Episodic dural stimulation in awake rats: a model for recurrent headache

Abstract

Objectives: To model, in rats, the development of chronic trigeminal nociceptive hypersensitivity seen in patients with recurrent headache.

Background: Pathophysiology studies suggest that patients with recurrent migraine headache experience repeated bouts of dural nociceptor activation. In some patients, the severity and frequency of headache attacks increase over time. Patients with recurrent headache are hypersensitive to nitric oxide donors, such as glyceryl trinitrate (GTN). Current trigeminal pain models do not reflect the repeated episodic nature of dural nociceptor activation in patients with recurrent headache. Repeated nociceptor activation creates long-lasting changes in the periphery and brain due to activity-dependent neuronal plasticity. An animal model of repeated activation of dural nociceptors will facilitate the study of the physiological changes caused by repeated, episodic pain and the factors important for the transition of episodic to chronic migraine.

Methods: We induced dural inflammation by infusing an inflammatory soup (IS) through a cannula on the dura in awake behaving rats. This was repeated 3 times per week for up to 4 weeks. Periorbital pressure sensory testing was used to monitor the change in trigeminal sensitivity. Rats were challenged with GTN to test the hypothesis that many dural stimulations are required to model the hypersensitivity of migraine patients. Quantitative trigeminal sensory testing and microdialysis in the trigeminal nucleus caudalis (TNC) were used to measure GTN hypersensitivity.

Results: Multiple infusions of IS (>8), over weeks, induced a long-lasting decrease in periorbital pressure thresholds that lasted >3 weeks after the last infusion. In contrast, IS infusion in IS-naive rats and those that received 3 IS infusions produced only short-lasting decreases in periorbital pressure thresholds. Rats that received more than 8 IS infusions showed a marked increase in their neurochemical and behavioral responses to GTN. In these rats, GTN induced a decrease in periorbital von Frey thresholds that lasted >5 hours. In contrast, in rats that received only 3 IS infusions, GTN caused a threshold decrease for 1.5 hour. In vivo microdialysis in the TNC showed that GTN increased extracellular glutamate levels in rats with more than 8 IS infusions to 7.7 times the basal levels. In IS-naive rats and those that received only 3 IS infusions, the extracellular glutamate levels rose to only 1.7 and 1.9 times the basal level, respectively.

Conclusions: Repeated IS stimulation of the dura produces a chronic state of trigeminal hypersensitivity and potentiates the response to GTN. This hyperresponsiveness outlasts the last IS infusion and is the basis of our rat model of recurrent headache. This model can be used to study the changes in the brain and periphery induced by repeated trigeminovascular nociceptor activation and has the potential to elucidate the mechanisms for the transition of episodic to chronic headache.

Fig. 1

Picture of a rat fitted with a dura cannula for IS infusion. This picture shows a rat fitted with a cannula (see small arrow) for IS infusion in the plastic tube restraint used for sensory testing on the face. The picture shows the von Frey monofilament (see large arrow) used to test the von Frey pressure threshold on the face. Threshold was noted when rats quickly retracted their head away from the bending von Frey monofilament. The restraint is atraumatic and it is used only to reduce the movement of the animal during sensory testing.

Fig. 2

(A) Basal threshold before IS infusion. The mean ± SD baseline von Frey periorbital pressure thresholds in 3 groups of rats. The testing was done before the animal received the inflammatory soup or saline infusion for that day. The dotted line shows the results from the continued monitoring of the rats without dural IS stimulation. The significant decrease in the thresholds noted in the more than 8 IS infusion group is due to the successive stimulation with IS. (^# P <.05, ^*P <.01 ANOVA). (B) Von Frey threshold change in response to IS infusion: The typical response to IS infusion in one rat on 2 different infusion days. The graph shows the periorbital von Frey thresholds for a rat during the second and the eighth times the rat received IS stimulation, which corresponds to the 7th and 24th day after implantation of the cannula. Thresholds from the eighth infusion are plotted in the inset with a different scale.

Fig. 3

Glyceryl trinitrate (GTN) von Frey response. The effects of GTN (0.1 mg/kg, ip) on the periorbital von Frey thresholds (mean ± SD) in 3 groups of rats: naive rats, rats that had received 3 IS infusions, and rats with a history of >10 IS infusions. Rats were tested 12–31 days after their last IS infusion. All of the rats reacted to the GTN injection with a decrease in periorbital thresholds. In the naive rats and rats with 3 IS infusions, the threshold decreased for ~1.5 hours and then recovered. In the rats with more than 8 IS infusions, the duration of the von Frey threshold decrease was >5 hours. There was a significant difference between the IS-naive rats and rats with few IS infusions vs those with a history more than 8 IS infusions at 2.5- and 5-hour time points (1.5 hour ^#P = .055, 2.5 and 5 hour P < .01, one-way ANOVA).

Fig. 4

GTN effects on the extracellular glutamate in the TNC. The effects of GTN (0.1 mg/kg, iv) on the extracellular glutamate levels in (i) naive rats that had not received IS, (ii) rats with a history of more than 8 IS infusions, and (iii) rats with 3 IS infusions. Glutamate levels increased in all of the rats following GTN. In the rats with more than 8 IS infusions, there was a significantly greater increase in glutamate after 1.5 hours compared to rats with 3 IS infusions. This difference was maintained for the duration of the experiment (^# P = .074, ^* P < .01, univariate ANOVA).