Asymmetry in D-2 binding in female rat striata

Becoming a mother entails anatomical changes in the ventral striatum of the human brain that facilitate its responsiveness to offspring cues

In mothers, offspring cues are associated with a powerful reinforcing value that motivates maternal care. Animal studies show that this is mediated by dopamine release into the nucleus accumbens, a core component of the brain's reward system located in the ventral striatum (VStr). The VStr is also known to respond to infant signals in human mothers. However, it is unknown whether pregnancy modifies the anatomy or functionality of this structure, and whether such modifications underlie its strong reactivity to offspring cues. Therefore, we analyzed structural and functional neuroimaging data from a unique pre-conception prospective cohort study involving first-time mothers investigated before and after their pregnancy as well as nulliparous control women scanned at similar time intervals. First, we delineated the anatomy of the VStr in each subject's neuroanatomical space and examined whether there are volumetric changes in this structure across sessions. Then, we tested if these changes could predict the mothers' brain responses to visual stimuli of their infants. We found decreases in the right VStr and a trend for left VStr reductions in the women who were pregnant between sessions compared to the women who were not. Furthermore, VStr volume reductions across pregnancy were associated with infant-related VStr responses in the postpartum period, with stronger volume decreases predicting stronger functional activation to offspring cues. These findings provide the first indications that the transition to motherhood renders anatomical adaptations in the VStr that promote the strong responsiveness of a mother's reward circuit to cues of her infant.

Age- and sex-dependent effects of methamphetamine on cognitive flexibility and 5-HT2C receptor localization in the orbitofrontal cortex of Sprague-Dawley rats

Adolescents and females experience worse outcomes of drug use compared to adults and males. This could result from age- and sex-specific consequences of drug exposure on brain function and cognitive behavior. In the current study, we examined whether a history of intravenous methamphetamine (METH) self-administration impacted cognitive flexibility and 5-HT_2CR localization in the orbitofrontal cortex (OFC) in an age- and sex-dependent manner. Strategy shifting was assessed in male and female Sprague-Dawley rats that had self-administered METH (0.08 mg/kg/inf) or received non-contingent infusions of saline during periadolescence or young adulthood. After all rats reached adulthood, they were tested in an operant strategy shifting task and their brains were subsequently analyzed using immunofluorescence to quantify co-localization of 5-HT_2C receptors with parvalbumin interneurons in the OFC. We found that adolescent-onset females were the only group impaired during discrimination and reversal learning, but they did not exhibit changes in localization of 5-HT_2C receptors. In contrast, adult-onset males exhibited a significant increase in co-localization of 5-HT_2C receptors within parvalbumin interneurons in the left hemisphere of the OFC. These studies reveal that age and sex differences in drug-induced deficits in reversal learning and 5-HT_2CR co-localization with parvalbumin interneurons are dissociable and can manifest independently. In addition, these data highlight the potential for certain treatment approaches to be more suitable in some populations compared to others, such as alleviating drug-induced cognitive deficits as a focus for treatment in adolescent females.

Anticlockwise or clockwise? A dynamic Perception-Action-Laterality model for directionality bias in visuospatial functioning

Orientation bias and directionality bias are two fundamental functional characteristics of the visual system. Reviewing the relevant literature in visual psychophysics and visual neuroscience we propose here a three-stage model of directionality bias in visuospatial functioning. We call this model the 'Perception-Action-Laterality' (PAL) hypothesis. We analyzed the research findings for a wide range of visuospatial tasks, showing that there are two major directionality trends in perceptual preference: clockwise versus anticlockwise. It appears these preferences are combinatorial, such that a majority of people fall in the first category demonstrating a preference for stimuli/objects arranged from left-to-right rather than from right-to-left, while people in the second category show an opposite trend. These perceptual biases can guide sensorimotor integration and action, creating two corresponding turner groups in the population. In support of PAL, we propose another model explaining the origins of the biases - how the neurogenetic factors and the cultural factors interact in a biased competition framework to determine the direction and extent of biases. This dynamic model can explain not only the two major categories of biases in terms of direction and strength, but also the unbiased, unreliably biased or mildly biased cases in visuosptial functioning.

Hemispheric differences in the mesostriatal dopaminergic system

The mesostriatal dopaminergic system, which comprises the mesolimbic and the nigrostriatal pathways, plays a major role in neural processing underlying motor and limbic functions. Multiple reports suggest that these processes are influenced by hemispheric differences in striatal dopamine (DA) levels, DA turnover and its receptor activity. Here, we review studies which measured the concentration of DA and its metabolites to examine the relationship between DA imbalance and animal behavior under different conditions. Specifically, we assess evidence in support of endogenous, inter-hemispheric DA imbalance; determine whether the known anatomy provides a suitable substrate for this imbalance; examine the relationship between DA imbalance and animal behavior; and characterize the symmetry of the observed inter-hemispheric laterality in the nigrostriatal and the mesolimbic DA systems. We conclude that many studies provide supporting evidence for the occurrence of experience-dependent endogenous DA imbalance which is controlled by a dedicated regulatory/compensatory mechanism. Additionally, it seems that the link between DA imbalance and animal behavior is better characterized in the nigrostriatal than in the mesolimbic system. Nonetheless, a variety of brain and behavioral manipulations demonstrate that the nigrostriatal system displays symmetrical laterality whereas the mesolimbic system displays asymmetrical laterality which supports hemispheric specialization in rodents. The reciprocity of the relationship between DA imbalance and animal behavior (i.e., the capacity of animal training to alter DA imbalance for prolonged time periods) remains controversial, however, if confirmed, it may provide a valuable non-invasive therapeutic means for treating abnormal DA imbalance.

Brain hemispheric differences in the neurochemical effects of lead, prenatal stress, and the combination and their amelioration by behavioral experience

Brain lateralization, critical to mediation of cognitive functions and to "multitasking," is disrupted in conditions such as attention deficit disorder and schizophrenia. Both low-level lead (Pb) exposure and prenatal stress (PS) have been associated with mesocorticolimbic system-mediated executive-function cognitive and attention deficits. Mesocorticolimbic systems demonstrate significant laterality. Thus, altered brain lateralization could play a role in this behavioral toxicity. This study examined laterality of mesocorticolimbic monoamines (frontal cortex, nucleus accumbens, striatum, midbrain) and amino acids (frontal cortex) in male and female rats subjected to lifetime Pb exposure (0 or 50 ppm in drinking water), PS (restraint stress on gestational days 16-17), or the combination with and without repeated learning behavioral experience. Control males exhibited prominent laterality, particularly in midbrain and also in frontal cortex and striatum; females exhibited less laterality, and this was primarily striatal. Lateralized Pb ± PS induced neurotransmitter changes were assessed only in males because of limited sample sizes of Pb + PS females. In males, Pb ± PS changes occurred in left hemisphere of frontal cortex and right hemisphere of midbrain. Behavioral experience modified the laterality of Pb ± PS-induced neurotransmitter changes in a region-dependent manner. Notably, behavioral experience eliminated Pb ± PS neurotransmitter changes in males. These findings underscore the critical need to evaluate both sexes and brain hemispheres for the mechanistic understanding of sex-dependent differences in neuro- and behavioral toxicity. Furthermore, assessment of central nervous system mechanisms in the absence of behavioral experience, shown here for males, may constitute less relevant models of human health effects.

Lateralized and sex-dependent behavioral and morphological effects of unilateral neonatal cerebral hypoxia-ischemia in the rat

Neonatal cerebral hypoxia-ischemia (HI) is an important cause of neurological deficits. The Levine-Rice model of unilateral HI is a useful experimental tool, but the resulting brain damage is mainly restricted to one hemisphere. Since the rat presents morphological and biochemical asymmetries between brain hemispheres, behavioral outcome from this model is probably dependent on which hemisphere is damaged. We here investigated the effects of sex and lesioned hemisphere on the outcome of open field, plus maze, inhibitory avoidance and water maze tasks in adult rats previously submitted to neonatal unilateral HI. Females were more active than males in some of studied parameters and males presented better spatial learning. Hypoxia-ischemia caused spatial deficits independently of sex or damaged hemisphere. Right-HI increased locomotion only in males and caused working memory in females and on aversive learning in both males and females. Morphological analysis showed that right-HI animals presented greater reduction of ipsilateral striatum area, with females being more affected. Interestingly, males showed greater hippocampal volume. These results show that task performance and cerebral damage extension are lateralized and sex-dependent, and that the right hemisphere, irrespective of sex, is more vulnerable to neonatal cerebral hypoxia-ischemia.

Loss of asymmetry in D2 receptors of putamen in unaffected family members at increased genetic risk for schizophrenia

OBJECTIVE

Dopamine dysregulation has been implicated in the pathophysiology of schizophrenia. The present study was performed to examine whether unaffected relatives at high genetic risk of schizophrenia have dopamine dysregulation in comparison with healthy controls.

METHOD

Eleven unaffected relatives from families with two or more first- or second-degree relatives with schizophrenia (n = 9) or with a monozygotic schizophrenic twin (n = 2) and 11 age- and sex-matched controls were examined using positron emission tomography (PET) with [(11)C] raclopride. Subjects also underwent extensive neuropsychological testing.

RESULTS

Subjects with high genetic risk showed a loss of asymmetry of D(2) receptors in the putamen in comparison with healthy controls. In addition, they showed significantly poorer performance on neuropsychological tests than controls.

CONCLUSION

Our results suggest that dopamine dysregulation and neuropsychological dysfunction may be present in subjects at high genetic risk of schizophrenia. However, further studies are required to confirm these findings.

Asymmetries of cerebral neuroanatomy

The mammalian cerebral cortex is asymmetrical. One hemisphere does not contain cortical areas or architectonic patterns, histological features, ultrastructural characteristics, or connectivities of the neurons that are not present in the other: homologous areas on the two sides may differ only in size. Asymmetry has directionality: two-thirds of human brains have plana temporale that are larger on the left. Conversely, roughly the same number of non-human brains show asymmetry in one direction as in the other. Asymmetry has magnitude: some brains show a large asymmetry, others show no asymmetry in a given area. Symmetrical areas are larger than their asymmetrical counterparts, which reflects fewer neurons in the latter. Indirect evidence points to variable asymmetry in the germinal zones in the production of symmetrical or asymmetrical cortical areas. These areas differ in their patterns of callosal connections. Fewer connections are seen in the asymmetrical cases, paralleling the smaller number of neurons. The symmetrical cases contain connections that are more widely distributed. These findings of different numbers of neurons and different proportions of callosal connections suggest that symmetrical and asymmetrical cortical areas may have different functional properties.

Mapping the amphetamine-evoked changes in [11C]raclopride binding in living rat using small animal PET: Modulation by MAO-inhibition

The performance of small animal PET for neuroreceptor studies in a psychopharmacological challenge paradigm is not yet well-described. Therefore, we used microPET and [(11)C]raclopride to map the availability of dopamine D(2/3) receptors in brain of anesthetized rats, first in a baseline condition, and again after challenge with saline or d-amphetamine. Parametric maps of the specific binding (binding potential, pB) were calculated using a reference tissue input from cerebellum, and spatially normalized to a digitized stereotaxic coordinate system for rat brain. In volumes of interest (VOIs), the mean baseline pB (n=6) was 2.05 in dorsal striatum (caudate-putamen), and 1.34 in ventral striatum (nucleus accumbens), and did not significantly differ upon retest 2 h later. The availability of [(11)C]raclopride binding sites at baseline was 8% higher in the right striatum. Challenge with amphetamine sulfate (1 mg/kg, i.v., n=4) decreased pB by 19% in both ventral and dorsal striatum. We have earlier predicted that blockade of monoamine oxidase (MAO) should potentiate the amphetamine-evoked dopamine release, thus enhancing the displacement of [(11)C]raclopride binding in vivo. However, pretreatment of rats with pargyline hydrochloride (4 mg/kg, n=4; 20 mg/kg, n=4) 1 day prior to PET did not potentiate the amphetamine-evoked reduction in dopamine receptor availability within the extended striatum. We conclude that small animal PET can be used to investigate stimulant-induced dopamine release, but that the spatial resolution is insufficient to detect differences between relative changes in dorsal vs. ventral divisions of the rat striatum. Furthermore, the present results do not reveal potentiation of the amphetamine-evoked release of dopamine in rats with MAO inhibition.

Effects of laterality and sex on cognitive strategy in a water maze place learning task and modification by nicotine and nitric oxide synthase inhibition in rats

The aim of the present study was to investigate sex differences in learning strategies and to elucidate the mechanisms, which may underlie these differences. In two separate experiments, rats were presented with different strategies that could be employed to learn the position of a platform in a water maze (WM); furthermore, rats received treatments that could influence these strategies. In the first experiment, we demonstrated that the response-learning paradigm can be applied to the WM and can be compared with visually cued learning and reversal learning. Naïve rats of either sex could acquire this protocol relatively easily. On the probe trial, where the rats are presented with a choice between using response versus visually cued learning, initially response learning was preferred, however, during these experiments, laterality emerged as a significant factor and rats trained to turn right had difficulty in reversing the learned pattern to find the platform. The second part of our study evaluated the effects of nicotine and nitric oxide synthase (NOS) inhibition on the aforementioned parameters. Drug treatments impaired acquisition compared to saline treatments and the effect was more pronounced with NOS inhibition. During the probe trial, while NOS inhibition enhanced the right-side bias in both sexes, nicotine treatment had the same effect only in males. In conclusion, naïve rats can acquire place learning using visible cues or response learning; however, there is a right side bias in both sexes and the laterality effect is more pronounced in male rats. In drug-treated animals, while NOS inhibition enhances laterality (right bias) in both sexes similarly, nicotine modifies the cognitive strategy in a sexually dimorphic manner by augmenting the right bias only in male rats.

Asymmetrical changes of dopamine receptors in the striatum after unilateral dopamine depletion

Dopamine plays an important role in modulating synaptic transmission in the striatum and has great influence on the function of the basal ganglia. Degeneration of dopamine neurons in the substantia nigra (SN) is the major cause of many neurological disorders, and the reduction of dopamine innervation results in alterations of dopamine receptors in the striatum. It has been shown that the nigrostriatal dopamine system has functional and neurochemical asymmetry. To investigate the lateralization of dopamine receptors in the striatum after dopamine denervation, the present study used quantitative autoradiography to compare the changes in dopamine receptor binding in the left and right striatum in rats after unilateral dopamine depletion. In comparison to control levels, dopamine D1)-like receptor binding, labeled with [3H]-SCH23390, in the dorsal striatum was reduced 2 weeks after unilateral lesions of the SN with 6-hydroxydopamine. D1-like receptor binding was decreased in the ipsilateral striatum following unilateral lesions of either the left or right SN. The left and right striatum responded similarly to unilateral SN lesions, as there were no significant differences in the percent decrease in D1-like binding in the two striata. In contrast, D2-like receptor binding, labeled with [3H]-spiroperidol, was significantly increased in the dorsal striatum following an ipsilateral SN lesion. Furthermore, the up-regulation of D2-like receptors in the right striatum was significantly greater than that in the left striatum after an ipsilateral lesion. The asymmetrical up-regulation of striatal D2 receptors after extensive dopamine depletion might contribute to the lateralization of the nigrostriatal system observed in some pathological conditions.

Hemispheric asymmetry in stress processing in rat prefrontal cortex and the role of mesocortical dopamine

The prefrontal cortex (PFC) is known to play an important role not only in the regulation of emotion, but in the integration of affective states with appropriate modulation of autonomic and neuroendocrine stress regulatory systems. The present review highlights findings in the rat which helps to elucidate the complex nature of prefrontal involvement in emotion and stress regulation. The medial PFC is particularly important in this regard and while dorsomedial regions appear to play a suppressive role in such regulation, the ventromedial (particularly infralimbic) region appears to activate behavioral, neuroendocrine and sympathetic autonomic systems in response to stressful situations. This may be especially true of spontaneous stress-related behavior or physiological responses to relatively acute stressors. The role of the medial PFC is somewhat more complex in conditions involving learned adjustments to stressful situations, such as the extinction of conditioned fear responses, but it is clear that the medial PFC is important in incorporating stressful experience for future adaptive behavior. It is also suggested that mesocortical dopamine plays an important adaptive role in this region by preventing excessive behavioral and physiological stress reactivity. The rat brain shows substantial hemispheric specialization in many respects, and while the right PFC is normally dominant in the activation of stress-related systems, the left may play a role in countering this activation through processes of interhemispheric inhibition. This proposed basic template for the lateralization of stress regulatory systems is suggested to be associated with efficient stress and emotional self-regulation, and also to be shaped by both early postnatal experience and gender differences.

Effects of acute nicotine on hemodynamics and binding of [11C]raclopride to dopamine D2,3 receptors in pig brain

Positive reinforcing properties of nicotine and the psychostimulants have been attributed to elevated dopamine release in the basal ganglia. It is well known that the specific binding of [(11)C]raclopride to dopamine D(2,3) receptors in living striatum is reduced by cocaine and amphetamines, revealing increased competition between endogenous dopamine and [(11)C]raclopride for dopamine D(2,3) receptors. However, the sensitivity of [(11)C]raclopride binding to nicotine-induced dopamine release is less well documented. In order to provide the basis for mapping effects of nicotine, we first optimized reference tissue methods for quantifying [(11)C]raclopride binding sites in striatum of living pigs (n = 16). In the same animals, the rate of cerebral blood flow (CBF) was mapped using [(15)O]water. Neither a low dose of nicotine (50 mu kg(-1), iv) nor a high dose of nicotine (500 microg kg(-1), iv) altered CBF in the pig brain, an important condition for calculating the binding of radioligands when using a reference tissue to estimate the free ligand concentration. The methods of Logan and of Lammertsma were compared using the cerebellum or the occipital cortex as reference tissues for calculating the binding potential (pB) of [(11)C]raclolpride in brain. Irrespective of the method used, the mean undrugged baseline pB in striatum (ca. 2.0) was significantly asymmetric, with highest binding in the left caudate and right putamen. Test-retest estimates of pB were stable. Subtraction of Logan pB maps revealed that the low dose of nicotine reduced the pB of [(11)C]raclopride by 10% in a cluster of voxels in the left anteroventral striatum, but this effect did not persist after correction for multiple comparisons. The high dose of nicotine (n = 9) acutely reduced pB by 10% bilaterally in the ventral striatum; 3 h after the high nicotine dose, the reductions had shifted dorsally and caudally into the caudate and putamen. Evidently, nicotine challenge enhances the competition between endogenous dopamine for [(11)C]raclopride binding sites with a complex temporal and spacial pattern in pig brain, initially presenting in the left ventral striatum.

Asymmetrical changes of excitatory synaptic transmission in dopamine-denervated striatum after transient forebrain ischemia

Spiny neurons in the neostriatum are highly vulnerable to cerebral ischemia. Recent studies have shown that the postischemic cell death in the right striatum was reduced after ipsilateral dopamine denervation whereas no protection was observed in the left striatum after dopamine denervation in the left side. In order to reveal the mechanisms of such asymmetrical protection, electrophysiological changes of dopamine-denervated striatal neurons were compared after ischemia between the left and right striatum using intracellular recording and staining techniques in vivo. No difference in cortically evoked initial excitatory postsynaptic potentials was found between the left and right striatum in intact animals after ipsilateral dopamine denervation. The initial excitatory postsynaptic potentials in the dopamine-denervated right striatum were suppressed after transient forebrain ischemia while no significant changes were found in the dopamine-denervated left striatum. Paired-pulse tests suggested that these changes involved presynaptic mechanisms. Although the incidence of a late depolarizing postsynaptic potential elicited by cortical stimulation increased after ischemia in both sides, the increase was greater in the left side. The analysis of current-voltage relationship of spiny neurons indicated that inward rectification in the left striatum transiently disappeared shortly after ischemia whereas that in the right side remained unchanged. The intrinsic excitability of spiny neurons in both sides were suppressed after ischemia, however, the suppression in the right side was stronger than in the left side. The above results demonstrate that after ipsilateral dopamine denervation, the depression of excitatory synaptic transmission and neuronal excitability in the right striatum is more severe than that in the left striatum following ischemia. The depression of excitatory synaptic transmission and neuronal excitability, therefore, might play an important role in neural protection after ischemic insult.

Gender difference in dopamine concentration and postischemic neuronal damage in neostriatum after unilateral dopamine depletion

Most neurons in the dorsal neostriatum die 1 day after 30 min of cerebral ischemia. Dopamine may play a role in the pathogenesis of neuronal injury in neostriatum following ischemia. It has been shown that the number of surviving neurons in the right neostriatum dramatically increased following ischemia after lesions were made in the right substantia nigra (SN), whereas no such protective effect was observed in the left neostriatum after left SN lesion. Using a voltammetric technique, the present study measured the dopamine concentration in neostriatum during ischemia after unilateral dopamine depletion and correlated it with the postischemic neuronal damage in neostriatum of male and female rats. In both genders, dopamine concentrations in the neostriatum of the intact side increased to 50-60 microM during ischemia while those of the lesion side were 15-30 microM. No difference in dopamine concentration was detected between animals with lesions in the left SN and those with lesions in right SN. In male rats, the number of surviving neurons in the right neostriatum (approximately 80% as control) was significantly greater than that in the left neostriatum (approximately 20%) after ipsilateral dopamine depletion, whereas in female animals, the number of surviving neurons in the right neostriatum (approximately 40%) was about the same as that in the left neostriatum (approximately 35%) after dopamine depletion. These results indicate that the asymmetry in ischemic outcome after unilateral dopamine depletion in male rats is not due to the difference in residual dopamine in neostriatum. The lateralization of D2 receptors in male rats may be responsible for the asymmetry of survivability of striatal neurons after transient forebrain ischemia.

Turning bias in humans is influenced by phase of the menstrual cycle

Previous studies have provided evidence that humans demonstrate subtle, but measurable, turning biases when tested in the absence of environmental constraints. Preferences for leftward or rightward rotation have been repeatedly demonstrated in rodents and appear to be modulated to a significant degree by ovarian hormones, particularly estrogen. In the present study, we examined the turning biases of adult women at the midluteal and menstrual phases of the menstrual cycle, associated with high and low levels of estradiol and progesterone, respectively. Saliva samples were collected during each test session, and salivary concentrations of estradiol and progesterone were measured using radioimmunoassays. Overall, a rightward-turning bias was evident; however, a minority of the women displayed consistent leftward biases. Among right-turning subjects, turning biases were significantly weaker at the midluteal phase than at the menstrual phase. These results suggest that the mechanisms underlying human turning biases are subject to modulation by ovarian hormones.

A sex difference in turning bias in humans

A large body of literature has documented the existence of individual preferences in turning direction among rodents which appear to be dependent on striatal dopaminergic mechanisms. Recent work has indicated that humans also demonstrate individual turning preferences, and that these preferences may also be related to the nigrostriatal dopamine system. We describe here a new method for measuring turning preferences in humans and report a sex difference in the magnitude of the directional preference. While both males and females tended to turn towards the right, this tendency was significantly stronger among females. Analyses of test-retest reliability across two sessions (1-2 weeks apart) indicated that, in general, the rotation task elicited consistent turning biases. However, the turning biases of males and of females using oral contraceptives were significantly more consistent than those of regularly cycling females. These results are compatible with the animal literature and provide indirect evidence that ovarian hormones may modulate the mechanism(s) underlying this motor asymmetry.

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.

Paw preference and brain dopamine asymmetries

The hemispheric content of dopamine and its metabolites in the frontal cortex, caudatus putamen and nucleus accumbens septi was evaluated in relation to behavioral lateralization assessed by paw preference. Three groups of C3H/He mice were selected on the basis of their performance in the paw preference test (left-handed, ambidextrous and right-handed) and levels of dopamine and its metabolites were measured in the two hemispheres of each group. Mice showed significant differences in hemispheric content of dopamine and 3-4 dihydroxyphenylacetic acid in the nucleus accumbens septi depending on the behavioral lateralization as expressed by paw preference. The hemispheric dominance (right hemisphere/right hemisphere + left hemisphere content of dopamine and metabolites x 100) was also calculated for each mouse. Significant differences in hemispheric dominance for dopamine, 3-4 dihydroxyphenylacetic acid and 3-methoxytyramine in the nucleus accumbens were found between right-handed and left-handed mice. This dominance was ipsilateral to the preferred paw: % right hemisphere/total content of dopamine and its metabolites were lowest in left-handed, highest in right-handed and intermediate in ambidextrous mice. Finally, individual % right hemisphere/total content for dopamine, 3-4 dihydroxyphenylacetic acid and 3-methoxytyramine in the nucleus accumbens positively correlated with individual paw preference scores. The analysis of the other brain areas did not reveal any significant effect. These results suggest a strong relationship between mesoaccumbens dopamine asymmetries and both the direction and the intensity of behavioral lateralization as expressed by paw preference in mice.

Investigations of fetal development models for prenatal drug exposure and schizophrenia. Prenatal d-amphetamine effects upon early and late juvenile behavior in the rat

Recent evidence suggests that mid-pregnancy is a critical period for production of fetal abnormalities that cause behavioral and neuropathological changes in adult offspring. The present experiments provide an animal model of these effects by treating pregnant Sprague-Dawley rats during gestational days 11-14 with d-amphetamine (AM). Offspring were tested for neurological signs, foraging activity, reversal learning, and sensitivity to amphetamine challenge. In the Early Juvenile period, postnatal days (PND) 20-30, female AM offspring initially showed reductions in rearing, holepoking, and midfield activity. On later trials, and as young adults, AM females showed signs of locomotor hyperactivity despite continued poor foraging efficiency, and were also more sensitive to a 1.0 mg/kg d-amphetamine challenge. AM males showed initially slower and more perseverative responding than controls, but then developed excessive response switching. These changes continued during tests for Retention, Reversal, and Extinction in the Late Juvenile/Early Adult stage (PND 50-90), when both AM-exposed sexes showed increased eating time, significantly more perseverative lateral turning preference (right or left), and slower reversal learning than controls. Behavioral data were consistent with aberrations in thalamo-frontal and mesolimbic/nigrostriatal projection systems that have been reported in AM animals and which are also affected by maternal drug abuse and schizophrenia.

Left/right nigrostriatal asymmetry in susceptibility to neurotoxic dopamine depletion with 6-hydroxydopamine in rats

Male Sprague-Dawley rats were investigated for the existence of a left/right hemispheric population asymmetry in the extent of striatal dopamine depletion following unilateral lesions with 6-hydroxydopamine (6-OHDA) in the left or right substantial nigra. Four doses of 6-OHDA were employed (with left and right lesion groups at each dose). Analysis of striatal dopamine depletion revealed significant main effects not only for dose of 6-OHDA, but also for side of lesion, with right-lesioned rats having greater dopamine depletion across doses. In a group of non-lesioned rats, randomly selected from this population, striatal dopamine turnover showed a significant right-sided bias. It is suggested that the latter functional asymmetry may have been responsible for the neurotoxic depletion asymmetry seen in lesioned animals, and that side of lesion should be considered when attempting to produce consistent and maximal dopamine depletion in large scale studies with the unilateral 6-OHDA lesion paradigm.

The ontogenesis of the forebrain commissures and the determination of brain asymmetries

We have reviewed the organization and development of the interhemispheric projections through the forebrain commissures, especially those of the CC, in connection with the development of brain asymmetries. Analyzing the available data, we conclude that the developing CC plays an important role in the ontogenesis of brain asymmetries. We have extended a previous hypothesis that the rodent CC may exert a stabilizing effect over the unstable populational asymmetries of cortical size and shape, and that it participates in the developmental stabilization of lateralized motor behaviors.

Lateralized deficits in visual attention in males with developmental dopamine depletion

Children with early treated phenylketonuria (ETPKU), a disorder associated with developmental dopamine depletion, were tested with a visual orienting paradigm to determine the existence of lateralized deficits in specific attentional operations. Male ETPKU subjects showed a right visual field impairment in disengaging attention, indicating left hemisphere dysfunction, and overall slowed reaction times. Female ETPKU and normal subjects did not differ. The results suggest that for males, dopamine depletion disrupts left hemisphere function. This finding has important implications for disorders with suspected developmental dopamine abnormalities, and may also illustrate how sex differences in functional lateralization develop in the normal brain.

Cerebral lateralization as a source of interindividual differences in behavior

Cerebral laterality can no longer be considered an exclusively human trait, as over the last 15 years there has been an emergence of data to suggest that animal brains are also lateralized. Morphologic, chemical and behavioral indices of brain asymmetry in the rodent have been reported, and it is suggested that variations in the magnitude and direction of these indices are determined by a complex interaction of genetic, hormonal and experiential factors. Interindividual differences in cerebral laterality have been shown to covary with, or predict, individual differences in spatial behavior and stress reactivity, as well as susceptibility to stress pathology and drug sensitivity. Such findings suggest that it is possible to study individual differences in lateralized brain function through the use of animal models.

Basal and amphetamine-induced asymmetries in striatal dopamine release and metabolism: bilateral in vivo microdialysis in normal rats

In vivo microdialysis was used to monitor bilaterally the release of dopamine and its metabolites 3,4-dihydroxyphenyl acetic acid (DOPAC) and homovanillic acid (HVA) in the striata of both anesthetized and awake, freely moving female rats. Under baseline conditions, an asymmetry in dopamine release was reciprocally related to an asymmetry in DOPAC. Baseline dopamine and DOPAC asymmetries were predictive of the preferred direction of amphetamine-induced rotation: the striatum having higher dopamine and lower DOPAC was contralateral to the preferred direction of rotation. Amphetamine (D-amphetamine sulfate, 1.25 mg/kg) enhanced dopamine release and decreased DOPAC and HVA; the increase in dopamine was greater in the ipsilateral striatum. Effects in anesthetized and awake rats were similar. Variations in rotation and in the dopamine asymmetry after amphetamine were correlated across time within individual awake rats.

Correlated asymmetries in striatal D1 and D2 binding: relationship to apomorphine-induced rotation

Long-Evans derived rats were tested for nocturnal, amphetamine-induced and apomorphine-induced rotation (circling behavior); the rats' left and right striata were subsequently dissected and D1 and D2 receptor densities (Bmax) were assayed in the same striatal homogenates using [3H]SCH-23390 and [3H]N-methylspiperone, respectively. D1 and D2 Bmax values were correlated (r = 0.68). Moreover, left-right asymmetries in D1 and D2 Bmax values were more highly correlated (r = 0.84). Although asymmetries in D1 and D2 binding were not by themselves related to rotational behavior, an asymmetry in the ratio or balance of D1 and D2 binding was associated with the direction of apomorphine-induced rotation: the D1/D2 ratio of Bmax values was significantly higher in the striatum ipsilateral to the preferred direction of apomorphine-induced rotation. These results suggest that normal variations in numbers of D1 and D2 receptors are determined by a common mechanism, that D1 and D2 receptors are functionally coupled, and that, with respect to activation of striatal receptors, D1 is inhibitory and D2 is excitatory. The effects of apomorphine, a mixed D1 and D2 agonist, appear to reflect the balance between D1 and D2 receptors.

Two kinds of nigrostriatal asymmetry: relationship to dopaminergic drug sensitivity and 6-hydroxydopamine lesion effects in Long-Evans rats

Recent work in this laboratory has provided evidence for a two-population model of normal rotational behavior: some rats circle predominantly away from the side containing the striatum with the greater dopamine levels and some rats circle predominantly towards the side containing the striatum with the greater dopamine levels. The two populations also respond differently to 6-hydroxydopamine lesions of the substantia nigra ipsilateral to the preferred direction of circling. The present study replicated these findings in another strain of rats and showed further that the two populations could be distinguished behaviorally by their relative responses to indirectly acting (D-amphetamine, cocaine) and directly acting (apomorphine, pergolide) dopamine agonists. The results suggest that the two populations differ with respect to the balance between pre- and postsynaptic elements within the striatum.

Lateral differences in GABA binding sites in rat brain

An asymmetric distribution of GABA binding sites was found in the cerebral cortex, hippocampus, cerebellar hemispheres, striatum, and thalamus. Higher levels of [3H]GABA binding were observed in the left-side of most brain areas and in a greater percentage of adult rats, but the opposite asymmetry was found in the thalamus. A similar left-right difference in cerebral hemispheres was also found in five day-old rats, suggesting the genetic predetermination of asymmetry.

Comparative distribution of dopamine D-1 and D-2 receptors in the basal ganglia of turtles, pigeons, rats, cats, and monkeys

The distribution and density of dopamine D-1 and D-2 receptors were studied in the basal ganglia of adult turtles, pigeons, rats, cats, and monkeys. Dopamine receptors were measured in vitro by quantitative autoradiography in alternate sections processed for D-1 and D-2 receptor subtypes and compared to adjacent sections stained for acetylcholinesterase (AChE) activity. [3H]-SCH 23390 and [3H]-spiroperidol were used to label the D-1 and D-2 dopamine receptor subtypes, respectively. The anatomic distribution of both D-1 and D-2 receptors in the basal ganglia was remarkably similar across all species examined. Whereas the absolute number of D-1 and D-2 receptors in the basal ganglia varied between species, the percentage of D-1 and D-2 receptors in a region was quite similar among species. The pattern of binding to the D-1 and D-2 receptor varied among the different species. The adult turtles, pigeons, and rats demonstrated nonpatchy D-1 and D-2 receptor binding in the striatum and pallidum. The adult cat and monkey caudate nucleus and putamen demonstrated mildly heterogeneous receptor binding in a pattern that differed from that seen with AChE staining, but did occasionally demonstrate similar patterns of the D-1 and D-2 receptor subtypes. The immature cat striatum was characterized by heterogeneous D-1 receptor binding that corresponded to heterogeneous AChE rich patches, whereas D-2 receptor binding was homogeneous. Heterogeneous binding was seen in other basal ganglia structures including the nucleus accumbens, olfactory tubercle, and substantia nigra pars compacta and reticulata. Complementary D-1 and D-2 receptor binding patterns were seen in the pallidum and substantia nigra of the mammals. The results of this study indicate that both D-1 and D-2 dopamine receptors are present in the basal ganglia of five different vertebrates. A common feature of dopamine receptors in the basal ganglia is their heterogeneity in distribution and density. The heterogeneity of dopamine receptors has similarities to and differences from the distribution of presynaptic dopamine and other neurotransmitter markers of the basal ganglia.