Short-lived diabetes in the young-adult ZDF rat does not exacerbate neuronal Ca(2+) biomarkers of aging

Periodontitis and diabetes reshape neuronal dendritic arborization in the thalamus and nucleus oralis in the rat

Diabetes is a metabolic disorder resulting in long-term hyperglycemia that could induce oxidative stress as well as neural modifications in the central nervous system. Periodontal disease is highly comorbid with diabetes and in some cases, with exacerbated pain responses. Periodontal tissue is innervated by trigeminal afferents which extend to the nucleus oralis (NO) that sends input to the ventral posterior lateral thalamic nuclei (VPL). The present study aimed to evaluate the consequences of periodontitis, diabetes and both conditions on the dendritic morphology, spine type, and density in neurons of the NO and VPL in male and female rats. A quantitative neuromorphological analysis was performed using the Cox-Golgi staining in male and female rats in four groups: naïve control, after a periodontitis procedure, diabetic, and diabetic with periodontitis. Periodontitis decreased the total dendritic length (TDL) in the NO of the male rat but no change in the female rat and no neuronal alterations were observed in the VPL of both male and female rats. In contrast, diabetes increased the number of spines in the NO and VPL and decreased TDL in the NO in both male and female rats. We observed that periodontitis induced a dimorphic effect in the NO, whereas diabetes induced a strong neuromorphological effect regardless of sex. Moreover, while periodontitis had a limited effect on the neuronal morphology, it dramatically modified the neural consequences in the VPL and NO when comorbid with diabetes. In conclusion, these neuroplastic modifications may be relevant to understand how diabetes exacerbates the outcome of periodontitis in humans, particularly in the female population.

Expression of a Constitutively Active Human Insulin Receptor in Hippocampal Neurons Does Not Alter VGCC Currents

Memory and cognitive decline are the product of numerous physiological changes within the aging brain. Multiple theories have focused on the oxidative, calcium, cholinergic, vascular, and inflammation hypotheses of brain aging, with recent evidence suggesting that reductions in insulin signaling may also contribute. Specifically, a reduction in insulin receptor density and mRNA levels has been implicated, however, overcoming these changes remains a challenge. While increasing insulin receptor occupation has been successful in offsetting cognitive decline, alternative molecular approaches should be considered as they could bypass the need for brain insulin delivery. Moreover, this approach may be favorable to test the impact of continued insulin receptor signaling on neuronal function. Here we used hippocampal cultures infected with lentivirus with or without IRβ, a constitutively active, truncated form of the human insulin receptor, to characterize the impact continued insulin receptor signaling on voltage-gated calcium channels. Infected cultures were harvested between DIV 13 and 17 (48 h after infection) for Western blot analysis on pAKT and AKT. These results were complemented with whole-cell patch-clamp recordings of individual pyramidal neurons starting 96 h post-infection. Results indicate that while a significant increase in neuronal pAKT/AKT ratio was seen at the time point tested, effects on voltage-gated calcium channels were not detected. These results suggest that there is a significant difference between constitutively active insulin receptors and the actions of insulin on an intact receptor, highlighting potential alternate mechanisms of neuronal insulin resistance and mode of activation.

Impact of Single or Repeated Dose Intranasal Zinc-free Insulin in Young and Aged F344 Rats on Cognition, Signaling, and Brain Metabolism

Novel therapies have turned to delivering compounds to the brain using nasal sprays, bypassing the blood brain barrier, and enriching treatment options for brain aging and/or Alzheimer's disease. We conducted a series of in vivo experiments to test the impact of intranasal Apidra, a zinc-free insulin formulation, on the brain of young and aged F344 rats. Both single acute and repeated daily doses were compared to test the hypothesis that insulin could improve memory recall in aged memory-deficient animals. We quantified insulin signaling in different brain regions and at different times following delivery. We measured cerebral blood flow (CBF) using MRI and also characterized several brain metabolite levels using MR spectroscopy. We show that neither acute nor chronic Apidra improved memory or recall in young or aged animals. Within 2 hours of a single dose, increased insulin signaling was seen in ventral areas of the aged brains only. Although chronic Apidra was able to offset reduced CBF with aging, it also caused significant reductions in markers of neuronal integrity. Our data suggest that this zinc-free insulin formulation may actually hasten cognitive decline with age when used chronically.