Sex behavior and the sexually dimorphic hypothalamic nucleus in male Zucker rats

To determine whether or not impaired male sex behavior in obese male Zucker rats is accompanied by any anatomical alterations in a hypothalamic area implicated in the control of sex behavior, 6 lean and 5 obese male Zucker rats were studied behaviorally and anatomically at 14 months of age. Obese males showed markedly decreased male sex behavior relative to lean males, in spite of serum levels of testosterone and testicular weights comparable to those of lean rats. Obese rats had significant decreases in brain weight and volumes of sexually dimorphic nuclei per g of brain, relative to lean rats; volumes per g brain of other structures (paraventricular and suprachiasmatic nuclei) were not different between groups. It is suggested that an incomplete expression of sexually dimorphic features of the preoptic area-anterior hypothalamus, due perhaps to an impaired process of perinatal brain androgenization, may contribute to decreased male sex behavior in adult obese rats.     

Evidence of incomplete behavioral sexual differentiation in obese male Zucker rats

Genetically obese male Zucker rats displayed lower rates of intromission behavior and ejaculated in significantly fewer tests than lean Zucker littermates. After castration and daily injections of a low dosage (5 micrograms/kg) or estradiol benzoate (EB) followed by progesterone (1 mg/kg), obese males displayed significantly higher lordosis quotients than lean controls. Continued daily administration of higher (10, 15 micrograms/kg) dosages of EB followed by progesterone caused significant reductions in lordotic responsiveness in obese males but not in lean controls. Thus, deficient masculine coital performance was correlated with altered receptive responsiveness to ovarian steroids in obese Zucker male rats. This suggests that the sexual differentiation of the developing brain may be deficient in obese Zucker males.     

High dosage testosterone propionate induces copulatory behavior in the obese male Zucker rat

Genetically obese male Zucker rats sire few litters compared with lean males. High dosage testosterone propionate (TP) dramatically improves their litter production. Here we report the effect of high dosage TP on the copulatory behavior of obese males. Subjects were placed with ovariectomized, hormonally-primed females 24 hr after receiving a subcutaneous injection of vehicle or 20 mg TP. Other obese males received three consecutive daily TP injections or TP once every three days for up to 30 days prior to being tested. While 90% of sham-injected lean males became sexually active, only 18.2% of sham-injected obese males did so. In contrast 50% to 62.5% of TP-injected obese males became active depending on TP exposure. The proportion of obese males which ejaculated was high for sham-injected males which copulated, but varied with TP exposure for TP-injected males. In other respects the copulatory performance of obese and lean males was similar. High dosage TP increases fertility by correcting a behavioral deficit. Three possible mechanisms of TP action are proposed.     

Plasma testosterone levels and sexual performance of young obese male Zucker rats

The sexual behavior of obese male Zucker rats was studied at the age of 7, 8, 9, 10, 11, 14, and 16 weeks. Between 12–14 weeks of age, obese rats were treated with 400 μg testosterone propionate daily. Plasma testosterone levels were determined at the age of 10, 14, 16, and 20 weeks. Compared to lean rats, the sexual performance of the obese rats was abnormal at all ages. Plasma testosterone levels of obese rats were significantly lower at the age of 20 weeks but not at the age of 10 and 16 weeks, compared to those of their lean littermates. At the age of 14 weeks, after 2 weeks of testosterone treatment, plasma testosterone concentration rose to levels significantly higher than those of their lean littermates. However, there was no improvement in their sexual behavior following this period of hormone treatment. This study strongly suggests that obese male Zucker rats have abnormal reproductive function at all ages and that testosterone deficiency is not a primary cause of this abnormality.     

Relationship of dietary intake to the development of glomerulosclerosis in obese Zucker rats

Obese Zucker rats are hyperphagic and develop premature glomerulosclerosis. The purpose of this study was to find out the effects of restriction of dietary intake on this glomerulosclerosis. The obese and lean male Zucker rats were fed with restricted amounts of balanced diet for periods of 40 and 50 weeks, sacrificed, and the body weight, the light and ultrastructural alterations of glomeruli, and the serum levels of blood urea nitrogen, triglyceride, and cholesterol were examined. Obese and lean male rats of identical ages were fed ad libitum with the diet and studied similarly. The dietary restriction significantly lessened the development of the glomerulosclerosis in obese rats, while those on the nonrestricted diet manifested an advanced glomerulosclerosis. The dietary restriction, however, did not normalize the obesity nor correct the elevated serum lipid level to the range of lean control rats. The spontaneous glomerular lesions of the obese rats were characterized by segmental mesangial expansion, disappearance of podocytes and endothelia, and obliteration of capillary lumina. The lean rats maintained essentially normal renal morphology. A similar study on the renal morphology done in female Zucker rats also revealed a preventive effect of dietary restriction on the development of glomerulosclerosis. In conclusion, there is a strong association between the glomerulosclerosis and the hyperphagia of the obese Zucker rats, both in males and females, and the emergence of this lesion is preventable to a significant degree.     

Decreased sensitivity of Zucker obese rats to the putative satiety agent cholecystokinin

In obese rodents increased daily food intake leading to accumulation of adipose tissue is frequently accompanied by increased meal size and loss of the normal diurnal variations in feeding pattern. Increased meal size of obese rats may be due to decreased sensitivity to factors which elicit satiety. We compared Zucker obese and lean rat feeding behavior responses to octapeptide of cholecystokinin (OP-CCK), a peptide shown to decrease meal size in several species. Obese rats were less sensitive than lean rats to OP-CCK (.06, .25 and 1.0 μg/kg/meal) injected before each of four consecutive scheduled meals in the light portion of the diurnal cycle, when obese meal size was larger than lean. However, neither obese nor lean rats responded to injection of the same doses of OP-CCK during meals in the dark, when average meal size was larger than during the light and when average meal size of the obese rats was similar to that of lean rats. In both obese and lean rats injection of OP-CCK affected daily feeding pattern. Obese and lean Zucker rats are less sensitive to OP-CCK when meal size is larger, whether this is due to phase of the diurnal cycle (dark vs. light in both obese and lean rats) or phenotype (obese vs. lean rats in the light).     

Nociception and opioid-induced analgesia in lean (Fa/-) and obese (fa/fa) Zucker rats

Previous research indicates a possible interrelationship between the endogenous opioids (EO), nociception and food-intake. We therefore considered the hyperphagic obese Zucker rat a good candidate for abnormal responses to nociceptive stimuli. Pairs of lean and obese sisters were tested for latency of response to nociceptive stimuli by tail-flick and tail-pinch methods. Obese rats exhibited shorter latencies in each test, (tail-flick, p less than 0.05 and tail-pinch, p less than 0.001). Dose/response curves for morphine analgesia indicate that morphine is less potent in obese than in lean rats (ED50's = 4.87 +/- 0.62 mg/kg and 3.12 +/- 0.41 mg/kg respectively, p less than 0.05). These data suggest a defect in the EO systems of obese Zucker rats.     

Ontogeny of feeding behavior in the Zucker obese rat

Increased food intake in Zucker obese rats has been reported as early as 16 days of age. To determine if the feeding behavior of Zucker obese rats differs from that of lean rats by weaning, or if the differences develop with age, feeding patterns of Zucker obese and lean rats were compared from 3–10 weeks of age. Increased food intakes of obese rats at 3 weeks of age were due to a trend toward increased frequency while at 4 weeks of age were due to increased meal size. Meal size subsequently increased at a faster rate in obese than lean rats and meal frequencies did not differ. While always greater in obese than lean rats meal durations decreased and rates of eating increased with age and were not different for obese and lean rats. The adult patterns of diurnal variation in obese and lean rats were apparent by 3 weeks of age. Thus, in the Zucker obese rat the characteristic of increased meal size did not occur until 4 weeks of age after increased food intake and body weight were evident, and the characteristic of decreased meal frequency did not occur by 10 weeks of age. While increased meal size is associated with early differences in feeding behaviors, decreased meal frequency may be a consequence of obesity.     

Adipose tissue lipoprotein lipase (LPL) and triglyceride uptake in Zucker rats

To investigate the relationship between elevated adipose tissue lipoprotein lipase (LPL) activity and triglyceride uptake in Zucker obese rats, fed and 12 hour fasted female obese and lean Zucker rats were given intrajugular infusion of radio-labelled triglyceride and label clearance and uptake were examined over 35 minutes. In the fed state, obese rats showed more rapid clearance of the label from the bloodstream and, in both fed and fasted states, greater uptake into retroperitoneal and parametrical fat pads than lean rats. Obese rats showed proportionally less uptake into heart and liver. Regardless of feeding condition, obese rats exhibited elevations in adipose tissue LPL, which was significantly correlated with label uptake in adipose tissue. These results show that, in Zucker obese rats, elevated adipose tissue LPL is associated with increased adipose tissue triglyceride uptake. A preferential “shunting of calories” into adipose tissue, which is presumably mediated by LPL, could underlie the intractability of the Zucker obesity syndrome as well as the altered feeding behavior of Zucker obese rats.     

Effects of anterior hypothalamic lesions on the sexual behavior of prenatally-stressed male rats

Sprague-Dawley females were exposed to the stress of heat, restraint and bright lights during the third trimester of gestation. Virtually all male offspring tested for masculine sexual behavior as adults ejaculated and copulated with lure females. Also prenatally-stressed males exhibited two to three times as many lordotic responses as did males from nonstressed mothers. Because animals were crossfostered, an in utero action of prenatal stress is supported. Anterior hypothalamic (AHA) lesions significantly reduced the number of lordotic responses observed in prenatally-stressed male rats compared to those observed in prenatally-stressed males bearing sham lesions of the AHA. The possibility is presented that prenatal stress may influence the developing male brain.     

Differential expression of nicotinamide adenine dinucleotide phosphate-diaphorase in hypothalamic areas of obese Zucker rats

Several studies have suggested that the activity of nitric oxide synthase (NOS) may be involved in the regulation of food intake in the genetically obese Zucker rats. In the present study, we investigated the expression of NOS in various hypothalamic regions of obese and lean Zucker rats using nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase histochemistry. Obese Zucker rats showed significantly lower staining intensities of NADPH-diaphorase-positive neurons in the paraventricular nucleus (PVN), lateral hypothalamic area (LHA) and ventromedial hypothalamic nucleus (VMH) than lean Zucker rats did. The differences in staining intensities between obese and lean Zucker rats were large in both the PVN and LHA, but such differences were relatively small in the VMH.     

Daily rhythms of feeding in the genetically obese and lean Zucker rats

Feeding patterns were examined in obese (fa/fa) and lean (Fa/-) adult Zucker rats over the light-dark cycle during 14 days. Obese rats eat more than lean rats especially during the dark phase. Light and dark feeding expressed as percentage of 24 hr intake showed no significant differences between the lean and obese groups. The higher food intake in obese rats is mainly caused by larger meals since obese rats ate fewer meals than lean rats. Only for the obese group differences were observed between mean meal size in light and dark phase. There is some indication that the circadian controlled temporal distribution of meals is different in obese rats compared to lean rats since obese rats eat fewer but larger meals during the first half of the dark phase. During this phase meal size increases gradually in the obese rats, suggesting that the circadian influence on feeding motivation is increased.     

In-Situ Mechanical Characteristics of the Tongue are not Altered in the Obese Zucker Rat

Study Objectives:

Obese Zucker rats have more collapsible isolated upper airways, compared with their lean counterparts. The functional characteristics of the tongue as a potential mechanism for the enhanced upper airway collapsibility in the obese Zucker rat are unknown. This study measured the functional characteristics of the tongue muscle in lean and obese Zucker rats.

Design:

In-situ tongue force (twitch and peak) and fatigability were measured in anesthetized obese and lean Zucker rats.

Setting:

Animal housing facility at the University of Buffalo.

Subjects:

Eight lean and eight obese Zucker rats.

Intervention:

Tongue force and fatigability were measured before, during, and following cocontraction of the tongue protrudor and retractor muscles via direct stimulation of the common hypoglossal nerve.

Measurements and Results:

Obese rats were significantly heavier than their lean counterparts (718 ± 101 gm vs. 545 ± 32, P < 0.05). Total force production at all stimulation frequencies was not different between lean and obese Zucker rats before or after fatigue (P = 0.436). Forces were significantly reduced at the end of the 5-minute stimulation period (P < 0.001) and returned to baseline within 1 minute after fatigue in both lean and obese rats. At the end of the fatigue protocol, tongue force averaged 63.3% ± 13.8% and 72.3% ± 17.8% of the initial force in obese and lean rats respectively (P = 0.85).

Conclusion:

Obesity does not alter the in-situ force production of the tongue muscle. Thus, increases in collapsibility of the isolated upper airway previously noted in obese Zucker rats cannot be ascribed to upper airway muscle dysfunction or enhanced fatigability.

Citation:

Ray AD; Farkas GA; Pendergast DR. In-situ mechanical characteristics of the tongue are not altered in the obese Zucker rat. SLEEP 2009;32(7):957-961.     

Tail pinch induces sexual behavior in olfactory bulbectomized male rats

Adult male Long-Evans rats were subjected to bilateral olfactory bulbectomy, sham surgery or no treatment. Of 34 bulbectomized rats, 24 failed to ejaculate on either of 2 tests with a primed ovariectomized female. All control animals exhibited normal sexual behavior, and 10 bulbectomized animals ejaculated at least once during the 2 tests. Later histological examination revealed a relationship between size of lesion and extent of behavioral deficits. After a third test, 16 nonejaculatory animals were subjected to a tail pinch (TP) procedure, immediately followed by a fourth test. The remaining 8 nonejaculatory animals were tested similarly, but without tail pinch. Ten of the 16 tail pinch animals showed complete sexual behavior on the first test, while 2 additional animals began to copulate after a second TP procedure 4 days later. Only 1 of the 8 animals not receiving TP ejaculated on these tests. Thus, TP applied shortly before sexual behavior tests can induce copulation in some males whose behavior had been disrupted by olfactory bulbectomy.     

Effects of prenatal morphine exposure on rat heterotypical sexual behavior.

Prenatal exposure to morphine inhibits ovarian steroid-dependent lordosis behavior in female rats, and enhances certain components of male sexual behavior in male rats. In the present study, the effects of mid to late gestational morphine exposure on male sexual behavior in females and on female sexual behavior in males were examined in adult offspring. Gonadectomized male rats were injected at weekly intervals with 30 or 60 microg estradiol benzoate and 1.0 mg progesterone and tested for female sexual behavior with stimulus males on 2 consecutive weekly tests. Ovariohysterectomized (OVX) females were injected with 500 microg testosterone propionate (TP) daily for 15 days and tested for male sexual behavior with stimulus females on the last day of TP injection and 1 week later, after TP withdrawal. Prenatal morphine exposure increased the expression of male sexual behaviors in female rats, but it did not increase lordosis behavior in male rats. Thus, exposure to morphine during gestation alters male and female sexual behavior in young adult animals. Because prenatal morphine exposure both defeminized and masculinized adult sexual behavior in female rats, it is possible that female brain development is more vulnerable to prenatal insult such as opiate exposure.     

Weight gain and food intake in corticotropin releasing factor immunized Zucker rats

In Zucker obese rats (fa/fa) there are disturbances in the regulation of ACTH and corticosterone. In addition, beta-endorphin concentrations are higher in the pituitary and hypothalamus in obese than in lean rats. Since ACTH and beta-endorphin are thought to be controlled by corticotropin releasing factor (CRF), these effects may be due to abnormalities in CRF regulation. This possibility was investigated by immunizing rats against CRF. Obese rats immunized against CRF developed higher titer antibodies than lean rats. Hypothalamic CRF concentrations were higher in CRF-immunized obese but not lean rats compared with those of control rats, suggesting that compensation for sequestration of peripheral CRF developed in obese rats. In obese, but not lean rats, immunization against CRF decreased weight gains during weeks 1-4 and increased gains during weeks 9-12 and food intakes were decreased during weeks 5-8 compared with those for obese rats immunized against bovine serum albumin (BSA). Adrenal glands weighed 30% less in both obese and lean rats immunized against CRF compared with those immunized against BSA. These responses to immunization against CRF occurred even though plasma, hypothalamic and pituitary concentrations of ACTH and beta-endorphin were unaffected at the end of the study.     

Food restriction neither improves nor exacerbates reproduction in obese female Zucker rats.

Obese female Zucker rats have several reproductive abnormalities, including delayed puberty, abnormal estrous cyclicity, and behavioral hyporesponsiveness to ovarian steroid hormones. To ascertain whether excessive body weight per se causes these reproductive abnormalities, obese Zucker female rats were fed ad lib or were food restricted to match their body weights to those of lean counterparts. Food restriction neither accelerated vaginal opening nor normalized estrous cyclicity in obese female rats. Following ovariectomy, an injection of estradiol benzoate (EB, 15 microg/kg, s.c.) induced extremely low sexual receptivity in all rats, and proceptive behaviors were never observed. After treatment with EB plus progesterone (P, 2 mg/kg, s.c.), lean rats were very receptive (lordosis quotient, LQ = 94 +/- 2%) and proceptive (PRO = 12.5 +/- 2 events/min) while both ad lib-fed and food-restricted obese rats were only marginally receptive and proceptive (LQ= 19 +/- 9%, PRO = 1.8 +/- 0.7 events/min; LQ = 31 +/- 15%, PRO = 4.7 +/- 3 events/min, respectively). A higher progesterone dose (20 mg/kg) elicited vigorous sexual receptivity (LQ = 88-99%) and proceptivity (PRO = 16.5-20.4 events/min) in all EB-treated rats. Adiposity was significantly lower in food-restricted obese rats as compared to ad lib-fed obese rats (36.5 +/- 1.7% vs. 69.4 +/- 2.7%), but greater than that observed in lean rats (24.4 +/- 1.1%). These data suggest that excessive body weight per se does not underlie reproductive abnormalities in obese Zucker rats, but do not rule out the possibility that excessive adiposity may contribute to their infertility.     

Variation in characteristics of islets of Langerhans in insulin‐resistant,diabetic and non‐diabetic‐rat strains

Assessment of the histopathological and plasma biochemical characteristics of diabetic and non‐diabetic rat strains [Han and AP Wistar, lean and obese Zucker Fatty (ZF), and lean and obese Zucker Diabetic Fatty (ZDF) rats] was performed at 6 or 14 weeks of age. Wistar and lean ZF and ZDF rats showed no or minimal islet pathology or plasma biochemical alterations at both timepoints. Obese ZFs were euglycaemic at both timepoints and mildly and severely hyperinsulinaemic at 6 and 14 weeks respectively. Islet morphology was normal at 6 weeks but at 14 weeks, islet hyperplasia was present with a minority showing degenerative changes namely, β‐cell vacuolation, vascular congestion and haemorrhage with minimal mononuclear cell and T lymphocytic infiltration. Obese ZDFs were euglycaemic and moderately hyperinsulinaemic at 6 weeks and severely hyperglycaemic with minor hypoinsulinaemia at 14 weeks. Obese ZDFs at 6 weeks showed mainly normal islets with some displaying degeneration (ranging from β‐cell vacuolation alone to the features described above). At 14 weeks, islet degeneration was more severe and widespread: β‐cell death was present in numerous islets at low level. Islet β‐cell numbers were reduced or absent (with associated reduction in insulin immunostaining) within the islets that now consisted predominantly of fibroblasts, collagen and mononuclear cells. Fibroproliferation consisting of smooth muscle actin‐α‐positive tissue was associated with mononuclear cell infiltration. Some fibrous scars were visible indicative of lost islets. Islet degeneration in obese ZF and ZDF rats was not accompanied by a reduction in β‐cell proliferation or in compensatory proliferation of β‐cell neogenic clusters. In the light of recent reports of adaptive and inflammation‐mediated degenerative changes in human non‐insulin dependent diabetes mellitus (NIDDM) islets, the hypertrophy/hyperplasia of β‐cells and islet degeneration involving infiltration by monocyte/macrophages in obese ZF and obese ZDF rats respectively offers substantial potential for elucidation of the processes involved.     

Feeding response of weanling zucker obese rats to cholecystokinin and bombesin

Zucker weanling obese rat meal size is greater than in lean litter-mates by 4 weeks of age, indicating a possible decreased sensitivity to satiety signals. Adult Zucker obese rats are less sensitive to the putative satiety signal octapeptide of cholecystokinin (OP-CCK) when injected after a normal intermeal interval. In these experiments were compared responses of Zucker lean and obese rats from 3–11 weeks of age to OP-CCK and bombesin (BBS), another recently reported putative satiety agent. Injection of 2.0 and 4.0 μg/kg OP-CCK in 4–5 week olds had no effect on food intake of obese rats while decreasing 60-min food intake in lean rats 29 and 28 percent, respectively. However, 8.0 μg/kg OP-CCK decreased food intake of obese and lean rats similarly, indicating decreased, rather than lack of, sensitivity in the obese. The doses of 2.0 and 4.0 μg/kg BBS decreased food intake similarly in the obese and lean rats, but 1.0 μg/kg, although having no effect in lean rats, increased food intake in obese rats approximately 17 percent. Thus, while Zucker obese weanling rats appear to be less sensitive to OP-CCK, shown to decrease food intake in lean rats, they appear to be equally sensitive to the satiety effect of similar doses of BBS, but at low doses BBS stimulated food intake in obese but not lean rats.     

Activity-induced GLP-1 release in lean and obese subjects

The aim of the study was to determine whether physical activity stimulates GLP-1 release on the short-term in normal weight and in obese subjects compared to rest and, furthermore, whether modest weight loss affects GLP-1 release or sensitivity in the obese. Normal weight (n=28; 12 males, 16 females; BMI 22.9+/-1.4; age 35+/-12.7), as well as obese subjects (n=27; 21 males, 6 females; BMI 30.9+/-2.7; age 47.1+/-11.86) were tested in a resting and a physical activity condition. Obese subjects were matched over two groups for a weight loss period of 3 months. After weight loss, the tests were repeated. The area under the curve (AUC pmol/lxmin) for GLP-1 concentrations was significantly increased in the physical activity condition compared to rest in lean subjects (P=0.05) as well as in the obese subjects after weight loss (P<0.05), but not in the obese subjects before weight loss. Physical activity-stimulated GLP-1 release in lean and obese subjects after a weight loss period supports the idea of a neuroendocrine loop in addition to distal-intestinal stimulation of GLP-1 release. Modest weight loss might be effective for increasing GLP-1 sensitivity to acute stimulation.