Amantadine reduces glucagon and enhances insulin secretion throughout the oral glucose tolerance test: central plus peripheral nervous system mechanisms

Glutamate as intracellular and extracellular signals in pancreatic islet functions

l-Glutamate is one of the most abundant amino acids in the body and is a constituent of proteins and a substrate in metabolism. It is well known that glutamate serves as a primary excitatory neurotransmitter and a critical neuromodulator in the brain. Recent studies have shown that in addition to its pivotal role in neural functions, glutamate plays many important roles in a variety of cellular functions, including those as intracellular and extracellular signals. In pancreatic islets, glutamate is now known to be required for the normal regulation of insulin secretion, such as incretin-induced insulin secretion. In this review, we primarily discuss the physiological and pathophysiological roles of glutamate as intracellular and extracellular signals in the functions of pancreatic islets.

Peripheral and Central Effects of Memantine in a Mixed Preclinical Mice Model of Obesity and Familial Alzheimer's Disease

There is growing evidence that obesity associated with type 2 diabetes mellitus (T2DM) and aging are risk factors for the development of Alzheimer's disease (AD). However, the molecular mechanisms through which obesity interacts with β-amyloid (Aβ) to promote cognitive decline remains poorly understood. Memantine (MEM), a N-methyl-D-aspartate receptor antagonist, is currently used for the treatment of AD. Nonetheless, few studies have reported its effects on genetic preclinical models of this neurodegenerative disease exacerbated with high-fat diet (HFD)-induced obesity. Therefore, the present research aims to elucidate the effects of MEM on familial AD HFD-induced insulin resistance and learning and memory impairment. Furthermore, it aspires to determine the possible underlying mechanisms that connect AD to T2DM. Wild type and APPswe/PS1dE9 mice were used in this study. The animals were fed with either chow or HFD until 6 months of age, and they were treated with MEM-supplemented water (30 mg/kg) during the last 12 weeks. Our study demonstrates that MEM improves the metabolic consequences produced by HFD in this model of familial AD. Behavioural assessments confirmed that the treatment also improves animals learning abilities and decreases memory loss. Moreover, MEM treatment improves brain insulin signalling upregulating AKT, as well as cyclic adenosine monophosphate response element binding (CREB) expression, and modulates the amyloidogenic pathway, which, in turn, reduced the accumulation of Aβ. Moreover, this drug increases the activation of molecules involved with insulin signalling in the liver, such as insulin receptor substrate 2 (IRS2), which is a key protein regulating hepatic resistance to insulin. These results provide new insight into the role of MEM not only in the occurrence of AD treatment, but also in its potential application on peripheral metabolic disorders where Aβ plays a key role, as is the case of T2DM.

Characterization of pancreatic NMDA receptors as possible drug targets for diabetes treatment

In the nervous system, NMDA receptors (NMDARs) participate in neurotransmission and modulate the viability of neurons. In contrast, little is known about the role of NMDARs in pancreatic islets and the insulin-secreting beta cells whose functional impairment contributes to diabetes mellitus. Here we found that inhibition of NMDARs in mouse and human islets enhanced their glucose-stimulated insulin secretion (GSIS) and survival of islet cells. Further, NMDAR inhibition prolonged the amount of time that glucose-stimulated beta cells spent in a depolarized state with high cytosolic Ca(2+) concentrations. We also noticed that, in vivo, the NMDAR antagonist dextromethorphan (DXM) enhanced glucose tolerance in mice, and that in vitro dextrorphan, the main metabolite of DXM, amplified the stimulatory effect of exendin-4 on GSIS. In a mouse model of type 2 diabetes mellitus (T2DM), long-term treatment with DXM improved islet insulin content, islet cell mass and blood glucose control. Further, in a small clinical trial we found that individuals with T2DM treated with DXM showed enhanced serum insulin concentrations and glucose tolerance. Our data highlight the possibility that antagonists of NMDARs may provide a useful adjunct treatment for diabetes.

Anorexia nervosa versus hyperinsulinism: therapeutic effects of neuropharmacological manipulation

Background:

We have demonstrated that anorexia nervosa is underpinned by overwhelming adrenal sympathetic activity which abolishes the neural sympathetic branch of the peripheral autonomic nervous system. This physiological disorder is responsible for gastrointestinal hypomotility, hyperglycemia, raised systolic blood pressure, raised heart rate, and other neuroendocrine disorders. Therefore, we prescribed neuropharmacological therapy to reverse this central and autonomic nervous system disorder, in order to normalize the clinical and neuroendocrine profile.

Methods:

The study included 22 female patients with anorexia nervosa (10 restricted type, 12 binge-eating type) who received three months of treatment with amantadine 100 mg/day. We measured blood pressure, heart rate, and circulating neurotransmitters, (noradrenaline, adrenaline, dopamine, platelet serotonin, free plasma serotonin) during supine resting, one minute of orthostasis, and a five-minute exercise test before and after one, two, and three months of treatment with amantadine, a drug which abrogates adrenal sympathetic activity by acting at the C1(Ad) medullary nuclei responsible for this branch of the peripheral sympathetic activity.

Results:

We found the amantadine abolished symptoms of anorexia nervosa from the first oral dose onwards. Normalization of autonomic and cardiovascular parameters was demonstrated within the early days of therapy. Abrupt and sustained increases in the plasma noradrenaline:adrenaline ratio and disappearance of abnormal plasma glucose elevation were registered throughout the three-month duration of the trial. Significant and sustained increases in body weight were documented in all cases. No relapses were observed.

Conclusion:

We have confirmed our previously published findings showing that the anorexia nervosa syndrome depends on the hypomotility of the gastrointestinal tract plus hyperglycemia, both of which are triggered by adrenal sympathetic hyperactivity. The above neuroendocrine plus neuroautonomic and clinical disorders which underpinned anorexia nervosa were abruptly suppressed since the first oral dose of amantadine, a drug able to revert the C1(Ad) over A5(NA) pontomedullary predominance responsible for adrenal and neural sympathetic activity, respectively.

Anorexia nervosa depends on adrenal sympathetic hyperactivity: opposite neuroautonomic profile of hyperinsulinism syndrome

OBJECTIVE

The aim of our study was to determine the central and peripheral autonomic nervous system profiles underlying anorexia nervosa (AN) syndrome, given that affected patients present with the opposite clinical profile to that seen in the hyperinsulinism syndrome.

DESIGN

We measured blood pressure and heart rate, as well as circulating neurotransmitters (noradrenaline, adrenaline, dopamine, plasma serotonin, and platelet serotonin), using high-performance liquid chromatography with electrochemical detection, during supine resting, one minute of orthostasis, and after five minutes of exercise. In total, 22 AN patients (12 binge-eating/purging type and 10 restricting type) and age-, gender-, and race-matched controls (70 ± 10.1% versus 98 ± 3.0% of ideal body weight) were recruited.

RESULTS

We found that patients with AN had adrenal sympathetic overactivity and neural sympathetic underactivity, demonstrated by a predominance of circulating adrenaline over noradrenaline levels, not only during the supine resting state (52 ± 2 versus 29 ± 1 pg/mL) but also during orthostasis (67 ± 3 versus 32 ± 2 pg/mL, P < 0.05) and after exercise challenge (84 ± 4 versus 30 ± 3 pg/mL, P < 0.01).

CONCLUSION

Considering that this peripheral autonomic nervous system disorder depends on the absolute predominance of adrenomedullary C1 adrenergic nuclei over A5 noradrenergic pontine nucleus, let us ratify the abovementioned findings. The AN syndrome depends on the predominance of overwhelming adrenal sympathetic activity over neural sympathetic activity. This combined central and autonomic nervous system profile contrasts with that registered in patients affected by hyperinsulinism, hypoglycemia, and bulimia syndrome which depends on the absolute predominance of neural sympathetic activity.

Effects of amantadine on circulating neurotransmitters in healthy subjects

Considering that glutamatergic axons innervate the C1(Ad) medullary nuclei, which are responsible for the excitation of the peripheral adrenal glands, we decided to investigate catecholamines (noradrenaline, adrenaline and dopamine) plus indolamines (plasma serotonin and platelet serotonin) at the blood level, before and after a small oral dose of amantadine, a selective NMDA antagonist. We found that the drug provoked a selective enhancement of noradrenaline plus a minimization of adrenaline, dopamine, plasma serotonin and platelet serotonin circulating levels. Significant enhancement of diastolic blood pressure plus reduction of systolic blood pressure and heart rate paralleled the circulating parameter changes. The above findings allow us to postulate that the drug was able to enhance the peripheral neural sympathetic activity. Minimization of both adrenal sympathetic and parasympathetic activities was also registered after the amantadine challenge. The above findings supported the postulation that this drug should be a powerful therapeutic tool for treating diseases affected by adrenal sympathetic hyperactivity.