No differential regulation of dopamine transporter (DAT) and vesicular monoamine transporter 2 (VMAT2) binding in a primate model of Parkinson disease

Abstract

Radioligands for DAT and VMAT2 are widely used presynaptic markers for assessing dopamine (DA) nerve terminals in Parkinson disease (PD). Previous in vivo imaging and postmortem studies suggest that these transporter sites may be regulated as the numbers of nigrostriatal neurons change in pathologic conditions. To investigate this issue, we used in vitro quantitative autoradioradiography to measure striatal DAT and VMAT2 specific binding in postmortem brain from 14 monkeys after unilateral internal carotid artery infusion of 1-Methyl-4-Phenyl-1,2,3,6-tetrahydropyridine (MPTP) with doses varying from 0 to 0.31 mg/kg. Quantitative estimates of the number of tyrosine hydroxylase (TH)-immunoreactive (ir) neurons in substantia nigra (SN) were determined with unbiased stereology, and quantitative autoradiography was used to measure DAT and VMAT2 striatal specific binding. Striatal VMAT2 and DAT binding correlated with striatal DA (r(s) = 0.83, r(s) = 0.80, respectively, both with n = 14, p<0.001) but only with nigra TH-ir cells when nigral cell loss was 50% or less (r = 0.93, n = 8, p = 0.001 and r = 0.91, n = 8, p = 0.002 respectively). Reduction of VMAT2 and DAT striatal specific binding sites strongly correlated with each other (r = 0.93, n = 14, p<0.0005). These similar changes in DAT and VMAT2 binding sites in the striatal terminal fields of the surviving nigrostriatal neurons demonstrate that there is no differential regulation of these two sites at 2 months after MPTP infusion.

Figure 1. SN outlined on a TH-immunostained transverse slice with 2× magnification.

Typical pictures were taken on the left (A) and right (B) side of the same slice separately with SN and ventral tegmental area (VTA) outlined. SN lies ventral and lateral to VTA. The 3^rd cranial nerve fibers are indicated by arrows. Scale bar: 1 cm.

Figure 2. Representative autoradiograms of [³H] WIN 35,428 and [³H] DTBZ.

[³H] WIN 35,428 binding to DAT (top row) (A–D), [³H] DTBZ binding to VMAT2 (bottom row) (E–H) in coronal sections from the injected side of control (A and E) and MPTP-treated monkeys (B–D and F–H). Images represent total binding, according to the pseudo-color bar on the right side of each image. Scale bar: 0.5 cm.

Figure 3. Relationship between VMAT2 (A) or DAT (B) Bmax and residual SNpc TH-ir neurons.

The value for each monkey was expressed as the ratio of the injected side to the control side in 13 monkeys.

Figure 4. Relationship between VMAT2 (A) or DAT (B) Bmax and residual striatal dopamine content.

The values for each monkey were expressed as the ratio of the injected side to the contralateral side, and the line is the linear fit of the data.

Figure 5. Relationship between striatal VMAT2 and DAT Bmax in 14 monkeys.

The value for each monkey was expressed as the ratio of the injected side to the control side, and the line is the linear fit of the data. The r value reflects the Spearman's correlation coefficient value obtained in striatum.