Malnutrition and reactivity to drugs acting in the central nervous system.

There is a well-established body of data demonstrating that protein or protein-calorie malnutrition experienced early in life is associated with neuroanatomical, neurochemical, as well as behavioral alterations in both animals and humans. A number of studies has focused on the following question: are the neuroanatomical and/or neurochemical changes produced by early malnutrition responsible for the altered behaviors reported in malnourished animals? A tool that has been used to help answer this question is the administration of drugs with specific actions in the various neurotransmitter systems in the central nervous system (CNS). This neuropharmacological approach has produced a considerable amount of data demonstrating that malnourished animals react to drugs differently from controls, suggesting that the altered behavioral expression of these animals could be partly explained by the alterations in the brain function following malnutrition. The present review will provide an overview of the literature investigating the reactivity of malnourished animals to psychoactive drugs acting through GABAergic, catecholaminergic, serotonergic, opioid and cholinergic neurotransmitter systems. Altered responsiveness to psychoactive drugs in malnourished animals may be especially relevant to understanding the consequences of malnutrition in human populations.

Prenatal malnutrition and development of the brain.

In this review, we have summarized various aspects as to how prenatal protein malnutrition affects development of the brain and have attempted to integrate several broad principles, concepts, and trends in this field in relation to our findings and other studies of malnutrition insults. Nutrition is probably the single greatest environmental influence both on the fetus and neonate, and plays a necessary role in the maturation and functional development of the central nervous system. Prenatal protein malnutrition adversely affects the developing brain in numerous ways, depending largely on its timing in relation to various developmental events in the brain and, to a lesser extent, on the type and severity of the deprivation. Many of the effects of prenatal malnutrition are permanent, though some degree of amelioration may be produced by exposure to stimulating and enriched environments. Malnutrition exerts its effects during development, not only during the so-called brain growth spurt period, but also during early organizational processes such as neurogenesis, cell migration, and differentiation. Malnutrition results in a variety of minimal brain dysfunction-type syndromes and ultimately affects attentional processes and interactions of the organism with the environment, in particular producing functional isolation from the environment, often leading to various types of learning disabilities. In malnutrition insult, we are dealing with a distributed, not focal, brain pathology and various developmental failures. Quantitative assessments show distorted relations between neurons and glia, poor formation of neuronal circuits and alterations of normal regressive events, including cell death and axonal and dendritic pruning, resulting in modified patterns of brain organization. Malnutrition insult results in deviations in normal age-related sequences of brain maturation, particularly affecting coordinated development of various cell types and, ultimately, affecting the formation of neuronal circuits and the commencing of activity of neurotransmitter cell types and, ultimately, affecting the formation of neuronal circuits and the commencing of activity of neurotransmitter systems. It is obvious that such diffuse type "lesions" can be adequately assessed only by interdisciplinary studies across a broad range of approaches, including morphological, biochemical, neurophysiological, and behavioral analyses.

Prenatal malnutrition-induced changes in blood pressure: dissociation of stress and nonstress responses using radiotelemetry.

A link between prenatal malnutrition and hypertension in human populations has recently been proposed. Rat models of prenatal malnutrition have provided major support for this theory on the basis of tail-cuff measurements. However, this technique requires restraint and elevated temperature, both potential sources of stress. To determine the effect of prenatal protein malnutrition on blood pressure under nonstress conditions, 24-hour radiotelemetric measurements were taken in the home cage. Male rats born to dams fed a 6% casein diet for 5 weeks before mating and throughout pregnancy were studied in early adulthood (from 96 days of age). During the waking phase of their cycle but not the sleep phase, prenatal malnutrition gave rise to small but significant elevations of diastolic blood pressure and heart rate compared with well-nourished controls. Direct effects of stress on blood pressure responses were determined in a second experiment using an olfactory stressor. Prenatally malnourished rats showed a greater increase in both systolic and diastolic pressures compared with well-nourished controls during the first exposure to ammonia. A different pattern of change of cardiovascular responses was also observed during subsequent presentations of the stressor. These findings of a small baseline increase in diastolic pressure consequent to prenatal malnutrition, but an augmented elevation of both systolic and diastolic pressures after first exposure to stress, suggest the need to reevaluate interpretation of the large elevations in blood pressure previously observed in malnourished animals using the stressful tail-cuff procedure.

Influence of prenatal protein malnutrition on behavioral reactivity to stress in adult rats.

In Experiment 1, adult prenatally protein malnourished and well-nourished male and female rats were tested in an open field after having been subjected to a 15-day regimen of varied uncontrollable and inescapable mild stress (experimental group). Their responses were compared with rats that had not been subjected to the stress regimen (control group). In the control group, females with a history of prenatal malnutrition made significantly fewer entries into the center of the arena than did well-nourished females, suggesting that baseline differences in anxiety exist between the two nutritional groups of females. In addition to open field, die experimental group of animals was also tested in a forced swim test conducted at the beginning (Day 5) and at the end of the stress regimen (Day 15). Significant differences were observed between nutritional groups on Day 15 only: prenatally malnourished males exhibited a lower latency to immobility than well-nourished males, whereas the opposite effect was found in malnourished females. In Experiment 2, separate groups of males were exposed to forced swim on two different occasions without the stress regimen between exposures. A somewhat different pattern of findings was generated. There was no significant difference in the latency to immobility between malnourished and well-nourished rats on the second forced swim. However, malnourished animals showed greater total immobility than the well-nourished controls in the second exposure to forced swim, providing further support for the interpretation that the malnourished males were less affected than well-nourished ones, or adapted more readily to the stress regimen in Experiment 1. Overall these results suggest that the relationship between prenatal malnutrition and stress depends on the level of stress (acute vs. chronic), the type of behavioral measure used to assess its effects, as well as gender.

Prenatal protein malnutrition and working memory performance in adult rats.

The performance of prenatally protein malnourished rats was examined in 2 different tasks of learning and memory beginning at 90 or 160 days of age. In Experiment 1, a rewarded alternation task, run as a spatial working memory procedure on an elevated T-maze, revealed no differences between the formerly malnourished (6/25) and control (25/25) rats when either a minimal or a 20-s inter-trial delay was used. Neither was there a difference when an additional, and conflicting, 'information' run was given to provide a source of proactive interference. In extinction, the 6/25 rats required significantly more sessions to abolish their learned alternation response than the controls. In Experiment 2, an operant equivalent of the T-maze task was applied which allowed greater control over the delay interval and task difficulty. Each trial consisted of a forced information response, for which a randomly selected lever was presented, followed by a free-choice stage, when both levers were presented. The rats were rewarded for pressing the lever not presented at the information stage. The inter-trial interval was always 30 s. When the information response requirement was 10 presses no group differences were found in acquisition of the alternation response or in task performance at delays of 5, 10, 15 or 30 s between information and choice stages. As task difficulty was increased, the performance of the 6/25 rats improved more than that of the 25/25 rats, such that they performed significantly better at the longest delay when the information response requirement was 2 presses. This superior performance of the experimental animals is discussed. No differences in reversal were detected. It was concluded that there is no straight-forward 'hippocampal syndrome' in prenatally malnourished adult rats. Working memory appears largely unaffected, whereas susceptibility to interference and extinction may be modified, depending upon the test parameters employed.

Section of the descending columns of the fornix produces delay- and interference-dependent working memory deficits.

Rats were allocated to one of three surgical treatment groups, and given either sham operations (SHAM), transection of the descending columns of the fornix (DCF), or transection of fibres at the base of the lateral septal nuclei (SC). They were trained on a rewarded alternation task, run as a spatial working memory task on an elevated T-maze, with a minimal intratrial interval. There were no differences between the SHAM and the SC rats. The DCF rats showed an initial impairment in acquisition, but their performance gradually improved to control levels. The imposition of a 20-s intratrial interval impaired choice accuracy in all groups, but the extent of the impairment was significantly greater in the DCF group. Addition of an extra, and conflicting, 'information run', to provide a source of proactive interference, also impaired performance in all groups, but the impairment was again greatest in the DCF group, suggesting that this lesion increases intrusion errors. Subsequent testing in the acquisition of 2-way active avoidance revealed that the DCF lesion facilitated learning with respect to the other two groups. The results indicate that there are clear functional differences between the different outputs from the subiculum, and that the descending subicular output plays a significant role in normal memory.

A phenomenon: left-biassed asymmetrical eye-opening in artificially reared rat pups.

From 4 or 5 postnatal days, rat pups were either reared by their mothers (MR) or artificially reared (AR) without their mothers by infusion of 'milk' through a gastric cannula. In AR, but not MR, pups the left eye was more likely to open first than the right eye (P less than 0.001). The left-biassed eye-opening of AR pups may be a manifestation of stress-induced lateralisation of their brains.

Effects of an every other day rapid kindling procedure in prenatally protein malnourished rats.

Prenatally protein (6/25) rats have been reported to require significantly more stimulations to attain a stage 5 seizure than well-nourished controls (25/25) when using either a traditional or rapid every day, kindling procedure. In the present study, a rapid kindling procedure was utilized where both prenatally malnourished and control rats received every other day perforant path kindling (50 Hz, 10 s train) 12 times a day at 5-min intervals. Using this procedure, stage 5 seizures and a fully state were attained in both nutritional groups at approximately the same rate. It is postulated that it is the every other day component of the present procedure which overcomes seizure-induced inhibition in the 6/25 subjects, thereby allowing them to attain stage 5 seizures at the same rate as controls.

Prenatal protein malnutrition and hippocampal function: partial reinforcement extinction effect.

Developing rats were either malnourished or well-nourished during the prenatal period by feeding their dams diets of low (6% casein) or adequate (25% casein) protein content 5 weeks prior to mating and throughout pregnancy. All pups were well-nourished from the day of birth onwards. Male offspring aged 107 days were gradually reduced to 80% of their free-feeding weight. When weights were stable they were trained to run in an alley for food rewards given on every trial (continuous reinforcement, CR) or on a random 50% of trials (partial reinforcement, PR), then the running response was extinguished. A very clear and similar partial reinforcement extinction effect (PREE) was demonstrated in the previously malnourished and control rats when reward was discontinued. That is, PR groups showed greater persistence as compared with CR groups. During acquisition, however, differences in running speed were observed in the goal section between 6-25 PR and 25-25 PR groups, and between 6-25 CR and 25-25 CR groups. The latter may be due to differences between the nutritional groups in food motivation.

Prenatal malnutrition effect on pyramidal and granule cell generation in the hippocampal formation.

The effects of prenatal malnutrition produced by protein deprivation on the neurogenesis of granule and pyramidal cells in the rat hippocampal formation was investigated by injecting pregnant rats with tritiated thymidine on E12, E16, or E20 and sacrificing the pups on P30. Granule cell neurogenesis was significantly decreased in the pups injected on E20, but not in E12 or E16 groups. There was no effect on the generation of pyramidal cells at the times noted, indicating a differential effect of prenatal malnutrition on the generation of these two different neuronal types in the hippocampal formation.

Effect of prenatal protein deprivation on postnatal granule cell generation in the hippocampal dentate gyrus.

The effect of prenatal malnutrition, produced by protein deprivation, on postnatal neurogenesis of granule cells in the fascia dentata of the rat hippocampal formation was examined by injecting tritiated thymidine on P8 and P15 and sacrificing the pups on P30, or by injecting on P30 and sacrificing on P90. The number of labeled granule cells was significantly decreased in prenatally malnourished rats injected on P8, and unaffected in those injected on P15. In contrast, the number of labeled granule cells in prenatally malnourished rats was significantly increased in animals injected in P30. The study shows that prenatal malnutrition significantly alters the postnatal pattern of granule cell neurogenesis in rat hippocampal formation and that the effect persists despite nutritional rehabilitation at birth.

Effects of early life undernutrition in artificially-reared rats. 2. Subsequent behaviour.

Four groups of rat pups were reared: mother-reared control (well-fed) and undernourished (MRC, MRU) and artificially-reared control and undernourished (ARC, ARU). AR rats were fed expressed rats' milk (days 5-7), mixtures of rats' milk and milk substitute (days 8-16), and milk substitute only (days 17-20) by intermittent infusion via a gastric cannula. Undernutrition, imposed from 5-25 days, was effected initially by underfeeding the mother (MRU) or by infusing restricted quantities of milk (ARU). Weaning was at 21 days and undernutrition from 21-25 days was by restricting the supply of solid food. All rats were fed ad lib from 25 days. From 18 weeks, male rats were subjected to 3 behavioural tests: open field, social behaviour (encounters between rats of like treatment group) and bar-pressing for food (CRF, FR5, VI 1', VI 2' and Extinction). Previously undernourished rats responded at higher rates on FR5, received fewer rewards on VI 2' and displayed more aggression during social interaction. These findings may reflect direct effects of nutrition on brain growth. In the open field and social situations ARU were generally less active than ARC animals, whereas MRU and MRC rats differed little. These results are discussed in terms of MRU pups being maternally "buffered" against the worst effects of undernutrition and ARC rats displaying characteristics of "early stimulated" animals.

Prenatal protein malnutrition affects the social interactions of juvenile rats.

The effects of prenatal protein malnutrition on juvenile social behavior was investigated in male and female rats. Animals were provided with 25% (control) or 6% (low protein) casein diets before and during pregnancy. After birth eight pups in each litter (six males and two females) were fostered to lactating control mothers. After weaning (21 days of age) all animals received a lab chow diet until behavioral testing began at 45 days of age. To assess social interaction, pairs of rats of the same gender, consisting of one malnourished and one control rat, were placed in a familiar rectangular arena on 3 consecutive days. Playful social behavior (pin), nonplayful social behaviors (anogenital sniff, walk-over, side-mount, and allogroom), and nonsocial behavior (rear) were recorded in 10-min sessions. Prenatal malnutrition significantly decreased both playful and nonplayful social behaviors, and increased nonsocial rearing. No significant gender differences were observed. The finding that early social behavior is altered by prenatal malnutrition opens the possibility that such changes may play an important role in determining some of the later behavioral differences described in the adult animal.

Prenatal protein malnutrition affects avoidance but not escape behavior in the elevated T-maze test.

An elevated T-maze was used to study the effects of prenatal protein deficiency on inhibitory avoidance and escape behaviors. Female rats were provided with a 25% (control) or a 6% (low protein) casein diets before and during pregnancy. After birth, eight pups in each litter (six males and two females) were fostered to a lactating well-nourished mother. After weaning (21 days of age) all animals received a lab chow diet. Behavioral testing of these offspring began at 70 days of age. To assess inhibitory avoidance, prenatally malnourished and control rats were placed individually at the end of an enclosed arm in an elevated T-maze (one enclosed and two open arms) and the time taken to emerge from this arm was recorded. The same procedure was repeated in 2 subsequent trials given at 30-s intervals. Thirty seconds after the last of these trials, the rat was placed at the end of one open arm and the time taken to withdraw from this arm was measured, thus estimating escape latency. To assess retention, inhibitory avoidance and escape were measured again 72 h later. Prenatally malnourished males and females did not significantly increase avoidance latency from trials 1-3, in contrast to male and female controls. Only control female rats significantly reduced their avoidance latency on the retention test. No significant differences in escape latency were found between diet groups. These results suggest that prenatal malnutrition results in a reduction of anxiety, and that there are gender-specific responses to this test.

An analysis of spatial navigation in prenatally protein malnourished rats.

Developing rats were either malnourished or adequately nourished during the prenatal period by feeding their dams diets of low (6% casein) or adequate (25% casein) protein content 5 weeks prior to mating and throughout pregnancy. All pups received adequate nutrition from the day of birth onwards. Male offspring were tested in one of two spatial navigation tests in the Morris water tank. In proximal-cue tests (postnatal days 16-20), the position of a platform, which provided a means to escape from swimming, was denoted by an obvious visual cue located directly on the platform. In distal-cue tests (postnatal days 20-27 and adult ages, days 70-71 and days 220-221), the escape platform was submerged below the surface of the water so that the rats were required to use extramaze visual cues to guide them to the platform. Neither proximal-cue nor distal-cue navigation was significantly impaired in the prenatally malnourished rats relative to controls, at any of the ages tested.

Differential effects of prenatal protein malnutrition and prenatal cocaine on a test of homing behavior in rat pups.

The effects of prenatal cocaine exposure and protein malnutrition on orientation to home nest material was assessed in rat pups. Sprague-Dawley dams were fed a diet of low protein content (6% casein), and isocaloric diet of adequate protein content (25% casein, control), or a laboratory chow diet prior to mating and throughout pregnancy. Within each diet group, rats received either cocaine injections (30 mg/kg IP 2 times per week prior to mating and the 30 mg/kg SC daily from day 3 to 18 of pregnancy) or saline injections. All litters were fostered on the day of birth to control mothers fed an adequate diet. On postnatal days 7, 9, and 11, a single pup from each litter (n = 11-15 per treatment) was tested repeatedly in a clean test cage for the rapidity of approach, and level of attraction to their own home (nest) bedding compared with fresh bedding. Prenatal malnutrition and prenatal cocaine exposure each gave rise to independent effects on performance, based upon factor analysis. Prenatal malnutrition, but not prenatal cocaine increased the time taken for rat pups to approach their nest bedding, reduced the time spent on this bedding, decreased the number of entries into the sector containing the home bedding and reduced pup weight. Prenatal cocaine, but not prenatal malnutrition, produced a reduction in activity, but had no effect on pup weight. The lower activity level was most pronounced on postnatal day 7. Surprisingly, interactive effects of prenatal cocaine and prenatal malnutrition were not observed on any behavior examined. Nevertheless, the co-existence of drug addiction and malnutrition in human populations raises the possibility that some of the effects generally attributed to drug exposure may, in fact, be due to malnutrition.

Prenatal protein malnutrition affects exploratory behavior of female rats in the elevated plus-maze test.

To study the effects of prenatal protein deficiency in the exploration of the elevated plus-maze, an ethological procedure was used. Female rats were provided with 25% (control) or with 6% (low-protein) casein diets before and during pregnancy. After birth eight pups in each litter (six males and two females) were fostered to a control mother. After weaning (21 days of age) all animals received a lab chow diet until behavioral testing began at 70 days of age. Individual prenatally malnourished (n = 12) and well-nourished (n = 12) females were placed at the center of the elevated plus-maze and allowed to explore for a 5-min session. One session was given per day for 6 consecutive days. The following variables were recorded: percentage of open arm entries; percentage of time spent in open arms; total arm entries; time in the center platform; latency to first open arm entry; number of attempts to enter an open arm; number of rearings; number of head-dips. The results showed a significant effect of malnutrition on six behaviors (percent open arm entries, percent time spent in open arms, attempts to enter open arms, rearings, head-dips, and latency to first open arm entry) and a significant diet by session interaction on two behaviors (attempts to enter open arms and head-dips). These results indicate increased exploration of the open arms in prenatally malnourished as compared with well-nourished control rats, suggestive of lower anxiety and/or a higher impulsiveness in these animals.

Differences between rats and undernourished preweaning, and controls in learning about a redundant stimulus during acquisition of a conditioned emotional response.

Male rats were undernourished during the first three weeks of life by restricting maternal food consumption. Following nutritional rehabilitation, previously undernourished (PU) and control (C) rats were trained to operate a lever to obtain food reward on a variable interval schedule. When rates of responding had stabilised, the rats were tested for suppression of lever-pressing during the illumination of a light which preceded footshock. There were not differences between PU and C groups in the acquisition of this conditioned emotional response, nor were there differences in suppression when a redundant, tone stimulus was presented contemporaneously with the light to predict shock. When the tone was subsequently tested alone for its ability to suppress lever-pressing it was found to have acquired this property in C, but not in PU animals. In a second experiment, PU and C rats were found not to differ in their response to the tone when it was presented as a novel stimulus, nor in suppression to the tone when it was made the sole predictor of footshock. It was concluded that PU and C rats differed in learning about a stimulus predicting footshock, only when that stimulus was redundant. Among the possible explanations for this behavioural difference between PU and control rats are differences in motivation, curiosity, or strength of conditioning. These possibilities are evaluated within the context of current formal theories of conditioning.

Mating behaviour of male rats following pre- and early postnatal undernutrition: a comparison of two outbred stocks.

Developing rats of two outbred stocks (Lister and Wistar) were either well-nourished or were undernourished by feeding their mothers a restricted quantity of food during pregnancy and lactation. All rats were fed ad lib from 25 days. Beginning at 14 months, the mating behaviour of males was investigated in five tests at weekly intervals using ovariectomised females brought into behavioural oestrus using injections of oestradiol and progesterone. Lister males were tested with Lister females, and Wistar males with Wistar females. Early life undernutrition reduced the numbers of non-penetrative mounts and shortened the latency to ejaculate in rats of the Lister but not the Wistar stock. Mount latency was reduced by early undernutrition in the Wistar stock, but was unaffected in the Lister stock. These results indicate that the effects of early life undernutrition on male mating behaviour differ in detail from one stock of rat to another. In general, however, the results suggest that infantile undernutrition facilitates, rather than impairs, the sexual behaviour of male rats in "middle-age."

Prenatal protein malnutrition and home orientation in the rat.

Home orienting behavior was studied during the first 11 days of life in prenatally protein-malnourished and well-nourished rat pups, both groups having been reared by well-nourished mothers since birth. On all days of testing, mean weights were compromised in the prenatally malnourished pups relative to their well-nourished age controls (body weight deficit = 19-29%). Eye opening was also significantly delayed in the malnourished group. On the test of homing behavior, malnourished pups were significantly impaired in their ability to locate the nest on days 9 and 11. The effect was not related to activity levels of the pups, which were generally similar in the two nutritional groups. Thus, prenatal malnutrition disturbs the development of the homing response in the early postnatal period.