Serotonin depletion, cortical spreading depression, and trigeminal nociception.

BACKGROUND: The attack of migraine has been observed to be associated with low level of serotonin (5-HT). Although the mechanism underlying this relationship is still unclear, change in cortical excitability or susceptibility of trigeminal system is a possible explanation.
OBJECTIVES: The aim was to study the effect of 5-HT depletion on the development of cortical spreading depression (CSD) and CSD-evoked trigeminal nociception.
METHODS: Wistar rats were separated into low 5-HT and control groups (eight rats each). 5-HT was depleted by administration of para-chlorophenylalanine, a tryptophan hydroxylase inhibitor. CSD was induced by applying 3 mg of potassium chloride on parietal cortex. Cortical activity was monitored for 1 hour. Trigeminal nociception was determined using number of Fos-immunoreactive (Fos-IR) neurons in trigeminal nucleus caudalis as the indicator.
RESULTS: Application of KCl led to the development of series of depolarization shift characteristics for CSD. The development of these CSD waves was enhanced in low 5-HT state. The area under curve of each CSD wave and the number of CSD waves occurring within 1 hour were greater in low 5-HT group. No significant change in peak amplitude and duration of CSD wave was observed. The numbers of Fos-IR cells on ipsilateral and contralateral trigeminal nucleus caudalis were significantly greater in the low 5-HT group than those of the controls.
CONCLUSION: Our findings indicate that 5-HT depletion enhances CSD-induced trigeminal nociception by increasing the cortical excitability and sensitivity of trigeminal nociceptive system. These findings may provide a better understanding regarding the relationship between low 5-HT and clinical headaches.