Some, which are nonselective in action and affect the dopamine and/or opioid systems, may be capable of inducing euphoria. Many dissociatives have general depressant effects and can produce sedation, respiratory depression, analgesia, anesthesia, and ataxia, as well as cognitive and memory impairment and amnesia. Arylcyclohexylamines are the oldest and most widely used dissociatives. The class includes the well known anaesthetic, ketamine.  began to appear on grey markets only as recently as 2013. Piperazine containing designer drugs have effects similar to MDMA (ecstasy). This class of drugs are mimics of serotonin that activate 5-HT receptor subtypes that release norepinephrine and dopamine. Empathogens are a class of psychoactive drugs that produce distinctive emotional and social effects similar to those of MDMA . Users of empathogens say the drugs often produce feelings of empathy, love, and emotional closeness to others. Substituted methylenedioxyphenethylamines (MDxx) are a large chemical class of derivatives of the phenethylamines, which includes many psychoactive drugs that act as entactogens, psychedelics, and/or stimulants, as well as entheogens. Benzofurans are similar in structure to MD(M)A but differ in that the methylenedioxy groups have been modified, removing one of the two oxygens in the methylenedioxy ring to render a benzofuran ring. Indane and tetralin-type phenethylamines are vaguely related to their amphetamine analogues. Only one non-tryptamine indole has been sold, 5-IT. It shows strong MAOI activity. Drugs containing the tryptamine moiety are typically substrates for the serotonin receptors, in keeping with their close structural resemblance to serotonin, a neurotransmitter. Substituted amphetamines are a chemical class of stimulants, entactogens, hallucinogens, and other drugs. They feature a phenethylamine core with a methyl group attached to the alpha carbon resulting in amphetamine, along with additional substitutions. Stimulants produce a variety of different kinds of effects by enhancing the activity of the central and peripheral nervous systems. Common effects, which vary depending on the substance and dosage in question, may include enhanced alertness, awareness, wakefulness, endurance, productivity, and motivation, increased arousal, locomotion, heart rate, and blood pressure, and the perception of a diminished requirement for food and sleep.