SHORT-DURATION AND LONG-DURATION RESPONSES TO LEVODOPA DURING THE FIRST YEAR OF LEVODOPA THERAPY

We examined the response to 2-hour levodopa infusions in 18 Parkinson' s disease subjects before starting long-term levodopa treatment and af ter 6 and 12 months of treatment using tapping speed as an index of br adykinesia. The long-duration response (LDR), measured as the increase in baseline (overnight without levodopa) tapping speed, increased by 29 +/- 18 at 6 months and by 35 +/- 24 at 12 months. The magnitude of the short-duration response (SDR) to a 2-hour levodopa infusion after an overnight levodopa withdrawal did not differ at 6 and 12 months (16 +/- 8 and 20 +/- 13 taps/min) from that before long-term levodopa (21 +/- taps/min). However, when levodopa was withheld for 3 days, it was evident that the SDR magnitude was increasing in magnitude (19, 23, a nd 31 taps/min at 0, 6, and 12 months). Duration of SDR did not change . A diurnal motor pattern with faster tapping speeds in the morning an d slower in the evening was apparent on the days no levodopa was admin istered. These observations indicate (1) the LDR is responsible for mu ch of the sustained response to levodopa during the first year of trea tment, (2) the SDR magnitude increases but is obscured by the LDR, and (3) a diurnal pattern of motor function is present that may be partia lly responsible for the poorer motor function in the afternoons and ev enings.