Identification and characterization of calcitonin gene-related peptide receptors in porcine renal medullary membranes

Molecular cloning and characterization of mouse calcitonin gene-related peptide receptor

The calcitonin gene-related peptide (CGRP) plays important roles as a neurotransmitter/neuromodulator in the central nervous system, and as a potent vasodilator when secreted from peripheral, perivascular nerves through its specific receptors. In this study, we cloned mouse cDNA counterparts of the human CGRP receptor composed of calcitonin receptor-like receptor (CRLR) and receptor activity modifying protein 1 (RAMP1) and examined the signal transduction mechanism through the CGRP receptor. Mouse CRLR (mCRLR) is a 462-amino acid G protein-coupled heptahelical receptor, and mouse RAMP1 (mRAMP1) is a 148-amino acid single membrane-spanning protein with a short cytoplasmic portion. Specific binding of (125)I-CGRP was detected only when both mCRLR and mRAMP1 cDNAs were cotransfected to COS-7 cells, and the Kd value of the receptor was 2.2 x 10(-10) M. CGRP induced a marked elevation of the intracellular cAMP levels in COS-7 cells cotransfected with mCRLR and mRAMP1. CGRP signaling through the mCRLR/mRAMP1 receptor complex was found to increase the promoter activities of cyclic AMP responsive element and serum responsive element in the co-transfected HeLa cells. These results indicate that mCRLR and mRAMP1 constitute a functional mouse CGRP receptor for the transduction of CGRP signaling by PKA and extracellular signal-regulated kinase signal transduction pathways.

Amylin compared with calcitonin: competitive binding studies in rat brain and antinociceptive activity

Binding studies for rat amylin (AMY) and salmon calcitonin (sCT) were performed on rat membranes prepared from pons and medulla oblongata of rats. The aim was to see whether specific binding sites for AMY and/or for sCT present in these areas could be relevant to some of the biological activities of the two peptides. Binding sites specific for [125I]AMY are present in the pons-medulla of rat brain as AMY, but not sCT, was able to displace radiolabeled AMY binding with an IC50 = 3.7+/-0.5x10(-10) M. In contrast, binding of [125I]sCT was displaced by both sCT and AMY, although with different potencies, the IC50 for sCT being 1+/-0.1x10(-11) M, and for AMY, 1.8+/-0.08x10(-7) M. The functional significance of the presence of these binding sites was evaluated in two different nociceptive tests, hot-plate and tail-flick. In the tail-flick test neither AMY (5-10 microg/rat, i.c.v.) nor sCT (10 microg/rat i.c.v.) showed antinociceptive activity, whereas in the hot-plate test AMY (10 microg/rat, i.c.v.) significantly increased the response latencies as did sCT (250 ng/rat, i.c.v.). These results demonstrated that a 40-fold greater dose of AMY is necessary to produce a comparable antinociceptive effect to that exerted by sCT. These findings are in accordance with the low affinity of AMY for sCT binding sites in rat pons-medulla. It is therefore suggested that the central inhibitory activity of AMY on pain perception involves interaction with sCT receptors whereas the selective AMY binding sites subserve other (as yet unknown) functions.

Comparative affinities of adrenomedullin (AM) and calcitonin gene-related peptide (CGRP) for [125I] AM and [125I] CGRP specific binding sites in porcine tissues

We have investigated the binding characteristics of rat [125I] adrenomedullin (AM) and human [125I] calcitonin gene-related peptide (CGRP) to membranes prepared from a number of porcine tissues including atrium, ventricle, lung, spleen, liver, renal cortex and medulla. These membranes displayed specific, high affinity binding for [125I] rat AM and [125I] human CGRP. Porcine lung displayed the highest density of binding sites for radiolabeled AM and CGRP followed by porcine renal cortex. Competition experiments performed with [125I] rat AM indicated that the rank order of potencies of various peptides for inhibiting [125I] rat AM binding to various tissues were rat AM > or = human AM > or = human AM(22-52) > h alpha CGRP > or = h alpha CGRP(8-37) >>>> sCT except spleen, atrium, renal cortex and renal medulla where rAM and hAM were 20-300 fold more potent than hAM (22-52). When the same experiments were performed using [125I] h alpha CGRP as the radioligand, the rank order potencies for various peptides were rAM = hAM > h alpha CGRP > h alpha CGRP(8-37) in most of the tissues except in spleen and liver where h alpha CGRP was the most potent ligand. In lung, h alpha CGRP was almost as potent as rAM and hAM in displacing [125I] h alpha CGRP binding. These data suggest the existence of distinct CGRP and AM specific binding sites in contrast to previous reports that showed that both peptides interact differently in rat tissues.

Calcitonin gene-related peptide receptor independently stimulates 3',5'-cyclic adenosine monophosphate and Ca2+ signaling pathways

Calcitonin gene-related peptide (CGRP) is a neuropeptide with diverse biological properties including potent vasodilating activity. Recently, we reported the cloning of complementary DNAs (cDNAs) encoding the human and porcine CGRP receptors which share significant amino acid sequence homology with the human calcitonin receptor, a member of the recently described novel subfamily of G-protein-coupled 7TM receptors. Activation of this family of receptors has been shown to result in an increase in intracellular cAMP accumulation and calcium release. In this study, we demonstrate that HEK-293 cells expressing recombinant CGRP receptors (HEK-293HR or PR) respond to CGRP with increased intracellular calcium release (EC50 = 1.6 nM) in addition to the activation of adenylyl cyclase (EC50 = 1.4 nM). The effect of CGRP on adenylyl cyclase activation and calcium release was inhibited by CGRP (8-37), a CGRP receptor antagonist. Both effects were mediated by cholera toxin-sensitive G-proteins, but these two signal transduction pathways were independent of each other. While cholera toxin pretreatment of HEK-293PR cells resulted in permanent activation of adenylyl cyclase, the same pretreatment resulted in an inhibition of CGRP-mediated [Ca2+]i release. Pertussis toxin was without effect on CGRP-mediated responses. In addition, CGRP-mediated calcium release appears to be due to release from a thapsigargin-sensitive intracellular calcium pool. These results show that the recombinant human as well as porcine CGRP receptor can independently increase both cAMP production and intracellular calcium release when stably expressed in the HEK-293 cell line.

Characterization of binding sites for calcitonin gene related peptide in abalone gill

Target organs for calcitonin gene related peptide were investigated in the abalone. To elucidate the function of this neuropeptide in the biomineralization process, we have localized, in different tissues from abalone, specific binding sites for human calcitonin gene related peptide (hCGRP). Highest binding was observed in gill membranes where two classes of affinity components were identified. The affinity constants and the number of binding sites per mg of proteins for the site I were 5 X 10(9)M(-1)and 8.8 x 10(10). For the site II, the affinity constant was 1.34 X 10(7) M(-1)and the number of binding sites per mg of proteins was 1.1 x 10(12). In contrast, no specific calcitonin binding could be detected in every tested tissue, but the similar displacement of the 125I-labeled CGRP binding with unlabeled hCGRP and sCT suggested that, in the abalone, the identified receptor could belong to a third class receptor subtype, that recognize both hCGRPalpha and sCT. These data suggest that, during evolution, the role of CGRP in gill function is particularly well conserved and that this neuropeptide is likely to participate in the control of hydromineral metabolism in aquatic species.

Uterine relaxation responses to calcitonin gene-related peptide and calcitonin gene-related peptide receptors decreased during labor in rats

OBJECTIVES

Our purpose was to investigate (1) whether uterine relaxation responses to calcitonin gene-related peptide are differentially regulated during pregnancy and labor, (2) the involvement of nitric oxide in smooth muscle relaxant action of calcitonin gene-related peptide in the rat uterus, (3) whether receptors for calcitonin gene-related peptide are expressed in rat uterus, and if so (4) whether the concentrations of these receptors are differently regulated during pregnancy and labor.

STUDY DESIGN

Rats were killed on day 18 of gestation, at the time of spontaneous labor, or postpartum day 2. The uteri were removed for in vitro contractility measurements, nitric oxide production, and calcitonin gene-related peptide receptor binding assay.

RESULTS

(1) Calcitonin gene-related peptide induced a dose-dependent relaxation in spontaneously contracting uterine strips from pregnant rats on day 18 of gestation; (2) the relaxation effects of calcitonin gene-related peptide on the uterus were decreased during spontaneous delivery at term and post partum compared with that during pregnancy; (3) calcitonin gene-related peptide-induced relaxation was inhibited by pretreatment of the uterine tissue with a calcitonin gene-related peptide receptor antagonist, calcitonin gene-related peptide(8-37); (4) nitric oxide synthesis inhibitor (N(G)-nitro-L-arginine methyl ester) and soluble guanylate cyclase inhibitor (LY83583) significantly decreased calcitonin gene-related peptide-induced relaxation of the rat uterus during pregnancy; (5) calcitonin gene-related peptide increased the uterine nitric oxide production in pregnant rats, and this increase was obliterated in the presence of calcitonin gene-related peptide(8-37); and (6) calcitonin gene-related peptide receptors are present in rat uterus, and the concentration of these receptors dramatically increases during pregnancy and decreases during labor at term.

CONCLUSIONS

Calcitonin gene-related peptide inhibits uterine spontaneous contractions in rats during pregnancy but not during labor and post partum. The inhibitory effects of calcitonin gene-related peptide on uterine contractility appear to be modulated, at least in part, by the activation of nitric oxide generation in the rat uterus. Changes in calcitonin gene-related peptide receptors could contribute to the changes in calcitonin gene-related peptide-mediated uterine relaxation during pregnancy and labor.

Interaction of adrenomedullin with calcitonin receptor in cultured human breast cancer cells, T 47D

Human adrenomedullin (hADM), human calcitonin gene-related peptide (hCGRP), and salmon calcitonin (sCT)-activated adenylyl cyclase with EC50 values of 132, 764, and 0.5 nM, respectively, in human breast cancer cell line, T 47D. Treatment of T 47D cell membranes with near maximal concentrations of sCT, hADM and hCGRP had no additive effect on adenylyl cyclase activity. Salmon calcitonin (8-32)[sCT (8-32)], selective antagonist of calcitonin receptor, inhibited the activation of adenylyl cyclase by these three peptides. On the other hand, the putative ADM receptor antagonist, ADM (22-52), and CGRP receptor antagonist, CGRP (8-37), failed to inhibit ADM-, CGRP- or sCT-activated adenylyl cyclase. These results suggest that in T47D cells, both ADM and CGRP activated adenylyl cyclase through sCT receptors.

Neuroanatomical localization, pharmacological characterization and functions of CGRP, related peptides and their receptors

Calcitonin generelated peptide (CGRP) is a neuropeptide discovered by a molecular approach over 10 years ago. More recently, islet amyloid polypeptide or amylin, and adrenomedullin were isolated from human insulinoma and pheochromocytoma respectively, and revealed between 25 and 50% sequence homology with CGRP. This review discusses findings on the anatomical distributions of CGRP mRNA, CGRP-like immunoreactivity and receptors in the central nervous system, as well as the potential physiological roles for CGRP. The anatomical distribution and biological activities of amylin and adrenomedullin are also presented. Based upon the differential biological activity of various CGRP analogs, the CGRP receptors have been classified in two major classes, namely the CGRP1 and CGRP2 subtypes. A third subtype has also been proposed (e.g. in the nucleus accumbens) as it does not share the pharmacological properties of the other two classes. The anatomical distribution and the pharmacological characteristics of amylin binding sites in the rat brain are different from those reported for CGRP but share several similarities with the salmon calcitonin receptors. The receptors identified thus far for CGRP and related peptides belong to the G protein-coupled receptor superfamily. Indeed, modulation of adenylate cyclase activity following receptor activation has been reported for CGRP, amylin and adrenomedullin. Furthermore, the binding affinity of CGRP and related peptides is modulated by nucleotides such as GTP. The cloning of various calcitonin and most recently of CGRP1 and adrenomedullin receptors was reported and revealed structural similarities but also significant differences to other members of the G protein-coupled receptors. They may thus form a new subfamily. The cloning of the amylin receptor(s) as well as of the other putative CGRP receptor subtype(s) are still awaited. Finally, a broad variety of biological activities has been described for CGRP-like peptides. These include vasodilation, nociception, glucose uptake and the stimulation of glycolysis in skeletal muscles. These effects may thus suggest their potential role and therapeutic applications in migraine, subarachnoid haemorrhage, diabetes and pain-related mechanisms, among other disorders.

Calcitonin gene-related peptide receptor in human placental syncytiotrophoblast brush-border and basal plasma membranes

Minerals, such as calcium and potassium, are essential for fetal development, but their transplacental transport, and in particular, the effect of hormones on this process has not been extensively studied. Human alpha-calcitonin gene-related peptide (h alpha CGRP), a hormone constituted of 37 amino acids, is obtained by the alternative splicing of the mRNA from the calcitonin gene, and could be implicated in placental ion transport. In order to study the presence of this receptor, brush-border and basal plasma membranes were purified, and membrane binding studies were conducted using [125I]h alpha CGRP. The initiation of binding of [125I]h alpha CGRP to both membranes was rapid and reached maximal value after 10 min of incubation at 37 degrees C. Scratchard analysis revealed single-affinity binding sites for h alpha CGRP with Kd equal to 4412.45 +/- 604.81 pM and 2673.24 +/- 552.51 pM for brush-border and basal plasma membranes, respectively, which were significantly different. Moreover, the maximal number of receptors was significantly different (P < 0.001) in both membranes, with Bmax of 627.94 +/- 31.40 fmol/mg protein for brush-border membranes and 343.70 +/- 43.52 fmol/mg protein in basal-plasma membranes. Competitive displacement of [125I]h alpha CGRP with other ligands showed the following potencies; h alpha CGRP approximately h beta CGRP approximately Cys (acm)2,7 CGRP > CGRP (8-37), but no competition was observed with human and salmon calcitonin. Half-maximal displacement for human alpha- and beta CGRP was reached at approximately 10(-10)M for brush-border and basal-plasma membranes. alpha- and beta CGRP, and their fragment and analogue, stimulated cyclic AMP production in placental homogenate ranging from 143-163 per cent. Thus, our results show the presence of CGRP-specific receptors in both the syncytiotrophoblast membranes of human placenta. The role(s) of this related peptide in placenta remains to be investigated.

Differential effects of guanine nucleotides on [125I]-hCGRP(8-37) binding to porcine lung and human neuroblastoma cell membranes

Calcitonin gene-related peptide (CGRP) mediates its effects by binding to specific receptors which are positively coupled to adenylyl cyclase. CGRP(8-37), a CGRP fragment devoid of the N-terminal region, was shown to be a competitive CGRP receptor antagonist. Only a limited amount of data exists on the usefulness of this ligand in studying CGRP receptors. In the present study, we used [125I]-hCGRP(8-37) to characterize CGRP receptors in porcine lung and human neuroblastoma cell (SK-N-MC) membranes. [125I]-hCGRP(8-37) displayed specific and high affinity binding in both membrane preparations. Displacement studies using [125I]-hCGRP(8-37) and the agonist CGRP revealed the presence of high and low affinity CGRP binding sites in SK-N-MC cell and porcine lung membranes. Addition of guanylimidodiphosphate [Gpp(NH)p] shifted the competition curve to the right and changed the two affinity states of the receptor to a single affinity in SK-N-MC cell membranes. On the other hand, in porcine lung membranes, the whole competition curve was shifted to the right while maintaining the two affinity states. Thus, our data indicate that the new radioligand [125I]-hCGRP(8-37) is a useful tool for characterizing CGRP receptors and their coupling to guanine nucleotide binding proteins.

A cDNA encoding the calcitonin gene-related peptide type 1 receptor

Calcitonin gene-related peptide (CGRP) is a neuropeptide with diverse biological effects including potent vasodilator activity. We report here the cloning of a complementary DNA (cDNA) encoding a human CGRP1 receptor, which shares significant peptide sequence homology with the human calcitonin receptor, a member of the G-protein-coupled receptor superfamily. Northern blot analysis revealed that the messenger RNA for this receptor is predominantly expressed in the lung and heart. In situ studies showed specific localization of the receptor mRNA to alveolar cells in the lung and to cardiac myocytes in the heart. Stable expression of the cDNA in human embryonic kidney 293 (HEK 293) cells produced specific, high affinity binding sites for CGRP that displayed pharmacological and functional properties very similar to native human CGRP1 receptor. Exposure of these cells to CGRP resulted in a 60-fold increase in cAMP production, which was inhibited in a competitive manner by the CGRP1 receptor antagonist, CGRP-(8-37).

Receptor subtypes mediating renal actions of calcitonin gene-related peptide

We previously reported that the renal arterial infusions of non-hypotensive doses of calcitonin gene-related peptide (CGRP) caused renal vasodilatation and increases in glomerular filtration rate at a low dose, but renal vasoconstriction, natriuresis and kaliuresis at a high dose. In the present study, we examined the effects of the specific CGRP1 receptor antagonist (CGRP-(8-37) (1 and 10 nmol/kg) and the putative CGRP receptor antagonist, [Tyr(0)]CGRP-(28-37)(3 and 30 nmol/kg), on the renal vascular and tubular effects of CGRP in inactin-anaesthetized Sprague-Dawley rats. Renal arterial infusion of single doses of CGRP (0.3-300 pmol/kg per min) did not significantly alter mean arterial pressure or heart rate. However, during the continuous renal arterial infusion of either CGRP-(8-37) or [Tyr(0)CGRP-(28-37) incompletely inhibited the vasoconstriction but did not inhibit diuresis, natriuresis and kaliuresis elicited by a high but non-hypotensive dose of CGRP. On the basis that CGRP-(8-37) is a competitive CGRP1 receptor antagonist, our results suggest: (1) the renal vascular effect of CGRP is completely mediated via the activation of CGRP1 receptors, (2) the renal tubular effects of CGRP are not mediated via CGRP1 receptors, and (3) [Tyr(0)]CGRP-(28-37) is a CGRP1 receptor antagonist with potency and efficacy less than those of CGRP-(8-37).

Multiple affinity and guanine nucleotide sensitive forms of the calcitonin gene related peptide (CGRP) receptor

Calcitonin gene related peptide (CGRP) is a novel neuropeptide with an impressive array of biological actions consistent with its diverse tissue distribution and suggested role as neurotransmitter or neuromodulator. Binding sites for CGRP with properties consistent with those of receptors are present in both central and peripheral tissues. Radioligand binding studies were performed to investigate the fundamental processes underlying CGRP receptor activation and signaling following agonist occupancy of the receptor. These studies documented the existence of a selective, high affinity, and homogeneous population of binding sites for CGRP in membranes prepared prepared from central and various peripheral tissues. The affinity of [125I]CGRP for these sites was regulated by GTP or its stable analog GTP--gamma S, indicating coupling of CGRP receptors to G-protein(s). Kinetic studies documented the existence of the CGRP receptor in multiple affinity states when both coupled to and uncoupled from G-protein(s). These findings suggest that CGRP occupancy of its receptor induces conformational changes in the receptor that may be involved in its coupling to G-proteins and that the resulting ligand--receptor---protein ternary complex exists in multiple affinity conformational states. It seems likely that the multiple affinity states of the CGRP receptor ternary complex are involved differentially in signaling by and desensitization of the receptor. This evidence for agonist-induced conformational changes in a G-protein-coupled receptor prior to its coupling with G-protein(s) and for the existence of the ligand--receptor--G-protein ternary complex in multiple affinity conformational states is novel and extends our current understanding of the nature of the processes involved in agonist-dependent activation of G-protein-coupled receptors.

Adrenomedullin increases cyclic AMP more potently than CGRP and amylin in rat renal tubular basolateral membranes

In rat renal tubular basolateral membranes, the potency to increase cAMP of adrenomedullin (AM), a novel vasorelaxant peptide originally isolated from human pheochromocytoma, was compared with those of calcitonin gene-related peptide (CGRP) and amylin. Although all three peptides raised cAMP in a time- and concentration-dependent manner with a 4-fold increase at 10(-6)-10(-5) M, the EC50 value (10(-9) M) of AM was 100-fold smaller than those of CGRP and amylin. CGRP[8-37], an antagonist for CGRP receptors, attenuated cAMP elevation induced by these peptides with the essentially similar concentration-inhibition curves. These results suggest that the receptors for AM, CGRP and amylin share a common structural homology, and that the receptors sensitive to AM are preferentially expressed in renal tubular basolateral membranes.

The calcitonin gene-related peptide antagonist CGRP8-37 increases the latency to withdrawal responses in rats

The present study explored the effects of calcitonin gene-related peptide (CGRP) and its antagonist CGRP8-37 on the latency to hindpaw withdrawal responses induced by both thermal and mechanical stimulation in rats. (1) Intrathecal injection of 10 nmol of CGRP had no effects on the latency to hindpaw withdrawal; intrathecal injection of 5 nmol of substance P (SP) decreased the latency to both withdrawal responses. (2) Intrathecal administration of 5 nmol or 10 nmol of CGRP8-37, but not 1 nmol, induced a significant increase in hindpaw withdrawal latency. (3) Intrathecal administration of CGRP8-37 not only reversed the SP-induced decrease in latency to both withdrawal responses but also mediated a significant increase in response latency compared to basal levels. The demonstrated results suggest that intrathecal administration of CGRP8-37 has a possible antinociceptive effect, and CGRP receptors in the spinal cord may be involved.

Characterization of CGRP receptors in various regions of gerbil brain

The distribution of specific calcitonin gene-related peptide (CGRP) binding sites was investigated in gerbil brain regions such as cortex, cerebellum, hypothalamus, and hippocampus. The binding of [125I]CGRP to membranes prepared from these regions of the gerbil brain was rapid, saturable and specific with dissociation constants (Kd) between 10-40 pM and maximum binding (Bmax) between 10-30 fmol/mg protein. Human and rat CGRP and hCGRP8-37 competed for [125I]hCGRP binding in a concentration-dependent manner with a Ki of 10-100 pM. Salmon calcitonin was very weak in competing for [125I]hCGRP binding (ki > 10 microM). CGRP did not stimulate adenylate cyclase in these brain regions. Cross-linking of [125I] hCGRP to the brain membrane fractions with disuccinimidyl suberate revealed specific incorporation of [125I] hCGRP to a protein band of approximate molecular weight 52 kDa.

Calcitonin gene-related peptide (CGRP)1 receptor mediates vasodilation in the rat isolated and perfused kidney

The vasodilator activity of rat(r) alpha and beta calcitonin gene-related peptide (CGRP) as compared to the structurally related peptide, rAmylin, [Cys(ACM)2,7]human (h)CGRP and salmon calcitonin, was investigated in the rat isolated and perfused kidney which vascular tone was increased by 3 microM noradrenaline. The order of potency in producing vasodilation was alpha CGRP > beta CGRP >> rAmylin. [Cys(ACM)2,7]hCGRP and salmon calcitonin were ineffective. The activity of alpha and beta forms of rCGRP and rAmylin was antagonized by hCGRP (8-37), a CGRP1 receptor antagonist, at a concentration which did not affect acetylcholine-induced vasodilation. These results indicate the involvement of CGRP1 receptor in the CGRP-induced vasodilation in the rat kidney.

Regional distribution and regulation of [125I]calcitonin gene-related peptide binding sites in coronary arteries

Quantitative in vitro autoradiographic techniques were used to localize and characterize 125I-labelled human calcitonin gene-related peptide ([125I]hCGRP) binding sites in sections of bovine left anterior descending coronary artery (LAD). Specific high affinity (Kd 0.4 nM) [125I]hCGRP binding sites were localized to the media of both epicardial and myocardial coronary arteries. Binding site density was greater in distal epicardial and myocardial arteries than in proximal epicardial regions of the LAD. Binding sites exhibited a significantly higher affinity for alpha-hCGRP (Ki 1.1 nM) than for hCGRP-(8-37) (Ki 7.0 nM) and [Cys(ACM)2,7]hCGRP (Ki 27.4 nM). Guanosine-5'-O-(3-thiotriphosphate) inhibited [125I]hCGRP binding in a concentration-dependent manner. Extrinsic denervation of the bovine heart resulted in a depletion of CGRP-like immunoreactive perivascular nerve fibres and an increase in the density of coronary artery [125I]hCGRP binding sites (P = 0.0092). The regional distribution of binding sites in human coronary arteries differed from that observed in bovine and porcine vessels. It is concluded that selective, G protein-coupled, CGRP receptors are present in the media of bovine coronary arteries; there are both regional and species differences in the distribution of CGRP binding sites in coronary arteries and endogenous CGRP may exert a tonic influence on coronary vasomotor tone.

Calcitonin gene-related peptide: multiple actions, multiple receptors

Calcitonin gene-related peptide (CGRP) shows diversity both in its effects and its receptors. It is likely to have roles as a neurotransmitter, neuromodulator, local hormone and trophic factor. Its effects include rapid changes in neuronal activity, relaxation of many types of smooth muscle, actions on metabolism and changes in gene expression. Receptor heterogeneity has been revealed from experiments comparing agonist potency ratios and antagonist affinities. The evidence from these approaches is reviewed in this article and a speculative receptor classification scheme is proposed. Some of the likely future directions for CGRP research are discussed.