Development of behavioral arousal and inhibition in the Swiss-Webster mouse

Paradoxical effects of D-amphetamine in infant and adolescent mice: role of gender and environmental risk factors

The psychostimulant D-amphetamine (AMPH) increases generalised activity in adult subjects, while exerting a paradoxical "calming effect" in children with Attention-deficit Hyperactivity Disorder (AD/HD). A number of animal models have been developed to characterise the neurobiological basis of this AMPH action. In this line, the present review summarises recent work on the effects of AMPH on behavioural and physiological parameters in developing mice with a special emphasis on the role of gender and environmental risk factors. Behavioural and neuroendocrine responses to AMPH administration (0, 1, or 3 mg/kg, IP) and their relation to changes in the environment, represented by social stimuli, were studied in infant CD-1 mouse pups of both sexes at three different developmental ages (3, 8, or 18 postnatal (pnd) days). Mouse pups were assessed either in baseline condition or following 24 h maternal deprivation. AMPH exerted a paradoxical effect on CORT secretion only in maternally deprived subjects while affecting behaviour mainly in deprived female subjects, which showed a generalised shift to the left in the dose-response curve to this drug. Unwanted perseverative motor effects and possible dependence states represent side effects of AMPH administration. Further knowledge on these aspects comes from another set of studies where a shortened conditioned place preference (CPP) paradigm was employed to assess the reinforcing properties of AMPH (0, 1, 3.3, or 10 mg/kg) in developing mice on 14-17, 21-24, and 28-31 pnd. Data indicate that AMPH-CPP develops early, mice being able, already at two weeks of age, to acquire a place preference that relies on adult-like sensory, motor, and associative capacities. AMPH-CPP appears earlier in females, compared to males. A detailed analysis of acute D-amphetamine effects evidenced that the drug produces a dose-dependent increase in locomotor activity and in several responses (including stereotypes). These effects appear much larger at both post weaning stages than in preweanlings and are significantly more pronounced in females than in males. Overall these data suggest that AMPH action is dependent on the baseline level of activity and indicate a strong role of gender in the effects of this drug measured early on during development, with females showing greater sensitivity to this drug. A better understanding of AMPH action during the early ontogenetic phases, particularly its interaction with environmental factors, might extend our knowledge on the neurobiological basis of AD/HD, possibly improving the clinical efficacy of psychostimulant drugs.

Behavioral effects of neonatal herpes simplex type 1 infection of mice

Neonatal infection with a mutant herpes simplex type 1 virus produced hyperactivity in mice. Activity was measured throughout a 24 hour period during adulthood, and the elevation of activity occurred during the period of the day when mice are normally inactive. In a second experiment, infected mice showed deficits in learning to inhibit behavior in a passive avoidance task, but no deficit in learning a complex spatial task. Virus was detected in the brain by 5 days of age. The peak percentage of mice infected was reached at 10 days of age and declined thereafter. Mortality due to the virus declined with age at which the mouse was infected, but rates of hyperactivity were not different when injection occurred within the first 4 days of life. The viral infection produced no deficit in body weight in suckling mice. Thus we have shown that a mild neonatal virus infection can produce specific behavioral deficits.

Ontogenetic and pharmacological dissociation of various components of locomotor activity and habituation in the rat

Sprague-Dawley-derived male rats were used to investigate locomotor activity and habituation in an open field as a joint function of developmental age (2-6 weeks), pattern of test exposure (single 30-min test vs three 5-min tests at 24-hr intervals), and treatment conditions (i.p. saline, d-amphetamine sulfate 1 mg/kg, or scopolamine hydrocloride 0.5 mg/kg). No-drug animals showed low activity levels in both tests at the end of the second week, intermediate response rates at the end of the third week, and a typical adult-like pattern at later ages (high initial activity followed by marked within-session or between-session habituation). Amphetamine effects varied considerably depending jointly on age and type of test. At the end of the second week, the drug hyperactivity was much more marked in successive brief tests than in the single extended test. One week later, the response increase was rather uniform in both tests. At the end of the fourth week, the sensitivity profile was reversed, consisting of a large drug effect in the extended test but not in successive brief tests. Scopolamine was still without effects at this age, while a typical hyperactivity was produced by the drug in 6-week-old animals. These data show that, at least in the rat strain used, the functional maturation of muscarinic regulatory systems is not a necessary condition either for the appearance of an adult-like response pattern, or for the occurrence of the age- and test-related changes of the amphetamine profile.(ABSTRACT TRUNCATED AT 250 WORDS)

Odor-aversion learning and retention span in neonatal mouse pups

One hundred and sixty-four litters of Swiss CD-1 random-bred mice were used to assess learning and retention capacities during the first postnatal week. In Experiment 1, whole 7-day litters were exposed for 65 min to commercial extracts of either mint or lemon sprinkled over wood shavings. Five minutes after the beginning of the exposure, half of the litters were injected ip with the illness-inducing agent lithium chloride (LiCl; 0.20 M, 2% of body weight); the other half was treated with saline solution (8% NaCl). On Postnatal Day 10, the animals were singly introduced in a warmed arena for a 180-s preference test, and the time spent in the mint- and lemon-scented areas of the apparatus was recorded. When compared with saline-injected pups, mice that experienced lemon-LiCl pairings showed a significant aversion for the lemon-scented area, while the mint aversion in the mint-LiCl group just missed statistical significance. Three additional control groups (unhandled on Day 7, or only LiCl- or saline-injected) did not show significant preferences for either the mint or the lemon odor. In Experiment 2, litters of 3, 5, or 7 days were similarly exposed to lemon-scented shavings for either 5 or 20 min, injected with LiCl or saline, and then exposed for an additional 60 min to the shavings. On Postnatal Day 10, tests like those of Experiment 1 showed a significant odor-aversion in animals conditioned on Day 7, but not in those conditioned on Day 3 or 5. In Experiment 3, 3- and 5-day old pups were exposed to lemon odor-LiCl or -NaCl pairings, and tested for aversion after 3 or 7 days (CS duration 5 min before injection and either 30 or 60 min after injection). Only when the conditioning-testing interval was limited to 3 days did LiCl-injected groups show a significant aversion, which did not depend on duration of CS exposure.

The ontogeny of independent ingestion in mice: or, why won't infant mice feed?

Unlike infant rats, which show deprivation-related ingestion in several different test situations, infant mice appeared to be relatively unwilling to feed independently of suckling until 12 days of age. We tested mouse pups that were deprived (of food, water, suckling, and maternal care) for 1, 7, or 24 hr in ingestive tests in which a milk diet was spread on the floor of their test container (Experiment 1). Pups at 3, 6, and 9 days of age consumed small amounts of the diet and showed little increase in intake when deprivation was increased. In contrast (and like rat pups of all ages), mouse pups 12 and 15 days of age actively ingested the diet and increased their intake with increased deprivation. Six-day-old mouse pups were similarly unwilling to ingest a 5% sucrose solution, though 12-day-old pups showed deprivation-related intake (Experiment 2). Cellular dehydration (produced by hypertonic saline injection), a potent stimulus for ingestion in infant rats, did not stimulate ingestion in mice younger than 12 days of age (Experiment 3). Finally, when ingestion was tested with diet infusions made through oral cannulas, mouse pups at 6 and 9 days of age showed only a slight increase in intake with increased deprivation. However, by 12 days of age, pups' ingestion increased markedly with deprivation (Experiment 4). Thus, mouse pups seem to be very different from rat pups with respect to the early existence of ingestive systems. The neural substrates for the ingestive responses that subserve independent ingestion are only minimally present in infant mice or are somehow inhibited.

Development of mouse activity, stimulus reactivity, habituation, and response to amphetamine and scopolamine

Twenty-four litters of non-inbred Swiss-derived mice were used to study the development of locomotion and tendency to approach a novel object in an open field, as well as the effects of dl-amphetamine sulfate and scopolamine hydrochloride (1 or 2 mg/kg IP). Brief (7 min) tests repeated for three consecutive days were preferred in order to obtain information on between-session habituation. Animals tested on days 14-16 showed low levels of activity without changes in successive sessions. In contrast, an adult-like pattern with a high initial activity and marked between-session decrements prevailed on days 21-23 and 28-30. Dl-amphetamine elevated activity only on days 14-16, while scopolamine produced hyperactivity and impaired habituation only on days 21-23 and 28-30. Latency to approach a novel object by untreated animals showed a substantial reduction between the end of the second week and subsequent developmental stages. This went hand in hand with an appearance of latency increases after dl-amphetamine treatments, while an opposite trend in the scopolamine data failed to reach statistical significance. Activity tests in a photocell apparatus at 61-72 days (without prior treatment) showed a reduction of locomotion relative to the level measured in animals from other litters raised in parallel and not subjected to early testing. Overall, the present data and those of the literature indicate that some developmental phenomena in small rodents are relatively insensitive to a variety of organismic, environmental, and test factors, while others (e.g., inverted U-shaped activity trends and successive modifications of the amphetamine profile) depend on complex interactions between several variables.

Heightened arousal in the 2-week-old rat: the importance of starvation

The 2-week-old rat has frequently been reported to be hyperactive; however, the causes of this are disputed. In 3 experiments, with split-litter designs, the following possibilities were eliminated as causative factors: (a) body temperature; (b) nest seeking; and (c) maturation of peripheral sensory, motor, and endocrine systems. In contrast, the isolated 2-week-old rat, in a novel environment, was shown to be very reactive to hunger, however mild. Further, this was true whether the hunger was self-generated during a forced switch in diet or external origin due to food deprivation. These data are consistent with and supportive of the hypothesis that the 2-week-old rat does not have functionally-mature forebrain inhibitory structures with which to modulate arousal. Thus, even a mild arousal stimulus results in extreme hyperactivity. Further, these results, together with previously published results, suggest that the concurrence of isolation, starvation, and novelty of environment are necessary, but none of these alone is sufficient, for the 2-week-old rat to become hyperactive.

Interactive effects of nutrition and cannabis upon rat perinatal development

In order to examine the effects of cannabinoids, malnutrition, and their possible interaction upon the developing rat fetus, female Wistar rats were exposed to cannabis smoke, placebo smoke, or no smoke while concurrently consuming 1 of 3 diets differing in protein concentration (8%, 24%, 64%). Both the diet and drug treatments were administered 20 days prior to and throughout gestation. Of the 12 variables affected by the low-protein diet, 8 were significantly potentiated when undernutrition was combined with cannabis treatment. Some dependent variables that were not altered either by the low-protein diet or by cannabis inhalation were affected by the combination of treatments. These included a lengthened gestation period, an increase in occurrence of stillbirths and litter destruction, and decreased activity in the rat pups. Cannabis coupled with a standard protein diet resulted in a number of developmental indices being delayed but combining the drug with an enriched protein diet ameliorated these effects. The evidence also suggests that the administration of cannabis both before and during gestation resulted in a degree of tolerance to some aspects of the drug effects.

Ontogeny of short- and long-term memory capacities for passive avoidance training in undernourished mice

To assess the effects of early postnatal undernutrition upon the developing abilities of infant and juvenile mice to acquire and retain a passive-avoidance response, we reared mice following birth in either "normally nourished" or "undernourished" conditions by maintaining litter sizes of 6 or 16, respectively. At ages ranging from 7 to 21 days, mice from each nutritional condition were trained to withhold stepping off from a small vibrating platform in order to avoid shock. Following training to a common criterion, separate groups of mice were retained to the same criterion either 1 or 24 hr later. From the comparisons with yoked control groups at each age and retention interval, we conclude that although undernutrition results in some age-related differences in the ability to withhold responding, these are quite small during acquisition within each age group. In contrast, the retention data suggest that undernutrition delayed the development of both short- and long-term memory abilities on this task. In general, these results are similar to earlier data involving discriminated shock-escape wherein undernutrition had little apparent effect upon acquisition at the early ages but rather marked effects upon developing memory system.

GABA-mediated behavioral inhibition during ontogeny in the mouse

Although immature rats and mice generally demonstrate poor behavioral inhibitory capacities, some recent evidence may indicate the presence of substantial inhibitory control. The present experiment investigated the possibility that gamma-aminobutyric acid (GABA) systems may mediate some behavioral inhibition during early development. Mice 9-100 days old were injected with the GABA-elevating agent amino-oxyacetic acid (AOAA) and tested for behavioral activity. High levels of locomotor activity characteristic of immature control mice were attenuated following AOAA injection, whereas AOAA had little effect on the activity of adult mice. Moreover, AOAA produced a period of rebound hyperactivity for young but not for adult mice. These findings suggest that although GABA systems may mediate early behavioral inhibition, coordination between excitatory and inhibitory capacities matures slowly. In a second experiment the dopamine-beta-hydroxylase inhibitor FLA-63 prevented rebound hyperactivity in young mice pretreated with AOAA, suggesting that the excitatory component may be mediated by noradrenergic systems.

Behavioral development in two genetically heterogeneous mouse stocks: Swiss-Webster and Binghamton-HET

Two genetically heterogeneous mouse stocks, Swiss-Webster and Binghamton-HET, were tested. Several measures assessing behavioral development revealed differences in rates of development between the stocks. Thus neither stock can be exclusively used to generalize about "typical" mouse behavior.

Relative aversion thresholds for shock in infant mice

Using a spatial-preference technique, we tested separate groups of mice, 5, 7, 9, 11, 13, and 15 days of age, for escape and avoidance of a range of shock intensities administered from AC contant current and fixed impedance shock sources. With intensities ranging from 0 to .2 mA and 0 to 70 V for the respective sources, near asymptotic escape and avoidance were obtained at .1 mA and at 50 V for ages tested. Although few differences in the relative aversiveness of particular shock intensities were noted across ages with each source, the fixed impedance source produced more consistent avoidance than did the constant source. The findings suggest that the motivational properties of shock remain relatively constant throughout the early development of the mouse and that the technique employed in this study should prove useful in assessing possible age-related alterations in sensitivity to shock as a result of physiological or pharmacological manipulations.

Acquisition and retention of a passive-avoidance task as a function of age in mice

In Experiment I groups of mice between 16 to 100 days of age were tested for retention of a passive-avoidance response between 1 min and 96 hr following a single training trial at 2 shock intensities. In general, although almost all age groups displayed reliable retention at all retest intervals, some retention losses were found among the youngest age groups at the longer retention intervals. Higher shock intensity resulted in longer retest latencies, primarily among the youngest mice. In Experiment II mice 16, 25, and 100 days of age were trained to criterion on the passive-avoidance task and retested on a single trial following retention intervals of 24, 96, 192, and 384 hr. Young mice exhibited severe retention losses relative to 100-day-old mice at the longer intervals, even though they did not show deficiencies in acquisition. Neurological maturity at the time of original training appears to account for the age-related memory differences.

Development of cholinergic inhibitory capacities in the hyperthyroid mouse

The ontogeny of behavioral arousal and inhibition, as measured by spontaneous locomotor activity, was compared in four experiments for controls and mice injected with thyroxine as neonates. Mice treated with thyroxine at 1-3 days of age had higher activity levels at 10-15 days of age than controls, suggesting potentiation of arousal systems by the hormone treatment. Although thyroxine-accelerated development had no reliable effect upon the age at which peak activity occurred, scopolamine injections increased activity as early as 15 days of age in thyroxine-treated mice, whereas saline-treated or unhandled controls did not show a similar increase until 16-17 days of age. The findings were interpreted as indicating both a potentiation of arousal and a compensatory acceleration of cholinergic inhibitory capacities as a result of the neonatal hyperthyroidism. In addition, the importance of the behaviorally suppressive effects of a novel injection experience in the neonatal mouse was demonstrated.

Escape learning in infant mice as a function of drive level and drive shifts during acquisition

Separate groups of 9-day-old Swiss-Webster mice began straight-alley escape training at .1 or .4 mA. After 12 trials, half of the mice in each group were shifted to .4 or .1 mA, respectively, whereas the remaining half continued at their original level for an additional 12 trials. Twenty-four hours later, half of each of the 4 shock-level groups were retested at .1 mA, half at .4 mA. The results indicated that those groups which made a large number of competing responses during early trials and showed a gradual reduction over training trials (.1-.1 and .1-.4) emitted the fewest number during retest at either shock level. In contrast, those groups with either limited (.4-.1) or no (.4-.4) opportunity to decrease competing responses during training showed evidence of poor (.4-.1) and no (.4-.4) retention of learned inhibition of that response. Running speed was clearly a performance measure, as it only reflected existing shock levels during both training and retention trials.