In this study, we investigated the histopathological effects of 6-OHDA infusion into the striatum of C57BL/6 mice. Using a novel image analysis technique to map brain-wide changes in TH expression and relate these to TH + neuron and projection densities, we found that neurodegeneration from 20 µg 6-OHDA at 14 days post-infusion was confined to the SNpc and associated with widespread, bilateral denervation of its striatal and extra-nigrostriatal targets, along with compensatory TH upregulation in subcortical regions. Male mice showed more severe pathology, indicating potential neuroprotective mechanisms in females. In male mice, lower doses (5 and 10 µg) led to a delayed reduction in striatal TH expression, primarily at day 14, while the highest dose (20 µg) caused a progressive, severe decrease in dopaminergic fibers in the striatum. Both optical fractionation and mean fluorescence intensity reliably indicated TH + cell loss in the SNpc, validating the latter as an efficient alternative to stereological counting. We observed robust glial activation (CD68, Iba1, LAMP1, GFAP) in the striatum three days after 6-OHDA infusion, along with an upregulation of inflammatory markers in the SNpc that began within a day and primarily sustained for at least a week preceding morphological cell death. Additionally, we found that 20 µg of 6-OHDA activated DAPK1 in the striatum, potentially promoting excitotoxicity, while GSK-3β and AKT did not regulate pro-apoptotic signaling.
We demonstrated that the 6-OHDA lesion can be extensively characterized with light-sheet fluorescence microscopy (LSFM) and quantitative 3D analysis of TH + neurons. Likewise, Roostalu, et al. observed a similar, albeit milder, pattern of TH loss after systemic injection of MPTP.35 Our catecholaminergic mapping data aligns with the conventional WB and 2D quantification techniques used in this and other studies.36,37 For example, 6-OHDA decreased the density of TH + cells 51–52% in a singular cluster localized 89–95% to the SNpc, 4–9% to the VTA, and 1–2% to the supramammillary nucleus (SUM). Conversely, loss of TH + fibers was widespread, impacting the striatum, isocortex, midbrain, hippocampal formation, olfactory areas, and amygdalar nuclei. As TH + fiber loss spreads beyond regions with dopaminergic innervation—such as the striatum, amygdala, hippocampus, prefrontal cortex, anterior cingulate area38, and rhinal cortices39—damage also affects noradrenergic projections. These projections extend widely across the brain, reaching areas including the isocortex, thalamus, hypothalamus, hippocampus, amygdala, and cerebellum.
In contrast to the widespread TH + fiber loss, compensatory increases were spatially confined to a few subcortical regions, including the thalamus, hypothalamus, midbrain, pallidum, and lateral septal nucleus. Consistent with this, bilateral remodeling of efferents and afferents connected to the dopaminergic A13 cell group in the hypothalamus has been reported following a unilateral infusion of 6-OHDA (3.6 µg) into the medial forebrain bundle, with mapping occurring 29 days later.40 Although the paraventricular hypothalamus (PVH), dopaminergic A13 group, the nucleus of reuniens (RE), and pontine central gray (PCG) appeared to show compensatory increases in TH + cell density, this likely indicates enhanced TH expression and/or TH fiber density, leading to the heightened detection of TH + cells.11,41,42 Interestingly, nuclei with noradrenergic neurons, such as the locus coeruleus (LC), were resistant to 6-OHDA toxicity. These regions may activate a compensatory response to counteract the catecholaminergic imbalance caused by 6-OHDA in the striatum.43 Given the projection patterns of noradrenergic neurons, the increased TH levels in regions with compensation may indicate heightened noradrenergic tone, mitigating the effects of reduced dopamine levels.44
Our brain-wide mapping enhances our understanding of regions implicated in motor and non-motor impairments in PD, extending beyond the traditional focus on nigrostriatal denervation. By leveraging machine learning and employing analytical tools from fMRI used in PD patients,45 our 3D analysis of immunolabeling in rodent brains efficiently screens anatomically analogous regions, identifying areas relevant to different PD stages and symptoms. Moreover, our approach provides a framework for spatially resolving neuronal morphology, glial reactivity, and disease markers.
Only two observational studies have explored the use of 6-OHDA in female C57BL/6 mice,11,18, with Masini and colleagues being the only ones considering sex differences at the histopathological level.11 Consistent with the findings of Masini et al., our results demonstrated a remarkable resistance of female mice to all doses of 6-OHDA. This resistance may be partially attributed to estrogen, which is known to support elevated levels of brain-derived nerve factor.46 It might also contribute to the lower incidence of PD in women. However, further investigation is needed to explore the neuroprotective mechanisms in female mice that underlie their resilience against the 6-OHDA neurotoxin at the doses studied here. Based on data from this and other studies, we strongly recommend conducting a priori power calculations when designing PD research involving female mice.11,18
In our study, the highest 6-OHDA dosage progressively reduced TH protein levels in the striatum and the prevalence of TH + cells in the SNpc within 14 days. Other studies have delivered 18 or 20 µg of 6-OHDA in the striatum, with results showing a 40% degeneration of the SNpc in one case17 and a more severe 60% reduction in another at 14 days.47 Considering the timing of TH downregulation and SNpc neuron loss, selecting the appropriate 6-OHDA dose is crucial based on the research question and histopathological endpoints. The lowest dose (5 µg) causes a gradual reduction in striatal TH + expression, which may not reach significance but still leads to SNpc neuron loss by day 14, indicating a milder injury, which could be easier to rescue. This makes it a sensitive readout for testing neurotherapeutics aimed at stabilizing pre-synaptic terminals or exploring striatal mechanisms, provided the studies are sufficiently powered. In contrast, the highest dose (20 µg) rapidly reduces TH + neurons in the SNpc within 7 days, making it better suited for efficient screening of experimental therapies attempting to preserve cell bodies and slow rostrocaudal degeneration of dopaminergic neurons. These findings should guide the dose selection in studies targeting specific neuroprotective strategies.
The optical fractionator stereology cell counting method is considered the gold standard for quantification of dopaminergic lesions in the SNpc.48–50 However, mean fluorescent intensity measurements have also been successfully employed in several studies,18,51 producing comparable results. Our data reinforces using mean fluorescent intensity as a proxy for TH cell densities, a more time-efficient alternative to the labor-intensive stereology cell counting method.
While many studies have documented astrocyte and microglial reactivity in rats and mice treated with 6-OHDA,17,52,53 none have previously examined cytokine/chemokine levels using a 48-plex immunoassay. Here, we report a distinct chemokine/cytokine signature in the SNpc on days 1, 3, and 7 following 6-OHDA infusion. The robust upregulation of chemokines from 6-OHDA is likely driven by proinflammatory, activated microglia, which promote chemoattraction (e.g., via CCL5) of CD8 + T helper cells, eosinophils, and monocytes. In turn, chemokines such as MIP-1α, MIP-1β, and MIP-2 create a sustained positive feedback loop of proinflammatory microglial activation and proliferation.54,55 IP-10/CXCL10 signaling leads to the activation of apoptotic pathways,56 while upregulation of the cytokine BAFF/TNFSF13B, primarily released by B cells, aligns with evidence of B and T cell infiltration in the brains of 6-OHDA-infused mice.57 This also explains the increase in RANTES/CCL5, predominantly related to T-cell infiltration.57 Our observation of a robust, rapid inflammatory response suggests early activation of microglia and astrocytes driven by damaged SNpc neurons.17 Glial reactivity is further evidenced by the increased protein levels of CD68, Iba1, LAMP1, and GFAP in the striatum on day 3, although the mechanisms underlying activation at the infusion site may differ. Notably, the cytokine fingerprint detected in the 6-OHDA model is not entirely detrimental; for example, RANTES/CCL5 has shown neuroprotective effects in Alzheimer's disease,58 while RANTES and MIP-1β have potent recruitment activity toward T lymphocytes, which are protective in 6-OHDA-treated mice.59 In contrast, the cytokine profile in PD patients is substantially different, with increased levels of TNF-α, IL-1β, IL-2, IL-4, IL-6, TGF-α, TGF-β1, and TGF-β2.60 These differences suggest that the 6-OHDA mouse model may be ideal for developing and testing immunomodulatory therapies aimed at rescuing SNpc neurons through the modulation of chemokine and cytokine expression.
We also examined the expression and phosphorylation levels of kinases (DAPK1, GSK-3β, and AKT) that regulate apoptosis.61,62 DAPK1 activity was increased in the striatum of 6-OHDA-treated mice at day 7, potentially indicating ongoing apoptosis due to excitotoxicity, which is associated with neuronal cell death in both in vivo and in vitro models.33 In contrast, GSK-3β and AKT were not activated, preventing them from influencing neuronal degeneration and survival.
In summary, this study provides a detailed characterization of the 6-OHDA mouse model, incorporating comprehensive catecholaminergic mapping, three different time points and dosages of 6-OHDA, and the inclusion of both male and female mice. Traditional histopathological methods effectively detected dose-dependent, progressive striatal TH + fiber loss and SNpc neurodegeneration, while also revealing resilience in female mice. These findings were complemented by an innovative approach for quantifying brain-wide iDISCO+/LSFM data. Specifically, differential TH immunolabeling was spatially resolved through voxel-wise analyses and carefully evaluated with FDR correction and precise measurements of TH + cell and fiber densities in the most affected regions. This study also highlights the early immune response preceding neuronal cell death in the striatum and SNpc, revealing a distinct cytokine/chemokine signature to that observed in PD patients. Additionally, we identified increased DAPK1 activity, which may drive excitotoxicity in SNpc neurons. Our findings offer valuable insights into the 6-OHDA mouse model, delineating brain-wide histopathology and the biochemical signaling profile in the nigrostriatal pathway. Furthermore, our methodologies can be applied to other animal models of neurodegenerative diseases, potentially advancing our understanding of disease processes and the development of new therapeutic strategies.