Relative normophagia and organ growth in growth-retarded weanling rats with dorsomedial hypothalamic lesions

Mechanisms by which lead depresses linear and ponderal growth in weanling rats

Lead is known to reduce linear and ponderal growth in children, even at levels of exposure common in the general population. The mechanism involved is not known. The present study was conducted in order to establish the role of food consumption and insulin-like growth factor (IGFI), also commonly known as somatomedin C (SmC), in lead-induced reduction in growth in weanling rats. A further purpose was to test the hypothesis that depressed growth could be prevented by administration of growth hormone and thyroxine, a procedure which prevents arrested growth due to hypophysectomy. Treatment with growth hormone in combination with thyroxine had no effect on depressed growth or food consumption. The reduction in linear and ponderal growth due to lead (approximately 17%) could be largely though not completely accounted for on the basis of reduced food consumption, apparently by a mechanism not involving growth hormone or thyroxine (see above). Plasma SmC was reduced in proportion to reduced food intake, regardless of whether due to Pb or to restriction of food intake in a pair-feeding experiment thereby ruling out an effect of lead on SmC synthesis or activity other than through reduced food consumption. The experiments leading to these conclusions all involved administration of lead in the drinking water, suggesting the possibility that reduced food consumption was peripherally mediated as a result of contact of lead with appetite-depressant receptors in the gastrointestinal tract, e.g., taste receptors. An additional experiment therefore was conducted comparing food consumption and growth resulting from oral lead administration to subcutaneous administration. Similar depressant effects on food consumption and growth resulted at similar concentrations of lead and zinc protoporphyrin in blood. These effects were significantly greater following oral administration, however, suggesting that lead depresses appetite by both a systemic mechanism and one operating at the level of the gastrointestinal tract.

Recovery of rats with dorsomedial hypothalamic nucleus lesions (DMNL rats) from body weight restriction: effect of duration of postoperative prerestriction period

The present study was performed to see whether somatic and underlying metabolic-adaptive responses of DMNL rats and sham-operated controls (CON) to body weight restriction and subsequent refeeding could be influenced by the duration of ad lib feeding between lesion production and start of restriction. In contrast to previous studies (42 and 55 days, respectively) this time was reduced to 25 days. Restriction was similar, i.e., 27 days. DMNL rats show the same adaptive capacity in most parameters as do restricted CON. However, this response was at a lower level of absolute body weight, appropriate, so it appears, for their DMNL-induced lower body weight. In some parameters different responses were noted, however, suggesting that the time of ad lib feeding following the DMNL does indeed affect adaptive responses. Notably, this is the case in both DMNL and CON commensurately. Linear growth was reduced by restriction in the present but not in the two previous studies. Food intake showed a pronounced "overshoot" on refeeding but did not previously. Efficiency of food utilization was normal in the present study but depressed previously. A rise in plasma free fatty acids was not evident but was so in previous experiments. We concluded that, although DMNL rats respond to food restriction and recovery like similarly-treated CON, the duration of the ad lib feeding before restriction and/or the absolute age of the animals at that time, do indeed affect some parameters. This may be related to the fact that different aspects of the DMNL syndrome declare themselves in a sequential rather than a simultaneous manner.

Meal patterns of rats with dorsomedial hypothalamic nuclei lesions or sham operations

Rats with bilateral dorsomedial hypothalamic electrolytic lesions (DMNL rats) are hypophagic, hypodipsic and have reduced linear and ponderal growth when compared to sham operated controls (SCON). Nevertheless, previous studies have shown that DMNL rats eat and drink adequate amounts for their size and have normal body composition. In the present study we investigated meal parameters: meal size, and frequency (both light and dark period), total intake and meal size per metabolic size (body weight 0.75). Compared to SCON, DMNL rats at twelve days post surgery weighed less, were shorter, but had a normal body composition as determined by the Lee Index, and were hypophagic (grams eaten/day). The animals were placed into individual, self-contained feeding modules and given powdered chow. After familiarization to the modules, meal parameters were recorded continuously by a computer for an eight day period. While dark phase meal frequency did not differ significantly between groups, the lesioned rats took more meals during the light period. Over the eight-day measurement period DMNL rats were hypophagic compared to SCON in absolute terms. However, when total intake and meal size were normalized to metabolic size, these two parameters did not differ significantly between groups. Upon refeeding, after a one-day fast, the initial meal size of the normally hypophagic DMNL rats exceeded that of SCON. Rats with DMNL have previously been shown to have deficits in some hypothesized short-term food intake control mechanism (e.g., cholecystokinin, glucose sensing). Thus overeating by the lesioned rats after a fast could possibly result from a specific short term control deficit.(ABSTRACT TRUNCATED AT 250 WORDS)

Ventromedial and dorsomedial hypothalamic syndromes in the weanling rat: is the "center" concept really outmoded?

This report juxtaposes findings from weanling rats with precise lesions in the ventromedial (VMNL rats) to data of weanling rats with lesions in the dorsomedial (DMNL) hypothalamic nuclei. Despite the proximity of the two nuclei their destruction produces opposite effects in most cases but similar responses in other parameters. Absolute and relative food intake are normal in VMNL rats yet they become obese in the face of normal body weight gains. DMNL rats show both reduced absolute food intake and body weight but normal relative food intake and body composition. Both VMNL and DMNL cause reduced linear growth and running wheel activity. DMNL rats defend their lower body weight set point against various challenges and maintain normal body composition. Organ growth in both absolute and relative terms is reduced in VMNL rats. In DMNL rats relative organ growth is normal. Pancreatic growth, protein/pancreas and content and concentrations of several pancreatic enzymes are normal in DMNL but reduced in VMNL rats. Mean 24-hour plasma growth hormone (GH) and corticosterone (B) levels are reduced and insulin levels are greatly elevated in VMNL rats; prolactin (PRL) levels are normal. In DMNL rats, GH, B, insulin and somatomedin activity are normal but PRL is elevated. Circadian rhythms of GH, insulin and triiodothyronine are normal in DMNL rats but B levels are disrupted, as they are in VMNL rats. Glucose incorporation and oxidation in adipose tissue of VMNL rats are enhanced in VMNL rats but normal in DMNL rats. Gluconeogenesis in VMNL rats is enhanced as early as 4 hours post-operatively; in DMNL rats it is normal at this time and several weeks thereafter. Basal lipolysis in epididymal fat pads is elevated in both VMNL and DMNL rats but epinephrine-stimulated lipolysis is elevated in VMNL and decreased in DMNL rats. Both VMNL and DMNL rats show normal basal and epinephrine-stimulated lipolysis in interscapular brown adipose tissue. Several hepatic enzymes are normal in DMNL and depressed in VMNL rats. The above data suggest that the DMN and its circuitry are part of an "organismic" set point system with a "true" body weight and no fat set point, as seems to be the case in the VMNL rat.

Plasma hormone levels in growth-retarded rats with dorsomedial hypothalamic lesions

Rats with lesions in the dorsomedial hypothalamic nuclei (DMNL rats) are hypophagic and growth-retarded. Since previous work had shown normal plasma growth hormone and insulin levels in DMNL rats we investigated the diurnal patterns of these and other hormones involved in growth. Trial 1: Rats received electrolytic DMNL or sham operations (SCON). The DMNL rats exhibited no differences from SCON rats in plasma triiodothyronine (T3), growth hormone (GH), insulin and somatomedin (SM) concentrations, Trial 2: kainic acid, a neurotoxin, was used for lesion production. Again, DMNL rats showed no deficiencies in plasma levels of T3, GH or insulin. Trial 3: In this experiment, diurnal hormone profiles were assessed. The GH profile and mean 24-hour secretion of both DMNL and SCON groups did not differ significantly. Both groups exhibited a diurnal release of T3, with the DMNL rats showing slightly higher levels. Plasma insulin rose after dark, i.e., at the onset of feeding, in SCON but not in DMNL rats; the later have a previously reported disrupted feeding rhythm. Glucose patterns were in keeping with insulin profiles. Controls showed a normal plasma corticosterone rhythm whereas DMNL rats had an altered pattern. The data suggest that deficiencies in the principal anabolic and growth-promoting hormones cannot be responsible for the retarded growth of DMNL rats.