(symbol: σ) a receptor that currently is not well understood but is thought to affect neurotransmitter release and function (particularly of dopamine and serotonin) as well as the immune and endocrine systems. Once identified as an opioid receptor in part because synthetic opioids such as dextromethorphan bind to it, it is now considered to be a separate class of receptor with two subtypes: sigma-1 (σ1) and sigma-2 (σ2). Sigma-1, the more fully investigated of these subtypes, is activated by some drugs of abuse (e.g., cocaine, methamphetamine), and it has been implicated in several disorders (e.g., depression, Alzheimer’s disease), with a possible role in their treatment.