Over the last decade, New Psychoactive Substances (NPS) have become a global phenomenon. The emergence of these substances have been reported in almost 100 countries and territories, and more than 500 NPS have been identified worldwide based on reports by national governments, as well as the EU, and international institutions (UNODC, 2014a, 2015) (Figure 1). In 2014, in Europe alone, 101 NPS have been detected showing an increase of 25%, as compared to 2013 [EMCDDA (New psychoactive substances in Europe), 2015b]. NPS are able to mimic the effects of controlled substances and are mainly synthetic cannabinoids, stimulants, hallucinogens, and opioids.

Previous studies show that the use of NPS occurs among different subject groups: school students, partygoers, psychonauts, prisoners, and injecting drug users. Motivations for use include factors such as legal status, availability, and cost, as well as the desire to avoid detection and user preferences for particular pharmacological properties [González et al., 2013; Helander et al., 2013, 2014; EMCDDA (European Drug Report), 2015a; EMCDDA (New psychoactive substances in Europe), 2015b]. Although global research is not available yet about NPS, prevalence of use among the population, single national surveys (with respect to substances and subpopulations) show that the use of NPS amongst the general adult population is relatively low compared with the use of other illicit drugs. However, adolescents use more NPS than illicit drugs mostly because many of them are legal and easily available on the web [Drug Policy Department Italian Presidency of the Council of Ministers, 2014; Fraser, 2014; Kikura-Hanajiri et al., 2014; EMCDDA (New psychoactive substances in Europe), 2015b; Hondebrink et al., 2015; Palamar et al., 2015; Wood et al., 2015].

Similar to many known illicit drugs, NPS can cause severe physical and psychological symptoms that can even result in death [Drug Policy Department Italian Presidency of the Council of Ministers, 2013b; Fraser, 2014; EMCDDA (European Drug Report), 2015a; UNODC, 2015]. A recent analysis by the European Drug Emergencies Network, monitoring emergency admissions in the last 5 years in 10 EU countries, found that 9% of all drug-related emergencies involved NPS, primarily synthetic cannabinoids and cathinones (Wood et al., 2014). Consequently, there is increasing evidence that NPS play a great role in hospital emergencies and some drug-induced deaths [EMCDDA (European Drug Report), 2015a]. However, the analytical detection of NPS for emergency services is not technically available so far; their recognition by means of second level analysis requires standards solution, methodologies and analytical equipment not accessible to every laboratory yet [Drug Policy Department Italian Presidency of the Council of Ministers, 2013b; UNODC, 2014a; EMCDDA (European Drug Report), 2015a]. Therefore, their identification in biological samples, as well as in seized or collected samples, represents one major difficulty.

Notably, the internet is an important marketplace for the sale of NPS. Evidence is emerging of so-called “gray marketplaces”-online sites selling NPS which operate on both the surface as well as the deep web (Deluca et al., 2012; Drug Policy Department Italian Presidency of the Council of Ministers, 2013b; Burns et al., 2014; Corazza et al., 2014). Therefore, NPS can be sold via the internet to everyone, including young, underage teenagers, with complete anonymity and an easy avoidance of law enforcement and health controls [Drug Policy Department Italian Presidency of the Council of Ministers, 2013b; UNODC, 2014a; EMCDDA (European Drug Report), 2015a]. The growth of online and virtual drug markets strongly contributes to the uncontrolled widespread use of these substances, increasing health risks for consumers, and challenging drug control policies.

The largest increase in terms of newly reported NPS involves synthetic cannabinoids, synthetic cathinones, and phenethylamines [EMCDDA (European Drug Report), 2015a; EMCDDA (New psychoactive substances in Europe), 2015b]. The first synthetic cannabinoids were identified in 2008 in preparations called “herbal mixtures” or “herbal blends” (i.e., Spice) and sold as incense or air fresheners. Their effects are similar, if not superior, to those caused by cannabis consumption (Hohmann et al., 2014; Khullar et al., 2014; Mills et al., 2015). Recently, a survey of the use of synthetic cannabinoids among US students showed that Spice products were the second most used drug after marijuana, with a prevalence of 7.4–7.9% in those aged between 15 and 18 years (Johnston et al., 2013). Adolescence, a critical developmental period commonly associated with an increase in drug abuse in the human population, may be a stage of particular vulnerability to the effects of the new psychoactive drugs (Johnston et al., 2013). In fact, most of the brain receptor systems have been shown to mature slowly, reaching maximal levels around age 20. Indeed, the use of these drugs might influence neurodevelopment inducing psychiatric disorders or other mental deficits (Paus, 2005; Sussman et al., 2008). Several NPS such as mephedrone, pentedrone and MDPV which mimic the effects of amphetamine-like stimulants (ATS), can be classified as synthetic cathinones with stimulant and empathogenic properties, or as phenethylamines which can induce stimulant and hallucinogenic effects (UNODC, 2014a, 2015). Similar to other NPS, synthetic cannabinoids and ATS are largely available online and are often sold as research chemical components. They are mainly produced in Eastern Europe, Central Asia and China, and then shipped and sold to Europe and the USA (UNODC, 2015).

Currently, not all NPS are under international control. Many countries worldwide have established permanent control measures for some substances or issued temporary bans [EMCDDA (New psychoactive substances in Europe), 2015b; UNODC, 2015]. Only a few NPS have been reviewed by the mechanisms established under the international drug conventions. Existing laws covering issues unrelated to controlled drugs, such as consumer safety legislation, have been used in some countries such as Poland and UK; in others (Hungary, Finland, Italy, France, Denmark, etc.) existing drug laws or processes have been extended or adapted; additionally, in Ireland, Austria, Portugal, Romania, and Sweden new legislation has been designed [EMCDDA (New psychoactive substances in Europe), 2015b; UNODC, 2015].

The forensic identification of NPS is very difficult. These may concern the lack of knowledge on NPS available to the professionals performing analytical analysis. In addition, analytical methodologies are still not sufficient to detect the presence of all of the NPS in the analyzed samples and many laboratories lack appropriate analytical equipment for their recognition (Drug Policy Department Italian Presidency of the Council of Ministers, 2013a). These are all important aspects to take into account when considering the legal, health, and social consequences related to NPS.

To date, several behavioral, neurochemical, and electrophysiological studies have helped us to understand the pharmacological mechanisms of action of NPS. However, many of them have been focused on the acute toxicological consequences of NPS use. As they are relatively new and novel, there are no epidemiological studies to show the long-term effects of these psychoactive compounds. Also, there is not a lot of evidence on the addictive properties of NPS.

This work has been divided into two main parts based on pharmacological classification of the most popular and public health-concerning NPS classes: amphetamine-like stimulants and cannabimimetic drugs. Moreover, specific references to recent papers by the authors have been presented. A thorough analysis of the rewarding and reinforcing properties of NPS and their abuse liability will hopefully, provide to be useful for understanding the new disturbing trends in the field of drug addiction and provide strategies to tackle this growing problem.

NPS can be divided into six chemical classes (Martinotti et al., 2015; Schifano et al., 2015): phenethylamines, piperazines, tryptamines, synthetic cathinones, alkylindoles (synthetic cannabinoids) and arylcyclohexylamines (see Table 1). Alternatively, a different classification is based on pharmacological and clinical effects: stimulants, entactogens, hallucinogens, and cannabis-like compounds.

Phenethylamines, piperazines, tryptamines, and synthetic catinones exhibit stimulant and hallucinogenic effects, making up the distinct class of entactogens, which are described as psychoactive substances that enhance feelings of empathy, love, and emotional closeness to others (Schifano et al., 2007). Entactogens can be chemically divided into phenethylamines, amphetamines, synthetic cathinones, piperazines, pipradrols/piperidines, aminoindanes, benzofurans, and tryptamines (see Table 2). Stimulant drugs usually inhibit monoamine reuptake, increasing the quantity of noradrenaline, dopamine and serotonin in the synaptic cleft leading to sympathomimetic effects (Schifano, 2013). Phenethylamines are synthetic compounds commercially known as “party pills” (e.g., tablets of different colors/shapes, capsules, powder/crystal). They act on serotoninergic receptors leading to psychedelic effects and some of them inhibit the monoamine reuptake as well (Nelson et al., 2014); 3,4-methylenedioxy-methamphetamine (MDMA), widely known as “ecstasy,” is one of the most popular drugs among young people because of its stimulant effects. But, recently a growing use of new dangerous molecules on the recreational drug scene, such as 2C and its derivatives (e.g., “N-Bomb,” “B-Fly,” and “Dr. Death”), 2-D series drugs, 3C-bromo-Dragonfly, 4-MTA, 6-APB, 4,4′-DMAR and MPA, that are novel derivatives of classic psychedelic phenethylamines/MDMA-like drugs (Nelson et al., 2014) has been reported; several cases of intoxications have been reported with symptoms such as hypertension, vomiting, hyperthermia, convulsions, dissociation, hallucinations, respiratory deficits, liver, and kidney failure and death in case of overdose (Winstock and Schifano, 2009; Schifano et al., 2010; Corazza et al., 2011; Dean et al., 2013; Bersani et al., 2014; Le Roux et al., 2015; Maas et al., 2015). The lead compound in piperazines, N-Benzylpiperazin (BZP), has a typical central nervous system stimulant structure so it triggers the release of dopamine and norepinephrine and inhibits the uptake of dopamine, norepinephrine and serotonin (Smith et al., 2015). Although BZP is structurally similar to amphetamine, it is reported to have only one-tenth the potency (Wikström et al., 2004). However, at higher dosages, hallucinations can be reported as well (Kersten and McLaughlin, 2015). Before legal restrictions were placed on it, BZP was used as a safe alternative to amphetamines such as MDMA (Monteiro et al., 2013). Tryptamines (the most common is the lysergic acid diethylamide-LSD) are a group of monoamine alkaloids, very similar to the endogenous neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) (Tittarelli et al., 2015), so they act both as 5HT2A receptor agonists and serotonin reuptake inhibitors (Lessin et al., 1965; Nichols, 2004; Fantegrossi et al., 2008; Cozzi et al., 2009; Fontanilla et al., 2010) provoking visual hallucinations, alterations in sensory perception, and depersonalization (Sogawa et al., 2007); novel tryptamines, as 5-MeO-AMT or 5-MeO-DMT, continue to appear on the online drug market and on the “dark net” (Araújo et al., 2015; Schifano et al., 2015; Teixeira-Gomes et al., 2014). Synthetic cathinones (mephedrone, methylone,butylone, MDPV, and α-PVP) are structural analogs of cathinones (a molecule present in the psychoactive plant Khat) and are available in tablets, capsules, powder/crystal and generally labeled as “bath salts” or “plant fertilizers” (Fass et al., 2012; German et al., 2014; Valente et al., 2014; Karila et al., 2015). Clinical effects most commonly reported with cathinones include anxiety, impaired concentration and memory, irritation of the nasal mucosa, headache, tachycardia, and hypertension. The typical clinical symptoms are indistinguishable from the acute effects of MDMA or cocaine (Prosser and Nelson, 2012; Baumann et al., 2013; Valente et al., 2014); among their psychoactive effects, agitation, restlessness, vertigo, abdominal pain, paranoia, rhabdomyolysis, convulsions, and death are included (Corkery et al., 2012, 2014; Schifano et al., 2012; Loi et al., 2015).

Synthetic cannabinoids belong to the alkylindoles and cyclohexylphenos classes which show high affinity for CB1 and CB2 cannabinoid receptors and act like Δ⁹-THC but with prolonged psychoactive effects and more side effects (Fattore and Fratta, 2011; Brents and Prather, 2014). As shown in Figure 2, they can be divided into naphtoylindoles (e.g., JWH- 018, JWH-073, JWH-210, WIN-55212), phenylacetylindoles (e.g., JWH-250 e JWH-251), benzoylindoles (e.g., WIN-48,098, AM-694, RSC-4), cyclohexylphenols (e.g., CP-47497, CP-55940, CP-55244) (Smith et al., 2015). They are generally consumed by inhalation through the consumption of cigarettes containing herbal substances as well as these synthetic molecules to obtain euphoria, anxiolytic, and antidepressant-like effects. However, reports presented by the EMCDDA (2009a) and by the Italian Early Warning System – N.E.W.S. (Anti-drug Policies Department) have shown effects like paranoia, tachycardia, panic, convulsions, psychosis, visual/auditory hallucinations, vomiting, and seizures (Hermanns-Clausen et al., 2013; Winstock and Barratt, 2013).

Finally, arylcyclohexylamine (ketamine, phencyclidine- PCP and methoxetamine) are dissociative anesthetics that distort perceptions of sight and sound and produce feelings of detachment (or dissociation) from the environment and self without hallucinations [Nishimura and Sato, 1999; ACMD (Advisory Council on the Misuse of Drugs), 2013]. Although present in the classification, the rewarding properties of the latter group will not be discussed in this review paper.

In the second part of the 90s, a global trend of escalating amphetamine-like stimulant use was observed and synthetic tryptamines appeared on illicit drug markets. Instead of replacing or displacing MDMA and cocaine, mephedrone, and other NPS from this group appear to have been added to the established repertoire of psychostimulant narcotics (Sanders et al., 2008; Zawilska, 2015).

In animal models of addiction, cathinones have displayed potential rewarding and reinforcing effects. For example, mephedrone produces conditioned place preference (CPP), facilitates intracranial self-stimulation and is self-administered in rats (Hadlock et al., 2011; Lisek et al., 2012; Motbey et al., 2013; Bonano et al., 2014; Gregg et al., 2015). Prior studies demonstrated that MDPV and methylone, another synthetic cathinone, increase locomotor activity in rodents (López-Arnau et al., 2012; Marusich et al., 2012; Aarde et al., 2013; Gatch et al., 2013) and also enhance intracranial self-stimulation (Watterson et al., 2012, 2014; Bonano et al., 2014) and engender conditioned place preference (Karlsson et al., 2014), effects that are indicators of high abuse potential (Schindler et al., 2015). This evidence suggests that each compound could produce behavioral effects consistent with psychostimulant drugs displaying high abuse liability, possibly higher than amphetamine. In fact, in rats trained to self-administer MDPV or methamphetamine, dose-substitution studies demonstrated that behavior was dose-sensitive for both drugs, but MDPV showed greater potency and efficacy than methamphetamine (Paillet-Loilier et al., 2014). Moreover, in mice models, mephedrone, methylone, and MDPV produce CPP equal or higher than amphetamine, strongly suggesting their addictive properties (Karlsson et al., 2014). In addition, MDMA, methylone, and mephedrone are self-administered in female rats with a higher intake in mephedrone-trained rats compared to methylone-trained animals. This seems to suggest that mephedrone might have greater reinforcing effects compared to methylone or MDMA (Creehan et al., 2015), despite their shared mechanism of action. They are non-selective transporter substrates that increase the release of dopamine, norepinephrine and 5-HT in vitro (Baumann et al., 2012; Eshleman et al., 2013; Simmler et al., 2013). Importantly, the 5-HT-releasing ability of these drugs is more prevalent than their effects on dopamine in vivo (Baumann et al., 2008, 2012; Kehr et al., 2011; Wright et al., 2012) suggesting empathogen-like effects. Such findings indicate that self-administration of MDMA-like drugs is influenced by 5-HT release, but also drug pharmacokinetics, effects on noradrenergic systems, or non-transporter sites of action (Schindler et al., 2015).

In humans, synthetic cathinones produce psychotropic effects similar to MDMA and cocaine (Simmler et al., 2013). The typical dose range varies according to the different cathinone derivatives. However, according to information released from users in drug forums, where people discuss their experiences with recreational drugs (e.g., “Drugs-Forum,” “Urban 75,” “Erowid”), they usually start with a small dose and gradually increase it. This is in line with scientific reports which show that an excessive increase of noradrenergic signals could promote the onset of adverse effects and that the potency of a substance to activate the noradrenergic system is inversely correlated to the doses typically used recreationally (Simmler et al., 2013). All cathinones exhibit higher dopaminergic activity when compared with their non β-keto amphetamines analogs. Recent studies of the mechanisms by which b-ketoamphetamines interact with DAT, offer significant insight into why these drugs have such divergent effects on neurotoxicity. “Bath salts” have been classified as substrates and non-substrates based on whether or not they are transported by the DAT (Anneken et al., 2015). This increased dopaminergic property of the cathinones suggests higher stimulant-type effects and a greater risk for dependence (Aarde et al., 2013). Stimulant and entactogenic properties are typical of cathinones. In fact, desired or pleasant effects most often described by users include euphoria, intensification of sensory senses, increased sociability, increased energy, mental stimulation, empathy connection, openness, increased sensory perception, decreased inhibition, and sexual arousal; but side effects such as cognitive confusion, cognitive impairment, psychiatric irritability, aggression that sometimes progresses to violent or even criminal behavior, and self-destructive behavior have also been reported [IACP (International Association of Chiefs of Police), 2012]. MDPV and mephedrone have been directly implicated in a number of fatalities in medical literature. In one case involving MDPV, the cause of death was consistent with excited delirium syndrome, a condition associated with stimulant drug overdose and attributable to excessive dopaminergic transmission (Baumann et al., 2012).

However, amphetamine remains the prototype of psychostimulants causing agitation, insomnia, loss of appetite and, at higher doses, “amphetamines psychosis” characterized by paranoia, hallucinations and delusion (Iversen et al., 2014). In experimental animals, low doses of amphetamine cause hyperactivity and higher doses lead to stereotyped repetitive behaviors (Whelpton, 2007). The neurotoxic effects of amphetamines has been vastly studied and their ability to damage brain monoaminergic cells was shown by long-term deficits in dopaminergic and serotoninergic systems in several brain areas of animals (Teixeira-Gomes et al., 2014). One of the major neurotoxic actions of amphetamines observed in laboratory animals is the sustained depletion of monoamine brain levels. In addition to the damage to dopaminergic and serotoninergic neuronal systems, amphetamines can also induce neuronal death. For example, in several studies, MDMA administration in mice and rats produced neuronal death in several brain areas including the cortex, hippocampus, amygdala, ventromedial/ventrolateral thalamus, and teniatecta (Teixeira-Gomes et al., 2014). MDMA is still one of the most widely used recreational drugs and many NPS were designed to mimic its effects or as substitutes for MDMA in ecstasy pills.

The potency of abused psychostimulants to activate the brain reward circuitry increases the risk of potential for abuse and addiction in humans (Table 3). In contrast, a relative activation of the serotonin system would be linked to a reduction in abuse potential. Thus, the DAT/SERT inhibition ratio and dopamine/serotonin release potency has been proposed to predict the effects of psychostimulants in humans (Paillet-Loilier et al., 2014). Data currently available has shown that the frequent consumption of high doses of synthetic cathinones induce tolerance, dependence, craving, and withdrawal syndrome after sudden suspension [NDIC (National Drug Intelligence Center), 2011]. Indeed, Kehr et al. (2011) showed that mephedrone induces a stimulation of the dopamine transmission in the nucleus accumbens, that might be a starting point for developing drug-addiction (Volkow et al., 2003; Di Chiara et al., 2004). Although the typical dose range of MDPV appears to be between 5 and 30 mg in a single administration, some users reported tolerance with the consumption of a single dose, higher than 200 mg (Coppola and Mondola, 2012). Several users have reported a withdrawal syndrome after abrupt cessation of long-term use of methcathinone, mephedrone and MDPV (Winstock et al., 2011). Moreover, Gatch et al. (2013) showed that all of the cathinone derivatives fully substituted for methamphetamine or cocaine in drug discrimination tests. Results suggest that these drugs are comparable to cocaine and methamphetamine, and are likely to induce dependence (Iversen et al., 2014).

The review of the biomedical data here presented, clearly demonstrates the unsafe nature of these new drugs of abuse. This is particularly alarming since adolescents seem to be the most exposed subjects to these dangerous NPS. Indeed, users are often unaware of the consequences of ingesting synthetic compounds that are sold as “legal alternative” to classical drugs, and their unexpected, sometimes fatal adverse effects. Only awareness can reduce NPS use since stopping their synthesis and diffusion seems to be an improbable task and, morbidity and mortality reports keep increasing as NPS gain popularity worldwide. Awareness campaigns about these substances and their devastating effects should be organized to inform everyone, including clinicians, who should be able to recognize symptoms of intoxication induced by NPS (Simonato et al., 2013; Papanti et al., 2013; Schifano et al., 2015).

Ultimately, this paper intends to be helpful to drive governments and civil society to not underestimate the NPS issue, and to encourage the scientific community to deeply evaluate the pharmacology and toxicological effects of NPS and to develop effective treatments for NPS intoxication. Additionally, this paper intends to be useful for advising law enforcement agencies, which need updated information for the prevention and fight against trafficking and sale of NPS, and in the long run, hopefully contribute to better protect public health and safety.

This is a review, different authors contributed as follows: CM: Section NPS: From Chemistry to Pharmacological Effects; Figures 1–3, Tables 1, 3, 4. GS and CR: Section–Introduction; MM and MM: Section–Human and Animal Studies on amphetamine-Like Stimulant Effects: Psychoactive Effects, Cognitive Deficits, Emotional Alterations, and Dependence -MM: Table 2. MDL: Section–Synthetic Marijuana and the Cannabimimetics, Section–Concluding Remarks and entire revision of the manuscript; Figures 4, 5 and Tables 3, 4.