D2 Dopamine receptor blockade results in sprouting of DA axons in the intact animal but prevents sprouting following nigral lesions

Persistent extrasynaptic hyperdopaminergia in the mouse hippocampus induces plasticity and recognition memory deficits reversed by the atypical antipsychotic sulpiride

Evidence suggests that subcortical hyperdopaminergia alters cognitive function in schizophrenia and antipsychotic drugs (APD) fail at rescuing cognitive deficits in patients. In a previous study, we showed that blocking D2 dopamine receptors (D2R), a core action of APD, led to profound reshaping of mesohippocampal fibers, deficits in synaptic transmission and impairments in learning and memory in the mouse hippocampus (HP). However, it is currently unknown how excessive dopamine affects HP-related cognitive functions, and how APD would impact HP functions in such a state. After verifying the presence of DAT-positive neuronal projections in the ventral (temporal), but not in the dorsal (septal), part of the HP, GBR12935, a blocker of dopamine transporter (DAT), was infused in the CA1 of adult C57Bl/6 mice to produce local hyperdopaminergia. Chronic GBR12935 infusion in temporal CA1 induced a mild learning impairment in the Morris Water Maze and abolished long-term recognition memory in novel-object (NORT) and object-place recognition tasks (OPRT). Deficits were accompanied by a significant decrease in DAT+ mesohippocampal fibers. Intrahippocampal or systemic treatment with sulpiride during GBR infusions improved the NORT deficit but not that of OPRT. In vitro application of GBR on hippocampal slices abolished long-term depression (LTD) of fEPSP in temporal CA1. LTD was rescued by co-application with sulpiride. In conclusion, chronic DAT blockade in temporal CA1 profoundly altered mesohippocampal modulation of hippocampal functions. Contrary to previous observations in normodopaminergic mice, antagonising D2Rs was beneficial for cognitive functions in the context of hippocampal hyperdopaminergia.

Synaptotagmin-1-dependent phasic axonal dopamine release is dispensable for basic motor behaviors in mice

In Parkinson's disease (PD), motor dysfunctions only become apparent after extensive loss of DA innervation. This resilience has been hypothesized to be due to the ability of many motor behaviors to be sustained through a diffuse basal tone of DA; but experimental evidence for this is limited. Here we show that conditional deletion of the calcium sensor synaptotagmin-1 (Syt1) in DA neurons (Syt1 cKODA mice) abrogates most activity-dependent axonal DA release in the striatum and mesencephalon, leaving somatodendritic (STD) DA release intact. Strikingly, Syt1 cKODA mice showed intact performance in multiple unconditioned DA-dependent motor tasks and even in a task evaluating conditioned motivation for food. Considering that basal extracellular DA levels in the striatum were unchanged, our findings suggest that activity-dependent DA release is dispensable for such tasks and that they can be sustained by a basal tone of extracellular DA. Taken together, our findings reveal the striking resilience of DA-dependent motor functions in the context of a near-abolition of phasic DA release, shedding new light on why extensive loss of DA innervation is required to reveal motor dysfunctions in PD.

Direct dopamine terminal regulation by local striatal microcircuitry

Regulation of axonal dopamine release by local microcircuitry is at the hub of several biological processes that govern the timing and magnitude of signaling events in reward-related brain regions. An important characteristic of dopamine release from axon terminals in the striatum is that it is rapidly modulated by local regulatory mechanisms. These processes can occur via homosynaptic mechanisms-such as presynaptic dopamine autoreceptors and dopamine transporters - as well heterosynaptic mechanisms such as retrograde signaling from postsynaptic cholinergic and dynorphin systems, among others. Additionally, modulation of dopamine release via diffusible messengers, such as nitric oxide and hydrogen peroxide, allows for various metabolic factors to quickly and efficiently regulate dopamine release and subsequent signaling. Here we review how these mechanisms work in concert to influence the timing and magnitude of striatal dopamine signaling, independent of action potential activity at the level of dopaminergic cell bodies in the midbrain, thereby providing a parallel pathway by which dopamine can be modulated. Understanding the complexities of local regulation of dopamine signaling is required for building comprehensive frameworks of how activity throughout the dopamine system is integrated to drive signaling and control behavior.

Increased vulnerability of nigral dopamine neurons after expansion of their axonal arborization size through D2 dopamine receptor conditional knockout

Parkinson’s disease (PD) is a neurodegenerative disorder characterized by the loss of dopamine (DA) neurons in the substantia nigra pars compacta (SNc). Rare genetic mutations in genes such as Parkin, Pink1, DJ-1, α-synuclein, LRRK2 and GBA are found to be responsible for the disease in about 15% of the cases. A key unanswered question in PD pathophysiology is why would these mutations, impacting basic cellular processes such as mitochondrial function and neurotransmission, lead to selective degeneration of SNc DA neurons? We previously showed in vitro that SNc DA neurons have an extremely high rate of mitochondrial oxidative phosphorylation and ATP production, characteristics that appear to be the result of their highly complex axonal arborization. To test the hypothesis in vivo that axon arborization size is a key determinant of vulnerability, we selectively labeled SNc or VTA DA neurons using floxed YFP viral injections in DAT-cre mice and showed that SNc DA neurons have a much more arborized axon than those of the VTA. To further enhance this difference, which may represent a limiting factor in the basal vulnerability of these neurons, we selectively deleted in mice the DA D2 receptor (D2-cKO), a key negative regulator of the axonal arbour of DA neurons. In these mice, SNc DA neurons have a 2-fold larger axonal arborization, release less DA and are more vulnerable to a 6-OHDA lesion, but not to α-synuclein overexpression when compared to control SNc DA neurons. This work adds to the accumulating evidence that the axonal arborization size of SNc DA neurons plays a key role in their vulnerability in the context of PD.

Parkinson’s disease motor symptoms have been linked to age-dependent degeneration of a class of neurons in the brain that release the chemical messenger dopamine. The reason for the selective loss of these neurons represents a key unsolved mystery. One hypothesis is that the neurons most at risk in this disease are those with the most extensive and complex connectivity in the brain, which would make these cells most dependent on high rates of mitochondrial energy production and expose them to higher rates of oxidative stress. Here we selectively deleted in dopamine neurons a key gene providing negative feedback control of the axonal arbor size of these neurons, in the objective of producing mice in which dopamine neurons have more extensive connectivity. We found that deletion of the dopamine D2 receptor gene in dopamine neurons leads to dopamine neurons with a longer and more complex axonal domain. We also found that in these mice, dopamine neurons in a region of the brain called the substantia nigra show increased vulnerability to a neurotoxin often used to model Parkinson’s disease in rodents. Our findings provide support for the hypothesis that the scale of a neuron’s connectivity directly influences its vulnerability to cellular stressors that trigger Parkinson’s disease.

Neuroprotection and neurorestoration as experimental therapeutics for Parkinson's disease

Disease-modifying treatments remain an unmet medical need in Parkinson's disease (PD). Such treatments can be operationally defined as interventions that slow down the clinical evolution to advanced disease milestones. A treatment may achieve this outcome by either inhibiting primary neurodegenerative events ("neuroprotection") or boosting compensatory and regenerative mechanisms in the brain ("neurorestoration"). Here we review experimental paradigms that are currently used to assess the neuroprotective and neurorestorative potential of candidate treatments in animal models of PD. We review some key molecular mediators of neuroprotection and neurorestoration in the nigrostriatal dopamine pathway that are likely to exert beneficial effects on multiple neural systems affected in PD. We further review past and current strategies to therapeutically stimulate these mediators, and discuss the preclinical evidence that exercise training can have neuroprotective and neurorestorative effects. A future translational task will be to combine behavioral and pharmacological interventions to exploit endogenous mechanisms of neuroprotection and neurorestoration for therapeutic purposes. This type of approach is likely to provide benefit to many PD patients, despite the clinical, etiological, and genetic heterogeneity of the disease.

Presynaptic D2 dopamine receptors control long-term depression expression and memory processes in the temporal hippocampus

BACKGROUND

Dysfunctional mesocorticolimbic dopamine signaling has been linked to alterations in motor and reward-based functions associated with psychiatric disorders. Converging evidence from patients with psychiatric disorders and use of antipsychotics suggests that imbalance of dopamine signaling deeply alters hippocampal functions. However, given the lack of full characterization of a functional mesohippocampal pathway, the precise role of dopamine transmission in memory deficits associated with these disorders and their dedicated therapies is unknown. In particular, the positive outcome of antipsychotic treatments, commonly antagonizing D2 dopamine receptors (D2Rs), on cognitive deficits and memory impairments remains questionable.

METHODS

Following pharmacologic and genetic manipulation of dopamine transmission, we performed anatomic, neurochemical, electrophysiologic, and behavioral investigations to uncover the role of D2Rs in hippocampal-dependent plasticity and learning. Naïve mice (n = 4-21) were used in the different procedures.

RESULTS

Dopamine modulated both long-term potentiation and long-term depression in the temporal hippocampus as well as spatial and recognition learning and memory in mice through D2Rs. Although genetic deletion or pharmacologic blockade of D2Rs led to the loss of long-term potentiation expression, the specific genetic removal of presynaptic D2Rs impaired long-term depression and performances on spatial memory tasks.

CONCLUSIONS

Presynaptic D2Rs in dopamine fibers of the temporal hippocampus tightly modulate long-term depression expression and play a major role in the regulation of hippocampal learning and memory. This direct role of mesohippocampal dopamine input as uncovered here adds a new dimension to dopamine involvement in the physiology underlying deficits associated with neuropsychiatric disorders.

Neuroprotective effects of tadalafil on gerbil dopaminergic neurons following cerebral ischemia

Impairment of dopamine function, which is known to have major effects on behaviors and cognition, is one of the main problems associated with cerebral ischemia. Tadalafil, a long-acting phosphodiesterase type-5 inhibitor, is known to ameliorate neurologic impairment induced by brain injury, but not in dopaminergic regions. We investigated the neuroprotective effects of treatment with tadalafil on cyclic guanosine monophosphate level and dopamine function following cerebral ischemia. Forty adult Mongolian gerbils were randomly and evenly divided into five groups (n = 8 in each group): Sham-operation group, cerebral ischemia-induced and 0, 0.1, 1, and 10 mg/kg tadalafil-treated groups, respectively. Tadalafil dissolved in distilled water was administered orally for 7 consecutive days, starting 1 day after surgery. Cyclic guanosine monophosphate assay and immunohistochemistry were performed for thyrosine hydroxylase expression and western blot analysis for dopamine D₂ receptor expression. A decrease in cyclic guanosine monophosphate level following cerebral ischemia was found with an increase in thyrosine hydroxylase activity and a decrease in dopamine D₂ receptor expression in the striatum and substantia nigra region. However, treatment with tadalafil increased cyclic guanosine monophosphate expression, suppressed thyrosine hydroxylase expression and increased dopamine D₂ receptor expression in the striatum and substantia nigra region in a dose-dependent manner. Tadalafil might ameliorate cerebral ischemia-induced dopaminergic neuron injury. Therefore, tadalafil has the potential as a new neuroprotective treatment strategy for cerebral ischemic injury.

A chemical genetic approach identifies piperazine antipsychotics as promoters of CNS neurite growth on inhibitory substrates

Injury to the central nervous system (CNS) can result in lifelong loss of function due in part to the regenerative failure of CNS neurons. Inhibitory proteins derived from myelin and the astroglial scar are major barriers for the successful regeneration of injured CNS neurons. Previously, we described the identification of a novel compound, F05, which promotes neurite growth from neurons challenged with inhibitory substrates in vitro, and promotes axonal regeneration in vivo (Usher et al., 2010). To identify additional regeneration-promoting compounds, we used F05-induced gene expression profiles to query the Broad Institute Connectivity Map, a gene expression database of cells treated with >1300 compounds. Despite no shared chemical similarity, F05-induced changes in gene expression were remarkably similar to those seen with a group of piperazine phenothiazine antipsychotics (PhAPs). In contrast to antipsychotics of other structural classes, PhAPs promoted neurite growth of CNS neurons challenged with two different glial derived inhibitory substrates. Our pharmacological studies suggest a mechanism whereby PhAPs promote growth through antagonism of calmodulin signaling, independent of dopamine receptor antagonism. These findings shed light on mechanisms underlying neurite-inhibitory signaling, and suggest that clinically approved antipsychotic compounds may be repurposed for use in CNS injured patients.

Chronic activation of the D2 autoreceptor inhibits both glutamate and dopamine synapse formation and alters the intrinsic properties of mesencephalic dopamine neurons in vitro

Dysfunctional dopamine (DA)-mediated signaling is implicated in several diseases including Parkinson's disease, schizophrenia and attention deficit and hyperactivity disorder. Chronic treatment with DA receptor agonists or antagonists is often used in pharmacotherapy, but the consequences of these treatments on DA neuron function are unclear. It was recently demonstrated that chronic D2 autoreceptor (D2R) activation in DA neurons decreases DA release and inhibits synapse formation. Given that DA neurons can establish synapses that release glutamate in addition to DA, we evaluated the synapse specificity of the functional and structural plasticity induced by chronic D2R activation. We show that chronic activation of the D2R with quinpirole in vitro caused a parallel decrease in the number of dopaminergic and glutamatergic axon terminals. The capacity of DA neurons to synthesize DA was not altered, as indicated by the lack of change in protein kinase A-mediated Ser(40) phosphorylation of tyrosine hydroxylase. However, the spontaneous firing rate of DA neurons was decreased and was associated with altered intrinsic properties as revealed by a prolonged latency to first spike after release from hyperpolarization. Moreover, D2R function was decreased after its chronic activation. Our results demonstrate that chronic activation of the D2R induces a complex neuronal reorganization involving the inhibition of both DA and glutamate synapse formation and an alteration in electrical activity, but not in DA synthesis. A better understanding of D2R-induced morphological and functional long-term plasticity may lead to improved pharmacotherapy of DA-related neurological and psychiatric disorders.

Running wheel exercise enhances recovery from nigrostriatal dopamine injury without inducing neuroprotection

Forced use of the forelimb contralateral to a unilateral injection of the dopaminergic neurotoxin 6-hydroxydopamine can promote recovery of motor function in that limb and can significantly decrease damage to dopamine terminals. The present study was conducted to determine (1) whether a form of voluntary exercise, wheel running, would improve motor performance in rats with such lesions, and (2) whether any beneficial effects of wheel running are attributable to ameliorating the dopaminergic damage. In experiment 1, rats were allowed to run in exercise wheels or kept in home cages for 2 1/2 weeks, then given stereotaxic infusions of 6-hydroxydopamine into the left striatum. The rats were replaced into their original environments (wheels or home cages) for four additional weeks, and asymmetries in forelimb use were quantified at 3, 10, 17, and 24 days postoperatively. After killing, dopaminergic damage was assessed by both quantifying 3 beta-(4-iodophenyl)tropan-2 beta-carboxylic acid methyl ester ([(125)I]RTI-55) binding to striatal dopamine transporters and counting tyrosine hydroxylase-positive cells in the substantia nigra. Exercised 6-hydroxydopamine-infused rats showed improved motor outcomes relative to sedentary lesioned controls, effects that were most apparent at postoperative days 17 and 24. Despite this behavioral improvement, 6-hydroxydopamine-induced loss of striatal dopamine transporters and tyrosine hydroxylase-positive nigral cells in exercised and sedentary groups did not differ. Since prior studies suggested that forced limb use improves motor performance by sparing nigrostriatal dopaminergic neurons from 6-hydroxydopamine damage, experiment 2 used a combined regimen of forced plus voluntary wheel running. Again, we found that the motor performance of exercised rats improved more rapidly than that of sedentary controls, but that there were no differences between these groups in the damage produced by 6-hydroxydopamine. It appears that voluntary exercise can facilitate recovery from partial nigrostriatal injury, but it does so without evident sparing of dopamine nerve terminals.

Estrogen down-regulates glial activation in male mice following 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine intoxication

Emerging evidence suggests beneficial effect of estrogen for Parkinson's disease (PD), yet the exact mechanisms implicated remain obscured. Activated glia observed in MPTP mouse model and in PD may participate in the cascade of deleterious events that ultimately leads to dopaminergic nigral neuronal death. In vitro studies demonstrate that estrogen can modify the microglial and astroglial expression of inflammatory mediator, such as cytokines and chemokines implicated in neuroinflammation and neurodegeneration. To determine whether estrogen-elicited neuroprotection in PD is mediated through glia, adult male C57Bl/6 mice were treated with 17beta-estradiol (E2) for a total of 11 days. Following 5 days of pretreatment with E2, they were injected with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on the sixth day. The brains were collected on day 11. Immunohistochemistry and quantitative study were used to assess the number of tyrosine hydroxylase-immunoreactive (TH-IR) neurons in the substantia nigra pars compacta (SNpc) and that of activated astrocytes and activated microglia in the SNpc and the striatum. Pretreatment with E2 decreased the loss of TH-IR nigral neurons and diminished the deficit of TH-IR striatal fibers triggered by MPTP. The neuroprotective effect of E2 was coincident with an attenuation of a glial response within the nigra and the striatum. These findings suggest that the neuroprotective effects of E2 evidenced in MPTP mouse model might mediate through an inhibition of reactive glia. However, direct neuroprotective effects of E2 upon TH-IR neurons cannot be excluded.

A transient expression of functional alpha2-adrenergic receptors in white matter of the developing brain

Norepinephrine is a neurotransmitter with peripheral and central actions mediated by alpha-1, alpha-2, and beta-adrenergic receptors. In this paper, we report an expression of alpha-2 adrenergic receptors in developing white matter tracts as revealed by [(3)H]RX821002 autoradiography. In rats, these receptors are present in the corpus callosum and anterior commissure at gestational day 20. Quantification of their postnatal expression reveals peak expression in the corpus callosum at postnatal day 1, which decreases with maturation and disappears by postnatal day 21. Expression in the anterior commissure is persistently elevated throughout the first ten days of postnatal development and then decreases to near background levels by postnatal day 21. Further characterization of the receptors by agonist-stimulated [(35)S]GTPgammaS binding verifies alpha-2 adrenergic receptors are functionally coupled to G proteins early in development and therefore are mature receptors. In situ hybridization did not detect mRNA for any of the alpha-2 adrenergic receptor subtypes (A, B, and C) in white matter tracts of postnatal day 5 brain. [(3)H]RX821002 emulsion autoradiography demonstrated autoradiographic grains that were of comparable density between cells and over cell bodies. Collectively, these data suggest that alpha-2 adrenergic receptors in neonatal commissures are synthesized at sites distant from their white matter expression and may be guiding the maturation of these brain commissures.

Levodopa therapy from the neuroprotection viewpoint

There are many reports indicating the existence of free radical adducts in the Parkinson's disease (PD) brain. However, levodopa may have two aspects in its characteristics, one is that levodopa is a source of free radicals in the pathogenesis of PD, and the other is that it is the precursor of the most efficient transmitter in dopaminergic neurons from the viewpoint of therapy. Dopamine (DA) supplemented by levodopa acts as a neural transmitter and as a dopaminergic receptor-mediated trophic agent. From studies using rodent models of parkinsonism, physical exercise potentially induces neurotrophins in the brain and protects or enhances the regeneration of nigrostriatal neurons. In our preliminary experiment, the potential effects of physical exercise on motor and psychological symptoms were observed, suggesting that physical exercise may, to a certain extent, modulate pathological conditions in the brain. On the basis of the above we conclude that DA supplementation in PD potentially increases the levels of neurotrophins in a dopaminergic receptor-mediated manner, and secondarily enhances mobility (physical exercise). In general, levodopa demand increases with disease progression, and physical therapy is more important in advanced stages. To obtain good outcomes of physical exercise, it is essential to maintain a good medical control, particularly at the advanced stages of PD. From these results, it can be speculated that levodopa exerts some neuroprotective effects as long as the dose is not excessively high.

Mice lacking the alpha4 nicotinic receptor subunit fail to modulate dopaminergic neuronal arbors and possess impaired dopamine transporter function

Neuronal nicotinic acetylcholine receptors (nAChRs) at presynaptic sites can modulate dopaminergic synaptic transmission by regulating dopamine (DA) release and uptake. Dopaminergic transmission in nigrostriatal and mesolimbic pathways is vital for the coordination of movement and is associated with learning and behavioral reinforcement. We reported recently that the D2 DA receptor plays a central role in regulating the arbor size of substantia nigra dopaminergic neurons. Given the known effects of nAChRs on dopaminergic neurotransmission, we assessed the ability of the alpha4 nAChR subunit to regulate arbor size of dopaminergic neurons by comparing responses of wild-type and alpha4 nAChR subunit knockout [alpha4(-/-)] mice to long-term exposure to cocaine, amphetamine, nicotine, and haloperidol, and after substantia nigra neurotoxic lesioning. We found that dopaminergic neurons in adult drug-naive alpha4(-/-) mice had significantly larger terminal arbors, and despite normal short-term behavioral responses to drugs acting on pre- and postsynaptic D2 DA receptors, they were unable to modulate their terminal arbor in response to pharmacological manipulation or after lesioning. In addition, although synaptosome DA uptake studies showed that the interaction of the D2 DA receptor and the dopamine transporter (DAT) was preserved in alpha4(-/-) mice, DAT function was found to be impaired. These findings suggest that the alpha4 subunit of the nAChR is an independent regulator of terminal arbor size of nigrostriatal dopaminergic neurons and that reduced functionality of presynaptic DAT may contribute to this effect by impairing DA uptake.

Modification of dopaminergic markers expression in the striatum by neonatal exposure to glutamate during development

Monosodium l-glutamate (MSG) was administered subcutaneously to male neonatal rats, and the effect on developmental profile of tyrosine hydroxylase (TH), D1, D2 receptors, and dopamine (DA) transporter expression in the striatum was examined using Western blot. In addition, TH-immunopositive neurons at substantia nigra (SN) were also examined. MSG treatment (4mg/g of body weight, administered on postnatal days 1, 3, 5, and 7) resulted in a reduction of D1 and D2 receptor expression from 30 days of age and persisted to adulthood (120 days of age), while DA transporter expression was significantly reduced from 14 days of age to adulthood. TH immunopositive neurons at SN showed a significant reduction, as well as TH expression on postnatal days 10, 30, 60, and 120 at striatum was reduced. No changes of TH were observed at 14 days of age. Results indicate that an over-stimulation of the glutamatergic system by neonatal exposure to a high glutamate concentration induces a partial loss in TH-positive neurons in the SN and an important reduction in dopaminergic markers expression in the striatum, suggesting that early excitotoxicity could contribute to developmental alterations in the nigrostriatal pathway, which may be associated with various disorders of the basal ganglia.

Glial responses associated with dopaminergic striatal reinnervation following lesions of the rat substantia nigra

Lesioning of dopaminergic substantia nigra pars compacta (SNpc) neurons leads to depletion of dopamine (DA) and dopaminergic axons in the dorsal striatum, followed by subsequent compensatory sprouting of dopaminergic fibers and striatal reinnervation. In this study, the response of striatal glia (microglia and astroglia) was compared with the degeneration and regeneration of dopaminergic axons following SNpc lesions. Following partial SNpc lesions, density of dopamine transporter (DAT) immunoreactive (-ir) terminals in the dorsal striatum returned to normal within 16 weeks of injury, suggesting that dopaminergic reinnervation of the striatum was complete. In conjunction, the glial responses in the dorsal striatum consisted of two peaks. The first peak in glial density occurred immediately after lesioning, peaking at 7 days, implying that it was likely to be associated with removal of debris from degenerating terminals. The second glial response commenced 8 weeks after lesioning and peaked some time after 16 weeks. The time of onset of the second peak suggests that it may be associated with the establishment of synapses rather than with axonal guidance.