Selective inhibition of amine oxidases differently potentiate the hypophagic effect of benzylamine in mice

Hyperglycemia and reduced adiposity of streptozotocin-induced diabetic mice are not alleviated by oral benzylamine supplementation

BACKGROUND

Benzylamine (Bza) oral administration delays the onset of hyperglycemia in insulin-resistant db^-/- mice; a genetic model of obesity and type 2 diabetes.

AIM

To extend the antihyperglycemic properties of oral benzylamine to a model of insulin-deficient type 1 diabetes.

METHODS

Male Swiss mice were rendered diabetic by streptozotocin treatment (STZ) and divided in two groups: one received 0.5% Bza as drinking solution for 24 d (STZ Bza-drinking) while the other was drinking water ad libitum. Similar groups were constituted in age-matched, nondiabetic mice. Food intake, liquid intake, body weight gain and nonfasting blood glucose levels were followed during treatment. At the end of treatment, fasted glycemia, liver and white adipose tissue (WAT) mass were measured, while glucose uptake assays were performed in adipocytes.

RESULTS

STZ diabetic mice presented typical features of insulin-deficient diabetes: reduced body mass and increased blood glucose levels. These altered parameters were not normalized in the Bza-drinking group in spite of restored food and water intake. Bza consumption could not reverse the severe fat depot atrophy of STZ diabetic mice. In the nondiabetic mice, no difference was found between control and Bza-drinking mice for any parameter. In isolated adipocytes, hexose uptake was partially activated by 0.1 mmol/L Bza in a manner that was obliterated in vitro by the amine oxidase inhibitor phenelzine and that remained unchanged after Bza supplementation. Oxidation of 0.1 mmol/L Bza in WAT was lower in STZ diabetic than in normoglycemic mice.

CONCLUSION

Bza supplementation could not normalize the altered glucose handling of STZ diabetic mice with severe WAT atrophy. Consequently, its antidiabetic potential in obese and diabetic rodents does not apply to lipoatrophic type 1 diabetic mice.

Oral Supplementation with Benzylamine Delays the Onset of Diabetes in Obese and Diabetic db-/- Mice

Substrates of semicarbazide-sensitive amine oxidase (SSAO) exert insulin-like actions in adipocytes. One of them, benzylamine (Bza) exhibits antihyperglycemic properties in several rodent models of diabetes. To further study the antidiabetic potential of this naturally occurring amine, a model of severe type 2 diabetes, the obese db-/- mouse, was subjected to oral Bza administration. To this end, db-/- mice and their lean littermates were treated at 4 weeks of age by adding 0.5% Bza in drinking water for seven weeks. Body mass, fat content, blood glucose and urinary glucose output were followed while adipocyte insulin responsiveness and gene expression were checked at the end of supplementation, together with aorta nitrites. Bza supplementation delayed the appearance of hyperglycemia, abolished polydypsia and glycosuria in obese/diabetic mice without any detectable effect in lean control, except for a reduction in food intake observed in both genotypes. The improvement of glucose homeostasis was observed in db-/- mice at the expense of increased fat deposition, especially in the subcutaneous white adipose tissue (SCWAT), without sign of worsened inflammation or insulin responsiveness and with lowered circulating triglycerides and uric acid, while NO bioavailability was increased in aorta. The higher capacity of SSAO in oxidizing Bza in SCWAT, found in the obese mice, was unaltered by Bza supplementation and likely involved in the activation of glucose utilization by adipocytes. We propose that Bza oxidation in tissues, which produces hydrogen peroxide mainly in SCWAT, facilitates insulin-independent glucose utilization. Bza could be considered as a potential agent for dietary supplementation aiming at preventing diabetic complications.

Methylamine-dependent release of nitric oxide and dopamine in the CNS modulates food intake in fasting rats

BACKGROUND AND PURPOSE

Methylamine is an endogenous aliphatic amine exhibiting anorexigenic properties in mice. The aim of this work was to show whether methylamine also modifies feeding behaviour in rats and, if so, to identify the mediator(s) responsible for such effects.

EXPERIMENTAL APPROACH

Microdialysis experiments with the probe inserted in the periventricular hypothalamic nucleus were carried out in 12 h starved, freely moving rats. Collected perfusate samples following methylamine injection (i.c.v.) were analysed for nitric oxide by chemiluminescence and for dopamine and 5-hydroxytryptamine content by HPLC. Kv1.6 potassium channel expression was reduced by antisense strategy and this decrease quantified by semi-quantitative RT-PCR analysis.

KEY RESULTS

Methylamine showed biphasic dose-related effects on rat feeding. At doses of 15-30 microg per rat, it was hyperphagic whereas higher doses (60-80 microg) were hypophagic. Methylamine stimulated central nitric oxide (+115% vs. basal) following hyperphagic and dopamine release (60% over basal values) at hypophagic doses, respectively. Treatment with L-N(G)-nitro-L-arginine-methyl ester (i.c.v. 2 microg 10 microl(-1)) or with alpha-methyl-p-tyrosine (i.p. 100 mg kg(-1)) before methylamine injection, reduced nitric oxide output and hyperphagia, or dopamine release and hypophagia respectively. Moreover, hypophagia and hyperphagia, as well as nitric oxide and dopamine release were significantly reduced by down-regulating brain Kv1.6 potassium channel expression.

CONCLUSIONS AND IMPLICATIONS

The effects of methylamine on feeding depend on the hypothalamic release of nitric oxide and dopamine as a result of interaction at the Kv1.6 channels. The study of methylamine levels in the CNS may provide new perspectives on the physiopathology of alimentary behaviour.

4-methyl benzylamine stimulates food consumption and counteracts the hypophagic effects of amphetamine acting on brain Shaker-like Kv1.1 channels

1.--4-methyl benzylamine (4-MBZ; 28 microg, 231 nmol) elicits a hyperphagic response in starved mice in contrast to the hypophagia induced by the parent compound benzylamine (BZ; 33 microg, 231 nmol) or by amphetamine (AMPH, 2 mug). 2.--In mice starved for only 4 h, and therefore with little stimulation to eat, the maximal increase in food consumption induced by intracerebroventricular (i.c.v.)-injected 4-MBZ was 190% over that of the controls (ED(50) 8.3+/-2.7 microg mouse(-1); 68+/-22 nmol mouse(-1)), whereas after i.p. administration, these values were 160% and approximately 129 mg kg(-1), respectively. 3.--The hyperphagic effect of 4-MBZ was reduced by more than 60% in mice pretreated with antisense oligodeoxyribonucleotide (aODN(1)) previously found to selectively inhibit (over 50%) the expression of Shaker-like Kv1.1 channels. 4.--In mice highly stimulated to eat after 12-h fasting, 4-MBZ (28 microg) significantly reduced (to about 70%) the hypophagic response by AMPH (2 microg) or BZ (33 microg). Conversely, these two compounds reduced (respectively, by 69 and 44%) the hyperphagic response of 4-MBZ in 4-h fasting mice. 5.--4-MBZ (28 microg) also reduced the hypermotility and the stimulation of inspection activity elicited by AMPH in mice and the release of DA stimulated by AMPH (2 microg) from the nucleus accumbens of rats. We hypothesize that 4-MBZ elicits hyperphagic effects probably by opening Shaker-like Kv1.1 subtypes in the brain, whereas AMPH and BZ are hypophagic by blocking these channels.

Effects of oral administration of benzylamine on glucose tolerance and lipid metabolism in rats

Repeated administration of benzylamine plus vanadate have been reported to exhibit anti-hyperglycemic effects in different models of diabetic rats. Likewise oral treatment with Moringa oleifera extracts which contain the alkaloïd moringine, identical to benzylamine, has also been shown to prevent hyperglycemia in alloxan-induced diabetic rats. With these observations we tested whether prolonged oral administration of benzylamine could interact with glucose and/or lipid metabolism. Seven week old male Wistar rats were treated for seven weeks with benzylamine 2.9 g/l in drinking water and were submitted to glucose tolerance tests. A slight decrease in water consumption was observed in benzylamine-treated animals while there was no change in body and adipose tissue weights at the end of treatment. Blood glucose and plasma insulin, triacylglycerol or cholesterol levels were not modified. However, benzylamine treatment resulted in a decrease in plasma free fatty acids in both fed and fasted conditions. Benzylamine treatment improved glucose tolerance as shown by the reduction of hyperglycemic response to intra-peritoneal glucose load. Oral benzylamine treatment did not alter the response of adipocytes to insulin nor to insulin-like actions of benzylamine plus vanadate, via in vitro activation of glucose transport or inhibition of lipolysis. This work demonstrates for the first time that oral administration of benzylamine alone influences glucose and lipid metabolism. However, these results obtained in normoglycemic rats require to be confirmed in diabetic models.

Dysregulation of striatal dopamine signaling by amphetamine inhibits feeding by hungry mice

Amphetamine (AMPH) releases monoamines, transiently stimulates locomotion, and inhibits feeding. Using a genetic approach, we show that mice lacking dopamine (DA-deficient, or DD, mice) are resistant to the hypophagic effects of a moderate dose of AMPH (2 microg/g) but manifest normal AMPH-induced hypophagia after restoration of DA signaling in the caudate putamen by viral gene therapy. By contrast, AMPH-induced hypophagia in response to the same dose of AMPH is not blunted in mice lacking the ability to make norepinephrine and epinephrine (Dbh(-/-)), dopamine D(2) receptors (D2r(-/-)), dopamine D(1) receptors (D1r(-/-)), serotonin 2C receptors (Htr2c(-/Y)), neuropeptide Y (Npy(-/-)), and in mice with compromised melanocortin signaling (A(y)). We suggest that, at this moderate dose of AMPH, dysregulation of striatal DA is the primary cause of AMPH-induced hypophagia and that regulated striatal dopaminergic signaling may be necessary for normal feeding behaviors.

Methylamine, but not ammonia, is hypophagic in mouse by interaction with brain Kv1.6 channel subtype

Ammonia and methylamine (MET) are endogenous compounds increased during liver and renal failure, Alzheimer's disease, vascular dementia and diabetes, where they alter some neurobehavioural functions probably acting as potassium channel blockers. We have already described that potassium channel blockers including tetraethylammonium (TEA), ammonia and MET are hypophagic in mice. Antisense oligonucleotides (aODNs) against Shaker-like Kv1.1 gene abolished the effect of TEA but not of ammonia and MET. The central effects elicited in fasted mice by ammonia and MET were further studied. For MET, an ED(50) value 71.4+/-1.8 nmol mouse(-1) was calculated. The slope of the dose-response curves for these two compounds and the partial hypophagic effect elicited by ammonia indicated a different action mechanism for these amines. The aODNs pretreatments capable of temporarily reducing the expression of all seven known subtypes of Shaker-like gene or to inactivate specifically the Kv1.6 subtype abolished the hypophagic effect of MET but not that of ammonia. Reverse transcription-polymerase chain reaction, Western blot and immunohistochemical results indicate that a full expression in the brain of Kv1.6 is required only for the activity of MET, and confirms the different action mechanism of ammonia and MET.

Antisense knockdown of the Shaker-like Kv1.1 gene abolishes the central stimulatory effects of amphetamines in mice and rats

Amphetamine (AMPH) is an indirect sympathomimetic compound classified as a substrate-type releaser that distinguishes it from other stimulants that act as uptake 1 blockers, such as cocaine (COC). In mammals, AMPH elicits central stimulation, hypermotility, anorexia, analgesia and analeptic activity, mainly through the increase of extracellular brain dopamine (DA). The inversion of vesicular transporters and/or intravesicular alkalinization is assumed to have a role in AMPH-induced exocytosis. However, the action mechanism of this compound has not yet been completely clarified. Recent evidence on the action of AMPHs indicates potassium channel-blocking properties in peripheral tissues. We investigated the possible involvement of a Shaker-like Kv1.1 channel subtype in the central effects of AMPH, using an antisense oligodeoxyribonucleotide (aODN) that specifically and reversibly inhibits the expression of these channels in the brain. The effect of aODN pretreatments was studied by evaluating the modification of behavioral effects induced in mice through the intracerebroventricular administration of AMPH, COC, or other compounds. The aODN in mice almost completely blocked the stimulatory effects of AMPH and other releasers but was ineffective in reducing the central activity of COC. In aODN-pretreated rats a strong reduction of the AMPH, but not of the COC-stimulated DA efflux from nucleus accumbens was observed. Our results suggest that the stimulant effects of AMPH and chemically related compounds, but not COC, require the presence of functionally active Kv1.1 channels in the brain.

Methylamine and benzylamine induced hypophagia in mice: modulation by semicarbazide-sensitive benzylamine oxidase inhibitors and aODN towards Kv1.1 channels

1. In starved mice, the anorectic activity of methylamine (MET) and benzylamine (BZ), both substrates of semicarbazide-sensitive benzylamine oxidases (Bz-SSAO), was compared with that of the potassium channel blocking agents charybdotoxin (ChTX), tetraethylammonium (TEA), gliquidone (GLI), ammonium chloride (NH(4)(+)) and of the anoressants amphetamine (AMPH) and nicotine (NIC). After i.c.v. administration, an approximate ranking order of potency was: ChTX> or =AMPH>NIC=TEA> or =GLI> or =MET>BZ>NH(4)(+). 2. Clorgyline (2.5 mg kg(-1) i.p.) or deprenyl (10 mg kg(-1) i.p.) potentiated the anorectic effect of i.c.v.-administered BZ, NIC and AMPH. The effect of TEA was increased only by deprenyl, while MET, NH(4)(+), ChTX and GLI were not affected by either of the inhibitors. 3. The Bz-SSAO inhibitors alpha-aminoguanidine (50 mg kg(-1) i.p.), B24 (100 mg kg(-1) i.p.) and MDL 72274 (2.5 mg kg(-1) i.p.) potentiated the effect of i.p., but not of i.c.v.-administered MET. 4. Antisense oligodeoxyribonucleotides (aODN) to Kv1.1 potassium channels abolished the effect of BZ and TEA, but was ineffective in reducing the activity of MET and other compounds. 5. These results suggest that MET is endowed with peculiar hypophagic effects at dosage levels that are not able to affect gross behaviour in mice. The effect of MET, differently from BZ, seems unrelated to an increase in the central release of monoaminergic mediators, as well as to a Kv1.1 blocking activity. Through a reduction of the endogenous breakdown of MET, Bz-SSAO inhibitors enhance the central pharmacological activity of this amine.