Abuse-Related Neurochemical Effects of Para-Substituted Methcathinone Analogs in Rats: Microdialysis Studies of Nucleus Accumbens Dopamine and Serotonin

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-CH3, 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.

Fig. 1.

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

Fig. 2.

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).

Fig. 3.

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.

Fig. 4.

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.

Fig. 5.

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 4. Error bars are omitted for clarity.

Fig. 6.

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).

Fig. 7.

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).