The opiates, opioid peptides (endorphins), and the opiate receptor.
D000818:Animals; D002490:Central Nervous System / Q000378:metabolism; D004723:Endorphins / Q000134:cerebrospinal fluid / Q000378:metabolism / Q000502:physiology*; D000882:Haplorhini; D006801:Humans; D008297:Male; D009294:Narcotics / Q000378:metabolism*; D018377:Neurotransmitter Agents / Q000502:physiology; D011817:Rabbits; D051381:Rats; D011957:Receptors, Opioid / Q000378:metabolism*
D004723:Endorphins; D009294:Narcotics; D018377:Neurotransmitter Agents; D011957:Receptors, Opioid