Ontogeny of behavioral arousal in the mouse: Effect of prior testing upon age of peak activity

Adult-like biphasic neurobehavioral changes induced by a GABA-A agonist in infant and weanling mice

The neurobehavioral responsivity to peripherally injected muscimol, a gamma-aminobutyric acid-A (GABA-A) agonist, was assessed in infant (14-day-old), weanling (20-day-old) and young adult (53-day-old) outbred male mice. In the first experiment, relatively high doses of muscimol (ranging from 0.05 to 0.40 mg/kg in developing and from 0.50 to 3 mg/kg in adult animals) were found to dose-dependently induced solid catalepsy and ataxia, evaluated 5 times at 20-min intervals. In the second experiment, the GABA agonist was injected in dose ranges which include relatively small concentrations in order to assess its excitatory properties, observable in adults, on rearing and locomotion in developing mice. It appeared that levels of rearing and especially locomotion were enhanced at the low doses (0.025 and 0.050 mg/kg in developing, and 1.3 and 1.9 mg/kg in adult mice) and inhibited at the higher ones (0.150 mg/kg in developing and 1.9 and 2.5 mg/kg in adult mice). This adult-like biphasic action of muscimol in developing mice--excitation at low and depression/sedation at high doses--strongly suggests a full maturation of the GABA-A-related behavioral functions at a period of ontogeny where adult-like locomotion emerges. Given that previous studies have shown that muscimol can biphasically affect behavioral activity in newborn murines as well, it is suggested that GABA-related behavioral functions mature near-monotonically during ontogeny, unlike those related to other major neurotransmitter systems.

Ontogeny of muscimol effects on locomotor activity, habituation, and pain reactivity in mice

Three hundred and twenty mouse pups of both sexes of the CD-1 outbred strain received IP muscimol and were subsequently assessed for locomotor activity (single Varimex 30-min session) and for hot-plate responding. Muscimol doses were 0.05, 0.1, or 0.2 mg/kg at 8 and 14 days, and 0.1, 0.5, or 1.0 mg/kg at 21, 28, and 35 days. Activity data showed a shift from an immature pattern at 8 and 14 days to an adult-like pattern from day 21 onwards (high initial activity followed by a marked within-session decrement). Muscimol was ineffective on day 8, and depressed activity from day 14 onwards. At 28 days, however, the higher-dose male group showed a non-monotonic trend of activity; that is an initial depression followed by a marked rebound hyperactivity. With regard to hot-plate exposure, muscimol was ineffective at 8 days, while it produced maximal analgesic effects on day 14, followed by a progressive decrease in drug sensitivity. Around day 70, mice of the former 0.1 mg/kg and saline groups were re-tested for locomotor activity and pain reactivity without additional drug treatment. Activity was generally higher in males than in females, and two groups habituated significantly less than the others (females tested in the muscimol state at 8 days and males tested in the saline state at 35 days). Moreover, prior testing at the earliest ages and prior muscimol exposure had additive attenuating effects on pain reactivity.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

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.

Early development in mice: II. Sensory motor behavior and genetic analysis

The genetic and environmental bases for differences in rate of development were investigated in 2 inbred strains of mice: C57BL/6By (B) and BALB/cBy (C). Twelve motor responses, aside from individual weight, were used to measure these differences. The Recombinant Inbred Strains method was chosen to perform the genetic analysis. Overdominance is shown to be present in 2 variables alone (eye opening, weight at 10 and 20 days). In most cases, each of the response reflexes was found to be associated with several genes (locomotion, hind limb, crossed extensor, righting, vibrissae placing, bar holding). Differences across strains are associated with one segregating unit for rate of disappearance of the rooting response. This unit is mapped on the part of the 4th chromosome including the loci b and H-21. The strain distribution pattern differs for each sensory motor response, consequently no one general genetic factor of development can be advanced. Maternal effects were found for 4 variables (grasping, fore limb placing, eye opening and weight). For two responses, the F1 pups developing the fastest were reared by mothers from the slowest developing parental strain. As regards this latter finding, the authors hypothesize that mothers differ as to the quality of the environment they furnish to their young and pups differ in their ability to benefit from these environments.

Development of locomotor activity of rat pups in figure-eight mazes

In a series of four experiments, social and experiential factors that influence the development of motor activity in rat pups were examined. Motor activity was monitored from postnatal Days 13 to 21 as photocell interruptions in figure-eight mazes and comparisons were made between pups maintained in a nest box containing a dam and siblings and allowed access to the maze for 23 hr/day, pups tested daily for 1 hr/day vs pups tested only on postnatal Days 15, 18, or 21, pups tested daily for either 5 min, 30 min, or 1 hr/day, and pups tested daily for 30 min/day either singly in a maze, paired with a littermate, or paired with an anesthetized pup of the same age. A monotonic increase in activity was seen for nest-box testing, minimal developmental change was seen for pups tested on only a single day or for pups tested with an anesthetized pup, whereas all other groups showed an inverted U-shaped profile of activity which was influenced by the duration of testing and/or the presence of a littermate. These data emphasize the relevance of environmental factors as determinants of preweaning behavior.

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.

Sex and strain differences in the developmental activity profile of the rat tested over clean vs home cage bedding

In the first of two experiments, CD rat litters were used to characterize activity patterns obtained in a size-adjustable, single photodetector chamber. Beginning on postnatal Day 10 or 12, pups were tested repeatedly over clean bedding (C) or over bedding removed from each pup's home cage (HC). In C rats of both sexes and in HC females, short-term activity levels peaked on Day 16. However, HC males displayed an earlier and even greater elevation in activity from Day 12 to 16. This overall pattern was found in rats tested either every second or fourth day. In the second experiment, Long-Evans pups were assigned to each testing condition (C vs HC) and activity measured beginning on Day 12. Peak levels were seen in all Long-Evans rats on Day 16 and only females showed significant alterations as a function of bedding condition. When overall activity levels of the two strains were compared, significant differences were found on Days 12, 24, 30, and 120 in males, and 12, 24, and 30 in females. Significant differences between strains in activity as a function of bedding condition were found in males on Days 12, 20, 24, and 120 and in females on Days 12, 30, and 60. These data confirm the generality of a developmental hyperactivity phase in isolated juvenile rats. However, different patterns of hyperactivity were found in male vs female rats across strains. CD males were more active in the presence of HC olfactory cues, while in Long-Evans rats, female activity was affected more by bedding condition.

Early development in mice: I. Genotype and post-natal maternal effects

The co-actions of genetic effects and the post-natal maternal rearing environment on the development of weight, 9 reflex responses, and survival have been tested by the cross-fostering method in two inbred mice strains--CBA/H and NZB. Pups of the two strains were not treated differentially by the mothers and experimental handling did not systematically affect pup development. Comparisons of unfostered, infostered, and cross-fostered pups show (1) in 16 cases out of 34, reflex development was affected by the pup strain, and in 10 cases out of 34 by the foster mother strain; (2) survival is only affected by the pup strain; (3) weight development is affected by strain of both the pup and the mother as well as their interactions. The adopted pups' scores were situated outside the range of the two non-adopted groups for certain reflexes as well as for weight. Two non-exclusive hypotheses are proposed: the mother strain can affect pup development (1) either through differences in stimulation provided by the mothers (2) or through differences in milk composition.

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.