Volodymyr I. Pidoplichko

Nicotine activates and desensitizes midbrain dopamine neurons

Tobacco use in developed countries is estimated to be the single largest cause of premature death. Nicotine is the primary component of tobacco that drives use, and like other addictive drugs, nicotine reinforces self-administration and place preference in animal studies. Midbrain dopamine neurons normally help toshape behaviour by reinforcing biologically rewarding events, but addictive drugs such as cocaine can inappropriately exert a reinforcing influence by acting upon the mesolimbic dopamine system. Here we show that the same concentration of nicotine achieved by smokers activates and desensitizes multiple nicotinic receptors thereby regulating the activity of mesolimbic dopamine neurons. Initial application of nicotine can increase the activity of the dopamine neurons, which could mediate the rewarding aspects of tobacco use. Prolonged exposure to even these low concentrations of nicotine, however, can cause desensitization of the nicotinic receptors, which helps to explain acute tolerance to nicotines effects. The effects suggest a cellular basis for reports that the first cigarette of the day is the most pleasurable, whereas the effect of subsequent cigarettes may depend on the interplay between activation and desensitization of multiple nicotinic receptors.

A receptor for protons in the nerve cell membrane

Abstract The neurones isolated from spinal ganglia and from the ganglion of trigeminal nerve of a rat were investigated by means of intracellular perfusion and voltage clamp. The extracellular solution was replaced in 0.1 s. Many cells responded to rapid shifts of external pH from 7.4 to 6.9 and lower by a pH-dependent inward current. Its amplitude saturated at pH 5.4 (p K a 6.2). This ‘H + -induced’ current was due to the increased permeability of the membrane to Na + and K + (P k :P Na

Differential Desensitization and Distribution of Nicotinic Acetylcholine Receptor Subtypes in Midbrain Dopamine Areas

˜0.1). The H + -induced current decay had a time constant about 0.5s and showed a desensitization which was removed within 10s. The H + -induced current was also found in the cells of mouse C-1300 neuroblastoma. It had similar pH and voltage dependence but a much slower kinetics of desensitization. It is suggested that this newly described conductance mechanism may serve as a pH-sensor in the sensory nerve endings throughout the body.

Receptor for ATP in the membrane of mammalian sensory neurones

Although many psychopharmacological factors contribute to nicotine addiction, midbrain dopaminergic systems have received much attention because of their roles in reinforcement and associative learning. It is generally thought that the mesocorticolimbic dopaminergic system is important for the acquisition of behaviors that are reinforced by the salient drives of the environment or by the inappropriate stimuli of addictive drugs. Nicotine, as obtained from tobacco, can activate nicotinic acetylcholine receptors (nAChRs) and excite midbrain neurons of the mesocorticolimbic system. Using midbrain slices from rats, wild-type mice, and genetically engineered mice, we have found differences in the nAChR currents from the ventral tegmental area (VTA) and the substantia nigra compacta (SNc). Nicotinic AChRs containing the α7 subunit (α7* nAChRs) have a low expression density. Electrophysiological analysis of nAChR currents, autoradiography of [125I]-α-bungarotoxin binding, and in situ hybridization revealed that α7* nAChRs are more highly expressed in the VTA than the SNc. In contrast, β2* nAChRs are move evenly distributed at a higher density in both the VTA and SNc. At the concentration of nicotine obtained by tobacco smokers, the slow components of current (mainly mediated by β2* nAChRs) become essentially desensitized. However, the minority α7* component of the current in the VTA/SNc is not significantly desensitized by nicotine in the range ≤100 nm. These results suggest that nicotine, as obtained from tobacco, can have multiple effects on the midbrain areas by differentially influencing dopamine neurons of the VTA and SNc and differentially desensitizing α7* and non-α7 nAChRs.

Variations in desensitization of nicotinic acetylcholine receptors from hippocampus and midbrain dopamine areas.

ATP-activated conductance has been found in a large number of neurones isolated from various sensory ganglia of the rat and cat. The inward current produced by ATP (Kd = 5 x 10(-6) M) is carried by cations and demonstrates rapid activation and slow desensitization. The sequence of agonists (ATP greater than ADP with AMP and adenosine ineffective) is different from those previously described for purinergic receptors P1 and P2.

Calcium inward current and related charge movements in the membrane of snail neurones.

This study addresses two issues arising from the desensitization of nicotinic acetylcholine receptors from the hippocampus, ventral tegmental area, and substantia nigra. First, biophysical studies can find potent and complete desensitization of nicotinic receptors; but in vivo studies often find that desensitization affecting a behavior is less than complete, or that desensitization is important over a different nicotine concentration range. Our results show that there can be significant differences in desensitization when comparing nearby neurons from the same area of the brain. Thus, nicotinic receptors on a minority of neurons may remain active and maintain a behavior under conditions that can produce significant desensitization. Second, agonist applications that are intended to active nicotinic receptors also cause desensitization. The prevailing conditions and the rate of agonist application and removal will control the degree of activation vs. desensitization. These and other factors regulate the efficacy of nicotinic agonists experimentally and physiologically.

Nicotine modifies the activity of ventral tegmental area dopaminergic neurons and hippocampal GABAergic neurons.

1. Isolated and intracellularly perfused neurones from the snail Helix pomatia have been investigated under voltage‐clamp conditions. Calcium inward current and asymmetric displacement current were examined. 2. Two components of the asymmetric displacement current have been distinguished. One of them was irreversibly blocked by intracellular fluoride together with the calcium inward current. Another component (about 20%) was not affected. The relation of the fluoride‐sensitive asymmetric displacement current to the activity of calcium channels was investigated. 3. The amounts of charge displaced by the on‐ and off‐responses of the asymmetric current were similar. The maximum charge displacement was between 1500 and 2000 electron charges/micrometers 2. 4. The normalized steady‐state voltage distribution curve of the displaced charge coincided with the square root of the normalized calcium conductance. The slope was 4 mV per e‐fold change for the calcium conductance and 8 mV for te charge distribution. Thus, the effective valencies were close to 6 (for the calcium conductance) and 3 (for the charge distribution), suggesting two gating particles per calcium channel. 5. The on‐process of the calcium inward current fitted m2 kinetics. The time constant tau m corresponded to the time constant tau ason of the asymmetric current in a wide range of tested voltages. This correspondence failed only at small depolarizations where tau m exceeded tau ason. 6. The ‘off’ time constants of the calcium inward current (tau Caoff) and the asymmetric current (tau asoff) examined at ‐40 mV did not depend significantly on the test pulse height. The ratio tau asoff/tau Caoff varied between 1.7 and 2.5. 7. Calcium inward‐current relaxation kinetics were measured for small shifts of membrane potential (Vtest = ‐2 mV) applied at the peaks of the current. The time constants were smaller than tau m and depended only weakly on voltage. 8. It is concluded that the fluoride‐sensitive asymmetric displacement current is related to the activation of calcium channels.

Ionic currents in the neuroblastoma cell membrane

While trying to mimic the dose and time course of nicotine as it is obtained by a smoker, we found the following results. The initial arrival of even a low concentration of nicotine increased the firing rate of dopaminergic neurons from the ventral tegmental area (VTA) and increased the spontaneous vesicular release of GABA from hippocampal neurons. Longer exposure to nicotine caused variable, but dramatic, desensitization of nicotinic receptors and diminished the effects of nicotine. The addictive properties of nicotine as well as its diverse effects on cognitive function could be mediated through differences in activation and desensitization of nicotinic receptors in various areas of the brain.

Potential‐dependent membrane current during the active transport of ions in snail neurones

Abstract Isolated neuroblastoma cells of the A1- clone (derived from the N-18 clone) were studied using the method of intracellular dialysis. The ions carrying the early inward (sodium) and the delayed outward (potassium) currents were directly determined by varying the ionic composition of the solutions inside and outside the cell. The sodium inward current was blocked by tetrodotoxin. Substitution of intracellular potassium by sodium ions showed that the P Na : P K ratio was 7:1. A calcium ionic current was not observed in the majority of the investigated cells. A relatively low potassium conductance was observed. Thus in some respects the neuroblastoma cell membrane resembles that of a vertebrate nerve fiber, but the lack of a readily demonstrable calcium inward current and of nonspecific outward currents implies some functional differences.

RDX Binds to the GABAA Receptor–Convulsant Site and Blocks GABAA Receptor–Mediated Currents in the Amygdala: A Mechanism for RDX-Induced Seizures

1. The membrane current caused by the iontophoretic injection of sodium into giant neurones of the snail Helix pomatia was investigated under a long lasting voltage clamp. The inhibition of this current by ouabain (10−4 M) and by cooling to + 7° C confirmed its link with the active transport of ions. Therefore this current is called the pump current.