Inhibitory transmission in locus coeruleus neurons expressing GABAA receptor epsilon subunit has a number of unique properties.

Fast inhibitory synaptic transmission in the brain relies on ionotropic GABA(A) receptors (GABA(A)R). Eighteen genes code for GABA(A)R subunits, but little is known about the epsilon subunit. Our aim was to identify the synaptic transmission properties displayed by native receptors incorporating epsilon. Immunogold localization detected epsilon at synaptic sites on locus coeruleus (LC) neurons. In situ hybridization revealed prominent signals from epsilon, and mRNAs, some low beta1 and beta3 signals, and no gamma signal. Using in vivo extracellular and in vitro patch-clamp recordings in LC, we established that neuron firing rates, GABA-activated currents, and mIPSC charge were insensitive to the benzodiazepine flunitrazepam (FLU), in agreement with the characteristics of recombinant receptors including an epsilon subunit. Surprisingly, LC provided binding sites for benzodiazepines, and GABA-induced currents were potentiated by diazepam (DZP) in the micromolar range. A number of GABA(A)R ligands significantly potentiated GABA-induced currents, and zinc ions were only active at concentrations above 1 muM, further indicating that receptors were not composed of only alpha and beta subunits, but included an epsilon subunit. In contrast to recombinant receptors including an epsilon subunit, GABA(A)R in LC showed no agonist-independent opening. Finally, we determined that mIPSCs, as well as ensemble currents induced by ultra-fast GABA application, exhibited surprisingly slow rise times. Our work thus defines the signature of native GABA(A)R with a subunit composition including epsilon: differential sensitivity to FLU and DZP and slow rise time of currents. We further propose that alpha(3,) beta(1/3,) and epsilon subunits compose GABA(A)R in LC.

Dopamine receptors set the pattern of activity generated in subthalamic neurons.

Information processing in the brain requires adequate background neuronal activity. As Parkinson's disease progresses, patients typically become akinetic; the death of dopaminergic neurons leads to a dopamine-depleted state, which disrupts information processing related to movement in a brain area called the basal ganglia. Using agonists of dopamine receptors in the D1 and D2 families on rat brain slices, we show that dopamine receptors in these two families govern the firing pattern of neurons in the subthalamic nucleus, a crucial part of the basal ganglia. We propose a conceptual frame, based on specific properties of dopamine receptors, to account for the dominance of different background firing patterns in normal and dopamine-depleted states.

[Phorbol myristate acetate promotes entry of calcium in hypophysial cells GH3 B6 via potential dependent calcium channels]. / Le phorbol myristate acétate favorise l'entrée de calcium dans les cellules hypophysaires GH3 B6 par les canaux calciques potentiel-dépendants.

10 GH3/B6 cells were patched-clamped using a pipette containing NMG as internal cation, 2 mM ATP and 100 microM leupeptin. Whole-cell calcium or barium currents were recorded prior and after PMA (10(-8) or 10(-7) M). PMA increased the inward calcium current at potential levels close to threshold in 8 cells; 7 cells only exhibited an increase in transitory calcium current at potential levels close to threshold; in one cell, both transitory and conventional calcium currents were increased. 2 cells did not respond to PMA.

The relationship between the transient inward current (TI) and other components of slow inward current in mammalian cardiac muscle.

When rabbit sino-atrial node preparations and isolated guinea-pig ventricular cells are subjected to Na-K pump blockade (either by reducing external K+ by a factor of 10: sinus node; or by the presence of 10(-7) M ouabain: ventricular cells) they develop oscillatory transient inward currents of the kind already recorded in Purkinje fibres and ventricular muscle strands. The time course of these transient currents, generally known as TI's, closely resembles that of the slow component of second inward current (isi,2) previously reported by us as occurring in rabbit sinus node when recorded near its threshold (-40 mV). Moreover, we have found that, under voltage clamp conditions, the 'envelope' of isi currents activated by depolarization from negative membrane potentials matches the outline of the iTI which develops during the initial hyperpolarization. In the sinus node, oscillations of iTI become smaller near O mV but are never flat and there is no clear cut reversal potential, whilst in ventricular cells oscillations and contractions cease at very positive membrane potentials (+35 mV) without the TI current ever becoming net outward. Replacing 75% of the external Na+ with Li+ reduces isi and iTI in the node by about the same proportion strongly suggesting that both are carried by a Na-Ca exchange mechanism. This idea is supported by reproducing the conditions of Na-K pump block in a computer model of the sinus node activity++, when oscillatory currents are generated by variations in activity of the Na-Ca exchange mechanism triggered by fluctuating levels of intracellular calcium. The same model when used to test the hypothesis that isi,2 might be carried by a non-specific ion channel showed that considerable distortion of the action potential would then occur. From the experimental and computed results it is concluded that the majority of isi,2 and iTI currents are both mediated by Na-Ca exchange.

Relationship between the transient inward current and slow inward currents in the sino-atrial node of the rabbit.

In low K+ (0.3 mM) solutions rabbit sinus node preparations show the oscillatory transient inward current, iTI, already recorded in these conditions in Purkinje and ventricular preparations. The time course of iTI closely resembles that of the slow component of the slow inward current (isi) previously reported by us (Brown, Kimura, Noble, Noble & Taupignon, 1984a) in rabbit sinus node, when recorded near its threshold (-40 mV). When the duration of voltage-clamp steps is varied there is a strong correlation between the 'envelope' of isi amplitudes on depolarization and the time course of iTI on hyperpolarization. Although oscillations of iTI become smaller near 0 mV, there is no potential at which the current records are completely flat, suggesting that there is no simple reversal potential. 75% substitution of Na+ by Li+ greatly reduces both iTI and slow isi in about the same proportion. Reducing the activity of the Na-K exchange pump by the amount expected in 0.3 mM-K+ solutions is sufficient to induce oscillatory iTI in a computer model of the sino-atrial node (Noble & Noble, 1984). The model reproduces the current as variations in the Na-Ca exchange current dependent on intracellular Ca2+ concentration ([ Ca]i). The model was also used to test the alternative hypothesis that the slow inward currents might be generated by [Ca]i-activated non-specific cation channels. It is shown that this would distort the shape of the repolarization phase of the action potential. It is concluded that the experiments and computations are consistent with the hypothesis that a large fraction of iTI and the slow component of isi could both be generated by Na-Ca exchange and that only a relatively small fraction might be generated by non-specific channels.