Monoamine Hypothesis Mcat

Chemical structure of the MCAT pharmacophore; R was systematically varied to generate MCAT analogs.

Department of Pharmacology and Toxicology (J.A.S., M.F.L., S.S.N., M.L.B.), Department of Medicinal Chemistry (F.S., R.K., R.A.G.), and Institute on Drug and Alcohol Studies (R.A.G., S.S.N., M.L.B.), Virginia Commonwealth University, Richmond, Virginia

Address correspondence to: Dr. Matthew L. Banks, Department of Pharmacology and Toxicology, Virginia Commonwealth University, 410 North 12th Street, P.O. Box 980613, Richmond, VA 23298., E-mail: [email protected]

Copyright © 2015 by The American Society for Pharmacology and Experimental Therapeutics

Associated Data

Abstract

Methcathinone (MCAT) is a monoamine releaser and parent compound to a new class of designer drugs that includes the synthetic cathinones mephedrone and flephedrone. Using MCAT and a series of para-substituted (or 4-substituted) MCAT analogs, it has been previously shown that expression of abuse-related behavioral effects in rats correlates both with the volume of the para substituent and in vitro neurochemical selectivity to promote monoamine release via the dopamine (DA) versus serotonin (5-HT) transporters in rat brain synaptosomes. The present study used in vivo microdialysis to determine the relationship between these previous measures and the in vivo neurochemical selectivity of these compounds to alter nucleus accumbens (NAc) DA and 5-HT levels. Male Sprague-Dawley rats were implanted with bilateral guide cannulae targeting the NAc. MCAT and five para-substituted analogs (4-F, 4-Cl, 4-Br, 4-CH_3, and 4-OCH₃) produced dose- and time-dependent increases in NAc DA and/or 5-HT levels. Selectivity was determined as the dose required to increase peak 5-HT levels by 250% divided by the dose required to increase peak DA levels by 250%. This measure of in vivo neurochemical selectivity varied across compounds and correlated with 1) in vivo expression of abuse-related behavioral effects (r = 0.89, P = 0.02); 2) in vitro selectivity to promote monoamine release via DA and 5-HT transporters (r = 0.95, P < 0.01); and 3) molecular volume of the para substituent (r = −0.85, P = 0.03). These results support a relationship between these molecular, neurochemical, and behavioral measures and support a role for molecular structure as a determinant of abuse-related neurochemical and behavioral effects of MCAT analogs.

Introduction

Methcathinone (MCAT) is the β-ketone analog of methamphetamine and serves as a parent compound to abused designer drugs that include mephedrone (4-CH₃ MCAT) and flephedrone (4-F MCAT) (Glennon et al., 1987; Spiller et al., 2011; De Felice et al., 2014). Like methamphetamine, MCAT functions as a monoamine transporter substrate that selectively promotes release of dopamine (DA) and norepinephrine over serotonin (5-HT) (Cozzi et al., 1999, 2013; Baumann et al., 2012). Consistent with MCAT’s Schedule I controlled substance classification in the United States, MCAT increases striatal and nucleus accumbens (NAc) DA levels as measured by in vivo microdialysis in rats (Gygi et al., 1997; Cozzi et al., 2013). Furthermore, MCAT functions as a reinforcer in nonhuman primate drug self-administration procedures (Kaminski and Griffiths, 1994) and produces an abuse-related facilitation of intracranial self-stimulation (ICSS) in rats (Bonano et al., 2015). Overall, the preclinical neurochemical and behavioral effects of MCAT are consistent with its accepted abuse liability in humans.

Results from our laboratory and others suggest that the identity of the para (or 4-position) substituent on the MCAT scaffold is a significant determinant of the abuse-related neurochemical and behavioral effects of synthetic MCAT analogs ( Fig. 1 ). In particular, the volume of the para substituent on the MCAT scaffold appears to influence its interaction with the DA transporter (DAT) and 5-HT transporter (SERT), such that DAT prefers small-volume substituents (e.g., MCAT itself, with a hydrogen atom at the para position), whereas SERT prefers substituents with larger volumes (e.g., mephedrone, with a methyl substituent at the para position) (Bonano et al., 2015; Sakloth et al., 2015). This shift in preference from DAT to SERT with increasing volume of the para substituent is evident from in vitro studies of DAT- and SERT-mediated monoamine release in rat brain synaptosomes, and this in vitro measure of declining DAT versus SERT selectivity correlates with in vivo evidence for declining expression of abuse-related ICSS facilitation (Bauer et al., 2013; Bonano et al., 2015). For example, mephedrone (4-CH₃ MCAT) has lower selectivity than MCAT to promote DAT- versus SERT-mediated monoamine release in rat brain synaptosomes and produces weaker ICSS facilitation than MCAT (Bonano et al., 2014).

Chemical structure of the MCAT pharmacophore; R was systematically varied to generate MCAT analogs.

The correlation of ICSS effects of para-substituted MCAT analogs (and other monoamine releasers) with in vitro DAT versus SERT selectivity suggests that these behavioral effects might also correlate with in vivo selectivity to promote DA versus 5-HT release. For example, it is well established that increased DA release in NAc plays a key role in mediating abuse-related effects of many classes of abused drugs (Di Chiara and Imperato, 1988). Conversely, drug-induced increases in 5-HT levels appear to oppose and limit DA-mediated effects that contribute to abuse (Czoty et al., 2002; Navailles et al., 2008; Baumann et al., 2011). The goal of the present study was to further evaluate the relationship between in vivo selectivity to increase NAc DA versus 5-HT levels and both 1) in vitro measures of DAT versus SERT selectivity and 2) expression of abuse-related behavioral effects. Toward this end, this study used an in vivo microdialysis procedure to determine the potency and time course of effects produced by MCAT and five para-substituted MCAT analogs on NAc DA and 5-HT levels in rats. We hypothesized that the selectivity, and not absolute DA or 5-HT changes, of these drugs to increase NAc DA versus 5-HT levels would correlate with our previously determined measures of both in vitro DAT versus SERT selectivity (using a rat brain synaptosome procedure) and in vivo expression of ICSS facilitation (Bonano et al., 2015; Sakloth et al., 2015). Moreover, we also hypothesized that in vivo selectivity to increase DA versus 5-HT levels would also correlate with the volume of the para substituent on the MCAT scaffold.

Materials and Methods

Subjects

Adult male Sprague-Dawley rats (Harlan, Frederick, MD) weighing a minimum of 300 g at the time of surgery were individually housed and maintained on a 12-hour light/dark cycle with lights on from 6:00 AM to 6:00 PM. Rats had ad libitum access to food and water except during microdialysis experiments. Animal facilities were accredited by the Association for the Assessment and Accreditation of Laboratory Animal Care (http://www.aalas.org/), and experimental procedures were approved by the Institutional Animal Care and Use Committee in accordance with guidelines (8th edition) for the care and use of animal subjects in research (National Research Council, 2011). All efforts were made to minimize animal suffering; to reduce the number of animals used; and to use alternatives to in vivo techniques, if available.

Drugs

This study determined the neurochemical effects of MCAT and five para-substituted MCAT analogs. For MCAT, the para substituent (the 4-position on the benzene ring of the MCAT scaffold) is hydrogen (H). For the purposes of this paper, and for consistency with previous studies by Bonano et al. (2015) and Sakloth et al. (2015), MCAT analogs are designated using the nomenclature 4-R MCAT, with R being the substituent at the para position (see Fig. 1 ). In some cases, these compounds also have generic names or other chemical names, and in these cases, alternative names are also listed subsequently. Specifically, the following compounds were studied: MCAT (PubChem CID: 1576), 4-F MCAT (flephedrone; PubChem CID: 49853406), 4-Cl MCAT (clephedrone), 4-CH₃ MCAT (mephedrone; PubChem CID: 45266826), 4-Br MCAT (brephedrone), and 4-OCH₃ MCAT (methedrone; PubChem CID: 216281). MCAT and MCAT analogs were synthesized as their racemic HCl salts using previously described methods (Bonano et al., 2015). S(+)-Amphetamine hemisulfate (PubChem CID: 3007) and (±)-fenfluramine HCl (PubChem CID: 3337) (Sigma Aldrich, St. Louis, MO) were tested as comparator phenethylamine controls with selectivity for DAT and SERT, respectively (Baumann et al., 2000). All compounds were dissolved in sterile saline for i.p. injection, and all drug doses are expressed as the salt forms listed previously.

Microdialysis

Surgery.

Rats (n = 63) were anesthetized with 3.0% isoflurane in oxygen until unresponsive to toe pinch and secured in a stereotaxic apparatus (Kopf Instruments, Tujunga, CA). Guide cannulae (8 mm long, 0.5 mm outer diameter; CXG-8, Eicom, San Diego, CA) were implanted bilaterally and terminated 1 mm above the NAc (coordinates: 1.5 mm anterior to bregma; 1.8 mm lateral to midsagittal suture; 6.0 mm ventral to dura). The guide cannulae were secured to the skull using screws (Plastics One, Inc., Roanoke, VA) and orthodontic resin (Butler Schein, Dublin, OH). A dummy cannula (CXD-8, Eicom) was inserted into each guide cannula to maintain cannula patency. Animals were allowed at least seven recovery days prior to initiating microdialysis testing.

Procedure.

On test days, rats were briefly anesthetized with 3.0% isoflurane in oxygen, one of the dummy cannula was removed, and a microdialysis probe (10 mm long, CX-I-8-2, Eicom) with a 2-mm artificial cellulose cuprophan membrane (50 kDa molecular weight cutoff) at its tip was inserted into an 8-mm guide cannula such that it extended 2 mm beyond the end of the guide cannula and into the NAc. The probe was connected to a two-channel liquid swivel (TCS2-23, Eicom), and the rat was placed into an acrylic experimental cage (30 cm 3 ). Microdialysis probes were perfused with artificial cerebrospinal fluid (147 mM NaCl, 2.8 mM KCl, 1.2 mM CaCl₂, 1.2 mM MgCl₂) at a rate of 1 μl/min. The mobile phase consisted of 2% methanol (EMD, Gibbstown, NJ), 100 mM phosphate buffer (Sigma Chemicals, St. Louis, MO), 500 mg/l 1-decane sodium sulfonate (TCI America, Montgomeryville, PA), and 50 mg/l EDTA-2Na + (Dojindo Laboratories, Kumamoto, Japan).

Dialysate samples were collected into a 50-μl injector loop at 10-minute intervals using an online autoinjector (EAS-20s, Eicom) and immediately analyzed for DA and 5-HT concentrations by high-pressure liquid chromatography coupled to electrochemical detection (HTEC-500, Eicom). DA and 5-HT were separated using a C₁₈-reverse phase column (PP-ODS II, Eicom) and detected using a graphite working electrode and an Ag + versus AgCl reference electrode with an applied potential of +450 mV. Preliminary experiments conducted by probe immersion into a known standard concentration of DA indicated a lag time of ∼20 minutes for dialysate to traverse the tubing from the probe to the electrochemical detector at the 1 μl/min flow rate. DA and 5-HT were identified according to characteristic retention times of the standard solution and concentrations were quantified by comparison with peak heights of the standard concentration curve (0.1–100 pg per 10 μl) generated prior to drug administration in each microdialysis experiment. The lower limit of neurotransmitter detection was 0.1 pg. DA and 5-HT levels were determined to be stable after six consecutive stable baseline samples were obtained with

Testing was conducted using drug doses based on previous behavioral studies from our laboratory (Bauer et al., 2013; Bonano et al., 2014, 2015). Specifically, saline vehicle, amphetamine (0.1–1.0 mg/kg), fenfluramine (1.0–3.2 mg/kg), MCAT (0.32–3.2 mg/kg), and five para-substituted MCAT analogs [4-F, 4-Cl, 4-CH₃, and 4-Br MCAT (1–10 mg/kg), and 4-OCH₃ MCAT (3.2–32 mg/kg)] were administered i.p., and dialysate samples were collected for 180 minutes after drug administration. Rats were tested no more than four times (twice per cannula; at least one week between reaccessing a given site), and each site was used to test a different experimental treatment. At the completion of all experiments, rats were euthanized with CO₂, and brains were removed and stored in 10% formalin. Probe placement was verified by visual inspection of cannula tracks in unstained brain sections as described previously (Bert et al., 2004; Miller et al., 2015). Only rats with correct probe placements were included in data analyses (data from n = 9 rats were discarded due to improper cannula placement).

Data Analysis.

The primary dependent variables were extracellular DA and 5-HT concentrations in each dialysate fraction expressed as a percent of the average of the six mean baseline concentrations before drug or vehicle administration for each experiment. The individual normalized DA and 5-HT concentrations were then averaged across rats to yield group mean results for graphical presentation. Results were analyzed for each drug dose using a repeated-measures one-way analysis of variance, with time as a fixed effect and subject as a random effect (JMP Pro 11, SAS, Cary, NC). Using this analytical method, within-subject comparisons using the Dunnet post hoc test were determined between monoamine concentrations at each time point and the 10-minute control monoamine concentration. This 10-minute time point represents a dialysis sample that was collected before drug administration, which had advanced into the cannula-to-injector-loop tubing at the time of drug administration and had reached the working electrode for analysis after drug injection due to the 20-minute lag time of tubing dead space. The criterion for statistical significance was set at P < 0.05.

Correlational Analysis.

Correlational analyses were conducted to compare selectivities of MCAT analogs to promote in vivo DA versus 5-HT release in the present study with their selectivities to promote DAT- versus SERT-mediated monoamine release in previous in vitro studies in rat brain synaptosomes (Bonano et al., 2015). For each drug in the present study, the maximum increase in mean DA and 5-HT was plotted as a function of drug dose, and log-linear regression was used to determine the drug dose producing a 250% increase in DA and 5-HT levels (ED₂₅₀). In vivo neurochemical selectivity was defined as ED₂₅₀ for 5-HT ÷ ED₂₅₀ for DA (i.e., higher numbers indicate higher potency to increase DA versus 5-HT and correspondingly higher DA versus 5-HT selectivity), and this measure of in vivo neurochemical selectivity was compared with previous measures of in vitro neurochemical selectivity (Bonano et al., 2015) using a Pearson correlation test.

The measure of in vivo neurochemical selectivity determined in this study was also correlated to two other measures for these compounds described in previous studies (Bonano et al., 2015; Sakloth et al., 2015). First, it had been found previously that in vitro selectivity of 4-R MCAT analogs to promote monoamine release mediated by DAT versus SERT correlated with a behavioral measure of abuse-related drug effects in ICSS (Bonano et al., 2015). Accordingly, the measure of in vivo neurochemical selectivity obtained in the present study was also correlated with these previously reported measures of maximal abuse-related ICSS effects. Second, it also had been previously reported that a structural predictor of neurochemical and behavioral effects of 4-R MCAT analogs was the volume of the para substituent in the MCAT scaffold (Sakloth et al., 2015). Accordingly, the measure of in vivo neurochemical selectivity obtained in the present study was correlated with the volume of the para substituent (Sakloth et al., 2015). Correlations and statistical analyses were carried out using Prism 6.0 for Mac (GraphPad Scientific, San Diego, CA), and correlations were considered statistically significant if P < 0.05.

Results

Baseline DA and 5-HT Levels and Effects of Amphetamine and Fenfluramine.

Across all microdialysis experiments, baseline (mean ± S.E.M.) NAc extracellular DA and 5-HT levels were 0.49 ± 0.03 and 0.44 ± 0.05 nM, respectively. Supplemental Figs. 1–5 show microdialysis probe placements for all rats included in data analyses.

Effects of saline vehicle, S(+)-amphetamine (0.1–1.0 mg/kg, i.p.) and (±)-fenfluramine (1.0–3.2 mg/kg, i.p.) on NAc DA and 5-HT levels expressed as a percentage of baseline neurotransmitter levels. Left panels indicate temporal changes in % baseline DA, while right panels indicate changes in % baseline 5-HT. Upward arrows indicate time of drug administration. Downward arrows indicate onset of drug effect. Filled symbols indicate statistical significance (P < 0.05) compared with 10 minute monoamine levels within a drug dose. All points show mean ± S.E.M. for seven rats (3.2 mg/kg fenfluramine) or five rats (all other treatments).

Effects of MCAT and Its Para-Substituted Analogs on NAc DA and 5-HT Levels.

Effects of MCAT, 4-F MCAT (flephedrone), and 4-Cl MCAT (clephedrone) on NAc DA and 5-HT levels expressed as a percentage of baseline neurotransmitter levels. All points show mean ± S.E.M. for two rats (3.2 mg/kg MCAT) or five rats (all other treatments); other details are the same as in Fig. 2 .

Effects of 4-CH₃ MCAT (mephedrone), 4-Br MCAT (brephedrone), and 4-OCH₃ MCAT (methedrone) on NAc DA and 5-HT levels expressed as a percentage of baseline neurotransmitter levels. All points show mean ± S.E.M. for five rats; other details are the same as in Fig. 2 .

Correlational Analyses.

Figure 5 shows the maximal % baseline increase in DA and 5-HT levels for MCAT and its five analogs. 4-Cl MCAT (10 mg/kg) produced the largest increase (1260%) in DA levels, whereas 4-OCH₃ MCAT (32 mg/kg) produced the largest increase (2428%) in 5-HT levels. The potencies and selectivities of each drug to increase NAc DA and 5-HT levels are given in Table 1 . Of all the MCAT series analogs, MCAT was the most potent compound to increase DA levels and the most DA versus 5-HT selective releaser. In contrast, 4-OCH₃ MCAT was the least potent compound to significantly alter DA and/or 5-HT levels, and it displayed the lowest selectivity to increase DA versus 5-HT levels.

Peak increases in NAc DA and 5-HT levels produced by each dose of MCAT and each para-substituted MCAT analog. Abscissae: Drug dose in mg/kg (log scale). Ordinates: Maximum increase in % baseline neurotransmitter levels observed at any time after each drug dose as reported in Figs. 3 and ​ and4. 4 . Error bars are omitted for clarity.

TABLE 1

Potency and selectivity of MCAT and its para-substituted analogs to produce a 250% increase in NAc DA and 5-HT levels (ED₂₅₀ values).

Drug	ED250 DA	ED250 5-HT	Selectivity (ED250 5-HT/ ED250 DA)
mg/kg	mg/kg
MCAT	0.37	4.65	12.56
4-F MCAT	0.86	1.07	1.24
4-Cl MCAT	0.93	1.15	1.23
4-Br MCAT	1.46	1.30	0.89
4-CH3 MCAT	1.82	1.12	0.62
4-OCH3 MCAT	11.12	3.61	0.32

Figure 6 shows correlations between the present measures of in vivo neurochemical selectivity and previously reported measures (Bonano et al., 2015) of in vitro neurochemical selectivity to promote monoamine release via DAT and SERT in rat brain synaptosomes (r = 0.95, P < 0.01; Fig. 6A ), and in vivo efficacy to produce abuse-related behavioral effects in an ICSS procedure in rats (r = 0.89, P = 0.02; Fig. 6B ). In addition, there was an inverse correlation between in vivo neurochemical selectivity and steric volume of the para substituent (r = −0.85, P = 0.03; Fig. 6C ) (Sakloth et al., 2015), such that para substituents with smaller volume (i.e., less steric bulk) were more selective to increase NAc DA relative to 5-HT levels.

Correlation of in vivo selectivity to increase NAc DA versus 5-HT with measures of in vitro selectivity to release monoamines via DAT versus SERT in rat brain synaptosomes (A), maximum abuse-related behavioral effects in an ICSS procedure in rats (B), and steric volume of the para substituent on the MCAT scaffold (C). In vitro DAT versus SERT selectivity, maximal ICSS facilitation, and physiochemical parameter volume values have been previously reported (Bonano et al., 2015; Sakloth et al., 2015).

Discussion

This study used an in vivo microdialysis procedure to compare the effects of MCAT and five para-substituted MCAT analogs on NAc DA and 5-HT levels in rats. There were two main findings. First, all MCAT analogs produced dose- and time-dependent changes in NAc DA and/or 5-HT levels. Second, the selectivity of these compounds to increase NAc DA versus 5-HT levels correlated with previously determined measures of 1) in vitro expression of DAT versus SERT selectivity in a rat brain synaptosome procedure; 2) in vivo expression of abuse-related behavioral effects using an ICSS procedure in rats; and 3) the volume of the para substituent on the MCAT scaffold. Results with this set of compounds support the utility of in vitro drug effects on monoamine release in rat brain synaptosomes to predict in vivo drug effects on monoamine release in rat NAc. These results also support the hypotheses that 1) the steric volume of the para substituent on the MCAT scaffold is one determinant of selectivity for DAT versus SERT and 2) that DAT versus SERT selectivity is a determinant of in vivo expression of abuse-related ICSS facilitation.

Consistent with their in vitro DAT versus SERT selectivity profiles (Rothman et al., 2001), S(+)-amphetamine produced a selective increase in NAc DA versus 5-HT levels, whereas fenfluramine produced a selective increase in NAc 5-HT versus DA levels. These results are consistent with previous microdialysis studies in both rats (Baumann et al., 2000; Kehr et al., 2011) and nonhuman primates (Laruelle et al., 1997; Murnane et al., 2010; Sawyer et al., 2012), and confirm the sensitivity of this microdialysis procedure to changes in NAc DA and 5-HT levels produced by agents that have been extensively evaluated. Also consistent with a previous MCAT microdialysis study (Cozzi et al., 2013), MCAT displayed a greater potency to increase NAc DA versus 5-HT levels. The present study extends these previous findings by examining a longer time course of MCAT-induced DA and 5-HT effects and using an i.p. rather than an i.v. route of administration. Also consistent with previous microdialysis results, the addition of a para-methyl group, 4-CH₃ MCAT (mephedrone), significantly altered in vivo DA versus 5-HT selectivity, such that 4-CH₃ MCAT preferentially increased 5-HT versus DA levels (Kehr et al., 2011; Baumann et al., 2012; Wright et al., 2012). The present 4-CH₃ MCAT results extended these previous findings by determining neurochemical effects via the i.p. route of administration. In summary, these MCAT and 4-CH₃ MCAT results provide the empirical framework for interpreting the effects of other para-substituted MCAT analogs and allow for the subsequent correlation with other in vitro neurochemical and in vivo behavioral measures.

The present results extended previous MCAT and 4-CH₃ MCAT findings by determining the in vivo neurochemical effects of four other para-substituted MCAT analogs, which to the best of our knowledge have not been previously reported. Relative to MCAT, the addition of a para-halogen (-F, -Cl, or -Br) produced qualitatively similar neurochemical effects across all three substituents by slightly lowering the potency to increase DA levels and raising the potency to increase 5-HT levels. The result of these changes was a net decrease in DA versus 5-HT selectivity. Addition of the 4-OCH₃ and largest-volume substituent to the MCAT scaffold (i.e., methedrone) produced a further decrease in potency to increase DA levels. Relative to 4-CH₃ and 4-halogenated analogs, 4-OCH₃ MCAT also displayed reduced potency to increase 5-HT levels; however, the decline in potency to release DA was larger than the decline in potency to release 5-HT, and as a result overall DA versus 5-HT selectivity was lower for 4-OCH₃ MCAT than for any other compound examined.

A major goal of the present study was to determine the degree to which in vivo measures of DA versus 5HT selectivity for these MCAT analogs might correlate with in vitro measures of their selectivity to promote monoamine release via DAT and SERT in a rat brain synaptosome preparation (Bonano et al., 2015), and a summary of these correlations is shown in Fig. 7 . The high and significant correlation between selectivity measures in these in vitro and in vivo procedures supports the utility of this in vitro procedure to predict in vivo neurochemical results. Moreover, selectivity of these MCAT analogs in both in vitro and in vivo neurochemical procedures correlates both with 1) the volume of the para substituent on the MCAT scaffold, and 2) the efficacy to produce abuse-related behavioral effects in an ICSS procedure (Bonano et al., 2015; Sakloth et al., 2015). Given the positive correlation between ICSS facilitation and other preclinical and clinical measures of abuse potential (Negus and Miller, 2014), the present results provide evidence for a mechanism whereby molecular features of a drug molecule (in this case the steric volume of the para substituent of the MCAT scaffold) can determine neurochemical selectivity to act at DAT and SERT and ultimately influence behavioral measures of abuse potential.

Correlation summaries of MCAT and its five para-substituted analogs between the steric volume, in vitro DAT versus SERT selectivity in a rat brain synaptosome procedure, in vivo DA versus 5-HT selectivity in an in vivo microdialysis procedure, and maximal facilitation in an ICSS procedure. Correlations between the physiochemical parameter volume, in vitro DAT versus SERT selectivity, and maximal ICSS facilitation have been previously reported (Bonano et al., 2015; Sakloth et al., 2015).

The present results with MCAT analogs also agree with studies that examined para-substituted analogs of either amphetamine or methamphetamine. For example, both 4-F amphetamine (para-fluroamphetamine, also known as PAL-303) and 4-CH₃ amphetamine (para-methylamphetamine, also known as PAL-313) were more potent to increase NAc 5-HT versus DA levels (Baumann et al., 2011). Furthermore, 4-OCH₃ methamphetamine (para-methoxymethamphetamine) had a DA versus 5-HT selectivity ratio of 0.28 (Matsumoto et al., 2014), which is consistent with the DA versus 5-HT selectivity ratio of 0.32 for 4-OCH₃ MCAT in the present study. Unfortunately, only DA levels have been reported for 4-Br amphetamine (Zsilla et al., 1981) and 4-Cl amphetamine (Hiramatsu and Cho, 1990; Johnson et al., 1990), thus precluding a direct comparison of in vivo selectivities between para-substituted amphetamine or methamphetamine analogs and the present results. Nonetheless, these results suggest that neurochemical and behavioral effects associated with para substitutions to the MCAT scaffold may also generalize to effects associated with effects of para-substituted analogs of amphetamine and methamphetamine.

In the present study, all compounds were administered i.p. to match the route of administration used in previous behavioral studies with these compounds in an ICSS procedure of abuse-potential assessment (Bauer et al., 2013; Bonano et al., 2014, 2015). A striking outcome using this route of administration was that DA effects displayed both a slower onset and longer duration than 5-HT effects, and this dissociation in time course of drug effects on DA and 5-HT may be related to route of administration. For example, i.v. 4-CH₃ MCAT produced coincident peak DA and 5-HT effects within 20 minutes (Baumann et al., 2011), whereas DA effects were more delayed compared with 5-HT effects after s.c. administration (Kehr et al., 2011; Wright et al., 2012), and DA effects were even more delayed after i.p. administration (present study). Moreover, the relatively slow onset of DA effects after i.p. administration of MCAT analogs in this study agrees with the slow onset of DA effects reported in previous studies of amphetamine and other para-halogenated or para-methoxy phenethylamines administered by the i.p. route (Marona-Lewicka et al., 1995; Matsumoto et al., 2014). The reason for the apparent route of administration–dependent differences in DA versus 5-HT time course effects remains to be elucidated.

The present results also afforded an opportunity to compare potency and time course of monoamine releaser effects in rats on in vivo NAc DA and 5-HT levels (present study) and previously published behavioral effects in our ICSS procedure (Bauer et al., 2013; Bonano et al., 2014, 2015). The abuse-related facilitation of low ICSS rates by monoamine releasers is thought to be mediated primarily by increases in DA, whereas abuse-limiting depression of high ICSS rates by these compounds is thought to be mediated primarily by increases in 5-HT (Wise, 1998; Negus and Miller, 2014). However, different relationships were observed between microdialysis measures of NAc DA and ICSS facilitation on the one hand and NAc 5HT and ICSS depression on the other hand. Specifically, DA-selective monoamine releasers such as amphetamine and MCAT displayed a lower potency, slower onset, and longer duration of action to increase NAc DA levels than to facilitate ICSS. Conversely, serotonin-selective releasers such as fenfluramine and 4-OCH₃ MCAT displayed a similar or higher potency, similar rate of onset, and shorter duration of action to increase NAc 5-HT levels than to depress ICSS. Thus, microdialysis measures of NAc DA served as a lagging and less-sensitive indicator than ICSS facilitation of monoamine releaser-induced DA effects, whereas measures of NAc 5-HT were a leading and more-sensitive indicator than ICSS depression of monoamine releaser-induced 5-HT effects.

In conclusion, the in vivo neurochemical effects of MCAT and five para-substituted MCAT analogs support the hypothesis that selectivity of monoamine releasers to elevate DA versus 5-HT levels via the respective monoamine transporters is a significant contributor to the abuse-related behavioral effects of monoamine releasers in ICSS studies. Although potency differences were noted, all six compounds examined produced similar maximal increases in extracellular DA levels. Thus, the in vivo selectivity differences observed in the present study were mostly driven by decreased potency to increase extracellular 5-HT levels. Furthermore, the present results provide an in vivo neurochemical connection with previous in vitro neurochemical and behavioral effects of these compounds (Bauer et al., 2013; Bonano et al., 2015). Given the purported direct and indirect connections between serotonergic pathways and the mesolimbic DA pathway (Alex and Pehek, 2007), the present results provide further neuropharmacological evidence for the abuse-limiting effects of 5-HT.

Supplementary Material

Abbreviations

Da	dopamine
DAT	dopamine transporter
5-HT	serotonin
ICSS	intracranial self-stimulation
MCAT	methcathinone
NAc	nucleus accumbens
SERT	serotonin transporter

Psychological Disorders for the MCAT: Everything You Need to Know

Learn key MCAT concepts about psychological disorders, plus practice questions and answers

(Note: This guide is part of our MCAT Psychology and Sociology series .)

Part 1: Introduction to psychological disorders

Part 2: Psychotic, depressive, and related disorders

a) Schizophrenia

b) Depressive disorders

c) Bipolar disorders

d) Personality disorders

e) Somatic symptom disorders

Part 3: Behavioral and related disorders

a) Obsessive-compulsive disorder

b) Body dysmorphic and eating disorders

c) Post-traumatic stress disorder

d) Anxiety disorders

e) Dissociative disorders

Part 4: Additional neurological disorders

a) Aphasia

b) Parkinson’s disease

c) Alzheimer’s disease

Part 5: High-yield terms

Part 6: Passage-based questions and answers

Part 7: Standalone questions and answers

Part 1: Introduction to psychological disorders

Psychological disorders are a characteristic set of feelings, thoughts, or behaviors that differ from the cultural norm and can cause distress to the individual suffering from them.

The Diagnostic and Statistical Manual of Mental Disorders (DSM) is a standardized resource used to assist clinicians in classifying and diagnosing these disorders. There are 20 different classes of psychological disorders defined by the DSM. In recent decades, we have learned a great amount about the physiology of these disorders and are able to classify them with increasing specificity. Within this guide, we’ll go over several key types of psychological disorders, their biological basis, and much more.

It may also be helpful to keep in mind the biomedical and biopsychosocial approaches to treatment. These are frameworks, or perspectives, used to guide professional therapies and treatment for a psychological disorder.

The biomedical approach views the root of these disorders as a physiological imbalance or disturbance. If this is true, then the method of treatment should also be biomedical in nature, aiming to reduce the symptoms of the disorder through a strictly scientific, neurochemical approach. This is a much more narrow scope of approach than the biopsychosocial approach because this approach fails to focus on additional stressors and factors in an individual’s life that may contribute to or exacerbate the disorder. For instance, an anxiety disorder could be due to neurotransmitter imbalance within emotional regions of the brain (a biomedical explanation) but could also be due to patterns of neglect in a family (an environmental factor).

The biopsychosocial approach is a more holistic view of psychological disorders. This approach considers biomedical, psychological, and social factors when considering stressors that may exacerbate the disorder. As you may have presumed, biomedical factors refer to an individual’s physiology; psychological factors refer to their thoughts and emotions; and social factors come from environmental, societal factors that are beyond the individual’s control. Under this framework, biopsychosocial treatment may combine direct therapy with a patient (such as prescribing medication or through one-on-one therapeutic sessions) and indirect therapy, which provides support for the individual with help from family, friends, or other components of the individual’s social network.

This is a high-yield topic for the MCAT. Let’s begin!

Part 2: Psychotic, depressive, and related disorders

Many psychological disorders appear to be somewhat hereditary or genetic. We will first address some of these disorders, starting with the psychotic disorders.

a) Schizophrenia

Individuals who suffer from psychotic disorders suffer from psychosis. Psychosis can manifest in feelings of paranoia, delusions, hallucinations, and a general loss of reality.

While the DSM lists several forms of psychotic disorders, the MCAT will focus on schizophrenia as the representative psychotic disorder. Schizophrenia is characterized by the presence of several positive symptoms that characterize psychosis, along with negative symptoms that further illustrate a deviation from normal behavior.

The prodromal phase, or prodrome, is a period of time directly before a diagnosis of schizophrenia. As this phase is primarily characterized by an abrupt change in behavior, family members and friends may notice “odd” behavior for a period of weeks or months, including a withdrawal from typical social activities and mood swings. The end of prodrome is marked by the onset of positive and negative symptoms of schizophrenia.

Positive symptoms refer to behaviors or thoughts that are exhibited in addition to an individual’s normal behavior, such as delusions, hallucinations, or nervous tics.

Positive symptoms come in many shapes and sizes. These behaviors are considered “symptoms” because they differ from the social norm of the culture in which they are observed. Hallucinations or delusions, for example, are considered commonplace in some Caribbean cultures as part of rituals, but in western culture, these may seem like symptoms.

Hallucinations are false observations that are not based on reality but are perceived as such. These can be auditory, like voices in someone’s head, or visual, like seeing a dead relative. Auditory hallucinations are more common than visual.

Delusions are held beliefs that directly contradict what is observed in reality. Delusions are usually firmly held onto by an individual and are generally not shared with multiple people.

Individuals may also begin to exhibit disorganized thoughts and behaviors. Patients with disorganized thought may be unable to express a cohesive narrative in conversation and instead express haphazard thoughts. Word salad is an extreme example of this, when the expressed thoughts are simply random words strung together. Disorganized behaviors follow a similar theme and refer to when an individual is unable to carry out their normal routine.

Negative symptoms refer to the lack or absence of an individual’s normal behavior, like the inability to eat or lack of emotion (or lack of affect). Affect refers to the display and conveying of emotion. Someone with flat affect displays virtually no emotion at all, while someone with inappropriate affect displays affect that is discordant with the individual’s speech or behavior.

Schizophrenia is also often associated with the downward drift hypothesis, stating that the symptoms of schizophrenia can lead to a decline in social wealth and resources, putting the individual at greater risk for experiencing worsening social factors and increasing the intensity of symptoms. This leads to a vicious cycle of worsening schizophrenia and socioeconomic status.

Based on genetic studies, the onset of schizophrenia seems to have a high hereditary factor. On a neurotransmitter level, individuals with schizophrenia appear to have elevated levels of dopamine in the brain. Neuroleptics (dopamine receptor antagonists) are used to treat schizophrenia.

b) Depressive disorders

While mood swings and natural sadness may last for hours or days, depressive disorders are characterized by an abnormally long period of sadness or intense feeling. Major depressive disorder is a mood disorder and is characterized by at least one major depressive episode. To be diagnosed with a major depressive episode, the patient must experience feelings of depression or sadness for at least two weeks and experience at least five symptoms from the following list:

• low or depressed mood
• anhedonia (loss of interest in previously interesting activities)
• changes in appetite and weight gain or loss
• sleep disturbances
• persistent feelings of guilt
• difficulty concentrating
• thoughts of death or suicide
• decreased energy in day-to-day activity

Depression is highly associated with an overactive amygdala: a small structure within the brain that controls basal emotions, such as fear and anxiety. The monoamine theory of depression describes a key physiological feature associated with depression: decreased levels of the neurotransmitters serotonin, dopamine, and norepinephrine levels.

Patients who experience depressed mood that is not severe enough to be diagnosed as a major depressive disorder may be diagnosed with dysthymia. Dysthymia is often diagnosed in individuals who experience a depressed and low mood for at least 2 years but who do not experience other symptoms of major depressive disorder.

Seasonal affective disorder (SAD) is not an isolated diagnosis in the DSM. Individuals who do not experience major depressive disorder or dysthymia may still experience a persistent depressive mood during the winter months. It is hypothesized that the lack of sunlight during the winter causes disruptions in melatonin metabolism, thereby affecting mood. As a result, bright light therapy is typically prescribed as a therapeutic method.

c) Bipolar disorders

Bipolar disorders are characterized by some combination of manic episodes (or hypomania) and depressive episodes.

Manic episodes are periods characterized by intensely high energy, high productivity, decreased need for sleep, and/or thoughts of grandeur. These episodes of elevated mood must last more than one week. Similarly, hypomania is an increased level of excitability; however, it is distinguished from mania as it does not inhibit the individual’s normal routines to the same degree. Depressive episodes will manifest similarly to the symptoms listed in the depressive disorders, with periods of persistent low mood and anhedonia.

There are three forms of bipolar disorder, each characterized by manic and depressive episodes occurring to differing degrees.

1. A diagnosis of bipolar I disorder requires documented manic episodes but may or may not require depressive episodes.
2. A diagnosis of bipolar II disorder requires documented hypomania, with at least one major depressive episode.
3. A diagnosis of cyclothymia requires a combination of hypomanic episodes and periods of dysthymia. Note that these periods of hypomania and dysthymia do not need to be as intense as periods of mania or depression.

Bipolar I disorder, bipolar II disorder, and cyclothymia all involve differing degrees of mania and depression.

The monoamine/catecholamine theory of depression explains the origin of mania and depression from a neurological transmitter. Under this theory, an overabundance or paucity of norepinephrine and serotonin leads to mania and depression, respectively.

d) Personality disorders

Personality disorders manifest in behavior patterns that appear to be erratic or strange by cultural standards. These disorders tend to warp the individual’s emotions, interpersonal functioning, and cognition in a way that results in impulsive actions and faulty explanations. Importantly, individuals with personality disorders tend to consider their behaviors and thoughts to be completely normal! They do not necessarily recognize these erratic thoughts and behaviors as abnormal or distressing.

While there are many subtypes of personality disorders, they are generally clustered into three overlapping groups. For the MCAT, it will be sufficient to distinguish disorders as belonging to Cluster A, Cluster B, or Cluster C.

Cluster A disorders include paranoid, schizotypal, and schizoid personality disorders. Paranoid personality disorder leads individuals to have a very strong distrust of others and their motives. Schizotypal personality disorders are characterized by eccentric thinking unaccepted by the cultural norm. Schizoid personality disorder applies to individuals who are disinterested in or detached from personal relationships with others. This cluster may be well-remembered as the “weird” disorders.