Why am I lost without dopamine? Effects of 6-OHDA lesion on the encoding of reward and decision process in CA3

Immunohistochemical field parcellation of the human hippocampus along its antero-posterior axis

The primate hippocampus includes the dentate gyrus, cornu ammonis (CA), and subiculum. CA is subdivided into four fields (CA1-CA3, plus CA3h/hilus of the dentate gyrus) with specific pyramidal cell morphology and connections. Work in non-human mammals has shown that hippocampal connectivity is precisely patterned both in the laminar and longitudinal axes. One of the main handicaps in the study of neuropathological semiology in the human hippocampus is the lack of clear laminar and longitudinal borders. The aim of this study was to explore a histochemical segmentation of the adult human hippocampus, integrating field (medio-lateral), laminar, and anteroposterior longitudinal patterning. We provide criteria for head-body-tail field and subfield parcellation of the human hippocampus based on immunodetection of Rabphilin3a (Rph3a), Purkinje-cell protein 4 (PCP4), Chromogranin A and Regulation of G protein signaling-14 (RGS-14). Notably, Rph3a and PCP4 allow to identify the border between CA3 and CA2, while Chromogranin A and RGS-14 give specific staining of CA2. We also provide novel histological data about the composition of human-specific regions of the anterior and posterior hippocampus. The data are given with stereotaxic coordinates along the longitudinal axis. This study provides novel insights for a detailed region-specific parcellation of the human hippocampus useful for human brain imaging and neuropathology.

Spatial learning deficits in Parkinson's disease with and without mild cognitive impairment

BACKGROUND

Several MRI studies have demonstrated hippocampal atrophy in Parkinson's disease (PD), a structure considered a key element in spatial learning. Despite this, no study has been undertaken to investigate spatial navigation in PD using a virtual version of the Morris water maze, which is the gold standard for testing hippocampal function in rodents.

METHODS

We studied 17 cognitively unimpaired PD patients, 12 PD patients with mild cognitive impairment (MCI) and 15 controls in a virtual water maze procedure.

RESULTS

Measured by the main outcome parameters latency to locate the target and heading error (average difference between direction of movement toward anticipated target and real direction toward the target), controls performed significantly better on the virtual water maze task than cognitively unimpaired PD patients or PD patients with MCI, while there was no significant difference between latter two groups.

CONCLUSIONS

The virtual water maze test differentiates PD patients from controls, but does not distinguish between cognitively normal and cognitively impaired PD patients, indicating a possible dopamine dependent component in spatial learning. Spatial performance deficits might thus constitute very early signs of dopamine depletion independent of the presence of MCI in Parkinson's disease.

Effects of drugs of abuse on hippocampal plasticity and hippocampus-dependent learning and memory: contributions to development and maintenance of addiction

It has long been hypothesized that conditioning mechanisms play major roles in addiction. Specifically, the associations between rewarding properties of drugs of abuse and the drug context can contribute to future use and facilitate the transition from initial drug use into drug dependency. On the other hand, the self-medication hypothesis of drug abuse suggests that negative consequences of drug withdrawal result in relapse to drug use as an attempt to alleviate the negative symptoms. In this review, we explored these hypotheses and the involvement of the hippocampus in the development and maintenance of addiction to widely abused drugs such as cocaine, amphetamine, nicotine, alcohol, opiates, and cannabis. Studies suggest that initial exposure to stimulants (i.e., cocaine, nicotine, and amphetamine) and alcohol may enhance hippocampal function and, therefore, the formation of augmented drug-context associations that contribute to the development of addiction. In line with the self-medication hypothesis, withdrawal from stimulants, ethanol, and cannabis results in hippocampus-dependent learning and memory deficits, which suggest that an attempt to alleviate these deficits may contribute to relapse to drug use and maintenance of addiction. Interestingly, opiate withdrawal leads to enhancement of hippocampus-dependent learning and memory. Given that a conditioned aversion to drug context develops during opiate withdrawal, the cognitive enhancement in this case may result in the formation of an augmented association between withdrawal-induced aversion and withdrawal context. Therefore, individuals with opiate addiction may return to opiate use to avoid aversive symptoms triggered by the withdrawal context. Overall, the systematic examination of the role of the hippocampus in drug addiction may help to formulate a better understanding of addiction and underlying neural substrates.

Long Term Recordings with Immobile Silicon Probes in the Mouse Cortex

A key experimental approach in neuroscience involves measuring neuronal activity in behaving animals with extracellular chronic recordings. Such chronic recordings were initially made with single electrodes and tetrodes, and are now increasingly performed with high-density, high-count silicon probes. A common way to achieve long-term chronic recording is to attach the probes to microdrives that progressively advance them into the brain. Here we report, however, that such microdrives are not strictly necessary. Indeed, we obtained high-quality recordings in both head-fixed and freely moving mice for several months following the implantation of immobile chronic probes. Probes implanted into the primary visual cortex yielded well-isolated single units whose spike waveform and orientation tuning were highly reproducible over time. Although electrode drift was not completely absent, stable waveforms occurred in at least 70% of the neurons tested across consecutive days. Thus, immobile silicon probes represent a straightforward and reliable technique to obtain stable, long-term population recordings in mice, and to follow the activity of populations of well-isolated neurons over multiple days.

The globus pallidus pars interna in goal-oriented and routine behaviors: Resolving a long-standing paradox

BACKGROUND

There is an apparent contradiction between experimental data showing that the basal ganglia are involved in goal-oriented and routine behaviors and clinical observations. Lesion or disruption by deep brain stimulation of the globus pallidus interna has been used for various therapeutic purposes ranging from the improvement of dystonia to the treatment of Tourette's syndrome. None of these approaches has reported any severe impairment in goal-oriented or automatic movement.

METHOD

To solve this conundrum, we trained 2 monkeys to perform a variant of a 2-armed bandit-task (with different reward contingencies). In the latter we alternated blocks of trials with choices between familiar rewarded targets that elicit routine behavior and blocks with novel pairs of targets that require an intentional learning process.

RESULTS

Bilateral inactivation of the globus pallidus interna, by injection of muscimol, prevents animals from learning new contingencies while performance remains intact, although slower for the familiar stimuli. We replicate in silico these data by adding lateral competition and Hebbian learning in the cortical layer of the theoretical model of the cortex-basal ganglia loop that provided the framework of our experimental approach.

CONCLUSION

The basal ganglia play a critical role in the deliberative process that underlies learning but are not necessary for the expression of routine movements. Our approach predicts that after pallidotomy or during stimulation, patients should have difficulty with complex decision-making processes or learning new goal-oriented behaviors. © 2016 Movement Disorder Society.

Relative contributions of severe dopaminergic neuron ablation and dopamine depletion to cognitive impairment

Parkinson's disease (PD) is characterized by the loss of dopaminergic neurons and produces a movement disorder and cognitive impairment that becomes more extensive with the duration of the disease. To what extent cognitive impairment in advanced PD can be attributed to severe loss of dopamine (DA) signaling is not well understood. Furthermore, it is unclear if the loss of DA neurons contributes to the cognitive impairment caused by the reduction in DA signaling. We generated genetic mouse models with equally severe chronic loss of DA achieved by either extensive ablation of DA neurons or inactivation of DA synthesis from preserved neurons and compared their motor and cognitive performance. Motor behaviors were equally blunted in both models, but we observed that DA neuron ablation caused more severe cognitive deficits than DA depletion. Both models had marked deficits in cue-discrimination learning. Yet, deficits in cue-discrimination learning were more severe in mice with DA neuron ablation and only mice with DA neuron ablation had drastically impaired performance in spatial learning, spatial memory and object memory tests. These results indicate that while a severe reduction in DA signaling results in motor and cognitive impairments, the loss of DA neurons promotes more extensive cognitive deficits and suggest that a loss of additional factors that depend on DA neurons may participate in the progressive cognitive decline found in patients with PD.

Involvement of the dopaminergic system in the consolidation of fear conditioning in hippocampal CA3 subregion

The hippocampus, the primary brain structure related to learning and memory, receives sparse but comprehensive dopamine innervations and contains dopamine D1 and D2 receptors. Systematic hippocampal dopaminergic dysfunction can cause deficits in spatial working memory and impair consolidation of contextual fear memories. CA3 is involved in the rapid acquisition of new memories and has extensive nerve fibre connections with other brain structures such as CA1, the amygdala, and the medial prefrontal cortex (mPFC). A bidirectional fibrous connection between CA3 and the amygdala reflects the importance of CA3 in fear conditioning. The present study evaluated the effects of a 6-OHDA lesion in CA3 on the acquisition and expression of conditioned fear. The results showed CA3 involvement in the expression but not the acquisition of conditioned fear. Injection of SCH23390 and quinpirole into the bilateral CA3 attenuated a conditioned fear-related freezing response, whereas SKF38393 and sulpiride were not associated with this effect. The present study found that a 6-OHDA lesion in CA3 up-regulated the expression of GluR1 in BLA and down-regulated NR2B in CA1 and the basolateral amygdala (BLA). Our data suggest that dopamine depletion in hippocampal subdivision CA3 may not be necessary for the acquisition of conditioned fear, but the expression of conditioned fear is likely dependent on the integrity of mesohippocampal dopaminergic connections. It is probable that both D1 and D2 dopaminergic receptors modulate the expression of conditioned fear. Changes in the expression of NR2B and GluR1 indicate that CA3 may modulate the activities of other brain structures.

The Michelin red guide of the brain: role of dopamine in goal-oriented navigation

Spatial learning has been recognized over the years to be under the control of the hippocampus and related temporal lobe structures. Hippocampal damage often causes severe impairments in the ability to learn and remember a location in space defined by distal visual cues. Such cognitive disabilities are found in Parkinsonian patients. We recently investigated the role of dopamine in navigation in the 6-Hydroxy-dopamine (6-OHDA) rat, a model of Parkinson’s disease (PD) commonly used to investigate the pathophysiology of dopamine depletion (Retailleau et al., 2013). We demonstrated that dopamine (DA) is essential to spatial learning as its depletion results in spatial impairments. Our results showed that the behavioral effect of DA depletion is correlated with modification of the neural encoding of spatial features and decision making processes in hippocampus. However, the origin of these alterations in the neural processing of the spatial information needs to be clarified. It could result from a local effect: dopamine depletion disturbs directly the processing of relevant spatial information at hippocampal level. Alternatively, it could result from a more distributed network effect: dopamine depletion elsewhere in the brain (entorhinal cortex, striatum, etc.) modifies the way hippocampus processes spatial information. Recent experimental evidence in rodents, demonstrated indeed, that other brain areas are involved in the acquisition of spatial information. Amongst these, the cortex—basal ganglia (BG) loop is known to be involved in reinforcement learning and has been identified as an important contributor to spatial learning. In particular, it has been shown that altered activity of the BG striatal complex can impair the ability to perform spatial learning tasks. The present review provides a glimpse of the findings obtained over the past decade that support a dialog between these two structures during spatial learning under DA control.