DIABETIC ENCEPHALOPATHY AND NEUROACTIVE STEROIDS: OBSERVATIONS IN FEMALE RATS

Diabetes mellitus (DM), a chronic metabolic disorder, may induce neurophysiological and structural changes in the central nervous system (i.e.,diabetic encephalopathy). Diabetic encephalopathy (DE) is one of the most common and severe chronic complications of DM, characterized by impaired cognitive and memory functions, and structural, neurochemical, and electrophysiological abnormalities. These alterations are associated with decline in cognitive processes, increased risk of cerebrovascular and Alzheimer’s disease, dementia, and psychiatric disorders. Memory and cognitive impairments are associated with hippocampal dysfunction, but the mechanisms behind these alterations are not fully known. Based on data collected in literature, the development of cognitive deficit seems to be the result of the concomitant presence of different processes such as mitochondrial dysfunction, oxidative stress, neuroinflammation and aberrant synaptogenesis in several brain areas of male rat model of type 1 DM (T1DM). Another important factor involved in the development of DE are steroid molecules because it is widely known that these molecules are regulators of the nervous function. Indeed, our previous data have shown that levels of many neuroactive steroids (i.e., steroids affecting the nervous system functionality) are decreased by three months (long-term) of DM but also by one month (short-term) in male streptozotocin (STZ)- induced rats. Moreover, DE presents differences, between two sexes, in term of incidence, progression and severity of the pathology. However, despite DE shows many sex dimorphic features, no observations in female rats have been so far reported in literature. On this basis, in the present study, firstly, we have explored the impact of one month of diabetes on memory abilities by Novel Object Recognition (NOR) test, on mitochondrial functionality, oxidative stress, synaptogenesis and neuroactive steroids levels in hippocampus and cerebral cortex of female STZ-rats in order to understand whether short-term DM affects these parameters also in females and whether possible changes are different in the two sexes. Data reported in this thesis indicate that short term of T1DM is able to significantly decrease memory abilities. This result is associated with aberrant synaptogenesis (i.e., decrease in the levels of synaptophysin, synapsin and syntaxin) and neuroinflammation (i.e., increased levels of IL-1b and IL-6) in hippocampus of female STZ-rats. Oxidative stress and mitochondrial functionality are not affected in this brain area, possibly due to protective effect of dihydroprogesterone (DHP) and allopregnanolone (ALLO), whose hippocampal levels are significantly increased. These effects are specific for hippocampus since the cerebral cortex does not present a similar increase in steroid levels and consequently shows increased neuroinflammation and oxidative stress possibly due to mitochondria dysfunction. T1DM induced not only a diabetic encephalopathy but also dysbiosis. On this basis, we focused our attention on the possible role of gut microbiota on the development of memory deficits in female STZ-rats in the contest of gut-brain axis. In particular, we explored, whether DM can affect the composition of gut microbiota, the levels of steroids in colon and gut permeability markers. In addition, we also evaluated whether correlations may occur among these parameters in female STZ-rats. Results obtained demonstrated that T1DM alters gut β-, but not α-diversity. The pathology is also associated with a significant decrease in pregnenolone (PREG) and increase in ALLO levels in the colon of female STZ-rats. In addition, T1DM altered gut permeability (i.e., decreased of zonulin-1 and claudin-1). Interestingly, we reported a significant correlation of PREG with Blautia, claudin-1 and the NOR index and of ALLO with Parasutterella, Gammaproteobacteria and claudin-1. Altogether, data obtained in this thesis suggest a pivotal role of neuroactive steroids and gut steroids in the development of memory disfunctions. In the central nervous system, increased levels of DHP and ALLO seem to be an attempt to counteract the negative effects of T1DM particularly in hippocampus. In periphery, the positive correlation between decreased level of PREG in colon and behavioural results, support the hypothesis that this steroid has an important role in the development of cognitive deficit observed in the CNS across the gut-brain-axis.