Alpha1-adrenoceptor subtypes mediating vasoconstriction in the carotid circulation of anaesthetized pigs: possible avenues for antimigraine drug development

Effects of the Novel High-affinity 5-HT(1B/1D)-receptor Ligand Frovatriptan on the Rat Carotid Artery

BACKGROUND

In blood vessels 5-HT stimulates sympathetic nerves, the endothelium and vascular smooth muscle cells. Triptans are specific anti-migraine drugs and they activate the serotoninergic 5HT1b/d receptors causing vasoconstriction of the cerebral vessels.

AIM

To evaluate the effect of frovatriptan on isolated rat carotid artery.

METHODS

Contractile activity of the preparations was registered isometrically. Krebs solution (pH = 7.4) was used for washing smooth muscle (SM) preparations aerated with 95% O2 and 5% CO2 at 37°C. The 60-minute adaptation of tone level of preparations was taken as a starting tone and the changes such as contraction or relaxation were calculated using it.

RESULTS

Frovatriptan (1×10-6 mol/l - 1×10-5 mol/l) induced a contraction, but at higher concentrations it caused relaxation of the carotid artery. The L-norepinephrine contractile reaction was enhanced in the presence of frovatriptan. In the presence of 5-HT2 receptor antagonist, methysergide, frovatriptan increased the relaxation. In the presence of the specific α-1 receptor antagonist, prazosin, the frovatriptan-induced relaxation decreased.

CONCLUSION

The observed contractile effect of frovatriptan is probably associated with the main effect of the drug - activation of the serotoninergic 5HT1B /1D receptors causing vasoconstriction of the cerebral vessels and their anti-migraine effect. At higher concentrations, frovatriptan, most likely via some non-specific mechanism, could activate the following intracellular chain reaction: stimulation of α1D could activate eNOS which may increase in the concentration of NO which results in the final effect of relaxation.

Current and prospective pharmacological targets in relation to antimigraine action

Migraine is a recurrent incapacitating neurovascular disorder characterized by unilateral and throbbing headaches associated with photophobia, phonophobia, nausea, and vomiting. Current specific drugs used in the acute treatment of migraine interact with vascular receptors, a fact that has raised concerns about their cardiovascular safety. In the past, alpha-adrenoceptor agonists (ergotamine, dihydroergotamine, isometheptene) were used. The last two decades have witnessed the advent of 5-HT(1B/1D) receptor agonists (sumatriptan and second-generation triptans), which have a well-established efficacy in the acute treatment of migraine. Moreover, current prophylactic treatments of migraine include 5-HT(2) receptor antagonists, Ca(2+) channel blockers, and beta-adrenoceptor antagonists. Despite the progress in migraine research and in view of its complex etiology, this disease still remains underdiagnosed, and available therapies are underused. In this review, we have discussed pharmacological targets in migraine, with special emphasis on compounds acting on 5-HT (5-HT(1-7)), adrenergic (alpha(1), alpha(2,) and beta), calcitonin gene-related peptide (CGRP(1) and CGRP(2)), adenosine (A(1), A(2), and A(3)), glutamate (NMDA, AMPA, kainate, and metabotropic), dopamine, endothelin, and female hormone (estrogen and progesterone) receptors. In addition, we have considered some other targets, including gamma-aminobutyric acid, angiotensin, bradykinin, histamine, and ionotropic receptors, in relation to antimigraine therapy. Finally, the cardiovascular safety of current and prospective antimigraine therapies is touched upon.

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.

Functional characterization of alpha-adrenoceptors mediating pupillary dilation in rats

Previously, we reported that the alpha(1A)-adrenoceptor, but not the alpha(1D)-adrenoceptor, mediates pupillary dilation elicited by sympathetic nerve stimulation in rats. This study was undertaken to further characterize the alpha-adrenoceptor subtypes mediating pupillary dilation in response to both neural and agonist activation. Pupillary dilator response curves were generated by intravenous injection of norepinephrine in pentobarbital-anesthetized rats. Involvement of alpha(1)-adrenoceptors was established as mydriatic responses were inhibited by systemic administration of nonselective alpha-adrenoceptor antagonists, phentolamine (0.3-3 mg/kg) and phenoxybenzamine (0.03-0.3 mg/kg), as well as by the selective alpha(1)-adrenoceptor antagonist, prazosin (0.3 mg/kg). The alpha(2)-adrenoceptor antagonist, rauwolscine (0.5 mg/kg), was without antagonistic effects. alpha(1A)-Adrenoceptor selective antagonists, 2-([2,6-dimethoxyphenoxyethyl]aminomethyl)-1,4-benzodioxane (WB-4101; 0.1-1 mg/kg) and 5-methylurapidil (0.1-1 mg/kg), the alpha(1B)-adrenoceptor selective antagonist, 4-amino-2-[4-[1-(benzyloxycarbonyl)-2(S)- [[(1,1-dimethylethyl)amino]carbonyl]-piperazinyl]-6,7-dimethoxyquinazoline (L-765314; 0.3-1 mg/kg), as well as the alpha(1D)-adrenoceptor selective antagonist, 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione (BMY-7378; 1 mg/kg), were used to delineate the adrenoceptor subtypes involved. Mydriatic responses to norepinephrine were significantly antagonized by intravenous administration of both WB-4101 and 5-methylurapidil, but neither by L-765314 nor by BMY-7378. L-765314 (0.3-3 mg/kg, i.v.) was also ineffective in inhibiting the mydriasis evoked by cervical sympathetic nerve stimulation. These results suggest that alpha(1B)-adrenoceptors do not mediate sympathetic mydriasis in rats, and that the alpha(1A)-adrenoceptor is the exclusive subtype mediating mydriatic responses in this species.

Possible role of alpha-adrenoceptor subtypes in acute migraine therapy

Even though the underlying mechanisms for the pathophysiology of migraine attacks are not completely understood, little doubt exists that the headache phase is explained by dilatation of cranial, extracerebral blood vessels. In this context, experimental models predictive for anti-migraine activity have shown that both triptans and ergot alkaloids, which abort migraine headache, produce vasoconstriction within the carotid circulation of different species. In contrast to the well-established role of serotonin (5-hydroxytryptamine; 5-HT) 5-HT1B receptors in the common carotid vascular bed, the role of alpha-adrenoceptors and their subtypes has been examined only relatively recently. Using experimental animal models and alpha1- and alpha2-adrenoceptor agonists (phenylephrine and BHT933, respectively) and antagonists (prazosin and rauwolscine, respectively), it was shown that activation of either receptor produces a cranioselective vasoconstriction. Subsequently, investigations employing relatively selective antagonists at alpha1- (alpha1A, alpha1B, alpha1D) and alpha2- (alpha2A, alpha2B, alpha2C) adrenoceptor subtypes revealed that specific receptors mediate the carotid haemodynamic responses in these animals. From these observations, together with the potential limited role of alpha1B- and alpha2C-adrenoceptors in the regulation of systemic haemodynamic responses, it is suggested that selective agonists at these receptors may provide a promising novel avenue for the development of acute anti-migraine drugs.