Development of spatial navigation following prenatal cocaine and malnutrition in rats: lack of additive effects

Prenatal protein malnutrition decreases neuron numbers in the parahippocampal region but not prefrontal cortex in adult rats

OBJECTIVE

Prenatal protein malnutrition produces anatomical and functional changes in the developing brain that persist despite immediate postnatal nutritional rehabilitation. Brain networks of prenatally malnourished animals show diminished activation of prefrontal areas and an increased activation of hippocampal regions during an attentional task [1]. While a reduction in cell number has been documented in hippocampal subfield CA1, nothing is known about changes in neuron numbers in the prefrontal or parahippocampal cortices.

METHODS

In the present study, we used unbiased stereology to investigate the effect of prenatal protein malnutrition on the neuron numbers in the medial prefrontal cortex and the cortices of the parahippocampal region that comprise the larger functional network.

RESULTS

Results show that prenatal protein malnutrition does not cause changes in the neuronal population in the medial prefrontal cortex of adult rats, indicating that the decrease in functional activation during attentional tasks is not due to a reduction in the number of neurons. Results also show that prenatal protein malnutrition is associated with a reduction in neuron numbers in specific parahippocampal subregions: the medial entorhinal cortex and presubiculum.

DISCUSSION

The affected regions along with CA1 comprise a tightly interconnected circuit, suggesting that prenatal malnutrition confers a vulnerability to specific hippocampal circuits. These findings are consistent with the idea that prenatal protein malnutrition produces a reorganization of structural and functional networks, which may underlie observed alterations in attentional processes and capabilities.

Differential Effects of Prenatal Protein Malnutrition and Prenatal Cocaine on Radial Arm Maze Performance in Adult Male Rats

The effects of prenatal cocaine and protein malnutrition were examined on acquisition of the radial arm maze in adult male Sprague-Dawley rats whose mothers were provided with a 6% casein, a 25% casein or a standard chow diet and cocaine (30mg/kg) or saline injections beginning 5 weeks prior to mating and continuing to parturition. Rats were tested using an 8-arm radial maze with 4 baited arms and were required to collect all 4 food pellets within 5 min to complete a trial. Subjects were tested for 1 trial/day until they met criterion for successful acquisition of the task. Criterion was attained when the rat collected 3 out of the 4 food pellets within their first 4 arm entries within a trial (while still completing the trial) with this level of performance being maintained for 3 consecutive trials. The results showed dissociation between the effects of prenatal protein malnutrition and prenatal cocaine. Prenatally protein malnourished adult male rats required a greater number of trials to criterion, made more reference memory (but not working memory) errors, and required a longer time to complete each trial when compared with control males. However, rats with prenatal cocaine exposure showed no significant impairments in the radial arm maze. These results contrast with our previous findings using the Morris maze task in which adult male subjects exhibited impaired acquisition following prenatal cocaine while there were no effects following prenatal protein malnutrition. Thus, the radial arm maze and water maze procedures appear to engage different processes that are differentially sensitive to the prenatal insults.

Gestational protein restriction induces CA3 dendritic atrophy in dorsal hippocampal neurons but does not alter learning and memory performance in adult offspring

Studies have demonstrated that nutrient deficiency during pregnancy or in early postnatal life results in structural abnormalities in the offspring hippocampus and in cognitive impairment. In an attempt to analyze whether gestational protein restriction might induce learning and memory impairments associated with structural changes in the hippocampus, we carried out a detailed morphometric analysis of the hippocampus of male adult rats together with the behavioral characterization of these animals in the Morris water maze (MWM). Our results demonstrate that gestational protein restriction leads to a decrease in total basal dendritic length and in the number of intersections of CA3 pyramidal neurons whereas the cytoarchitecture of CA1 and dentate gyrus remained unchanged. Despite presenting significant structural rearrangements, we did not observe impairments in the MWM test. Considering the clear dissociation between the behavioral profile and the hippocampus neuronal changes, the functional significance of dendritic remodeling in fetal processing remains undisclosed.

Sex-dependent cognitive performance in baboon offspring following maternal caloric restriction in pregnancy and lactation

In humans a suboptimal diet during development has negative outcomes in offspring. We investigated the behavioral outcomes in baboons born to mothers undergoing moderate maternal nutrient restriction (MNR). Maternal nutrient restriction mothers (n = 7) were fed 70% of food eaten by controls (CTR, n = 12) fed ad libitum throughout gestation and lactation. At 3.3 ± 0.2 (mean ± standard error of the mean [SEM]) years of age offspring (controls: female [FC, n = 8], male [MC, n = 4]; nutrient restricted: female [FR, n = 3] and male [MR, n = 4]) were administered progressive ratio, simple discrimination, intra-/extra-dimension set shift and delayed matching to sample tasks to assess motivation, learning, attention, and working memory, respectively. A treatment effect was observed in MNR offspring who demonstrated less motivation and impaired working memory. Nutrient-restricted female offspring showed improved learning, while MR offspring showed impaired learning and attentional set shifting and increased impulsivity. In summary, 30% restriction in maternal caloric intake has long lasting neurobehavioral outcomes in adolescent male baboon offspring.

Pre- and/or postnatal protein restriction in rats impairs learning and motivation in male offspring

Suboptimal developmental environments program offspring to lifelong health complications including affective and cognitive disorders. Little is known about the effects of suboptimal intra-uterine environments on associative learning and motivational behavior. We hypothesized that maternal isocaloric low protein diet during pregnancy and lactation would impair offspring associative learning and motivation as measured by operant conditioning and the progressive ratio task, respectively. Control mothers were fed 20% casein (C) and restricted mothers (R) 10% casein to provide four groups: CC, RR, CR, and RC (first letter pregnancy diet and second letter lactation diet), to evaluate effects of maternal diet on male offspring behavior. Impaired learning was observed during fixed ratio-1 operant conditioning in RC offspring that required more sessions to learn vs. the CC offspring (9.4±0.8 and 3.8±0.3 sessions, respectively, p<0.05). Performance in fixed ratio-5 conditioning showed the RR (5.4±1.1), CR (4.0±0.8), and RC (5.0±0.8) offspring required more sessions to reach performance criterion than CC offspring (2.5±0.5, p<0.05). Furthermore, motivational effects during the progressive ratio test revealed less responding in the RR (48.1±17), CR (74.7±8.4), and RC (65.9±11.2) for positive reinforcement vs. the CC offspring (131.5±7.5, p<0.05). These findings demonstrate negative developmental programming effects due to perinatal isocaloric low protein diet on learning and motivation behavior with the nutritional challenge in the prenatal period showing more vulnerability in offspring behavior.

A meta-analysis of animal studies on disruption of spatial navigation by prenatal cocaine exposure

Water-maze testing has been used to assess prenatal cocaine (PCOC)-induced deficits in behavioral studies of spatial navigation and memory abilities. Effects of PCOC in acquisition or in probe trials over water-maze testing days were rarely detected. Despite an absence of effects of PCOC when data were collapsed over multiple days, there was a potential difference when examined during the first day of acquisition training, characterized by a PCOC-associated decrease in learning efficiency but not capacity. Here, we review studies of PCOC-related changes in day-1 water-maze acquisition training and examine the relationship between experimental methodologies and PCOC-treatment procedures and the variability in effect size estimates across studies. The results revealed a significant increase in latencies to goal platform on acquisition training day-1 in PCOC-exposed offspring vs. controls (effect size: r=0.44). Significant effects attributable to variations in the PCOC-treatment procedures across studies were also identified. The moderating variable of PCOC "dose" was significant as lower doses of PCOC exposure yielded larger treatment effects. "Duration" of PCOC exposure was not significant, although a trend for greater effects was observed in studies that employed longer daily treatment schedules or schedules administered in later gestational periods. This analysis identified a consistent difference in acquisition training day-1 of water-maze testing in PCOC-exposed offspring indicating a PCOC-induced deficiency in spatial learning. These findings of impaired spatial learning efficiency are of particular interest given clinical scenarios involving acutely impaired spatial memory and related learning in PCOC-exposed children that highlight the potential consequences in classroom learning.

Global undernutrition during gestation influences learning during adult life

Intrauterine growth restriction can lead to significant long-term health consequences such as metabolic and cardiovascular disorders, but less is known about its effects on choice and behavioral adaptation in later life. Virgin Wistar rats were time mated and randomly assigned to receive either ad-libitum access to chow or 30% of that level of nutrition during pregnancy to generate growth-restricted offspring. At 60 days of age, 6 female offspring from each group were trained on concurrent variable-interval schedules. Sessions consisted of seven randomly arranged concurrent-schedule components, each with a different reinforcer ratio that varied from 27:1 to 1:27, and each component lasting for 10 reinforcer deliveries. Behavioral change across reinforcers in components, measured by sensitivity to reinforcement, was consistently lower for offspring of undernourished mothers, showing that their behavior was less adaptable to environmental change. These results provide direct experimental evidence for a link between prenatal environmental conditions and reduced behavioral adaptability--learning--in later life.

Prenatal intravenous cocaine and the heart rate-orienting response: a dose-response study

Attentional dysfunction is a persistent behavioral abnormality that is emerging as one of the cardinal features in the investigations of the teratogenic effects of cocaine in humans and rodents. The present study sought to extend this work by using a dose-response design with an alternate strain of rat. Virgin Long-Evans female rats, implanted with an IV access port prior to breeding were administered saline, 0.5, 1.0, or 3.0 mg/kg of cocaine HCl from gestational day (GD) GD8-21 (1x per day-GD8-14, 2x per day-GD15-21). Cocaine had no significant effect on maternal or litter parameters. At 14-15 days of age, 1 male and 1 female from each litter were tested to evaluate the heart rate orienting response (HR-OR). Following 20 min for acclimation, pups were presented an olfactory stimulus for 20s per trial, across four trials, and with an intertrial interval of 2 min. The initial baseline HR was not significantly different across the treatment groups, although cocaine did alter the stability of the QRS complex duration. The magnitude of the HR-OR averaged across trials increased as a linear function of dosage of cocaine. A more complex (quadratic) interaction between cocaine dose and sex of the offspring was also noted. When examined across trials, the controls failed to display any significant within-session variation in the HR-OR; in contrast all of the prenatal cocaine treated groups displayed either sensitization (low and high dose) or habituation of the response (middle dose). Analysis of the peak HR-OR confirmed that the controls were indeed displaying the response on at least one trial of the session, albeit not consistently on any specific trial. The more vigorous HR-OR of the prenatal cocaine groups, relative to vehicle controls, most likely reflects an alteration in development of the neural basis of response; as previously shown, the most vigorous response to the olfactory stimulus is seen early (12 days of age) and progressively decreases across the preweaning period. In sum, prenatal exposure to cocaine, at least when administered by the IV route, provides reproducible alterations in attentional processes, as indexed by the noradrenergically-mediated HR-OR. The documentation of a linear dose-response function suggests that there is likely no threshold for the drug-induced alteration. Moreover, the sex of the animal also appears to play some role in the nature of the expression of the altered HR-OR.

Beneficial effects of the sigma1 receptor agonists igmesine and dehydroepiandrosterone against learning impairments in rats prenatally exposed to cocaine

In utero cocaine (IUC) exposure results in offspring rats in complex neurochemical and behavioral alterations, particularly affecting learning and memory processes. We examined here the impact of IUC exposure on memory functions in male and female offspring rats and report that selective sigma(1) (sigma(1)) receptor agonists are effective in reversing the deficits. Dams received a daily cocaine, 20 mg/kg ip, injection between gestational days E17 to E20. Learning was examined in offspring between day P30 and P41 using delayed alternation in the T-maze, water-maze learning and passive avoidance. Both male and female rats prenatally exposed to cocaine showed delayed alternation deficits and impairments of acquisition of a fixed platform position in the water maze, as shown by higher acquisition latencies and diminutions of time spent in the training quadrant during the probe test. The acquisition of a daily changing platform position also demonstrated impaired working memory. Finally, passive avoidance deficits were observed. Pretreatment with the synthetic sigma(1) agonist igmesine (0.1-1 mg/kg ip) or the neuroactive steroid dehydroepiandrosterone (DHEA 10-40 mg/kg ip) reversed the prenatal cocaine-induced learning deficits in offspring rats for each test. The sigma(1) antagonist BD1063 (1 mg/kg ip) failed to affect performances alone but blocked the igmesine and DHEA effects, confirming the involvement of the sigma(1) receptor. IUC exposure thus results in marked memory deficits, affecting spatial and nonspatial short- and long-term memories in juvenile male and female offspring rats. The activation of the sigma(1) neuromodulatory receptor allows a complete behavioral recovery of the memory functions in prenatally cocaine-exposed rats.

Prenatal cocaine exposure disrupts non-spatial, short-term memory in adolescent and adult male rats

The rise in the recreational use of cocaine in the last two decades has resulted in a growing health concern about fetal drug exposure. In exposed children, investigators have noted altered cognitive performance in complex or distracting, but not more controlled, situations. In rodent models, deficits in short-term memory have been noted in some, but not all, paradigms, although these studies also differ in routes of administration and dosing models. Here, we report short-term memory deficits in rats prenatally exposed to cocaine using an intravenous administration model to closer mimic cocaine doses and pharmacokinetics seen with human use. A spontaneous two object recognition task was used to avoid (1) clearly aversive or rewarding components, (2) reference memory component and (3) the use of external motivators, such as swimming stress or food deprivation/rewards. In this task, adolescent and adult male rats exposed to cocaine in utero demonstrated deficits in short-term memory compared with saline controls. No difference in the time spent exploring the objects or the number of failures was noted between the prenatal cocaine and saline rats. This study suggests that prenatal exposure to cocaine can result in a long-lasting deficit in non-spatial, short-term memory in a spontaneously performed task.

Prenatal cocaine exposure increases mesoprefrontal dopamine neuron responsivity to mild stress

Children whose mothers used cocaine during pregnancy appear to have an increased incidence of certain neurobehavioral deficits. Rodent models of prenatal cocaine exposure have mimicked these deficits in the offspring, yet the biochemical basis of the behavioral abnormalities is unknown. We have been able to reproduce short-term memory deficits in our rat intravenous model of prenatal cocaine exposure, and as short-term memory is dependent on the function of dopamine neurons innervating the medial prefrontal cortex, we hypothesized that prenatal cocaine induces a dysfunction in the regulation of this pathway. Here we report that mild footshock stress, which preferentially activates the mesoprefrontal dopamine system, leads to an enhanced increase in dopamine turnover in the ventromedial prefrontal cortex of adolescent (postnatal day 35-37) rats exposed to cocaine in utero, suggesting that the dopamine neurons innervating this region are hyperresponsive in these rats. Thus, this biochemical alteration may be central to some of the cognitive deficits exhibited by offspring that were exposed to cocaine during fetal development.

Effects of neonatal corticosterone treatment on maze performance and HPA axis in juvenile rats

Previous research has indicated that administering corticosterone to dams' drinking water for 21 days produced persistent alterations in physiology and behavior. We investigated whether 4 days of corticosterone exposure would have similar effects, and whether greater effects would be found when corticosterone was administered early in neonatal life than later in neonatal life. Sprague-Dawley dams were given either corticosterone (250 microg/ml) in their water bottles for postnatal days (PND) 5-9 (early corticosterone treatment: ECT), PND 13-17 (late corticosterone treatment: LCT) or no treatment (NT). At the end of treatment, corticosterone levels were higher in pups of corticosterone drinking dams. However, at weaning, ECT and LCT pups had lower basal corticosterone levels than NT pups. As juveniles, ECT pups learned to navigate to a visible and then to a nonvisible platform in a Morris water maze more quickly than did LCT and NT pups. Among females, ECT pups had higher corticosterone release in response to stress than LCT and NT pups. There were no differences in hippocampal corticosteroid receptor levels among the groups. The pattern of results is similar to, but not identical to, that found for pups exposed to corticosterone for 21 days. The results also suggest that there is a critical or sensitive period for corticosterone treatment in that early treatment was more effective than later treatment.

Prenatal exposure to cocaine reduces the number and enhances reactivity of A10 dopaminergic neurons to environmental stress

Prenatal exposure to cocaine has been shown to result in poor cognitive performance in the resulting offspring in humans and laboratory animals. The underlying biochemical changes that contribute to these behavioral effects are not known but have been proposed to involve changes in dopaminergic function. In these studies, we exposed rats to cocaine in utero using the clinically relevant intravenous model and report a mean loss of 24.8% of the tyrosine hydroxylase immunoreactive, presumed dopaminergic, neurons in the A10, but not A9 and A8, cell groups of the young adult offspring. Additionally, in prenatal cocaine-exposed rats dopaminergic neurons in the ventral, midline A10, and lateral A9 regions demonstrated a hyperreactivity to environmental stress, as measured by activation of the immediate-early gene, Fos. Mild, intermittent footshock did not further increase the number of dopamine neurons expressing Fos in prenatal cocaine-exposed rats, as it did in the prenatal saline controls. Because the exposure to cocaine took place during development, other potential changes in dopaminergic and nondopaminergic neuronal systems could result from the cocaine-induced reduction in numbers of A10 dopamine neurons. We hypothesize that a perinatal loss of A10 dopamine neurons, and subsequent developmental changes, contributes to a dysregulation of the adult mesoprefrontal system, resulting in the reported cognitive deficits.

3,4-methylenedioxymethamphetamine (ecstasy)-induced learning and memory impairments depend on the age of exposure during early development

Use of 3,4-methylenedioxymethamphetamine (MDMA; ecstasy) has increased dramatically in recent years, yet little is known about its effects on the developing brain. Neonatal rats were administered MDMA on days 1-10 or 11-20 (analogous to early and late human third trimester brain development). MDMA exposure had no effect on survival but did affect body weight gain during treatment. After treatment, body weight largely recovered to 90-95% of controls. MDMA exposure on days 11-20 resulted in dose-related impairments of sequential learning and spatial learning and memory, whereas neonatal rats exposed on days 1-10 showed almost no effects. At neither stage of exposure did MDMA-treated offspring show effects on swimming ability or cued learning. Brain region-specific dopamine, serotonin, and norepinephrine changes were small and were not correlated to learning changes. These findings suggest that MDMA may pose a previously unrecognized risk to the developing brain by inducing long-term deleterious effects on learning and memory.

Effects of prenatal cocaine on Morris and Barnes maze tests of spatial learning and memory in the offspring of C57BL/6J mice

Cocaine was administered to gravid C57BL/6J mice on embryonic days E8-18 at doses of either 17.5 or 20 mg/kg x 2 per day; controls received equal volumes of vehicle. The two cocaine dose groups were indistinguishable in their effects on maternal weight gain, offspring survival or body weight; therefore, the two groups were combined. Offspring were assessed as adults in straight channel swimming, cued and spatial reference-memory and working memory versions of the Morris water maze (MWM), and in the Barnes spatial maze to escape from a light, tone and fan. Cocaine offspring had shorter latencies in the straight channel and increased cumulative distance from the platform and path length in the spatial version of the Morris maze, but only when the platform size was reduced, not under standard platform conditions. In the working memory test, cocaine offspring showed deficits in acquisition and, following random trials, on relearning during a final test phase. In the Barnes maze, cocaine offspring were delayed in utilizing more efficient search strategies and took longer to find the goal. Taken together, the data suggest that prenatal cocaine induces modest but significant long-term alterations in both reference and working memory-based spatial learning and memory.

Effects of age and gender but not prenatal cocaine on random ratio and delayed spatial alternation responding in rats

This investigation employed a longitudinal analysis of rat operant behavior under two different schedules of reinforcement following prenatal exposure to cocaine. Offspring were derived from four maternal exposure groups: 50 mg/kg cocaine, their pair-fed controls, 25 mg/kg cocaine, and freely fed controls. Cocaine was administered via gavage from gestation day 6-20. A maternal fostering procedure was used. Pairs of male and female littermates were assigned to a 7-, 14-, or 21-month cohort and at the appropriate age were trained to respond on one lever in a two-lever operant chamber. Reinforcement was delivered with a series of random ratio (RR) schedules where the RR value was increased across sessions. After RR training, animals were examined with a delayed spatial alternation (DSA) procedure in the same chambers. Male offspring responded at higher rates than females during high-probability RR schedules, whereas advancing age was associated with lower response rates during low-probability RR schedules in both males and females. Prenatal cocaine exposure exerted only limited effects on RR responding during transition and did not affect DSA behavior. The results of this longitudinal analysis suggest that prenatal cocaine does not exert global or far-reaching learning deficits in prenatally exposed rats.

Chlordiazepoxide-induced spatial learning deficits: dose-dependent differences following prenatal malnutrition

The sensitivity of prenatally protein-malnourished rats to the amnestic properties of the benzodiazepine (BZ) receptor agonist, chlordiazepoxide (CDP), was studied in the male offspring of rats provided with a protein-deficient diet (6% casein) for 5 weeks prior to mating and throughout pregnancy. Rats were tested during acquisition of the submerged platform version of the Morris water maze task using three systemic doses of CDP (3.2, 5.6, and 7.5 mg/kg i.p.) at two ages (day 30 and day 90). At 30 days, prenatally malnourished rats showed less sensitivity to the amnestic effect of the 5.6-mg/kg dose when compared with well-nourished controls by displaying shorter swim paths during acquisition and a more selective search of the target quadrant upon removal of the platform (probe trial). At 90 days, prenatally malnourished rats again showed less sensitivity to CDP at a dose of 5.6 mg/kg, but more sensitivity to the 3.2-mg/kg dose (indicated on the probe trial). No obvious relationship was identified between the nutritional group differences in behavioral sensitivity to CDP at 90 days and their BZ receptor density in the hippocampus or medial septum. It can be concluded that prenatal malnutrition alters the amnestic response to CDP in a dose-dependent and developmentally specific manner, thus providing further support for functional changes within the GABAergic system subsequent to malnutrition.

Prenatal intravenous cocaine adversely affects attentional processing in preweanling rats

Perhaps the sole, clinically reported, deficit in infants of women that abused cocaine (COC) during pregnancy that persists through early childhood is that of an attentional disorder. Using the heart rate orienting response (HR-OR), a putative valid and reliable measure of attention, we examined the offspring of rats exposed to COC in utero via the clinically relevant intravenous (IV) route. Sprague-Dawley females, implanted with IV access ports prior to breeding, were administered saline or 3 mg/kg COC HC1, 1X/day on gestational day (GD) 8-14 and 2X/day on GD15-21. No significant effects of prenatal COC were apparent for maternal or litter parameters. Six pups/litter were tested: one of each sex on postnatal day (PD) 12, PD16, and PD21. Following 20 min of adaptation, pups were exposed to a novel odor (20 s amyl acetate) for a set of four acquisition trials; after a 4-h retention interval, the same procedure was again employed. At PD12, both prenatal COC and control pups demonstrated a significant HR-OR on the acquisition trials and both groups showed significant within-session habituation. Across the 4-h retention interval, prenatal COC-exposed pups showed habituation whereas control pups did not. At PD16, the magnitude of the HR-OR was significantly greater in prenatal COC-exposed pups relative to control pups. Within-session habituation also characterized the HR-OR of the COC, but not control, pups. For the retention data, within-subject and regression analyses suggested the COC-exposed pups displayed greater between and within-session habituation, respectively. At PD21, the prenatal COC-treated pups displayed an HR-OR that did not habituate across acquisition trials; the control pups displayed a significant HR-OR only during the initial 5 s of the first two trials. During the retention trials, regression analyses again suggested the COC-exposed pups displayed greater evidence of within-session habituation. Collectively, these data demonstrate that prenatal exposure to COC alters attention throughout the preweanling period of development. Given the putative role of norepinephrine, but not dopamine or serotonin, in central mediation of the HR-OR of preweanling rats, the effects of prenatal IV COC exposure in this task are consistent with a noradrenergically based attentional disorder.