Dopamine depletion, stimulation or blockade in the rat disrupts spatial navigation and locomotion dependent upon beacon or distal cues

Modulation of Spatial Memory Deficit and Hyperactivity in Dopamine Transporter Knockout Rats via α2A-Adrenoceptors

Attention deficit hyperactivity disorder (ADHD) is manifested by a specific set of behavioral deficits such as hyperactivity, impulsivity, and inattention. The dopamine neurotransmitter system is postulated to be involved in the pathogenesis of ADHD. Guanfacine, a selective α2A-adrenoceptor agonist, is prescribed for ADHD treatment. ADHD also is known to be associated with impairment of multiple aspects of cognition, including spatial memory, however, it remains unclear how modulation of the norepinephrine system can affect these deficits. Hyperdopaminergic dopamine transporter knockout (DAT-KO) rats are a valuable model for investigating ADHD. The DAT-KO rats are hyperactive and deficient in spatial working memory. This work aimed to evaluate the effects of noradrenergic drugs on the fulfillment of spatial cognitive tasks by DAT-KO rats. The rats were tested in the Hebb - Williams maze during training and following noradrenergic drugs administration. The efficiency of spatial orientation was assessed as to how fast the animal finds an optimal way to the goal box. Testing in a new maze configuration allowed us to evaluate the effects of drug administration after the acquisition of the task rules. The behavioral variables such as the distance traveled, the time to reach the goal box, and the time spent in the error zones were analyzed. It has been observed that α2A-adrenoceptor agonist Guanfacine (0.25 mg/kg) had only a minimal inhibitory effect on hyperactivity of DAT-KO rats in the maze but significantly ameliorated their perseverative pattern of activity and reduced the time spent in the error zones. In contrast, α2A-adrenoceptor antagonist Yohimbine, at the dose of 1 mg/kg, increased the distance traveled by DAT-KO rats and elevated the number of perseverative reactions and the time spent in the error zones. Guanfacine caused minimal effects in wild-type rats, while Yohimbine altered several parameters reflecting a detrimental effect on the performance in the maze. These data indicate that modulation of α2A-adrenoceptor activity potently affects both dopamine-dependent hyperactivity and cognitive dysfunctions. Similar mechanisms may be involved in the beneficial effects of Guanfacine on cognitive deficits in ADHD patients. This study further supports the translational potential of DAT-KO rats for testing new pharmacological drugs.

Deficit in working memory and abnormal behavioral tactics in dopamine transporter knockout rats during training in the 8-arm maze

Understanding the role of the dopamine system in learning and memory processes is very important for uncovering central mechanisms underlying complex behavioral responses that can be impaired in patients with neuropsychiatric disorders caused by dopamine system dysfunction. One of the most useful animal models for dopaminergic dysregulation is the strain of dopamine transporter knockout (DAT-KO) rats that have no dopamine re-uptake and thus elevated extracellular dopamine levels. It is known that dopamine is involved in various cognitive processes such as learning, memory and attention. This investigation was focused on the ability of DAT-KO rats to learn and perform a behavioral task in the 8-arm radial maze test. It was found that DAT-KO rats are able to learn the behavioral task, but the level of task performance did not reach that of WT group. The behavioral tactics used by animals during training significantly differ in mutants. The behavioral tactics used by DAT-KO rats involved perseverations and resulted in worse task fulfillment in comparison to wild-type controls. The data obtained indicate that deficient dopamine reuptake results in an impairment of working memory and perseverative behavioral tactics in DAT-KO rats.

The role of cytokines in modulating learning and memory and brain plasticity

Cytokines are proteins secreted in the central nervous system by neurons, microglia, astrocytes and infiltrating peripheral immune cells under physiological and pathological conditions. Over the last 20 years, a growing number of reports have investigated the effects of these molecules on brain plasticity. In this review, we describe how the key cytokines interleukin 1β, interleukin 6 and tumour necrosis factor α were found to support long-term plasticity and learning and memory processes in physiological conditions. In contrast, during inflammation where cytokines levels are elevated such as in models of brain injury or infection, depression or neurodegeneration, the effects of cytokines are mostly detrimental to memory mechanisms, associated behaviours and homeostatic plasticity.

Closed-Loop Implantable Therapeutic Neuromodulation Systems Based on Neurochemical Monitoring

Closed-loop or intelligent neuromodulation allows adjustable, personalized neuromodulation which usually incorporates the recording of a biomarker, followed by implementation of an algorithm which decides the timing (when?) and strength (how much?) of stimulation. Closed-loop neuromodulation has been shown to have greater benefits compared to open-loop neuromodulation, particularly for therapeutic applications such as pharmacoresistant epilepsy, movement disorders and potentially for psychological disorders such as depression or drug addiction. However, an important aspect of the technique is selection of an appropriate, preferably neural biomarker. Neurochemical sensing can provide high resolution biomarker monitoring for various neurological disorders as well as offer deeper insight into neurological mechanisms. The chemicals of interest being measured, could be ions such as potassium (K⁺), sodium (Na⁺), calcium (Ca²⁺), chloride (Cl⁻), hydrogen (H⁺) or neurotransmitters such as dopamine, serotonin and glutamate. This review focusses on the different building blocks necessary for a neurochemical, closed-loop neuromodulation system including biomarkers, sensors and data processing algorithms. Furthermore, it also highlights the merits and drawbacks of using this biomarker modality.

Behavioral and neurochemical responses derived from dopaminergic intrastriatal grafts in hemiparkinsonian rats engaged in a novel motor task

BACKGROUND

Putative treatments derived from in vivo stem cell transplant-derived dopamine (DA) in hemiparkinsonian rats have been assessed via DA-agonist-induced rotations involving imbalanced intra-hemispheric striatal DA receptor stimulation. However, such tests obscure the natural responses of grafts to sensory stimuli, and drug-induced plasticity can modify the circuit being tested. Thus, we propose an alternative testing strategy using a novel water tank swimming apparatus.

NEW METHOD

Microdialysis was used to compare striatal DA levels when rats were: (1) in a rest-phase within a bowl-shaped apparatus, or (2) in an active forced-swim phase within a specially-equipped water tank. Resting-phase DA release levels were compared with active-phase levels obtained while rats were required to swim in the water-tank task. Behavioral variables such as asymmetric circling while swimming (rotations), front-limb strokes, and front-limb reaches were captured by a camera for analysis.

RESULTS AND COMPARISON WITH EXISTING METHODS

Transplanted cells had a very modest effect on percentage of contralateral front-limb strokes, but did not reduce lesion-induced rotational asymmetry in the swim task. Neither striatal DA levels, nor their breakdown products, were significantly different between transplanted and sham-transplanted groups. Our new behavioral test eliminates the need for pharmacological stimulation, enabling simultaneous assessment of DA released in resting and active phases to explore graft control.

CONCLUSIONS

Our new method allows for accurate assessments of stem cell therapy for PD as an alternative to "rotation" tests. Use of natural motivations to engage in sensory-driven motor tasks provides more accurate insights into ongoing graft-derived behavioral support.

Depression in Parkinson's Disease: The Contribution from Animal Studies

Besides being better known for causing motor impairments, Parkinson's disease (PD) can also cause many nonmotor symptoms, like depression and anxiety, which can cause significant loss of life quality and may not respond to regular drugs treatment. In this review, we discuss the depression in PD, based on data from studies in humans and rodents. Depression frequency seems higher in PD patients than in general population, despite high variation in data due to diagnosis disparities. Development of depression in PD seems more likely to be caused by the nigrostriatal pathway degeneration than as a consequence of the awareness of disease prognostic, and it seems to be related to dopaminergic, noradrenergic, and serotoninergic synapses deficits. The dopaminergic role could be more significant, since it can modulate the release of the others, and its depletion is progressive, due to the degenerative feature of PD. Highly regarded in major depression, serotonin can be depleted in rats after nigrostriatal damage, but data from human patients are more conflicting. Animal studies can help in understanding the neurobiological mechanisms of depression in PD and the pursuit for more effective drugs for its treatment, but they lack the complexity of the disease progression, especially the nondopaminergic degeneration.

The group II metabotropic glutamate receptor agonist LY354740 and the D2 receptor antagonist haloperidol reduce locomotor hyperactivity but fail to rescue spatial working memory in GluA1 knockout mice

Group II metabotropic glutamate receptor agonists have been suggested as potential anti‐psychotics, at least in part, based on the observation that the agonist LY354740 appeared to rescue the cognitive deficits caused by non‐competitive N‐methyl‐d‐aspartate receptor (NMDAR) antagonists, including spatial working memory deficits in rodents. Here, we tested the ability of LY354740 to rescue spatial working memory performance in mice that lack the GluA1 subunit of the AMPA glutamate receptor, encoded by Gria1, a gene recently implicated in schizophrenia by genome‐wide association studies. We found that LY354740 failed to rescue the spatial working memory deficit in Gria1^−/− mice during rewarded alternation performance in the T‐maze. In contrast, LY354740 did reduce the locomotor hyperactivity in these animals to a level that was similar to controls. A similar pattern was found with the dopamine receptor antagonist haloperidol, with no amelioration of the spatial working memory deficit in Gria1^−/− mice, even though the same dose of haloperidol reduced their locomotor hyperactivity. These results with LY354740 contrast with the rescue of spatial working memory in models of glutamatergic hypofunction using non‐competitive NMDAR antagonists. Future studies should determine whether group II mGluR agonists can rescue spatial working memory deficits with other NMDAR manipulations, including genetic models and other pharmacological manipulations of NMDAR function.

6-Hydroxydopamine-Induced Dopamine Reductions in the Nucleus Accumbens, but not the Medial Prefrontal Cortex, Impair Cincinnati Water Maze Egocentric and Morris Water Maze Allocentric Navigation in Male Sprague-Dawley Rats

The nucleus accumbens (Nacc) and medial prefrontal cortex (mPFC) receive dopaminergic innervation from the ventral tegmental area and are involved in learning. Male rats with 6-hydroxydopamine (6-OHDA)-induced dopaminergic and noradrenergic reductions in the Nacc or mPFC were tested for allocentric and egocentric learning to determine their role in these forms of neuroplasticity. mPFC dopaminergic and noradrenergic reductions did not result in changes to either type of learning or memory. Nacc dopaminergic and noradrenergic reductions resulted in allocentric learning and memory deficits in the Morris water maze (MWM) on acquisition, reversal, and probe trials. MWM cued performance was also affected, but straight-channel swim times and swim speed during hidden platform trials in the MWM were not affected. Nacc dopaminergic and noradrenergic reductions also impaired egocentric learning in the Cincinnati water maze (CWM). Nacc-lesioned animals tested in the CWM in an alternate path through the maze were not significantly affected. 6-OHDA injections in the Nacc resulted in 63 % dopamine and 62 % norepinephrine reductions in the Nacc and 23 % reductions in adjacent dorsal striatum. 6-OHDA injections in the mPFC resulted in 88 % reductions in dopamine and 59 % reductions in norepinephrine. Hence, Nacc dopamine and/or norepinephrine play a role in egocentric and allocentric learning and memory, while mPFC dopamine and norepinephrine do not.

Dopamine depletion in either the dorsomedial or dorsolateral striatum impairs egocentric Cincinnati water maze performance while sparing allocentric Morris water maze learning

Both egocentric route-based learning and spatial learning, as assessed by the Cincinnati water maze (CWM) and Morris water maze (MWM), respectively, are impaired following an 80% dopamine (DA) loss in the neostriatum after 6-hydroxydopamine (6-OHDA) administration in rats. The dorsolateral striatum (DLS) and the dorsomedial striatum (DMS) are implicated in different navigational learning types, namely the DLS is implicated in egocentric learning while the DMS is implicated in spatial learning. This experiment tested whether selective DA loss through 6-OHDA lesions in the DMS or DLS would impair one or both types of navigation. Both DLS and DMS DA loss significantly impaired route-based CWM learning, without affecting spatial or cued MWM performance. DLS 6-OHDA lesions produced a 75% DA loss in this region, with no changes in other monoamine levels in the DLS or DMS. DMS 6-OHDA lesions produced a 62% DA loss in this region, without affecting other monoamine levels in the DMS or DLS. The results indicate a role for DA in DLS and DMS regions in route-based egocentric but not spatial learning and memory. Spatial learning deficits may require more pervasive monoamine reductions within each region before deficits are exhibited. This is the first study to implicate DLS and DMS DA in route-based egocentric navigation.

The dopamine antagonist cis-flupenthixol blocks the expression of the conditioned positive but not the negative effects of cocaine in rats

Human cocaine users report that the initial "high" produced by cocaine administration is followed by an anxiogenic "crash". Given that cocaine has such robust and opposing properties, it is likely that both positive and negative effects of cocaine contribute to an individual's motivation to administer the drug. Despite this likelihood, the neurobiology underlying cocaine's dual processes remains unclear. While much literature supports a role for dopamine (DA) in cocaine reward, it is uncertain if DA also contributes to the drug's negative effects. Our laboratory has extensively utilized a modified conditioned place test to explore cocaine's opponent processes. In this paradigm rats develop conditioned place preferences (CPPs) for an environment paired with the immediate/positive effects of cocaine, and conditioned place aversions (CPAs) for an environment paired with the delayed/negative effects present 15-min after i.v. injection. In the current study rats were conditioned to associate an environment with either the immediate or delayed effects of i.v. cocaine (1mg/kg/0.1ml) 3h after i.p. pre-treatment with either the DA D1/D2 receptor antagonist cis-flupenthixol (0.5mg/kg/ml) or saline vehicle. As expected, vehicle-treated control animals developed the normal pattern of CPPs for cocaine's immediate effects or CPAs for the delayed effects of cocaine. However, while DA receptor antagonism prevented the expression of cocaine CPPs it did not alter the expression of cocaine-induced CPAs. These data confirm a role for DA transmission in cocaine reward but suggest that different neural pathways mediate the drug's negative/anxiogenic properties.

Prolonged dopamine signalling in striatum signals proximity and value of distant rewards

Predictions about future rewarding events have a powerful influence on behaviour. The phasic spike activity of dopamine-containing neurons, and corresponding dopamine transients in the striatum, are thought to underlie these predictions, encoding positive and negative reward prediction errors^1–5. Many behaviours, however, are directed toward distant goals, for which transient signals might fail to provide sustained drive. Here we report a novel, extended mode of reward-predictive dopamine signalling in the striatum that emerged as rats moved toward distant goals. These dopamine signals, which were detected with fast-scan cyclic voltammetry (FSCV), gradually increased or--in rare instances--decreased as the animals navigated mazes to reach remote rewards, rather than having phasic or steady tonic profiles. These dopamine increases (ramps) scaled flexibly with both the distance and size of the rewards. During learning, these dopamine signals exhibited spatial preferences for goals in different locations and readily changed in magnitude to reflect changing values of the distant rewards. Such prolonged dopamine signalling could provide sustained motivational drive, a control mechanism that may be important for normal behaviour and that can be impaired in a range of neurologic and neuropsychiatric disorders.

Behavioral characterization of the 6-hydroxidopamine model of Parkinson's disease and pharmacological rescuing of non-motor deficits

BACKGROUND

Parkinson's disease (PD) is a chronic neurodegenerative condition that is characterized by motor symptoms as a result of dopaminergic degeneration, particularly in the mesostriatal pathway. However, in recent years, a greater number of clinical studies have focused on the emergence of non-motor symptoms in PD patients, as a consequence of damage on the mesolimbic and mesocortical dopaminergic networks, and on their significant impact on the quality of life of PD patients. Herein, we performed a thorough behavioral analysis including motor, emotional and cognitive dimensions, of the unilateral medial forebrain bundle (MFB) 6-hydroxidopamine (6-OHDA)-lesioned model of PD, and further addressed the impact of pharmacological interventions with levodopa and antidepressants on mood dimensions.

RESULTS

Based on apomorphine-induced turning behaviour and degree of dopaminergic degeneration, animals submitted to MFB lesions were subdivided in complete and incomplete lesion groups. Importantly, this division also translated into a different severity of motor and exploratory impairments and depressive-like symptoms; in contrast, no deficits in anxiety-like and cognitive behaviors were found in MFB-lesioned animals. Subsequently, we found that the exploratory and the anhedonic behavioural alterations of MFB-lesioned rats can be partially improved with the administration of both levodopa or the antidepressant bupropion, but not paroxetine.

CONCLUSIONS

Our results suggest that this model is a relevant tool to study the pathophysiology of motor and non-motor symptoms of PD. In addition, the present data shows that pharmacological interventions modulating dopaminergic transmission are also relevant to revert the non-motor behavioral deficits found in the disease.

Cognitive dysfunction and depression in Parkinson's disease: what can be learned from rodent models?

Parkinson's disease (PD) has for decades been considered a pure motor disorder and its cardinal motor symptoms have been attributed to the loss of dopaminergic (DAergic) neurons in the substantia nigra pars compacta and to nigral Lewy body pathology. However, there has more recently been a shift in the conceptualization of the disease, and its pathological features have now been recognized as involving several other areas of the brain and indeed even outside the central nervous system. There are a corresponding variety of intrinsic non-motor symptoms such as autonomic dysfunction, cognitive impairment, sleep disturbances and neuropsychiatric problems, which cannot be explained exclusively by nigral pathology. In this review, we will focus on cognitive impairment and affective symptoms in PD, and we will consider whether, and how, these deficits can best be modelled in rodent models of the disorder. As only a few of the non-motor symptoms respond to standard DA replacement therapies, the quest for a broader therapeutic approach remains a major research effort, and success in this area in particular will be strongly dependent on appropriate rodent models. In addition, better understanding of the different models, as well as the advantages and disadvantages of the available behavioural tasks, will result in better tools for evaluating new treatment strategies for PD patients suffering from these neuropsychological symptoms.

Animal foraging and the evolution of goal-directed cognition

Foraging- and feeding-related behaviors across eumetazoans share similar molecular mechanisms, suggesting the early evolution of an optimal foraging behavior called area-restricted search (ARS), involving mechanisms of dopamine and glutamate in the modulation of behavioral focus. Similar mechanisms in the vertebrate basal ganglia control motor behavior and cognition and reveal an evolutionary progression toward increasing internal connections between prefrontal cortex and striatum in moving from amphibian to primate. The basal ganglia in higher vertebrates show the ability to transfer dopaminergic activity from unconditioned stimuli to conditioned stimuli. The evolutionary role of dopamine in the modulation of goal-directed behavior and cognition is further supported by pathologies of human goal-directed cognition, which have motor and cognitive dysfunction and organize themselves, with respect to dopaminergic activity, along the gradient described by ARS, from perseverative to unfocused. The evidence strongly supports the evolution of goal-directed cognition out of mechanisms initially in control of spatial foraging but, through increasing cortical connections, eventually used to forage for information.

Dorsal striatal dopamine depletion impairs both allocentric and egocentric navigation in rats

Successful navigation requires interactions among multiple but overlapping neural pathways mediating distinct capabilities, including egocentric (self-oriented, route-based) and allocentric (spatial, map-based) learning. Route-based navigation has been shown to be impaired following acute exposure to the dopaminergic (DA) drugs (+)-methamphetamine and (+)-amphetamine, but not the serotoninergic (5-HT) drugs (±)-3,4-methylenedioxymethamphetamine or (±)-fenfluramine. The dopaminergic-rich neostriatum is involved in both allocentric and egocentric navigation. This experiment tested whether dorsal striatal DA loss using bilateral 6-hydroxydopamine (6-OHDA) injections impaired one or both types of navigation. Two weeks following 6-OHDA injections, rats began testing in the Cincinnati water maze (CWM) followed by the Morris water maze (MWM) for route-based and spatial navigation, respectively. 6-OHDA treatment significantly increased latency and errors in the CWM and path length, latency, and cumulative distance in the MWM with no difference on cued MWM trials. Neostriatal DA levels were reduced by 80% at 2 and 7 weeks post-treatment. In addition, 6-OHDA increased DA turnover and decreased norepinephrine (NE) levels. 6-OHDA injections did not alter monoamine levels in the prefrontal cortex. The data support that neostriatal DA modulates both types of navigation.

Nigral grafts in animal models of Parkinson's disease. Is recovery beyond motor function possible?

Parkinson's disease (PD) has long been considered predominantly to be a "movement disorder," and it is only relatively recently that nonmotor symptoms of PD have been recognized to be a major concern to patients. Consequently, there has been surprisingly little investigation into the feasibility of utilizing cell replacement therapies to ameliorate any of the nonmotor dysfunctions of PD. In this chapter, we identify nonmotor impairments associated predominately with dopaminergic dysmodulation, evaluate the few emerging studies that have identified a role for dopamine and nigral transplantation in nonmotor performance, and consider a number of outstanding questions and considerations dominating the field of nigral transplantation today. Preliminary results obtained from rodent models of PD, despite being limited in number, give clear indications of graft effects on striatal processing beyond the simple activation of motor output and promise a major, exciting, and fruitful new avenue of research for the next decade. We can now consider the prospect of rewriting the opportunities for treating patients, with new stem cell sources to be complemented by new targets for therapeutic benefit.

Three-dimensional motion analysis of postural adjustments during over-ground locomotion in a rat model of Parkinson's disease

Postural instability, a symptom of Parkinson's disease (PD) patients, leads to frequent falls and difficulty in forward motion during gait. These motor deficits are mainly caused by neurodegenerative processes in the brain leading to reduced levels of the neurotransmitter dopamine. Postural studies involving animal models of PD are mainly based on movement scores or descriptive approaches to discerning differences in behaviour or function. Our aim was to describe postural adjustments in a rat model of PD utilising a quantitative three dimensional motion analysis technique during gait to investigate the effects of unilateral dopamine depletion on rat locomotion while walking on beams of varying widths (wide, narrow and graduated). Tail orientation, limb positions on the beam, range of motion and kinematic waveforms of the Roll, Pitch and Yaw of male Lister Hooded rats were investigated using passive markers placed in locations that were representative of their body axis. Hemiparkinsonian rats moved on the wide beam with a significantly higher Roll range of motion coupled with a positively biased Roll kinematic waveform during one gait cycle. While walking on the narrow beam they displayed an increased use of the ledge and placed their tail towards the right. These results are brought about by the rats' inability to shift body posture using the impaired limb. Our data demonstrate that marker-based motion capture can provide an effective and simple approach to quantifying postural adjustments for rat models of PD.

Associative learning and CA3-CA1 synaptic plasticity are impaired in D1R null, Drd1a-/- mice and in hippocampal siRNA silenced Drd1a mice

Associative learning depends on multiple cortical and subcortical structures, including striatum, hippocampus, and amygdala. Both glutamatergic and dopaminergic neurotransmitter systems have been implicated in learning and memory consolidation. While the role of glutamate is well established, the role of dopamine and its receptors in these processes is less clear. In this study, we used two models of dopamine D(1) receptor (D(1)R, Drd1a) loss, D(1)R knock-out mice (Drd1a(-/-)) and mice with intrahippocampal injections of Drd1a-siRNA (small interfering RNA), to study the role of D(1)R in different models of learning, hippocampal long-term potentiation (LTP) and associated gene expression. D(1)R loss markedly reduced spatial learning, fear learning, and classical conditioning of the eyelid response, as well as the associated activity-dependent synaptic plasticity in the hippocampal CA1-CA3 synapse. These results provide the first experimental demonstration that D(1)R is required for trace eyeblink conditioning and associated changes in synaptic strength in hippocampus of behaving mice. Drd1a-siRNA mice were indistinguishable from Drd1a(-/-) mice in all experiments, indicating that hippocampal knockdown was as effective as global inactivation and that the observed effects are caused by loss of D(1)R and not by indirect developmental effects of Drd1a(-/-). Finally, in vivo LTP and LTP-induced expression of Egr1 in the hippocampus were significantly reduced in Drd1a(-/-) and Drd1a-siRNA, indicating an important role for D(1)R in these processes. Our data reveal a functional relationship between acquisition of associative learning, increase in synaptic strength at the CA3-CA1 synapse, and Egr1 induction in the hippocampus by demonstrating that all three are dramatically impaired when D(1)R is eliminated or reduced.

Behavioral analysis of motor and non-motor symptoms in rodent models of Parkinson's disease

Alongside the classical motor symptoms, non-motor symptoms are increasingly recognised to play a major role in the disability associated with Parkinson's disease in humans. Animal models based on experimental depletion of forebrain dopamine have traditionally focussed on the simple and easy to measure motor impairments, and they reproduce well the bradykinesia, rigidity and impairments in the initiation and sequencing of voluntary goal-directed movement. However, a more comprehensive analysis is now urgently required. In this chapter we summarise the predominant unilateral and bilateral dopamine lesion, toxin and genetic models of human parkinsonism, and review the consequences in more complex cognitive, motor learning and psychiatric ('behavioural') domains. Theoretical and experimental advances in our understanding of information processing and associative plasticity within the striatum are not only revolutionising our understanding of normal striatal function but also bear directly on our understanding of the processes that underlie non-motor as well as motor disability in human disease, including in Parkinson's disease.

Capturing dopaminergic modulation and bimodal membrane behaviour of striatal medium spiny neurons in accurate, reduced models

Loss of dopamine from the striatum can cause both profound motor deficits, as in Parkinson's disease, and disrupt learning. Yet the effect of dopamine on striatal neurons remains a complex and controversial topic, and is in need of a comprehensive framework. We extend a reduced model of the striatal medium spiny neuron (MSN) to account for dopaminergic modulation of its intrinsic ion channels and synaptic inputs. We tune our D1 and D2 receptor MSN models using data from a recent large-scale compartmental model. The new models capture the input-output relationships for both current injection and spiking input with remarkable accuracy, despite the order of magnitude decrease in system size. They also capture the paired pulse facilitation shown by MSNs. Our dopamine models predict that synaptic effects dominate intrinsic effects for all levels of D1 and D2 receptor activation. We analytically derive a full set of equilibrium points and their stability for the original and dopamine modulated forms of the MSN model. We find that the stability types are not changed by dopamine activation, and our models predict that the MSN is never bistable. Nonetheless, the MSN models can produce a spontaneously bimodal membrane potential similar to that recently observed in vitro following application of NMDA agonists. We demonstrate that this bimodality is created by modelling the agonist effects as slow, irregular and massive jumps in NMDA conductance and, rather than a form of bistability, is due to the voltage-dependent blockade of NMDA receptors. Our models also predict a more pronounced membrane potential bimodality following D1 receptor activation. This work thus establishes reduced yet accurate dopamine-modulated models of MSNs, suitable for use in large-scale models of the striatum. More importantly, these provide a tractable framework for further study of dopamine's effects on computation by individual neurons.

The ventral basal ganglia, a selection mechanism at the crossroads of space, strategy, and reward

The basal ganglia are often conceptualised as three parallel domains that include all the constituent nuclei. The 'ventral domain' appears to be critical for learning flexible behaviours for exploration and foraging, as it is the recipient of converging inputs from amygdala, hippocampal formation and prefrontal cortex, putatively centres for stimulus evaluation, spatial navigation, and planning/contingency, respectively. However, compared to work on the dorsal domains, the rich potential for quantitative theories and models of the ventral domain remains largely untapped, and the purpose of this review is to provide the stimulus for this work. We systematically review the ventral domain's structures and internal organisation, and propose a functional architecture as the basis for computational models. Using a full schematic of the structure of inputs to the ventral striatum (nucleus accumbens core and shell), we argue for the existence of many identifiable processing channels on the basis of unique combinations of afferent inputs. We then identify the potential information represented in these channels by reconciling a broad range of studies from the hippocampal, amygdala and prefrontal cortex literatures with known properties of the ventral striatum from lesion, pharmacological, and electrophysiological studies. Dopamine's key role in learning is reviewed within the three current major computational frameworks; we also show that the shell-based basal ganglia sub-circuits are well placed to generate the phasic burst and dip responses of dopaminergic neurons. We detail dopamine's modulation of ventral basal ganglia's inputs by its actions on pre-synaptic terminals and post-synaptic membranes in the striatum, arguing that the complexity of these effects hint at computational roles for dopamine beyond current ideas. The ventral basal ganglia are revealed as a constellation of multiple functional systems for the learning and selection of flexible behaviours and of behavioural strategies, sharing the common operations of selection-by-disinhibition and of dopaminergic modulation.

Prefrontal cortical D1 dopamine receptors modulate subcortical D2 dopamine receptor-mediated stress responsiveness

Increased responsiveness to stress plays an important role in the manifestation of schizophrenia symptoms. Evidence indicates that the prefrontal cortex (PFC), and dopamine neurotransmission in the PFC in particular, is involved in the modulation of stress responsiveness. Decreased dopaminergic activity and loss of dopamine fibres have been reported in PFC in schizophrenia patients. Consequently, it was hypothesized that depletion of dopamine in PFC may facilitate increased stress responsiveness. Adult Sprague-Dawley rats received injections of 6-hydroxydopamine or saline bilaterally into the medial PFC (mPFC) following desipramine pretreatment to selectively deplete dopaminergic fibres. Following a 3-wk recovery period, the lesioned and control rats received injections of a D1 or D2 dopamine receptor agonist or vehicle into the mPFC and were immediately subjected to forced swimming as a stressor. Results showed that frequency of locomotion and rearing, behavioural measures indicative of increased dopaminergic activity in the nucleus accumbens (NAc), were significantly increased following stress in prefrontal cortical dopamine-depleted rats. This effect was significantly ameliorated by infusions of a D1 dopamine receptor agonist directly into the mPFC in a dose-dependent manner but not by infusion of a D2 dopamine receptor agonist. In addition, stress-induced behavioural changes in prefrontal cortical dopamine-depleted rats were significantly reduced following selective discrete infusions of a D2 dopamine receptor antagonist into the NAc shell. The results suggest that dopaminergic transmission via D1 receptors in the mPFC modulates D2 dopamine receptor-mediated stress responsiveness in the NAc, a feature that may be disrupted in schizophrenia patients.

Dopamine-modulated dynamic cell assemblies generated by the GABAergic striatal microcircuit

The striatum, the principal input structure of the basal ganglia, is crucial to both motor control and learning. It receives convergent input from all over the neocortex, hippocampal formation, amygdala and thalamus, and is the primary recipient of dopamine in the brain. Within the striatum is a GABAergic microcircuit that acts upon these inputs, formed by the dominant medium-spiny projection neurons (MSNs) and fast-spiking interneurons (FSIs). There has been little progress in understanding the computations it performs, hampered by the non-laminar structure that prevents identification of a repeating canonical microcircuit. We here begin the identification of potential dynamically-defined computational elements within the striatum. We construct a new three-dimensional model of the striatal microcircuit's connectivity, and instantiate this with our dopamine-modulated neuron models of the MSNs and FSIs. A new model of gap junctions between the FSIs is introduced and tuned to experimental data. We introduce a novel multiple spike-train analysis method, and apply this to the outputs of the model to find groups of synchronised neurons at multiple time-scales. We find that, with realistic in vivo background input, small assemblies of synchronised MSNs spontaneously appear, consistent with experimental observations, and that the number of assemblies and the time-scale of synchronisation is strongly dependent on the simulated concentration of dopamine. We also show that feed-forward inhibition from the FSIs counter-intuitively increases the firing rate of the MSNs. Such small cell assemblies forming spontaneously only in the absence of dopamine may contribute to motor control problems seen in humans and animals following a loss of dopamine cells.

Metabotropic glutamate receptors 5 blockade reverses spatial memory deficits in a mouse model of Parkinson's disease

Visuo-spatial deficits are the most consistently reported cognitive abnormalities in Parkinson's disease (PD), and they are frequently associated to motor symptoms in the early stages of the disease when dopamine loss is moderate and still restricted to the caudate-putamen. The metabotropic glutamate receptor 5 (mGluR5) antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), has beneficial effects on motor symptoms in animal models of PD. However, the effects of MPEP on the cognitive deficits of the disease have never been investigated. Thus, the purpose of this study was to explore its therapeutic potentials by investigating its effects on the visuo-spatial deficits induced by 6-hydroxydopamine (6-OHDA) lesions of dorsal striatum in CD1 mice. The results demonstrated that systemic injections of MPEP (6, 12, and 24 mg/kg, i.p.) impair visuo-spatial discrimination in intact mice at high concentrations, whereas lower doses (1.5 and 3 mg/kg, i.p.) were void of effects. Nevertheless, when an ineffective dose (MPEP 3 mg/kg) was injected, either acutely or subchronically (8 days), it antagonized the visuo-spatial discrimination deficit induced by bilateral dopamine lesion of the striatum. Furthermore, the same treatment increased contralateral turning induced by L-DOPA in mice bearing unilateral 6-OHDA lesion. These results confirm the therapeutic potential of mGluR5 blockade on motor symptoms induced by reduced striatal dopamine function. Further, they demonstrate that mGluR5 blockade may also have beneficial effects on cognitive deficits induced by dopamine depletion.

MDMA modifies active avoidance learning and recall in mice

RATIONALE

Several studies have suggested the existence of cognitive deficits after repeated or high doses of 3,4-methylenedioxymethamphetamine (MDMA) in humans and experimental animals. However, the extent of the impairments observed in learning or memory tasks remains unclear.

OBJECTIVE

The objective of this study was to evaluate the effects of different dosing regimens of MDMA on the ability of mice to learn and recall an active avoidance task.

MATERIALS AND METHODS

Animals were treated with MDMA (0, 1, 3, 10 and 30 mg/kg) under four different experimental conditions, and active avoidance acquisition and recall were evaluated. In experiments 1 and 2, MDMA was administered 1 h before different active avoidance training sessions. In experiments 3 and 4, mice received a repeated treatment with MDMA before or after active avoidance training, respectively. Changes in presynaptic striatal dopamine transporter (DAT) binding sites were evaluated at two different time points in animals receiving a high dose of MDMA (30 mg/kg) or saline twice a day over 4 days.

RESULTS

MDMA administered before the active avoidance sessions interfered with the acquisition and the execution of a previously learned task. A repeated treatment with high doses of MDMA administered before training reduced acquisition of active avoidance in mice, while pre-treatment with both high and low doses of MDMA impaired recall of this task. A reduction in DAT binding was observed 4 days but not 23 days after the last MDMA administration.

CONCLUSIONS

Acute MDMA modifies the acquisition and execution of active avoidance in mice, while repeated pre-treatment with MDMA impairs acquisition and recall of this task.

Spatial deficits in a mouse model of Parkinson disease

RATIONALE

Accumulating evidence in humans demonstrated that visuo-spatial deficits are the most consistently reported cognitive abnormalities in Parkinson disease (PD). These deficits have been generally attributed to cortical dopamine degeneration. However, more recent evidence suggests that dopamine loss in the striatum is responsible for the visuo-spatial abnormalities in PD. Studies based on animal models of PD did not specifically address this question.

OBJECTIVES

Thus, the first goal of this study was to analyze the role of dopamine within the dorsal striatum in spatial memory. We tested bilateral 6-OHDA striatal lesioned CD1 mice in an object-place association spatial task. Furthermore, to see whether the effects were selective for spatial information, we measured how the 6-OHDA-lesioned animals responded to a non-spatial change and learned in the one-trial inhibitory avoidance task.

RESULTS

The results demonstrated that bilateral (approximately 75%) dopamine depletion of the striatum impaired spatial change discrimination. On the contrary, no effect of the lesion was observed on non-spatial novelty detection or on passive avoidance learning.

CONCLUSIONS

These results confirm that dopamine depletion is accompanied by cognitive deficits and demonstrate that striatal dopamine dysfunction is sufficient to induce spatial information processing deficits.

Parallel loss of hippocampal LTD and cognitive flexibility in a genetic model of hyperdopaminergia

Dopamine-mediated neurotransmission has been implicated in the modulation of synaptic plasticity and in the mechanisms underlying learning and memory. In the present study, we tested different forms of activity-dependent neuronal and behavioral plasticity in knockout mice for the dopamine transporter (DAT-KO), which constitute a unique genetic model of constitutive hyperdopaminergia. We report that DAT-KO mice exhibit slightly increased long-term potentiation and severely decreased long-term depression at hippocampal CA3-CA1 excitatory synapses. Mutant mice also show impaired adaptation to environmental changes in the Morris watermaze. Both the electrophysiological and behavioral phenotypes are reversed by the dopamine antagonist haloperidol, suggesting that hyperdopaminergia is involved in these deficits. These findings support the modulation by dopamine of synaptic plasticity and cognitive flexibility. The behavioral deficits seen in DAT-KO mice are reminiscent of the deficits in executive functions observed in dopamine-related neuropsychiatric disorders, suggesting that the study of DAT-KO mice can contribute to the understanding of the molecular basis of these disorders.

D1 but not D5 Dopamine Receptors Are Critical for LTP, Spatial Learning, and LTP-Induced arc and zif268 Expression in the Hippocampus

Recent evidence suggests that glutamatergic and dopaminergic afferents must be activated to induce persistent long-term potentiation (LTP) in the hippocampus. Whereas extensive evidence supports the role of glutamate receptors in long-lasting synaptic plasticity and spatial learning and memory, there is less evidence regarding the role of dopamine receptors in these processes. Here, we used dopamine D(1) receptor knockout (D(1)R(-/-)) mice to explore the role of D(1)R in hippocampal LTP and its associated gene expression. We show that the magnitude of early and late phases of LTP (E-LTP and L-LTP) was markedly reduced in hippocampal slices from D(1)R(-/-) mice compared with wild-type mice. SCH23390, a D(1)/D(5)R antagonist, did not further reduce L-LTP in D(1)R(-/-) mice, suggesting that D(5)Rs are not involved. D(1)R(-/-) mice also showed a significant reduction of D(1)R-induced potentiation of N-Methyl-D-aspartic acid-mediated currents, via protein kinase activated by cyclic adenosine 3',5'-monophosphate activation. Finally, LTP-induced expression of the immediate early genes zif268 and arc in the hippocampal CA1 area was abolished in D(1)R(-/-) mice, and these mice showed impaired learning. These results indicate that D(1)R but not D(5)R are critical for hippocampal LTP and for the induction of Zif268 and Arc, proteins required for the transition from E-LTP to L-LTP and for memory consolidation in mammals.

Apomorphine effects on frog locomotor behavior

The neuroanatomical pathways of the DA systems have been shown to be largely conserved across many vertebrate taxa. It is less certain whether the structural similarities seen between mammals and amphibians reflect a similar functional homology. DA is well known for its role in facilitating motor behaviors in mammals. We examined whether a similar role for DA exists in amphibians using the Northern Leopard Frog (Rana pipiens). We investigated the effects of the nonspecific DA agonist, apomorphine (APO) on a complex motor task that included two distinct components known to be differentially modulated by DA in mammals: swimming and climbing. We demonstrated that a high single dose of APO (20 mg/kg, body weight) strongly increased the amount of time spent completing the motor task. Furthermore, we showed that although APO did not significantly alter several aspects of swimming behavior, two aspects of climbing behavior were disrupted. Both climbing speed and climbing ability were impaired by APO treatment. These results increase our understanding of DA function in amphibians and add to our understanding of structure-function homologies of dopamine function across vertebrate taxa.

D1 and D2 receptor antagonist injections in the prefrontal cortex selectively impair spatial learning in mice

The prefrontal cortex (PFC) is a cortical area involved in selecting and retaining information to produce complex behaviors. Within the PFC, the dopaminergic system plays an important role in information processing. Thus, the objective of this study was to test whether bilateral administration of the D1 and D2 receptor antagonists in the prelimbic region of the PFC influenced the performance of mice in a non-associative spatial learning task. CD1 mice were bilaterally microinjected in the PFC with either the D1 receptor antagonist, SCH23390 (SCH 6.25; 12.5; 50 ng), or the D2 receptor antagonist, sulpiride (SULP 12.5; 50; 100 ng) and placed into an open field containing five different objects. After three sessions of habituation two objects were repositioned (spatial change) and in the subsequent session one of the objects was substituted (non-spatial change). No significant alteration was observed in the habituation pattern of the animals after D1 or D2 receptor blockade. When two of the objects were displaced, control mice explored the displaced objects far more than the non-displaced ones, while mice treated with SCH or SULP spent a comparable amount of time re-exploring the two object categories. Conversely, DA antagonists had no effects on the discrimination of the new object. Thus, the administration of both SCH and SULP selectively impaired the ability of mice to discriminate a spatial change, without affecting any other behavioral parameter. These findings could provide a model to study the role of the PFC dopaminergic system in spatial learning and to study the neural mechanisms underlying cognitive and attention deficits often observed in psychiatric disorders.

Insights into the Role of Dopamine Receptor Systems in Learning and Memory

It is well established that learning and memory are complex processes involving and recruiting different brain modulatory neurotransmitter systems. Considerable evidence points to the involvement of dopamine in various aspects of cognition, and interest has been focused on investigating the clinical relevance of dopamine systems to age-related cognitive decline and manifestations of cognitive impairment in schizophrenia, Alzheimer's disease, Parkinson's disease and other neurodegenerative diseases. In the past decade or so, in spite of the molecular cloning of the five dopamine receptor subtypes, their specific roles in brain function remained inconclusive due to the lack of completely selective ligands that could distinguish between the members of the D1-like and D2-like dopamine receptor families. One of the most important advances in the field of dopamine research has been the generation of mutant mouse models permitting evaluation of the dopaminergic system using gene targeting technologies. These mouse models represent an important approach to explore the functional roles of closely related receptor subtypes. In this review, we present and discuss evidence on the role of dopamine receptors in different aspects of learning and memory at the cellular, molecular and behavioral levels. We compare evidence using conventional pharmacological, lesion or electrophysiological studies with results from mice with targeted deletions of different subtypes of dopamine receptor genes. We particularly focus on dopamine D1 and D2 receptors in an effort to delineate their specific roles in various aspects of cognitive function. We provide strong evidence, from our own recent work as well as others, that dopamine is part of the network that plays a very important role in cognitive function, and that although multiple dopamine receptor subtypes contribute to different aspects of learning and memory, the D1 receptor seems to play a more prominent role in mediating plasticity and specific aspects of cognitive function, including spatial learning and memory processes, reversal learning, extinction learning, and incentive learning.

Dopaminergic modulation of the persistence of one-trial hippocampus-dependent memory

The persistence of new memory traces in the hippocampus, encoded following appropriate activation of glutamatergic receptors and the induction of synaptic plasticity, can be influenced by heterosynaptic activation of neuromodulatory brain systems. We therefore investigated the effects of a hippocampus-specific blockade of dopamine D1/D5 receptors on the persistence of spatial memory encoded in one trial using a delayed matching-to-place (DMP) task in a watermaze in which rats learn a new escape location each day. A within-subjects design was used such that both short (20 min) and long (6 h) retention intervals, and both drug (SCH23390, a D1/D5 receptor antagonist) and vehicle (aCSF) infusions were tested on different days in the same animals. Bilateral intrahippocampal infusion of SCH23390 (5 microg in 1 microL per side) prior to trial 1 (encoding) caused a differential impairment as a function of memory delay-with no effect during trial 2 (memory retrieval) after a 20-min interval, but a block of memory at 6 h. Further experiments revealed that infusion of SCH23390 immediately after trial 1 had no effect on retention 6 h later, and the poor memory seen at long retention intervals when the drug was present at encoding was not due to a state-dependent failure of retrieval. These results suggest that activation of D1/D5 receptors during memory encoding is necessary for the formation of a persistent memory trace in the hippocampus. The complementary effects of D1/D5 receptor blockade on the persistence of LTP and the duration of memory are consistent with the idea that changes in synaptic strength underlie memory.

The role of the extrapersonal brain systems in religious activity

The neuropsychology of religious activity in normal and selected clinical populations is reviewed. Religious activity includes beliefs, experiences, and practice. Neuropsychological and functional imaging findings, many of which have derived from studies of experienced meditators, point to a ventral cortical axis for religious behavior, involving primarily the ventromedial temporal and frontal regions. Neuropharmacological studies generally point to dopaminergic activation as the leading neurochemical feature associated with religious activity. The ventral dopaminergic pathways involved in religious behavior most closely align with the action-extrapersonal system in the model of 3-D perceptual-motor interactions proposed by . These pathways are biased toward distant (especially upper) space and also mediate related extrapersonally dominated brain functions such as dreaming and hallucinations. Hyperreligiosity is a major feature of mania, obsessive-compulsive disorder, schizophrenia, temporal-lobe epilepsy and related disorders, in which the ventromedial dopaminergic systems are highly activated and exaggerated attentional or goal-directed behavior toward extrapersonal space occurs. The evolution of religion is linked to an expansion of dopaminergic systems in humans, brought about by changes in diet and other physiological influences.

A Novel Mutant Mouse, joggle, with Inherited Ataxia

While establishing a new mouse strain, we discovered a novel mutant mouse that exhibited ataxia. Mating experiments showed that the mutant phenotype was due to a single autosomal recessive gene, which we have termed joggle (gene symbol: jog). The ataxia becomes apparent around postnatal day 12, when the mice first attempt to walk, and worsens thereafter. The life span of the mutant mouse is comparable to that of the wild-type mouse. After 21 days of age, the cerebellum weights of the jog/jog mice are significantly lower than those of the wild-type mice. These observations indicate that jog/jog mutant mice could be useful models for biomedical research.

Spatial learning in rats is impaired after degeneration of the nigrostriatal dopaminergic system

We investigated spatial learning in rats with unilateral and bilateral lesions of the nigrostriatal dopaminergic system. We used the Morris water maze paradigm, which tests spatial forms of learning and memory and allows discrimination between sensory-motor and learning disabilities. Animals were trained preoperatively to learn the location of a spatially fixed hidden platform to escape from the swimming pool (acquisition training). A visual and a probe test were used before and after the acquisition training, respectively. Our results show that animals with unilateral lesions, although displaying longer escape latencies, have normal spatial memory abilities. Animals with bilateral lesions were able to swim as fast or even faster than animals with unilateral lesion. Despite the fact that these animals had learned the spatial navigation tasks preoperatively, bilateral dopaminergic lesions led to a profound deficit in ability to find a hidden platform during an acquisition task. In general, animals with bilateral lesions persisted in swimming along the pool walls and their spatial navigation performance during a probe test was very poor. These results suggest that deficit of the nigrostriatal dopaminergic system can affect the selection and maintenance of behavioral strategies in spatial navigation.

Modeling Parkinson's disease in rats: an evaluation of 6-OHDA lesions of the nigrostriatal pathway

Human idiopathic Parkinson's disease (PD) is a progressive neurodegenerative disorder that is primarily characterized by degeneration of the dopaminergic neurons of the nigrostriatal pathway. Different 6-OHDA rat models of PD have been developed in which this toxin has been injected into different parts of the nigrostriatal pathway: (a) the medial forebrain bundle which leads to extensive dopamine (DA) depletion; (b) the substantia nigra pars compacta, which leads to more specific and moderate DA depletions; and (c) subregions of the caudate-putamen complex (CPu), which also leads to specific DA depletions. In this article we review the dopaminergic depletion and behavioral consequences of 6-OHDA lesions in the rat. It was examined whether the relation between DA depletion and behavioral deficits mimic idiopathic PD. In addition, it was evaluated which model most closely approximates the human situation, especially in relation to the stage of this progressive disease. It was concluded that with respect to the site of the lesion, rats with partial lesions of the ventrolateral CPu are the most appropriate models to study early and late stages of PD. The choice of the behavioral parameters determines the use of unilateral or bilateral lesions, although it is obvious that the bilateral model mimics the human situation more closely.

Differential effects of a small, unilateral, 6-hydroxydopamine-induced nigral lesion on behavior in high and low responders to novelty

The goal of this study was to develop an animal model that evaluates striatal-specific behavior after partial, unilateral destruction of nigrostriatal neurons. 6-OHDA (1 microg) was injected intranigrally (day 0) to reduce dopaminergic innervation of the dorsal striatum (DS); 6-OHDA (5 microg) was injected to reduce innervation of DS and nucleus accumbens (ACC). We analyzed changes in (a) behavior regulated by dopamine (DA) release in the DS (hindpaw preference from day 5 to day 19, every other day) and the ACC (novelty-induced locomotion on day 16) and (b) apomorphine-induced rotation (on day 21). We used two types of rat that show differences in structure and function of the dopaminergic neurons, namely high (HR) and low (LR) responders to novelty. 6-OHDA (1 microg) significantly decreased TH immunoreactivity (TH-ir) in the DS and increased preference for the hindpaw controlled by the nonlesioned side in HRs and LRs in time. Only in LRs was the significant increase of novelty-induced locomotion accompanied by a significant increase in TH-ir density in the ACC: this suggests a lesion-induced shift in nigrostriatal/mesolimbic balance toward a dominance of the mesolimbic system. The higher 6-OHDA dose significantly decreased TH-ir in the DS and the ACC and increased preference for the hindpaw controlled by the nonlesioned side in HRs and LRs in time. However, this increase occurred significantly earlier in LRs than in HRs. Apomorphine elicited contralateral rotations solely in LRs, and not in HRs, indicating development of supersensitive dopamine receptors in the DS of LRs, but not HRs. The data show that LRs are more susceptible to 6-OHDA than HRs. The relevance of the present data for Parkinson's disease is discussed.

Mutant mice as a model for cerebellar ataxia

Not later than two synapses after their arrival in the cerebellar cortex all excitatory afferent signals are subsequently transformed into inhibitory ones. Guaranteed by the exceedingly ordered and stereotyped synaptic arrangement of its cellular elements, the cerebellar cortex transmits this inhibitory result of cerebellar integration exclusively via Purkinje cells (PCs) in a precise temporal succession directly onto the target neurons of the deep cerebellar and vestibular nuclei. Thus the cerebellar cortex seems to produce a temporal pattern of inhibitory influence on these target neurons that modifies their excitatory action in such a way that an activation of muscle fibers occurs which progressively integrates the intended motion into the actual condition of the motoric inventory. In consequence, disturbances that affect this cerebellar inhibition will cause uncoordinated, decomposed and ataxic movements, commonly referred to as cerebellar ataxia. Electrophysiological investigations using different cerebellar mouse mutants have shown that alterations in the cerebellar inhibitory input in the target nuclei lead to diverse neuronal responses and to different consequences for the behavioural phenotype. A dependence between the reconstitution of inhibition and the behavioural outcome seems to exist. Obviously two different basic mechanisms are responsible for these observations: (1) ineffective inhibition on target neurons by surviving PCs; and (2) enhancement of intranuclear inhibition in the deep cerebellar and vestibular nuclei. Which of the two strategies evolves is dependent upon the composition of the residual cell types in the cerebellum and on the degree of PC input loss in a given area of the target nuclei. Motor behaviour seems to deteriorate under the first of these mechanisms whereas it may benefit from the second. This is substantiated by stereotaxic removal of the remaining PC input, which eliminates the influence of the first mechanism and is able to induce the second strategy. As a consequence, motor performance improves considerably. In this review, results leading to the above conclusions are presented and links forged to human cerebellar diseases.

Super-stereotypy I: enhancement of a complex movement sequence by systemic dopamine D1 agonists

Peripheral administration of D1 dopamine agonists elicits grooming behavior from rodents. The present study examined grooming behavior and the relative probability and stereotypy of a natural sequence of grooming movements (called a syntactic grooming chain) that follows a predictable fixed pattern of serial order. We compared the amount of grooming behavior vs. the stereotypy of sequential patterns after peripheral administration of either a partial D1 agonist (SKF 38393; 2.5, 5.0, 10, 20 mg/kg), a full D1 agonist (SKF 82958; 0.1, 0.2, 0.5, 1.0 mg/kg; i.p.), a D2 agonist (quinpirole; 5.0, 10 mg/kg), or ACTH (2.0, 5.0 mg/kg). There was a dissociation between the elicited grooming amount, the pattern frequency, and the pattern completion or sequential stereotypy after these drugs. Quinpirole and ACTH both reduced the likelihood that the sequential pattern would be completed in the normal pattern (and reduced the overall amount of grooming). Administration of either SKF 38393 or SKF 82958 increased the tendency to engage in complex stereotyped sequential patterns of grooming (even though only the partial D1 agonist increased the total amount of grooming). In addition, SKF 38393 increased the sequential stereotypy of the already-stereotyped pattern itself (as measured by the probability of completing the stereotyped sequence once it began). Thus, dopamine D1 receptor activation appears to contribute to a kind of sequential super-stereotypy in which a complex, stereotyped behavioral sequence is initiated more frequently and more often goes to completion.

Systemic administration of the propargylamine CGP 3466B prevents behavioural and morphological deficits in rats with 6-hydroxydopamine-induced lesions in the substantia nigra

The ability of CGP 3466B to attenuate the behavioural and morphological consequences of experimentally induced cell death was investigated in a recently updated animal model of Parkinson's disease. 6-Hydroxydopamine was infused bilaterally into the substantia nigra pars compacta of rats that were pretreated with desimipramine. Treatment with CGP 3466B (0.0014-1.4 mg/kg, injected subcutaneously) or its solvent was begun 2 h after the 6-OHDA injection, and maintained twice daily for 14 days. After a washout period of 14 days, changes in motor behaviour were evaluated, using the open field test (analysis of normal and abnormal stepping, e.g.) and the paw test (analysis of retraction time of limbs). Changes in learning and memory were evaluated with the help of the Morris water maze task. Following immunocytochemical staining of tyrosine hydroxylase, the extent of the lesion was quantified using a computerized system. CGP 3466B prevented all deficits produced by 6-hydroxydopamine (6-OHDA), though at different doses. It prevented: abnormal stepping (0.0014-0.014 mg/kg); increased forelimb and hindlimb retraction time (0.014-0.14 mg/kg and 0.0014-0.14 mg/kg, respectively); delayed learning (1.4 mg/kg); and reduced tyrosine hydroxylase immunoreactivity in the substantia nigra (0.0014-0.014 mg/kg). CGP 3466B (0.0014-0.14 mg/kg) induced no deficits in sham-treated rats. CGP 3466B (1.4 mg/kg), however, did not show any benefit on motor deficits in 6-OHDA-lesioned rats, and induced abnormal movements and decreased the tyrosine hydroxylase immunoreactivity in the substantia nigra pars compacta and the ventral tegmental area of sham-lesioned animals. It is concluded that CGP 3466B prevents all 6-OHDA-induced behavioural and immunocytochemical deficits, though at different doses. CGP 3466B is suggested to be a valuable agent for inhibiting the dopaminergic degeneration in patients with Parkinson's disease.

A cautionary note regarding drug and brain lesion studies that use swimming pool tasks: partial reinforcement impairs acquisition of place learning in a swimming pool but not on dry land

Spatial tasks are used widely in neurobiological studies because it is thought that they provide an unbiased assessment of the integrity of neural structures that mediate spatial learning. For example, in the Morris swimming pool place task, animals are required to locate a hidden platform in a swimming pool in relation to environmental cues. Treatments that result in an animal's failure to find the platform are assumed to reflect defects in the function of neural systems involved in spatial learning. The present study demonstrates, however, that an animal's reinforcement history can contribute to its spatial performance. Animals were trained in the Morris place task with the platform present on 100, 75 or 50% of trials. Relative to the 100% group, the 75% group was impaired in place acquisition, and the 50% group failed to learn. Even placing the 50% group animals onto the platform at the completion of an unsuccessful trial failed to improve acquisition. Animals trained to search for food on an identical dry maze problem were not affected by similar reinforcement schedules. The present findings demonstrate that the Morris swimming pool place task does not provide an unbiased assessment of spatial learning: A treatment effect may be confounded with reinforcement history. The results are discussed in relation to widespread applications of the Morris place task to neurobiological problems.

Spatial learning deficit in dopamine D(1) receptor knockout mice

Dopamine D(1) receptors are expressed in the hippocampus and prefrontal cortex, suggesting a role in cognition. Dopamine D(1) receptor-deficient mice (D(1)-/-) were used to investigate the role of this receptor in spatial learning and memory. Using the Morris water maze, mice were trained to locate a hidden platform. Subsequently, the platform was removed from the maze and mice were scored for the percentage of time spent in the target quadrant and the number of crossings through the target position. D(1)-/- mice had significantly longer escape latencies compared to wild-type (D(1)+/+) and heterozygous (D(1)+/-) littermates and showed absence of spatial bias during the probe trials. In a visually cued task, D(1)-/- mice performed better than on the hidden platform trials, but maintained slightly higher escape latencies than D(1)+/+ and D(1)+/- mice. Naive D(1)-/- mice exposed only to the cued task eventually acquired identical escape latencies as the D(1)+/+ and D(1)+/- mice. Sensorimotor reflexes, locomotor activity, spontaneous alternation and contextual learning were not different among the groups. These results indicate that D(1)-/- mice have a deficit in spatial learning without visual or motor impairment, suggesting that dopamine D(1) receptors are involved in at least one form of the cognitive processes.

Incomplete nigrostriatal dopaminergic cell loss and partial reductions in striatal dopamine produce akinesia, rigidity, tremor and cognitive deficits in middle-aged rats

The present study was conducted to determine if the full array of parkinsonian symptoms could be detected in rats with nigrostriatal cell loss and striatal dopamine depletions similar to levels reported in the clinical setting, and to determine if older rats exhibit more robust parkinsonian deficits than younger rats. Young (2 months old) and middle-aged (12 months old) rats received bilateral striatal infusions of 6-OHDA, over the next 3 months they were assessed with a battery of behavioral tests, and then dopaminergic nigrostriatal cells and striatal dopamine and DOPAC levels were quantified. The results of the present study suggest that: (1) the full array of parkinsonian symptoms (i.e. akinesia, rigidity, tremor and visuospatial cognitive deficits) can be quantified in rats with incomplete nigrostriatal dopaminergic cell loss and partial reductions in striatal dopamine levels (2) parkinsonian symptoms were more evident in middle-aged rats with 6-OHDA infusions, and (3) there was evidence of substantial neuroplasticity in the older rats, but regardless of the age of the animal, endogenous compensatory mechanisms were unable to maintain striatal dopamine levels after rapid, lesion-induced nigrostriatal cell loss. These results suggest that using older rats with nigrostriatal dopaminergic cell loss and reductions in striatal dopamine levels similar to those in the clinical condition, and measuring behavioral deficits analogous to parkinsonian symptoms, might increase the predictive validity of pre-clinical rodent models.

Vermectomy enhances parvalbumin expression and improves motor performance in weaver mutant mice: an animal model for cerebellar ataxia

In the Weaver mutant mouse (wv/wv), an animal model for hereditary cerebellar ataxia, electrophysiological experiments have revealed a disorganized output of cerebellar Purkinje cells (the latter using GABA as an inhibitory transmitter) which, by a cascade of mechanisms, was thought to be the cause of the poor motor abilities. In Purkinje cell degeneration mice (pcd/pcd) lacking nearly all Purkinje cells and displaying milder motor deficiencies than wv, in comparison to wild-type mice, a strong increase in parvalbumin- and (co-localized with parvalbumin) glycine-immunopositive somata in the deep cerebellar and vestibular nuclei has recently been found. It was therefore intriguing to investigate whether motor performance in weaver mutants could be ameliorated by applying cerebellar lesions to eliminate the faulty output and to look for a change in transmitter weighting, indicated by a strong increase in parvalbumin-positive somata in areas (the respective target areas) which were formerly devoid of it. Ten Weaver mutants were subjected to cerebellar lesions. After removal of the vermis a total abolition of tremor, a definite improvement in the balance of affected body parts, an increase in locomotor activity when tested in an open-field matrix, and a strong increase in parvalbumin expression in Weaver mutant deep cerebellar and vestibular nuclei in comparison to wild-types have indeed been found. Increase in motor activity (or explorative behaviour) has been placed in relation to learning mechanisms. The increase in parvalbumin expression and the observed improvement in motor abilities and mechanisms probably related to learning underline the hypothesis that any change in the physiological equilibrium of the brain function by removal of input or output related to an assembly of nerve cells leads to a cascade of changes at the transmitter and neuronal level in near or distant connected brain structures.

Effects of medial and lateral caudate-putamen lesions on place- and cue-guided behaviors in the water maze: relation to thigmotaxis

Rats with dorsomedial or dorsolateral caudate-putamen lesions and sham-operated controls were trained on the standard hidden platform (place) task in the water maze. Compared to controls, rats with dorsomedial, but not dorsolateral lesions were slower to escape to the hidden platform and spent significantly more time swimming near the wall of the pool (thigmotaxis) on the early trials, but eventually achieved control levels of performance. When the platform was removed from the pool, all groups exhibited a significant bias for swimming in the training quadrant and crossing the former location of the platform. In the second phase of the experiment rats were given visible platform (cue) training in a different room/pool with the platform moved to a new location each day. Rats with dorsomedial, but not dorsolateral lesions required more trials to reach criterion; again, thigmotaxis was observed on the early trials. The third phase, carried out in the original room/pool, included a place-retention trial followed by a place-cue competition test, (i.e. a choice between the learned spatial location of the hidden platform and the visible platform in a new location). The rats with dorsomedial, but not dorsolateral lesions swam to the visible platform more frequently than the controls. In the final phase, the rats in both lesion groups exhibited slightly lower thigmotactic tendencies than controls in a standard dry-land open field, a finding inconsistent with the hypothesis that thigmotaxis in the water maze is due to increased fear or anxiety. Taken together with other behavioral and anatomical findings, the results suggest that the dorsomedial caudate-putamen, by virtue of its connections with limbic and prefrontal cortical regions, may mediate a response selection process that integrates cognitive information with stimulus-response tendencies.