Use of imaging photoplethysmography for assessing intracranial blood flow in response to trigemino-vascular activation in a rat migraine model
Migraine is a primary headache, which is characterized by intense, predominantly one-sided, throbbing headache and may be accompanied by autonomic disturbances such as nausea and vomiting, as well as photo- and phonophobia. The pathogenesis of migraine remains unclear, however, according to modern concepts, a migraine attack is a consequence of the activation of the trigemino-vascular system (TVS) - a morphofunctional complex formed by cranial vessels and central and peripheral formations of the trigeminal nerve. As a result of such activation, a number of vasoactive substances are released from trigeminal afferents into the perivascular space, which, in turn, can lead to the occurrence of cranial vasodilation, which is probably accompanied by aseptic perivascular neurogenic inflammation.
Those changes in vascular tone that are likely to occur in patients during a migraine attack can be simulated in animal experiments using a model of trigemino-vascular nociception with measuring the vascular reactions at the level of the dura mater. Electrical stimulation of the dura mater, where trigeminal afferents are located, leads to TVS activation and, as a consequence, to neurogenic dural vasodilation. Today the most useful method to assess changes in vascular tone is an intravital microscopy, which allows to visualize the dilatation of the branches of the middle meningeal artery in response to electrostimulation. This method has a high degree of subjectivity due to the low accuracy of definition of vessel walls boundaries, it is also time-consuming due to the long-term selection of the target artery and, finally, it ignores processes occurring in the whole vascular bed. The laser Doppler fluometry technique was also applied to evaluate changes in intracranial blood flow. However, this is a contact method and it has limited informativeness and low reproducibility.
Above shortcomings of the existing methods led us to look for an alternative way of assessing vascular reactions associated with the TVS activation. In this sense, an imaging photoplethysmography (PPG) operating at green light illumination has great prospects as a method that allows for quantitative estimation of the blood-flow parameters varying both in time and space by registering changes in intensity of the light after its interaction with tissues of a living organism. The aim of the study was to demonstrate feasibility of the imaging PPG technique for analyzing changes in intracranial blood flow under conditions of trigemino-vascular activation in experimental modeling of migraine in rats.
A series of experiments was performed on anesthetized (urethane + alpha-chloralose 800 mg / kg + 60 mg / kg, i.p. - ICN, USA) male Wistar rats (n = 12, m = 350-450 g). During each experiment, the vital parameters of the animal were monitored (blood pressure, heart rate, end-tidal CO2, body temperature). To visualize meningeal vessels, a closed cranial window (CCW) was formed by thinning the parietal bone of the skull. Continuous recording of brain images through CCW was accomplished by a monochrome CMOS digital camera under brain illumination by incoherent green light (LEDs @ 530 ± 25 nm). Simultaneous ECG monitoring was provided by a digital electrocardiograph synchronized with the camera using needle electrodes inserted into the muscles of the rats’ limbs. To stimulate meningeal trigeminal afferents, bipolar electrode was placed on the CCW surface. To assess changes in blood flow, video recording of the state of intracranial vessels was made before, during and after three consecutive CCW electrical stimulation with a duration of 15 seconds (50V, 10 Hz, 2 ms) with an interval of 10 minutes. The experimental data were analyzed using an original software implemented in the Matlab® platform.
We have found that the DC-component of the PPG waveform (also referred to as optical intrinsic signal) is diminishing in response to CCW electrical stimulation indicating an increase in the blood filling of the tissues. After switching off the stimulation, the DC-component returned to its initial value. In addition, during electrical stimulation, a significant increase (more than 1.5 times) in amplitude of pulsatile component of the PPG waveform was observed. This parameter characterizes an instant change in vascular tone of the intracranial arteries. The reproducibility of the effect was found in all 12 animals .
In conclusion, the results of our experiments have shown that the imaging PPG system allows for quantitative assessment of intracranial blood-flow changes. The data obtained in the migraine model indicated the severity of vascular reactions during trigeminal activation. In the case of successful pharmacological validation, this method can be used in further experimental work in order to screen new and study the mechanism of action of already existing anti-migraine interventions.
Anastasiia V. Osipchuk
Department of Neuropharmacology, Valdman Institute of Pharmacology, Pavlov First Saint Petersburg State Medical University, L'va Tolstogo str. 6-8, 197022 Saint Petersburg, Russia
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