[53] Chronic medication can affect cerebral cortical activity. In rats, chronic exposure to acetaminophen increases the frequency of cortical Tanespimycin spreading depression (CSD), an analog of migraine aura.[54] CSD-evoked increases of 5-HT2A serotonin receptor expression and c-Fos-immunoreactivity in the cerebral cortex and TNC have been found in rats after chronic acetaminophen treatment.[55] Increased CSD development and increased TNC c-Fos immunoreactivity were also shown in rats chronically treated with dihydroergotamine.[56]

These findings suggest that chronic exposure to either antimigraine drugs or nonspecific analgesics can increase the excitability of cortical neurons, thus increasing susceptibility to develop CSD, facilitating the trigeminal nociceptive process. Preclinical studies support clinical findings of an altered 5-HT system in patients with MOH. Chronic administration of acetaminophen resulted in the upregulation of 5-HT2A receptors

in the cerebral cortex.[57] Changes in the expression of 5-HT receptors and transporters in several subcortical areas, including the PAG and the locus coeruleus, were also reported in animals after chronic triptan exposure.[58, 59] A derangement in the endogenous 5-HT-dependent control system may underlie the cortical selleck products hyperexcitation and pain facilitation seen in MOH. Animals with decreased 5-HT levels show an increase in CSD susceptibility and CSD-evoked c-Fos expression in the TNC.[60] Inhibition of NO production can attenuate this cortical hyperexcitability.[61] Low levels of 5-HT may subsequently upregulate the expression of pronociceptive 5-HT2A receptors in the cortex medchemexpress and trigeminal system. Activation of this pronociceptive receptor can upregulate NOS expression[62] and increase susceptibility to CSD. Dysfunction of the 5-HT system also

facilitates the trigeminal nociceptive process. The expression of c-Fos and phosphorylation of the NR1 NMDA-receptor subunit in TNC neurons evoked by meningeal inflammation is increased in animals with levels of 5-HT depleted by tryptophan hydroxylase inhibition.[63] Animals with depleted 5-HT levels also showed an increase in CGRP expression in the TG and an increase of CGRP release evoked by CSD.[64, 65] The evidence presented above shows that the central modulating control has a strong influence on the function of the trigeminal system. Derangement of this control system, either decreasing nociceptive inhibition or increasing nociceptive facilitation, may enhance the process of central sensitization. The clinical and preclinical studies described above indicate an increased excitability of neurons in the cerebral cortex and trigeminal system after chronic headache medication. The cortical hyperexcitability may increase the probability of developing CSD, while increased excitability of trigeminal neurons may facilitate peripheral and central sensitization.