A Memory for Extracellular Ca²⁺by Speeding Recovery of P2X Receptors from Desensitization

Abstract

Nerve endings of nociceptors (pain-sensing neurons) express an unusual subtype of ATP-gated ion channel, the P2X3 receptor, that rapidly desensitizes (Ͻ100 msec) and slowly recovers (Ͼ20 min). Here we show that Ca 2ϩ , or certain other polyvalent cations, binds to an extracellular site on rat sensory neurons and can increase current through P2X3 channels more than 10-fold. Importantly, Ca 2ϩ facilitates P2X3 current to precisely the same level whether a transient Ca 2ϩ change occurred just before or several minutes before activating the channels with ATP. This memory for past changes in Ca 2ϩ is integrative in that a 90 sec Ca 2ϩ stimulus delivered just before an ATP application has the same effect as an earlier series of three, separated 30 sec Ca 2ϩ stimuli. These diverse phenomena are explained by a single mechanism: Ca 2ϩ speeds recovery of P2X channels from desensitization. Recovery follows an exponential growth curve that depends on the duration, but not the timing, of changes in recovery rate. Modulation of desensitization underlies a well described short-term memory in bacteria, and it might be similarly used in the nervous system.Key words: P2X3; purinergic; Ca 2ϩ ; desensitization; recovery rate; rat; sensory neuron; nociceptor; learning; memory; short term memory; pain; hyperalgesia; dorsal root ganglia ATP-gated ion channels (P2X receptors) are distributed throughout somatic and nervous tissues (Bean, 1992;Abbracchio et al., 1994;Surprenant et al., 1995). In response to micromolar extracellular ATP, P2X receptors pass cations nonselectively. The resulting depolarization triggers processes such as secretion, smooth muscle contraction, or neuronal excitation. Sensory neurons of rat dorsal root ganglia contain mRNA for six of the seven cloned members of the P2X family (Collo et al., 1996), and one, P2X3, is found only in sensory neurons (Chen et al., 1995;Lewis et al., 1995). P2X3 protein is localized both to nociceptive peripheral nerve endings , where it may mediate ATP-driven nociception (Bleehen and Keele, 1977;Burnstock and Wood, 1996), and to central presynaptic terminals , where it facilitates neurotransmitter release (Gu and MacDermott, 1997).ATP-activated current in nociceptors has transient (desensitizing) and persistent components. Channels made only of P2X3 subunits account for transient current, and channels combining P2X3 and P2X2 subunits account for persistent current . Homomeric P2X3 channels desensitize to sustained applications of ATP within 100 msec, yet they recover from desensitization in Ͼ20 min (Chen et al., 1995;Lewis et al., 1995, Robertson et al., 1996). These radically different time scales are unusual for ion channels (see Table 1). They suggest that cellular functions controlled by P2X3 decrease after the channels are desensitized by recent exposure to ATP. However, P2X3 current is so large in sensory neurons , and see below] that activation of a small fraction of the total P2X3 channels should be sufficient to excite the cell. Therefore, P2X3 may influence sensory physio...

Extracellular Cations Sensitize and Gate Capsaicin Receptor TRPV1 Modulating Pain Signaling

“…Although the brain NaX channel is gated by monovalent sodium ions (Hiyama et al, 2002), we are not aware of a previous report that describes a channel being gated by extracellular divalent cations. Several ion channels are instead known to be sensitized by cations, including P2X3 receptors (Cook et al, 1998), nicotinic acetylcholine receptors (Mulle et al, 1992), and acid-sensitive ion channels (Babini et al, 2002;Immke and McCleskey, 2003). Interestingly, TRPV1 shares functional homology not with an ion channel but with a G-protein-coupled receptor, the Ca 2ϩ -sensing receptor, in that it is directly activated by Ca 2ϩ and Mg 2ϩ ions (Hofer and Brown, 2003).…”

Use-Dependent Inhibition of P2X3Receptors by Nanomolar Agonist

“…The equation (Cook et al, 1998) was fit to recovery and unbinding data using the program NFIT (University of Texas Medical Branch, Galveston, TX), a least-squares algorithm. Rates of entry into the desensitized state ( entry ) by 3 nM ATP and ␣␤ methyleneATP (␣␤meATP) (see Fig.…”

“…Brief application of micromolar ATP results in a rapid inward current that desensitizes in Ͻ30 ms (Pankratov Yu et al, 2001). In contrast, recovery of P2X 3 current takes Ͼ10 min and is Ͼ10,000-fold slower than the rate of desensitization ( desens ) (Cook et al, 1998). The reason for such slow recovery is unknown, but it is likely an intrinsic property of the channel (Cook et al, 1998;Alexander et al, 1999;Fabbretti et al, 2004).…”

“…In contrast, recovery of P2X 3 current takes Ͼ10 min and is Ͼ10,000-fold slower than the rate of desensitization ( desens ) (Cook et al, 1998). The reason for such slow recovery is unknown, but it is likely an intrinsic property of the channel (Cook et al, 1998;Alexander et al, 1999;Fabbretti et al, 2004). Using site-directed mutagenesis and chimeric P2X receptors, researchers have identified several regions that influence the rate at which P2X receptors desensitize (Werner et al, 1996;Fabbretti et al, 2004;Zemkova et al, 2004).…”

Ca 2+ /calmodulin-dependent protein kinase II potentiates ATP responses by promoting trafficking of P2X receptors

“…Peak current ratios (I 2 ͞I 1 ), where I 1 is the peak ATP current obtained from the first ATP application and I 2 is the current from the second ATP application, were plotted as a function of time separation between ATP pulses within each pair. The curve was fitted with the equation (20). All of the data are expressed as mean Ϯ SEM or as percentage.…”