Decline in the acute insulin response in relationship to plasma glucose concentrations

BACKGROUND

To investigate (1) whether the decline in acute insulin response (AIR) precedes or coincides with defined glucose regulation categories and whether acute insulin response decline varies by race and adiposity, (2) linearity of the relationship between acute insulin response and increasing plasma glucose concentrations, (3) longitudinal changes in acute insulin response accounting for changes in insulin action across categories of glucose tolerance.

METHODS

Clinical cross-sectional and longitudinal study of nondiabetic subjects. Inpatient assessment of oral glucose tolerance (2-h PG, fasting PG), and acute insulin response (intravenous glucose tolerance test) in 326 and 84 Native Americans of full and ≤6/8th Southwestern heritage, respectively, and 115 Whites. Linearity of acute insulin response vs plasma glucose concentrations investigated using spline analyses. Follow-up (average = 2.07 years) glucose tolerance, acute insulin response, and insulin action (hyperinsulinemic-euglycemic clamp) assessed in 230 full Native Americans.

RESULTS

In certain groups, the relationship between acute insulin response and increasing plasma glucose levels was non-linear. In all groups, acute insulin response decline preceded the cut-offs for traditional glucose regulation categories, although the timing with respect to increasing plasma glucose varied by race and adiposity. Longitudinal data indicated that improvement in insulin action is the key factor to preserve insulin secretion, underlying the reversion of glucose tolerance in prediabetic individuals.

CONCLUSIONS

With worsening insulin action, the decline in insulin secretion occurred prior to current diagnostic guidelines for impaired glucose regulation. However, the relationship between acute insulin response and increasing plasma glucose varies and was not always non-linear. Understanding the dynamics of this relationship may determine when to initiate preventive pharmacotherapy directed at the preservation of β-cell failure.

Early administration of RS 67333, a specific 5-HT4 receptor agonist, prevents amyloidogenesis and behavioral deficits in the 5XFAD mouse model of Alzheimer's disease

Amyloid β (Aβ) accumulation is considered the main culprit in the pathogenesis of Alzheimer’s disease (AD). Recent studies suggest that decreasing Aβ production at very early stages of AD could be a promising strategy to slow down disease progression. Serotonin 5-HT₄ receptor activation stimulates α-cleavage of the amyloid precursor protein (APP), leading to the release of the soluble and neurotrophic sAPPα fragment and thus precluding Aβ formation. Using the 5XFAD mouse model of AD that shows accelerated Aβ deposition, we investigated the effect of chronic treatments (treatment onset at different ages and different durations) with the 5-HT₄ receptor agonist RS 67333 during the asymptomatic phase of the disease. Chronic administration of RS 67333 decreased concomitantly the number of amyloid plaques and the level of Aβ species. Reduction of Aβ levels was accompanied by a striking decrease in hippocampal astrogliosis and microgliosis. RS 67333 also transiently increased sAPPα concentration in the cerebrospinal fluid and brain. Moreover, a specific 5-HT₄ receptor antagonist (RS 39604) prevented the RS 67333-mediated reduction of the amyloid pathology. Finally, the novel object recognition test deficits of 5XFAD mice were reversed by chronic treatment with RS 67333. Collectively, these results strongly highlight this 5-HT₄ receptor agonist as a promising disease modifying-agent for AD.

Prenatal pharmacotherapy rescues brain development in a Down's syndrome mouse model

Intellectual impairment is a strongly disabling feature of Down's syndrome, a genetic disorder of high prevalence (1 in 700-1000 live births) caused by trisomy of chromosome 21. Accumulating evidence shows that widespread neurogenesis impairment is a major determinant of abnormal brain development and, hence, of intellectual disability in Down's syndrome. This defect is worsened by dendritic hypotrophy and connectivity alterations. Most of the pharmacotherapies designed to improve cognitive performance in Down's syndrome have been attempted in Down's syndrome mouse models during adult life stages. Yet, as neurogenesis is mainly a prenatal event, treatments aimed at correcting neurogenesis failure in Down's syndrome should be administered during pregnancy. Correction of neurogenesis during the very first stages of brain formation may, in turn, rescue improper brain wiring. The aim of our study was to establish whether it is possible to rescue the neurodevelopmental alterations that characterize the trisomic brain with a prenatal pharmacotherapy with fluoxetine, a drug that is able to restore post-natal hippocampal neurogenesis in the Ts65Dn mouse model of Down's syndrome. Pregnant Ts65Dn females were treated with fluoxetine from embryonic Day 10 until delivery. On post-natal Day 2 the pups received an injection of 5-bromo-2-deoxyuridine and were sacrificed after either 2 h or after 43 days (at the age of 45 days). Untreated 2-day-old Ts65Dn mice exhibited a severe neurogenesis reduction and hypocellularity throughout the forebrain (subventricular zone, subgranular zone, neocortex, striatum, thalamus and hypothalamus), midbrain (mesencephalon) and hindbrain (cerebellum and pons). In embryonically treated 2-day-old Ts65Dn mice, precursor proliferation and cellularity were fully restored throughout all brain regions. The recovery of proliferation potency and cellularity was still present in treated Ts65Dn 45-day-old mice. Moreover, embryonic treatment restored dendritic development, cortical and hippocampal synapse development and brain volume. Importantly, these effects were accompanied by recovery of behavioural performance. The cognitive deficits caused by Down's syndrome have long been considered irreversible. The current study provides novel evidence that a pharmacotherapy with fluoxetine during embryonic development is able to fully rescue the abnormal brain development and behavioural deficits that are typical of Down's syndrome. If the positive effects of fluoxetine on the brain of a mouse model are replicated in foetuses with Down's syndrome, fluoxetine, a drug usable in humans, may represent a breakthrough for the therapy of intellectual disability in Down's syndrome.