Methamphetamine-Associated Psychosis

D-amino acid oxidase activator (DAOA)

DAOA is the gene encoding the d-amino acid oxidase activator. DAOA is expressed in multiple tissues including the amygdala, caudate nucleus, spinal cord and testes (Chumakov et al. 2002). Although the functional mechanisms of DAOA are not fully understood, DAOA activates D-amino acid oxidase, which oxidizes d-serine, an endogenous ligand for the glycine site of the N-Methyl-D-aspartate (NMDA) type glutamate receptor (Chumakov et al. 2002). D-serine levels are low in schizophrenic patients and administration of D-serine has been shown to reduce some of the symptoms of this disease (Kantrowitz et al. 2010). This provides a potential link between DAOA and the glutamate hypo-function hypothesis of schizophrenia, which integrates environmental influences and causative genes, is based on clinical and neuropathological evidence of the hypo-function of glutamatergic signaling via NMDA receptors. The DAOA gene is located on chromosome 13q and has been found to be a susceptibility locus for schizophrenia (Badner and Gershon 2002), and many studies have found an association between genetic variants in DAOA and schizophrenia (Chumakov et al. 2002; Schumacher et al. 2004; Yue et al. 2007). In a case-controlled study, Kotaka et al. (2009) demonstrated an association between a DAOA polymorphism (rs778293; p = 0.0002) and psychosis among METH-dependent subjects. This polymorphism was previously shown to be associated with schizophrenia; however, its functionality is unknown.

Dystrobrevin-binding protein 1 (DTNBP1)

DTNBP1 encodes dystrobrevin-binding protein 1 also referred to as dysbindin. This ubiquitously expressed, coiled-coil-containing protein is a subunit of the biogenesis of lysosome-related organelles complex 1 (BLOC-1), which regulates trafficking to lysosome-related organelles (Li et al. 2003). In muscle and non-muscle cells, DTNBP1 binds to α- and β-dystrobrevins, components of the dystrophin-associated protein complex (DPC) (Benson et al. 2001). In muscle, the DPC is required for the maintenance of muscle integrity and normal muscle function. In the brain, dysbindin is most prevalent in axons, particularly those with large synaptic terminals such as the mossy fiber synaptic terminals in the cerebellum and hippocampus (Benson et al. 2001). In a case controlled study, Kishimoto et al. (2008a) demonstrated an association of a polymorphism in the DTNBP1 gene and psychosis among METH-dependent individuals (rs3213207; p = 0.000025). Furthermore, two haplotypes at these loci were also significantly associated with MAP (p values < 0.0015). Significant associations between schizophrenia and DTNBP1 also have been reported (Edwards et al. 2008; Straub et al. 2002). Consistent with this, reduced levels of dysbindin expression have been associated with the pathogenesis of schizophrenia (Talbot et al. 2004; Weickert et al. 2008) and thought to be related to glutamatergic neurotransmission. In schizophrenic patients, dysbindin-1 is reduced in intrinsic, glutamatergic terminals of the hippocampus which is inversely correlated with increased vesicular glutamate transporter (Talbot et al. 2004). As evidence is mounting that glutamate hypo-function may be related to the etiology of schizophrenia (Konradi and Heckers 2003), it is postulated that DTNBP1 variants may contribute to MAP through pathways involving glutamatergic neurotransmission. Despite the many genetic studies of this gene, the link between functionality of these polymorphisms and dysbindin expression has not been investigated.

Frizzled 3 (FZD3)

Frizzled proteins are cell surface receptors for secreted Wnt proteins (Wang et al. 2006a). Both frizzled and Wnt proteins are thought to be important in central nervous system (CNS) development. FZD3, the human Frizzled-3 gene, is widely expressed in the developing nervous system and is involved in axonal growth and guidance (Wang et al. 2006b). Wnt signaling plays a role in axonal remodeling and synaptic differentiation in the cerebellum (Cadet and Krasnova 2009; Lucas and Salinas 1997) and abnormal Wnt signaling has been linked to schizophrenia (Cotter et al. 1998). FZD3 maps to chromosome 8p21 and consists of eight exons. The 8p22-21 region has been identified as a susceptibility locus for schizophrenia in several studies (Blouin et al. 1998; Gurling et al. 2001; Pulver et al. 1995). However, genetic studies with the Frizzled-3 gene and schizophrenia have been contradictory, with both positive and negative associations among Han Chinese (Yang et al. 2003), Japanese (Zhang et al. 2004), and Korean (Jeong et al. 2006) populations. Interestingly, the association between Frizzled-3 and MAP has only been observed with haplotypes (p < 0.00002) and not at the individual loci, which may indicate a required synergism between the polymorphisms for a phenotypic effect (Kishimoto et al. 2008b).

Metabotropic glutamate receptor 2 (GRM2)

GRM2 is another gene involved in glutamatergic neurotransmission and found to be significantly associated with MAP (Tsunoka et al. 2010). GRM2 is located on chromosome 3p in a region linked to schizophrenia in several studies (Badner and Gershon 2002; Pulver et al. 1995). GRM2 encodes the group II metabotropic glutamate receptor 2 (mGlu2). mGlu2 is a G-protein coupled receptor involved in inhibition of adenylate cyclase and cAMP formation (Cartmell et al. 1999) and the major presynaptic group II autoreceptor activated by synaptically released glutamate (Kew et al. 2002). Although another group II metabotropic glutamate receptor, glutamate receptor 3 (mGlu3), has been implicated in etiological, pathophysiological and pharmacotherapeutic aspects of schizophrenia (Harrison et al. 2008; Lyon et al. 2008; Sartorius et al. 2008), no statistically significant association with GRM2 and schizophrenia was found in the Japanese population (Joo et al. 2001).

5-Hydroxytryptamine (Serotonin) receptor 1A (5-HT1A)

5-HT1A, the gene encoding the 5-HT receptor 1A, is a G protein-coupled receptor which is widely expressed in the brain, including in the hypothalamus, hippocampus, and cortex. Through serotonin (5-HT) binding, this receptor mediates inhibitory neurotransmission. The serotonin system has been shown to be important in the neural processing of anxiety and the neurobiological control of learning and memory. Similar to the glutamatergic pathway, altered serotonergic neurotransmission is speculated to contribute to schizophrenia susceptibility. Evidence suggests that METH, which acts as a substrate for the 5-HT transporter, elevates extracellular 5-HT levels by both promoting the efflux via transporter-mediated exchange and by increasing cytoplasmic levels by disrupting storage of 5-HT in vesicles (Cadet and Krasnova 2009; Rothman and Baumann 2003). Although studies have explored whether there is an association between several of the genes involved in 5-HT regulation and MAP, only the rs878567 polymorphism in the 5-HT1A receptor was significantly associated using an alpha level of 0.001 as significant (Kishi et al. 2010). Polymorphisms in the 5-HT1A receptor have been associated with schizophrenia in several studies (Huang et al. 2004; Le Francois et al. 2008; Lemonde et al. 2003, 2004), including the rs878567 polymorphism. Currently, 5-HT(1A) receptor agonists are being considered for the treatment of schizophrenia (McCreary and Jones 2010).

Prokineticin receptor 2 (PROKR2)

PROKR2 encodes prokineticin receptor 2 (PK-R2), an integral membrane G-protein coupled receptor for prokineticins. Prokineticins and their receptors are involved in a wide range of biological functions in multiple organ systems. In the CNS, prokineticin 2 modulates neurogenesis, circadian rhythms, and migration of the subventricular zone-derived neuronal progenitors (Cheng et al. 2002; Ng et al. 2005). The involvement of PK-R2 in circadian rhythm raises the question of whether this gene may be associated with mood disorders, a phenotype often observed in patients with drug addiction. Several animal studies have shown that METH increases expression of circadian genes in the brain (Iijima et al. 2002). The PROKR2 gene is located on chromosome 20p12.3, which was shown to be linked to bipolar disorder in three studies (Detera-Wadleigh et al. 1997; Fanous et al. 2008; Ross et al. 2008). An association between PROKR2 gene variants and major depressive disorder and bipolar disorder was reported in a Japanese population (Kishi et al. 2009b); however, this study was small and confirmatory studies will be necessary to validate this finding. Furthermore, these same investigators have shown that 3 individual SNPs in the PROKR2 gene (rs6085086, rs4815787, rs3746682), as well their haplotypes (p ≤ 0.00019), were associated with MAP (Kishi et al. 2010).

Glycine transporter 1 (SLC6A9)

SLC6A9, which encodes the glycine transporter type 1 (GLYT1), is involved in glutamatergic neurotransmission, particularly at NMDA-type glutamate receptors. It is currently believed that termination of the different synaptic actions of glycine is produced by rapid re-uptake through two sodium- and chloride-coupled transporters, GLYT1 (located in the plasma membrane of glial cells) and GLYT2 (located in pre-synaptic terminals). GLYT1 regulates both glycinergic and glutamatergic neurotransmission by controlling the reuptake of glycine at synapses. The NMDA receptors are regulated in vivo by the amino acids glycine and D-serine. Glycine levels, in turn, are regulated by GLYT1, which serves to maintain low sub-saturating glycine levels in the vicinity of the NMDA receptors. Competitive antagonists of NMDA receptors produce a psychotic state in healthy subjects and exacerbate symptoms in schizophrenics. SLC6A9 has emerged as a key novel target for the treatment of schizophrenia. Morita et al. examined SLC6A9 among Japanese subjects with METH-dependence and psychosis (Morita et al. 2008). Two SNPs conferred an increased risk for MAP (rs2486001 and rs2248829) in addition to the haplotype T-G (p = 0.000037). It is speculated that variants of the SLC6A9 gene may affect susceptibility to MAP by modulating NMDA receptor function.

In summary, there is reasonably strong evidence that genetic variation in neurotransmitter systems and in neural development or growth is associated with risk for MAP. Of the seven genes with strong empirical support for association with MAP, four are involved in glutamatergic neurotransmission (DAOA, DTNBP1, GRM2 and SLC6A9). Potential epistatic (i.e. gene x gene) interactions among these glutamatergic genes should be investigated. Polymorphism in a key gene in serotonin system regulation and signaling (HTR1A) is also associated with risk for MAP, which suggests that other 5-HT system genes may also be good candidates. Finally, two genes with roles in CNS development (FZD3) and neurogenesis (PROKR2) may help to identify other genes as well as developmental processes that mediate vulnerability to MAP.

Behavioral characteristics of MAP in nonhuman primates

Most of this work has been carried out by Japanese researchers since the early 1970s beginning during the decade immediately after World War II when METH abuse occurred in epidemic proportions in Japan (Ujike and Sato 2004). Japanese psychiatrists had observed an increased number of cases of MAP in chronic METH users and began to investigate the behavioral characteristics of the psychosis and associated biochemical mechanisms in animals. Nonhuman primate models are thought to be better than other species models in capturing complex and fine-grained behavioral abnormalities resembling human MAP, especially aspects of perceptual aberrations, social interaction, and interpersonal relationship. This is because monkeys, especially Japanese monkeys (Macaca fuscata) are well known to form a stable and intricate hierarchical society in which each member follows a certain rank order appropriate in interacting with others.

An early study provided a vivid description of acute and chronic effects of METH treatment on a group of Japanese monkeys (Machiyama 1992). They gave monkeys intramuscular METH injection at 1.0 mg/kg from Monday to Saturday for 3–6 months. At the same time, physiological saline was given by intramuscular injection to the other animals that served as controls. Upon acute treatment, some monkeys showed motor excitation, whereas other showed motor suppression. They identified that this marked individual difference was due to the different behavioral traits of monkeys in the group. Active monkeys that showed enhanced repetitive motor activity to acute METH were those aggressive individuals occupying higher ranks while non-active monkeys, at the bottom in the ranking order, demonstrated a decrease in motor activity to acute METH treatment. Over the course of repeated drug treatment, some monkeys developed behavioral abnormalities in a variety of psychological domains. In the sensorimotor domain, after about 1 month treatment, some monkeys displayed a stereotypical body-fingering behavior which entailed continuous fingering and investigating certain parts of the body, such as the wrist, thigh, abdomen, penis, or scrotum (Machiyama 1992). In the perception domain, after about 2 months of repeated treatment, some monkey displayed hallucination-like perceptual distortions (e.g., “staring” at vacant space, floor or certain body parts of their own or cage mates, and at times touching with fingers). In the social behavior domain, some monkeys gradually lost interest in social interaction (e.g., grooming and mounting), and withdrew to stay in a location in the cage, and could suddenly show inexplicable aggression and fear, and broken behavioral response patterns (termed “splitting”). There were also marked individual differences. Monkeys, with middle and high ranks that responded actively to acute METH injection, demonstrated the most severe behavioral abnormalities in response to chronic METH treatment. Monkeys with lower ranks fared better and only exhibited mild changes. Two high ranked monkeys even died before the entire experiment was completed. After the termination of daily injections, some behavioral abnormalities disappeared, while others persisted for an extended period. In general, perceptual aberrations (e.g., staring, fixation) diminished rather rapidly and to a greater extent than deficits in sensorimotor (e.g., fixating) and social functioning (e.g., social withdrawal). Also, some monkeys who exhibited persistent behavioral changes that were easily identifiable by uninformed observers recovered better than others. Most interestingly, the psychotic behaviors of chronically treated monkeys could be re-triggered by a saline injection or a METH injection, mimicking clinical phenomena of stress and drug priming-induced relapse of METH psychosis.

One of the issues with these observational studies is the lack of formal assessments of psychological functions. Therefore, it is unclear what psychological function(s) (e.g., attention, working memory, episodic memory, executive functioning, emotional regulation, etc.) were impaired by chronic METH treatment that contributed to the observed behavioral abnormalities. The second issue is that the experimenter-controlled METH administration regimen did not mimic human METH use (via self-administration) and the constant dosing regimen used in the monkey studies also did not reflect the typical pattern of human METH use. Most METH abusers start with very low doses of the drug and have had a long history of progressively escalating their doses. Thus, human METH psychosis often appears during the course of escalating dosage of drug administration (i.e., “binges” or “runs”), and discontinuation of drug usage usually results in a rapid decline of the psychosis, closely paralleling urine drug levels (Angrist 1994; Davis and Schlemmer 1980; Kuczenski et al. 2009). This aspect of METH use pattern was not mimicked in these early studies.

There are several issues that may hinder the effort to delineate the neurobiological underpinnings of METH psychosis. The first is the lack of validated rodent behavioral models of METH psychosis. Unlike non-human primate models, which provide richer behavioral repertoires sensitive to the psychotomimetic effect of METH, most rodent studies focus on motor activity and stereotypy. Behavioral sensitization is taken as signs of METH psychosis (Martinez et al. 2005; Nestler 2001; Robinson and Becker 1986; Segal and Mandell 1974; Segal et al. 1981). One issue with psychomotor sensitization and stereotypy as behavioral indices of METH psychosis is that they do not seem to capture the emotional, cognitive, and perceptual disturbances that characterize human METH psychosis disorders. Other behavioral abnormalities induced by repeated METH treatment, such as disruption of PPI of acoustic startle response, may provide a better model (Braff et al. 2001). The following section discusses this rodent research in more detail.

Behavioral characteristics of METH psychosis in mice and rats

To date, the behavioral models explicitly developed for studying symptoms of MAP in rodents fall into three broad categories: i) METH-induced deficits in PPI, ii) METH attenuation of social interactions, and iii) METH induced stereotypy and alterations in locomotion. The following narrative will provide an overview of this research and highlight some key findings that indicate their possible utility for understanding aspects of MAP in humans.

Pre-pulse inhibition (PPI)

Rodents will startle to a sudden loud sound such as a 40 ms, 120-dB white noise. Under typical conditions, this startle reaction is reduced if this startle stimulus is preceded by a shorter and less intense pre-pulse stimulus (e.g., 20 ms, 72-dB white noise). This inhibition of the startle is thought to measure sensorimotor gating (Braff and Geyer 1990). Individuals diagnosed with schizophrenia show deficits in PPI; an effect that can be simulated by acute and repeated treatment with METH. For example, Arai and colleagues found that mice pretreated with 3 mg/kg METH showed reduced PPI (Arai et al. 2008). A similar acute METH treatment protocol has been shown to be effective at producing PPI deficits in rats (Maehara et al. 2008). Of note, this deficit in PPI was not seen in mice treated acutely with 1 mg/kg METH (Arai et al. 2008). However, in that same report by Arai et al., repeated treatment with METH (1 mg/kg) for 7 days disrupted PPI and this deficit persisted after 7 days of abstinence from METH. More recently, Hadamitzky et al. (2011) report deficits in PPI in rats that had an extended history of self-administering intravenous METH. This research reflects an important advance in developing a translational model to study aspects of MAP. That is, like humans, this alteration in sensorimotor gating was self-induced by the rat.

Behavioral pharmacology research investigating the mechanism of the METH-induced deficits in PPI have focused either on the effects of established antipsychotic medications or on the effects of ligands thought to act on receptor processes involved in aspects of the psychosis. Along the former lines, the typical antipsychotic medication haloperidol has been shown to alleviate PPI deficits induced by acute treatment with METH (Maehara et al. 2008). Further, the atypical antipsychotics olanzapine and risperidone alleviated PPI deficits induced by repeated METH treatment (Abekawa et al. 2008). Arai and colleagues found that the GABA_B agonist baclofen alleviated deficits in PPI induced by acute and chronic METH exposure (Arai et al. 2009). Further, the cholinergic system appears to be important for METH-induced deficits in PPI. For example, pretreatment with the muscarinic agonist oxotremorine blocked METH-induced deficits in PPI (Maehara et al. 2008). In this same report, the investigators did not find an effect of the nicotinic agonist nicotine. This result contrasts with (Mizoguchi et al. 2009) who found that pretreatment with nicotine alleviated PPI deficits produced by METH. Further, the nicotinic antagonist dihydro-β-erythroidine (DHβE) and METHyllycaconitine both blocked this ameliorative effect of nicotine indicating a role for α4β2- and α7-containing nicotinic acetylcholine receptors. The most notable difference in protocol between these discrepant reports was that Maehara et al. 2008 used rats; whereas, Mizoguchi et al. 2009 used mice. Regardless, the comorbidity between METH and tobacco use, as well as schizophrenia and tobacco use, makes this an important area for future research.

Social interaction

When a rat is paired with a conspecific, they show a variety of species-specific behaviors. These behaviors include sniffing, grooming, crawling over or under, and nosing (Barnett 1963). Some researchers have suggested that alternations in social interaction resulting from METH exposure may be a useful model to study the paranoid and social anxiety symptoms of humans with MAP (Clemens et al. 2004; Rawson et al. 2002). Previous research has shown that acute METH treatment can alter social interaction. For example, Shinba et al. 1996 found that rats treated with either 0.1 or 1 mg/kg METH on average spent their time further away from the conspecific in the environment than saline controls [see also (Arakawa 1994)]. Perhaps of more interest from a modeling or simulation perspective is the more recent work by Clemens and colleagues showing ‘social withdrawal’ in rats following an abstinence period from METH (Clemens et al. 2007a, b). In one study, Clemens and colleagues administered METH at 2.5 or 5 mg/kg every 2 h for a total of 4 injections. METH increased general activity in the chamber across the entire 7.5 h treatment/measurement period regardless of dose (Clemens et al. 2004). Rats treated with the highest dose of METH (20 mg/kg total) also developed head weaving at the end of the treatment period, suggesting stereotypy with this protocol (see next section). Acute high dose METH increased activity and head-weaving. Albeit interesting, the most notable finding from the perspective of this report is that following 4 weeks of abstinence from METH these rats had significantly lower social interaction score than controls. Subsequently, this social withdrawal finding was extended to a treatment regimen in which rats were administered 8 mg/kg METH once per week for 16 weeks (Clemens et al. 2007a, b). In fact, the alterations in social interaction were seen after 7 weeks of abstinence.

Stereotypy and alterations in locomotion

As the acute dose of METH increases, its behavioral effect in rats and mice generally shifts from inducing hyperactivity to inducing stereotypy. These effects of METH can become exaggerated (i.e., sensitization) with repeated treatment (Bevins and Peterson 2004; Kuczenski et al. 2009; Maehara et al. 2008; Ujike et al. 1989). Further, a handful of papers published in the 1960s showed that spontaneous wheel running, activity, and reactivity to external stimuli are blunted long after (e.g., 10 weeks) chronic treatment with METH has ceased [see Utena 1961, 1966); Yagi 1963; Yui et al. 2000 and Machiyama 1992 for more detailed discussion of this and related research]. The presentation of such behavioral alterations that sensitize with repeated treatment and persist with abstinence has been considered a model for some of the symptoms of psychosis (Machiyama 1992; Takigawa et al. 1993; Yui et al. 2000). Converging support for this notion comes from the overlap in the neurobiological processes underlying these motor effects of METH and that of psychosis in humans, as well as behavioral pharmacologic work indicating the effectiveness of antipsychotic medications. For instance, METH-induced locomotor stimulation is blocked by clozapine, haloperidol, and chlorpromazine (Maehara et al. 2008; Okuyama et al. 1997). Notably, METH-induced stereotypy in the work of Okuyama et al. 1997 was blocked by haloperidol and chlorpromazine, but not clozapine. Additional pharmacologic research has implicated the serotonergic, dopaminergic, and muscarinic systems in these effects (Balsara et al. 1979; Maehara et al. 2008; Ujike et al. 1989). For instance, Ujike et al. 1989 found that repeated daily METH treatment in rats (4 mg/kg for 14 days) increased locomotion and stereotypy. Pretreatment with dopamine D1 receptor antagonists SCH23390 or the D2 receptor antagonist YM-09151-2 blocked this sensitization.

Most of the repeated or chronic METH exposure research in this area involves a daily injection of METH for a prescribed number of days. Notably, a recent study by Kuczenski et al. (2009) sought to investigate these shifts in locomotor stimulation and stereotypy in rats, using an intravenous infusion protocol designed to more closely mimic the chronic escalating use of METH seen in humans and simulate the longer half-life of METH in humans (ca. 12 h in human versus 1 h in rats). Their findings confirm and extend earlier research describing the development of stereotypy, as well as disruption of sleep, in what seems to be a more translationally relevant model of MAP. Whether this is the case or not will need to await further research. However, one limitation of this model that will need to be overcome, if this model is to be more widely adopted, is the high mortality rate of the rats in the escalating METH exposure phase.