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American Journal of Physiology - Regulatory, Integrative and Comparative Physiology logoLink to American Journal of Physiology - Regulatory, Integrative and Comparative Physiology
. 2020 Nov 18;320(2):R173–R181. doi: 10.1152/ajpregu.00027.2020

Chronic CNS-mediated cardiometabolic actions of leptin: potential role of sex differences

Alexandre A da Silva 1,✉, Mark A Pinkerton 1, Frank T Spradley 2, Ana C Palei 2, John E Hall 1, Jussara M do Carmo 1
PMCID: PMC7948126  PMID: 33206555

Abstract

Previous studies using male rodents showed the adipocyte-derived hormone leptin acts in the brain to regulate cardiovascular function, energy balance, and glucose homeostasis. The importance of sex differences in cardiometabolic responses to leptin, however, is still unclear. We examined potential sex differences in leptin’s chronic central nervous system (CNS)-mediated actions on blood pressure (BP), heart rate (HR), appetite, and glucose homeostasis in normal and type 1 diabetic rats. Female and male Sprague-Dawley (SD) rats were instrumented with intracerebroventricular cannulas for continuous 7-day leptin infusion (15 µg/day), and BP and HR were measured by telemetry 24 h/day. At baseline, females had lower mean arterial pressure (MAP) (96 ± 3 vs. 104 ± 4 mmHg, P < 0.05) but higher HR (375 ± 5 vs. 335 ± 5 beats/min, P < 0.05) compared with males. After leptin treatment, we observed similar increases in BP (∼3 mmHg) and HR (∼25 beats/min) in both sexes. Females had significantly lower body weight (BW, 283 ± 2 vs. 417 ± 7 g, P < 0.05) and caloric intake (162 ± 20 vs. 192 ± 9 kcal/kg of body wt, P < 0.05) compared with males, and leptin infusion reduced BW (−10%) and caloric intake (−62%) similarly in both sexes. In rats with streptozotocin-induced diabetes (n = 5/sex), intracerebroventricular leptin treatment for 7 days completely normalized glucose levels. The same dose of leptin administered intraperitoneally did not alter MAP, HR, glucose levels, or caloric intake in normal or diabetic rats. These results show that leptin’s CNS effects on BP, HR, glucose regulation, and energy homeostasis are similar in male and female rats. Therefore, our results provide no evidence for sex differences in leptin’s brain-mediated cardiovascular or metabolic actions.

Keywords: blood pressure, diabetes, food intake, glucose, heart rate

INTRODUCTION

Leptin, an adipocyte-derived peptide, controls appetite, thermogenesis, and glucose and lipid metabolism, and it is one of the most important hormones for body weight regulation (1). Disruption of leptin receptor signaling or inability of adipocytes to produce leptin causes severe early-onset, morbid obesity in experimental animals and in humans (1, 2), whereas increased leptin levels in subjects that exhibit normal sensitivity to leptin are accompanied by weight loss and increased metabolic rate (1, 2).

In addition to modulating appetite and energy expenditure, leptin is also an important regulator of glucose and lipid metabolism. Acute and chronic leptin infusions enhance lipid and glucose uptake and metabolism in various tissues including skeletal muscles (3–7). Leptin also suppresses liver glucose production and output into the circulation, helping to regulate glucose levels under normal conditions (8–10). A major part of leptin’s chronic effects on lipid and glucose metabolism appears to be mediated by activation of leptin receptors in the central nervous system (CNS) (11–14), although leptin can also directly act on peripheral tissues to acutely promote lipid and glucose utilization (3, 5, 14). We demonstrated, for instance, that chronic infusion of leptin in the cerebral ventricles of insulin-deficient diabetic rats at a very small dose that does not alter circulating leptin levels restored blood glucose concentration all the way to normal despite almost undetectable plasma insulin levels (12). We also demonstrated that activation of the brain melanocortin system is necessary for leptin to exert its antidiabetic actions (11, 15). Subsequently, German et al. (8) showed that the CNS-mediated antidiabetic effects of leptin involve increased glucose uptake in peripheral tissues as well as normalization of glucose production by the liver in insulin-deficient rats.

Leptin not only regulates appetite, body weight, and metabolic functions but also has numerous other important physiological actions including modulation of sympathetic nervous system (SNS) activity to tissues important for controlling blood pressure (BP) (16). Chronic increases in circulating leptin levels cause elevations in BP that can be abolished by adrenergic receptor blockade (16, 17). Conversely, leptin deficiency is associated with normal or reduced SNS activity and BP despite morbid obesity that normally causes SNS activation and elevated BP (2, 18, 19). These previous studies, however, were performed almost exclusively in male rodents, and recent studies suggest that some of the effects of leptin observed in males are not always demonstrated in female animals. For instance, the ability of increased plasma leptin levels to evoke SNS activation appears to be blunted in female mice, whereas leptin-mediated increase in mineralocorticoid receptor (MR) activation seems to contribute importantly to the rise in BP in female but not male mice (18, 20, 21).

In the present study, we examined the chronic CNS-mediated effects of leptin on food intake, body weight, BP regulation, and glucose homeostasis in nondiabetic male and female Sprague-Dawley rats. We also examined the CNS-mediated effects of leptin on glucose regulation in type 1 diabetic rats. Our results indicate that the brain-mediated cardiovascular and metabolic actions of leptin examined in our study were comparable in male and female rats, suggesting no major sex differences for these effects of leptin. These results suggest that the sex differences observed for leptin’s actions on BP regulation are likely to be caused by peripheral and not CNS-mediated actions of leptin.

MATERIALS AND METHODS

All experimental protocols and procedures were approved by the Institutional Animal Care and Use Committee of the University of Mississippi Medical Center, Jackson, Mississippi. The rats were kept in a 12-h dark (6:00 PM to 6:00 AM) and light (6:00 AM to 6:00 PM) cycle and given free access to food and water throughout the study.

Animals

Male and female Sprague-Dawley (SD) rats at 12 wk of age were purchased from Harlan/Envigo (Houston, TX) and acclimatized to our Laboratory Animals Facility for at least 7 days before use in this study.

Surgeries

The rats were anesthetized with isoflurane (5.0% induction and 1.5%–2.0% maintenance), and a telemetry blood pressure transmitter (model TA11PAC40; Data Sciences International) was inserted in the abdominal aorta distal to the renal arteries under aseptic conditions, as previously described (22). At the end of the telemetry implantation, rats were moved to a stereotaxic apparatus and a steel intracerebroventricular (ICV) cannula (26 gauge, 10 mm long) was inserted in the lateral ventricle (23). The rats were allowed to recover for 8 days before control measurements were recorded. Body weight and food intake were measured daily as well as BP and heart rate (HR), where 24-h averages were derived from bursts of 10 s every 10 min using Dataquest 4.0 software. The ICV cannula placement was confirmed by a dipsogenic response to angiotensin II (11) and by visual inspection postmortem. All rats received water and food (Cat. No. 170955; Harlan/Envigo, WI) ad libitum.

Experimental Protocols

Protocol 1.

After recovery from surgical procedures and stable baseline control measurements for 5 days, male (n = 5) and female (n = 6) rats were briefly anesthetized with isoflurane, and an osmotic minipump (model 2001; Durect Corp.) was implanted subcutaneously between the scapulas and connected to the ICV cannula to deliver leptin (0.62 µg/h, 1.0 µL/h) continuously for 7 days, as previously described (11, 12, 15). This dose of leptin corresponds to 2% of the dose used in our previous studies in which intravenous leptin infusion raised plasma leptin to levels found in obesity (12). Taking into account that cerebrospinal fluid volume is ∼3%–4% of plasma volume, the dose of leptin chosen was expected to produce brain levels similar to those in severe obesity. Despite differences in body weight in male and female rats of the same age, their brain weights were comparable (2.11 ± 0.03 g in males and 1.91 ± 0.02 g in females). Therefore, we infused leptin ICV at the same rates in males and females, rather than adjusting for body weight, which would have resulted in much higher concentrations of leptin in the cerebrospinal fluid of male than in female rats. On the last day of intracerebroventricular leptin infusion, the cannula connecting the osmotic minipump to the ICV cannula was severed, and we continued to follow the animals for an additional 5-day recovery (posttreatment) period. BP, HR, and food intake were measured daily, and body weight was determined at baseline, day 7 of leptin ICV infusion, and at the end of the recovery posttreatment period. Tail blood samples (0.1 mL) were obtained under fasting conditions between 9:00 and 10:00 AM for determination of plasma insulin, leptin, and glucose concentrations. Plasma insulin (Cat. No. 90080; Crystal Chem) and leptin (Cat. No. MBOO; R&D Systems) concentrations were determined using ELISA kits, and blood glucose concentrations were determined using a glucose meter (ReliOn).

Protocol 2.

In a different set of male (n = 5) and female (n = 5) SD rats, insulin-deficient diabetes was induced by a single intraperitoneal injection of streptozotocin (STZ, 50 mg/kg; Sigma-Aldrich, dissolved in 0.5 mL of 0.05 M citrate buffer, pH 4.5). After confirmation of diabetes (glucose levels >250 mg/dL) 5 days later, the animals were anesthetized, and a minipump was implanted to infuse leptin intracerebroventricularly for 7 days as described in protocol 1. Blood glucose concentration was measured each morning between 9:00 and 11:00 AM for determination of blood glucose levels using glucose strips (ReliOn).

Protocol 3.

To test whether the metabolic and cardiovascular responses to chronic intracerebroventricular leptin infusion are mediated by potential spillover of leptin into the systemic circulation, leading to activation of leptin receptors in peripheral tissues, we administered leptin intraperitoneally using osmotic minipump implanted in the intraperitoneal (IP) cavity of male and female rats (n = 5/sex) to deliver leptin for 7 days at the same dose infused intracerebroventricularly (0.62 µg/h, 1.0 µL/h). Additional groups of male and female rats (n = 5/sex) were also made diabetic by STZ injection as described in protocol 2 and implanted with IP osmotic minipumps to deliver leptin at the same dose infused intracerebroventricularly to test if potential spillover of leptin into the systemic circulation of type 1 diabetic rats could contribute to leptin’s antidiabetic effects. For IP minipump implantation, after stable control measurements for 5 days, rats were anesthetized, and under aseptic conditions, a small abdominal incision (1 cm) was made 2 cm to the right of the midline where the BP telemeter device was implanted, as described earlier, and an Alzet minipump model 2001 filled with leptin was inserted, and the muscle and skin quickly sutured. BP, HR, and blood collection for insulin, glucose, and leptin measurements were performed as described in protocols 1 and 2.

Statistical Analyses

Data are expressed as means ± SE. Significant differences between two groups were determined by Student’s t test. Significant differences between two groups over time were determined by two-way ANOVA followed by Bonferroni’s post hoc test. Comparisons between control, experimental, and recovery periods within the same group were performed by one-way ANOVA followed by Dunnett’s post hoc test. A P value of <0.05 indicates a significant difference.

RESULTS

Baseline Characteristics of Male and Female SD Rats

As expected, female SD rats were lighter at 16–18 wk of age and consumed fewer calories, even after adjustment for body weight, compared with age-matched males (Fig. 1, A and B). Although female rats weighed less and ate fewer calories per day, their fasting glucose concentration was ∼20% higher than in males, while insulin concentration was similar (102 ± 6 vs. 84 ± 5 mg/dL and 1.9 ± 0.3 vs. 1.9 ± 0.2 ng/mL, respectively, Fig. 1C).

Figure 1.

Figure 1.

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Baseline characteristics of male (n = 5) and female Sprague-Dawley (SD) rats (n = 6). A: body weight. B: calorie (Cal.) intake per kilogram body weight (BW). C: fasting blood glucose and insulin concentrations. D: mean arterial pressure (MAP). E: heart rate (HR). Detection of statistical differences was done by using an unpaired Student’s t test. n.s., not significant.

Female SD rats also exhibited significantly lower BP but higher HR compared with age-matched male rats (Fig. 1, D and E).

Body Weight, Food Intake, Glucose, and Insulin Levels in Response to Chronic Intracerebroventricular Leptin Infusion in Nondiabetic Male and Female Rats

Despite reduced caloric intake at baseline, female SD rats when infused intracerebroventricularly with leptin at a dose that does not alter circulating leptin levels (Fig. 2A) responded with similar reductions in caloric intake compared with male rats with higher baseline caloric intake (Fig. 2B). However, the full effect of leptin’s anorexic action appeared to be slightly delayed in male compared with female rats. This delayed response was also observed during the recovery period after leptin infusion was terminated, with female rats showing a slightly faster return of caloric intake to baseline levels compared with male rats (Fig. 2B). This reduction in caloric intake was accompanied by a rapid weight loss of almost 10% in both sexes by the end of the 7-day treatment period (males: from 417 ± 7 to 378 ± 6 g and females: from 283 ± 2 to 256 ± 7 g, respectively, for baseline and day 7 of intracerebroventricular leptin infusion).

Figure 2.

Figure 2.

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Plasma leptin levels, caloric intake, blood glucose and insulin levels, and blood pressure (BP) and heart rate (HR) responses to chronic intracerebroventricular (ICV) leptin infusion (15 µg/day) in nondiabetic male (n = 5) and female Sprague-Dawley (SD) rats (n = 6). A: plasma leptin levels at baseline and day 7 of ICV leptin infusion. B: percent delta (Δ) change in calorie intake, compared with baseline, during ICV leptin infusion. C: fasting blood glucose. D: plasma insulin concentrations. E and F: delta (Δ) change in mean arterial pressure (MAP) (E) and heart rate (HR) (F) in response to chronic ICV leptin infusion for 7 days. *P < 0.05 compared with baseline values using one-way ANOVA followed by Dunnett’s post hoc test or using a paired Student’s t test (for blood glucose and insulin levels). n.s., not significant.

Leptin infusion markedly improved glucose regulation in both male and female animals, as indicated by a significant reduction in fasting blood glucose levels (Fig. 2C), while causing a marked reduction in plasma insulin concentration (Fig. 2D).

Blood Pressure and Heart Rate Responses to Chronic Intracerebroventricular Leptin Infusion in Nondiabetic Male and Female Rats

Chronic intracerebroventricular leptin infusion for 7 days caused a gradual, albeit nonsignificant, increase in BP in female rats, whereas in male rats, BP spiked transiently during the first 2 days of infusion followed by a gradual rise in pressure, reaching the same small nonsignificant increase in BP as observed in female rats by day 7 of leptin treatment (Fig. 2E). A similar pattern was observed for HR (Fig. 2F), except that the magnitude of the increase in HR observed in male and female rats during intracerebroventricular leptin infusion was more pronounced than leptin’s impact on BP. After leptin infusion was stopped, BP and HR gradually returned to baseline values (Fig. 2, E and F).

Impact of Chronic Intraperitoneal Leptin Infusion in Nondiabetic Male and Female Rats

Although chronic intracerebroventricular leptin infusion at the dose used in the present study did not significantly increase circulating leptin levels (Fig. 2A), we examined the impact of the same dose infused intraperitoneally on caloric intake, plasma glucose, insulin, and leptin levels as well as on BP and HR in male and female rats to test the possibility that part of leptin’s intracerebroventricular effects described earlier could be mediated by potential spillover of leptin into the systemic circulation.

Chronic intraperitoneal leptin infusion at the small dose of 0.62 µg/h did not alter caloric intake or body weight (Fig. 3A) and did not improve insulin sensitivity, as evidenced by no significant reductions in glucose or insulin levels (Fig. 3, B and C), despite causing a small but significant increase in plasma leptin levels in male and female rats (Fig. 3D). IP leptin administration also did not alter BP or HR in male and female rats (Fig. 3, E and F).

Figure 3.

Figure 3.

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Caloric intake, body weight (BW), blood glucose, insulin and leptin levels, and blood pressure (BP) and heart rate (HR) responses to chronic intraperitoneal (IP) leptin infusion (15 µg/day) in nondiabetic male (n = 5) and female Sprague-Dawley (SD) rats (n = 5). A: caloric intake and BW; B: fasting blood glucose; C and D: plasma insulin (C) and leptin (D) concentrations at baseline and day 7 of IP leptin infusion; E and F: delta (Δ) change in mean arterial pressure (MAP) (E) and heart rate (HR) (F) in response to chronic IP leptin infusion for 7 days. Statistical differences observed in plasma leptin levels were assessed using a paired Student’s t test. n.s., not significant.

Impact of Chronic Intracerebroventricular or Intraperitoneal Leptin Infusion on Glucose Tolerance in Nondiabetic Rats and on Blood Glucose Levels, Appetite, BP, and HR in STZ-Diabetic Male and Female Rats

We previously showed that leptin exerts powerful CNS-mediated effects on glucose homeostasis, including antidiabetic effects that are independent of insulin (12, 24). These previous studies, however, were performed only in male animals. Therefore, we first tested whether intracerebroventricular leptin infusion or the same dose administered intraperitoneally for 7 days in nondiabetic male and female rats alters tolerance to an acute load of glucose during an oral glucose tolerance test (GTT). The animals were fasted for 7 h (7:30 AM to 14:30 PM), and glucose (3 mg/g of body wt) was given by oral gavage 1 day before leptin treatment was initiated and repeated on day 7 of intracerebroventricular or intraperitoneal leptin infusion. Glucose levels were measured at 0, 15, 30, 60, and 90 min after the oral gavage. Fasting glucose levels were remarkably low at time 0 before the gavage in intracerebroventricular leptin-treated males and females compared with time 0 values at baseline (Fig. 4A). However, no significant differences in male or female rats were observed for the area under the glucose curve during the GTT at baseline or at day 7 of intracerebroventricular leptin treatment (Fig. 4, A and B). Intraperitoneal leptin infusion did not alter glucose levels at time 0 or during the GTT when compared with baseline, despite a tendency for improved tolerance in IP leptin-treated male rats (Fig. 4, C and D).

Figure 4.

Figure 4.

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Impact of chronic intracerebroventricular (ICV) or intraperitoneal (IP) leptin infusion (15 µg/day) on glucose tolerance in nondiabetic rats and on fasting blood glucose levels in streptozotocin (STZ)-induced diabetic male and female rats (n = 5/sex). A and B: oral glucose tolerance test (GTT) (A) and area under the curve (AUC) (B) of blood glucose levels during the GTT in nondiabetic male and female rats at baseline and on day 7 of ICV leptin infusion; C and D: oral GTT (C) and AUC (D) of blood glucose levels during the GTT in nondiabetic male and female rats at baseline and on day 7 of IP leptin infusion; E: fasting blood glucose levels in STZ-diabetic male and female rats at baseline and day 7 of ICV leptin infusion; F: fasting blood glucose levels in STZ-diabetic male and female rats at baseline and day 7 of IP leptin infusion. Statistical differences observed in fasting blood glucose levels were assessed using a paired Student’s t test. n.s., not significant.

Although we did not observe significant differences in acute glucose tolerance during the GTT at baseline compared with day 7 of intracerebroventricular leptin infusion, likely due to very low glucose levels at time 0 and leptin’s known actions to suppress insulin secretion, a similar CNS-mediated powerful antidiabetic effect of leptin was observed in male and female SD rats. As demonstrated in previous studies, chronic intracerebroventricular leptin infusion completely normalized glucose levels in male insulin-deficient diabetic rats (Fig. 4E). In female diabetic rats, intracerebroventricular leptin infusion also restored euglycemia (Fig. 4E), exhibiting the same efficacy in lowering glucose levels as in male diabetic rats. In animals where leptin was infused intraperitoneally at the same dose given intracerebroventricularly, leptin failed to attenuate hyperglycemia in male and female diabetic rats (Fig. 4F).

Since STZ-induced diabetes is associated with hyperphagia and cardiovascular alterations including pronounced bradycardia in rats (25), we also investigated potential sex differences in leptin’s effects to suppress the hyperphagia and the bradycardia in this model of diabetes. Chronic intracerebroventricular leptin infusion caused similar reductions in caloric intake (∼62%) and body weight (∼5%) in diabetic male and female rats (Table 1). Intracerebroventricular leptin treatment also reversed the bradycardia in both sexes (Table 1), raising HR to values comparable with those observed in nondiabetic rats (Fig. 1E). No significant changes were observed in BP (Table 1). Intraperitoneal leptin infusion at the same dose given intracerebroventricularly had no significant effects on body weight, caloric intake, BP, or HR in male or female diabetic rats (Table 1).

Table 1.

Body weight, caloric intake, mean arterial pressure, and heart rate in STZ-induced diabetic rats treated ICV or IP with leptin for 7 days

Treatment Body Weight, g Caloric Intake, kcal/day MAP, mmHg Heart Rate, beats/min
Males
 Baseline 352 ± 8 155 ± 13 109 ± 6 288 ± 11
 Leptin-ICV 336 ± 9 59 ± 8* 110 ± 9 348 ± 12*
Females
 Baseline 267 ± 9 125 ± 16 102 ± 5 308 ± 7
 Leptin-ICV 253 ± 10 48 ± 6* 104 ± 5 355 ± 9*
Males
 Baseline 360 ± 8 142 ± 11 108 ± 4 279 ± 2
 Leptin-IP 357 ± 7 145 ± 5 107 ± 3 275 ± 4
Females
 Baseline 275 ± 4 107 ± 3 98 ± 2 276 ± 12
 Leptin-IP 286 ± 5 103 ± 4 95 ± 3 274 ± 9
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Values represent means ± SE obtained on days 4 to 5 of the control period and days 6 to 7 of intraperitoneal (IP) or intracerebroventricular (ICV) leptin infusion (15 µg/day). *P < 0.05 compared with baseline using a paired Student’s t test (n = 5 rats per sex/group). MAP, mean arterial pressure; STZ, streptozotocin.

DISCUSSION

In the past 25 years, the view of leptin’s physiological importance has changed significantly from an adipokine contributing primarily to appetite and body weight regulation to a hormone with a myriad of important actions on many distinct physiological functions. Most previous studies focused on leptin’s effects in male animals since, until recently, sex differences were thought to be less common for most aspects of animal physiology. An increasing number of studies, however, have demonstrated that sex differences in cardiometabolic regulation may be more important than originally anticipated (26–28). Therefore, in this study, we examined potential sex differences in the CNS-mediated cardiovascular and metabolic actions of leptin under normal conditions as well as in the setting of insulin-deficient type 1 diabetes. We found that despite being leaner and consuming fewer calories than male controls, female SD rats exhibited similar responses to chronic CNS hyperleptinemia on food intake, BP, HR, and glucose regulation. These observations provide no evidence for major sex differences in the CNS-mediated effects of leptin on cardiometabolic regulation in young rats.

Leptin is a major regulator of body weight via its modulation of energy balance including the input (i.e., food intake) and output (i.e., energy expenditure and energy storage). Although female rats used in our study were lighter and consumed ∼15% fewer calories, their chronic anorexic response to leptin did not differ from what was observed in age-matched males. Cote et al. (29) also showed similar reductions in food intake in male and female rats with adenoviral-mediated overexpression of leptin in the CNS during the first week postadenoviral injection, with increased sensitivity to the anorexic effects of leptin in female rats on weeks 2–4 of treatment. Thus, it is possible that females may be more sensitive to leptin’s modulation of appetite over a longer period.

Although female SD rats in our study appeared to be less insulin sensitive at baseline, as evidenced by higher fasting glucose levels but similar plasma insulin concentrations, compared with age-matched males, leptin’s impact on glucose regulation was not different in male and female rats. Shek et al. (30) showed that chronic carotid artery or intravenous administration of leptin improves glucose homeostasis in lean male rats, as evidenced by small reductions in blood glucose levels while circulating insulin concentration were markedly reduced.

In the present study, we demonstrated that a low-dose intracerebroventricular leptin infusion to raise leptin levels only in the brain but not in the systemic circulation markedly reduced plasma insulin levels and caused a small decrease in blood glucose levels in lean male and female nondiabetic rats. These data indicate that leptin’s CNS effects are important for glucose homeostasis in both sexes of lean nondiabetic animals. We previously showed that leptin’s powerful CNS actions on glucose regulation are even more evident in type 1 diabetic male animals that cannot produce insulin (11, 24, 31); therefore, we also examined if leptin’s antidiabetic effects are equally potent in female insulin-deficient diabetic rats. We found that intracerebroventricular leptin administration completely normalized glucose levels in male and female STZ-treated diabetic rats, demonstrating that leptin’s powerful insulin-independent CNS-mediated antidiabetic actions were not substantially influenced by the sex of the rats. Furthermore, we showed that although chronic systemic infusion of leptin, at the same dose administered into the brain, caused a small but significant increase in circulating leptin levels, it did not alter fasting glucose or insulin levels in nondiabetic rats, nor did it reduce the hyperglycemia in male and female STZ-diabetic rats. These observations indicate that leptin’s antidiabetic effect observed during intracerebroventricular leptin infusion is mediated by CNS actions of leptin and not by activation of leptin receptors in peripheral tissues due to potential spillover of leptin from the brain into the systemic circulation.

Surprisingly, intracerebroventricular or intraperitoneal leptin infusion did not alter glucose tolerance during an oral GTT. The lack of improved glucose tolerance during oral GTT in male and females rats during leptin intracerebroventricular infusion may be explained, at least in part, by the very low baseline fasting glucose levels at time 0, which in addition to leptin’s known effect to lower insulin levels may have buffered leptin’s effect to enhance acute glucose disposal. It is also possible that leptin’s chronic effect on glucose homeostasis relies more on modulation of liver glucose production, a major factor regulating fasting glucose levels, than on glucose disposal by peripheral tissues. The current study, however, was not designed to test this possibility.

Recent studies in mice suggest that leptin may modulate BP via distinct mechanisms in a sex-dependent manner (18, 20, 32). In male rodents, leptin acts on the CNS to increase SNS activity and raise BP and HR (18, 33), whereas in female mice, leptin appears to rely less on SNS activation and more on modulating aldosterone secretion and MR activation (18, 20, 32). In the present study, we observed a small but nonsignificant increase in BP of similar magnitude in male and female rats during chronic intracerebroventricular leptin infusion, although male rats may have exhibited a greater initial rapid rise in mean arterial pressure (MAP) and HR than observed in females. Thus, it is possible that leptin’s CNS effects on SNS activity may be more robust in males, or that male rats are more sensitive to stress-induced acute increases in BP (i.e., minipump implantation for intracerebroventricular leptin delivery), although additional studies are needed to test this possibility.

Previous studies have shown that obese women exhibit greater reductions in BP following adrenergic receptor blockade therapy than lean women (34), which may suggest that in women, SNS activation in obesity may play a more important role in BP regulation than in female rodents. Nevertheless, our data indicate that the CNS effects of leptin on BP and HR are similar in male and female rats, and that potential sex differences of leptin’s impact on BP regulation are likely mediated by activation of peripheral rather than CNS leptin receptors.

It is also important to note that although the impact of intracerebroventricular leptin infusion on BP in lean male and female rats in our study was small, it occurred despite significant weight loss that is usually associated with reductions in BP and may have blunted the full effect of leptin to raise BP.

Perspectives and Significance

The importance of examining sex differences when studying physiological responses to various perturbations has been highlighted in recent studies. Therefore, whenever possible, males and females should be included when designing an experimental protocol. In the present study, we observed similar dietary, metabolic and cardiovascular responses in male and female Sprague-Dawley rats when leptin was chronically infused directly into the CNS. We previously found significant sex differences in BP regulation in mice lacking leptin receptors in the CNS. These findings suggest that leptin’s CNS cardiometabolic actions do not appear to exhibit major sex differences, whereas activation of leptin receptors in peripheral tissues may account for the differential impact of leptin in males and females. The mechanisms involved in the potential sex differences evoked by peripheral leptin receptors are still elusive but may involve the interaction of sex hormones with factors that regulate sodium handling by the kidneys. Our current findings, however, do not rule out the possibility that there may be differences between males and females in access of adipocyte-derived leptin into the brain, which could also contribute to sex differences observed in previous studies. We also acknowledge that the dose of leptin used in the present study may be supraphysiological, perhaps in the pathophysiological range.

Unraveling the mechanisms responsible for sex differences in obesity-induced hypertension as well as in other forms of hypertension is critical for improved management of BP with potential to effectively reduce mortality and morbidity in men and women. Our current studies also highlight the powerful antidiabetic effects of leptin that are mediated via CNS actions of leptin in male and female rodents, although the mechanisms involved are still unclear and deserve further investigation.

GRANTS

The authors were supported by Grants from the National Heart, Lung, and Blood Institute (P01 HL51971 and R00HL130577), the National Institute of General Medical Sciences (P20 GM104357, P20GM121334 and U54 GM115428), and the National Institute of Diabetes and Digestive and Kidney Diseases (R01 DK121411).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

A.A.d.S. conceived and designed research; A.A.d.S., M.A.P., and J.M.d.C. performed experiments; A.A.d.S. and M.A.P. analyzed data; A.A.d.S., F.T.S., A.C.P., J.E.H., and J.M.d.C. interpreted results of experiments; A.A.d.S. prepared figures; A.A.d.S. drafted manuscript; A.A.d.S., F.T.S., A.C.P., J.E.H., and J.M.d.C. edited and revised manuscript; A.A.d.S., M.A.P., F.T.S., A.C.P., J.E.H., and J.M.d.C. approved final version of manuscript.

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