Regional difference in spontaneous firing inhibition by GABA(A) and GABA(B) receptors in nigral dopamine neurons

GABAergic control over dopamine (DA) neurons in the substantia nigra is crucial for determining firing rates and patterns. Although GABA activates both GABA(A) and GABA(B) receptors distributed throughout the somatodendritic tree, it is currently unclear how regional GABA receptors in the soma and dendritic compartments regulate spontaneous firing. Therefore, the objective of this study was to determine actions of regional GABA receptors on spontaneous firing in acutely dissociated DA neurons from the rat using patch-clamp and local GABA-uncaging techniques. Agonists and antagonists experiments showed that activation of either GABA(A) receptors or GABA(B) receptors in DA neurons is enough to completely abolish spontaneous firing. Local GABA-uncaging along the somatodendritic tree revealed that activation of regional GABA receptors limited within the soma, proximal, or distal dendritic region, can completely suppress spontaneous firing. However, activation of either GABA(A) or GABA(B) receptor equally suppressed spontaneous firing in the soma, whereas GABA(B) receptor inhibited spontaneous firing more strongly than GABA(A) receptor in the proximal and distal dendrites. These regional differences of GABA signals between the soma and dendritic compartments could contribute to our understanding of many diverse and complex actions of GABA in midbrain DA neurons.

Hypoxic Pulmonary Vasoconstriction and Change in Outward K Current in Pulmonary Arterial Smooth Muscle Cell of the Rabbit

Hypoxic pulmonary vasoconstriction (HPV) is well-known characteristic of pulmonary artery during the exposure of low O2 conditions. It has been demonstrated that hypoxia inhibits an outward K current, thus causing membrane depolarization and calcium influx through the voltage-dependent Ca channels in pulmonary artery smooth muscle cells. Hypoxia induces a reducing condition within the cell. The change of the redox (reduction and oxidation) state in the cells can also modulate the kinetics of ion channels which is influential to membrane potential. Under the assumption that the reducing milieu during hypoxia inhibits the outward K current and vasodilation, we have studied the effects of redox state on the K currents in the pulmonary artery smooth muscle cells of the rabbit using patch-clamp technique. The outward K current was induced by depolarization from holding potential -80 mV. A reducing agent dithiothreitol inhibited the outward K current, whereas an oxidizing agent dithio-bis (5-nitropyridine) potentiated it. The tetraethyl ammonium, a K channel blocker, mainly suppressed the inactivating component of the K current. Another blocker of voltage dependent K channel E4031 had no effect. This results suggest that the reducing state within the cell during hypoxia modulates the outward K current, which leads vasoconstriction through depolarization and Ca influx. This may be one of mechanisms of HPV in contrast to the other arteries of the body to dilate during low O2 tension.

Evidence for adenosine triphosphate (ATP) as an excitatory neurotransmitter in guinea-pig gastric antrum

We explore the question of whether adenosine 5'-triphosphate (ATP) acts as an excitatory neurotransmitter in guinea-pig gastric smooth muscle. In an organ bath system, isometric force of the circular smooth muscle of guinea-pig gastric antrum was measured in the presence of atropine and guanethidine. Under electrical field stimulation (EFS) at high frequencies (>20 Hz), NO-mediated relaxation during EFS was followed by a strong contraction after the cessation of EFS (a "rebound-contraction"). Exogenous ATP mimicked the rebound-contraction. A known P2Y-purinoceptor antagonist, reactive blue 2 (RB-2), blocked the rebound-contraction while selective desensitization of P2x-purinoceptor with alpha, beta-MeATP did not affect it. ATP and 2-MeSATP induced smooth muscle contraction, which was effectively blocked by RB-2 and suramin, a nonselective P2-purinoceptor antagonist. Particularly, in the presence of RB-2, exogenous ATP and 2-MeSATP inhibited spontaneous phasic contractions, suggestingthe existence of different populations of purinoceptors. Both the rebound-contraction and the agonist-induced contraction were not inhibited by indomethacin. The rank orders of agonists' potency were 2-MeSATP > ATP gtoreq UTP for contraction and alpha, beta-MeATP gtoreq beta, gamma-MeATP for inhibition of the phasic contraction, that accord with the commonly accepted rank order of the classical P2Y-purinoceptor subtypes. Electrical activities of smooth muscles were only slightly influenced by ATP and 2-MeSATP, whereas alpha, beta-MeATP attenuated slow waves with membrane hyperpolarization. From the above results, it is suggested that ATP acts as an excitatory neurotransmitter, which mediates the rebound-contraction via P2Y-purinoceptor in guinea-pig gastric antrum.

The role of intracellular Mg2+ in regulation of Ca2+-activated K+ channel in pulmonary arterial smooth muscle cells of the rabbit

Although the Ca2+-activated K+ (IK,Ca) channel is known to play an important role in the maintenance of resting membrane potential, the regulation of the channel in physiological condition is not completely understood in vascular myocytes. In this study, we investigated the role of cytoplasmic Mg2+ on the regulation of IK,Ca channel in pulmonary arterial myocytes of the rabbit using the inside-out patch clamp technique. Mg2+ increased open probability (Po), but decreased the magnitude of single channel current. Mg2+-induced block of unitary current showed strong voltage dependence but increase of Po by Mg2+ was not dependent on the membrane potential. The apparent effect of Mg2+ might, thus, depend on the proportion between opposite effects on the Po and on the conductance of IK,Ca channel. In low concentration of cytoplasmic Ca2+, Mg2+ increased IK,Ca by mainly enhancement of Po. However, at very high concentration of cytoplasmic Ca2+, such as pCa 5.5, Mg2+ decreased IK,Ca. through the inhibition of unitary current. Moreover, Mg2+ could activate the channel even in the absence of Ca2+. Mg2+ might, therefore, partly contribute to the opening of IK,Ca channel in resting membrane potential. This phenomenon might explain why IK,Ca contributes to the resting membrane potential where membrane potential and concentration of free Ca2+ are very low.