Thermogenic and lipolytic effect of yohimbine in the dog

Metabolic and cardiovascular benefits and risks of 4-hydroxy guanabenz hydrochloride: α2-adrenoceptor and trace amine-associated receptor 1 ligand

BACKGROUND

α2-adrenoceptor ligands have been investigated as potential therapeutic agents for the treatment of obesity. Our previous studies have shown that guanabenz reduces the body weight of obese rats, presumably through its anorectic action. This demonstrates an additional beneficial effect on selected metabolic parameters, including glucose levels. The purpose of this present research was to determine the activity of guanabenz's metabolite-4-hydroxy guanabenz hydrochloride (4-OH-Guanabenz).

METHODS

We performed in silico analyses, involving molecular docking to targets of specific interest as well as other potential biological targets. In vitro investigations were conducted to assess the selectivity profile of 4-OH-Guanabenz binding to α-adrenoceptors, along with intrinsic activity studies involving α2-adrenoceptors and trace amine-associated receptor 1 (TAAR1). Additionally, the effects of 4-OH-Guanabenz on the body weight of rats and selected metabolic parameters were evaluated using the diet-induced obesity model. Basic safety and pharmacokinetic parameters were also examined.

RESULTS

4-OH-guanabenz is a partial agonist of α2A-adrenoceptor. The calculated EC50 value for it is 316.3 nM. It shows weak agonistic activity at TAAR1 too. The EC50 value for 4-OH-Guanabenz calculated after computer simulation is 330.6 µM. Its primary mode of action is peripheral. The penetration of 4-OH-Guanabenz into the brain is fast (tmax = 15 min), however, with a low maximum concentration of 64.5 ng/g. 4-OH-Guanabenz administered ip at a dose of 5 mg/kg b.w. to rats fed a high-fat diet causes a significant decrease in body weight (approximately 14.8% compared to the baseline weight before treatment), reduces the number of calories consumed by rats, and decreases plasma glucose and triglyceride levels.

CONCLUSIONS

The precise sequence of molecular events within the organism, linking the impact of 4-OH-Guanabenz on α2A-adrenoceptor and TAAR1 with weight reduction and the amelioration of metabolic disturbances, remains an unresolved matter necessitating further investigation. Undoubtedly, the fact that 4-OH-Guanabenz is a metabolite of a well-known drug has considerable importance, which is beneficial from an economic point of view and towards its further development as a drug candidate.

Adipose and non-adipose perspectives of plant derived natural compounds for mitigation of obesity

ETHNOPHARMACOLOGICAL RELEVANCE

Phyto-preparations and phyto-compounds, by their natural origin, easy availability, cost-effectiveness, and fruitful traditional uses based on accumulated experiences, have been extensively explored to mitigate the global burden of obesity.

AIM OF THIS REVIEW

The review aimed to analyse and critically summarize the prospect of future anti-obesity drug leads from the extant array of phytochemicals for mitigation of obesity, using adipose related targets (adipocyte formation, lipid metabolism, and thermogenesis) and non-adipose targets (hepatic lipid metabolism, appetite, satiety, and pancreatic lipase activity). Phytochemicals as inhibitors of adipocyte differentiation, modulators of lipid metabolism, and thermogenic activators of adipocytes are specifically discussed with their non-adipose anti-obesogenic targets.

MATERIALS AND METHODS

PubMed, Google Scholar, Scopus, and SciFinder were accessed to collect data on traditional medicinal plants, compounds derived from plants, their reported anti-obesity mechanisms, and therapeutic targets. The taxonomically accepted name of each plant in this review has been vetted from "The Plant List" (www.theplantlist.org) or MPNS (http://mpns.kew.org).

RESULTS

Available knowledge of a large number of phytochemicals, across a range of adipose and non-adipose targets, has been critically analysed and delineated by graphical and tabular depictions, towards mitigation of obesity. Neuro-endocrinal modulation in non-adipose targets brought into sharp dual focus, both non-adipose and adipose targets as the future of anti-obesity research. Numerous phytochemicals (Berberine, Xanthohumol, Ursolic acid, Guggulsterone, Tannic acid, etc.) have been found to be effectively reducing weight through lowered adipocyte formation, increased lipolysis, decreased lipogenesis, and enhanced thermogenesis. They have been affirmed as potential anti-obesity drugs of future because of their effectiveness yet having no threat to adipose or systemic insulin sensitivity.

CONCLUSION

Due to high molecular diversity and a greater ratio of benefit to risk, plant derived compounds hold high therapeutic potential to tackle obesity and associated risks. This review has been able to generate fresh perspectives on the anti-diabetic/anti-hyperglycemic/anti-obesity effect of phytochemicals. It has also brought into the focus that many phytochemicals demonstrating in vitro anti-obesogenic effects are yet to undergo in vivo investigation which could lead to potential phyto-molecules for dedicated anti-obesity action.

Activation of Anxiogenic Circuits Instigates Resistance to Diet-Induced Obesity via Increased Energy Expenditure

Anxiety disorders are associated with body weight changes in humans. However, the mechanisms underlying anxiety-induced weight changes remain poorly understood. Using Emx1^Cre/+ mice, we deleted the gene for brain-derived neurotrophic factor (BDNF) in the cortex, hippocampus, and some amygdalar subregions. The resulting mutant mice displayed impaired GABAergic transmission and elevated anxiety. They were leaner when fed either a chow diet or a high-fat diet, owing to higher sympathetic activity, basal metabolic rate, brown adipocyte thermogenesis, and beige adipocyte formation, compared to control mice. BDNF re-expression in the amygdala rescued the anxiety and metabolic phenotypes in mutant mice. Conversely, anxiety induced by amygdala-specific Bdnf deletion or administration of an inverse GABA_A receptor agonist increased energy expenditure. These results reveal that increased activities in anxiogenic circuits can reduce body weight by promoting adaptive thermogenesis and basal metabolism via the sympathetic nervous system and suggest that amygdalar GABAergic neurons are a link between anxiety and metabolic dysfunction.

Pyrrolidin-2-one derivatives may reduce body weight in rats with diet-induced obesity

Obesity affects an increasing number of individuals in the human population and significant importance is attached to research leading to the discovery of drug which would effectively reduce weight. The search for new drugs with anorectic activity and acting within the adrenergic system has attracted the interest of researchers. This study concerns the experimental effects on body weight of α2-adrenoceptor antagonists from the group of pyrrolidin-2-one derivatives in rats with diet-induced obesity.

METHODS

The intrinsic activity of the test compounds at the α-adrenoreceptors was tested. Obesity in rats was obtained by the use of fatty diet and then the influence of the test compounds on body weight, food and water intakes, lipid and glucose profiles and glycerol and cortisol levels were determinated. The effects of the compounds on locomotor activity, body temperature, blood pressure and heart rate were tested.

RESULTS

One of the test compounds (1-(3-(4-phenylpiperazin-1-yl)propyl)pyrrolidin-2-one) reduces the animal's body weight and the amount of peritoneal adipose tissue during chronic administration, at the same time it does not cause significant adverse effects on the cardiovascular system. This compound decreases temperature and elevates glycerol levels and does not change the locomotor activity and cortisol level at anti-obese dose.

CONCLUSIONS

Some derivatives of pyrrolidin-2-one that act as antagonists of the α2-adrenoreceptor may reduce body weight. Reducing body weight for 1-(3-(4-phenylpiperazin-1-yl)propyl)pyrrolidin-2-one can be associated with decrease in food intake, body fat reduction, reduction of blood glucose, and increased thermogenesis and lipolysis. This effect cannot be the result of changes in spontaneous activity or stress.

A Comparison of the Anorectic Effect and Safety of the Alpha2-Adrenoceptor Ligands Guanfacine and Yohimbine in Rats with Diet-Induced Obesity

The search for drugs with anorectic activity, acting within the adrenergic system has attracted the interest of researchers. Partial α₂-adrenoceptor agonists might offer the potential for effective and safe treatment of obesity. We compared the effectiveness and safety of α₂-adrenoceptor ligands in reducing body mass. We also analyzed if antagonist and partial agonists of α₂-adrenoceptor––yohimbine and guanfacine––act similarly, and determined which course of action is connected with anorectic activity. We tested intrinsic activity and effect on the lipolysis of these compounds in cell cultures, evaluated their effect on meal size, body weight in Wistar rats with high-fat diet-induced obesity, and determined their effect on blood pressure, heart rate, lipid profile, spontaneous locomotor activity, core temperature and glucose, as well as glycerol and cortisol levels. Both guanfacine and yohimbine showed anorectic activity. Guanfacine was much more effective than yohimbine. Both significantly reduced the amount of intraperitoneal adipose tissue and had a beneficial effect on lipid profiles. Decreased response of α_2A-adrenoceptors and partial stimulation of α_2B-receptors seem to be responsible for the anorectic action of guanfacine. The stimulation of α₁-adrenoceptors by guanfacine is responsible for cardiovascular side effects but may also be linked with improved anorexic effect. α₁-adrenoceptor blockade is connected with the side effects of yohimbine, but it is also associated with the improvement of lipid profiles. Guanfacine has been approved by the Food and Drug Administration (FDA) to treat hypertension and conduct disorder, but as it reduces body weight, it is worth examining its effectiveness and safety in models of obesity.

Clinical safety assessment of oral higenamine supplementation in healthy, young men

OBJECTIVE

Higenamine, an herbal agent also known as norcoclaurine, is thought to stimulate β-androgenic receptors and possess lipolytic activity. It is currently making its way into the dietary supplement market. To our knowledge, no studies have been conducted to determine the safety profile of oral higenamine when used alone and in conjunction with other commonly used lipolytic agents.

METHODS

Forty-eight men were assigned to ingest either a placebo, higenamine, caffeine, or higenamine + caffeine + yohimbe bark extract daily for a period of 8 weeks. Before and after 4 and 8 weeks of supplementation, the following variables were measured: resting respiratory rate, heart rate, blood pressure, urinalysis, complete blood count, metabolic panel, liver enzyme activity, and lipid panel.

RESULTS

No interaction effects were noted for any variable (p > 0.05), with no changes of statistical significance occurring across time for any of the four conditions (p > 0.05).

CONCLUSION

This is the first study to determine the safety profile of oral higenamine intake in human subjects. Our data indicate that 8 weeks of daily higenamine supplementation, either alone or in conjunction with caffeine and yohimbe bark extract, does not result in a statistically significant change in any of the measured outcome variables. Additional studies, inclusive of a larger sample size, are needed to extend these initial findings.

Acute oral intake of a higenamine-based dietary supplement increases circulating free fatty acids and energy expenditure in human subjects

Background

Higenamine, also known as norcoclaurine, is an herbal constituent thought to act as a beta-2 adrenergic receptor agonist—possibly stimulating lipolysis. It was the purpose of this study to determine the impact of a higenamine-based dietary supplement on plasma free fatty acids and energy expenditure following acute oral ingestion.

Methods

Sixteen healthy subjects (8 men; 26.1 ± 2.5 yrs; 8 women 22.4 ± 3.1 yrs) ingested a dietary supplement containing a combination of higenamine, caffeine (270 mg), and yohimbe bark extract or a placebo, on two separate occasions in a double-blind, randomized, cross-over design, separated by 6–8 days. Blood samples were collected immediately before ingestion, and at 30, 60, 120, and 180 minutes post ingestion, and analyzed for plasma free fatty acids (FFA) and glycerol. Breath samples were collected at the same times for a measure of kilocalorie expenditure and respiratory exchange ratio (RER) using indirect calorimetry. Heart rate and blood pressure were recorded at all times. Data collection occurred in the morning following a 10 hour overnight fast.

Results

A condition effect was noted for both FFA (p < 0.0001) and kilocalorie expenditure (p = 0.001), with values higher for supplement compared to placebo at 60, 120, and 180 minutes post ingestion. No statistically significant effects were noted for glycerol or RER (p > 0.05). A condition effect was noted for heart rate (p = 0.03) and systolic blood pressure (p < 0.0001), with values higher for supplement compared to placebo.

Conclusion

Ingestion of a higenamine-based dietary supplement stimulates lipolysis and energy expenditure, as evidenced by a significant increase in circulating FFA and kilocalorie expenditure. The same supplement results in a moderate increase in heart rate (~3 bpm) and systolic blood pressure (~12 mmHg), which is consistent with previous studies evaluating moderate doses of caffeine and yohimbine, suggesting that higenamine contributes little to the increase in these hemodynamic variables. These findings are in reference to young, healthy and active men and women.

Pharmacological characterisation of the thermogenic effect of bupropion

The pharmacological mechanism of bupropion's thermogenic effect has been investigated in female Wistar rats by measuring oxygen consumption at thermoneutrality (29 degrees C). Bupropion (30 mg/kg) rapidly increased oxygen consumption (VO2) with a maximum effect at 30 min, and VO2 remained elevated throughout the 4-h experimental period. The nonselective 5-hydroxytryptamine (5-HT or serotonin) receptor antagonist, metergoline (1 mg/kg), and the alpha1-adrenoceptor antagonist, prazosin (1 mg/kg), had no effect on the VO2 response to bupropion, whereas the alpha2-adrenoceptor antagonist, RS79948 [(8aR, 12aS, 13aS)-5,8,8a,9,10,11,12,12a,13,13a-decahydro-3-methoxy-12-(ethylsulphonyl)-6H-isoquino[2,1-g][1,6]-naphthyridine hydrochloride] (1 mg/kg), potentiated the response. The VO2 response to bupropion during the first 60 min was significantly inhibited by a high dose of the nonselective beta-adrenoceptor antagonist, propranolol (20 mg/kg), but it had no effect at a low dose (1 mg/kg). Pretreatment with the dopamine D2/D1 receptor antagonist, (+)butaclamol (200 microg/kg), caused a partial, but significant, inhibition (P<0.01) of the VO2 response to bupropion during the first 60 min, and this antagonist abolished the effect of bupropion between 90 and 240 min. Pretreatment with a combination of a high dose of propranolol (20 mg/kg) and (+)butaclamol (200 microg/kg) prevented any increase in VO2 induced by bupropion. It is concluded that the beta3-adrenoceptor subtype, as well as dopamine D2/D1 receptors, is responsible for the increase in oxygen consumption induced by bupropion. We have previously demonstrated that bupropion did not significantly reduce food intake in rats. Hence, in this species, its weight-reducing action predominantly results from thermogenesis mediated via activation of beta3-adrenergic and dopamine D2/D1 receptors. Because bupropion has also been reported not to alter food intake in the clinic, thermogenesis may also contribute to its antiobesity effect in man.

Mechanism of the thermogenic effect of Metabolite 2 (BTS 54 505), a major pharmacologically active metabolite of the novel anti-obesity drug, sibutramine

OBJECTIVE

To investigate the pharmacological mechanisms underlying the induction of thermogenesis by Metabolite 2 (M2; BTS 54 505), a major pharmacologically active metabolite of the anti-obesity drug, sibutramine.

DESIGN

Adult female Wistar rats were treated with M2 or vehicle, with or without various monoamine receptor antagonists, prazosin, RS79948, metergoline, propranolol and (+)butaclamol.

MEASUREMENTS

Colonic temperature and food intake at room temperature (21+/-1 degrees C), thermoregulatory behavioural response, operant responding for exogenous heat at -8 degrees C and oxygen consumption at thermoneutrality (29 degrees C).

RESULTS

M2 (10 mg/kg, p.o.) significantly increased colonic temperature during the 4.5 h period following drug administration. This effect was abolished by the non-selective 5-HT receptor antagonist, metergoline (1 mg/kg, p.o.), and alpha(1)-adrenoceptor antagonist, prazosin (1 mg/kg, p.o.), measured at 1.5-2.5 h post-M2 administration, and was partially antagonized by each antagonist at 3.5-4.5 h. The non-selective beta-adrenoceptor antagonist, propranolol (1 mg/kg, p.o.), had no effect on the M2-induced increase in colonic temperature, whereas at 20 mg/kg (p.o.), propranolol partially inhibited the effect of M2 on colonic temperature. By contrast, the selective alpha(2)-adrenoceptor antagonist, RS79948 (1 mg/kg, p.o.), and the D2/D1 receptor antagonist, (+)butaclamol (200 micro g/kg, p.o.), did not alter the effect of M2 on colonic temperature. In the thermoregulatory study, M2 (10 mg/kg, i.p.)-treated rats required significantly less radiant heat at -8 degrees C to maintain body temperature, and this effect was not affected by the D2/D1 receptor antagonist (+)butaclamol (100 micro g/kg(-1), i.p.). The hypophagia induced by M2 (10 mg/kg) measured up to 24 h was partially antagonized by the alpha(1)-adrenoceptor antagonist, prazosin, whereas metergoline, RS79948, propranolol and (+)butaclamol had no effect on M2-induced hypophagia.

CONCLUSION

It is concluded that 5-HT, alpha(1)- and beta(3)-adrenoceptors are involved in the induction of thermogenesis by M2, whereas the hypophagic effect is mainly mediated via alpha(1)-adrenoceptors. These findings are consistent with M2 increasing 5-HT and noradrenaline tone via potent reuptake inhibition which subsequently results in increased efferent sympathetic activity to brown adipose tissue (BAT).

Pharmacological approaches for the treatment of obesity

The high incidence of obesity, its multifactorial nature, the complexity and lack of knowledge of the bodyweight control system, and the scarcity of adequate therapeutics have fuelled anti-obesity drug development during a considerable number of years. Irrespective of the efforts invested by researchers and companies, few products have reached a minimum level of effectiveness, and even fewer are available in medical practice. As a consequence of anti-obesity research, our knowledge of the bodyweight control system increased but, despite this, the pharmacological approaches to the treatment of obesity have not resulted yet in effective drugs. This review provides a panoramic of the multiple different approaches developed to obtain workable drugs. These approaches, however, rely in only four main lines of action: control of energy intake, mainly through modification of appetite;control of energy expenditure, essentially through the increase of thermogenesis;control of the availability of substrates to cells and tissues through hormonal and other metabolic factors controlling the fate of the available energy substrates; andcontrol of fat reserves through modulation of lipogenesis and lipolysis in white adipose tissue. A large proportion of current research is centred on neuropeptidic control of appetite, followed by the development of drugs controlling thermogenic mechanisms and analysis of the factors controlling adipocyte growth and fat storage. The adipocyte is also a fundamental source of metabolic signals, signals that can be intercepted, modulated and used to force the brain to adjust the mass of fat with the physiological means available. The large variety of different approaches used in the search for effective anti-obesity drugs show both the deep involvement of researchers on this field and the large amount of resources devoted to this problem by pharmaceutical companies. Future trends in anti-obesity drug research follow closely the approaches outlined; however, the increasing mass of information on the molecular basis of bodyweight control and obesity will in the end prevail in our search for effective and harmless anti-obesity drugs.

Interactions between an alpha2-adrenergic antagonist and a beta3-adrenergic agonist on the expression of UCP2 and UCP3 in rats

This experimental trial was devised to assess whether selective beta3-adrenergic receptor (AR) stimulation and simultaneous blockade of alpha2-AR would affect thermoregulation. With this purpose, the individual and combined administration of a beta-AR agonist, trecadrine, and an alpha2-AR antagonist, yohimbine, were evaluated. Yohimbine produced a marked decrease (p < 0.001) in body temperature one hour after administration (5 mg kg(-1), i.p.) and blocked the thermogenic effect of trecadrine (1 mg kg(-1), i.p.) when simultaneously administered. Uncoupling protein-2 expression in skeletal muscle was downregulated (p < 0.05) by trecadrine, while yohimbine had no effect. White adipose tissue UCP2 and muscle UCP3 were not modified by either trecadrine or yohimbine administration. Liver UCP2 mRNA expression was significantly decreased by yohimbine (p < 0.05). However, this downregulation does not seem to explain the reduction in temperature produced by yohimbine given the fact that trecadrine produced a similar downregulation of hepatic UCP2 (p < 0.05). The present work indicates that alpha2-AR antagonism blocks the thermogenic effects mediated by beta3-AR stimulation, contrary to our expectations, suggesting a possible interplay between both mechanisms. Moreover, these effects are not apparently explained by changes in UCP2 and UCP3.

Glucose-induced sympathetic activity and energy expenditure during acute alpha2-adrenergic antagonism in obese subjects

OBJECTIVE

To determine the effect of an alpha2-adrenoceptor antagonist, idazoxan, on the sympathetic nervous system and on energy expenditure responses after an oral glucose load, in obese patients. (idazoxan acts as an indirect sympathomimetic drug through blockade of presynaptic alpha2-adrenoceptors).

DESIGN

Double-blind randomized placebo-controlled cross-over study. Idazoxan (40 mg) or placebo were administered orally 90 min before a 100 g oral glucose load.

SUBJECTS

Twelve long-standing obese subjects (six men and six women, age range from 24 to 45 y, body mass index range from 30.2 to 41.3 kg/m2).

MEASUREMENTS

Energy expenditure was derived from oxygen consumption and carbon dioxide production according to indirect calorimetry. Plasma samples were obtained for plasma adrenaline and noradrenaline, glucose, non-esterified fatty acid (NEFA), glycerol and insulin determinations.

RESULTS

The plasma noradrenaline concentration response to the glucose load was significantly higher after idazoxan than after placebo administration. The time-course of glucose load-induced thermogenesis was not significantly different after administration of idazoxan nor placebo. Idazoxan administration did not modify the insulin, non-esterified fatty acids or glycerol concentration responses to the glucose load. Neither heart rate nor blood pressure values were modified by idazoxan when compared to placebo. However, idazoxan significantly improved glucose tolerance.

CONCLUSION

The alpha2-adrenergic antagonist idazoxan increases glucose-induced sympathetic activity but not energy expenditure in obese subjects. These data do not argue for the development of alpha2AR antagonist compounds as anti-obesity treatment.

Anti-obesity effect of MPV-1743 A III, a novel imidazoline derivative, in genetic obesity

MPV-1743 A III ((+/-)-4-(5-fluoro-2,3-dihydro-1H-inden-2-yl)-1H-imidazole) is a novel imidazoline derivative. In this study, it was shown to bind with high affinity to alpha2-adrenoceptor subtypes alpha2A (IC50) = 0.66 +/- 0.06 nM), alpha2B (IC50) = 3.8 +/- 0.53 nM), alpha2C (IC50) = 3.1 +/- 0.61 nM) in the recombinant S115 cells and to alpha2D (IC50 = 0.94 +/- 0.10 nM) in the rat submandibular gland. MPV-1743 A III also showed remarkably high affinity to alpha1-adrenoceptors (IC50 = 150 +/- 12 nM) in the rat cerebral cortex and to imidazoline I2b-binding sites (IC50) = 150 +/- 5.0 nM) in the rat liver. The functional alpha2-adrenoceptor antagonistic effect of MPV-1743 A III was demonstrated by studying the ability of orally administered MPV-1743 A III to reverse and prevent the alpha2-adrenoceptor agonist detomidine-induced mydriasis in rat. The anti-obesity effect of MPV-1743 A III was investigated in genetically obese (fa/fa) Zucker rats in two different phases of obesity. Chronic treatment with MPV-1743 A III (0.3 3 mg/kg per day p.o. for 3 weeks) dose dependently decreased weight gain in early-phase obesity. In fully established obesity, GDP binding to mitochondria and expression of uncoupling protein mRNA were increased in brown adipose tissue by MPV-1743 A III indicating an activation of non-shivering thermogenesis. The present study shows that MPV- 1743 A III has a modest anti-obesity effect in the genetic rodent model of obesity. The relative importance of alpha2- and alpha1-adrenoceptors and imidazoline I2b-binding sites in mediating the effects of MPV-1743 A III needs further evaluation.

Temperature-dependent effects of alpha-adrenergic agonists and antagonists in the cold

This series of experiments examined whether temperature-dependent effects of the alpha-antagonists prazosin and yohimbine compromised their use as blockers of alpha-adrenergic agonist responses in cold-exposed rats. An operant leverpressing task was used to measure the demand for heat in a cold environment. The alpha 1-antagonist prazosin had modest effects, but the alpha 2-antagonist yohimbine was thermolytic in that it dose dependently increased operant responding but decreased posttest colonic temperature (Tc). These potent effects of the alpha 2-antagonist led to tests of the alpha 2-agonist clonidine. Clonidine increased operant responding for heat to an extraordinary degree, resulting in significant increases in posttest Tc. However, clonidine was found to be a hypothermic agent when tested in rats at 5 degrees C but denied the opportunity to increase body temperature by operant lever pressing, suggesting a central effect on the control of thermal balance. Measurement of changes in metabolic rate at 5 and 23 degrees C showed that yohimbine increased metabolism at 23 degrees C but decreased it in the cold. Prazosin had little effect on metabolism or Tc at either temperature. Prazosin inhibited the decrease in Tc induced by norepinephrine (NE), but had little effect on the lever-pressing response. Yohimbine had no significant antagonistic effect on NE-induced changes in lever-pressing behavior or posttest Tc, but neither did the thermolytic effects of yohimbine exacerbate those of NE. These results show that alpha-antagonist interactions with agonists can be complicated by temperature-dependent effects of each.

Effects of epinephrine on thermoregulatory behavior in lean and obese Zucker rats in the cold

This series of experiments examined whether epinephrine (EPI) produces the same thermoregulatory effects in the cold that have been reported for norepinephrine and isoproterenol. Lean and obese Zucker rats were trained to press a lever to activate infrared heat lamps in a cold (-8 degrees C) environment. Operant thermoregulatory behavior increased dose-dependently following EPI (0-100 micrograms/kg), but posttest colonic temperature (Tc) fell. Thermal balance calculations showed a substantial increase in net heat loss, more so in obese than lean animals. EPI is therefore thermolytic--i.e., disrupts thermal balance. A low dose (100 micrograms/kg) of the alpha-antagonist phentolamine produced a marked improvement in operant behavior, Tc, and thermal balance, whereas a comparable dose of the beta-antagonist propranolol had no beneficial effect. Increasing the dose of phentolamine worsened the responses with respect to the 100-micrograms/kg dose. The selective alpha 1-antagonist prazosin ameliorated the decrease in Tc induced by EPI but had little effect on operant behavior or thermal balance; the selective alpha 2-antagonist yohimbine had no effect on any parameter compared to EPI alone. These results suggest that the paradoxical effects of EPI in the cold are mediated by alpha-adrenoceptors, but definitive identification of the subclass of receptor involved cannot be determined.

Potentiation of the anti-obesity effect of the selective beta 3-adrenoceptor agonist BRL 35135 in obese Zucker rats by exercise

1. The effects of chronic treatments with a selective beta 3-adrenoceptor agonist and a selective alpha 2-adrenoceptor antagonist and their interactions with physical exercise training were studied in experimental obesity. 2. BRL 35135 (beta 3-agonist, 0.5 mg kg-1 day-1 p.o.), atipamezole (alpha 2-antagonist, 4.0 mg kg-1 day-1 p.o.) and placebo were given to genetically obese male Zucker rats. Half of the rats were kept sedentary whereas the other half were subjected to moderate treadmill exercise training. Body weight gain, cumulative food intake, the neuropeptide Y content of the hypothalamic paraventricular nucleus, brown adipose tissue thermogenic activity (measured as GDP binding), plasma insulin and glucose levels were measured after 3 weeks' treatment and exercise. 3. Treatment with BRL 35135 reduced weight gain by 19%, increased brown adipose tissue thermogenic activity 45-fold and reduced plasma insulin by 50%. Atipamezole slightly increased food intake and neuropeptide Y content in the paraventricular hypothalamic nucleus but had no effect on the other measured parameters. Exercise alone had no effect on weight gain, food intake or thermogenic activity, whereas it reduced plasma insulin and glucose levels. 4. The effect of BRL 35135 on weight gain and thermogenic activity was significantly potentiated by exercise; the reduction in weight gain was 56% in comparison with 19% in sedentary animals. Food intake was significantly reduced in the BRL 35135-treated-exercise-trained animals, although neither beta 3-agonist nor exercise alone affected it. 5. Based on the present results in genetically obese Zucker rats, combination of 03-agonist treatment with a moderate physical training may offer a new feasible approach to the therapy of obesity.-

KEYWORDS

BRL 35135; atipamezole; P3-adrenoceptor agonism; M2-adrenoceptor antagonism; brown adipose tissue; thermogenesis;genetic obesity; Zucker rat; exercise; neuropeptide Y

Physiological and environmental aspects of drinking stimulated by chronic exposure to amphetamine in rats

1. To examine whether the hyperdipsic response to chronic administration of d,l-amphetamine (AMPH) is associated with modification of salt appetite, rats were allowed to choose between tap water and a 1.7% NaCl solution. 2. Under AMPH rats preferred water to saline throughout the experiment. 3. By testing rats in a distinct test cage environmental influences on AMPH-mediated hyperdipsia were also evaluated. 4. In the test cage hyperdipsia was suppressed, but preference for tap water was preserved. 5. Finally, the role of alpha 2-adrenoceptors in the drinking response to AMPH was evaluated by studying the effects of clonidine and yohimbine on water intake. 6. We conclude that AMPH-induced preference for tap water over saline is unrelated to hyperdipsia but, being also induced by yohimbine, it may depend on noradrenergic mechanisms.

Is yohimbine-induced increase in salivary secretion a kinin-dependent mechanism?

We have previously reported that the alpha 2-adrenoceptor antagonist yohimbine induced a significant increase in both salivary flow rate and kallikrein output. In order to assess the possible role of the kinin-kallikrein system in the increase in salivary secretion elicited by yohimbine, the effects of aprotinin, an inhibitor of kallikrein activity, were investigated in yohimbine-treated conscious dogs. Aprotinin (at a dose, 5000 IU/kg iv, which reduced both resting and yohimbine-induced increase in kininogenase and amidolytic activities of saliva) which remained inactive alone, failed to modify the increase in salivary volume elicited by yohimbine (0.5 mg/kg iv). These results show that the rise in salivary flow rate observed under alpha 2-adrenoceptor antagonist is not induced by the kinin-kallikrein system. The release of kallikrein into saliva observed after yohimbine is rather the consequence than the cause of the increase in salivary secretion.