Objective.— To explore whether pharmacological stimulation of the 5-hydroxytryptamine7 (5-HT7) receptor modulates Fos-like immunoreactivity in the trigeminal nucleus caudalis of rats.
Background.— The serotonin 5-HT7 receptor was proposed to be involved in migraine pathogenesis and evidence suggests it plays a role in peripheral nociception and hyperalgesia through an action on sensory afferent neurons.
Methods.— The potential activating or sensitizing role of 5-HT7 receptors on trigeminal sensory neurons, as visualized by Fos-like immunoreactivity in the superficial layers of the trigeminal nucleus caudalis in rats, was investigated using the 5-HT7 receptor agonist, LP-211, in the absence and the presence of intracisternal capsaicin, respectively. The agonist effect was characterized with the 5-HT7 receptor antagonist, SB-656104. Male Wistar rats received a subcutaneous injection of LP-211, SB-656104, and SB-656104 + LP-211. They were then anesthetized and prepared to receive an intracisternal injection of capsaicin or its vehicle. Animals were perfused and brains removed; sections of the brain stem from the area postrema to the CI level were obtained and processed for Fos immunohistochemistry.
Results.— Capsaicin but not its vehicle induced Fos-like immunoreactivity within laminae I and II of trigeminal nucleus caudalis. Pretreatment with LP-211 had no effect on Fos-like immunoreactivity but strongly increased the response produced by capsaicin; this effect was abolished by SB-656104. Interestingly, capsaicin-induced Fos-like immunoreactivity was abolished by SB-656104 pretreatment thus suggesting involvement of endogenous 5-HT.
Conclusions.— Data suggest that 5-HT7 receptors increase activation of meningeal trigeminovascular afferents and/or transmission of nociceptive information in the brain stem. This mechanism could be relevant in migraine and its prophylactic treatment.