Aversive prenatal stimulation: effects on behavioral, biochemical, and somatic ontogeny in the rat

Determining normal variability in a developmental neurotoxicity test: a report from the ILSI Research Foundation/Risk Science Institute expert working group on neurodevelopmental endpoints

With the implementation of the Food Quality Protection Act in 1996, more detailed evaluations of possible health effects of pesticides on developing organisms have been required. As a result, considerable developmental neurotoxicity (DNT) data have been generated on a variety of endpoints, including developmental changes in motor activity, auditory startle habituation, and various learning and memory parameters. One issue in interpreting these data is the level of variability for the measures used in these studies: excessive variability can obscure treatment-related effects, or conversely, small but statistically significant changes could be viewed as treatment related, when they might in fact be within the normal range. To aid laboratories in designing useful DNT studies for regulatory consideration, an operational framework for evaluating observed variability in study data has been developed. Elements of the framework suggest how an investigator might approach characterization of variability in the dataset; identification of appropriate datasets for comparison; evaluation of similarities and differences in variability between these datasets, and of possible sources of the variability, including those related to test conduct and test design. A case study using auditory startle habituation data is then presented, employing the elements of this proposed approach.

Does prenatal stress affect the motoric development of rat pups?

Pregnant rats were exposed to an acute or a repeated stress (presence of a cat) either at the 10th or the 14th gestational day, and the development of their offspring was studied during the first 2 weeks of life. Motor development was measured by different tests: rooting reflex, vibrissae placing response, righting reflex, negative geotaxis. Other landmarks such as eye opening and spontaneous locomotor activity were also recorded. The results showed that, except for the rooting reflex which was most often enhanced (while not significantly) in prenatally stressed rats, the development of the vibrissae placing response, the righting reflex and the negative geotaxis behavior was delayed in the offspring of dams stressed at the 10th gestational day and not (or almost not) in the offspring of dams stressed at the 14th gestational day, the delay being more severe when the prenatal stress was repeated than when it was acutely administered. The spontaneous motor activity was also altered in repeatedly prenatally stressed rats, whatever the day of pregnancy when it was administered, while it was unaffected in acutely prenatally stressed animals. The delay in motor reflexes development was interpreted as alterations in maturation of nervous structures sustaining motor skills, while permanent decrease of spontaneous motor activity was explained by emotional and motivational alterations due to prenatal stress.

Effects of perinatal visual stimulation on preference, growth, and mortality in African clawed frogs (Xenopus Laevis)

Two exploratory experiments examined the effects of flashing light stimulation on growth, mortality, and behavioral preferences of Xenopus laevis tadpoles. Experiment 1 showed that tadpoles exposed to continuous visual stimulation, from egg-laying through postnatal day 40, had significantly higher mortality rates and weighed significantly less than controls. In contrast to controls, experimental tadpoles showed a preference for visual stimulation throughout early development. Results support the notion that augmented visual stimulation during early development affects species-typical development and the creation of postnatal preferences. Experiment 2 exposed subjects to propranolol in their water to investigate a potential sympathetic nervous system (SNS) mechanism responsible for the previous results. Tadpoles exposed to propranolol and visual stimulation simultaneously did not show a preference for the visual stimulation. Although this preliminary finding suggests SNS involvement, this notion deserves further investigation.

Prenatal cocaine and/or nicotine exposure produces depression and anxiety in aging rats

The adult use of cocaine and nicotine has been linked to depression and/or anxiety. Changes in emotional behavior were assessed using behavioral paradigms developed as animal analogs of psychiatric disorders in 12-14 month old Sprague-Dawley rats exposed daily on gestational days 8-20 to cocaine and nicotine, either alone or in combination. Results from the elevated plus maze (EPM), used to assess anxiety-related behaviors, indicated that offspring prenatally exposed to either high-dose cocaine (40 mg/kg/day) or high-dose nicotine (5.0 mg/kg/day) were less timid/more impulsive. Animals from these two groups spent the most time on the open arms, and had the highest percentage of entries into the open arms of the EPM. Combined in utero exposure to cocaine and nicotine nullified these effects. Cocaine challenge (20 mg/kg) did not interact with prenatal treatment, but increased activity on all arms of the EPM in all groups. Sucrose preference was used as a measure of anhedonia, a cardinal symptom of depressive illness. Reduced sucrose preference was seen only in the group of offspring prenatally exposed to high-dose cocaine (40 mg/kg) plus low-dose nicotine (2.5 mg/kg/day). Exposure to a water-deprivation stress normalized sucrose preference in this group, without altering preference or intake in the other prenatal treatment groups. Transient hyperactivity was seen in the offspring of dams treated with high-dose nicotine, an effect that was again reversed in combined drug groups. Traditional gender differences in activity levels and sucrose intake, that is, females greater than males, were still evident in this population of aging rats. These data indicate that prenatal exposure to cocaine and/or nicotine has long-term effects on emotional behavior. Combined drug exposure contributed to the development of depressive symptoms, but not anxiety-like behavior, in a dose-dependent manner. In contrast, exposure to high doses of either drug alone reduced cautionary behavior. Data from this line of research could provide insight into the pathogenesis of emotional disorders, especially during the aging process.

Effects of pre- and postnatal stimulation on developmental, emotional, and cognitive aspects in rodents: a review

Interactions between the organism and its environment, during pregnancy as well as during the postnatal period, can lead to important neurobehavioral changes. We briefly review the literature, and successively present the main results from our laboratory concerning the behavioral effects of prenatal stress, differential rearing conditions, and postnatal handling. We show that submitting primiparous DA/HAN rats to an acute emotional stress (exposure to a cat) at gestational day10, 14, or 19 leads to greatly increased mortality of pups and to decreased body weight of surviving animals. The effects of such a stressor on emotional reactivity are less obvious. Cognitive processes are impaired depending on the learning task. Enriched environments restore abnormal behaviors (emotional reactivity, motor skills, motor and spatial learning) due to brain trauma or genetic deficiencies. In any case, environmental enrichment does prevent or slow down aging effects. The effects of postnatal handling noted when using classical tests of emotional reactivity also are clear when defensive reaction paradigms are used. Furthermore, pregnant females that are early handled are less anxious than nonhandled females. We hypothesize that, when subjected to a stressor, the offspring of early-handled females would be protected from the deleterious effects of this stress compared to pups of nonhandled females.

Prenatal stress effects are partially ameliorated by prenatal administration of the neurosteroid allopregnanolone

This study examined the effects of exposure to prenatal stress on young and adult rats, and whether the concomitant administration of an anxiolytic neurosteroid, allopregnanolone (3-alpha-hydroxy-5 alpha-pregnan-20-one), could ameliorate some of the behavioral dysfunctions associated with prenatal stress. Pregnant dams were assigned to one of five treatment groups on gestational day 14. These groups were exposed to either 1) restraint for 45 min three times daily; 2) a vehicle injection twice daily; 3) 5 mg/kg allopregnanolone twice daily; 4) restraint with allopregnanolone injections; or 5) nonhandled controls. Assays for plasma allopregnanolone concentrations indicated that exogenous allopregnanolone injections significantly raised circulating levels to a comparable degree in gestational day 20 dams and their fetuses. At 7 days of age, however, subjects prenatally exposed to allopregnanolone either alone or with restraint now had lower circulating levels compared to the other groups, suggesting some negative compensatory change. Behavioral results suggested that the effects of prenatal stress on affective behaviors (ultrasonic vocalizations emitted after a brief maternal separation at 7 days of age, and plus-maze behavior at 70 days of age) could be reversed by coadministration of allopregnanolone. When locomotor activity was assessed at 16 and 60 days of age, no comparable reversal effect was observed. In fact, the allopregnanolone groups had results similar to those of the restraint alone group. Thus, for some neuronal systems, allopregnanolone may exert either a direct teratogenic effect or an indirect effect due to neurosteroid-induced behavioral changes in the pregnant dam.

Influence of prenatal maternal stress on the immunocompetence of the offspring

To evaluate the effects of prenatal maternal stress on the development of humoral immunocompetence in the offspring and on their hormonal and immunologic responses to postnatal stress, gravid Sprague-Dawley rats were exposed daily on gestational days 15-21 to prenatal environmental stress [(PES) 15 unsignaled, inescapable electric foot-shocks (0.05 mA for 0.5 s)] or prenatal psychological stress [(PPS) pregnant rats were placed in the nonelectrified section of the apparatus and allowed to see, hear, and smell a nonpregnant partner being environmentally stressed]. Pregnant controls (PC) were placed in the apparatus for 30 min. Serum corticosterone (CCS) and immunoglobulin G (IgG) levels were measured in the offspring every 7 days from birth to postnatal day (PND) 28. On PND 29-33, offspring were environmentally stressed; hormonal and immune status were determined on PND 34. Levels of IgG were reduced in PES and PPS offspring on PND 0 and in PES offspring on PND 7 and 28. These changes were unrelated to differences in CCS and did not reflect altered maternal-pup interactions or nutritional factors. Postnatal stress was immunosuppressive in PC pups but did not alter immune parameters in PPS offspring. In PES females, postnatal stress was also immunosuppressive. However, in PES males with already reduced IgG levels postnatal stress enhanced immune function. These data provide the first experimental evidence that prenatal maternal stress can alter immune parameters in the rat offspring.

Neurobehavioral and immunological effects of prenatal cocaine exposure in rat

Time-pregnant Sprague-Dawley rats were injected subcutaneously with 20 mg/kg of cocaine HCl or 0.9% saline daily from gestation days 15 through 21. Maternal plasma levels of approximately 720 ng/ml of cocaine did not alter maternal weight gain during treatment, duration of pregnancy, any of the litter variables or several indices of maternal behavior. Offsprings' body weight from birth to 30 days of age and physical maturation were not generally affected by prenatal cocaine exposure. While the development of surface righting, cliff avoidance, and the startle response was accelerated in cocaine-exposed offspring, acquisition of a preference for a social odor was unaltered. Prenatal cocaine also attenuated the locomotor response of the offspring to d-amphetamine and cocaine at PND 15; at PND 30 both of these catecholaminergic agonists increased activity in prenatal saline and prenatal cocaine offspring. However, the difference in plasma levels of cocaine at PND 30 suggests a possible down-regulation of adrenergic receptors following prenatal cocaine exposure. Decreased thymus/body weight ratios and splenomegaly were observed in prenatal cocaine animals at 55 days of age. Although complete neutralization of herpes simplex virus-type 1 was not observed, sera from prenatal cocaine offspring showed an increased rate of appearance of cytopathic effect, while sera from animals given cocaine postnatally showed a reduction in the rate at which viral infectivity was expressed in culture. These results indicate that prenatal cocaine exposure can alter neurobehavioral ontogeny and humoral immune responsitivity in the offspring.

The effects of stress on plasma ACTH and corticosterone in young and aging pregnant rats and their fetuses

Compared to younger rats, old rats exhibit prolonged elevations of plasma ACTH and corticosterone (CORT) in response to stress. In addition, CORT crosses the placenta. To investigate whether fetuses of older rats may be exposed to higher concentrations of CORT during development than fetuses of young rats, we compared the effects of stress on hypothalamic-pituitary-adrenal (HPA) axis function in young and aging pregnant rats and their 19-day-old fetuses. The plasma of the mothers and fetuses was assayed for ACTH and CORT by radioimmunoassay. Both young and aging pregnant rats showed a significant increase in plasma ACTH and CORT immediately after exposure to stress. However, aging rats had more prolonged elevations of ACTH and CORT than young rats. This suggests that, like old male rats, aging pregnant rats have an alteration in feedback inhibition of the HPA axis. Prolonged elevation of CORT was also seen in fetuses of aging mothers. These results have important implications concerning the effects of stress during pregnancy at different maternal ages, and for the potential deleterious consequences of prolonged prenatal elevation in stress hormones on the offspring of aging females.

Prenatal ethanol and stress in mice: 1. Pup behavioral development and maternal physiology

On days 12 to 17 of pregnancy, B6D2F1 mice were pair-fed liquid diets containing either 25% ethanol-derived calories or an isocaloric amount of maltose-dextrin. During this period, half the mice in each dietary condition also underwent two daily one-hour periods of restraint stress. A fifth group, given lab chow and water ad lib, was left undisturbed throughout gestation. Neither treatment affected offspring body weight on days 22 or 32 postconception, but undernutrition produced by the pair feeding procedure reduced day 32 body weight in all groups relative to the ad lib-fed group. Both prenatal ethanol and pair feeding led to delayed neurobehavioral development on day 32, while prenatal stress significantly reduced the degree of developmental delay caused by these factors. In a second study, restraint stress significantly reduced blood alcohol concentrations in pregnant dams on day 15 of gestation while elevating plasma corticosterone concentrations, and this elevation was consistent regardless of the dietary condition of the dam. The pair feeding procedure also produced corticosterone elevations but the effect of ethanol was not significant. These results suggest that prenatal stress in the presence of other physiological insults may act to counter the actions of those insults.

Acute and repeated gestational stress affect offspring learning and activity in rats

This study assessed possible long-lasting effects of mild, indirect prenatal stress upon offspring. Dams were restrained for 30 minutes either once or four times during the third trimester of gestation. Their male offspring were challenged in adulthood with a series of appetitive operant learning tasks. Both acute and repeated prenatal maternal restraint retarded the performance of the offspring in a selective manner: deficits appeared during the reversal stage of an operant discrimination task, with no effect on acquisition, discrimination or extinction. Repeated, but not acute, maternal stress was also associated with offspring hyperactivity. This highlights the differential impact of varying the stress schedule. Furthermore, use of multiple measures of learning uncovered a long-lasting, selective effect of relatively mild, indirect prenatal manipulation.

Restraint stress during late pregnancy in rats elicits early hypermyelination in the offspring

Female rats were subjected to a regular, daily schedule of 2 hr of restraint stress during the final 6 days of pregnancy. During the first 2 postnatal weeks, adrenal weights were greater than normal in the offspring of the stressed dams. The concentration of brain myelin was higher than control at 14 and 21 days of age but similar to normal by day 40. Early hypermyelination may be partly responsible for early motor development, as previously observed in prenatally stressed rats.

Prenatal antiepileptic drug exposure alters seizure susceptibility in rats

An animal model is used to address the issue of prenatal exposure to certain antiepileptic drugs and seizure susceptibility in the offspring. Administration of doses established as median therapeutic doses in humans of phenobarbital, valproate and clonazepam to pregnant rats during the last third of gestation produced sexually dimorphic alterations in pentylenetetrazol (PTZ)-induced seizures as well as in non-convulsive (spontaneous alternation and cliff avoidance) behaviors in the offspring. Altered seizure susceptibility occurred in the absence of overtly recognizable morphological abnormalities and did not appear to reflect differences in the status of circulating drug-binding plasma proteins. Possible neural and/or metabolic mechanisms responsible for these behavioral changes are discussed.

Prenatal stress: effect on development of rat brain adrenergic receptors

Female rats were subjected to stress treatments during pregnancy and the offspring were studied at several different ages. The ligands [3H]WB-4101, [3H]clonidine and [3H]dihydroalprenolol were used to measure alpha 1, alpha 2 and beta receptor binding in several brain regions. At 16 but not at 23, 40 or 60 days of age the offspring showed reduced alpha 1 and beta receptor binding in cerebral cortex whereas a previous study had shown a similarly transient elevation of norepinephrine (NE) level at 16 days of age. The 60 day-old offspring showed only a reduced alpha 2 binding which appeared to have no regional specificity. Consistent with our previous finding that in 60 day-old offspring NE levels were not significantly affected by maternal stress exposure in almost all brain regions studied, we find no effect on the ability of nerve endings to synthesise catecholamines. These data provide additional support for the proposal that prenatal stress results in permanent neurochemical changes and suggest that there may be a delayed or impaired development of the postsynaptic elements of noradrenergic neurons.

Prenatal stress: effects on brain biogenic amine and plasma corticosterone levels

Pregnant rats were subjected to once daily stress treatments consisting of handling and a saline injection. The offspring showed region-specific changes in brain 5-hydroxytryptamine (5-HT), 5-hydroxyindoleacetic acid (5-HIAA) and norepinephrine (NE) levels in infancy but only the hypothalamus still showed significant changes at 60 days of age. In a reaction-to-stress test 23-day-old offspring in the prenatal stress group showed a greater elevation in plasma corticosterone level but smaller changes in hypothalamic NE and 5-HIAA levels than control offspring suggesting that prenatal stress may have altered the functioning of the hypothalamic-pituitary-adrenal axis. It is suggested that changes in the development of specific monoamine-containing neurons may be associated with the reported behavioral deficits in offspring of female rats stressed during pregnancy.

Infant rat hyperactivity elicited by home cage bedding is unaffected by neonatal telencephalic dopamine or norepinephrine depletion

Newborn rats received either stereotaxically guided bilateral injections of 6-hydroxydopamine (6-OHDA) in the neostriatum so as to deplete dopamine (DA) there, or subcutaneous 6-OHDA to deplete forebrain norepinephrine (NE). Both the DA and NE depleted rats as well as their respective control rats were significantly more active at 15-16 days of age when tested in a novel environment containing soiled bedding from their home cages, than when tested in the presence of clean bedding material. Furthermore, under both the home cage and clean bedding conditions the DA depleted rats were more active at this age than their controls. Thus while transiently elevated locomotor activity is one consequence of neonatal, neostriatal DA depletion, inattention to olfactory stimuli (which occurs after adult neostriatal DA depletion) is not another. The NE depleted rats in both home cage and clean bedding test conditions showed activity levels equivalent to that of their control groups. Furthermore, the NE depletion did not affect hyperactivity elicited by artificially (peppermint) scented bedding like that in which the rats had been reared. Thus, contrary to expectations based on the reported reduction of preference for conspecific odor after neonatal and adult forebrain NE depletion, these data show that the locomotor activating effects of neither conspecific nor artificial odors associated with the nest odors are attenuated by neonatal NE depletion. The mortality rates among NE depleted rats raised in the peppermint scented shavings was unusually high and overall, these rats were less active than similarly raised controls.

Experimental methods in behavioral teratology

The possibility that the exposure of the embryo to certain chemical substances can lead to behavioral disturbances is known from human epidemiological studies, e.g., in chronic poisoning with mercury and ethanol. Therefore, efforts are made to develop toxicological techniques with which new behavioral teratogens can be recognized. The review describes the most important experimental methods which are presently explored, and which are based on a rich body of knowledge accumulated by experimental psychologists. Most of the tests were developed with small animals, mostly with rats. They range from a rather straightforward determination of various reflexes to complex behavioral situations involving mechanical devices, operant conditioning techniques and procedures evaluating social behavior. In applying these methods in routine toxicology, it is important to remember, that many behavioral effects determined in newborn and adult animals are subtle. Moreover, they are influenced by a large variety of environmental factors affecting the health and the behavior of the mothers and of the offspring in the early and later phases of development. Therefore, the experiments must be conducted under highly standardized conditions and must be controlled rigorously. It is concluded that the best experimental strategy for the evaluation of potential behavioral teratogens is not yet established. Therefore, it would be premature to decide on a fixed protocol to be included in routine animal safety experiments for drugs and other chemical substances.

Failure of severe maternal stress or ACTH during pregnancy to affect emotionality of male rat offspring: implications of litter effects for prenatal studies

Daily maternal neck restraint, whole body restraint, hyperthermia, and ACTH treatment during the last 3rd of gestation had no reliable effect on open-field and cage-emergence behavior in male Sprague-Dawley offspring. Many of these treatments, however, produced considerable maternal pathology and evidence for maternal adrenocorticoid release. Significant litter effects were found for almost every morphological and behavioral measure. Failure to control for the litter variable may account for many previously reported effects of prenatal stress on emotionality in rats. Female rats showed greater activity and less defecation than males on postpubertal open-field and cage emergence tests.

Prenatal morphine administration alters behavioral development in the rat

Female rats were administered increasing doses of morphine sulfate 5 days prior to mating and during gestation until 4-6 days before the birth of their litters. Prenatal morphine exposure altered the normal during the 3rd and 4th postnatal weeks. This disruption in behavioral ontogeny did not coincide with changes in physical parameters. Decreased body weight ant postnatal week. The appearance of behavioral disturbances in the absence of physical abnormalities stresses the need for follow-up studies of infants born to narcotic-dependent mothers after signs of physical withdrawal or retarded growth have disappeared.