L dopa

Diphtheria toxin (List Biological Laboratories, Campbell, CA, USA), with a half-life of ~18 h, was dissolved in 0.9% saline and administered intramuscularly [13 (link)]. L-Dopa (10 mg; Abcam #Ab142497), with an estimated half-life of 1 h, was dissolved in phosphate-buffered saline (PBS) (with 25 mg of ascorbic acid) for a concentration of 1 mg/mL [38 (link)]. The decarboxylase inhibitor benserazide (Abcam #Ab143181) was combined with the L-Dopa solution for a concentration of 0.48 mg/mL. benserazide was used reduce some of L-Dopa’s untoward effects such as nausea. It may also prolong the half-life of L-Dopa. Propranolol HCl (Tocris # 0624), with a half-life of 3–6 h, was dissolved in 0.9% saline at concentrations of 0.4 mg/mL and 1.6 mg/mL. Propranolol and L-Dopa/benserazide were administered by separate i.p. injections. The vehicle consisted of an equal volume of 0.9% saline (0.01 mL/gm mouse). Unless specified in the text, chemicals and drugs were obtained from Sigma (St. Louis, MO, USA).

Rats were trained to consume controlled doses of L-DOPA suspended in sweetened, water-based vehicle from a syringe (Chesler et al, unpublished data, July 2019). Once trained, L-DOPA–treated rats consumed 20 mg/kg L-DOPA/5 mg/kg carbidopa voluntarily from a syringe. The L-DOPA was prepared with 10 mg/mL L-DOPA (Sigma-Aldrich, St. Louis, MO) and 2.5 mg/mL carbidopa monohydrate (Abcam, Cambridge, MA) to emulate a commonly prescribed drug combination for Parkinson's disease.³³ (link) Carbidopa is a peripheral decarboxylase inhibitor that minimizes the amount of L-DOPA that oxidizes to dopamine in the periphery to maximize the amount that can pass the blood–brain barrier and act on retinal and neuronal tissue. Once combined, the solution was vortexed thoroughly in a sweetened, water-based vehicle solution (MediDrop Sucralose; Clear H2O, Portland, ME) to create a homogenous suspension of L-DOPA/carbidopa. The L-DOPA/carbidopa cocktail was made fresh weekly, stored in a refrigerated, light-proof container, thoroughly remixed before use and kept cold during use. We were cautious to select a dosage that was lower than the typical dosage used to induce dyskinesia (25 mg/kg)³⁴ (link) yet could still increase retinal dopamine levels.³⁵ (link)