The concentration of dopamine, 5-hydroxytryptamine, and some of their acid metabolites in the brain of genetically diabetic rats

Dopamine in the Regulation of Glucose Homeostasis, Pathogenesis of Type 2 Diabetes, and Chronic Conditions of Impaired Dopamine Activity/Metabolism: Implication for Pathophysiological and Therapeutic Purposes

Dopamine regulates several functions, such as voluntary movements, spatial memory, motivation, sleep, arousal, feeding, immune function, maternal behaviors, and lactation. Less clear is the role of dopamine in the pathophysiology of type 2 diabetes mellitus (T2D) and chronic complications and conditions frequently associated with it. This review summarizes recent evidence on the role of dopamine in regulating insular metabolism and activity, the pathophysiology of traditional chronic complications associated with T2D, the pathophysiological interconnection between T2D and chronic neurological and psychiatric disorders characterized by impaired dopamine activity/metabolism, and therapeutic implications. Reinforcing dopamine signaling is therapeutic in T2D, especially in patients with dopamine-related disorders, such as Parkinson's and Huntington's diseases, addictions, and attention-deficit/hyperactivity disorder. On the other hand, although specific trials are probably needed, certain medications approved for T2D (e.g., metformin, pioglitazone, incretin-based therapy, and gliflozins) may have a therapeutic role in such dopamine-related disorders due to anti-inflammatory and anti-oxidative effects, improvement in insulin signaling, neuroinflammation, mitochondrial dysfunction, autophagy, and apoptosis, restoration of striatal dopamine synthesis, and modulation of dopamine signaling associated with reward and hedonic eating. Last, targeting dopamine metabolism could have the potential for diagnostic and therapeutic purposes in chronic diabetes-related complications, such as diabetic retinopathy.

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.

Reduced insulin-receptor mediated modulation of striatal dopamine release by basal insulin as a possible contributing factor to hyperdopaminergia in schizophrenia

Schizophrenia is a severe and chronic neuropsychiatric disorder which affects 1% of the world population. Using the brain imaging technique positron emission tomography (PET) it has been demonstrated that persons with schizophrenia have greater dopamine transmission in the striatum compared to healthy controls. However, little progress has been made as to elucidating other biological mechanisms which may account for this hyperdopaminergic state in this disease. Studies in animals have demonstrated that insulin receptors are expressed on midbrain dopamine neurons, and that insulin from the periphery acts on these receptors to modify dopamine transmission in the striatum. This is pertinent given that several lines of evidence suggest that insulin receptor functioning may be abnormal in the brains of persons with schizophrenia. Post-mortem studies have shown that persons with schizophrenia have less than half the number of cortical insulin receptors compared to healthy persons. Moreover, these post-mortem findings are unlikely due to the effects of antipsychotic treatment; studies in cell lines and animals suggest antipsychotics enhance insulin receptor functioning. Further, hyperinsulinemia - even prior to antipsychotic use - seems to be related to less psychotic symptoms in patients with schizophrenia. Collectively, these data suggest that midbrain insulin receptor functioning may be abnormal in persons with schizophrenia, resulting in reduced insulin-mediated regulation of dopamine transmission in the striatum. Such a deficit may account for the hyperdopaminergic state observed in these patients and would help guide the development of novel treatment strategies. We hypothesize that, (i) insulin receptor expression and/or function is reduced in midbrain dopamine neurons in persons with schizophrenia, (ii) basal insulin should reduce dopaminergic transmission in the striatum via these receptors, and (iii) this modulation of dopaminergic transmission by basal insulin is reduced in the brains of persons with schizophrenia.

Reduced insulin sensitivity is related to less endogenous dopamine at D2/3 receptors in the ventral striatum of healthy nonobese humans

BACKGROUND

Food addiction is a debated topic in neuroscience. Evidence suggests diabetes is related to reduced basal dopamine levels in the nucleus accumbens, similar to persons with drug addiction. It is unknown whether insulin sensitivity is related to endogenous dopamine levels in the ventral striatum of humans. We examined this using the agonist dopamine D2/3 receptor radiotracer [(11)C]-(+)-PHNO and an acute dopamine depletion challenge. In a separate sample of healthy persons, we examined whether dopamine depletion could alter insulin sensitivity.

METHODS

Insulin sensitivity was estimated for each subject from fasting plasma glucose and insulin using the Homeostasis Model Assessment II. Eleven healthy nonobese and nondiabetic persons (3 female) provided a baseline [(11)C]-(+)-PHNO scan, 9 of which provided a scan under dopamine depletion, allowing estimates of endogenous dopamine at dopamine D2/3 receptor. Dopamine depletion was achieved via alpha-methyl-para-tyrosine (64mg/kg, P.O.). In 25 healthy persons (9 female), fasting plasma and glucose was acquired before and after dopamine depletion.

RESULTS

Endogenous dopamine at ventral striatum dopamine D2/3 receptor was positively correlated with insulin sensitivity (r(7)=.84, P=.005) and negatively correlated with insulin levels (r(7)=-.85, P=.004). Glucose levels were not correlated with endogenous dopamine at ventral striatum dopamine D2/3 receptor (r(7)=-.49, P=.18). Consistently, acute dopamine depletion in healthy persons significantly decreased insulin sensitivity (t(24)=2.82, P=.01), increased insulin levels (t(24)=-2.62, P=.01), and did not change glucose levels (t(24)=-0.93, P=.36).

CONCLUSION

In healthy individuals, diminished insulin sensitivity is related to less endogenous dopamine at dopamine D2/3 receptor in the ventral striatum. Moreover, acute dopamine depletion reduces insulin sensitivity. These findings may have important implications for neuropsychiatric populations with metabolic abnormalities.

Hypoglycemia induced changes in cholinergic receptor expression in the cerebellum of diabetic rats

Glucose homeostasis in humans is an important factor for the functioning of nervous system. Hypoglycemia and hyperglycemia is found to be associated with central and peripheral nerve system dysfunction. Changes in acetylcholine receptors have been implicated in the pathophysiology of many major diseases of the central nervous system (CNS). In the present study we showed the effects of insulin induced hypoglycemia and streptozotocin induced diabetes on the cerebellar cholinergic receptors, GLUT3 and muscle cholinergic activity. Results showed enhanced binding parameters and gene expression of Muscarinic M1, M3 receptor subtypes in cerebellum of diabetic (D) and hypoglycemic group (D + IIH and C + IIH). α7nAchR gene expression showed a significant upregulation in diabetic group and showed further upregulated expression in both D + IIH and C + IIH group. AchE expression significantly upregulated in hypoglycemic and diabetic group. ChAT showed downregulation and GLUT3 expression showed a significant upregulation in D + IIH and C + IIH and diabetic group. AchE activity enhanced in the muscle of hypoglycemic and diabetic rats. Our studies demonstrated a functional disturbance in the neuronal glucose transporter GLUT3 in the cerebellum during insulin induced hypoglycemia in diabetic rats. Altered expression of muscarinic M1, M3 and α7nAchR and increased muscle AchE activity in hypoglycemic rats in cerebellum is suggested to cause cognitive and motor dysfunction. Hypoglycemia induced changes in ChAT and AchE gene expression is suggested to cause impaired acetycholine metabolism in the cerebellum. Cerebellar dysfunction is associated with seizure generation, motor deficits and memory impairment. The results shows that cerebellar cholinergic neurotransmission is impaired during hyperglycemia and hypoglycemia and the hypoglycemia is causing more prominent imbalance in cholinergic neurotransmission which is suggested to be a cause of cerebellar dysfunction associated with hypoglycemia.

Neuroprotective role of curcumin in the cerebellum of streptozotocin-induced diabetic rats

AIMS

Chronic hyperglycaemia in diabetes involves a direct neuronal damage caused by intracellular glucose which leads to altered neurotransmitter functions and reduced motor activity. The present study investigated the effect of curcumin in the functional regulation of muscarinic and alpha7 nicotinic acetylcholine receptors, insulin receptors, acetylcholine esterase and Glut3 in the cerebellum of streptozotocin (STZ)-induced diabetic rats.

MAIN METHODS

All studies were done in the cerebellum of male Wistar rats. Radioreceptor binding assays were done for total muscarinic, M(1) and M(3) receptors using specific ligands, and the gene expression was also studied using specific probes.

KEY FINDINGS

Our results showed an increased gene expression of acetylcholine esterase, Glut3, muscarinic M1, M3, alpha7 nicotinic acetylcholine and insulin receptors in the cerebellum of diabetic rats in comparison to control. Scatchard analysis of total muscarinic, M1 and M3 receptors showed an increased binding parameter, B(max) in diabetic rats compared to control. Curcumin and insulin inhibited diabetes-induced elevation in the gene expression of acetylcholine esterase, Glut3, insulin and cholinergic receptors in the cerebellum of diabetic rats.

SIGNIFICANCE

Our studies suggest that curcumin plays a vital role in regulating the activity of cholinergic and insulin receptors and mechanism of glucose transportation through Glut3, which results in normalizing the diabetes-mediated cerebellar disorders. Thus, curcumin has a significant role in a therapeutic application for the prevention or progression of diabetic complications in the cerebellum.

Enhanced muscarinic M1 receptor gene expression in the corpus striatum of streptozotocin-induced diabetic rats

Acetylcholine (ACh), the first neurotransmitter to be identified, regulate the activities of central and peripheral functions through interactions with muscarinic receptors. Changes in muscarinic acetylcholine receptor (mAChR) have been implicated in the pathophysiology of many major diseases of the central nervous system (CNS). Previous reports from our laboratory on streptozotocin (STZ) induced diabetic rats showed down regulation of muscarinic M1 receptors in the brainstem, hypothalamus, cerebral cortex and pancreatic islets. In this study, we have investigated the changes of acetylcholine esterase (AChE) enzyme activity, total muscarinic and muscarinic M1 receptor binding and gene expression in the corpus striatum of STZ--diabetic rats and the insulin treated diabetic rats. The striatum, a neuronal nucleus intimately involved in motor behaviour, is one of the brain regions with the highest acetylcholine content. ACh has complex and clinically important actions in the striatum that are mediated predominantly by muscarinic receptors. We observed that insulin treatment brought back the decreased maximal velocity (Vmax) of acetylcholine esterase in the corpus striatum during diabetes to near control state. In diabetic rats there was a decrease in maximal number (Bmax) and affinity (Kd) of total muscarinic receptors whereas muscarinic M1 receptors were increased with decrease in affinity in diabetic rats. We observed that, in all cases, the binding parameters were reversed to near control by the treatment of diabetic rats with insulin. Real-time PCR experiment confirmed the increase in muscarinic M1 receptor gene expression and a similar reversal with insulin treatment. These results suggest the diabetes-induced changes of the cholinergic activity in the corpus striatum and the regulatory role of insulin on binding parameters and gene expression of total and muscarinic M1 receptors.

A short-term diabetes induced changes of catecholamines and p38-MAPK in discrete areas of rat brain

Chronic diabetes is associated with the alteration of second messengers and CNS disorders. We have recently identified that protein kinases (CaMKII and PKC-alpha) and brain neurotransmitters are altered during diabetes as well as in hyperglycemic and acidotic conditions. In this study, we investigated the effects of acute diabetes on the levels of dopamine (DA), norepinephrine (NE), epinephrine (E) and p38-Mitogen-Activated Protein Kinase (p38-MAPK) in striatum (ST), hippocampus (HC), hypothalamus (HT), midbrain (MB), pons medulla (PM), cerebellum (CB) and cerebral cortex (CCX). Alloxan (45 mg/kg) diabetic untreated rats that showed hyperglycemia (>260 mg%), revealed significant increases of DA level in ST (1.5 fold), HC (2.2 fold) and PM (2.0 fold) and the E level also found to be increased significantly in HT (2.4 fold), whereas the NE level was decreased in CB (0.5 fold), after 7 days of diabetes. Immunoblotting study of p38-MAPK expression under identical conditions showed significant alterations in ST, HC, HT and PM (p<0.05) correlated with the changes of catecholamines (DA and E). On the other hand, the above changes were reversed in insulin-treated diabetic rats maintained under normal glucose level (80 -110 mg %). These results suggest that p38-MAPK may regulate the rate of either the synthesis or release of DA and E in corresponding brain areas, but not NE, under these conditions.

PKC-alpha mediated alterations of indoleamine contents in diabetic rat brain

We previously have reported that acute or chronic diabetes in animals resulted in altered neurotransmitter levels. In this study, we investigated the concentrations of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in discrete areas of brain viz. striatum (ST), hippocampus (HC), hypothalamus (HT), midbrain (MB), pons medulla (PM), cerebellum (CB) and cerebral cortex (CCX) of control, untreated diabetic and insulin treated diabetic rats after 30 days. Alloxan (45 mg/kg) diabetic untreated rats, which showed hyperglycemia (>250 mg%), revealed significant increases of 5-HT level in ST, MB, PM, CB and CCX and the 5-HIAA level found to be increased significantly in ST, HC and MB. Whereas the insulin treated rats, which was maintained under normal glucose level (80-110 mg%), showed no significant changes in any of the areas studied. The expressions of PKC-alpha studied by immunoblotting also showed significant changes in ST, HC, MB, PM, CB and CCX that is identical to the changes of both 5-HT and 5-HIAA under similar condition, suggesting that the PKC-alpha may regulate the synthesis and release of indoleamines in diabetic animals.

Streptozotocin-induced diabetes provokes changes in serotonin concentration and on 5-HT1A and 5-HT2 receptors in the rat brain

Since reduced levels of brain serotonin are known to cause behavioural abnormalities, to which diabetics are also prone, we investigated the effect, in rats, of chronic diabetes on brain serotonin concentration and on the numbers of 5-HT(1A) and 5-HT2 receptors in cerebral cortex and brainstem. Our data show that streptozotocin induces a longlasting hyperglicemia that is associated with a decrease in cerebral concentration of serotonin and with an accompanying increase in the maximum number of 5-HT(1A) and 5-HT2 receptors in the brain areas studied. Our results may suggest that changes in serotonergic transmission in the CNS play a role in diabetes-related behavioural abnormalities.

Diabetes decreases limbic extracellular dopamine in rats

Mesolimbic dopamine (DA) was measured by ventral striatum (including nucleus accumbens) microdialysis in freely moving streptozotocin (STZ)-diabetic male rats. DA and dihydroxyphenylacetic acid (DOPAC) basal levels and amphetamine-induced DA increase were lower in diabetic than in normal rats. These results are discussed in terms of decreased DA neuron activity and DA receptor hypersensitivity in the mesolimbic system of STZ-diabetic rats.

Alteration in hypothalamic monoamine metabolism of freely moving diabetic rat

Changes in hypothalamic monoamine metabolism were investigated in freely moving streptozotocin (STZ)-induced diabetic rats by using in vivo microdialysis technique. Six weeks later, the animals were implanted with microdialysis probe (molecular weight cut-off index: 12,000-14,000) into the ventromedial portion of the hypothalamus (VMH). The dialysate was collected and loaded onto HPLC to be assayed for norepinephrine (NE), dopamine (DA), serotonin (5-HT) and their metabolites (MHPG, DOPAC and 5-HIAA). The concentration of NE was decreased in the dialysate from the VMH of diabetic rats, whereas there was no significant change in MHPG level. The concentrations of 5-HT and 5-HIAA were reduced in diabetic rats. The DA concentration was obviously increased accompanied by the reduction of DOPAC level. The observed changes in hypothalamic monoamine metabolism, especially the reduced NE release, may play an important role in the induction of hyperphagia in freely moving STZ-induced diabetic rats.

Effect of long-lasting diabetes mellitus on rat and human brain monoamines

Experimental alloxan- or streptozotocin-produced diabetes in rats was accompanied by an increase in the levels of norepinephrine, dopamine, and serotonin, whereas the contents of metabolites, i.e., 5-hydroxyindoleacetic acid and homovanillic acid, in the whole brain gradually decreased with the duration of diabetes. Among the striatum, thalamus, and hypothalamus of alloxan diabetic rats, monoamine alterations were observed only in the hypothalamus; after 1 week an increase of norepinephrine content and after 13 weeks an increase of norepinephrine and dopamine contents were found. Tissues of 11 brain regions of 10 diabetic and 12 control patients post mortem were investigated for monoamine concentrations. Patients were all male, of similar age and interval between death and autopsy. Diabetic patients had an increase in the content of serotonin in the medial and lateral hypothalamus. The content of dopamine increased in the medial hypothalamus, putamen, and medial and lateral pallidus. In diabetic patients, the content of norepinephrine increased in the lateral pallidus and decreased in the nucleus accumbens and claustrum. Thus, it seems that diabetes mellitus in rats, as well as in humans is associated with regionally specific changes in brain monoamines.

Streptozotocin and alloxan produce alterations in rat brain monoamines independently of pancreatic beta cells destruction

In recent years the effect of experimental diabetes mellitus on brain neurochemistry has been under an intensive investigation. In most of these studies diabetes was produced by a peripheral administration of streptozotocin or alloxan. In line with previous reports, a week after such an application of alloxan (200 mg/kg s.c.) we found the concentration of serotonin, dopamine and norepinephrine to be increased in the brain of a diabetic rat. Accumulation of these monoamines, produced by inhibition of monoamine oxydase with pargyline (100 mg/kg i.p.) decreased in animals made diabetic by alloxan or streptozotocin (100 mg/kg i.p.) suggesting a decrease in deamination rate. Surprisingly, however, one week after an intracerebroventricular administration of non-diabetogenic doses of streptozotocin (5-20 mg/kg) or alloxan (20 mg/kg), changes in brain monoamines were similar to those observed in diabetic animals. This observation apparently suggests that the CNS effect of streptozotocin or alloxan is not necessarily related to a diabetogenic, beta-cytotoxic action of these substances.

Diabetes mellitus: stress, neurochemistry and behavior

Neurochemical alterations in several rodent models of insulin-dependent diabetes are compared and their relevance to behavioral and physiological pathology in the clinical disorder is discussed. In the majority of rodent models, reductions in metabolism of norepinephrine (NE), dopamine (DA) and serotonin (5HT) in the central nervous system (CNS) have been reported. While there are two reports of increased 5HT turnover in CSF or post-mortem brains of diabetic humans experiencing severe ketoacidosis, these patients were receiving insulin therapy. Insulin appears to have effects on monoamines opposite to that of chronic hyperglycemia. Both in rodent models and in clinical populations, there is widespread evidence of enhanced hormonal and behavioral responsiveness to stress. There are findings in rodent models indicating that hormonal responses to stress are related to CNS monoamine activity. The mechanisms responsible for both hormonal and CNS alterations in diabetes, as well as their involvement in behavioral pathology, can best be investigated further using animal models.

Effects of insulin on rat brain noradrenaline

A single intracardial injection of streptozotocin produced a significant increase in rat hypothalamic noradrenaline while no changes were observed in the olfactory tubercles. The parenteral administration of a single dose of insulin decreased rat hypothalamic noradrenaline; the effect had a rapid onset and lasted for at least six hours. Similar noradrenaline reductions were observed in the olfactory tubercles but in this tissue the depletion started later and recovered earlier. In addition, in olfactory tubercles after insulin injection, tyrosine level and dopamine metabolism were increased. The results show that the increases in hypothalamic NA observed in streptozotocin diabetic rats are counteracted by insulin administration and possibly the consequence of changes in noradrenaline turnover.

Behavioral and neurochemical profile of the spontaneously diabetic Wistar BB rat

The overall objective of the present investigation was to examine the behavioral and neurochemical profile of long-term diabetes (2-4 months), in the spontaneously diabetic Wistar BB rat (SDR). This animal model mimics the salient symptomatology of Type-I diabetes in man and circumvents confounds attributed to non-specific effects encountered in the chemically-induced models of diabetes. The first set of experiments were designed to investigate the effects of dopamine (DA) agonists and circadian cycle on the following spontaneous behaviors: locomotion, floor activity, rearing frequency and rearing duration. The results demonstrated that the SDR manifests (1) a blunted response to D-amphetamine (0.5-3.0 mg/kg; i.p.), and (2) lower levels of spontaneous locomotor and rearing activity in the latter part of the dark cycle, particularly at the transition of the cycle from dark to light. The next set of experiments assessed the status of brain catecholamine and metabolite levels in the insulin maintained and deprived SDR. The regional catecholamine and metabolite levels of the insulin-maintained SDR were not significantly different from those of the non-diabetic or the genetically distinct controls. However, the cessation of insulin administration to the SDR for 4 days resulted in significant increases in the levels of norepinephrine in the cortex and the hypothalamus, DA in the hippocampus, and homovanillic acid in the striatum.

Regional brain monoamines and their metabolites after portacaval shunting

Disturbances in brain monoamine neurotransmitter metabolism have been implicated in the development of hepatic encephalopathy produced by portacaval shunting or liver disease. We have measured the content of serotonin, norepinephrine and dopamine, as well as their metabolites 5-hydroxyindoleacetic acid, dihydroxyphenylacetic acid and homovanillic acid in nine selected brain areas of rats with portacaval shunts and sham-operated control rats. All substances were measured in single samples using high performance liquid chromatography with electrochemical detection, after a simple extraction procedure. In shunted rats serotonin content was 26% higher in the raphe nuclei area, and 5-hydroxyindoleacetic acid throughout the brain (by 51 to 137%), suggesting increased serotonin turnover. Norepinephrine content was higher by 26% in the frontal cortex. Dopamine content was unaffected; however its metabolites were higher in a few areas including the caudate and ventral tegmentum. Brain content of the monoamine precursor amino acids tryptophan, tyrosine and phenylalanine was higher throughout the brain in the shunted rats. The results suggest that serotonin metabolism is altered throughout the brain after portacaval shunting, which could be related to some of the characteristic behavioral abnormalities found in this condition. Catecholamine metabolism appears to be more selectively and less extensively affected.

Diabetes-induced changes in monoamine concentrations of rat hypothalamic nuclei

Streptozotocin-induced diabetes produced marked alterations of monoamine concentrations in several hypothalamic nuclei of male and female rats. Norepinephrine (NE) concentrations were significantly elevated in the median eminence (ME), supraoptic nucleus (SON) and periventricular nucleus (PEVN) in both sexes of diabetic rats. NE concentrations in the suprachiasmatic nucleus (SCN) and ventromedial nucleus (VMN) of male and female diabetic animals remained unaltered. Serotonin (5-HT) concentrations were increased in PEVN of male and female diabetic rats. No significant changes in hypothalamic dopamine (DA) concentrations were observed. Insulin treatment reversed the diabetes-related changes in monoamine concentrations in most of the nuclei. The significance of these biochemical changes relative to the endocrine and behavioral abnormalities in diabetes is discussed.