Inhibitory action of methadone and its metabolites on erg-mediated K+ current in GH₃ pituitary tumor cells

Evaluating the expression pattern of the opioid receptor in pituitary neuroendocrine tumors (PitNET) and the role of morphine and naloxone in the regulation of pituitary cell line growth and apoptosis

PURPOSE

The expression pattern of the opioid receptor (MOR) in pituitary neuroendocrine tumors (PitNET) and the possible effect of morphine and naloxone on GH3 cell growth and apoptosis were evaluated.

METHODS

The 114 pituitary tissues including non-functioning, GH-producing and ACTH-producing PitNET and healthy cadaver pituitary tissues were included. The expression level of the MOR gene and protein was assessed using real-time PCR and Western blot. The association with patient demographic characteristics was assessed. Morphine and naloxone were applied to assess their possible pharmacological role in GH3 pituitary adenoma cell death. The cytotoxic effect, the apoptosis rate, the cell cycle distribution, the content of reactive oxygen species and the caspase 3 activity were measured.

RESULTS

MOR gene levels increased significantly in pituitary neuroendocrine tumors (PitNET) compared to the healthy pituitary samples. The increased level of MOR gene expression was prominent in invasive functional and non-functional pituitary tumors. A consistent expression pattern was demonstrated for MOR protein levels in PitNET samples. A dose- and time-dependent reduction in the rate of GH3 pituitary cells was observed after morphine treatment with an IC50 of 483 µM after 24 h of incubation. Morphine induced early apoptosis, accumulation of cells in sub-G1 phase, increase in cellular ROS levels and caspase-3 activity. The observed effects of morphine were reversed after MOR blockade using 10 and 25 µM naloxone.

CONCLUSION

The possible contributing role of the MOR in pituitary tumor cell growth and the putative pharmaceutical effect of morphine in pituitary neuroendocrine tumor cell death (PitNET) is illustrated.

The Effectiveness of Isoplumbagin and Plumbagin in Regulating Amplitude, Gating Kinetics, and Voltage-Dependent Hysteresis of erg-mediated K+ Currents

Isoplumbagin (isoPLB, 5-hydroxy-3-methyl-1,4-naphthoquinone), a naturally occurring quinone, has been observed to exercise anti-inflammatory, antimicrobial, and antineoplastic activities. Notably, whether and how isoPLB, plumbagin (PLB), or other related compounds impact transmembrane ionic currents is not entirely clear. In this study, during GH3-cell exposure to isoPLB, the peak and sustained components of an erg (ether-à-go-go related gene)-mediated K+ current (IK(erg)) evoked with long-lasting-step hyperpolarization were concentration-dependently decreased, with a concomitant increase in the decaying time constant of the deactivating current. The presence of isoPLB led to a differential reduction in the peak and sustained components of deactivating IK(erg) with effective IC50 values of 18.3 and 2.4 μM, respectively, while the KD value according to the minimum binding scheme was estimated to be 2.58 μM. Inhibition by isoPLB of IK(erg) was not reversed by diazoxide; however, further addition of isoPLB, during the continued exposure to 4,4′-dithiopyridine, did not suppress IK(erg) further. The recovery of IK(erg) by a two-step voltage pulse with a geometric progression was slowed in the presence of isoPLB, and the decaying rate of IK(erg) activated by the envelope-of-tail method was increased in its presence. The strength of the IK(erg) hysteresis in response to an inverted isosceles-triangular ramp pulse was diminished by adding isoPLB. A mild inhibition of the delayed-rectifier K+ current (IK(DR)) produced by the presence of isoPLB was seen in GH3 cells, while minimal changes in the magnitude of the voltage-gated Na+ current were demonstrated in its presence. Moreover, the IK(erg) identified in MA-10 Leydig tumor cells was blocked by adding isoPLB. Therefore, the effects of isoPLB or PLB on ionic currents (e.g., IK(erg) and IK(DR)) demonstrated herein would be upstream of our previously reported perturbations on mitochondrial morphogenesis or respiration. Taken together, the perturbations of ionic currents by isoPLB or PLB demonstrated herein are likely to contribute to the underlying mechanism through which they, or other structurally similar compounds, result in adjustments in the functional activities of different neoplastic cells (e.g., GH3 and MA-10 cells), presuming that similar in vivo observations occur.

Silver(i)-catalyzed tandem reaction of enynones and 4-alkynyl isoxazoles: regioselective synthesis of highly functionalized 4H-furan[3,4-c]pyrroles

This work reports a silver(i)-catalyzed tandem reaction of enynones with 4-alkynyl isoxazoles.

Effective block by pirfenidone, an antifibrotic pyridone compound (5-methyl-1-phenylpyridin-2[H-1]-one), on hyperpolarization-activated cation current: An additional but distinctive target

Pirfenidone (PFD), a pyridone compound, is well recognized as an antifibrotic agent tailored for the treatment of idiopathic pulmonary fibrosis. Recently, through its anti-inflammatory and anti-oxidant effects, PFD based clinical trial has also been launched for the treatment of coronavirus disease (COVID-19). To what extent this drug can perturb membrane ion currents remains largely unknown. Herein, the exposure to PFD was observed to depress the amplitude of hyperpolarization-activated cation current (I_h) in combination with a considerable slowing in the activation time of the current in pituitary GH₃ cells. In the continued presence of ivabradine or zatebradine, subsequent application of PFD decreased I_h amplitude further. The presence of PFD resulted in a leftward shift in I_h activation curve without changes in the gating charge. The addition of this compound also led to a reduction in area of voltage-dependent hysteresis evoked by long-lasting inverted triangular (downsloping and upsloping) ramp pulse. Neither the amplitude of M-type nor erg-mediated K⁺ current was altered by its presence. In whole-cell potential recordings, addition of PFD reduced the firing frequency, and this effect was accompanied by the depression in the amplitude of sag voltage elicited by hyperpolarizing current stimulus. Overall, this study highlights evidence that PFD is capable of perturbing specific ionic currents, revealing a potential additional impact on functional activities of different excitable cells.

Evidence for the Effectiveness of Remdesivir (GS-5734), a Nucleoside-Analog Antiviral Drug in the Inhibition of I K(M) or I K(DR) and in the Stimulation of I MEP

Remdesivir (RDV, GS-5734), a broad-spectrum antiviral drug in the class of nucleotide analogs, has been particularly tailored for treatment of coronavirus infections. However, to which extent RDV is able to modify various types of membrane ion currents remains largely uncertain. In this study, we hence intended to explore the possible perturbations of RDV on ionic currents endogenous in pituitary GH3 cells and Jurkat T-lymphocytes. The whole-cell current recordings of ours disclosed that upon membrane depolarization in GH3 cells the exposure to RDV concentration-dependently depressed the peak or late components of I K(DR) elicitation with effective IC50 values of 10.1 or 2.8 μM, respectively; meanwhile, the value of dissociation constant of RDV-induced blockage of I K(DR) on the basis of the first-order reaction was yielded to be 3.04 μM. Upon the existence of RDV, the steady-state inactivation curve of I K(DR) was established in the RDV presence; moreover, the recovery became slowed. However, RDV-induced blockage of I K(DR) failed to be overcome by further addition of either α,β-methylene ATP or cyclopentyl-1,3-dipropylxanthine. The RDV addition also lessened the strength of M-type K+ current with the IC50 value of 2.5 μM. The magnitude of voltage hysteresis of I K(M) elicited by long-lasting triangular ramp pulse was diminished by adding RDV. Membrane electroporation-induced current in response to large hyperpolarization was enhanced, with an EC50 value of 5.8 μM. Likewise, in Jurkat T-lymphocytes, adding RDV declined I K(DR) amplitude concomitantly with the raised rate of current inactivation applied by step depolarization. Therefore, in terms of the RDV molecule, there appears to be an unintended activity of the prodrug on ion channels. Its inhibition of both I K(DR) and I K(M) occurring in a non-genomic fashion might provide additional but important mechanisms through which in vivo cellular functions are seriously perturbed.

Against Repurposing Methadone for Glioblastoma Therapy

Methadone, which is used as maintenance medication for outpatient treatment of opioid dependence or as an analgesic drug, has been suggested by preclinical in vitro and mouse studies to induce cell death and sensitivity to chemo- or radiotherapy in leukemia, glioblastoma, and carcinoma cells. These data together with episodical public reports on long-term surviving cancer patients who use methadone led to a hype of methadone as an anti-cancer drug in social and public media. However, clinical evidence for a tumoricidal effect of methadone is missing and prospective clinical trials, except in colorectal cancer, are not envisaged because of the limited preclinical data available. The present article reviews the pharmacokinetics, potential molecular targets, as well as the evidence for a tumoricidal effect of methadone in view of the therapeutically achievable doses in the brain. Moreover, it provides original in vitro data showing that methadone at clinically relevant concentrations fails to impair clonogenicity or radioresistance of glioblastoma cells.

Effectiveness in the inhibition of dapagliflozin and canagliflozin on M-type K+ current and α-methylglucoside-induced current in pituitary tumor (GH3) and pheochromocytoma PC12 cells

Dapagliflozin (DAPA) or canagliflozin (CANA), Na⁺-dependent glucose co-transporter type 2 (SGLT2) inhibitors, were used for treatment of type II diabetes mellitus. Addition of DAPA or CANA suppressed M-type K⁺ current (I_K(M)) in pituitary tumor (GH₃) and pheochromocytoma PC12 cells. The IC₅₀ value for DAPA- or CANA-mediated inhibition of I_K(M) in GH₃ cells was 0.11 or 0.42 μM, respectively. The presence of DAPA (0.1 μM) shifted the steady-state activation of I_K(M) to less depolarized potential without changing the gating charge of the current. During high-frequency depolarizing pulses, I_K(M) magnitude was reduced by DAPA; however, DAPA-induced block of I_K(M) remained effective. The amplitude of neither erg-mediated K⁺ current nor hyperpolarization-activated cation current in GH₃ cells was modified in the presence of 1 μM DAPA. Alternatively, addition of DAPA, CANA, phlorizin or chlorotoxin effectively suppressed α-methylglucoside-(αMG-) induced current (I_αMG) in GH₃ cells, albeit inability of tefluthrin (activator of I_Na) to suppress this current. DAPA shifted the charge-voltage relation of presteady-state I_αMG in a rightward and downward direction with no change in the gating charge of the I_αMG. Under current-clamp recordings, subsequent additions of DAPA, but still in the continued presence of αMG, increased the firing rate of spontaneous action potentials stimulated by αMG. Our results suggested that activity of SGLT was expressed functionally in GH₃ and PC12 cells. Therefore, inhibitory actions of DAPA or CANA on the amplitude and gating of I_K(M) might provide a yet unidentified mechanism through which the SGLT1 or SGLT2 activity were attenuated in unclamped cells occurring in vivo.

Characterization of Convergent Suppression by UCL-2077 (3-(Triphenylmethylaminomethyl)pyridine), Known to Inhibit Slow Afterhyperpolarization, of erg-Mediated Potassium Currents and Intermediate-Conductance Calcium-Activated Potassium Channels

UCL-2077 (triphenylmethylaminomethyl)pyridine) was previously reported to suppress slow afterhyperpolarization in neurons. However, the information with respect to the effects of UCL-2077 on ionic currents is quite scarce. The addition of UCL-2077 decreased the amplitude of erg-mediated K⁺ current (I_K(erg)) together with an increased deactivation rate of the current in pituitary GH₃ cells. The IC₅₀ and K_D values of UCL-2077-induced inhibition of I_K(erg) were 4.7 and 5.1 μM, respectively. UCL-2077 (10 μM) distinctly shifted the midpoint in the activation curve of I_K(erg) to less hyperpolarizing potentials by 17 mV. Its presence decreased the degree of voltage hysteresis for I_K(erg) elicitation by long-lasting triangular ramp pulse. It also diminished the probability of the opening of intermediate-conductance Ca²⁺-activated K⁺ channels. In cell-attached current recordings, UCL-2077 raised the frequency of action currents. When KCNH2 mRNA was knocked down, a UCL-2077-mediated increase in AC firing was attenuated. Collectively, the actions elaborated herein conceivably contribute to the perturbating effects of this compound on electrical behaviors of excitable cells.

Concerted suppression of Ih and activation of IK(M) by ivabradine, an HCN-channel inhibitor, in pituitary cells and hippocampal neurons

Ivabradine (IVA), a heart-rate reducing agent, is recognized as an inhibitor of hyperpolarization-activated cation current (I_h) and also reported to ameliorate inflammatory or neuropathic pain. However, to what extent this agent can perturb another types of membrane ion currents in neurons or endocrine cells remains to be largely unknown. Therefore, the I_h or other types of ionic currents in pituitary tumor (GH₃) cells and in hippocampal mHippoE-14 neurons was studied with or without the presence of IVA or other related compounds. The IVA addition caused a time- and concentration-dependent reduction in the amplitude of I_h with an IC₅₀ value of 0.64 μM and a K_D value of 0.68 μM. IVA (0.3 μM) shifted the I_h activation curve to a more negative potential by approximately 8 mV, despite no concomitant change in the gating charge. Additionally, IVA was found to increase M-type K⁺ current (I_K(M)) together with a rightward shift in the activation curve. In cell-attached current recordings, IVA (3 μM) applied to the bath increased the open probability of M-type K⁺ channels; however, it did not modify single-channel conductance of the channel. In current-clamp voltage recordings, IVA suppressed the firing of spontaneous action potentials in GH₃ cells; and, further addition of linopirdine attenuated its suppression of firing. In hippocampal mHippoE-14 neurons, IVA also effectively increased I_K(M) amplitude. In summary, both inhibition of I_h and activation of I_K(M) caused by IVA can synergistically combine to influence electrical behaviors in different types of electrically excitable cells occurring in vivo.

Bisoprolol, Known to Be a Selective β₁-Receptor Antagonist, Differentially but Directly Suppresses IK(M) and IK(erg) in Pituitary Cells and Hippocampal Neurons

Bisoprolol (BIS) is a selective antagonist of β₁ adrenergic receptors. We examined the effects of BIS on M-type K⁺ currents (I_K(M)) or erg-mediated K⁺ currents (I_K(erg)) in pituitary GH_3, R1220 cells, and hippocampal mHippoE-14 cells. As GH₃ cells were exposed to BIS, amplitude of I_K(M) was suppressed with an IC₅₀ value of 1.21 μM. The BIS-induced suppression of I_K(M) amplitude was not affected by addition of isoproterenol or ractopamine, but attenuated by flupirtine or ivabradine. In cell-attached current, BIS decreased the open probability of M-type K⁺ (K_M) channels, along with decreased mean opening time of the channel. BIS decreased I_K(erg) amplitude with an IC₅₀ value of 6.42 μM. Further addition of PD-118057 attenuated BIS-mediated inhibition of I_K(erg). Under current-clamp conditions, BIS depolarization increased the firing of spontaneous action potentials in GH₃ cells; addition of flupirtine, but not ractopamine, reversed BIS-induced firing rate. In R1220 cells, BIS suppressed I_K(M); subsequent application of ML-213(Kv7.2 channel activator) reversed BIS-induced suppression of the current. In hippocampal mHippoE-14 neurons, BIS inhibited I_K(M) to a greater extent compared to its depressant effect on I_K(erg). This demonstrated that in pituitary cells and hippocampal neurons the presence of BIS is capable of directly and differentially suppressing I_K(M) and I_K(erg), despite its antagonism of β₁-adrenergic receptors.

Activation of voltage-gated sodium current and inhibition of erg-mediated potassium current caused by telmisartan, an antagonist of angiotensin II type-1 receptor, in HL-1 atrial cardiomyocytes

Telmisartan (TEL) is a non-peptide blocker of angiotensin II type-1 (AT₁ ) receptor. However, the mechanisms through which this drug interacts directly with ion currents in hearts remain largely unclear. Herein, we aim to investigate the effects of TEL the on ionic currents and membrane potential of murine HL-1 cardiomyocytes. In whole-cell recordings, addition of TEL stimulated the peak and late components of voltage-gated Na⁺ currents (I_Na ) with different potencies. The EC₅₀ values required to achieve the stimulatory effect of this drug on peak and late I_Na were 0.2 and 1.2 μmol/L, respectively, and the current-voltage relationship of peak I_Na shifted toward less-depolarized potentials during exposure to TEL. Telmisartan not only increased peak I_Na but also prolonged the inactivation time course of late I_Na . Amiodarone (Amio) or ranolazine (Ran), but not angiotensin II, could reverse TEL-mediated effects. The drug enhanced the recovery rate of I_Na inactivation and exerted an inhibitory effect on erg-mediated K⁺ and L-type Ca²⁺ currents. In whole-cell current-clamp recordings, addition of the drug resulted in prolongation of the duration of action potentials (APs) in a dose-dependent manner in HL-1 cells; Amio or Ran could reverse this increase in AP durations. Telmisartan-mediated prolongation of AP was attenuated in KCNH2 siRNA-transfected HL-1 cells. In cultured smooth muscle cells of the human coronary artery, TEL enhanced I_Na amplitudes and slowed current inactivation. Stimulation by TEL of I_Na in HL-1 cells did not simply increase current magnitude but altered current kinetics, thereby suggesting state-dependent activation. Telmisartan may have greater affinity to the open/inactivated state than to the resting state residing in Na_V channels. Collectively, TEL-mediated stimulation of I_Na and inhibition of I_K(erg) could be an important ionic mechanism underlying the increased cell excitability of HL-1 cells; these actions, however, cannot be entirely explained by its blockade of AT₁ receptor.

Stimulatory actions of a novel thiourea derivative on large-conductance, calcium-activated potassium channels

In this study, we examine whether an anti-inflammatory thiourea derivative, compound #326, actions on ion channels. The effects of compound #326 on Ca²⁺ -activated K⁺ channels were evaluated by patch-clamp recordings obtained in cell-attached, inside-out or whole-cell configuration. In pituitary GH₃ cells, compound #326 increased the amplitude of Ca²⁺ -activated K⁺ currents (I_K(Ca) ) with an EC₅₀ value of 11.6 μM, which was reversed by verruculogen, but not tolbutamide or TRAM-34. Under inside-out configuration, a bath application of compound #326 raised the probability of large-conductance Ca²⁺ -activated K⁺ (BK_Ca ) channels. The activation curve of BK_Ca channels was shifted to less depolarised potential with no modification of the gating charge of the curve; consequently, the difference of free energy was reduced in the presence of this compound. Compound #326-stimulated activity of BK_Ca channels is explained by a shortening of mean closed time, despite its inability to alter single-channel conductance. Neither delayed-rectifier nor erg-mediated K⁺ currents was modified. Compound #326 decreased the peak amplitude of voltage-gated Na⁺ current with no clear change in the overall current-voltage relationship of this current. In HEK293T cells expressing α-hSlo, compound #326 enhanced BK_Ca channels effectively. Intriguingly, the inhibitory actions of compound #326 on interleukin 1β in lipopolysaccharide-activated microglia were significantly reversed by verruculogen, whereas BK_Ca channel inhibitors suppressed the expressions of inducible nitric oxide synthase. The BK_Ca channels could be an important target for compound #326 if similar in vivo results occur, and the multi-functionality of BK_Ca channels in modulating microglial immunity merit further investigation.

Ability of naringenin, a bioflavonoid, to activate M-type potassium current in motor neuron-like cells and to increase BKCa-channel activity in HEK293T cells transfected with α-hSlo subunit

Background

Naringenin (NGEN) is a citrus bioflavonoid known to have beneficial health properties; however, the ionic mechanism of its actions remains largely unclear. In this study, we attempted to evaluate the possible effects of NGEN on K⁺ currents in NSC-34 neuronal cells and in HEK293T cells expressing α-hSlo.

Results

NGEN increased M-type K⁺ current (I_K(M)) in a concentration-dependent manner with an EC₅₀ value of 9.8 μM in NSC-34 cells. NGEN shifted the activation curve of I_K(M) conductance to the more negative potentials. In cell-attached recordings, NGEN or flupirtine enhanced the activity of M-type K⁺ (K_M) channels with no changes in single-channel amplitude. NGEN (10 μM) had minimal effect on erg-mediated K⁺ currents. Under cell-attached voltage-clamp recordings, NGEN decreased the frequency of spontaneous action currents and further application of linopirdine can reverse NGEN-induced inhibition of firing. In HEK293T cells expressing α-hSlo, this compound increased the amplitude of Ca²⁺-activated K⁺ current (I_K(Ca)). Under inside-out recordings, NGEN applied to the intracellular side of the detached patch enhanced the activity of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels. Moreover, from the study of a modeled neuron, burst firing of simulated action potentials (APs) was reduced in the presence of the increased conductances of both K_M and K_Ca channels. Fast-slow analysis of AP bursting from this model also revealed that as the conductances of both K_M and BK_Ca channels were increased by two-fold, the voltage nullcline was shifted in an upward direction accompanied by the compression of burst trajectory.

Conclusions

The present results demonstrate that activation of both K_M and BK_Ca channels caused by NGEN might combine to influence neuronal activity if similar channels were functionally co-expressed in central neurons in vivo.

Effects of midazolam on ion currents and membrane potential in differentiated motor neuron-like NSC-34 and NG108-15 cells

Midazolam (MDL) was known to act through stimulation of benzodiazepine receptors (GABA). Whether midazolam affects ion currents and membrane potential in neurons remains largely unclear. Electrophysiological studies of midazolam actions were performed in differentiated motor neuron-like (NSC-34 and NG108-15) cells. Midazolam suppressed the amplitude of delayed rectifier K(+) current (IK(DR)) in a time- and concentration-dependent manner with an IC50 value of 10.4 µM. Addition of midazolam was noted to enhance the rate of IK(DR) inactivation. On the basis of minimal binding scheme, midazolam-induced block of IK(DR) was quantitatively provided with a dissociation constant of 9.8 µM. Recovery of IK(DR) from inactivation in the presence of midazolam was fitted by a single exponential. midazolam had no effect on M-type or erg-mediated K(+) current in these cells. Midazaolam (30 µM) suppressed the peak amplitude of voltage-gated Na(+) current (INa) with no change in the current-voltage relationships of this current. Inactivation kinetics of INa remained unaltered in the presence of this agent. In current-clamp configuration, midazolam (30 µM) prolonged the duration of action potentials (APs) and reduce AP amplitude. Similarly, in differentiated NG108-15 cells, the exposure to midazolam also suppressed IK(DR) with a concomitant increase in current inactivation. Midazolam can act as an open-channel blocker of delayed-rectifier K(+) channels in these cells. The synergistic blocking effects on IK(DR) and INa may contribute to the underlying mechanisms through which midazolam affects neuronal function in vivo.

Effects of ketamine and its metabolites on ion currents in differentiated hippocampal H19-7 neuronal cells and in HEK293T cells transfected with α-hslo subunit

Ketamine (KT), a dissociative anesthetic, is known to induce schizophrenia-like psychosis. The percentage of KT abuse has recently grown fast despite KT being a controlled drug. The mechanism of KT actions is related to the inhibition of NMDA receptors. Whether KT produces other effects on ion currents in hippocampal neurons remains unclear. In this study, we attempted to evaluate the possible effects of KT and other related compounds on ion currents in hippocampal neuron-derived H19-7 cells. This drug exerted an inhibitory effect on Ca(2+)-activated K(+) current (IK(Ca)) in these cells with an IC(50) value of 274 μM. Pimaric acid (30 μM) or abietic acid (30 μM), known to stimulate large-conductance Ca(2+)-activated K(+) channels, reversed KT-induced inhibition of I(K)(Ca). In HEK293T cells expressing a-humans low poke, KT-induced inhibition of I(K)(Ca) still existed. Dehydronorketamine (300 μM) had little or no effect on the IK(Ca) amplitude, while norketamine (300 μM) slightly but significantly suppressed it. In inside–out configuration, KT applied to the intracellular face of the membrane did not alter single channel conductance of large-conductance Ca(2+)-activated K(+) (BKCa) channels; however, it did significantly reduce the probability of channel openings. Addition of KT was effective in depressing the peak amplitude of voltage-gated Na(+) current. Moreover, the presence of KT was noted to enhance the amplitude of membrane electroporation-induced inward currents (IMEP) in differentiated H19-7 cells. KT-stimulated IMEP was reversed by further application of LaCl(3) (100 μM), but not by NMDA (30 μM). The modulations by this compound of ion channels may contribute to the underlying mechanisms through which KT and its metabolites influence the electrical behavior of hippocampal neurons if similar findings occur in vivo.

Endogenous opiates and behavior: 2011

This paper is the thirty-fourth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2011 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

The inhibition by di(2-ethylhexyl)-phthalate of erg-mediated K⁺ current in pituitary tumor (GH₃) cells

DEHP (bis(2-ethylhexyl)-phthalate) known to be an endocrine-disrupting chemical is a widely used phthalate. Little information regarding the effects of phthalate esters on ion currents is available. In this study, the effects of DEHP and other phthalate esters (DBEP: di(2-butoxyethyl)-phthalate and DMGP: di(2-methylglycol)-phthalate) on ion currents were investigated in pituitary GH₃ cells. Hyperpolarization-elicited K⁺ currents in GH3 cells bathed in high-K⁺, Ca²⁺-free solution were examined to evaluate the effects of DEHP, DBEP, and DMGP on the ether-a`-go-go-related-gene (erg) K⁺ current (IK(erg)). Addition of DEHP to GH₃ cells suppressed the amplitude of IK(erg) in a concentration-dependent manner with an IC₅₀ value of 16.3 μM. With a two-pulse protocol, addition of DEHP shifted the activation curve of IK(erg) to a depolarized potential by approximately 10 mV with no change in the rate of IK(erg) deactivation. This compound did not have any effects on delayed rectifier K⁺ current in GH₃ cells, while 4-aminopyridine-3-methanol (100 μM) suppressed this current significantly. DBEP (30 μM) had little or no effect on IK(erg), while DMGP (30 μM) slightly reduced it. In inside-out configuration, DEHP (30 μM) applied to the bath slightly reduced the activity of large-conductance Ca²⁺-activated K⁺ channels. DEHP (30 μM) increased the frequency of spontaneous action potentials (APs); however, this compound at the same concentration had no effect on AP firing in KCNH2 siRNA-transfected GH₃ cells. The effects described herein can contribute to their actions on functional activity of endocrine or neuroendocrine cells if similar results are found in vivo.

The effects of magnetite (Fe₃O₄) nanoparticles on electroporation-induced inward currents in pituitary tumor (GH₃) cells and in RAW 264.7 macrophages

Aims

Fe₃O₄ nanoparticles (NPs) have been known to provide a distinct image contrast effect for magnetic resonance imaging owing to their super paramagnetic properties on local magnetic fields. However, the possible effects of these NPs on membrane ion currents that concurrently induce local magnetic field perturbation remain unclear.

Methods

We evaluated whether amine surface-modified Fe₃O₄ NPs have any effect on ion currents in pituitary tumor (GH₃) cells via voltage clamp methods.

Results

The addition of Fe₃O₄ NPs decreases the amplitude of membrane electroporation-induced currents (I_MEP) with a half-maximal inhibitory concentration at 45 μg/mL. Fe₃O₄ NPs at a concentration of 3 mg/mL produced a biphasic response in the amplitude of I_MEP, ie, an initial decrease followed by a sustained increase. A similar effect was also noted in RAW 264.7 macrophages.

Conclusion

The modulation of magnetic electroporation-induced currents by Fe₃O₄ NPs constitutes an important approach for cell tracking under various imaging modalities or facilitated drug delivery.

Evidence for aconitine-induced inhibition of delayed rectifier K(+) current in Jurkat T-lymphocytes

Aconitine (ACO) is a highly toxic diterpenoid alkaloid and known to exert the immunomodulatory action. However, whether it has any effects on ion currents in immune cells remains unknown. The effects of ACO and other related compounds on ion currents in Jurkat T-lymphocytes were investigated in this study. ACO suppressed the amplitude of delayed-rectifier K(+) current (I(K(DR))) in a time- and concentration-dependent manner. Margatoxin (100 nM), a specific blocker of K(V)1.3-encoded current, decreased the I(K(DR)) amplitude in these cells and the ACO-induced inhibition of I(K(DR)) was not reversed by 1-ethyl-2-benzimidazolinone (30 μM) or nicotine (10 μM). The IC(50) value for ACO-mediated inhibition of I(K(DR)) was 5.6 μM. ACO accelerated the inactivation of I(K(DR)) with no change in the activation rate of this current. Increasing the ACO concentration not only reduced the I(K(DR)) amplitude, but also accelerated the inactivation time course of the current. With the aid of minimal binding scheme, the inhibitory action of ACO on I(K(DR)) was estimated with a dissociation constant of 6.8 μM. ACO also shifted the inactivation curve of I(K(DR)) to a hyperpolarized potential with no change in the slope factor. Cumulative inactivation for I(K(DR)) was enhanced in the presence of ACO. In Jurkat cells incubated with amiloride (30 μM), the ACO-induced inhibition of I(K(DR)) remained unaltered. In RAW 264.7 murine macrophages, ACO did not modify the kinetics of I(K(DR)), although it suppressed I(K(DR)) amplitude. Taken together, these effects can significantly contribute to its action on functional activity of immune cells if similar results are found in vivo.

Inhibitory effect of memantine, an NMDA-receptor antagonist, on electroporation-induced inward currents in pituitary GH3 cells

The membrane electroporation-induced inward current (IMEP) in pituitary tumor (GH3) cells was characterized. This current emerges irregularly when membrane hyperpolarizations to -200 mV with a holding potential of -80 mV were elicited. Neither E-4031 (10 μM), glibenclamide (30 μM), nor ZD7288 (30 μM) caused any effects on IMEP. The single-channel conductance and pore radius were estimated to be around 1.12 nS and 1.7 nm, respectively. LaCl3- and memantidine (MEM)-induced block of this current was also examined. The IC50 value for LaCl3- and MEM-induced inhibition of IMEP was 35 and 75 μM, respectively. However, unlike LaCl3, MEM (300 μM) did not exert any effect on voltage-gated Ca2+ current. In inside-out configuration, MEM applied to either external or internal surface of the excised patch did not suppress the activity of ATP-sensitive K+ channels expressed in GH3 cells, although glibenclamide significantly suppressed channel activity. This study provides the first evidence to show that MEM, a non-competitive antagonist of N-methyl D-aspartate receptors, directly inhibits the amplitude of IMEP in pituitary GH3 cells. MEM-mediated block of IMEP in these cells is unlinked to its inhibition of glutamate-induced currents or ATP-sensitive K+ currents. The channel-suppressing properties of MEM might contribute to the underlying mechanisms by which it and its structurally related compounds affect neuronal or neuroendocrine function.