Ontogeny of forebrain inhibition of behavioral arousal in the rat

Modification of dendritic development

Since 1890 Ramón y Cajal strongly defended the theory that dendrites and their processes and spines had a function of not just nutrient transport to the cell body, but they had an important conductive role in neural impulse transmission. He extensively discussed and supported this theory in the Volume 1 of his extraordinary book Textura del Sistema Nervioso del Hombre y de los Vertebrados. Also, Don Santiago significantly contributed to a detailed description of the various neural components of the hippocampus and cerebral cortex during development. Extensive investigation has been done in the last Century related to the functional role of these complex brain regions, and their association with learning, memory and some limbic functions. Likewise, the organization and expression of neuropsychological qualities such as memory, exploratory behavior and spatial orientation, among others, depend on the integrity and adequate functional activity of the cerebral cortex and hippocampus. It is known that brain serotonin synthesis and release depend directly and proportionally on the availability of its precursor, tryptophan (TRY). By using a chronic TRY restriction model in rats, we studied their place learning ability in correlation with the dendritic spine density of pyramidal neurons in field CA1 of the hippocampus during postnatal development. We have also reported alterations in the maturation pattern of the ability for spontaneous alternation and task performance evaluating short-term memory, as well as adverse effects on the density of dendritic spines of hippocampal CA1 field pyramidal neurons and on the dendritic arborization and the number of dendritic spines of pyramidal neurons from the third layer of the prefrontal cortex using the same model of TRY restriction. The findings obtained in these studies employing a modified Golgi method, can be interpreted as a trans-synaptic plastic response due to understimulation of serotoninergic receptors located in the hippocampal Ammon's horn and, particularly, on the CA1 field pyramidal neurons, as well as on afferences to the hippocampus which needs to be further investigated.

Long term neurological and behavioral effects of graded perinatal asphyxia in the rat

Perinatal hypoxic-ischemic states can cause irreversible damage to the brain, ranging from minimal brain dysfunction to death. Only few studies have been reported describing neurological, cognitive and behavioral deficits following perinatal asphyxia. We therefore decided to study long term effects of perinatal asphyxia in a well-documented animal model resembling the clinical situation. Caeserean section in rats was performed and the pups, still in the uterus horns, were placed into a water bath at 37 degrees C for periods of 5-20 min; pups were then given to surrogate mothers and examined at three month of age. Examinations consisted of a battery of motor and reflex tests, Morris water maze, multiple T-maze, elevated plus maze and open field studies. No abnormalities were found in rats even with long periods of perinatal asphyxia by neurological examination, in the open field and in mazes. Interestingly, in the elevated plus maze rats with long lasting exposure to hypoxia (15 and 20 min of asphyxia) showed reduced anxiety-related behavior. This finding may be relevant for the explanation of anxiety related disorders in adulthood with a tentative history in the perinatal period.

Serotonin involvement in the spontaneous alternation ability: a behavioral study in tryptophan-restricted rats

Spontaneous alternation (SA) is controlled by septal cholinergic terminals in the hippocampus. Serotoninergic terminals end on cholinergic nerve endings in the hippocampus, and their possible role in SA was investigated in rats fed with a tryptophan-deficient diet, from weaning to 60 days of age. A T-maze was used for the test. At the age of 40 days, an increase in SA occurred in the tryptophan deficient rats, although this effect disappeared by 60 days of age. A modulatory role of serotonin in the psychoneural control of SA is suggested, and it may be through presynaptic inhibition of hippocampal cholinergic terminals.

Long-term memory for DRL: a comparison between weanling, adult and senescent rats

Weanling (24-day-old), adult (3- and 7-month-old) and senescent rats (24-month-old) were submitted for 10 days to a DRL 20 seconds schedule, at a pace of 4 sessions a day. After a 3 month interval, the subjects were tested for retention in an identical experimental setting. In training, the median Interresponse-Time (IRT) is directly related to the age of the subjects, whereas response rates are inversely related to it. Age-related differences subside at the end of training. In testing, adult rats improve their efficiency, whereas senescent and particularly former weanling rats do not catch up with training scores. These data are discussed in relationship with developmental variables in the temporal regulation of behavior, collateral activity, contextual and methodological variables in the study of memory.

Behavioral differences in the developing rat following postnatal anoxia or postnatally injected AF-64A, a cholinergic neurotoxin

Rat pups were submitted postnatally to one of two procedures: a 25-min exposure to 100% nitrogen or an i.c.v. bilateral injection of AF-64A, 2 nmol contained in 1-microliter saline. Throughout further development of either group, their performance in passive and active avoidance tests and in amphetamine-induced stereotype behavior was followed and compared. Both groups exhibited hyperactivity which persisted until 42 days of age in the anoxia group and beyond 120 days in the AF-64A group. Both groups were equally inferior to controls in the passive avoidance test, but only the anoxia group was inferior to controls in the active avoidance test. Amphetamine-induced stereotype behavior was much less pronounced in the anoxia group relative to AF-64A-treated rats or to controls. The results suggest that the lesion induced by the neurotoxin is more specific and less widespread than the one caused by anoxia.

DRL performance in the weanling rat: a comparison with adult subjects

Weanling (21-day) and adult (3-month-old) albino rats were reinforced for lever pressing on a Differential Reinforcement of Low rates 20 seconds schedule. After 2 sessions at DRL 5, 10, 15 and 20 seconds, 40 DRL 20 seconds sessions were performed, at a pace of 4 sessions a day. Adult rats emit lower response rates, higher median Inter-Response-Times (IRTs) and obtain more reinforcers than weanling rats. Furthermore, their coefficients of variation (ratio between semi-interquartile range and median of the IRT distribution) are significantly lower than those of the young subjects. Age-related differences are discussed in relationship with general activity, food motivation and timing mechanisms. It is suggested that weanling and adult timing devices do not basically differ, but that age-related differences exist at the level of the translation of time estimates into overt behavior.

Fixed interval performance in weanling rats: a comparison with adult and senile subjects

Weanling (21 days), adult (100 days) and senescent (26 months) albino rats were submitted for 8 days to a fixed interval 60 second schedule, at a pace of 5 half-hour sessions a day. Temporal regulation indices (Curvature Index of Fry, Kelleher and Cook), overall response rates and running rates depend upon the age of the subjects and rank orderly (weanling greater than adult greater than senile). No significant difference was found for post-reinforcement pause durations. Two weeks after the end of the FI 60 second sessions, the senile subjects were submitted to a FI 120 second schedule for 10 days. The inverse relationship between age and temporal regulation accuracy evidenced in FI 60 seconds is discussed in relationship with classical FI data, factors involved in developmental studies and models for time regulated behavior.

Sodium valproate: effects on social behaviour and physical development in the mouse

Sodium valproate given in drinking fluid at 600 mg/l (160-180 mg/kg daily) to breeding mice did not affect fertility, birth weights or physical development of pups. Postnatal and postweaning administration of this dose also had no effects upon development or weight gain. The offspring ingested 103-158 mg/kg valproate daily after weaning. Behaviour was examined in a neutral enclosure by ethological methods. Offspring exposed to valproate in utero and throughout postnatal life showed no behavioural changes at 5 weeks, although at 15 weeks Immobility was reduced in females and Social Investigation increased. At 25 weeks when encountering mice of the opposite sex, treated males showed increase in Social Investigation and treated females increases in Other Non-social Activity. Postnatal and postweaning treatment with valproate caused behavioural changes both in juveniles and adults. After postnatal exposure, reduced Immobility with increased Social Investigation and Explore and Scan occurred at 5 and 15 weeks, at 25 weeks valproate increased Social Investigation in males encountering females and at 30 weeks enhanced Aggression in pair-housed males. Stimulation of Social Investigation was the only significant behavioural effect after postweaning exposure. Overall valproate appears to enhance behaviour stimulated by the test situation; urinary pheromones do not appear to play a part in this behavioural action.

Periadolescence: age-dependent behavior and psychopharmacological responsivity in rats

The behavior and psychopharmacological sensitivity of periadolescent rats are examined in this review. Periadolescent rats are hyperactive and engage in more conspecific play behavior than younger or older rats. When compared with other-aged rats, periadolescents exhibit enhanced performance in simple active-avoidance learning tasks, but perform poorly in more complex appetitive and avoidance learning tasks in which increases in locomotor activity do not improve performance, perhaps as a result of age-specific alterations in selective attention or stimulus processing. Such behavioral "anomalies" of periadolescent animals observed in traditional laboratory situations may be in some way adaptive when considered in the context of the animals' natural habitat. In terms of psychopharmacological responsiveness, periadolescent rats, when compared with younger or older animals, are less sensitive to catecholaminergic agonists but are more responsive to the catecholaminergic antagonist haloperidol. This pattern of psychopharmacological sensitivity suggests that the catecholaminergic systems may be temporarily hyposensitive during the periadolescent period. Evidence is presented that a negative feedback system in the form of dopamine autoreceptors may become functionally mature in mesolimbic brain regions during the periadolescent period. The possibility is presented that maturation of these self-inhibitory autoreceptors might result in a temporary decrease in the efficacy of mesolimbic dopamine projections, perhaps contributing to the psychopharmacological and behavioral characteristics of periadolescent animals. In support of this suggestion, evidence is reviewed indicating that the behavior of adult animals with lesions of the ventral tegmental area, a region containing cell bodies from which these mesolimbic dopaminergic projections originate, resembles that of periadolescent rats.

Effect of propranolol treatment in pregnant rats on motor activity and avoidance learning of the offspring

Rats born to mothers treated with propranolol, during days 8-22 of gestation, displayed hyperactivity in the open field which lasted up to 60 days of age and an impairment of avoidance in the shuttle box which was more marked in the male rats. Females exhibited hyperactivity in the open field but developed impaired avoidance learning only when exposed prenatally to both propranolol and hypoxia. Propranolol administration during the last term of pregnancy (days 18-22) affected mostly shuttle box performance. In contrast, hyperactivity could be induced by treatment during various stages of pregnancy, (days 8-22, 8-18, or 18-22) with the duration of hyperactivity being directly related to the length of treatment of the mothers. The possible mechanism of the disruptive effect of propranolol in the fetus and newborn is discussed.

Behavioural effects of phenobarbitone. 1. Effects in the offspring of laboratory mice

Phenobarbitone at a concentration of 187.5 mg/l in drinking fluid of breeding mice and their offspring after weaning did not affect gestation period, litter size, litter weight or pup development before weaning, although a slight retardation of weight gain after weaning occurred. This level of phenobarbitone given to mice after weaning did not affect weight gain. The average daily intake of phenobarbitone ranged from 30 to 52 mg/kg body weight depending on age and sex. Behaviour of offspring and mice treated after weaning was examined by ethological analysis of encounters between unfamiliar mice of the same sex and treatment group in a neutral enclosure. After lifelong exposure to phenobarbitone mice at 5 and 15 weeks of age showed an increased amount of scanning and exploration of the unfamiliar cage coupled with a decrease of time spent in immobility. Difference from control levels was more pronounced at 15 than at 5 weeks of age, in part because controls showed more immobility and explored less as they matured. No behavioural changes were detected in mice treated with this level of phenobarbitone after weaning. Lifelong exposure to phenobarbitone did not affect agonistic behaviour in pair-housed males at 30 weeks of age, and under these circumstances no longer stimulated exploration to a significant extent.

Ontogenesis of glucose sensitivity in the rat lateral hypothalamus: a brain slice study

The development of glucose-sensitive neurons in the lateral hypothalamus (LHA) brain slices was examined. In 60-100-day-old rats, 20% of LHA neurons were sensitive, that is, their firing rates decreased upon increase in the glucose concentration in the perfusate and increased upon decrease in glucose concentration. In 2-, 3- and 7-day-old rats, 16% of the neurons were glucose sensitive. Results confirm glucose-sensitivity in the LHA at a very early stage of ontogeny.

Maze exploration in juvenile rats treated with corticosteroids during development

The possibility of permanent damage to brain function after developmental corticosteroid treatment has been raised in connection with therapeutic use of potent synthetic corticosteroids during the period of brain development in the human fetus and infant. In the present study, brain function was evaluated in rats treated during brain development with triamcinolone acetonide (TAC). A subtoxic dose (0.9 mg/kg), which did not retard growth, was used. TAC treatment at two periods of post-embryonic brain development (day 0 and day 10 postnatal) led to deviations from normal exploration strategies in a radial maze. Treatment on day 16 gestation was effective only in males. Neonatal TAC-treatment also affected spontaneous alternation in a T-maze. Measures which reflected arousal-activity levels during behavioral tests, such as time to initiate exploration and time to complete exploration, were not affected by treatment. These results suggest that the development of complex brain function in rats can be altered by subtoxic corticosteroid treatment in the perinatal period.

Evidence for a glycinergic cortico-lateral hypothalamic inhibitory pathway in the rat

To determine the neurotransmitter involved in the frontal cortex (FC)-lateral hypothalamic (LHA) inhibitory pathway, the effects of transmitter candidates and their antagonists on FC-evoked inhibitions in LHA neurons were analyzed using multibarrel iontophoretic and extracellular recording methods. The result suggests that glycine is involved in this inhibitory pathway.

Postnatal ontogenesis of hippocampal CA1 area in rats. I. Development of dendritic arborisation in pyramidal neurons

Hippocampal pyramidal neurons (area CA1) in rats, 5, 10, 15, 24, 48 and 90 days old were studied by means of the Golgi-Cox impregnation. Using a video-computer microscope the development of dendritic arborisation was analysed and descriptive as well as quantitative data were obtained. During the suckling period (5 and 10 days) the basal dendrites, the main shaft and within the stratum moleculare branching terminal fibers of the apical dendrite were found to develop. At the weaning period (15 and 24 days) the development progressed with proliferation and ramification of the apical dendrite's lateral branches in the stratum radiatum and of its preterminal fibers branching in the stratum lacunosum. The lateral branches of the apical dendrite developed in a consequent order from the neuronal soma towards fissura hippocampi. Adult numbers of dendrites were set in first, followed by elongation and ramification of the fibers. The final number of segments was in the whole dendrite system, except of the main shaft, established before the maximum of the total dendritic length was reached. The afferent fibers termination upon the pyramidal neurons are known to be distinctly divided within the hippocampal layers. Step by step development of the individual groups of dendrites therefore probably eventuates the linkage of afferent fibers in a successive order.

Postnatal ontogenesis of hippocampal CA1 area in rats. II. Development of ultrastructure in stratum lacunosum and moleculare

Fine structure of stratum moleculare and lacunosum of the hippocampal CA1 area was studied in rats 5, 10, 15, 24, 48 and 90 days old. By means of an electron microscope the detailed description of both layers as well as quantitative data about the development of neuropile were obtained. The density of dendrites per 1 mm3 decreased during the development. Adult values were reached in both layers at the age of 24 days. Formation of axon terminals extended in the stratum lacunosum till the 24th day; in the stratum moleculare the density of terminals was highest on the 15th day, but decreased during the later period. The density of axo-dendritic synapses did not change in the whole postnatal period while that of axo-spine synapses considerably increased. Dendritic spines increased their density in both layers, having reached their maximum on the 24th day in the stratum moleculare and on the 48th day in the stratum lacunosum. The significance of fine structure development becomes apparent in light of the distribution of afferent fibers and the functional activity of pyramidal neurons. The gradual development of individual afferent systems probably results from the input information pattern conveyed by the afferents. The resulting integration of pyramidal neurons should thus be subjected to a substantial change during development.

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.

Ontogeny of ultradian and nocturnal activity rhythms in the isolated albino rat

The activity of isolated rats from 6 to 25 days of age was analyzed at 10-min intervals. Ultradian peak activity rhythms, with a rough 1-2-hr periodicity, developed between 9 and 12 days of age, and reached an extreme at 15 days. Ultradian periods of attenuated activity were however detectable even in 6-day-olds. Isolated rats at 15 days of age displayed greatly accentuated profiles characterized not only by heightened activity levels, but by an extreme cyclicity. These subjects spent most of their time oscillating between periods of marked activity and marked inactivity. Both the average and total duration of these extreme periods were greater in 15-day-olds than any of the other ages. Free-running nocturnal activity rhythms were also observed to develop in isolated rats tested in continuous darkness by the 15th day.

Ontogeny of behavioral development, arousal and stereotypes in two strains of mice

A number of reflexes and amphetamine-induced locomotor and stereotyped behavior were assesses in 8, 16, 32, 90 and 360 day old C57BL/6J and SEC/1ReJ inbred mice. The data indicate that C57 mice are more precocious for a number of neuronal and behavioral mechanisms while SEC mice are less mature at birth. In addition, there are appreciable fluctuations of these behavioral patterns throughout life. A rise in arousal levels was evident in both strains between 8 and 16 days and between 32 and 90 days of age. Three-hundred-sixty days old mice presented a general decrease in the levels of arousal. These findings are discussed in terms of neuronal and behavioral plasticity and in relation to the ontogeny of the different catecholaminergic systems which modulate excitory and inhibitory different behavioral patterns at different ages.

Deficits in passive-avoidance learning following atropine in the developing rat

The maturation of cholinergic inhibitory mechanisms that may be involved in passive-avoidance learning was studied in rats 14, 17, 21, 25, 28, and 34 days of age. Acquisition and extinction of the conditioned response were examined under saline and atropine sulfate (5 mg/kg). Learning was also tested following scopolamine hydrobromide injections (1, 4, 8 mg/kg) in rats 17 days of age and following alpha-methylatropine (5 mg/kg) in 17- and 34-day-old groups. In normal animals the rate of acquisition increased during ontogenesis, with a significant improvement between postnatal days 17 and 21, whereas the rate of extinction did not vary with age. Acquisition was impaired by atropine sulfate at all ages and even totally prevented in younger groups (14 and 17 days of age). It was also completely disrupted by scopolamine in 17-day-old rats. Extinction following acquisition under atropine was more rapid than after normal acquisition. Methyl-atropine was without effect. These results support the hypothesis of central cholinergic mechanisms involved in response suppression, already functioning in the rat 14 days of age and maturing mainly between the 17th and the 21st postnatal days.

Genetic and ontogenetic variations in locomotor activity following treatment with scopolamine or d-amphetamine

Highly inbred mice of 3 strains (A/J, DBA/2J, and C57BL/6J) were tested in an open field at 14, 21, or 28 days of age. Ten minutes prior to testing, mice received treatment of saline, scopolamine (.5 or 1.0 mg/kg of body weight), or d-amphetamine (.5, 1.0, or 5.0 mg/kg). The d-amphetamine (5.0 mg/kg) increased activity in all strains at 14 days and 28 days of age, and at 21 days significantly increased activity in all except the C5BL/6. In contrast, increased activity with the scopolamine treatment was seen in DBA/2 at 21 days, but not in A and C57BL/6 until 28 days postnatally. The data support a caudal-rostral gradient of brain development with the inhibitory cholinergic system developing more slowly than the excitatory catecholamine system. In addition, strain-specific differences in activity levels are discussed in relation to the differential rates of chloinergic maturation.

Brain damage and associated behavioral deficits following the administration of L-cysteine to infant rats

L-cysteine in doses of 1.2-1.3 mg/g, was administered to 4-day-old rats which were tested throughout development and as adults on a variety of behavioral tasks. Lesions of the type previously described from L-cysteine treatment were confirmed in several cortical and limbic structures. No neurodegenerative changes were observed in NaCl (10 mmoles/kg) controls. Surviving L-cysteine treated animals displayed no obvious impairments in motor ability or growth rate, but did show behavioral deficits when tested as adults on 3 behavioral tasks: spontaneous alternation; Lashley III maze; and pattern discrimination. Activity in the open field was significantly higher in the L-cysteine group at 20 days of age. Amphetamine, administered in doses of 1.5 to 3.0 mg/kg, had no differential effects on open-field activity in the two groups. The behavioral changes observed in L-cysteine treated animals is similar to that which has been reported in adult rats with extensive hippocampal damage.

Effect of atropine on behavioral arousal in the developing rat

Atropine-induced changes in behavioral arousal were examined in the developing rat between 21 and 40 days of age using a single exploratory trial on an elevated T-maze. Under atropine (10 mg/kg, i.p.) spontaneous activity as measured by numbers of errors, rearing responses, and stereotyped responses was significantly decreased in animals less than 25 days of age. It was significantly increased, however, between 26 and 30 days, with maximal action on the 28th day. After 30 days the effect of atropine was negligible. These results support the hypothesis of a cholinergic inhibitory mechanism controlling behavioral arousal and becoming efficient after the 25th day of life.