Nicotinic acetylcholine receptors in cultured cells as targets of neurotoxic compounds

Using electrophysiological techniques, effects of neurotoxicants were studied on mammalian neuronal and endplate type nicotinic acetylcholine receptors (nAChR) in N1E-115 cells and in BC3H(1) cells, respectively, and insect nAChR in locust neurons. Neuronal nAChR are highly sensitive to inorganic lead (Pb(2+)). Between 1 nm and 3 mum-Pb(2+) the ACh-induced inward current is blocked in a concentration-dependent manner (IC(50) = 19 nm; maximal effect (E(max)) = 90%). In contrast, the serotonin 5-HT(3) receptor is far less sensitive to Pb(2+) (IC(50) = 49 mum). Surprisingly, between 10 mum and 100 mum Pb(2+) the blocking effect on the nAChR is reversed, and the kinetics of the ACh-induced inward current are delayed. Nitromethylene heterocyclic (NMH) compound constitute a new class of selective insecticides, that presumably affect insect nAChR. The effect of the NMH compound 1-(pyridin-3-yl-methyl)-2-nitromethylene-imidazolidine (PMNI) on the different subtypes of nAChR has been analysed. Distinct agonistic effects of PMNI on nAChR are observed on insect neurons only. Further, PMNI blocks nicotinic responses mediated by the different subtypes of nAChR in the following potency order: locust å neuronal type endplate type nAChR. These results demonstrate that the analysis of electrophysiological endpoints in cultured cells is a valuable approach to the investigation of target site selectivity and species specificity of neurotoxic compounds.

Calcium oscillations in human T and natural killer cells depend upon membrane potential and calcium influx

With the use of single-cell digital imaging of the fluorescent Ca2+ indicator dye fura-2 we investigated Ca2+ signaling in human T lymphocytes and NK cells during activation by a variety of stimuli. A low percentage of resting T cells or T cell blasts displayed oscillations in cytosolic Ca2+ when stimulated with the mitogenic lectin PHA or by the addition of OKT3 mAb followed by a secondary cross-linking antibody. Lymphokine-activated T killer cells were more responsive than resting cells. A comparison of PHA, cross-linked anti-CD3, and a heteroconjugate mAb showed that at least 20% of the cells from these T cell preparations oscillated. Addition of PHA or cross-linked anti-CD16 caused NK cells to oscillate. In contrast, thapsigargin, a microsomal ATPase blocker, resulted in a relatively uniform, slowly rising and sustained Ca2+ response in all cell types studied. The maintenance of both thapsigargin- and receptor-induced responses required Ca2+ influx driven by a negative membrane potential. Because Ca2+ oscillations occurred in response to stimuli which mimic the normal activation of lymphocytes, and inasmuch as the percentage of oscillating cells increases with state of activation, these oscillations may play an important role in mitogenic activation.

Calcium oscillations in human T and natural killer cells depend upon membrane potential and calcium influx

With the use of single-cell digital imaging of the fluorescent Ca2+ indicator dye fura-2 we investigated Ca2+ signaling in human T lymphocytes and NK cells during activation by a variety of stimuli. A low percentage of resting T cells or T cell blasts displayed oscillations in cytosolic Ca2+ when stimulated with the mitogenic lectin PHA or by the addition of OKT3 mAb followed by a secondary cross-linking antibody. Lymphokine-activated T killer cells were more responsive than resting cells. A comparison of PHA, cross-linked anti-CD3, and a heteroconjugate mAb showed that at least 20% of the cells from these T cell preparations oscillated. Addition of PHA or cross-linked anti-CD16 caused NK cells to oscillate. In contrast, thapsigargin, a microsomal ATPase blocker, resulted in a relatively uniform, slowly rising and sustained Ca2+ response in all cell types studied. The maintenance of both thapsigargin- and receptor-induced responses required Ca2+ influx driven by a negative membrane potential. Because Ca2+ oscillations occurred in response to stimuli which mimic the normal activation of lymphocytes, and inasmuch as the percentage of oscillating cells increases with state of activation, these oscillations may play an important role in mitogenic activation.

Neuronal-type nicotinic receptors in human neuroblastoma and small-cell lung carcinoma cell lines

A beta subunit of the neuronal nicotinic receptor, sharing 88% homology with the rat beta 4 subunit, has been cloned from a human neuroblastoma cell line. The gene encoding the human beta 4 subunit is expressed in association with the alpha 3 gene in neuroblastoma and small-cell lung carcinoma cell lines. Patch-clamp experiments and radioligand binding assays confirm that these neuroendocrine tumor cell lines express functional neuronal nicotinic receptors. We suggest that these receptors might play a crucial role in the control of neurotransmitter and hormone secretion from neurosecretory human tumors.

The nitromethylene heterocycle 1-(pyridin-3-yl-methyl)-2-nitromethylene-imidazolidine distinguishes mammalian from insect nicotinic receptor subtypes

Effects of the insecticidal compound 1-(pyridin-3-yl-methyl)-2-nitromethylene-imidazolidine (PMNI) on different subtypes of the nicotinic acetylcholine receptor were studied in voltage-clamped locust thoracic ganglion neurons, mouse BC3H1 muscle cells and mouse N1E-115 neuroblastoma cells. In locust neurons 10 microM PMNI induced agonistic effects and subsequent complete block of 1 mM acetylcholine-induced inward currents. The same concentration of PMNI produced no significant agonistic and antagonistic effects on the endplate type nicotinic receptor in BC3H1 cells. Neuronal type nicotinic receptor operated ion currents in N1E-115 cells were blocked by 10 microM PMNI to 50% of the control value, while agonistic effects were not observed. The differential action of PMNI designates this compound as a potential new tool to distinguish between subtypes of the nicotinic acetylcholine receptor.

Block of deltamethrin-modified sodium current in cultured mouse neuroblastoma cells: local anesthetics as potential antidotes

Effects of local anesthetics and anticonvulsants on the pyrethroid-modified sodium current in cultured mouse neuroblastoma cells have been investigated using the suction pipette voltage clamp technique. In the presence of 10 microM of the pyrethroid deltamethrin the sodium current consists of an enhanced peak current during membrane depolarization and a slowly decaying, deltamethrin-induced tail current remaining after repolarization. At the onset of block the local anesthetics tetracaine, lidocaine and QX 314 reduced the deltamethrin-induced tail current more effectively than the peak current. Lidocaine, but not phenytoin, caused a time-dependent block of tail currents evoked by membrane depolarizations lasting 10-1000 ms. Both lidocaine- and phenytoin-induced blocks were independent of the membrane potential during the tail current. The anticonvulsants phenytoin, phenobarbital and valproate blocked the tail and the peak sodium current to the same extent, but diazepam, mephenesin and urethane blocked the peak current more effectively. Vitamin E, which suppresses pyrethroid-induced paresthesia of the skin, had no effect on the voltage-dependent sodium current. It is concluded that indirect effects of anticonvulsants on pyrethroid-induced toxic symptoms predominate, whereas local anesthetics preferentially block the pyrethroid-induced tail current. Therefore, local anesthetics are potentially useful pyrethroid antidotes.

Nanomolar concentrations of lead selectively block neuronal nicotinic acetylcholine responses in mouse neuroblastoma cells

The effects of inorganic lead (Pb2+) on the ion currents mediated by (1) neuronal nicotinic acetylcholine (ACh) receptors, (2) serotonin 5-HT3 receptors, as well as (3) voltage-dependent Ca2+ and Na+ channels have been investigated in voltage clamped mouse neuroblastoma cells. The nicotinic ACh receptor-ion channel complex appeared more sensitive to Pb2+ than the other ion channels investigated. Low concentrations of Pb2+ (1 nM - 3 microM) reduced the peak amplitude of the ACh-induced inward current to 74%-10% of the control value in a concentration-dependent manner. However, between 10 microM and 100 microM Pb2+ the blocking effect was reversed, while the decay of the ACh-induced inward current was delayed. These effects of Pb2+ on the nicotinic receptor-mediated inward current can be described by the sum of two sigmoidal concentration-effect curves with an IC50 value of 19 nM and an EC50 of 21 microM and with slope factors of -0.5 and 0.8, respectively. The current mediated by 5-HT3 receptors was less potently blocked by Pb2+ (IC50 = 49 microM; slope factor = -0.3). In addition, Pb2+ blocked the ion current through voltage-dependent Ca2+ channels. The IC50 value of the concentration-effect curve of block of transient type Ca2+ channels by Pb2+ is 4.8 microM and the slope factor is -0.9. Voltage-dependent Na+ channels were not affected by Pb2+ up to 100 microM. At concentrations greater than 1 microM, Pb2+ also induced a noninactivating inward current. The present results show that modification of neuronal nicotinic receptor function may contribute to neurotoxic effects of Pb2+ poisoning.

Novel type of ion channel activated by Pb2+, Cd2+, and Al3+ in cultured mouse neuroblastoma cells

Superfusion with Pb2+ induces a slow, noninactivating and reversible inward current in voltage-clamped N1E-115 neuroblastoma cells. The amplitude of this inward current increases in the range of 1-200 microM Pb/+. Single-channel patch-clamp experiments have revealed that this inward current is mediated by discrete ion channels. Reversal potentials from linear I-V relationships are close to 0 mV for whole-cell and single-channel currents and the single-channel conductance amounts to 24 pS. The Pb2(+)-induced membrane current is not mediated by various known types of ion channels, since it is not blocked by external tetrodotoxin, tetraethylammonium, D-tubocurarine, atropine, ICS 205-930 and by internal EGTA. In Na(+)-free solutions superfusion with Pb2+ neither evokes a whole-cell inward current, nor single-channel openings. At -80 mV the open-time distribution of the single channels activated by 1 microM Pb2+ is dual exponential with time constants of 17 and 194 msec. When the Pb2+ concentration is increased from 1 to 20 microM these time constants decrease to 2 and 13 msec, but the amplitude of single-channel currents remains -1.9 nA. Cd2+ and Al3+ induce inward currents and single-channel openings similar to Pb2+. Time constants fitted to the open-time distribution of single channels are 14 and 135 msec in the presence of 1 microM Cd2+ and 15 and 99 msec in the presence of 50 microM Al3+. Conversely, Cu2+ induces an irreversible inward current in neuroblastoma cells. Single-channel openings are undetected in the presence of Cu2+ and in Na(+)-free solutions Cu2+ is still able to induce an inward current. It is concluded that Pb2+, Cd2+ and possibly Al3+ activate a novel type of metal ion-activated (MIA) channel in N1E-115 cells.

Properties of neuronal type acetylcholine receptors in voltage clamped mouse neuroblastoma cells

The pharmacological and physiological characteristics of nicotinic acetylcholine receptor-mediated ion current in mouse neuroblastoma N1E-115 cells have been investigated by superfusion of voltage clamped cells with known concentrations of agonists and antagonists for short periods. The acetylcholine-induced inward current is blocked by d-tubocurarine and by kappa-bungarotoxin with IC50 values of 0.5 microM and 30 nM, respectively. The inward current is resistant to alpha-bungarotoxin up to a concentration of 0.5 microM. This allows classification of the acetylcholine receptors of N1E-115 cells as neuronal type nicotinic receptors. The amplitude of the inward current increases with increasing concentration of the agonists acetylcholine and carbamylcholine, resulting in concentration-effect curves with EC50 values of 53 and 240 microM and slope factors slightly below unity. Conversely, at the highest concentrations of the agonists the amplitude of the inward current is reduced and a transient increase of the current appears when the agonist is removed. The characteristics of this transient tail current indicate that the agonists cause rapid ion channel block by interacting with a low affinity site. In the continued presence of acetylcholine the peak inward current is reduced by desensitization. The IC50 value and the slope factor of the steady-state desensitization curve are 1.1 microM and 0.58, respectively. At a low concentration of acetylcholine both the onset of desensitization and the inward current decay are described by similar dual exponential kinetics, but the steady-state inward current is smaller than expected from the degree of desensitization. Neuronal nicotinic receptors in N1E-115 cells and end-plate nicotinic receptors have several characteristics in common. However, the present results indicate that these receptors are distinct, not only in their sensitivity to snake toxins, but also with respect to functional properties.