Kynurenic acid

All drugs were obtained from Sigma-Aldrich (St. Louis, MO, USA) except tetrodotoxin citrate (TTX), kynurenic acid, and 6,7-dinitroquinoxaline-2,3-dione disodium salt (DNQX), which were purchased from Tocris Bioscience (Ellisville, MO, USA). Stock solutions of bicuculline methyl bromide, allopregnanolone, and picrotoxin were dissolved in DMSO, while stock solutions of kynurenic acid and mefenamic acid were made up in 0.1 M sodium hydroxide. The final bath concentrations of these solvents did not exceed 0.1% of the recording solutions. Stock solutions of DNQX, phenobarbital sodium salt, chlordiazepoxide hydrochloride, d-2-amino-5-phosphonovalerate (AP5), tetraethylammonium chloride (TEA), and strychnine hydrochloride were made up in bath solution. All drug solutions were made fresh immediately prior to experiments except for TTX, which was stored at −20 °C in small aliquots of stock solution made up in deionized water. Drugs were applied directly to cells under voltage-clamp from the tip of a 250-μm pipette. Fresh bath solution was also perfused through the bath (at 1–2 mL/min) using a gravity-feed system to prevent any build-up of drug solutions in the bath. At least 2 or 3 control responses were recorded before the addition of any drug. Drugs were washed out once a stable effect was observed and control responses then re-established before further drug applications.

To confirm that evoked IPSCs were, in fact, GABAergic, 1 μM gabazine (SR 95531, Tocris) was applied at the end of STDP recordings. To see the effect of GABAergic synaptic transmission on EPSP kinetics, 1 μM gabazine was applied for 5 min to a separate set of mouse neurons while evoking EPSPs at a rate of 0.14 Hz, as described above. To confirm that evoked EPSCs were, in fact, glutamatergic, 2 mM kynurenic acid (Tocris) was applied at the end of STDP recordings (Supplementary Figure 8F).
To see the effect of DA bath application on EPSP kinetics and basic cellular properties, 20 μM DA was prepared in a light protected beaker to prevent oxidation by light. Dopamine was then applied for 5 min to mouse and human neurons while evoking EPSPs at a rate of 0.14 Hz, as described above.

Baclofen, CGP55845, kynurenic acid were from Tocris (Avonmouth, UK), while pyruvic acid was from MP Biomedicals (Irvine, CA, USA). All other drugs and reagents were from Sigma (St. Louis, MO, USA).