Dopamine concentrations in the rat brain following injections into the substantia nigra of baclofen, γ-aminobutyric acid, γ-hydroxybutyric acid, apomorphine and amphetamine

Neuropharmocology.

1978. 17. 169-174.

Pergammon Press Printed in Great Britain

DOPAMINE CONCENTRATIONS IN THE RAT BRAIN FOLLOWING INJECTIONS INTO THE SUBSTANTIA NIGRA OF BACLOFEN, y-AMINOBUTYRIC ACID, ?/-HYDROXYBUTYRIC ACID, APOMORPHINE AND AMPHETAMINE P. H. KELLY and K. E. MDDRE Department of Pharmacology, Michigan State University, Fast Lansing, Michigan 48824, U.S.A. (Accepted 2 August 1977)

Summary-Systemic administration of dl-baclofen (2C80 mg/kg) and y-butyrolactone (25CrSOOmg/kg) increased the concentration of dopamine but not of norepinephrine in the rat forebrain. These drugs caused an equivalent increase in the concentration of dopamine in the striatum and olfactory tubercle, regions containing terminals of nigrostriatal and mesolimbic dopaminergic neurones, respectively. When injected directly into substantia nigra (A9), GABA, y-hydroxybutyrate and baclofen increased the dopamine concentration in the forebrain; apomorphine and d-amphetamine were without effect. When injected into the medial portion of the ventral tegmentum (A IO) baclofen increased the dopamine concentration in the olfactory tubercle and nucleus accumbens. Following intracerebral injections, d/-baclofen was several hundred times more potent than GABA, and most of the activity of baclofen resided in the [-isomer. Intracerebral injections of picrotoxin antagonized the ability of GABA but not of baclofen to increase the striatal concentration of dopamine. After unilateral intranigral injections of GABA, y-hydroxybutyrate and baclofen rats circled away from the side of the drug injection.

A reduction in the firing of dopaminergic nigrostriatal nerves as a result of acute lesions or the systemic administration of drugs such as y-hydroxybutyrate (GBH), y-butyrolactone (GBL) or HA-966 (l-hydroxy3-amino-pyrrolidone-2) causes a rapid increase in the concentration of dopamine in the terminals of these neurones in the striatum (Gessa, Vargiu, Crabai, Boero, Caboni and Camba, 1966; Hillen and Noach, 1971; Walters, Roth and Aghajanian, 1973; Stock, Magnusson and Anden, 1973). The activity of these dopaminergic nerves appears to be regulated, in part, by descending striatonigral or pallidonigral neurones which use y-aminobutyric acid (GABA) (Kim, Bak, Hassler and Okada. 1971; Fonnum, Grofova, Rinvik, Storm-Mathisen and Walberg, 1974; McGeer, Fibiger, McGeer and Brooke, 1973) or substance P (Kanazawa, Emson and Cuello, 1977) as inhibitory and excitatory neurotransmitters, respectively. The drugs cited above may reduce impulse flow in the dopamine neurones by mimicking GABA or blocking the actions of substance P. It has been proposed that baclofen P-p-chlorophenyl-GABA; Lioresal), a chemical derivative of GABA, inhibits neuronal activity by mimicking GABA (Fuxe, Hiikfelt, Ljungdahl, Agnati, Johansson and Perez de la Mora, 1975; Anden and Wachtel, 1977) by antagonizing substance P (Saito, Konishi and Otsuka, 1975) or by a non-specific depressant action (Phillis, 1976; Henry and Ben-Ari, 1976). This drug reduces the firing rate of nigrostriatal dopaminergic nerves when administered microiontophore-

tically (Davies and Dray, 1976) or intravenously (Bernard, Edwards, Fielding, Robson, Saelens, Simke and Welch, 1975). Systemically administered baclofen antagonizes the increase in dopamine turnover produced by neuroleptics (Fuxe et al., 1975; Da Prada and Keller, 1976) and increases the concentration of dopamine, but not of norepinephrine in the brains of rats and mice, presumably by inhibiting the firing rate of dopaminergic nerves (Da Prada and Keller. 1976; And&n and Wachtel, 1977; Gianutsos and Moore, 1977). Microinjections of GABA and GHB into the substantia nigra increase the brain concentration of dopamine (Anden and Stock, 1973). A similar technique was used in the present study to determine the effects of baclofen on mesolimbic and nigrostriatal dopaminergic neurons. The effects of intracerebral injections of baclofen were compared with those of GABA, GBL and GBH. In addition, the effects of d-amphetamine and apomorphine were examined since it has been suggested that these drugs can also reduce the firing of dopaminergic neurones by activating autoreceptors on the cell bodies in the substantia nigra (Aghajanian and Bunney, 1973; Bunney and Aghajanian, 1975; Groves, Wilson, Young and Rebec, 1975). METHODS Subjects

and surgery

Experiments SpragueDawley 169

were performed on adult, male rats. In experiments involving the

170

P. H.

KELLY and

intracranial application of drugs to the substantia nigra. stainless-steel guide cannulae were implanted bitateraliy at least 1 day before the experiment. The rats were anaesthetized with 3mljkg of Equithesin (Jensen-Salsbery) and positioned in a stereotaxic apparatus. Guide cannulae, made from 23 gauge stainless-steel hypodermic needle tubing, were implanted bilaterally to a point 1 mm above the substantia nigra pars compacta. Level-head co-ordinates of the cannula tips were 4.8 mm posterior to bregma. 2.0 mm lateral and 6.0 mm below dura. Guide cannulae were anchored to the skull by stainless-steel screws and dental cement. When not in use they were occluded by stainless-steel stylets. In similar experiments involving injections into the medial portion of the ventral tegmentum (A10 region of mesolimbic dopaminergic cell bodies) the co-ordinates of the guide cannula tip were 5.5 mm posterior to bregma. 0.5-0.8 mm lateral and 6.1 mm below dura. Iufracranial injections Drug or saline injections were made from a 10-d microsyringe connected to a 30”gauge stainless-steel injector which protruded 1 mm beyond the tip of the guide cannula. The injection volume was 1 ~1 except for the 1000 pg dose of GABA which was delivered in 2 ~1. Infusion rate was 1 &‘min, and the injector was kept in place for 30 set following an injection. In all experiments a drug was injected into one substantia nigra while saline was injected concurrently into the other. The pH of all drug solutions injected intracranially was between 6.8 and 7.8 except apomorphine (pH = 5.3). Drugs Jl-Baciofen and d- and I-baclofen hydrochloride were kindly supplied by Dr R. D. Robson, CibaGeigy Corp., Summit. N.J. y-Aminobutyric acid, butyric acid, sodium y-hydroxybutyrate, y-butyrolactone. d-amphe~mine sulphate, a~morphine hydrochloride and picrotoxin were purchased from commercial sources. Ail drugs were freshly dissolved in 0.90/, saline before use except butyric acid which was brought to pH 7.3 with QN NaOH and diluted with distilled water. Doses of drugs refer to the forms listed above. and, unless specified otherwise. baclofen refers to the racemic mixture.

Rats were killed by decapitation, their brains rapidly removed and chilled on an ice-cold glass plate. In regional studies, the olfactory tubercles, nuclei accumbens and striata were dissected as previously described (Glowinski and Iversen, 1966; Horn, Cue110 and Miller, 1974). These regions were homogenized in 0.4N perchloric acid containing 0.1 mg/ml EDTA. Dopamine was assayed by a modification (Moore and Phillipson. 1975) of the radioenzymatic method of Cuello, Hiley and Iversen (1973). In other experiments. the pons-medulla and cerebel-

K. E.

MOORE

lum were discarded and the remaining forebrain, or the two separate hemiforebrains were homogenized in 0.4N perchloric acid containing 0.1 mg/ml EDTA and the catecholamine content determined by the fluorimetric assay of Chang (1964). Behauioural methods For measurement of rotation, rats were placed in hemispherical plastic bowls as previously described (Kelly and Moore. 1976). The number of complete 360” turns made in 1 min was determined by direct observation at S-min intervals. Statistical methods The statistical significance of bioche~cal differences was determined by Student’s t-test. When it was possible to compare drug- and saline-treated hemiforebrains in the same animal, the Student’s t-test for paired observations was employed (Goldstein, 1964). RESULTS

~~traper~toneal injection of bacfofen and GBL The effects of various doses of baclofen and GBL on the catecholamine content of the forebrain. measured 1 hr following injection are depicted in Figure 1. Baclofen, in doses of 20, 40 or 8Omg/kg sig~ficantIy increased the forebrain content of dopamine without affecting that of norepinephrine. Similar effects were observed following 250 or SOOmg/kg of GBL. To examine the possible differences in the sensitivity of mesolimbic or nigrostriatal neurones to this dopamine-elevating effect of these drugs, dopamine was measured in the olfactory tubercle and striatum 1 hr following various doses of baclofen or GBL. Both drugs increased the dopamine content of these regions (Fig. 2) but there was no difference in the dose

Dose, w/kg Fie. 1, Rat forebrain concentrations of dopamine (DA) and no;epinephrine (NE) 1 hr after intraperitoneal injections of various doses of baclofen (a1 or GBL (01. Symbols and vertical lines represent me% + 1 SE. cak&ted from 5 to 8 determinations. Solid symbols indicate those values that are significantly different (P <:0.05)from saline. . .* trearecl comfois.

Feedback

regulation

of striatal

I-II

dopamine 160-

(u 0 ,” 150.

Boclofen

t” u

I hboclofen

.%I40

7 E - 130. : ‘;

a?120. 5 E 110. :: z_! L,

*O IO

20

40

6380

Dose)

125

250

,J 500

::

w/kg

.g 100. 8

Fig. 2. Dopamine concentrations in olfactory tubercle (A) and striatum (0)1 hr after intraperitoneal injection of baclofen or GBL. Symbols and vertical lines represent means and 1 S.E. calculated from 5 determinations. Solid symbols

indicate those values that are significantly different (P < 0.05) from saline-treated controls. The dopamine concentrations in the olfactory tubercle and striatum of salineinjected

controls

were

3.91 f 0.17 respectively.

and

4.97 f 0.1 I &g,

required to produce the effect in me~limbic ni~ostriatal dopaminergic neurones. ~ntr~ranial

injection

go-

d- baclofen .;$;:

25

of drugs

630

2500

ng

Fig. 4. Dopamine concentrations in rat hemiforebrains 30 min after microinjections of I-baclofen (0) or d-baclofen (A) into substantia nigra. Symbols and vertical lines represent means & I SE. calculated from 3 to 4 determinations. Solid symbols indicate those values on the drug injected side which are significantly different (P <: 0.05) from the saline-injected side. The dopamine concentration in all control hemiforebrains was 0.54 + 0.02 pggig (mean &- I SE., K = 34).

I-isomer. This is also the case when these isomers are injected systemically (Gianutsos and Moore, 1977). Table 1 shows the hemiforebrain dopamine content 30 min following the unilateral injection of drugs into one substantia nigra and saline into the other. A significant increase in the hemiforebrain dopamine content of the drug-injected side was produced by GBH (250 pg) whereas sodium butyrate (250 pg), apomorphine (10 pg) and d-amphetamine (IO pg) were ineffective. When in,jected into the region of the mesolimbic dopamine cell bodies (area AlO), baclofen (2.5 pg) significantly increased (P < 0.01) the dopamine content of the nucleus accumbens and otfactory tubercle, to 132 and 153% of control. respectively. An unexpected behaviourat finding following the unilateral intranigral injection of GABA, GHB and baclofen is shown in Figure 5. All three compounds produced circling away from the drug-injected side and towards the saline-injected side. Intranigral injections of apomorphine (10 pg) and d-amphetamine (10 pg) produced no circling. of picrotoxin

intranigra~ GAB.4 25 Dose,

160

Dose,

Effect

063

40

or

Figure 3 shows the elevation of hemiforebrain dopamine content 30min following injection of various doses of baclofen or GABA into the substantia nigra. Baclofen is clearly more potent than GABA by a factor of several hundred. The potencies of the d- and I-isomers of baclofen following intranigral injections were also compared. The results depicted in Figure 4 show that most activity resides in the

0 16

IO

w

Fig. 3. Dopamine concentrations in rat hemiforebrains 30 min after microinjections of various doses of baclofen (A) or GABA (0)into substantia nigra. Symbols and vertical lines represent means k 1 S.E. calculated from 3 to 7 determinations. Solid symbols indicate those values on the drug-injected side which are significantly different (P < 0.05) from the saline-injected side. The dopamine concentration in all control hemiforebrains was 0.63 k 0.01 pg/g (mean * 1 SE., iV = 73).

on

the

hioehemicai

actions

C$

and baclofen

Picrotoxin was injected into the substantia nigra immediately before doses of GABA (250~t&) or baclofen (1.25pg) chosen to have approximately equivalent effects in elevating forebrain dopamine. The results summarized in Table 2 show that picrotoxin (4 pg) did not alter forebrain dopamine concentrations when injected alone but antagonized the elevation of forebrain dopamine produced by GABA. The same dose of picrotoxin (4 pg) which antagonized the effect of GABA did not affect the elevation of

172

P. H. KELLYand K. E. MOORE Table

1. Concentration

Treatment

of dopamine in hemiforebrains of rats 30 min after of drugs or saline into the substantia nigra

Saline-injected side

GHB, 25Opg

0.72 0.60 0.70 0.54

Sodium butyrate, 250 pg d-Amphetamine, 10 pg Apomorphine, IOpg

the injection

Drug-injected

+_ 0.04 + 0.03 * 0.04 * 0.02

0.88 0.68 0.71 0.54

side

f 0.08* k 0.05 + 0.03 _+ 0.02

Values represent the mean + 1 SE. of the dopamine concentration &g/g) as determined from 5 to 6 animals. * Indicates those dopamine concentrations on the drug-injected side which are significantly greater (P < 0.05) than those on the saline-injected side.

forebrain dopamine produced by baclofen. That is, the concentration of dopamine in the forebrain after GABA was significantly greater than that after GABA and picrotoxin (0.81 vs 0.68 pg/g, P < 0.01 ), whereas the dopamine concentration after baciofen and picrotoxin was not different from that after baclofen alone (0.75 vs 0.76 pg/g). The most consistent behavioural effect of picrotoxin was to produce rolling towards the injected side. This behaviour occurred with a latency of t&20 min and continued until the animal was sacrificed at 30 min. It has been reported (Tarsy, Pycock, Meldrum and Marsden, 1975) that intranigral injection of a lower dose of picrotoxin (0.25 pg) caused contralateral circling in rats. In the present study, this dose of picrotoxin produced no consistent ~havioural effect and failed to prevent the elevation of dopamine in the striatum produced by 250 pg GABA injected into substantia nigra.

concentration is increased to about the same extent in regions containing terminals of nigrostriatal or mesolimbic neurones. Intracerebral injections of baclofen share some of the characteristics of GABA and GHB, which is believed to be the active metabolite of GBL. When injected unilaterally into the substantia nigra all three drugs elevated the content of dopamine, but not of norepinephrine, in the ipsilateral hemiforebrain, and all caused the rats to circle away from the drug-injected side. When applied to the nigral cells by iontophoresis, GABA inhibits their firing (Feltz, 1971; Crossman, Walker and Woodruff, 1973), and the increase in forebrain concentration of dopamine produced by intranigral injections of GABA and GHB has been attributed to the reduction in impulse flow (Andkn and Stock, 1973). The increased forebrain dopamine following intranigral injections of baclofen suggests that this drug also inhibits the activity of dopaminergic nigrostriatal neurones and this has been observed directly by Davies and Dray (1976) and Bernard et al. (1975). It is not clear, at present, if the depression in the firing rate of the nigrostriatal neurones produced by GABA, GHB and baclofen is due to the same or to a different mechanism. Depression of firing could be produced by activating inhibitory mechanisms or by antagonizing excitatory influences. Whereas the effects of GABA were antagonized by picrotoxin pre-

DISCUSSION

Confirming reports of previous experiments in rats (DaPrada and Keller, 1976; And& and Wachtel, 1977) and in mice (Gianutsos and Moore, 1977) systemic administration of baclofen, like GBL, increases the brain concentration of dopamine but not of norepinephrine. As illustrated in Figure 2, the dopamine

lo

GABA(250&

IO-

lo GHB(2fQ.q)

I E-

ofen BOCI

(8pg)

’

6-

.C 6E > : $ 4-

O’OW Minutes

after

injectio

Fig. 5. Circling induced by unilateral intranigral injections of GABA, GHB and bactofen. Symbols and vertical lines represent means & 1 S.E. calculated from 5 to 6 animals. All turns are away from the side of drug injection.

Feedback Table 2. Effects of picrotoxin by injections

regulation

of striatal

on the elevation of forebrain dopamine concentrations of GABA or baclofen into the substantia nigra

injections

Saline-injected

Picrotoxin GABA GABA + picrotoxin Baclofen Baclofen + picrotoxin

0.68 0.63 0.61 0.54 0.54

Drug

173

dopamine

+ + + * *

0.10 0.02 0.03 0.03 0.02

side

Drug-injected 0.60 0.81 0.68 0.76 0.75

+ f f f f

induced

side

0.05 0.04’ 0.03 0.06* 0.03;

Drugs were injected into the right substantia nigra simultaneously with injections of saline into the left substantia nigra; all injections were made in a volume of 1 ~1. In the right substantia nigra picrotoxin (4 pg) or saline was injected immediately before injections of GABA (250 p(p) or baclofen (1.25 pg). Rats were decapitated 30 min later and hemiforebrains were assayed for dopamine. Values represent means + 1 S.E. of the forebrain concentration of dopamine (pg/g) calculated from 6 determinations. * Significantly different from saline injected side (P < 0.01)

treatment, the effects of baclofen were not. This suggests that baclofen and GABA are acting by different mechanisms. The results of the present study are consistent with the inability of bicuculline (Davies and Watkins, 1974; Curtis, Game, Johnston and McCullogh, 1974) to antagonize the inhibitory effects of iontophoretically-applied baclofen on cortical and other neurones. The markedly greater potency of baclofen compared to GABA upon intranigral injection (Fig. 3) contrasts with the estimated potency, slightly less than that of GABA, in inhibiting cortical neurones upon iontophoretic application (Davies and Watkins, 1974; Curtis et al., 1974). This difference might be due to the different methodologies since the amount of drug administered is not accurately known in iontophoretic studies. Alternatively, it might indicate a difference in the sensitivity to baclofen between neurones in the substantia nigra and in other areas of the brain. For example, substance P is present in high concentrations in substantia nigra (Kanazawa et al., 1977; Hong, Yang, Racagni and Costa, 1977). This compound excites nigral neurones when applied iontophoretically and baclofen can antagonize this action (Davies and Dray, 1976). Phillis (1976) and Henry and Ben-Ari (1976). however, have presented evidence that baclofen is not a specific substance P antagonist, but rather that it is a non-specific depressant. Using a different experimental approach, based primarily on biochemical criteria, Naik, Guidotti and Costa (1976) have concluded that baclofen is not a GABA agonist. In addition to its effects on nigrostriatal neurones, baclofen also elevates the dopamine content of mesolimbic dopamine terminal regions when injected into the region of A10 cell bodies. This finding is consistent with the suggestion that baclofen can inhibit the firing of mesolimbic dopamine neurones (Fuxe er al., 1975; Bernard et al., 1975). After parenteral administration of baclofen or GBL, dopamine increased in regions of both mesolimbic and nigrostriatal dopamine terminals. By this measure, mesolimbic dopamine neurones were not more sensitive than nigrostriatal neurones to either drug. In contrast, the pimozideinduced increase in turnover of mesolimbic dopamine neurons is reported to be more sensitive to reversal

by baclofen than the corresponding change in nigrostriatal neurones (Fuxe et al., 1975). It has been reported that injections of d-amphetamine into the substantia nigra can produce a longlasting depression of the firing of neurones in the pars compacta of the substantia nigra (Groves et al., 1975). These changes were reported following nanogram doses of amphetamine. Using much larger amounts of this drug no increase in forebrain dopamine content was observed in the present experiments suggesting that there is no inhibition of nigrostriatal neurones Aghajanian and Bunney (1973) found only a weak depression of firing of nigrostriatal neurones by microiontophoresis of d-amphetamine but apomorphine caused a depression of nigrostriatal firing which showed tachyphylaxis. In the present experiments, however, apomorphine injected into the substantia nigra produced no increase in forebrain dopamine. Negative data in the present study should be viewed with caution since the elevation of dopamine concentration is only an indirect indication of decreased nerve activity. Finally, the mechanism of the circling produced by GABA and its derivatives following unilateral intranigral application is unclear. When there is an asymmetry of striatal dopaminergic activity animals turn away from the side of the greatest activity (Ungerstedt and Arbuthnott, 1970) provided mesolimbic dopamine receptors are also activated (Kelly and Moore, 1976). Possibly, therefore, there is increased release of dopamine from the nigrostriatal neurones on the drug-stimulated side, even though their firing rate is reduced. This possibility receives support from the observation that baclofen simultaneously increases dopamine and homovanillic acid concentrations in the brain (DaPrada and Keller, 1976). The latter compound is believed to be an extraneuronal metabolite of dopamine suggesting that baclofen does cause the release of dopamine. Alternatively, the circling could result from an action on non-dopaminergic neurones. REFERENCES Aghajanian, minergic

G. K. and Bunney, B. S. (1973). Central dopaneurons: Neurophysiological identification and

174

P. Ii.

KELLY

and K. E. MOORE

to drugs. In: Fronriers in Catechofamine Research (Usdin, E. and Snyder, S. H., Eds), pp. 643-648.

responses

Pergamon Press, Oxford. And&n, N.-E. and Stock, G. (1973). Inhibitory effect of gamma-hydroxybutyri~ acid and gamma-~inobutyric acid on the dopamine cells in the substantia nigra. Naunyn Schmiedehergs 89-92.

Arch exp Path. Pharmak. 279:

Anden, N.-E. and Wachtel, H. (1977). Biochemical effects of baclofen (@-parachlorophenyl-GABA) on the dopamine and the noradrenaline in the rat brain. Acfa pharmix. fox. 90: 310-320.

Bernard. P.. Edwards, S. J., Fielding. S., Robson, R. D., Saelens, J. K.. Simke, J. P. and Welch, J. (1975). Baclofen: An unusual CNS-active agent. Pharmncologisr 17: 256. Bunney, B. S. and Aghajanian, 6. K. (1975). Evidence for drug actions on both pre- and postsynaptic catecholamine receptors in the CNS. In: Pre- and Posfsynuprie Receptors (Usdin, E. and Bunney, W. E. Jr. Eds), pp. 89-120. Marcel Dekker. New York. Chang, C. C. (1964). A sensitive method for spectrofluorometric assay of catecholamines. Inr. J. Neuropharmac. 3: 643-649.

Crossman, A. R.. Walker, R. J. and Woodruff, G. N. (1973). Picrotoxin antagonism of y-aminobutyric acid inhibitory responses and synaptic inhibition in the rat substantia nigra. Br. J. Pharmac. 49: 696-698. Cuello, A. C., Hiley, R. and Iversen, L. L. (1973). The use of catechol-o-methyhransferase for the enzyme radiochemical assay of dopamine. J. Neurochem. 21: 133-F-1340.

Groves, P. M., Wilson, C. J,, Young. S. J. and Rebec, G. V. (1975). Self-inhibition by dopaminergic neurons. Science 190: 522-529. Henry, J. L. and Ben-Ari, Y. (1976). Actions of the pchiorophenyl derivative of GABA, lioresal, on nociceptive and non-nociceptive units in the spinal cord of the cat. Brain Res. 117: 54@544. Hillen, F. C. and Noach, E. L. (1971). The influence of 1-hydroxy-3-amino-pyrrolidinone-2 (HA-966) on dopamine metabolism in the rat corpus striatum. Eur. J. Pharmac. 16: 222-224,

Hong, J. S.. Yang, H-Y. T., Racagni. G. and Costa, E. (1977). Projections of substance P containing neurons from neostriatum to substantia nigra. Brain Res. 122: 541-544. Horn, A. S., Cuello, A. C. and Miller, R. J. (1974). Dopamine in the mesolimbic system of the rat brain: Endogenous levels and the effects of drugs on the uptake mechanism and stjmulation of adenvlate cyclase activity. J. Neurochem. 22: 265-270. _ * Kanazawa. I.. Emson. P. C. and Cuello. A. C. 319771. Evidence for the existence of substance P-containing’fibers in striato-nigral and pallido-nigral pathways in rat brain. Brain Res. 119: 441-453.

Kelly, P. H. and Moore, K. E. (1976). Mesolimbic dopamine neurons in the rotational model of nigrostriatal function. Nature, Land. 263: 695-696 Kim. J. S., Bak, 1. J.. Hassler. R. and Okada, Y. (19711, Role of ~-aminobutyric acid (GABA) in the extrapyramida1 motor system. 2. Some evidence for the-existence of a type of GABA-rich strio-nigral neuron. Expl Brain Res. 14: 95-104.

Curtis, D. R., Game, C. J. A., Johnston. G. A. R. and McGeer, E. G., Fibiger, H. C., McGeer. P. L. and Brooke, McCulloch, R. M. (1974). Central e&c% of /I-@-chloroS. (1973). Temporal changes in amine synthesizing phenyl~y-a~nobutyric acid. Brain Res. 70: 493499. enzymes of rat extmpyram~dal structures after hemitransections or &hydroxydopamine administration. Brain Da Prada. M. and Keller. H. H. (1976). Baciofen and hydroxybutyrate: similar effects on cerebral dopamine neurRes. 52: 289-300. Moore, K. E. and Phillipson, 0. T. (1975). Effects of dexaones. Life Sci. 19: 1253-1264. methasone on phenylethanolamine N-methyltransferase Davies, J. and Dray, A. (1976). Substance P in the substanand adrenaline in brain and superior cervical ganglia tia nigra. Bruin Res. 107: 623-627. of adult and neonatal rats. J. Neurochem. 25: 289-294. Davies, J. and Watkins, J. C. (1974). The action of Naik, S. R., Guidotti, A. and Costa, E. (1976). Central /?-phenyl-GABA derivatives on neurons of cat cerebral GABA receptor agonists: Comparison of muscimol and cortex. Brain Res. 70: 501-505. baclofen. ~earophur~c~logy 15: 479-484. Feltz, P. (197t). y-Aminobuty~c acid and a caudato-nigral inhibition. Can. J. Physiol. Pharmac. 4% 1113-l 115. Phillis, J. W. (1976) is /?-(4-chlorophenyl)-GABA a specific antagonist of substance P on cerebral cortical neurons’? Fonnum, F., Grofova. I.. Rinvik, E., Storm-Mathisen, J, and Walberg, F. (1974) Origin and distribution of glutaExperientiu 32: 593-594. Saito, K., Konishi, S. and Otsuka, M. (1975). An~gonism mate decarboxylase in substantia nigra of the cat. Brain between Lioresal and substance P in rat spinal cord. Res. 71: 77-92. Brain Res. 9% 177-l 80. Fuxe. K.. Hiikfelt. T., Ljungdahl, A., Agnati, L., JohansStock, G., Mannusson, T. and And&. N.-E. (1973). Inson. 0. and Perez de la Mora, M. (1975). Evidence for crease in br& dopamine after axotomy or treatment an inhibitory gabergic control of the mesolimbic dopawith gammahydroxybutyric acid due to elimination of mine neurons: Possibility of improving treatment of nerve impulse flow. Naunyn-Schmiedehergs Arch. exp. schizophrenia by combined treatment with neuroleptics Path. Pharmak. 278: 347-361. and gabergic drugs. Med. Biol. 53: 177-183. Tarsy, D., Pycock, C., Meldrum, B. and Marsden, C. D. Gessa,C. L.rVargi;, L., Crabai, F., Boero, G. C.. Caboni. (1975). Rotational behavior induced in rats by intraF. and Camba. R. (19663. Selective increase of brain &ral picrotoxin. Bruin Res. 89: 160-165. dopamine induced b; gamma-hydroxybuiyrate. Lije Sei. Ungerstedt. U. and Arbuthnott, G. W. (1970). Quantitative 5: 1921-1930. recording of rotational behavior in rats after 6-hydroxyGianutsos, G. and Moore, K. E. (i977). Increase in mouse dopamine lesions of the nigrostriatal dopamine system. brain dopamine content by baclofen: Effects of apoBrain RPS. 24: 4851194. mornhine and neuroleptics. Psvchopharmucolouv . . _. 52: Walters. J. R.. Roth, R. H. and Aghajanian, 6. K. (1973). 217-221. Do~minergic neurons: Similar biochemical and histoGfowinski, J. and Iversen, L. L. (1966). Regional studies chemical effects of ~-hydroxybutyrate and acute lesions of ~t~holamines in the brain. J. ~earochem. 13~ of the nigro-neostriatai pathway. J. Pharmuc. exp. Ther. 655-669. 186: 630-639. Goldstein, A. (1964). Biostatisfics, Macmillan, New York.