Rat and Monkey Models of Parkinsons Disease

We have extensively studied the 6-OHDA rat PD model using phMRI [5,8,12]. A schematic of such a model was shown in Figure 10.11. Our studies have been geared towards multiple objectives. First, we wished to show that the hemodynamic effects of amphetamine or p>-CFT (a selective dopamine transporter blocker similar to cocaine or methylphenidate) were due to dopamine release itself, and not some other neurotransmitter. This aspect of the experiment was discussed above in the beginning of Section 10.5. Careful lesioning leads to selective dopamine cell loss, whereas cholinergic, serotonergic, and noradrenergic neurons are preserved. Thus, we showed that there was loss of the hemodynamic response on the lesioned side, whereas the phMRI response was preserved on the intact side [5,8]. This effect is shown in Figure 10.12. In this figure there is a loss of response to amphetamine on the lesioned side. Conversely, there is an increased response to apomorphine. Second, we showed that the profile of the rotational behavior after administration of amphetamine correlated well with the phMRI time course. This lends credence to the utility of the technique for assessment of the behavioral consequences of the disease model. Such a phenomenon helps provide evidence that phMRI could be a useful marker for following potential therapies. We showed such a possibility where recovery of the dopaminergic response occurs after transplantation with fetal dopamine cells in the striatum (see also Chapter 25, Section 25.2 and Section 25.6). This recovery of the phMRI correlated with both the temporal pattern of the rotational behavior and with the spatial distribution of nC-CFT binding (a ligand selective for the DAT), as well as subsequent histology [8]. These experiments were conducted using BOLD phMRI. Newer experiments show that the IRON phMRI experiments (as expected) are even more sensitive and can show recovery of poststriatal cortical circuitry after the stem cell graft is able to successfully innervate the host [143].

A third issue we wished to investigate was the effect of receptor supersensitivity. It is well known that after unilateral lesioning with 6-OHDA, there is postsynaptic upregulation of dopamine receptors. This is referred to as supersensitivity, because it leads to unusual behavioral sensitivity to the effects of dopamine (this phenomenon has also been shown for other neurotransmitter systems). We showed that injection of amphetamine in unilaterally lesioned animals led to the expected loss of phMRI response on the lesioned side. Injection of apomorphine, a nonselective dopamine agonist, led to increased rCBV on the lesioned side only, and very little on the intact side [12]. This finding correlated well with PET measurements, in the same animals, of upregulation in D2 receptors postsynaptically (Figure 10.12). These studies, therefore, lend great credence to the belief that phMRI may be of considerable use for investigating dopamine receptor dynamics in PD models, and in PD itself. These studies amply demonstrate that phMRI is a useful tool for following dopamine cell loss, dopamine cell recovery using fetal and stem cell transplantation, and also show that receptor supersensitivity can be measured. The primate studies discussed below lead to further evidence that this is the case.

We also studied primates given chronic, low-dose MPTP to replicate the symptoms of PD. These animals showed many of the same behavioral and histopathological hallmarks of PD. We studied the response to amphetamine injection before and after MPTP had induced stable Parkinsonian symptoms. These studies, similar to the rat models, showed a loss of response to amphetamine, with preservation of the signal in the accumbens. The latter feature mirrors the well-known preservation of the A10 neurons innervating the accumbens in both PD as well as MPTP models [172]. An illustration of the exquisite spatial resolution of the IRON technique is shown in Figure 10.13. This figure demonstrates the loss of amphetamine-induced activation in the substantia nigra and caudate/putamen, while the response to amphetamine is preserved in the accumbens.

FIGURE 10.13 (See color insert following page 328.) Maps of amphetamine stimulation in cynomolgus macaques. The response to 2.5 mg/kg amphetamine is shown on the top. Note the large increases in rCBV in the dopaminergic circuitry including the caudate, putamen, nucleus accumbens, parafascicular thalamus, substantia nigra, and ventral tegmental area. The bottom of the figure shows the response to amphetamine after long-term chronic treatment with low-dose MPTP. Note the response to amphetamine is lost in most regions except for the nucleus accumbens and the parafascicular thalamus. The data were acquired at 3 T using the IRON technique and spatial resolution of 0.7 mm in plane with 1.5 mm slices.

FIGURE 10.13 (See color insert following page 328.) Maps of amphetamine stimulation in cynomolgus macaques. The response to 2.5 mg/kg amphetamine is shown on the top. Note the large increases in rCBV in the dopaminergic circuitry including the caudate, putamen, nucleus accumbens, parafascicular thalamus, substantia nigra, and ventral tegmental area. The bottom of the figure shows the response to amphetamine after long-term chronic treatment with low-dose MPTP. Note the response to amphetamine is lost in most regions except for the nucleus accumbens and the parafascicular thalamus. The data were acquired at 3 T using the IRON technique and spatial resolution of 0.7 mm in plane with 1.5 mm slices.

One question that remains at the end of this section is the comparison with PET. The hemodynamic measurements made using phMRI are indirect, while PET can directly measure dopamine receptor binding, as well as dopamine metabolism using 18F-labeled dopa. We will address the above issues comparing the PET and phMRI for measuring dopamine cell loss, dopamine cell recovery, and receptor supersensitivity. A snapshot of the comparisons of the parameters measured using either PET or phMRI to study dopamine cell loss in PD is presented in Table 10.5. PET measurements of dopamine cell loss seem to be best reflected in ligands specific for the DAT [173]. These ligands provide a marker for presynaptic dopamine terminals that are depleted in PD. By comparison, amphetamine stimulus, for example, reflects the amount of dopamine that can be released. Therefore, loss of the dopamine transporter will lead to a loss of an effect after either amphetamine stimulus or stimulation with a DAT blocker such as |3-CFT. The lack of an effect from an amphetamine stimulus arises because not only are dopamine concentrations depleted (meaning less for release) but the reverse transport of dopamine into the synaptic cleft induced by amphetamine requires invagination of the DAT. Compounds such as

TABLE 10.5

Dopaminergic Decline in PD as Assessed Using PET or phMRI

Physical Effect or

Symptom Receptor Parameter PET Marker phMRI Marker

Loss of presynaptic Loss of DAT C-CFT binding decreased DA terminals

Loss of DA synthesis Loss of DA cell bodies Decreased 18F-Dopa uptake

Supersensitivity or Upregulation of postsynaptic Increased binding potential

Dyskinesias D2 receptors of 11C-raclopride

Amphetamine or Cocaine phMRI response decreased Decreased response to amphetamine ? Large phMRI response to apomorphine or L-dopa

P-CFT are also ineffective at evoking an increase in the dopamine levels due to the lack of the target DAT. We showed a good correlation between PET measures of decreased CFT binding in unilaterally lesioned rats with both amphetamine and CFT-induced phMRI changes [5,152]. A comparison between CFT binding and amphetamine-induced rCBV changes in monkeys showed a weak, but statistically significant correlation between the two parameters. This reflects the fact that phMRI after amphetamine probably represents the remaining capacity to release dopamine, whereas CFT binding assessed by PET represents the remaining DAT. These two parameters are not necessarily correlated in a one-to-one fashion.

Another parameter of interest is that of supersensitivity caused by upregulation of postsynaptic dopamine receptors. In this instance, one can inject the nonspecific dopamine agonist apomorphine to elicit a large response on the lesioned side, while the intact side shows no response. In this case, the endogenous dopamine competes with apomorphine for binding to receptors on the intact side, whereas this is not possible on the lesioned side, therefore there is a large increase in the CBV after apomorphine injection. We showed this was a very sensitive marker for supersensitivity [12]. Other studies have shown decreases in R2 after administration of L-dopa in an MPTP-lesioned primate model [11]. This effect is similar to that noted with apomorphine, although the changes are smaller due to the relatively low increases in synaptic dopamine levels caused by L-dopa.

These results clearly indicate the utility of phMRI to assess various components of dopaminergic function. Separating some of these effects into their components may be difficult, but carefully designed experiments in the animal models may well do this. For instance, deciding whether there is loss of DAT or just loss of presynaptic dopamine stores is made by considering the differences between CFT and amphetamine. If both compounds cannot produce a signal increase in the lesioned brain then that must mean DAT targets are depleted, however, the sum of those experiments does not indicate whether or not the synthesis and presynaptic dopamine stores are intact (although in these models we know they are not). The use of L-dopa as a challenge may prove to be quantitatively related to the amount of presynaptic dopamine that can be released. In order to potentiate the effects of L-dopa it could be administered in conjunction with a DAT blocker such as cocaine or CFT.

PhMRI also makes a good tool for following therapeutic interventions in lesion models. We showed a good recovery of dopamine cell loss following transplantation of fetal dopamine cells or even stem cells [8,143]. The size of the grafts were shown by the phMRI to be consistent with the grafts seen in PET scans as well as subsequent histology. This has the potential to turn into a useful clinical tool when, and if, such therapies prove to be generally efficacious in PD.

An Addict's Guide To Freedom

An Addict's Guide To Freedom

Get All The Support And Guidance You Need To Be A Success At Understanding And Getting Rid Of Addictions. This Book Is One Of The Most Valuable Resources In The World When It Comes To New Ways To Understand Addicts And Get Rid Of Addictions.

Get My Free Ebook


Post a comment