Serotonin Depletion Leads to Cortical Hyperexcitability and Trigeminal Nociceptive Facilitation via the Nitric Oxide Pathway

Abstract

To investigate the role of nitric oxide (NO) in the development of cortical hyperexcitability and trigeminal nociceptive facilitation induced by serotonin (5-HT) depletion. Nitric oxide and 5-HT are important in the pathogenesis of primary headaches. An increase in cortical excitability and trigeminal nociception has been demonstrated in animals with low 5-HT levels. Although the mechanism underlying this increase is unclear, an alteration of the NO system is one possible explanation. Male Wistar rats were divided into control and 5-HT-depleted groups. 5-HT was depleted by i.p. injection of parachlorophenylalanine (100 mg/kg). Three days after injection, a microelectrode was inserted into the cerebral cortex for electrocorticograph recording and waves of cortical spreading depression (CSD) were triggered with KCl application. N-nitro-L-arginine methyl ester (L-NAME; 10 mg/kg by i.v. injection) or saline was given after the second CSD wave. Following the experiment, the cerebral cortex and brain stem were removed for anti-neuronal nitric oxide synthase (nNOS) and anti-Fos immunohistochemistry. Relative to the control group, the 5-HT-depleted group exhibited a higher frequency of CSD waves, more nNOS-immunoreactive cells in both the cerebral cortex and brainstem and more Fos-immunoreactive cells in the trigeminal nucleus caudalis (TNC). In the control group, L-NAME application led to fewer nNOS-immunoreactive cells in the cerebral cortex and TNC, and fewer Fos-immunoreactive cells in the TNC; however, L-NAME was without effect on the CSD pattern. By contrast, in addition to decreased nNOS and Fos expression, L-NAME significantly reduced the frequency of CSD events in the 5-HT-depleted group. Inhibition of NO production can counter both the cortical hyperexcitability and facilitation of trigeminal nociception that develop in the depleted 5-HT state. Therefore, NO is likely involved in the increase in both CSD events and CSD-evoked trigeminal nociception under decreased 5-HT conditions.