European Journal of Pharmacology, 186 (1990) 133- 136
133
Elsevier F.JP 20692 Short communication
Effect of y-butyro|actone on the retrograde axonal transport of neurotensin in dopaminergic neurones M a r i e - N o , l i e Castel, A l a i n Boireau and Pierre M. L a d u r o n RhOne-Poulenc Santd, Centre de Recherches de Vitry-AlforviUe, 13 Quai Jules Guesde. 94403 Vitry-sur-Seine Cddex. France
Received28 May 1990. accepted 10 July 1990
y-Butyrolactone is a drug known to inhibit the flow of electrical impulses in dopaminergic neurones and to prevent dopamine release in the striatum. In this study, we investigated the effects of this compound on the retrograde transport of neurotensin in the nigrostriatal dopanfinergic pathway. The amount of radioactivity measured in the substantia nigra after injection of iodinated neurotensin into the striatum and the time course of its accumulation were not found to be modified in y-butyrolactone-treated rats. We therefore conclude that the retrograde axonal transport of neurotensin in the nigrostriatal path,~_y is not affected by reduced dopaminergic firing or by inhibition of the striatal release of dopamine. Neurotensin; Axonal transport; y-Butyrolactone; Neuronal activity; Dopamine release
1. Introduction
Interactions between neurotensin and dopamine in the rat brain are now well documented (Stoessl, 1989). It has recently been demonstrated that iodinated neurotensin injected into the striarum appears in the ipsilateral substantia nigra after fast retrograde axonal transport (Castel et al., 1989; in press). This process occurs in the dopaminergic nigrostriatal pathway and was found to be dependent on the concentration of the active ligand and on the integrity of the microtubules (Castel et al., 1989; in press). The accumulation of radioactivity in the substantia nigra was maximal 3 and 4 h after injection (Castel et al., 1989; in press). The aim of the present study was to examine the effects of y-butyrolactone on the retrograde
Correspondence to: M.N. Castel, Rh6ne-PoulencSant6, Centre de Recherchesde Vitry-Alforville,13 Quai Jules Guesde,94403 Vitry-sur-SeineC6dex, France.
axonal transport of neurotensin in the nigrostriatal dopaminergic pathway, y-Butyrolactone, a drug which interacts with dopaminergic neurones (Roth et al., 1973; Imperato and Di Chiara, 1984; Westerink and Spaan, 1982), was used at subanaesthetic (200 m g / k g i.v.) and anaesthetic (750 m g / k g i.p.) doses, both of which depress the firing rate of dopaminergic neurones (Roth et al., 1973); the anaesthetic dose also prevents the striatal release of dopamine (Imperato and Di Chiara, 1984).
2. Materials and methods
Monoiodo-Tyr 3 neurotensin was purchased from Amersham (France). Unlabelled neurotensin and y-butyrolactone were obtained from Sigma (St. Louis) and chloral hydrate from Prolabo (France). Kelatorphan was a generous gift from Pr. B.P. Roques (University of Pans V). Male Sprague-Dawley rats (Charles River, France, n = 74) weighing 280-320 g were housed in a temperature (24 _+ 2" C) and fight ~on 07:00-
0014-2999/90/$03.50 © 1990 ElsevierScience Publishers B.V. (Biomedical Division)
19:00 h) controlled environment, and had free a~'ess to commerci',d rat chow and water. The rats were anaesthetized with either chloral hydrate (400 m g / k g i.p.) or y-butyrolactone (750 m g / k g i.p.). Five minutes later, 30 pg of kelatorphan in a total volume of 2 ~1 was injected at a rate of 0.2 ~al/min into the right striatum (coordinates AP 7.5 nun, L 3 ram, V 0 mm according to K~nig and Klippel (1967)) to prevent neurotensin degradation. Ten minutes later, 0.16 pmol of monoiodoTyr 3 neurotensin (2000 Ci/mmol), dissolved in saline in a total volume of 3/tl, was infused into three different areas of the right striatum (coordinates AP 9.4 ram, L 2 mm and V 0.2 mm; AP 6.6 ram, L 3.5 mm and V - 0 . 4 ram; AP 4.6 mm, L 4.4 mm and V - 0 . 6 ram). In chloral hydrateanaesthetized rats, saline and y-butyrolactone (200 mg/kg) were administered i.v. 5 rain before the injection of iodinated-neurotensin. The coordinates were verified by dye injection. Rats were killed 1, 2, 4 or 6 h after the end of the neurotensin injection, and their brains dissected out. The radioactivity in the ipsi- and the C~n 350-
(7)
300'
contralateral substantia nigra was measured with an LKB gamma counter. Wilcoxon's test was used to compare data between groups.
3. Results
No radioacti,Aty was detected 1 h after inj~tion of iodinated neurotensin into the striatum (data not shown, n = 4 for each treatment). Some radioactivity was detected in the ipsilateral substantia nigra 2 h after injection of iodinated-neurotensin. The amount of radioactivity was maximal at 4 h and decreased at 6 h (fig. 1). This time course of the appearance of radioactivity in the ipsilateral substantia nigra was the same regardless of the treatment used. No significant difference was observed in the amount of radioactivity measured in the substantia nigra when chloral hydrate-anaesthetized rats received saline or ybutyrolactone (200 m g / k g i.v.), or when animals were anaesthetized with a high dose of y-butyrolactone (750 m g / k g i.p.). Radioactivity was undetectable in the contralateral substantia nigra.
4. Discussion
(7)
250'
161
•i]I
200'
150' 100
50
(4)
161 151
0 2 Hours
4 Hours
6 Hours
Fig. 1. Effect of ¥-butyrolactone on the retrograde transport of neurotensin in the nigrostriatal dopaminergic pathway. Radio-. activity was measured in the fight substantia nigra 2, 4 or 6 h after the injection of monoiodo-Tyr3 neurotensin into the striatum. Rats were anaesthetized either with chloral hydrate (400 mg/',,g i.p.) or with ~,-butyrolactone (GBL) at the dose of 750 mg/kg i.p. Chloral hydrate (Chloral, 400 mm/kg i.p.)anaesthetized rats were treated with saline 5 rain before iodinated neurotensin injection into the striaturn, or with ¥butyrolactone (200 mg/kg i.v., Chloral + GBL). The numbers between parentheses represent the number of rats u,~,d. Results are expressed in dpm+S.E.M. [] Chloral 400 mg/kg i.p. g! GBL 750 mg/kg i.p. m Chloral 400 mg/kg i.p. +GBL 200 mg/kg i.v.
We recently demonstrated that neurotensin moves retrogradely from the striatum towards the ipsilateral substantia nigra inside dopaminergic neurones (Castei et al., 1989; in press). In order to study the effects of cessation of impulse flow and blockade of dopamine release on the retrograde axonal transport of neurotensin in the nigrostriatal dopaminergic pathway, we used two sets of experimental conditions. Many authors have shown that ~,-butyrolactone interferes with electrochemical and biochemical parameters under the experimental conditions used (Roth et al., 1973; lmperato and Di Chiara, 1984; Westerink and Spaan, 1982). In the first set of experiments, we studied the effect of inhibition of dopaminergic impulse flm.v on the retrograde transport of neurotensin in the nigrostfiatal pathway, y-Butyrolactone was injected i.v. at a dose of 200 mg/kg; it has pi'eviously been demonstrated that this drug completdy
135
inhibits the dopaminergic firing rate at a dose of 100 m g / k g i.v. in chloral hydrate-anaesthet/zed rats (Roth et al., 1973). This inhibitory effect of ~,-butyrolactone was found to appear rapidly, less than 5 rain after its peripheral injection, and recovery of the dopaminergic firing rate occurred gradually and was dose-depe~dent (Roth et al., 1973). For instance, a 25% recovery of dopaminergic cells in the substanti~ nigra pars compacta was obtained 35 min after the i.v. administration of 150 m g / k g of v-butyrolactone (Roth et al., 1973). Since our experimental conditions were similar to those used by Roth et al. (1973), we may assume that the injection of neurotensin into the striatum and its subsequent binding and internalization occurred when dopaminergic firing was totally inhibited. Our results show that the amount and the time course of accumulation of radioactivity in the substantia nigra were the same in control and in y-butyrolactone-treated rats, indicating that the time needed for neurotensin to bind to its receptors and to be internalized was not altered. Since no radioactivity was detected in the substantia nigra 4 h after co-injection of iodinated neurotensin and a 1000-fold excess of unlabeiled neurotensin into the striata of chloral hydrateand ~,-butyrolactone-treated rats, we can conclude "that this transport was not the result of a diffusion phenomenon, but involved neurotensin receptors. Thus, we have demonstrated that, under experimental procedures where the dopaminergic firing rate has been previously demonstrated to be interrupted (Roth et al., 1973), no change in the quantity of internalized neurotensin is observed, since there was no difference in the levels of radioactivity measured in the substaatia nigra of control and treated rats. In addition, no modification of the time course of transport in the nigrostriatal pathway occurred. Our results are the first to demonstrate the influence of nervous activity on the retrograde axonal transport of molecules in the CNS. Some investigators (Geffen and Rush, 1968; Keen and McLean, 1974) have studied the interaction between firing rate and axonal transport and have demonstrated that neuronal activity is not neces-
sary for anterograde axonal transport in peripheral nerves. No data is currently available concerning the effects of a 200 m g / k g dose of ~,-butyrolactone on dopamine release in anaesthetized rats. However, it is established that at an anaesthetic dose of 750 m g / k g i.p., y-butyrolactone completely inhibits both the dopaminergic firing rate and the release of dopamine in the striatum (Roth et al., 1973; Imperato and Di Chiara, 1984; Westerink and Spaan, 1982). Thus, in the second set of experiments, using a dose of 750 mg/kg i.p., we studied the effects of an inhibition of dopamine release on the retrograde transport of neurotensin. According to Imperato and Di Chiara (1984), v-butyrolactone (700 m g / k g i.p.) produces a sharp drop in the amount of dopamine released. This inhibitory effect begins during the first half-hour and lasts for approximately 6 h (lmperato and Di Chiara, 1984). In our study, ~,-butyrolactone was injected 30 min before iodinated neurotensin. Thus, it can be predicted that dopamine was no longer being released in the striatum. No significant difference was observed between the amount of radioactivity measured in the substantia nigra and the time course of label accumulation in this structure between chloral hydrate- and ~,-butyrolactoneanaesthetized rats. The retrograde transport of neurotensin seemed to be independent of the inhibition of dopamine release in the striatum. The results presented here indicated that dopaminergic firing rate and dopamine release in the striatum are not essential for the retrograde axonai transport of neurotens'.'n in the nigrostriatal dopaminergic pathway. It would be of great interest to study the influence of electrical nerve stimulation on the retrograde axonal transport of neurotensin because it is known that when the nigrostriatal pathway is stimulated, there is an immediate release of dopamine in the striatum (Imperato and Di Chiara, 1984), which could modulate the binding and internalization of neurotensin in striatum. Moreover, such an experiment could give information on the effect of an increased amount of fast-transport proteins, induced by a nerve stimulation, on the retrograde axonal transport of neurotensin.
|36
Ac~ovdedgements We are grateful to Pr. B.P. Roques for the gift of kelatorphan and Dr. D. GLrdlestone for stimulating discussion. We wish to thank Karen Birmingham fo~"her help in preparing the manuscript.
References CasteL M.N., C. Malgouri~ J.C. Blanchard and P.M. Laduron, 1989, Retrograde axonal transport of neurotensin in the rat brain, European J. Pharmacol. 166, 353. Castel, M.N., C. Malgouris, J.C. Blanchard and P.M. Laduron, Retrograde axonai transport of neuroteusin in the dopaminergic nigrostriatal pathway in the rat, Neuroscience (in press). Geffen, LB. and ILA. Rush, 1968, Transport of noradrenafine in sympathetic nerves and the effect of nerve impulses on its contribution to transmitter stores, J. Neurochem. 15, 925. Imperato, A. and G. Di Chiara, 1984, Trans-striatai dialysis
coupled to reverse phase high performance liquid chromatography with electrochemical detection: a new method for the study of the in vivo release of endogenous dopamine and metabolites, J. Neurosci. 4, 966. Keen, P. and W.G. McLean, 1974, The effect of nerve stimulation on the axonai transport of noradrenaline and dopamine-/~-hydroxylase, Br. J. Pharmacol. 52, 527. K~nig, F.R.G. and R.A. Klippel, 1967, The Rat Brain. A Stereotaxic Atlas of the Forebrain and the Lower Parts of the Brain Stem (Robert E. Krieger Publishing Co. Inc., New-York). Roth, R.H., J.R. Waiters and G.K. Aghajanian, 1973, Effect of impulse flow on the release and synthesis of dopamine in thG rat striatum, in: Frontiers in Catecholamines Research (Pergamon Press, Great Britain) p. 567. Stoessl, AJ., 1989, Peptide-dopamine interactions in the central nervous system: implications for neuropsychiatric disorders, J. Psychopharmacol. 3, 99. Westerink, B.H.C. and S.J. Spaan, 1982, On the significance of endogenous 3-methoxytyramine for the effects of centrally acting drugs on dopamine release in the rat brain, J. Neurochem. 38, 680.
133
Elsevier F.JP 20692 Short communication
Effect of y-butyro|actone on the retrograde axonal transport of neurotensin in dopaminergic neurones M a r i e - N o , l i e Castel, A l a i n Boireau and Pierre M. L a d u r o n RhOne-Poulenc Santd, Centre de Recherches de Vitry-AlforviUe, 13 Quai Jules Guesde. 94403 Vitry-sur-Seine Cddex. France
Received28 May 1990. accepted 10 July 1990
y-Butyrolactone is a drug known to inhibit the flow of electrical impulses in dopaminergic neurones and to prevent dopamine release in the striatum. In this study, we investigated the effects of this compound on the retrograde transport of neurotensin in the nigrostriatal dopanfinergic pathway. The amount of radioactivity measured in the substantia nigra after injection of iodinated neurotensin into the striatum and the time course of its accumulation were not found to be modified in y-butyrolactone-treated rats. We therefore conclude that the retrograde axonal transport of neurotensin in the nigrostriatal path,~_y is not affected by reduced dopaminergic firing or by inhibition of the striatal release of dopamine. Neurotensin; Axonal transport; y-Butyrolactone; Neuronal activity; Dopamine release
1. Introduction
Interactions between neurotensin and dopamine in the rat brain are now well documented (Stoessl, 1989). It has recently been demonstrated that iodinated neurotensin injected into the striarum appears in the ipsilateral substantia nigra after fast retrograde axonal transport (Castel et al., 1989; in press). This process occurs in the dopaminergic nigrostriatal pathway and was found to be dependent on the concentration of the active ligand and on the integrity of the microtubules (Castel et al., 1989; in press). The accumulation of radioactivity in the substantia nigra was maximal 3 and 4 h after injection (Castel et al., 1989; in press). The aim of the present study was to examine the effects of y-butyrolactone on the retrograde
Correspondence to: M.N. Castel, Rh6ne-PoulencSant6, Centre de Recherchesde Vitry-Alforville,13 Quai Jules Guesde,94403 Vitry-sur-SeineC6dex, France.
axonal transport of neurotensin in the nigrostriatal dopaminergic pathway, y-Butyrolactone, a drug which interacts with dopaminergic neurones (Roth et al., 1973; Imperato and Di Chiara, 1984; Westerink and Spaan, 1982), was used at subanaesthetic (200 m g / k g i.v.) and anaesthetic (750 m g / k g i.p.) doses, both of which depress the firing rate of dopaminergic neurones (Roth et al., 1973); the anaesthetic dose also prevents the striatal release of dopamine (Imperato and Di Chiara, 1984).
2. Materials and methods
Monoiodo-Tyr 3 neurotensin was purchased from Amersham (France). Unlabelled neurotensin and y-butyrolactone were obtained from Sigma (St. Louis) and chloral hydrate from Prolabo (France). Kelatorphan was a generous gift from Pr. B.P. Roques (University of Pans V). Male Sprague-Dawley rats (Charles River, France, n = 74) weighing 280-320 g were housed in a temperature (24 _+ 2" C) and fight ~on 07:00-
0014-2999/90/$03.50 © 1990 ElsevierScience Publishers B.V. (Biomedical Division)
19:00 h) controlled environment, and had free a~'ess to commerci',d rat chow and water. The rats were anaesthetized with either chloral hydrate (400 m g / k g i.p.) or y-butyrolactone (750 m g / k g i.p.). Five minutes later, 30 pg of kelatorphan in a total volume of 2 ~1 was injected at a rate of 0.2 ~al/min into the right striatum (coordinates AP 7.5 nun, L 3 ram, V 0 mm according to K~nig and Klippel (1967)) to prevent neurotensin degradation. Ten minutes later, 0.16 pmol of monoiodoTyr 3 neurotensin (2000 Ci/mmol), dissolved in saline in a total volume of 3/tl, was infused into three different areas of the right striatum (coordinates AP 9.4 ram, L 2 mm and V 0.2 mm; AP 6.6 ram, L 3.5 mm and V - 0 . 4 ram; AP 4.6 mm, L 4.4 mm and V - 0 . 6 ram). In chloral hydrateanaesthetized rats, saline and y-butyrolactone (200 mg/kg) were administered i.v. 5 rain before the injection of iodinated-neurotensin. The coordinates were verified by dye injection. Rats were killed 1, 2, 4 or 6 h after the end of the neurotensin injection, and their brains dissected out. The radioactivity in the ipsi- and the C~n 350-
(7)
300'
contralateral substantia nigra was measured with an LKB gamma counter. Wilcoxon's test was used to compare data between groups.
3. Results
No radioacti,Aty was detected 1 h after inj~tion of iodinated neurotensin into the striatum (data not shown, n = 4 for each treatment). Some radioactivity was detected in the ipsilateral substantia nigra 2 h after injection of iodinated-neurotensin. The amount of radioactivity was maximal at 4 h and decreased at 6 h (fig. 1). This time course of the appearance of radioactivity in the ipsilateral substantia nigra was the same regardless of the treatment used. No significant difference was observed in the amount of radioactivity measured in the substantia nigra when chloral hydrate-anaesthetized rats received saline or ybutyrolactone (200 m g / k g i.v.), or when animals were anaesthetized with a high dose of y-butyrolactone (750 m g / k g i.p.). Radioactivity was undetectable in the contralateral substantia nigra.
4. Discussion
(7)
250'
161
•i]I
200'
150' 100
50
(4)
161 151
0 2 Hours
4 Hours
6 Hours
Fig. 1. Effect of ¥-butyrolactone on the retrograde transport of neurotensin in the nigrostriatal dopaminergic pathway. Radio-. activity was measured in the fight substantia nigra 2, 4 or 6 h after the injection of monoiodo-Tyr3 neurotensin into the striatum. Rats were anaesthetized either with chloral hydrate (400 mg/',,g i.p.) or with ~,-butyrolactone (GBL) at the dose of 750 mg/kg i.p. Chloral hydrate (Chloral, 400 mm/kg i.p.)anaesthetized rats were treated with saline 5 rain before iodinated neurotensin injection into the striaturn, or with ¥butyrolactone (200 mg/kg i.v., Chloral + GBL). The numbers between parentheses represent the number of rats u,~,d. Results are expressed in dpm+S.E.M. [] Chloral 400 mg/kg i.p. g! GBL 750 mg/kg i.p. m Chloral 400 mg/kg i.p. +GBL 200 mg/kg i.v.
We recently demonstrated that neurotensin moves retrogradely from the striatum towards the ipsilateral substantia nigra inside dopaminergic neurones (Castei et al., 1989; in press). In order to study the effects of cessation of impulse flow and blockade of dopamine release on the retrograde axonal transport of neurotensin in the nigrostriatal dopaminergic pathway, we used two sets of experimental conditions. Many authors have shown that ~,-butyrolactone interferes with electrochemical and biochemical parameters under the experimental conditions used (Roth et al., 1973; lmperato and Di Chiara, 1984; Westerink and Spaan, 1982). In the first set of experiments, we studied the effect of inhibition of dopaminergic impulse flm.v on the retrograde transport of neurotensin in the nigrostfiatal pathway, y-Butyrolactone was injected i.v. at a dose of 200 mg/kg; it has pi'eviously been demonstrated that this drug completdy
135
inhibits the dopaminergic firing rate at a dose of 100 m g / k g i.v. in chloral hydrate-anaesthet/zed rats (Roth et al., 1973). This inhibitory effect of ~,-butyrolactone was found to appear rapidly, less than 5 rain after its peripheral injection, and recovery of the dopaminergic firing rate occurred gradually and was dose-depe~dent (Roth et al., 1973). For instance, a 25% recovery of dopaminergic cells in the substanti~ nigra pars compacta was obtained 35 min after the i.v. administration of 150 m g / k g of v-butyrolactone (Roth et al., 1973). Since our experimental conditions were similar to those used by Roth et al. (1973), we may assume that the injection of neurotensin into the striatum and its subsequent binding and internalization occurred when dopaminergic firing was totally inhibited. Our results show that the amount and the time course of accumulation of radioactivity in the substantia nigra were the same in control and in y-butyrolactone-treated rats, indicating that the time needed for neurotensin to bind to its receptors and to be internalized was not altered. Since no radioactivity was detected in the substantia nigra 4 h after co-injection of iodinated neurotensin and a 1000-fold excess of unlabeiled neurotensin into the striata of chloral hydrateand ~,-butyrolactone-treated rats, we can conclude "that this transport was not the result of a diffusion phenomenon, but involved neurotensin receptors. Thus, we have demonstrated that, under experimental procedures where the dopaminergic firing rate has been previously demonstrated to be interrupted (Roth et al., 1973), no change in the quantity of internalized neurotensin is observed, since there was no difference in the levels of radioactivity measured in the substaatia nigra of control and treated rats. In addition, no modification of the time course of transport in the nigrostriatal pathway occurred. Our results are the first to demonstrate the influence of nervous activity on the retrograde axonal transport of molecules in the CNS. Some investigators (Geffen and Rush, 1968; Keen and McLean, 1974) have studied the interaction between firing rate and axonal transport and have demonstrated that neuronal activity is not neces-
sary for anterograde axonal transport in peripheral nerves. No data is currently available concerning the effects of a 200 m g / k g dose of ~,-butyrolactone on dopamine release in anaesthetized rats. However, it is established that at an anaesthetic dose of 750 m g / k g i.p., y-butyrolactone completely inhibits both the dopaminergic firing rate and the release of dopamine in the striatum (Roth et al., 1973; Imperato and Di Chiara, 1984; Westerink and Spaan, 1982). Thus, in the second set of experiments, using a dose of 750 mg/kg i.p., we studied the effects of an inhibition of dopamine release on the retrograde transport of neurotensin. According to Imperato and Di Chiara (1984), v-butyrolactone (700 m g / k g i.p.) produces a sharp drop in the amount of dopamine released. This inhibitory effect begins during the first half-hour and lasts for approximately 6 h (lmperato and Di Chiara, 1984). In our study, ~,-butyrolactone was injected 30 min before iodinated neurotensin. Thus, it can be predicted that dopamine was no longer being released in the striatum. No significant difference was observed between the amount of radioactivity measured in the substantia nigra and the time course of label accumulation in this structure between chloral hydrate- and ~,-butyrolactoneanaesthetized rats. The retrograde transport of neurotensin seemed to be independent of the inhibition of dopamine release in the striatum. The results presented here indicated that dopaminergic firing rate and dopamine release in the striatum are not essential for the retrograde axonai transport of neurotens'.'n in the nigrostriatal dopaminergic pathway. It would be of great interest to study the influence of electrical nerve stimulation on the retrograde axonal transport of neurotensin because it is known that when the nigrostriatal pathway is stimulated, there is an immediate release of dopamine in the striatum (Imperato and Di Chiara, 1984), which could modulate the binding and internalization of neurotensin in striatum. Moreover, such an experiment could give information on the effect of an increased amount of fast-transport proteins, induced by a nerve stimulation, on the retrograde axonal transport of neurotensin.
|36
Ac~ovdedgements We are grateful to Pr. B.P. Roques for the gift of kelatorphan and Dr. D. GLrdlestone for stimulating discussion. We wish to thank Karen Birmingham fo~"her help in preparing the manuscript.
References CasteL M.N., C. Malgouri~ J.C. Blanchard and P.M. Laduron, 1989, Retrograde axonal transport of neurotensin in the rat brain, European J. Pharmacol. 166, 353. Castel, M.N., C. Malgouris, J.C. Blanchard and P.M. Laduron, Retrograde axonai transport of neuroteusin in the dopaminergic nigrostriatal pathway in the rat, Neuroscience (in press). Geffen, LB. and ILA. Rush, 1968, Transport of noradrenafine in sympathetic nerves and the effect of nerve impulses on its contribution to transmitter stores, J. Neurochem. 15, 925. Imperato, A. and G. Di Chiara, 1984, Trans-striatai dialysis
coupled to reverse phase high performance liquid chromatography with electrochemical detection: a new method for the study of the in vivo release of endogenous dopamine and metabolites, J. Neurosci. 4, 966. Keen, P. and W.G. McLean, 1974, The effect of nerve stimulation on the axonai transport of noradrenaline and dopamine-/~-hydroxylase, Br. J. Pharmacol. 52, 527. K~nig, F.R.G. and R.A. Klippel, 1967, The Rat Brain. A Stereotaxic Atlas of the Forebrain and the Lower Parts of the Brain Stem (Robert E. Krieger Publishing Co. Inc., New-York). Roth, R.H., J.R. Waiters and G.K. Aghajanian, 1973, Effect of impulse flow on the release and synthesis of dopamine in thG rat striatum, in: Frontiers in Catecholamines Research (Pergamon Press, Great Britain) p. 567. Stoessl, AJ., 1989, Peptide-dopamine interactions in the central nervous system: implications for neuropsychiatric disorders, J. Psychopharmacol. 3, 99. Westerink, B.H.C. and S.J. Spaan, 1982, On the significance of endogenous 3-methoxytyramine for the effects of centrally acting drugs on dopamine release in the rat brain, J. Neurochem. 38, 680.
