Effect of acute and chronic administration of 1,2,3,4-tetrahydroisoquinoline on muscle tone, metabolism of dopamine in the striatum and tyrosine hydroxylase immunocytochemistry in the substantia nigra, in rats.

The effects of acute and chronic administration of 1,2,3,4-tetrahydroisoquinoline, an endogenous substance suspected of producing parkinsonism in humans, on the muscle tone and metabolism of dopamine in the striatum, and on the number of tyrosine hydroxylase-immunoreactive cells in the substantia nigra were investigated in rats. Muscle tone was examined using a combined mechanomyographic and electromyographic method which measured simultaneously the muscle resistance of the rat's hind foot to passive extension and flexion in the ankle joint and electromyographic activity of the antagonistic muscles of that joint: gastrocnemius and tibialis anterior. 1,2,3,4-Tetrahydroisoquinoline administered at doses of 50 and 100 mg/kg intraperitoneally for 19 days increased muscle resistance 1 h after the first injection (acute treatment), 1 h after the last injection (chronic treatment) and three days after compound withdrawal. Rigidity observed on the third day of 1,2,3,4-tetrahydroisoquinoline withdrawal was accompanied by an increased tonic (resting) electromyographic activity of the gastrocnemius and tibialis anterior muscles. At the same time, a significant reduction in the number of tyrosine hydroxylase-immunoreactive neurons in the substantia nigra and a decrease in the dopamine level in the striatum were also found. A declining number of tyrosine hydroxylase-immunoreactive neurons in the whole substantia nigra showed a significant negative correlation with the enhanced muscle resistance, as well as with the tonic electromyographic activity recorded at rest, i.e. before the start of movements, from the gastrocnemius and tibialis anterior muscles. Our results suggest that 1,2,3,4-tetrahydroisoquinoline may be one of the endogenous substances involved in the progress of Parkinson's disease.