Modulation of the hypothermic and hyperglycaemic effects of 8-OH-DPAT by alpha 2-adrenoceptor antagonists

Effects of alpha-2-adrenoceptor agonism and antagonism on equine blood insulin and glucose concentrations after oral carbohydrate load

Alpha-2-adrenoceptor agonist detomidine is a commonly used sedative agent in horses. In addition to the sedative effect, detomidine has been reported to elicit changes in energy metabolism such as hypoinsulinaemia and hyperglycaemia. This study aimed to investigate the effects of detomidine with and without peripherally acting alpha-2-adrenoceptor antagonist vatinoxan on insulin and blood glucose (BG) concentrations in horses after a standard dose of oral carbohydrates. Sixteen horses were assigned to four intravenous treatments in a randomised cross-over design: saline (SAL), detomidine (0.02 mg/kg; DET), vatinoxan (0.2 mg/kg; VAT), and a combination of detomidine and vatinoxan (DET+VAT). Horses were administered corn syrup (0.45 mL/kg) immediately before each treatment. Blood samples were collected until 360 min. The differences between treatments were evaluated with repeated measures analysis of covariance and change from baseline was used as a response. P<0.05 was considered significant. After oral carbohydrate load, DET reduced insulin (median 30 min nadir 3.7, min-max 0.6-7.4 µIU/mL) significantly compared with SAL (P<0.0001; 17.4, 9.3-65.4 µIU/mL) and DET+VAT (P=0.0005; 6.4, 2.9-12.9 µIU/mL). BG increased significantly after DET (peak; 130.5, 8.8-15.8 mmol/L) compared with SAL (P<0.0001; 8.7, 6.9-12.4 mmol/L) and DET+VAT (P<0.0001; 8.5, 6.8-10.6 mmol/L). Vatinoxan alone reduced BG (peak median 7.6, 7.0-9.9 mmol/L) compared with SAL (P=0.02) and delayed insulin responses to carbohydrates. In conclusion, vatinoxan alleviated the detomidine-induced changes (DET+VAT compared to DET) in insulin and BG after oral carbohydrate load. Additionally, vatinoxan is potentially able to modulate BG concentration and insulin response after oral carbohydrate administration in horses, but more research is warranted.

Altered glucose homeostasis in alpha2A-adrenoceptor knockout mice

To elucidate the functions of alpha2-adrenoceptor subtypes in metabolic regulation, we determined plasma glucose and insulin levels and tissue uptake of the glucose analogue 2-[18F]fluoro-2-deoxy-d-glucose ([18F]FDG) in C57Bl/6J wild-type (WT) and alpha2A-adrenoceptor knockout (alpha2A-KO) mice at baseline and following alpha2-adrenoceptor agonist ((+)-4-(S)-[1-(2,3-dimethylphenyl)ethyl]-1H-imidazole (dexmedetomidine)) and antagonist (4-[2-ethyl-2,3-dihydro-1H-inden-2-yl]-1H-imidazole (atipamezole)) administration. Basal glucose levels were 30% lower in alpha2A-KO mice than in WT mice. In WT mice, dexmedetomidine lowered insulin and elevated glucose levels, and atipamezole reduced glucose levels. In alpha2A-KO mice, neither drug affected the glucose or insulin levels. [18F]FDG uptake was investigated in plasma, heart, liver, kidney, pancreas, lung, fat, and skeletal muscle. Cardiac [18F]FDG uptake was a sensitive indicator of sympathetic function. Liver [18F]FDG uptake conformed to the plasma glucose levels. In alpha2A-KO mice, drug effects on [18F]FDG tissue uptake were absent. Thus, the alpha2A-adrenoceptor is the alpha2-adrenoceptor subtype primarily involved in the regulation of blood glucose homeostasis in vivo.

Inhibitory Effects of the 5-HT1A Receptor Agonist Buspirone on Stress-Induced Hyperglycemia in Mice: Involvement of Insulin and a Buspirone Metabolite, 1-(2-Pyrimidinyl)piperazine (1-PP)

Effects of serotonergic anxiolytic buspirone on immobilization-induced hyperglycemia were studied in mice. Stress elicited hyperglycemia in mice. Pretreatment with buspirone significantly reduced immobilization-induced hyperglycemia. Buspirone increased serum insulin levels in both non- and stressed mice. The major metabolite of buspirone, 1-(2-pyrimidinyl)piperazine (1-PP) also increased and this further inhibited immobilization-induced hyperglycemia, since 1-PP increased serum insulin levels in both non-stressed and stressed mice, similar to the increases induced by buspirone. These results suggest that buspirone can reduce stress-induced hyperglycemia by facilitating insulin release. Moreover, 1-PP, a metabolite of buspirone may participate in the effects of buspirone. Since 1-PP is an antagonist of alpha(2) receptors, alpha(2) receptors may be related to effects of 1-PP.

Hyperglycemia induced by acute central fluoxetine administration: role of the central CRH system and 5-HT3 receptors

Brain serotonin and CRH systems participate in the control of blood glucose levels. We have previously demonstrated that the pharmacological stimulation of central 5-HT3 receptors, the target for several therapeutic agents used as antiemetics in the course of chemotherapy, induces hyperglycemia. The aim of the present study was to investigate the participation of the brain CRH component and 5-HT3 receptors in basal blood glucose levels as well as in the hyperglycemia induced by third ventricle injections of fluoxetine, a serotonin reuptake inhibitor with a broad range of clinical use. In this study, we used fasted adult Wistar male rats (220 +/- 20 g) whose third ventricles were cannulated 7 days prior to the experiments. Acute third ventricle injections of fluoxetine caused a significant increase in plasma glucose levels throughout the experiment. Pretreatment with alpha-helical CRH, a selective CRH antagonist, significantly blunted fluoxetine-induced hyperglycemia. Also, pretreatment with two distinct selective 5-HT3 receptor antagonists (LY-278,584 and ondansetron) significantly impaired the rise in plasma glucose levels observed in fluoxetine-treated animals pretreated with isotonic saline solution. None of these antagonists was able to modify blood glucose levels when injected alone into the third ventricle. Animals receiving third ventricle injections of fluoxetine, in spite of being hyperglycemic, presented plasma insulin levels similar to those displayed by normoglycemic, saline-treated controls. It is suggested that the acute increase in brain serotonergic activity caused by third ventricle injections of fluoxetine induces a hyperglycemic response that requires the functional integrity of the brain CRH system and 5-HT3 receptors. Also, it is proposed that the absence of a compensatory increase in plasma insulin levels may contribute to the generation of a hyperglycemic response after central fluoxetine administration.

The 5-HT1A receptor agonist 8-hydroxy-2-di-n-(propylamino)tetralin (8-OH-DPAT) induces hyperglucagonemia in rats

Effects of the 5-HT1A receptor agonist 8-hydroxy-2-di-n-(propylamino)tetralin (8-OH-DPAT) on plasma glucagon levels were investigated. 8-OH-DPAT increased plasma glucose and glucagon levels in rats. Both hyperglycemia and hyperglucagonemia elicited by 8-OH-DPAT were prevented by the 5-HT1A receptor antagonist pindolol and prior adrenodemedullation. These results suggest that increases in plasma glucagon levels induced by 8-OH-DPAT were based on the adrenaline release from the adrenal gland and its effects may contribute to its hyperglycemic effects.

Selective serotonin reuptake inhibitors fluoxetine and fluvoxamine induce hyperglycemia by different mechanisms

The effects of the selective serotonin reuptake inhibitors, fluoxetine and fluvoxamine, on plasma glucose levels were investigated in mice. Both fluoxetine and fluvoxamine elicited significant hyperglycemia, while a selective noradrenaline reuptake inhibitor maprotiline had no effect. Fluoxetine and fluvoxamine did not change serum insulin levels, although they elicited hyperglycemia. Pretreatment with the serotonin (5-hydroxytryptamine, 5-HT) depleter, p-chlorophenylalanine (pCPA), abolished fluvoxamine-induced hyperglycemia, although pCPA did not affect the fluoxetine-induced glycemic effects. These results suggest that the selective serotonin reuptake inhibitors fluoxetine and fluvoxamine induce hyperglycemia by inhibition of insulin release. Moreover, our findings indicate that the glycemic effects of these drugs are differentially associated with serotonergic mechanisms.

Effects of the non-selective 5-HT receptor agonist, 5-carboxamidotryptamine, on plasma glucose levels in rats

The effects of the 5-HT1A/1B/1D/5/7 receptor agonist, 5-carboxamidotryptamine (5-CT), on blood glucose, insulin and glucagon levels in rats were investigated. 5-CT above the dosage of 0.05 mg/kg elicited significant hyperglycemic effects and 0.1 mg/kg, induced a 35% increase in plasma glucose levels. 5-CT did not affect plasma glucagon, and serum insulin levels increased following the high dose of 5-CT. Adrenodemedullation abolished the 5-CT-induced hyperglycemia. Hyperglycemia induced by 5-CT was prevented by pretreatment with the 5-HT1/2/7 receptor antagonist, metergoline, and the 5-HT1/2/5/7 receptor antagonist, methysergide, although the 5-HT2A receptor antagonist, ketanserin, the 5-HT2A/2B/2C receptor antagonist, ritanserin, and the 5-HT3/4 receptor antagonist, tropisetron, had no effect. Although 5-CT has a high affinity with 5-HT1A receptors, the 5-HT1A and 5-HT1B and beta receptor antagonist, (-)-popranolol, did not affect 5-CT-induced hyperglycemia. These results indicate that 5-CT-induced hyperglycemia is elicited by facilitation of adrenaline release from the adrenal gland and that 5-CT-induced hyperglycemia is mediated by the 5-HT7 receptor unrelated to 5-HT1A, 5-HT1B, 5-HT2, 5-HT3, 5-HT4 or 5-HT5 receptors.

Intraventricular galanin modulates a 5-HT1A receptor-mediated behavioural response in the rat

The present studies have examined whether the neuropeptide galanin can modulate brain serotoninergic (5-HT) neurotransmission in vivo and, particularly, 5-HT1A receptor-mediated transmission. For that purpose, we studied the ability of galanin (given bilaterally into the lateral ventricle, i.c.v.) to modify the impairment of passive avoidance retention induced by the selective 5-HT1A agonist 8-hydroxy-2-(di-n-propyloamino)tetralin (8-OH-DPAT) when injected prior to training. This impairment appears to be mainly related to activation of 5-HT1A receptors in the CNS. Galanin dose-dependently (significant at 3.0 nmol/rat) attenuated the passive avoidance impairment (examined 24 h after training) induced by the 0.2 mg/kg dose of 8-OH-DPAT. This 8-OH-DPAT dose produced signs of the 5-HT syndrome indicating a postsynaptic 5-HT1A receptor activation. Furthermore, both the impairment of passive avoidance and the 5-HT syndrome were completely blocked by the 5-HT1A receptor antagonist WAY 100635 (0.1 mg/kg). Galanin (0.3 or 3.0 nmol) or WAY 100635 (0.1 mg/kg) failed by themselves to affect passive avoidance retention. 8-OH-DPAT given at a low dose 0.03 mg/kg, which presumably stimulates somatodendritic 5-HT1A autoreceptors in vivo, did not alter passive avoidance retention or induce any visually detectable signs of the 5-HT syndrome. Galanin (0.3 or 3.0 nmol) given i.c.v. in combination with the 0.03 mg/kg dose of 8-OH-DPAT, did not modify passive avoidance. The immunohistochemical study of the distribution of i.c.v. administered galanin (10 min after infusion) showed a strong diffuse labelling in the periventricular zone (100-200 microm) of the lateral ventricle. Furthermore, in the dorsal and ventral hippocampus galanin-immunoreactive nerve cells appeared both in the dentate gyrus and the CA1, CA2 and CA3 layers of the hippocampus. In the septum only endogenous fibres could be seen while in the caudal amygdala also galanin-immunoreactive nerve cells were visualized far away from the labelled periventricular zone. At the level of the dorsal raphe nucleus a thin periventricular zone of galanin immunoreactivity was seen but no labelling of cells. These results suggest that galanin can modulate postsynaptic 5-HT1A receptor transmission in vivo in discrete cell populations in forebrain regions such as the dorsal and ventral hippocampus and parts of the amygdala. The indication that galanin administered intracerebroventrically may be taken up in certain populations of nerve terminals in the periventricular zone for retrograde transport suggests that this peptide may also affect intracellular events.

Effects of galanin on 8-OH-DPAT induced decrease in body temperature and brain 5-hydroxytryptamine metabolism in the mouse

Central administration of galanin dose-dependently (minimum effective dose, M.E.D. = 1 nmol) blocked the hypothermia induced by the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 0.5 mg/kg s.c.), in mice. This inhibitory effect was reversed by pretreatment with the galanin receptor antagonist galantide (0.3 nmol) and also by pretreatment with the ATP-sensitive potassium channel blockers glibenclamide (10 nmol) and gliquidone (10 nmol). The hypothermic response to 8-OH-DPAT was also blocked by the 5-HT1A receptor antagonist (N-(2,4(2-methoxyphenyl)-1-piperazinyl)ethyl-N-(2-pyridinyl)cyclohexane, (WAY 100,635, M.E.D. = 0.01 mg/kg s.c.), and the centrally acting muscarinic receptor antagonist scopolamine (M.E.D. = 10 mg/kg i.p.) but not the peripheral muscarinic receptor antagonist N-methylscopolamine. 8-OH-DPAT (0.5 mg/kg s.c.) also decreased cortical and hypothalamic 5-HT (5-hydroxytryptamine, serotonin) metabolism, an effect which was not blocked by pretreatment with galanin (0.3-3 nmol intracerebroventricular, i.c.v.). Neither did galanin (0.03-3 nmol/5 microliters i.c.v.) affect basal 5-HT metabolism in these brain regions. Furthermore, pretreatment in vitro of mouse cortical membranes with galanin (10 or 1000 nM) had no effect on 5-HT1A receptor affinity, Bmax or pharmacology determined using [3H]8-OH-DPAT. These results suggest that the inhibition of 8-OH-DPAT induced hypothermia by galanin is probably not mediated by an interaction with 5-HT1A receptors but more likely by blocking the indirect activation by 8-OH-DPAT of central cholinergic pathways involved in temperature regulation.

Resolution of multiple affinity states of 5-hydroxytryptamine-1A receptors labelled by [3H]-8-hydroxy-2-(di-n-propilamino) tetralin in rat hippocampal membranes

1. We examined the binding of [3H]-8-hydroxy-2-(DI-n-propilamino)tetralin ([3H]-8OH-DPAT) to 5-hydroxytriptamine-1A (5-HT1A) receptors in rat hippocampal membranes. 2. Computer analysis of [3H]-8OH-DPAT displacement curves in the absence and in the presence of 100 microM guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S) were best fitted with a three-site model with apparent dissociation constants (Kd) of 0.45, 2.8 and 30 nM; the corresponding binding capacity (Bmax) existing in the three affinity states were 4, 2 and 12 pmol/g of tissue (wet weight), respectively. 3. These results suggest that [3H]-8OH-DPAT binding can be resolved as complex isotherms and we provided evidence that [3H]-8OH-DPAT labels 2 high-affinity GTP gamma S-sensitive and one low-affinity GTP gamma S-sensitive state of 5-HT1A receptors.

8-OH-DPAT stimulates gastric acid secretion through a vagal-independent, adrenal-mediated mechanism

Serotonin (5-hydroxytryptamine, 5-HT) is a neuroendocrine component of the gastrointestinal tract. 5-HT1A receptors exist both in the brain and have been demonstrated autoradiographically in high density in the rat stomach. However, the physiologic role of 5-HT1A receptors in modulating gastric function is not known. The effect of the selective 5-HT1A receptor agonist, (+/-)-8-hydroxy-2-(n-dipropylamino)tetralin (8-OH-DPAT), on gastric acid secretory function was compared to 5-HT in acute, urethane-anesthetized gastric-fistulated rats during pentagastrin infusion. 5-HT inhibited, but 8-OH-DPAT stimulated, gastric acid secretion in a dose-dependent manner. Bilateral cervical vagotomy or celiac ganglionectomy did not reverse the effect of 8-OH-DPAT on acid secretion. However, the enhancement of acid by 8-OH-DPAT was attenuated by acute adrenalectomy or close intra-arterial administration of spiperone, but not idazoxan. Thus, the data suggest that the selective 5-HT1A receptor agonist 8-OH-DPAT may augment gastric secretory function via an adrenal-dependent mechanism.

The effects of ethanol in combination with the alpha 2-adrenoceptor agonist dexmedetomidine and the alpha 2-adrenoceptor antagonist atipamezole on brain monoamine metabolites and motor performance of mice

The time course of the effects of ethanol alone and in combination with the selective alpha 2-adrenoceptor agonist dexmedetomidine and the alpha-adrenoceptor antagonist atipamezole was studied in NIH-Swiss mice. Core body temperature, rotarod performance, motility and changes in the noradrenaline, dopamine, and 5-hydroxytryptamine (5-HT) metabolite contents of different brain parts (limbic forebrain, striatum, lower brainstem, the rest of the forebrain + midbrain and hypothalamus) were measured. Atipamezole (3 mg/kg) attenuated the hypothermia induced by either ethanol (3 g/kg) alone or ethanol in combination with dexmedetomidine (0.3 mg/kg). Atipamezole shortened the duration of the ethanol-impaired and ethanol + dexmedetomidine-impaired rotarod performance. Further, atipamezole prevented the decreased motility due to the combined treatment with ethanol and dexmedetomidine. Ethanol increased 3-methoxy-4-hydroxyphenylethylene glycol (MHPG), homovanillic acid (HVA) and 3,4-dihydroxyphenylacetic acid (DOPAC) values. Dexmedetomidine alone decreased MHPG and 5-hydroxyindoleacetic acid (5-HIAA) concentrations and increased DOPAC and HVA values. Dexmedetomidine combined with ethanol resulted in a further increase in DOPAC and HVA values. Pharmacokinetic parameters did not contribute to this antagonism of ethanol's effects by atipamezole, nor did the antagonism observed in rotarod performance or hypothermia seem to correlate with the changes seen in the brain noradrenaline and dopamine or 5-HT metabolism. In conclusion, these findings suggest that several ethanol effects are not mediated via direct activation of alpha 2-adrenoceptors, even though some of ethanol's behavioral and physiological effects may be antagonized by coadministration of alpha 2-adrenoceptor antagonists.

Physiopharmacological interactions between stress hormones and central serotonergic systems

The present review tries to delineate some mechanisms through which the sympathetic nervous system (SNS) and the hypothalamo-pituitary-adrenal (HPA) interact with central serotonergic systems. The recent progress in 5-hydroxytryptamine (5-HT) receptor pharmacology has helped to define the means by which central serotonergic activity may alter the respective activities of the SNS (sympathetic nerves and adrenomedulla) and of the HPA axis. These pharmacological findings have also helped to characterize the differential effects of central 5-HT upon different branches of the SNS and the numerous sites at which 5-HT exerts stimulatory influences upon the HPA axis. Although relevant to stress-related neuroendocrinology, the extent to which these interactions are involved in the antidepressant/anxiolytic properties of some serotonergic agents still remains to be clarified. Beside these findings, there is also abundant evidence for a tight control of central serotonergic systems by stress hormones. Activation of the SNS increases, by numerous means, central availability of tryptophan, whereas glucocorticoids exert differential actions upon the intra- and the extraneuronal regulation of 5-HT function. Actually, a significant number of these mechanisms is involved in the maintenance of homeostasis during stressful events, thereby conferring to these mechanisms a key role in adaptation processes.

Opioid receptor mediation of the hypothermic response to caffeine

Caffeine and other methylxanthines induce a dose-dependent reduction in core body temperature in mice. These experiments investigated the effects of neurotransmitter and neuromodulator antagonists on caffeine-induced hypothermia. Pretreatment with the alpha 2-adrenoceptor antagonist, atipamezole; the beta-adrenoceptor antagonist, propranolol; the dopamine antagonist, haloperidol; or the benzodiazepine receptor antagonist, flumazenil had no intrinsic effects on core body temperature nor did they interact significantly with the hypothermic effects of caffeine. The alpha 1-adrenoceptor antagonist, prazosin and the 5-HT receptor antagonist, metergoline significantly enhanced the hypothermic effects of caffeine, probably involving a combined effect with their intrinsic hypothermic actions. Pretreatment with the opiate receptor antagonist, naloxone (3 mg/kg i.p.), had no intrinsic effect on core body temperature but attenuated the hypothermic effect of caffeine reflected in a parallel shift to the right in the caffeine dose-effect curve. The naloxone-induced attenuation of the hypothermic effects of caffeine was also seen to be dose-dependent. The results reveal that opiate receptors (but not adrenoceptors, 5-HT, dopamine or benzodiazepine receptors) may play a role in modulating the hypothermic action of caffeine and possibly other methylxanthines.

Hyperglycemic properties of serotonin receptor antagonists

Several serotonin (5-HT) receptor antagonists with varying specificities for the 5-HT receptor types, were studied with regard to their effects on blood glucose levels in mice. The non-selective antagonists, metergoline and methysergide, proved to be hyperglycemic at doses commonly used to antagonize 5-HT receptors. In contrast, ritanserin (a 5-HT2 and 5-HT1c antagonist) and MDL 72222 (a 5-HT3 antagonist) were effective only at doses which surpassed the dose range considered to be selective for their respective receptors. The results suggest that 5-HT systems play a role in maintaining glucose homeostasis and that 5-HT1 receptors may be particularly important in this function. Furthermore, the inherent hyperglycemic properties of non-selective serotonin antagonists described here, are pertinent to studies using these agents to investigate glucose metabolism.