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Undernutrition and the development of brain neurotransmitter systems
Life Sciences, Vol. 35, pp. 2085-2094 Printed in the U.S.A.
Pergamon Pres
MINIREVIEW
UNDERNUTRITION AND THE DEVELOPMENT NEUROTRANSMITTER SYSTEMS
R.C.
Wiggins,
Gregory
Fuller
OF
and S.J.
BRAIN
Enna
D e p a r t m e n t s of P h a r m a c o l o g y and of N e u r o b i o l o g y and A n a t o m y U n i v e r s i t y of Texas M e d i c a l School at H o u s t o n P.O. Box 20708, Houston, Texas 77025
SUMMARY
While there are h o m e o s t a t i c m e c h a n i s m s to p r o t e c t the brain against wide fluctuations in the availability of essential nutrients, food deprivation is known to influence brain neurochemistry. Given the growing problem of infant undernutrition and the fact that the d e v e l o p i n g n e r v o u s system appears to be e s p e c i a l l y v u l n e r a b l e to this type of insult, numerous studies have been c o n d u c t e d to d e f i n e the relationship between n u t r i t i o n a l f a c t o r s and c e l l u l a r g r o w t h and maturation in the brain. The data suggest that the d e v e l o p m e n t of both neural and nonneural e l e m e n t s are significantly affected by undernutrition. This includes p r o c e s s e s and s u b s t a n c e s important for n e u r o t r a n s m i s s i o n such as t r a n s m i t t e r synthesis, degradation and r e c e p t o r sites. B e c a u s e m a n y n e u r o p s y c h i a t r i c c o n d i t i o n s can be traced to d y s f u n c t i o n s in synaptic neurochemistry, it is possible that some of the c e n t r a l n e r v o u s s y s t e m abnormalities which result from c h i l d h o o d u n d e r n u t r i t i o n may be a consequence of a m o d i f i c a t i o n in synaptic b i o c h e m i s t r y . The p r e s e n t report reviews data r e l a t i n g to this issue with the aim of a s s e s s i n g its r e l e v a n c e to d e v e l o p m e n t a l n e u r o b i o l o g y .
INTRODUCTION U n d e r n u t r i t i o n is a w o r l d w i d e p r o b l e m c u r r e n t l y a f f e c t i n g an estimated 300 m i l l i o n infants w i t h a p r o j e c t e d increase to 2 b i l l i o n in the near future. D i e t a r y d e p r a v a t i o n is known to cause a significant n u m b e r of gross a b n o r m a l i t i e s in the developing c e n t r a l n e r v o u s s y s t e m (I), w i t h p a r t i c u l a r l y striking effects on m y e l i n f o r m a t i o n (2). There are u n d o u b t e d l y m a n y subtle effects that have yet to be c h a r a c t e r i z e d , as well as d r a m a t i c changes that may be r e s t r i c t e d to d i s c r e t e areas of the brain. The long-term n e u r o l o g i c c o n s e q u e n c e s of an i n a d e q u a t e or u n b a l a n c e d diet during early development is therefore of considerable interest to n e u r o b i o l o g i s t s . In rodent b r a i n it a p p e a r s that the c o n c e n t r a t i o n s of most n e u r o c h e m i c a l m a r k e r s (enzymes, transmitters, u p t a k e sites and receptors) are r e l a t i v e l y low at b i r t h and i n c r e a s e d r a m a t i c a l l y during the first 3 or 4 w e e k s of postnatal life (3). This 0024-3205/84 $3.00 + .00 Copyright (c) 1984 Pergamon Press Ltd.
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sequence c o r r e s p o n d s to the rapid p r o l i f e r a t i o n and development of various cell types and has been termed the postnatal brain growth spurt. As with myelin, synaptic n e u r o c h e m i s t r y may be most vulnerable to m o d i f i c a t i o n during this time making it conceivable that any factor w h i c h i n t e r f e r s w i t h the systematic progression of n e u r o t r a n s m i t t e r d e v e l o p m e n t may have a p e r m a n e n t effect on brain function. Given the e v i d e n c e that a l t e r a t i o n s in synaptic n e u r o c h e m i s t r y are r e l a t e d to the symptoms of a v a r i e t y of neuropsychiatric conditions (4,5), it is possible that n u t r i t i o n a l l y d e p r i v e d i n d i v i d u a l s may he prone to such d i s o r d e r s especially if u n d e r n o u r i s h m e n t o c c u r r e d during a c r i t i c a l phase of brain d e v e l o p m e n t . There have been a n u m b e r of attempts to examine the relationship between nutrition and brain development (6-10). Many of these have been aimed at defining the neurochemicsl a b n o r m a l i t i e s w h i c h ensue as a result of n u t r i t i o n a l d e p r i v a t i o n . While the animal m o d e l s e m p l o y e d in u n d e r n u t r i t i o n r e s e a r c h vary (11,12), there are now s u f f i c i e n t data for making tentative conclusions about the major features of transmitter system vulnerability to u n d e r n u t r i t i o n . Although a detailed comparison of n u t r i t i o n a l r e g i m e n s i s beyond the scope of this review, it should be borne in m i n d that apparant discrepancies in the literature can often be traced to d i f f e r e n c e s in the model employed. ACETYLCHOLINE Severe undernutrition b e f o r e (13) or after (14) weaning significantly reduces w h o l e brain a c e t y l c h o l i n e (ACh) levels in rat. This deficit is c o r r e c t e d w h e n u n d e r n o u r i s h e d animals are placed on a n o r m a l diet (13), s u g g e s t i n g that the synthesizing and storage c a p a c i t i e s for this t r a n s m i t t e r are not permanently damaged. Relatively mild p o s t n a t a l u n d e r n o u r i s h m e n t appears to have no effect on brain ACh content (14). Choline acetyltransferase (CHAT) a c t i v i t y is initially reduced when rats are u n d e r n o u r i s h e d during suckling, although total brain enzyme a c t i v i t y is w i t h i n the n o r m a l range by 21 days of age even with c o n t i n u e d n u t r i t i o n a l d e p r i v a t i o n (15). On the other hand, ChAT a c t i v i t y remains d e p r e s s e d in the olfactory bulbs and h y p o t h a l a m u s (15), but is above n o r m a l levels in these brain regions f o l l o w i n g five weeks of n u t r i t i o n a l r e h a b i l i t a t i o n . In contrast, brain stem ChAT a c t i v i t y does not recover in rehabilitated rats, i n d i c a t i n g that this s t r u c t u r e is severely affected by u n d e r n o u r i s h m e n t (16). Undernourishment initiated after weaning does not alter ChAT a c t i v i t y in the brain stem, s u g g e s t i n g that the time of exposure is an important v a r i a b l e in the response. It is u n k n o w n w h e t h e r the m o d i f i c a t i o n s in ChAT a c t i v i t y are due to specific changes in enzyme content, a loss of cholinergic neurons, or to alterations in enzyme kinetics. N e v e r t h e l e s s , w h i l e u n d e r n u t r i t i o n causes r e g i o n a l a l t e r a t i o n s in ChAT activity, these data indicate that the u n d e r n u t r i t i o n induced r e d u c t i o n in total brain ACh c o n t e n t is p r o b a b l y not due solely to a d e c r e a s e in synthesis.
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The influence of undernutrition on acetylcholinesterase activity (ACHE) has been studied e x t e n s i v e l y , but w i t h mixed results. Elevations in rat brain AChE activity have been reported when undernutrition was initiated prior to weaning (17,18), a l t h o u g h most i n v e s t i g a t o r s have o b s e r v e d no effect on the enzyme under this c o n d i t i o n ( 1 6 , 1 9 , 2 0 ) . It is more g e n e r a l l y agreed, however, that AChE is elevated above normal following continued (postweaning) undernourishment (21) or during n u t r i t i o n a l r e h a b i l i t a t i o n (16,22). A d e t a i l e d c o m p a r i s o n of the effects of undernourishment during gestational, suckling or postweaning periods shows that any combination of these treatments causes a s i g n i f i c a n t increase in brain stem AChE activity (23). Likewise, u n d e r n o u r i s h m e n t appears to increase n o n s p e c i f i c c h o l i n e s t e r a s e a c t i v i t y in brain ( 1 9 , 2 4 - 2 6 ) . As with CHAT, there is as yet n o e x p l a n a t i o n for the a l t e r a t i o n in AChE activity. Some p o s s i b i l i t i e s include the sparing of s t r u c t u r e s containing this enzyme c o m p a r e d to other cells, an increase in the number of enzyme m o l e c u l e s or an alteration in enzyme kinetics. It is feasable that the reported r e d u c t i o n in brain ACb content might result from the increase in d e g r a d a t i v e enzyme activity. However, there is not a perfect c o r r e s p o n d e n c e b e t w e e n the two markers since ACh levels recover following r e h a b i l i t a t i o n , while AChE a c t i v i t y remains elevated. There have been few studies examining the influence of undernutrition on brain cholinergic receptor binding or activity. It has been found that when rats are undernourished during the suckling period and then given a n o r m a l diet for one week, c h o l i n e r g i c m u s c a r i n i c r e c e p t o r b i n d i n g is s i g n i f i c a n t l y reduced in the corpus s t r i a t u m and h y p o t h a l a m u s , and sllghtly increased in t h e m i d b r a i n (Table i). No changes in b i n d i n g were noted in the cerebral cortex or cerebellum. Receptor binding site saturation analysis revealed that these a l t e r a t i o n s were due solely to a change in the c o n c e n t r a t i o n of b i n d i n g sites, with no m o d i f i c a t i o n in the a f f i n i t y of the receptor for the r a d i o l i g a n d . While a change in r e c e p t o r number may reflect an altertion in p r e s y n a p t i c a c t i v i t y (27), it is impossible to conclude whether this is the case under these c o n d i t i o n s . Thus it is c o n c e i v a b l e that early u n d e r n u t r i t i o n m o d i f i e s the rate of r e c e p t o r synthesis or d e g r a d a t i o n i n d e p e n d e n t of any effect on p r e s y n a p t i c activity. Alternatively, the d e c r e a s e in r e c e p t o r number could be due to a delay in the m a t u r a t i o n of some cells. It is i n t e r e s t i n g that, as w i t h enzyme activity, receptor binding changes appear to be region-specific, indicating d i f f e r e n c e s in v u l n e r a b i l i t y among brain areas. It is unknown whether these binding site alterations are i n d i c a t i v e of a change in c h o l i n e r g i c receptor function.
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Table The
Influence
of U n d e r n u t r i t i o n
Binding
Vol. 35, No. 21, 19~4
i on N e u r o t r a n s m i t t e r
Regions
Receptor
of the Rat Brain
Receptor Binding (% of Control)
Brain R e g i o n
Cerebral Corpus
Cortex
Striatum
Midbrain Hypothalamus Cerebellum
Cholinergic Muscarinic 3H-QNB
GABA 3H-muscimol
I00
Dopamine 3H-spiroperidol
136+
80+
149+
111+
150+
75+
156+
i00
78+
i00
On the day of b i r t h l i t t e r m a t e s were r a n d o m l y a s s i g n e d to control or u n d e r n o u r i s h e d groups. The undernourished animals were seperated from the dams each day for p r o g r e s s i v e l y longer periods of time such that by two weeks of age the a n i m a l s had no access to nourishment for 10-12 hrs/day. Control animals remained w i t h the dams at all times (11,56). All of the pups were w e a n e d at 21 days of age and g i v e n free access to food and water. The animals were d e c a p i t a t e d at 30 days of age and receptor binding a n a l y s e d using e s t a b l i s h e d p r o c e d u r e s (57,58). Each value r e p r e s e n t s the m e a n of 8-10 animals c o m p a r e d to an equal number of l i t t e r m a t e controls. + p < 0.05
(two-tailed
t-test)
CATECHOLAMINES Tyrosine accumulation and s t e a d y - s t a t e c o n c e n t r a t i o n s are either u n c h a n g e d or slightly e l e v a t e d in the u n d e r n o u r i s h e d rat brain (28-32) suggesting that there is an ample supply of precursor for catecholamine synthesis under this condition. Moreover, t y r o s i n e h y d r o x y l a s e a c t i v i t y appears i n c r e a s e d in the undernourished developing brain (33,34) and the in vivo conversion of t y r o s i n e to d o p a m i n e is enhanced (35). Tyrosine hydroxylase activity remains elevated in previously undernourished animals even after 90 days of nutritional r e h a b i l i t a t i o n (35). While in adult rats food d e p r i v a t i o n has no effect on brain d o p a m i n e content (36), most studies indicate that
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the overall brain d o p a m i n e c o n c e n t r a t i o n is reduced undernourished from birth (30-32, 34,37,38) a l t h o u g h
in rats there are
contradictory
that
reports
(19,39).
It
has been
suggested
the
d e c r e a s e in b r a i n d o p a m i n e is p r i m a r i l y due to a catecholamine deficit in the corpus striatum, the most d o p a m i n e - r i c h area of the b r a i n (30). However, it has been r e p o r t e d that a 70 day period of n u t r i t i o n a l r e h a b i l i t a t i o n returns the d o p a m i n e content to normal t h r o u g h o u t the b r a i n (39). Early undernutrition has a variable effect on brain norepinephrine content depending to some extent on the brain region (19,20,30,32,34,35,37,38,40-43). It is possible that the significant reductions in brain stem or telencephalon norepinephrine concentration are p r i m a r i l y r e s p o n s i b l e for the d e c r e a s e found w h e n the w h o l e brain is analysed (31,42). Shortterm (48 hr) s t a r v a t i o n in the adult rat has no effect on brain norepinephrine levels (36). Data suggest that following nutritional conditions where norepinephrine concentrations are reduced, several months of a n o r m a l diet can reverse this n e u r o c h e m i c a l deficit (20,39). G i v e n the findings that tyrosine levels are u n c h a n g e d , that tyrosine hydroxylase activity may be elevated, and that the c o n v e r s i o n of tyrosine to dopa is enhanced, it seems curious that steady-state c a t e c h o l a m i n e levels are g e n e r a l l y r e d u c e d in the undernourisheddeveloping rat brain. These findings suggest that undernourishment may m o d i f y c a t e c h o l a m i n e storage, or enhance c a t e c h o l a m i n e turnover or m e t a b o l i s m . In this regard it w o u l d be interesting to know whether monoamine oxidase activity is increased in the undernourished condition. Given the data suggesting that the specific a c t i v i t y of a number of enzyme systems is i n c r e a s e d by u n d e r n o u r i s h m e n t , it is c o n c e i v a b l e that the reduction in catecholamine levels may be traced to an e n h a n c e m e n t in the m e t a b o l i s m of these substances. Ligand binding assays have revealed that early undernutrition modifies the n u m b e r of catecholamine receptor sites in rat brain (44). When studying adult rats that had been u n d e r n o u r i s h e d during early d e v e l o p m e n t , a significant reduction was found in the number, but not the affinity, of alpha- and beta-adrenergic receptor b i n d i n g sites in h o m o g e n a t e s of whole brain. These authors s u g g e s t e d that the d e c r e a s e in receptor number may be indicative of an increase in presynaptic n o r a d r e n e r g i c activity, though they p r o v i d e no direct e v i d e n c e to support this h y p o t h e s i s . It has also been found that the number of striatal dopamine receptors is significantly reduced in the brains of animals undernourished during suckling (Table I). This finding is particularly noteworthy in light of the previously reported reduction in r e s p o n s e to a p o m o r p h i n e , a direct-acting dopamine receptor agonist, in rats exposed to a p r o t e i n d e f i c i e n t diet during d e v e l o p m e n t (45). Taken together these results suggest that the striatal dopaminergic system is modified by undernutrition. Since the r e d u c t i o n in d o p a m i n e r e c e p t o r b i n d i n g
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may be an adaptive response to a hyperactive nigro-striatal d o p a m i n e system, it is i m p o s s i b l e to c o n c l u d e that the change in r e c e p t o r number reflects a f u n c t i o n a l a l t e r a t i o n in this system. Nevertheless, the decreases in receptor binding for both norepinephrine and d o p a m i n e may be taken as evidence that the reduction in the steady-state concentrations of these neurotransmitters may be s e c o n d a r y to an increase in m o n o a m l n e turnover.
SEROTONIN
The m a j o r i t y of reports suggest that s e r o t o n e r g i c activity is enhanced in the d e v e l o p i n g brain of u n d e r n o u r i s h e d animals. It has been found that u n d e r n o u r i s h m e n t at any stage of life increases brain concentrations of tryptophan, the serotonin precursor (46-48). The increase in brain t r y p t o p h a n is most likely a c o n s e q u e n c e of the increase in the amount of unbound amino acid in blood. It is b e l i e v e d that this increase is due to the liberation of bound t r y p t o p h a n from a l b u m i n by nonesterified fatty acids, the concentrations of which increase in the undernourished state. These fatty acids are known to compete with tryptophan for the albumin binding sites. Moreover, undernourishment reduces the plasma albumin c o n c e n t r a t i o n which further serves to increase the circulating levels of free tryptophan. Early undernutrition increases the activity of tryptophan hydroxylase in brain (33). By 35 days of age brain tryptophan hydroxylase activity is 3-fold greater in undernourished rats than in controls, It is t h e r e f o r e not surprising that most investigators find an increase in the c o n c e n t r a t i o n of brain serotonin in undernourished animals (18,31,41,47-51). This occurs w h e t h e r the u n d e r n o u r i s h m e n t is during the prenatal or immediate postnatal period. While others have found a decrease or no change in s e r o t o n i n content (19,38,40), these d i f f e r e n c e s are p r o b a b l y not due to the type of d e p r i v a t i o n since increases have been reported using both the protein deficiency and dietary r e s t r i c t i o n models. A l t h o u g h increases in s e r o t o n i n content have been observed in virtually every brain region examined, the midbrain, brain stem and c e r e b e l l u m c o n s i s t e n t l y show the greatest enhancement in serotonin content. Interestingly, brain serotonin levels remain elevated into adulthood if the animals are c o n t i n u o u s l y undernourished from birth (41,51), and the s e r o t o n i n synthesis rate remains high even if the animals were only deprived for a brief period during infancy (50). Brain s e r o t o n i n content can also be elevated in at least some regions of adult brain by dietary restriction, indicating that the change in n e u r o t r a n s m i t t e r content is not strictly a function of d e v e l o p m e n t (36). The concentration of the serotonin metabolite 5hydroxyindoleacetic acid is also increased in undernourished animals ( 1 8 , 3 1 , 3 6 , 4 1 , 4 6 - 4 8 ) . This may be taken as evidence of an increase in s e r o t o n i n turnover, though it does not necessarily
Vol.
35, No. 21,
prove However, likelihood release
Undernutrition
1984
there is an increase when viewed in their undernutrition that of brain serotonin.
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in serotonergic transmission. entirety, these data suggest increases the synthesis
Y -AMINOBUTYRIC
ACID
the and
( GABA)
There have been few studies investigating the influence of undernutrition on the GABAerg ic system. Available evidence suggests that dietary restriction, during either the neonatal period or immediately following weaning, causes a reduction in the activity of glutamic acid decarboxylase (GAD), the enzyme primarily responsible for the synthesis of brain GABA (14,521. Regional studies indicate that the reduction in GAD activity is found in all brain areas except for the cerebellum and brainstem, and that enzyme activity begins to normalize with time even though the animals are continued on a restricted diet (15). This suggests that undernutrition delays rather than arrests the development of this enzyme. Indeed, GAD activity is completely rehabilitated in undernourished animals placed on a normal diet at a time when GAD activity is known to be reduced (15,52,53). These data suggest that undernutrition causes a transient decrease in GABA synthesis. IU contrast, undernourishment appears to have no effect on GABA transaminase, the enzyme responsible for the metabolism of this neurotransmitter (54). Significant changes in GABA receptor binding have been noted in rat brain following early undernutrition, with GABA binding being significantly increased in the cerebral cortex, corpus hypothalamus (Table 1) (55). striatum, midbrain and These may be related to the possible diminution in GABAergic increases activity resulting from the decrease in GAD activity. Saturation the change in binding was due to an analysis revealed that number of receptors, with no alteration in the increase in affinity. model of different using a obtained data were Similar that (55). In this case it was found undernutrition undernutrition enhances binding by increasing the availability of receptors. affinity sites and the number of low affinity high were were abolished when undernourished rats differences These that suggested It was a normal diet for 4 weeks. exposed to substance some may decrease the production of undernourishment GABAsuch as the higher affinity site, masks normally that the data indicate that In either case, modulin or GABA itself. undernourished the sensitive in be more GABA receptors may synthesis perhaps as a consequence of a decrease in the animal, data are no there As transmitter. of release yet or apparent these consequences of functional the demonstrating receptor modifications. CONCLUSIONS While
there
are
several
forms
and
different
degrees
of
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undernutrition, it would appear that any type of dietary restriction has an effect on rat brain neurochemistry if it occurs during the p e r i o d of rapid brain d e v e l o p m e n t . Given the n u m b e r of p o s s i b l e v a r i a b l e s , it is not s u r p r i s i n g that there are some conflicting data as to the type of neurochemical changes w h i c h may occur. What is certain, however, is that w h i l e there are physiologic mechanisms protecting the brain against radical shifts in the c o n c e n t r a t i o n of important nutrients, u n d e r n u t r i t i o n can alter the n e u r o c h e m i c a l b a l a n c e in brain. In this regard it is i n t e r e s t i n g that not all transmitter systems appear to be s i m i l a r l y altered. While the results suggest that cholinergic and G A B A e r g i c t r a n s m i s s i o n may be reduced in the d e v e l o p i n g brain, c a t e c h o l a m i n e and s e r o t o n i n systems would seem to be more active in the u n d e r n o u r i s h e d state than in controls. This implies that u n d e r n u t r i t i o n has a s e l e c t i v e effect on brain n e u r o t r a n s m i t t e r activity, w h i c h may express itself in a v a r i e t y of ways. Of p a r t i c u l a r note is the finding that m a n y of the neurochemical perturbations can be r e v e r s e d if the animal is placed on a proper diet, suggesting that the neurochemical changes are not n e c e s s a r i l y p e r m a n e n t . Up to now most of the work in this area has been primarily descriptive. Given these data, however, it may be p o s s i b l e to f o r m u l a t e t e s t a b l e h y p o t h e s i s r e l a t i n g to m e c h a n i s t i c issues. For example, it would be of value to k n o w w h e t h e r the changes in enzyme a c t i v i t y are due to a change in the production of these p r o t e i n s or to the s y n t h e s i s of an a l t e r e d form of the enzyme. Are the r e c e p t o r b i n d i n g changes always s e c o n d a r y to an alteration in t r a n s m i t t e r activity, or might they be due to an alteration in the production or degradation of receptor molecules? Which nutritional factor is most important for regulating these neurochemical events? At what stage of d e v e l o p m e n t is the system most s u s c e p t a b l e to m o d i f i c a t i o n ? The answers to these and other q u e s t i o n s may provide fundamental i n f o r m a t i o n about brain n e u r o c h e m i s t r y and d e v e l o p m e n t a l biology, as well as yield insights into the b i o l o g i c a l basis of brain dysfunction following undernutrition.
ACKNOWLEDGEMENTS Preparation of this m a n u s c r i p t was made p o s s i b l e in part by grants from the N a t i o n a l I n s t i t u t e of H e a l t h (NS-13799, N S - 0 0 4 7 4 ) , the N a t i o n a l Science F o u n d a t i o n ( B N S - 8 2 1 5 4 2 7 ) and Bristol-Myers Corporation. REFERENCES I.
M. Winick, Oxford Univ.
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R.C. Wiggins, Brain Res. 5 : 1 5 1 - 1 7 5 (1982). S.D. Telang and S.J. Enna, Develo~gntal Neurochemistr~, R.C. Wiggins, D.W. M c C a n d l e s s and S.J. Enna (eds.), Univ. Press, Austin, in press.
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M a l n u t r i t i o n and Brain Devel__qop__men~, pp. Press, L o n d o n (1976).
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4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
21. 22. 23. 24. 25.
26. 27.
28, 29. 30. 31.
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S.J. Enna, L.Z. Stern, G.J. Wastek and H.I. Yamamura, Life Sci 20:205-212 (1977). S.J. Enna, S e r o t o n l n ~ N e u r o ~ and Functiopj B. Haber, S. Gabay, M.R. Issidorides and S.G. Alivisatos (eds.), pp. 347-258, Plenum Press, Hew York, (1981). M.A.B. Brazier (ed.), Growth and Development of the Brain. Nutritiona__lj Genetic and Envirqonmgntal Factors, pp. 1-399, M.A. Corner, R.E. Baker, N.E. Van De Poll, D.F. Swaab and H.B.M. Uylings (Eds.), Prgg. i_._~nB._~rai__~nReso 48:1-425 (1978). P.R. Dodge, A.L. Prensky, R.D. Feigin and S.J. Holmes (Eds.), Nut____._Xrj~onand the Devel_oping Nervous STstem , pp. 1538, C.V. Mosby, St. Louis (1975). S.L. Manocha, Malnutrit~o__n and Retarded Human Deve!9~ment , pp. 1-382, Charles Thomas, Springfield (1972). R.J. Wurtman and J.J. Wurtman (Eds.), Nutrition and the Brain (Vols. 1-6), Raven Press, New York (1977-1983). R.C. Wiggins and G.N. Fuller, N e u r o ~ h g m ~ Res___~.¢:827-830 (1979). L.S. Crnic and H.P. Chase, Dev. Neur_._OS__CCi~.3:49-58 (1980). A.B. K u l k a r n i and B.B. Gaitonde, Ex_Re__rie.____nnti____aa 38 :377-378 (1982). R. Rajalakshmi, M. P a r a m e s w a r a n and C.V. Ramakrishnan, J_.~. N e u r o c h e m 23:123-127 (1974). A.J. Pate1, M. Del Vecchlo and D.J. Atkinson, Dev. Neurosci. 1:41-53 ( 1 9 7 8 ) C.D. Eckhert, R.H. Barnes and D.A. Levitsky, Brain R es= 101:372-377 (1976). B.P.F. Adlard and J. Dobbing, Brain Res. 28:97-107 (1971). T.J, S o b o t k a , M . P . Cook and R.E. Brodie, B r a i n Res 6 5 : 4 4 3 457 ( 1 9 7 4 ) . G. Ahmad a n d M.A. H a h m a n , J . N u t r . 1 0 5 : 1 0 9 0 - 1 1 0 3 (1975). F. Serini, N. Principi, L. Perletti and L.P. Serini, Biol. Neonat. 10:254-265 (1966). B.F.F. Adlard and J. Dobbing, Br. J. Nutr. 28:139-143 (1972). B.P.F. Adlard and J, Dobbing, Brain Res. 30:198-199 (1971). C.D. Eckhert, R.H. Barnes and D.A. Levitsky, J~ Neurochem. " 27:277-283 (1976). H.S. Im, R.H. B a r n e s and D.A. L e v i t s k y , Nature 233:269-270 (1971). H . S . Im, R.H. B a r n e s and D.A. L e v i t s k y , J__~. N u t r . 1 0 6 : 3 4 2 - 3 4 9 (1976). H.S. Im, R.H. Barnes, D.A. Levitsky and W.G. Pond, Brain Res. 63:461-465 (1973). S.J. Enna and S.J. Strada, Clinical N e u r o s c i e n c ~ ~ (Vol. V__!), R. Rosenberg, R. Grossman, S. Schochet, E.R. Heinz and W. Willis (eds.), pp. 145-170, Churchill Livingstone, New York, (1983). M. Miller, J.P. Leahy, F. McConville, J.P. Morgane and O. Resnick, Brai~n Res_~. Buli. 3:681-686 (1978). M. Miller, R. Hasson and O. Resnick, Expt!= Ne.__~ur_~!.77:163178 (1982). W.J. Shoemaker and R.J. Wurtman, J__~. Nu__~tr__~.103:1537-1547 (1973). K. Hisatomi, Y. Niiyama, J__~.Nutr. S~ci__~.V_~tami__nno.~l~26:279-292 (1980).
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Vol. 35, No. 21, 1984
K. Hisatomi, S. Sasaki and Y. Niiyama J__~. N__uut__Kr~ Sci. Vitaminol. 25:243-253 (1979). S. Kalyanasundaram and P.S.V. Ramanamurthy, J__~. Neuxo~hem, 36:1580-1582 (1981). W .J. Shoemaker and R.J. Wurtman, Science 1 7 1 : 1 0 1 7 - 1 0 1 9 (1971). E.S. Marichich, V.A. Molina and O.A. Orsingher, J. Nutr. 1 0 9 : 1 0 4 5 - 1 0 5 0 (1979). C.C. Loullis, D.L. Felten and P.A. Shea, Fharmacol. Biochem. & B e h a y ~ 11:89-93 (1979). C.J. Lee and R. Duobos, J__~. E _ x ~ ! l . M__eed_~.1 3 6 : 1 0 3 1 - 1 0 4 2 (1972). P.S.V. Ramanamurthy, J__L. N__eeu_ro_cchem.28:253-254 ( 1 9 7 7 ) . R.C. Wiggins, G.N. Fuller, W.E. Seifert, I.J. Butler and Z. Gottesfeld, J__~. N__eeuzos~ R e s . 8 : 6 5 1 - 6 5 6 (1982). J.W.T. Dickerson and S.K. Pao, Biol. Neonate 25:114-124 (1975). W.C. Stern, M. Miller, W.B. Forbes, P.J. Morgane and O. Resnick, E x ~ = Neurol. 49:314-326 (1975). W.C. Stern, P.J. Morgane, M. Miller and O. Resnick, Expt!~ Neurol. 47:56-76 (1956). E.M. Burns and K.B. Brown, Brain Res. B ul__!l~ 2:313-316 (1977). E.A. Keller, N.I. Munaro and O.A. Orsingher, Science 2 1 5 : 1 2 6 9 - 1 2 7 0 (1982). J.P. Leahy, W.C. Stern, O. Resnick and P.J. Morgane D ev. Ps_%ychobiol. 11:361-370 (1978). S. Kohsaka, K. Takamatsu a n d Y. T s u k a d a , Neurochem___~. Res___~. 5:69-93(1980). M. Miller, J.P. Leahy, W.C. Stern, P.J. Morgan and O. Resnick, E x p t l = Neurol. 57:142-157 (1977). M. Miller and O. Resnick, Exptl= Neu______Krol=67:298-314 (1980). R.J. Hernandez, Biol. Neonate 30:181-186 (1976). J.L. Smart, M.D. Tricklebank, B.P.F. Adlard and J. Dohbing, Pediat. Res. 10:807-811 (1976). W.C. Stern, W.B. Forbes, O. Resnick and P.J. Morgane, Brain Res. 79:375-384 (1974). R. Rajalakshmi, M. Parameswaran, S.D. Telang and C.V. Ramakrishnan, j__=.Neurochem. 23:129-133 (1974). R. Rajalakshmi, M. P a r a m e s w a r a n and C.V. Ramakrishnan, J. Neurochem_~. 23:123-127 (1974). R. Rajakshmi, K.R. Pillai and C.V. Ramakrishnan, ~. Neurochem. 16:599-606 (1969). S. Telang, G. Fuller, R. Wiggins and S.J. Enna, Neurochem__~., 43:640-645 (1984). R.C. Wiggins and G.N. Fuller, J__~_.N__eeur_ochem. 30:1231-1237 (1978). T.D. Reisine, J.Z. Fields, H.I. Yamamura, E. Bird, E. Spokes, P. Schreiner and S.J. Enna, Life Scl. 21:335-344 (1977). K. Beaumont, W. Chilton, H.I. Yamamura and S.J. Enna, Brain Res. 148:153-162 (1978).
Pergamon Pres
MINIREVIEW
UNDERNUTRITION AND THE DEVELOPMENT NEUROTRANSMITTER SYSTEMS
R.C.
Wiggins,
Gregory
Fuller
OF
and S.J.
BRAIN
Enna
D e p a r t m e n t s of P h a r m a c o l o g y and of N e u r o b i o l o g y and A n a t o m y U n i v e r s i t y of Texas M e d i c a l School at H o u s t o n P.O. Box 20708, Houston, Texas 77025
SUMMARY
While there are h o m e o s t a t i c m e c h a n i s m s to p r o t e c t the brain against wide fluctuations in the availability of essential nutrients, food deprivation is known to influence brain neurochemistry. Given the growing problem of infant undernutrition and the fact that the d e v e l o p i n g n e r v o u s system appears to be e s p e c i a l l y v u l n e r a b l e to this type of insult, numerous studies have been c o n d u c t e d to d e f i n e the relationship between n u t r i t i o n a l f a c t o r s and c e l l u l a r g r o w t h and maturation in the brain. The data suggest that the d e v e l o p m e n t of both neural and nonneural e l e m e n t s are significantly affected by undernutrition. This includes p r o c e s s e s and s u b s t a n c e s important for n e u r o t r a n s m i s s i o n such as t r a n s m i t t e r synthesis, degradation and r e c e p t o r sites. B e c a u s e m a n y n e u r o p s y c h i a t r i c c o n d i t i o n s can be traced to d y s f u n c t i o n s in synaptic neurochemistry, it is possible that some of the c e n t r a l n e r v o u s s y s t e m abnormalities which result from c h i l d h o o d u n d e r n u t r i t i o n may be a consequence of a m o d i f i c a t i o n in synaptic b i o c h e m i s t r y . The p r e s e n t report reviews data r e l a t i n g to this issue with the aim of a s s e s s i n g its r e l e v a n c e to d e v e l o p m e n t a l n e u r o b i o l o g y .
INTRODUCTION U n d e r n u t r i t i o n is a w o r l d w i d e p r o b l e m c u r r e n t l y a f f e c t i n g an estimated 300 m i l l i o n infants w i t h a p r o j e c t e d increase to 2 b i l l i o n in the near future. D i e t a r y d e p r a v a t i o n is known to cause a significant n u m b e r of gross a b n o r m a l i t i e s in the developing c e n t r a l n e r v o u s s y s t e m (I), w i t h p a r t i c u l a r l y striking effects on m y e l i n f o r m a t i o n (2). There are u n d o u b t e d l y m a n y subtle effects that have yet to be c h a r a c t e r i z e d , as well as d r a m a t i c changes that may be r e s t r i c t e d to d i s c r e t e areas of the brain. The long-term n e u r o l o g i c c o n s e q u e n c e s of an i n a d e q u a t e or u n b a l a n c e d diet during early development is therefore of considerable interest to n e u r o b i o l o g i s t s . In rodent b r a i n it a p p e a r s that the c o n c e n t r a t i o n s of most n e u r o c h e m i c a l m a r k e r s (enzymes, transmitters, u p t a k e sites and receptors) are r e l a t i v e l y low at b i r t h and i n c r e a s e d r a m a t i c a l l y during the first 3 or 4 w e e k s of postnatal life (3). This 0024-3205/84 $3.00 + .00 Copyright (c) 1984 Pergamon Press Ltd.
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sequence c o r r e s p o n d s to the rapid p r o l i f e r a t i o n and development of various cell types and has been termed the postnatal brain growth spurt. As with myelin, synaptic n e u r o c h e m i s t r y may be most vulnerable to m o d i f i c a t i o n during this time making it conceivable that any factor w h i c h i n t e r f e r s w i t h the systematic progression of n e u r o t r a n s m i t t e r d e v e l o p m e n t may have a p e r m a n e n t effect on brain function. Given the e v i d e n c e that a l t e r a t i o n s in synaptic n e u r o c h e m i s t r y are r e l a t e d to the symptoms of a v a r i e t y of neuropsychiatric conditions (4,5), it is possible that n u t r i t i o n a l l y d e p r i v e d i n d i v i d u a l s may he prone to such d i s o r d e r s especially if u n d e r n o u r i s h m e n t o c c u r r e d during a c r i t i c a l phase of brain d e v e l o p m e n t . There have been a n u m b e r of attempts to examine the relationship between nutrition and brain development (6-10). Many of these have been aimed at defining the neurochemicsl a b n o r m a l i t i e s w h i c h ensue as a result of n u t r i t i o n a l d e p r i v a t i o n . While the animal m o d e l s e m p l o y e d in u n d e r n u t r i t i o n r e s e a r c h vary (11,12), there are now s u f f i c i e n t data for making tentative conclusions about the major features of transmitter system vulnerability to u n d e r n u t r i t i o n . Although a detailed comparison of n u t r i t i o n a l r e g i m e n s i s beyond the scope of this review, it should be borne in m i n d that apparant discrepancies in the literature can often be traced to d i f f e r e n c e s in the model employed. ACETYLCHOLINE Severe undernutrition b e f o r e (13) or after (14) weaning significantly reduces w h o l e brain a c e t y l c h o l i n e (ACh) levels in rat. This deficit is c o r r e c t e d w h e n u n d e r n o u r i s h e d animals are placed on a n o r m a l diet (13), s u g g e s t i n g that the synthesizing and storage c a p a c i t i e s for this t r a n s m i t t e r are not permanently damaged. Relatively mild p o s t n a t a l u n d e r n o u r i s h m e n t appears to have no effect on brain ACh content (14). Choline acetyltransferase (CHAT) a c t i v i t y is initially reduced when rats are u n d e r n o u r i s h e d during suckling, although total brain enzyme a c t i v i t y is w i t h i n the n o r m a l range by 21 days of age even with c o n t i n u e d n u t r i t i o n a l d e p r i v a t i o n (15). On the other hand, ChAT a c t i v i t y remains d e p r e s s e d in the olfactory bulbs and h y p o t h a l a m u s (15), but is above n o r m a l levels in these brain regions f o l l o w i n g five weeks of n u t r i t i o n a l r e h a b i l i t a t i o n . In contrast, brain stem ChAT a c t i v i t y does not recover in rehabilitated rats, i n d i c a t i n g that this s t r u c t u r e is severely affected by u n d e r n o u r i s h m e n t (16). Undernourishment initiated after weaning does not alter ChAT a c t i v i t y in the brain stem, s u g g e s t i n g that the time of exposure is an important v a r i a b l e in the response. It is u n k n o w n w h e t h e r the m o d i f i c a t i o n s in ChAT a c t i v i t y are due to specific changes in enzyme content, a loss of cholinergic neurons, or to alterations in enzyme kinetics. N e v e r t h e l e s s , w h i l e u n d e r n u t r i t i o n causes r e g i o n a l a l t e r a t i o n s in ChAT activity, these data indicate that the u n d e r n u t r i t i o n induced r e d u c t i o n in total brain ACh c o n t e n t is p r o b a b l y not due solely to a d e c r e a s e in synthesis.
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Undernutrition and Brain Development
2087
The influence of undernutrition on acetylcholinesterase activity (ACHE) has been studied e x t e n s i v e l y , but w i t h mixed results. Elevations in rat brain AChE activity have been reported when undernutrition was initiated prior to weaning (17,18), a l t h o u g h most i n v e s t i g a t o r s have o b s e r v e d no effect on the enzyme under this c o n d i t i o n ( 1 6 , 1 9 , 2 0 ) . It is more g e n e r a l l y agreed, however, that AChE is elevated above normal following continued (postweaning) undernourishment (21) or during n u t r i t i o n a l r e h a b i l i t a t i o n (16,22). A d e t a i l e d c o m p a r i s o n of the effects of undernourishment during gestational, suckling or postweaning periods shows that any combination of these treatments causes a s i g n i f i c a n t increase in brain stem AChE activity (23). Likewise, u n d e r n o u r i s h m e n t appears to increase n o n s p e c i f i c c h o l i n e s t e r a s e a c t i v i t y in brain ( 1 9 , 2 4 - 2 6 ) . As with CHAT, there is as yet n o e x p l a n a t i o n for the a l t e r a t i o n in AChE activity. Some p o s s i b i l i t i e s include the sparing of s t r u c t u r e s containing this enzyme c o m p a r e d to other cells, an increase in the number of enzyme m o l e c u l e s or an alteration in enzyme kinetics. It is feasable that the reported r e d u c t i o n in brain ACb content might result from the increase in d e g r a d a t i v e enzyme activity. However, there is not a perfect c o r r e s p o n d e n c e b e t w e e n the two markers since ACh levels recover following r e h a b i l i t a t i o n , while AChE a c t i v i t y remains elevated. There have been few studies examining the influence of undernutrition on brain cholinergic receptor binding or activity. It has been found that when rats are undernourished during the suckling period and then given a n o r m a l diet for one week, c h o l i n e r g i c m u s c a r i n i c r e c e p t o r b i n d i n g is s i g n i f i c a n t l y reduced in the corpus s t r i a t u m and h y p o t h a l a m u s , and sllghtly increased in t h e m i d b r a i n (Table i). No changes in b i n d i n g were noted in the cerebral cortex or cerebellum. Receptor binding site saturation analysis revealed that these a l t e r a t i o n s were due solely to a change in the c o n c e n t r a t i o n of b i n d i n g sites, with no m o d i f i c a t i o n in the a f f i n i t y of the receptor for the r a d i o l i g a n d . While a change in r e c e p t o r number may reflect an altertion in p r e s y n a p t i c a c t i v i t y (27), it is impossible to conclude whether this is the case under these c o n d i t i o n s . Thus it is c o n c e i v a b l e that early u n d e r n u t r i t i o n m o d i f i e s the rate of r e c e p t o r synthesis or d e g r a d a t i o n i n d e p e n d e n t of any effect on p r e s y n a p t i c activity. Alternatively, the d e c r e a s e in r e c e p t o r number could be due to a delay in the m a t u r a t i o n of some cells. It is i n t e r e s t i n g that, as w i t h enzyme activity, receptor binding changes appear to be region-specific, indicating d i f f e r e n c e s in v u l n e r a b i l i t y among brain areas. It is unknown whether these binding site alterations are i n d i c a t i v e of a change in c h o l i n e r g i c receptor function.
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Undernutrition and Brain Development
Table The
Influence
of U n d e r n u t r i t i o n
Binding
Vol. 35, No. 21, 19~4
i on N e u r o t r a n s m i t t e r
Regions
Receptor
of the Rat Brain
Receptor Binding (% of Control)
Brain R e g i o n
Cerebral Corpus
Cortex
Striatum
Midbrain Hypothalamus Cerebellum
Cholinergic Muscarinic 3H-QNB
GABA 3H-muscimol
I00
Dopamine 3H-spiroperidol
136+
80+
149+
111+
150+
75+
156+
i00
78+
i00
On the day of b i r t h l i t t e r m a t e s were r a n d o m l y a s s i g n e d to control or u n d e r n o u r i s h e d groups. The undernourished animals were seperated from the dams each day for p r o g r e s s i v e l y longer periods of time such that by two weeks of age the a n i m a l s had no access to nourishment for 10-12 hrs/day. Control animals remained w i t h the dams at all times (11,56). All of the pups were w e a n e d at 21 days of age and g i v e n free access to food and water. The animals were d e c a p i t a t e d at 30 days of age and receptor binding a n a l y s e d using e s t a b l i s h e d p r o c e d u r e s (57,58). Each value r e p r e s e n t s the m e a n of 8-10 animals c o m p a r e d to an equal number of l i t t e r m a t e controls. + p < 0.05
(two-tailed
t-test)
CATECHOLAMINES Tyrosine accumulation and s t e a d y - s t a t e c o n c e n t r a t i o n s are either u n c h a n g e d or slightly e l e v a t e d in the u n d e r n o u r i s h e d rat brain (28-32) suggesting that there is an ample supply of precursor for catecholamine synthesis under this condition. Moreover, t y r o s i n e h y d r o x y l a s e a c t i v i t y appears i n c r e a s e d in the undernourished developing brain (33,34) and the in vivo conversion of t y r o s i n e to d o p a m i n e is enhanced (35). Tyrosine hydroxylase activity remains elevated in previously undernourished animals even after 90 days of nutritional r e h a b i l i t a t i o n (35). While in adult rats food d e p r i v a t i o n has no effect on brain d o p a m i n e content (36), most studies indicate that
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Undernutrition and Brain Development
2089
the overall brain d o p a m i n e c o n c e n t r a t i o n is reduced undernourished from birth (30-32, 34,37,38) a l t h o u g h
in rats there are
contradictory
that
reports
(19,39).
It
has been
suggested
the
d e c r e a s e in b r a i n d o p a m i n e is p r i m a r i l y due to a catecholamine deficit in the corpus striatum, the most d o p a m i n e - r i c h area of the b r a i n (30). However, it has been r e p o r t e d that a 70 day period of n u t r i t i o n a l r e h a b i l i t a t i o n returns the d o p a m i n e content to normal t h r o u g h o u t the b r a i n (39). Early undernutrition has a variable effect on brain norepinephrine content depending to some extent on the brain region (19,20,30,32,34,35,37,38,40-43). It is possible that the significant reductions in brain stem or telencephalon norepinephrine concentration are p r i m a r i l y r e s p o n s i b l e for the d e c r e a s e found w h e n the w h o l e brain is analysed (31,42). Shortterm (48 hr) s t a r v a t i o n in the adult rat has no effect on brain norepinephrine levels (36). Data suggest that following nutritional conditions where norepinephrine concentrations are reduced, several months of a n o r m a l diet can reverse this n e u r o c h e m i c a l deficit (20,39). G i v e n the findings that tyrosine levels are u n c h a n g e d , that tyrosine hydroxylase activity may be elevated, and that the c o n v e r s i o n of tyrosine to dopa is enhanced, it seems curious that steady-state c a t e c h o l a m i n e levels are g e n e r a l l y r e d u c e d in the undernourisheddeveloping rat brain. These findings suggest that undernourishment may m o d i f y c a t e c h o l a m i n e storage, or enhance c a t e c h o l a m i n e turnover or m e t a b o l i s m . In this regard it w o u l d be interesting to know whether monoamine oxidase activity is increased in the undernourished condition. Given the data suggesting that the specific a c t i v i t y of a number of enzyme systems is i n c r e a s e d by u n d e r n o u r i s h m e n t , it is c o n c e i v a b l e that the reduction in catecholamine levels may be traced to an e n h a n c e m e n t in the m e t a b o l i s m of these substances. Ligand binding assays have revealed that early undernutrition modifies the n u m b e r of catecholamine receptor sites in rat brain (44). When studying adult rats that had been u n d e r n o u r i s h e d during early d e v e l o p m e n t , a significant reduction was found in the number, but not the affinity, of alpha- and beta-adrenergic receptor b i n d i n g sites in h o m o g e n a t e s of whole brain. These authors s u g g e s t e d that the d e c r e a s e in receptor number may be indicative of an increase in presynaptic n o r a d r e n e r g i c activity, though they p r o v i d e no direct e v i d e n c e to support this h y p o t h e s i s . It has also been found that the number of striatal dopamine receptors is significantly reduced in the brains of animals undernourished during suckling (Table I). This finding is particularly noteworthy in light of the previously reported reduction in r e s p o n s e to a p o m o r p h i n e , a direct-acting dopamine receptor agonist, in rats exposed to a p r o t e i n d e f i c i e n t diet during d e v e l o p m e n t (45). Taken together these results suggest that the striatal dopaminergic system is modified by undernutrition. Since the r e d u c t i o n in d o p a m i n e r e c e p t o r b i n d i n g
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may be an adaptive response to a hyperactive nigro-striatal d o p a m i n e system, it is i m p o s s i b l e to c o n c l u d e that the change in r e c e p t o r number reflects a f u n c t i o n a l a l t e r a t i o n in this system. Nevertheless, the decreases in receptor binding for both norepinephrine and d o p a m i n e may be taken as evidence that the reduction in the steady-state concentrations of these neurotransmitters may be s e c o n d a r y to an increase in m o n o a m l n e turnover.
SEROTONIN
The m a j o r i t y of reports suggest that s e r o t o n e r g i c activity is enhanced in the d e v e l o p i n g brain of u n d e r n o u r i s h e d animals. It has been found that u n d e r n o u r i s h m e n t at any stage of life increases brain concentrations of tryptophan, the serotonin precursor (46-48). The increase in brain t r y p t o p h a n is most likely a c o n s e q u e n c e of the increase in the amount of unbound amino acid in blood. It is b e l i e v e d that this increase is due to the liberation of bound t r y p t o p h a n from a l b u m i n by nonesterified fatty acids, the concentrations of which increase in the undernourished state. These fatty acids are known to compete with tryptophan for the albumin binding sites. Moreover, undernourishment reduces the plasma albumin c o n c e n t r a t i o n which further serves to increase the circulating levels of free tryptophan. Early undernutrition increases the activity of tryptophan hydroxylase in brain (33). By 35 days of age brain tryptophan hydroxylase activity is 3-fold greater in undernourished rats than in controls, It is t h e r e f o r e not surprising that most investigators find an increase in the c o n c e n t r a t i o n of brain serotonin in undernourished animals (18,31,41,47-51). This occurs w h e t h e r the u n d e r n o u r i s h m e n t is during the prenatal or immediate postnatal period. While others have found a decrease or no change in s e r o t o n i n content (19,38,40), these d i f f e r e n c e s are p r o b a b l y not due to the type of d e p r i v a t i o n since increases have been reported using both the protein deficiency and dietary r e s t r i c t i o n models. A l t h o u g h increases in s e r o t o n i n content have been observed in virtually every brain region examined, the midbrain, brain stem and c e r e b e l l u m c o n s i s t e n t l y show the greatest enhancement in serotonin content. Interestingly, brain serotonin levels remain elevated into adulthood if the animals are c o n t i n u o u s l y undernourished from birth (41,51), and the s e r o t o n i n synthesis rate remains high even if the animals were only deprived for a brief period during infancy (50). Brain s e r o t o n i n content can also be elevated in at least some regions of adult brain by dietary restriction, indicating that the change in n e u r o t r a n s m i t t e r content is not strictly a function of d e v e l o p m e n t (36). The concentration of the serotonin metabolite 5hydroxyindoleacetic acid is also increased in undernourished animals ( 1 8 , 3 1 , 3 6 , 4 1 , 4 6 - 4 8 ) . This may be taken as evidence of an increase in s e r o t o n i n turnover, though it does not necessarily
Vol.
35, No. 21,
prove However, likelihood release
Undernutrition
1984
there is an increase when viewed in their undernutrition that of brain serotonin.
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in serotonergic transmission. entirety, these data suggest increases the synthesis
Y -AMINOBUTYRIC
ACID
the and
( GABA)
There have been few studies investigating the influence of undernutrition on the GABAerg ic system. Available evidence suggests that dietary restriction, during either the neonatal period or immediately following weaning, causes a reduction in the activity of glutamic acid decarboxylase (GAD), the enzyme primarily responsible for the synthesis of brain GABA (14,521. Regional studies indicate that the reduction in GAD activity is found in all brain areas except for the cerebellum and brainstem, and that enzyme activity begins to normalize with time even though the animals are continued on a restricted diet (15). This suggests that undernutrition delays rather than arrests the development of this enzyme. Indeed, GAD activity is completely rehabilitated in undernourished animals placed on a normal diet at a time when GAD activity is known to be reduced (15,52,53). These data suggest that undernutrition causes a transient decrease in GABA synthesis. IU contrast, undernourishment appears to have no effect on GABA transaminase, the enzyme responsible for the metabolism of this neurotransmitter (54). Significant changes in GABA receptor binding have been noted in rat brain following early undernutrition, with GABA binding being significantly increased in the cerebral cortex, corpus hypothalamus (Table 1) (55). striatum, midbrain and These may be related to the possible diminution in GABAergic increases activity resulting from the decrease in GAD activity. Saturation the change in binding was due to an analysis revealed that number of receptors, with no alteration in the increase in affinity. model of different using a obtained data were Similar that (55). In this case it was found undernutrition undernutrition enhances binding by increasing the availability of receptors. affinity sites and the number of low affinity high were were abolished when undernourished rats differences These that suggested It was a normal diet for 4 weeks. exposed to substance some may decrease the production of undernourishment GABAsuch as the higher affinity site, masks normally that the data indicate that In either case, modulin or GABA itself. undernourished the sensitive in be more GABA receptors may synthesis perhaps as a consequence of a decrease in the animal, data are no there As transmitter. of release yet or apparent these consequences of functional the demonstrating receptor modifications. CONCLUSIONS While
there
are
several
forms
and
different
degrees
of
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undernutrition, it would appear that any type of dietary restriction has an effect on rat brain neurochemistry if it occurs during the p e r i o d of rapid brain d e v e l o p m e n t . Given the n u m b e r of p o s s i b l e v a r i a b l e s , it is not s u r p r i s i n g that there are some conflicting data as to the type of neurochemical changes w h i c h may occur. What is certain, however, is that w h i l e there are physiologic mechanisms protecting the brain against radical shifts in the c o n c e n t r a t i o n of important nutrients, u n d e r n u t r i t i o n can alter the n e u r o c h e m i c a l b a l a n c e in brain. In this regard it is i n t e r e s t i n g that not all transmitter systems appear to be s i m i l a r l y altered. While the results suggest that cholinergic and G A B A e r g i c t r a n s m i s s i o n may be reduced in the d e v e l o p i n g brain, c a t e c h o l a m i n e and s e r o t o n i n systems would seem to be more active in the u n d e r n o u r i s h e d state than in controls. This implies that u n d e r n u t r i t i o n has a s e l e c t i v e effect on brain n e u r o t r a n s m i t t e r activity, w h i c h may express itself in a v a r i e t y of ways. Of p a r t i c u l a r note is the finding that m a n y of the neurochemical perturbations can be r e v e r s e d if the animal is placed on a proper diet, suggesting that the neurochemical changes are not n e c e s s a r i l y p e r m a n e n t . Up to now most of the work in this area has been primarily descriptive. Given these data, however, it may be p o s s i b l e to f o r m u l a t e t e s t a b l e h y p o t h e s i s r e l a t i n g to m e c h a n i s t i c issues. For example, it would be of value to k n o w w h e t h e r the changes in enzyme a c t i v i t y are due to a change in the production of these p r o t e i n s or to the s y n t h e s i s of an a l t e r e d form of the enzyme. Are the r e c e p t o r b i n d i n g changes always s e c o n d a r y to an alteration in t r a n s m i t t e r activity, or might they be due to an alteration in the production or degradation of receptor molecules? Which nutritional factor is most important for regulating these neurochemical events? At what stage of d e v e l o p m e n t is the system most s u s c e p t a b l e to m o d i f i c a t i o n ? The answers to these and other q u e s t i o n s may provide fundamental i n f o r m a t i o n about brain n e u r o c h e m i s t r y and d e v e l o p m e n t a l biology, as well as yield insights into the b i o l o g i c a l basis of brain dysfunction following undernutrition.
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