Behavioral lateralization in the T-maze and monoaminergic brain asymmetries

Zebrafish (Danio rerio) behavioral laterality predicts increased short-term avoidance memory but not stress-reactivity responses

Once considered a uniquely human attribute, behavioral laterality has proven to be ubiquitous among non-human animals, and is associated with several neurophenotypes in rodents and fishes. Zebrafish (Danio rerio) is a versatile vertebrate model system widely used in translational neuropsychiatric research owing to their highly conserved genetic homology, well-characterized physiological responses, and extensive behavioral repertoire. Although spontaneous left- and right-biased responses, and associated behavioral domains (e.g., stress reactivity, aggression, and learning), have previously been observed in other teleost species, no information relating to whether spontaneous motor left–right-bias responses of zebrafish predicts other behavioral domains has been described. Thus, we aimed to investigate the existence and incidence of natural left–right bias in adult zebrafish, exploiting an unconditioned continuous free movement pattern (FMP) Y-maze task, and to explore the relationship of biasedness on performance within different behavioral domains. This included learning about threat cues in a Pavlovian fear conditioning test, and locomotion and anxiety-related behavior in the novel tank diving test. Although laterality did not change locomotion or anxiety-related behaviors, we found that biased animals displayed a different search strategy in the Y-maze, making them easily discernable from their unbiased counterparts, and increased learning associated to fear cues. In conclusion, we showed, for the first time, that zebrafish exhibit a natural manifestation of motor behavioral lateralization which can influence aversive learning responses.

Molecular basis of chronic stress-induced hippocampal lateral asymmetry in rats and impact on learning and memory

Neurochemical lateralization has been demonstrated in the rat brain suggesting that such lateralization might contribute to behavior. Thus, the aim of the present study was to examine neurochemical asymmetry in the hippocampus, molecular basis of neurochemical lateralization and its impact on spatial learning and memory. Changes in noradrenaline content, tyrosine hydroxylase (TH) were studied in the right and left hippocampus of naive control and chronically isolated rats, by applying TaqMan RT-PCR and Western blot analysis. Hippocampal-based spatial learning and memory were evaluated using the Barnes maze. In control rats an asymmetrical right-left distribution of noradrenaline content and gene expression of catecholamine synthesizing enzyme was found. Chronic psychosocial stress further emphasized asymmetry. Isolation stress reduced noradrenaline content only in the right hippocampus. No changes were observed in gene expression and protein levels of TH in the right hippocampus, whereas expression of catecholamine synthesizing enzyme was elevated in the left hippocampus. Reduced noradrenaline content in the right hippocampus did not cause impairment in spatial learning and memory. Our findings suggest that chronic psychosocial stress reduces noradrenaline stores in the right hippocampus which may be caused by molecular asymmetry, but it does not affect spatial learning and memory.

Cerebral correlates of visual lateralization in Sepia

The common cuttlefish, Sepia officinalis (cephalopod mollusc) has recently become a relevant model for studying the setting-up of brain asymmetry among invertebrates. As the animals age from 3 to 30 days post hatching, they progressively develop a left-turning bias resulting from an eye-use preference. The aim of this study is to investigate whether anatomical (vertical, peduncle, inferior buccal, and optic lobes) or neurochemical (monoamines in optic lobes) brain asymmetries are present in the cuttlefish brain at 3 or at 30 post hatching days; and whether these correlate with side-turning preferences. We here find brain and behavioral asymmetry only at 30 post hatching days. Cuttlefish displayed a significant population bias towards a larger right peduncle lobe, and higher monoamine concentration in the left optic lobe (i.e. serotonin, dopamine and noradrenaline). None of these brain asymmetries were correlated to the studied side-turning bias. However, we found individual variation in the magnitude of the vertical and optic lobes asymmetry. A striking correlation was found with the behavioral results: the larger the right optic lobe and the right part of the vertical lobe, the stronger the bias to turn leftwards. To our knowledge, this is the first study to demonstrate a relationship at the individual level between brain and behavioral asymmetries in invertebrates.

Analysis of behavioral asymmetries in the elevated plus-maze and in the T-maze

When studying functional asymmetries in normal laboratory rats, several behavioral tests have been applied and proven their utility, including turning in rotometers or open-fields, handedness in paw usage, T-maze alternation, and others. Here, we analyzed male Wistar rats in two tests, namely the elevated plus-maze and the T-maze. In these tests, behavioral asymmetries are rather likely to occur, since the animals have to show several types of turns towards the left or right when ambulating through these environments. In a first study using the plus-maze, we provide detailed data on (A) the types of turns which the animals showed when changing their direction within arms (i.e., 180 degrees turns), and (B) the types of turns when proceeding from one arm to an adjacent one (i.e., 90 degrees turns). With respect to asymmetry, we found moderate biases in favor of the right. On the 1st day of plus-maze testing, there was a trend for more rightward turns within arms. On the 2nd day of testing, there was a trend for turns towards the right when alternating between arms of the plus-maze. In a 2nd study, we asked for asymmetries in the plus-maze in animals, which had been treated acutely with the psychostimulatory amphetamine analogue 3,4-methylene-dioxymethamphetamine (MDMA). Psychostimulants drugs, especially amphetamine, have repeatedly been used before in work on functional asymmetry, since they can enhance or reveal asymmetries in normal rats. MDMA had dose-dependent effects on activity, which affected turns within arms, and turns between arms; however, there was only sparse evidence with respect to asymmetry. Interestingly, and if at all, asymmetry was in favor of the right. Finally, we present data for behavior in the T-maze, where we used a spontaneous test version, that is, the animals could explore the maze but had no task to solve. Asymmetries were measured as turns within the start arm (180 degrees), and as left- or rightward turns between arms (90 degrees ) at the T-point of the maze. In both measures, we again obtained evidence for asymmetries in favor of the right. This work supports previous studies showing that the T-maze is suitable to analyze behavioral asymmetries in rats. In addition, it provides new evidence with respect to the elevated plus-maze, indicating that this standard tool of anxiety research may also be useful in research on behavioral asymmetries and their underlying brain mechanisms. Behavioral biases in favor of the right, as shown here, have often been reported before, especially with Wistar rats. Such biases should be taken into account, since they can serve as an approach to study brain/behavior relationships, and since they may affect the outcome of physiological manipulations or behavioral trainings.

Asymmetrical hatching behaviors: The development of postnatal motor laterality in three precocial bird species

The effects of asymmetrical hatching behaviors on the development of turning bias and footedness in domestic chicks, bobwhite quail, and Japanese quail chicks were examined. Control tests with incubator reared domestic chicks and bobwhite quail revealed significant individual and population left-side turning bias and right footedness. When late stage hatching behaviors were disrupted, population laterality was not evident and individual laterality was reduced. By contrast, Japanese quail chicks demonstrated no population turning bias or footedness and only weak individual biases. Disruption of hatch behaviors further decreased laterality. Examination of discarded eggshells showed significant differences in the degree of rotation made to cut out of the egg by Japanese quail versus domestic chicks and bobwhite quail. Taken together these findings suggest that the counterclockwise hatching behaviors that are characteristic of many precocial bird species serve to facilitate the development of motor laterality at both the individual and population level.

Postnatal maturation of cortical serotonin lateral asymmetry in gerbils is vulnerable to both environmental and pharmacological epigenetic challenges

Long-term effects of postnatal differential rearing conditions and/or early methamphetamine (MA) application on serotonin (5-HT) fibre density were investigated in several cortical areas of both hemispheres of gerbils. The aim of this study was twofold: (1) Is the 5-HT fibre innervation of the cerebral cortex lateralised, and (2) if so, do postnatal environmental conditions and/or an early drug challenge interfere with development of 5-HT cerebral asymmetries? For that purpose, male gerbils were reared either under semi-natural or restricted environmental and social conditions, under both conditions once (on postnatal day 14) being treated with either a single dose of MA (50 mg/kg, i.p.) or saline. On postnatal day 110, 5-HT fibres were immunohistochemically stained and innervation densities quantified in prefrontal cortex, insular cortex, frontal cortex, parietal cortex, and entorhinal cortex. It was found that (1) 5-HT innervation in the cerebral cortex was clearly lateralised; (2) direction and extent of this asymmetry were not uniformly distributed over the different areas investigated; (3) both early methamphetamine challenge and rearing condition differentially interfered with adult 5-HT cerebral asymmetry; (4) combining MA challenge with subsequent restricted rearing tended to reverse the effects of MA on 5-HT cerebral asymmetry in some of the cortical areas investigated; and (5) significant responses in 5-HT cerebral asymmetry only occurred in prefrontal and entorhinal association cortices. The present findings suggest that the ontogenesis of cortical laterality is influenced by epigenetic factors and that disturbances of the postnatal maturation of lateralised functions may be associated with certain psychopathological behaviours.

Response of the GABAergic and dopaminergic mesostriatal projections to the lesion of the contralateral dopaminergic mesostriatal pathway in the rat

Although dopamine is the main neurotransmitter in the mesostriatal system, recent studies indicate the existence of two nigrostriatal GABAergic projections: one arising from neurons immunoreactive for GABA, glutamic acid decarboxylase (GAD67), and parvalbumin (PV) lying in the substantia nigra pars reticulata (nigrostriatal GABA cells) and the other arising from a subpopulation of dopaminergic neurons lying in the substantia nigra pars compacta and ventral tegmental area, which under normal conditions, contains mRNA for GAD65 (one of the two isoforms of glutamic acid decarboxylase), but which is not immunoreactive for GABA and GAD65 (nigrostriatal dopaminergic [DA]/GABA cells). With the aim of improving our knowledge about the interaction between the nigrostriatal system of both brain hemispheres, we have studied the response of these three components of the mesostriatal system (GABA, DA/GABA, and DA) to the lesion of the contralateral mesostriatal DA pathway, by using morphological and neurophysiological techniques. Our findings show that, in the side contralateral to the lesion, (1) the number of nigrostriatal GABA cells increases from 6% to 17% with respect to the total number of nigrostriatal cells, (2) the soma of DA/GABA cells becomes immunoreactive for GABA and GAD65, and (3) there is an increase in the firing rate and burst activity of DA-neurons, except in those projecting to the striatum, which may be under the action of the GABA hyperactivity. Taken together, our results suggest that the GABAergic components of the mesostriatal projection play a regulatory role on the DA component, activated or upregulated after contralateral DA lesion and are probably addressed to restoring the functional symmetry in basal ganglia and to slowing down the contralateral progression of DA-cell degeneration in Parkinson's disease.

Response of GABAergic cells in the deep mesencephalic nucleus to dopaminergic cell degeneration: an electrophysiological and in situ hybridization study

The deep mesencephalic nucleus (DMN) is a large midbrain reticular region located between the substantia nigra compacta and the superior colliculus. It contains GABAergic cells that share striatal afferents, thalamic and collicular efferents, as well as neurochemical and electrophysiological similarities, with those of the substantia nigra reticulata. In the present paper we used electrophysiological (firing rate and firing pattern) and morphological (densitometric analysis of in situ hybridization histochemical labeling for glutamic acid decarboxylase (GAD)65 and GAD67 mRNA) techniques, to study the response of DMN GABAergic cells to the degeneration of nigral dopaminergic cells. Our results showed that unilateral dopaminergic cell loss (after injection of 6-hydroxydopamine in the medial forebrain bundle) induces a bilateral and symmetrical increase in both firing rate and GAD67 mRNA levels and a decrease in GAD65 mRNA levels. These findings support the involvement of DMN GABAergic cells in the basal ganglia modifications that follow dopaminergic cell loss, also suggesting its participation in the pathophysiology of Parkinson's disease. The symmetry of effects, together with its recently reported bilateral projections to the thalamus and superior colliculus, suggest that unlike substantia nigra reticulata, DMN is involved in the interhemispheric regulation of basal ganglia, probably keeping their functional symmetry even after asymmetric lesions.

Effect of innate direction bias on T-maze learning in rats: implications for research

Adult, male Wistar rats showed substantial left (22.2%) or right (52.8%) bias in spontaneous arm preference in the T-maze; this bias was consistent over 2 days of testing separated by a 30 day interval. Left and right biased rats learnt very rapidly when trained to enter the arm ipsilateral to the bias; learning was significantly poorer or did not occur in the contralateral arm. This contralateral learning difficulty was particularly evident when transfer of learning was assessed. Right-biased rats were more impaired in contralateral learning than left-biased rats. Unbiased rats (25%) also showed learning difficulties. This study has important implications for spatial tasks of learning and memory; with specific reference to the T maze, it is concluded that animals should be preselected for capacity to learn in both arms, randomization into experimental and control groups should be stratified for spontaneous arm bias, and original learning should be directed towards the contralateral arm while transfer of learning, if required, can be directed into the ipsilateral arm.

The deep mesencephalic nucleus as an output center of basal ganglia: morphological and electrophysiological similarities with the substantia nigra

The deep mesencephalic nucleus (DMN) is a large midbrain reticular region between the superior colliculus, the substantia nigra compacta, the periaqueductal gray, and the medial geniculate body. Although some data suggest that it is involved in nociception and visceral control, its functions remain unclear. In the present study, by using morphological (combination of anterograde and retrograde tracers with immunocytochemistry and in situ hibrydization) and electrophysiological (firing activity and transynaptic response to striatal stimulation) methods, we show that a subpopulation of DMN cells shares many morphological and electrophysiological characteristics with those of the substantia nigra reticulata (SNR). These similarities include the following: 1) firing rate, firing pattern, and conduction velocity; 2) expression of GAD65, GAD67, and PV; 3) excitatory and inhibitory inputs from the striatum; and 4) projections to the ventral thalamus, superior colliculus, and pedunculopontine tegmental nucleus. Some differences were also found. In comparison with SN, DMN cells and striatal afferents are more sparsely distributed and they show conspicuous contralateral projections to the thalamus and superior colliculus. This suggests that, similarly to the SNR, the DMN acts as an output center of basal ganglia and probably facilitates the inter-hemispheric regulation of these centers.

Asymmetrical hatching behaviors influence the development of postnatal laterality in domestic chicks (Gallus gallus)

Lateralized motor behaviors have been reported in some avian species. For instance, footedness has been reported in parrots and domestic chicks, and turning biases have been reported in such species as quail and domestic chicks. This study examined the effects of asymmetrical hatching behaviors on the development of lateralized turning bias and footedness in domestic chicks. Asymmetrical hatching behaviors are counter-clockwise full body turns that many precocial birds make to escape the egg. To study the role of such coordinated prenatal motor behaviors in the development of lateralization, hatching behaviors were systematically disrupted following pipping. Subjects were subsequently tested on two measures of laterality: footedness and turning bias. Results indicated a significant reduction in individual and group lateralization for both measures. These findings suggest that the hatching behaviors found in domestic chicks serve to induce the development of strong motor biases at both the individual and population level.

Arousal mediates relations among medial paw preference, lateral paw preference, and spatial preference in the mouse

Rodents exhibit two well-documented behavioural lateralities: spatial preference and paw preference. Waters and Denenberg [36] have identified two seemingly independent factors of paw preference: medial and lateral paw preference. In the present work, the relations among spatial preference (SP), medial paw preference (MPP), and lateral paw preference (LPP) during states of high and low arousal were examined. These preferences were measured in terms of direction, which describes the side of the preference regardless of strength, and degree, which describes the strength of the preference regardless of direction. A strong positive correlation between LPP and SP was found during periods of high, but not low, arousal. A negative correlation between the degree components of LPP and MPP was found during the low, but not high, arousal periods.

Right-left asymmetry of D1- and D2-receptor density is lost in the basal ganglia of old rats

In the present study a left-right asymmetry in both D1- and D2-receptor density in the caudate-putamen nucleus is shown and a lateralisation of D2-receptor distribution in the accunbens nucleus is also described. In old animals in which D1- and D2-receptors density is decreased, the dopamine receptor asymmetries are lost.

Permanent dopaminergic alterations in the n. accumbens after prenatal stress

It has been suggested that stress during the initial stages of human life may serve as a predisposing factor to mental illness. Recently, we reported that in pregnant rats, stress induces an increase of behavioral depression in the female offsprings when adult. This article describes the effect of prenatal stress on central dopaminergic transmission during adulthood. The offspring of stressed mothers showed an increase of behavioral depression in the Porsolt test and a reduction of DOPAC, HVA, and DOPAC/DA index in the n. accumbens. The effect on the right accumbens was more marked than on the left. A great body of information exists to suggest that depression is related to a decrease of dopaminergic neurotransmission, and the present data provide new evidence in support of the hypothesis that maternal stress during gestation increases the risk of depression in the offspring. We are also reporting a hitherto uncommented relationship between behavioral depression in the Porsolt test and the decrease of dopamine transmission in the n. accumbens.

Asymmetrical modulation of immune reactivity in left- and right-biased rats after ipsilateral ablation of the prefrontal, parietal and occipital brain neocortex

We report here on the lateralized brain immunomodulation in male Wistar rats, a phenomenon related to the rotational bias of animal and the site of cortical lesion. Rats assigned to left- and right-rotators in a cylindrical Plexiglass rotometer were subjected to the ablation of the ipsilateral prefrontal cortex (PFC), parietal cortex (PC) and occipital cortex (OC) and sensitized with bovine serum albumin (BSA) in complete Freund's adjuvant. Intact and sham-lesioned left-biased animals demonstrated increased Arthus and delayed hypersensitivity skin reactions and antibody production to BSA in comparison with corresponding right-biased animals. PFC ablation decreased humoral and cellular immune responses to BSA in left- but increased in right-biased rats. Lesioning of PC decreased humoral immune reactions in left- but increased in right-rotating animals. OC ablation failed to produce immunological abnormalities. These results suggest that immunopotentiation is associated with the left neocortex, and immunosuppression with the right neocortex. The prefrontal cortex appears to be particularly associated with immune reactions.

Maternal ingestion of tyrosine during rat pregnancy modifies the offspring behavioral lateralization

It was previously reported that oral administration of tyrosine (500 mg/kg) to pregnant rats increases tyrosine and monoamines level in the fetal brain and modifies locomotion during postnatal life. In the present study, it was found that this treatment alters behavioral lateralization in the offspring. Neonatal rats whose mothers received tyrosine during the second half of gestation showed a low level of absolute and population laterality in both tail and head movements. The alteration of behavioral lateralization was also found during postnatal development and during adulthood. The T-maze behavioral ontogeny was different for tyrosine-mother and sham-treated or untreated mother rats. During adulthood, the T-max lateralization after stress sessions (a procedure that decreases alternation behavior and facilitates the quantification of behavioral lateralization) was also different in control and tyrosine-mother groups. Neonatal and adult rats showed an increase in right-side movements probability. These data provide evidence that maternal ingestion of a catecholamine precursor during gestation may induce a long-lasting modification of the behavioral lateralization of the offspring.

Ontogenic development of brain asymmetry in dopaminergic neurons

In the present study the right-left brain asymmetry of central dopamine (DA) systems during postnatal brain development is evaluated. DA and dihydroxyphenylacetic acid (DOPAC) levels increased from neonatal to adult life in both the forebrain and mesencephalon. This increase was not similar in the right and left brain sides. From neonatal life to adulthood a fall was observed in (a) DA percentage in the DA high-brain side in the mesencephalon and (b) DOPAC percentage in the DOPAC high-brain side in both the forebrain and mesencephalon. The percentage of lateralized rats (more than 65% of DA or DOPAC levels in either brain side) also decreased during ontogeny. Thus, biochemical lateralization decreases during ontogeny. The right-left brain correlation for DA level and DA turnover was used to evaluate the inter-hemispheric regulation of dopaminergic systems. The correlation coefficient was near to 0 during postnatal life and around -0.8 during adulthood in both forebrain and mesencephalon. Taken together, these data suggest that the ontogenic decrease of in brain asymmetry for DA or DOPAC levels is related to the postnatal development of an inter-hemispheric regulatory system that control dopaminergic neurons activity.

Ontogeny of T-maze behavioral lateralization in rats

Behavioral lateralization has been reported in both humans and animals. In humans, lateralization can be detected in neonates and increases to adult levels during postnatal development. Recently we reported lateralization of head and tail movements in neonatal rats. However, the postnatal ontogeny of lateralization in animals has not been previously studied. This work presents a study of rat behavioral lateralization in the T-maze test during postnatal development (from day 30 to day 60). A decrease was found in absolute (percent preferred-side choices) and population (right-left arm choices) laterality between day 30 and day 45 of postnatal life. The lateralization degree remained unchanged between days 45 and 60. Because behavioral alternation increases from day 30 to day 45, the present data suggest that animal lateralization of behavior is a phenomenon that remains throughout the subject's life span, but whose behavioral quantification could be concealed by the ontogenic increases of other phenomena such us behavioral alternation. This hypothesis could explain the high level of lateralization in neonatal rats, the low level of rat lateralization during adulthood, and the increases in lateralization induced by stress.

Neonatal lateralization of behavior and brain dopaminergic asymmetry

Behavioral lateralization has previously been reported in adult animals. This work presents a study of behavioral laterality and spontaneous alternation behavior in 156 neonatal rats (39 litters with two males and two females per litter). The initial tail and head lateral movements (axis body-tail or head higher than 30 degrees) were recorded after neonatal rats were gently placed along a straight line drawn on a glass surface. This test was repeated 10 times. A leftward population lateralization was found for tail movement while head movement was rightward. The alternation behavior was lower (10-25%) than that previously reported for adult rats (80-90%) and than that expected if movement was made at random (50%). Males were more lateralized than females for head and tail movements. No sex differences were observed for behavioral alternation. In 2-day-old rats (10 litters with one male and one female per litter), the right brain side had a higher content in dopamine (mesencephalon) and DOPAC (proencephalon) than the left brain side. Thus, we concluded that behavioral and biochemical asymmetries in animals are conditioned by phenomena present during prenatal or early neonatal (first hours after birth) life and that spontaneous alternation behavior is not present during the earlier stages of postnatal development.

Nigrostriatal dopaminergic cell activity is under control by substantia nigra of the contralateral brain side: electrophysiological evidence

The biochemical balance between right- and left-ascending DA systems is an essential factor to regulate behavioral lateralization. However, there is no electrophysiological evidence for the regulation of interhemispheric DA systems. In the present paper we report electrophysiological evidence supporting the hypothesis that the A9 DA cells are under control of the contralateral substantia nigra. The activity of more than 80% of the A9 cells recorded was affected by contralateral SN stimulation. This is a very high proportion because the previously reported response of A9 cells to ipsilateral caudate stimulation is proportionally lower. The potency of a stimulus, estimated as the number of action potentials induced or inhibited by each electrical stimulation of the contralateral substantia nigra or by the percentage of modification in the number of action potentials induced or inhibited in relation to the spontaneous potential expected, was also high. The response to the contralateral stimulation was complex. Fifty-four percent of all the DA cells studied showed more than a single response. Forty-four percent showed at least one stimulation and at least one inhibition. Because 1) the percentage of cells with at least one stimulation (70%) was higher than the percentage of cells with at least one inhibition (56%), and 2) the potency of stimulations was higher than the inhibition potency, the present data provide evidence that contralateral control of A9 cells is mainly excitatory. The percentage of cells activated by contralateral stimulation was high, between 30 ms and 220 ms and between 400 ms and 700 ms. The probability of inhibition was higher than the probability of activation between 10 ms and 30 ms and between 230 and 380 ms.(ABSTRACT TRUNCATED AT 250 WORDS)

Behavioral lateralization in rats: prenatal stress effects on sex differences

The possible relationship between human and animal asymmetries is currently a controversial point. In the present study we report that, after practice, rat behavioral asymmetry presents important similarities with human laterality. Thus, rat behavior presents: (1) absolute and population laterality to the same degree and with the same right-bias as that reported for humans; (2) the female has less absolute laterality but similar population laterality to the male; (3) male-female differences for behavioral laterality are modified by prenatal stress as occurs with similar other hormone-regulated behavior.

Asymmetries in thigmotactic scanning: evidence for a role of dopaminergic mechanisms

In two experiments, the influence of spontaneous asymmetries in thigmotactic scanning was analyzed on spontaneous and drug-induced behavior in the rat. The side of the face with which an animal performed more scanning in a baseline test was defined as the dominant vibrissae side. In experiment 1, repeated testing of either spontaneous thigmotactic scanning, or scanning after apomorphine or amphetamine, yielded no evidence that rats would preferably use one side of the face for scanning, when re-exposed to the same environment. However, an asymmetry in turning was observed both under apomorphine and amphetamine, that is, turning away from the dominant vibrissae side. In experiment 2, an influence of spontaneous asymmetries in scanning was found on behavioral asymmetries induced by unilateral vibrissae removal. Only animals, in which the vibrissae of the non-dominant side had been removed, showed more scanning with the intact vibrissae side, both undrugged and after apomorphine. Turning under apomorphine was more pronounced in animals in which the vibrissae on the dominant side had been removed. These animals showed an asymmetry in turning towards the intact vibrissae side. Furthermore, in both experiments we found evidence for left/right differences in turning or scanning. The results are discussed with respect to possible endogenous substrates of asymmetry, such as within the mesostriatal dopamine system.

Right-sided population bias in male rats: role of stress

Recent studies have shown that rats develop a pronounced right-sided population bias when tested repeatedly over several days in a shock-escape T-maze paradigm. In the present study we sought to determine if this bias was the result of learning or of the repeated exposure to a stressor (footshock). Rats were tested in the T-maze for 5 trials on each of 6 consecutive days. One group of rats (HS-C) was allowed a free left or right choice on each trial whereas another group of rats (HS-A) was forced to alternate left and right turns on each trial for the first 5 days of testing and then allowed a free choice on Day 6. The population and individual laterality of the HS-A group did not differ from that of the HS-C group, indicating that learning does not account for the biases. A third group of rats (LS-C) was tested using a lower shock level; this decrease in stressor intensity delayed the development of a right-sided population bias without affecting the development of individual laterality. These results indicate that repeated exposure to a stressor, rather than learning, is primarily responsible for the marked right-sided population bias observed in the T-maze.

Interhemispheric regulation of dopaminergic ascending systems

There are previous evidences of biochemical and functional interhemispheric asymmetry for central dopaminergic systems. The purpose of the current study was to establish an evidence of the existence of mechanisms for interhemispheric regulation of dopaminergic systems. The initial hypothesis was that to compensate the spontaneous interhemispheric asymmetry, the brain side with lower dopamine (DA) concentration show a higher DA turnover. The ratio of metabolite/neurotransmitter was computed as an index of presynaptic turnover rate. Both striatum and hippocampus showed a DOPAC/DA index higher in the brain side with a lower DA concentration. In addition, we found an inverse relation between the interhemispheric index of DA and interhemispheric index of DA turnover. Taken together the present data provide evidence of an interhemispheric regulation for the DA-innervation of both striatum and hippocampus in normal unlesioned rats.