he relative roles of D1 and D2 dopamine receptor stimulation in mediating the antiparkinsonian effects of dopaminergic drugs remain unclear. To determine the functional metabolic consequences of selective dopamine receptor stimulation, we used 2-deoxyglucose (2-DG) autoradiography to examine the effects of the D1 agonist SKF-38393 and the D2 agonist LY-171555 on regional cerebral glucose utilization (RCGU) in rats with unilateral 6-hydroxydopamine (6-OHDA) substantia nigra lesions. SKF-38393 (0.5-25.0 mg/kg) and LY-171555 (0.01-5.0 mg/kg) produced indistinguishable behavioral responses, including vigorous contralateral rotation. Treatment with each drug similarly increased glucose utilization, dose-dependently, in the parafascicular thalamus, subthalamic nucleus, deep layers of the superior colliculus, and lateral midbrain reticular formation ipsilateral to the nigral lesion; glucose utilization was decreased in the ipsilateral lateral habenula. By contrast, the D1 and D2 agonists differentially altered glucose utilization in the entopeduncular nucleus (EP) and the substantia nigra pars reticulata (SNr). SKF-38393, 5.0 and 25.0 mg/kg, increased glucose utilization 127 and 275%, respectively, in the pars reticulata ipsilateral to the lesion. LY-171555, 1.0 and 5.0 mg/kg, caused maximal contralateral turning, yet did not alter glucose utilization in the ipsilateral SNr. The glucose utilization response of the ipsilateral EP paralleled that of the SNr demonstrating large increases following administration of SKF-38393 and minimal change following the use of LY-171555. The results demonstrate that the selective D1 agonist reproduces the marked glucose utilization increases (2-3-fold above control values) in the EP and SNr that were previously observed using L-DOPA and apomorphine in this model, whereas the selective D2 agonist does not.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopaminergic denervation supersensitivity has been implicated in the pathogenesis of levodopa-induced dyskinesias, the most common and limiting side effect in the drug treatment of Parkinson's disease, yet the mechanisms that mediate altered drug sensitivity remain poorly understood. In animals models, one key component of denervation supersensitivity is the enhanced efficacy of selective D1 agonists to stimulate locomotion. In rats with chronic dopamine depletion induced by 6-hydroxydopamine nigral lesion, the increased ability of D1 agonists to stimulate regional cerebral glucose utilization (RCGU) in the substantia nigra pars reticulata (SNr) has provided a metabolic correlate to the heightened motor response. In this study, we used the stimulation of RCGU in the SNr as a sensitive in vivo assay of D1 agonist effect to examine the time course of development of supersensitivity in rats following acute dopamine depletion with single doses of reserpine (5.0 mg/kg, i.p.) and alpha-methyl-p-tyrosine (AMPT; 100 mg/kg, i.p.). The stimulatory effect of the D1 agonist SKF 38393 (30 mg/kg) on RCGU in the SNr was first enhanced 6 hr after reserpine/AMPT injection and was maximally enhanced at 12-24 hr (relative 2-deoxyglucose uptake increased 32-51%; P less than 0.05). The response to SKF 38393 returned to control values 5 d after reserpine/AMPT injection. The single reserpine/AMPT injections depleted striatal dopamine to 1-2% of control values from 3-48 hr postinjection, whereas D1 and D2 dopamine receptor densities were unchanged at 24 hr.(ABSTRACT TRUNCATED AT 250 WORDS)

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