Nadja Freund

Dopamine modulation of learning and memory in the prefrontal cortex: insights from studies in primates, rodents, and birds

In this review, we provide a brief overview over the current knowledge about the role of dopamine transmission in the prefrontal cortex during learning and memory. We discuss work in humans, monkeys, rats, and birds in order to provide a basis for comparison across species that might help identify crucial features and constraints of the dopaminergic system in executive function. Computational models of dopamine function are introduced to provide a framework for such a comparison. We also provide a brief evolutionary perspective showing that the dopaminergic system is highly preserved across mammals. Even birds, following a largely independent evolution of higher cognitive abilities, have evolved a comparable dopaminergic system. Finally, we discuss the unique advantages and challenges of using different animal models for advancing our understanding of dopamine function in the healthy and diseased brain.

Depressive-like behavior in adolescents after maternal separation: sex differences, controllability, and GABA.

Exposure to adversity during development is an identified risk factor for depression later in life. In humans, early adversity accelerates the onset of depressive symptoms, which manifest during adolescence. Animal studies have used maternal separation as a model of early adversity to produce adult depressive-like behaviors, but have yet to examine these behaviors during adolescence. Moreover, the nature of depressive-like behaviors has not been well characterized in this model. Here, we used the triadic model of learned helplessness to understand controllability, helplessness, and motivational factors following maternal separation in male and female adolescent rats. We found sex-dependent changes in the effects of separation, with males demonstrating loss of controllability in an escapable shock condition, whereas females demonstrated motivational impairment in a no-shock condition. The effect, however, did not endure as adult females were no longer helpless. Reductions in parvalbumin, a GABAergic marker, in the prefrontal cortex of separated subjects relative to age-matched controls were evident and paralleled depressive-like behavior. Understanding the risk factors for depression, the nature of depressive-like behaviors, and their unique sex dependency may ultimately provide insight into improved treatments.

ORGANIZATION OF TELENCEPHALOTECTAL PROJECTIONS IN PIGEONS: IMPACT FOR LATERALIZED TOP-DOWN CONTROL

Birds display hemispheric specific modes of visual processing with a dominance of the right eye/left hemisphere for detailed visual object analysis. In pigeons, this behavioral lateralization is accompanied by morphological left-right differences in the ascending tectofugal pathway. This system is also asymmetrically modulated by descending telencephalotectal input whereby the left forebrain displays a much more pronounced physiological control over ipsilateral left and contralateral right visual thalamic processes. In the present study we aimed to answer the question if this top-down asymmetry that up to now had been demonstrated in single cell recording studies is due to anatomical asymmetries in the size of the fiber systems descending from the telencephalon to the tectum. We approached this question by means of a quantitative retrograde tracing study. Cholera toxin subunit B (CtB) was injected unilaterally into either the left or right optic tectum of adult pigeons. After immunohistochemical detection of CtB-positive cells, the number of ipsi- and contralaterally projecting neurons was estimated. Retrogradely labeled cells were located within the arcopallium, the hyperpallium apicale (HA) and the temporo-parieto-occipital area (TPO). Descending projections from HA, arcopallium, and TPO were mainly or exclusively ipsilateral with the contralateral projection being extremely small. Moreover, there was no difference between left and right hemispheric projections. These anatomical data sharply contrast with behavioral and electrophysiological ones which reveal an asymmetric and bilateral top down control. Therefore, contralateral and lateralized forebrain influences onto tectofugal processing are possibly not the direct result of asymmetrical descending axon numbers. Those influences emerge by a lateralized intra- and/or interhemispheric integration of ascending and descending input onto the rotundus.

Viral over-expression of D1 dopamine receptors in the prefrontal cortex increase high-risk behaviors in adults: Comparison with adolescents

RationaleAdolescents are often described as “lacking brakes” resulting in an increase in several behaviors associated with risk for addiction. Prefrontal cortex dopamine and cortico-limbic interaction play an important role in addiction, and we have previously shown that the dopamine D1 receptor is elevated on prelimbic prefrontal output neurons in adolescent rats. We hypothesized that a constellation of risk-related behaviors is mediated by prefrontal output neuron expression of D1.ObjectivesWe aimed to determine the role of the dopamine D1 receptor in behavioral and neural correlates of risk for addiction that are often observed in adolescents. Therefore, high-risk behaviors as well as subcortical D2 receptor expression were investigated in adult animals with experimentally elevated D1 on prefrontal glutamatergic neurons.MethodsA lentiviral vector that selectively expressed the D1 receptor within glutamate neurons was injected in the prelimbic prefrontal cortex of adult male rats. Place conditioning to cocaine, alcohol, and nicotine, as well as delay discounting, novelty preferences, anxiety, cocaine self-administration, and sucrose preferences were assessed.ResultsVirally mediated D1 over-expression in adults leads to stronger drug-cue associations and greater consumption of sweet solutions, elevates bias towards immediate satisfaction rather than delaying gratification, decreases anxiety, and causes rats to work harder for and take more cocaine. Furthermore, elevated cortical D1 reduces D2 receptors in the accumbens (a putative risk marker).ConclusionsTogether, these data suggest a common mechanism for increased motivational drive to seek and consume substances with hedonic value, consistent with adolescent addictive processes.

Windows of vulnerability: maternal separation, age, and fluoxetine on adolescent depressive-like behavior in rats.

Early exposure to stressful life events plays a significant role in adolescent depression. Clinical studies have identified a number of factors that increase the risk of depression, including sex of the subject, duration of the stressor, and genetic polymorphisms that elevate serotonin levels. In this study we used the maternal separation (MS) model to investigate to what extent these factors interacted during development to manifest in depressive-like behavior in male and female rats. The triadic model of learned helplessness parses depressive-like behavior into aspects of controllable, uncontrollable, and motivational behaviors. This model was used to investigate how the timing of MS between the ages of postnatal day (P) 2-9 and P9-16 interacted with either simultaneous vehicle (saline; 1ml/kg; i.p.) or fluoxetine (10mg/kg) exposure, which was used to enhance serotonin levels; these experiments also compared the effect of a vehicle injection during these developmental periods to a no injection control. Vehicle injections alone increased helplessness in the controllable condition in male rats when injected between P9-16 only, and did not interact further with MS. MS at both ages decreased controllability in male adolescents; females demonstrated an increase in controllability after MS. Elevated serotonin at P2-9 increased escape latencies in male and female control and MS subjects. Fluoxetine exposure at P9-16 increased helplessness in controls. Fluoxetine decreased helplessness in MS males independent of age, but increases helplessness in MS females. This study highlights the importance of age of MS (MS between P2-9 increases helplessness in males more than females), the duration of the stressor (previous results show females are effected by longer MS [P2-20], but not shorter [this study]), and that elevated serotonin increases escape latencies to a greater extent in females.

Single forebrain neurons represent interval timing and reward amount during response scheduling

Climbing activity, the gradual increase of neural discharge rate across a delay, has been suggested to play a crucial role in interval timing. However, most previous studies demonstrated climbing activity only in conjunction with tasks that involved a passive tracking of the passage of time, but that did not necessitate to actively time an event, e.g. a motor response. To demonstrate the significance of climbing activity for action timing, we trained pigeons in a self‐control task requiring either immediate responding to a key after the onset of a light cue (‘rapid‐response’ trials), or waiting for a fixed interval after cue presentation before responding to the key (‘wait’ trials). The cue also indicated whether a correctly timed response would be rewarded with a large or a small reward. Single‐cell recordings in the Nidopallium caudolaterale, the avian prefrontal cortex, revealed that some neurons showed climbing activity between cue onset and response. Their increase in firing rate was flatter and reached the peak later in wait compared with rapid‐response trials. An error analysis confirmed that, relative to correct responses, premature responses were accompanied by steeper, and tardy responses by flatter ramps. In addition, the climbing discharge pattern was modulated by the amount of the anticipated reward, suggesting that timing is an intrinsic property of neurons encoding other task‐related information. These results demonstrate the behavioural and motivational significance of climbing activity in prospective information encoding. Our study supports a recent paradigm shift in our understanding of the vertebrate brain evolution, and it provides further evidence for the similarity between the mammalian cortex and the avian pallium.

Visual asymmetries and the ascending thalamofugal pathway in pigeons

The lateralized visual systems of pigeons and chickens are excellent models to study neural asymmetries at the functional and anatomical level. The aim of the current study was to reveal why these two species closely resemble each other with respect to left–right differences in behavior but not with respect to the pathways involved: While pigeons show an asymmetrically organized tectofugal system, only transient lateralizations of the thalamofugal system have been observed in chickens. Four possible explanations are conceivable. (1) Adult pigeons might also show a hitherto undiscovered thalamofugal asymmetry like chickens. (2) The thalamofugal asymmetry might be transient in both species. (3) Prehatch light stimulation could differentially affect the two visual pathways of chickens and pigeons that mature with different speeds. (4) Tecto- and thalamofugal asymmetries represent species differences, independent of developmental factors. To test these explanations, we injected retrograde tracers into the Wulst of adult pigeons, of hatchlings, and of dark reared pigeons which were monocularly deprived on their left or right eye for one week after hatch. Subsequently we counted labeled cells within the ipsi- and contralateral n. geniculatus lateralis pars dorsalis in search for possible lateralizations of ascending pathways. None of the experimental groups displayed significant differences in the thalamofugal projection pattern. This indicates that visual lateralization in pigeons and chickens depends on tectofugal and thalamofugal asymmetries, respectively. Thus, in different species a highly similar pattern of behavioral asymmetries can be subserved by diverse neural systems.

A method for the evaluation of intracranial tetrodotoxin injections

Tetrodotoxin is one of the most potent and oldest known neurotoxins. It acts by blocking the voltage-gated sodium channels in nerve cell membranes, leading to a transient silencing of neural activity. TTX is among the most widely used pharmacological agents for the temporary and selective blocking of neural structures. As such, an exact knowledge of the spatial and temporal diffusion gradient of TTX is important when planning pharmacological interventions. Here we report a method for the direct assessment of spatio-temporal TTX diffusion gradients using immunohistochemistry. TTX injections were performed in vivo via chronically implanted guiding cannulae, placed bilaterally in the dorsal entopallium of pigeons. To determine the temporal spread, animals were perfused at different time points after TTX injections. For visualization of the TTX affected area an immunohistochemical protocol was developed. The extension of staining was assessed 1h after injection when TTX was diffused over 3mm in all directions. TTX immunolabeling could be detected for up to 32 h; after 48 h no staining was found. Our findings provide a better understanding of the temporal decay and spread of intracranial TTX injections, thereby allowing a more reliable estimation of size and duration of TTX-effects.

Sex-dependent changes in ADHD-like behaviors in juvenile rats following cortical dopamine depletion

Reduced cortical dopamine levels have been observed in individuals with attention deficit hyperactivity disorder (ADHD). Global dopamine depletions by 6-hydroxydopamine (6-OHDA; with noradrenergic protection) in neonatal rats produces locomotor hyperactivity, with less known about how cortical depletion modulates risky behaviors. Here, we determined the effect of a medial prefrontal cortex (PFC) 6-OHDA depletions (30-60%) or sham microinjection at postnatal day 11 on behavior in male and female juvenile rats. Separate groups were studied for delay discounting (impulsive choice), novelty-preference, and preferences for cues and environments associated with cocaine (10, 20, and 40 mg/kg), their extinction, and reinstatement with place conditioning. Because PFC D1 receptors play a role in these behaviors, confocal microscopy was used to measure D1-immunoreactive projections to the nucleus accumbens core. Both 6-OHDA males and females increased delay discounting relative to sham controls, although only 6-OHDA females increased novelty preferences. Preferences for cocaine-associated environments, their extinction, and reinstatement with a priming dose of cocaine were reduced in 6-OHDA subjects overall. However, impulsive choice at 5s positively correlated with preferences for cocaine-associated environments in 6-OHDA subjects, but not sham controls. As possible compensation for low dopamine levels, D1-immunoreactivity on traced neurons increased in 6-OHDA females; dopamine levels did not remain low in adolescent 6-OHDA males and D1 did not change. We believe that these modest depletions restricted to the PFC demonstrate the role of dopamine, and not norepinephrine, in understanding these behaviors in other animal models where cortical dopamine is reduced during development.

Asymmetric top-down modulation of ascending visual pathways in pigeons.

Cerebral asymmetries are a ubiquitous phenomenon evident in many species, incl. humans, and they display some similarities in their organization across vertebrates. In many species the left hemisphere is associated with the ability to categorize objects based on abstract or experience-based behaviors. Using the asymmetrically organized visual system of pigeons as an animal model, we show that descending forebrain pathways asymmetrically modulate visually evoked responses of single thalamic units. Activity patterns of neurons within the nucleus rotundus, the largest thalamic visual relay structure in birds, were differently modulated by left and right hemispheric descending systems. Thus, visual information ascending towards the left hemisphere was modulated by forebrain top-down systems at thalamic level, while right thalamic units were strikingly less modulated. This asymmetry of top-down control could promote experience-based processes within the left hemisphere, while biasing the right side towards stimulus-bound response patterns. In a subsequent behavioral task we tested the possible functional impact of this asymmetry. Under monocular conditions, pigeons learned to discriminate color pairs, so that each hemisphere was trained on one specific discrimination. Afterwards the animals were presented with stimuli that put the hemispheres in conflict. Response patterns on the conflicting stimuli revealed a clear dominance of the left hemisphere. Transient inactivation of left hemispheric top-down control reduced this dominance while inactivation of right hemispheric top-down control had no effect on response patterns. Functional asymmetries of descending systems that modify visual ascending pathways seem to play an important role in the superiority of the left hemisphere in experience-based visual tasks.