WASHINGTON -- Serotonin receptors are well-known for theirapparent involvement in depression; a variety of anti-depressants act upon the serotonin system or on the receptorsthemselves.
But a better understanding of how the receptors and theirantagonists work could open the door to the development ofdrugs for treating disorders associated with appetite, sex,learning and memory, sleep, pain, anxiety, stress, drug abuse,schizophrenia and contraction of the aorta, said Jean Shih of theDepartment of Molecular Pharmacology and Toxicology at theUniversity of Southern California School of Pharmacology.
Shih told members of the National Institutes of Health'sStructural Biology Interest Group on Wednesday that she andher colleagues have been dissecting the structure and functionof the serotonin 5-HT2 receptor.
This receptor is located mostly within the membrane of thepost-synaptic neuron and has seven transmembrane domains.To find regions that might be involved in receptor binding, Shihsearched for homologies between human and rat 5-HT2receptors. The homologies -- about 30 percent of the molecules-- included areas that correspond to pieces of the receptor thatcould have chemical affinity for active groups on serotonin andthat crystalographic studies have suggested are well-placed todo this.
Shih tested these regions for binding by converting singlepositions in the protein from one amino acid to another andthen measuring changes in the binding strength. She used site-directed mutagenesis to make the conversions. (The hardestpart of the operation, she said, was checking the clones to makesure the mutations were correct.)
Most of the mutations she tested weakened binding affinity,including one at position 155 in which she replaced thenegative aspartic acid -- which is probably the main counter-ion to the positive amine group of serotonin -- with the non-negative asparagine. However, she cautioned that the particularpositions in the wild type could be indirectly involved inbinding -- by altering confirmation to improve strength, forexample.
More recently, Shih has searched the receptor promoter foractive sites. "Our ultimate goal is to try to see whether thesegenes are relevant to disease," said Shih. "To do that, we needto go to the level of the gene."
To identify the active portion of the promoter region, Shih andher colleagues made chimeras of the promoter using differentportions of promoter. One small fragment provided 100 percentof promoter activity, while longer fragments resulted in lesspromotion, indicating that some repressor was present in thosesites, said Shih. The researchers also found several initiationsites.
"We are now looking for important transcription factors in thepromoter region. ... With current technologies we can ask, 'Whatdoes it mean?' " Clearly, a plethora of interesting possibilitiescould lead to relief of many ills. The different aspects ofserotonin receptor structure and function Shih has studiedcould all be candidates for causal agents.
-- David C. Holzman Washington Editor
(c) 1997 American Health Consultants. All rights reserved.