Central 5-HT3 receptor stimulation by m-CPBG increases blood glucose in rats.

The aim of the present study was to investigate the role of central 5-HT3 receptors on the control of blood glucose in stressed and non-stressed rats in both fasted and fed states. Adult Wistar male rats had each their third ventricle cannulated 7 days before the experiments. Injections of m-CPBG, a selective 5-HT3 receptor agonist, induced a significant increase in blood glucose in non-stressed rats in both fasted and in fed states. The same procedure was unable to modify stress-induced hyperglycemia. The hyperglycemic effect of m-CPBG central administration was blocked by pretreatment with ondansetron, a specific 5-HT3 receptor antagonist, indicating that the effects here obtained with m-CPBG were a result of its interaction with 5-HT3 receptors. Third ventricle injections of ondansetron alone were not able to modify blood glucose in non-stressed animals and did not change the hyperglycemic responses observed after immobilization stress. We conclude that pharmacological activation of the central 5-HT3 receptor induces a hyperglycemic effect in non-stressed animals.     

Acute hyperglycemia induced by ketamine/xylazine anesthesia in rats: mechanisms and implications for preclinical models

The effects of anesthetic agents, commonly used in animal models, on blood glucose levels in fed and fasted rats were investigated. In fed Sprague-Dawley rats, ketamine (100 mg/kg)/xylazine (10 mg/kg) (KX) produced acute hyperglycemia (blood glucose 178.4 +/- 8.0 mg/dl) within 20 min. The baseline blood glucose levels (104.8 +/- 5.7 mg/dl) reached maximum levels (291.7 +/- 23.8 mg/dl) at 120 min. Ketamine alone did not elevate glucose levels in fed rats. Isoflurane also produced acute hyperglycemia similar to KX. Administration of pentobarbital sodium did not produce hyperglycemia in fed rats. In contrast, none of these anesthetic agents produced hyperglycemia in fasted rats. The acute hyperglycemic effect of KX in fed rats was associated with decreased plasma levels of insulin, adrenocorticotropic hormone (ACTH), and corticosterone and increased levels of glucagon and growth hormone (GH). The acute hyperglycemic response to KX was dose-dependently inhibited by the specific alpha2-adrenergic receptor antagonist yohimbine (1-4 mg/kg). KX-induced changes of glucoregulatory hormone levels such as insulin, GH, ACTH, and corticosterone were significantly altered by yohimbine, whereas the glucagon levels remained unaffected. In conclusion, the present study indicates that both KX and isoflurane produce acute hyperglycemia in fed rats. The effect of KX is mediated by modulation of the glucoregulatory hormones through stimulation of alpha2-adrenergic receptors. Pentobarbital sodium did not produce hyperglycemia in either fed or fasted rats. Based on these findings, it is suggested that caution needs to be taken when selecting anesthetic agents, and fed or fasted state of animals in studies of diabetic disease or other models where glucose and/or glucoregulatory hormone levels may influence outcome and thus interpretation. However, fed animals are of value when exploring the hyperglycemic response to anesthetic agents.     

The roles of glucagon and adrenal epinephrine in mediating hyperglycemia induced by third cerebroventricular injection of bombesin

The roles of glucagon and adrenal epinephrine in mediating bombesin-induced central hyperglycemia were further studied in anesthetized rats. Bombesin (10(-9) mol) injected into the third cerebral ventricle produced an increase in plasma concentrations of glucose, glucagon, and epinephrine. Prior bilateral adrenalectomy completely prevented the hyperglucagonemic and hyperglycemic responses to third cerebral ventricle injection of bombesin. These results support the view that bombesin-induced increases in plasma glucose and glucagon are fully dependent on adrenal epinephrine secretion. Furthermore, during constant intravenous infusion of somatostatin, the hyperglycemic response to third cerebral ventricle injection of bombesin was not significantly influenced despite complete inhibition of the increase in plasma glucagon. Therefore, it is suggested that bombesin-induced central hyperglycemia is mainly mediated by epinephrine itself rather than via epinephrine-stimulated glucagon secretion.     

Catecholamine-induced changes in plasma glucose, glucagon and insulin in rabbits: effects of somatostatin.

The present study was conducted to determine if glucagon release is involved in the hyperglycemic response to epinephrine and isoproterenol in the fasted and fed, unanesthetized rabbit. Epinephrine produced dose-related increases in plasma glucose and glucagon levels in fed and fasted rabbits whereas isoprotereol produced modest hyperglycemia without hyperglucagonemia. Infusion of somatostatin suppressed epinephrine-induced glucagon release and this was correlated with a 50% reduction in the hyperglycemic response. These data suggest that epinephrine-induced glucagon release is the primary reason for the difference in hyperglycemic activity between epinephrine and isoproterenol in the unanesthetized rabbit.     

Serotonergic mechanism in the control of β-endorphin and acth release in male rats

The role of the serotonergic mechanism in the regulation of β-endorphin (β-EP) and adrenocorticotropin (ACTH)-like immunoreactivity in plasma was investigated. Increases in β-EP and ACTH-LI produced by quipazine maleate (QPZ), a serotonergic agonist, 1 hr after injection could be completely prevented by the serotonin (5-HT) antagonist, cinanserin (CIN), which when injected alone, decreased basal plasma concentrations of both β-EP-LI and ACTH-LI. Concurrent injections of L-5-HTP with the 5-HT reuptake inhibitor, fluoxetine, produced an additive increase in plasma β-EP-LI 1 hr after injection. Injection of the 5-HT antagonist, cyproheptadine, significantly decreased plasma β-EP-LI. Stress by immobilization for 30 min or exposing the rats to 40° ± 1°C for 30 min produced an approximate 4-fold increase in plasma β-EP-LI and ACTH-LI, which was potentiated by I.P. injections of fluoxetine. Furthermore, the stress induced increases in plasma concentrations of β-EP-LI and ACTH-LI were significantly reduced by the serotonin antagonists metergoline and cinanserin. These results suggest that 5-HT is a potent stimulator of both β-EP and ACTH release and the increase in plasma concentrations of ACTH and β-EP induced by stress are probably mediated, at least in part, by central serotonergic mechanisms.     

Brain Indoleamines in Alloxan- and Streptozotocin-Induced Diabetic Rats

Previous work by other authors has shown that alloxan-induced diabetes increases whereas streptozotocin-induced diabetes does not alter nonesterified fatty acid (NEFA) plasma levels. The present study replicates these results and demonstrates that fasted, streptozotocin-induced diabetic animals also have increased NEFA levels. In addition, brain levels of 5-hydroxytryptamine (5-HT) and of its immediate precursor and metabolite were measured. Alloxan- and fasted, streptozotocin-induced diabetic rats showed significant increases in brain indoleamine concentrations, whereas fed, streptozotocin-induced diabetic rats had unchanged levels of the same compounds. Levels of brain indoleamines exhibited a strong positive correlation with wet-dog shakes (an index of 5-HT activity) elicited by hippocampal stimulation. Blockade of wet-dog shakes by 5-HT receptor antagonists strengthens the proposal that this behavior is a good index of central 5-HT activity. The increased content of brain indoleamines in alloxan- and fasted, streptozotocin-induced diabetic rats may be related to the increased NEFA plasma levels seen in the same animals. This hypothesis is supported by the positive correlation demonstrated between NEFA and 5-HT levels. In conclusion, it is suggested that alloxan-induced diabetes may represent a useful model for studying the various behavioral changes known to occur in diabetics.     

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 withMoringa 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, viain 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.     

Effects of imipramine and serotonin-2 agonists and antagonists on serotonin-2 and beta-adrenergic receptors following noradrenergic or serotonergic denervation

The effects of chronic (14 day) administration of the tricyclic antidepressant imipramine, the serotonin-2 (5-HT2) antagonist ketanserin, and the serotonin agonist quipazine on 5-HT2 receptor binding parameters and 5-HT2-mediated behavior were examined in rats with or without prior serotonergic denervation [via 5,7-dihydroxytryptamine (5,7-DHT)] or noradrenergic denervation [via N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4)]. Chronic administration of imipramine, ketanserin, or quipazine produced a marked reduction in the number of 5-HT2 binding sites which was accompanied by reductions in the 5-HT2-mediated quipazine-induced head shake response. In animals receiving DSP4 or 5,7-DHT lesions and continuous vehicle treatment, beta-adrenergic receptor binding sites were significantly up-regulated while 5-HT2 receptor binding sites did not change. Imipramine normalized the lesion-induced increases in beta-adrenergic binding observed in DSP4 and 5,7-DHT-lesioned rats but failed to down-regulate beta-adrenergic binding sites below non-lesioned control levels. Chronic imipramine, ketanserin, and quipazine reduced quipazine-induced head shakes and down-regulated 5-HT2 binding sites in rats with noradrenergic denervation. While imipramine, ketanserin, and quipazine all down-regulated 5-HT2 binding sites in animals with serotonergic denervation, only imipramine's ability to reduce quipazine-induced head shakes was attenuated in 5,7-DHT-lesioned rats. The present results suggest that imipramine-induced down-regulation of 5-HT2 receptors may not involve presynaptic 5-HT mechanisms, and imipramine-induced alterations in 5-HT2 sensitivity as reflected in the quipazine-induced head shake may, in part, be influenced by beta-adrenergic receptors.     

Reciprocal changes of plasma glucose and ketone bodies in fasted and acutely diabetic rats after CNS stimulation

To assess the effect of chemical stimulation of the central nervous system (CNS) on ketogenesis, we injected neostigmine (5 x 10(-8)mol) into the third cerebral ventricle in normal rats fasted for 48 h and fed rats with diabetes induced by streptozotocin (STZ, 80 mg/kg). The hepatic venous plasma levels of ketone bodies (3-hydroxybutyrate and acetoacetate), free fatty acids (FFA), and glucose were measured for 120 min after the injection of neostigmine under pentobarbital anesthesia. In the normal rats, plasma glucose levels were significantly increased but neither ketone bodies nor FFA were affected by CNS stimulation with neostigmine. In contrast the plasma levels of ketone bodies and FFA were significantly increased in STZ-diabetic rats, while glucose levels remained unchanged. The intravenous infusion of somatostatin (1.0 microgram/kg/min) suppressed the increase in plasma ketone bodies following CNS stimulation in STZ-diabetic rats. These findings suggest that CNS stimulation with neostigmine may accelerate ketogenesis by promoting the lipolysis, which may be induced by glucagon, in fed diabetic rats but not in normal fasted rats.     

Hypoinsulinemic and hyperglycemic effects of beta-endorphin in rabbits

The present study was designed to investigate the in vivo effects of beta-endorphin on plasma levels of glucagon, insulin and glucose in rabbits, and to elucidate some of the mechanisms involved. beta-Endorphin (50 micrograms) injected intravenously into fasted rabbits, decreased plasma levels of insulin (-4.5 +/- 1.3 microU/ml, P less than 0.05) and increased plasma levels of glucose (+2.7 +/- 0.4 mmol/l, P less than 0.05). Similar hypoinsulinemic and hyperglycemic effects were observed for 25 and 2.5 micrograms beta-endorphin in fasted and 50 and 0.5 micrograms beta-endorphin in fed rabbits. beta-Endorphin produced slight and transient increases in plasma levels of glucagon at the highest dose in fed rabbits, only (+80 +/- 9 pg/ml, P less than 0.05). The beta-endorphin-induced hypoinsulinemia was not inhibited by phentolamine, yohimbine, propranolol or atropine, which is in consistency with a direct inhibitory effect of beta-endorphin on the beta-cell in rabbits. The beta-endorphin-induced hyperglycemia was reduced by naloxone (+0.8 +/- 0.1 mmol/l) but not by N-methyl-naloxone (ORG 10908) a peripheral opiate receptor blocking drug (+2.2 +/- 0.2 mmol/l), suggesting a central nervous action on opiate receptors. This central action of beta-endorphin was probably not mediated by catecholamine release or other stimulation of adrenergic or muscarinic receptors, since the beta-endorphin-induced hyperglycemia was not inhibited by phentolamine, yohimbine, propranolol or atropine. These results suggest that the beta-endorphin-induced hyperglycemia was caused, at least in part, by a peripheral inhibition of insulin release and a central stimulation on glucoregulation.     

Complex role of 5-HT in the regulation of TSH secretion in the male rat

The role of 5-hydroxytryptamine (5-HT) in the regulations of TSH secretion was studied in male rats using both peripheral and central administration of the drugs. Basal TSH levels were not modified by moderate doses of 5-HT (subcutaneously) or its precursors or antagonists (intraperitoneally) given 1 h before decapitation. The cold-stimulated TSH secretion was decreased by L-tryptophan (L-TRP, 400 mg/kg i.p.), quipazine (10 mg/kg i.p.) and 5-HT (1 or 5 mg/kg s.c. or i.v.) as well as by p-chlorophenylalanine (pCPA, 20 or more mg/kg i.p.) when the drugs were given 1 h before sampling. pCPA (100-400 mg/kg i.p.) was active 24-48 h after the injection but repetitive administration did not affect TSH levels. 5-HT (5 mg/kg s.c.) was effective also in pinealectomized animals. L-TRP and 5-hydroxytryptophan potentiated the TRH-stimulated TSH secretion when given 1 h before killing. 5-HT (10 microgram/rat) infused into the third ventricle enhanced the cold-stimulated TSH secretion when given 30-45 min before sampling. When injected into the medial basal hypothalamus, 50-HT (1-10 microgram/rat) had no effect on basal or stimulated TSH levels. The results suggest: (1) 5-HT does not play any role in the regulation of basal TSH secretion; (2) in the cold-stimulated TSH secretion 5-HT has a stimulatory action evidently inside the blood-brain barrier and also an inhibitory effect obviously outside this barrier.     

Unique modulation of central 5-HT2 receptor binding sites and 5-HT2 receptor-mediated behavior by continuous gepirone treatment.

The effect of continuous treatment with the selective 5-HT1A agonist gepirone upon 5-HT2-mediated behavior and cortical 5-HT2 receptor binding sites was examined in naive rats or rats receiving noradrenergic (DSP4) or serotonergic (5,7-DHT) lesions. Continuous administration of gepirone in non-lesioned rats for 3, 7, or 14 days enhanced the head shake response to the 5-HT agonist quipazine. This enhancement of 5-HT2-mediated behavior occurred despite concomitant down-regulation of cortical 5-HT2 binding sites. However, 28 days of gepirone administration significantly reduced behavioral responsiveness to quipazine. The gepirone-induced facilitation of 5-HT2-mediated behavior observed after 7 days of continuous treatment was blocked in both DSP4 and 5,7-DHT-lesioned rats. However, both noradrenergic and serotonergic denervation failed to modify the down-regulation of 5-HT2 receptor binding sites produced by continuous gepirone administration. These results suggest that the curious dissociation of behavioral and biochemical indices of 5-HT2 receptor function produced by continuous gepirone treatment may be the result of a dual yet separate action of the drug on central presynaptic noradrenergic and serotonergic mechanisms and postsynaptic 5-HT receptors. Furthermore, the postsynaptic action of gepirone which reduces the maximal number of cortical 5-HT2 receptor binding sites may be the result of gepirone's agonist action at postsynaptic 5-HT1A receptors.     

Evidence that acute serotonergic activation potentiates the locomotor-stimulating effects of corticotropin-releasing hormone in juvenile chinook salmon (Oncorhynchus tshawytscha)

The present study investigated whether the serotonergic system is involved in mediating the behavioral effects of corticotropin-releasing hormone (CRH) in juvenile spring chinook salmon, Oncorhynchus tshawytscha. An intracerebroventricular (ICV) injection of CRH induced hyperactivity. The effect of CRH was potentiated in a dose-dependent manner by the concurrent administration of the serotonin (5-HT) selective reuptake inhibitor fluoxetine. However, administration of fluoxetine alone had no effect on locomotor activity, suggesting that the locomotor-stimulating effect of CRH is mediated by the activation of the serotonergic system. Conversely, ICV injections of the 5-HT(1A) receptor antagonist NAN-190 attenuated the effect of CRH on locomotor activity when given in combination with CRH but had no effect when administered alone. These results provide the first evidence to support the hypothesis that the effect of CRH on locomotor activity in teleosts is mediated by activating the serotonergic system.     

Amylin is more potent and more effective than glucagon in raising plasma glucose concentration in fasted, anesthetized rats.

Amylin is a 37 amino-acid peptide secreted from the pancreatic beta-cells. It has actions on carbohydrate metabolism in vivo, including elevation of blood glucose. In this study, the hyperglycemic effect of intravenous bolus injections of amylin was compared with similar injections of glucagon in 20-hour fasted rats lightly anesthetized with halothane. Administered doses ranged from 0.01 micrograms to 1000 micrograms (about 7 pmol/kg--750 nmol/kg for amylin and 8 pmol/kg--800 pmol/kg for glucagon). Control animals received an equal volume of saline. A single intravenous injection of amylin or glucagon led to an increase of plasma glucose levels, which peaked approximately at 1 hour after treatment. The calculated ED50 for amylin was 1.48 nmol whereas that for glucagon was 7.46 nmol; the maximum glucose increment was 4.3 mM for amylin, and 2.9 mM for glucagon. These results show that amylin is a more potent and more effective hyperglycemic agent than glucagon under these experimental conditions.     

Serotonergic regulation of blood glucose levels in the crayfish, Procambarus clarkii: site of action and receptor characterization

The present study investigated the site of action of 5-hydroxytryptamine (5-HT) and pharmacologically characterized the receptors involved in regulating blood glucose levels in the crayfish, Procambarus clarkii. Injection of 5-HT into intact animals increased glucose levels in a dose-dependent manner. In contrast, 5-HT failed to elicit a hyperglycemic response in eyestalk-ablated animals. Effects of several 5-HT receptor agonists and antagonists were examined. 5-CT, oxymetazoline (both 5-HT(1) receptor agonists) and alpha-methyl-5-HT (a 5-HT(2) receptor agonist), but not 1-phenylbiguanide, m-CPBG (both 5-HT(3) receptor agonists), or RS 67333 (a 5-HT(4) receptor agonist), induced hyperglycemic responses in a dose-dependent manner. In addition, 8-OH-DPAT (a 5-HT(1A) receptor agonist), L-694,247 (a 5-HT(1B/1D) receptor agonist), and DOI (a 5-HT(2A) receptor agonist) were effective in significantly increasing the glucose levels, whereas both BW 723C86 (a 5-HT(2B) receptor agonist) and m-CPP (a 5-HT(2C) receptor agonist) were ineffective. Finally, ketanserin (a 5-HT(2A) receptor antagonist), but not p-MPPF (a 5-HT(1A) receptor antagonist), GR 55562 (a 5-HT(1B/1D) receptor antagonist), SB 206553 (a 5-HT(2B/2C) receptor antagonist), or tropisetron (a 5-HT(3) receptor antagonist), was able to block 5-HT-induced hyperglycemia. The combined results support the hypothesis that 5-HT exerts its hyperglycemic effect by enhancing the release of hyperglycemic factor(s) from the eyestalks, and suggest that 5 HT-induced hyperglycemia is mediated by 5-HT(1)- and 5-HT(2)-like receptors.     

Hypothalamic insulin signaling is required for inhibition of glucose production

Circulating insulin inhibits endogenous glucose production. Here we report that bidirectional changes in hypothalamic insulin signaling affect glucose production. The infusion of either insulin or a small-molecule insulin mimetic in the third cerebral ventricle suppressed glucose production independent of circulating levels of insulin and of other glucoregulatory hormones. Conversely, central antagonism of insulin signaling impaired the ability of circulating insulin to inhibit glucose production. Finally, third-cerebral-ventricle administration of inhibitors of ATP-sensitive potassium channels, but not of antagonists of the central melanocortin receptors, also blunted the effect of hyperinsulinemia on glucose production. These results reveal a new site of action of insulin on glucose production and suggest that hypothalamic insulin resistance can contribute to hyperglycemia in type 2 diabetes mellitus.     

A comparative study of several serotonin receptor antagonists on basal and stimulus induced release of insulin and glucagon in the intact rat.

Several commonly used serotonin receptor antagonists were studied for their ability to influence basal plasma insulin and glucagon (using 30K antibody) levels as well as the response of these hormones to a glucose or arginine challenge administered systematically to overnight fasted rats. Cyproheptadine, in contrast to other antagonists employed, induced large increases of insulin, glucagon and glucose, although this hyperinsulinemia was of a smaller magnitude when compared with hormone levels observed during an equivalent hyperglycemia resulting from glucose administration. The pancreatic response to a glucose load (increased insulin and decreased glucagon release) and an arginine load (increased insulin and glucagon release) were prevented by cyproheptadine pretreatment. Basal insulin levels were bot consistently altered by methysergide or cinanserin and were slightly elevated by metergoline. Basal glucagon levels were unaffected by these drugs. These three agents potentiated the insulinotropic effect of an arginine load whereas only metergoline exerted a similar effect on the response to glucose loading. Glucagon release in response to these stimuli was not significantly altered by drug pretreatment.     

The role of hepatic glycogen stores in anaerobic metabolism in rats

Plasma levels of lactate and oxypurines markedly increased in both fed and fasted rats exposed to three acute anoxic states, cyanide poisoning, carbon monoxide poisoning and inhalation of oxygen-deficient gas, suggesting that the transition of aerobic to anaerobic metabolism occurred similarly in both groups. Plasma glucose level of fed rats increased 1.8-2.5 times after exposure to anoxia, whereas a remarkable hypoglycemia was induced by the exposure of fasted rats to anoxia. Hepatic glycogen stores in fed rats induced hyperglycemia, while exhaustion of the stores in fasted rats resulted in severe hypoglycemia during acute anoxia.     

Hepatic glycogen breakdown is implicated in the maintenance of plasma mannose concentration

D-mannose is an essential monosaccharide constituent of glycoproteins and glycolipids. However, it is unknown how plasma mannose is supplied. The aim of this study was to explore the source of plasma mannose. Oral administration of glucose resulted in a significant decrease of plasma mannose concentration after 20 min in fasted normal rats. However, in fasted type 2 diabetes model rats, plasma mannose concentrations that were higher compared with normal rats did not change after the administration of glucose. When insulin was administered intravenously to fed rats, it took longer for plasma mannose concentrations to decrease significantly in diabetic rats than in normal rats (20 and 5 min, respectively). Intravenous administration of epinephrine to fed normal rats increased the plasma mannose concentration, but this effect was negated by fasting or by administration of a glycogen phosphorylase inhibitor. Epinephrine increased mannose output from the perfused liver of fed rats, but this effect was negated in the presence of a glucose-6-phosphatase inhibitor. Epinephrine also increased the hepatic levels of hexose 6-phosphates, including mannose 6-phosphate. When either lactate alone or lactate plus alanine were administered as gluconeogenic substrates to fasted rats, the concentration of plasma mannose did not increase. When lactate was used to perfuse the liver of fasted rats, a decrease, rather than an increase, in mannose output was observed. These findings indicate that hepatic glycogen is a source of plasma mannose.