Low dose pramipexole is neuroprotective in the MPTP mouse model of Parkinson's disease, and downregulates the dopamine transporter via the D3 receptor.
BACKGROUND
Our aim was to determine if pramipexole, a D3 preferring agonist, effectively reduced dopamine neuron and fiber loss in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model when given at intraperitoneal doses corresponding to clinical doses. We also determined whether subchronic treatment with pramipexole regulates dopamine transporter function, thereby reducing intracellular transport of the active metabolite of MPTP, 1-methyl-4-phenylpyridinium (MPP+).


METHODS
Ten 12-month old C57BL/6 mice were treated with MPTP (or saline) twice per day at 20 mg/kg s.c. (4 injections over 48 h). Mice were pretreated for 3 days and during the 2-day MPTP regimen with pramipexole (0.1 mg/kg/day) or saline. Stereological quantification of dopamine neuron number and optical density measurement of dopamine fiber loss were carried out at 1 week after treatment, using immunostaining for dopamine transporter (DAT) and tyrosine hydroxylase (TH). Additional wild-type (WT) and D3 receptor knockout (KO) mice were treated for 5 days with pramipexole (0.1 mg/kg/day) or vehicle. The kinetics of [3H]MPP+ and [3H]DA uptake (Vmax and Km) were determined 24 h later; and at 24 h and 14 days dopamine transporter density was measured by quantitative autoradiography.


RESULTS
Pramipexole treatment completely antagonized the neurotoxic effects of MPTP, as measured by substantia nigra and ventral tegmental area TH-immunoreactive cell counts. MPTP- induced loss of striatal innervation, as measured by DAT-immunoreactivity, was partially prevented by pramipexole, but not with regard to TH-IR. Pramipexole also reduced DAT- immunoreactivity in non-MPTP treated mice. Subchronic treatment with pramipexole lowered the Vmax for [3H]DA and [3H]MPP+ uptake into striatal synaptosomes of WT mice. Pramipexole treatment lowered Vmax in WT but not D3 KO mice; however, D3 KO mice had lower Vmax for [3H]DA uptake. There was no change in DAT number in WT with pramipexole treatment or D3 KO mice at 24 h post-treatment, but there was a reduction in WT-pramipexole treated and not in D3 KO mice at 14 days post-treatment.


CONCLUSIONS
These results suggest that protection occurs at clinically suitable doses of pramipexole. Protection could be due to a reduced amount of MPP+ taken up into DA terminals via DAT. D3 receptor plays an important role in this regulation of transporter uptake and availability.
Thomas H Christopher Center for Parkinson's Disease Research, Sun Health Research Institute, 10515 West Santa Fe Dr, Sun City, AZ 85352, USA. Jeff.Joyce@sunhealth.org
Jeffrey N
JN
Joyce
Cheryl
C
Woolsey
Han
H
Ryoo
Sabine
S
Borwege
Diane
D
Hagner
D000818:Animals; D000978:Antiparkinson Agents / Q000493:pharmacokinetics / Q000627:therapeutic use*; D052160:Benzothiazoles; D002452:Cell Count; D003342:Corpus Striatum / Q000187:drug effects / Q000378:metabolism / Q000473:pathology; D004195:Disease Models, Animal; D004298:Dopamine / Q000378:metabolism; D018491:Dopamine Agonists / Q000493:pharmacokinetics / Q000627:therapeutic use*; D050483:Dopamine Plasma Membrane Transport Proteins / Q000187:drug effects* / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D015536:Down-Regulation; D007279:Injections, Subcutaneous; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009412:Nerve Fibers / Q000187:drug effects / Q000473:pathology; D009474:Neurons / Q000187:drug effects / Q000378:metabolism / Q000473:pathology; D018696:Neuroprotective Agents / Q000493:pharmacokinetics / Q000627:therapeutic use*; D010300:Parkinson Disease; D000077487:Pramipexole; D050637:Receptors, Dopamine D3 / Q000378:metabolism*; D013844:Thiazoles / Q000493:pharmacokinetics / Q000627:therapeutic use*
D000978:Antiparkinson Agents; D052160:Benzothiazoles; D018491:Dopamine Agonists; D050483:Dopamine Plasma Membrane Transport Proteins; D018696:Neuroprotective Agents; D050637:Receptors, Dopamine D3; D013844:Thiazoles; D000077487:Pramipexole; D004298:Dopamine