The receptor concept: pharmacology's big idea.

Chemical signalling is the main mechanism by which biological function is controlled at all levels, from the single cell to the whole organism. Chemical recognition is the function of receptors, which, in addition to recognising endogenous chemical signals, are also the target of many important experimental and therapeutic drugs. Receptors, therefore, lie at the heart of pharmacology. This article describes the way in which the receptor concept originated early in the 20th century, and evolved through a highly innovative stage of quantitative theory based on chemical kinetics, to the point where receptors were first isolated and later cloned, until we now have a virtually complete catalogue of all the receptors present in the genome. Studies on signal transduction are revealing great complexity in the events linking ligand binding to the physiological or therapeutic response. Though some simple quantitative rules of 'receptor theory' are still useful, the current emphasis is on unravelling the pathways that link receptors to responses, and it will be some time before we know enough about them to embark on the next phase of 'receptor theory'.

Comparison of intracellular calcium signals evoked by heat and capsaicin in cultured rat dorsal root ganglion neurons and in a cell line expressing the rat vanilloid receptor, VR1.

The cloning of the receptor for capsaicin, vanilloid receptor 1, has shown it to be non-selective cation channel with a high calcium permeability which can be opened by noxious heat as well as capsaicin. Here we compare the calcium signals produced by native and recombinant capsaicin receptors when activated by either heat or capsaicin by imaging intracellular calcium levels ([Ca2+](i)) in rat dorsal root ganglion neurons and Chinese hamster ovary cells transfected with the rat vanilloid receptor, vanilloid receptor 1. Vanilloid receptor 1 transfected cells and a subset of dorsal root ganglion neurons responded to both capsaicin and to heating to 50 degrees C with rapid, substantial and reversible rises in [Ca2+](i). All except one of the dorsal root ganglion neurons responsive to capsaicin also showed sensitivity to heat, and most, but not all, heat-sensitive neurons also responded to capsaicin. Both capsaicin and heat responses were dependent on the presence of extracellular Ca2+. Non-transfected Chinese hamster ovary cells and non-responsive dorsal root ganglion neurons showed only small rises in [Ca2+](i) in response to heat which did not depend on the presence of external Ca2+. Responsive dorsal root ganglion neurons and vanilloid receptor 1 transfected cells showed a clear temperature threshold, above which [Ca2+](i) increased rapidly. This was estimated to be 42.6+/-0.3 degrees C for vanilloid receptor 1 transfected cells and 42.0+/-0.6 degrees C for dorsal root ganglion neurons. The competitive capsaicin antagonist capsazepine (10microM) abolished [Ca2+](i) increases stimulated by capsaicin in both dorsal root ganglion neurons and vanilloid receptor 1 transfected cells. However, responses to heat of a similar magnitude in the same cells were inhibited by only 37% by capsazepine (10microM). In vanilloid receptor 1 transfected cells, Ruthenium Red (10microM) blocked responses to both capsaicin and heat. These results demonstrate that imaging of [Ca2+](i) can identify dorsal root ganglion neurons which are responsive to both heat and capsaicin. They show that heat and capsaicin responses mediated by native and recombinant capsaicin receptors are similar with respect to the characteristics and pharmacology examined, suggesting that expression of recombinant vanilloid receptor 1 in cell lines accurately reproduces the properties of the native receptor.

Pharmacological differences between the human and rat vanilloid receptor 1 (VR1).

Vanilloid receptors (VR1) were cloned from human and rat dorsal root ganglion libraries and expressed in Xenopus oocytes or Chinese Hamster Ovary (CHO) cells. Both rat and human VR1 formed ligand gated channels that were activated by capsaicin with similar EC(50) values. Capsaicin had a lower potency on both channels, when measured electrophysiologically in oocytes compared to CHO cells (oocytes: rat=1.90+/-0.20 microM; human=1.90+/-0.30 microM: CHO cells: rat=0.20+/-0.06 microM; human=0.19+/-0.08 microM). In CHO cell lines co-expressing either rat or human VR1 and the calcium sensitive, luminescent protein, aequorin, the EC(50) values for capsaicin-induced responses were similar in both cell lines (rat=0.35+/-0.06 microM, human=0.53+/-0.03 microM). The threshold for activation by acidic solutions was lower for human VR1 channels than that for rat VR1 (EC(50) pH 5.49+/-0.04 and pH 5.78+/-0.09, respectively). The threshold for heat activation was identical (42 degrees C) for rat and human VR1. PPAHV was an agonist at rat VR1 (EC(50) between 3 and 10 microM) but was virtually inactive at the human VR1 (EC(50)>10 microM). Capsazepine and ruthenium red were both more potent at blocking the capsaicin response of human VR1 than rat VR1. Capsazepine blocked the human but not the rat VR1 response to low pH. Capsazepine was also more effective at inhibiting the noxious heat response of human than of rat VR1.

Comparison of currents activated by noxious heat in rat and chicken primary sensory neurons.

The vanilloid receptor 1 (VR1) gene is responsible for both capsaicin-, and low threshold (LT) noxious heat-sensitivity in mammalian primary sensory neurons. Although, birds lack capsaicin-sensitivity they express LT noxious heat-sensitivity. Here, we compared LT noxious heat-activated whole-cell currents produced by rat and chicken cultured dorsal root ganglion neurons in order to find out the similarities and differences in the LT noxious heat transduction mechanisms between the two species. No significant differences between rat and chicken neurons were found in the mean cell diameter of the LT noxious heat-sensitive cells (20.4+/-0.8 microm, n=19 and 20.6+/-0.6 microm, n=11, respectively) and the average threshold (45.7+/-0.5 degrees C, n=19 and 46.1+/-0.7 degrees C, n=11, respectively) and peak amplitude (-2.9+/-0.6 nA, n=19 and -2.1+/-0.6 nA, n=11, respectively) of the heat-evoked responses. The current-voltage curves of the responses both in rat and chicken cells reversed at the same range (-19.5+/-3.8 mV, n=4 and -15.5+/-1. 2 mV, n=3, respectively) and showed strong outward rectification at negative membrane potentials. While all LT noxious heat-sensitive rat cells responded to capsaicin, none of the chicken neurons produced detectable response to it. These findings suggest that a VR1 homologue which lacks to sequence for capsaicin-sensitivity is possibly the LT noxious heat transducer in chicken.

Bradyzide, a potent non-peptide B(2) bradykinin receptor antagonist with long-lasting oral activity in animal models of inflammatory hyperalgesia.

Bradyzide is from a novel class of rodent-selective non-peptide B(2) bradykinin antagonists (1-(2-Nitrophenyl)thiosemicarbazides). Bradyzide has high affinity for the rodent B(2) receptor, displacing [(3)H]-bradykinin binding in NG108-15 cells and in Cos-7 cells expressing the rat receptor with K(I) values of 0.51+/-0.18 nM (n=3) and 0.89+/-0.27 nM (n=3), respectively. Bradyzide is a competitive antagonist, inhibiting B(2) receptor-induced (45)Ca efflux from NG108-15 cells with a pK(B) of 8.0+/-0.16 (n=5) and a Schild slope of 1.05. In the rat spinal cord and tail preparation, bradyzide inhibits bradykinin-induced ventral root depolarizations (IC(50) value; 1.6+/-0.05 nM (n=3)). Bradyzide is much less potent at the human than at the rodent B(2) receptor, displacing [(3)H]-bradykinin binding in human fibroblasts and in Cos-7 cells expressing the human B(2) receptor with K(I) values of 393+/-90 nM (n=3) and 772+/-144 nM (n=3), respectively. Bradyzide inhibits bradykinin-induced [(3)H]-inositol trisphosphate (IP(3)) formation with IC(50) values of 11.6+/-1.4 nM (n=3) at the rat and 2.4+/-0.3 microM (n=3) at the human receptor. Bradyzide does not interact with a range of other receptors, including human and rat B(1) bradykinin receptors. Bradyzide is orally available and blocks bradykinin-induced hypotension and plasma extravasation. Bradyzide shows long-lasting oral activity in rodent models of inflammatory hyperalgesia, reversing Freund's complete adjuvant (FCA)-induced mechanical hyperalgesia in the rat knee joint (ED(50), 0.84 micromol kg(-1); duration of action >4 h). It is equipotent with morphine and diclofenac, and 1000 times more potent than paracetamol, its maximal effect exceeding that of the non-steroidal anti-inflammatory drugs (NSAIDs). Bradyzide does not exhibit tolerance when administered over 6 days. In summary, bradyzide is a potent, orally active, antagonist of the B(2) bradykinin receptor, with selectivity for the rodent over the human receptor. British Journal of Pharmacology (2000) 129, 77 - 86

Similarities and differences between the responses of rat sensory neurons to noxious heat and capsaicin.

We have compared the membrane response of rat primary sensory neurons to capsaicin and noxious heat, using electrophysiological and ion flux measurements. Our aim was to determine whether, as recently proposed, the same molecular entity accounts for excitation by both types of stimulus. The properties of the ion channels activated by heat and capsaicin show many similarities but also important differences. The calcium permeability of heat-activated channels is lower than that of capsaicin-activated channels. Distinct single channels respond to heat or capsaicin, and only a few show dual sensitivity. At the whole-cell level, individual cells invariably show dual sensitivity, but the amplitudes of the responses show little correlation. We conclude that distinct molecular entities, which are both likely to be derived from the VR1 gene product, account for the membrane responses to heat and capsaicin.

Noxious heat activates all capsaicin-sensitive and also a sub-population of capsaicin-insensitive dorsal root ganglion neurons.

A sub-population of primary afferent fibres comprising mainly Adelta and C polymodal nociceptors specifically detects high intensity heat stimuli. These fibres are also sensitive to high threshold mechanical stimulation and different chemicals including inflammatory mediators. C-polymodal nociceptors are also activated by capsaicin. Recent findings show that noxious heat induces inward currents in a sub-population of cultured dorsal root ganglion neurons by opening nonselective cation-channels. It has been suggested that noxious heat is transduced by the recently cloned capsaicin-gated ion-channel since oocytes and HEK 293 cells expressing this channel respond to heat as well as capsaicin. In agreement with this suggestion Kirschstein et al. found in a small sample of dorsal root ganglion cells that all heat-sensitive neurons were also sensitive to capsaicin. In this study we examine further, by whole-cell voltage-clamp recording from adult rat dorsal root ganglion neurons grown in culture, the relationship between heat- and capsaicin-sensitivity. Our results show the existence of two kinds of heat-sensitive neurons, distinguished by their temperature thresholds. The low-threshold cells, which comprise the small-medium diameter population, are capsaicin-sensitive, whereas the high-threshold (mainly large-diameter) cells are not, and we postulate the existence of a heat transducer distinct from capsaicin receptor in the latter group.

Pathological and epidemiological aspects of skin lesions in histoplasmosis. Observations in an AIDS patient and badgers outside endemic areas of histoplasmosis.

As a consequence of HIV infection, histoplasmosis is increasingly occurring as an opportunistic infection with a systemic course outside histoplasmosis-endemic areas, e.g. in Europe. Accordingly, questions concerning the epidemiology of this mycosis arise. Two incidents involving histoplasmosis in man and badgers with prevailing involvement of the skin encouraged us to review the pathogenesis and epidemiology of this mycosis in Germany, where so far Histoplasma capsulatum has not been endemic. With a view to prevention, attention is drawn to the avoidance of microfoci of H. capsulatum in the newly introduced concept of biowaste and its composting plants in countries with modern waste management.

Capsaicin and carbon dioxide act by distinct mechanisms on sensory nerve terminals in the cat cornea.

The discharge of single afferent units recorded from filaments of the mixed ciliary nerve of anaesthetised cats was used to study the effects of CO2, capsaicin and the capsaicin antagonist, capsazepine, on sensory nerve terminals in the cornea. Units were selected on the basis of their sensitivity to CO2 (98.5% applied to the cornea for 30 s in a moist gas stream). Of these units, about 50% (18/38) also responded to capsaicin (0.1 microM applied as droplet and washed off after 20 s), with a discharge of similar magnitude to that produced by CO2. The other CO2-sensitive units (20/38) did not respond to capsaicin. Capsaicin-sensitive units responded more rapidly to CO2 (mean latency to first spike 0.7 +/- 0.2 s) than capsaicin-insensitive units (mean latency to first spike 5.1 +/- 1.2 s). On the basis of their conduction velocity, both the capsaicin-sensitive and the capsaicin-insensitive groups included both A- and C-fibres. Application of capsazepine (10 microM for 5-20 min) to the cornea reversibly blocked the response of the units to capsaicin without affecting responses to CO2. Units that were acutely desensitised by exposure to capsaicin (0.1 microM) for 30 s, during which time the discharge evoked by capsaicin declined to zero, still responded to CO2, though the response was reduced by 44% compared with controls. It is concluded that activation of sensory afferents in the cat cornea by capsaicin and by CO2 appear to involve distinct mechanisms, since: (a) many CO2- sensitive units are not excited by capsaicin; (b) capsazepine selectively blocks excitation by capsaicin without affecting responses to CO2; and (c) desensitisation to capsaicin impairs only partially the responsiveness to CO2.