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Evidence that opioid receptors in the substantia nigra pars reticulata are relevant in regulating the function of striatal efferent pathways
Behavioural Brain Research, 5 ( 1982) 415-422
415
Elsevier Biomedical Press
Evidence that opioid receptors in the substantia nigra pars reticulata are relevant in regulating the function of striatal efferent pathways
LECHOSLAW TURSKI*, U R S U L A HAVEMANN** and KLAUS K U S C H I N S K Y
Dept. of Biochemical Pharmacology, Max Planck Institute for Experimental Medicine, Hermann Rein Str. 3, D-3400 G6ttingen (F.R.G.) (Received February 25th, 1982) (Revised version received March 29th, 1982) (Accepted April 26th, 1982)
Key words: electromyogram - morphine - naloxone - substantia nigra pars reticulata - striatum
A tonic activity in the electromyogram (EMG) was induced in conscious rats by injections of morphine either systemically or into the caudate nucleus. This activity was antagonized by injections of naloxone into the substantia nigra pars reticulata. These findings suggest that opioid receptors in this brain area are relevant in regulating the function of striatal efferent pathways.
Several lines of evidence indicate that the muscular rigidity ('lead pipe rigidity'), observed after administration of morphine in rats, is mediated mainly by opioid receptors in the striatum [8-10]. Accordingly, muscular rigidity, measured as a tonic activity in the electromyogram (EMG), might be used as a reliable and convenient tool for delineating the function of the striatum and its efferent pathways. One of the important efferent pathways relaying striatally evoked motor functions to the substantia nigra [5] is the striatonigral GABAergic pathway [4, 15]. In particular, behavioural studies suggested that the pars reticulata of the substantia nigra (SNR) is an important relay station for striatal efferent GABAergic neurones [2, 5, 6], which seems to be relevant for the regulation of posture and motility. Part of the terminals of these neurones were found in the SNR, but part of them were also found in the pars compacta of the substantia nigra (SNC) [ 14]. In addition, Jessell et al. [ 14] found that substance P-containing neurones, originating in the caudate nucleus, project to the substantia nigra, probably to
* On leave from Department of Pharmacology, Institute of Clinical Pathology, Medical School, Lublin, Poland. ** To whom correspondence should be addressed. 0166-4328/82/0000-0000/$02.75 © 1982 Elsevier Biomedical Press
416 both parts of it. Enkephalins [ 11 ] and opioid receptors [3] were also found in the substantia nigra, but the information about the distribution of both opioid par> meters in the pars compacta and pars reticulata is scanty. Lesion studies suggested that opioid receptors in the substantia nigra are located mainly on endings of nigrostriatal GABAergic neurones rather than on endings of substance P-containing neurones [ 16]. Since the role of enkephalinergic mechanisms in the regulation of the activity of striatal efferent pathways, relayed in the SNR, was unclear, we studied and characterized the effects of naloxone, an opioid antagonist, after injection into the SNR on the tonic activity in the EMG induced by systemic or intrastriatat administration of morphine [9]. In a previous study [ 17] applying injections of morphine into the SNC or SNR, we could functionally discriminate both brain areas, since injections of morphine into the SNC antagonized the tonic activity in the EMG produced by systemic administration of morphine, whereas injections into the SNR enhanced it. The present study was performed in order to evaluate the relevance of opioid receptors in the SNR for mediating the tonic activity in the EMG induced either by systemic administration of morphine or by intrastriatal injection of the drug. For this purpose, the opioid receptors in the SNR were blocked by naloxone. For intracerebral injections, male albino Wistar rats ( T N O / W 70, F. Winkelmann, Borchen, F.R.G.) weighing 220-240 g were stereotaxically implanted with permanent guide cannulas under pentobarbital anaesthesia, as described in detail elsewhere [9]. The cannulas were implanted vertically above the caudate nucleus and/or the SNR, the tip of the cannula being at the surface of the brain. Four to 6 days after surgery, the unilateral (left side) injections of the drugs (morphine, naloxone, or saline) were performed into the SNR and/or into the caudate nucleus of non-anaesthetized rats with a fine injection cannula ( ~ 0.4 mm) according to the coordinates of Fifkovfl and Margala [7]: for the SNR, AP + 4.8, L 1.9, V 9.0; and for the caudate nucleus, AP -1.25, L 2.6, V 4.6. The drugs were injected into the SNR (volume of 0.5 #1 in saline) and into the caudate nucleus (volume of 1.5 btl in saline) at the rate of 0.25 ltl/min, either alone or in combination. The injection cannula was maintained in position for an additional 2.5 min in order to allow adequate absorption by the surrounding tissue and thus lessen the likelihood of the injected drug being drawn back by capillary forces and thus diffusing to other sites [13]. Each rat was injected once only. The electromyogram (EMG) was recorded from the gastrocnemiussoleus (GS) muscle of the hindlimb of non-anaesthetized rats ipsilaterally to the SNR injected, as described elsewhere [9]. The recording of the EMG was performed ipsilaterally, since And6n et al. [ 1] found that, in rats, the rigidity appears preferentially on the same side as the alteration in neostriatal function, e.g. of a shift in the dopaminergic/cholinergic balance in the neostriatum. The signals were amplified, bandpass-filtered (5 Hz-10 kHz), rectified and fed into an integrator,
417
\ Fig. 1. Histologicaldemonstration of an injection into the SNR (cresyl-violet stain) x 10. which was automatically reset after a preset voltage was reached. The reciprocal of the reset-time of the integrator was the measure of the activity in the EMG. The E M G was recorded continuously and the values of the mean activities of 5 min periods were calculated. Recording was performed using pairs of percutaneously inserted, teflon-insulated stainless steel fine wire electrodes (Cooner Wire, AS 632 SS) sampling changes in the E M G from the motor units of the muscle under investigation. After completion of each experiment, the location of the injection site was verified histologically in cresyl violet-stained serial sections (an example is given in Fig. 1). The following drugs were used: morphine hydrochloride (E. Merck, Darmstadt, F.R.G.) and naloxone hydrochloride (Endo Labs., Inc., Garden City, NY, U.S.A.). The doses are expressed as free base. Systemic administration of morphine (15 mg/kg i.p.) induced muscular rigidity, which was recorded as a tonic activity in the E M G of the GS muscle. No tonic activity could be observed before injection of i.p. morphine or after injection of i.p. saline. Intranigral injection was performed ipsilaterally to the E M G recording site, 45 min after the systemic administration of morphine, when the activity in the E M G was maximal. After injection of saline into the SNR, the
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Fig. 2. Effect of an injection of naloxone, naloxone + morphine, morphine, or saline into the SNR on the EMG activity induced by systemic administration of morphine (15 mg/kg i.p.). The intranigral injection was performed 45 min after systemic administration of morphine. Abscissa: time (min) after systemic administration (upper scale) or after intranigral administration (lower scale); ordinate: EMG activity in the GSmuscle. Median values. Significances: * ~ < 0 . 0 5 ; ** ~<0.01, *** ~ < 0.002 (Mann-Whitney U-test) compared with saline controls at identical times. MO, morphine; NAL, naloxone.
419 activity in the E M G slowly decreased during the following 45 min, but was still evident 120 min after the systemic administration of morphine (Fig. 2). Injections of morphine into the SNR at the dose of 1 #g, and in a more pronounced way at 5/~g, immediately and significantly enhanced the activity in the EMG. This enhancement was maximal 45 min after the intranigral injection and disappeared after slightly more than 1 h. In contrast, the administration ofnaloxone at the dose of 1/~g into the SNR rapidly and significantly decreased the activity in the E M G (Fig. 2). Naloxone at the dose of 5/~g almost completely abolished the activity in the EMG. These effects were maximal after 5-10 min and lasted for about 50-60 min after intranigral injection. Co-administration of morphine and naloxone (5/~g each) did not significantly affect the tonic activity in the E M G observed after systemic administration of morphine. Injections of naloxone alone (1 or 5/~g) into the SNR did not induce any tonic E M G activity, whereas injections of morphine alone (1-5 #g) led to a tonic E M G activity in a dose-dependent way (own, unpublished observations). No effects on activity in the E M G were observed following injection of morphine or naloxone into areas adjacent to the SNR, i.e. the red nucleus or sites located dorsally to the crus cerebri and the lemniscus medialis. Although the rats were kept in a special cage during the E M G recording procedure, some behavioural phenomena could be observed. Injections of naloxone (5/~g) into the SNR of rats pretreated with morphine systemically, induced restlessness, mild licking and gnawing, which started about 10-20 min after the injection into the SNR and lasted for more than 30 min. When the rats were removed from the cages, they showed contralateral posturing or circling. A similar circling behaviour was observed immediately after a simultaneous administration of morphine into the striatum and of naloxone into the SNR. Injection of morphine (15/~g), but not of saline, into the head of the caudate nucleus induces a tonic activity in the E M G (Fig. 3). Simultaneous administration of naloxone (5/ag) (but not of saline) into the ipsilateral SNR completely suppressed (for up to 40 min) the tonic activity in the E M G evoked by morphine administered into the caudate nucleus. Naloxone at the dose of 1 pg was less effective (Fig. 3). These results demonstrate that an activation or inhibition of opioid receptors located in the SNR strongly modifies the tonic activity in the EMG, which is observed after systemic administration of morphine and is, at least to a major part, due to an activation of opioid receptors in the striatum [9, 10]. In addition, the activity in the E M G produced by exclusively activating striatal opioid receptors was inhibited in a similar way as the activity in the E M G observed after systemic administration of morphine. The SNR is an output station of dopaminemediated striatal syndromes, and striatonigral GABAergic neurones seem to play an essential role in mediating these dopaminergic responses (postural component of turning, locomotor component of turning, stereotypic gnawing, catalepsy [5]).
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Our results suggest that enkephalinergic mechanisms in the SNR can modulate or even mediate the striatal output. This is shown in the morphine-induced muscular rigidity (measured as a tonic activity in the EMG) which is probably not due to a decrease in striatal dopaminergic neurotransmission, but due to actions of morphine 'down-stream' of the dopaminergic neurones [8, 10]. It seems unlikely that the nigro-striato-nigral loop, including the nigrostriatal dopaminergic neurones, is involved in the inhibition by naloxone, injected into the SNR, of the morphine-induced tonic activity in the EMG, since the effect of naloxone is in a direction opposite to that of morphine injected into the SNR. In a previous paper [ 17], we have presented evidence that the inhibitory effect of morphine, injected into the SNC, on the tonic activity in the E M G should involve the nigro-striato-nigral loop by activating the nigrostriatal dopaminergic neurones. The observation that administration of morphine into the SNR enhances the tonic activity observed after systemic administration of morphine, suggests that the nigro-striato-nigral loop is not involved in the latter case. Opioids might affect the release of GABA in the SNR, since there is evidence that opioid receptors are located on GABAergic terminals in the substantia nigra [16], although in the latter finding, no discrimination was done between pars compacta and pars reticulata. Efferent neurones of the SNR are probably also GABAergic and are ending
421 in the deep layers of the superior colliculus and the dorsal mesencephalic reticular formation; other pathways are relayed in the ventromedial thalamus [12]. It seems likely that one or several of these pathways mediate the morphine-induced tonic activity in the EMG, and that an activation or inhibition of opioid receptors in the SNR modifies the activity of these pathways. This study was supported by a grant (B 10) of the SFB 33 'Nervensystem und biologische Information' of the Deutsche Forschungsgemeinschaft. The skillful technical assistance of C. Bode, H. Kt~gler, H. Ropte and R. Meseke is gratefully acknowledged. Naloxone was kindly donated by Endo Labs., Inc., Garden
City, NY, U.S.A. And6n, N.-E., Dahlstr0m, A., Fuxe, K. and Larsson, K., Functional role of the nigro-neostriatal dopaminergic neurons, Acta pharmacol, toxicol., 24 (1966) 263-274. Arnt, J. and Scheel-Krt~ger, J., Behavioral differences induced by muscimol selectively injected into the pars compacta and pars reticulata of substantia nigra, Naunyn-Schmiedeberg~ Arch. Pharmacol., 310 (1979) 43-51. Atweh, S.F. and Kuhar, M.J., Autoradiographic localization of opiate receptors in rat brain. II. The brain stem, Brain Res., 129 (1977) 1-12. Crossman, A.R., Walker, R.J. and Woodruff, G.N., Picrotoxin antagonism of ~-aminobutyric acid inhibitory responses and synaptic inhibition in the rat substantia nigra, Brit. J. Pharmacol., 49 (1973) 696-698. Di Chiara, G., Morelli, M., Imperato, A. and Porceddu, M.L., Substantia nigra as an efferent station for dopaminergic behavioural syndromes arising in the striatum. In G.L. Gessa and G.U. Corsini (Eds.), Apomorphine and Other Dopaminomimetics, Vol. 1, Raven Press, New York, 1981, pp. 41-64. Di Chiara, G., Porceddu, M.L., Morelli, M., Mulas, M.L and Gessa, G.L., Substantia nigra as an out-put station for striatal dopaminergic responses: role of a GABA-mediated inhibition of pars reticulata neurons, Naunyn-Schmiedeberg's Arch. Pharmacol., 306 (1979) 153-159. Fifkov& E. and MarSala, J., Stereotaxic atlases for the cat, rabbit and rat. In J. Bures, M. Petran and J. Zachar (Eds.), Electrophysiological Methods in Biological Research, Academia, Prague and Academic Press, New York, 1967, pp. 653-696. Havemann, U. and Kuschinsky, K., Further characterization ofopioid receptors in the striatum mediating muscular rigidity in rats, Naunyn-Schmiedeberg~. Arch. Pharmacol., 317 (1981) 321-325. Havemann, U., Winkler, M. and Kuschinsky, K., Opioid receptors in the caudate nucleus can mediate EMG-recorded rigidity in rats, Naunyn-Schmiedeberg~ Arch. Pharmacol., 313 (1980) 139-144. 10 Havemann, U., Winkler, M., Genq, E. and Kuschinsky, K., Effects of striatal lesions with kainic acid on morphine-induced 'catatonia' and increase of striatal dopamine turnover, Nauto,nSchmiedeberg's Arch. Pharmacol., 317 ( 1981) 44-50. 11 Hong, J.S., Yang, H.-Y., Fratta, W. and Costa, E., Determination of methionine enkephalin in discrete regions of rat brain, Brain Res., 134 (1977) 383-386. 12 Imperato, A. and Di Chiara, G., Behavioural effects of GABA-agonists and antagonists infused in the mesencephalic reticular formation - deep layers of superior colliculus, Bra#t Res., 224 (1981) 185-194. 13 Jacquet, Y.F. and Lajtha, A., Morphine action at central nervous system sites in rat: analgesia or hyperalgesia depending on site and dose, Science, 182 (1973) 490492.
422 14 Jessell, T.M., Emson, P.C., Paxinos, G. and Cuello, A.C., Topographic projections of substance P and GABA pathways in the striato- and pallido-nigral system: a biochemical and immunohistochemical study, Brain Res., 152 (1978) 487-498. 15 Kim, J.S., Bak, J.J., Hassler, R. and Okada, Y., Role of ~aminobutyric acid in the extrapyramidal motor system. II. Some evidence for existence of a type of GABA-rich strionigral neurons, Exp. Brain Res., t4 (1971) 95-104. Llorens-Cortes, C., Pollard, H. and Schwartz, J.C., Localization of opiate receptors in substantia nigra evidence by lesion studies, Neurosci. Lett., 12 (1979) 165-170. 17 Turski, L., Havemaun, U. and Kuschinsky, K., Evidence for functional interactions of morphine in substantia nigra and striatum in relation to muscular rigidity in rats, Neurosci. Lett.,
16
28 (1982) 291-296.
415
Elsevier Biomedical Press
Evidence that opioid receptors in the substantia nigra pars reticulata are relevant in regulating the function of striatal efferent pathways
LECHOSLAW TURSKI*, U R S U L A HAVEMANN** and KLAUS K U S C H I N S K Y
Dept. of Biochemical Pharmacology, Max Planck Institute for Experimental Medicine, Hermann Rein Str. 3, D-3400 G6ttingen (F.R.G.) (Received February 25th, 1982) (Revised version received March 29th, 1982) (Accepted April 26th, 1982)
Key words: electromyogram - morphine - naloxone - substantia nigra pars reticulata - striatum
A tonic activity in the electromyogram (EMG) was induced in conscious rats by injections of morphine either systemically or into the caudate nucleus. This activity was antagonized by injections of naloxone into the substantia nigra pars reticulata. These findings suggest that opioid receptors in this brain area are relevant in regulating the function of striatal efferent pathways.
Several lines of evidence indicate that the muscular rigidity ('lead pipe rigidity'), observed after administration of morphine in rats, is mediated mainly by opioid receptors in the striatum [8-10]. Accordingly, muscular rigidity, measured as a tonic activity in the electromyogram (EMG), might be used as a reliable and convenient tool for delineating the function of the striatum and its efferent pathways. One of the important efferent pathways relaying striatally evoked motor functions to the substantia nigra [5] is the striatonigral GABAergic pathway [4, 15]. In particular, behavioural studies suggested that the pars reticulata of the substantia nigra (SNR) is an important relay station for striatal efferent GABAergic neurones [2, 5, 6], which seems to be relevant for the regulation of posture and motility. Part of the terminals of these neurones were found in the SNR, but part of them were also found in the pars compacta of the substantia nigra (SNC) [ 14]. In addition, Jessell et al. [ 14] found that substance P-containing neurones, originating in the caudate nucleus, project to the substantia nigra, probably to
* On leave from Department of Pharmacology, Institute of Clinical Pathology, Medical School, Lublin, Poland. ** To whom correspondence should be addressed. 0166-4328/82/0000-0000/$02.75 © 1982 Elsevier Biomedical Press
416 both parts of it. Enkephalins [ 11 ] and opioid receptors [3] were also found in the substantia nigra, but the information about the distribution of both opioid par> meters in the pars compacta and pars reticulata is scanty. Lesion studies suggested that opioid receptors in the substantia nigra are located mainly on endings of nigrostriatal GABAergic neurones rather than on endings of substance P-containing neurones [ 16]. Since the role of enkephalinergic mechanisms in the regulation of the activity of striatal efferent pathways, relayed in the SNR, was unclear, we studied and characterized the effects of naloxone, an opioid antagonist, after injection into the SNR on the tonic activity in the EMG induced by systemic or intrastriatat administration of morphine [9]. In a previous study [ 17] applying injections of morphine into the SNC or SNR, we could functionally discriminate both brain areas, since injections of morphine into the SNC antagonized the tonic activity in the EMG produced by systemic administration of morphine, whereas injections into the SNR enhanced it. The present study was performed in order to evaluate the relevance of opioid receptors in the SNR for mediating the tonic activity in the EMG induced either by systemic administration of morphine or by intrastriatal injection of the drug. For this purpose, the opioid receptors in the SNR were blocked by naloxone. For intracerebral injections, male albino Wistar rats ( T N O / W 70, F. Winkelmann, Borchen, F.R.G.) weighing 220-240 g were stereotaxically implanted with permanent guide cannulas under pentobarbital anaesthesia, as described in detail elsewhere [9]. The cannulas were implanted vertically above the caudate nucleus and/or the SNR, the tip of the cannula being at the surface of the brain. Four to 6 days after surgery, the unilateral (left side) injections of the drugs (morphine, naloxone, or saline) were performed into the SNR and/or into the caudate nucleus of non-anaesthetized rats with a fine injection cannula ( ~ 0.4 mm) according to the coordinates of Fifkovfl and Margala [7]: for the SNR, AP + 4.8, L 1.9, V 9.0; and for the caudate nucleus, AP -1.25, L 2.6, V 4.6. The drugs were injected into the SNR (volume of 0.5 #1 in saline) and into the caudate nucleus (volume of 1.5 btl in saline) at the rate of 0.25 ltl/min, either alone or in combination. The injection cannula was maintained in position for an additional 2.5 min in order to allow adequate absorption by the surrounding tissue and thus lessen the likelihood of the injected drug being drawn back by capillary forces and thus diffusing to other sites [13]. Each rat was injected once only. The electromyogram (EMG) was recorded from the gastrocnemiussoleus (GS) muscle of the hindlimb of non-anaesthetized rats ipsilaterally to the SNR injected, as described elsewhere [9]. The recording of the EMG was performed ipsilaterally, since And6n et al. [ 1] found that, in rats, the rigidity appears preferentially on the same side as the alteration in neostriatal function, e.g. of a shift in the dopaminergic/cholinergic balance in the neostriatum. The signals were amplified, bandpass-filtered (5 Hz-10 kHz), rectified and fed into an integrator,
417
\ Fig. 1. Histologicaldemonstration of an injection into the SNR (cresyl-violet stain) x 10. which was automatically reset after a preset voltage was reached. The reciprocal of the reset-time of the integrator was the measure of the activity in the EMG. The E M G was recorded continuously and the values of the mean activities of 5 min periods were calculated. Recording was performed using pairs of percutaneously inserted, teflon-insulated stainless steel fine wire electrodes (Cooner Wire, AS 632 SS) sampling changes in the E M G from the motor units of the muscle under investigation. After completion of each experiment, the location of the injection site was verified histologically in cresyl violet-stained serial sections (an example is given in Fig. 1). The following drugs were used: morphine hydrochloride (E. Merck, Darmstadt, F.R.G.) and naloxone hydrochloride (Endo Labs., Inc., Garden City, NY, U.S.A.). The doses are expressed as free base. Systemic administration of morphine (15 mg/kg i.p.) induced muscular rigidity, which was recorded as a tonic activity in the E M G of the GS muscle. No tonic activity could be observed before injection of i.p. morphine or after injection of i.p. saline. Intranigral injection was performed ipsilaterally to the E M G recording site, 45 min after the systemic administration of morphine, when the activity in the E M G was maximal. After injection of saline into the SNR, the
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Fig. 2. Effect of an injection of naloxone, naloxone + morphine, morphine, or saline into the SNR on the EMG activity induced by systemic administration of morphine (15 mg/kg i.p.). The intranigral injection was performed 45 min after systemic administration of morphine. Abscissa: time (min) after systemic administration (upper scale) or after intranigral administration (lower scale); ordinate: EMG activity in the GSmuscle. Median values. Significances: * ~ < 0 . 0 5 ; ** ~<0.01, *** ~ < 0.002 (Mann-Whitney U-test) compared with saline controls at identical times. MO, morphine; NAL, naloxone.
419 activity in the E M G slowly decreased during the following 45 min, but was still evident 120 min after the systemic administration of morphine (Fig. 2). Injections of morphine into the SNR at the dose of 1 #g, and in a more pronounced way at 5/~g, immediately and significantly enhanced the activity in the EMG. This enhancement was maximal 45 min after the intranigral injection and disappeared after slightly more than 1 h. In contrast, the administration ofnaloxone at the dose of 1/~g into the SNR rapidly and significantly decreased the activity in the E M G (Fig. 2). Naloxone at the dose of 5/~g almost completely abolished the activity in the EMG. These effects were maximal after 5-10 min and lasted for about 50-60 min after intranigral injection. Co-administration of morphine and naloxone (5/~g each) did not significantly affect the tonic activity in the E M G observed after systemic administration of morphine. Injections of naloxone alone (1 or 5/~g) into the SNR did not induce any tonic E M G activity, whereas injections of morphine alone (1-5 #g) led to a tonic E M G activity in a dose-dependent way (own, unpublished observations). No effects on activity in the E M G were observed following injection of morphine or naloxone into areas adjacent to the SNR, i.e. the red nucleus or sites located dorsally to the crus cerebri and the lemniscus medialis. Although the rats were kept in a special cage during the E M G recording procedure, some behavioural phenomena could be observed. Injections of naloxone (5/~g) into the SNR of rats pretreated with morphine systemically, induced restlessness, mild licking and gnawing, which started about 10-20 min after the injection into the SNR and lasted for more than 30 min. When the rats were removed from the cages, they showed contralateral posturing or circling. A similar circling behaviour was observed immediately after a simultaneous administration of morphine into the striatum and of naloxone into the SNR. Injection of morphine (15/~g), but not of saline, into the head of the caudate nucleus induces a tonic activity in the E M G (Fig. 3). Simultaneous administration of naloxone (5/ag) (but not of saline) into the ipsilateral SNR completely suppressed (for up to 40 min) the tonic activity in the E M G evoked by morphine administered into the caudate nucleus. Naloxone at the dose of 1 pg was less effective (Fig. 3). These results demonstrate that an activation or inhibition of opioid receptors located in the SNR strongly modifies the tonic activity in the EMG, which is observed after systemic administration of morphine and is, at least to a major part, due to an activation of opioid receptors in the striatum [9, 10]. In addition, the activity in the E M G produced by exclusively activating striatal opioid receptors was inhibited in a similar way as the activity in the E M G observed after systemic administration of morphine. The SNR is an output station of dopaminemediated striatal syndromes, and striatonigral GABAergic neurones seem to play an essential role in mediating these dopaminergic responses (postural component of turning, locomotor component of turning, stereotypic gnawing, catalepsy [5]).
420 EMB ACTIVITY in GS 0.3~
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o o MO 15pg (CN)+ Saline (SNR) ,x......zx MO lSpg (CN)+ NAL lpg (SNR) CN)+ NAL 5pg(SNR)
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0.1-
'"..., , . . .• . ~. ~ .. . , ~, ~ ~....- ~ - ~ ~! 0
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'
20'
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Fig. 3. Effect of an injection of naloxone or saline into the SNR on the EMG activity induccd by simultaneous injection of morphine into the caudate nucleus (CN). Abscissa: time (min) alter beginning of the injections. For further explanations, see Fig. 2.
Our results suggest that enkephalinergic mechanisms in the SNR can modulate or even mediate the striatal output. This is shown in the morphine-induced muscular rigidity (measured as a tonic activity in the EMG) which is probably not due to a decrease in striatal dopaminergic neurotransmission, but due to actions of morphine 'down-stream' of the dopaminergic neurones [8, 10]. It seems unlikely that the nigro-striato-nigral loop, including the nigrostriatal dopaminergic neurones, is involved in the inhibition by naloxone, injected into the SNR, of the morphine-induced tonic activity in the EMG, since the effect of naloxone is in a direction opposite to that of morphine injected into the SNR. In a previous paper [ 17], we have presented evidence that the inhibitory effect of morphine, injected into the SNC, on the tonic activity in the E M G should involve the nigro-striato-nigral loop by activating the nigrostriatal dopaminergic neurones. The observation that administration of morphine into the SNR enhances the tonic activity observed after systemic administration of morphine, suggests that the nigro-striato-nigral loop is not involved in the latter case. Opioids might affect the release of GABA in the SNR, since there is evidence that opioid receptors are located on GABAergic terminals in the substantia nigra [16], although in the latter finding, no discrimination was done between pars compacta and pars reticulata. Efferent neurones of the SNR are probably also GABAergic and are ending
421 in the deep layers of the superior colliculus and the dorsal mesencephalic reticular formation; other pathways are relayed in the ventromedial thalamus [12]. It seems likely that one or several of these pathways mediate the morphine-induced tonic activity in the EMG, and that an activation or inhibition of opioid receptors in the SNR modifies the activity of these pathways. This study was supported by a grant (B 10) of the SFB 33 'Nervensystem und biologische Information' of the Deutsche Forschungsgemeinschaft. The skillful technical assistance of C. Bode, H. Kt~gler, H. Ropte and R. Meseke is gratefully acknowledged. Naloxone was kindly donated by Endo Labs., Inc., Garden
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16
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