Pharmacological analysis of the haemodynamic effects of 5-HT1B/D receptor agonists in the normotensive rat

Effects of new generation triptans - frovatriptan and almotriptan - on hemodynamic parameters in intact male and female rats

The introduction of the second generation triptans in clinical and experimental practice was a major progress in the pharmacotherapy of migraine. Frovatriptan is a second generation triptan with strong 5-HT1B/1D serotonergic agonism and low 5-HT1A/7 receptor affinity, while almotriptan possesses not only the typical 5-HT1B/1D receptor agonist activity, but shows an affinity to the 5-HT1F receptor. The aim of our study was to assess the impact of frovatriptan and almotriptan on hemodynamics in male and female rats. We used a non-invasive "tail-cuff" method to measure the arterial blood pressure. Female and male Wistar rats were treated separately with high and low dosages of frovatriptan and almotriptan. Male and female rats showed reduction in all hemodynamic parameters, but only male rats showed an increase in the heart rate. In general, we could say that both almotriptan and frovatriptan potentiate cardiovascular safety.

Effect of TRPA1 activator allyl isothiocyanate (AITC) on rat dural and pial arteries

BACKGROUND

Transient receptor potential ankyrin 1 (TRPA1) channels may have a role in migraine as some substances known to cause headache activate the channel. In the craniovascular system such activation causes a calcitonin gene-related peptide (CGRP)-dependent increase in meningeal blood flow. TRPA1 channels in the endothelium of cerebral arteries cause vasodilation when activated. The headache preventive substance feverfew inhibits activation of TRPA1 channels. In this study we aim to compare and characterize the effect of the TRPA1 agonist allyl isothiocyanate (AITC) on the diameter of rat dural and pial arteries in vivo.

METHODS

The genuine closed-cranial window technique in rats was used to examine changes in dural and pial artery diameter and mean arterial blood pressure (MABP) after intracarotid infusion of AITC. Blockade experiments were performed by intravenous infusion of olcegepant, HC-030031, sumatriptan or capsazepine immediately after infusion of AITC, in four different groups of rats.

RESULTS

AITC caused a significant dilation of dural arteries, which was inhibited by HC-030031, olcegepant and sumatriptan, but not by capsazepine. In pial arteries AITC caused a significant dilation, which was not inhibited by any of the pre-treatments, suggesting a poor penetration of the blood-brain barrier or autoregulation due to dimethyl sulfoxide (DMSO) mediated decrease in MABP during HC-030031 infusion. AITC did not cause a significant change in MABP.

CONCLUSION

AITC causes dilation of dural arteries via a mechanism dependent on CGRP and TRPA1 that is sensitive to sumatriptan. AITC causes a small but significant dilation of pial arteries.

Serotonin and blood pressure regulation

5-Hydroxytryptamine (5-HT; serotonin) was discovered more than 60 years ago as a substance isolated from blood. The neural effects of 5-HT have been well investigated and understood, thanks in part to the pharmacological tools available to dissect the serotonergic system and the development of the frequently prescribed selective serotonin-reuptake inhibitors. By contrast, our understanding of the role of 5-HT in the control and modification of blood pressure pales in comparison. Here we focus on the role of 5-HT in systemic blood pressure control. This review provides an in-depth study of the function and pharmacology of 5-HT in those tissues that can modify blood pressure (blood, vasculature, heart, adrenal gland, kidney, brain), with a focus on the autonomic nervous system that includes mechanisms of action and pharmacology of 5-HT within each system. We compare the change in blood pressure produced in different species by short- and long-term administration of 5-HT or selective serotonin receptor agonists. To further our understanding of the mechanisms through which 5-HT modifies blood pressure, we also describe the blood pressure effects of commonly used drugs that modify the actions of 5-HT. The pharmacology and physiological actions of 5-HT in modifying blood pressure are important, given its involvement in circulatory shock, orthostatic hypotension, serotonin syndrome and hypertension.

Contribution of thromboxane a₂ in rat common carotid artery response to serotonin

Serotonin is a vasoactive substance that in different blood vessels mostly induces vasoconstriction. Considering the important role of common carotid artery in brain blood supply, the aims of this study were to investigate the effect of serotonin on isolated rat common carotid artery and also to examine participation of intact endothelium, cyclooxygenase products, Ca⁺⁺ channels and 5-HT₂ receptors in serotonin-evoked action. Endothelium was mechanically removed from some vascular rings. Circular artery segments were placed in organ baths containing Krebs–Ringer bicarbonate solution. Cumulative concentration-contraction curves for serotonin were obtained in rings previously equilibrated at basal tone. Serotonin produced concentration-dependent contraction, which was unaltered by endothelial denudation. Serotonin-induced effect was notably and comparably reduced by indomethacin (cyclooxygenase inhibitor) or OKY–046 (thromboxane A₂-synthase inhibitor) on intact or denuded rings. Nifedipine (Ca⁺⁺ channel blocker) or ketanserin (5-HT₂ receptor antagonist) strongly reduced serotonin-evoked effect. Our results suggest that serotonin produced concentration-dependent and endothelium-independent contraction of carotid artery, which was initiated by activation of 5-HT₂ receptors located on smooth muscle cells and mediated via L-type Ca⁺⁺ channels. Thromboxane A₂ from smooth muscle cells notably contributed to the overall contraction of carotid artery induced by serotonin.

Donitriptan decreases jugular venous oxygen saturation in rats in the absence of cranial vasoconstriction: an overlooked mechanism of antimigraine action?

The aim of the present study was to determine whether donitriptan and sumatriptan decreased jugular venous oxygen saturation and increased carbon dioxide partial pressure in venous blood. However, previous studies conducted with these compounds cannot discriminate whether the decrease of venous oxygen saturation is dependent of cranial vasoconstrictor. In the present study, vehicle (n = 10), donitriptan (2.5, 10, and 40 microg/kg; n = 8) or sumatriptan (630 microg/kg; n = 8) were infused into the carotid artery in the anesthetized rat. Regional blood flows were evaluated in the presence of donitriptan (10 microg/kg; n = 6) or vehicle (n = 6). Jugular venous oxygen saturation was significantly decreased by donitriptan (from 10 microg/kg) with maximal changes of -32.9 +/- 8.0%. Jugular carbon dioxide partial pressure was increased by donitriptan, reaching maximal changes of 17.7 +/- 4.6% (P < 0.05 versus vehicle). Similarly, sumatriptan significantly decreased venous oxygen saturation and increased jugular carbon dioxide partial pressure. These changes induced by donitriptan are abolished by the 5-hydroxytryptamine (5-HT)(1B/1D) receptor antagonist GR 127935 (N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2-[-methyl-4(5-methyl-1,2,4)-oxadiazol-3-yl]-(1,1 biphenyl)-4-carboxamide dihydrochloride). In addition, donitriptan was devoid of significant effects on systemic arterial pressure, heart rate, or regional blood flows, including systemic arterial-jugular venous anastomotic, systemic, or cranial. The results demonstrate that donitriptan increases cerebral oxygen consumption by 5-HT(1B/1D) receptor activation in the absence of cranial vasoconstriction.

Triptans in migraine: a comparative review of pharmacology, pharmacokinetics and efficacy

Triptans are a new class of compounds developed for the treatment of migraine attacks. The first of the class, sumatriptan, and the newer triptans (zolmitriptan, naratriptan, rizatriptan, eletriptan, almotriptan and frovatriptan) display high agonist activity at mainly the serotonin 5-HT1B and 5-HT1D receptor subtypes. As expected for a class of compounds developed for affinity at a specific receptor, there are minor pharmacodynamic differences between the triptans. Sumatriptan has a low oral bioavailability (14%) and all the newer triptans have an improved oral bioavailability and for one, risatriptan, the rate of absorption is faster. The half-lives of naratriptan, eletriptan and, in particular, frovatriptan (26 to 30h) are longer than that of sumatriptan (2h). These pharmacokinetic improvements of the newer triptans so far seem to have only resulted in minor differences in their efficacy in migraine. Double-blind, randomised clinical trials (RCTs) comparing the different triptans and triptans with other medication should ideally be the basis for judging their place in migraine therapy. In only 15 of the 83 reported RCTs were 2 triptans compared, and in 11 trials triptans were compared with other drugs. Therefore, in all placebo-controlled randomised clinical trials, the relative efficacy of the triptans was also judged by calculating the therapeutic gain (i.e. percentage response for active minus percentage response for placebo). The mean therapeutic gain with subcutaneous sumatriptan 6mg (51%) was more than that for all other dosage forms of triptans (oral sumatriptan 100mg 32%; oral sumatriptan 50mg 29%: intranasal sumatriptan 20mg 30%; rectal sumatriptan 25mg 31%; oral zolmitriptan 2.5mg 32%; oral rizatriptan 10mg 37%; oral eletriptan 40mg 37%; oral almotriptan 12.5mg 26%). Compared with oral sumatriptan 100mg (32%), the mean therapeutic gain was higher with oral eletriptan 80mg (42%) but lower with oral naratriptan 2.5mg (22%) or oral frovatriptan 2.5mg (16%). The few direct comparative randomised clinical trials with oral triptans reveal the same picture. Recurrence of headache within 24 hours after an initial successful response occurs in 30 to 40% of sumatriptan-treated patients. Apart from naratriptan, which has a tendency towards less recurrence, there appears to be no consistent difference in recurrence rates between the newer triptans and sumatriptan. Rizatriptan with its shorter time to maximum concentration (tmax) tended to produce a quicker onset of headache relief than sumatriptan and zolmitriptan. The place of triptans compared with non-triptan drugs in migraine therapy remains to be established and further RCTs are required.

Characterisation of the 5-HT receptor binding profile of eletriptan and kinetics of [3H]eletriptan binding at human 5-HT1B and 5-HT1D receptors

The affinity of eletriptan ((R)-3-(1-methyl-2-pyrrolidinylmethyl)-5-[2-(phenylsulphonyl )ethyl]-1H-indole) for a range of 5-HT receptors was compared to values obtained for other 5-HT1B/1D receptor agonists known to be effective in the treatment of migraine. Eletriptan, like sumatriptan, zolmitriptan, naratriptan and rizatriptan had highest affinity for the human 5-HT1B, 5-HT1D and putative 5-ht1f receptor. Kinetic studies comparing the binding of [3H]eletriptan and [3H]sumatriptan to the human recombinant 5-HT1B and 5-HT1D receptors expressed in HeLa cells revealed that both radioligands bound with high specificity (>90%) and reached equilibrium within 10-15 min. However, [3H]eletriptan had over 6-fold higher affinity than [3H]sumatriptan at the 5-HT1D receptor (K(D)): 0.92 and 6.58 nM, respectively) and over 3-fold higher affinity than [3H]sumatriptan at the 5-HT1B receptor (K(D): 3.14 and 11.07 nM, respectively). Association and dissociation rates for both radioligands could only be accurately determined at the 5-HT1D receptor and then only at 4 degrees C. At this temperature, [3H]eletriptan had a significantly (P<0.05) faster association rate (K(on) 0.249 min(-1) nM(-1)) than [3H]sumatriptan (K(on) 0.024 min(-1) nM(-1)) and a significantly (P<0.05) slower off-rate (K(off) 0.027 min(-1) compared to 0.037 min(-1) for [3H]sumatriptan). These data indicate that eletriptan is a potent ligand at the human 5-HT1B, 5-HT1D, and 5-ht1f receptors and are consistent with its potent vasoconstrictor activity and use as a drug for the acute treatment of migraine headache.