Activation of retinal tyrosine hydroxylase: tolerance induced by chronic treatment with haloperidol does not modify...

A single dose of haloperidol administered to rats in the dark increases the activity of retinal tyrosine hydroxylase. The ability of haloperidol to activate the enzyme is diminished 24 hr after terminating 22 to 30 days of treatment with haloperidol. The retinal enzyme is also tolerant to activation by treatment with chlorpromazine. In contrast, exposure of the animals to light activates the enzyme to the same extent in chronic haloperidol-treated and control animals. Thus, chronic haloperidol treatment does not modify the ability of the retinal enzyme system to respond to the physiological stimulus, light. Apparently, activation of retinol tyrosine hydroxylase by haloperidol and light occurs by independent mechanisms.