Comparison of the potency of MDL 100,907 and SB 242084 in blocking the serotonin (5-HT)2 receptor agonist-induced increases in rat serum corticosterone concentrations: evidence for 5-HT2A receptor mediation of the HPA axis

Direct-acting serotonin (5-HT) receptor agonists increase serum corticosterone in rats by activating receptors of the 5-HT1A or the 5-HT2A/2C subtypes. While involvement of 5-HT1A receptors in activation of the hypothalamic–pituitary–adrenocortical (HPA) axis is clear, the 5-HT2 receptor subtype—5-HT2A or 5-HT2C—responsible for activation of the HPA axis by direct-acting 5-HT2 receptor agonists has been difficult to determine due to the lack of selective pharmacologic agents. Recently, however, 5-HT2 receptor antagonists with high selectivity for 5-HT2A and 5-HT2C receptor subtypes have been discovered. The selective 5-HT2A receptor antagonist MDL 100,907 and the selective 5-HT2C receptor antagonist SB 242084 were used to block the increases in rat serum corticosterone elicited by 5-HT2 receptor agonists with varying degrees of affinity for 5-HT2A and 5-HT2C receptors. MDL 100,907 was fully effective in blocking the increases in corticosterone concentrations produced by quipazine, DOI, m-CPP and Ro 60-0175, whereas SB 242084 was ineffective or was only marginally effective. Our findings implicate 5-HT2A receptors rather than 5-HT2C receptors in mediating increases in rat serum corticosterone produced by direct-acting 5-HT2 receptor agonists in vivo.