Normal plasma growth hormone levels in growth-retarded weanling rats with dorsomedial hypothalamic lesions

Summary Weanling rats received bilateral electrolytic lesions destroying the areas of the ventromedial (VMN) and dorsomedial (DMN) hypothalamic nuclei, respectively. Trunk blood was collected at sacrifice (Experiment 1 and 2 31 days, Experiment 3 14 days) for the determination of plasma growth hormone (GH) by radioimmunoassay. Rats with DMN lesions showed consistently normal to slightly elevated plasma GH levels while animals with VMN lesions showed significantly reduced plasma GH levels. Linear growth was reduced by both types of hypothalamic lesions but body weight gains and food intake were reduced only in the DMN lesioned rats. Carcass fat was greatly elevated in VMN lesioned rats, moderately elevated in one experiment in the DMN animals and normal in another experiment. These data confirm a previous hypothesis that growth retardation in DMN rats is not due to low circulating GH level but rather represents a hypocaloric-type dwarfism.This investigation was supported in part by U.S.P.H.S. grants HD 03331 and AM 14118, N.I.H. and by Natl. Inst. of Gen. Med. Sciences, Grant GM 15768.     

Feed efficiency in growth-retarded rats with ventromedial and dorsomedial hypothalamic lesions produced shortly after weaning

Weanling male rats received electrolytic lesions (L) in the ventromedial (VMN) and dorsomedial (DMN) hypothalamic nuclei; sham-operated rats served as controls. The animals were maintained for various periods up to 50 postoperative days on lab chow under standard conditions. In a short-term study, food intake and body weights were measured daily and body weight gain was divided by the food eaten to obtain the efficiency of food utilization. Also, gains in metabolic size ($\text{kg}\text{3}\text{4}$) were divided by food eaten. Both manipulations showed that DMNL rats, except for the first two postoperative days, utilized food normally. The same changes were obtained for weanling VMNL rats, except that they did not show a decline in utilization during the first two postoperative days. Computation of efficiency of food utilization for both VMNL and DMNL rats over postoperative durations ranging from 7 to 50 days showed that among 12 out of 14 experiments DMNL rats utilized food as well as controls. Out of 8 experiments, rats with VMNL utilized food better for weight gained than controls in one and poorer than controls also in one experiment; in reference to metabolic size they utilized food normally. The foregoing data, gathered from 225 DMNL, 64 VMNL and 181 control rats show quite convincingly that food utilization in both types of experimental animals is unimpaired despite profound reductions in ponderal and linear growth and food intake in the DMNL rat and reduced circulating growth hormone levels in the VMNL rat.     

Feeding studies in weanling rats with dorsomedial hypothalamic lesions: maintenance of competence to compensate for additional calories.

Weanling rats received bilateral electrolytic lesions in the dorsomedial hypothalamus primarily destroying the dorsomedial hypothalamic nuclei (DMN). Sham-operated rats served as controls. After a 14 day postoperative period during which food intake (lab chow) and body weight were recorded, each of the above groups were subdivided into 2 groups. One DMN group and one sham-operated control group were continued on lab chow alone throughout the remainder of the study. The other DMN group and the second control group were given additional calories in the form of a liquid diet by stomach tube during 2 separate periods of 10 and 14 days, respectivly, to increase their caloric intake beyond that taken in spontaneously. Both tube-fed groups reduced their ad lib caloric intake from chow considerably and to the same extent. Body weight gains were similar in tube-fed versus non-tube-fed rats, whether with or without DMN lesions. After the second, 14-day-long tube feeding period, however, DMN rats regulated their body weight somewhat less precisely than the controls. This may be related to their reduced food intake during that time period. The data indicate that weanling rats with DMN lesions, despite their basic hypophagia, do not show a deficit in caloric metering and gross body weight regulation.     

The weanling rat ventromedial syndrome: males get just as fat as females

Weanling female and male Holtzman rats received bilateral electroytic lesions in the ventromedial hypothalamic nucleus (VMN) at the age of 27 and 26 days, respectively. Sham-operated rats served as controls. The animals were maintained on a synthetic diet (4.2 Cal/g) and tap water ad lib, and body weight, Lee (obesity) Index, tail length and food and water intake were recorded weekly for 6 weeks. The only parameters in which a significant sex difference could be demonstrated were body weight gains and water intake which were greater in the male VMN rats. Female and male VMN rats also utilized food energy for fat deposition, body weight and body length change to the same extent. The data also show that in growing animals, body weight is a poor criterion for the accurate assessment of obesity status and true growth.     

Long-term effects of insulin in weanling rats with dorsomedial hypothalamic hypophagia: Food intake,efficiency of food utilization,body weight and composition

Twenty-five day-old Sprague-Dawley rats received electrolytic lesions in the dorsomedial hypothalamic nuclei (DMNL rats); sham-operated rats served as controls. Two weeks after the operation the DMNL rats showed reduced (p<0.001) body weight and food intake but normal body composition (Lee Index) and efficiency of food utilization (EFU). During the following 32 days subcutaneous administration twice daily of intermediary-acting insulin in increasing doses (mean daily dose 2.64 IU/kg) caused highly significant increases in food intake in both groups. Injection for the subsequent 14 days of higher doses of insulin (mean daily dose 5.64 IU/kg) caused dramatic increase in both food intake and Lee Index and equalized the rate of weight gain with that of the controls. However, in absolute terms the DMNL rats remained consistently hypophagic and weighed significantly less than the controls. Both DMNL rats and controls showed the same EFU during both periods of insulin administration. On discontinuation of hormone treatment during the subsequent 20 days, food intake and body weight gains returned to pretreatment values and the insulin-induced increased Lee Index returned into the low-normal range. However, EFU was significantly (p<0.05) decreased during this period. At sacrifice, plasma glucose, glycerol, free fatty acids and total protein and carcass lipid and protein were normal in the DMNL rats. Absolute and relative (per 100 g body weight and per metabolic size) weight of epididymal fat pads, pituitaries, adrenals and kidneys were normal in the DMNL rats but testes weight per 100 g body weight was higher (p<0.05) in the DMNL rats. Although DMN lesions may remove some glucose-sensitive elements within the hypothalamus, the animals are still capable of responding to the food intake and weight-promoting properties of insulin, as do intact animals.     

The effect of dorsomedial hypothalamic nuclei lesions on body weight regulation.

Prior to hypothalamic surgery 1 group of male rats was placed on a partial starvation regimen to lower their body weight. A second group was fed ad libitum. Just before surgery the 2 groups were divided into 2 sub groups. Bilateral electrolytic lesions were then placed in the dorsomedial hypothalamic nucleus in some of the partially starved and some of the ad libitum fed rats. The remaining animals were sham operated. After surgery all groups were fed ad libitum. Following hypothalamic operation the group that before surgery had been fed ad libitum showed the previously reported postoperative hypophagia and reduced body weight. On the other hand, the rats that were partially starved prior to placement of the lesion, ate significantly more than ad libitum fed lesioned animals during the first 9 days after surgery. The body weights of the partially starved, lesioned rats increased steadily from the day of the operation while lesioned rats fed ad libitum showed an initial delay of ponderal growth. The data suggest that the transient postoperative increase in food intake of the partially starved, lesioned rats is an active process to bring the body weight of these animals up to a new but lowered ‘body weight set point’ initiated by the hypothalamic destruction. Since previous studies had shown that rats with a lesion in the dorsomedial nucleus have a normal body composition, it is suggested that the lesions change the animals' ‘body weight set point’ and not the ‘body fat set point’, as has been suggested after ventromedial and lateral hypothalamic lesions.     

Effects of ventromedial nucleus lesions on the display of lordosis behavior in the male rat. Interactions with facilitory effects of male urine

Previous observations showed that exposure to the odor of male urine prior to mating could enhance the display of lordosis behavior in male rats feminized with ovarian hormones. This study was performed to determine in feminized male rats whether the control of lordosis behavior by the olfactory system was mediated by the ventromedial nucleus (VMN) of the hypothalamus. Male rats were orchidectomized (ORCH) as adults and primed with 25 micrograms estradiol benzoate (EB) and 150 micrograms progesterone (P) 40 hr apart. Lordosis behavior was tested 9 +/- 1 hr after P injection. VMN lesions were shown to completely suppress the display of lordosis behavior as compared to sham VMN operated and dorsomedial nucleus (DMN) lesioned animals. Exposure of feminized rats to the odor of male urine by 9 +/- 1 hr before mating significantly increased the proportion of ORCH rats that displayed lordosis behavior in response to male mounts. This effect was abolished by VMN lesions but was maintained in the sham VMN operated and DMN lesioned animals. These results were discussed in the light of the present knowledge on the neuroendocrine and olfactory structures which mediate lordosis behavior in the male rat.     

Effect of insulin in rats with lesions of the dorsomedial hypothalamic nucleus.

Four hours after insulin injection Dorsomedial Hypothalamic Nuclei (DMN) lesioned rats consumed an amount of food that was comparable to that eaten by injected sham-operated animals. However, the DMN lesioned rats are not as initially responsive to the food intake stimulating properties of insulin as are the controls. A second study showed ad lib fed and fasted lesioned animals displayed a lower plasma glucose concentration after insulin challenge than did respectively treated controls. This suggests the initial insulin-induced feeding of the lesioned rats was blunted when compared to the controls even in the face of lower plasma glucose levels. Although a previous investigation revealed that DMN lesions destroy glucoreceptor tissue, the present data shows that DMN lesioned rats will increase their food intake in the face of insulin challenge, albeit their initial feeding response to insulin challenge is somewhat blunted. Finally, the present study confirms a previous report in that DMN lesioned rats can competently meter their 24 hour calorie intake.     

Feeding after nutrient infusions: effects of hypothalamic lesions and vagotomy.

Normal rats, and rats with lesions in the lateral hypothalamic area (LHA), ventromedial hypothalamus (VMH), or with subdiaphragmatic vagotomies were duodenally infused with nutrient solutions representative of all three macronutrients, and the effect on food intake for a period up to 3 hr was assessed. Glucose reduced consumption in the control and VMH controls, but not in the LHA or vagotomized groups. Amino acids decreased intake in the control and LHA groups and showed a trend toward a decrease in the VMH group, but not in the vagotomized group. Fat suppressed intake significantly in control, VMH and LHA groups and there was a trend toward suppression in vagotomized rats. The subcutaneous injection of insulin to the same animals elicited eating in all but LHA animals. These results suggest that different nutrients reduce food consumption in the short term through different anatomical substrates involving the peripheral and central nervous system.     

Area postrema lesions produce feeding deficits in the rat: Effects of preoperative dieting and 2-deoxy-D-glucose

To investigate the possible role of the area postrema (AP) in the control of food intake and body weight, male albino rats were divided into four groups: (a) animals dieted to 80% of their original body weights prior to receiving AP lesions, (b) nondieted animals with AP lesions, (c) animals dieted to 80% prior to receiving sham lesions, and (d) nondieted animals with sham lesions. Lesions of the AP in nondieted rats resulted in hypophagia, hypodipsia and body weight loss followed by recovery of normal intake and maintenance of body weight at a fixed percentage of the sham operated animals' weight. Reducing body weight prior to surgery led to body weight maintenance levels equivalent to those of the nondieted groups. We also tested the animals for sensitivity to glucoprivation caused by intraperitoneal injections of 2-deoxy-D-glucose (2-DG). Injections of 2-DG produced hyperphagia in sham lesioned rats, but not in rats with AP lesions. Our data suggest that the effects of AP lesions on intake and body weight are similar, in several important respects, to the lateral hypothalamic feeding syndrome and to the effects of subdiaphragmatic vagotomy. We discuss the results with respect to hierarchical levels of neural circuitry involved in controlling feeding behavior.     

Transient aphagia produced following bilateral destruction of the lateral hypothalamic area and quinto-frontal tract of chicks

Six groups of broiler chicks, Gallus domesticus, sustained bilateral lesions to specific neural structures residing in the lateral hypothalamic and thalamic areas. In contrast to past data reported for the albino rat, the pigeon, Columba livia and barbary dove, Streptopelia risoria, bilateral destruction of the chick lateral hypothalamic area (LHy), quinto-frontal tract (QF), and stratum cellulare externum (SCE) resulted in transient aphagia and rapid recovery of lost body weight. Similarly, bilateral destruction of the nucleus reticularis superior (RS) and nucleus intercalatus (ICT) resulted in a temporary 1–3 day period of aphagia with body weight returning to pre-operative levels in approximately 4 days. Bilateral destruction of the ansa lenticularis (AL) resulted in a more prolonged period of aphagia (4 days) and an average 8-day period to recover lost body weight. Additional data suggest that more persistent aphagia can be induced following lesions to the posterior hypothalamus and midbrain. Specifically, bilateral lesions which destroyed the following combination of neural structures resulted in prolonged aphagia: AL, QF and posterior LHy; AL and posterior nucleus of the AL (ALp); and AL, ALp and QF. It is suggested that the AL and ALp contain neurons which are part of a more complex system that modulates or controls motor activity and feeding behavior in birds.     

Naloxone suppression of food and water intake and cholecystokinin reduction of feeding is attenuated in weanling rats with dorsomedial hypothalamic lesions

In Experiment 1, sham operated (SCON) and dorsomedial hypothalamic nuclei (DMN) lesioned (L) rats were given saline or naloxone (0.1, 1.0 or 2.0 mg/kg) just prior to the onset of the dark cycle, lights out. Compared to saline injections, naloxone at all doses suppressed the cumulative food intake of the SCON during the second and third hr of measurement. Naloxone was without significant effect on the food intake of DMNL rats. Similar results were obtained in Experiment 2, except that naloxone at 2.0 mg/kg significantly suppressed the DMNL rats' food intake by the fourth hr of measurement. Cumulative water intake of both groups was significantly suppressed by naloxone in both experiments but its effects appeared to be attenuated in the DMNL group. In a preliminary trial cholecystokinin octapeptide (3.0 and 6.0 micrograms/kg) given at the onset of the dark cycle significantly suppressed the food intake of the SCON group but had no significant effect on the DMNL rats. The possibility exists that the reduced food intake and lower body weight of DMNL rats may partially result from damage to an opioid system. The data also tentatively suggest that DMN may play a role in cholecystokinin-induced satiety.     

Liquid diet preference in weanling rats with dorsomedial hypothalamic lesions

Two groups of weanling rats received bilateral electrolytic lesions in the ventromedial and dorsomedial hypothalamic nuclei, respectively. Sham-operated animals served as controls. During 14 days post-operation, the intake of standard laboratory chow was measured. The rats then received, in addition to laboratory chow and tap water, a sweet, eggnog-type liquid diet for 17 days, during 14 of which food intake from both diets was measured daily. The liquid diet was then withdrawn and for two weeks laboratory chow was again the sole source of calories. During the availability of the liquid diet, both groups of rats with hypothalamic lesions profoundly reduced their intake of laboratory chow but conspicuously increased their consumption of liquid diet. The rats with dorsomedial lesions consumed as much liquid diet as the controls, while on laboratory chow alone they were highly significantly hypophagic compared with the controls. During the availability of the liquid diet the rats with dorsomedial lesions also became obese, but after the return to a period in which laboratory chow was the sole source of calories they lost this extra fat while again being hypophagic.It is suggested that the hypophagia observed in rats lesioned in the dorsomedial hypothalamic nucleus is not due to a deficiency in caloric metering but rather, alternatively, to (a) aversion to dry food, (b) taste preference for a sweet, liquid diet, (c) a motivational deficit or (d) a motor deficit.     

Serotonin-depleting midbrain lesions fail to mitigate hyperphagia and obesity in the Zucker fatty rat

Previous research has shown that damage to the dorsal and median raphe nuclei of rats can impede the subsequent development of hypothalamic hyperphagia and obesity as well as impair the defense of established hypothalamic obesity in response to food deprivation. The present study sought to determine if raphe injury might alter the development of another form of obesity, namely that which occurs spontaneously in the Zucker fatty rat. Subjects were 20 obese females (fafa; mean weight of 200 g) and 20 lean littermate controls (FaFa females; mean weight of 150 g). Following 10 days of baseline intake and weight recordings, half of each group received radio-frequency heat lesions of the dorsal and median raphe nuclei while the other half received sham surgery. Except for a mild suppression of food intake and weight gain during the first few days after lesioning, raphe injury did not alter the hyperphagia or obesity shown by fatties over the 7 week ad lib feeding period studied. Additional 24-hr intake tests of varying sucrose and quinine solutions revealed reduced sucrose acceptance and enhanced quinine rejection by fatties much as has been seen in previous studies of hypothalamic obese rats. Terminal assays of forebrain monoamine levels confirmed that raphe lesions were effective in depleting serotonin (-71% compared to controls) without producing major changes in norepinephrine or dopamine (-14% and +2%, respectively). The inability of raphe lesions to mitigate this form of hyperphagia and obesity suggests that earlier observations of their attenuating effects on hypothalamic obesity were not due to non-specific impairments of behavioral or metabolic factors necessary to permit overeating and weight gain.     

Digestibility in male rats with lateral hypothalamic lesions

The body weight of male rats with lateral hypothalamic lesions remained approximately 20% below that maintained by nonlesioned controls. One month following surgery, both food intake and energy losses in feces and urine were measured in all animals for five days. The percentage of ingested food absorbed by the gut (“digestibility”) was determined from calorimetric analysis of the feces to be slightly, though significantly, higher for the lesioned animals (79.7% vs 77.4%). Resorptive capacity, as indicated by the percentage of absorbed energy lost in the urine, was determined to be the same for the lesioned and control animals (4.8% vs 4.5%). These results indicate that the digestive, absorptive, and resorptive processes of LH-lesioned animals are at least as efficient as those of nonlesioned animals maintaining normal body weights. Thus, though LH-lesioned animals are reported to display various gastrointestinal dysfunctions, alterations in digestive or resorptive efficiency are apparently not responsible for their chronically-reduced body weights.     

Regulation of feeding behavior in the prepubertal female rat.

Responses to challenges of long-term regulation of feeding behavior were compared between adult and weanling female rats. Adulteration of a high fat diet with NaCl caused both adult and weanling rats to reduce their food intake, but neither group refused to eat. Food deprivation for 24 hr was followed by an increase in feeding for both adult and weanling animals during a period when food intake is normally very low. Continued limited food access to 2 hr during the light period was compensated for by an increase in the normal food intake for this period for both adult and young female rats. It was observed that both adult and weanling rats showed a marked preference for the more dilute glucose solution when given a choice. In addition, both groups maintained a constant caloric intake during presentation of the glucose solutions by adjusting their intake of a solid food source. In each challenge of long-term regulation of feeding behavior, the response of weanling animals was as good or superior to that shown by adults. It is concluded that weanling female rats regulate their feeding just as adults to maintain long-term energy balance. It was also observed that bilateral lesions placed in the ventromedial hypothalamus (VMH) at 21 days of age resulted in reduced daily food intake and retarded body weight gain. Furthermore, young rats with VMH lesions failed to respond to 24 hr of food deprivation or limited food access. These data suggest an important role for the VMH in the long-term regulation of feeding in young rats.     

Persistent depression of wood-gnawing following ventromedial hypothalamic lesions in the rat

Female rats with ventromedial hypothalamic lesions that resulted in hyperphagia and obesity displayed chronically reduced levels of wood gnawing that persisted for more than 200 days post-operatively. Reduced wood gnawing was apparent even when the animals were totally deprived of food, thus ruling out the possibility that the effect was secondary to hyperphagia.     

Meal-induced decreases in serum corticosterone in the lateral hypothalamic syndrome

Decreases in serum corticosterone levels were used to quantify positive emotional responses to meals in rats that survived bilateral lateral hypothalamic lesions (n=9), intraventricular 6-hydroxydopamine injections with pargyline pretreatment (n=3), or without it (n=8) and operated controls that were fed either ad lib (n=19) or were pair-fed with brain-damaged rats throughout the study (n=11). Meals of dry powdered chow and tap water, offered in the evening after 8 hr of food and water deprivation, did not reliably reduce corticosterone levels within 30 min. When the rats were forced to eat their entire daily food ration during a 1-hr morning period for over 2 weeks, corticosterone levels were lower within 10 min of meal onset than they were prior to feeding. Placement of the rats into chambers in which they had received their daily meals was also effective in reducing corticosterone levels. Lateral hypothalamic or 6-hydroxydopamine lesions did not affect the magnitude of the corticosterone decreases, but did affect changes in glucose levels associated with meals. The data were discussed in terms of mechanisms involved in the permanent hypophagia of the lateral hypothalamic syndrome.     

Preoperative experience with insulin enhances glucoprivic feeding in rats with lateral hypothalamic lesions

Adult male Sprague-Dawley rats received either one injection per week of regular insulin (IP, 5 Units) or saline for 4 weeks prior to destruction of the LH or sham-operations. During this preoperative period, animals given insulin consumed significantly more food in a 6-hr test period than animals given saline. Following surgery, animals were given 3 weeks to recover from the acute effects of LH lesions and then tested for responsiveness to glucoprivic challenges. Sham-operated animals from both pre-operative injection groups consumed significantly more food during a 6-hour period when injected with either insulin (5 and 7.5 Units) or 2-DG (400 mg/kg) than when given saline injections. Similarly, LH-lesioned rats with preoperative experience with insulin significantly increased food intake when given insulin or 2-DG. In contrast, LH-lesioned rats without preoperative experience with insulin failed to increase feeding in response to the administration of either insulin or 2-DG. Differences in feeding responses following glucoprivation between LH-lesioned rats with and without preoperative exposure to insulin were not a function of differences in the extent of central nervous system damage. The present data indicate that experimental conditions play an important role in determining the presence or absence of regulatory deficits following brain damage.     

Food intake and utilization in lateral hypothalamically lesioned rats

The daily food intake of rats with lateral hypothalamic (LH) lesions was monitored in two experiments. Confirming earlier reports, LH-lesioned animals were found to ingest nearly the same amount of food per day as nonlesioned controls even though their body weights remained substantially below those of the controls. In view of this result, an experiment was conducted to compare the efficiency of food utilization of LH-lesioned and control animals at different body weights. First, the daily food intakes of seven LH-lesioned rats and seven nonlesioned controls were determined. The body weights of these animals were then lowered by restricting food intake in four successive weekly periods to 85%, 70%, 55% and 40% of their ad lib level. Finally, all animals were refed for one week at their prerestriction levels of food intake reduced in proportion to the intervening loss in metabolic mass (body weight_kg^0.75). At each level of caloric restriction, the weight losses observed in the LH-lesioned and control animals were equivalent. Likewise, though given only prerestriction amounts (indexed to their reduced metabolic mass), LH-lesioned and nonlesioned animals both gained weight rapidly and at equivalent rates during refeeding. Thus, LH-lesioned animals appear to utilize food in a normal fashion and, as do controls, adapt to weight loss by increasing their efficiency of food utilizazion. In the case of LH-lesioned animals, however, such adjustments occur around a reduced level of maintained body weight, or set-point.