D1 Dopamine Receptor Supersensitivity in the Dopamine-Depleted Striatum Animal Model of Parkinson’S Disease CHARLES R

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REVIEW ■ D1 Dopamine Receptor Supersensitivity in the Dopamine-Depleted Striatum Animal Model of Parkinson’s Disease CHARLES R. GERFEN Section on Neuroanatomy, Laboratory of Systems Neuroscience National Institute of Mental Health

Dopamine acts in the striatum principally through the D1 and D2 dopamine receptor subtypes, which are segregated to the direct and indirect striatal projection neurons, respectively. As a consequence, degeneration of the dopamine input to the striatum results in opposing affects in these pathways. The resulting functional imbalance is thought to be responsible for the bradykinesia of Parkinson’s disease, which may be temporarily normalized by dopamine replacement therapy. However, direct striatal projection neurons become irreversibly supersensitive to D1 dopamine receptor activation, despite the fact that there is an actual decrease in receptor number. Recent studies show that this D1-supersensitive response results from a switch from the normal D1-mediated activation of protein-kinase A to an aberrant activation of ERK1/2/MAPkinase. This switch in D1-receptor-mediated regulation of protein kinase systems responsible for neuronal plasticity is suggested to underlie dyskinesia produced by L-DOPA treatment of Parkinson’s disease. NEUROSCIENTIST 9(6):455–462, 2003. DOI: 10.1177/1073858403255839

KEY WORDS Dopamine, Gene regulation, Protein kinase, Receptor supersensitivity, Parkinson’s disease, Dyskinesia

A cornerstone of animal models of neurologic disease is rons. Neurons of this type share a common morphology, the unilateral nigrostriatal dopamine lesion rodent model with dendrites densely studded with spines, which are of Parkinson’s disease (Ungerstedt 1971). In this para- the target of glutamatergic, excitatory synaptic input digm, the dopamine-specific neurotoxin 6-hydroxy- from the cerebral cortex. These neurons are subdivided dopamine is used to lesion the nigrostriatal dopamine into two main subtypes based on the distribution of their pathway on one side of the brain, leaving the other side axonal projections out of the striatum (Kawaguchi and intact. When such lesioned animals are treated with others 1990). One type, the direct striatal projection neu- dopamine-receptor agonists, animals rotate contralater- ron, distributes axon collaterals sparsely to the external ally, away from the side of the lesion. This robust behav- segment of the globus pallidus and more robust projec- ioral paradigm still serves, some 30 years since its intro- tions to the medial segment of the globus pallidus and duction, as the standard for determining the affect of substantia nigra. These latter two nuclei are the main dopamine depletion in the striatum. The traditional output nuclei of the basal ganglia. The other medium explanation for this behavior was that striatal neurons spiny neuron type, the indirect striatal projection neuron, responded to the loss of dopamine by producing more extends a projection axon only to the external segment of receptors, such that the response to dopamine receptor the globus pallidus. These neurons are indirectly con- agonists of neurons in the lesioned striatum was super- nected with the output nuclei of the basal ganglia sensitive compared with the dopamine-intact striatum. through connections of the external segment of the However, the relationship between dopamine-depletion globus pallidus and substantia nigra and through con- and the supersensitivity responsible for altered behavior nections with the subthalamic nucleus. Although both is not a result of a simple compensatory response of stri- direct and indirect striatal projection neurons use GABA atal neurons to increase receptor expression. as their main neurotransmitter, they express different To appreciate the affect of dopamine depletion on stri- dopamine receptor subtypes and neuropeptides (Gerfen atal dopamine receptor function requires an understand- and Young 1988; Gerfen and others 1990). Direct striatal ing of the functional organization of the striatum relative projection neurons express the D1 dopamine receptor, as to the distribution of dopamine receptor subtypes well as the neuropeptides dynorphin and substance P. On (Gerfen and Wilson 1996). More than 90% of striatal the other hand, indirect striatal projection neurons neurons are classified as medium spiny projection neu- express the D2 dopamine receptor and the neuropeptide enkephalin (Fig. 1). Address correspondence to: C. R. Gerfen, Bldg. 36, Room 2D-30, 36 The segregation of D1 and D2 dopamine receptor sub- Convent Drive, MSC 4075, Bethesda, MD 20817-4075 (e-mail: ger- types, respectively, to direct and indirect striatal projec- [email protected]). tion neurons was established on the basis of expression

Volume 9, Number 6, 2003 THE NEUROSCIENTIST 455 This article is a U.S. Government work and is not protected by copyright in the United States. © 2003 Sage Publications outside of the United States. ISSN 1073-8584 the neuropeptides (Gerfen and others 1990). Moreover, functional studies, demonstrating that D1 agonists selectively altered gene expression in direct projection neurons (Robertson and others 1990; Gerfen and others 1995), whereas D2 agonists and antagonists selectively altered gene expression in indirect striatal neurons (Dragunow and others 1990), substantiated the segregation of dopamine receptor subtypes. However, although a number of studies challenged this view, suggesting that D1 and D2 receptors were co-localized in most striatal neurons (Surmeier and others 1992), further in situ hybridization histochemical studies (LeMoine and others 1990; LeMoine and Bloch 1995), as well as immuno- histochemical studies (Hersch and others 1995) of the distribution of the receptor proteins, have led to a con- sensus view that D1 and D2 dopamine receptors are segregated to direct and indirect striatal neurons.

Dopamine Receptor-Mediated Gene Regulation Alterations in gene expression in striatal neurons have been instructive in understanding the functional changes that follow dopamine depletion in the striatum and subsequent dopamine agonist treatment. The analytic tools used to study the function of dopamine in the striatum have developed somewhat in reverse of the time sequence of their affects. The first studies examining gene regulation focused on the expression levels of neuropeptides and dopamine receptors themselves (Young and others 1986; Gerfen and others 1990, 1991). These gene products are late-response genes in that their expression is the result of dopamine receptor-mediated activation of protein kinase signaling pathways (Sheng and Greenberg 1990; Konradi and others 1994). The Fig. 1. The medium spiny striatal projection neuron. A) next set of tools that were used were the so-called imme- Photomicrograph of a single medium spiny projection neuron diate early genes (IEGs), such as c-fos and zif268 and ′ filled with biocytin. A , High magnification of intracellularly filled other transcription factors that are responsible for regu- medium spiny neuron. B) Tracings of an indirect and direct striatal projection neuron drawn in place on a sagittal brain dia- lating the expression levels of the late response genes gram (from Kawaguchi and others 1990). The indirect striatal (Robertson and others 1990; Dragunow and others 1990; pathway neuron has a projection axon that extends into the lat- Graybiel and others 1990). Most recently, it has been eral globus pallidus, where it arborizes and does not extend possible to analyze the activation of the protein kinase beyond this nucleus. The direct striatal pathway neuron has a projection axon that extends some collaterals into the lateral signaling pathways themselves with immunohistochem- globus pallidus (GP) and extends to the medial globus pallidus ical visualization of the phosphorylation of the proteins (MGP) and substantia nigra. Higher magnification of the indirect that are involved in the protein kinase signaling path- and direct striatal pathway neurons shows their dendrites ways (Cole and others 1994; Sgambato and others (black) and local axon collaterals within the striatum (gray). C) 1998). In temporal sequence, the phosphorylation events Diagrammatic representation of the direct and indirect striatal pathway neurons. Both neurons are GABAergic and receive a occur within 5 to 15 min of pharmacologic treatment, glutamatergic corticostriatal input. Direct pathway neurons followed at 30 to 60 min by induction of the IEGs, which express the D1 dopamine receptor subtype, the Gs and Golf stim- lead to changes in gene expression of the late-response ulatory G proteins, as well as the peptides substance P (SP) and genes such as the neuropeptides. Use of all these tools dynorphin (DYN). These neurons project to the lateral globus pallidus (MGP), medial globus pallidus (MGP), and substantia has provided insight into the function of dopamine in the nigra pars reticulata (SNR). Indirect striatal pathway neurons normal and dopamine-depleted Parkinson’s disease express the D2 dopamine receptor, which is coupled to the model. inhibitory Gi G protein, as well as the A2Adenosine receptor, The first studies showed that following lesions of the which is coupled to the stimulatory G G protein. These neu- olf nigrostriatal dopamine system, gene expression in the rons also express the peptide enkephalin (ENK). direct and indirect striatal neurons is affected in opposite directions. In direct projecting neurons, the neuropep- of the mRNA encoding of these receptors in connection- tides substance P and dynrophin, as well as the D1 ally identified striatal neurons and co-expression with dopamine receptor, show decreased levels of expression

456 THE NEUROSCIENTIST D1 Dopamine and Parkinson’s Disease (Young and others 1986; Gerfen and others 1990, 1991). D1 Dopamine Receptor–Mediated Gene Conversely, in indirect striatal projection neurons, the Regulation in the Dopamine-Intact Striatum neuropeptide enkephalin and the D2 dopamine receptor show increased levels of expression (Young and others Analysis of IEGs provided new insights into functional 1986; Gerfen and others 1990, 1991). These changes in response of striatal neurons to dopamine (Robertson and gene expression following striatal dopamine depletion others 1990; Dragunow and others 1990; Graybiel and are instructive in that neurons do not simply respond to others 1990). Induction of IEGs, many of which, such as the loss of dopamine with increased expression of c-fos, are transcription factors, precede changes in regu- dopamine receptors. Rather, the response of striatal neu- lation of the late-response genes such as the neuropep- rons reflects the loss of the normal affect that dopamine tides. In the dopamine-intact striatum, treatment with has on these neurons. D1 and D2 dopamine receptors are indirect dopamine agonists, such as cocaine, which G-protein-coupled receptors that are linked, respectively, results in excessive dopamine activation in the striatum, to stimulatory and inhibitory G proteins that regulate the results in the robust induction of IEGs in the dorsal stria- activity of adenylate cyclase, which regulates gene tum, followed by increased dynorphin expression expression in striatal neurons through activation of pro- (Steiner and Gerfen 1993). These affects of cocaine on tein kinase signaling pathways. Thus, following gene regulation are mediated by activation of D1 dopamine depletion, the absence of D1-mediated stimu- dopamine receptors. Repeated daily treatment with lation of protein kinase signaling results in decreased cocaine results in a diminishing c-fos response on the gene expression in direct striatal projection neurons. 2nd and 3rd days of treatment, with little induction of the Importantly, there is actually a decrease in the expres- 4th day. Interestingly, it was found that the diminishing sion of the D1 dopamine receptor (Marshall and others cocaine induction of c-fos and other IEGs was correlat- 1989; Gerfen and others 1990). On the other hand, the ed with increased dynorphin expression in the dorsal absence of D2-mediated inhibition of protein kinase sig- striatum in direct striatal projection neurons (Steiner and naling results in increased gene expression in indirect Gerfen 1993). The normal expression of dynorphin dis- striatal projection neurons. plays high levels in the nucleus accumbens and ventral Treatments with dopamine receptor agonists follow- striatum and low levels in the dorsal striatum. This pat- ing dopamine depletion provide further understanding of tern is the inverse of the pattern of cocaine-induced c-fos the function of dopamine in the striatum. In indirect stri- induction, which, after the initial treatment, is highest in atal projection neurons, the elevated levels of gene the dorsal striatum. Thus, there appeared to be a rela- expression are reversed with continuous treatment with tionship between dynorphin expression in the normal D2 dopamine receptor agonists (Gerfen and others striatum and the pattern of cocaine induction of c-fos 1990). Thus, elevated levels of enkephalin and D2 recep- and the diminished cocaine induction of c-fos correlated tor mRNAs that occur as a result of dopamine depletion with increased dynorphin expression in the dorsal stria- are normalized by continuous D2 agonist treatments. tum. These findings led to the suggestion that in the Interestingly, repeated daily injections of D2 receptor normal dopamine-intact striatum, that dynorphin agonists do not result in reversal of the dopamine- expression blunts cocaine, D1-mediated c-fos induction. depletion affects on gene expression. Although each Subsequent studies demonstrating that dynorphin ago- acute treatment reverses gene expression of enkephalin nists were able to block the initial cocaine induction of and D2 dopamine receptors, these changes are transient c-fos in the dorsal striatum substantiated this idea. Thus, and continuous occupancy of the D2 dopamine receptor it appears that in the dopamine-intact striatum, direct is required to maintain levels of gene expression. These striatal projection neurons display an adaptive response results point to gene expression in indirect striatal pro- to excessive D1 dopamine receptor stimulation by projection neurons being regulated in a straightforward ducing dynorphin, which functions to blunt the response manner by the inhibitory effect of D2 receptor function of these neurons. on indirect striatal neurons. Treatment of D1 receptor agonists on gene expression in direct striatal neurons D1 Dopamine Receptor– displays a very different response profile. The decreased Mediated Gene Regulation in gene expression of substance P and D1 dopamine recep- the Dopamine-Depleted Striatum tors that occurs following dopamine depletion in direct The adaptive response to repeated daily excessive stim- striatal projection neurons is reversed by single and ulation of the D1 dopamine receptor is in marked con- repeated daily treatments with D1 dopamine receptor trast to the response following dopamine depletion of the agonists. Continuous treatment is ineffective. striatum (Steiner and Gerfen 1996). In the dopamine- Interestingly, repeated daily treatments result in greatly depleted striatum, D1 agonist treatment results in a elevated levels of dynorphin in direct striatal projection robust IEG induction, including c-fos and zif268 among neurons, reaching levels in excess of 300% above con- some 40 other IEGs (Berke and others 1998). This IEG trol levels. These elevated levels of dynorphin expression response is supersensitive in that doses of D1 dopamine were the first indication that D1 dopamine–mediated agonists that do not elicit an IEG response in the regulation of gene expression in direct striatal projection dopamine-intact striatum result in a quite robust neurons is abnormal in the dopamine-depleted striatum. response in the dopamine-depleted striatum. As dis-

Volume 9, Number 6, 2003 THE NEUROSCIENTIST 457 cussed above, in the dopamine-depleted striatum, repeated D1 agonist treatment results in an overexpression of dynorphin in direct striatal projection neurons, far in excess of the levels achieved with repeated cocaine treatment in the dopamine-intact striatum. However, in contrast to the affect of repeated D1 receptor stimulation in the dopamine-intact striatum, which results in diminished IEG induction, IEG induction in the dopamine- depleted striatum remains as robust and even more robust following repeated D1 dopamine receptor agonist treatment. This suggests that following dopamine depletion there is a switch in the signal transduction mechanisms that regulate gene expression.

Dopamine-Depletion D1-Receptor Supersensitivity With the myriad of studies of gene regulation changes, we have focused on the supersensitive response of direct striatal neurons to D1 dopamine receptors following dopamine depletion. A time course study following unilateral 6-hydroxydopamine-induced lesions of the nigrostriatal dopamine system shows that the D1 dopamine receptor agonist treatment results in induction of IEGs in the lesioned striatum within days of the degeneration of the dopamine input to the striatum (Fig. 2). For the IEG response to occur, dopamine degenera- Fig. 2. Time course of the development of degeneration of stri- tion in the striatum must be nearly complete. atal dopamine following unilateral 6-hydroxydopamine (6- OHDA) induced lesions of the nigrostriatal dopamine system Significantly, such dopamine depletion results in a and the development of D1 dopamine receptor agonist induc- decrease of D1 dopamine receptors in the lesioned stria- tion of the immediate early gene c-fos. Animals received injec- tum (Marshall and others 1989; Gerfen and others tions of 6-OHDA into the right nigrostriatal dopamine pathway 1990). Nonetheless, the first time the animals are treat- and were treated with the D1 dopamine receptor agonist ed with D1 agonists, striatal D1 expressing direct pro- SKF38393 (2 mg/kg/ip) and killed 1 h later. Photographs on the left show coronal sections through the striatum labeled with jection neurons display an IEG response. Thus, in the 3H-mazindol, which binds to dopamine terminals. Selections case of the D1 dopamine receptor, the supersensitive from animals 1 day, 3 days, and 5 days following infusions of 6- response following dopamine depletion is not due to OHDA show that degeneration begins at 1 day and is nearly increased receptor expression. complete by 5 days postlesion. Photographs on the right show ISHH labeling of the mRNA encoding c-fos in these animals. C- The IEG response to D1 dopamine receptor stimula- fos induction becomes apparent in the lesioned striatum at 1 tion is considered supersensitive as doses of D1 agonists day postlesion and increases at 3 days and becomes robust at that result in little or no IEG induction in the dopamine- 5 days postlesion. intact striatum induce a robust IEG response in the dopamine-depleted striatum. More than 40 IEGs have been identified that are induced by D1 agonist treatment We investigated the involvement of ERK1/2 in the dopamine-depleted striatum (Fig. 3) (Berke and MAPKinase in the D1 dopamine receptor supersensitiv- others 1998). Induction of IEGs occurs as a result of ity model (Gerfen and others 2002). In animals with uni- activation of G-coupled protein kinase signaling path- lateral lesions of the nigrostriatal dopamine system, ways. In the striatum, a number of protein kinase signal- treatment with a D1 dopamine receptor agonist ing pathways have been shown to be active. One involves (SKF38393) at a dose (1 mg/kg) that results in IEG the stimulation of adenylate cyclase, resulting in cAMP induction in the dopamine-depleted striatum but not in activation of protein kinase A (PKA), which phosphory- the dopamine-intact striatum results in phosphorylation lates the transcription factor CREB. Phosphorylated of ERK1/2 (Gerfen and others 2002). In the dopamine- CREB is responsible for inducing the transcription of a depleted striatum, phosphorylated ERK1/2 immunore- number of IEGs including c-fos. Induction of c-fos in activity is localized to direct striatal projection neurons, the normal dopamine-intact striatum by indirect ago- which express the D1 dopamine receptor, and only rarely nists, such as amphetamine, has been shown to involve in indirect striatal projection neurons (Fig. 4). Inhibitors PKA activation of phosphorylated CREB (Konradi and of the MAPKinase kinase, MEK (Alessi and others others 1994). Another protein kinase pathway involving 1995; Dudley and others 1995; Favata and others 1998), the MAPKinase ERK1/2 have been shown to be activat- which is responsible for phosphorylating ERK1/2, ed in striatal neurons by stimulation of corticostriatal infused either directly into the striatum or administered glutamatergic input (Sgambato and others 1998). systemically, block D1 receptor supersensitive induction

458 THE NEUROSCIENTIST D1 Dopamine and Parkinson’s Disease Fig. 4. D1 dopamine receptor–mediated phosphorylation of ERK1/2 (p-ERK1/2) in the dopamine-depleted striatum. Unilateral lesion of the nigro-striatal dopamine system is demonstrated by the loss of tyrosine hydroxylase immunoreactivity in the right lesioned striatum (A). Following treatment (15 min) with the partial D1 dopamine agonist SKF38393 (2 Fig. 3. Panel showing ISHH labeling of mRNAs encoding 28 mg/kg/ip), p-ERK1/2 is not evident in the dopamine-intact stria- immediate early genes that are induced in the dopamine- tum (B) but is present in numerous neurons in the dopamine- depleted striatum following D1 agonist treatment. Coronal sec- depleted striatum (C). To determine the type of striatal neuron in tions through the striatum of an animal with a unilateral lesion of which p-ERK1/2 is present, sections are processed to display the nigrostriatal dopamine system, which was treated with a D1 p-ERK1/2 with a green fluorescent label (D) and enkephalin with dopamine receptor agonist (SKF38393), are on the left. From a red fluorescent label (D′). Nearly all p-ERK1/2 immunoreactive Berke and others (1998). neurons (green arrows) are enkephalin negative. Only a small number of enkephalin-positive neurons display p-ERK1/2 immunoreactivity (yellow arrow), whereas the vast majority are p-ERK1/2 negative (orange arrows). The graph provides quanti- of IEGs in the dopamine-depleted striatum (Gerfen and tative data of the average number of pERK-positive/enkephalin- others 2002). Given the induction of a large number of negative (green), p-ERK-positive/enkephalin-positive (yellow), and pERK-negative/enkephalin-positive (red) neurons in a 500 IEGs in response to D1 agonist treatment in the µm2 area from the lateral striatum of four animals. Enkephalin dopamine-depleted striatum, the activation of ERK1/2 provides a marker of indirect projection neurons, with any given might seem unremarkable. However, a number of addi- striatal area having an equal number of direct projecting, tional experiments point to this activation of ERK1/2 as enkephalin-negative neurons (Gerfen and Young 1988). Data being the key determinant of D1 dopamine receptor indicate that in the dopamine-intact striatum, there are few p- ERK1/2 immunoreactive neurons, whereas in the dopamine- supersensitivity in the dopamine-depleted striatum. depleted striatum, D1-agonist-induced p-ERK1/2 occurs selec- Comparison with D1-mediated induction of IEGs in tively in enkephalin-negative, direct striatal projection neurons. the dopamine-intact striatum demonstrates that in the dopamine-depleted striatum, there is a dramatic switch in the regulation of activation of ERK1/2 (Gerfen and others 2002). First, in the dopamine-intact striatum, with higher doses of SKF38393 or with a full D1 stimulation of the nigro-striatal dopamine pathway dopamine receptor agonist such as SKF81297 results in results in induction of c-fos in D1 receptor direct path- IEG induction in the dopamine-intact striatum. However, way neurons throughout the striatum and nucleus such treatments result in activation of ERK1/2 accumbens. However, such stimulation does not activate MAPKinase only in the dopamine-depleted striatum. ERK1/2, except in parts of the nucleus accumbens (Fig. Combining treatments of the full D1 agonist with D2 5). Second, in the unilateral striatal dopamine depletion agonist treatment and a muscarinic antagonist results in model, treatment with the partial D1 receptor agonist IEG induction in the direct projection neurons that is as SKF38393 (0.5–2 mg/kg) results in activation of robust in the dopamine-intact striatum as in the ERK1/2 and induction of IEGs only in the dopamine- dopamine-depleted striatum. However, again, activation depleted striatum, with no activation of ERK1/2 in the of ERK1/2 MAPKinase occurs only in the dopamine- dopamine-intact striatum. Treatment of such animals depleted striatum (Fig. 6). In addition, phosphorylation

Volume 9, Number 6, 2003 THE NEUROSCIENTIST 459 of c-jun also occurs only in the dopamine-depleted striatum and not in the dopamine-intact striatum. Interestingly, activation of ERK1/2 MAPKinase with these treatments does occur in parts of the nucleus accumbens. All of these studies indicate that experimen- tal paradigms that produce very robust IEG induction in dopamine-intact striatum do not involve activation of ERK1/2 MAPKinase, with the exception of parts of the nucleus accumbens. Therefore, it appears that dopamine depletion results in a switch in the protein kinase signaling mechanisms that regulate IEG induction, such that ERK1/2 MAPKinase, which is normally not involved in the dopamine-intact striatum, is abnormally activated following dopamine deafferentation. The results of the studies discussed above demon- Fig. 5. Electrical stimulation of the nigrostriatal pathway results strate that the induction of IEGs, such as c-fos, does not in the induction of the immediate early gene (IEG) c-fos throughout the striatum and nucleus accumbens, but activation distinguish the D1 dopamine response in the dopamine- of ERK1/2 occurs only in the nucleus accumbens. Electrodes depleted striatum from the dopamine-intact striatum. A were placed in the junction between dopamine neurons in the number of protein kinase signaling pathways converge ventral tegmental area (VTA) and substantia nigra pars com- on transcriptional regulation of c-fos, whose promoter pacta (SNc) and stimulated (A, E). In animals killed 45 min after contains serum response element (SRE), TPA responsive stimulation onset (A–D), the IEG c-fos is induced throughout the dorsal striatum and nucleus accumbens (B) Higher power element/activator protein complex-1 (TRE/AP-1), and photomicrographs reveal c-fos immunoreactive nuclei in the cyclic AMP response element (CRE) (Sheng and nucleus accumbens (C) and in the dorsal striatum (D). In ani- Greenberg 1990; Ghosh and Greenberg 1995; Karin mals killed 15 min after stimulation onset (E–H), the time point 1995; Montminy 1997; Gutkind 1998). ERK1/2/ that is optimal for detecting phosphorylated ERK1/2, immunoreactive neurons is observed only in the nucleus MAPKinase phosphorylates transcription factors bind to accumbens (F). Higher power photomicrographs reveal numer- the SRE site JNkinase phosphorylates c-jun, which ous immunoreactive neurons in the nucleus accumbens (G), binds to the TRE/AP-1 site, and both MAPKinase and whereas in the dorsal striatum, only scattered large immunore- PKA phosphorylates CREB, which binds to the CRE active neurons are observed (H) and not medium-sized projec- site. In the dopamine-intact striatum, D1 receptor sig- tion neurons. naling involves adenynlate cyclase coupled to PKA to regulate gene expression. A recent study has shown that way is involved in the dopamine-depleted striatum. inhibition of CREB blocks D1 dopamine receptor induc- Many questions remain. One is, What are the differences tion of c-fos in the dopamine-intact striatum but not in in these different forms of neuronal plasticity mediated the dopamine-depleted striatum (Andersson and others by the PKA and ERK1/2 MAPKinase pathways in direct 2001). This finding supports the idea that in the striatal projection neurons? dopamine-intact striatum, the PKA pathway is involved, Among the receptor-coupled protein kinase signaling through activation of CREB, in D1-mediated induction systems, the ERK1/2 MEK pathway is emerging as crit- of c-fos. Moreover, induction of IEGs in the dopamine- ical to activity-dependent enhancement of synaptic neu- intact striatum in response to D1 agonist receptor ago- rotransmission underlying learning and memory nists does not involve activation of ERK1/2 (Kornhauser and Greenberg 1997; Silva and others MAPKinase, whereas it does in the dopamine-depleted 1998; Impey and others 1999). Recent studies demon- striatum (Gerfen and others 2002). These results suggest strate that the protein kinase signaling pathways, includ- that the ERK1/2 MAPKinase signaling pathway is criti- ing PKA, ERK1/2 MAPKinase, and JNkinase, are nor- cal for the D1 dopamine receptor supersensitive mally differentially regulated in the direct and indirect response in the dopamine-depleted striatum. striatal projection neurons. Moreover, dopamine may function to inhibit activation of the ERK1/2 MAPKinase Functional Implications of D1 Dopamine signaling pathway. In indirect striatal neurons, in which Receptor Supersensitivity ERK1/2 MAPKinase is activated in response to stimulation of cortico-striatal afferent input, dopamine, acting Receptor-mediated activation of protein kinase path- through D2 dopamine receptors, appears to inhibit such ways, resulting in the activation of IEGs and late- activation. In the dopamine-intact striatum, the direct response genes, are thought to underlie alterations in the striatal pathway neurons do not appear to normally use response of neurons, which constitute neuronal plastici- the ERK1/2 MAPKinase signaling pathway. Whether ty. The main implication of the findings discussed above dopamine, acting on D1 dopamine receptors, is normal- is that D1 dopamine receptor–mediated neuronal plas- ly responsible for inhibiting activation of ERK1/2 ticity is distinct in the dopamine-intact compared with MAPKinase remains to be determined. What is clear the dopamine-depleted striatum. In the dopamine-intact from recent studies is that in the dopamine-depleted striatum, the PKA protein kinase pathway mediates neu- striatum, D1 dopamine receptor activation results in the ronal plasticity, whereas the ERK1/2 MAPKinase path- aberrant activation of the ERK1/2 MAPKinase signaling

460 THE NEUROSCIENTIST D1 Dopamine and Parkinson’s Disease normal regulation of the ERK1/2 MAPKinase signaling, by which it is normally restricted to the indirect striatal projection pathway. ERK1/2 MAPKinase activation appears to be an evolutionary conserved mechanism underlying learning and memory (Brambilla and others 1997; Atkins and others 1998; Blum and others 1999). A reasonable speculation follows that the learning and memory of habitual movements that is attributed to the basal ganglia (Graybiel and others 1994; Knowlton and others 1996) involves activation of ERK1/2 MAPKinase in the indirect striatal projection pathway. Depletion of dopamine in the striatum results in the aberrant activation of ERK1/2 MAPKinase by dopamine agonist treatments that activate the D1 dopamine receptor in direct striatal pathway neurons. In the treatment of Parkinson’s disease, L-DOPA is effective at reversing bradykinesia Fig. 6. Demonstration of distinct mechanisms of D1 dopamine in the short term, but long-term treatment invariably receptor–mediated gene regulation in the dopamine (DA)-intact leads to the development of uncontrolled dyskinetic and -depleted striatum, using the full D1 agonist SKF81297, movements (Bergmann and others 1987). We propose alone or combined with other drugs. (A–D) In situ hybridization histochemical localization of mRNA encoding c-fos 45 min fol- that the development of dyskinesias results from the lowing different drug combinations: A) SKF81297 (0.5 mg/kg), repeated aberrant activation of ERK1/2 MAPKinase in B) SKF81297 (2.0 mg/kg), C) SKF81297 (2.0 mg/kg) combined direct striatal pathway neurons in response to L-DOPA with the muscarinic receptor agonist scopolamine (5mg/kg), or activation of the D1 dopamine receptor. These recent D) SKF81297 (2.0 mg/kg) combined with the D2 dopamine receptor agonist (1 mg/kg) and scopolamine. The low dose of studies suggest that inhibitors of MEK, which blocks the agonist alone (A) demonstrates the supersensitive response by aberrant supersensitive response of direct striatal pathway the selective induction of c-fos in the dopamine-deleted stria- neurons to D1 dopamine receptor agonists, may provide a tum. Bilateral induction of c-fos IEG in both the dopamine- novel therapeutic adjunct to the use of L-DOPA in the treat- intact and dopamine-depleted striatum follows treatment with ment of Parkinson’s disease. the high dose of the full D1 agonist alone (B) or in combination with other drugs (C, D). However, when animals receiving any of these treatments are killed at 15 min, p-ERK1/2 immunoreac- References tive neurons are evident only in the dopamine-depleted striatum and not in the dopamine-intact striatum (not shown). The Albin RL, Young AB, Penney JB. 1989. The functional anatomy of treatment combining the full D1 agonist with both the D2 ago- basal ganglia disorders. Trends Neurosci 12:366–75. nist and scopolamine produces the most robust c-fos IEG Alessi DR, Cuenda A, Cohen P, Dudley DT, Saltiel AR. 1995. PD response in the dopamine-intact striatum at 45 min (E ). This 098059 is a specific inhibitor of the activation of mitogen-activat- treatment also results in persistent p-ERK1/2 (H) and phos- ed protein kinase kinase in vitro and in vivo. J Biol Chem phorylated-c-jun (J) in the dopamine-depleted striatum but 270:27489–94. does not activate p-ERK1/2 (G) or phosphorylate-c-jun (I) in Andersson M, Konradi C, Cenci MA. 2001. cAMP response element- neurons in the dopamine-intact striatum. These results demon- binding protein is required for dopamine-dependent gene expres- strate that although D1 dopamine receptor–mediated induction sion in the intact but not the dopamine-denervated striatum. 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462 THE NEUROSCIENTIST D1 Dopamine and Parkinson’s Disease

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