In vivo imaging neurotransmitter function. The rat 6-hydroxydopamine model and its relevance for human Parkinson's disease

DAT versus D2 receptor binding in the rat striatum: l-DOPA-induced motor activity is better predicted by reuptake than release of dopamine

The reuptake and release of dopamine (DA) can be estimated using in vivo imaging methods by assessing the competition between endogenous DA and an administered exogenous DA transporter (DAT) and D2 receptor (D2 R) radioligand, respectively. The aim of this study was to investigate the comparative roles of DA release vs DA reuptake in the rat striatum with small animal SPECT in relation to l-DOPA-induced behaviors. DAT and D2 R binding, together with behavioral measures, were obtained in 99 rats in response to treatment with either 5 or 10 mg/kg l-DOPA or vehicle. The behavioral parameters included the distance travelled, and durations and frequencies of ambulation, sitting, rearing, head-shoulder motility, and grooming. Data were subjected to a cluster analysis and to a multivariate principal component analysis. The highest DAT binding (i.e., the lowest DA reuptake) was associated with the highest, and the lowest DAT binding (i.e., the highest DA reuptake) was associated with the lowest motor/exploratory activity. The highest and the lowest D2 R binding (i.e., the lowest and the highest DA release, respectively) were merely associated with the second highest and second lowest levels of motor/exploratory activity. These findings indicate that changes in DA reuptake in response to fluctuating DA levels offer a better prediction of motor activity than the release of DA into the synaptic cleft. This dissociation, as reflected by in vivo DAT and D2 R binding data, may be accounted for by the regulatory sensitization meachnisms that occur at D2 R binding sites in response to altered levels of DA. Synapse 70:369-377, 2016. © 2016 Wiley Periodicals, Inc.

Relation between microPET imaging and rotational behavior in a parkinsonian rat model induced by medial forebrain bundle axotomy

The purpose of the current study was to examine the relation between apomorphine (APO) induced rotational behavior and the pre- and post-synaptic dopaminergic function in a parkinsonian rat model induced by medial forebrain bundle (MFB) axotomy. The brains of these rats were unilaterally lesioned by mechanical transection of the nigrostriatal dopamine pathway at the MFB. Behavioral studies were carried out by APO challenge prior to and 1, 3, and 5 weeks after MFB axotomy. MicroPET scans with [(11)C]CFT and [(11)C]raclopride were performed 2 days after the behavioral test. The two PET scans were separated by an interval of 24-48 h. Immunohistochemistry was conducted 4 days after the last PET scan. Our data showed that [(11)C]CFT binding decreased progressively 1, 3, and 5 weeks postlesion, and there was a significant nonlinear correlation between [(11)C]CFT uptake ratio (right/left) and APO induced rotations. In contrast, [(11)C]raclopride binding only increased significantly 3 weeks postlesion, and there was a positive linear correlation between [(11)C]raclopride uptake ratio (right/left) and APO induced rotations. Postmortem immunohistochemical studies confirmed the loss of both striatal dopamine fibers and nigral neurons on the lesioned side. These findings not only demonstrate the relation between APO induced rotational behavior and the pre- and post-synaptic dopamine function but also indicate the utility and validity of in vivo PET imaging in understanding disease mechanisms and progression, which should in turn lead to development of new therapies.

High correlation between in vivo [123I]β-CIT SPECT/CT imaging and post-mortem immunohistochemical findings in the evaluation of lesions induced by 6-OHDA in rats

Background

6-Hydroxydopamine (6-OHDA) is widely used in pre-clinical animal studies to induce degeneration of midbrain dopamine neurons to create animal models of Parkinson's disease. The aim of our study was to evaluate the potential of combined single-photon emission computed tomography/computed tomography (SPECT/CT) for the detection of differences in 6-OHDA-induced partial lesions in a dose- and time-dependent manner using the dopamine transporter (DAT) ligand 2β-carbomethoxy-3β-(4-[¹²³I]iodophenyl)tropane ([¹²³I]β-CIT).

Methods

Rats were unilaterally lesioned with intrastriatal injections of 8 or 2 × 10 μg 6-OHDA. At 2 or 4 weeks post-lesion, 40 to 50 MBq [¹²³I]β-CIT was administered intravenously and rats were imaged with small-animal SPECT/CT under isoflurane anesthesia. The striatum was delineated and mean striatal activity in the lesioned side was compared to the intact side. After the [¹²³I]β-CIT SPECT/CT scan, the rats were tested for amphetamine-induced rotation asymmetry, and their brains were immunohistochemically stained for DAT and tyrosine hydroxylase (TH). The fiber density of DAT- and TH-stained striata was estimated, and TH-immunoreactive cells in the rat substantia nigra pars compacta (SNpc) were stereologically counted.

Results

The striatal uptake of [¹²³I]β-CIT differed significantly between the lesion groups and the results were highly correlated to both striatal DAT- and TH-immunoreactive fiber densities and to TH-immunoreactive cell numbers in the rat SNpc. No clear progression of the lesion could be seen.

Conclusions

[¹²³I]β-CIT SPECT/CT is a valuable tool in predicting the condition of the rat midbrain dopaminergic pathway in the unilateral partial 6-OHDA lesion model of Parkinson's disease and it offers many advantages, allowing repeated non-invasive analysis of living animals.

Key players in major and bipolar depression--a retrospective analysis of in vivo imaging studies

In the present study, we evaluated the contribution of the individual synaptic constituents of all assessed neurotransmitter systems by subjecting all available in vivo imaging studies on patients with unipolar major depressive disorder (MDD) and bipolar depression (BD) to a retrospective analysis. In acute MDD, findings revealed significant increases of prefrontal and frontal DA synthesis, decreases of thalamic and midbrain SERT, increases of insular SERT, decreases of midbrain 5-HT(1A) receptors and decreases of prefrontal, frontal, occipital and cingulate 5-HT(2A) receptors, whereas, in remission, decreases of striatal D₂ receptors, midbrain SERT, frontal, parietal, temporal, occipital and cingulate 5-HT(1A) receptors and parietal 5-HT(2A) receptors were observed. In BD, findings indicated a trend towards increased striatal D₂ receptors in depression and mania, decreased striatal DA synthesis in remission and decreased frontal D₁ receptors in all three conditions. Additionally, there is some evidence that ventrostriatal and hippocampal SERT may be decreased in depression, whereas in remission and mania elevations of thalamic and midbrain SERT, respectively, were observed. Moreover, in depression, limbic 5-HT(1A) receptors were elevated, whereas in mania a decrease of both cortical and limbic 5-HT(2A) receptor binding was observed. Furthermore, in depression, prefrontal, frontal, occipital and cingulate M2 receptor binding was found to be reduced. From this, a complex pattern of dysregulations within and between neurotransmitter systems may be derived, which is likely to be causally linked not only with the subtype and duration of disease but also with the predominance of individual symptoms and with the kind and duration of pharmacological treatment(s).