In vivo research studies on neurotransmission alterations are crucial to provide biological-based evidence of molecular alterations in neurodegenerative diseases, supporting available symptomatic therapeutic strategies and fostering drug discovery. In this study, we provide unique in vivo evidence for specific alterations of the dopaminergic pathways along the AD stages in a well-characterized sample of amyloid-positive individuals with AD-D and AD-MCI. We demonstrated that the dopaminergic projections arising from the VTA are the most vulnerable in AD, with significant reduction of DAT density in the major mesocorticolimbic targets, since the prodromal disease phases (Fig. 2). We also found extensive alterations of the molecular architecture of the mesocorticolimbic pathway (Fig. 4). The dopaminergic projections arising from the SN were instead spared, with loss of DAT density limited to the head of the caudate nucleus (Fig. 3), and no network alteration in its molecular circuitry (Fig. 4).
The first finding of this study pertains to the presence of significant loss of DAT density in several dopaminergic targets in AD (Fig. 2–3). These results are supported by recent results obtained on a validated mouse model of AD[32, 33] and corroborated by previous post-mortem[2, 7–11] and in vivo imaging evidence[19, 34], reporting pre-synaptic dopaminergic dysfunction in AD-D. We also provide a new remarkable finding of system-level dopaminergic alterations, with disruption of the pattern of regional connectivity observed in HC (Fig. 4). Abnormal patterns of dopaminergic connectivity were previously reported in neurodegenerative disease characterized by known dopaminergic deficits, such as Parkinson’s disease and DLB, showing prominent alterations within the nigrostriatal system (cf.[25]). The present fnew in vivo findings demonstrate alterations of the neurotransmission architecture in AD as well, but within the mesocorticolimbic system.
The second main finding of this study pertains to the disease stages according to which dopaminergic deficits occur in AD. Available in vivo studies provided evidence for dopaminergic alterations exclusively in advanced disease stages[19–23]. No in vivo studies are available directly assessing dopamine pathophysiology in AD-MCI. Here, we found, already at the stage of MCI, a significant loss of DAT density, at a degree comparable to that observed in patients with dementia, in several dopaminergic targets (Fig. 3). The lack of significant differences in regional DAT density between participants with AD-MCI vs. AD-D, suggests that dopaminergic dysfunction is an early event, also plateauing early along the disease course. Subjects with AD-MCI showed extensive alterations in the molecular architecture of the mesocorticolimbic dopaminergic circuitry, with loss of inter-connections at the level of the ventral striatum, amygdala, hippocampus and anterior and middle cingulate gyri. The widespread derangement of molecular connectivity, exceeding the loss of local DAT density (Fig. 5), indicates an early dysfunction of the dopaminergic circuitry in AD, possibly contributing to its pathophysiology, also at consistency with previous structural and functional connectivity studies[31, 35, 36].
The third main finding of this study pertains to the different vulnerability of the two main dopaminergic pathways in AD. Previous in vivo and post mortem studies focused mainly on the nigrostriatal dopaminergic pathway, reporting alterations in the SN[2–6] and in its major subcortical targets, i.e. putamen[8, 11, 15, 16, 23, 37] and caudate nucleus[7, 11, 16, 23, 37]. There are however several studies reporting lack of alterations in the aforementioned regions[1, 5, 8–10, 12, 13, 17, 23, 34]. Of note, most of the available evidence reports dopaminergic alterations without partitioning the striatum into its ventral and dorsal components, with few exceptions[21]. Our findings partially support the involvement of the nigrostriatal pathway in AD, with loss of DAT density limited to the caudate nucleus, but no alterations in molecular connectivity (Fig. 3–4). The caudate nucleus, part of the “associative-cognitive loop”, receives its major dopaminergic input from the dorsomedial portions of the SN pars compacta[38]. Notably, previous post mortem evidence reported alterations in pre-synaptic dopaminergic function specifically in the dorsal tier of SN pars compacta in AD, at difference to what is observed in Parkinson’s disease, where the ventral portion of the SN pars compacta (and relative projections) are mainly affected[2]. Altogether, this evidence suggests a certain degree of vulnerability of the nigrostriatal pathway in AD, with a very different topography compared to Parkinson’s disease.
Compared to the nigrostriatal pathway, the mesocorticolimbic pathway has been far less studied in AD. Previous post mortem and in vivo neuroimaging evidence suggests an involvement of the VTA[3] and of its targets, i.e. nucleus accumbens[5, 10, 16], limbic striatum[21], hippocampus[18, 23], amygdala[8, 11, 18] and cingulate gyrus[11], with some studies reporting however negative results[1, 8, 10, 12, 23, 37]. Alterations of the mesocorticolimbic targets have been associated with neuropsychiatric symptoms in AD[21, 31], present in up to 90% of patients with dementia[39]. Depression and apathy are the most frequently reported symptoms in both AD-MCI and AD-D[40]. Results from the current study further and strongly support the involvement of the mesocorticolimbic pathway in AD, with loss of DAT activity and molecular connectivity in its major targets. Specifically, we observed loss of DAT activity in the ventral striatum and hippocampus, and in the middle cingulate gyrus, receiving dopaminergic input from the ventral and dorsal portions of VTA, respectively[41] (Fig. 5). Widespread molecular connectivity alterations affect both subcortical and cortical targets, indicating involvement of the major VTA projections, including the meso-limbic, meso-cortical, meso-hippocampal and meso-amygdala routes (cf.[31]) (see Fig. 5). The finding of a prominent involvement of the mesocorticolimbic system in AD is particularly relevant, as some of the VTA targets, and most prominently the ventral striatum, represent crucial hubs in the complex feed-forward organization of the dopaminergic circuitry. Notably, the ventral striatum, while receiving inputs from a relatively small populations of neurons in the VTA, sends back widespread projections to both the VTA and the SN, hence being able to affect dopaminergic activity in widespread portions of the dorsal striatum, particularly in its associative regions (cf. [38]). This evidence raises the question of whether our findings of altered dopaminergic activity in the caudate nucleus might be consequential to the finding of altered dopaminergic function in the ventral striatum.
While our findings show a prominent involvement of the mesocorticolimbic system and a very limited involvement of the nigrostriatal targets, namely the caudate, the pathogenic mechanisms underlying dopaminergic vulnerability in AD remain unknown. Some authors have suggested that the peculiar physiology of midbrain ventrategmental dopaminergic neurons, per se, might render those dopaminergic neurons particularly vulnerable to amyloid pathology in AD[32].
A link between dopaminergic dysfunction and tau pathology has also been hypothesized (cf.[35]). In this direction, a recent post-mortem study on human brain tissue reported an association between tau pathology and dysregulation of genetic pathways linked to dopaminergic neurotransmission[42]. Notably, the earliest tau-site in AD, namely the locus coeruleus (LC), sends substantial noradrenergic afferents to midbrain dopaminergic nuclei, providing trophic support to both the VTA and the medial portions of SN pars compacta[43]. While the hypothesis that tau pathology in the LC is at the basis of the dopaminergic deficits observed in AD remains speculative, it is worth noting that the topography of dopaminergic alteration detected in our cohorts, specifically encompassing projections from the VTA and the medial portions of the SN pars compacta (Fig. 5), closely matches that of the noradrenergic projections from the LC to the midbrain dopaminergic nuclei[44]. Alternatively, it can also be hypothesized a direct effect of LC on the dopaminergic function of mesocorticolimbic targets, such as the hippocampus, that receive substantial dopaminergic input directly from LC’styrosine hydroxylase -immunoreactive neurons[45].
This study has some limitations. While all participants were within the AD spectrum and underwent an extensive biomarker assessment[28], we do not have post mortem pathology confirmation. Thus, we cannot exclude that some of them had additional co-pathologies. We also acknowledge that while [123I]FP-CIT primarily measures DAT density, bindings to the serotonergic transporter should be considered in the extra-striatal regions. However, since we limited our analysis exclusively to regions belonging to the nigrostriatal and mesocorticolimbic structures and pathways, very rich in dopamine transporters [46], we believe the interpretation of our findings in terms of alterations in DAT density is solid. Last, we acknowledge the absence of correction for partial volume effects might represent a limit, however, the combined use of: i) anatomical and functional probabilistic atlases for ROIs segmentation, ii) the strategy of picking only the center of each volume, and iii) non-smoothed SPECT images, should minimize the influence of partial volume effects. This strategy represents in addition the only solution that can be applied without the need of structural MRI, not available for all our subjects.