IDENTIFICATION, MOLECULAR-CLONING, AND DISTRIBUTION OF A SHORT VARIANT OF THE 5-HYDROXYTRYPTAMINE(2C) RECEPTOR PRODUCED BY ALTERNATIVE SPLICING

The actions of the neurotransmitter 5-hydroxytryptamine (5-HT) (seroto nin) are mediated by multiple receptor subtypes. One of the prominent serotonin receptors in the brain is the 5-HT2C receptor (5-HT2C-R). We report the occurrence of a second 5-HT2C-R transcript, first identifi ed using S1 nuclease protection of total RNA isolated from the choroid plexus. Analyses of the distribution of these two RNAs revealed that the short form is expressed in the same structures as the 5-HT2C-R mRN A, including choroid plexus, striatum, hippocampus, hypothalamus, olfa ctory tubercles, and spinal cord. Cloning and sequence analyses reveal ed a second cDNA with a 95-nt deletion in the region coding for the pu tative second intracellular loop and the fourth transmembrane domain o f the 5-HT2C-R, This deletion leads to a frameshift in the coding sequ ence and the introduction of a premature stop codon. The predicted tru ncated protein (5-HT2C-tr) contains 172 amino acids, with 153 residues at the amino terminus, identical to the 5-HT2C-R, and 19 carboxyl-ter minal amino acids that are unique. Although antibodies specific to the 5-HT2C-tr protein showed that the truncated form is expressed in a tr ansfected fibroblast cell model system, there was no serotonergic liga nd binding activity or phosphoinositide hydrolysis. Analyses of the 5- HT2C-R gene revealed that the two transcripts arise from a single gene by differential splicing using alternative donor sites and a common 3 '-splice acceptor. Polymerase chain reaction amplification of mouse an d human brain cDNAs demonstrated the occurrence of the same splicing p atterns in these species, Although this study demonstrates tissue-spec ific expression of two 5-HT2C mRNA splice variants in rat, mouse, and human, the significance of the truncated form in these three species r emains to be established.