Striatal But Not Extrastriatal Dopamine Receptors Are Critical to Dopaminergic Motor Stimulation

Dopaminergic inhibition of the inwardly rectifying potassium current in direct pathway medium spiny neurons in normal and parkinsonian striatum

Despite the profound behavioral effects of the striatal dopamine (DA) activity and the inwardly rectifying potassium channel ( Kir ) being a key determinant of striatal medium spiny neuron (MSN) activity that also profoundly affects behavior, previously reported DA regulations of Kir are conflicting and incompatible with MSN function in behavior. Here we show that in normal mice with an intact striatal DA system, the predominant effect of DA activation of D1Rs in D1-MSNs is to cause a modest depolarization and increase in input resistance by inhibiting Kir, thus moderately increasing the spike outputs from behavior-promoting D1-MSNs. In parkinsonian (DA-depleted) striatum, DA increases D1-MSN intrinsic excitability more strongly than in normal striatum, consequently strongly increasing D1-MSN spike firing that is behavior-promoting; this DA excitation of D1-MSNs is stronger when the DA depletion is more severe. The DA inhibition of Kir is occluded by the Kir blocker barium chloride (BaCl 2 ). In behaving parkinsonian mice, BaCl 2 microinjection into the dorsal striatum stimulates movement but occludes the motor stimulation of D1R agonism. Taken together, our results resolve the long-standing question about what D1R agonism does to D1-MSN excitability in normal and parkinsonian striatum and strongly indicate that D1R inhibition of Kir is a key ion channel mechanism that mediates D1R agonistic behavioral stimulation in normal and parkinsonian animals.

Dopamine-independent development and maintenance of mouse striatal medium spiny neuron dendritic spines

Striatal medium spiny neurons (MSNs) and striatal dopamine (DA) innervation are profoundly important for brain function such as motor control and cognition. A widely accepted theory posits that striatal DA loss causes (or leads to) MSN dendritic atrophy. However, examination of the literature indicates that the data from Parkinson's disease (PD) patients and animal PD models were contradictory among studies and hard to interpret. Here we have re-examined the potential effects of DA activity on MSN morphology or lack thereof. We found that in 15-day, 4- and 12-month old Pitx3 null mutant mice that have severe DA denervation in the dorsal striatum while having substantial residual DA innervation in the ventral striatum, MSN dendrites and spine numbers were similar in dorsal and ventral striatum, and also similar to those in normal mice. In 15-day, 4- and 12-month old tyrosine hydroxylase knockout mice that cannot synthesize L-dopa and thus have no endogenous DA in the entire brain, MSN dendrites and spine numbers were also indistinguishable from age-matched wild-type (WT) mice. Furthermore, in adult WT mice, unilateral 6-OHDA lesion at 12 months of age caused an almost complete striatal DA denervation in the lesioned side, but MSN dendrites and spine numbers were similar in the lesioned and control sides. Taken together, our data indicate that in mice, the development and maintenance of MSN dendrites and spines are DA-independent such that DA depletion does not trigger MSN dendritic atrophy; our data also suggest that the reported MSN dendritic atrophy in PD may be a component of neurodegeneration in PD rather than a consequence of DA denervation.

D2R signaling in striatal spiny neurons modulates L-DOPA induced dyskinesia

Degeneration of dopaminergic neurons leads to Parkinson's disease (PD), characterized by reduced levels of striatal dopamine (DA) and impaired voluntary movements. DA replacement is achieved by levodopa treatment which in long-term causes involuntary movements or dyskinesia. Dyskinesia is linked to the pulsatile activation of D1 receptors of the striatal medium spiny neurons (MSNs) forming the direct output pathway (dMSNs). The contribution of DA stimulation of D2R in MSNs of the indirect pathway (iMSNs) is less clear. Using the 6-hydroxydopamine model of PD, here we show that loss of DA-mediated inhibition of these neurons intensifies levodopa-induced dyskinesia (LID) leading to reprogramming of striatal gene expression. We propose that the motor impairments characteristic of PD and of its therapy are critically dependent on D2R-mediated iMSNs activity. D2R signaling not only filters inputs to the striatum but also indirectly regulates dMSNs mediated responses.

Mice hypomorphic for Pitx3 show robust entrainment of circadian behavioral and metabolic rhythms to scheduled feeding

Pitx3ak mice lack a functioning retina and develop fewer than 10% of dopamine neurons in the substantia nigra. Del Río-Martín et al. (2019) reported that entrainment of circadian rhythms to daily light-dark (LD) cycles is absent in these mice, and that rhythms of locomotor activity, energy expenditure, and other metabolic variables are disrupted with food available ad libitum and fail to entrain to a daily feeding. The authors propose that retinal innervation of the suprachiasmatic nucleus is required for development of cyclic metabolic homeostasis, but methodological issues limit interpretation of the results. Using standardized feeding schedules and procedures for distinguishing free-running from entrained circadian rhythms, we confirm that behavioral and metabolic rhythms in Pitx3ak mice do not entrain to LD cycles, but we find no impairment in circadian organization of metabolism with food available ad libitum and no impairment in entrainment of metabolic or behavioral rhythms by daily feeding schedules. This Matters Arising paper is in response to Del Río-Martín et al. (2019), published in Cell Reports. See also the response by Fernandez-Perez et al. (2022), published in this issue.

Role of the Dopaminergic System in the Striatum and Its Association With Functional Recovery or Rehabilitation After Brain Injury

Disabilities are estimated to occur in approximately 2% of survivors of traumatic brain injury (TBI) worldwide, and disability may persist even decades after brain injury. Facilitation or modulation of functional recovery is an important goal of rehabilitation in all patients who survive severe TBI. However, this recovery tends to vary among patients because it is affected by the biological and physical characteristics of the patients; the types, doses, and application regimens of the drugs used; and clinical indications. In clinical practice, diverse dopaminergic drugs with various dosing and application procedures are used for TBI. Previous studies have shown that dopamine (DA) neurotransmission is disrupted following moderate to severe TBI and have reported beneficial effects of drugs that affect the dopaminergic system. However, the mechanisms of action of dopaminergic drugs have not been completely clarified, partly because dopaminergic receptor activation can lead to restoration of the pathway of the corticobasal ganglia after injury in brain structures with high densities of these receptors. This review aims to provide an overview of the functionality of the dopaminergic system in the striatum and its roles in functional recovery or rehabilitation after TBI.

Marijuana improved motor impairments and changes in synaptic plasticity-related molecules in the striatum in 6-OHDA-treated rats

Using marijuana has become popular and is allowed for medical purposes in some countries. The effect of marijuana on Parkinson's disease is controversial and Medical marijuana may benefit for motor and non-motor symptoms of patients with Parkinson's disease. No research has been conducted to fully prove the benefits, risks, and uses of marijuana as a treatment for patients with Parkinson's disease. In the present study, several different approaches, including behavioral measures and the western blot method for protein level assay, were used to investigate whether exposure to marijuana affects the motor and synaptic plasticity impairment induced by 6-OHDA. Marijuana consumption significantly decreased apomorphine-induced contralateral rotation, beam travel time, beam freeze time, and catalepsy time, but significantly increased latency to fall in the rotarod test, balance time, and protein level of PSD-95 and dopamine receptor D1 in the 6-OHDA + marijuana group. These results suggest that marijuana may be helpful for motor disorders and synaptic changes in patients with Parkinson's disease.

Effects of intrastriatal injection of the dopamine receptor agonist SKF38393 and quinpirole on locomotor behavior in hemiparkinsonism rats

Dopamine (DA) in the striatum is essential to influence motor behavior and may lead to movement impairment in Parkinson's disease (PD). The present study examined the different functions of the DA D1 receptor (D1R) and DA D2 receptor (D2R) by intrastriatal injection of the D1R agonist SKF38393 and the D2R agonist quinpirole in 6-hydroxydopamine (6-OHDA)-lesioned and control rats. All rats separately underwent dose-response behavior testing for SKF38393 (0, 0.5, 1.0, and 1.5 μg/site) or quinpirole (0, 1.0, 2.0, and 3.0 μg/site) to determine the effects of the optimal modulating threshold dose. Two behavior assessment indices, the time of latency to fall and the number of steps on a rotating treadmill, were used as reliable readouts of motor stimulation variables for quantifying the motor effects of the drugs. The findings indicate that at threshold doses, SKF38393 (1.0 μg/site) and quinpirole (1.0 μg/site) produce a dose-dependent increase in locomotor activity compared to vehicle injection. The ameliorated behavioral responses to either SKF38393 or quinpirole in lesioned rats were greater than those in unlesioned control rats. Moreover, the dose-dependent increase in locomotor capacity for quinpirole was greater than that for SKF38393 in lesioned rats. These results can clarify several key issues related to DA receptors directly and may provide a basis for exploring the potential of future selective dopamine therapies for PD in humans.

Dopamine D1 + D3 receptor density may correlate with parkinson disease clinical features

Objective

Dopamine D2‐like receptors – mainly dopamine D2 receptors (D2R) and dopamine D3 receptors (D3R) – are believed to be greatly involved in the pathology of Parkinson disease (PD) progression. However, these receptors have not been precisely examined in PD patients. Our aim was to quantitatively calculate the exact densities of dopamine D1 receptors (D1R), D2R, and D3R in control, Alzheimer disease (AD), and Lewy body disease (LBD) patients (including PD, Dementia with Lewy bodies, and Parkinson disease dementia); and analyze the relationship between dopamine receptors and clinical PD manifestations.

Methods

We analyzed the densities of D1R, D2R, and D3R in the striatum and substantia nigra (SN) using a novel quantitative autoradiography procedure previously developed by our group. We also examined the expression of D2R and D3R mRNA in the striatum by in situ hybridization.

Results

The results showed that although no differences of striatal D1R were found among all groups; D2R was significantly decreased in the striatum of PD patients when compared with control and AD patients. Some clinical manifestations: age of onset, PD stage, dopamine responsiveness, and survival time after onset; showed a better correlation with striatal D1R + D3R densities combined compared to D1R or D3R alone.

Interpretation

There is a possibility that we may infer the results in diagnosis, treatment, and prognosis of PD by detecting D1R + D3R as opposed to using dopamine D1 or D3 receptors alone. This is especially true for elderly patients with low D2R expression as is common in this disease.

The antiparkinson drug ropinirole inhibits movement in a Parkinson's disease mouse model with residual dopamine neurons

The dopamine (DA) D2-like receptor (D2R) agonist ropinirole is often used for early and middle stage Parkinson's disease (PD). However, this D2-like agonism-based strategy has a complicating problem: D2-like agonism may activate D2 autoreceptors on the residual DA neurons in the PD brain, potentially inhibiting these residual DA neurons and motor function. We have examined this possibility by using systemic and local drug administration in transcription factor Pitx3 null mutant (Pitx3Null) mice that mimic the DA denervation in early and middle stage PD and in DA neuron tyrosine hydroxylase (TH) gene knockout (KO) mice that mimic the severe DA loss in late stage PD. We found that in Pitx3Null mice with residual DA neurons and normal mice with normal DA system, systemically injected ropinirole inhibited locomotion, whereas bilateral dorsal striatal-microinjected ropinirole stimulated movement in Pitx3Null mice; bilateral microinjection of ropinirole into the ventral tegmental area also inhibited movement in Pitx3Null mice; we further determined that ropinirole inhibited nigral DA neuron spike firing in WT mice. In contrast, both systemically and striatum-locally administered ropinirole increased movements in TH KO mice, but produced relatively more dyskinesia than L-dopa. Although requiring confirmation in non-human primates and PD patients, these data suggest that while activating D2-like receptors in striatal projection neurons and hence stimulating movements, D2-like agonists can inhibit residual DA neurons and cause akinesia when the residual DA neurons and motor functions are still substantial, and this motor-inhibitory effect disappears when almost all DA neurons are lost such as in late stage PD.

GABAergic imbalance is normalized by dopamine D1 receptor activation in the striatum contralateral to the cortical injury in motor deficit-recovered rats

RATIONALE

The sensorimotor cortex and the striatum are interconnected by the corticostriatal pathway, suggesting that cortical injury alters the striatal function, which may be modulated by dopamine.

OBJECTIVES

We studied whether the activation of dopamine D₁ receptors (D₁Rs) modulates the γ-aminobutyric acid (GABA) and glutamate levels in the striatum of recovered rats at 192 h after cortical injury.

METHODS

The D₁R agonist SKF-38393 (0, 2, 3, or 4 mg/kg) was administered at 24, 48, 96, and 192 h post-injury, and then rats were decapitated to determine GABA and glutamate levels and the levels of D₁R mRNA on both sides of the striatum.

RESULTS

GABAergic imbalance in the striatum contralateral to the injury site was normalized by the administration of the D₁R agonist, but this treatment did not produce a significant effect on glutamate levels, suggesting that glutamate was metabolized into GABA. The administration of SKF-38393 (2 mg/kg) decreased the levels of D₁R mRNA in the striatum contralateral to the injury, and this effect was blocked by the coadministration of the D₁R antagonist SCH-23390 (2 mg/kg). In the striatum ipsilateral to the injury, the D₁R agonist increased the D₁R mRNA levels, an effect that was blocked by SCH-23390.

CONCLUSION

The reversal of the GABAergic imbalance in the striatum contralateral to the cortical injury can be modulated by extrastriatal D₁R activation, and the D₁R agonist-induced increases in the D₁R mRNA levels in the striatum ipsilateral to the injury suggest that the striatum may be necessary to achieve functional recovery.

Implicit time-place conditioning alters Per2 mRNA expression selectively in striatum without shifting its circadian clocks

Animals create implicit memories of the time of day that significant events occur then anticipate the recurrence of those conditions at the same time on subsequent days. We tested the hypothesis that implicit time memory for daily encounters relies on the setting of the canonical circadian clockwork in brain areas involved in the formation or expression of context memories. We conditioned mice to avoid locations paired with a mild foot shock at one of two Zeitgeber times set 8 hours apart. Place avoidance was exhibited only when testing time matched the prior training time. The suprachiasmatic nucleus, dorsal striatum, nucleus accumbens, cingulate cortex, hippocampal complex, and amygdala were assessed for clock gene expression. Baseline phase dependent differences in clock gene expression were found in most tissues. Evidence for conditioned resetting of a molecular circadian oscillation was found only in the striatum (dorsal striatum and nucleus accumbens shell), and specifically for Per2 expression. There was no evidence of glucocorticoid stress response in any tissue. The results are consistent with a model where temporal conditioning promotes a selective Per2 response in dopamine-targeted brain regions responsible for sensorimotor integration, without resetting the entire circadian clockwork.

Hyperactive Response of Direct Pathway Striatal Projection Neurons to L-dopa and D1 Agonism in Freely Moving Parkinsonian Mice

Dopamine (DA) profoundly stimulates motor function as demonstrated by the hypokinetic motor symptoms in Parkinson's disease (PD) and by the hyperkinetic motor side effects during dopaminergic treatment of PD. Dopamine (DA) receptor-bypassing, optogenetics- and chemogenetics-induced spike firing of striatal DA D1 receptor (D1R)-expressing, direct pathway medium spiny neurons (dSPNs or dMSNs) promotes movements. However, the endogenous D1R-mediated effects, let alone those of DA replacement, on dSPN spike activity in freely-moving animals is not established. Here we show that using transcription factor Pitx3 null mutant (Pitx3Null) mice as a model for severe and consistent DA denervation in the dorsal striatum in Parkinson's disease, antidromically identified striatonigral neurons (D1R-expressing dSPNs) had a lower baseline spike firing rate than that in DA-intact normal mice, and these neurons increased their spike firing more strongly in Pitx3Null mice than in WT mice in response to injection of L-dopa or the D1R agonist, SKF81297; the increase in spike firing temporally coincided with the motor-stimulating effects of L-dopa and SKF81297. Taken together, these results provide the first evidence from freely moving animals that in parkinsonian striatum, identified behavior-promoting dSPNs become hyperactive upon the administration of L-dopa or a D1 agonist, likely contributing to the profound dopaminergic motor stimulation in parkinsonian animals and PD patients.