Properties of native P2X receptors in large multipolar neurons dissociated from rat hypothalamic arcuate nucleus

ATP, the ligand of P2X receptors, is a candidate of neurotransmitter or co-transmitter in the peripheral and the central nervous systems. Anatomical studies have revealed the wide distribution of P2X receptors in the brain. So far, P2X-mediated small synaptic responses have been recorded in some brain regions. To determine the physiological significance of postsynaptic ATP receptors in the brain, we have investigated the P2X responses in rat dissociated hypothalamic arcuate neurons by using the patch-clamp technique. ATP evoked inward currents in a concentration-dependent manner (EC ₅₀=42 μM) at a holding potential of -70 mV. The current-voltage relationship showed a marked inward rectification starting around -10 mV. Although neither 300 μM αβ-methylene-ATP nor 300 μM βγ-methylene-ATP induced any currents, 100 μM ATPγS and 100 μM 2-methylthio-ATP evoked inward currents of which amplitude was about 60% of the control currents evoked by 100 μM ATP. PPADS, one of P2 receptor antagonists, inhibited the ATP-evoked currents in a time- and a concentration-dependent manners (IC₅₀=19 μM at 2 min). Permeant Ca²⁺ inhibited the ATP-evoked currents in the range of millimolars (IC₅₀=7 mM); however, Cd²⁺ (1-300 μM), a broad cation channel blocker, facilitated the currents with slow off-response. Zn ²⁺ in the range of 1-100 μM facilitated the currents whereas Zn²⁺ at the concentrations over 100 μM inhibited the currents. These observations suggest that functional P2X receptors are expressed in the hypothalamic arcuate nucleus. The most likely subunit combinations of the P2X receptors are P2X₂-homomultimer and P2X₂/P2X ₆-heteromultimer.