Involvement of ventrolateral striatal dopamine in movement initiation and execution: a microdialysis and behavioral investigation

Partnered dance evokes greater intrinsic motivation than home exercise as therapeutic activity for chemotherapy-induced deficits: secondary results of a randomized, controlled clinical trial

Introduction

Dance has been proposed to support superior intrinsic motivation over non-dance forms of therapeutic physical activity. However, this hypothesis has yet to be evaluated empirically, particularly among populations living with neuropathology such as survivors of cancer with neurologic complications from chemotherapy treatment. Questions about motivation are relevant to clinical outcomes because motivation mediates neuroplasticity. We conducted this secondary analysis of a randomized-controlled study to begin to investigate the relationships between personal motivation and neurophysiologic effects of dance-based intervention for healthy aging among populations with neurologic complications of cancer.

Methods

We measured motivation using the Intrinsic Motivation Inventory, a validated patient-reported outcome from the psychological approach of Self Determination Theory. We assessed intrinsic motivation, extrinsic motivation, and satisfaction with intervention within a randomized controlled trial of dance versus exercise designed to alleviate symptoms of chemotherapy-induced impairment. Fifty-two survivors of breast cancer with chemotherapy-induced neuropathy diagnosis and associated sensorimotor functional deficits were randomized (1:1) to 8 weeks of partnered dance or home exercise, performed biweekly (NCT05114005; R21-AG068831).

Results

While satisfaction did not differ between interventions, intrinsic motivation was higher among participants randomized to dance than those randomized to exercise (p < 0.0001 at all timepoints: 2 weeks, 4 weeks, 6 weeks, and 8 weeks of intervention), as was extrinsic motivation at 2 weeks (p = 0.04) and 8 weeks (p = 0.01).

Discussion

These data provide evidence that social dance is more motivating than the type of home exercise generally recommended as therapeutic physical activity. The results inform directions for future study of the effect of dance-based therapeutics on embodied agency, neuroplastic changes, and clinically-relevant neuropathic improvement.

Simulated Dopamine Modulation of a Neurorobotic Model of the Basal Ganglia

The vertebrate basal ganglia play an important role in action selection-the resolution of conflicts between alternative motor programs. The effective operation of basal ganglia circuitry is also known to rely on appropriate levels of the neurotransmitter dopamine. We investigated reducing or increasing the tonic level of simulated dopamine in a prior model of the basal ganglia integrated into a robot control architecture engaged in a foraging task inspired by animal behaviour. The main findings were that progressive reductions in the levels of simulated dopamine caused slowed behaviour and, at low levels, an inability to initiate movement. These states were partially relieved by increased salience levels (stronger sensory/motivational input). Conversely, increased simulated dopamine caused distortion of the robot's motor acts through partially expressed motor activity relating to losing actions. This could also lead to an increased frequency of behaviour switching. Levels of simulated dopamine that were either significantly lower or higher than baseline could cause a loss of behavioural integration, sometimes leaving the robot in a 'behavioral trap'. That some analogous traits are observed in animals and humans affected by dopamine dysregulation suggests that robotic models could prove useful in understanding the role of dopamine neurotransmission in basal ganglia function and dysfunction.

Rat Models of Vocal Deficits in Parkinson's Disease

Parkinson's disease (PD) is a progressive, degenerative disorder that affects 10 million people worldwide. More than 90% of individuals with PD develop hypokinetic dysarthria, a motor speech disorder that impairs vocal communication and quality of life. Despite the prevalence of vocal deficits in this population, very little is known about the pathological mechanisms underlying this aspect of disease. As such, effective treatment options are limited. Rat models have provided unique insights into the disease-specific mechanisms of vocal deficits in PD. This review summarizes recent studies investigating vocal deficits in 6-hydroxydopamine (6-OHDA), alpha-synuclein overexpression, DJ1-/-, and Pink1-/- rat models of PD. Model-specific changes to rat ultrasonic vocalization (USV), and the effects of exercise and pharmacologic interventions on USV production in these models are discussed.

D2 dopamine receptor activation induces female preference for male song in the monogamous zebra finch

The evolutionary conservation of neural mechanisms for forming and maintaining pair bonds is unclear. Oxytocin, vasopressin and dopamine (DA) transmitter systems have been shown to be important in pair-bond formation and maintenance in several vertebrate species. We examined the role of dopamine in formation of song preference in zebra finches, a monogamous bird. Male courtship song is an honest signal of sexual fitness; thus, we measured female song preference to evaluate the role of DA in mate selection and pair-bond formation, using an operant conditioning paradigm. We found that DA acting through the D2 receptor, but not the D1 receptor, can induce a song preference in unpaired female finches and that blocking the D2 receptor abolished song preference in paired females. These results suggest that similar neural mechanisms for pair-bond formation are evolutionarily conserved in rodents and birds.

Ex Vivo Measurement of Electrically Evoked Dopamine Release in Zebrafish Whole Brain

Zebrafish (Danio rerio) have recently emerged as useful model organism for the study of neuronal function. Here, fast-scan cyclic voltammetry (FSCV) at carbon-fiber microelectrodes was used to measure locally evoked dopamine release and uptake in zebrafish whole brain preparations and results were compared with those obtained from brain slices. Evoked dopamine release ([DA]_max) was similar in whole brain and sagittal brain slice preparations (0.49 ± 0.13 μM in whole brain and 0.59 ± 0.28 μM in brain slices). Treatment with α-methyl-p-tyrosine methyl ester (αMPT), an inhibitor of tyrosine hydroxylase, diminished release and the electrochemical signal reappeared after subsequent drug washout. No observed change in stimulated release current occurred after treatment with desipramine or fluoxetine in the whole brain. Treatment with the uptake inhibitors, nomifensine or GBR 12909 increased [DA]_max, while treatment with sulpiride, a D2 dopamine autoreceptor antagonist, resulted in increased stimulated dopamine release in whole brain, but had no effect on release in slices. Dopamine release in whole brains increased progressively up to an electrical stimulation frequency of 25 Hz, while release in slices increased up to a frequency of only 10 Hz and then plateaued, highlighting another key difference between these preparations. We observed a lag in peak dopamine release following stimulation, which we address using diffusion models and pharmacological treatments. Collectively, these results demonstrate the electrochemical determination of dopamine release in the whole, intact brain of a vertebrate species ex vivo and are an important step for carrying out further experiments in zebrafish.

Sexual dimorphism in striatal dopaminergic responses promotes monogamy in social songbirds

In many songbird species, males sing to attract females and repel rivals. How can gregarious, non-territorial songbirds such as zebra finches, where females have access to numerous males, sustain monogamy? We found that the dopaminergic reward circuitry of zebra finches can simultaneously promote social cohesion and breeding boundaries. Surprisingly, in unmated males but not in females, striatal dopamine neurotransmission was elevated after hearing songs. Behaviorally too, unmated males but not females persistently exchanged mild punishments in return for songs. Song reinforcement diminished when dopamine receptors were blocked. In females, we observed song reinforcement exclusively to the mate’s song, although their striatal dopamine neurotransmission was only slightly elevated. These findings suggest that song-triggered dopaminergic activation serves a dual function in social songbirds: as low-threshold social reinforcement in males and as ultra-selective sexual reinforcement in females. Co-evolution of sexually dimorphic reinforcement systems can explain the coexistence of gregariousness and monogamy.

While monogamy is rare within the animal kingdom, some species – including humans and many birds – can be highly social and yet sustain monogamous relationships. Zebra finches, for example, are among a number of species of songbirds in which numerous males and females live closely together but maintain monogamous partnerships. Male songbirds use their songs to attract females, who do not themselves sing. But if female birds are attracted to any male song, how do they manage to remain monogamous when surrounded by potential suitors?

In songbirds, and in humans too, a region of the brain called the striatum regulates both social and sexual behaviors. It does this by modulating the release of a molecule called dopamine, which is the brain’s reward signal. Tokarev et al. show that hearing songs triggers dopamine release within the striatum of unattached male zebra finches, but has no such effect in single females. Unattached male songbirds will also put up with irritating puffs of air in exchange for being able to watch videos of singing birds, whereas unattached females will not. Female songbirds with partners will tolerate the air puffs, but only if the videos are accompanied with the songs of their own mate.

These findings suggest that song serves as a generic social stimulus for zebra finch males, helping large numbers of birds to live together. By contrast, for a female zebra finch, the song of her partner is a highly selective sexual stimulus. These sex-specific responses to the same socially-relevant stimuli may explain how gregarious animals are able to maintain monogamous pair bonds. More generally, these results are a step towards understanding how brain reward systems regulate social interactions. Studying these mechanisms in songbird species with different social and mating systems could ultimately provide insights into social and sexual behavior in people.

Calcium-activated SK channels control firing regularity by modulating sodium channel availability in midbrain dopamine neurons

Dopamine neurons in the substantia nigra pars compacta and ventral tegmental area regulate behaviours such as reward-related learning, and motor control. Dysfunction of these neurons is implicated in Schizophrenia, addiction to drugs, and Parkinson’s disease. While some dopamine neurons fire single spikes at regular intervals, others fire irregular single spikes interspersed with bursts. Pharmacological inhibition of calcium-activated potassium (SK) channels increases the variability in their firing pattern, sometimes also increasing the number of spikes fired in bursts, indicating that SK channels play an important role in maintaining dopamine neuron firing regularity and burst firing. However, the exact mechanisms underlying these effects are still unclear. Here, we develop a biophysical model of a dopamine neuron incorporating ion channel stochasticity that enabled the analysis of availability of ion channels in multiple states during spiking. We find that decreased firing regularity is primarily due to a significant decrease in the AHP that in turn resulted in a reduction in the fraction of available voltage-gated sodium channels due to insufficient recovery from inactivation. Our model further predicts that inhibition of SK channels results in a depolarisation of action potential threshold along with an increase in its variability.

Reassessing wanting and liking in the study of mesolimbic influence on food intake

Humans and animals such as rats and mice tend to overconsume calorie-dense foods, a phenomenon that likely contributes to obesity. One often-advanced explanation for why we preferentially consume sweet and fatty foods is that they are more "rewarding" than low-calorie foods. "Reward" has been subdivided into three interdependent psychological processes: hedonia (liking a food), reinforcement (formation of associations among stimuli, actions, and/or the food), and motivation (wanting the food). Research into these processes has focused on the mesolimbic system, which comprises both dopamine neurons in the ventral tegmental area and neurons in their major projection target, the nucleus accumbens. The mesolimbic system and closely connected structures are commonly referred to as the brain's "reward circuit." Implicit in this title is the assumption that "rewarding" experiences are generally the result of activity in this circuit. In this review, I argue that food intake and the preference for calorie-dense foods can be explained without reference to subjective emotions. Furthermore, the contribution of mesolimbic dopamine to food intake and preference may not be a general one of promoting or coordinating behaviors that result in the most reward or caloric intake but may instead be limited to the facilitation of a specific form of neural computation that results in conditioned approach behavior. Studies on the neural mechanisms of caloric intake regulation must address how sensory information about calorie intake affects not just the mesolimbic system but also many other forms of computation that govern other types of food-seeking and food-oriented behaviors.

Activation of Dopamine Receptors in the Nucleus Accumbens Promotes Sucrose-Reinforced Cued Approach Behavior

Dopamine receptor activation in the nucleus accumbens (NAc) promotes vigorous environmentally-cued food-seeking in hungry rats. Rats fed ad libitum, however, respond to fewer food-predictive cues, particularly when the value of food reward is low. Here, we investigated whether this difference could be due to differences in the degree of dopamine receptor activation in the NAc. First, we observed that although rats given ad libitum access to chow in their home cages approached a food receptacle in response to reward-predictive cues, the number of such approaches declined as animals accumulated food rewards. Intriguingly, cued approach to food occurred in clusters, with several cued responses followed by successive non-responses. This pattern suggested that behavior was dictated by transitions between two states, responsive and non-responsive. Injection of D1 or D2 dopamine receptor agonists into the NAc dose-dependently increased cue responding by promoting transitions to the responsive state and by preventing transitions to the non-responsive state. In contrast, antagonists of either D1 or D2 receptors promoted long bouts of non-responding by inducing transitions to the non-responsive state and by preventing transitions to the responsive state. Moreover, locomotor behavior during the inter-trial interval was correlated with the responsive state, and was also increased by dopamine receptor agonists. These results suggest that activation of NAc dopamine receptors plays an important role in regulating the probability of approach to food under conditions of normative satiety.

Electrochemical Detection of Dopamine via Assisted Ion Transfer at Nanopipet Electrode Using Cyclic Voltammetry

We present here the detection of dopamine (DA) at nanopipet electrodes with radii of hundreds of nanometers ranging from 160 nm to 480 nm. Dibenzo-18-crown-6 (DB18C6) was employed as an ionophore to facilitate DA transfer, resulting in a half-wave transfer potential, E1/2, DA, of -0.322 (±0.020) V vs. E1/2, TBA. Well-defined steady-state sigmoidal cyclic voltammograms were observed for the transfer of DA. High resolution scanning electron microscopy was used to measure the size and taper angle of the nanopipet electrodes. The detection is linear with concentration of DA ranging from 0.25 mM to 2 mM; calculated diffusion coefficient at nanopipet electrodes with above mentioned sizes is 4.87 (±0.28) × 10-10 m2/s. The effect of the common interferent ascorbic acid on DA detection with nanopipet electrodes was evaluated, where DA detection still shows linear behavior with well-defined sigmoidal CVs with E1/2, DA being -0.328 (±0.029) V vs. E1/2, TBA. The diffusion coefficient for DA transfer in MgCl2 with the presence of 2 mM AA was measured to be 1.93 (±0.59) × 10-10 m2/s on nanoelectrodes with radii from 161 nm to 263 nm, but the physiological concentration of 0.1 mM AA had no effect on DA's diffusion coefficient.

Modeling falls in Parkinson's disease: Slow gait, freezing episodes and falls in rats with extensive striatal dopamine loss

Falls in patients with Parkinson's disease (PD) are a major and levodopa-unresponsive source of morbidity. We previously described an animal model of falls resulting from impairments in attentional-motor interactions. Reproducing the multisystem dopaminergic-cholinergic cell loss in patients with a history for falls, partial loss of striatal dopamine innervation interacted with loss of forebrain cholinergic neurons to generate falls that was hypothesized to reflect impairments in the attentional control of gait and balance and the sequencing of complex movements [1]. As clinical evidence also indicates that basal ganglia dopamine (DA) loss per se is associated with severe discoordination and thus a greater risk for falls, here we demonstrate that relatively extensive striatal DA loss, in contrast to the lack of effects of smaller, dorsal striatal DA losses and sham lesions, increased falls and slips and caused slowing while traversing dynamic surfaces. Falls in large DA rats were associated specifically with spontaneous or slip-triggered stoppages of forward movement. Collectively, the evidence suggests that low motivation or vigor for movement in general, and for initiating corrective movements in particular, are major sources for falls in rats with large DA losses. Falls are a result of complex cognitive-motor interactions, and rats with large DA losses model the impact of a propensity for freezing of gait when traversing dynamic surfaces.

Contribution of the mGluR7 receptor to antiparkinsonian-like effects in rats: a behavioral study with the selective agonist AMN082

BACKGROUND

Metabotropic glutamate receptors (mGluRs) have been shown to be potential targets for numerous neurological diseases, including Parkinson's disease (PD). We previously reported that ACPT-1, a non-selective group III mGluRs agonist, injected locally into the globus pallidus, striatum or substantia nigra pars reticulata (SNr), significantly attenuated the haloperidol-induced catalepsy in rats. N,N'-dibenzhydryl-ethane-1,2-diamine dihydrochloride (AMN082) is a potent, brain penetrating mGluR7 agonist, selective over other mGluRs.

METHODS

The aim of the present study was to determine whether (1) activation of mGluR7 by systemic administration of AMN082 may produce antiparkinsonian-like effects in the haloperidol-induced catalepsy and reserpine-induced akinesia models in rats; (2) striatal and nigral mGluR7 is likely to contribute to such an effect.

RESULTS

We found that AMN082 (1 and 3 mg/kg) decreased the haloperidol (0.25 mg/kg)-induced catalepsy, but was not efficient in attenuating the reserpine (2.5 mg/kg)-induced akinesia. When given locally, AMN082 also significantly diminished catalepsy in rats; however, its effective striatal doses were 10-fold lower than those used in the SNr (2.5 and 7.5 pmol/0.5 μl/ side vs. 25 and 75 pmol/0.5 μl/side, respectively).

CONCLUSION

The above findings support the idea that the activation of mGluR7 can produce antiparkinsonian-like effects in rats. Furthermore, our results indicate contribution of both striatal and nigral mGluR7 to the anticataleptic effects of AMN082.

Beyond the basal ganglia: cFOS expression in the cerebellum in response to acute and chronic dopaminergic alterations

The suggestion of an anatomical and functional relationship between the basal ganglia and cerebellum is recent. Traditionally, these structures were considered as neuronal circuits working separately to organize and control goal-directed movements and cognitive functions. However, several studies in rodents and primates have described an anatomical interaction between cortico-basal and cortico-cerebellar networks. Most importantly, functional changes have been observed in one of these circuits when altering the other one. In this context, we aimed to accomplish an extensive description of cerebellar activation patterns using cFOS expression (cFOS-IR) after acute and chronic manipulation of dopaminergic activity. In the acute study, substantia nigra pars compacta (SNc) activity was stimulated or suppressed by intra cerebral administration of picrotoxin or lidocaine, respectively. In addition, we analyzed cerebellar activity after the induction of a parkinsonism model, the tremulous jaw movements. In this model, tremulous jaw movements were induced in male rats by IP chronic administration of the dopamine antagonist haloperidol (1.5mg/kg). Acute stimulation of SNc by picrotoxin increased cFOS-IR in the vermis and cerebellar hemispheres. However, lidocaine did not produce an effect. After 14days of haloperidol treatment, the vermis and cerebellar hemispheres showed an opposite regulation of cFOS expression. Chronic dopaminergic antagonism lessened cFOS expression in the vermis but up-regulated such expression in the cerebellar hemisphere. Overall, the present data indicate a very close functional relationship between the basal ganglia and the cerebellum and they may allow a better understanding of disorders in which there are dopamine alterations.

Amelioration of behavioral abnormalities in BH(4)-deficient mice by dietary supplementation of tyrosine

This study reports an amelioration of abnormal motor behaviors in tetrahydrobiopterin (BH4)-deficient Spr (-/-) mice by the dietary supplementation of tyrosine. Since BH4 is an essential cofactor for the conversion of phenylalanine into tyrosine as well as the synthesis of dopamine neurotransmitter within the central nervous system, the levels of tyrosine and dopamine were severely reduced in brains of BH4-deficient Spr (-/-) mice. We found that Spr (-/-) mice display variable 'open-field' behaviors, impaired motor functions on the 'rotating rod', and dystonic 'hind-limb clasping'. In this study, we report that these aberrant motor deficits displayed by Spr (-/-) mice were ameliorated by the therapeutic tyrosine diet for 10 days. This study also suggests that dopamine deficiency in brains of Spr (-/-) mice may not be the biological feature of aberrant motor behaviors associated with BH4 deficiency. Brain levels of dopamine (DA) and its metabolites in Spr (-/-) mice were not substantially increased by the dietary tyrosine therapy. However, we found that mTORC1 activity severely suppressed in brains of Spr (-/-) mice fed a normal diet was restored 10 days after feeding the mice the tyrosine diet. The present study proposes that brain mTORC1 signaling pathway is one of the potential targets in understanding abnormal motor behaviors associated with BH4-deficiency.

Dopamine-rich grafts alleviate deficits in contralateral response space induced by extensive dopamine depletion in rats

Unilateral infusion of 6-hydroxydopamine into the nigro-striatal pathway in the rat is the most common dopamine lesion model of Parkinson's disease. In the present study, we explore the impact of near complete unilateral loss of dopamine along the nigro-striatal pathway and subsequent cell replacement therapy in a choice reaction time task in rats, with assessment of spatial responding towards either side of the body (ipsilateral or contralateral to the lesion) on alternate days. Results indicated a stable contralateral deficit in response accuracy, reaction times and motor function for 50 consecutive days of testing, with no signs of recovery or compensation. All lesioned rats developed a near-hole bias and displayed prolonged movement and reaction times when responses had to be directed towards a distal response location on the side of the body contralateral to the lesion, as well as a smaller ipsilateral impairment in response accuracy and movement times. Grafts of dopamine-rich tissue into the denervated striatum improved some, but not all, of the deficits induced by the lesion. Specifically, grafted rats performed at a similar level to control animals when assessed on the ipsilateral side, they demonstrated a partial restitution of their ability to respond to far contralateral stimuli, and they exhibited a marked reduction in the time to complete all lateralised responses on both sides. The present characterisation of the task and the effects of cell replacement via primary fetal mesencephalic tissue demonstrate restorative properties in alleviating the marked spatial response bias induced by unilateral loss of dopamine.

Characterisation of spatial neglect induced by unilateral 6-OHDA lesions on a choice reaction time task in rats

Unilateral dopamine depletion and excitotoxic lesions of the striatum have been shown to induce a contralateral neglect when rats have to respond in a choice reaction time setting. Whereas, in a lateralised setting when response options are to either side of the animal's head all contralateral responding is impaired, testing animals only on one side of the head per day but with a near and far response option, rats are able to correctly respond to contralateral stimuli, but rather bias their responses towards the near hole. Here, we further investigated the nature of the contralateral neglect in egocentric space coding in more detail. Firstly, we tested the effects of near-complete unilateral dopamine depletion on this type of task. Secondly, previous observations suggested that lesioned rats shifted their response strategy which resulted in a response bias towards the most proximal location in contralateral space. In order to "encourage" dopamine depleted rats to respond to the neglected response location we implemented an error correction procedure to the task. Near-complete unilateral dopamine depletion, via 6-hydroxydopamine infusions into the medial forebrain bundle of female Lister Hood rats, resulted in a reduction of usable trials, a near hole bias when animals were tested on the side contralateral to the lesion, as well as increased reaction and movement time latencies. The introduction of an error-correction procedure had no effect on the animals' response bias towards the near contralateral location. Probe trials showed that the bias is most likely the result of responses being misdirected when in a choice situation. The findings further highlight the role of dopamine and an intact striatum to code responses into egocentrically defined space.

Pharmacological and physiological characterization of the tremulous jaw movement model of parkinsonian tremor: potential insights into the pathophysiology of tremor

Tremor is a cardinal symptom of parkinsonism, occurring early on in the disease course and affecting more than 70% of patients. Parkinsonian resting tremor occurs in a frequency range of 3-7 Hz and can be resistant to available pharmacotherapy. Despite its prevalence, and the significant decrease in quality of life associated with it, the pathophysiology of parkinsonian tremor is poorly understood. The tremulous jaw movement (TJM) model is an extensively validated rodent model of tremor. TJMs are induced by conditions that also lead to parkinsonism in humans (i.e., striatal DA depletion, DA antagonism, and cholinomimetic activity) and reversed by several antiparkinsonian drugs (i.e., DA precursors, DA agonists, anticholinergics, and adenosine A(2A) antagonists). TJMs occur in the same 3-7 Hz frequency range seen in parkinsonian resting tremor, a range distinct from that of dyskinesia (1-2 Hz), and postural tremor (8-14 Hz). Overall, these drug-induced TJMs share many characteristics with human parkinsonian tremor, but do not closely resemble tardive dyskinesia. The current review discusses recent advances in the validation of the TJM model, and illustrates how this model is being used to develop novel therapeutic strategies, both surgical and pharmacological, for the treatment of parkinsonian resting tremor.

Progressive impairment in motor skill learning at 12 and 20 weeks post 6-OHDA- SNc lesion in rats

Deficiency in skilled motor activity is primarily attributed to the loss of dopaminergic neurons in the pars compacta of substantia nigra (SNc), which can be detected by performance of the rotarod test. Previous reports have demonstrated impaired skilled motor behavior in rats during the pre-motor stage of Parkinson's disease (PD) (3-8 weeks post 6-OHDA lesion of striatum). We studied skilled motor learning in 6-hydroxydopamine (6-OHDA) SNc lesion rats at 12 and 20 weeks by rotarod task after providing sufficient training to give allowance for ageing (3 sessions/day for 14 consecutive days). On each day, the stay duration on rotarod was noted and compared between the groups (Group 1 = Control, Group 2 = Post lesion (PL) week 12, Group 3 = PL week 20). In Group 2 rats, the duration of stay on rotarod gradually increased from day 1 through 7 {day 7 = 193.1 (81.8-247.4) vs. control group day 7 = 202.1 (87.7-279.8), p = 0.771} and declined thereafter. While, the stay duration in Group 3 rats remained lower {day 7 = 32.5 (20.4-52.1), p = 0.011} than that of the control rats throughout the study period. The results of our study suggest a slower brief learning of skilled motor tasks at post lesion week 12 whereas no learning at all at post-lesion week 20.

Extracellular dopamine levels in striatal subregions track shifts in motivation and response cost during instrumental conditioning

Tonic dopamine (DA) signaling is widely regarded as playing a central role in effort-based decision making and in the motivational control of instrumental performance. The current study used microdialysis to monitor changes in extracellular DA levels across subregions of the nucleus accumbens and dorsal striatum of rats as they lever pressed for food reward on a probabilistic schedule of reinforcement, a procedure that ensured they would experience variation in the amount of effort needed to earn rewards across tests. Each rat was given three tests. Rats were hungry for the first and last test, but were sated on food before the middle test, allowing us to assess the effects of a downshift in motivational state on task performance and conditioning-induced DA efflux. During hungry tests, DA levels rose in both the shell and core of the accumbens and, to a lesser degree, in both the medial and lateral divisions of the dorsal striatum. Interestingly, changes in DA efflux across hungry tests in the accumbens core were negatively correlated with changes in the effort required to obtain rewards. We also found that--across regions--the DA response to instrumental conditioning was attenuated when rats were sated before testing. Furthermore, the effect of satiety on DA efflux in the accumbens shell was positively correlated with its effect on task performance. Together, the results indicate that tonic DA contributes to the control of instrumental performance by conveying information about the costs and benefits of responding to different striatal subregions.

Within-session analysis of amphetamine-elicited rotation behavior reveals differences between young adult and middle-aged F344/BN rats with partial unilateral striatal dopamine depletion

Preclinical modeling of Parkinson's disease using 6-hydroxydopamine (6-OHDA) has been valuable in developing and testing therapeutic strategies. Recent efforts have focused on modeling early stages of disease by infusing 6-OHDA into the striatum. The partial DA depletion that follows intrastriatal 6-OHDA is more variable than the near-complete depletion following medial forebrain bundle infusion, and behavioral screening assays are not as well characterized in the partial lesion model. We compared relationships between amphetamine-elicited rotation behavior and DA depletion following intrastriatal 6-OHDA (12.5 microg) in 6 month vs. 18 month F344/BN rats, at 2-weeks and 6-weeks post-lesion. We compared the total number of rotations with within-session (bin-by-bin) parameters of rotation behavior as indicators of DA depletion. Striatal DA depletion was greater in the young adult than in the middle-aged rats at 2 weeks but not at 6 weeks post-lesion. The total number of rotations for the whole session and striatal DA depletion did not differ between the two age groups. Regression analysis revealed a greater relationship between within-session parameters of rotation behavior and DA depletion in the middle-aged group than in the young adult group. These results have implications for estimating DA depletion in preclinical studies using rats of different ages.

The application of Russell and Burch 3R principle in rodent models of neurodegenerative disease: the case of Parkinson's disease

Currently, the accepted ethical standards for the regulation of animal experimentation are provided by the 3R principle (Replacement, Reduction and Refinement). The development of alternative methods to the use of animals (Replacement), the design of adequate experimental protocols to reduce the number of animals (Reduction), the application of refinement practices (Refinement) are all aspects to be considered to ensure ethical and scientific validity to animal experimentation. This review intends to address these issues, using experimental research on Parkinson's disease (PD) as a paradigmatic example of the use of animal models to improve knowledge on a devastating human pathology. In particular, current rodent models of PD and their validity are reviewed and discussed, and methodologies that may ultimately reduce animal's suffering emphasized. Although procedures referring to with 3R principle can be traced in the literature reviewed, they are not considered yet an important part of the methodological information. The formal inclusion in scientific papers of a section devoted to 3Rs may increase knowledge and eventually adherence to this principle by scientists.

Dopamine/adenosine interactions related to locomotion and tremor in animal models: possible relevance to parkinsonism

Adenosine A(2A) antagonists can exert antiparkinsonian effects in animal models. Recent experiments studied the ability of MSX-3 (an adenosine A(2A) antagonist) to reverse the locomotor suppression and tremor produced by dopamine antagonists in rats. MSX-3 reversed haloperidol-induced suppression of locomotion, and reduced the tremulous jaw movements induced by haloperidol, pimozide, and reserpine. Infusions of MSX-3 into the nucleus accumbens core increased locomotion in haloperidol-treated rats, but there were no effects of infusions into the accumbens shell or ventrolateral neostriatum. In contrast, MSX-3 injected into the ventrolateral neostriatum reduced pimozide-induced tremulous jaw movements. Dopamine/adenosine interactions in different striatal subregions are involved in distinct aspects of motor function.

Long-term functional and protective actions of preconditioning with hypoxia, cobalt chloride, and desferrioxamine against hypoxic-ischemic injury in neonatal rats

Preconditioning with hypoxia and hypoxia-mimetic compounds cobalt chloride (CoCl2) and desferrioxamine (DFX) protects against hypoxic-ischemic (HI) injury in neonatal rat brain. We examined long-term functional and protective actions of preconditioning induced by hypoxia, CoCl(2) and DFX in a neonatal rat model of HI. Postnatal day six rat pups were exposed to preconditioning with hypoxia (8% oxygen) or injections of CoCl(2), DFX or saline vehicle and 24 h later rats underwent HI or sham surgery. Behavioral tests were performed and at the conclusion of experiments, brains removed for morphologic analyses. HI resulted in a large unilateral lesion in the ipsilateral hemisphere compared with sham control rats. All preconditioning treatments significantly reduced the total lesion volume. Behavioral deficits were observed in HI rats compared with sham controls. The reduction in forelimb grasping strength in HI rats was attenuated by preconditioning with hypoxia, CoCl(2) and DFX. HI increased the number of foot faults in a grid-walking test and resulted in forelimb asymmetry in the cylinder test. Only preconditioning with hypoxia reversed all three functional deficits after HI. These findings indicate that preconditioning, especially when induced by hypoxia, has the potential to minimize the morphologic and functional effects of neonatal HI injury.

Injections of the selective adenosine A2A antagonist MSX-3 into the nucleus accumbens core attenuate the locomotor suppression induced by haloperidol in rats

There is considerable evidence of interactions between adenosine A2A receptors and dopamine D2 receptors in striatal areas, and antagonists of the A2A receptor have been shown to reverse the motor effects of DA antagonists in animal models. The D2 antagonist haloperidol produces parkinsonism in humans, and also induces motor effects in rats, such as suppression of locomotion. The present experiments were conducted to study the ability of the adenosine A2A antagonist MSX-3 to reverse the locomotor effects of acute or subchronic administration of haloperidol in rats. Systemic (i.p.) injections of MSX-3 (2.5-10.0 mg/kg) were capable of attenuating the suppression of locomotion induced by either acute or repeated (i.e., 14 day) administration of 0.5 mg/kg haloperidol. Bilateral infusions of MSX-3 directly into the nucleus accumbens core (2.5 microg or 5.0 microg in 0.5 microl per side) produced a dose-related increase in locomotor activity in rats treated with 0.5 mg/kg haloperidol either acutely or repeatedly. There were no overall significant effects of MSX-3 infused directly into the dorsomedial nucleus accumbens shell or the ventrolateral neostriatum. These results indicate that antagonism of adenosine A2A receptors can attenuate the locomotor suppression produced by DA antagonism, and that this effect may be at least partially mediated by A2A receptors in the nucleus accumbens core. These studies suggest that adenosine and dopamine systems interact to modulate the locomotor and behavioral activation functions of nucleus accumbens core.

Striatal dopamine transporters correlate with simple reaction time in elderly subjects

The decline in motor performance that accompanies advanced age has unclear neurobiological substrates but may relate, in part, to degeneration of the nigrostriatal dopamine system. This research tested the hypothesis that striatal dopamine transporter (DAT) availability in healthy elderly individuals was related to measures of motor performance. Thirty-six healthy volunteers (18 male, 18 female) who ranged in age from 68 to 88 (75.4+/-4.9 years) received a neuropsychological evaluation that included two primary motor measures (tested with dominant hand): (1) simple reaction time (SRT); and (2) finger tapping (FT). Subjects underwent SPECT scanning with [(123)I]2beta-carbomethoxy-3beta-(4-iodophenyl)tropane ([(123)I]beta-CIT) for measurement of striatal DAT availability. A ratio of specific to nondisplaceable brain uptake (i.e., radical V3 =[striatal-occipital]/occipital), a measure proportional to the binding potential (B(max)/K(D)), was derived. SRT was significantly correlated with striatal DAT availability with or without controlling for the contribution of age. However, contrary to hypothesis, FT was not correlated with striatal DAT availability. Comparison measures, including episodic memory and general intelligence, were also unrelated to striatal DAT availability. These results demonstrate that a loss of nigrostriatal dopaminergic function likely contributes to slowing of reaction speed with advancing age.

Rat model of Parkinson's disease with bilateral motor abnormalities, reversible with levodopa, and dyskinesias

Parkinson's disease (PD) is characterized by the bilateral degeneration of the midbrain dopamine-containing neurons with the most severe lesion in the posterolateral part of the substantia nigra pars compacta (SNpc). In humans, such lesions lead to specific motor abnormalities (i.e., akinesia, rigidity, and tremor) that are greatly improved by levodopa treatment. After a few years, the beneficial effect of the treatment is frequently offset by the development of dyskinesias. To improve treatment strategies, an animal model showing most of the histological and clinical characteristics of the human disease is mandatory. Ten rats received a bilateral injection of small doses of 6-OHDA in the medial forebrain bundle (MFB) and were compared with five sham-lesioned rats. The 6-OHDA-lesioned rats progressively developed abnormal motor behavior (assessed by the stepping test) compared with the sham-lesioned rats. The lesioned rats greatly improved under levodopa treatment, but developed concomitant dyskinesias. All 6-OHDA-lesioned animals had bilateral partial lesions of the SNpc, with the most severe lesion being in its posterolateral part. There was a significant correlation between the severity of the dopaminergic cell loss and the severity of the levodopa-induced dyskinesias. These rats constitute an interesting model of PD, sharing some of the main characteristics of the human disease.

Quantitative measurement of neurological deficit after mild (30 min) transient middle cerebral artery occlusion in rats

Although 30-min transient middle cerebral artery occlusion (30-min tMCAo) causes reproducible subcortical infarction in rats, it is difficult to evaluate the resulting neurological deficit using common behavioral tests such as the rota-rod test, adhesive-removal test, or narrow beam test. Establishment of a method of quantitative evaluation would help to develop a novel therapeutic approach to treat cerebral infarction. To solve this problem, we examined whether the neurological deficit could be detected by the Montoya staircase test or methamphetamine-induced rotation, which are commonly used in a Parkinson disease model induced by intrastriatal injection of 6-hydroxydopamine (6-OHDA). From 10 to 14 days after tMCAo, the Montoya staircase test showed significant clumsiness in forelimb tasks contralateral to the lesion side, whereas sham-operated rats showed no significant clumsiness in both forelimbs. The number of ipsilateral rotations induced by methamphetamine was also increased in tMCAo-rats at 21 days after tMCAo. Although Pearson's correlations coefficient showed that the results of these tests were correlated with the infarction volume, there was no significant correlation between the results of these two tests. These findings imply that the neurological deficit detected by both tests might reflect the severity of ischemic injury, but each test might evaluate different aspects of neurological deficit. Thus, the Montoya staircase test and methamphetamine-induced rotation are useful to evaluate neurological deficit in the chronic stage of subcortical infarction induced by 30-min tMCAo.

Comparison of incremental and accelerating protocols of the rotarod test for the assessment of motor deficits in the 6-OHDA model

The rotarod test, in which animals must balance on a rotating drum, is widely used to assess motor deficit in neurodegenerative disease models in rodents. Performance is measured by the duration that an animal stays on the rod as a function of drum speed. Two different protocols are widely used, incremental fixed speeds or an accelerating protocol, but there is little information on their equivalence or the relative power, reliability and sensitivity of the two protocols. The present study was undertaken to compare the incremental fixed-speed and accelerating rotarod protocols on two different lesions of the ascending forebrain dopamine pathways. Three groups of rats were used, controls, rats with 6-OHDA lesions of nigrostriatal bundle, and rats with terminal 6-OHDA lesions within the striatum. Rats were tested at different time points after the lesion. We report that whereas the incremental protocol is more sensitive to detect the presence of a lesion, the accelerating protocol provides a more discriminative test to correlate motor deficits against lesion size.

Comparison between multiple behavioral effects of peripheral ethanol administration in rats: Sedation, ataxia, and bradykinesia

Although low doses of systemic ethanol stimulate locomotion in mice, in rats the typical response to peripheral ethanol administration is a dose-dependent suppression of motor activity. In the present study, male rats received acute doses of ethanol IP (0.0, 0.25, 0.5, 1.0 or 2.0 g/kg) and were tested on several behavioral tasks related to the motor suppressive or sedative effects of the drug. This research design allowed for comparisons between the effects of ethanol on different behavioral tasks in order to determine which tasks were most sensitive to the drug (i.e., which tasks would yield deficits that appear at lower doses). In the first two experiments, rats were evaluated on a sedation rating scale, and ataxia/motor incoordination was assessed using the rotarod apparatus. Administration of 2.0 g/kg ethanol produced sedation as measured by the sedation scale, and also impaired performance on the rotarod. In a third experiment, ethanol reduced locomotion in the stabilimeter at several doses and times after IP injection, with 0.25 g/kg being the lowest dose that produced a significant decrease in locomotion. Finally, experiment four studied the effects of ethanol on operant lever pressing reinforced on a fixed ratio 5 (FR5) schedule for food reinforcement. Data showed suppressive effects on lever pressing at doses of 1.0, and 2.0 g/kg ethanol. Analysis of the interresponse time distribution showed that ethanol produced a modest slowing of operant responding, as well as fragmentation of the temporal pattern of responding and increases in pausing. Taken together, these results indicate that rats can demonstrate reduced locomotion and slowing of operant responding at doses lower than those that result in sedation or ataxia as measured by the rotarod. The detection of subtle changes in different motor test across a broad range of ethanol doses is important for understanding ethanol effects in other cognitive, motivational or sensory processes.

Lentivector-mediated delivery of GDNF protects complex motor functions relevant to human Parkinsonism in a rat lesion model

Although viral vector-mediated delivery of glial cell-line derived neurotrophic factor (GDNF) to the brain has considerable potential as a neuroprotective strategy in Parkinson's disease (PD), its ability to protect complex motor functions relevant to the human condition has yet to be established. In this study, we used an operant task that assesses the selection, initiation and execution of lateralized nose-pokes in Lister Hooded rats to assess the efficacy with which complex behaviours are protected against neurotoxic lesions by prior injection of a lentiviral vector expressing GDNF. Unilateral injection of 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle (MFB) caused rats to attempt fewer trials and to make more procedural errors. Lesioned rats also developed a pronounced ipsilateral bias, with a corresponding drop in contralateral accuracy. They were also slower to react to contralateral stimuli and to execute movements bilaterally. Rats that were pre-treated 4 weeks prior to lesion surgery with an equine infectious anaemia virus (EIAV) vector carrying GDNF [EIAV-GDNF, injected into the striatum and above the substantia nigra (SN)] performed significantly better on all of these parameters than control rats. In addition to the operant task, EIAV-GDNF successfully rescued contralateral impairments in the corridor, staircase, stepping and cylinder tasks, and prevented drug-induced rotational asymmetry. This study confirms that GDNF can protect against 6-OHDA-induced impairments in complex as well as simple behaviours, and reinforces the use of EIAV-based vectors for the treatment of PD.

Behavioral differences in a rotenone-induced hemiparkinsonian rat model developed following intranigral or median forebrain bundle infusion

A mitochondrial complex-I inhibitor, rotenone was unilaterally infused into the substantia nigra pars compacta (SNpc) or median forebrain bundle (MFB) to create hemiparkinsonian animal models and investigated spontaneous and drug-induced stereotypic rotations, as well as certain postural behaviors in Sprague-Dawley rats. Animals infused intranigrally, but not intra-MFB, with rotenone exhibited spontaneous contralateral rotations immediately after recovery from anesthesia. Head position bias and elevated body swing test showed insignificant contralateral bias in animals with nigral damage but a significant ipsilateral bias in MFB-lesioned rats. General motor activity of the animals was reduced in both the groups as indicated by reduced performance on a Plus-Maze. Intranigrally, rotenone-infused animals exhibited progressive ipsilateral rotations when challenged with d-amphetamine on the 7th, 14th, 21st, and 28th days or with apomorphine on 9th, 16th, 23rd, and 30th days. However, animals that received rotenone in MFB exhibited ipsilateral or contralateral rotations when challenged respectively with d-amphetamine or apomorphine only in the 5th week (28th and 30th days). Stereotaxic administration of rotenone into SNpc or MFB caused a significant loss of dopamine in the ipsilateral striatum (>80% in SNpc; >95% in MFB), when assayed employing an HPLC equipped with electrochemical detector on the 32nd day. Neuronal loss in SNpc was confirmed in coronal sections stained with cresyl violet and revealed extension of lesion towards SN pars reticulata, in SNpc-infused animals. Our results demonstrate that rotenone-induced neurodegeneration is a slow, yet progressive process similar to that in idiopathic Parkinson's disease and unlike that observed in other classical neurotoxin-mediated lesions which are abrupt and developed in few hours to days. Thus, intranigral or intra-MFB infusion of rotenone could be used for producing hemiparkinsonian animal models in rats. These findings further suggest that, while both d-amphetamine and apomorphine-induced stereotypic rotations could be used as a valuable behavioral assay procedure to test novel drugs against Parkinson's disease, yet apomorpine-induced contralateral bias in turning is a reliable indicator of specific destruction in nigrostriatal pathway and development of postsynaptic dopamine receptor supersensitivity.

Voltammetric study of the control of striatal dopamine release by glutamate

The central dopamine systems are involved in several aspects of normal brain function and are implicated in a number of human disorders. Hence, it is important to understand the mechanisms that control dopamine release in the brain. The striatum of the rat receives both dopaminergic and glutamatergic projections that synaptically target striatal neurons but not each other. Nevertheless, these afferents do form frequent appositional contacts, which has engendered interest in the question of whether they communicate with each other despite the absence of a direct synaptic connection. In this study, we used voltammetry in conjunction with carbon fiber microelectrodes in anesthetized rats to further examine the effect of the ionotropic glutamate antagonist, kynurenate, on extracellular dopamine levels in the striatum. Intrastriatal infusions of kynurenate decreased extracellular dopamine levels, suggesting that glutamate acts locally within the striatum via ionotropic receptors to regulate the basal extracellular dopamine concentration. Infusion of tetrodotoxin into the medial forebrain bundle or the striatum did not alter the voltammetric response to the intrastriatal kynurenate infusions, suggesting that glutamate receptors control a non-vesicular release process that contributes to the basal extracellular dopamine level. However, systemic administration of the dopamine uptake inhibitor, nomifensine (20 mg/kg i.p.), markedly decreased the amplitude of the response to kynurenate infusions, suggesting that the dopamine transporter mediates non-vesicular dopamine release. Collectively, these findings are consistent with the idea that endogenous glutamate acts locally within the striatum via ionotropic receptors to control a tonic, impulse-independent, transporter-mediated mode of dopamine release. Although numerous prior in vitro studies had suggested that such a process might exist, it has not previously been clearly demonstrated in an in vivo experiment.

Neurobehavioral evaluation of Relnrl-orl mutant mice and correlations with cytochrome oxidase activity

The Reln-rl-orl mutation is characterized by a marked deficit in cerebellar granule cell and Purkinje cell number as well as ectopias in cerebellum, hippocampus, and neocortex. By comparison to Balb/c controls, Reln-rl-orl mutants did not alternate spontaneously in a T-maze and were deficient for visuomotor guidance in a water maze. Despite cerebellar ataxia and motor coordination impairments on stationary beam, coat-hanger, and rotorod tests, the horizontal motor activity of Reln-rl-orl mutants was not reduced in an open-field. The elevated cytochrome oxidase (CO) activity in Purkinje cells and the reduced CO activity in the roof nuclei (interpositus and dentate) of the mutants were associated with poor performance on the small stationary beam. In addition, deficient CO activity of the granular layer of the motor cortex was associated with shorter latencies before falling from the larger stationary beam and a lower number of rears in the open-field. Conversely, elevated CO activity in the polymorphic layer of primary somatosensory cortex was congruent with higher latencies before falling from the same apparatus, indicating functional compensation.

Dorsal striatum and stimulus-response learning: lesions of the dorsolateral, but not dorsomedial, striatum impair acquisition of a stimulus-response-based instrumental discrimination task, while sparing conditioned place preference learning

While some evidence suggests that the dorsal striatum is important for stimulus-response learning, disagreement exists about the relative contribution of the dorsolateral and dorsomedial striatum to this form of learning. In the present experiment, the effects of lesions of the dorsolateral and dorsomedial striatum were investigated on two tasks that differentially require the development of stimulus-response learning. The first task utilized an operant conditional discrimination task, which is likely to rely heavily upon stimulus-response learning. The second task looked conditioned place preference learning, a task that is unlikely to require the development of stimulus-response associations. Animals with lesions of the dorsolateral striatum were impaired on the operant conditional discrimination task, but retained the ability to learn the conditioned place preference task. In contrast, animals with lesions of the dorsomedial striatum were not found to be impaired on either task used in the present experiment. These results suggest that the dorsolateral striatum is necessary for the successful acquisition of tasks that place a demand upon stimulus-response learning, while the dorsomedial striatum is not involved in this type of learning.

Dorsal striatum and stimulus-response learning: lesions of the dorsolateral, but not dorsomedial, striatum impair acquisition of a simple discrimination task

In the present experiment, the effects of neurotoxic lesions (quinolinic acid) of the dorsolateral or dorsomedial striatum were investigated on a simple instrumental discrimination task (CS+/CS-). Rats with lesions of the dorsolateral striatum were found to be impaired in the acquisition of this task, as compared to rats with either dorsomedial striatal or sham lesions. Furthermore, dorsolateral striatal lesioned animals had significantly lower levels of responding across the course of discrimination training, as assessed both by overall rate of response during CS+ presentations and number of CS+ trials without a response, despite having shown levels of responding during variable interval training that did not differ from that of sham lesioned animals. In contrast, animals with lesions of the dorsomedial striatum did not show an impairment in acquisition of the present task, but had slightly higher rates of responding during CS- presentations. It is argued that the poor acquisition and low response rates observed in animals with dorsolateral striatal lesions reflect a failure in stimulus-response learning, while the performance of animals with dorsomedial striatal lesions may have been the result of an increase in overall activity rate.

Long-lasting functional disabilities in middle-aged rats with small cerebral infarcts

Recommendations from experts and recently established guidelines on how to improve the face and predictive validity of animal models of stroke have stressed the importance of using older animals and long-term behavioral-functional endpoints rather than relying almost exclusively on acute measures of infarct volume in young animals. The objective of the present study was to determine whether we could produce occlusions in older rats with an acceptable mortality rate and then detect reliable, long-lasting functional deficits. A reversible intraluminar suture middle cerebral artery occlusion (MCAO) procedure was used to produce small infarcts in middle-aged rats. This resulted in an acceptable mortality rate, and robust disabilities were detected in functional assays, although the degree of total tissue loss measured 90 d after MCAO was quite modest. Infarcted animals were functionally impaired relative to sham control animals even 90 d after the occlusions, and when animals were subgrouped based on amount of tissue loss, MCAO animals with only 4% tissue loss exhibited enduring neurological-behavioral impairments relative to sham-operated controls, and the functional impairments in the group with the largest infarcts (20% tissue loss) were more severe than the functional impairments in the rats with 4% tissue loss. These results suggest that this model, using reversible MCAO to produce small infarcts and long-lasting functional-behavioral deficits in older rats, may represent an advance in the relatively higher-throughput modeling of stroke and its recovery in rodents and may be useful in the development and characterization of future stroke therapies.

Behavioral effects of intraventricular injections of low doses of ethanol, acetaldehyde, and acetate in rats: studies with low and high rate operant schedules

Although ethanol is typically classed as a sedative-hypnotic, low doses of ethanol have been shown to stimulate locomotor activity in mice. However, in rats the typical response to peripheral administration of ethanol is a dose-dependent suppression of motor activity and operant responding. The present study was undertaken to determine the effects of intraventricular (ICV) infusions of ethanol, acetaldehyde, and acetate on operant performance in rats. ICV injections of ethanol, acetaldehyde, or acetate were given to rats previously trained on either a differential-reinforcement-of-low-rates-of-responding (DRL) 30-s schedule, which generates low rates of responding, or a fixed ratio 5 (FR5) schedule, which generates relatively high rates. Ethanol, acetaldehyde, and acetate all produced a rate-increasing effect in rats on the DRL 30-s schedule at moderate doses (2.8 and 1.4 micromol, respectively). Acetate also produced a rate-decreasing effect on the DRL 30-s schedule at a larger dose (8.8 micromol). Performance on the FR5 schedule was unaltered by ethanol and acetaldehyde, even at doses as high as 17.6 micromol. However, acetate produced a rate-decreasing effect on the FR5 schedule at doses of 4.4, 5.6, and 8.8 micromol. Central administration of low doses of ethanol and its metabolites can increase operant responding on some schedules in rats. Acetate is the substance that is most potent for producing rate-suppressing effects. These results indicate that the major metabolites of ethanol are pharmacologically active when injected into the brain, and suggest that acetate may mediate some of the rate-suppressing effects of ethanol, such as sedation, ataxia or motor slowing.

Substantia nigra pars reticulata GABA is involved in the regulation of operant lever pressing: pharmacological and microdialysis studies

Substantia nigra pars reticulata (SNr) is an important mesencephalic nucleus that functions as a relay area for basal ganglia output. SNr receives GABAergic inputs from the neostriatum and globus pallidus, and in turn sends projections to a variety of motor areas. Although a large number of studies have focused upon the behavioral functions of basal ganglia dopamine, much less is known about the behavioral functions of SNr GABA. The present studies were undertaken to investigate the role of SNr GABA in lever pressing behavior. In the first experiment, the GABA(A) antagonist bicuculline was infused locally into SNr to determine if blockade of GABA receptors interfered with the performance of lever pressing on a fixed ratio 5 schedule. SNr injections of bicuculline produced a dose-related suppression of operant responding. Analysis of interresponse time bins showed that SNr bicuculline produced a response slowing characterized by a relative reduction in the number of fast interresponse times, and an increase in the relative number of pauses. In an additional experiment, microdialysis methods were used to determine if extracellular GABA is elevated during the performance of fixed ratio five lever pressing. During the 30 min lever pressing session, extracellular GABA showed a significant and substantial increase relative to baseline levels. These data support the hypothesis that SNr GABA is involved in the regulation of motor output, and indicate that GABA release in this structure is increased during behavioral stimulation.

Brain implantations of engineered GABA-releasing cells suppress tremor in an animal model of Parkinsonism

Traditional approaches in the treatment of Parkinson's disease have typically been directed at restoring dopaminergic tone in the neostriatum of the basal ganglia. Nevertheless, the vast majority of neostriatal efferent projections use GABA as their neurotransmitter. Substantia nigra pars reticulata (SNr) is a major basal ganglia output area that is a target of these GABAergic projections, and research from animal models has indicated that stimulation of GABA receptors in SNr can produce motor effects consistent with an antiparkinsonian action. In the present study, implantation of engineered GABA-releasing cells into SNr reduced tremulous movements in an animal model of parkinsonian tremor. These results suggest that implantation of GABA cells into SNr, or possibly into other basal ganglia structures, could provide an alternative transplantation strategy for the treatment of Parkinsonism.

A 'single toxin-double lesion' rat model of striatonigral degeneration by intrastriatal 1-methyl-4-phenylpyridinium ion injection: a motor behavioural analysis

Previous attempts to reproduce striatonigral degeneration, the core pathology underlying Parkinsonism in multiple system atrophy, have been impeded by interactions in the neurotoxins used to replicate striatal and nigral degeneration in rodents. To overcome these interactions, we have developed a new model of striatonigral degeneration which uses a single unilateral administration of 1-methyl-4-phenylpyridinium ion (MPP(+)) into the rat striatum. Spontaneous and drug-induced rotational behaviour, thigmotactic scanning, stepping adjusting steps and paw reaching deficits were compared in four groups of animals: group 1 (control), group 2 (20 microg quinolinic acid), group 3 (20 microg 6-hydroxydopamine), and group 4 (90 nmol MPP(+)). MPP(+) administration resulted in the absence of the amphetamine-induced ipsilateral bias observed in the 6-hydroxydopamine group and of the apomorphine-induced ipsilateral bias observed in the quinolinic acid group. There was no thigmotactic scanning asymmetry in the MPP(+)-injected rats compared to the quinolinic acid- and the 6-hydroxydopamine-injected rats. MPP(+) elicited a bilateral stepping adjustment deficit similar to that found in the quinolinic acid group when compared to controls. MPP(+) also elicited a more severe and significant contralateral deficit in paw reaching compared to controls, 6-hydroxydopamine and quinolinic acid groups. Histopathology revealed a significant reduction of the lesioned striatal surface (-47.53%) with neuronal loss and increased astrogliosis in the MPP(+) group grossly similar to that found in the quinolinic acid group. Contrary to the latter group, however, loss of intrastriatal and striatal-crossing fibre bundles was observed in the MPP(+) group as there was also some retrograde degeneration in the ipsilateral thalamic parafascicular nucleus. The mean loss of dopaminergic cells in the ipsilateral substantia nigra pars compacta in MPP(+) rats was less marked (-48.8%) than in the 6-hydroxydopamine rats (-63.6%) and was not significant in quinolinic acid rats (-5.2%). This study shows that a single unilateral intrastriatal administration of MPP(+) induces a unique motor behaviour resulting from both nigral and striatal degeneration, but also from possible extrastriatal damage. This 'single toxin-double lesion' paradigm may thus serve as a rat model of striatonigral degeneration.

Motivational views of reinforcement: implications for understanding the behavioral functions of nucleus accumbens dopamine

Although the Skinnerian 'Empirical Law of Effect' does not directly consider the fundamental properties of stimuli that enable them to act as reinforcers, such considerations are critical for determining if nucleus accumbens dopamine systems mediate reinforcement processes. Researchers who have attempted to identify the critical characteristics of reinforcing stimuli or activities have generally arrived at an emphasis upon motivational factors. A thorough review of the behavioral literature indicates that, across several different investigators offering a multitude of theoretical approaches, motivation is seen by many as being fundamental to the process of reinforcement. The reinforcer has been described as a goal, a commodity, an incentive, or a stimulus that is being approached, self-administered, attained or preserved. Reinforcers also have been described as activities that are preferred, deprived or in some way being regulated. It is evident that this 'motivational' or 'regulatory' view of reinforcement has had enormous influence over the hypothesis that DA directly mediates 'reward' or 'reinforcement' processes. Indeed, proponents of the DA/reward hypothesis regularly cite motivational theorists and employ their language. Nevertheless, considerable evidence indicates that low/moderate doses of DA antagonists, and depletions of DA in nucleus accumbens, can suppress instrumental responding for food while, at the same time, these conditions leave fundamental aspects of reinforcement (i.e. primary or unconditioned reinforcement; primary motivation or primary incentive properties of natural reinforcers) intact. Several complex features of the literature on dopaminergic involvement in reinforcement are examined below, and it is argued that the assertions that DA mediates 'reward' or 'reinforcement' are inaccurate and grossly oversimplified. Thus, it appears as though it is no longer tenable to assert that drugs of abuse are simply turning on the brain's natural 'reward system'. In relation to the hypothesis that DA systems are involved in 'wanting', but not 'liking', it is suggested in the present review that 'wanting' has both directional aspects (e.g. appetite to consume food) and activational aspects (e.g. activation for initiating and sustaining instrumental actions; tendency to work for food). The present paper reviews findings in support of the hypothesis that low doses of DA antagonists and accumbens DA depletions do not impair appetite to consume food, but do impair activational aspects of motivation. This suggestion is consistent with the studies showing that low doses of DA antagonists and accumbens DA depletions alter the relative allocation of instrumental responses, making the animals less likely to engage in instrumental responses that have a high degree of work-related response costs. In addition, this observation is consistent with studies demonstrating that accumbens DA depletions make rats highly sensitive to ratio requirements on operant schedules. Although accumbens DA is not seen as directly mediating appetite to consume food, principles of behavioral economics indicate that accumbens DA could be involved in the elasticity of demand for food in terms of the tendency to pay work-related response costs. Future research must focus upon how specific aspects of task requirements (i.e. ratio requirements, intermittence of reinforcement, temporal features of response requirements, dependence upon conditioned stimuli) interact with the effects of accumbens DA depletions, and which particular factors determine sensitivity to the effects of DA antagonism or depletion.

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.