Extracellular ATP activates a cation conductance and a K+ conductance in cultured microglial cells from mouse brain

Abstract

Microglial cells have important functions during regenerative processes after brain injury. It is well established that they rapidly respond to damage to the brain tissue. Stages of activation are associated with changes of cellular properties such as proliferation rate or expression of surface antigens. Yet, nothing is known about signal substances leading to the rapid changes of membrane properties, which may be required to initiate the transition from one cell stage into another. From our present study, using the patch-clamp technique, we report that cultured microglial cells obtained from mouse or rat brain respond to extracellularly applied ATP with the activation of a cation conductance. Additionally, in the majority of cells an outwardly directed K+ conductance was activated with some delay. Since ADP, AMP, and adenosine (in descending order) were less potent or ineffective in inducing the cation conductance, the involvement of a P2 purinergic receptor is proposed. The receptor activation is accompanied by an increase of cytosolic Ca2+ as determined by a fura-2-based Ca(2+)-imaging system. This ATP receptor could enable microglial cells to respond to transmitter release from nerve endings with ATP as a transmitter or cotransmitter or to the death of cells with resulting leakage of ATP.

Activation of purinergic P2X receptors inhibits P2Y-mediated Ca2+ influx in human microglia

“…It is interesting that the inward non-selective cationic current mediated by P 2X purinoceptors exhibits relatively rapid inactivation in both rodent [2][3][4]14] and human microglia [10]. However, current clamp studies in mouse microglia have documented a large, prolonged ATPinduced cell depolarization [2].…”

“…The maintained depolarization may be a consequence of a low level of expression of outward K + channels in the cells [2]. It should also be noted that P 2X responses have been shown to exhibit desensitization with repeated applications of ATP [3,10,14].…”

“…In addition, for both rodent and human microglia, activation of the P 2Y pathway has been found to enhance an outward K + current [3,4,10,14]. Thus, the net change of [Ca 2+ ] i in response to ATP is dependent on a number of modulatory factors affecting cell membrane potential and the status of intracellular Ca 2+ stores and store-operated channels.…”

“…However, recent work has documented a diversity of agents which cause transient increases in levels of cytoplasmic Ca 2+ in rodent microglia; such responses may be involved in coupling activating stimuli to particular cellular functions. Among the spectrum of stimuli found to elicit transient Ca 2+ responses in cultured rodent cells are complement [1], ATP [2][3][4], carbachol [5], platelet-activating factor (PAF [6]) and endothelin [7]. All of these stimuli (complement has not yet been tested) have also been found to cause transient Ca 2+ responses in human microglia [8][9][10][11].…”

“…In addition a P 2Z receptor, associated with a high conductance pore permeable to large hydrophylic solutes, has also been reported in microglia [15,16]; activation of this receptor may require concentrations of ATP near 1 mM [16]. The P 2X purinoceptor is coupled to a non-selective cationic channel allowing influx of Na + and Ca 2+ leading to a transient cell depolarization [14], whereas activation of the P 2Y purinoceptor leads to an increase in cytoplasmic [Ca 2+ ] due to emptying of IP 3 -sensitive intracellular stores [10,14]. Depletion of intracellular stores following ATP…”