BIBN4096BS and CGRP(8-37) antagonize the relaxant effects of alpha-CGRP more than those of beta-CGRP in human extracranial arteries

Possible site of action of CGRP antagonists in migraine

BACKGROUND

The calcitonin gene-related peptide (CGRP) receptor antagonists olcegepant and telcagepant are very potent drugs. Both are effective in migraine but in doses much higher than would be predicted from receptor binding and other in vitro results. This could perhaps suggest an effect of CGRP antagonists behind the blood-brain barrier (BBB), i.e. in the central nervous system (CNS).

METHODS

Comparison of doses needed for CGRP blocking effect in vitro with dose needed in vivo in man and monkeys. Discussion of these doses in relation to doses needed for anti-migraine activity.

RESULTS

In vivo studies in monkeys and man showed that high doses compared to doses needed in vitro are needed to block capsaicin-induced in skin blood flow, a CGRP-mediated reaction. These doses are close to those needed for anti-migraine activity.

CONCLUSION

The apparently high doses of CGRP receptor antagonists, olcegepant and telcagepant needed for anti-migraine effect are not so high after all. They do not allow a conclusion as to whether CGRP antagonists act on peripheral sites or central sites in migraine.

The involvement of CGRP, adrenomedullin, and sensory nerves in remote vasomotor responses within the hamster cheek pouch microcirculation

Previous work from our laboratory demonstrated a role for sensory nerves in remote dilations to microapplied methacholine by blocking the response with CGRP8-37 and concluded CGRP was the neurotransmitter. Recently, a more specific CGRP receptor antagonist, BIBN4096BS, was developed. The goals of the present study are to characterize the effects of BIBN4096BS on vasomotor responses in the hamster cheek pouch microcirculation, and to verify the role of CGRP in remote dilations to capsaicin and methacholine and to test adrenomedullin as an alternative neurotransmitter. BIBN4096BS pretreatment inhibits dilation to CGRP while having no significant effect on baseline diameter, it shifts the EC(50) to superfused CGRP from 1.5+/-0.3 pM to 2.5+/-0.6 nM and it shifts the apparent EC(50) to capsaicin from 31.5 nM to 171 nM. Local and remote dilations caused by the microapplication of methacholine are not inhibited by 300 nM BIBN4096BS (Local: 9.7+/-1.2 versus 9.7+/-1.5; 500:5.5+/-0.4 versus 5.7+/-0.5; 1000:4.4+/-0.6 versus 4.8+/-0.5). Remote dilations to methacholine were significantly inhibited however when adrenomedullin receptor antagonist adrenomedullin-(26-52) was microapplied to the remote site. Perivascular neurons containing adrenomedullin can be detected with immunohistochemistry. The results, combined with previous work, suggest that adrenomedullin, and not CGRP, is involved in remote dilations to methacholine.

What makes a CGRP2 receptor?

1. Heterogeneity in the receptors for the neuropeptide calcitonin gene-related peptide (CGRP) has been apparent for nearly 20 years. This is most clearly manifested in the observation of CGRP(8-37)-sensitive and -insensitive populations of CGRP-activated receptors. The pA(2) values for CGRP(8-37) in excess of 7 are widely considered to be the result of antagonism of CGRP(1) receptors, whereas those below 7 are believed to be the consequence of antagonism of a second population of receptors, namely CGRP(2) receptors. 2. However, a multitude of pA(2) values exist for CGRP(8-37), spanning several log units, and as such no obvious clusters of values are apparent. Understanding the molecular nature of the receptors that underlie this phenomenon is likely to aid the development of selective pharmacological tools to progress our understanding of the physiology of CGRP and related peptides. Because there is active development of CGRP agonists and antagonists as therapeutics, such information would also further this pursuit. 3. The CGRP(1) receptor is pharmacologically and molecularly well defined as a heterodimer of the calcitonin receptor-like receptor (CL) and receptor activity modifying protein (RAMP) 1. The CL/RAMP1 complex is highly sensitive to CGRP(8-37). Conversely, the constituents of the CGRP(2) receptor have not been identified. In fact, there is little evidence for a distinct molecular entity that represents the CGRP(2) receptor. 4. Recent pharmacological characterization of receptors related to CGRP(1) has revealed that some of these receptors may explain CGRP(2) receptor pharmacology. Specifically, AMY(1(a)) (calcitonin receptor/RAMP1) and AM(2) (CL/RAMP3) receptors can be activated by CGRP but are relatively insensitive to CGRP(8-37). 5. This, along with other supporting data, suggests that the 'CGRP(2) receptor' that has been extensively reported in the literature may, in fact, be an amalgamation of contributions from a variety of CGRP-activated receptors. The use of appropriate combinations of agonists and antagonists, along with receptor expression studies, could allow such receptors to be separated.

Temporal Thermometry Fails to Track Body Core Temperature during Heat Stress

PURPOSE

The aim of this study was to assess the accuracy of temporal scanning thermometry in monitoring internal temperature increases during passive heating.

METHODS

Sixteen subjects (5 males and 11 females) underwent a whole-body passive heat stress (water-perfused suit) to increase internal temperature. Temperatures were obtained with a temporal scanner and with an ingestible-pill telemetry system that tracks intestinal temperature. Temperatures were recorded while subjects were normothermic (34 degrees C water-perfusing suit) and every 10 min during passive heating (48 degrees C water-perfusing suit).

RESULTS

Heart rate (ECG), mean skin temperature (weighted six-site average), skin blood flow (laser Doppler flowmetry), and sweat rate (capacitance hygrometry) were all significantly elevated at the end of heating (all P < 0.001). Pre-heat stress temporal-derived temperature was not different from intestinal temperature (36.98 +/- 0.09 vs 37.01 +/- 0.09 degrees C, respectively, P = 0.76). However, after 30 min of heating (the greatest duration of heating completed by all subjects), temporal-derived temperature decreased to below the pre-heat stress baseline (-0.22 +/- 0.11), whereas intestinal temperature increased by 0.39 +/- 0.07 degrees C (P < 0.001 between the two methods). After 50 min of heating (N = 11), intestinal-derived internal temperature increased by 0.70 +/- 0.09 degrees C, whereas temporal-derived temperature decreased by 0.29 +/- 0.10 degrees C (P < 0.001). The group average (+/- SEM) R2 and slope between the two methods were 0.29 +/- 0.08 and -0.34 +/- 0.14, respectively.

CONCLUSION

These results demonstrate that temporal scanning does not track internal temperature, as measured via intestinal temperature, during passive heating. Given these findings, it is recommended that this technique not be used to assess temperature in hyperthermic diaphoretic subjects.

Effects of current and prospective antimigraine drugs on the porcine isolated meningeal artery

Vasoconstriction to agonists at serotonin (5-hydroxytryptamine; 5-HT) receptors and alpha-adrenoceptors, as well as vasodilatation induced by alpha-CGRP, have been well described in the porcine carotid circulation in vivo. The present study sets out to investigate the effects of current and prospective antimigraine drugs on porcine meningeal artery segments in vitro. Sumatriptan, ergotamine, dihydroergotamine, isometheptene and clonidine failed to contract the meningeal artery, but 5-HT, noradrenaline and phenylephrine induced concentration-dependent contractions. The contractions to 5-HT were competitively antagonized by the 5-HT(2A) receptor antagonist ketanserin, whilst those to noradrenaline were antagonized by alpha(1)-(prazosin), alpha(2)-(rauwolscine and yohimbine) and alpha(2C/2B)-(OPC-28326) adrenoceptor antagonists. Whilst dobutamine and salbutamol were ineffective, alpha-CGRP produced concentration-dependent relaxations that were antagonized by the CGRP(1) receptor antagonist olcegepant. In agreement with their lack of contractile effect, sumatriptan and ergotamine failed to influence forskolin-stimulated cyclic AMP accumulation in the porcine meningeal artery; in contrast, both compounds decreased forskolin-stimulated cyclic AMP accumulation in the human isolated saphenous vein, where they induced contractions. Finally, using RT-PCR, we could demonstrate the presence of mRNAs encoding for several 5-HT receptors (5-HT(1B), 5-HT(1D), 5-HT(1F), 5-HT(2A) and 5-HT(7)) and adrenoceptors (alpha(1A), alpha(1B), alpha(1D), alpha(2A), alpha(2B), alpha(2C), beta(1) and beta(2)), as well as that for the calcitonin receptor like receptor, a component of the CGRP(1) receptor. These results suggest that: (i) the porcine meningeal artery may not be involved in the vasoconstriction of the carotid vascular bed elicited by antimigraine drugs in anaesthetized pigs, and (ii) the mismatch between the presence of receptor mRNA and the lack of response to sumatriptan, dobutamine and salbutamol implies that mRNAs for the 5-HT(1B) receptor and beta(1)- and beta(2)-adrenoceptors are probably unstable, or that their density is too low for being translated as receptor protein in sufficient quantities.

Identification of specific calcitonin-like receptor residues important for calcitonin gene-related peptide high affinity binding

Background

Calcitonin gene-related peptide (CGRP) is a vasoactive neuropeptide whose biological activity has potential therapeutic value for many vascular related diseases. CGRP is a 37 amino acid neuropeptide that signals through a G protein-coupled receptor belonging to the secretin receptor family. Previous studies on the calcitonin-like receptor (CLR), which requires co-expression of the receptor-activity-modifying protein-1 (RAMP1) to function as a CGRP receptor, have shown an 18 amino acid N-terminus sequence important for binding CGRP. Moreover, several investigations have recognized the C-terminal amidated phenylalanine (F37) of CGRP as essential for docking to the mature receptor. Therefore, we hypothesize that hydrophobic amino acids within the previously characterized 18 amino acid CLR N-terminus domain are important binding contacts for the C-terminal phenylalaninamide of CGRP.

Results

Two leucine residues within this previously characterized CLR N-terminus domain, when mutated to alanine and expressed on HEK293T cells stably transfected with RAMP1, demonstrated a significantly decreased binding affinity for CGRP compared to wild type receptor. Additional decreases in binding affinity for CGRP were not found when both leucine mutations were expressed in the same CLR construct. Decreased binding characteristic of these leucine mutant receptors was observed for all CGRP ligands tested that contained the necessary amidated phenylalanine at their C-terminus. However, there was no difference in the potency of CGRP to increase cAMP production by these leucine mutant receptors when compared to wild type CLR, consistent with the notion that the neuropeptide C-terminal F37 is important for docking but not activation of the receptor. This observation was conserved when modified CGRP ligands lacking the amidated F37 demonstrated similar potencies to generate cAMP at both wild type and mutant CLRs. Furthermore, these modified CGRP ligands displayed a significant but similar loss of binding for all leucine mutant and wild type CLR because the important receptor contact on the neuropeptide was missing in all experimental situations.

Conclusion

These results are consistent with previous structure-function investigations of the neuropeptide and are the first to propose specific CLR binding contacts for the amidated F37 of CGRP that are important for docking but not activation of the mature CGRP receptor.

Characterisation of CGRP receptors in the human isolated middle meningeal artery

Although the understanding of migraine pathophysiology is still incomplete, there seems to be little doubt that dilatation of cranial blood vessels, including meningeal arteries, is involved in the headache phase of migraine. Since calcitonin gene-related peptide (CGRP) has been implicated in this vasodilatation, the present study set out to compare the relaxant effects of the endogenous ligand h-alphaCGRP, and [ethylamide-Cys(2,7)]h-alphaCGRP ([Cys(Et)(2,7)]h-alphaCGRP), a CGRP(2) receptor agonist, on human isolated middle meningeal artery segments, precontracted with KCl. Classical Schild plot analysis was used to characterise the receptor population in this artery using BIBN4096BS and h-alphaCGRP(8-37) as antagonists. h-alphaCGRP relaxed arterial segments more potently than [Cys(Et)(2,7)]h-alphaCGRP (pEC(50): 8.51+/-0.16 and 7.48+/-0.24, respectively), while the maximal responses to these agonists were not significantly different. BIBN4096BS equipotently blocked the relaxations induced by both agonists with a pA(2) of approximately 10 and with a Schild plot slope not significantly different from unity. h-alphaCGRP(8-37) also antagonised the response to h-alphaCGRP with a pA(2) of 6.46+/-0.16 and a Schild plot slope not different from unity. Furthermore, the results obtained from RT-PCR studies confirmed the presence of all the essential components required for a functional CGRP(1) receptor in these arteries. Considering the high antagonist potency of BIBN4096BS, coupled to the lower agonist potency of [Cys (Et)(2,7)]h-alphaCGRP, it is reasonable to suggest a predominant role of CGRP(1) receptors in the human middle meningeal artery. This view is reinforced by Schild plot analysis, which revealed a slope of unity in all experiments, giving further evidence for a homogeneous CGRP receptor population in this vascular preparation.