Dopaminergic mediation of the discriminative stimulus functions of modafinil in rats

Characterization of the Neurochemical and Behavioral Effects of Solriamfetol (JZP-110), a Selective Dopamine and Norepinephrine Reuptake Inhibitor

Excessive sleepiness (ES) is associated with several sleep disorders, including narcolepsy and obstructive sleep apnea (OSA). A role for monoaminergic systems in treating these conditions is highlighted by the clinical use of US Food and Drug Administration-approved drugs that act on these systems, such as dextroamphetamine, methylphenidate, modafinil, and armodafinil. Solriamfetol (JZP-110) is a wake-promoting agent that is currently being evaluated to treat ES in patients with narcolepsy or OSA. Clinical and preclinical data suggest that the wake-promoting effects of solriamfetol differ from medications such as modafinil and amphetamine. The goal of the current studies was to characterize the mechanism of action of solriamfetol at monoamine transporters using in vitro and in vivo assays. Results indicate that solriamfetol has dual reuptake inhibition activity at dopamine (DA; IC₅₀ = 2.9 μM) and norepinephrine (NE; IC₅₀ = 4.4 μM) transporters, and this activity is associated in vivo with increased extracellular concentration of DA and NE as measured by microdialysis. Solriamfetol has negligible functional activity at the serotonin transporter (IC₅₀ > 100 μM). Moreover, the wake-promoting effects of solriamfetol are probably owing to activity at DA and NE transporters rather than other neurotransmitter systems, such as histamine or orexin. The dual activity of solriamfetol at DA and NE transporters and the lack of significant monoamine-releasing properties of solriamfetol might explain the differences in the in vivo effects of solriamfetol compared with modafinil or amphetamine. Taken together, these data suggest that solriamfetol may offer an important advancement in the treatment of ES in patients with narcolepsy or OSA.

The unique psychostimulant profile of (±)-modafinil: investigation of behavioral and neurochemical effects in mice

Blockade of dopamine (DA) reuptake via the dopamine transporter (DAT) is a primary mechanism identified as underlying the therapeutic actions of (±)-modafinil (modafinil) and its R-enantiomer, armodafinil. Herein, we explored the neurochemical and behavioral actions of modafinil to better characterize its psychostimulant profile. Swiss-Webster mice were implanted with microdialysis probes in the nucleus accumbens shell (NAS) or core (NAC) to evaluate changes in DA levels related to acute reinforcing actions of drugs of abuse. Additionally, subjective effects were studied in mice trained to discriminate 10 mg/kg cocaine (i.p.) from saline. Modafinil (17-300 mg/kg, i.p.) significantly increased NAS and NAC DA levels that at the highest doses reached ~300% at 1 h, and lasted > 6 h in duration. These elevated DA levels did not show statistically significant regional differences between the NAS and NAC. Modafinil produced cocaine-like subjective effects at 56-100 mg/kg when administered at 5 and 60 min before the start of the session, and enhanced cocaine effects when the two were administered in combination. Despite sharing subjective effects with cocaine, modafinil's psychostimulant profile was unique compared to that of cocaine and like compounds. Modafinil had lower potency and efficacy than cocaine in stimulating NAS DA. In addition, the cocaine-like subjective effects of modafinil were obtained at lower doses and earlier onset times than expected based on its dopaminergic effects. These studies suggest that although inhibition of DA reuptake may be a primary mechanism underlying modafinil's therapeutic actions, non DA-dependent actions may be playing a role in its psychostimulant profile.

Identification of Treatment Targets in a Genetic Mouse Model of Voluntary Methamphetamine Drinking

Methamphetamine has powerful stimulant and euphoric effects that are experienced as rewarding and encourage use. Methamphetamine addiction is associated with debilitating illnesses, destroyed relationships, child neglect, violence, and crime; but after many years of research, broadly effective medications have not been identified. Individual differences that may impact not only risk for developing a methamphetamine use disorder but also affect treatment response have not been fully considered. Human studies have identified candidate genes that may be relevant, but lack of control over drug history, the common use or coabuse of multiple addictive drugs, and restrictions on the types of data that can be collected in humans are barriers to progress. To overcome some of these issues, a genetic animal model comprised of lines of mice selectively bred for high and low voluntary methamphetamine intake was developed to identify risk and protective alleles for methamphetamine consumption, and identify therapeutic targets. The mu opioid receptor gene was supported as a target for genes within a top-ranked transcription factor network associated with level of methamphetamine intake. In addition, mice that consume high levels of methamphetamine were found to possess a nonfunctional form of the trace amine-associated receptor 1 (TAAR1). The Taar1 gene is within a mouse chromosome 10 quantitative trait locus for methamphetamine consumption, and TAAR1 function determines sensitivity to aversive effects of methamphetamine that may curb intake. The genes, gene interaction partners, and protein products identified in this genetic mouse model represent treatment target candidates for methamphetamine addiction.