Interaction Profiles of Central Nervous System Active Drugs at Human Organic Cation Transporters 1–3 and Human Plasma Membrane Monoamine Transporter

Many psychoactive compounds have been shown to primarily interact with high-affinity and low-capacity solute carrier 6 (SLC6) monoamine transporters for norepinephrine (NET; norepinephrine transporter), dopamine (DAT; dopamine transporter) and serotonin (SERT; serotonin transporter). Previous studies indicate an overlap between the inhibitory capacities of substances at SLC6 and SLC22 human organic cation transporters (SLC22A1–3; hOCT1–3) and the human plasma membrane monoamine transporter (SLC29A4; hPMAT), which can be classified as high-capacity, low-affinity monoamine transporters. However, interactions between central nervous system active substances, the OCTs, and the functionally-related PMAT have largely been understudied. Herein, we report data from 17 psychoactive substances interacting with the SLC6 monoamine transporters, concerning their potential to interact with the human OCT isoforms and hPMAT by utilizing radiotracer-based in vitro uptake inhibition assays at stably expressing human embryonic kidney 293 cells (HEK293) cells. Many compounds inhibit substrate uptake by hOCT1 and hOCT2 in the low micromolar range, whereas only a few substances interact with hOCT3 and hPMAT. Interestingly, methylphenidate and ketamine selectively interact with hOCT1 or hOCT2, respectively. Additionally, 3,4-methylenedioxymethamphetamine (MDMA) is a potent inhibitor of hOCT1 and 2 and hPMAT. Enantiospecific differences of R- and S-α-pyrrolidinovalerophenone (R- and S-α-PVP) and R- and S-citalopram and the effects of aromatic substituents are explored. Our results highlight the significance of investigating drug interactions with hOCTs and hPMAT, due to their role in regulating monoamine concentrations and xenobiotic clearance.

Overlap and Specificity in the Substrate Spectra of Human Monoamine Transporters and Organic Cation Transporters 1, 2, and 3

Human monoamine transporters (MATs) are cation transporters critically involved in neuronal signal transmission. While inhibitors of MATs have been intensively studied, their substrate spectra have received far less attention. Polyspecific organic cation transporters (OCTs), predominantly known for their role in hepatic and renal drug elimination, are also expressed in the central nervous system and might modulate monoaminergic signaling. Using HEK293 cells overexpressing MATs or OCTs, we compared uptake of 48 compounds, mainly phenethylamine and tryptamine derivatives including matched molecular pairs, across noradrenaline, dopamine and serotonin transporters and OCTs (1, 2, and 3). Generally, MATs showed surprisingly high transport activities for numerous analogs of neurotransmitters, but their substrate spectra were limited by molar mass. Human OCT2 showed the broadest substrate spectrum, and also the highest overlap with MATs substrates. Comparative kinetic analyses revealed that the radiotracer meta-iodobenzylguanidine had the most balanced uptake across all six transporters. Matched molecular pair analyses comparing MAT and OCT uptake using the same methodology could provide a better understanding of structural determinants for high cell uptake by MATs or OCTs. The data may result in a better understanding of pharmacokinetics and toxicokinetics of small molecular organic cations and, possibly, in the development of more specific radiotracers for MATs.

Effects of intranasal instillation of nanoparticulate matter in the olfactory bulb

Nanoparticulate matter activates the aryl hydrocarbon receptor (AhR) pathway in the respiratory system in a process involving the AhR nuclear translocator (ARNT) and cytochrome P450 family 1, member A1 (CYP1A1). We examined changes in AhR-related pathways following intranasal instillation of nanoparticulate matter in the olfactory bulb and cerebral cortex. Twice a day for 5 days per week for 1 week or 2 weeks, 8-week-old Sprague–Dawley rats were intranasally instilled with 10 µL nanoparticulate matter (nano group; n = 36). An equal volume of saline was intranasally instilled in control rats (n = 36). One week after intranasal instillation, olfactory function and Y-maze tests were performed. The expression levels of AhR in the olfactory bulb and temporal cortex were analyzed using western blotting and immunofluorescence assays. The expression levels of AhR, CYP1A1, inducible nitric oxide synthase (iNOS), and five genes encoding cation transporters (ARNT, ATP7B, ATPB1, OCT1, and OCT2) in the olfactory bulb were analyzed using quantitative reverse transcription. The olfactory discrimination capability was reduced in the nano group compared with the control group. Proportional changes in the Y-maze test were not significantly different between the nano and control groups. AhR mRNA and protein expression in the olfactory bulb increased 1.71-fold (P < 0.001) and 1.60-fold (P = 0.008), respectively. However, no significant changes were observed in the temporal cortex. In the olfactory bulb, the expression of ARNT, ATP7B, ATPB1, and OCT2 was downregulated. CYP1A1 and iNOS expression in the olfactory bulb was upregulated compared with that in the temporal cortex. The intranasal instillation of nanoparticulate matter decreased the olfactory discrimination ability, which was accompanied by upregulation of AhR expression and downregulation of cation transporters in the olfactory bulb.