonist of the P2Y purinoceptor, purinoceptor, at concentrations of 0.3-10 microM and suramin, a non-selective P2 purinoceptor antagonist, at 1-30 microM blocked the ATP-induced oscillation in a concentration-dependent manner. 3. RB-2 and suramin also blocked the ADP-induced K(+)-current oscillation at the same concentration range as in the case of ATP. However, both suramin and RB-2 had no effect on thrombin- and inositol 1,4,5-trisphosphate (IP3)-induced K+ current oscillation, indicating that they act as specific purinoceptor antagonists. 4. Thus, the purinoceptors on megakaryocytes show the properties of the P2 subtype according to their blockade by antagonists.

Nystatin-perforated patch recordings disclose KA-operated outward currents in rat cortical neurons

Kainate (KA)-induced responses were studied in acutely dissociated rat cortical neurons. KA elicited the inward currents with conventional whole-cell and nystatin-perforated patch recordings under voltage-clamp condition. An additional outward current was observed only with the perforated patch recording. The outward current was due to the activation of K+ current by Ca2+ passing through the KA channels. This K+ channel was sensitive to both iberiotoxin and tetraethylammonium (TEA)-Cl. This K+ channel may serve to limit the depolarization during excessive KA-mediated excitation.

Alterations of Ca2+ currents and their sensitivities to Ca2+ antagonists during application and after deprivation of nerve growth factor

The alterations in morphology and Ca2+ currents (ICa) were assessed in PC12 cells, which were (1) control (untreated), (2) treated with nerve growth factor (NGF), and (3) deprived of NGF. NGF treatment induced neuronal differentiation morphologically and increased the current density of ICa, and especially that of the omega-conotoxin (omega-CgTX)-sensitive component. NGF-deprived cells manifested morphological deterioration and did not exhibit remarkable changes in the current density of the total ICa and dihydropyridine components, whereas the omega-CgTX-sensitive ICa component was decreased significantly compared with that in NGF-treated cells. The results suggest that the total function of Ca2+ channels, including the dihydropyridine component, does not seem to be lost at the initial stage of neuronal deterioration, whereas omega-CgTX-sensitive Ca2+ channels lost their function early.

Cytoplasmic Ca2+ oscillation in rat megakaryocytes evoked by a novel type of purinoceptor

1. The responses of megakaryocytes isolated from rat bone marrow to externally applied adenosine triphosphate (ATP) were investigated in the whole-cell mode by the use of nystatin perforated patch-clamp technique. 2. ATP at 1-100 microM evoked periodic outward currents at a holding potential of -40 mV. The reversal potential of the currents was close to K+ equilibrium potential (EK) and the K+ channel blockers such as quinine and quinidine suppressed the currents, indicating that the outward currents are predominantly carried by K+. 3. Since it has been reported that adenosine diphosphate (ADP) evoked monophasic K+ current using a conventional whole-cell recording, we compared the results obtained by perforated and conventional patch-clamp techniques. The crucial difference between our results and previous results was due to the intracellular perfusion with internal solution containing a high concentration of EGTA by which both current shape and concentration response were modified. 4. The membrane permeable Ca2+ chelator, 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (acetoxy methyl ester; BAPTA AM), inhibited the K+ current concentration dependently, suggesting that ATP-induced oscillatory K+ currents are caused by changes in cytoplasmic free Ca2+ concentration ([Ca2+]i). 5. With increasing ATP concentration, the frequency and the maximum amplitude of K+ current oscillation increased and the latency of current, which is the period required to activate the first K+ current after ATP application, decreased. 6. ADP, 2-methylthio-ATP and ATP-gamma-S could also evoke the periodic K+ currents, but adenosine, uridine triphosphate (UTP) and alpha-beta-methylene adenosine 5'-triphosphate (AMP-CPP) failed. 2-Methylthio-ATP was the most potent agonist; next was ADP which showed a 10-30 times stronger effect than ATP. Cross-desensitization was observed between ATP and ADP, but not between ATP or ADP and thrombin. 7. Extracellular Ca2+ was not required for the ATP-induced K+ current activation, indicating that Ca2+ released from intracellular pools induced the oscillatory response. In addition, the agonist potency increased when extracellular Ca2+ concentration ([Ca2+]o) decreased, suggesting that the principal agonists might be ATP4- and ADP3-. 8. The results suggest the presence of a novel subtype of purinoceptor in the megakaryocyte plasma membrane which induces cytoplasmic Ca2+ oscillation and evokes periodic K+ current flux.

Cytoplasmic pH regulates ATP-induced Ca(2+)-dependent K(+)-current oscillation in rat megakaryocytes

The effect of cytoplasmic pH (pH(i)) on ATP-induced Ca2+ oscillation of rat megakaryocytes was investigated by the whole-cell patch-clamp technique. Megakaryocytes responded to extracellular ATP and showed the periodic activation of K+ channels, which reflects the oscillation in intracellular free Ca2+ concentration. Intracellular alkalinization by 20 mM NH4Cl resulted in inhibition of the oscillatory response, and intracellular acidification by 20 mM sodium acetate enhanced the response. NH4Cl also inhibited the Ins(1,4,5)P3-induced oscillation. Sodium acetate had no effect on the InsP3-induced oscillation, but enhanced the guanosine 5'-[gamma-thio]triphosphate-induced response. These results indicate that pH(i) modulates the ATP-induced cellular response at least at two points.

Rolipram enhances the development of voltage-dependent Ca2+ current and serotonin-induced current in rat pheochromocytoma cells

The effects of chronic treatment (6-8 days) with a phosphodiesterase inhibitor, rolipram, on the expression of voltage-dependent Ca2+ channels, nicotinic acetylcholine (ACh) receptors and 5-hydroxytryptamine (5-HT) receptors were investigated in PC12 cells. The results were compared with the effects of nerve growth factor (NGF), 8-bromo-cyclic AMP (8-Br-cAMP) and phorbol 12-myristate 13-acetate (PMA). In the morphological study rolipram, at a high concentration (100 microM) induced the extension of neurites. A similar result was obtained in 8-Br-cAMP (1 mM)-treated cells. Rolipram, at a low concentration (10 microM) or PMA (10(-7) M) did not induce obvious morphological change. NGF (100 ng/ml) induced the extension of long neurites and the formation of neural networks. Rolipram (100 microM) increased the current density (pA/pF) of voltage dependent Ca2+ current (ICa). Both NGF and 8-Br-cAMP also increased the current density of ICa, whereas PMA did not. NGF increased the current density of the nicotinic ACh response whereas rolipram, 8-Br-cAMP and PMA decreased. Rolipram (100 microM), NGF (100 ng/ml), and 8-Br-cAMP (1 mM) increased the current density of the 5-HT response whereas the effect of PMA (100 nM) was slight. The results suggest that rolipram is able to contribute to the neuronal development by increasing intracellular cAMP as well as 8-Br-cAMP. Consequently, rolipram behaves like a neurotrophic factor in cultured PC12 cells.

Nicotinic acetylcholine receptor in dissociated rat nucleus tractus solitarii neurons

The ACh-activated response of the acutely dissociated neuron from the rat nucleus tractus solitarii (NTS) was investigated using conventional and perforated-patch techniques. In the present preparation, ACh and nicotine evoked inward transient currents in approximately 30% of NTS neurons tested. The ACh-activated inward current reversed the direction near 0 mV and was inhibited by D-tubocurarine in a dose-dependent manner. In contrast, muscarine resulted in no detectable changes in the NTS neurons. Some populations of the NTS had nicotinic but no muscarinic acetylcholine receptors.

Regional variation of excitatory and inhibitory amino acid-induced responses in rat dissociated CNS neurons

Regional differences in glutamate (Glu), aspartate (Asp), gamma-aminobutyric acid (GABA) and glycine (Gly) responses in CNS neurons were investigated by means of the whole-cell mode of the patch-clamp technique. The neurons were freshly dissociated from rat cortex, limbic system (hippocampal CA1 region), diencephalon (ventromedial hypothalamus), medulla (nucleus paragigantocellularis lateralis) and spinal cord (spinal dorsal horn). The current amplitudes induced by Glu and GABA did not show any regional differences whereas those of Asp- and Gly-induced responses were significantly different among CNS regions. The enhancement of Asp response by Gly was observed in all regions, and the facilitatory ratio did not differ among these regions. Even though the NMDA response in cortical neurons was significantly greater than that in spinal neurons, the ratios of NMDA response facilitation by Gly were also the same in both regions. When the current amplitudes induced by individual amino acids were estimated for the unit surface area of respective neurons (current density), the Glu, Asp and Gly responses showed regional heterogeneity whereas the GABA response did not.

Effects of Ca2+ antagonists and aminoglycoside antibiotics on Ca2+ current in isolated outer hair cells of guinea pig cochlea

The effects of various Ca2+ antagonists and aminoglycoside antibiotics on the Ca2+ channel in isolated outer hair cells of the guinea pig were investigated using a whole-cell patch-clamp technique. The inhibitory action was in the order of La3+ much greater than Cd2+ much greater than Ni2+ greater than Co2+ for inorganic Ca2+ antagonists, and flunarizine = nicardipine greater than omega-conotoxin greater than methoxyverapamil = diltiazem much greater than amiloride for organic ones. Aminoglycoside antibiotics also had antagonistic effects on the Ca2+ channel.

Direct modulation of GABAA receptor by intracellular ATP in dissociated nucleus tractus solitarii neurones of rat

1. Effect of intracellular ATP on Cl- current (ICl) mediated by the GABA (gamma-aminobutyric acid) receptor subtype, GABAA, was studied in dissociated nucleus tractus solitarii (NTS) neurones using the whole-cell mode of patch clamp. A concentration-jump technique termed 'Y tube' was used to rapidly apply agents externally. Dissociated neurones were obtained from 1- to 3-week-old rats. 2. When the patch-pipette solution contained 2 mM-ATP, the amplitude of ICl elicited by 10(-5) M-GABA did not show any time-dependent decrease (apparent run-down), for more than 60 min after the initial recording. In the presence of ATP, the half-maximum concentration (KD) and Hill coefficient calculated from the GABA concentration-response curve were 9.12 microM and 1.47, respectively. 3. In the absence of intracellular ATP, the amplitude of GABA-induced ICl decreased with time. The relative peak amplitudes after 20 and 60 min from the initial recording were 0.40 +/- 0.09 (n = 11) and 0.16 +/- 0.05 (n = 8) with respect to the initial response. 4. Removal of Mg2+ from the internal solution induced run-down of the GABA response even in the presence of 2 mM-intracellular ATP, suggesting that both intracellular ATP (2 mM or more) and Mg2+ are necessary to prevent run-down of the GABA response. 5. Activation of dephosphorylation processes by alkaline phosphatase (100-200 microM) did not affect the GABA response in neurones perfused with internal solution containing 2 mM-ATP and 3 mM-Mg2+. Blocking the dephosphorylation process by okadaic acid, a phosphatase inhibitor, did not prevent the run-down of the GABA response. 6. Calcium influxes passing through both the voltage-dependent L-type Ca2+ channel and the glutamate receptor-operated cation channel did not affect ICl induced by GABA. 7. GABA-induced ICl was also maintained by adding 2 mM-ADP or ATP gamma S (adenosine-5'-O-3-thiotriphosphate) to the internal solution containing Mg2+. Addition of 2 mM-adenosine, AMP, cyclic AMP, AMP-PNP (adenylimido-diphosphate) or ADP beta S (adenosine-5'-O-2-thiodiphosphate) to the internal solution did not prevent the run-down of the GABA response even in the presence of 3 mM-intracellular Mg2+. Based on the chemical specificity of these ATP analogues, it is suggested that there is an ATP-sensitive binding site (ATP receptor) in the cytoplasmic side of the cell membrane.(ABSTRACT TRUNCATED AT 400 WORDS)

Glycine response in isolated dorsal cochlear nucleus of C57BL/6J mouse

Pharmacological properties of glycine (Gly)-induced Cl- current (ICl) in the dorsal cochlear nucleus (DCN) neurons acutely dissociated from C57BL/6J mouse were investigated in the whole-cell configuration of the patch-clamp technique. Gly-induced ICl increased in a sigmoidal manner with higher Gly concentrations. Strychnine blocked the Gly response competitively at low and non-competitively at high concentrations. Both glutamate (Glu) and N-methyl-D-aspartate (NMDA) responses were augmented by adding 10(-6) M Gly, at which concentration Gly did not induce any ICl. This facilitation was not affected by strychnine. Our results clearly show the existence of strychnine-sensitive and -insensitive glycine receptors in the DCN neurons.