In vivo extracellular recording of striatal neurons in the awake rat following unilateral 6-hydroxydopamine lesions

Bilateral effects of unilateral intrastriatal GDNF on locomotor-excited and nonlocomotor-related striatal neurons in aged F344 rats

In order to determine its effects on locomotor-related striatal electrophysiology in aged rats, glial cell line-derived neurotrophic factor (GDNF) was infused (vehicle or 30mug) into the right striatum of 24-25-month-old Fischer 344 (F344) rats. Multi-wire electrode arrays were then chronically implanted in striatum bilaterally. Thirty days later, striatal electrophysiological activity was recorded during freely moving conditions. Individual neurons were classified as locomotor-excited if they exhibited significant increases in firing rates during locomotor bouts versus periods of nonmovement. GDNF produced a significant increase in overall firing rates in locomotor-excited striatal neurons. This effect was observed in both the infused and the contralateral striatum. GDNF also attenuated the bursting activity of nonlocomotor-related striatal neurons, an effect that was also present bilaterally. These results suggest that GDNF's antiparkinsonism effects are associated with increased excitability of motor-related striatal neurons and diminished activity of neurons that do not exhibit explicit motor-related changes in activity. Such studies may aid in understanding the mechanism of potential therapies for movement disorders seen in aging and Parkinson's disease.

Chronic high-frequency stimulation of the subthalamic nucleus and L-DOPA treatment in experimental parkinsonism: effects on motor behaviour and striatal glutamate transmission

Hyperactivity of striatal glutamatergic synaptic transmission in response to dopamine depletion plays a major role in the pathogenesis of parkinsonian motor symptoms. In the present study we investigated the impact, on this hyperactivity, of chronic dyskinesiogenic L-DOPA treatment, combined or not with high-frequency stimulation (HFS) of the subthalamic nucleus (STN). In vitro patch-clamp recordings were performed from striatal spiny neurons of hemiparkinsonian rats (intranigral 6-OHDA injection). Here we show that dyskinesiogenic L-DOPA treatment exacerbated striatal glutamatergic hyperactivity induced by 6-OHDA lesion. Chronic 5-day STN HFS had the opposite effect, reducing striatal glutamatergic transmission in both parkinsonian and dyskinetic animals. Consistently, chronic HFS stimulation could progressively ameliorate motor parkinsonian signs (akinesia) but, conversely, did not improve L-DOPA-induced dyskinesia (LID). Thus, the effects of L-DOPA and HFS on corticostriatal transmission seem to be dissociated. These data show for the first time that dyskinesiogenic L-DOPA treatment and chronic STN HFS with antiakinetic effects induce opposite plastic rearrangements in the striatum. The interaction between these two treatments provides further evidence that striatal glutamatergic hyperactivity is a pathophysiological correlate of akinesia rather than LID.

Cortical inputs and GABA interneurons imbalance projection neurons in the striatum of parkinsonian rats

The striatum receives massive cortical excitatory inputs and is densely innervated by dopamine. Striatal projection neurons form either the direct or indirect pathways. Models of Parkinson's disease propose that dopaminergic degeneration imbalances both pathways, although direct electrophysiological evidence is lacking. Here, striatal neurons were identified by electrophysiological criteria and Neurobiotin labeling combined with either immunohistochemistry or in situ hybridization. Their spontaneous discharge activity and spike response to cortical stimulation were recorded in vivo in anesthetized rats rendered hemi-parkinsonian by 6-hydroxydopamine. We showed that striatonigral neurons (direct pathway) were inhibited whereas striatopallidal neurons (indirect pathway) were activated by dopaminergic lesion. We also identified, with antidromic stimulations, corticostriatal neurons that preferentially innervate striatonigral or striatopallidal neurons and showed that dopaminergic depletion selectively decreased the spontaneous activity of the former. Therefore, dopamine degeneration induces a cascade of imbalances that spread out of the basal ganglia and affect the whole basal ganglia-thalamo-cortical circuits. Fast-spiking GABA interneurons provide potent feedforward inhibition of striatal projection neurons. We showed here that these interneurons narrowed the time window of the responses of projection neurons to cortical stimulation. In the dopamine-depleted striatum, because the intrinsic activity of these interneurons was not altered, their feedforward inhibition worsened the striatal imbalance. Indeed, the time window of the evoked responses was narrower for striatonigral neurons and wider for striatopallidal neurons. Therefore, after dopaminergic depletion, cortical inputs and GABA interneurons might imbalance striatal projection neurons and represent two novel nondopaminergic mechanisms that might secondarily contribute to the pathophysiology of Parkinson's disease.

Bradykinesia induced by dopamine D2 receptor blockade is associated with reduced motor cortex activity in the rat

Disruption of motor cortex activity is hypothesized to play a major role in the slowed movement (bradykinesia) associated with reduced dopaminergic function. We recorded single neurons in the motor cortex of free-moving rats performing a forelimb-reaching task. The same neurons were examined before and after induction of bradykinesia with the D2 dopamine receptor antagonist haloperidol. Within-cell changes in the firing rate and firing pattern of individual cells and the correlation between simultaneously recorded cells after injection of haloperidol were statistically compared with vehicle-only control experiments. During haloperidol-induced bradykinesia (mean movement time increase, +231%), there was an average 11% decrease in baseline firing rate. Movement-related peaks in firing rate were more dramatically affected, with an overall reduction in peak amplitudes of 40%. Bradykinesia was also associated with decreased intensity of bursting and amplitude of cross-correlation peaks at rest. The results show for the first time that significant reductions can be detected in motor cortex activity at rest in animals with impaired ability to generate movements induced by reduced dopamine action and confirm that impaired movements are associated with reduced cortical activation. Together, these changes in neural activity may reduce recruitment and rate modulation of motor units in the spinal cord.

Neural responses in multiple basal ganglia regions following unilateral dopamine depletion in behaving rats performing a treadmill locomotion task

To investigate basal ganglia (BG) neural responses to dopamine (DA) depletion, multiple channel, single unit recording was carried out in freely moving rats performing a treadmill locomotion task. Single unit activity from 64 microelectrodes in the striatum (STR), globus pallidus (GP), subthalamic nucleus (STN) and substantia nigra pars reticulata (SNr) was recorded simultaneously before and after a unilateral DA lesion induced by microinjection of 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle. The DA lesion resulted in an impairment of treadmill walking manifested by a significant decrease in swing time of both forelimbs. The stance time, however, increased significantly only in ipsilateral (good) forelimbs, reflecting compensatory changes in the good limb for motor deficits. Neural activity in the STR and GP ipsilateral to the lesion decreased during the 7-day period following the DA lesion. Conversely, an increase in spike discharges appeared in the ipsilateral SNr and STN several days after the DA lesion. Changes in the type of neural response associated with treadmill locomotion were also found in some neurons after DA depletion. Such changes were most prominent in the STR. Limb movement-related neural activity increased significantly mainly in the SNr. Additionally, neural responses to the tone cue associated with the onset of the treadmill diminished greatly in the lesioned side of the BG. Increased activity in SNr neurons is consistent with the concept that inhibition of thalamus contributes to hypokinesis in the absence of DA. Substantial decrease in striatal activity supports a concept that DA loss leads to a global suppression of recurrent cortical striatal thalamic activity that degrades normal information flow in Parkinson's diseases.

Firing properties of dopamine neurons in freely moving dopamine-deficient mice: effects of dopamine receptor activation and anesthesia

To examine the regulation of midbrain dopamine neurons, recordings were obtained from single neurons of freely moving, genetically engineered dopamine-deficient (DD) mice. DD mice were tested without dopamine signaling (basal state) and with endogenous dopamine signaling (after L-dopa administration). In the basal state, when dopamine concentration in DD mice is <1% of that in control animals, the firing properties of midbrain dopamine neurons were remarkably similar among genotypes. However, L-dopa treatment, which restores dopamine and feeding and locomotor behavior in DD mice, profoundly inhibited the firing rate and bursting of dopamine neurons in DD mice. In addition, dopamine neurons in DD mice were hypersensitive to the dopamine receptor agonists quinpirole and SKF 81297. Anesthesia markedly reduced the firing rate of dopamine neurons in DD mice but did not significantly decrease the firing rate in control dopamine neurons. These data suggest that restoration of endogenous dopamine signaling activates hypersensitive long-loop feedback pathways that serve to limit dopamine release and underscore the importance of recording from awake animals.

Contrast enhancement: a physiological effect of striatal dopamine?

Dopamine functions as an important neuromodulator in the dorsal striatum and ventral striatum/nucleus accumbens. Evidence is accumulating for the idea that striatal neurons compete with each other for control over the animal's motor resources, and that dopamine plays an important modulatory role that allows a particular subset of neurons, encoding a specific behavior, to predominate in this competition. One means by which dopamine could facilitate selection among competing neurons is to enhance the contrast between stronger and weaker excitations (or to increase the "signal to noise ratio" among neurons, where the firing of the most excited neurons is assumed to transmit signal and the firing of the least excited to transmit noise). Here, we review the electrophysiological evidence for this hypothesis and discuss potential cellular mechanisms by which dopamine-mediated contrast enhancement could occur.

Striatal plasticity and extrapyramidal motor dysfunction

Knowledge of molecular events contributing to motor dysfunction in Parkinson's disease has advanced rapidly during the past decade. Studies in animal models as well as in patients afflicted by this disorder suggest that the nonphysiologic stimulation of striatal dopamine receptors, first as a result of dopaminergic denervation and later as a consequence of the intermittent high-intensity stimulation produced by standard therapeutic regimens, leads to plastic changes in striatal medium spiny neurons. The clinical appearance of parkinsonism and subsequently of motor response complications is associated with the aberrant activation of signaling cascades within medium spiny neurons that modify the phosphorylation state of their ionotropic glutamatergic receptors. Resultant NMDA and AMPA receptor sensitization augments cortical excitatory input to these spiny efferent neurons, thus altering striatal output in ways that compromise motor function. These findings have already yielded new insight into mechanisms subserving motor memory and synaptic integration as well as accelerated development of novel approaches to the improved treatment of motor disability.

Aged F344 rats exhibit altered electrophysiological activity in locomotor-unrelated but not locomotor-related striatal neurons

Multi-wire electrode arrays were chronically implanted and striatal electrophysiological activity was recorded in young (4-9 months) versus aged (24-29 months) Fischer 344 (F344) rats in order to determine whether locomotor-related striatal neurons exhibit age-related changes in electrophysiological activity during freely-moving conditions. Individual neurons were classified as locomotor-related if they exhibited significant differences in their firing rates between periods of locomotion versus periods of non-movement. While the activity of locomotor-related striatal neurons did not differ between young and aged rats, neurons that were not related to locomotion exhibited significantly greater activity in the aged rats during both periods of non-movement and bouts of locomotion. These results suggest that in the aged striatum, increased activity of nonlocomotor-related neurons may contribute to hypokinesia through their influence on basal ganglia output nuclei. Such studies may aid in the understanding of movement disorders seen in aging and Parkinson's disease.

A2A antagonist prevents dopamine agonist-induced motor complications in animal models of Parkinson’s disease

Adenosine A(2A) receptors, abundantly expressed on striatal medium spiny neurons, appear to activate signaling cascades implicated in the regulation of coexpressed ionotropic glutamatergic receptors. To evaluate the contribution of adenosinergic mechanisms to the pathogenesis of the response alterations induced by dopaminergic treatment, we studied the ability of the selective adenosine A(2A) receptor antagonist KW-6002 to prevent as well as palliate these syndromes in rodent and primate models of Parkinson's disease. In rats, KW-6002 reversed the shortened motor response produced by chronic levodopa treatment while reducing levodopa-induced hyperphosphorylation at S845 residues on AMPA receptor GluR1 subunits. In primates, KW-6002 evidenced modest antiparkinsonian activity when given alone. Once-daily coadministration of KW-6002 with apomorphine prevented the development of dyskinesias, which appeared in control animals 7-10 days after initiating apomorphine treatment. Animals initially given apomorphine plus KW-6002 for 3 weeks did not begin to manifest apomorphine-induced dyskinesias until 10-12 days after discontinuing the A(2A) antagonist. These results suggest that KW-6002 can attenuate the induction as well as the expression of motor response alterations to chronic dopaminergic stimulation in parkinsonian animals, possibly by blocking A(2A) receptor-stimulated signaling pathways. Our findings strengthen the rationale for developing A(2A) antagonists as an early treatment strategy for Parkinson's disease.

Dopamine depletion causes fragmented clustering of neurons in the sensorimotor striatum: evidence of lasting reorganization of corticostriatal input

Firing during sensorimotor exam was used to categorize single neurons in the lateral striatum of awake, unrestrained rats. Five rats received unilateral injection of 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle to deplete striatal dopamine (DA; >98% depletion, postmortem assay). Three months after treatment, rats exhibited exaggerated rotational behavior induced by L-dihydroxyphenylalanine (L-DOPA) and contralateral sensory neglect. Electrode track "depth profiles" on the DA-depleted side showed fragmented clustering of neurons related to sensorimotor activity of single body parts (SBP neurons). Clusters were smaller than normal, and more SBP neurons were observed in isolation, outside of clusters. More body parts were represented per unit volume. No recovery in these measures was observed up to one year post lesion. Overall distributions of neurons related to different body parts were not altered. The fragmentation of SBP clusters after DA depletion indicates that a percentage of striatal SBP neurons switched responsiveness from one body part to one or more different body parts. Because the specific firing that characterizes striatal SBP neurons is mediated by corticostriatal inputs (Liles and Updyke [1985] Brain Res. 339:245-255), the data indicate that DA depletion resulted in a reorganization of corticostriatal connections, perhaps via unmasking or sprouting of connections to adjacent clusters of striatal neurons. After reorganization, sensory activity in a localized body part activates striatal neurons that have switched to that body part. In turn, switched signals sent from basal ganglia to premotor and motor neurons, which likely retain their original connections, would create mismatches in these normally precise topographic connections. Switched signals could partially explain parkinsonian deficits in motor functions involving somatosensory guidance and their intractability to L-DOPA therapy-particularly if the switching involves sprouting.