Comparison of more and less lipophilic serotonin (5HT1B/1D) agonists in a model of trigeminovascular nociception in cat.

The trigeminovascular system consists of bipolar neurons innervating pain-producing intracranial structures, such as the superior sagittal sinus (SSS), and projecting to the medullary and upper cervical dorsal horn second order neurons. Zolmitriptan is a newly developed 5HT1B/1D receptor agonist with both peripheral and central sites of action in the trigeminovascular system due to greater lipophilicity relative to the more hydrophilic antimigraine compound sumatriptan. Given that we have seen electrophysiological and autoradiographic binding data to suggest that the compound may inhibit activity at second-order neurons this study was designed to examine whether such an effect could be demonstrated in a population of trigeminal neurons using Fos immunohistochemistry. Cats were anesthetised with alpha-chloralose (60 mg/kg intraperitoneal then 20 mg/kg intravenous maintenance) with all surgery being conducted using halothane (1-3%). The animals were prepared for physiological monitoring, including blood pressure, heart rate, rectal temperature, and end-expiratory CO2. They were intubated, ventilated, and paralyzed with gallamine triethiodide (6 mg/kg i.v.). A midline craniotomy was performed to expose the sinus for electrical stimulation using hook electrodes. Twenty-four hours after completion of the surgical procedures the animal was ready for treatment. Vehicle, sumatriptan (85 micrograms/kg), or zolmitriptan (30 micrograms/kg) was administered and the SSS was stimulated (250 microseconds, 100 V at 0.3 Hz) for 1 h. Following an additional 1 h the animal was perfused and immunohistochemistry was used to detect the protein product of the immediate early gene c-Fos. We compared the dorsal horns of the medulla (trigeminal nucleus caudalis) and the C1 and C2 cervical spinal cords in control animals with those receiving zolmitriptan or sumatriptan. We noted a significant reduction in Fos expression after treatment with zolmitriptan but no effect with sumatriptan. Given that zolmitriptan accesses central neurons and that the method of stimulation we have employed would bypass peripheral trigeminal mechanisms it is likely that the reduction in second-order trigeminal neuronal activity was due to a direct inhibitory effect of the compound on those cells. These neurons form a possible site for the treatment of acute attacks of migraine.