Allelic variation in the 5-HT2C receptor (HTR2C) and functional responses to the 5-HT2C receptor agonist, m-chlorophenylpiperazine

Serotonin (5-HT) pathways utilising postsynaptic 5-HT2C receptors have been implicated in the control of neuroendocrine function, body temperature and food intake (Khan and Wetzler 1991). The human 5-HT2C receptor gene has been localised to the X chromosome and contains a C-G polymorphism at codon 23 (nucleotide 68) such that serine is substituted for cysteine in the receptor (Lappalainen et al. 1995). The population allele frequencies are approximately 85% cysteine and 15% serine (Sodhi et al. 1995). The pharmacological consequences of the polymorphism are unknown, but altered binding affinity for agonists or antagonists has been advocated (Goldman 1995). That the polymorphism has functional significance is demonstrated by the association of serine alleles with therapeutic response to clozapine (Sodhi et al. 1995), exemplifying how genetic variation may contribute to individual differences in response to psychoactive agents. Given these considerations, the aim of the present study waste assess whether functional responses to the selective 5-HT2C receptor agonist, m-chlorophenylpiperazine (m-CPP) differed in female subjects according to their 5-HT2C receptor genotype. DNA was extracted from buccal swabs or blood samples using standard methods and amplified using PCR with primers directed at nucleotides 40-60 and 324-345 of the human 5-HT2C receptor gene. PCR products were analysed by dot blotting using allele-specific 5'-gamma(32)P-labelled oligonucleotides. Genotype was independently confirmed by restriction analysis of a 104 bp fragment using the Hsp92II isoschizomer of NlaIII as described (Burnet et al. 1997). From these procedures, we identified 13 women with a serine allele (5-HT(2C)Ser) (two homozygote, 11 heterozygote). Their mean age was 38.8 years (22-60 years). As a comparison group, we studied 15 cysteine homozygotes (5-HT(2C)Cys). Their mean age was 31 years (range 18-56 years). Subjects were tested after an overnight fast 0900 hours on two occasions (mean +/- SEM interval, 5.0 +/- 1.1 days) in a double-blind, placebo-controlled, balanced order, cross-over design. Premenopausal subjects were tested in the follicular stage of the menstrual cycle, After a 30-min rest period for baseline sampling, they received either m-CPP (0.4 mg/kg) or placebo in matching capsules. Venous blood samples were taken and oral temperature measured for the next 3 h, following which a test meal of pre-selected sandwiches of known calorie content was offered. Plasma samples were analysed for prolactin (PRL) cortisol (CORT) and m-CPP and calculated as area under the curve (AUC) as previously described (Quested et al. 1997; Sargent et al. 1998). Placebo values were subtracted from those for m-CPP to yield a net response to rn-CPP. Data were analysed by factorial ANOVA with genotype as the between-subjects. factor. Plasma m-CPP levels were a significant covariate of all the functional responses and adjusted means are shown in Table 1. None of the responses varied with genotype. However, the hypophagic effect of m-CPP was significantly less in subjects with a 5-HT(2C)Ser receptor allele prior to covarying out m-CPP levels (P < 0.05).