Ethanol's effects on tremor and positioning in squirrel monkeys

Ethanol ingestion markedly reduces tremor in patients with essential tremor. This clinical observation prompted the present experiments, which were designed to investigate ethanol's reduction of tremor in squirrel monkeys trained to execute a bar-holding task. A lever was attached to the hub of a rotary variable differential transformer (RVDT) and three squirrel monkeys were trained to position this lever within a 4.5 cm band for 8 seconds for a fruit juice reward. Behavior was maintained by a random ratio 2 schedule of reinforcement. Angular position of the lever was sampled for 5.12 seconds while the monkey held the bar, differentiated twice and analyzed to obtain a spectral description of tremor in units of acceleration 2/Hz. During control and vehicle sessions a spectral peak appeared at about 6-8 Hz and the magnitude of this peak varied from 25 to 150 milli-g2/Hz (where g is the acceleration due to gravity). A second peak appeared in two animals at greater than 15 Hz. For one animal this high-frequency peak was dominant during control sessions but the 6-8 Hz peak was dominant after intubation with water or ethanol. Ethanol produced consistent and dose-related decreases in the amplitude of the spectrum describing tremor but the location of the spectral peaks did not differ from vehicle sessions. The doses that altered tremor also produced an increase in the number of short-duration holds as well as other, less consistent, alterations in the form of the response. These data confirm and quantify ethanol's potency as a tremorolytic agent.