Following the completion of PCA training and the selection of STs and GTs, chronic indwelling catheters were implanted into the right jugular veins, as described previously (e.g. Pitchers et al., 2017b (link)). Briefly, animals were anesthetized initially with 5% isoflurane in an anaesthetic chamber (Anesco/Surgivet) then maintained with 2% isoflurance via nose cone. Gas was carried via oxygen at a flow rate of 0.6 L/min. Animals’ body temperatures were maintained at 37 °C using Deltaphase isothermal pads (Braintree Scientific) and ophthalmic ointment were provided for lubrication of the eyes. Post-operative pain was managed with carprofen (5 mg/kg, s.c.), given prior to surgery and for 48 h thereafter. The catheter exited through the dorsal skin surface between scapulae. Following surgery, catheters were flushed daily with 0.2 mL of sterile saline containing 5 mg/mL gentamicin sulphate (Vedco) to prevent occlusions and minimize infections. Catheter patency was tested periodically by intravenous (IV) injection of 0.15 mL of methohexital sodium (10 mg/mL in sterile water; JHP Pharmaceuticals). Two animals were removed because they failed to become ataxic after infusion; the data from these animals were discarded from analyses.