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The role of striatal adenosine A2A receptors in regulation of the muscle tone in rats
Neuroscience Letters 276 (1999) 79±82 www.elsevier.com/locate/neulet
The role of striatal adenosine A2A receptors in regulation of the muscle tone in rats Jadwiga Wardas*, Jolanta Konieczny, ElzÇbieta Lorenc-Koci Department of NeuroPsychopharmacology, Institute of Pharmacology, Polish Academy of Sciences, 12 SmeËtna Street, 31-343 KrakoÂw, Poland Received 12 August 1999; received in revised form 23 September 1999; accepted 24 September 1999
Abstract The aim of the present study was to assess contribution of striatal adenosine A2A receptors to regulation of the muscle tone in rats. The muscle tone was examined by a combined mechano- and electromyographic method, which measured simultaneously muscle resistance (MMG) of a rats hind foot to passive extension and ¯exion in the ankle joint and the electromyographic activity (EMG) of the antagonistic muscles: gastrocnemius and tibialis anterior. CGS 21680 (1 and 2 mg/0.5 ml), injected bilaterally into the rostral part of the striatum, dose-dependently increased both MMG and the EMG. The present results show that stimulation of striatal adenosine A2A receptors by CGS 21680 evokes parkinsonian-like muscle rigidity which may be due to activation of the GABAergic strio-pallidal pathway. q 1999 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Adenosine A2A receptors; Striatum; Muscle rigidity; Electromyogram; Mechanomyogram
Adenosine A2A receptors that are positively coupled to adenylate cyclase are densely and selectively located in such dopamine-innervated areas as the striatum, nucleus accumbens and olfactory tubercle [2]. Recently it has been shown that in experimental animals adenosine A2A agonists have behavioural effects similar to those produced by neuroleptics. Systemic administration of adenosine A2A agonists to rodents induces pronounced depression of the spontaneous and dopamine agonists-induced motor activity [5,6,12]. Moreover, when used in high doses, A2A adenosine receptor agonists produce catalepsy after both systemic and intracerebral injection [3,5,6]. Since Ferre et al. [2] have shown that stimulation of adenosine A2A receptors decreases the af®nity of dopamine D2 receptors for dopamine agonists and impairs the D2 signal transduction in rat striatum, it has been suggested that a major mechanism responsible for the adenosine A2A agonists-induced counteraction of dopamine neurotransmission is the existence of a speci®c antagonistic interaction between A2A - and D2 receptors in the striatum. It is well known that in the striatum A2A receptors are selectively co-located with dopamine D2 receptors on GABAergic neurons projecting to the globus pallidus [2]. * Corresponding author. Tel.: 148-12-637-4022; fax: 148-12637-4500. E-mail address: [email protected] (J. Wardas)
Overactivity of this pathway has been postulated to produce such extrapyramidal effects as catalepsy and muscle rigidity. Previous studies showed that the blockade of dopamine D2 receptors by neuroleptics injected into the rostral part of the striatum induced potent catalepsy and muscle rigidity [1,13]. Furthermore, Hauber and MuÈnkle [5] found that stimulation of adenosine A2A receptors by CGS 21680 injected into the anterodorsal striatum induced catalepsy. Therefore the aim of the present study was to ®nd out whether CGS 21680, a selective adenosine A2A receptor agonist, injected directly into the ventro-lateral part of the rostral striatum affected the muscle tone in rats, measured by a combined mechano-electromyographic method (MMG/EMG) [9]. The experiment was carried out on male Wistar rats (260± 350 g), implanted bilaterally with guide cannulae (0.4 mm o.d.) in the rostral part of the striatum (A 9650±8920, L 1.6±2.4, H 21.6 to 10.8) (according to Ref. [8]) under pentobarbital anaesthesia (Vetbutal, 30 mg/kg i.p.; Biowet, Poland). Mechano- and electromyographic experiments were conducted 1 week after stereotaxic surgery. As described in detail previously [9], a conscious rat was placed in a metaplex cage with its hind foot protruding from an opening at the bottom of the cage. Two pairs of ¯exible stainless steel wire electrodes (Cooner Wire, Chatsworth, CA,
0304-3940/99/$ - see front matter q 1999 Elsevier Science Ireland Ltd. All rights reserved. PII: S03 04 - 394 0( 9 9) 00 77 9- X
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J. Wardas et al. / Neuroscience Letters 276 (1999) 79±82
USA), which were te¯on-insulated (external diameter 0.1 mm), were inserted percutaneously into the gastrocnemius and tibialis anterior muscles. The experiment involved succesive passive extension and ¯exion (30-s apart) of the rats foot in the ankle joint by 258 with a velocity of 100 deg/ s. The maximum resistance (torque (gcm); in comparison with the premovement value) of hind leg muscles for each movement was determined. The EMG activity of both muscles was recti®ed and averaged with a time constant of 20 ms for each movement. The following parameters were estimated for each movement: (1) EMG baseline ± the mean of a premovement amplitude as a value of the resting EMG activity; (2) components computed as differences between the maximum amplitude of the averaged EMG curves at four time periods after the start of a movement and the EMG baseline: EMG-A (0±20 ms), EMG-B (20±40 ms), EMG-C (60±140 ms) and EMG-D (220±320 ms). Cycles disturbed by voluntary movements of an animal were discarded. CGS 21680-HCl (RBI) was dissolved in redistilled water and the pH was balanced with 0.1 N NaOH to a value of 6± 7. CGS 21680-HCl was injected bilaterally in doses of 1 and 2 mg/0.5 ml /side into the ventro-lateral part of the rostral striatum within 2 min via an inner cannula (0.3 mm o.d.) which protruded by 0.7 mm from the guide cannula. Control rats were injected with 0.5 ml of the appropriate vehicle instead of CGS 21680. The measurement of the muscle tone started 30 min before intrastriatal injections of CGS 21680 or vehicle and lasted 60 min after the injection. After the experiment, the placement of the cannulae was checked histologically. Only rats with cannulae implanted properly on both sides in the rostral part of the striatum were subjected to a statistical analysis. Statistical analyse of MMGmax values was carried out using the means obtained from all the correct cycles calculated every 10 min (MMG), as well as the means calculated for the whole experimental session (60 min after CGS 21680 injection, EMG). The statistical signi®cance (P , 0:05) of differences was estimated using the Kruskal±Wallis and Wilcoxon tests. Bilateral injection of CGS 21680 in a dose of 1 mg/0.5 ml (1.86 nmol) into the rostral striatum (Fig. 1) did not change the muscle resistance to passive ¯exion; however, a nonsigni®cant increasing tendency was observed during passive extension of a rats hind foot (Fig. 2). Injection of CGS 21680 in a dose of 2 mg/0.5 ml (3.73 nmol) into the rostral striatum enhanced the muscle resistance developed in response to passive extension of a hind foot in the ankle joint (Fig. 2). Such an effect was observed during the ®rst 10 min after intrastriatal injection and lasted until the end of measurement. Control rats, that were injected with the vehicle did not show any changes in the muscle resistance (Fig. 2). Injection of CGS 21680 in a higher dose of 2 mg/0.5 ml into the rostral striatum potently affected the EMG activity in both the gastrocnemius and tibialis anterior muscles,
Fig. 1. Localization of cannula tips in frontal sections of rat brain. Filled circles: cannula tips located in the rostral part of the striatum; each circle denotes a cannula placed in one animal on one side. CP, caudate-putamen; NA, nucleus accumbens; A, anterior plane, according to KoÈnig and Klippel [8].
mainly during extension of the hind foot (Fig. 3). That dose of CGS 21680 produced a statistically signi®cant (P , 0:05 vs. control) increase in both short- (Fig. 3A) and long-latency (Fig. 3C,D) components in both muscles during ankle joint extension of the hind foot. Moreover, a statistically signi®cant (P , 0:05 vs. control) increase in
Fig. 2. The effect of CGS 21680 (1 and 2 mg/0.5 ml) administration into the rostral part of the striatum on the physiological muscle tone (MMG) developed during passive extension (A) and ¯exion (B) of the hind foot in the ankle joint. The measurement started 30 min before injection and lasted 60 min after. Abscissa, time in min; ordinate, maximum torque (torque) in gcm. The results are presented as mean ^ SEM; *P , 0:05 vs. control animals. The number of animals in experimental groups: control n 10; CGS 21680 1 mg n 10; CGS 21680 2 mg n 9.
J. Wardas et al. / Neuroscience Letters 276 (1999) 79±82
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Fig. 3. The electromyographic activity (EMG), recorded in the gastrocnemius (A,B) and tibialis anterior (C,D) muscles, during ankle joint extension and ¯exion of the hind foot in control - and CGS 21680-treated rats. The EMG activity was recti®ed and averaged with a time constant of 20 ms. Curves were obtained by superimposing the EMG curves of all the undisturbed individual cycles, recorded for all rats of each group. (A±C) Vertical lines denote the start and the end of a movement, respectively. (D) Five components are shown: EMG ± baseline (estimated before the movement), EMG ± (A±D) (estimated at 0±20, 20±40, 60±140 and 220±320 ms, respectively, after the start of a movement). The measurement was carried out 60 min after CGS 21680 or vehicle administration. Abscissa, time in ms; ordinate ± EMG activity in mV. The number of animals in experimental groups: control n 10; CGS 21680 1 mg n 10; CGS 21680 2 mg n 9.
component D in the gastrocnemius muscle, but not in the tibialis anterior, during ankle joint ¯exion was seen. The present results show that stimulation of striatal adenosine A2A receptors by the selective agonist CGS 21680 enhances the muscle resistance (MMG) developed in response to passive extension. This enhanced MMG response is accompanied with an increase in long-latency EMG re¯ex responses to both passive movements in the gastrocnemius muscle but not in the tibialis anterior muscle. It has been shown previously that the increase in the EMG activity, elicited in the gastrocnemius muscle during ankle joint ¯exion and in the tibialis anterior muscle during ankle joint extension seems to be a response to the stretching of these muscles [14]. However, at the same time, burst of EMG activity appeared in antagonistic muscles which are being shortened [14]. Simultaneous occurrence of EMG activity in both sets of antagonistic muscles was observed in both monkeys and rats [14]. In the present experiment the EMG activity in both the gastrocnemius and tibialis anterior muscles was signi®cantly increased during passive extension but during passive ¯exion the statistically signi®cant increase was seen only in the gastrocnemius muscle (increased long-latency component D). It may be speculated that the lack of an EMG effect in the tibialis anterior muscle during ¯exion accounts for the observed absence of an MMG effect during this movement of a rats foot after CGS 21680 administered into the striatum. At present we cannot offer any explanation of the lack of an enhanced
MMG response during passive ¯exion. However, we also observed a relatively smaller MMG effect during passive ¯exion than during extension of a rats foot after systemic injection of very high doses of haloperidol [9]. We have recently demonstrated that administration of some model substances which evoke parkinsonian symptoms, such as haloperidol or bilaterally injected 6-hydroxydopamine (6-OHDA) into the substantia nigra pars compacta increases the muscle resistance (MMG) to passive movements and produces a rise in long-latency EMG re¯ex responses in rats [9,18]. The observed enhanced MMG/ EMG response in rats closely resembles the muscle rigidity in parkinsonian patients. Therefore it is suggested that ± like in the case of other neuroleptics - stimulation of adenosine A2A receptors by CGS 21680 in the rostral striatum produces parkinsonian-like muscle rigidity. Moreover, Hauber and MuÈnkle [5] have shown that injection of CGS 21680 in doses similar to ours to the rostral part of the striatum induces catalepsy in rats. Hence, the above results suggest that this compound has a pro®le similar to that of other classic neuroleptics such as haloperidol. These results contrast with ®ndings of other authors [15,16] who show that CGS 21680 injected systemically shows a distinct atypical antipsychotic pro®le in animal models. However, in the present paper CGS 21680 was administered in high doses directly to the rostral striatum in which the strio-pallidal projection originates. The observed muscle rigidity seems to be due to direct
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activation of the GABA-ergic strio-pallidal pathway, where adenosine A2A - and dopamine D2 receptors are co-localized [2]. Moreover, in these neurons a speci®c antagonistic interaction between A2A/D2 receptors occurs in the striatum [2]. In line with this view, stimulation of striatal adenosine A2A receptors or blockade of dopamine D2 receptors was found to increase GABA release in the globus pallidus, a structure to which the strio-pallidal projection sends its terminals [2,10,11]. In agreement with the above-cited results, intrapallidal injection of muscimol, a selective agonist of GABAA receptors, induced both catalepsy and muscle rigidity [17]. Hence, the above data indicate that stimulation of adenosine A2A receptors affects dopamine D2 responses via their opposite effects on the transduction mechanism in the strio-pallidal neurons [2] and through decrease of dopamine D2mediated neurotransmission may enhance the activity of the strio-pallidal pathway and thus provoke motor inhibition, e.g. catalepsy or muscle rigidity. Additionally, further contribution to the effect of adenosine A2A receptor agonists might be related to a cholinergic mechanism. This suggestion is based on the fact that stimulation of adenosine A2A receptors leads to enhancement of the ACh release in the striatum [7]. Thus the increased release of ACh which affects the activity of the strio-pallidal pathway may be involved in the expression of muscle rigidity or catalepsy, observed after direct stimulation of adenosine A2A receptors in the rostral striatum. However, the role of striatal adenosine A2A receptors in the regulation of ACh release is still under debate [4]. Hence elucidation of the mechanisms underlying the observed muscle rigidity requires further studies. In conclusion, the present results show that stimulation of adenosine A2A receptors in the striatum produces parkinsonian-like muscle rigidity; moreover, they support the idea that adenosine A2A agonists may have a therapeutic pro®le similar to that of other dopamine D2 receptor antagonists. This study was supported by the KBN (State Commitee for Scienti®c Research) grant No. 4.POA5.035.12. [1] Ellenbroek, B., Klockgether, T., Turski, L. and Schwarz, M., Distinct sites of functional interaction between dopamine, acetylcholine and gamma-aminobutyrate within the neostriatum: an electromyographic study in rats. Neuroscience, 17 (1986) 79±88. [2] Ferre, S., Fredholm, B.B., Morelli, M., Popoli, P. and Fuxe, K., Adenosine-dopamine receptor-receptor interactions as an integrative mechanism in the basal ganglia. Trends Neurosci., 20 (1997) 482±487. [3] Ferre, S., Rubio, A. and Fuxe, K., Stimulation of adenosine
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A2 receptors induces catalepsy. Neurosci. Lett., 130 (1991) 162±165. Fredholm, B.B. and Svenningson, P., Striatal adenosine A2A a receptors ± where are they? What do they do? Trends Pharmacol. Sci., 19 (1998) 46±47. Hauber, W. and MuÈnkle, M., Stimulation of adenosine A2A receptors in the striatum induces catalepsy that is reversed by antagonists of N-methyl-D-aspartate receptors. Neurosci. Lett., 196 (1995) 205±208. Kafka, S.H. and Corbett, R., Selective adenosine A2A receptor/dopamine D2 receptor interaction in animal models of schizophrenia. Eur. J. Pharmacol., 95 (1996) 147±154. Kirkpatrick, K.A. and Richardson, P.J., Adenosine receptormediated modulation of acetylcholine release from rat striatal synaptosomes. Br. J. Pharmacol., 110 (1992) 949± 954. KoÈnnig, J.F.R. and Klippel, R.A., The Rat Brain. A Sterotaxic Atlas of the Forebrain and Lower Parts of the Brain Stem, Williams and Wilkins, Baltimore, MD, 1963. Lorenc-Koci, E., Wolfarth, S. and Ossowska, K., Haloperidol-increased muscle tone in rats as a model of parkinsonian rigidity. Exp. Brain Res., 109 (1996) 268±276. May®eld, R.D., Larson, G., Orona, R.A. and Zahniser, N.R., Opposing action of adenosine A2A and dopamine D2 receptor activation on GABA release in the basal ganglia: evidence for an A2A/D2 receptor interaction in globus pallidus. Synapse, 22 (1996) 132±138. Marco, E., Mao, C.C., Cheney, C.L., Revuelta, A. and Costa, E., The effects of antipsychotics on the turnover rate of GABA and acetylcholine in rat brain nuclei. Nature, 264 (1976) 263±266. Morelli, M., Fenu, S., Pinna, A. and Di Chiara, G., Adenosine A2 receptors interact negatively with dopamine D2 and D2 receptors in unilaterally 6-hydroxydopamine-lesioned rats. Eur. J. Pharmacol., 251 (1994) 21±25. Ossowska, K., Karcz-Kubicha, M., Wardas, J. and Wolfarth, S., Striatal and nucleus accumbens D1/D2 receptors in neuroleptic catalepsy. Eur. J. Pharmacol., 182 (1990) 327± 334. Ossowska, K., Lorenc-Koci, E., Schulze, G., Konieczny, J., Wolfarth, S., Bojarski, M. and Coper, H., The role of re¯ex activity in the regulation of muscle tone in rats. Exp. Physiol., 81 (1996) 211±223. Pinna, A., Wardas, J., Cristalli, G. and Morelli, M., Adenosine A2A receptor agonists increase Fos-like immunoreactivity in mesolimbic areas. Brain Res., 759 (1997) 41±49. Rimondini, R., Ferre, S., Ogren, S.O. and Fuxe, K., Adenosine A2a agonists: a potential new type of a typical antipsychotic. Neuropsychopharmacology, 17 (1997) 82±91. Turski, L., Havemann, U. and Kuschinsky, K., GABAergic mechanisms in mediating muscular rigidity, catalepsy and postural asymmetry in rats: differences between dorsal and ventral striatum. Brain Res., 322 (1984) 49±57. Wolfarth, S., Konieczny, J., SÂmial¤owska, M., Schulze, G. and Ossowska, K., In¯uence of 6-hydroxydopamine lesion of the dopaminergic nigrostriatal pathway on the muscle tone and electromyographic activity measured during passive movements. Neuroscience, 74 (1996) 985±996.
The role of striatal adenosine A2A receptors in regulation of the muscle tone in rats Jadwiga Wardas*, Jolanta Konieczny, ElzÇbieta Lorenc-Koci Department of NeuroPsychopharmacology, Institute of Pharmacology, Polish Academy of Sciences, 12 SmeËtna Street, 31-343 KrakoÂw, Poland Received 12 August 1999; received in revised form 23 September 1999; accepted 24 September 1999
Abstract The aim of the present study was to assess contribution of striatal adenosine A2A receptors to regulation of the muscle tone in rats. The muscle tone was examined by a combined mechano- and electromyographic method, which measured simultaneously muscle resistance (MMG) of a rats hind foot to passive extension and ¯exion in the ankle joint and the electromyographic activity (EMG) of the antagonistic muscles: gastrocnemius and tibialis anterior. CGS 21680 (1 and 2 mg/0.5 ml), injected bilaterally into the rostral part of the striatum, dose-dependently increased both MMG and the EMG. The present results show that stimulation of striatal adenosine A2A receptors by CGS 21680 evokes parkinsonian-like muscle rigidity which may be due to activation of the GABAergic strio-pallidal pathway. q 1999 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Adenosine A2A receptors; Striatum; Muscle rigidity; Electromyogram; Mechanomyogram
Adenosine A2A receptors that are positively coupled to adenylate cyclase are densely and selectively located in such dopamine-innervated areas as the striatum, nucleus accumbens and olfactory tubercle [2]. Recently it has been shown that in experimental animals adenosine A2A agonists have behavioural effects similar to those produced by neuroleptics. Systemic administration of adenosine A2A agonists to rodents induces pronounced depression of the spontaneous and dopamine agonists-induced motor activity [5,6,12]. Moreover, when used in high doses, A2A adenosine receptor agonists produce catalepsy after both systemic and intracerebral injection [3,5,6]. Since Ferre et al. [2] have shown that stimulation of adenosine A2A receptors decreases the af®nity of dopamine D2 receptors for dopamine agonists and impairs the D2 signal transduction in rat striatum, it has been suggested that a major mechanism responsible for the adenosine A2A agonists-induced counteraction of dopamine neurotransmission is the existence of a speci®c antagonistic interaction between A2A - and D2 receptors in the striatum. It is well known that in the striatum A2A receptors are selectively co-located with dopamine D2 receptors on GABAergic neurons projecting to the globus pallidus [2]. * Corresponding author. Tel.: 148-12-637-4022; fax: 148-12637-4500. E-mail address: [email protected] (J. Wardas)
Overactivity of this pathway has been postulated to produce such extrapyramidal effects as catalepsy and muscle rigidity. Previous studies showed that the blockade of dopamine D2 receptors by neuroleptics injected into the rostral part of the striatum induced potent catalepsy and muscle rigidity [1,13]. Furthermore, Hauber and MuÈnkle [5] found that stimulation of adenosine A2A receptors by CGS 21680 injected into the anterodorsal striatum induced catalepsy. Therefore the aim of the present study was to ®nd out whether CGS 21680, a selective adenosine A2A receptor agonist, injected directly into the ventro-lateral part of the rostral striatum affected the muscle tone in rats, measured by a combined mechano-electromyographic method (MMG/EMG) [9]. The experiment was carried out on male Wistar rats (260± 350 g), implanted bilaterally with guide cannulae (0.4 mm o.d.) in the rostral part of the striatum (A 9650±8920, L 1.6±2.4, H 21.6 to 10.8) (according to Ref. [8]) under pentobarbital anaesthesia (Vetbutal, 30 mg/kg i.p.; Biowet, Poland). Mechano- and electromyographic experiments were conducted 1 week after stereotaxic surgery. As described in detail previously [9], a conscious rat was placed in a metaplex cage with its hind foot protruding from an opening at the bottom of the cage. Two pairs of ¯exible stainless steel wire electrodes (Cooner Wire, Chatsworth, CA,
0304-3940/99/$ - see front matter q 1999 Elsevier Science Ireland Ltd. All rights reserved. PII: S03 04 - 394 0( 9 9) 00 77 9- X
80
J. Wardas et al. / Neuroscience Letters 276 (1999) 79±82
USA), which were te¯on-insulated (external diameter 0.1 mm), were inserted percutaneously into the gastrocnemius and tibialis anterior muscles. The experiment involved succesive passive extension and ¯exion (30-s apart) of the rats foot in the ankle joint by 258 with a velocity of 100 deg/ s. The maximum resistance (torque (gcm); in comparison with the premovement value) of hind leg muscles for each movement was determined. The EMG activity of both muscles was recti®ed and averaged with a time constant of 20 ms for each movement. The following parameters were estimated for each movement: (1) EMG baseline ± the mean of a premovement amplitude as a value of the resting EMG activity; (2) components computed as differences between the maximum amplitude of the averaged EMG curves at four time periods after the start of a movement and the EMG baseline: EMG-A (0±20 ms), EMG-B (20±40 ms), EMG-C (60±140 ms) and EMG-D (220±320 ms). Cycles disturbed by voluntary movements of an animal were discarded. CGS 21680-HCl (RBI) was dissolved in redistilled water and the pH was balanced with 0.1 N NaOH to a value of 6± 7. CGS 21680-HCl was injected bilaterally in doses of 1 and 2 mg/0.5 ml /side into the ventro-lateral part of the rostral striatum within 2 min via an inner cannula (0.3 mm o.d.) which protruded by 0.7 mm from the guide cannula. Control rats were injected with 0.5 ml of the appropriate vehicle instead of CGS 21680. The measurement of the muscle tone started 30 min before intrastriatal injections of CGS 21680 or vehicle and lasted 60 min after the injection. After the experiment, the placement of the cannulae was checked histologically. Only rats with cannulae implanted properly on both sides in the rostral part of the striatum were subjected to a statistical analysis. Statistical analyse of MMGmax values was carried out using the means obtained from all the correct cycles calculated every 10 min (MMG), as well as the means calculated for the whole experimental session (60 min after CGS 21680 injection, EMG). The statistical signi®cance (P , 0:05) of differences was estimated using the Kruskal±Wallis and Wilcoxon tests. Bilateral injection of CGS 21680 in a dose of 1 mg/0.5 ml (1.86 nmol) into the rostral striatum (Fig. 1) did not change the muscle resistance to passive ¯exion; however, a nonsigni®cant increasing tendency was observed during passive extension of a rats hind foot (Fig. 2). Injection of CGS 21680 in a dose of 2 mg/0.5 ml (3.73 nmol) into the rostral striatum enhanced the muscle resistance developed in response to passive extension of a hind foot in the ankle joint (Fig. 2). Such an effect was observed during the ®rst 10 min after intrastriatal injection and lasted until the end of measurement. Control rats, that were injected with the vehicle did not show any changes in the muscle resistance (Fig. 2). Injection of CGS 21680 in a higher dose of 2 mg/0.5 ml into the rostral striatum potently affected the EMG activity in both the gastrocnemius and tibialis anterior muscles,
Fig. 1. Localization of cannula tips in frontal sections of rat brain. Filled circles: cannula tips located in the rostral part of the striatum; each circle denotes a cannula placed in one animal on one side. CP, caudate-putamen; NA, nucleus accumbens; A, anterior plane, according to KoÈnig and Klippel [8].
mainly during extension of the hind foot (Fig. 3). That dose of CGS 21680 produced a statistically signi®cant (P , 0:05 vs. control) increase in both short- (Fig. 3A) and long-latency (Fig. 3C,D) components in both muscles during ankle joint extension of the hind foot. Moreover, a statistically signi®cant (P , 0:05 vs. control) increase in
Fig. 2. The effect of CGS 21680 (1 and 2 mg/0.5 ml) administration into the rostral part of the striatum on the physiological muscle tone (MMG) developed during passive extension (A) and ¯exion (B) of the hind foot in the ankle joint. The measurement started 30 min before injection and lasted 60 min after. Abscissa, time in min; ordinate, maximum torque (torque) in gcm. The results are presented as mean ^ SEM; *P , 0:05 vs. control animals. The number of animals in experimental groups: control n 10; CGS 21680 1 mg n 10; CGS 21680 2 mg n 9.
J. Wardas et al. / Neuroscience Letters 276 (1999) 79±82
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Fig. 3. The electromyographic activity (EMG), recorded in the gastrocnemius (A,B) and tibialis anterior (C,D) muscles, during ankle joint extension and ¯exion of the hind foot in control - and CGS 21680-treated rats. The EMG activity was recti®ed and averaged with a time constant of 20 ms. Curves were obtained by superimposing the EMG curves of all the undisturbed individual cycles, recorded for all rats of each group. (A±C) Vertical lines denote the start and the end of a movement, respectively. (D) Five components are shown: EMG ± baseline (estimated before the movement), EMG ± (A±D) (estimated at 0±20, 20±40, 60±140 and 220±320 ms, respectively, after the start of a movement). The measurement was carried out 60 min after CGS 21680 or vehicle administration. Abscissa, time in ms; ordinate ± EMG activity in mV. The number of animals in experimental groups: control n 10; CGS 21680 1 mg n 10; CGS 21680 2 mg n 9.
component D in the gastrocnemius muscle, but not in the tibialis anterior, during ankle joint ¯exion was seen. The present results show that stimulation of striatal adenosine A2A receptors by the selective agonist CGS 21680 enhances the muscle resistance (MMG) developed in response to passive extension. This enhanced MMG response is accompanied with an increase in long-latency EMG re¯ex responses to both passive movements in the gastrocnemius muscle but not in the tibialis anterior muscle. It has been shown previously that the increase in the EMG activity, elicited in the gastrocnemius muscle during ankle joint ¯exion and in the tibialis anterior muscle during ankle joint extension seems to be a response to the stretching of these muscles [14]. However, at the same time, burst of EMG activity appeared in antagonistic muscles which are being shortened [14]. Simultaneous occurrence of EMG activity in both sets of antagonistic muscles was observed in both monkeys and rats [14]. In the present experiment the EMG activity in both the gastrocnemius and tibialis anterior muscles was signi®cantly increased during passive extension but during passive ¯exion the statistically signi®cant increase was seen only in the gastrocnemius muscle (increased long-latency component D). It may be speculated that the lack of an EMG effect in the tibialis anterior muscle during ¯exion accounts for the observed absence of an MMG effect during this movement of a rats foot after CGS 21680 administered into the striatum. At present we cannot offer any explanation of the lack of an enhanced
MMG response during passive ¯exion. However, we also observed a relatively smaller MMG effect during passive ¯exion than during extension of a rats foot after systemic injection of very high doses of haloperidol [9]. We have recently demonstrated that administration of some model substances which evoke parkinsonian symptoms, such as haloperidol or bilaterally injected 6-hydroxydopamine (6-OHDA) into the substantia nigra pars compacta increases the muscle resistance (MMG) to passive movements and produces a rise in long-latency EMG re¯ex responses in rats [9,18]. The observed enhanced MMG/ EMG response in rats closely resembles the muscle rigidity in parkinsonian patients. Therefore it is suggested that ± like in the case of other neuroleptics - stimulation of adenosine A2A receptors by CGS 21680 in the rostral striatum produces parkinsonian-like muscle rigidity. Moreover, Hauber and MuÈnkle [5] have shown that injection of CGS 21680 in doses similar to ours to the rostral part of the striatum induces catalepsy in rats. Hence, the above results suggest that this compound has a pro®le similar to that of other classic neuroleptics such as haloperidol. These results contrast with ®ndings of other authors [15,16] who show that CGS 21680 injected systemically shows a distinct atypical antipsychotic pro®le in animal models. However, in the present paper CGS 21680 was administered in high doses directly to the rostral striatum in which the strio-pallidal projection originates. The observed muscle rigidity seems to be due to direct
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activation of the GABA-ergic strio-pallidal pathway, where adenosine A2A - and dopamine D2 receptors are co-localized [2]. Moreover, in these neurons a speci®c antagonistic interaction between A2A/D2 receptors occurs in the striatum [2]. In line with this view, stimulation of striatal adenosine A2A receptors or blockade of dopamine D2 receptors was found to increase GABA release in the globus pallidus, a structure to which the strio-pallidal projection sends its terminals [2,10,11]. In agreement with the above-cited results, intrapallidal injection of muscimol, a selective agonist of GABAA receptors, induced both catalepsy and muscle rigidity [17]. Hence, the above data indicate that stimulation of adenosine A2A receptors affects dopamine D2 responses via their opposite effects on the transduction mechanism in the strio-pallidal neurons [2] and through decrease of dopamine D2mediated neurotransmission may enhance the activity of the strio-pallidal pathway and thus provoke motor inhibition, e.g. catalepsy or muscle rigidity. Additionally, further contribution to the effect of adenosine A2A receptor agonists might be related to a cholinergic mechanism. This suggestion is based on the fact that stimulation of adenosine A2A receptors leads to enhancement of the ACh release in the striatum [7]. Thus the increased release of ACh which affects the activity of the strio-pallidal pathway may be involved in the expression of muscle rigidity or catalepsy, observed after direct stimulation of adenosine A2A receptors in the rostral striatum. However, the role of striatal adenosine A2A receptors in the regulation of ACh release is still under debate [4]. Hence elucidation of the mechanisms underlying the observed muscle rigidity requires further studies. In conclusion, the present results show that stimulation of adenosine A2A receptors in the striatum produces parkinsonian-like muscle rigidity; moreover, they support the idea that adenosine A2A agonists may have a therapeutic pro®le similar to that of other dopamine D2 receptor antagonists. This study was supported by the KBN (State Commitee for Scienti®c Research) grant No. 4.POA5.035.12. [1] Ellenbroek, B., Klockgether, T., Turski, L. and Schwarz, M., Distinct sites of functional interaction between dopamine, acetylcholine and gamma-aminobutyrate within the neostriatum: an electromyographic study in rats. Neuroscience, 17 (1986) 79±88. [2] Ferre, S., Fredholm, B.B., Morelli, M., Popoli, P. and Fuxe, K., Adenosine-dopamine receptor-receptor interactions as an integrative mechanism in the basal ganglia. Trends Neurosci., 20 (1997) 482±487. [3] Ferre, S., Rubio, A. and Fuxe, K., Stimulation of adenosine
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