You will be shown an abstract from a biomedical research paper. Given this abstract, your task is to extract all unique entities of the following types: ["Chemical"].

Please return the output as a JSON object of the format: {"Chemical": ["Arsenic", ...]}. The keys should be entity types and values should be lists of extracted entities belonging to the corresponding type. If you cannot find entities belonging to a specific type, the value should be [].

Only output the JSON object and do not include any additional text.

Abstract:

Recent developments in the study of opioid receptors. It is now about 40 years since Avram Goldstein proposed the use of the stereoselectivity of opioid receptors to identify these receptors in neural membranes. In 2012, the crystal structures of the four members of the opioid receptor family were reported, providing a structural basis for understanding of critical features affecting the actions of opiate drugs. This minireview summarizes these recent developments in our understanding of opiate receptors. Receptor function is also influenced by amino acid substitutions in the protein sequence. Among opioid receptor genes, one polymorphism is much more frequent in human populations than the many others that have been found, but the functional significance of this single nucleotide polymorphism (SNP) has been unclear. Recent studies have shed new light on how this SNP might influence opioid receptor function. In this minireview, the functional significance of the most prevalent genetic polymorphism among the opioid receptor genes is also considered.
{"Chemical": ["amino acid"]}