Protein malnutrition during gestation alters brain development and produces specific behavioral and cognitive changes that persist into adulthood and increase the risks of neuropsychiatric disorders. Given evidence for the role of the prefrontal cortex in such diseases, it is significant that studies in humans and animal models have shown that prenatal protein malnutrition specifically affects functions associated with prefrontal cortex. However, the neural basis underlying these changes is unclear. In the current study, prenatally malnourished and control rats performed a sustained attention task with an unpredictable distractor, a task that depends on intact prefrontal cortical function. Radiolabeled 2-deoxyglucose was used to measure neural and brain network activity during the task. Results confirmed that adult prenatally malnourished rats were more distractible than controls and exhibited lower functional activity in prefrontal cortices. Thus, prefrontal activity was a predictor of task performance in controls but not prenatally malnourished animals. Instead, prenatally malnourished animals relied on different brain networks involving limbic structures such as the hippocampus. These results provide evidence that protein reduction during brain development has more wide-reaching effects on brain networks than previously appreciated, resulting in the formation of brain networks that may reflect compensatory responses in prenatally malnourished brains.
There is increasing evidence that the maternal environment exerts enduring influences on the fetal brain. In response to certain environmental stimuli such as reduced protein content, the fetus changes the course of its brain development, which leads to specific and programed changes in brain anatomy and physiology. These alterations produce a brain with a fundamentally altered organization, which then translates to alterations in adult cognitive function. The effects on brain and behavior may be linked, such that a prenatal stimulus relays a signal to alter brain development and encourage the selection and development of brain circuits and behaviors that would be beneficial for the environment in which the animal was anticipated to emerge. At the same time, the signal would deselect behaviors unlikely to be adaptive. We draw on evidence from rodent models to suggest that the brain that develops after a reduction in protein during the prenatal phase is not uniformly dysfunctional, but simply different. This perspective has implications for the role of prenatal factors in the production and expression of behavior, and may account for the elevation of risk factors for neurological and psychiatric illnesses.
Exposure to prenatal protein malnutrition (PPM) leads to a reprogramming of the brain, altering executive functions involving the prefrontal cortex (PFC). In this study we used microdialysis to assess the effects of PPM on extracellular concentrations of norepinephrine (NE), dopamine (DA) and serotonin (5-HT) bilaterally in the ventral portion of the medial prefrontal cortex (vmPFC; ventral prelimbic and infralimbic cortices) of adult Long-Evans rats. Female Long-Evans rats were fed either a low protein (6%) or adequate protein diet (25%) prior to mating and throughout pregnancy. At birth, all litters were culled and fostered to dams fed a 25% (adequate) protein diet. At 120 days of age, 2 mm microdialysis probes were placed into left and right vmPFC. Basal extracellular concentrations of NE, DA, and 5-HT were determined over a 1-h period using HPLC. In rats exposed to PPM there was a decrease in extracellular concentrations of NE and DA in the right vmPFC and an increase in the extracellular concentration of 5-HT in the left vmPFC compared to controls (prenatally malnourished: = 10, well-nourished: = 20). Assessment of the cerebral laterality of extracellular neurotransmitters in the vmPFC showed that prenatally malnourished animals had a significant shift in laterality from the right to the left hemisphere for NE and DA but not for serotonin. In a related study, these animals showed cognitive inflexibility in an attentional task. In animals in the current study, NE levels in the right vmPFC of well-nourished animals correlated positively with performance in an attention task, while 5-HT in the left vmPFC of well-nourished rats correlated negatively with performance. These data, in addition to previously published studies, suggest a long-term reprogramming of the vmPFC in rats exposed to PPM which may contribute to attention deficits observed in adult animals exposed to PPM.
Exposure to malnutrition early in development increases likelihood of neuropsychiatric disorders, affective processing disorders, and attentional problems later in life. Many of these impairments are hypothesized to arise from impaired development of the prefrontal cortex. The current experiments examine the impact of prenatal malnutrition on the noradrenergic and cholinergic axons in the prefrontal cortex to determine if these changes contribute to the attentional deficits seen in prenatal protein malnourished rats (6% casein vs. 25% casein). Because prenatally malnourished animals had significant decreases in noradrenergic fibers in the prelimbic cortex with spared innervation in the anterior cingulate cortex and showed no changes in acetylcholine innervation of the prefrontal cortex, we compared deficits produced by malnutrition to those produced in adult rats by noradrenergic lesions of the prelimbic cortex. All animals were able to perform the baseline sustained attention task accurately. However, with the addition of visual distractors to the sustained attention task, animals that were prenatally malnourished and those that were noradrenergically lesioned showed cognitive rigidity, i.e., were less distractible than control animals. All groups showed similar changes in behavior when exposed to withholding reinforcement, suggesting specific attentional impairments rather than global difficulties in understanding response rules, bottom-up perceptual problems, or cognitive impairments secondary to dysfunction in sensitivity to reinforcement contingencies. These data suggest that prenatal protein malnutrition leads to deficits in noradrenergic innervation of the prelimbic cortex associated with cognitive rigidity.
Adolescence is a period during which many aspects of executive function are maturing. Much of the literature has focused on discrepancies between sub-cortical and cortical development that is hypothesized to lead to over-processing of reinforcement related stimuli unchecked by fully matured response inhibition. Specifically, maturation of sub-cortical dopaminergic systems that terminate in the nucleus accumbens has been suggested to occur prior to the full maturation of corticopetal dopaminergic systems. However, converging evidence supports the hypothesis that many aspects of cognitive control are critically linked to cortical noradrenergic systems, that the effectiveness of drugs used to treat disorders of executive function, e.g. ADHD, may result primarily from increases in cortical norepinephrine (NE) and that cortical noradrenergic systems mature across adolescence. However, little attention has been given to the development of this system during adolescence or to its influence in executive function. In the present paper, we discuss the developmental trajectory of the noradrenergic system of the forebrain, highlight the interactions between noradrenergic and dopaminergic systems, and highlight the contribution of the immature corticopetal noradrenergic systems in the ontogeny of several aspects of executive function. Finally we compare data from adolescent rats to those gathered after selective depletion of NE in sub-regions of the prefrontal cortex with an emphasis on the similarities in performance of NE lesioned rats and adolescents.
Exposure to general anesthetic agents during development has been associated with neurotoxicity and long-term behavioral impairments in rodents and non-human primates. The phenotype of anesthetic-induced cognitive impairment has a robust learning and memory component, however less is known about other psychological domains. Data from retrospective human patient studies suggest that children undergoing multiple procedures requiring general anesthesia are at increased risk of attention deficit hyperactivity disorder. We therefore assessed whether single or repeated exposures of neonatal rats to general anesthesia caused long-term attentional impairments. Female or male Long-Evans pups were exposed to 2.5% sevoflurane for 2h on postnatal day (P) 7, or for 2h each on P7, P10 and P13. Rats were behaviorally tested in late adolescence on the sustained attention task and on the attentional set shifting task. There was no compelling evidence for anesthetic-induced impairment in attentional processing in adult rats exposed to general anesthesia as neonates. These results suggest that, at least at the developmental stage tested here, the phenotype of anesthetic-induced cognitive impairment does not involve disruptions to attentional processing.
BACKGROUND: Early childhood malnutrition affects 113 million children worldwide, impacting health and increasing vulnerability for cognitive and behavioral disorders later in life. Molecular signatures after childhood malnutrition, including the potential for intergenerational transmission, remain unexplored.
METHODS: We surveyed blood DNA methylomes (~483,000 individual CpG sites) in 168 subjects across two generations, including 50 generation 1 individuals hospitalized during the first year of life for moderate to severe protein-energy malnutrition, then followed up to 48 years in the Barbados Nutrition Study. Attention deficits and cognitive performance were evaluated with the Connors Adult Attention Rating Scale and Wechsler Abbreviated Scale of Intelligence. Expression of nutrition-sensitive genes was explored by quantitative reverse transcriptase polymerase chain reaction in rat prefrontal cortex.
RESULTS: We identified 134 nutrition-sensitive, differentially methylated genomic regions, with most (87%) specific for generation 1. Multiple neuropsychiatric risk genes, including COMT, IFNG, MIR200B, SYNGAP1, and VIPR2 showed associations of specific methyl-CpGs with attention and IQ. IFNG expression was decreased in prefrontal cortex of rats showing attention deficits after developmental malnutrition.
CONCLUSIONS: Early childhood malnutrition entails long-lasting epigenetic signatures associated with liability for attention and cognition, and limited potential for intergenerational transmission.
OBJECTIVE: This study assessed the effect of varying prenatal protein levels on the development of homing behavior in rat pups.
METHODS: Long-Evans rats were fed one of the four isocaloric diets containing 6% (n = 7 litters), 12% (n = 9), 18% (n = 9), or 25% (n = 10) casein prior to mating and throughout pregnancy. At birth, litters were fostered to well-nourished control mothers fed a 25% casein diet during pregnancy, and an adequate protein diet (25% casein) was provided to weaning. On postnatal days 5, 7, 9, 11, and 13, homing behaviors, including activity levels, rate of successful returns to the nest quadrant and latencies to reach the nest over a 3-minute test period were recorded from two starting positions in the home cage. Adult body and brain weights were obtained at sacrifice (postnatal day 130 or 200).
RESULTS: Growth was impaired in pups whose mothers were fed a 6% or, to a lesser extent, a 12% casein diet relative to pups whose mothers were fed the 18 and 25% casein diets. The 6 and 12% prenatal protein levels resulted in lower activity levels, with the greatest reduction on postnatal day 13. However, only the 6% pups had reduced success and higher latencies in reaching the nest quadrant when compared with pups from the three other nutrition groups. Latency in reaching the nest quadrant was significantly and negatively associated with adult brain weight.
DISCUSSION: Home orientation is a sensitive measure of developmental deficits associated with variations in prenatal protein levels, including levels of protein deficiency that do not lead to overt growth failure.
Converging evidence supports the hypothesis that the prefrontal cortex is critical for cognitive control. One prefrontal subregion, the anterior cingulate cortex, is hypothesized to be necessary to resolve response conflicts, disregard salient distractors and alter behavior in response to the generation of an error. These situations all involve goal-oriented monitoring of performance in order to effectively adjust cognitive processes. Several neuropsychological disorders, e.g., schizophrenia, attention deficit hyperactivity and obsessive compulsive disorder, are accompanied by morphological changes in the anterior cingulate cortex. These changes are hypothesized to underlie the impairments on tasks that require cognitive control found in these subjects. A novel conflict monitoring task was used to assess the effects on cognitive control of excitotoxic lesions to anterior cingulate cortex in rats. Prior to surgery all subjects showed improved accuracy on the second of two consecutive, incongruent trials. Lesions to the anterior cingulate cortex abolished this. Lesioned animals had difficulty in adjusting cognitive control on a trial-by-trial basis regardless of whether cognitive changes were increased or decreased. These results support a role for the anterior cingulate cortex in adjustments in cognitive control.
Prenatal protein malnutrition (PPM) in rats causes enduring changes in brain and behavior including increased cognitive rigidity and decreased inhibitory control. A preliminary gene microarray screen of PPM rat prefrontal cortex (PFC) identified alterations in KCNJ3 (GIRK1/Kir3.1), a gene important for regulating neuronal excitability. Follow-up with polymerase chain reaction and Western blot showed decreased KCNJ3 expression in the PFC, but not hippocampus or brainstem. To verify localization of the effect to the PFC, baseline regional brain activity was assessed with (14)C-2-deoxyglucose. Results showed decreased activation in the PFC but not hippocampus. Together these findings point to the unique vulnerability of the PFC to the nutritional insult during early brain development, with enduring effects in adulthood on KCNJ3 expression and baseline metabolic activity.
The variability of walking gait timing increases with age and is strongly related to fall risk. The purpose of the study was to examine the interaction of age, cognitive function, and gait performance during dual-task walking. Forty-two, healthy men and women, 50-80 years old, completed the Mini-Mental State Exam (MMSE) and Trail Making Test (TMT) to assess cognitive performance and were separated into groups by decade of life. They then performed dual-task walking, at a self-selected pace, on an instrumented treadmill during three cognitive loading conditions: (1) no cognitive load, (2) subtraction from 100 by 1s, and (3) subtraction from 100 by 3s. The treadmill recorded spatiotemporal gait parameters that were used to calculate the mean and coefficient of variation for each variable over ten strides. Time to complete the TMT was positively correlated with age, stride time, double-limb support time, and mediolateral instability and was inversely correlated with single-limb support time. Subjects in their 70s increased their stride time and double-limb support time during the most challenging dual-task condition (subtraction by 3s), whereas subjects in their 50s and 60s did not. Across conditions, the variability of stride length, stride time, and single-limb support time was greatest in the 70s. Mediolateral instability increased only for subjects in their 70s in the subtraction by 3s condition. Reduced cognitive function with age makes it difficult for older adults to maintain a normal, rhythmical gait pattern while performing a cognitive task, which may place them at greater risk for falling.
OBJECTIVE: To compare neuropsychological profiles of adults who had experienced an episode of moderate to severe protein-energy malnutrition confined to the first year of life with that of a healthy community comparison group.
METHOD: We assessed neuropsychological functioning in a cohort of Barbadian adults, all of whom had birth weight >2268 g. The previously malnourished group (N = 77, mean age = 38 years, 53% male) had been hospitalized during the first year of life for moderate to severe protein energy malnutrition and subsequently enrolled in a program providing nutrition education, home visits and subsidized foods to 12 years of age. They also had documented, adequate nutrition throughout childhood and complete catch-up in growth by the end of adolescence. The healthy comparison group (N = 59, mean age = 38 years, 54% male) were recruited as children from the same classrooms and neighborhoods.
RESULTS: Adjusted for effects of standard of living during childhood and adolescence and current intellectual ability level, there were nutrition group differences on measures of cognitive flexibility and concept formation, as well as initiation, verbal fluency, working memory, processing speed, and visuospatial integration. Behavioral and cognitive regulation were not affected.
CONCLUSIONS: Postnatal malnutrition confined to the first year of life is associated with neurocognitive compromise persisting into midlife. Early malnutrition may have a specific neuropsychological signature, affecting response initiation to a somewhat greater extent than response inhibition.
Globally, over 25% of all children under the age of 5 years experience malnutrition leading to cognitive and emotional impairments that can persist into adulthood and beyond. We use a rodent model to determine the impact of prenatal protein malnutrition on executive functions in an attentional set-shifting task and metabolic activity in prefrontal cortex (PFC) subregions critical to these behaviors. Long-Evans dams were provided with a low (6% casein) or adequate (25% casein) protein diet 5 weeks before mating and during pregnancy. At birth, the litters were culled to 8 pups and fostered to control dams on the 25% casein diet. At postnatal day 90, prenatally malnourished rats were less able to shift attentional set and reverse reward contingencies than controls, demonstrating cognitive rigidity. Naive same-sexed littermates were assessed for regional brain activity using the metabolic marker (14)C-2-deoxyglucose (2DG). The prenatally malnourished rats had lower metabolic activity than controls in prelimbic, infralimbic, anterior cingulate, and orbitofrontal cortices, but had comparable activity in the nearby piriform cortex and superior colliculus. This study demonstrates that prenatal protein malnutrition in a well-described animal model produces cognitive deficits in tests of attentional set shifting and reversal learning, similar to findings of cognitive inflexibility reported in humans exposed to early childhood malnutrition.
OBJECTIVE: To compare neuropsychological profiles of adults who had experienced an episode of moderate to severe protein-energy malnutrition confined to the first year of life with that of a healthy community comparison group. METHOD: We assessed neuropsychological functioning in a cohort of Barbadian adults, all of whom had birth weight >2268 g. The previously malnourished group (N = 77, mean age = 38 years, 53% male) had been hospitalized during the first year of life for moderate to severe protein energy malnutrition and subsequently enrolled in a program providing nutrition education, home visits and subsidized foods to 12 years of age. They also had documented, adequate nutrition throughout childhood and complete catch-up in growth by the end of adolescence. The healthy comparison group (N = 59, mean age = 38 years, 54% male) were recruited as children from the same classrooms and neighborhoods. RESULTS: Adjusted for effects of standard of living during childhood and adolescence and current intellectual ability level, there were nutrition group differences on measures of cognitive flexibility and concept formation, as well as initiation, verbal fluency, working memory, processing speed, and visuospatial integration. Behavioral and cognitive regulation were not affected. CONCLUSIONS: Postnatal malnutrition confined to the first year of life is associated with neurocognitive compromise persisting into midlife. Early malnutrition may have a specific neuropsychological signature, affecting response initiation to a somewhat greater extent than response inhibition.
BACKGROUND: Psychostimulants improve a variety of cognitive and behavioral processes in patients with attention-deficit/hyperactivity disorder (ADHD). Limited observations suggest a potentially different dose-sensitivity of prefrontal cortex (PFC)-dependent function (narrow inverted-U-shaped dose-response curves) versus classroom/overt behavior (broad inverted U) in children with ADHD. Recent work in rodents demonstrates that methylphenidate (MPH; Ritalin) elicits a narrow inverted-U-shaped improvement in performance in PFC-dependent tests of working memory. The current studies first tested the hypothesis that PFC-dependent tasks, in general, display narrow dose sensitivity to the beneficial actions of MPH.
METHODS: The effects of varying doses of MPH were examined on performance of rats in two tests of PFC-dependent cognition, sustained attention and attentional set shifting. Additionally, the effect of pretreatment with the α₁-antagonist prazosin (.5 mg/kg) on MPH-induced improvement in sustained attention was examined.
RESULTS: MPH produced a broad inverted-U-shaped facilitation of sustained attention and attentional set shifting. Prior research indicates α₁-receptors impair, whereas α₂-receptors improve, working memory. In contrast, attentional set shifting is improved with α₁-receptor activation, whereas α₂-receptors exert minimal effects in this task. Given the similar dose sensitivity of sustained attention and attentional set-shifting tasks, additional studies examined whether α₁-receptors promote sustained attention, similar to attentional set shifting. In these studies, MPH-induced improvement in sustained attention was abolished by α₁-receptor blockade.
CONCLUSIONS: PFC-dependent processes display differential sensitivity to the cognition-enhancing actions of psychostimulants that are linked to the differential involvement of α₁- versus α₂-receptors in these processes. These observations have significant preclinical and clinical implications.
As neuropsychiatric disorders such as schizophrenia, attention deficit disorder, and mood disorders all impact executive function and are likely to be diagnosed prior to adulthood, it is important to understand the normal ontogeny of executive function. Previous behavioral research has shown that adolescents' executive function is different than that of adults. In the present study, we use a previously validated cognitive test, the intradimensional/extradimensional (ID/ED) set-shifting task, to assess attentional set shifting and reversal learning in adolescent and adult, male, Long-Evans rats. These data suggest that adolescent rats are more cognitively rigid than adult rats and have impairments in the shifting, but not formation, of an attentional set. Adolescent rats are also more susceptible to distraction than adult rats when an irrelevant stimulus dimension is introduced as part of a complex stimulus. Moreover, we find that attentional set shifting becomes adult-like at an earlier age than reversal learning. As these functions are mediated by distinct prefrontal subregions, that is, the prelimbic and orbitofrontal cortices, respectively, we hypothesize that prefrontal cortical subregions show slightly different developmental trajectories.
Adolescent rats show immaturities in executive function and are less able than adult rats to learn reinforcement reversals and shift attentional set. These two forms of executive function rely on the functional integrity of the orbitofrontal and prelimbic cortices respectively. Drugs used to treat attention deficit disorder, such as atomoxetine, that increase cortical catecholamine levels improve executive functions in humans, non-human primates and adult rats with prefrontal lesions. Cortical noradrenergic systems are some of the last to mature in primates and rats. Moreover, norepinephrine transporters (NET) are higher in juvenile rats than adults. The underdeveloped cortical noradrenergic system and higher number of NET are hypothesized to underlie the immaturities in executive function found in adolescents. We assessed executive function in male Long-Evans rats using an intra-dimensional/extradimensional set shifting task. We administered the NET blocker, atomoxetine (0.0, 0.1, 0.9 mg/kg/ml; i.p.), prior to the test of attentional set shift and a reinforcement reversal. The lowest dose of drug facilitated attentional set shifting but had no effect on reversal learning. These data demonstrate that NET blockade allows adolescent rats to more easily perform attentional set shifting.
BACKGROUND: The majority of studies assessing executive function in attention deficit disorder (ADD) have shown deficits in attentional set shifting using either the Wisconsin card sorting task or the intra-dimensional/extra-dimensional set-shifting task (ID/ED). Damage to the prefrontal cortex in humans, primates, and rodents impairs extra-dimensional (ED) shifts. Noradrenergic depletion of the medial prefrontal cortex in rats is sufficient to impair attentional set shifting. Atomoxetine, a selective norepinephrine (NE) re-uptake inhibitor, is hypothesized to produce beneficial effects in patient with ADD by augmenting NE release in prefrontal cortex.
MATERIALS AND METHODS: We assessed the effects of systemic administration of atomoxetine (0.0, 0.1, 0.3, and 0.9 mg/kg/ml) in normal and noradrenergically lesioned (NE-LX) rats on attentional-set shifts. We replicated findings showing NE-LX rats are selectively impaired on the ED shifts but not reversals or other discriminations.
RESULTS: Atomoxetine remediated the attentional set-shifting impairments in NE-LX rats but impaired ED performance of non-lesioned rats.
DISCUSSION: Though atomoxetine is neurochemically selective, it is not wholly specific at doses >0.3 mg/kg. All doses of the drug were similar in their efficacy in reversing the ED deficit, but the effectiveness of the 0.1 mg/kg dose supports the hypothesis that increases in prefrontal NE alone are sufficient to improve attention in NE-LX rats. Moreover, the detrimental effects of the drug in non-lesioned rats support the hypothesis that optimal levels of NE in prefrontal cortex are critical to attentional set shifting with both supra- and sub-optimal levels producing attentional impairments.
The effects of restricted cholinergic deafferentation of prefrontal cortex in rats on sustained attention were assessed. Attentional demands were increased by presentation of distractor stimuli in a different modality (auditory) or the same modality (visual) as target stimuli. Additionally, the effects of the regularity of the distractor on rats' ability to disregard this stimulus were assessed by testing different frequencies of stimuli for each modality. Cholinergically lesioned rats were more sensitive to the effects of auditory distractors than nonlesioned rats, whereas visual distractors of any frequency potently impaired the performance of all subjects. The effects of the auditory stimuli on attentional performance varied depending on the frequency of the tone. A tone with a predictable pattern enhanced signal detection in all rats. An irregular tone selectively impaired performance of rats with cholinergic lesions. Additional tests suggest that rats use the regular tone to time when to attend. Lesioned rats were impaired when the regular tone was presented with a more variable intertrial interval in a subsequent testing session, suggesting impairments in top-down control. In addition to changes in top-down control of attention, differential effects on performance based on the regularity of the tone suggest that stimulus properties encoded by bottom-up processes are also altered after lesioning. The current data suggest that cholinergic deafferentation of prefrontal cortex alters top-down and bottom-up processing of stimuli.
Both norepinephrine and acetylcholine have been shown to be critically involved in mediating attention but there remains debate about whether they serve similar or unique functions. Much of what is known about the role of these neurochemicals in cognition is based on manipulations done at the level of the cell body but these findings are difficult to reconcile with data regarding the unique contribution of cortical subregions, e.g. the dorsolateral prefrontal cortex, to attention. In the current study, we directly compared the effects of noradrenergic and cholinergic deafferentation of the rat medial prefrontal cortex, the homologue of primate dorsolateral prefrontal cortex, using an intradimensional/extradimensional attentional set shifting task, a task previously shown to be able to dissociate the function of the primate dorsolateral prefrontal cortex from orbitofrontal cortex. We found that noradrenergic, but not cholinergic, deafferentation produces specific impairments in the ability to shift attentional set. We also clarified the nature of the attentional deficits by assessing the ability of rats to disregard irrelevant stimuli. Noradrenergic lesions did not alter the ability of rats to ignore irrelevant stimuli, suggesting that the attentional deficit results from an overly focused attentional state that retards learning that a new stimulus dimension predicts reward.