There are several rat models that may have translational benefits when using MRI to study T2DM [33]. These include the GK rat [34], Zuker Diabetic Fatty (ZDF) rat [35], HFD/STZ rat [36] and BBBZK/Wor, to list a few. This exploratory study began the process of evaluating the BBBZK/Wor model of T2DM with multimodal imaging with the expectation of finding common neuroradiological measures of cerebral neuropathy as reported in the clinic. If so, the model would have the potential to provide valuable imaging data on disease progression from prediabetic to late stage diabetes and a non-invasive means of assessing the efficacy of therapeutic intervention on brain structure and function.
Voxel-Based Morphometry
A reduction in brain volume is a consistent finding across all imaging studies in 2TDB [11]. Cortical volumes show a decrease in gray [6, 37-40] and white matter [6, 40]. The underlying cause of the global reduction in brain volumes is unknown but is thought to be the consequence of small vessel disease [41]. The general brain atrophy and reduction in gray matter is associated with diminished cognitive function [37, 40, 42, 43]. Hippocampal volumes are smaller in T2DM versus age-matched controls [40, 44, 45] and maybe a contributing factor to the cognitive decline. The BBBZK/Wor rat presented with several brain areas that were significantly smaller as compared to age-matched controls. These areas included the somatosensory, entorhinal and temporal cortices but not CA1, CA3, dentate, and subiculum, the primary components of the hippocampal complex.
Diffusion weighted imaging
DWI is an indirect way of assessing the integrity of white and gray matter microarchitecture. A recently published study by Moghaddam and colleagues [46] provides a comprehensive review of the literature on T2DM and DWI. The literature is very consistent reporting changes in microarchitecture in the areas of the frontal-temporal cortex, hippocampus, cerebellum, thalamus and all of the major white matter tracts. The cognitive decline in T2DM is strongly associated with alterations in DWI in white matter tracts [47-49]. With few exceptions, the general DWI profile is a decrease in FA and increase in ADC. This inverse relationship suggests a loss of microstructural integrity and network organization [46]. The BBBZK/Wor rat also presented with global and pervasive changes in measures of DWI. The FA and ADC values were inversely related with FA being greater than ADC a potential sign of global neuroinflammation and cytotoxic edema. The changes in DWI were extensive affecting much of the brain including the hippocampus, thalamus, amygdala, cerebellum and white matter tracts.
Functional Connectivity
A review by Macpherson and colleagues covers much of the literature on rsFC in T2DM [50]. There is general agreement across multiple studies that T2DM presents with a reduction in connectivity in the default mode network, interconnections between the prefrontal cortex, parietal cortex, and hippocampus. Thalamic coupling to cortical and cerebellar regions is also reduced [51]. The effect on global connectivity in this BBZDR/Wor diabetes model is contrary to that reported in the clinical literature. As shown in the connectivity matrix Fig 2 there is no clear reduction in connectivity when looking at the clusters highlighted. If anything, there are areas of the brain that show hyperconnectivity with diabetes. Intra-thalamic connections are enhanced (area A), as are thalamic connections to the dorsal hippocampus (area D) and cerebellar/pontine connections to the brainstem reticular activating system (area G). The increase in connectivity is not unprecedented and can occur between some brain areas in human T2DM [52], but that would appear to be the exception. The hyperconnectivity observed in BBBZK/Wor maybe a compensatory response to underlying pathology as reported in traumatic brain injury [53] or in young children with early type 1 diabetes [54].
Limitations and Considerations
As an exploratory study we recognize its many limitations. 1) There were no female rats, an issue of concern given data shows differences in diabetic pathology between females and males [55, 56] 2) There were no measures of cognitive function to correlation with the MRI data as is routine with clinical studies. As noted in the Methods, we were unable to collect behavioral data due to the severity of the obesity in the BBBKZ/Wor rat. 3) We did not image for white matter hyperintensities, lesions associated with microvascular insult. Again, this is routine in clinical studies and would have aided in our analysis of BBBKZ/Wor as a relevant imaging model for T2DM. 4) Our rsFC studies were done under light isoflurane anesthesia. These studies could have been done under awake conditions as we have done so in many other task-related BOLD imaging [57-59] or phMRI studies [60, 61] in rodents. However, “resting state” poses a dilemma in awake animal imaging no matter the level of acclimation prior to imaging [62]. Any physical restraint will most likely have some level of stress; hence, the rsFC data were collected under light anesthesia. Nonetheless, numerous studies comparing the anesthetized and conscious states show similar rsFC data [63, 64]