Effects of L-threo-dihydroxyphenylserine on spinal reflexes and decerebrate rigidity in rats.

1. The effects of a noradrenaline precursor, L-threo-3,4-dihydroxyphenylserine (L-DOPS), on spinal mono-(MSR) and polysynaptic reflexes (PSR) and decerebrate rigidity, were studied. 2. Although a low dose of L-DOPS (10 mg/kg, i.v.) did not affect MSR or PSR in C1 spinal rats, high doses of L-DOPS (50 and 100 mg/kg, i.v.) moderately enhanced the amplitudes of both MSR and PSR. 3. Clorgyline-HCl (1 mg/kg, i.v.), an MAO inhibitor, enhanced the excitatory effects of low-dose L-DOPS (10 mg/kg, i.v.) on both reflexes. 4. Benserazide-HCl (1 mg/kg, i.v.), an L-aromatic amino acid decarboxylase inhibitor, decreased the pressor effect, but not the stimulatory effects, of L-DOPS (100 mg/kg, i.v.) on MSR and PSR. 5. L-DOPS (300 mg/kg, i.p. or i.d.) did not affect the muscle tone of rigid hindlimbs caused by radio frequency lesioning of the midbrain. 6. These results suggest that the moderate enhancing effects of L-DOPS on MSR and PSR are due to conversion of L-DOPS to noradrenaline in the spinal cord.