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Zinc deficiency and anorexia in rats: The effect of central administration of norepinephrine, muscimol and bromerogocryptine
Physiology &Behavior, Vol. 32, pp. 479--482.Copyright©PergamonPress Ltd., 1984.Printedin the U.S.A.
0031-9384/84$3.00 + .00
Zinc Deficiency and Anorexia in Rats: The Effect of Central Administration of Norepinephrine, Muscimol and Bromerogocryptine M'B. E S S A T A R A , C. J. M c C L A I N , j A. S. L E V I N E A N D J. E. M O R L E Y
Department o f Food Science and Nutrition, University o f Minnesota Neuroendocrine Research Laboratory and Departments o f Medicine Minneapolis Veterans Administration Medical Center and University of Minnesota R e c e i v e d 4 O c t o b e r 1982 ESSATARA, M'B., C. J. McCLAIN, A. S. LEVINE AND J. E. MORLEY. Zinc deficiency and anorexia in rats." The effect of central administration of norepinephrine, muscimol and bromerogocryptine. PHYSIOL BEHAV 32(3) 479-482, 1984.--Anorexia is a major manifestation of zinc deficiency, but the mechanism(s) for this anorexia are not well defined. In this study we investigated the effects of three modulators of feeding response on the food consumption of zinc deficient rats. Zinc deficient rats showed partial resistance to norepinephrine, eating significantly less at the 20/~g dose than the zinc sufficient ad lib controls, and food ingestion could not be induced at the 10/zg dose. Similarly, higher doses of the GABA agonist, muscimol, were requiredto induce feeding in the zinc deficient animals compared to the zinc sufficient controls. The dopamine agonist, bromergocryptine, failed to induce feeding in the zinc deficient animals. These findings are compatible with the concept that zinc deficiency produces a generalized decrease in receptor responsitivity, possibly secondary to alterations in membrane fluidity. Zinc
Appetite
Feeding
GABA
Muscimol
ONE of the major and initial manifestations of zinc deficiency is anorexia [22]. The mechanism(s) by which zinc deficiency induces anorexia is unknown. Both alterations in taste acuity [4,7] and in circulating amino acid levels [i, 22, 25] have been implicated as etological factors. Zinc deficiency in the rat has been reported to affect total brain catecholamine levels [6,26], and catecholamine levels in specific regions of the hypothalamus [21]. Norepinephrine [1 i,17], the GABA agonist, muscimol [16,20], and bromergocryptine [14] have been shown to induce feeding in rats after intracerebroventricular (ICV) injection. In this study we report the effects of central administration of these substances on food ingestion in zinc deficient rats.
Dopamine
Norepinephrine
Anorexia
received the diet ad lib, and weight restricted animals were weight paired with zinc deficient animals and fed an amount of diet so as to keep the body weights equal. The rats were maintained in an environment especially designed for trace metal studies described by us previously [19]. Rats were housed individually in stainless steel wire-bottom cages, stainless steel feeders were used, and plastic water bottles were equipped with silicone stoppers. Distilled and deionized water with 30 ppm replacement of zinc as zinc acetate was supplied ad lib to weight restricted and ad lib control groups, while there was no zinc replacement in the water of zinc deficient animals. After five weeks on their respective diets, the animals were placed under light ether anesthesia and chronic indwelling cannulas were inserted into their right ventricles as previously described by us [13]. The animals were allowed a 5-7 day post operative recovery period after which testing procedures were undertaken in a randomized order.
METHOD
Animals and Diet Fifty-six male weanling Sprague-Dawley rats (Biolab, St. Paul, MN) weighing from 40-46 g were randomly divided into three groups. All animals were fed a commercially prepared zinc deficient pelleted diet (Ziegler Bros., Inc., Gardners, PA) which contained 0.7 ppm zinc and had no added phytate [19]. The diet was fed with the following between group variations: zinc deficient and ad lib controls
Pharmacological Testing All animals had free access to food and water until the experiments were performed. All testing was carried out between 1300 to 1500 hours. Immediately after drug adminis-
~Requests for reprints should be addressed to Craig J. McClain, Gastroenterology Division, University of Kentucky Medical Center, 800 Rose Street, Lexington, KY 40536.
479
481)
ESSATARA, McCLAIN, LEVINE AND M()RI,t,.Y TABLE 1 BODY W E I G H T S A N D Z I N C C O N C E N T R A T I O N S
Body weight(g) Serum zinc (/,tg/dl) Femur zinc (~g/g dry weight)
ZD (18)
WR (19)
AL (19)
F
72 + 4.1 54 + 6 99 + 12
71 + 3.9 158 + 7 314 ± 12
186 + 1 4 . 1 164 ± 8 321 + 11
55.69p- 0.005 23.39p<0.005 114.82 p,0.005
Number of rats in parenthesis.
The Effect of Muscimol on Food intake in Zinc Deficient Rats
The Effect of Norepinephrine on Food Intake in Zinc Deficient Rots p~O.Ol
p,OOt
,
p'(0,01 '--'--"
[]
AL
'
3
p(O0~
~AL
,
~ZD
llz
0
I0
20
[ ] w.
0
I00
250
500
N~epinephrine (jag ICV)
FIG. I. The effect of norepinephrine on food intake in zinc deficient rats and appropriate controls (*p<0.01) compared to animals receiving vehicle. F=5.55. p<0.005,
FIG. 2. The effect of muscimol on food intake in zinc deficient animals and appropriate controls (tp>0.05, *p<0.01) compared to animals receiving vehicle. Muscimol doses ranged from 0-500 ng. F=4.93, p <0.005.
tration animals were returned to their home cage together with 2 pellets of pre-weighed zinc deficient diet (7-10 g). In all studies, food intake is expressed as grams eaten (to the nearest 0.1 g)/time period. After norepinephrine and muscimol administration, testing was carried out for 30 minutes, and after bromergocryptine for 60 minutes. All drugs were administered in a 5 /zl volume. Norepinephrine (Sigma Chemical Co., St. Louis, MO) was freshly dissolved in slightly acidified physiological saline. Muscimol (Sigma Chemical Co., St. Louis, MO) was dissolved in physiological saline. Bromergocryptine (Sandoz Pharmaceutical, Hanover, N J) was dissolved in propylene glycol with 0.01% ascorbic acid and physiological saline 2:3 (v/v). Control animals received the appropriate vehicle alone. All results are expressed as mean+-SEM. Results were compared using analysis of variance and the two-tailed unpaired Student's t-test.
contamination. The specimens for zinc analysis were prepared and dry ashed as described previously [19].
Measurement o f Zin(" ('on(entrations Zinc concentrations were determined applying atomic absorption techniques using a Varian AA375 Atomic Absorption Spectrophotometer [12]. All specimens for zinc analysis were stored in polyethylene containers that had been previously soaked in a I% solution of EDTA and deionized water, and then rinsed with deionized water in order to avoid zinc
RESULTS Zinc deficiency was assessed by growth rate, the development of skin lesions typical for zinc deficiency, and tissue zinc concentrations. Zinc deficient rats developed crusting skin lesions around the eyes, nose and over the extremities and had mild alopecia, findings that are typical for zinc deficiency. Zinc deficient rats had severe growth retardation as shown in Table 1. Tissue zinc concentrations were significantly depressed in zinc deficient rats compared to the two control groups (Table I). Zinc deficient animals displayed partial resistance to norepinephrine, in that they ate significantly less food than the ad lib controls at the 20/zg dose, and food ingestion could not be induced in them at the 10/.tg dose (Fig. I). Similarly. the zinc deficient rats were extremely resistant to muscimol induced eating, i.e., they ate less food at the 500 ng dose and were resistant to the 250 and 100 ng doses (Fig. 2). The zinc sufficient ad lib controls showed enhanced food ingestion after 80/.Lg of bromocryptine ICV, whereas the zinc deficient animals showed no food intake at all (Table 2). Because we have previously shown that food intake after bromergocryptine occurs over an extremely narrow dose range [14], we
ZINC I N D U C E D ANOREXIA
481
TABLE 2 EFFECT OF ICV BROMERGOCRYPTINEON FEEDINGIN ZINC DEFICIENT ANIMALS n
g/60 min
Controls: Vehicle Bromergocryptine 80 p.g
6 11
0.2 ± 0.1 0.8 ± 0.1"
Zinc Deficient: Vehicle Bromergocryptine 40 p.g 80/xg 100/zg 120/xg 200/xg
6 5 7 7 5 5
0.1 0.2 0 0 0.1 0.1
± ± ± ± ± ±
0.1 0.2 0 0 0.1 0.1
*p<0.01 vs. vehicle. Fc = 14.97 p <0.005. Fz~)=0.65 NS. extended the dose range in the zinc deficient animals in order to examine whether there had been a resetting of the effective dose. Feeding could not be induced at 40, 100, 120 and 200 ~g in the zinc deficient animals. The zinc sufficient weight restricted animals proved to be poor controls because these semi-starved animals ate voraciously during the control period after receiving the vehicle. The rate at which they ate during the control periods already appeared to be close to maximal as injection of norepinephrine failed to produce any statistically significant increase in food intake and only the lowest dose of muscimol produced a significant increase in food intake. DISCUSSION Historically, norepinephrine has been implicated as an important hypothalamic factor in the activation of feeding [11,18], although after injection into some areas of the lateral hypothalamus it can produce the opposite effect [11]. Zinc deficient rats have been reported to have increased total brain concentration of norepinephrine [26], and increased norepinephrine levels in the anterior hypothalamus but decreased dopamine levels in the posterior hypothalamus [21]. Norepinephrine, when administered into the ventricles of rats, is a relatively weak stimulator of feeding, in that its effect can be antagonized by the majority of known satiety
factors [ 17]. Thus it is perhaps not surprising to find that zinc deficient animals are partially resistant to the effects of norepinephrine. The GABA agonist, muscimol, is a potent inducer of feeding in rats [16,20]. It has been suggested that muscimol binds perferentially to GABA receptors within the satietycontrolling areas, exerting an inhibitory effect on this system [20], Norepinephrine induced feeding has been postulated to be secondary to alterations in this GABA system [16,20]. The effects of zinc deficiency on GABA have not been studied. Our zinc deficient animals demonstrated a similar partial resistance to muscimol, as was seen with norepinephrine. It is well recognized that destruction of the dopaminergic fibers in the nigrostriatal tract inhibits feeding [24]. Administration of dopamine agonists is well recognized to result in increased locomotor activity and sterotypic behavior including gnawing [5,9]. We have shown that the dopamine agonist, bromergocryptine, induces spontaneous eating over an extremely narrow range with a bell shaped dose response curve [ 14]. Zinc deficiency in rats has been reported to cause a modest increase in total brain dopamine concentration [26], but a decrease in posterior hypothalamus dopamine levels [21]. Our zinc deficient rats were completely resistant to bromergocryptine induced feeding. Because of the narrow dose range effective in producing feeding in normal animals [14] we extended our dose range over a broad area in an attempt to determine whether zinc deficiency had resulted in a shift of the dose response curve. We could not induce feeding at any of the doses tested. Our results showed that the anorexia of zinc deficiency is resistant to the effects of a number of food inducing substances. Although our results do not provide an absolute answer to the etiology of zinc deficiency anorexia, they do when taken together with the known biochemical effects of zinc, allow us to suggest at least one possible mechanism. Zinc has been demonstrated to play an important role in the maintenance of membrane structure and function [3]. Alterations in membrane fluidity has been demonstrated to result in alterations in receptors for serotonin [8], the catecholamines [2], and possibly the opiates [10,15]. The studies reported here are compatible with the concept that there is a decrease in receptor responsitivity involving a variety of receptors. Should one of the primary effects of zinc deficiency be to produce profound alterations in membrane fluidity this would lead to a secondary decrease in the responsitivity of multiple receptors. Such decreased receptor responsitivity may be sufficient to explain the anorexia of zinc deficiency.
REFERENCES I. Ashley, D. V. M. and G. H. Anderson. Correlation between the plasma tryptophan to neutral amino acid ratio and protein intake in the self-selecting weanling rat. J Nutr 105: 1412-1421, 1975. 2. Bakardjieva, A., M. J. Yalla and E. J. M. Helmreich. Modulation of the B-receptor adenylate cyclase interaction in cultured chong liver cells by phospholipid enrichment. Bh~chemistry 18: 3016-3023, 1979. 3. Bettger, W. J. and B. L. O'DelI. A critical physiological role of zinc in the structure and function of biomembranes. Lift~ Sci 28: 1625-1638, 1981. 4. Catalunalto, F. A. and P. Lacy. Effects of a zinc deficient diet upon fluid intake in the rat. J Nutr 107: 436-442, 1977. 5. Flemenbaum, A. Rat dopamine hypersensitivity. I. Effect of age. Nettropsy~hobiology 5:213-22 I, 1979.
6. Halas, E. S., J. C. Wallwork and H. H. Sandstead. Mild zinc deficiency and undernutrition during the prenatal and postnatal periods in rats: Effect on weight, food consumption, and brain catecholamine concentrations. J Nutr 112:542-551, 1982. 7. Henkin, R. I., P. P. G. Graziadei and D. F. Bradley. The molecular basis of taste and its disorders. Ann Intern Med 71: 791-819. 1969. 8. Heron, D. S., M. Shinitsky, M. Heshkowitz and D. Samuel. Lipid fluidity markedly modulates the binding of serotonin to mouse brain membranes. Proc Natl Acad St'i USA 77: 74637467, 1980. 9. Hicks, P., R. Strong, J. C. Schoolar and T. Samorajski. Aging alters amphetamine-induced sterotyped gnawing and neostriatal dimination of amphetamine in mice. Lift, St'i 27: 715-722, 1980.
482
10. Hiller, J. M., L. M. Angel and E. J. Simon. Multiple opiate receptors: Alcohol selectively inhibits binding to the delta receptors. Science 214: 468-469, 1981. I I. Leibowitz, S. F. Neurochemical systems of the hypothalamus in control of feeding and drinking behavior and water electrolyte excretion. In: Handbook ~['the Hypothalamus. vol 3, Part A. New York: Marcel Dekker, 1980, pp. 299-437. 12. McClain, C. J.. C. Soutor, N. Steele, A. S. Levine and S. E. Silvis. Severe zinc deficiency presenting with acrodermatitis during hyperalimentation: Diagnosis. pathogenesis, and treatment..I ('fin Gastrocnter~l 2: 125-131, 1980. 13. Morley, J. E. and A. S. Levine. Thyrotropin releasing hormone (TRH) suppresses stress-induced eating. LiJ~~ Sci 27: 26%274, 1980. 14. Morley, J. E., A. S, Levine, M. Grace and J. Kneip. Dynorphin( 1-13) dopamine and feeding in rats. Pharmacol Biochem Behav 16: 701-705, 1982. 15. Morley, J. E., A. S. Levine and S. Hess. Alcohol and the opiate receptor. Clin Res 29: 757A, 1981. 16. Morley, J. E., A. S. Levine and J. Kneip. Muscimol-induced feeding: A model to study the hypothalamic regulation of appetite. l, til~' Sci 29: 1213-1218, 1981. 17. Morley, J. E., A. S. Levine, S. S. Murray and J. Kneip. Peptidergic regulation of norepinephrine-induced feeding. Pharma~ ol Biochem Behav 16: 225-228, 1982. 18. Myers, R. D. Handbook ~[" Dru~,, and Chemical Stimulation o f the Braipt. New York: Van Nostrand Reinhold, 1974.
E S S A T A R A , M c C 1 , A I N . L E V I N E A N [ ) M()RI~EY
19. O'Leary, M. J., C. J. McClain and P. V. J. Hegarly. Effect of zinc deficiency on the weight, cellularity and zinc concentration of different skeletal muscles in the post-weanling rat. Br . / N u t r 42: 487-495, 1979. 20. Olgiati, V. R., C. Netti, F. Cuidobono and A. Pecile. l'he central GABAergic system and control of food intake under the different experimental conditions. Psychopharma('olo~,y (Bet = litt) 68: 163-167, 1980. 21. Reeves, P. G. and B. 14. O'Dell. Trace Element Metabolism in Man and Animals. edited by J. McC. Howell, J. M. Gawthorne and C. L. White. Canberra, Australia: Australian Academy of Science, 1981, pp. 338-340. 22. Reeves, P. G. and B. L. ()'Dell. Short-term zinc deficiency in the rat and self-selection of dietary protein level..I Nutr 111: 375-383, 1981. 23. Underwood, E. J. Trace lzTements in H u m a n attd Animal Nutritiara. New York: Academic Press, 1977, pp. 196-242. 24. Ungerstedt, U. Adipsia and aphagia after 6-hydroxydopamine induced degeneration of the nigrostriatal dopamine system. Acta Physiol S t a n d Suppl 367: 95-122, 1971. 25. Wallwork, J. C., G. J. Fosmire and H. H. Sandstead. Cyclic feeding patterns and plasma amino acid concentrations in zinc deficient rats. Fed Proc 38: 606, 1979. 26. Wallwork, J. C. and H. H. Sandstead. Effect of zinc deficiency on brain catecholamine concentrations in the rat. Fed Proc 40: 939, 198 I.
0031-9384/84$3.00 + .00
Zinc Deficiency and Anorexia in Rats: The Effect of Central Administration of Norepinephrine, Muscimol and Bromerogocryptine M'B. E S S A T A R A , C. J. M c C L A I N , j A. S. L E V I N E A N D J. E. M O R L E Y
Department o f Food Science and Nutrition, University o f Minnesota Neuroendocrine Research Laboratory and Departments o f Medicine Minneapolis Veterans Administration Medical Center and University of Minnesota R e c e i v e d 4 O c t o b e r 1982 ESSATARA, M'B., C. J. McCLAIN, A. S. LEVINE AND J. E. MORLEY. Zinc deficiency and anorexia in rats." The effect of central administration of norepinephrine, muscimol and bromerogocryptine. PHYSIOL BEHAV 32(3) 479-482, 1984.--Anorexia is a major manifestation of zinc deficiency, but the mechanism(s) for this anorexia are not well defined. In this study we investigated the effects of three modulators of feeding response on the food consumption of zinc deficient rats. Zinc deficient rats showed partial resistance to norepinephrine, eating significantly less at the 20/~g dose than the zinc sufficient ad lib controls, and food ingestion could not be induced at the 10/zg dose. Similarly, higher doses of the GABA agonist, muscimol, were requiredto induce feeding in the zinc deficient animals compared to the zinc sufficient controls. The dopamine agonist, bromergocryptine, failed to induce feeding in the zinc deficient animals. These findings are compatible with the concept that zinc deficiency produces a generalized decrease in receptor responsitivity, possibly secondary to alterations in membrane fluidity. Zinc
Appetite
Feeding
GABA
Muscimol
ONE of the major and initial manifestations of zinc deficiency is anorexia [22]. The mechanism(s) by which zinc deficiency induces anorexia is unknown. Both alterations in taste acuity [4,7] and in circulating amino acid levels [i, 22, 25] have been implicated as etological factors. Zinc deficiency in the rat has been reported to affect total brain catecholamine levels [6,26], and catecholamine levels in specific regions of the hypothalamus [21]. Norepinephrine [1 i,17], the GABA agonist, muscimol [16,20], and bromergocryptine [14] have been shown to induce feeding in rats after intracerebroventricular (ICV) injection. In this study we report the effects of central administration of these substances on food ingestion in zinc deficient rats.
Dopamine
Norepinephrine
Anorexia
received the diet ad lib, and weight restricted animals were weight paired with zinc deficient animals and fed an amount of diet so as to keep the body weights equal. The rats were maintained in an environment especially designed for trace metal studies described by us previously [19]. Rats were housed individually in stainless steel wire-bottom cages, stainless steel feeders were used, and plastic water bottles were equipped with silicone stoppers. Distilled and deionized water with 30 ppm replacement of zinc as zinc acetate was supplied ad lib to weight restricted and ad lib control groups, while there was no zinc replacement in the water of zinc deficient animals. After five weeks on their respective diets, the animals were placed under light ether anesthesia and chronic indwelling cannulas were inserted into their right ventricles as previously described by us [13]. The animals were allowed a 5-7 day post operative recovery period after which testing procedures were undertaken in a randomized order.
METHOD
Animals and Diet Fifty-six male weanling Sprague-Dawley rats (Biolab, St. Paul, MN) weighing from 40-46 g were randomly divided into three groups. All animals were fed a commercially prepared zinc deficient pelleted diet (Ziegler Bros., Inc., Gardners, PA) which contained 0.7 ppm zinc and had no added phytate [19]. The diet was fed with the following between group variations: zinc deficient and ad lib controls
Pharmacological Testing All animals had free access to food and water until the experiments were performed. All testing was carried out between 1300 to 1500 hours. Immediately after drug adminis-
~Requests for reprints should be addressed to Craig J. McClain, Gastroenterology Division, University of Kentucky Medical Center, 800 Rose Street, Lexington, KY 40536.
479
481)
ESSATARA, McCLAIN, LEVINE AND M()RI,t,.Y TABLE 1 BODY W E I G H T S A N D Z I N C C O N C E N T R A T I O N S
Body weight(g) Serum zinc (/,tg/dl) Femur zinc (~g/g dry weight)
ZD (18)
WR (19)
AL (19)
F
72 + 4.1 54 + 6 99 + 12
71 + 3.9 158 + 7 314 ± 12
186 + 1 4 . 1 164 ± 8 321 + 11
55.69p- 0.005 23.39p<0.005 114.82 p,0.005
Number of rats in parenthesis.
The Effect of Muscimol on Food intake in Zinc Deficient Rats
The Effect of Norepinephrine on Food Intake in Zinc Deficient Rots p~O.Ol
p,OOt
,
p'(0,01 '--'--"
[]
AL
'
3
p(O0~
~AL
,
~ZD
llz
0
I0
20
[ ] w.
0
I00
250
500
N~epinephrine (jag ICV)
FIG. I. The effect of norepinephrine on food intake in zinc deficient rats and appropriate controls (*p<0.01) compared to animals receiving vehicle. F=5.55. p<0.005,
FIG. 2. The effect of muscimol on food intake in zinc deficient animals and appropriate controls (tp>0.05, *p<0.01) compared to animals receiving vehicle. Muscimol doses ranged from 0-500 ng. F=4.93, p <0.005.
tration animals were returned to their home cage together with 2 pellets of pre-weighed zinc deficient diet (7-10 g). In all studies, food intake is expressed as grams eaten (to the nearest 0.1 g)/time period. After norepinephrine and muscimol administration, testing was carried out for 30 minutes, and after bromergocryptine for 60 minutes. All drugs were administered in a 5 /zl volume. Norepinephrine (Sigma Chemical Co., St. Louis, MO) was freshly dissolved in slightly acidified physiological saline. Muscimol (Sigma Chemical Co., St. Louis, MO) was dissolved in physiological saline. Bromergocryptine (Sandoz Pharmaceutical, Hanover, N J) was dissolved in propylene glycol with 0.01% ascorbic acid and physiological saline 2:3 (v/v). Control animals received the appropriate vehicle alone. All results are expressed as mean+-SEM. Results were compared using analysis of variance and the two-tailed unpaired Student's t-test.
contamination. The specimens for zinc analysis were prepared and dry ashed as described previously [19].
Measurement o f Zin(" ('on(entrations Zinc concentrations were determined applying atomic absorption techniques using a Varian AA375 Atomic Absorption Spectrophotometer [12]. All specimens for zinc analysis were stored in polyethylene containers that had been previously soaked in a I% solution of EDTA and deionized water, and then rinsed with deionized water in order to avoid zinc
RESULTS Zinc deficiency was assessed by growth rate, the development of skin lesions typical for zinc deficiency, and tissue zinc concentrations. Zinc deficient rats developed crusting skin lesions around the eyes, nose and over the extremities and had mild alopecia, findings that are typical for zinc deficiency. Zinc deficient rats had severe growth retardation as shown in Table 1. Tissue zinc concentrations were significantly depressed in zinc deficient rats compared to the two control groups (Table I). Zinc deficient animals displayed partial resistance to norepinephrine, in that they ate significantly less food than the ad lib controls at the 20/zg dose, and food ingestion could not be induced in them at the 10/.tg dose (Fig. I). Similarly. the zinc deficient rats were extremely resistant to muscimol induced eating, i.e., they ate less food at the 500 ng dose and were resistant to the 250 and 100 ng doses (Fig. 2). The zinc sufficient ad lib controls showed enhanced food ingestion after 80/.Lg of bromocryptine ICV, whereas the zinc deficient animals showed no food intake at all (Table 2). Because we have previously shown that food intake after bromergocryptine occurs over an extremely narrow dose range [14], we
ZINC I N D U C E D ANOREXIA
481
TABLE 2 EFFECT OF ICV BROMERGOCRYPTINEON FEEDINGIN ZINC DEFICIENT ANIMALS n
g/60 min
Controls: Vehicle Bromergocryptine 80 p.g
6 11
0.2 ± 0.1 0.8 ± 0.1"
Zinc Deficient: Vehicle Bromergocryptine 40 p.g 80/xg 100/zg 120/xg 200/xg
6 5 7 7 5 5
0.1 0.2 0 0 0.1 0.1
± ± ± ± ± ±
0.1 0.2 0 0 0.1 0.1
*p<0.01 vs. vehicle. Fc = 14.97 p <0.005. Fz~)=0.65 NS. extended the dose range in the zinc deficient animals in order to examine whether there had been a resetting of the effective dose. Feeding could not be induced at 40, 100, 120 and 200 ~g in the zinc deficient animals. The zinc sufficient weight restricted animals proved to be poor controls because these semi-starved animals ate voraciously during the control period after receiving the vehicle. The rate at which they ate during the control periods already appeared to be close to maximal as injection of norepinephrine failed to produce any statistically significant increase in food intake and only the lowest dose of muscimol produced a significant increase in food intake. DISCUSSION Historically, norepinephrine has been implicated as an important hypothalamic factor in the activation of feeding [11,18], although after injection into some areas of the lateral hypothalamus it can produce the opposite effect [11]. Zinc deficient rats have been reported to have increased total brain concentration of norepinephrine [26], and increased norepinephrine levels in the anterior hypothalamus but decreased dopamine levels in the posterior hypothalamus [21]. Norepinephrine, when administered into the ventricles of rats, is a relatively weak stimulator of feeding, in that its effect can be antagonized by the majority of known satiety
factors [ 17]. Thus it is perhaps not surprising to find that zinc deficient animals are partially resistant to the effects of norepinephrine. The GABA agonist, muscimol, is a potent inducer of feeding in rats [16,20]. It has been suggested that muscimol binds perferentially to GABA receptors within the satietycontrolling areas, exerting an inhibitory effect on this system [20], Norepinephrine induced feeding has been postulated to be secondary to alterations in this GABA system [16,20]. The effects of zinc deficiency on GABA have not been studied. Our zinc deficient animals demonstrated a similar partial resistance to muscimol, as was seen with norepinephrine. It is well recognized that destruction of the dopaminergic fibers in the nigrostriatal tract inhibits feeding [24]. Administration of dopamine agonists is well recognized to result in increased locomotor activity and sterotypic behavior including gnawing [5,9]. We have shown that the dopamine agonist, bromergocryptine, induces spontaneous eating over an extremely narrow range with a bell shaped dose response curve [ 14]. Zinc deficiency in rats has been reported to cause a modest increase in total brain dopamine concentration [26], but a decrease in posterior hypothalamus dopamine levels [21]. Our zinc deficient rats were completely resistant to bromergocryptine induced feeding. Because of the narrow dose range effective in producing feeding in normal animals [14] we extended our dose range over a broad area in an attempt to determine whether zinc deficiency had resulted in a shift of the dose response curve. We could not induce feeding at any of the doses tested. Our results showed that the anorexia of zinc deficiency is resistant to the effects of a number of food inducing substances. Although our results do not provide an absolute answer to the etiology of zinc deficiency anorexia, they do when taken together with the known biochemical effects of zinc, allow us to suggest at least one possible mechanism. Zinc has been demonstrated to play an important role in the maintenance of membrane structure and function [3]. Alterations in membrane fluidity has been demonstrated to result in alterations in receptors for serotonin [8], the catecholamines [2], and possibly the opiates [10,15]. The studies reported here are compatible with the concept that there is a decrease in receptor responsitivity involving a variety of receptors. Should one of the primary effects of zinc deficiency be to produce profound alterations in membrane fluidity this would lead to a secondary decrease in the responsitivity of multiple receptors. Such decreased receptor responsitivity may be sufficient to explain the anorexia of zinc deficiency.
REFERENCES I. Ashley, D. V. M. and G. H. Anderson. Correlation between the plasma tryptophan to neutral amino acid ratio and protein intake in the self-selecting weanling rat. J Nutr 105: 1412-1421, 1975. 2. Bakardjieva, A., M. J. Yalla and E. J. M. Helmreich. Modulation of the B-receptor adenylate cyclase interaction in cultured chong liver cells by phospholipid enrichment. Bh~chemistry 18: 3016-3023, 1979. 3. Bettger, W. J. and B. L. O'DelI. A critical physiological role of zinc in the structure and function of biomembranes. Lift~ Sci 28: 1625-1638, 1981. 4. Catalunalto, F. A. and P. Lacy. Effects of a zinc deficient diet upon fluid intake in the rat. J Nutr 107: 436-442, 1977. 5. Flemenbaum, A. Rat dopamine hypersensitivity. I. Effect of age. Nettropsy~hobiology 5:213-22 I, 1979.
6. Halas, E. S., J. C. Wallwork and H. H. Sandstead. Mild zinc deficiency and undernutrition during the prenatal and postnatal periods in rats: Effect on weight, food consumption, and brain catecholamine concentrations. J Nutr 112:542-551, 1982. 7. Henkin, R. I., P. P. G. Graziadei and D. F. Bradley. The molecular basis of taste and its disorders. Ann Intern Med 71: 791-819. 1969. 8. Heron, D. S., M. Shinitsky, M. Heshkowitz and D. Samuel. Lipid fluidity markedly modulates the binding of serotonin to mouse brain membranes. Proc Natl Acad St'i USA 77: 74637467, 1980. 9. Hicks, P., R. Strong, J. C. Schoolar and T. Samorajski. Aging alters amphetamine-induced sterotyped gnawing and neostriatal dimination of amphetamine in mice. Lift, St'i 27: 715-722, 1980.
482
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