Sigma receptor

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Schematic σ opioid receptor
opioid receptor, sigma 1
Identifiers
Symbol	OPRS1
HUGO	8157
Entrez	10280
OMIM	601978
RefSeq	NM_005866
UniProt	Q2TSD1
Other data
Locus	Chr. 9 [1]

The sigma receptors σ₁ and σ₂ were once thought to be a type of opioid receptor, because the d stereoisomers of the benzomorphan class of opioid drugs had no effects at μ, κ, and δ receptors, but reduced coughing. However, pharmacological testing indicated that the sigma receptors were activated by drugs completely unrelated to the opioids, and their function was unrelated to the function of the opioid receptors. For example, phencyclidine (PCP), and the antipsychotic haloperidol may interact with these receptors. Neither phencyclidine nor haloperidol have any appreciable chemical similarity to the opioids. When the σ₁ receptor was isolated and cloned, it was found to have no structural similarity to the opioid receptors. At this point, they were designated as a separate class of receptors. The functions of these receptors are poorly understood and any endogenous ligands have yet to be identified. Sigma receptors may induce hallucinogenic effects and also may be responsible for the paradoxical convulsions sometimes seen in opiate overdose. Drugs known to be sigma agonists in addition to their major mechanisms of action include cocaine, heroin, PCP and dextromethorphan, however the exact role of sigma receptors is difficult to establish as many sigma agonists also bind to other targets such as the κ-opioid receptor and the NMDA receptor. In animal experiments, sigma antagonists such as rimcazole were able to block convulsions from cocaine overdose. Sigma antagonists are also under investigation for use as antipsychotic medications.