Animal Studies
Many limitations exist in animal studies evaluating migraine. First, the majority of studies use predominantly male animals, which may not be representative of a predominantly female disorder. Secondly, despite a variety of different migraine animal models, none are able to fully model the complexities of the various pain and pain-related symptoms of migraine. Finally, most animal models of migraine have been formed based on our current understanding of migraine pathophysiology, and rely less on the nociceptive behavior of the animals to corroborate that headache was the result of the interventions. With these limitations in mind, two studies using two different rodent models of migraine were recently published and are reviewed below (see Table 1).
Using the nitroglycerin (NTG)-induced neuronal activation (as detected by fos activation) rodent migraine model, Greco et al. attempted to evaluate the influence of estrogen on migraine in rats. Intact females were tested during the proestrus phase of the estrus cycle (generally corresponding to a period of estrogen peak, see Fig. 1a), and were demonstrated to have an increased NTG-induced activation in the paraventricular nucleus and supraoptic nucleus of the hypothalamus, as well as the nucleus trigeminalis caudalis of the brainstem as compared with intact male rats. Further, although ovariectomy significantly reduced neuronal activation in all above-mentioned areas in the female rat, chronic intraperitoneal administration of estrogen restored the activation levels to those of an intact female. In contrast, in male gonadectomized rats, neuronal suppression was demonstrated only in the nucleus trigeminalis caudalis as compared with intact males.
(Enlarge Image)
Figure 1.
Relative fluctuations of estradiol and progesterone in the human menstrual cycle and rats estrous cycle. Adapted with permission from Figure 3 of [20].
In a second study, using the migraine rodent model of meningeal inflammation as measured by dural mast cell activation, Boes and Levy demonstrated that dural mast cell activation in intact female rats was consistently higher than in intact males, with the exception of the proestrus phase (i.e. peak estrogen levels). In addition, overall mast cell density was higher in estrus (a period of somewhat lower estrogen concentrations, see Fig. 1a) (P < 0.01) and even higher in diestrus 1 (when estrogen concentrations are low) (P < 0.01) as compared with the proestrus (peak estrogen) or the diestrus 2 (rising estrogen) phases. Further, although ovariectomized females had decreased mast cell density, those ovariectomized females that received a subsequent subcutaneous injection of E2 showed an increase in dural mast cell density compared with those treated with placebo. Interestingly, administration of progesterone together with E2 blunted the increase in dural mast cell density seen after E2 treatment alone. Boes and Levy also reported that the effect of E2 on mast cell density was time-specific – being apparent at 24 and 48 h after E2 injection and disappearing after 72 h.
The above studies appear to report contradictory findings. Greco et al. demonstrated elevated pain sensitivity in females (increased NTG-induced fos activation) corresponding to the proestrus phase, whereas Boes and Levy showed a relatively lower state of pain-sensitivity (decreased mast cell activation) in proestrus. Further, even though Boes and Levy showed a lower pain response during a phase of high estrogen (proestrus) in normal cycling rats, they showed an increased pain response when subcutaneous estrogen was given to animals of a low estrogen state (ovariectomized rats). These conflicting data may be largely because of the difficulty in determining the specific hormonal levels in rats (Fig. 1a). Notably, neither study directly evaluated the status of sex hormones in the animals (i.e. obtaining serum estrogen levels), but instead determined the estrus cycle based on vaginal smears, a method extremely variable depending on the specific timing of the studies (Fig. 1). These studies also demonstrate the difficulty in extrapolating the results of animal studies to humans, as rats have brief 4-day estrus cycles in which a difference of a few hours can produce either peak or trough estrogen concentrations (Fig. 1).
In addition to estrogen itself, different estrogen receptors have been studied recently in animal models of general pain, although not specifically in headache or migraine. As the differential effect of estrogen may very well be receptor-specific, further studies on estrogen receptors in the headache or migraine rodent models would be of interest.