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The present study was designed to determine the effects of p-chlorophenylalanine (PCPA) and p-chloroamphetamine (PCA) administration on (1) the levels of serotonin (5-hydroxytryptamine, 5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in rat brain, (2) the sensitivity of LSD-trained rats to the stimulus effects of LSD, and (3) the maximal levels of 5-HT2A and 5-HT2C receptor mediated phosphoinositide (PI) hydrolysis in rat brain. PCA and PCPA both produced a significant depletion of whole brain 5-HT and 5-HIAA concentrations. The depletion of serotonin with PCPA, but not PCA, resulted in supersensitivity of LSD-trained subjects to the stimulus effects of LSD. Neither PCPA nor PCA treatment altered the maximal level of 5-HT2A receptor-mediated PI hydrolysis. However, PCPA, but not PCA, treatment resulted in a significant upregulation (46%, P < 0.05) of the maximal level of 5-HT2C receptor mediated PI hydrolysis. These data suggest that upregulation of the 5-HT2C receptor mediates the supersensitivity to LSD discriminative stimulus which follows the depletion of central nervous system serotonin by PCPA.

The present study was designed to investigate the hypothesis that agonist interactions at 5-HT2C receptors mediate the discriminative stimulus properties of m-chlorophenylpiperazine (mCPP). Three structural classes of compounds have been described to stimulate increases in phosphoinositide (PI) hydrolysis at the 5-HT2C receptor site: phenylpiperazines, phenylalkylamines, and indolamines. Four representative phenylpiperazines, mCPP, TFMPP, MK-212 and quipazine, one phenylalkylamine, (-)DOM, and one indolamine, LSD, were employed in the present study. The efficacies of these compounds were defined (1) in vitro, with respect to their abilities to stimulate increases in PI hydrolysis in the choroid plexus, and (2) in vivo with respect to their abilities to substitute for the mCPP discriminative stimulus. In vitro intrinsic activity at the 5-HT2C site was expressed as a fraction of the maximal PI hydrolysis response elicited by serotonin (5-HT). MK-212 (fractional efficacy = 1.1) and (-)DOM (0.77) were full agonists, while mCPP (0.72), LSD (0.27), quipazine (0.24), and TFMPP (0.22) were partial agonists with respect to the stimulation of PI hydrolysis at the 5-HT2C receptor. In vivo, each of the phenylpiperazines fully substituted for the mCPP stimulus, while (-)DOM (75%), and LSD (67%) elicited only partial substitution. While compounds with agonist activity at the 5-HT2C receptor in vitro substitute for the mCPP stimulus in vivo, no clear relationship exists between in vitro intrinsic activity at the 5-HT2C receptor with respect to the stimulation of PI turnover and maximal substitution for the mCPP stimulus in vivo. The present data suggest that mCPP elicits a compound stimulus which is mediated by agonist interactions at the 5-HT2C receptor and possibly additional interactions with 5-HT2A, 5-HT3, and/or 5-HT1B receptors.

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