Kl. Double et Ad. Crocker, DOPAMINE-RECEPTORS IN THE SUBSTANTIA-NIGRA ARE INVOLVED IN THE REGULATION OF MUSCLE TONE, Proceedings of the National Academy of Sciences of the United Statesof America, 92(5), 1995, pp. 1669-1673
The aim of the present study was to localize the dopamine receptors in
volved in the regulation of muscle tone. A strategy was used whereby t
he effects on muscle tone of injecting the irreversible dopamine recep
tor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) i
n discrete brain regions were assessed. Increases in muscle tone were
measured as changes in electromyographic activity of the gastrocnemius
and tibialis muscles of conscious, unrestrained rats. No increases in
muscle tone were found after injections of EEDQ into the anterior and
posterior striatum, which produced marked reductions in dopamine rece
ptor concentration. The effects on muscle tone of injecting EEDQ into
the substantia nigra pars reticulata were also assessed. Large increas
es in muscle tone were observed associated with inactivation of either
D-1 or D-2 dopamine receptors in the substantia nigra. The increased
muscle tone was not reduced by subcutaneous administration of apomorph
ine, despite the presence of a normal population of striatal dopamine
receptors. These findings provide evidence that dopamine receptors in
the substantia nigra play an important role in the regulation of muscl
e tone. Further, they challenge the hypothesis that the muscle rigidit
y of Parkinson disease results primarily from loss of striatal dopamin
e receptor stimulation.