Dorsomedial hypothalamic hypophagia: self-selection of diets and macronutrients, efficiency of food utilization, "stress eating", response to high-protein diet and circulating substrate concentrations

Opposite influence of carbohydrates and fat on hypothalamic neurotensin in Long-Evans rats

Neurotensin inhibits food intake when injected in the central nervous system and is released after fat ingestion. The aim of the present study was to measure it in different brain areas and to determine if it is involved in the long-term variations in food intake induced by the ingestion of a high-fat (HF) diet. We compared the results with those obtained with 2 low-fat [high-carbohydrates (HC)] diets and a well-balanced diet. For this purpose, weanling male Long-Evans rats were fed ad libitum for 14 weeks either on a control diet, a HF diet or a HC diet. The rats with the HC (high-starch) diet were divided into 2 subgroups: the first (HC) drank water and the second (HCS) drank a 25% sucrose solution. During the last week of the experiment, energy intake of the HCS rats was significantly greater than that of the 3 other groups of rats (+17.2%; p < 0.01; +27.1%; p < 0.001 and +34.6%; P < 0.001 vs the control, HC and HF rats respectively). NT did not vary in the midbrain and particularly in the ventral tegmental area. Its concentrations were significantly higher in the 2 HC groups than in the HF rats both in the paraventricular (PVN; p < 0.02) and dorsomedial nuclei (DMN; p < 0.03). In the DMN, they were positively correlated with energy intake (r = 0.39; p = 0.027). These results indicate that hypothalamic neurotensin is indeed involved in the long-term modulation of feeding behavior by diet composition and that fat is the more potent macronutrient for its regulation.

Aging and the hypothalamus: research perspectives

There are several hypothalamic theories of aging, none of which has been validated. An approach to validation is to search for consequences of anatomic ablations of hypothalamic regions that are functional hallmarks of aging, or consequences of ablation that postpone the appearance of hallmarks of aging or extend longevity. Ablation of the hypothalamic ventromedial nucleus (VMN) in the weanling rat is associated with subsequent increased body fat, glucose intolerance, hyperlipidemia, and decreased renal function. Each of these consequences is characteristic of aging in humans and in several animal models of aging. Ablation of the hypothalamic dorsomedial nucleus (DMN) in the weanling rat leads to a symmetrically smaller animal with normal glucose and lipid metabolism, decreased body fat for size, and reduced risk of decreased renal function and circulating IGF-I levels. These are findings consistent with calorie restriction models in rodents that significantly extend life span. This review compares outcomes of lesions in the VMN, DMN, and lateral hypothalamic area (LHA) for relevance to aging. To establish a relationship between these anatomic areas of the hypothalamus and aging, it is concluded that the VMN, DMN, and LHA lesions should be examined for impact on longevity and compared with data obtained from simultaneously studied intact ad-lib-fed and 40% calorie-restricted animals. Lesioned animals also should be rigorously studied for neurotransmitters (e.g., neuropeptide Y, beta-endorphin, serotonin, corticotropin-releasing factor, and galanin), and for behavioral changes consistent with aging, for accumulation of specific tissue lipofuscin and amyloid that are associated with normal aging and for other age-dependent findings, such as incidence of tumors and cataract.

Hypophagic rats with dorsomedial hypothalamic lesions produce lighter and smaller pups with a lower survival rate at weaning than offspring of sham-operated controls

Weanling and mature rats with dorsomedial hypothalamic nucleus lesions (DMNL rats) show reduced ponderal and linear growth and hypophagia and hypodipsia in the presence of normal body composition and anabolic hormone levels. The present study was conducted to assess their reproductive/parenting capacity and some offspring parameters. Four groups were used: DMNL mothers and fathers, DMNL mothers and control (SCON) fathers, SCON mothers and DMNL fathers, and SCON mothers and SCON fathers. The constituent rats of each group were bred to yield between 14 and 22 litters. The smallest litter size, litter weight, mean pup weight, percent of live-born and percent of weaned pups and greatest percentage of still-born pups were recorded when both parents were DMNL rats. The latter parents also cannibalized the majority of litters. The above parameters improved when only one parent was a DMNL rat, but this was still significantly below the offspring of SCON x SCON parents. The DMN is not part of the classical hypophysiotropic area (HTA), but earlier findings indicate hyperprolactinemia in DMNL rats. Therefore, the lesion-induced hypophagia during gestation and the postpartum neuroendocrine profile of the DMNL mothers may be the cause of the observed litter deficiencies and poor survival.

Somatic, endocrine and metabolic changes in controls pair-fed for six weeks to rats with dorsomedial hypothalamic nucleus lesions (DMNL rats)

One group of weanling male Sprague-Dawley rats received lesions in the dorsomedial hypothalamic nuclei (DMNL rats), whereas two additional groups of rats were sham-operated (CON). One of these CON groups was allowed to feed ad lib (CON-ADLIB) while the other CON group was pair-fed for 6 weeks to the DMNL rats (CON-PF). Despite eating the same amount of food as DMNL rats. CON-PF animals had consistently lower body weights and also utilized food energy more poorly than DMNL rats. The CON-PF group also had smaller kidneys and less percent liver protein but more epididymal fat pad percent protein than DMNL rats. Whereas plasma glucose concentrations were comparable among the three groups, insulin levels were significantly higher, and free fatty acid levels lower in CON-PF than in DMNL rats. The CON-PF group incorporated less glucose-U-C14 carbon into liver glycogen but more of the tracer into liver lipid than the DMNL group. Glucose carbon was also incorporated more avidly into epididymal fat pad lipid by CON-PF than by DMNL rats. The data not only confirm previous findings in DMNL rats but in addition show that the neurologically intact rats fed the same amount of food that is eaten spontaneously by DMNL rats show somatic and metabolic alterations that suggest that they cannot cope with this low amount of substrate. The normalcy of the DMNL rats, compared to ad lib-fed sham-operated controls, in all metabolic parameters suggests that the low food intake is indeed "normal" for this preparation and may be the reflection of an "organismic" set point.

Normal catch-up growth in rats severely food-restricted prior to lesions of the dorsomedial hypothalamic nucleus: the first 48 hours

Following a brief period of ad lib (AL) feeding, 45-day-old male Sprague-Dawley rats were either fed AL or food-restricted (REST) for 21 days to 50% of the intake of the AL rats. At this time, some AL and some REST rats received electrolytic lesions in the dorsomedial hypothalamic nuclei (DMNL), whereas other AL and REST rats were sham-operated (CON). Following this, all rats were refed (REF) AL and killed two days later. At this time, DMNL-REST + REF and DMNL-AL weighed as much as CON-REST + REF and CON-AL, whereas the body weight of the DMNL-AL group began to separate from the CON-AL group; carcass lipid and protein were normal among the groups. DMNL-AL laid down more % lipid and % protein/g food eaten than CON-AL; this was not the case in the REST + REF groups. DMNL-AL were hypophagic vs. CON-AL, but DMNL-REST + REF ate as much as CON-REST + REF. Compared to DMNL-AL, DMNL-REST + REF increased their food intake more than four-fold and also utilized food energy more efficiently than DMNL-AL rats. Epididymal fat pads and kidneys were smaller in REST + REF vs. AL groups, irrespective of brain manipulation. Plasma glucose and growth hormone were normal among the groups, but plasma insulin concentrations were higher in REST + REF DMNL and CON groups vs. DMNL-AL and CON-AL, respectively. Glucose incorporation into epididymal fat pad lipid and CO2 and liver lipid was elevated in REST-REF groups vs. respective AL groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Further evidence for the existence of an "organismic" set point in rats with dorsomedial hypothalamic nucleus lesions (DMNL rats): normal catch-up growth

The present study was performed to assess the capacity of rats with dorsomedial hypothalamic nucleus lesions (DMNL rats) and sham-operated controls (CON) for catch-up growth following body weight (b.wt.) reduction prior to DMNL (and sham-lesion) production. Male SD rats (45 days, 157 +/- 1.3 g) were maintained for 11 days ad lib (ADLIB) after arrival and then divided into two groups. One group continued to feed ADLIB, the other group was fed half of the ration eaten by ADLIB rats for 32 days. At this point each group was divided into two subgroups. One subgroup received DMNL, the other subgroup consisted of CON. From then on all rats were fed ADLIB [except for one group of CON that was pair-fed to the ADLIB DMNL rats (PF-CON)] for 37 days (69th day of experiment) and then killed. DMNL rats lesioned at normal b.wt. (ADLIB DMNL) showed a precipitous drop in food intake, b.wt. and efficiency of food utilization (EFU). In striking contrast, rats that had received DMNL after b.wt. restriction (REST DMNL) and were then refed ADLIB showed a dramatic rise in food intake, b.wt., change in b.wt. and EFU, the latter being almost twice that of the ADLIB DMNL. Notably, the PF-CON weighed less than the ADLIB CON and utilized food poorer than ADLIB CON, REST CON and ADLIB DMNL. Liver weight (both absolute and relative (per kg 3/4 b.wt.) was reduced in DMNL irrespective of dietary treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Body weight set point studies in weanling rats with dorsomedial hypothalamic lesions (DMNL rats)

In order to further characterize a previously postulated "organismic" set point, weanling DMNL and control (CON) rats were maintained on lab chow ad lib (AL) for 55 post-operative days. Subsequently, some DMNL and CON rats were food-restricted (REST) to 80% of the food intake of their AL-fed counterparts for 24 days. At this point, representative rats from each group were killed by decapitation and the remaining animals were re-fed AL and killed 7 and 22 days thereafter. At the end of REST, both DMNL and CON showed significant weight loss, which was greater in CON than in DMNL rats. After 7 days of refeeding, DMNL rats normalized their body weights but re-fed CON still weighed less than AL-fed CON 22 days after refeeding. Food intake in formerly REST groups overshot on refeeding for 7 days, but this was significant only in DMNL rats. Notably, during this time formerly REST-DMNL ate as much as AL-fed CON. Efficiency of food utilization was normal in DMNL during AL feeding and became reduced on REST as it did in REST-CON. Notably, on refeeding formerly REST-DMNL rats overshot that of AL-fed DMNL rats by the same magnitude as previously REST-CON overshot the values of AL-fed CON. After 22 days of refeeding, this overshoot was still evident in DMNL but not in CON. At the end of the REST period, plasma insulin and glucose were similar in AL-fed DMNL and AL-fed CON. They were significantly and comparably reduced in both REST-DMNL and REST-CON compared to the AL-fed DMNL and AL-fed CON. On refeeding these changes normalized within seven days. At the end of REST, plasma free fatty acid concentrations were higher in REST-DMNL and REST-CON than in AL-fed DMNL and AL-fed CON. After seven days of refeeding they normalized only in formerly REST-CON. Plasma glycerol and total protein were normal throughout all groups, as was carcass protein. Carcass fat was equivalently reduced in both DMNL and CON at the end of REST and normalized 7 days after refeeding. AL-DMNL had the same carcass fat as AL-CON and REST-DMNL had the same carcass fat as REST-CON. In conjunction with previously reported normal anabolic hormone levels the data suggest that DMNL rats are not growth-retarded but are merely scaled down in size without compromise of their homeostatic competence. We take this as strong evidence for the existence of an "organismic" set point.

Medial hypothalamic lesions in Syrian hamsters: characterization of hyperphagia and weight gain

Syrian golden hamsters (Mesocricetus auratus) received bilateral lesions aimed at the ventromedial hypothalamus (VMH) or a sham lesion. In the first study, some of the animals in each surgical group were housed in standard sedentary conditions while others had free access to running wheels. The lesions produced a 30% increase in the daily intake of chow, and this was accomplished exclusively by increased meal sizes. As a result, lesioned hamsters gained body weight relative to controls both on the chow diet and in a subsequent high fat diet phase. The effects were comparable in both sedentary and exercising groups. The lesions produced increases in body length and fat content. In the second study, lesions were made in the VMH or in adjacent nuclei and, after an initial period on chow, the hamsters were then given a choice between chow and high fat diet. The lesioned hamsters showed no unusual preference for the high fat diet but, as before, those with damage to the VMH or paraventricular nucleus (PVN) showed exaggerated body weight gain. Hamsters with these lesions were hyperinsulinemic in both fed and fasted conditions at the end of the study.

Clonidine-induced feeding: analysis of central sites of action and fiber projections mediating this response

Clonidine (CLON), an alpha-adrenergic agonist, was used in conjunction with norepinephrine (NE) to elicit feeding in satiated rats that had sustained hypothalamic electrolytic lesions, or coronal knife cuts at the hypothalamic, midbrain or pontine level of the brainstem. Electrolytic lesions of the paraventricular nucleus (PVN) of the hypothalamus significantly attenuated feeding normally stimulated by intraperitoneal injection of CLON. This contrasts with lesions in the dorsomedial or perifornical hypothalamic regions which had no effect on CLON-elicited eating. Knife cuts placed in the posterior hypothalamus and throughout the midbrain tegmentum also left intact the CLON eating response, in contrast to specific cuts in the dorsal pontine tegmentum which disrupted feeding elicited by PVN injections of NE and CLON, as well as by peripheral administration of CLON. Analyzed together, these results with effective and ineffective cuts relative to CLON and NE feeding provide evidence for an alpha-adrenergic feeding circuit which originates in the PVN and descends from this nucleus, via a dorsal periventricular course, through the diencephalon and midbrain. Further caudally, these fibers mediating NE and CLON feeding then appear to traverse ventrolaterally into the dorsolateral pontine tegmentum on their way to the dorsal medulla.

Minireview. Stress induced eating

The relationship of oral behaviors to stress has long been recognized both in humans and in wild animals. In the last decade numerous advances have been made in our understanding of stress-induced feeding predominately because of the development of the simple tail-pinch model of stress induced feeding in rats. Present evidence strongly implicates monoamines and the endogenous opioid peptides as well as other neuropeptides as playing a role in the central regulation of stress-induced eating.