Selective and divergent regulation of cortical 5-HT(2A) receptors in rabbit

It is well established that repeated administration of both 5-hydroxytryptamine(2) (5-HT(2)) receptor agonists and antagonists decreases the density of 5-HT(2A) and 5-HT(2C) receptors. However, the regulation of these two receptors has not been studied in the same tissue. Therefore, we examined the effects of repeated daily injections of the 5-HT(2) receptor agonists (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI) and D-lysergic acid diethylamide (LSD) and the antagonists d-2-bromolysergic acid diethylamide hydrogen tartrate (BOL) and alpha-phenyl-2-(2-phenylethyl)-4-piperidinemethanol (MDL 11,939) on rabbit cortical 5-HT(2A) and 5-HT(2C) receptors. Repeated administration of DOI, LSD, or BOL decreased cortical 5-HT(2A) receptor density but had no effect on the density of cortical 5-HT(2C) receptors. Repeated administration of the selective 5-HT(2A) receptor antagonist MDL 11,939 significantly increased 5-HT(2A) receptor density. This unexpected outcome also occurred without any change in cortical 5-HT(2C) receptor density. The down-regulation of 5-HT(2A) receptors produced by chronic administration of BOL was associated with a decrease in DOI-elicited head bobs, whereas 5-HT(2A) receptor up-regulation produced by MDL 11,939 was associated with an increase in DOI-elicited head bobs compared with controls. These studies demonstrate that 5-HT(2A) receptor antagonists can both down- and up-regulate the density of cortical 5-HT(2A) receptors and these changes in receptor density have functional consequences for 5-HT(2A) receptor-mediated behaviors. Furthermore, because DOI, LSD, and BOL have approximately equal affinities for the 5-HT(2A) and 5-HT(2C) receptors, these results suggest that different mechanisms regulate 5-HT(2A) and 5-HT(2C) receptor density, in that chronic occupation of 5-HT(2C) receptors does not modulate their density in rabbit frontal cortex.