Effects of donitriptan on carotid haemodynamics and cardiac output distribution in anaesthetized pigs

Animal models of migraine headache and aura

PURPOSE OF REVIEW

Over the past 30 years, animal models of migraine have led to the identification of novel drug targets and drug treatments as well as helped to clarify a mechanism for abortive and prophylactic drugs. Animal models have also provided translational knowledge and a framework to think about the impact of hormones, genes, and environmental factors on migraine pathophysiology. Although most acknowledge that these animal models have significant shortcomings, promising new drugs are now being developed and brought to the clinic using these preclinical models. Hence, it is timely to provide a short overview examining the ways in which animal models inform us about underlying migraine mechanisms.

RECENT FINDINGS

First generation migraine models mainly focused on events within pain-generating intracranial tissues, for example, the dura mater and large vessels, as well as their downstream consequences within brain. Upstream events such as cortical spreading depression have also been modeled recently and provide insight into mechanisms of migraine prophylaxis. Mouse mutants expressing human migraine mutations have been genetically engineered to provide an understanding of familial hemiplegic migraine and possibly, by extrapolation, may reflect on the pathophysiology of more common migraine subtypes.

SUMMARY

Animal models of migraine reflect distinct facets of this clinically heterogeneous disorder and contribute to a better understanding of its pathophysiology and pharmacology.

Neurovascular pharmacology of migraine

Migraine is a paroxysmal neurovascular disorder, which affects a significant proportion of the population. Since dilation of cranial blood vessels is likely to be responsible for the headache experienced in migraine, many experimental models for the study of migraine have focussed on this feature. The current review discusses a model that is based on the constriction of carotid arteriovenous anastomoses in anaesthetized pigs, which has during the last decades proven of great value in identifying potential antimigraine drugs acting via a vascular mechanism. Further, the use of human isolated blood vessels in migraine research is discussed. Thirdly, we describe an integrated neurovascular model, where dural vasodilatation in response to trigeminal perivascular nerve stimulation can be studied. Such a model not only allows an in-depth characterization of directly vascularly acting drugs, but also of drugs that are supposed to act via inhibition of vasodilator responses to endogenous neuropeptides, or of drugs that inhibit the release of these neuropeptides. We discuss the use of this model in a study on the influence of female sex hormones on migraine. Finally, the implementation of this model in mice is considered. Such a murine model allows the use of genetically modified animals, which will lead to a better understanding of the ion channel mutations that are found in migraine patients.

Experimental migraine models and their relevance in migraine therapy

Although the understanding of migraine pathophysiology is incomplete, it is now well accepted that this neurovascular syndrome is mainly due to a cranial vasodilation with activation of the trigeminal system. Several experimental migraine models, based on vascular and neuronal involvement, have been developed. Obviously, the migraine models do not entail all facets of this clinically heterogeneous disorder, but their contribution at several levels (molecular, in vitro, in vivo) has been crucial in the development of novel antimigraine drugs and in the understanding of migraine pathophysiology. One important vascular in vivo model, based on an assumption that migraine headache involves cranial vasodilation, determines porcine arteriovenous anastomotic blood flow. Other models utilize electrical stimulation of the trigeminal ganglion/nerve to study neurogenic dural inflammation, while the superior sagittal sinus stimulation model takes into account the transmission of trigeminal nociceptive input in the brainstem. More recently, the introduction of integrated models, namely electrical stimulation of the trigeminal ganglion or systemic administration of capsaicin, allows studying the activation of the trigeminal system and its effect on the cranial vasculature. Studies using in vitro models have contributed enormously during the preclinical stage to characterizing the receptors in cranial blood vessels and to studying the effects of several putative antimigraine agents. The aforementioned migraine models have advantages as well as some limitations. The present review is devoted to discussing various migraine models and their relevance to antimigraine therapy.

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.

Investigation of the effects of naratriptan, rizatriptan, and sumatriptan on jugular venous oxygen saturation in anesthetized pigs: implications for their mechanism of acute antimigraine action

The effects of naratriptan, rizatriptan, and sumatriptan on arteriovenous oxygen saturation difference and carotid hemodynamics were compared in the anesthetized pig. Oxygen and carbon dioxide partial pressures in systemic arterial and jugular venous blood as well as hemoglobin oxygen saturation were determined by conventional blood gas analysis. Vehicle (n = 19) or naratriptan, rizatriptan, or sumatriptan (0.63, 2.5, 10, 40, 160, 630, and 2,500 microg/kg i.v.; n = 7/group) were infused cumulatively. In naratriptan-, rizatriptan-, and sumatriptan-treated animals, jugular venous oxygen saturation decreased dose dependently (geometric mean ED50 values of 3.1, 17.9, and 16.0 microg/kg, respectively) concomitantly with increases in carotid vascular resistance. Rizatriptan significantly and dose dependently, from 160 microg/kg, increased PvCO2 (P < 0.05 versus vehicle). Naratriptan and sumatriptan also tended to increase PvCO2 albeit nonstatistically significantly. All three triptans consistently evoked quantitatively similar carotid vasoconstriction, whereas decreases in jugular venous oxygen saturation (VOS) and increases in PvCO2 had different magnitudes and occurred only in around one-half of the animals studied. Maximal variations in PvCO2 were found to correlate highly with those in PvO2 (P = 0.002), but maximal variations in carotid resistance failed to correlate with those in PvCO2 (P = 0.76) or PvO2 (P = 0.28). The results demonstrate that the triptans investigated robustly produced carotid vasoconstriction, but elicited less consistent decreases in VOS and increases in jugular PvCO2, possibly suggestive of distinct mechanisms. Collectively, the data suggest that triptan-induced increases in arteriovenous oxygen saturation difference and carbon dioxide partial pressure in venous blood draining the head are class effects.

Donitriptan selectively decreases jugular venous oxygen saturation in the anesthetized pig: further insights into its mechanism of action relevant to headache relief

The effects of donitriptan on systemic arterial-jugular venous oxygen saturation difference were evaluated in pentobarbitone-anesthetized pigs. Oxygen and carbon dioxide partial pressures in systemic arterial and jugular venous blood as well as hemoglobin oxygen saturation were determined by conventional blood gas analysis. Vehicle (40% polyethyleneglycol in saline, n = 9) or donitriptan (0.01, 0.04, 0.16, 0.63, 2.5, 10, and 40 microg/kg, n = 7) were cumulatively infused over 15 min/dose. The involvement of 5-hydroxytryptamine(1B) (5-HT(1B)) receptors was assessed in the presence of the 5-HT(1B/1D) receptor antagonist, GR 127935. Donitriptan decreased markedly and dose dependently jugular venous oxygen saturation [ED(50) 0.5 (0.3-1.1) microg/kg], in parallel with increases in carotid vascular resistance [ED(50) 0.9 (0.7-1.1) microg/kg]. Since arterial oxygen saturation and partial pressure remained unchanged, donitriptan significantly increased arteriovenous oxygen saturation difference from 0.63 microg/kg (maximal variation: 57 +/- 18%, P < 0.05 compared with vehicle). Unexpectedly, donitriptan from 2.5 microg/kg induced marked and significant increases in carbon dioxide partial pressure (pVCO(2)) in venous blood (maximal increase 18.8 +/- 5.7%; P < 0.05 compared with vehicle). Pretreatment with GR 127935 (0.63 mg/kg, n = 5) abolished the fall in venous oxygen saturation and the increase in carotid vascular resistance and reduced the increases in pVCO(2) induced by donitriptan. The results demonstrate that donitriptan, via 5-HT(1B) receptor activation, decreases the oxygen saturation of venous blood draining the head, concomitantly with cranial vasoconstriction. Since donitriptan also increased pVCO(2), an effect upon cerebral oxygen consumption and metabolism is suggested in addition to cranial vasoconstriction, which may be relevant to its headache-relieving effects.

Therapies in development for the treatment of migraine

While oral formulations of agonists at the serotonin (5-HT) receptor subdivisions 5-HT(1B/1D)--the "triptan" class of drugs--dominate the market for migraine therapy and while still more triptans are under development, recent research efforts have also explored other pharmacological avenues, which are mostly based on our extended insights into the neuropathology and genetics of migraine. Genetic research, in particular, continues to reveal new potential drug targets, indicating that the migraine treatment options that may become available by 2010 might be substantially different from today's scenery. Meanwhile, pharmaceutical companies make increasing use of drug delivery and formulation technologies to improve the action profile and convenience of use for current triptans and this has even led to a partial revival of "old" migraine drugs, such as ergot alkaloids. The broadest potential, however, lies in a better understanding of the molecular pharmacology of migraine.