Brain 3-methyoxytyramine varies inversely with blood glucose in decapitated rats

Insulin effects on core neurotransmitter pathways involved in schizophrenia neurobiology: a meta-analysis of preclinical studies. Implications for the treatment

Impairment of insulin action and metabolic dysregulation have traditionally been associated with schizophrenia, although the molecular basis of such association remains still elusive. The present meta-analysis aims to assess the impact of insulin action manipulations (i.e., hyperinsulinemia, hypoinsulinemia, systemic or brain insulin resistance) on glutamatergic, dopaminergic, γ-aminobutyric acid (GABA)ergic, and serotonergic pathways in the central nervous system. More than one hundred outcomes, including transcript or protein levels, kinetic parameters, and other components of the neurotransmitter pathways, were collected from cultured cells, animals, or humans, and meta-analyzed by applying a random-effects model and adopting Hedges'g to compare means. Two hundred fifteen studies met the inclusion criteria, of which 180 entered the quantitative synthesis. Significant impairments in key regulators of synaptic plasticity processes were detected as the result of insulin handlings. Specifically, protein levels of N-methyl-D-aspartate receptor (NMDAR) subunits including type 2A (NR2A) (Hedges' g = -0.95, 95%C.I. = -1.50, -0.39; p = 0.001; I2 = 47.46%) and 2B (NR2B) (Hedges'g = -0.69, 95%C.I. = -1.35, -0.02; p = 0.043; I2 = 62.09%), and Postsynaptic density protein 95 (PSD-95) (Hedges'g = -0.91, 95%C.I. = -1.51, -0.32; p = 0.003; I2 = 77.81%) were found reduced in insulin-resistant animal models. Moreover, insulin-resistant animals showed significantly impaired dopamine transporter activity, whereas the dopamine D2 receptor mRNA expression (Hedges'g = 3.259; 95%C.I. = 0.497, 6.020; p = 0.021; I2 = 90.61%) increased under insulin deficiency conditions. Insulin action modulated glutamate and GABA release, as well as several enzymes involved in GABA and serotonin synthesis. These results suggest that brain neurotransmitter systems are susceptible to insulin signaling abnormalities, resembling the discrete psychotic disorders' neurobiology and possibly contributing to the development of neurobiological hallmarks of treatment-resistant schizophrenia.

Glucose regulation and cognitive functions: relation to Alzheimer's disease and diabetes

Glucose has been found to improve memory in animals and humans. Animal research has revealed that glucose may improve memory through a facilitation of acetylcholine (ACh) synthesis and release in the brain. This glucose-related memory improvement has prompted research in elderly humans. These studies have shown that the memory-improving action of glucose depends on each individuals' blood glucose regulation. Based on these data, researchers have evaluated the effect of glucose on memory in patients with Alzheimer's disease (AD). Results demonstrated that glucose could improve memory in a subset of patients that had abnormalities in their blood glucose regulation. Interestingly, these alterations in blood glucose regulation were believed to depend on the severity of the disease process. Another line of investigation has focused on alterations in brain glucose metabolism. Both animal models and studies with Type II diabetic elderly patients have shown that altered glucose regulation impairs learning and memory processes. It is possible that in AD patients, hyperglycemia exerts a deleterious effect by potentiating the neuronal death produced by other pathological processes taking place such as amyloid deposition. Based on these data, it appears important to find the prevalence of altered glucoregulation at various stages of AD. Secondly, it may be of interest to determine prospectively whether altered glucoregulation is linked to a faster progression of the disease. Finally, if such a relationship is observed, the next logical step would be to determine whether AD patients could benefit from treatments aimed at normalizing blood glucose regulation and improving insulin sensitivity.

Anorexia following the systemic injection of amylin

The intravenous injection of 100 micrograms/kg of rat amylin reduced food intake in schedule-fed rats for 1 h of an 8 h measurement period. Associated with this brief anorexia was a hyperglycemic response, observed 30 min after a subsequent amylin administration. Determination of neurochemical alterations revealed increased concentration of serotonin in the hypothalamus and decreased level of the dopamine metabolite, 3-methoxytyramine, in the corpus striatum. Since similar neurochemical alterations were observed following the systemic injection of glucose, both the neurochemical changes and anorexia following intravenous amylin treatment may be secondary to hyperglycemia.

Glucose concentrations in brain and blood: regulation by dopamine receptor subtypes

The stimulation of D1 and D2 dopamine (DA) receptors by selective agonists produced large increases in brain glucose concentrations. D2 receptor stimulation also produced large increases in blood glucose. The D1-induced increases were somewhat variable in normal animals, but were more reliably observed and greatly increased in animals with brain DA depletions. Blockade of D1 receptors prevented these increases. Likewise, centrally acting D2 antagonists, but not the peripheral D2 antagonist domperidone, prevented D2 agonist-induced increases in brain and blood glucose. These observations point to an important role for DA in the regulation of glucose homeostasis.