Dorsal root ganglion sensory neurons associated with C-fibres, many of which are activated by tissue-damage, express an unusual voltage-gated sodium channel that is resistant to tetrodotoxin. We report here that we have identified a 1,957 amino-acid sodium channel in these cells that shows 65% identity with the rat cardiac tetrodotoxin-insensitive sodium channel, and is not expressed in other peripheral and central neurons, glia or non-neuronal tissues. In situ hybridization shows that the channel is expressed only by small-diameter sensory neurons in neonatal and adult dorsal root and trigeminal ganglia. The channel is resistant to tetrodotoxin when expressed in Xenopus oocytes. The electrophysiological and pharmacological properties of the expressed channel are similar to those described for the small-diameter sensory neuron tetrodotoxin-resistant sodium channels. As some noxious input into the spinal cord is resistant to tetrodotoxin, block of expression or function of such a C-fibre-restricted sodium channel may have a selective analgesic effect.

ATP is known to depolarize sensory neurons, and may play a role in nociceptor activation when released from damaged tissue. Here we report the molecular cloning and characterization of a new member of the P2X receptor family, P2X3, expressed by these cells. The channel transcript was present in a subset of rat dorsal-root-ganglion sensory neurons, some of which express nociceptor-associated markers; it was absent in other tissues that were tested, including sympathetic, enteric and central nervous system neurons. Moreover, when expressed in Xenopus oocytes, the channel showed an ATP-dependent cation flux. P2X3 is the only ligand-gated channel known to be expressed exclusively by a subset of sensory neurons. The remarkable selectivity of expression of the channel coupled with its sensory neuron-like pharmacology suggests that this channel may transduce ATP-evoked nociceptor activation.

Partial agonists are ligands which bind to receptors but produce only a small maximum response even at concentrations where all receptors are occupied. In the case of ligand-activated ion channels, it has been supposed ever since 1957 that partial agonists evoke a small response because they are inefficient at eliciting the change of conformation between shut and open states of the channel. We have investigated partial agonists for two members of the nicotinic superfamily, the muscle nicotinic acetylcholine receptor and the glycine receptor and find that the open-shut reaction is similar for both full and partial agonists, but the response to partial agonists is limited by an earlier conformation change ('flipping') that takes place while the channel is still shut. This has implications for the interpretation of structural studies, and in the future, for the design of partial agonists for therapeutic use.Agonists are small molecules that bind to a receptor and activate it. The best understood receptors are ligand-gated ion channels. When neurotransmitters bind to their extracellular domain, the resulting change of conformation opens an ion channel, which carries current through the cell membrane, allowing electrical signals to propagate. The natural neurotransmitters, acetylcholine and glycine, are very efficacious agonists: when they are bound, the channel is open for 95-98% of the time1,2. In the 1950s, agonists were discovered that could not produce such a large response even when they saturated the binding sites. These were called partial agonists. Here we find that taurine can hold the glycine receptor channel open for at most about 54% of the time. del Castillo and Katz3 were the first to propose for an ion channel that when the receptor is occupied by a partial agonist, the 'gating' equilibrium between open and shut conformations lies towards the shut side. This is equivalent to saying that agonists work because they have a higher affinity for the open state than the shut state4,5 so their binding shifts the equilibrium towards the open state. The more efficacious the agonist, the greater is its selectivity for the open state. These views of partial agonism have persisted, essentially unchanged, for 50 years. However, recent findings suggest another possibility. Φ-value analysis indicates that, after binding, nicotinic receptors move through a number of brief intermediate shut states, a 'conformational wave', before the channel opens6-10. Our own work on glycine receptors suggested that it is possible to detect and measure the properties of an intermediate conformation, which we refer to as 'flip'2. The flipped receptor has a higher affinity for the agonist than the resting receptor, so it is a sort of activated, pre-open state. Higher affinity could result from domain closure around the bound agonist, a phenomenon that is clear in structures of extracellular domains of glutamate channels11,12, but less obvious in the nicotinic superfamily13,14, where binding sites are at the interface between sub...

An alternative, and novel, explanation is that agonist binding stabilizes a higher affinity form of the receptor that is produced by a conformational change ("flip") that is separate from, and precedes, channel opening. Both the "interaction" scheme and the flip scheme describe our data well, but the latter has fewer free parameters and above all it offers a mechanism for the affinity increase. Distinguishing between the two mechanisms will be important for our understanding of the structural dynamics of activation in the nicotinic superfamily and is important for our understanding of mutations in these receptors.

1. Pain hypersensitivity is characterized by an increase in the response to noxious stimuli (hyperalgesia) and a reduction in threshold such that innocuous stimuli begin to elicit pain (allodynia). These sensitivity changes can be produced by an increase in excitability of dorsal horn neurons; the phenomenon of central sensitization. We have now examined whether a reduction in local segmental inhibitory mechanisms produces similar changes. The model system used for studying touch-evoked allodynia has been the recruitment of a low-threshold mechanoreceptor input to the nociceptive flexion withdrawal reflex in the decerebrate-spinal rat. 2. Hamstring flexor alpha motoneurons are characterized by high-threshold cutaneous receptive fields. Mechanical stimuli (pinch or firm pressure) evoke a brisk firing response in these cells, whereas low-intensity stimuli (light touch or brush) produce little or no effect, as expected for the output neurons of the nociceptive flexion withdrawal reflex. 3. Primary afferent C fiber conditioning inputs have previously been shown to produce prolonged increases in the excitability of the flexion reflex, as measured by the augmentation of the response to high-intensity peripheral stimuli. We have now examined whether these conditioning inputs and segmental disinhibition modify the responsiveness of the reflex to low-threshold inputs. 4. Brief (20 s), low-frequency (1 Hz), C fiber conditioning stimuli to the sural nerve increased the response of the hamstring flexor motor neurons to low-intensity cutaneous touch stimuli, reduced the cutaneous mechanical threshold, and increased the response to A beta inputs from the sural nerve. 5. Intrathecal injections of subconvulsant doses of the glycine receptor antagonist, strychnine (7 nmol) or the gamma-aminobutyric acid-A (GABAA) receptor antagonist, bicuculline (8 nmol) produced similar but longer lasting changes. The GABAB antagonist P-(3-aminopropyl)-P-diethoxymethyl-phosphonic acid (CGP 35348) had no significant effects. 6. The nociceptive flexion withdrawal reflex is under the control, therefore, of segmental inhibitory mechanisms mediated by glycine and GABAA receptors. Removal of this inhibition enables the reflex to be activated by low-intensity cutaneous stimuli. Given the similarities between the stimulus-response profiles of the nociceptive flexion reflex and the production of pain in man, these findings indicate that a decrease in the efficacy of spinal inhibitory circuits may contribute to the touch-evoked allodynia that occurs in pain hypersensitivity states, where A beta inputs begin to produce pain.

No abstract

The Concise Guide to PHARMACOLOGY 2019/20 is the fourth in this series of biennial publications. The Concise Guide provides concise overviews of the key properties of nearly 1800 human drug targets with an emphasis on selective pharmacology (where available), plus links to the open access knowledgebase source of drug targets and their ligands (http://www.guidetopharmacology.org/), which provides more detailed views of target and ligand properties. Although the Concise Guide represents approximately 400 pages, the material presented is substantially reduced compared to information and links presented on the website. It provides a permanent, citable, point‐in‐time record that will survive database updates. The full contents of this section can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.14749. Ion channels are one of the six major pharmacological targets into which the Guide is divided, with the others being: G protein‐coupled receptors, nuclear hormone receptors, catalytic receptors, enzymes and transporters. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. The landscape format of the Concise Guide is designed to facilitate comparison of related targets from material contemporary to mid‐2019, and supersedes data presented in the 2017/18, 2015/16 and 2013/14 Concise Guides and previous Guides to Receptors and Channels. It is produced in close conjunction with the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC‐IUPHAR), therefore, providing official IUPHAR classification and nomenclature for human drug targets, where appropriate.

1. In order to establish the subunit composition of neuronal nicotinic receptors in rat superior cervical ganglia (SCG), their single-channel properties were compared with those of recombinant receptors expressed in Xenopus oocytes, using outside-out excised patch recording. 2. The mean main conductance of SCG channels from adult and 1-day-old rats was 34-8 and 36-6 pS, respectively. Less frequent openings to lower conductances occurred both as isolated bursts and as events connected to the main level by direct transitions. There was considerable interpatch variability in the values of the lower conductances.3. Nicotinic receptors from oocytes expressing a3/14 and a4&4 subunits had chord conductances lower than that of SCG neurones (22 pS for oc3fi4 and 29 pS for a4#4).4. Prolonged recording from both native and recombinant channels was precluded by 'rundown', i.e. channel activity could be elicited for only a few minutes after excision. Nevertheless, SCG channel openings were clearly seen to occur as short bursts (slowest component, 38 ms), whereas recombinant channels opened in very prolonged bursts of activity, the major component being the slowest (480 ms). 5. Addition of the a5 subunit to the a314 pair produced channels with a higher conductance than those observed after injection of the pair alone (24l9 vs. 22 pS), suggesting incorporation of a5 into the channel. Addition of the 82 subunit did not change a3/14 singlechannel properties. In one out of fourteen a3a5fl4 patches, both ganglion-like, high conductance, short burst openings and recombinant-type, low conductance, slow burst openings were observed. 6. Channels produced by expression in Xenopus oocytes of neuronal nicotinic subunits present in rat SCG as a rule differ from native ganglion receptors in single-channel conductance and gross kinetics. While it is possible that an essential nicotinic subunit remains to be cloned, it is perhaps more likely that oocytes either cannot assemble neuronal nicotinic subunits efficiently into channels with the correct composition and stoichiometry, or that they produce post-translational channel modifications which differ from those of mammalian neurones.