High dietary fructose does not exacerbate the detrimental consequences of high fat diet on basilar artery function

The objective of the study was to determine the effects of a high fat (HF) diet alone or with high fructose (HF/F) on functional and structural changes in the basilar arteries and cardiovascular health parameters in rats. Male Sprague Dawley rats were fed either a HF (30%) or HF/F (30/40%) diet for 12 weeks. The basilar artery was cannulated in a pressurized system (90 cm H2O) and vascular responses to KCl (30 - 120 mM), endothelin (10(-11) - 10(-7) M), acetylcholine (ACh) (10(-10) - 10(-4) M), diethylamine (DEA)-NONO-ate (10(-10) - 10(-4) M), and papaverine (10(-10) - 10(-4) M) were evaluated. Rats were also monitored for food intake, body weight, blood lipids, blood pressure, and heart rate. At death, asymmetrical dimethyl arginine level (ADMA) and leptin were assayed in serum. Although there was no significant difference in weight gain and food intake, HF and HF/F diets increased body fat composition and decreased the lean mass. HF/F diet accelerated the development of dyslipidemia. Although resting blood pressure remained unchanged, stress caused a significant elevation in blood pressure and a modest increase in heart rate in HF fed rats. Both HF and HF/F diet resulted in decreased response to endothelium-dependent and -independent relaxation, whereas increased basilar artery wall thickness was observed only in HF group. Serum leptin levels positively correlated with wall thickness. Moreover serum ADMA was increased and eNOS immunofluorescence was significantly decreased with both diets. These data suggest that the presence of high fructose in a HF diet does not exacerbate the detrimental consequences of a HF diet on basilar artery function.

Targeted leptin receptor blockade: role of ventral tegmental area and nucleus of the solitary tract leptin receptors in body weight homeostasis

The present investigation examined whether leptin stimulation of ventral tegmental area (VTA) or nucleus of the solitary tract (NTS) has a role in body weight homeostasis independent of the medial basal hypothalamus (MBH). To this end, recombinant adeno-associated viral techniques were employed to target leptin overexpression or overexpression of a dominant negative leptin mutant (leptin antagonist). Leptin antagonist overexpression in MBH or VTA increased food intake and body weight to similar extents over 14 days in rats. Simultaneous overexpression of leptin in VTA with antagonist in MBH resulted in food intake and body weight gain that were less than with control treatment but greater than with leptin alone in VTA. Notably, leptin overexpression in VTA increased P-STAT3 in MBH along with VTA, and leptin antagonist overexpression in the VTA partially attenuated P-STAT3 levels in MBH. Interestingly, leptin antagonist overexpression elevated body weight gain, but leptin overexpression in the NTS failed to modulate either food intake or body weight despite increased P-STAT3. These data suggest that leptin function in the VTA participates in the chronic regulation of food consumption and body weight in response to stimulation or blockade of VTA leptin receptors. Moreover, one component of VTA-leptin action appears to be independent of the MBH, and another component appears to be related to leptin receptor-mediated P-STAT3 activation in the MBH. Finally, leptin receptors in the NTS are necessary for normal energy homeostasis, but mostly they appear to have a permissive role. Direct leptin activation of NTS slightly increases UCP1 levels, but has little effect on food consumption or body weight.

Leptin overexpression in VTA trans-activates the hypothalamus whereas prolonged leptin action in either region cross-desensitizes

High-fat feeding or CNS leptin overexpression in chow-fed rats results in a region-specific cellular leptin resistance in medial basal hypothalamic regions and the ventral tegmental area (VTA). The present investigation examined the effects of targeted chronic leptin overexpression in the VTA as compared with the medial basal hypothalamus on long-term body weight homeostasis. The study also examined if this targeted intervention conserves regional leptin sensitivity or results in localized leptin resistance. Cellular leptin resistance was assessed by leptin-stimulated phosphorylation of signal transducers and activators of transcription 3 (STAT3). Tyrosine hydroxylase was measured in hypothalamus and VTA along with brown adipose tissue uncoupling protein 1. Leptin overexpression in VTA tempered HF-induced obesity, but to a slightly lesser extent than that with leptin overexpression in the hypothalamus. Moreover, the overexpression of leptin in the VTA stimulated cellular STAT3 phosphorylation in several regions of the medial basal hypothalamus, whereas verexpression in the hypothalamus did not activate STAT3 signaling in the VTA. This unidirectional trans-stimulation did not appear to involve migration of either the vector or the gene product. Long-term leptin overexpression in either the medial basal hypothalamus or VTA caused desensitization of leptin signaling in the treated region and cross-desensitization of leptin signaling in the untreated region. These results demonstrate a role of leptin receptors in the VTA in long-term body weight regulation, but the trans-activation of the hypothalamus following VTA leptin stimulation suggests that an integrative response involving both brain regions may account for the observed physiological outcomes.

Simultaneous introduction of a novel high fat diet and wheel running induces anorexia

Voluntary wheel running (WR) is a form of physical activity in rodents that influences ingestive behavior. The present report describes an anorexic behavior triggered by the simultaneous introduction of a novel diet and WR. This study examined the sequential, compared with the simultaneous, introduction of a novel high-fat (HF) diet and voluntary WR in rats of three different ages and revealed a surprising finding; the simultaneous introduction of HF food and voluntary WR induced a behavior in which the animals chose not to eat although food was available at all times. This phenomenon was apparently not due to an aversion to the novel HF diet because introduction of the running wheels plus the HF diet, while continuing the availability of the normal chow diet did not prevent the anorexia. Moreover, the anorexia was prevented with prior exposure to the HF diet. In addition, the anorexia was not related to extent of WR but dependent on the act of WR. The introduction a HF diet and locked running wheels did not induce the anorexia. This voluntary anorexia was accompanied by substantial weight loss, and the anorexia was rapidly reversed by removal of the running wheels. Moreover, the HF/WR-induced anorexia is preserved across the age span despite the intrinsic decrease in WR activity and increased consumption of HF food with advancing age. The described phenomenon provides a new model to investigate anorexia behavior in rodents.

Region-specific diet-induced and leptin-induced cellular leptin resistance includes the ventral tegmental area in rats

Diet-induced obesity (DIO) results in region-specific cellular leptin resistance in the arcuate nucleus (ARC) of the hypothalamus in one strain of mice and in several medial basal hypothalamic regions in another. We hypothesized that the ventral tegmental area (VTA) is also likely susceptible to diet-induced and leptin-induced leptin resistance in parallel to that in hypothalamic areas. We examined two forms of leptin resistance in F344xBN rats, that induced by 6-months of high fat (HF) feeding and that induced by 15-months of central leptin overexpression by use of recombinant adeno-associated viral (rAAV)-mediated gene delivery of rat leptin. Cellular leptin resistance was assessed by leptin-stimulated phosphorylation of signal transducers and activators of transcription 3 (STAT3) in medial basal hypothalamic areas and the VTA. The regional pattern and degree of leptin resistance with HF was distinctly different than that with leptin overexpression. Chronic HF feeding induced a cellular leptin resistance that was identified in the ARC and VTA, but absent in the lateral hypothalamus (LH), ventromedial hypothalamus (VMH), and dorsomedial hypothalamus (DMH). In contrast, chronic central leptin overexpression induced cellular leptin resistance in all areas examined. The identification of leptin resistance in the VTA, in addition to the leptin resistance in the hypothalamus, provides one potential mechanism, underlying the increased susceptibility of leptin resistant rats to HF-induced obesity.

Wheel running eliminates high-fat preference and enhances leptin signaling in the ventral tegmental area

Voluntary wheel running (WR) is a form of physical activity in rodents that influences ingestive behavior. This study examined the effects of WR on dietary preference and the potential role of leptin in mediating these effects. In a two-diet choice paradigm in which both palatable high-fat (HF) food and standard laboratory chow were provided ad libitum, rats displayed a strong preference for the former and chose to eat almost exclusively the HF diet over chow in sedentary conditions. With free access to running wheels, however, rats exhibited no preference for the HF food and consumed equal gram amounts of both chow and HF diets. The total daily caloric consumption during WR in the dietary choice protocol was equivalent to the amount of calories consumed daily by WR rats with only chow or only HF diet available, yet significantly less than sedentary chow caloric consumption. Two days after initiating WR, leptin signal transduction was examined in multiple selected brain sites following leptin injection into the third cerebral ventricle. The maximal leptin-stimulated STAT3 phosphorylation was enhanced only in the ventral tegmental area (VTA), but not in the arcuate nucleus, lateral hypothalamus, dorsal medial or ventral medial hypothalamus, or substantia nigra. In conclusion, wheel running appears to act either as an independent reinforcing factor or as a more favored activity to substitute for the consumption of a palatable HF diet, thus eliminating the preference for the HF food. Moreover, WR enhances leptin signaling specifically in the VTA, suggestive of a WR-evoked mechanism of heightened leptin function in the VTA to curb the drive to consume palatable HF foods.

Leptin resistance exacerbates diet-induced obesity and is associated with diminished maximal leptin signalling capacity in rats

AIMS/HYPOTHESIS

Leptin resistance is generally considered a consequence of obesity. We postulated that leptin resistance is associated with diminished hypothalamic leptin signalling capacity and that leptin resistance is causal to obesity. We assessed maximal leptin-mediated binding of the transcription factor signal transducer and activator of transcription 3 (STAT3), and the response to high-fat feeding in lean leptin-resistant rats.

MATERIALS AND METHODS

Recombinant adeno-associated virus encoding rat leptin cDNA (rAAV-leptin) or control vector were administered by intracerebroventricular injection to lean F344 x BN rats for up to 150 days, and food consumption, body weight, serum leptin and glucose tolerance were measured. Leptin-mediated hypothalamic transcription factor binding was assessed at day 150 following an intracerebroventricular injection of 2 mug leptin. Rats pretreated with either control or rAAV-leptin vector for 94 days were given a high-fat diet, and energy intake, body weight gain and adiposity were examined.

RESULTS

The rAAV-leptin-treated rats initially responded to leptin gene delivery then became leptin-resistant. They displayed persistent submaximal hypothalamic leptin signalling and enhanced insulin sensitivity, yet maximal hypothalamic signalling capacity was decreased by more than 50%. On a high-fat diet, the leptin-resistant rats consumed more energy, gained more weight and accumulated greater visceral fat mass than controls.

CONCLUSIONS/INTERPRETATION

The maximal hypothalamic leptin signalling capacity was diminished in leptin-resistant rats receiving central rAAV-leptin gene therapy. Moreover, this leptin-invoked leptin resistance perturbs the regulation of energy homeostasis in response to high fat exposure, producing augmented energy consumption. This, coupled with potential hypersensitivity to insulin, creates a milieu favouring fat deposition. Our data suggest that leptin resistance is both a consequence and cause of obesity.

Induction of uncoupling protein 1 by central interleukin-6 gene delivery is dependent on sympathetic innervation of brown adipose tissue and underlies one mechanism of body weight reduction in rats

Interleukin-6 (IL-6) is a multifunctional cytokine that may have a role in energy regulation. Using a recombinant adeno-associated viral vector expressing murine interleukin-6 (rAAV-IL-6), we examined the chronic effects of centrally expressed IL-6 on food intake, body weight and adiposity in male Sprague-Dawley rats, and investigated the underlying mechanisms. Direct delivery of rAAV-IL-6 into rat hypothalamus suppressed weight gain and visceral adiposity without affecting food intake over a 5-week period. rAAV-IL-6 enhanced uncoupling protein 1 (UCP1) protein levels in interscapular brown adipose tissue (BAT). To investigate if the induction of UCP1 and the reduction in body weight are dependent on sympathetic innervation of BAT, we administered rAAV-IL-6 or a control vector into the hypothalamus of rats in which the interscapular BAT was unilaterally denervated. Over 21 days, there was no difference in food consumption or body weight between rAAV-IL-6- and control vector-treated rats. rAAV-IL-6 delivery increased UCP1 mRNA and protein levels in innervated BAT pads but not denervated BAT pads. Hypothalamic IL-6 signal transduction, indicated by phosphorylated signal transducer and activator of transcription 3 (P-STAT3) levels, was elevated by 2.6-fold at day 21, but returned to control levels by day 35. However, the suppressor of cytokine signaling-3 mRNA level was significantly elevated both at day 21 and day 35. These data demonstrate that chronic elevation of IL-6 in the CNS reduces body weight gain and visceral adiposity without affecting food intake. The mechanism involves sympathetic induction of UCP1 in BAT and, presumably, enhanced thermogenesis in BAT. Furthermore, chronic central IL-6 stimulation desensitizes IL-6 signal transduction characterized by reversal of elevated P-STAT3 levels.

Central leptin gene delivery evokes persistent leptin signal transduction in young and aged-obese rats but physiological responses become attenuated over time in aged-obese rats

The purpose of this study was to determine if long-term leptin treatment desensitizes leptin signal transduction and the subsequent downstream anorexic and thermogenic responses in normal and leptin-resistant age-related obese rats. To this end, we administered, i.c.v., recombinant adeno-associated virus encoding rat leptin cDNA (rAAV-leptin) or control virus into young and aged-obese rats and after 9 or 46 days, examined food intake, oxygen consumption, body weight, serum leptin, STAT3 phosphorylation, hypothalamic NPY and POMC mRNAs, and UCP1 expression and protein level in brown adipose tissue (BAT). In young rats, rAAV-leptin depleted body fat and both anorexic and thermogenic mechanisms contributed to this effect. Moreover, leptin signal transduction was not desensitized, and there were persistent physiological responses. Similarly, in the aged-obese rats, there was unabated leptin signal transduction, however, both the anorexic and thermogenic responses completely attenuated sometime after day 9. This attenuation, downstream of the leptin receptor, may be contributing to the leptin-resistance and age-related weight gain in these aged-obese rats. Finally, in young rats, although the initial responses to rAAV-leptin were dominated by anorexic responses, by 46 days, the predominant response was thermogenic rather than anorexic, suggesting that energy expenditure may be an important component of long-term weight maintenance.

Hypothalamic leptin resistance is associated with impaired leptin signal transduction in aged obese rats

Leptin contributes to the regulation of both food intake and energy expenditure. We previously demonstrated that the F344xBN rat, a rodent model for late-onset obesity, is leptin-resistant and that leptin signal transduction following peripheral administration of leptin is impaired in these aged, overweight rats. To determine if leptin signal transduction is impaired in response to central administration of leptin and whether reduced hypothalamic leptin receptors may be contributing to the impaired signal transduction, we examined the in vivo dose-response leptin-induced STAT3 activation (phosphorylation and binding activity to the SIE M67 oligonucleotide) in response to i.c.v. administration of leptin along with the level of hypothalamic leptin receptor protein in young and older, late-onset obese rats. The leptin-induced maximum phosphorylation of STAT3 was 41% greater in young compared with older obese rats, but the dose required for half-maximal phosphorylation of STAT3 was similar in both the young (41 ng) and old-obese (47 ng) rats. There were no changes in total STAT3 protein with leptin or age, and leptin did not increase phosphorylation of STAT1. Leptin increased phosphorylation of STAT3 transcription factor binding eight-fold in the young but only four-fold in the aged-obese rats, and leptin receptor protein was 50% greater in the young compared with aged rats. These data indicate that aged-overweight rats demonstrate reduced signal transduction in response to centrally administered leptin that may be the result of the diminished leptin receptor protein observed in the aged-obese rats. The diminished leptin receptors and impaired leptin signal transduction may explain the diminished physiological responses observed following leptin administration in older rats. This impaired leptin signal transduction may be due either to the elevated obesity with age or to age itself, or to both.

Impaired leptin signal transduction with age-related obesity

Leptin contributes to the regulation of both food intake and energy expenditure. We previously demonstrated that the F-344xBN rat, a rodent model for late-onset obesity, is leptin resistant, suggesting that leptin signal transduction may be impaired in these aged, overweight rats. To test this hypothesis, we examined the in vivo dose-response and time-course response of leptin-induced STAT3 activation (phosphorylation and binding activity to the SIE M67 oligonucleotide) in the hypothalamus of young rats along with the dose-response leptin-induced STAT3 phosphorylation (P-STAT3) and maximum increase in binding activity in young and aged rats. In young rats there was a dose (0-1 mg, iv) and time dependent increase in P-STAT3 and in P-STAT3 binding activity. P-STAT3 paralleled the rise and fall in serum leptin levels with P-STAT3 elevated for at least 4 h with return to basal levels by 14 h after 1 mg leptin. The maximum level of leptin-induced P-STAT3 was unchanged with age, but the dose for half maximal phosphorylation was greater in aged (138 microg) compared with young (26 microg) rats. In addition, the leptin-induced increase in P-STAT3 transcription factor binding was diminished in aged rats. These data suggest that leptin signal transduction, in vivo, demonstrate a time and dose response increase paralleling the rise and fall in serum leptin, suggesting that serum leptin levels are the most important factor in determining leptin-induced phosphorylation of STAT3 in the hypothalamus. In addition, aged, overweight rats demonstrate reduced signal transduction in response to leptin, with reduced sensitivity for STAT3 phosphorylation and diminished leptin-induced P-STAT3 transcription factor binding. This impaired leptin signal transduction may be due to either the elevated obesity with age or due to age itself or both.

Otolaryngologic aspects of oral-facial-digital syndrome

The oral-facial-digital (OFD) syndromes are a heterogeneous group of hereditary disorders which have in common the findings of oral abnormalities, facial dysmorphism, and hand/feet malformations. We report the case history of an 18-month-old male with cerebellar cysts, hydrocephalus, tongue hamartomas, and polydactyly. These findings are most consistent with OFD VI. The clinical features of eight different types of OFD are discussed, with particular attention to the characteristics of the most interest to the otolaryngologist.

Impaired leptin responsiveness in aged rats

We previously reported that adiposity and serum leptin levels increase with age in male F-344xBN rats and that when physiological levels of serum leptin are manipulated by fasting, there is a corresponding reciprocal change in hypothalamic neuropeptide Y (NPY) mRNA in young rats, but there are no changes in older rats. These findings suggest that the regulation of hypothalamic NPY mRNA by leptin may be impaired with age. To test this hypothesis, we infused saline or leptin for 7 days into ad libitum-fed rats and compared these with saline-infused rats that were pair-fed the amount of food consumed by the leptin-treated rats. We examined daily food consumption, body weight, whole-body oxygen consumption, serum leptin, and NPY mRNA in the hypothalamus. Food consumption decreased by 50% in the leptin-infused compared with the saline-infused young rats but only decreased by 20% in the aged rats. In the leptin-treated young rats, there was a 24% increase in oxygen consumption compared with the pair-fed rats, but there were no changes in oxygen consumption in the aged rats. Leptin infusion diminished hypothalamic NPY levels by nearly 50% compared with pair-fed young rats, whereas there were no changes in the hypothalamic NPY mRNA levels in senescent rats. In summary, aged rats demonstrate a reduced responsiveness to leptin, including a diminished decrease in food intake and no increase in energy expenditure. These diminished responses to leptin were associated with and may be the result of an impaired suppression of hypothalamic NPY mRNA levels. This leptin resistance may be due to either the elevated obesity and serum leptin with age or due to age itself, or both.

Modulation of uncoupling protein 2 and uncoupling protein 3: regulation by denervation, leptin and retinoic acid treatment

We recently reported that the leptin-induced increase in uncoupling protein 1 (UCP1) mRNA in brown adipose tissue (BAT) is prevented by the denervation of BAT. We also reported that retinoic acid (RA) increases UCP1 mRNA in BAT. To extend these finding to UCP2 and UCP3 in BAT, we examined UCP2 and UCP3 mRNA after unilateral denervation of BAT, as well as after leptin, beta(3)-adrenergic agonist, RA, and glucocorticoid administration to rats. UCP3 mRNA was 20% less in the denervated compared with the intact BAT, whereas UCP2 mRNA was unchanged with denervation. The beta(3)-adrenergic agonist, CGP-12177 (0.75 mg/kg), increased UPC3 mRNA by 40% in the innervated and by 85% in the denervated BAT. Leptin (0.9 mg/day for 3 days) increased both UCP2 and UCP3 mRNA by 30% in the innervated and, surprisingly, in the denervated BAT. RA (7.5 mg/kg) increased UCP1 mRNA but decreased UCP2 and UCP3 mRNA by 50%, whereas methylprednisolone (65 mg/kg, two doses 24 h apart) suppressed all three uncoupling proteins by greater than 60%. The present findings indicate that: sympathetic innervation is necessary to maintain basal levels of UCP3 mRNA; beta(3)-adrenergic agonist stimulation induces UCP3 mRNA; leptin induces UCP2 and UCP3 mRNA and this induction is not dependent on sympathetic innervation; RA increases UCP1 but decreases UCP2 and UCP3 mRNA; and methylprednisolone suppresses UCP1, UCP2, and UCP3 mRNA equally. These data suggest that there are distinct patterns of regulation between UCP1, UCP2, and UCP3, and there may be at least two modes by which leptin could modulate thermogenesis in BAT; first, by increasing sympathetic stimulation of BAT and induction of UCP1 mRNA and, secondly, by increasing UCP2 and UCP3 mRNA by a mechanism independent of sympathetic stimulation.

Differential down-regulation of beta3-adrenergic receptor mRNA and signal transduction by cold exposure in brown adipose tissue of young and senescent rats

Diminished beta3 adrenergic-stimulated thermogenesis in brown adipose tissue (BAT) in senescent rats is restored by a short period of cold exposure. To investigate if the mechanism of this restoration involves preferential up-regulation of beta3-adrenergic signal transduction in senescent compared with young rats, we examined the steady state levels of beta3-adrenergic receptor messenger ribonucleic acid (beta3AR mRNA) and the dose/response relationship for the activation of adenylyl cyclase by the selective beta3-adrenergic agonist, BRL 37344 in BAT membranes from young and senescent rats with or without 2 days cold exposure at 8 degrees C. beta3AR mRNA declined by 30% with age and by 60% with cold exposure in young rats. In contrast, cold exposure did not down-regulate beta3AR mRNA in senescent rats. BRL 37344-stimulated adenylyl cyclase activity was 38% less in senescent rats and was desensitized by cold exposure decreasing maximum stimulation by 78% and increasing, the dissociation constant 2.5-fold. Cold exposure decreased BRL 37344-stimulated adenylyl cyclase activity to a lesser extent in senescent rats (50%), such that the activity was similar in cold-exposed young and old rats. These data indicate that although cold exposure did not, as we hypothesized, up-regulate beta3-adrenergic signal transduction in the senescent rats, cold exposure preferentially down-regulated beta3AR mRNA and beta3AR-mediated adenylyl cyclase in the young compared with senescent rats.

UCP2, UCP3 and leptin gene expression: modulation by food restriction and leptin

To determine the effects of food restriction and leptin administration on several transcripts involved in energy homeostasis, we examined leptin, uncoupling proteins (UCP) 1, 2 and 3, lipoprotein lipase (LPL), beta3-adrenergic receptors (beta3AR) and hormone-sensitive lipase (HSL) mRNA levels in brown adipose tissue (BAT) and epididymal (EWAT) and perirenal (PWAT) white adipose tissue in three groups of rats. The groups were administered leptin for 1 week, or had food restricted to the amount of food consumed by the leptin-treated animals, or had free access to food. Leptin administration increased serum leptin concentrations 50-fold and decreased food consumption by 43%, whereas serum insulin and corticosterone concentrations were unchanged. Leptin increased LPL mRNA by 80%, UCP1 mRNA twofold, and UCP3 mRNA levels by 62% in BAT, and increased UCP2 mRNA levels twofold in EWAT. In contrast, UCP2 mRNA levels were unchanged in PWAT and BAT. In WAT from food-restricted rats, leptin gene expression was diminished by 40% compared with those fed ad libitum. With leptin administration, there was a further 50% decrease in leptin expression. LPL mRNA levels were decreased by food restriction but not by leptin in WAT, whereas beta3AR and HSL mRNA levels were unchanged with either food restriction or leptin treatment. The present study indicates that leptin increases the gene expression of UCP2 in EWAT and that of UCP1, UCP3 and LPL in BAT, whereas reduced food consumption but not leptin, decreases LPL expression in WAT. In addition, with leptin administration there is a decrease in leptin gene expression in WAT, independent of food intake and serum insulin and corticosterone concentrations.

Aging and fasting regulation of leptin and hypothalamic neuropeptide Y gene expression

To investigate the role of aging on the fasting-induced suppression of leptin gene expression and increase in hypothalamic neuropeptide Y (NPY) gene expression, we fasted or fed ad libitum male F-344xBN rats aged 3, 24, and 31 mo for 2 days. We examined leptin mRNA levels in retroperitoneal, inguinal, and epididymal white adipose tissue (WAT); serum leptin levels; and NPY mRNA levels in the hypothalamus. We found that leptin mRNA levels were increased from 3 to 24 mo and leveled off between 24 and 31 mo in both retroperitoneal WAT and inguinal WAT but were unchanged with age in epididymal WAT. Serum leptin levels increased with age, whereas hypothalamic NPY mRNA levels did not change with age. Fasting suppressed leptin gene expression in all three WATs and serum leptin. Moreover, this suppression of serum leptin and of leptin message in retroperitoneal WAT was less in aged rats. Conversely, fasting increased hypothalamic NPY message, again to a lesser extent in aged rats. In both fed (ad libitum) and fasted rats, there was a strong correlation between serum leptin and hypothalamic NPY mRNA levels in the young but not in either of the two aged groups. These data suggest that aged F-344xBN rats are leptin resistant and that the fasting regulation of serum leptin, leptin mRNA, and hypothalamic NPY mRNA is impaired in aged rats.

Leptin induction of UCP1 gene expression is dependent on sympathetic innervation

We previously demonstrated that leptin increases uncoupling protein 1 (UCP1) and lipoprotein lipase (LPL) gene expression in brown adipose tissue (BAT) of rats. To determine whether the induction of these transcripts is dependent on sympathetic innervation of BAT, we unilaterally surgically denervated interscapular BAT in both pair-fed and leptin (0.9 mg/day by infusion)-treated rats. In pair-fed rats, the level of UCP1 mRNA in the denervated BAT pad was 30-47% less than in the innervated pad. In the intact BAT pad, leptin administration increased UCP1 mRNA levels by nearly 2.5-fold compared with pair-fed rats. In contrast, in the denervated BAT pad, there was no increase in UCP1 gene expression. When LPL mRNA was examined in pair-fed rats, there was no difference between innervated and denervated BAT pads. With leptin administration, LPL gene expression increased by 75% in both the innervated and denervated BAT pads. beta3-Adrenergic receptor mRNA was unaffected by either denervation or leptin, whereas uncoupling protein 2 mRNA levels were increased in epididymal white adipose tissue (WAT) but not in perirenal WAT. CGP-12177, a specific beta3-adrenergic receptor agonist, induced nearly a fourfold increase in UCP1 and a twofold increase in LPL gene expression in both the innervated and denervated BAT pads. These data indicate that the leptin induction of UCP1 gene expression in BAT is dependent on sympathetic innervation but that the leptin induction of LPL gene expression is not.

Leptin gene expression increases with age independent of increasing adiposity in rats

Humans and rats tend to gain weight as they age. Leptin is one regulator of food intake and energy expenditure. To determine if the increase in adiposity with age is related to altered leptin gene expression, we assessed adiposity levels, leptin mRNA levels in epididymal and inguinal white adipose tissue (EWAT and IWAT), and uncoupling protein (UCP1) mRNA levels in interscapular brown adipose tissue (IBAT) from F344 x BN rats ages 3, 12, 18, 24, and 30 months (n = 8/age). Levels of adiposity determined by the adiposity index and the Lee index increased between ages 3 and 24 months, with a decrease at age 30 months. There were parallel increases with age in body weight, EWAT, and IWAT depot size up to age 24 months, followed by a nonsignificant decrease at age 30 months. Daily food intake was unchanged with age. In EWAT, leptin mRNA per microgram of RNA was unchanged with age, whereas in IWAT, it increased up to 24 months, then declined at 30 months. Total leptin mRNA levels in both IWAT and EWAT depots increased with age, peaking at age 24 months, and were correlated with adiposity. Serum leptin levels increased with age, also peaking at age 24 months, and were correlated with total leptin mRNA in WAT pads and adiposity. The rate of increase in serum leptin was greater than the increase in adiposity with age, suggesting contributions from both the increase in leptin expression per unit of WAT and the increase in WAT depot size. In addition, UCP1 mRNA levels in IBAT did not change with age. These data suggest that adiposity increases with age and cannot be attributed to increased food intake, impaired leptin gene expression, or decreased UCP1 mRNA level in IBAT. Furthermore, leptin gene expression in IWAT increases with age independent of increasing adiposity.

Leptin increases uncoupling protein expression and energy expenditure

In ob/ob mice, leptin increases energy expenditure and sympathetic outflow to brown adipose tissue (BAT). To test whether the mechanism of increased energy expenditure may involve increased thermogenesis in BAT, we acclimated normal rats to thermoneutrality for 2 wk followed by leptin administration for 1 wk. Some rats were food restricted for 1 wk to the level of food consumption in the leptin-treated ad libitum-fed rats, and the same rats were both food restricted and administered leptin for a second week. We examined oxygen consumption and uncoupling protein (UCP) expression in BAT. Leptin increased oxygen consumption after the 5th and 6th days in ad libitum-fed rats and after the 4th, 5th, and 6th days in food-restricted rats. Leptin increased BAT UCP mRNA levels greater than twofold in both ad libitum-fed and food-restricted rats. These data demonstrate a leptin-induced increase in energy expenditure in nonmutant rodents and suggest that one mechanism by which leptin increases energy expenditure is through increased thermogenesis in BAT, including increased expression of UCP.

beta 3-Adrenergic-mediated suppression of leptin gene expression in rats

To investigate the role of beta 3-adrenergic receptors in the suppression of leptin gene expression, we fasted F-344 rats to decrease leptin mRNA levels, refed the rats to stimulate leptin mRNA production, and examined the ability of the beta 3-adrenergic agonist CGP-12177 to prevent the rise in leptin mRNA levels. In the initial 2 h after CGP-12177 (0.75 mg/kg), there were significant reductions in both food consumption and leptin mRNA levels in epididymal, perirenal, and interscapular white adipose tissue. We were unable to detect leptin mRNA in interscapular brown adipose tissue (IBAT), whereas there was a significant increase in uncoupling protein mRNA levels in IBAT after CGP-12177. The suppression of leptin mRNA and food intake by CGP-12177 was confirmed in a second experiment using another rat strain, the F-344 x BN. Furthermore, refeeding after a period of fasting increased leptin mRNA, which was prevented by CGP-12177. These data indicate a role for beta 3-adrenergic-mediated regulation of leptin gene expression in nonmutant rodents and are consistent with other reports suggesting that beta 3-adrenergic agonists suppress food intake.

Effects of age on beta adrenergic subtype activation of adenylyl cyclase in brown adipose tissue

Thermogenesis in brown adipose tissue (BAT) is believed to be mediated mainly by beta3 adrenergic receptors. We previously demonstrated that the specific beta3 adrenergic agonist CGP-12177 increases whole body oxygen consumption and BAT GDP binding to a greater extent in young than in senescent rats. In contrast, the forskolin-induced increases were maintained with age, suggesting that early events in beta3 adrenergic signal transduction are impaired with age. To investigate whether beta1 or beta3 adrenergic function is decreased with age, we assessed beta1 and beta3 adrenergic receptor mRNA levels and the ability of beta1 and beta3 adrenergic receptors to activate adenylyl cyclase in BAT membranes from 4- and 24-month-old F-344 rats. Both beta1 and beta3 adrenergic receptor mRNA levels decreased by 50% with age. Adenylyl cyclase stimulated by the nonspecific agonist, isoproterenol, and by the specific beta3 agonist, BRL 37344, also declined by 50% with age, whereas glucagon stimulation decreased by more than 70%. The isoproterenol-stimulated adenylyl cyclase activation curves were resolved by two-site regression analysis to determine the contribution of beta1 and beta3 adrenergic receptors. The Vmax for both beta1 and beta3 adrenergic receptors decreased by 50% with age. However, stimulation of adenylyl cyclase by NaF and forskolin was also diminished by the same amount as beta adrenergic stimulation, suggesting that the activation with age may be limited by the amount of adenylyl cyclase catalytic unit rather than by receptor number. These data suggest both beta1 and beta3 adrenergic receptors and adenylyl cyclase catalytic units are deficient with age in rodent BAT.

Thermoregulation with age: restoration of beta(3)-adrenergic responsiveness in brown adipose tissue by cold exposure

The beta(3)-adrenergic-stimulated thermogenic response in brown adipose tissue (BAT) is impaired in senescent rats, whereas cold-induced thermogenesis is not. To determine if cold exposure can restore beta(3)-adrenergic receptor responsiveness in senescent rats, we examined BAT mitochondrial GDP binding in young and old rats, and UCP mRNA levels in young rats following stimulation by the beta(3)-adrenergic agonist CGP-12177 with and without prior cold exposure. F-344 male rats were maintained at thermoneutrality or exposed to 8 degrees C for 48 hr, followed by a 24-hr period of rewarming before administration of 0.75 mg/kg CGP-12177 or vehicle solution. During the rewarming period, GDP binding remained elevated but UCP mRNA levels with a half-life of 11 hr returned to levels observed in the thermoneutral controls. In young rats, both cold exposure and administration of the beta(3)-adrenergic agonist to thermoneutral controls increased GDP binding 2-fold and UCP mRNA levels 4-fold. However, in cold-exposed young rats, there was no further increase with beta(3)-agonist treatment. In senescent control rats, CGP-12177 did not increase GDP binding, but cold exposure did. However, in cold-exposed old rats, the beta(3)-agonist was now able to increase GDP binding. The induction of UCP mRNA by CGP-12177 was also investigated and found to be 25% less in senescent compared with young rats. These observations indicate that cold exposure restores the impaired beta(3)-adrenergic signal transduction in BAT from senescent rats. One possibility is that cold exposure induces the synthesis of one or more components in the beta(3)-adrenergic pathway in senescent rats.

Thermogenesis in brown adipose tissue with age: post-receptor activation by forskolin

beta3-Adrenergic-stimulated thermogenesis in brown adipose tissue (BAT) is diminished with age. beta3-Adrenergic receptors are positively coupled to adenylyl cyclase in BAT. To determine whether thermo- genesis, in response to direct activation of adenylyl cyclase, is also impaired with age, we examined whole body oxygen consumption, mitochondrial guanosine diphosphate (GDP) binding and BAT mitochondrial uncoupling protein (UPC) mRNA levels in 4- and 24-month-old F-344 rats following forskolin administration. We also examined the forskolin-induced change in body temperature in 4-month-old rats. In some instances, the results were compared with administration of the specific beta3-adrenergic agonist, CGP-12177. Forskolin (3.5 mg/kg) increased oxygen consumption but decreased body temperature. In subsequent experiments the BAT was unilaterally denervated. In these rats, the forskolin-(1.8 mg/kg) stimulated increase in oxygen consumption was similar in young and old rats. Forskolin increased GDP binding and UCP mRNA levels in both the denervated and innervated BAT pads. The increases were equal or greater in the BAT from senescent rats. These findings, coupled with our previous report of an impaired CGP-12177-stimulated increase in GDP binding in senescent rats, suggests beta3-adrenergic-stimulated, but not post-receptor-stimulated, thermogenesis is diminished with age.

Myocardial adenylyl cyclase type V and VI mRNA: differential regulation with age

Myocardial beta-adrenergic signal transduction diminishes with age. This decrease is not due to a decrease in the number of beta-adrenoceptors, but may be a result of an impaired capacity to activate adenylyl cyclase (AC). Forskolin-stimulated AC activity is diminished, and the number of forskolin binding sites is decreased, suggesting that the decrease in signal transduction with age is a result of fewer AC catalytic units. To investigate whether the decrease in AC with age is associated with diminished AC mRNA, we assessed AC type V and type VI mRNA in ventricles from 6-, 11-, and 24-month-old F-344 rats. The predominant mRNA species, type V, increased by 45% between 6 and 11 months of age but decreased to just below the 6-month level by age 24 months. In contrast, type VI mRNA decreased by 44% between 6 and 11 months of age and then increased to the 6-month level at age 24 months. The changes in type V and type VI mRNA did not parallel the decreases in the AC activity or forskolin binding sites with senescence. These data indicate that the steady-state levels of type V and VI AC mRNA are not reliable predictors of the efficacy of forskolin stimulation of AC activity at different ages.

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Thermoregulation with age: role of thermogenesis and uncoupling protein expression in brown adipose tissue

To investigate whether attenuation of thermogenesis in interscapular brown adipose tissue (IBAT) may account for the loss of thermoregulation with age, we examined two indices of thermogenesis after two types of cold exposure: one in which the senescent rats maintained homeothermy and the other in which the senescent rats became hypothermic. To this end, we assessed body temperature, guanosine 5'-diphosphate (GDP) binding to the IBAT mitochondrial uncoupling protein (UCP) and the induction of UCP mRNA after both 1-hr and 48-hr mild cold exposures at 8 degrees C and after a more severe, 1-hr cold exposure at 4 degrees C in 3- and 24-month-old F-344 rats. Thermoneutrality was determined to occur at an ambient temperature of 26 degrees C in rats of both ages. In the 1-hr mild cold-exposed rats, there was no significant increase in GDP binding to IBAT UCP. However, after 48 hr of mild cold exposure, there was a 3-fold increase in GDP binding and a 5-fold increase in the expression of UCP mRNA despite no hypothermia in either the young or old rats. During the more severe cold exposure, the senescent rats, but not the young rats, became hypothermic. GDP binding to UCP increased 75% following cold exposure and, surprisingly was the same in young and old rats. UCP transcripts did not increase during the 1-hr cold exposure. These data, coupled with our previous findings of diminished beta 3-agonist-stimulated IBAT thermogenesis, suggest that (i) IBAT thermogenesis, at least in the senescent rats, may be mediated by other than beta 3-adrenergic receptors, and (ii) that altered heat dissipation or impaired thermogenesis at some site other than interscapular BAT is responsible for the observed hypothermia.

Impaired febrile response with age: role of thermogenesis in brown adipose tissue

We demonstrated previously that in Escherichia coli-infected rats, the heat necessary for the febrile response is a result of thermogenesis in brown adipose tissue (BAT). To investigate whether senescent rats have an impaired febrile response to infection and whether such an impairment is a result of attenuated sympathetically activated thermogenesis in BAT, we assessed body temperature and the increase in mitochondrial guanosine 5'-diphosphate (GDP) binding sites in interscapular BAT in response to E. coli administration in young and senescent male F-344 rats. There was a significant delay of 2 hr in the onset of fever in the older animals. In addition, in senescent rats, the peak fever (1.0 +/- 0.1 delta degrees C vs 2.2 +/- 0.1) and the cumulative fever (383 +/- 43 delta degrees C.min vs 775 +/- 69) were significantly less than in the young rats (P less than 0.005). Baseline levels of GDP binding were the same in young and old rats. In young rats, during the rising phase of the fever, E. coli infection resulted in a 50% increase in the density of GDP binding sites in BAT mitochondria. In contrast, there was no increase in GDP binding in the older rats following infection. The failure to increase GDP binding may be a result of a reduced ability to unmask reserve GDP binding sites. Alternatively, there may be fewer total GDP binding sites (masked and unmasked) in senescent rats and these sites may already be unmasked. Collectively, these data suggest that the impaired febrile response with age is due to reduced thermogenesis in BAT.

Thermogenesis and mitochondrial GDP binding with age in response to the novel agonist CGP-12177A

The ability to regulate body temperature diminishes with age in both humans and rodents. To investigate whether attenuation of sympathetically activated thermogenesis in brown adipose tissue (BAT) may account for the loss of thermoregulation with age, we assessed O2 consumption and body temperature in response to norepinephrine and the specific BAT beta-adrenergic agonist CGP-12177A in 6-, 18-, and 24-mo-old rats. In addition, the effects of this agonist on interscapular BAT mitochondrial GDP binding in young and senescent rats were determined. CGP-12177A rapidly induced an elevation in O2 consumption, which peaked at 25 min, followed by a decline over 4 h. The peak increase in O2 consumption over baseline and the cumulative 4-h response were decreased with age [P less than 0.02, analysis of variance (ANOVA)]. CGP-12177A induced an increase in body temperature that paralleled but appropriately lagged behind the increase in O2 consumption and that was decreased with age (P less than 0.02, ANOVA). The norepinephrine-induced increase in O2 consumption was also reduced with age but was not paralleled by a change in body temperature and was associated with a four- to fivefold increase in physical activity. In young rats CGP-12177A increased the number of available BAT mitochondrial GDP binding sites at 20 and 60 min post-injection, but in senescent rats GCP-12177A was unable to increase GDP binding. These data indicate that CGP-12177A is a novel agonist for BAT thermogenesis. With age there is a reduced capacity for thermogenesis that involves a failure to increase GDP binding, either due to a diminished amount of uncoupling protein with age or a failure to unmask reserve GDP binding sites.

Adenylate cyclase agonist properties of CGP-12177A in brown fat: evidence for atypical beta-adrenergic receptors

Thermogenesis in brown adipose tissue (BAT) is stimulated by catecholamine activation of adenylate cyclase through the beta-adrenergic receptor. Recently it was reported that the beta-adrenergic antagonist CGP-12177A stimulates oxygen consumption in BAT. To investigate the mechanism of action of CGP-12177A in BAT, we assessed the inhibitory and stimulatory affects of CGP-12177A on the adenylate cyclase system in myocardial and BAT membranes from rats. CGP-1277A inhibited isoproterenol-stimulated adenylate cyclase activity in a dose-dependent manner, with an inhibitory constant (Ki) of 1.94 +/- 0.18 microM in BAT and 0.49 +/- 0.11 microM in the heart. However, in the absence of isoproterenol, CGP-12177A stimulated adenylate cyclase in BAT with two components of activation, and half-maximal stimulation occurred at 1 microM and 1.5 mM. In contrast, CGP-12177A did not stimulate adenylate cyclase activity in heart membranes. Propranolol inhibited the isoproterenol-stimulated activity with a potency that was one log less in BAT compared with heart. Propranolol fully blocked the high-affinity component but only weakly blocked the low-affinity component of CGP-12177A-stimulated activity in BAT. Pindolol was also less potent in BAT but inhibited the CGP-12177A-stimulated activity in a manner similar to the inhibition of the isoproterenol-stimulated activity, suggesting the CGP-12177A activation was beta-receptor mediated. Binding curves of [125I]iodocyanopindolol ([125I]ICYP) in competition with CGP-12177A demonstrated a shift to lower affinity in the presence of beta,gamma-imidoguanosine 5'-triphosphate, indicating that CGP-12177A has agonist properties with respect to the [125I]ICYP binding site.(ABSTRACT TRUNCATED AT 250 WORDS)