Elecrtophysiological properties of voltage-gated Ca^<2+> channel and other cation channels.

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Ca^<2+> controls diverse cellular processes, including muscle contraction, neurotransmitter release and other forms of secretion, gene expression, and cell proliferation. To evoke these cellular responses, Ca^<2+> influx across the plasma membrane makes a major contribution to augmenting the cytosolic free Ca^<2+> concentration ([Ca^<2+>]_i). In addition to the well-characterized voltage-gated Ca^<2+> channels, ligand-gated channels are the major transmembrane pathways. ATP, the ligand of P2X receptors, is a candidate of neurotransmitter or co-transmitter in the peripheral and central nervous system. Anatomical studies have revealed the wide distribution of P2X receptors in the brain. So far, P2X-mediated synaptic responses have been recorded in a few region of brain. To determine the physiological significance of postsynaptic ATP receptors in the brain, we have investigated the P2X responses in neurons acutely dissociated from rat hypothalamic arcuate nucleus by using the whole-cell m … More ode of the patch-clamp technique. ATP evoked inward currents in a concentration-dependent manner (K_d=42μM) at a holding potential of -70 mV. The current-voltage relationship showed a marked inward rectification starting around -10 mV. Although P2 receptor agonists, 300μM αβ-methylene-ATP and 300μM βγ-methylene-ATP did not induced any current, 100 μM ATPγS and 100μM 2-methylthio-ATP evoked inward currents of which amplitude was 60% of that evoked by 100 μM ATP. PPADS, P2 receptor antagonist, inhibited the ATP-evoked currents in a time- and concentration-dependent manner (K_i=19μM at 2 min). Permeant Ca^<2+> inhibited the ATP-evoked currents in the range of milimolars (K_i=6.9mM), however, Cd^<2+> (1-300 μM), a broad cation channel blocker, facilitated the currents with slow off-rate. Zn^<2+> in the range of 1-20 μM facilitated the current whereas Zn^<2+> at the concentrations over 50 μM inhibited the currents. These observations suggest that functional P2X receptors are expressed in hypothalamic arcuate nucleus. Less