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Effects of MK-801 on D1 dopamine receptor-mediated immediate early gene expression in the dopamine-depleted striatum
Brain Research 871 (2000) 156–159 www.elsevier.com / locate / bres
Short communication
Effects of MK-801 on D1 dopamine receptor-mediated immediate early gene expression in the dopamine-depleted striatum Anindita Ganguly, Kristen A. Keefe* Department of Pharmacology and Toxicology, University of Utah, 30 South, 2000 East [201, Salt Lake City, UT 84112 -5820, USA Accepted 18 April 2000
Abstract Previous work indicates that intrastriatal administration of MK-801 does not completely block D1 agonist-induced gene expression in dopamine-depleted rats. The present study examined the effects of systemic MK-801 on such gene expression. A low dose of MK-801 did not affect induction of c-fos or zif268. A high dose completely blocked induction of c-fos, but only slightly suppressed zif268. The data suggest that NMDA receptor activity may not always be necessary for D1-induced gene expression. 2000 Elsevier Science B.V. All rights reserved. Theme: Motor systems and sensorimotor integration Topic: Basal ganglia Keywords: NMDA; Basal ganglia; zif268 ; c-fos; Parkinson; Glutamate
The basal ganglia are subcortical structures, important for motor and cognitive functions. The striatum is the main input nucleus of the basal ganglia, receiving dopamine (DA) input from substantia nigra pars compacta and glutamate input from the cortex and thalamus [4,7,12– 14,19,24]. These inputs synapse onto spiny efferent neurons, and are anatomically positioned to interact with each other. Striatal efferent neurons regulate the output of the basal ganglia via striatonigral and striatopallidal pathways, which express the D1 and D2 subtypes of DA receptors, respectively [5,6,17,18]. Numerous findings provide support for interactions between DA and N-methyl-D-aspartate (NMDA) receptormediated processes in the regulation of striatal neuron function. For example, amphetamine-induced immediate early gene (IEG) expression in the striata of intact animals requires activation of D1 receptors [2,20,26]. This D1 receptor-mediated IEG expression is also dependent on glutamate input via NMDA receptors, as MK-801 blocks it *Corresponding author. Tel.: 11-801-585-7989; fax: 11-801-5855111. E-mail address: [email protected] (K.A. Keefe)
completely [16,27]. Our data also indicate that induction of c-fos and zif268 in the intact striatum by the full D1 receptor agonist SKF 82958 is blocked by antagonists of NR2A subunit-containing NMDA receptors [10]. Together, these data indicate that D1-mediated IEG expression is fully dependent on ongoing NMDA receptor activity in the intact striatum. Data from studies conducted on DA-depleted animals, however, suggest that D1 receptor-mediated IEG expression is not as dependent on NMDA receptors. Intrastriatal infusion of NMDA receptor antagonists has no effect on induction of zif268 in the DA-depleted striatum by the partial D1 agonist SKF38393 and only partially (20–40%) attenuates c-fos expression [11]. Dopamine depletion therefore seems to alter the involvement of NMDA receptors in D1 receptor-mediated regulation of striatonigral neurons. However, in the studies on intact rats reviewed above, the NMDA receptor antagonists were administered systemically, whereas in the study on DA-depleted rats, the antagonists were administered intrastriatally. Furthermore different types of DA agonists were used in the studies described above. Thus, the route of administration or the type of drug used could account for the discrepant observa-
0006-8993 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0006-8993( 00 )02435-5
A. Ganguly, K. A. Keefe / Brain Research 871 (2000) 156 – 159
tions obtained from intact versus lesioned rats. Therefore, to evaluate the hypothesis that D1 DA receptor-mediated IEG expression is independent of NMDA receptors in DA-depleted animals, we examined the effects of systemic NMDA receptor blockade on the induction of IEGs in 6-hydroxydopamine (6-OHDA)-treated rats by systemic administration of the full D1 agonist SKF82958. Male Sprague–Dawley rats (Charles River, Wilmington, MA, USA) weighing 225–250 g were used in all experiments. Rats were housed in groups of four in hanging wire-mesh cages in a temperature-controlled room on a 12:12 light:dark cycle. Rats had free access to food and water. All animal care and experimental manipulations were approved by the Institutional Animal Care and Use Committee of the University of Utah and were in accordance with the NIH Guide for the Care and Use of Laboratory Animals. All drugs were obtained from Research Biochemicals International (Natick, MA, USA). The doses of SKF82958 and MK-801 were calculated as the salt, whereas the doses of desipramine HCl and 6-OHDA were calculated as the free base. Rats were anesthetized with sodium pentobarbital (50 mg / kg, i.p.) and then pre-treated with desipramine (25 mg / kg, i.p.) 30 min prior to infusion of 6-OHDA. A 29-gauge needle connected to a 25-ml Hamilton syringe on a syringe pump was lowered into the right medial forebrain bundle at the following coordinates relative to bregma and the skull (mm): AP 24.0, ML 11.5, DV 28.5. An aliquot of 2 ml of 6-OHDA (8 mg) in 0.02% ascorbic acid–0.9% saline was infused over 5 min. The canula was left in place for 5 min after the infusion. Experiments were performed 3 weeks post lesion. On the day of the experiment, the rats were rehoused in plastic tub cages (4–5 per cage) and transferred to the laboratory. The rats were weighed and then injected with either MK801 (0.01 or 1 mg / kg, i.p.) or the vehicle (deionized water). Fifteen min later, each rat received an injection of SKF82958 (1 mg / kg, i.p.) or the vehicle (deionized water). Control rats received two vehicle injections. One hour after the second injection, the rats were euthanized by exposure to CO 2 for 1 min and then decapitated. The brains were rapidly removed and frozen in isopentane chilled on dry ice. The brains were stored at 2208C until they were cut in 12-mm sections in a cryostat (Cryocut 1800, Cambridge Instruments, Germany). Subsequently, slides from all animals were postfixed and processed for in situ hybridization for c-fos and zif268 as previously described [10]. The extent of DA depletion was determined by measuring preproenkephalin (PPE) mRNA in striatum and tyrosine hydroxylase (TH) mRNA in substantia nigra. In the case of PPE, an antisense ribonucleotide probe complementary to bases 51–987 (courtesy of Dr. C.R. Gerfen) was synthesized using 35 S-UTP and SP6 RNA polymerase. For the detection of TH mRNA, a 48-base oligonucleotide probe complementary to bases 1441–1488 was synthesized by the DNA / peptide synthesis
157
facility at the University of Utah. The purified probe was end-labeled with 35 S-dATP and terminal deoxynucleotidyl transferase (Boehringer Mannheim, Indianapolis, IN, USA). All data were analyzed using the image analysis program Image (NIH) as previously described [10]. Images of sections from all groups in the experiment were captured and measured under constant lighting and camera conditions. Measurements were made over medial and lateral thirds of the DA-depleted striatum. Data from the densitometric analysis of film autoradiograms were analyzed with a one-way analysis of variance for both medial and lateral striatum. Post-hoc analysis was performed with the Tukey–Kramer test. Paired Student’s t-tests were used to compare the effects of DA-depletion on PPE and TH mRNA in the ipsilateral versus contralateral striata and substantia nigra, respectively, of the 6-OHDA-treated animals. Statistical significance was set at P,0.05. All animals chosen for analysis had a greater than 90% loss of DA, as reflected by a significant reduction (.90%, P,0.0001) in TH mRNA in the ipsilateral substantia nigra and a significant increase (31%, P,0.0001) in PPE mRNA in the lesioned striatum compared to the contralateral side [21]. Systemic administration of SKF82958 (1 mg / kg, i.p.) increased the expression of both c-fos and zif268 in the medial and lateral striatum (Fig. 1). Administration of the low dose of MK-801 (0.01 mg / kg, i.p.) had no effect on the D1-induced increases (Fig. 1a and b). Administration of the high dose of MK-801 (1.0 mg / kg, i.p.) completely blocked the D1-induced increase in c-fos expression (Fig. 1a). However, this high dose only slightly suppressed the D1 agonist-induced increase in zif268 expression (Fig. 1b). We have shown that systemic NMDA receptor blockade does not fully block D1 agonist-induced IEG expression in the DA-depleted striatum. A low dose of MK-801 had no effect on either D1 agonist-induced c-fos or zif268 expression. A high dose of MK-801, on the other hand, markedly reduced c-fos expression, but only very slightly attenuated zif268 expression. These data stand in marked contrast to our previous work in intact rats [10], as well as other reports in the literature [16,27], demonstrating that these doses of MK-801 block D1 receptor-induced increases in both c-fos and zif268 expression. Taken together, these data indicate that the involvement of NMDA receptors in D1 agonist-induced gene expression is altered after DAdepleting brain lesions, especially in the case of zif268 expression. Increased levels of activated protein kinase A (PKA) may be responsible for the lack of effect of NMDA receptor blockade on D1-mediated IEG expression in the DA-depleted striatum. Konradi et al. [22] have postulated that under low levels of activation of PKA, increased Ca 11 influx through the NMDA receptor is primarily responsible for mediating gene expression. On the other hand, high levels of activation of PKA are thought to be
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A. Ganguly, K. A. Keefe / Brain Research 871 (2000) 156 – 159
Fig. 1. Effects of systemic administration of MK-801 (‘MK’; 0.01 mg / kg, i.p.; 1.0 mg / kg, i.p.) on c-fos (a) and zif268 (b) expression induced in the DA-depleted striatum by systemic administration of the D1 DA receptor agonist SKF82958 (‘SKF’; 1.0 mg / kg, i.p.). MK-801 was injected 15 min before the injection of SKF82958. The control rats were given two injections of the vehicle 15 min apart. The rats treated with MK-801 alone received MK-801 followed 15 min later by the vehicle. Rats receiving SKF82958 alone received the vehicle followed 15 min later by an injection of SKF82958. All rats were sacrificed 1 h after the second injection. The graphs show average gray values (6S.E.M.; arbitrary units) in medial and lateral thirds of the mid-striatum obtained from densitometric analysis of film autoradiograms. The numbers in parentheses indicate the number of animals in each group. *P,0.05 vs. control; 1P,0.05 vs. SKF82958.
capable of increasing transcription factor activity independent of NMDA receptors. We think that the same phenomenon occurs in the DA-depleted striatum, since D1 DA receptors are supersensitive under such conditions [3,25]. Presumably, the dose of D1 agonist used produced more pronounced activation of PKA in the DA-depleted animals, overshadowing the contribution of Ca 11 influx through NMDA receptors. Other studies from our lab also show differential involvement of NMDA receptors in D2 DA receptor antagonist-induced gene expression depending on the degree of gene induction [1,9]. Further studies examining the second messenger pathways involved in D1-induced IEG expression in the DA-depleted striatum should help clarify the basis for the differential involvement of NMDA receptors in D1 receptor-mediated changes in striatal gene expression. Our previous [11] and present data also demonstrate a differential regulation by MK-801 of D1-mediated c-fos and zif268 expression. Several different signal transduction pathways either independently or via cross-talk with other signaling pathways can lead to the induction of these two genes [15,22,28]. Furthermore, within the promoter regions of these genes, several regulatory elements have been defined that are important for the transcriptional response, including the serum response element and the calcium / cAMP response element [8,15,23,28]. It is therefore likely that different mechanisms are involved in the induction of c-fos and zif268 by D1 receptor activation. The present data suggest that the pathways involved in zif268 expression are more significantly altered as a consequence of the
DA depletion. Further experiments to delineate the alterations occurring in these pathways as a consequence of DA depletion may provide a greater insight into the basis of symptoms associated with DA-depleting disorders such as Parkinson’s disease. In conclusion, our data indicate that in the DA-depleted striatum, the involvement of NMDA receptors in D1 DA receptor-mediated regulation of striatonigral neuron function is altered. Further studies are warranted to understand the signal transduction pathways involved and the alterations occurring in these pathways as a consequence of DA depletion.
References [1] A.C. Adams, K.A. Keefe, Zif268 expression induced by a low dose of eticlopride is dependent on NMDA receptors, Soc. Nsci. Abstr. 25 (1999) 1653. [2] S. Berretta, H.A. Robertson, A.M. Graybiel, Dopamine and glutamate agonists stimulate neuron-specific expression of fos-like protein in the striatum, J. Neurophys. 68 (1992) 767–777. [3] D.G. Cole, L.A. Kobierski, C. Konradi, S.E. Hyman, 6-Hydroxydopamine lesions of rat substantia nigra up-regulate dopamineinduced phosphorylation of the cAMP-response element-binding protein in striatal neurons, P.N.A.S., USA 91 (1994) 9631–9635. [4] L. Dube, A.D. Smith, J.P. Bolam, Identification of synaptic terminals of thalamic or cortical origin in contact with distinct medium-size spiny neurons in the rat neostriatum, J. Comp. Neurol. 267 (1988) 455–471. [5] C.R. Gerfen, W.S. Young, Distribution of striatonigral and striatopallidal peptidergic neurons in both patch and matrix compartments: an
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[17] C. Le Moine, E. Normand, A.F. Guitteny, B. Fouque, R. Teoule, B. Bloch, Dopamine receptor gene expression by enkephalin neurons in rat forebrain, P.N.A.S., USA 87 (1990) 230–234. [18] C. Le Moine, E. Normand, B. Bloch, Phenotypical characterization of the rat striatal neurons expressing the D1 dopamine receptor gene, P.N.A.S., USA 88 (1991) 4205–4209. [19] P.L. McGeer, E.G. McGeer, U. Scherer, K. Singh, A glutamatergic corticostriatal path?, Brain Res. 128 (1977) 369–373. [20] T.V. Nguyen, B.E. Kosofsky, R. Birnbaum, B.M. Cohen, S.E. Hyman, Differential expression of c-fos and zif268 in rat striatum after haloperidol, clozapine and amphetamine, P.N.A.S., USA 89 (1992) 4270–4274. [21] L.K. Nisenbaum, W.R. Crowley, S.T. Kitai, Partial striatal dopamine depletion differentially affects striatal substance P and enkephalin messenger RNA expression, Mol. Brain Res. 37 (1996) 209–216. [22] A. Rajadhyaksha, J.C. Leveque, W. Macias, A. Barczak, C. Konradi, Molecular components of striatal plasticity, Dev. Neurosci. 20 (1998) 204–215. [23] V. Sgambato, C. Pages, M. Rogard, M.J. Besson, J. Caboche, Extracellular signal-regulated kinase (ERK) controls immediate early gene induction on corticostriatal stimulation, J. Neurosci. 18 (1998) 8814–8825. [24] Y. Smith, B.D. Bennett, J.P. Bolam, A. Parent, A.F. Sadikot, Synaptic relationships between dopaminergic afferents and cortical or thalamic input in the sensorimotor territory of the striatum in monkey, J. Comp. Neurol. 344 (1994) 1–19. [25] J.M. Trugman, C.L. James, D1 dopamine agonist and antagonist effects on regional cerebral glucose utilization in rats with intact dopaminergic innervation, Brain Res. 607 (1993) 270–274. [26] J.Q. Wang, J.F. McGinty, Differential effects of D1 and D2 dopamine receptor antagonists on acute amphetamine- or methamphetamine-induced up-regulation of zif268 mRNA expression in rat forebrain, J. Neurochem. 65 (1995) 2706–2715. [27] J.Q. Wang, J.B. Daunais, J.F. McGinty, NMDA receptors mediate amphetamine-induced upregulation of zif/268 and preprodynorphin mRNA expression in rat striatum, Synapse 18 (1994) 343–353. [28] Z. Xia, H. Dudek, C.K. Miranti, M.E. Greenberg, Calcium influx via the NMDA receptor induces immediate early gene transcription by a MAP kinase / ERK-dependent mechanism, J. Neurosci. 16 (1996) 5425–5436.
Short communication
Effects of MK-801 on D1 dopamine receptor-mediated immediate early gene expression in the dopamine-depleted striatum Anindita Ganguly, Kristen A. Keefe* Department of Pharmacology and Toxicology, University of Utah, 30 South, 2000 East [201, Salt Lake City, UT 84112 -5820, USA Accepted 18 April 2000
Abstract Previous work indicates that intrastriatal administration of MK-801 does not completely block D1 agonist-induced gene expression in dopamine-depleted rats. The present study examined the effects of systemic MK-801 on such gene expression. A low dose of MK-801 did not affect induction of c-fos or zif268. A high dose completely blocked induction of c-fos, but only slightly suppressed zif268. The data suggest that NMDA receptor activity may not always be necessary for D1-induced gene expression. 2000 Elsevier Science B.V. All rights reserved. Theme: Motor systems and sensorimotor integration Topic: Basal ganglia Keywords: NMDA; Basal ganglia; zif268 ; c-fos; Parkinson; Glutamate
The basal ganglia are subcortical structures, important for motor and cognitive functions. The striatum is the main input nucleus of the basal ganglia, receiving dopamine (DA) input from substantia nigra pars compacta and glutamate input from the cortex and thalamus [4,7,12– 14,19,24]. These inputs synapse onto spiny efferent neurons, and are anatomically positioned to interact with each other. Striatal efferent neurons regulate the output of the basal ganglia via striatonigral and striatopallidal pathways, which express the D1 and D2 subtypes of DA receptors, respectively [5,6,17,18]. Numerous findings provide support for interactions between DA and N-methyl-D-aspartate (NMDA) receptormediated processes in the regulation of striatal neuron function. For example, amphetamine-induced immediate early gene (IEG) expression in the striata of intact animals requires activation of D1 receptors [2,20,26]. This D1 receptor-mediated IEG expression is also dependent on glutamate input via NMDA receptors, as MK-801 blocks it *Corresponding author. Tel.: 11-801-585-7989; fax: 11-801-5855111. E-mail address: [email protected] (K.A. Keefe)
completely [16,27]. Our data also indicate that induction of c-fos and zif268 in the intact striatum by the full D1 receptor agonist SKF 82958 is blocked by antagonists of NR2A subunit-containing NMDA receptors [10]. Together, these data indicate that D1-mediated IEG expression is fully dependent on ongoing NMDA receptor activity in the intact striatum. Data from studies conducted on DA-depleted animals, however, suggest that D1 receptor-mediated IEG expression is not as dependent on NMDA receptors. Intrastriatal infusion of NMDA receptor antagonists has no effect on induction of zif268 in the DA-depleted striatum by the partial D1 agonist SKF38393 and only partially (20–40%) attenuates c-fos expression [11]. Dopamine depletion therefore seems to alter the involvement of NMDA receptors in D1 receptor-mediated regulation of striatonigral neurons. However, in the studies on intact rats reviewed above, the NMDA receptor antagonists were administered systemically, whereas in the study on DA-depleted rats, the antagonists were administered intrastriatally. Furthermore different types of DA agonists were used in the studies described above. Thus, the route of administration or the type of drug used could account for the discrepant observa-
0006-8993 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0006-8993( 00 )02435-5
A. Ganguly, K. A. Keefe / Brain Research 871 (2000) 156 – 159
tions obtained from intact versus lesioned rats. Therefore, to evaluate the hypothesis that D1 DA receptor-mediated IEG expression is independent of NMDA receptors in DA-depleted animals, we examined the effects of systemic NMDA receptor blockade on the induction of IEGs in 6-hydroxydopamine (6-OHDA)-treated rats by systemic administration of the full D1 agonist SKF82958. Male Sprague–Dawley rats (Charles River, Wilmington, MA, USA) weighing 225–250 g were used in all experiments. Rats were housed in groups of four in hanging wire-mesh cages in a temperature-controlled room on a 12:12 light:dark cycle. Rats had free access to food and water. All animal care and experimental manipulations were approved by the Institutional Animal Care and Use Committee of the University of Utah and were in accordance with the NIH Guide for the Care and Use of Laboratory Animals. All drugs were obtained from Research Biochemicals International (Natick, MA, USA). The doses of SKF82958 and MK-801 were calculated as the salt, whereas the doses of desipramine HCl and 6-OHDA were calculated as the free base. Rats were anesthetized with sodium pentobarbital (50 mg / kg, i.p.) and then pre-treated with desipramine (25 mg / kg, i.p.) 30 min prior to infusion of 6-OHDA. A 29-gauge needle connected to a 25-ml Hamilton syringe on a syringe pump was lowered into the right medial forebrain bundle at the following coordinates relative to bregma and the skull (mm): AP 24.0, ML 11.5, DV 28.5. An aliquot of 2 ml of 6-OHDA (8 mg) in 0.02% ascorbic acid–0.9% saline was infused over 5 min. The canula was left in place for 5 min after the infusion. Experiments were performed 3 weeks post lesion. On the day of the experiment, the rats were rehoused in plastic tub cages (4–5 per cage) and transferred to the laboratory. The rats were weighed and then injected with either MK801 (0.01 or 1 mg / kg, i.p.) or the vehicle (deionized water). Fifteen min later, each rat received an injection of SKF82958 (1 mg / kg, i.p.) or the vehicle (deionized water). Control rats received two vehicle injections. One hour after the second injection, the rats were euthanized by exposure to CO 2 for 1 min and then decapitated. The brains were rapidly removed and frozen in isopentane chilled on dry ice. The brains were stored at 2208C until they were cut in 12-mm sections in a cryostat (Cryocut 1800, Cambridge Instruments, Germany). Subsequently, slides from all animals were postfixed and processed for in situ hybridization for c-fos and zif268 as previously described [10]. The extent of DA depletion was determined by measuring preproenkephalin (PPE) mRNA in striatum and tyrosine hydroxylase (TH) mRNA in substantia nigra. In the case of PPE, an antisense ribonucleotide probe complementary to bases 51–987 (courtesy of Dr. C.R. Gerfen) was synthesized using 35 S-UTP and SP6 RNA polymerase. For the detection of TH mRNA, a 48-base oligonucleotide probe complementary to bases 1441–1488 was synthesized by the DNA / peptide synthesis
157
facility at the University of Utah. The purified probe was end-labeled with 35 S-dATP and terminal deoxynucleotidyl transferase (Boehringer Mannheim, Indianapolis, IN, USA). All data were analyzed using the image analysis program Image (NIH) as previously described [10]. Images of sections from all groups in the experiment were captured and measured under constant lighting and camera conditions. Measurements were made over medial and lateral thirds of the DA-depleted striatum. Data from the densitometric analysis of film autoradiograms were analyzed with a one-way analysis of variance for both medial and lateral striatum. Post-hoc analysis was performed with the Tukey–Kramer test. Paired Student’s t-tests were used to compare the effects of DA-depletion on PPE and TH mRNA in the ipsilateral versus contralateral striata and substantia nigra, respectively, of the 6-OHDA-treated animals. Statistical significance was set at P,0.05. All animals chosen for analysis had a greater than 90% loss of DA, as reflected by a significant reduction (.90%, P,0.0001) in TH mRNA in the ipsilateral substantia nigra and a significant increase (31%, P,0.0001) in PPE mRNA in the lesioned striatum compared to the contralateral side [21]. Systemic administration of SKF82958 (1 mg / kg, i.p.) increased the expression of both c-fos and zif268 in the medial and lateral striatum (Fig. 1). Administration of the low dose of MK-801 (0.01 mg / kg, i.p.) had no effect on the D1-induced increases (Fig. 1a and b). Administration of the high dose of MK-801 (1.0 mg / kg, i.p.) completely blocked the D1-induced increase in c-fos expression (Fig. 1a). However, this high dose only slightly suppressed the D1 agonist-induced increase in zif268 expression (Fig. 1b). We have shown that systemic NMDA receptor blockade does not fully block D1 agonist-induced IEG expression in the DA-depleted striatum. A low dose of MK-801 had no effect on either D1 agonist-induced c-fos or zif268 expression. A high dose of MK-801, on the other hand, markedly reduced c-fos expression, but only very slightly attenuated zif268 expression. These data stand in marked contrast to our previous work in intact rats [10], as well as other reports in the literature [16,27], demonstrating that these doses of MK-801 block D1 receptor-induced increases in both c-fos and zif268 expression. Taken together, these data indicate that the involvement of NMDA receptors in D1 agonist-induced gene expression is altered after DAdepleting brain lesions, especially in the case of zif268 expression. Increased levels of activated protein kinase A (PKA) may be responsible for the lack of effect of NMDA receptor blockade on D1-mediated IEG expression in the DA-depleted striatum. Konradi et al. [22] have postulated that under low levels of activation of PKA, increased Ca 11 influx through the NMDA receptor is primarily responsible for mediating gene expression. On the other hand, high levels of activation of PKA are thought to be
158
A. Ganguly, K. A. Keefe / Brain Research 871 (2000) 156 – 159
Fig. 1. Effects of systemic administration of MK-801 (‘MK’; 0.01 mg / kg, i.p.; 1.0 mg / kg, i.p.) on c-fos (a) and zif268 (b) expression induced in the DA-depleted striatum by systemic administration of the D1 DA receptor agonist SKF82958 (‘SKF’; 1.0 mg / kg, i.p.). MK-801 was injected 15 min before the injection of SKF82958. The control rats were given two injections of the vehicle 15 min apart. The rats treated with MK-801 alone received MK-801 followed 15 min later by the vehicle. Rats receiving SKF82958 alone received the vehicle followed 15 min later by an injection of SKF82958. All rats were sacrificed 1 h after the second injection. The graphs show average gray values (6S.E.M.; arbitrary units) in medial and lateral thirds of the mid-striatum obtained from densitometric analysis of film autoradiograms. The numbers in parentheses indicate the number of animals in each group. *P,0.05 vs. control; 1P,0.05 vs. SKF82958.
capable of increasing transcription factor activity independent of NMDA receptors. We think that the same phenomenon occurs in the DA-depleted striatum, since D1 DA receptors are supersensitive under such conditions [3,25]. Presumably, the dose of D1 agonist used produced more pronounced activation of PKA in the DA-depleted animals, overshadowing the contribution of Ca 11 influx through NMDA receptors. Other studies from our lab also show differential involvement of NMDA receptors in D2 DA receptor antagonist-induced gene expression depending on the degree of gene induction [1,9]. Further studies examining the second messenger pathways involved in D1-induced IEG expression in the DA-depleted striatum should help clarify the basis for the differential involvement of NMDA receptors in D1 receptor-mediated changes in striatal gene expression. Our previous [11] and present data also demonstrate a differential regulation by MK-801 of D1-mediated c-fos and zif268 expression. Several different signal transduction pathways either independently or via cross-talk with other signaling pathways can lead to the induction of these two genes [15,22,28]. Furthermore, within the promoter regions of these genes, several regulatory elements have been defined that are important for the transcriptional response, including the serum response element and the calcium / cAMP response element [8,15,23,28]. It is therefore likely that different mechanisms are involved in the induction of c-fos and zif268 by D1 receptor activation. The present data suggest that the pathways involved in zif268 expression are more significantly altered as a consequence of the
DA depletion. Further experiments to delineate the alterations occurring in these pathways as a consequence of DA depletion may provide a greater insight into the basis of symptoms associated with DA-depleting disorders such as Parkinson’s disease. In conclusion, our data indicate that in the DA-depleted striatum, the involvement of NMDA receptors in D1 DA receptor-mediated regulation of striatonigral neuron function is altered. Further studies are warranted to understand the signal transduction pathways involved and the alterations occurring in these pathways as a consequence of DA depletion.
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