Longitudinal monoaminergic PET imaging of chronic proteasome inhibition in minipigs

Abstract

Impairment of the ubiquitin proteasome system has been implicated in Parkinson's disease. We used positron emission tomography to investigate longitudinal effects of chronic intracerebroventricular exposure to the proteasome inhibitor lactacystin on monoaminergic projections and neuroinflammation. Göttingen minipigs were implanted in the cisterna magna with a catheter connected to a subcutaneous injection port. Minipigs were imaged at baseline and after cumulative doses of 200 and 400 μg lactacystin, respectively. Main radioligands included [¹¹C]-DTBZ (vesicular monoamine transporter type 2) and [¹¹C]-yohimbine (α2-adrenoceptor). [¹¹C]-DASB (serotonin transporter) and [¹¹C]-PK11195 (activated microglia) became available later in the study and we present their results in a smaller subset of animals for information purposes only. Striatal [¹¹C]-DTBZ binding potentials decreased significantly by 16% after 200 μg compared to baseline, but the decrease was not sustained after 400 μg (n = 6). [¹¹C]-yohimbine volume of distribution increased by 18-25% in the pons, grey matter and the thalamus after 200 μg, which persisted at 400 μg (n = 6). In the later subset of minipigs, we observed decreased [¹¹C]-DASB (n = 5) and increased [¹¹C]-PK11195 (n = 3) uptake after 200 μg. These changes may mimic monoaminergic changes and compensatory responses in early Parkinson's disease.

Figure 1

Behavioural assessment of the animals at baseline and after lactacystin injections. (a) Average velocity and (b) stride length were measured based on 4 quality readings for each minipig on a Gait mat. (c) Distance travelled was measured by videotracking the minipigs in their cages at baseline and after 200 μg and 400 μg lactacystin.

Figure 2

VMAT2 availability determined by in vivo [¹¹C]-DTBZ PET. (a) [¹¹C]-DTBZ BP_ND values calculated using Logan reference tissue model in striatum, ventral midbrain and anterior pons in 6 minipigs. Values are shown for naïve minipigs and for minipigs inserted with the injection port following a few saline injections (baseline), and after a cumulative dose of 200 and 400 μg lactacystin. **< 0.01 (b) BP_ND parametric maps are shown for a representative minipig at baseline, 200 and 400 μg in coronal view at the level of striatum, ventral midbrain and anterior pons.

Figure 3

α2-adrenoceptor distribution determined by in vivo [¹¹C]-yohimbine PET. (a) [¹¹C]-yohimbine V_T values are displayed for thalamus, cortical grey matter and anterior pons after saline (baseline) and doses of 200 μg and 400 μg lactacystin in 6 minipigs. *< 0.05, **< 0.01 (b) V_T parametric maps are shown for a representative minipig at baseline, 200 and 400 μg in coronal view at the level of thalamus and anterior pons and transverse view for cortical grey matter.

Figure 4

SERT distribution determined by in vivo [¹¹C]-DASB PET. (a) [¹¹C]-DASB BP_ND values are displayed for striatum, thalamus and anterior pons in 5 minipigs after saline (baseline) and doses of 200 μg and 400 μg lactacystin. (b) BP_ND parametric maps are shown for a representative minipig at the three different conditions in coronal view at the level of striatum, thalamus and the anterior pons.

Figure 5

Microligal activation determined by in vivo [¹¹C]-PK11195 PET. (a) [¹¹C]-PK11195 V_T values are displayed for thalamus, ventral midbrain and anterior pons after saline (baseline) and a dose of 200 μg lactacystin. (b) V_T parametric maps are shown for a representative minipig at the baseline and 200 μg dose at coronal sections corresponding to the level of thalamus, ventral midbrain and anterior pons.

Figure 6

Preliminary immunohistochemistry of tyrosine hydroxylase positive fibers in the striatum (a,d) and neurons in the substantia nigra (b,e) and locus coeruleus (c,f) in a control minpig (top row) compared to a lactacystin-injected minipig (bottom row).