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Intrapallidal D2 dopamine receptors control globus pallidus neuron activity in the rat
Neuroscience Letters 300 (2001) 79±82
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Intrapallidal D2 dopamine receptors control globus pallidus neuron activity in the rat E. Querejeta a, A. Delgado a, R. Valdiosera a, D. Erlij b, J. Aceves a,* a
Departamento de Fisiologia, BiofõÂsica y Neurociencias, Centro de InvestigacioÂn y de Estudios Avanzados del Instituto PoliteÂcnico Nacional, Apartado Postal 14140, 07000 MeÂxico City, MeÂxico b Department of Physiology, SUNY Downstate Medical Center, Brooklyn, NY 11203, USA Received 20 November 2000; received in revised form 14 December 2000; accepted 28 December 2000
Abstract Because activation of D2 dopamine receptors inhibits g-aminobutyric acid (GABA) release from intrapallidal nerve terminals, we measured the effects of modi®ers of dopamine D2 receptors on the ®ring rate of single neurons in the globus pallidus (GP) of the anesthetized rat. The predominant effect of intrapallidal administration of the selective D2 agonist quinpirole was an increase in the rate of spontaneous ®ring while the D2 blocker sulpiride caused a decrease. The spontaneous ®ring of GP neurons is inhibited by stimulation of the GABAergic striatopallidal projection. We therefore measured the effects of modi®ers of D2 receptors on striatal inhibition of GP neurons and found that intrapallidal quinpirole blocked the inhibitory effects of striatal stimulation while sulpiride enhanced them. These experiments show that both the spontaneous rate of ®ring of pallidal neurons and its modi®cation by striatopallidal inputs is controlled by intrapallidal dopamine D2 receptors. In addition, taken together with other ®ndings in the literature, our results suggest that activation of dopamine D2 receptors within the globus pallidus leads to inhibition of GABA release from presynaptic terminals. q 2001 Published by Elsevier Science Ireland Ltd. Keywords: Basal ganglia; Dopamine receptors; g-aminobutyric acid release and transmission; Quinpirole; Striatopallidal transmission; Sulpiride
Dopamine D2 receptors in the rat globus pallidus (GP) appear to have a major role in the control of transmitter release by g-aminobutyric acid (GABA)-ergic terminals within this nucleus. Studies in slices show that activation of D2 receptors with the selective agonist quinpirole inhibits GABA release induced by either electrical stimulation [15] or K 1 depolarization [6]. Furthermore, blockade of D2 receptors by the intrapallidal administration of sulpiride causes a marked increase in GABA release [6,12]. To determine whether the control of pallidal output by D2 dopamine receptors in vivo [5,7] is consistent with the effects on GABA release from presynaptic terminals, we measured the effects of intrapallidal injection of modi®ers of D2-like receptors on the rate of ®ring of single neurons. Previous experiments have already shown that dopaminergic agents modulate the discharge of pallidal neurons, however, the receptors mediating these effects appeared to
* Corresponding author. Tel.: 152-5-7477000 ext. 5119; fax: 152-5-7477105. E-mail address: jaceves@®sio.cinvestav.mx (J. Aceves).
be located within the striatum [2,3,8,10] A preliminary account of our results has already been published [16]. Extracellular single-unit recordings were performed in ketamine anesthetized (150 mg/kg intraperitoneally (i.p.)) male Wistar rats weighing 180±200 g, maintained and handled according with the guidelines of the CINVESTAV-IPN Animal Care Committee. In addition, all experimental procedures were in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals. Supplemental ketamine (50 mg/kg) was given as needed. Once anesthetized, the rat was mounted on a David Kopf stereotaxic frame. Body temperature was maintained at 36±388C with a heating pad. Glass microelectrodes ®lled with 2 M NaCl with tip resistance 2±6 MV were used. Microelectrodes were stereotactically guided through drilled skull holes to the following co-ordinates: 0.0 mm anterior to bregma, 3.7 mm lateral to the midline and 4.5 mm ventral to the dura. Electrical signals were ampli®ed by a DAM-80 preampli®er (WPI) and processed online using a program for spike data analysis [18]. The signals were also monitored with an oscilloscope and stored on an audiocas-
0304-3940/01/$ - see front matter q 2001 Published by Elsevier Science Ireland Ltd. PII: S03 04 - 394 0( 0 1) 01 55 0- 6
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E. Querejeta et al. / Neuroscience Letters 300 (2001) 79±82
Fig. 1. Intrapallidal quinpirole infusion (2 nmol) increases the spontaneous rate of pallidal neuron ®ring. Frequency histograms of two neurons (A, B) are shown to illustrate that the effect is seen in neurons with very different initial rates of spontaneous ®ring. Horizontal bar indicates the period of infusion. (C) Illustrates the dose-dependence of the effect. *P , 0:001 as compared with basal values.
sette. Single-unit activity was isolated with a window discriminator (WPI-121) and ®ring frequency was analyzed off line in a computer using the software SPS-8701 (Signal Processing Systems). All selected units had biphasic (^) type II waveforms [10] and amplitudes greater than 200 mV. The drugs were infused into the globus pallidus (GP) through cannulas made of 30 gauge syringe needles. The cannulas were stereotactically implanted at an angle of 608 in the latero-medial direction to the following co-ordinates: 0.2 mm anterior to bregma, 4.5 mm lateral to the midline and 4.6 mm ventral to the dura. The tips of the recording microlectrode and injection cannula were within a range of not greater than 0.2 mm. The drugs were dissolved in physiological saline, and were infused into the GP by means of a 5 ml syringe. The piston of the syringe was connected to a precision micrometer that allowed the slow (<3 nl/s) infusion of the drugs in a volume of 200 nl. The slow infusion of this volume of saline did not affect the spontaneous ®ring rate of pallidal neurons. For the experiments in which the striatum was electrically stimulated, a monopolar tungsten wire electrode was stereotactically placed in the striatum with an angle of 708 in the anteroposterior direction to the following co-ordinates: 3.5 mm anterior to bregma, 3.5 mm lateral to the midline and 4.5 mm ventral to dura. Rectangular pulses of 0.1 ms duration
with intensities of 25±100 mA and frequencies of 5±20 Hz were selected in each individual experiment to produce a response suitable to the experimental goals (see Fig. 3). At the end of the experiment the brain was removed, after an intracardiac administration of 200 ml of 4% formaldehyde for ®xation. After overnight incubation in formaldehyde, the brains were sliced to verify the position of the recording microlectrode and the injection cannula in the GP and the stimulating electrode in the striatum. When either the recording microelectrode or the injection cannula was not positioned in the globus pallidus the experiment was discarded. The spontaneous rates of discharge of pallidal neurons varied from 0.1 to 93 Hz in the 382 cells examined (average ^ SEM 19 ^ 1 Hz). Individual examples can be seen in the control portions of the frequency histograms of Figs. 1±3. The predominant effect of intrapallidal administration of the D2 receptor agonist quinpirole was an increase in the rate of spontaneous ®ring rate (Fig. 1). Two examples are illustrated in Fig. 1 to show that the increase in ®ring caused by quinpirole was observed in cells with widely different basal rates of discharge. In 11 out of 24 neurons in which quinpirole was tested at a dose of 2 nmol the frequency increased by 112 ^ 32%. In two neurons it decreased by 46 ^ 11% and in 11 neurons the ®ring rate was not affected. As shown in Fig. 1C, the effect of quinpirole was dose-dependent. The predominant effect of intrapallidal infusion of the D2 antagonist sulpiride was a decrease in the spontaneous ®ring. Responses in two neurons with different spontaneous patterns of discharge are illustrated, one with a regular (Fig. 2A), and the other with a bursting ®ring pattern (Fig. 2B). In 14 of 29 neurons tested, sulpiride decreased the rate of ®ring to 66 ^ 7% of the basal. In 12 neurons the frequency was not affected and in three the ®ring rate increased (39 ^ 1%). The GP receives a major input from GABAergic ®bers
Fig. 2. Intrapallidal sulpiride infusion (200 pmol) decreases the spontaneous rate of pallidal neuron ®ring. (A) The effect of sulpiride on a neuron with regular ®ring rate. (B) The effect of sulpiride on a neuron with bursting ®ring pattern. Horizontal bar indicates the period of infusion
E. Querejeta et al. / Neuroscience Letters 300 (2001) 79±82
Fig. 3. The effects of striatal stimulation on globus pallidus neuron ®ring are modi®ed by injection of either quinpirole or sulpiride. The upper tracing in (A) shows the inhibitory effects of electrical stimulation (100 mA, 0.1 ms, 20 Hz) to the striatum. Horizontal bar indicates the period of stimulation (St). Between the upper and lower tracing 200 pmol of quinpirole were injected into the pallidum. The lower tracing shows the discharge of the same neuron; when the electrical stimulation was repeated after the injection, the inhibitory effects were diminished considerably. (B) Another example of electrical stimulation to the striatum; the stimulation parameters (25 mA, 0.1 ms, 10 Hz) were selected to produce a minimal inhibitory response. Between the upper and lower tracing 200 pmol of sulpiride were injected into the pallidum. Repeating the stimulation after the injection (lower tracing) produced a clear-cut inhibitory effect.
originating in the striatum. To test whether pallidal neurons in our experiments were under tonic GABAergic in¯uences we infused bicuculline (100 pmol) into the pallidum. In all 11 neurons tested, spontaneous ®ring rate increased (154 ^ 86 %, mean ^ SEM). Thus, the effects described above of the dopaminergic modi®ers may be mediated by changes in these tonic GABAergic in¯uences. Additional evidence of dopaminergic control of GABAergic striatopallidal in¯uences can be obtained by measuring the effects of dopaminergic agents on the
81
response of GP to striatal stimulation. Fig. 3A shows that quinpirole completely blocked the inhibitory effects of electrical stimulation of the striatum. The tracings illustrate an example of the procedure used to collect these data. Stimulation parameters that produced a clear-cut inhibitory effect were ®rst selected (upper tracing). Then quinpirole (200 pmol) was injected, and the stimulation was again applied (lower tracing). Quinpirole reduced the inhibition from 89 ^ 3 to 30 ^ 6% (n 14) of the basal frequency. Fig. 3B shows that sulpiride (200 pmol) increased the inhibitory effects of the stimulation from 21 ^ 7 to 72 ^ 9% of basal frequency. For these experiments we selected stimulation parameters that produced a minimal inhibitory response during the control period (upper tracing). Between the upper and lower tracing 200 pmol of sulpiride were injected into the GP. Repeating the stimulation after the injection produced a clear-cut inhibitory effect. The marked effects of intrapallidal injections of quinpirole and sulpiride show that the spontaneous rate of discharge of pallidal neurons in the anesthetized rat is under tonic control of D2-like dopaminergic receptors located within this nucleus. The bulk of the evidence suggest that this intrapallidal dopaminergic control of neuron ®ring is mediated through the same mechanism previously described in brain slices, namely, the modulation of GABA release from the presynaptic terminal [6,15]. The effects of both quinpirole and sulpiride on the response to electrical stimulation of the striatum, described here, are consistent with a presynaptic site of action. Moreover, it has been shown recently [4] that dopamine depresses striatal GABAergic inhibitory postsynaptic currents in the globus pallidus through a presynaptic mechanism. In these experiments dopamine was without effect on postsynaptic conductances. This is a very intriguing observation because receptor autoradiography and in situ hybridization reveal that D2 binding sites and the corresponding mRNA coexist in the globus pallidus indicating that pallidal cell bodies express the D2 receptor [14]. Prior investigators already observed effects of activation of dopaminergic receptors on pallidal neuron ®ring. Systemic administration of dopamine agonists increased the ®ring rate of most pallidal neurons [2,3,8,10] The increase in ®ring was interpreted as due to activation of striatal receptors regulating the output to the globus pallidus. However, the results described here, taken together with previous GABA release experiments [6,15] and the presence of dopamine D2 receptors within the pallidum [1,9,14] suggest that dopaminergic terminals that originate in the substantia nigra and terminate in the globus pallidus [13] play an important role in controlling the output of this nucleus. The ®nding that a few cells decrease basal ®ring after quinpirole administration and a few cells increase ®ring after sulpiride administration remains to be explained. One possibility for the inhibitory effects seen in a few neurons when quinpirole was used is suggested by some
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E. Querejeta et al. / Neuroscience Letters 300 (2001) 79±82
recent observations of Kita and Kitai [11] who found that axons of GP neurons emit multiple collaterals with large boutons en-passant and boutons terminaux within the globus pallidus. Accordingly, excitation of some pallidal neurons would inhibit the ®ring of the pallidal neurons receiving GABAergic collaterals. This mechanism could also explain the increase in ®ring of some neurons caused by sulpiride; the effect could be caused by disinhibition of the GABAergic input via these collaterals. In conclusion, these data show that intrapallidal dopamine D2 receptors play an important role in controlling the output of globus pallidus neurons. The predominant effect of the activation of intrapallidal D2 receptors is an increase in the ®ring rate. This increase is in agreement with predictions derived from release experiments since these showed that activation of pallidal D2 receptors causes inhibition of GABA release from presynaptic terminals. The inhibition of pallidal ®ring caused by a D2 antagonist, as described here, would lead to increased activity of neurons in the subthalamic nucleus and the output nuclei of the basal ganglia [17], which would in turn depress motor behavior. This work was supported by a grant (3539P-N9607) from CONACYT (MeÂxico). E.Q. was a CONACYT fellow. [1] Beckstead, R.M., Wooten, G.F. and Trugman, J.M., Distribution of D1 and D2 dopamine receptors in the basal ganglia of the cat determined by quantitative autoradiography, J. Comp. Neurol., 268 (1988) 131±145. [2] Bergstrom, D.A., Bromley, S.D. and Walters, J.R., Apomorphine increases the activity of rat globus pallidus neurons, Brain Res., 238 (1982) 266±271. [3] Bergstrom, D.A., Bromley, S.D. and Walters, J.R., Dopamine agonists increase pallidal unit activity: attenuation by agonist pretreatment and anesthesia, Eur. J. Pharmacol., 100 (1984) 3±12. [4] Cooper, A. and Stanford, I.M., Dopamine-mediated inhibition of striatal GABAA IPSCs in the rat globus pallidus in vitro, J. Physiol., 525P (2000) 52P±53P. [5] Costall, B., Naylor, R.J. and Olley, J.E., Catalepsy and circling behaviour after intracerebral injections of neuroleptic, cholinergic and anticholinergic agents into the caudateputamen, globus pallidus and substantia nigra of rat brain, Neuropharmacology, 11 (1972) 645±663. [6] Floran, B., Floran, L., Sierra, A. and Aceves, J., D2 receptormediated inhibition of GABA release by endogenous dopa-
[7] [8]
[9]
[10]
[11] [12]
[13] [14]
[15]
[16]
[17] [18]
mine in the rat globus pallidus, Neurosci. Lett., 237 (1997) 1±4. Hauber, W. and Lutz, S., Dopamine D1 or D2 receptor blockade in the globus pallidus produces akinesia in the rat, Behav. Brain Res., 106 (1999) 143±150. Hooper, K.C., Banks, D.A., Stordahl, L.J., White, I.M. and Rebec, G.V., Quinpirole inhibits striatal and excites pallidal neurons in freely moving rats, Neurosci. Lett., 237 (1997) 69±72. Huang, N., Ase, A.R., Hebert, C., van Gelder, N.M. and Reader, T.A., Effects of chronic neuroleptic treatments on dopamine D1 and D2 receptors: homogenate binding and autoradiographic studies, Neurochem. Int., 30 (1997) 277± 290. Kelland, M.D., Soltis, R.P., Anderson, L.A., Bergstrom, D.A. and Walters, J.R., In vivo characterization of two cell types in the rat globus pallidus which have opposite responses to dopamine receptor stimulation: comparison of electrophysiological properties and responses to apomorphine, dizocilpine, and ketamine anesthesia, Synapse, 20 (1995) 338± 350. Kita, H. and Kitai, S.T., The morphology of globus pallidus projection neurons in the rat: an intracellular staining study, Brain Res., 636 (1994) 308±319. LimoÂn, I.D., Martinez-Fong, D., NunÄez, A., Rosales, M.G. and Aceves, J., Differential modulation of GABA and glutamate release by D2 and D1 dopamine receptors in the globus pallidus, Soc. Neurosci. Abst., 24 (1998) 6474. Lindvall, O. and BjoÈrklund, A., Dopaminergic innervation of the globus pallidus by collaterals from the nigrostriatal pathway, Brain Res., 172 (1979) 169±173. Mansour, A., Meador-Woodruff, J.H., Bunzow, J.R., Civelli, O., Akil, H. and Watson, S.J., Localization of dopamine D2 receptor and D1 and D2 receptor binding in the rat brain and pituitary: an in situ hybridization-receptor autoradiographic analysis, J. Neurosci., 10 (1990) 2587±2600. May®eld, R.D., Larson, G., Orona, R.A. and Zahniser, N.R., Opposing actions 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. Querejeta, E., Sierra, A., Delgado, A., Valdiosera, R. and Aceves, J., D2 receptor-mediated inhibition of the striatopallidal transmission in the rat, Soc. Neurosci. Abst., 23 (1997) 193. Smith, Y., Bevan, M.D., Shink, E. and Bolam, J.P., Microcircuitry of the direct and indirect pathways of the basal ganglia, Neuroscience, 86 (1998) 353±387. Soto, E. and Vega, R.A., Turbo Pascal program for on line spike data analysis using a standard serial port, J. Neurosci. Methods, 19 (1987) 61±68.
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Intrapallidal D2 dopamine receptors control globus pallidus neuron activity in the rat E. Querejeta a, A. Delgado a, R. Valdiosera a, D. Erlij b, J. Aceves a,* a
Departamento de Fisiologia, BiofõÂsica y Neurociencias, Centro de InvestigacioÂn y de Estudios Avanzados del Instituto PoliteÂcnico Nacional, Apartado Postal 14140, 07000 MeÂxico City, MeÂxico b Department of Physiology, SUNY Downstate Medical Center, Brooklyn, NY 11203, USA Received 20 November 2000; received in revised form 14 December 2000; accepted 28 December 2000
Abstract Because activation of D2 dopamine receptors inhibits g-aminobutyric acid (GABA) release from intrapallidal nerve terminals, we measured the effects of modi®ers of dopamine D2 receptors on the ®ring rate of single neurons in the globus pallidus (GP) of the anesthetized rat. The predominant effect of intrapallidal administration of the selective D2 agonist quinpirole was an increase in the rate of spontaneous ®ring while the D2 blocker sulpiride caused a decrease. The spontaneous ®ring of GP neurons is inhibited by stimulation of the GABAergic striatopallidal projection. We therefore measured the effects of modi®ers of D2 receptors on striatal inhibition of GP neurons and found that intrapallidal quinpirole blocked the inhibitory effects of striatal stimulation while sulpiride enhanced them. These experiments show that both the spontaneous rate of ®ring of pallidal neurons and its modi®cation by striatopallidal inputs is controlled by intrapallidal dopamine D2 receptors. In addition, taken together with other ®ndings in the literature, our results suggest that activation of dopamine D2 receptors within the globus pallidus leads to inhibition of GABA release from presynaptic terminals. q 2001 Published by Elsevier Science Ireland Ltd. Keywords: Basal ganglia; Dopamine receptors; g-aminobutyric acid release and transmission; Quinpirole; Striatopallidal transmission; Sulpiride
Dopamine D2 receptors in the rat globus pallidus (GP) appear to have a major role in the control of transmitter release by g-aminobutyric acid (GABA)-ergic terminals within this nucleus. Studies in slices show that activation of D2 receptors with the selective agonist quinpirole inhibits GABA release induced by either electrical stimulation [15] or K 1 depolarization [6]. Furthermore, blockade of D2 receptors by the intrapallidal administration of sulpiride causes a marked increase in GABA release [6,12]. To determine whether the control of pallidal output by D2 dopamine receptors in vivo [5,7] is consistent with the effects on GABA release from presynaptic terminals, we measured the effects of intrapallidal injection of modi®ers of D2-like receptors on the rate of ®ring of single neurons. Previous experiments have already shown that dopaminergic agents modulate the discharge of pallidal neurons, however, the receptors mediating these effects appeared to
* Corresponding author. Tel.: 152-5-7477000 ext. 5119; fax: 152-5-7477105. E-mail address: jaceves@®sio.cinvestav.mx (J. Aceves).
be located within the striatum [2,3,8,10] A preliminary account of our results has already been published [16]. Extracellular single-unit recordings were performed in ketamine anesthetized (150 mg/kg intraperitoneally (i.p.)) male Wistar rats weighing 180±200 g, maintained and handled according with the guidelines of the CINVESTAV-IPN Animal Care Committee. In addition, all experimental procedures were in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals. Supplemental ketamine (50 mg/kg) was given as needed. Once anesthetized, the rat was mounted on a David Kopf stereotaxic frame. Body temperature was maintained at 36±388C with a heating pad. Glass microelectrodes ®lled with 2 M NaCl with tip resistance 2±6 MV were used. Microelectrodes were stereotactically guided through drilled skull holes to the following co-ordinates: 0.0 mm anterior to bregma, 3.7 mm lateral to the midline and 4.5 mm ventral to the dura. Electrical signals were ampli®ed by a DAM-80 preampli®er (WPI) and processed online using a program for spike data analysis [18]. The signals were also monitored with an oscilloscope and stored on an audiocas-
0304-3940/01/$ - see front matter q 2001 Published by Elsevier Science Ireland Ltd. PII: S03 04 - 394 0( 0 1) 01 55 0- 6
80
E. Querejeta et al. / Neuroscience Letters 300 (2001) 79±82
Fig. 1. Intrapallidal quinpirole infusion (2 nmol) increases the spontaneous rate of pallidal neuron ®ring. Frequency histograms of two neurons (A, B) are shown to illustrate that the effect is seen in neurons with very different initial rates of spontaneous ®ring. Horizontal bar indicates the period of infusion. (C) Illustrates the dose-dependence of the effect. *P , 0:001 as compared with basal values.
sette. Single-unit activity was isolated with a window discriminator (WPI-121) and ®ring frequency was analyzed off line in a computer using the software SPS-8701 (Signal Processing Systems). All selected units had biphasic (^) type II waveforms [10] and amplitudes greater than 200 mV. The drugs were infused into the globus pallidus (GP) through cannulas made of 30 gauge syringe needles. The cannulas were stereotactically implanted at an angle of 608 in the latero-medial direction to the following co-ordinates: 0.2 mm anterior to bregma, 4.5 mm lateral to the midline and 4.6 mm ventral to the dura. The tips of the recording microlectrode and injection cannula were within a range of not greater than 0.2 mm. The drugs were dissolved in physiological saline, and were infused into the GP by means of a 5 ml syringe. The piston of the syringe was connected to a precision micrometer that allowed the slow (<3 nl/s) infusion of the drugs in a volume of 200 nl. The slow infusion of this volume of saline did not affect the spontaneous ®ring rate of pallidal neurons. For the experiments in which the striatum was electrically stimulated, a monopolar tungsten wire electrode was stereotactically placed in the striatum with an angle of 708 in the anteroposterior direction to the following co-ordinates: 3.5 mm anterior to bregma, 3.5 mm lateral to the midline and 4.5 mm ventral to dura. Rectangular pulses of 0.1 ms duration
with intensities of 25±100 mA and frequencies of 5±20 Hz were selected in each individual experiment to produce a response suitable to the experimental goals (see Fig. 3). At the end of the experiment the brain was removed, after an intracardiac administration of 200 ml of 4% formaldehyde for ®xation. After overnight incubation in formaldehyde, the brains were sliced to verify the position of the recording microlectrode and the injection cannula in the GP and the stimulating electrode in the striatum. When either the recording microelectrode or the injection cannula was not positioned in the globus pallidus the experiment was discarded. The spontaneous rates of discharge of pallidal neurons varied from 0.1 to 93 Hz in the 382 cells examined (average ^ SEM 19 ^ 1 Hz). Individual examples can be seen in the control portions of the frequency histograms of Figs. 1±3. The predominant effect of intrapallidal administration of the D2 receptor agonist quinpirole was an increase in the rate of spontaneous ®ring rate (Fig. 1). Two examples are illustrated in Fig. 1 to show that the increase in ®ring caused by quinpirole was observed in cells with widely different basal rates of discharge. In 11 out of 24 neurons in which quinpirole was tested at a dose of 2 nmol the frequency increased by 112 ^ 32%. In two neurons it decreased by 46 ^ 11% and in 11 neurons the ®ring rate was not affected. As shown in Fig. 1C, the effect of quinpirole was dose-dependent. The predominant effect of intrapallidal infusion of the D2 antagonist sulpiride was a decrease in the spontaneous ®ring. Responses in two neurons with different spontaneous patterns of discharge are illustrated, one with a regular (Fig. 2A), and the other with a bursting ®ring pattern (Fig. 2B). In 14 of 29 neurons tested, sulpiride decreased the rate of ®ring to 66 ^ 7% of the basal. In 12 neurons the frequency was not affected and in three the ®ring rate increased (39 ^ 1%). The GP receives a major input from GABAergic ®bers
Fig. 2. Intrapallidal sulpiride infusion (200 pmol) decreases the spontaneous rate of pallidal neuron ®ring. (A) The effect of sulpiride on a neuron with regular ®ring rate. (B) The effect of sulpiride on a neuron with bursting ®ring pattern. Horizontal bar indicates the period of infusion
E. Querejeta et al. / Neuroscience Letters 300 (2001) 79±82
Fig. 3. The effects of striatal stimulation on globus pallidus neuron ®ring are modi®ed by injection of either quinpirole or sulpiride. The upper tracing in (A) shows the inhibitory effects of electrical stimulation (100 mA, 0.1 ms, 20 Hz) to the striatum. Horizontal bar indicates the period of stimulation (St). Between the upper and lower tracing 200 pmol of quinpirole were injected into the pallidum. The lower tracing shows the discharge of the same neuron; when the electrical stimulation was repeated after the injection, the inhibitory effects were diminished considerably. (B) Another example of electrical stimulation to the striatum; the stimulation parameters (25 mA, 0.1 ms, 10 Hz) were selected to produce a minimal inhibitory response. Between the upper and lower tracing 200 pmol of sulpiride were injected into the pallidum. Repeating the stimulation after the injection (lower tracing) produced a clear-cut inhibitory effect.
originating in the striatum. To test whether pallidal neurons in our experiments were under tonic GABAergic in¯uences we infused bicuculline (100 pmol) into the pallidum. In all 11 neurons tested, spontaneous ®ring rate increased (154 ^ 86 %, mean ^ SEM). Thus, the effects described above of the dopaminergic modi®ers may be mediated by changes in these tonic GABAergic in¯uences. Additional evidence of dopaminergic control of GABAergic striatopallidal in¯uences can be obtained by measuring the effects of dopaminergic agents on the
81
response of GP to striatal stimulation. Fig. 3A shows that quinpirole completely blocked the inhibitory effects of electrical stimulation of the striatum. The tracings illustrate an example of the procedure used to collect these data. Stimulation parameters that produced a clear-cut inhibitory effect were ®rst selected (upper tracing). Then quinpirole (200 pmol) was injected, and the stimulation was again applied (lower tracing). Quinpirole reduced the inhibition from 89 ^ 3 to 30 ^ 6% (n 14) of the basal frequency. Fig. 3B shows that sulpiride (200 pmol) increased the inhibitory effects of the stimulation from 21 ^ 7 to 72 ^ 9% of basal frequency. For these experiments we selected stimulation parameters that produced a minimal inhibitory response during the control period (upper tracing). Between the upper and lower tracing 200 pmol of sulpiride were injected into the GP. Repeating the stimulation after the injection produced a clear-cut inhibitory effect. The marked effects of intrapallidal injections of quinpirole and sulpiride show that the spontaneous rate of discharge of pallidal neurons in the anesthetized rat is under tonic control of D2-like dopaminergic receptors located within this nucleus. The bulk of the evidence suggest that this intrapallidal dopaminergic control of neuron ®ring is mediated through the same mechanism previously described in brain slices, namely, the modulation of GABA release from the presynaptic terminal [6,15]. The effects of both quinpirole and sulpiride on the response to electrical stimulation of the striatum, described here, are consistent with a presynaptic site of action. Moreover, it has been shown recently [4] that dopamine depresses striatal GABAergic inhibitory postsynaptic currents in the globus pallidus through a presynaptic mechanism. In these experiments dopamine was without effect on postsynaptic conductances. This is a very intriguing observation because receptor autoradiography and in situ hybridization reveal that D2 binding sites and the corresponding mRNA coexist in the globus pallidus indicating that pallidal cell bodies express the D2 receptor [14]. Prior investigators already observed effects of activation of dopaminergic receptors on pallidal neuron ®ring. Systemic administration of dopamine agonists increased the ®ring rate of most pallidal neurons [2,3,8,10] The increase in ®ring was interpreted as due to activation of striatal receptors regulating the output to the globus pallidus. However, the results described here, taken together with previous GABA release experiments [6,15] and the presence of dopamine D2 receptors within the pallidum [1,9,14] suggest that dopaminergic terminals that originate in the substantia nigra and terminate in the globus pallidus [13] play an important role in controlling the output of this nucleus. The ®nding that a few cells decrease basal ®ring after quinpirole administration and a few cells increase ®ring after sulpiride administration remains to be explained. One possibility for the inhibitory effects seen in a few neurons when quinpirole was used is suggested by some
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recent observations of Kita and Kitai [11] who found that axons of GP neurons emit multiple collaterals with large boutons en-passant and boutons terminaux within the globus pallidus. Accordingly, excitation of some pallidal neurons would inhibit the ®ring of the pallidal neurons receiving GABAergic collaterals. This mechanism could also explain the increase in ®ring of some neurons caused by sulpiride; the effect could be caused by disinhibition of the GABAergic input via these collaterals. In conclusion, these data show that intrapallidal dopamine D2 receptors play an important role in controlling the output of globus pallidus neurons. The predominant effect of the activation of intrapallidal D2 receptors is an increase in the ®ring rate. This increase is in agreement with predictions derived from release experiments since these showed that activation of pallidal D2 receptors causes inhibition of GABA release from presynaptic terminals. The inhibition of pallidal ®ring caused by a D2 antagonist, as described here, would lead to increased activity of neurons in the subthalamic nucleus and the output nuclei of the basal ganglia [17], which would in turn depress motor behavior. This work was supported by a grant (3539P-N9607) from CONACYT (MeÂxico). E.Q. was a CONACYT fellow. [1] Beckstead, R.M., Wooten, G.F. and Trugman, J.M., Distribution of D1 and D2 dopamine receptors in the basal ganglia of the cat determined by quantitative autoradiography, J. Comp. Neurol., 268 (1988) 131±145. [2] Bergstrom, D.A., Bromley, S.D. and Walters, J.R., Apomorphine increases the activity of rat globus pallidus neurons, Brain Res., 238 (1982) 266±271. [3] Bergstrom, D.A., Bromley, S.D. and Walters, J.R., Dopamine agonists increase pallidal unit activity: attenuation by agonist pretreatment and anesthesia, Eur. J. Pharmacol., 100 (1984) 3±12. [4] Cooper, A. and Stanford, I.M., Dopamine-mediated inhibition of striatal GABAA IPSCs in the rat globus pallidus in vitro, J. Physiol., 525P (2000) 52P±53P. [5] Costall, B., Naylor, R.J. and Olley, J.E., Catalepsy and circling behaviour after intracerebral injections of neuroleptic, cholinergic and anticholinergic agents into the caudateputamen, globus pallidus and substantia nigra of rat brain, Neuropharmacology, 11 (1972) 645±663. [6] Floran, B., Floran, L., Sierra, A. and Aceves, J., D2 receptormediated inhibition of GABA release by endogenous dopa-
[7] [8]
[9]
[10]
[11] [12]
[13] [14]
[15]
[16]
[17] [18]
mine in the rat globus pallidus, Neurosci. Lett., 237 (1997) 1±4. Hauber, W. and Lutz, S., Dopamine D1 or D2 receptor blockade in the globus pallidus produces akinesia in the rat, Behav. Brain Res., 106 (1999) 143±150. Hooper, K.C., Banks, D.A., Stordahl, L.J., White, I.M. and Rebec, G.V., Quinpirole inhibits striatal and excites pallidal neurons in freely moving rats, Neurosci. Lett., 237 (1997) 69±72. Huang, N., Ase, A.R., Hebert, C., van Gelder, N.M. and Reader, T.A., Effects of chronic neuroleptic treatments on dopamine D1 and D2 receptors: homogenate binding and autoradiographic studies, Neurochem. Int., 30 (1997) 277± 290. Kelland, M.D., Soltis, R.P., Anderson, L.A., Bergstrom, D.A. and Walters, J.R., In vivo characterization of two cell types in the rat globus pallidus which have opposite responses to dopamine receptor stimulation: comparison of electrophysiological properties and responses to apomorphine, dizocilpine, and ketamine anesthesia, Synapse, 20 (1995) 338± 350. Kita, H. and Kitai, S.T., The morphology of globus pallidus projection neurons in the rat: an intracellular staining study, Brain Res., 636 (1994) 308±319. LimoÂn, I.D., Martinez-Fong, D., NunÄez, A., Rosales, M.G. and Aceves, J., Differential modulation of GABA and glutamate release by D2 and D1 dopamine receptors in the globus pallidus, Soc. Neurosci. Abst., 24 (1998) 6474. Lindvall, O. and BjoÈrklund, A., Dopaminergic innervation of the globus pallidus by collaterals from the nigrostriatal pathway, Brain Res., 172 (1979) 169±173. Mansour, A., Meador-Woodruff, J.H., Bunzow, J.R., Civelli, O., Akil, H. and Watson, S.J., Localization of dopamine D2 receptor and D1 and D2 receptor binding in the rat brain and pituitary: an in situ hybridization-receptor autoradiographic analysis, J. Neurosci., 10 (1990) 2587±2600. May®eld, R.D., Larson, G., Orona, R.A. and Zahniser, N.R., Opposing actions 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. Querejeta, E., Sierra, A., Delgado, A., Valdiosera, R. and Aceves, J., D2 receptor-mediated inhibition of the striatopallidal transmission in the rat, Soc. Neurosci. Abst., 23 (1997) 193. Smith, Y., Bevan, M.D., Shink, E. and Bolam, J.P., Microcircuitry of the direct and indirect pathways of the basal ganglia, Neuroscience, 86 (1998) 353±387. Soto, E. and Vega, R.A., Turbo Pascal program for on line spike data analysis using a standard serial port, J. Neurosci. Methods, 19 (1987) 61±68.