The Acute and Early Effects of Whole-Brain Irradiation on Glial Activation, Brain Metabolism, and Behavior: a Positron Emission Tomography Study

Cranial irradiation is a frequently applied treatment modality for primary brain tumors and brain metastases, and as prophylactic treatment to prevent brain metastases. Despite its undisputed therapeutic importance, irradiation of normal brain tissue can lead to complications [1‐4]. Depending on the time of onset, complications (intended as clinically apparent and tissue reaction) can be classified into (i) acute (days to weeks: sickness, nausea, vomiting, edema); (ii) early-delayed (1–4 months: neuroinflammation, transient demyelination, somnolence, cognitive deficits); and (iii) late-delayed (4–6 months to 1 year: neuroinflammation, vascular abnormalities, demyelination, radiation necrosis, irreversible cognitive decline) [3].

Neuroinflammation appears to be an important mechanistic link in the cascade leading towards delayed complications [5‐7]. Neurons damaged by ionizing radiation release various chemokines, cytokines, and purine metabolites that activate microglia [8]. Activated microglia in turn secrete a panel of pro-inflammatory cytokines, which disrupt neurogenic signaling and neurogenesis, meanwhile stimulating infiltration of T lymphocytes and peripheral monocytes/macrophages [9] and inducing astrocyte activation, leading to gliosis and glial scar formation [10]. Selective inhibition of microglia-mediated neuroinflammation was shown to alleviate radiation-induced cognitive impairment [11].

Another radiation-induced adverse effect is decreased glucose metabolism. Radiotherapy was found to reduce the glucose metabolic rate in long-term survivors of childhood cancer [12, 13]. Indeed, radiation damage to normal cells causes electrophysiological and biochemical alterations, alterations in redox-sensitive processes, and direct mitochondrial damage resulting from the inflammatory response, which can all decrease cellular metabolism. Metabolic changes are likely to precede any anatomic changes like atrophy or radionecrosis [14].

In recent years, there has been a growing awareness that neuroimaging may be used to investigate early physiological effects after radiotherapy, thus helping to clarify the mechanisms underlying radiation induced-brain injury [15‐19]. Positron emission tomography (PET) offers the opportunity to noninvasively measure physiological processes like neuroinflammation and glucose metabolism. Radiolabeled ligands targeting the 18-kDa translocator protein (TSPO) are tools to detect glial activation [20, 21]. TSPO expression is increased in activated microglia, activated astrocytes, infiltrating/perivascular macrophages, and T lymphocytes. TSPO overexpression is considered an in vivo biomarker for neuroinflammation and can be measured with 1-(2-chlorophenyl)-N-[¹¹C]methyl-(1-methylpropyl)-3-isoquinoline carboxamide ([¹¹C]PK11195). Brain glucose metabolism can be assessed with 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG).

The aim of this study was to investigate the dose dependency of brain irradiation on acute and early-delayed glial cell activation (neuroinflammation) and changes in cerebral glucose metabolism using PET imaging. Since such molecular changes could culminate into behavioral abnormalities, our secondary aim was to assess whether our molecular imaging findings could be linked to behavioral changes assessed by the open-field test (OFT).

The primary aim of this study was to investigate the acute and early-delayed, dose-dependent effects of brain irradiation on glial cell activation and cerebral glucose metabolism. We observed that whole-brain irradiation in healthy rats caused a dose-dependent reduction in body weight; a transient metabolic flair in the brain, especially in the 10Gy group; and a delayed neuroinflammatory response, in particular in the high-dose group.

The radiolabeled TSPO ligand [¹¹C]PK11195 was selected as in vivo brain inflammation marker [22, 27], because TSPO expression is increased in activated microglia/astrocytes, infiltrating/perivascular macrophages/lymphocytes, and infiltrating neutrophils in response to neuroinflammatory stimuli. Radiotherapy can induce damage to the brain, and stress signals from damaged cells can trigger the activation of glial cells, which is accompanied by an increase in TSPO expression. Moreover, radiotherapy can induce the production of reactive oxygen species (ROS). TSPO is known to form polymers in the presence of ROS and these polymers bind [¹¹C]PK11195 with higher affinity than the monomer [28]. For these reasons, we expected cranial irradiation to increase [¹¹C]PK11195 binding in the brain. Although whole-brain [¹¹C]PK11195 uptake in the acute phase was not significantly different between groups, to our surprise, whole-brain irradiation caused an acute, transient decrease in [¹¹C]PK11195 accumulation in the cerebellum and several cortical regions in the 25 Gy group. It is known that, under normal physiological conditions, low levels of TSPO expression can be detected in glial cells, but also in other cells in the brain, such as cerebrovascular/endothelial cells, and smooth muscle cells [29, 30]. The TSPO-expressing cells in the brain can be damaged by high doses of radiation and go into apoptosis. A possible explanation for the acute reduction in the [¹¹C]PK11195 signal in these brain regions early after irradiation could therefore be a temporary reduction in the number of glial and/or endothelial cells due to radiation-induced cell death [31‐34]. This result is in agreement with a flow cytometry study in mice that demonstrated that the number of CD11b-expressing microglia was significantly reduced 7 and 14 days after cranial radiation (10Gy), but had recovered to control levels on day 28 [33]. Besides brain regions with reduced [¹¹C]PK11195 uptake, the 25 Gy group also has regions in the brain with increased tracer uptake on day 3. The regions with increased tracer uptake are more centrally located than the regions with reduced tracer uptake. So, an alternative explanation for our [¹¹C]PK11195 PET findings on day 3 could be glial cells migrating from cortical brain regions to the midbrain in response to 25-Gy irradiation.

On day 31, our study showed that [¹¹C]PK11195 uptake was significantly higher in almost all brain regions in the 25-Gy-irradiated animals than in controls, with the exception of the cerebellum and some cortical areas. Interestingly, these exceptions were the brain regions that had shown reduced tracer uptake on day 3. These data suggest a delayed activation of glial cells in most brain regions and a normalization of the concentration of glial cells in regions that were depleted of TSPO-expressing cells on day 3 after the acute phase.

In contrast to the 25-Gy group, rats that were irradiated with 10 Gy had [¹¹C]PK11195 uptake similar to that in controls in almost all regions, both on days 3 and 31. Apparently, the lower irradiation dose is not sufficient to induce any detectable glial activation. Dose-dependent differences in the severity of the radiation-induced tissue damage also seem to be reflected in the effect of brain irradiation on body weight, as the high 25-Gy dose caused significant weight loss (Fig. 1), whereas the lower 10-Gy dose had a much smaller effect on body weight. Similar effects of cranial irradiation on rodent body weight have previously been reported [35, 36].

Noteworthy, the decline in [¹¹C]PK11195 uptake between days 3 and 31 in control and 10 Gy treated could be pointing to a combination of repeated animal handling and stress induced to them in a short time due to the experimental procedures, i.e., pre-whole-brain (sham-)irradiation anesthesia by intraperitoneal injection, (sham-)irradiation procedures (rats hanging by upper incisors for about 18 min), the slow recovery from anesthesia, and transport to the PET center, as well the xylazine/ketamine administration for the irradiation procedure. Indeed, a single administration of ketamine increased the protein levels of IL-6, IL-1β, and TNF-α, in mice brain 6 h post intraperitoneal injection [37].

Taken together, all the procedures before the first PET scan could stress the animals resulting in a microglia imprinting that leads to some inflammatory response in the brain that gradually disappeared, or was not detectable anymore on day 31, in healthy and 10-Gy irradiated animals. This is also in line with the higher [¹⁸F]FDG value found on day 3 vs 31, where the inflammatory response could increase [¹⁸F]FDG uptake on day 3 [38]. That this decline was not observed in the rats irradiated with 25Gy might be related to an inflammatory response to the irradiation, resulting in increased [¹¹C]PK11195 uptake compensating the decline observed in the control and 10-Gy-treated group.

Whole-brain irradiation increased brain glucose metabolism on day 3. This could be ascribed to an early adaptive response of the brain to meet the energy demand for repair of radiation-induced tissue damage [39] or induction of GLUT1 transporter expression [40]. Remarkably, the effect of cranial irradiation on glucose metabolism was more pronounced in the 10-Gy group than in the 25-Gy group. This difference in metabolic response might be explained by the extent of damage induced by the treatment [41]. Radiation induces a peak in apoptosis 6 h after irradiation, and apoptosis continues for another 24–48 h, while radiation also can increase cell proliferation between 1 and 7 days after irradiation [42, 43]. It seems plausible that the impact of apoptosis is more important in the 25-Gy group than in the 10-Gy group, resulting in a smaller net energy demand for repair and thus a smaller increase in glucose metabolism in the high-dose group. Between days 3 and 31, cerebral [¹⁸F]FDG uptake in all groups decreased. This reduction in cerebral glucose metabolism may be due to a general reduction in activity of the rats upon aging. This is in line with our findings of reduced mobility in the OFT.

The PET data suggest that whole-brain irradiation causes a transient “flare” response of glucose metabolism, in combination with delayed activation of glial cells in the brain. These physiological effects, however, did not seem to be associated with behavioral changes as assessed by the OFT. The OFT tests did not show any significant radiation-induced behavioral differences between control and irradiated animals. So, either the functional changes observed by PET are too subtle to induce behavioral changes, or the statistical power of the experiment was insufficient to reveal the subtle behavioral changes. In general, inter-individual variability in behavioral studies is larger than that in PET studies. Consequently, a larger group size would be required for behavioral tests. Other studies, however, have shown late behavioral changes after cranial irradiation [44, 45]. It is possible that other types of behavioral changes (e.g., impairment cognition) were induced than could be detected in the OFT. Therefore, future research should aim to correlate PET imaging with late behavioral changes, in particular with more appropriate behavioral tests, like memory tests such as Y-maze, Morrison maze, or novel object/spatial recognition tests. The OFT in this study did show a reduction in locomotion between days 6 and 27 in all groups, but no differences between groups. The reduction in exploratory behavior between both time points could be ascribed to recognition of the arena in the repeated test. Moreover, animals tend to move less when they get older. The latter explanation is in line with the general reduction in glucose metabolism in all groups observed by [¹⁸F]FDG PET, suggesting a general reduction in activity.

A limitation of our study is that imaging findings were not confirmed by ex vivo histology or immunohistochemistry, due to the longitudinal study design. Another limitation is the lack of cell-type (e.g., microglia, macrophage, astrocyte) and phenotype specificity (M1, M2) of [¹¹C]PK11195 PET. Attempts to develop more specific tracers are in progress. Finally, the sample size in this study may have been too small to reveal statistically significant behavioral changes, and the open-field behavioral test may not have been suitable to demonstrate radiation-induced cognitive impairment.

Taken together, this study has demonstrated dose-dependent acute and early-delayed changes in cerebral physiology after radiotherapy. PET proved an adequate method to monitor these changes in a longitudinal manner, as non-invasive PET imaging revealed that whole-brain irradiation of healthy rats induces a transient metabolic flare and delayed neuroinflammation (glial activation), without causing acute or early-delayed behavioral changes. Additional studies, using different (fractionated) dosing schemes, are required to evaluate the impact of the observed physiological changes on the development of late-delayed clinical complications and the mechanisms that are involved in this process.