Functional investigations into the role of dopamine and serotonin in partial bilateral striatal 6-hydroxydopamine lesioned rats

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.

6-hydroxydopamine lesions in the rat neostriatum impair sequential learning in a serial reaction time task

Sequential behavior has been intensively investigated in humans using so-called serial reaction time tasks(SRTT), in which visual stimuli are either presented in a random or sequential order. Typically, when the stimulus presentation follows a previously learned sequential order,reaction times are decreased compared to random stimulus presentation and become partly automated. A vast amount of SRTT findings indicates that sequential learning and performance seem to be mediated amongst others by the basal ganglia-especially the striatum-and the neurotransmitter dopamine therein. In this study we used an operant rat version of the human four choice SRTT to investigate the effect of bilateral neostriatal dopamine lesions induced by 6-hydroxydopamine on sequential learning. The rats' task was to respond rapidly to illuminated holes by nose-poking into them. During extensive training, the position of the illuminated hole followed a 12-item sequence. The outcome of this sequential training was also investigated in two tests, namely an interference test, where stimulus presentation switched between this sequential and a pseudo random order every five minutes, and a violation test, in which only one sequence item was eventually skipped. The neurotoxic lesions, which was placed before the start of training, led to the expected sub-total dopamine depletions (i.e. residual levels around 34-56% of controls), especially in the medial neostriatum. These lesions did not lead to general motor deficits in a catalepsy task, but moderate deficits in locomotion in an activity box, which largely recovered with time after lesion. In the SRTT, rats with lesions showed impaired learning, that is, less response accuracy and slower reaction times than the control group.During a subsequent test with alternating phases of sequential and random stimulus presentations, reaction times and accuracy of the control group were superior during sequential as compared to random stimulus phases. In the lesion group, only a moderate advantage in accuracy was observed. In the violation test, another outcome measure, the control group showed an expected increase in reaction times on the violated positions. By contrast, the lesion group showed no such increase, which indicates less automation of sequential behavior in these animals. For one, these findings support previous evidence in showing that neostriatal dopamine plays an important role for instrumental behavior, in general. Furthermore,and most importantly, they suggest that dopaminergic-striatal networks also play an important role in sequential behavior, especially its acquisition.

Alterations in 5-HT2A receptor binding in various brain regions among 6-hydroxydopamine-induced Parkinsonian rats

The serotonergic system has close interactions with the dopaminergic system and is strongly implicated in the pathophysiological mechanisms and therapeutic paradigms of Parkinson's disease (PD). This study aims to investigate regional changes in 5-hydroxytryptamine (5-HT) 2A receptors in the rat brain 3 weeks after unilateral medial forebrain bundle lesion by 6-hydroxydopamine (6-OHDA). 5-HT 2A receptor distributions and alterations in the postmortem rat brain were detected by [(3)H]ketanserin-binding autoradiography. In the 6-OHDA-induced Parkinson's rat model, nigrostriatal dopaminergic neuron loss significantly mediated the decreased [(3)H]ketanserin binding, predominantly in the agranular insular cortex (17.3%, P = 0.03), cingulate cortex (18.2%, P < 0.001), prefrontal cortex (8%, P = 0.043), primary somatosensory cortex (17.7%, P = 0.002), and caudate putamen (14.5%, P = 0.02) compared to controls while a profound reduction of tyrosine hydroxylase (TH) immunostaining in the striatum was also observed. Alterations in [(3)H]ketanserin binding in the examined brain areas may represent the specific regions that mediate cognitive dysfunctions via the serotonin system. The downregulation of 5-HT(2A) receptor binding in this study also provides indirect evidence for plasticity in the serotonergic system in the rat brains. This study contributes to a better understanding of the critical roles of 5-HT(2A) receptors in treating neurodegenerative disorders and implicates 5-HT(2A) receptors as a novel therapeutic target in the treatment of PD.

Time-dependent alterations of peripheral immune parameters after nigrostriatal dopamine depletion in a rat model of Parkinson's disease

Dysfunction of the central dopaminergic system is associated with neurodegenerative disorders and mental illnesses such as Parkinson's disease and schizophrenia. Patients suffering from these diseases were reported to exhibit altered immune functions compared to healthy subjects and imbalance of the central dopaminergic system has been suggested as one causative factor for the immune disturbances. However, it is unclear whether the observed immune changes are primary or secondary to the disease. Here we demonstrate that central dopamine (DA) depletion in a rat model of Parkinson's disease induced transient changes in blood leukocyte distribution and cytokine production that were apparent until four weeks after bilateral intrastriatal administration of the neurotoxin 6-hydroxydopamine (6-OHDA). Eight weeks after treatment, no differences in blood immune parameters were anymore evident between neurotoxin-treated and control animals. Nevertheless, animals with a widespread damage of dopaminergic neurons in the nigrostriatal system showed an exacerbated pro-inflammatory response following in vivo challenge with bacterial lipopolysaccharide. Our data indicate that peripheral immune perturbations in the early phase after intrastriatal 6-OHDA administration might have been related to the neurodegenerative process itself whereas the increased sensitivity to the inflammatory stimulus seems to have resulted from an impaired dopaminergic control of prolactin (PRL) and corticosterone (CORT) secretion. The findings demonstrate that the brain dopaminergic system is involved in peripheral immune regulation and suggest that central dopaminergic hypoactivity bears the risk of excessive inflammation, e.g., during infection or tissue injury.

Neuroanatomical localization of an internal clock: A functional link between mesolimbic, nigrostriatal, and mesocortical dopaminergic systems

The effects of selective dopamine (DA) depleting lesions with 6-hydroxydopamine microinjection into the SN, CPu, and NAS, as well as radiofrequency lesions of the CPu on the performance characteristics of rats trained on a single-valued 20-s peak-interval (PI) timing procedure or a double-valued 10-s and 60-s PI procedure were evaluated. A double dissociation in the performance of duration discriminations was found. Rats with CPu lesions were unable to exhibit temporal control of their behavior suggesting complete insensitivity to signal duration but were able to show discrimination of the relative reward value of a signal by differentially modifying their response rates appropriately. In contrast, rats with NAS lesions were able to exhibit temporal control of their behavior by differentially modifying their response rates as a function of signal duration(s), suggesting no impairment of sensitivity to signal duration, but were unable to show discrimination of the relative reward value of a signal.