Area postrema lesions cause overconsumption of palatable foods but not calories

Physiology& Behavior, Vol. 32, pp. 923-927. Copyright©PergamonPress Ltd., 1984. Printedin the U.S.A.

0031-9384/84$3.00 + .00

Area Postrema Lesions Cause Overconsumption of Palatable Foods But Not Calories R O B E R T C. R I T T E R A N D G A Y L E N L. E D W A R D S

WOI Regional Program in Veterinary Medical Education, University o f Idaho, Moscow, ID 83843 and Department o f Veterinary and Comparative Anatomy, Pharmacology and Physiology College o f Veterinary Medicine, Washington State University, Pullman, WA 99164 R e c e i v e d 2 N o v e m b e r 1983 RITTER, R. C. AND G. L. EDWARDS. Area postrema lesions cause overconsurnption of palatable foods but not calories. PHYSIOL BEHAV 32(6) 923-927, 1984.--Rats with lesions of the area postrema and immediately adjacent solitary nucleus consume greater amounts of highly palatable food during short exposures than do control rats. When a highly palatable substance (cookies or glucose solutions) is available continuously along with laboratory chow, lesioned rats exhibit average 24 hour calorie intakes which are not different from those of control rats. Nevertheless, the lesioned animals ingest a significantly greater proportion of total calories as the highly palatable substances than do control rats. The data suggest that lesions involving the area postrema and adjacent nucleus of the solitary tract enhance intake of highly palatable food without causing overconsumption of calories. Area postrema lesions

Palatable foods

Calorie intake

Food intake

(cookies or glucose solution). We have found that rats with lesions of the AP and adjacent NST exhibit total caloric consumptions similar to control rats, while consuming a significantly greater proportion of their calories as the highly palatable food.

THE area postrema (AP) and the immediately adjacent nucleus of the solitary tract (NST) possess neural and vascular connections which seem appropriate for the control of ingestive behavior [6-10, 12]. In fact, we have previously reported that lesions which destroy the AP and damage the adjacent NST caused rats to double their consumption of preferred foods during short duration feeding tests [2]. Ingestion of laboratory chow ad lib or after food deprivation did not differ between lesioned and control rats. Furthermore, rats with lesions of the AP and adjacent NST were not different from controls with regard to the suppression of food intake by intragastric preloads, intraperitoneal injections of cholecystokinin octapeptide (CCK-8), or quinine adulteration of the diet. The overtly normal response of the lesioned rats to satiety cues and to quinine adulteration of the diet led us to speculate that lesions caused overresponsiveness specifically to positive orosensory properties of a food. Increased responsiveness to palatable foods could potentially influence 24-hour food intake by exaggerating the increase in total caloric consumption which normally occurs during continuous exposure to a highly palatable food. Alternatively, increased responsiveness to palatable foods might alter diet choice without changing total caloric consumption. Hyde and Miselis [5] have reported that APlesioned rats and control rats gain weight at similar rates. These authors, however, did not measure food intake or preference. In order to determine whether rats with lesions of the AP and the adjacent NST overconsume highly palatable foods and fluids when they are available on a 24-hour basis, we offered lesioned and control rats a choice between our ground laboratory rodent diet and a palatable food

METHOD Male Sprague-Dawley derived rats were the subjects in these experiments. The rats were housed individually in a temperature-controlled room with an automatically maintained 12:12 light dark cycle. Ten rats received thermal lesions directed at the AP according to a procedure described previously [11]. Briefly, each rat was anesthetized with Metafane and placed in a stereotagic frame with head ventroflexed. The foramen magnum was exposed and slightly enlarged using rongeurs. Following removal of the overlying meninges, the AP was visualized with an operating microscope, blotted dry with a cotton applicator, and touched briefly (1 second of less) with a triangular thermocautery probe measuring 0.7 mm/side. Eight sham-operated control rats underwent similar surgery but were not touched with the cautery probe. Incisions were closed with gut and silk sutures. One month elapsed from the time of surgery until experiments were begun. Ground laboratory rodent diet was available ad lib in spill-proof cups throughout the postsurgical recovery period and during all experimental trials. Tap water was always available ad lib in calibrated drinking tubes. Solutions of 5, 15, or 35% glucose of 0.13% saccharin, made in distilled water, were also presented in calibrated drinking tubes. Cookies (Nilla Wafers, 4.75 kcal/g) were pre-

923

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RITTER AND EDWARDS

FIG. I. Panel A shows a section through the center of the area postrema of an intact control rat. Panel B shows a section through the center of the area postrema of a typical lesioned animal. At this anterior-posterior level, all lesions were maximal in size. The photomicrographs were taken at 10x magnification. The lesions produced in this study were similar to those which we have described previously in detail [2,3]. Abbreviations: Area postrema, AP; solitary tract, ST; nucleus of the solitary tract, NST; dorsal motor nucleus of the vagus, DMV; hypoglossal nucleus, N XII. s e n t e d on the cage floor. All ingestibles w e r e m a d e available in sufficient q u a n t i t i e s to p e r m i t ad lib c o n s u m p t i o n . U n l e s s o t h e r w i s e n o t e d , p r e s e n t a t i o n s o f different s w e e t solutions or c o o k i e s were always s e p a r a t e d by at least t w o w e e k s , d u r i n g w h i c h time the rats r e c e i v e d only p o w d e r e d c h o w a n d tap water. In our first diet c h o i c e e x p e r i m e n t , the rats were divided into two lesioned g r o u p s of five a n i m a l s e a c h a n d two c o n t r o l g r o u p s o f four and five animals, r e s p e c t i v e l y . A f t e r m o n i t o r ing intake of g r o u n d c h o w a n d w a t e r for t h r e e d a y s , o n e

lesioned a n d o n e control g r o u p was g i v e n a c c e s s to 35% glucose, while the o t h e r t w o g r o u p s had a c c e s s to 5% glucose. T w e n t y - f o u r - h o u r c o n s u m p t i o n o f glucose, c h o w , a n d w a t e r was m o n i t o r e d daily for five days. O n day 6, the glucose solution offered to e a c h g r o u p was e x c h a n g e d . T h e original 5% glucose g r o u p r e c e i v e d 35% glucose and the original 35% g r o u p was g i v e n 5% glucose. I n t a k e s were t h e n m o n i t o r e d for a n additional five days. F o l l o w i n g a t w o - w e e k rest, all of the rats w e r e g i v e n a c c e s s to s a c c h a r i n (0.13%) a n d intake of c h o w , water, a n d s a c c h a r i n was m o n i t o r e d for five

AREA POSTREMA LESIONS AND FOOD INTAKE

925

TABLE 1 FIVE DAY A V E R A G E C A L O R I C I N T A K E (Cal) F O R L E S I O N E D A N D S H A M - L E S I O N E D RATS

Chow

Chow + Sacch.

Chow + 5% Glucose

Chow + 15% Glucose

AP Lesion

76.8 _+ 1.5

76.5 _+ 2.8

61.7 _+ 4.7

123.1 _+ 5.6

It0

Sham Lesion

77.4 _+ 2.1

79.5 _+ 3.0

73.4 _+ 4.0

121.1 _+ 4.5

104.7 _ 6.6

days. Subsequently, the rats were given a similar five-day exposure to 15% glucose and chow. Finally, the rats were offered a choice of cookies and chow for five days. Spillage of cookies and chow was air-dried for 48 hours and then was weighed along with uningested foods remaining in the cages. After completion of the behavioral experiments, the rats were deeply anesthetized with pentabarbitol and perfused transcardially with buffered formalin. The brains were removed, postfixed in formalin and embedded in paraffin. Subsequently, 20/~m coronal sections were cut and stained with Weil and Darrow red stains. The stained sections of caudal brainstem were examined microscopically and the lesions reconstructed using the atlas of Pellegrino et al. Differences between lesioned and control groups were tested using appropriate analyses of variance and nondirectional t-tests.

-+ 6.8

TABLE 2 A V E R A G E P E R C E N T A G E O F C A L O R I E S T A K E N AS G L U C O S E OR C O O K I E S O V E R FIVE DAY TEST PERIODS

5% Glucose + Chow

15% Glucose + Chow

35% Glucose + Chow

Cookies + Chow

AP Lesion

40 _+ 6

67 _+ 3t

93 _+ 1"

95t _+ 2

Sham Lesion

40 _+ 3

54 _+ 1

76 _+ 4

83 _+ 2

*p<0.01.

tp<0.001.

RESULTS

Histological examination of the brains of the lesioned animals revealed that in two rats a substantial portion of the AP remained intact. Data from these rats has been excluded from the results reported here. The remaining eight lesioned rats all sustained total destruction of the AP and varying amounts of damage to the immediately adjacent NST. In two of these rats, the dorsal motor nucleus of the vagus nerve was also damaged unilaterally. Figure 1 shows photomicrographs of a typical lesions. With the exception of the first day of exposure to vanilla wafers (see Fig. 3), the total daily caloric intake of the lesioned rats was statistically indistinguishable from that of controls under all dietary regimens (Table 1). However, the daily proportion of calories consumed as 15% glucose, 35% glucose, or wafers was always significantly greater for lesioned rats than for controls. The average proportions of the daily caloric intake taken as glucose or cookies are shown in Table 2. Figures 2 and 3 show the daily average total caloric intakes and the percentages taken as 35% glucose or cookies, respectively, for lesioned and control rats. Daily body weight gain of the lesioned rats was not statistically different from that of controls except when 35% glucose was offered. Under this condition, control rats gained 2.0_+0.4 g/day, whereas lesioned rats did not gain weight (-0.2_+0.6 g/day). The highest rates of weight gain were attained when the rats had access to vanilla wafers. Under this condition, the lesioned rats averaged 2.8_+1.0 g/day and the controls averaged 3.0_+0.8 g/day. Water intakes for the two groups of rats were not statistically different except when the rats had access to 35% glucose. Under this condition, the lesioned rats drank an average of 20-+7 ml of water per day, while controls drank only 4_+2 ml per day. Both lesioned and control rats consumed large amounts of 5% glucose and 0.13% saccharin. Nevertheless, the two groups did not differ either in the volume (data not shown) of these solutions consumed nor in their total caloric consumptions (see Table 1).

Chow + 35% Glucose

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Time (days) FIG. 3. Daily caloric consumption and percent of consumption as vanilla wafers by rats with lesions of the area postrema and adjacent nucleus of the solitary tract (APL) and sham-lesioned rats (sham). Each point represents 24 hr intake averaged for 8 lesioned and 9 sham-lesioned rats. Asterisks (*) indicate significant differences from sham value at p<0.01 confidence limit.

DISCUSSION

We have previously demonstrated that thermal lesions of the AP and adjacent NST cause rats to overcome highly palatable solid and liquid foods when these are offered in 30-minute to six-hour exposures [2]. In fact during such exposures to palatable food, lesioned rats will often consume two times as much as control rats. In this study, we made a palatable food or solution available continuously. Under these conditions, lesioned rats did not increase their caloric consumption more than the controls. If the palatable diet was sufficiently concentrated (cookies, 4.75 Cal/g; 35% glucose, 1.33 Cal/g; or 15% glucose, 0.57 Cal/g), the lesioned rats reduced their intake of lab chow to a greater extent than did controls. However, if the palatable food was dilute (5% glu-

926

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RITTER A N D E D W A R D S

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FIG. 2. Daily caloric consumption and percent consumption as 35% glucose solution by rats with lesions of the area postrema and adjacent nucleus of the solitary tract (APL) and sham-lesioned rats (sham). Each point represents 24 hr intake averaged for 8 lesioned and 9 sham-lesioned rats. Asterisks (*) indicate significant differences from sham value at p<0.01 confidence limit.

cose, 0.19 cal/g) or non-nutritive (saccharin), then the lesioned and control rats consumed similar amounts of palatable solutions and similar amounts of laboratory chow. The apparent failure to overconsume 5% glucose or saccharin in this experiment was probably due to the fact that both groups consumed very large volumes of these solutions and attained a level of intake beyond which further consumption was uncomfortable. The availability of a calorically dense palatable food or solution increased total caloric intake in both lesioned and control rats. Nevertheless, the size of this increase generally was not significantly different between the two groups, regardless of the dietary regimen. Therefore, these data demonstrate that the exaggerated ingestion observed for lesioned rats during brief exposures to highly palatable foods does not precipitate overconsumption of calories when these diets are continuously available. In this respect, our data support the findings of Hyde and Miselis [5] who measured weight gain by lesioned rats on a "supermarket diet." Our direct measurements of intake as well as weight gain indicate that this lesion alters diet choice without causing increased caloric intake. Although total caloric intake and rate of weight gain by lesioned rats and controls was similar under most conditions, the fact that lesioned rats gained less weight than controls, while eating 35% glucose, suggests that lesioned rats assimilated fewer calories than controls while consuming this solution. Such an impairment of weight gain while consuming a high sugar diet is compatible with the presence of diabetes mellitus. However, when we measured plasma glucose in the lesioned rats during the period when they were receiving 35% glucose, their plasma glucose concentrations were not elevated above those of controls. Furthermore, extensive measurements of plasma glucose and insulin in similarly lesioned rats has revealed no evidence of diabetes (Edwards and Ritter, unpublished observations). At this time, we have no explanation for why the lesioned rats gain weight less efficiently than controls when they are consuming 35% glucose solution. Rats with lesions of the AP and adjacent NST overcon-

sumed solid foods as well as solutions, during brief exposure tests. Furthermore, lesioned rats consumed a larger proportion of total calories as cookies, when these were made available continuously. These observations indicate that overconsumption of palatable substances by rats with AP and NST damage is related to feeding behavior and is not due to the changes in thirst or fluid-electrolyte balance which have been reported following lesions of the AP and the adjacent NST [3,4]. Nevertheless, when 35% glucose was consumed, water intake by the lesioned rats increased significantly more than that of the control animals. It is possible that the lesioned rats consumed excess water as a means of diluting the sweet taste of the concentrated sugar solution. Alternatively, it may be that the increased water intake by lesioned rats was a response to an extracellular thirst challenge provided by ingestion of the 35% glucose solution. We have previously reported that rats with lesions of the AP and adjacent NST over-drink in response to angiotensin II and extracellular thirst challenges [3]. Hyde and Miselis [4] have reported that rats with aspiration lesions in this area excrete excessive amounts of water and sodium. It is possible that 35% glucose solution acted as an extracellular thirst stimulus either by causing a solute diuresis or by drawing extracellular fluid into the gut. Further experiments must be performed in order to elucidate the mechanism by which concentrated glucose solution causes over-drinking by lesioned rats. The results of these experiments suggest that rats with lesions of the AP and adjacent NST are sensitive to postingestive consequences of caloric consumption and under our experimental conditions control their total caloric intake in a quantitatively similar manner to intact rats. This is not to say that rats with these lesions do not have deficits in the metabolic control of food intake or body weight, which are apparent under the other experimental conditions. Indeed, Hyde and Miselis [5] have emphasized the fact that lesions of the AP and adjacent NST cause a pronounced loss of body weight, which they attribute to a regulatory deficit. Contreras et al. [1] have reported and we have also observed (unpublished data) that lesions in the vicinity of the AP impair feeding in response to 2-deoxyglucose. There is, however, no evidence that these deficits are related directly to the over-consumption of palatable foods by lesioned rats. In fact, we have recently demonstrated that over-consumption of palatable foods can be produced by injection of the neurotoxin, capsaicin, directly into the region of the AP [13]. Such capsaicin treatment does not cause a chronic reduction of body weight, overdrinking in response to angiotensin, or impaired feeding in response to 2-deoxyglucose. Thus, the regulatory deficits associated with thermal lesions of the AP and the adjacent NST appear to be dissociable from exaggerated consumption of palatable foods, suggesting that thermal lesions damage several distinct neural substrates associated with control of ingestion. In conclusion, rats with lesions of the AP and the adjacent NST exhibit exaggerated intake of highly palatable foods during brief exposures to such foods and when such foods are continuously available. This exaggerated intake does not lead to a relative over-ingestion of calories on the part of lesioned rats. Rather, rats with lesions reduce their intake of laboratory chow and thereby compensate for their enhanced intake of highly palatable food. The data suggest that lesions of the AP and the adjacent NST cause enhanced responsiveness to highly palatable foods without disabling the animals" ability to control total caloric intake.

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927

ACKNOWLEDGEMENTS We thank Carol Dugger for typing the manuscript. We also thank Dr. Sue Ritter for reading the initial drafts and providing useful comments. This research was supported by the National Institutes of Health Grant AM-20035 to R. Ritter.

REFERENCES 1. Contreras, R. J., E. Fox and M. L. Dragovich. Area postrema lesions produce feeding deficits in the rat: Effects of preoperative fasting and 2-deoxy-D-glucose. Physiol Behav 29: 875-884, 1982. 2. Edwards, G. L. and R. C. Ritter. Ablation of the area postrema causes exaggerated consumption of preferred foods in the rat. Brain Res 216: 265-276, 1981. 3. Edwards, G. L. and R. C. Ritter. Area postrema lesions increase drinking to angiotensin and extracellular dehydration. Physiol Behav 29: 943-947, 1982. 4. Hyde, T. M. and R. R. Miselis. Regulatory deficits in feeding and drinking following lesions of the area postrema with little or no damage to the nucleus of the solitary tract. Eastern Psychological Association Abstracts 33, 1981. 5. Hyde, T. M. and R. R. Miselis. Effects of area postrema/caudal medial nucleus of solitary tract lesions on food intake and body weight. Am J Physiol 244: R577-R587, 1983. 6. Kalia, M. and M. J, Sullivan. Brainstem projections of the sensory and motor components of the vagus nerve in the rat. J Comp Neurol 211: 248-264, 1982. 7. Leslie, R. A., D. G. Gwyn and D. A. Hopkins. The central distribution of the cervical vagus nerve and gastric afferent and efferent projections in the rat. Brain Res Bull 8: 37-43, 1982.

8. Morest, D. K. Experimental study of the projections of the nucleus of the tractus solitarius and the area postrema in the cat. J Comp Neurol 130: 277-300, 1967. 9. Ottersen, O. P. Afferent pr.ojections to the amygdaloid complex of the rat with some observations in the cat. III. Afferents from the lower brainstem. J Comp Neurol 202: 335-356, 1981. I0. Ricardo, J. A. and E. T. Koh. Anatomical evidence of direct projections from the nucleus of the solitary tract to the hypothalamus, amygdala and other forebraln structures in the rat. Brain Res 153: 1-26, 1978. I i. Ritter, S., J. J. McGlone and K. W. Kelley. Absence of lithiuminduced taste aversion after area postrema lesion. Brain Res 201: 501-506, 1980. 12. Roth, G. I. and W. S. Yamamoto. The microcirculation of the area postrema in the rat. J Comp Neurol 133: 329-340, 1968. 13. South, E. H. and R. C. Ritter. Overconsumption of preferred foods following capcaicin-pretreatment of the area postrema and adjacent nucleus of the solitary tract. Brain Res 288: 243251, 1983.