Pain-producing heat is detected by several classes of nociceptive sensory neuron that differ in their thermal response thresholds. The cloned capsaicin receptor, also known as the vanilloid receptor subtype 1 (VR1), is a heat-gated ion channel that has been proposed to mediate responses of small-diameter sensory neurons to moderate (43 degrees C) thermal stimuli. VR1 is also activated by protons, indicating that it may participate in the detection of noxious thermal and chemical stimuli in vivo. Here we identify a structurally related receptor, VRL-1, that does not respond to capsaicin, acid or moderate heat. Instead, VRL-1 is activated by high temperatures, with a threshold of approximately 52 degrees C. Within sensory ganglia, VRL-1 is most prominently expressed by a subset of medium- to large-diameter neurons, making it a candidate receptor for transducing high-threshold heat responses in this class of cells. VRL-1 transcripts are not restricted to the sensory nervous system, indicating that this channel may be activated by stimuli other than heat. We propose that responses to noxious heat involve these related, but distinct, ion-channel subtypes that together detect a range of stimulus intensities.

The adenosine-5'-triphosphate (ATP) molecule is an extracellular messenger in neural and non-neural tissues, where it activates several cell-surface-receptor subtypes, including G-protein-coupled receptors and ligand-gated ion channels. ATP-gated channels (termed P2x receptors) have been characterized on smooth muscle cells and autonomic and sensory neurons, where they mediate membrane depolarization and, in some cases, Ca2+ entry. P2x receptors are functionally heterogeneous, but resemble acetylcholine- and serotonin-gated channels with respect to ion selectivity and kinetic parameters of channel gating. We report here that despite such close functional similarities, the deduced sequence of a cloned P2x receptor predicts an unusual subunit structure resembling voltage-insensitive cation channels. Thus, the P2x receptor provides a striking example of convergent evolution, whereby proteins have been fashioned with similar functional properties from subunits having very different structural characteristics. There is sequence similarity between the ATP receptor and RP-2, a gene activated in thymocytes undergoing programmed cell death. RP-2 may encode a receptor for ATP or another metabolite released during apoptosis.

The neurotransmitter serotonin (5HT) activates a variety of second messenger signaling systems and through them indirectly regulates the function of ion channels. Serotonin also activates ion channels directly, suggesting that it may also mediate rapid, excitatory responses. A complementary DNA clone containing the coding sequence of one of these rapidly responding channels, a 5HT3 subtype of the serotonin receptor, has been isolated by screening a neuroblastoma expression library for functional expression of serotonin-gated currents in Xenopus oocytes. The predicted protein product has many of the features shared by other members of the ligand-gated ion channel family. The pharmacological and electrophysiological characteristics of the cloned receptor are largely consistent with the properties of native 5HT3 receptors. Messenger RNA encoding this receptor is found in the brain, spinal cord, and heart. This receptor defines a new class of excitatory ligand-gated channels.

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Extracellular ATP activates cell-surface metabotropic and ionotropic nucleotide (P2) receptors in vascular, neural, connective, and immune tissues. These P2 receptors mediate a wealth of physiological processes, including nitric oxide-dependent vasodilation of vascular smooth muscle and fast excitatory neurotransmission in sensory afferents. Although ATP is now recognized as a signaling molecule, the cellular and molecular mechanisms underlying its actions have been difficult to study due to the absence of selective P2 receptor antagonists and cloned receptor genes. Nonetheless, five mammalian P2 receptor subtypes have been tentatively assigned based solely on agonist specificity and signaling properties.Here we report the cloning of a mouse cDNA encoding a P2 receptor that shares striking homology with several G proteincoupled peptide receptors. When expressed in Xenopus laevis oocytes, the cloned receptor resembles a metabotropic P2U receptor; activation by either ATP or UTP elicits the mobilization of intracellular calcium. mRNA encoding the P2U purinergic receptor is found in neural and nonneural tissues.There is considerable evidence suggesting that ATP functions as an extracellular signaling molecule in neural and nonneural mammalian tissues (1, 2). In central and peripheral synapses, ATP mediates fast excitatory neurotransmission (3, 4). In the autonomic nervous system, ATP is a major purinergic cotransmitter that is often colocalized in secretory vesicles with norepinephrine or acetylcholine (5, 6). In the vascular system, aggregating platelets secrete ATP and ADP, which stimulate the release of nitric oxide and other vasodilators from the endothelium (7). In the immune system, ATP modulates macrophage phagocytosis (8) and mast cell degranulation (9). In the human airway epithelium, ATP stimulates transepithelial ion transport (10), an effect that may underlie the therapeutic effect of ATP and UTP in the treatment of cystic fibrosis-related lung disease (11).It has been postulated that these responses to extracellular ATP are mediated by specific plasma membrane receptors, called P2 purinergic receptors (12, 13). Based on agonist selectivity and signaling properties, five subclasses of P2 receptor have been tentatively defined: three subclasses of receptors (P2T, P2U, and P2y) that are believed to signal through G proteins, one subclass (P2X) that is believed to be a ligand-gated cation channel, and one subclass (P2z) that is present on mast cells, macrophages, and fibroblasts, but whose signaling mechanism is less well understood (1, 14-16). G protein-coupled P2 receptors are found in numerous cultured cell lines, where they have been shown to activate signal transduction systems that involve the breakdown of membrane phospholipids and the elevation of cytoplasmic free Ca2+ (17,18). Ionotropic P2X receptors carry Na+, K+, and Ca2+ currents and appear to be predominantly expressed in neural and neuromuscular tissues (16).A more complete characterization ofthis putative family of P2 purinergic rec...