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Turning behavior after unilateral lesion of the subthalamic nucleus in the rat
Behavioural Brain Research, 8 (1983) 217-223
217
Elsevier Biomedical Press
Turning behavior after unilateral lesion of the subthalamic nucleus in the rat
EVANGELOS KAFETZOPOULOS and GEORGE PAPADOPOULOS*
Department of Experimental Pharmacology, University of Athens, Medical School, Athens 609 (Greece) (Received August 2nd, 1982) (Revised version received January 10th, 1983) (Accepted January 18th, 1983)
Key words: turning behavior - subthalamic nucleus - kainic acid - apomorphine - amphetamine haloperidol - rat
Unilateral stereotaxic lesions of the subthalamic nucleus of rats with kainic acid induced a transient spontaneous ipsiversive turning which was present for several days, and which could be blocked by i.p. administration ofhaloperidol. After the 20th postoperative day, when no spontaneous turning activity was present, i.p. administration of apomorphine or amphetamine in the kainic acid lesioned rats induced ipsiversive turning, while i.p. haloperidol in the same rats induced contraversive turning. These findings suggest that the subthalamic nucleus is involved in the dopaminergic mechanisms mediating turning behavior of rats.
According to numerous authors [ 1, 3, 4, 15, 16] the subthalamic nucleus (STN) of the monkey is connected with the globus pallidus (GP), forming a GP-STN-GP loop, through which it was presumed to exert its physiological role in elaborating motor activities. Recently, Kanazawa et al. [ 11 ] in an HRP study and Hammond et al. [9] in a single unit recording study, reported evidence of a massive STN projection to the substantia nigra (SN), both pars compacta and pars reticulata, in the rat. Meibach and Katzman [14] suggested that the pars reticulata of the SN many project back to the STN, forming another short loop STN-SN-STN. The same authors reported evidence that the STN in the cat contains both catecholaminergic cell bodies and terminals. These findings concerning the anatomical relationships of the STN with two structures involved in the dopaminergic mechanisms of the brain led us to examine the possible role of the STN in turning behavior of rats, which represents a useful model in the study of dopaminergic activity within the basal ganglia [22]. Male Wistar rats (n = 10) weighing 260-300 g were lesioned unilaterally in the subthalamic nucleus by locally injected kainic acid. The rats were anesthetized
* To whom correspondence should be addressed. 0166-4328/83/0000-0000/$03.00 © 1983 Elsevier Science Publishers
218 with pentobarbital sodium (40 mg/kg, i.p.) and placed into a stereotaxic frame. After trephining of the skull an injection of 0.25 #g of kainic acid dissolved in 0.5/zl of physiological saline was made with a 28-gauge injection cannula connected through a polyethylene tube to a hand-driven 5 #1 Hamilton microsyringe. The injection coordinates were according to the stereotaxic atlas of De Groot [6]: 4.0 mm anterior, 2.5 mm lateral and 2.0 mm dorsal to the midpoint of the interaural line. Injections were made over a period of 3 min and the injection cannula was left in place for 3 min following each injection in order to minimize leakage of kainic acid up the cannula track. The site of lesion was then verified at the end of the experiments by histological examination. After intracardial perfusion with formalin the brains were embedded in paraffin and cut into serial 15 #m slices which were stained with toluidine blue. The extent of the lesions was verified under microscopic examination and the percentage of the neurons destroyed was calculated after transferring the lesions on the corresponding plates of De Groot atlas and comparing the intact and the lesioned side on the same anteroposterior level. Upon awakening from the anesthesia all rats exhibited a postural asymmetry with the head and the tail turned towards the intact side. As soon as the animals could move, contraversive turning appeared, but after a few hours they exhibited postural asymmetry towards the lesioned side and spontaneous ipsiversive turning which lasted 4-8 days at decreasing intensity. ~2
t
38
3"
Fig. 1. Serial sections showing the localization and extent of the kainic acid lesions of the subthalamus as seen on light microscopyand transferred to diagrams redrawn from De Groot [5] by schematicallydelineatingthe area of complete neuronal loss. The black area represents the smallest lesion and the hatched area the largest lesion, the limits of the remaining 8 lesions lying between them. The numbers represent mm anterior to interaural line.
219 mln 5
15
30
60
A -1
B
-2
Fig. 2. Turning behavior after haloperidol administration (2 mg/kg i.p.) in rats with kainic acid lesions of the subthalamic nucleus, presented as rpm versus time (means + S.E.M.). A: during the period of spontaneous turning. B: after the 20th postoperative day, when no turning activity was spontaneously present. Negative y-values indicate turning contralateral to the lesioned side.
During the period of spontaneous turning (on the second postoperative day) the animals were injected i.p. with haloperidol (2 mg/kg), which blocked turning in 2 rats and induced contralateral turning in 8 rats (Fig. 2A). The period of spontaneous turning was considered as ended when no turning activity was present, even after behavioral stimulation of the rats. Behavioral stimulations were induced either by placing the animals in a new environment, or by tail pinch, i.e. by pressing the tail between the thumb and the index of the gloved right hand, which is known to raise the arousal state of rats, increasing locomotor activity [ 12]. Twenty days after the lesion, when no such spontaneous turning activity could be observed, all rats were injected i.p. with apomorphine (0.5 and 2 mg/kg), amphetamine (2 and 4 mg/kg) and haloperidol (2 mg/kg) at 48 h intervals in a random fashion and tested for turning activity for the following 60 min. To test turning, the animals were put in a cylindrical gray colored metal cage (35 cm diam., 50 cm high) immediately after the injection of the drug and the rate of turning (revolutions per min (rpm)) was measured for 3 min 5, 15, 30 and 60 min after the injection. Apomorphine caused postural asymmetry towards the lesioned side and dose-dependent ipsiversive turning. A significant difference (P < 0.05) between the rates of turning was found 30 min after the injection with the two different doses of the drug (Fig. 3A). Amphetamine also caused postural asymmetry towards the lesioned side and ipsilateral turning, but no significant differences were found between the two doses of the drug (Fig. 3B). Finally, haloperidol induced an asymmetry towards the intact side. Since haloperidol injected in this amount sedated the animals, the rate of turning was recorded after behavioral stimulation of the rats induced by tail-pinch. By this procedure a weak contraversive turning appeared (Fig. 2B). The histological examination of the brains showed a loss of STN neurons, which were replaced by glial cells. The animals in which the observed gliosis
220 7.
.5 rncj / k9
t3
13 2 r n c j / k 9
2 mglkc J
o
o
4 mcj l k g
6¸ 5 / E
~3 2 1
5
15
30
60 min
5
15
30
60 min
Fig. 3. Turning behavior after apomorphine (0.5 and 2 mg/kg i.p.) (A) and amphetamine(2 and 4 mg/kg i.p.) (B) administrationin kainic acid lesionedrats after spontaneous turning activity had subsided. Results are presented as rpm versus time (means + SIE.M.). Positive y-valuesindicate turning ipsilateral to the lesioned side. * P < 0.05 in respect to the rate of turning after 0.5 mg/kg administration.
occupied adjacent areas such as the substantia nigra or the ventromediat thalamus were excluded from the study, since they are critically involved in the causation of turning behavior. The small size of the STN and the rigorous criteria of the extent of the lesions restricted the success of the operation to 10 rats, in which the neuronal loss and the concomitant gliosis remained r o u ~ t y within the limits of the STN (Fig. 1). In only 3 rats the lesion extended to the overlying zona incerta, but in no case the amount of the neurons destroyed exceeded the 20 % of the total neuronal population of the zona incerta, while the neuronal loss of the STN varied between 70 and 100%. Previous studies suggested that the STN is involved in motor control, since unilateral lesion of STN induces hemiballism in the monkey [4, 10], but there exists no report concerning behavioral or motor disturbances after STN lesions in the rat. The fmdings of the present study and particularly the opposite effects of apomorphine and haloperidol suggest that the STN participates in the dopaminergic motor mechanisms controlling turning behavior in rats. The best known of such mechanisms is that which includes the nisrostriatal dopaminergic (DA) neurons and the corresponding DA receptors in the corpus striatum. Lesion or pharmacological manipulation in the nigrostriatal bundle or in the corpus striatum induce turning towards the lesioned side or away from the higher D A activity respectively [22, for review see 20]. According to recent data the turning caused by activation of this system is mediated through a nigro-strio-nigral loop [17-19]. The nigrostriatal DA neurons, through unknown interneurons, activate a descending strio-nigral GABA~gic pathway, which in turn inhibits the GABAergic cells of the substantia nigra pars reticulata. The inhibition o f these GABAergic cells, which project mainly to the ventromediat thalamus [8], seems to be the final striatal mechanism through which the turning behavior is expressed [ 19].
221 Given the electrophysiological evidence that the STN is connected with both pars compacta and pars reticulata of the SN which are two main stations of this system, it seems possible that the turning after STN lesions is brought about by direct or indirect blocking of the DA activity within the ipsilateral nigro-strionigral system. On the other hand, the turning after administration of DA related drugs could be mediated by the preferential activation of the same system on the intact side. Although this is a valuable explanation, two other possibilities must be considered in the interpretation of the observed results. According to a recent study [23] the majority of STN neurons project simultaneously to both the substantia nigra and globus pallidus in the rat. Globus pallidus is known to receive a DA innervation from collaterals of the nigrostriatal axons [13] and is also involved in the expression of the turning behavior, since electrolytic lesion of it induces turning in the rat [26]. Although the palUidal mechanisms mediating turning behavior are unknown, a possible influence of the STN on these mechanisms might be responsible for the observed turning after STN lesion. Another possibility could be the direct action of DA related drugs on the subthalamic nucleus neurons. Significant quantities of dopamine [24] and dopamine-sensitive adenylate cyclase [27] have been found in the STN. In addition, administration of D-amphetamine in the conscious rat increases the cerebral glucose utilization in the STN [25], indicating a high dopaminergic activity. The site of origin of the DA innervation of the STN is still unknown, but it cannot be excluded that DA systems directly connected to STN may play a role in the causation of turning after STN lesion. However, the exact role of these systems cannot be assessed until catecholaminergic innervation or projections of the STN will be anatomically identified. According to studies in the monkey the subthalamic nucleus is traversed by nigro-striatal ascending [2] and by pallidofugal fibers [16], but histological examination of the brains did not show any cellular loss in both the zona compacta of the substantia nigra and globus pallidus. Absence of retrograde degeneration supports the conclusion that axons of these neurons have been spared by kainic acid. This is in agreement with the general hypothesis that kainic acid acts rather selectively, destroying nerve cell bodies while sparing axons and axon terminals [ 5, 7]. The acute period of contralateral turning in the lesioned rats for a few hours after the operation may be due to the powerful stimulant activity of kainic acid on neurons in or around the STN, since a similar phenomenon is observed after kainic acid injections into other structures as striatum or substantia nigra [ 5 ]. The results of the present study are also in agreement with a recent report, which suggested that the STN participate in mediating DA striatal output functions, through a well defined pallidosubthalamic GABAergic pathway [21]. Local injection of GABA agonists and antagonists into the STN induced contraversive or ipsiversive turning in apomorphine-treated rats, indicating that the STN
222
is critically involved in motor effects induced after striatal DA receptor stimulation. But further experiments are required to establish whether the turning induced by STN lesions is due to alteration in striatal output activity through this pallidosubthalamic pathway, or to alteration in STN outputs which indirectly affected the nigrostriatal DA activity. REFERENCES 1 Carpenter, M.B., Frazer, R.A.R. and Shriver, J.E., The organization of paUido-subthalamic fibers in the monkey, Brain Res., 11 (1968) 522-559. 2 Carpenter, M.B. and Peter, P., Nigrostriatal and nigrothalamic fibers in the rhesus monkey, J. comp. Neurol., 144 (1972) 93-116. 3 Carpenter, M.B. and Strominger, N.L., Efferent fibers of the subthalamic nucleus in the monkey. A comparison of the efferent projections ofthe subthalamic nucleus, substantia nigra and globus pallidus, Amer. J. Anat., 121 (1967) 41-72. 4 Carpenter, M.B., Whittier, J.R. and Merrier, F.A., Analysis of choreoid hyperkinesia in rhesus monkey. Surgical and pharmacological analysis of hyperkinesia resulting after lesions in the subthalamic nucleus of Luys, J. comp. Neurol., 92 (1950) 293-331. 5 Coyle, J.T., Schwarz, R., Bennet, J.P. and Campochiaro, P., Clinical, neuropathologic and pharmacological aspects of Huntington's disease: correlates with a new animal model, Progr. Neuropsychopharmaeol., 1 (1979) 13-30. 6 De Groot, J., The Rat Forebrain in Stereotaxic Coordinates, 4th edn., Elsevier/North-Holland, Amsterdam, t972. 7 Di Chiara, G., Olianas, M., Del Fiacco, M., Spano, R.F. and Tagliamonte, A., Intranigral kainic acid is evidence that nigral non-dopaminergic neurons control posture, Nature (Lond.), 268 (1977) 143-145. 8 Di Chiara, G., Porceddu, M.L., Morelli, M., Mulas, M.L. and Gessa, G.L., Evidence of a GABAergic projection from the substantia nigra to the ventromedial thalamus and to the superior colliculus of the rat, Brain Res., 176 (1979) 273-284. 9 Hammond, C., Deniau, LM., Rizk, A. and Feger, J., Electrophysiological demonstration of an excitatory subthalamonigral pathway in the rat, Brain Res., 151 (1978) 235-244. I0 Hammond, C., Feger, J., Bioulac, B. and Souteyrand, J.P., Experimental hemiballism in the monkey produced by unilateral kainic acid lesion in corpus Luysii, Brain Res., 171 (1979) 577-580. 11 Kanazawa, I., Marshall, G.R. and Kelly, J.S., Afferents of the rat substantia nigra studied with horseradish peroxidase, with special reference to fibers from the subthalamic nucleus, Brain Res., 115 (1976) 485-491. 12 Katz, R.J. and Roth, K., Tail pinch induced stress-arousal facilitates brain stimulation reward, Physiol. Behav., 22 (1979) 193-194. 13 Lindvall, O. and Bjorklund, A., Dopaminergic innervation of the globus patlidus by collaterals from the nigrostriatal pathway, Brain Res., 172 (1979) 169-173. 14 Meibach, R.C. and Katzman, R., Catecholaminergic innervation of the subthalamic nucleus: evidence for a rostral continuation of the A9 (substantia nigra) dopaminergic cell group, Brain Res., 173 (1979) 364-368. 15 Nakamura, S. and Sutin, J., The pattern of termination of pallidal axons upon cells of the subthalamic nucleus, Exp. Neurol., 35 (1972)254-264. 16 Nauta, W.J.H. and Mehler, W.R., Projections of the lentiform nucleus in the monkey, Brain Res., 1 (1966) 3-42.
223 17 Oberlander, G., Dumont, C. and Boissier, J.R., Rotational behavior after unilateral intranigral injection of muscimol in rats. Europ. J. Pharmacol., 43 (1977) 389-390. 18 Olpe, H.R., Schellenberg, H. and Koella, W.P., Rotational behavior induced in rats by intranigral application of GABA related drugs and GABA antagonists. Europ. ,I. Pharmacol., 45 (1977) 291-294. 19 Papadopoulos, G. and Huston, J.P., Contralateral turning after electrolytic lesion of substantia nigra in thalamic rats, Neurosci. Leg., 13 (1979) 63-67. 20 Pycock, C.J., Turning behavior in animals, Neuroscience, 5 (1980) 461-514. 21 Scheel-Kriiger, J. and Magelund, G., GABA in the entopenduncular nucleus and the subthalamic nucleus participates in mediating dopaminergic striatal output functions, Life Sci., 29 (1981) 1555-1562. 22 Ungerstedt, U., Striatal dopamine release after amphetamine or nerve degeneration revealed by rotational behaviour, A cta physiol, scand., Suppl., 367 (1971 ) 49-68. 23 Van der Kooy, D. and Hattori, T., Single subthalamic nucleus neurons project to both the globus pallidus and substantia nigra in rat, J. comp. Neurol., 192 (1980) 751-768. 24 Versteeg, D.H.G., Gugten, J.V.D., De Jong, W. and Palkovits, M., Regional concentrations of noradrenaline and dopamine in rat brain, Brain Res., 113 (1976) 563-574. 25 Wechsler, L.R., Savaki, H.E. and Sokoloff, L., Effects of D- and L-amphetamine on local cerebral glucose utilization in the conscious rat, J. Neurochem., 32 (1979) 15-22. 26 Westermann, K.H. and Funk, K.F., Pharmacological evidence for dopaminergic pallido-striatal interaction, Pharmacol. Biochem. Behav., 8 (1978) 645-649. 27 Wolfson, L.I., Brown, L., Makman, M., Gardner, E., Dvorkin, B. and Katzman, R., Evidence of dopamine receptors in subthalamic nucleus in rats, Proc. 6th int. Soc. Neurochem., p. 453.
217
Elsevier Biomedical Press
Turning behavior after unilateral lesion of the subthalamic nucleus in the rat
EVANGELOS KAFETZOPOULOS and GEORGE PAPADOPOULOS*
Department of Experimental Pharmacology, University of Athens, Medical School, Athens 609 (Greece) (Received August 2nd, 1982) (Revised version received January 10th, 1983) (Accepted January 18th, 1983)
Key words: turning behavior - subthalamic nucleus - kainic acid - apomorphine - amphetamine haloperidol - rat
Unilateral stereotaxic lesions of the subthalamic nucleus of rats with kainic acid induced a transient spontaneous ipsiversive turning which was present for several days, and which could be blocked by i.p. administration ofhaloperidol. After the 20th postoperative day, when no spontaneous turning activity was present, i.p. administration of apomorphine or amphetamine in the kainic acid lesioned rats induced ipsiversive turning, while i.p. haloperidol in the same rats induced contraversive turning. These findings suggest that the subthalamic nucleus is involved in the dopaminergic mechanisms mediating turning behavior of rats.
According to numerous authors [ 1, 3, 4, 15, 16] the subthalamic nucleus (STN) of the monkey is connected with the globus pallidus (GP), forming a GP-STN-GP loop, through which it was presumed to exert its physiological role in elaborating motor activities. Recently, Kanazawa et al. [ 11 ] in an HRP study and Hammond et al. [9] in a single unit recording study, reported evidence of a massive STN projection to the substantia nigra (SN), both pars compacta and pars reticulata, in the rat. Meibach and Katzman [14] suggested that the pars reticulata of the SN many project back to the STN, forming another short loop STN-SN-STN. The same authors reported evidence that the STN in the cat contains both catecholaminergic cell bodies and terminals. These findings concerning the anatomical relationships of the STN with two structures involved in the dopaminergic mechanisms of the brain led us to examine the possible role of the STN in turning behavior of rats, which represents a useful model in the study of dopaminergic activity within the basal ganglia [22]. Male Wistar rats (n = 10) weighing 260-300 g were lesioned unilaterally in the subthalamic nucleus by locally injected kainic acid. The rats were anesthetized
* To whom correspondence should be addressed. 0166-4328/83/0000-0000/$03.00 © 1983 Elsevier Science Publishers
218 with pentobarbital sodium (40 mg/kg, i.p.) and placed into a stereotaxic frame. After trephining of the skull an injection of 0.25 #g of kainic acid dissolved in 0.5/zl of physiological saline was made with a 28-gauge injection cannula connected through a polyethylene tube to a hand-driven 5 #1 Hamilton microsyringe. The injection coordinates were according to the stereotaxic atlas of De Groot [6]: 4.0 mm anterior, 2.5 mm lateral and 2.0 mm dorsal to the midpoint of the interaural line. Injections were made over a period of 3 min and the injection cannula was left in place for 3 min following each injection in order to minimize leakage of kainic acid up the cannula track. The site of lesion was then verified at the end of the experiments by histological examination. After intracardial perfusion with formalin the brains were embedded in paraffin and cut into serial 15 #m slices which were stained with toluidine blue. The extent of the lesions was verified under microscopic examination and the percentage of the neurons destroyed was calculated after transferring the lesions on the corresponding plates of De Groot atlas and comparing the intact and the lesioned side on the same anteroposterior level. Upon awakening from the anesthesia all rats exhibited a postural asymmetry with the head and the tail turned towards the intact side. As soon as the animals could move, contraversive turning appeared, but after a few hours they exhibited postural asymmetry towards the lesioned side and spontaneous ipsiversive turning which lasted 4-8 days at decreasing intensity. ~2
t
38
3"
Fig. 1. Serial sections showing the localization and extent of the kainic acid lesions of the subthalamus as seen on light microscopyand transferred to diagrams redrawn from De Groot [5] by schematicallydelineatingthe area of complete neuronal loss. The black area represents the smallest lesion and the hatched area the largest lesion, the limits of the remaining 8 lesions lying between them. The numbers represent mm anterior to interaural line.
219 mln 5
15
30
60
A -1
B
-2
Fig. 2. Turning behavior after haloperidol administration (2 mg/kg i.p.) in rats with kainic acid lesions of the subthalamic nucleus, presented as rpm versus time (means + S.E.M.). A: during the period of spontaneous turning. B: after the 20th postoperative day, when no turning activity was spontaneously present. Negative y-values indicate turning contralateral to the lesioned side.
During the period of spontaneous turning (on the second postoperative day) the animals were injected i.p. with haloperidol (2 mg/kg), which blocked turning in 2 rats and induced contralateral turning in 8 rats (Fig. 2A). The period of spontaneous turning was considered as ended when no turning activity was present, even after behavioral stimulation of the rats. Behavioral stimulations were induced either by placing the animals in a new environment, or by tail pinch, i.e. by pressing the tail between the thumb and the index of the gloved right hand, which is known to raise the arousal state of rats, increasing locomotor activity [ 12]. Twenty days after the lesion, when no such spontaneous turning activity could be observed, all rats were injected i.p. with apomorphine (0.5 and 2 mg/kg), amphetamine (2 and 4 mg/kg) and haloperidol (2 mg/kg) at 48 h intervals in a random fashion and tested for turning activity for the following 60 min. To test turning, the animals were put in a cylindrical gray colored metal cage (35 cm diam., 50 cm high) immediately after the injection of the drug and the rate of turning (revolutions per min (rpm)) was measured for 3 min 5, 15, 30 and 60 min after the injection. Apomorphine caused postural asymmetry towards the lesioned side and dose-dependent ipsiversive turning. A significant difference (P < 0.05) between the rates of turning was found 30 min after the injection with the two different doses of the drug (Fig. 3A). Amphetamine also caused postural asymmetry towards the lesioned side and ipsilateral turning, but no significant differences were found between the two doses of the drug (Fig. 3B). Finally, haloperidol induced an asymmetry towards the intact side. Since haloperidol injected in this amount sedated the animals, the rate of turning was recorded after behavioral stimulation of the rats induced by tail-pinch. By this procedure a weak contraversive turning appeared (Fig. 2B). The histological examination of the brains showed a loss of STN neurons, which were replaced by glial cells. The animals in which the observed gliosis
220 7.
.5 rncj / k9
t3
13 2 r n c j / k 9
2 mglkc J
o
o
4 mcj l k g
6¸ 5 / E
~3 2 1
5
15
30
60 min
5
15
30
60 min
Fig. 3. Turning behavior after apomorphine (0.5 and 2 mg/kg i.p.) (A) and amphetamine(2 and 4 mg/kg i.p.) (B) administrationin kainic acid lesionedrats after spontaneous turning activity had subsided. Results are presented as rpm versus time (means + SIE.M.). Positive y-valuesindicate turning ipsilateral to the lesioned side. * P < 0.05 in respect to the rate of turning after 0.5 mg/kg administration.
occupied adjacent areas such as the substantia nigra or the ventromediat thalamus were excluded from the study, since they are critically involved in the causation of turning behavior. The small size of the STN and the rigorous criteria of the extent of the lesions restricted the success of the operation to 10 rats, in which the neuronal loss and the concomitant gliosis remained r o u ~ t y within the limits of the STN (Fig. 1). In only 3 rats the lesion extended to the overlying zona incerta, but in no case the amount of the neurons destroyed exceeded the 20 % of the total neuronal population of the zona incerta, while the neuronal loss of the STN varied between 70 and 100%. Previous studies suggested that the STN is involved in motor control, since unilateral lesion of STN induces hemiballism in the monkey [4, 10], but there exists no report concerning behavioral or motor disturbances after STN lesions in the rat. The fmdings of the present study and particularly the opposite effects of apomorphine and haloperidol suggest that the STN participates in the dopaminergic motor mechanisms controlling turning behavior in rats. The best known of such mechanisms is that which includes the nisrostriatal dopaminergic (DA) neurons and the corresponding DA receptors in the corpus striatum. Lesion or pharmacological manipulation in the nigrostriatal bundle or in the corpus striatum induce turning towards the lesioned side or away from the higher D A activity respectively [22, for review see 20]. According to recent data the turning caused by activation of this system is mediated through a nigro-strio-nigral loop [17-19]. The nigrostriatal DA neurons, through unknown interneurons, activate a descending strio-nigral GABA~gic pathway, which in turn inhibits the GABAergic cells of the substantia nigra pars reticulata. The inhibition o f these GABAergic cells, which project mainly to the ventromediat thalamus [8], seems to be the final striatal mechanism through which the turning behavior is expressed [ 19].
221 Given the electrophysiological evidence that the STN is connected with both pars compacta and pars reticulata of the SN which are two main stations of this system, it seems possible that the turning after STN lesions is brought about by direct or indirect blocking of the DA activity within the ipsilateral nigro-strionigral system. On the other hand, the turning after administration of DA related drugs could be mediated by the preferential activation of the same system on the intact side. Although this is a valuable explanation, two other possibilities must be considered in the interpretation of the observed results. According to a recent study [23] the majority of STN neurons project simultaneously to both the substantia nigra and globus pallidus in the rat. Globus pallidus is known to receive a DA innervation from collaterals of the nigrostriatal axons [13] and is also involved in the expression of the turning behavior, since electrolytic lesion of it induces turning in the rat [26]. Although the palUidal mechanisms mediating turning behavior are unknown, a possible influence of the STN on these mechanisms might be responsible for the observed turning after STN lesion. Another possibility could be the direct action of DA related drugs on the subthalamic nucleus neurons. Significant quantities of dopamine [24] and dopamine-sensitive adenylate cyclase [27] have been found in the STN. In addition, administration of D-amphetamine in the conscious rat increases the cerebral glucose utilization in the STN [25], indicating a high dopaminergic activity. The site of origin of the DA innervation of the STN is still unknown, but it cannot be excluded that DA systems directly connected to STN may play a role in the causation of turning after STN lesion. However, the exact role of these systems cannot be assessed until catecholaminergic innervation or projections of the STN will be anatomically identified. According to studies in the monkey the subthalamic nucleus is traversed by nigro-striatal ascending [2] and by pallidofugal fibers [16], but histological examination of the brains did not show any cellular loss in both the zona compacta of the substantia nigra and globus pallidus. Absence of retrograde degeneration supports the conclusion that axons of these neurons have been spared by kainic acid. This is in agreement with the general hypothesis that kainic acid acts rather selectively, destroying nerve cell bodies while sparing axons and axon terminals [ 5, 7]. The acute period of contralateral turning in the lesioned rats for a few hours after the operation may be due to the powerful stimulant activity of kainic acid on neurons in or around the STN, since a similar phenomenon is observed after kainic acid injections into other structures as striatum or substantia nigra [ 5 ]. The results of the present study are also in agreement with a recent report, which suggested that the STN participate in mediating DA striatal output functions, through a well defined pallidosubthalamic GABAergic pathway [21]. Local injection of GABA agonists and antagonists into the STN induced contraversive or ipsiversive turning in apomorphine-treated rats, indicating that the STN
222
is critically involved in motor effects induced after striatal DA receptor stimulation. But further experiments are required to establish whether the turning induced by STN lesions is due to alteration in striatal output activity through this pallidosubthalamic pathway, or to alteration in STN outputs which indirectly affected the nigrostriatal DA activity. REFERENCES 1 Carpenter, M.B., Frazer, R.A.R. and Shriver, J.E., The organization of paUido-subthalamic fibers in the monkey, Brain Res., 11 (1968) 522-559. 2 Carpenter, M.B. and Peter, P., Nigrostriatal and nigrothalamic fibers in the rhesus monkey, J. comp. Neurol., 144 (1972) 93-116. 3 Carpenter, M.B. and Strominger, N.L., Efferent fibers of the subthalamic nucleus in the monkey. A comparison of the efferent projections ofthe subthalamic nucleus, substantia nigra and globus pallidus, Amer. J. Anat., 121 (1967) 41-72. 4 Carpenter, M.B., Whittier, J.R. and Merrier, F.A., Analysis of choreoid hyperkinesia in rhesus monkey. Surgical and pharmacological analysis of hyperkinesia resulting after lesions in the subthalamic nucleus of Luys, J. comp. Neurol., 92 (1950) 293-331. 5 Coyle, J.T., Schwarz, R., Bennet, J.P. and Campochiaro, P., Clinical, neuropathologic and pharmacological aspects of Huntington's disease: correlates with a new animal model, Progr. Neuropsychopharmaeol., 1 (1979) 13-30. 6 De Groot, J., The Rat Forebrain in Stereotaxic Coordinates, 4th edn., Elsevier/North-Holland, Amsterdam, t972. 7 Di Chiara, G., Olianas, M., Del Fiacco, M., Spano, R.F. and Tagliamonte, A., Intranigral kainic acid is evidence that nigral non-dopaminergic neurons control posture, Nature (Lond.), 268 (1977) 143-145. 8 Di Chiara, G., Porceddu, M.L., Morelli, M., Mulas, M.L. and Gessa, G.L., Evidence of a GABAergic projection from the substantia nigra to the ventromedial thalamus and to the superior colliculus of the rat, Brain Res., 176 (1979) 273-284. 9 Hammond, C., Deniau, LM., Rizk, A. and Feger, J., Electrophysiological demonstration of an excitatory subthalamonigral pathway in the rat, Brain Res., 151 (1978) 235-244. I0 Hammond, C., Feger, J., Bioulac, B. and Souteyrand, J.P., Experimental hemiballism in the monkey produced by unilateral kainic acid lesion in corpus Luysii, Brain Res., 171 (1979) 577-580. 11 Kanazawa, I., Marshall, G.R. and Kelly, J.S., Afferents of the rat substantia nigra studied with horseradish peroxidase, with special reference to fibers from the subthalamic nucleus, Brain Res., 115 (1976) 485-491. 12 Katz, R.J. and Roth, K., Tail pinch induced stress-arousal facilitates brain stimulation reward, Physiol. Behav., 22 (1979) 193-194. 13 Lindvall, O. and Bjorklund, A., Dopaminergic innervation of the globus patlidus by collaterals from the nigrostriatal pathway, Brain Res., 172 (1979) 169-173. 14 Meibach, R.C. and Katzman, R., Catecholaminergic innervation of the subthalamic nucleus: evidence for a rostral continuation of the A9 (substantia nigra) dopaminergic cell group, Brain Res., 173 (1979) 364-368. 15 Nakamura, S. and Sutin, J., The pattern of termination of pallidal axons upon cells of the subthalamic nucleus, Exp. Neurol., 35 (1972)254-264. 16 Nauta, W.J.H. and Mehler, W.R., Projections of the lentiform nucleus in the monkey, Brain Res., 1 (1966) 3-42.
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