m-chlorophenylpiperazine and m-trifluoromethylphenylpiperazine are partial agonists at cloned 5-HT2A receptors expressed in fibroblasts.

Serotonin2A (5-HT2A) and 5-HT2C receptors share numerous pharmacological properties. Two compounds thought to discriminate between these two receptor subtypes are m-chlorophenypiperazine (mCPP) and m-trifluoromethylphenylpiperazine (TFMPP). These two drugs have been classified as antagonists at 5-HT2A receptors but as agonists at 5-HT2C receptors on the basis of phosphoinositide hydrolysis studies in cerebral cortex and choroid plexus, respectively. To determine more fully the properties of mCPP and TFMPP at 5-HT2A receptors, NIH 3T3 fibroblasts transfected with the 5-HT2A receptor complementary DNA (GF6 cells) were used as a model system of receptor function. These cells express approximately 15-fold higher 5-HT2A receptor density than is found in cerebral cortex. In GF6 cells, mCPP and TFMPP dose-dependently stimulated phosphoinositide hydrolysis with maximal effects less than that of 5-HT. This agonist activity was blocked by 5-HT2A receptor antagonists but not by prior treatment with pertussis toxin. Partial inactivation of 5-HT2A receptors with phenoxybenzamine decreased the maximal effects of mCPP and TFMPP but did not eliminate agonist activity. Thus mCPP and TFMPP are partial agonists at 5-HT2A receptors in GF6 cells, and these agonist properties are retained even under conditions where receptor density is comparable to that of cerebral cortex. Although it has not yet been demonstrated that mCPP and TFMPP are agonists at central 5-HT2A receptors, this possibility should be considered when evaluating in vivo effects of these drugs.