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

Rats that differentially respond to cocaine differ in their dopaminergic storage capacity of the nucleus accumbens

Cocaine (COC) inhibits the re-uptake of dopamine. However, the dopamine response to COC also depends on dopamine inside storage vesicles. The aim of this study was to investigate whether rats that differentially respond to COC differ in their dopaminergic storage capacity of the nucleus accumbens. Total and vesicular levels of accumbal dopamine as well as accumbal vesicular monoamine transporter-2 levels were established in high (HR) and low responders (LR) to novelty rats. Moreover, the effects of reserpine (RES) on the COC-induced increase of extracellular accumbal dopamine were investigated. HR displayed higher accumbal levels of total and vesicular dopamine than LR. Moreover, HR displayed more accumbal vesicular monoamine transporters-2 than LR. COC increased extracellular accumbal dopamine more strongly in HR than in LR. A low dose of RES prevented the COC-induced increase of accumbal dopamine in LR, but not in HR. A higher dose of RES was required to inhibit the COC-induced increase of accumbal dopamine in HR. These data demonstrate that HR were marked by a larger accumbal dopaminergic storage pool than LR. It is hypothesized that HR are more sensitive to COC than LR, because COC can release more dopamine from accumbal storage vesicles in HR than in LR.

J. Neurochem. (2008) 105, 2122–2133.

Nigral degeneration with inclusion body formation and behavioral changes in rats after proteasomal inhibition

OBJECTIVE

We were interested in studying nigral degeneration with inclusion body formation and behavioral changes in rats after proteasomal inhibition.

METHODS

Observation of progressive behavioral and pathological changes in rats following a unilateral nigral injection of lactacystin, a selective proteasome inhibitor.

RESULTS

After administration at a concentration of 10 microg (2 microl) of lactacystin, when tyrosine hydroxylase (TH) immunostaining decreased gradually in the substantia nigra pars compacta (SNc) and corpus striatum, alpha-synuclein-immunopositive inclusion appeared extensively in the surviving neurons. We also observed the degeneration of diverse cellular organelles by transmission electron microscopy. The effect of cellular organelle degeneration on behavior, a clinical index, was striking and was statistically significant. Over the 3 weeks following the administration of lactacystin, a highly significant decrease in TH immunostaining was observed and alpha-synuclein-immunopositive inclusions gradually appeared. Interestingly, there was a strong correlation in behavioral changes and the increase in alpha-synuclein-immunopositive inclusions whereas the decrease in TH immunostaining did not seem to induce any behavioral changes.

CONCLUSIONS

Our results reveal that unilateral nigral proteasome inhibition induces degeneration in the SNc and corpus striatum as well as behavioral changes demonstrating strong time dependence. Behavioral changes were driven by the formation of alpha-synuclein inclusions, but not by decreased TH neurons.

Nonmotor symptoms in Parkinson's disease: investigating early-phase onset of behavioral dysfunction in the 6-hydroxydopamine-lesioned rat model

To investigate the psychiatric symptoms accompanying the early phases of Parkinson's disease (PD), we injected adult rats with 10.5 microg 6-hydroxydopamine (6-OHDA) bilaterally into the dorsal striatum. The resulting neurodegeneration led, 12 weeks after injection, to a mild (36%) reduction of striatal dopamine. We tested the behavioral response of sham and 6-OHDA-lesioned animals at different time points after injection to evaluate the onset and progression of behavioral abnormalities. The results showed that such a mild reduction of dopamine levels was associated with a decrease in anxiety-like behavior, an increase in "depression"-like behavior, and a marked change in social behavior. Learning and memory abilities were not affected. Overall, the PD rat model used here displays behavioral alterations having face validity with psychiatric symptoms of the pathology and thus appears to be a valuable tool for investigating the neural bases of the early phases of PD.

Role of Dopamine in the Motivational and Cognitive Control of Behavior

Brain dopamine has often been implicated in impulsive and/or inflexible behaviors, which may reflect failures of motivational and/or cognitive control. However, the precise role of dopamine in such failures of behavioral control is not well understood, not least because they implicate paradoxical changes in distinct dopamine systems that innervate dissociable neural circuits. In addition, there are large individual differences in the response to dopaminergic drugs with some individuals benefiting from and others being impaired by the same drug. This complicates progress in the understanding of dopamine's role in behavioral control processes, but also provides a major problem for neuropsychiatry, where some individuals are disproportionately vulnerable to the adverse effects of dopamine-enhancing drugs on motivation and cognition. Recent progress is reviewed from cognitive and behavioral neuroscience research on motivation and cognitive control, which begins to elucidate the factors that mediate the complex roles of mesolimbic, mesocortical, and nigrostriatal dopamine in behavioral control. NEUROSCIENTIST 14(4):381–395, 2008. DOI: 10.1177/1073858408317009

High and low responders to novelty show differential effects in striatal glutamate

The goal of this study was to determine whether there was a difference in glutamate within the dorsolateral striatum in mice exhibiting either a high (HR) or low (LR) locomotor response to a novel environment. The number of line crossings over a 30-min-period when the mice were placed in a novel environment was determined, and those mice for which the values were above the mean were in the HR group and those with the values below the mean were in the LR group. In vivo microdialysis was carried out to determine the basal extracellular level of striatal glutamate, and the contralateral striatum was taken to measure the density of glutamate immunolabeling within nerve terminals making an asymmetrical (excitatory) synaptic contact using quantitative immuno-gold electron microscopy. There was a statistically significant difference (35%) in the basal extracellular level of striatal glutamate between the HR and LR groups, with the HR group having a lower level, compared with that of the LR group. There was a 25% difference in the density of nerve terminal glutamate immuno-gold labeling associated with the synaptic vesicle pool in the HR, compared with that in the LR group, but this difference was not statistically significant. There was no change in the basal extracellular level of striatal dopamine between the two groups, but there was a statistically significant difference (73%) in the basal turnover ratio of striatal dopamine and its metabolites in the HR, compared with that in the LR group. The data suggests that the difference in extracellular striatal glutamate between the HR and LR groups is not due to an alteration in basal extracellular dopamine but could be due to an increase in dopamine turnover.

Bilateral nigral 6-hydroxydopamine lesions increase the amount of extracellular dopamine in the nucleus accumbens

The goal of the present study was to provide neurochemical evidence for a shift in the functional balance between the nigrostriatal and mesolimbic pathway in favour of the mesolimbic pathway by investigating the effects of a partial, nigral, bilateral 6-hydroxydopamine lesion on basal and novelty-induced extracellular dopamine release in the accumbens of Low responder rats to novelty (LR). Low responders were chosen because the above-mentioned shift was seen in LR rats, but not in rats that have a high response to novelty (HR). About 1 microg/microl of 6-hydroxydopamine was injected bilaterally into the substantia nigra pars compacta and a guide cannula was placed into the right accumbens. Changes in extracellular dopamine in response to novelty, a new cage, were measured using a microdialysis probe inserted into the accumbens. The lesion size was determined by quantification of tyrosine hydroxylase immunoreactivity of the substantia nigra and the ventral tegmental area. This revealed that the lesion partly destroyed the dopaminergic cells of the nigrostriatal pathway, thereby relatively sparing the dopaminergic cells of the mesolimbic pathway. The lesion significantly increased the amount of extracellular dopamine in the accumbens during both basal and novelty conditions. We suggest that the experimentally induced neuronal death in the substantia nigra pars compacta with subsequent removal of lateral inhibition of adjacent neurons underlies the observed changes in the amount of extracellular dopamine in the accumbens.

Morphological and functional effects of PACAP in 6-hydroxydopamine-induced lesion of the substantia nigra in rats

Pituitary adenylate cyclase activating polypeptide (PACAP) has several different actions in the nervous system, including neuroprotective effects. In the present study, we investigated the effects of different doses of PACAP on the functional and morphological outcome in a rat model of Parkinson's disease. Rats were given unilateral injections of 6-hydroxydopamine (6-OHDA) into the substantia nigra. PACAP-treated animals received 1, 0.1 or 0.01 microg PACAP as a pretreatment. Control animals without PACAP treatment displayed severe hypokinesia at 1 and 10 days post-lesion when compared to normal animals or those receiving saline only. PACAP treatment resulted in less severe acute hypokinesia, and complete recovery by 10 days. Asymmetrical signs were observed in all lesioned animals 1 day post-lesion. PACAP-treated animals, however, showed better recovery as they ceased to display asymmetrical signs 10 days later and showed markedly less apomorphine-induced rotations. Best behavioral outcome was observed in animals treated with 0.1 microg PACAP. Tyrosine-hydroxylase (TH) immunohistochemistry revealed increased number of dopaminergic neurons in the substantia nigra pars compacta and in the ventral tegmental area in all PACAP-treated rats in contrast to the severe cell loss in control animals. These results indicate that PACAP may be a promising therapeutic agent in Parkinson's disease.

Repeated sucrose access influences dopamine D2 receptor density in the striatum

A decrease in D2 dopamine receptor subtype (D2R) binding in the striatum has been reported in obese individuals and drug addicts. We examined D2R density in the striatum of food-restricted rats that had contingent access to food with different incentive values. Results showed that animals receiving limited access to 0.3 M sucrose paired 2 h with a chow meal for 7 days had a significantly lower D2R binding in nucleus accumbens shell and dorsolateral striatum compared with animals that had limited access to chow. There was no differential binding, however, in the accumbens core in any of the groups. These findings indicate that feeding conditions and sucrose intake influence D2R density specifically in subregions of the striatum.