Effects of peripheral and local administration of picrotoxin on the release of newly synthesized 3H-dopamine in the caudate nucleus of the cat

Regulation of the prefrontal cortical dopamine release by GABAA and GABAB receptor agonists and antagonists

The gamma-aminobutyric acid-dopamine (GABA-DA) relationship was studied by intracerebral microdialysis in the prefrontal cortex. Nomifensine (5 microM) was included in the Ringer solution during all the dialysis experiments. Muscimol, a GABAA receptor agonist (50 and 500 microM) did not affect the extracellular output of DA and 3,4-dihydroxyphenylacetic acid (DOPAC). Baclofen, a GABAB receptor agonist (50 microM) significantly decreased the extracellular output of DA and DOPAC. On the other hand, picrotoxin and phaclofen, GABAA and GABAB receptor antagonists respectively, at a concentration of 50 microM, both significantly increased the release of DA. While the DOPAC level was affected only by picrotoxin perfusion. The present study indicates that GABA could control the release of DA in the prefrontal cortex.

The role of GABA receptors in the control of nigrostriatal dopaminergic neurons: dual-probe microdialysis study in awake rats

A microdialysis probe implanted into the substantia nigra was used to infuse gamma-aminobutyric acid-ergic (GABAergic) compounds onto cell bodies/dendrites of dopaminergic neurons, while a second microdialysis probe was used to record the extracellular concentrations of dopamine and 3,4-dihydroxy-phenylacetic acid (DOPAC) in the ipsilateral striatum. The GABAA receptor agonist muscimol (10 mumol/l) increased the release of dopamine in the ipsilateral striatum to 120% of the control values. The GABAB receptor agonist, (Z)-3[(aminoiminomethyl)-thiol]-prop-2- enoic acid (500 mumol/l), was without effect. Infusion of the GABAA receptor antagonists, bicuculline (50 mumol/l) and picrotoxin (50 mumol/l), stimulated the release of dopamine in the ipsilateral striatum to 160 and 130% of the controls, respectively. The GABAB receptor agonist, baclofen (10 and 50 mumol/l), strongly inhibited the release of striatal dopamine, whereas infusion of the GABAB receptor antagonist, 2-hydroxy-saclofen (100 mumol/l), was without effect. The results indicate that, in the substantia nigra, GABAA as well as GABAB receptors participate in controlling the activity of dopaminergic neurons.

Protein kinase C and dopamine release--II. Effect of dopamine acting drugs in vivo

The hypothesis that protein kinase C (PKC) plays a role in the release of dopamine (DA) in the nigrostriatal pathway was examined. It was found that injections of apomorphine, SKF 38393 (D1 agonist), LY 171555 (D2 agonist) or gamma-butyrolactone (GBL) (which decreases impulse-induced release of DA) resulted in a decrease in particulate, and an increase in soluble, PKC activity. Injections of fluphenazine, haloperidol, SCH 23390 (D1 antagonist), sulpiride (D2 antagonist) or picrotoxin (gamma-aminobutyric acid antagonist which increases DA release transneuronally) had the opposite effect of increasing particulate and decreasing soluble PKC activity. The total activity was not changed. These effects were receptor mediated since the effect of each agonist could be reversed by its specific antagonist. These drugs influenced PKC in the striatum in a dose-dependent manner. In contrast, no effects were seen in the cerebellum, a region with sparse dopaminergic innervations. The change in PKC activity was mediated via a change in the Km for calcium, while the Vmax was unchanged. The phosphorylation of endogenous substrate proteins by PKC was also altered by injections of these drugs. Besides affecting PKC, these DA acting drugs also affected the calmodulin-dependent protein kinase activity, but the direction of change was opposite to that for PKC. In a synaptosomal preparation, PKC acting drugs also affected the depolarization-induced release of DA. Adriamycin and melittin decreased the potassium-induced release of DA, whereas tetradecanoyl-phorbol-13-acetate (TPA) enhanced this release. These results showed that there was a good correlation between the ability of drugs to alter the impulse-induced release of DA in vivo and their ability to affect changes in particulate and soluble PKC activity. They lend support to the hypothesis that PKC, together with calmodulin, plays a key role in the release of DA in the nigrostriatal pathway.

Effects of avermectin B1a and picrotoxin on striatal release of dopamine with reference to replacement of extracellular chloride with nitrate

The present study was an attempt to elucidate the effects of avermectin B1a (AVM) and picrotoxin, an anion channel opener and blocker, respectively, on the release of endogenous dopamine from the slices of caudate nucleus of the rat, using a superfusion method in order to determine the interaction between these agents with the gamma-aminobutyric acid (GABA) receptor-anion channel complex. Avermectin (1.14-11.4 microM) reduced the Ca2+-dependent release of dopamine stimulated by 40 mM KCl without affecting the basal release of dopamine. In contrast, picrotoxin in doses larger than 20 microM facilitated the K+-stimulated release of dopamine. The inhibitory effect of avermectin was completely antagonized by 10 microM picrotoxin and 0.1 mM bicuculline; these doses of both agents did not change the K+-stimulated release of dopamine. Replacement of chloride (Cl-) in the superfusion medium with nitrate (NO3-) markedly facilitated the K+-stimulated release of dopamine and the increase was antagonized by verapamil (10 microM) and tetrodotoxin (1 microM). In the nitrate medium, avermectin reduced the K+-stimulated release of dopamine and the inhibitory effect was antagonized by bicuculline. However, picrotoxin up to 100 microM did not affect the K+-stimulated release of dopamine either in the presence or absence of bicuculline. These results suggest that the dopaminergic nerve terminals in the caudate nucleus receive inhibitory regulation through the facilitation of anion channels. This regulation is apparently altered depending on the main anion in the extracellular fluid.

Nafion-coated electrodes with high selectivity for CNS electrochemistry

A major improvement in the selectivity of small graphite electrodes used for in vivo electrochemistry is described. The electrodes are coated with Nafion, a perfluorosulfonated polymer. This coating is practically impermeable to ascorbic acid and anionic biogenic amine metabolites and only slightly responsive to neutral metabolites. Thus it becomes selective for the cationic primary neurotransmitters, dopamine, norepinephrine and 5-hydroxytryptamine. Responses of Nafion-coated and untreated electrodes in vivo are compared.

Alterations in apomorphine concentration in spinal cord and brain follow the time course of catalepsies induced by different treatments

Because evidence for the neurotransmitter role of dopamine in the gray matter of the spinal cord is accumulating, a question arises of whether or not spinal dopamine receptors are also involved in the effects of dopaminomimetics which are believed to induce beneficial effects in Parkinson's disease through an action thought to be mediated mainly by striatal dopamine receptors. To test this hypothesis muscimol and picrotoxin were injected unilaterally into the posterior part of the substantia nigra of rabbits permanently implanted with stainless-steel cannulae. Muscimol (a GABA-mimetic) enhanced locomotor activity, evoked a stereotyped behavior and contralateral rotations, and increased apomorphine-induced gnawing. Picrotoxin, a substance which inhibits GABA transmission, induced ipsilateral rotations, evoked catalepsy and muscle rigidity, and inhibited locomotor activity. Picrotoxin abolished apomorphine-induced gnawing, and increased haloperidol-mediated catalepsy. The catalepsy induced by an intranigral injection of picrotoxin, and the picrotoxin-evoked blockade of the apomorphine-induced gnawing disappeared within 16 h after the intranigral injection. Alterations in the apomorphine concentration in brain structures (n. caudatus and cerebral cortex) and in spinal cord after picrotoxin injection followed the same time course as the behavioral changes, and returned to the control values 16 h after injection of picrotoxin. Apomorphine was always injected 30 min before the rabbits were killed. Moreover, the substantial increase (to 300%) in apomorphine concentration in the spinal cord probably reflects the antagonism between behavioral changes induced by picrotoxin and the haloperidol catalepsy, rather than the decreased apomorphine concentrations observed in the brain structures. We suggest, therefore, that there exists a correlation between the behavioral effects, which are generally accepted as laboratory models of Parkinson's disease, and the enhanced apomorphine concentration in the spinal cord.

Evoked release of proteins from central neurons in vivo

Push-pull cannulae were implanted in both substantiae nigrae and caudate nuclei of the halothane-anesthetized cat. The release of total protein, acetylcholinesterase, and nonspecific cholinesterases was examined. Following direct application of potassium to one substantia nigra, changes occurred in the local release of total protein and acetylcholinesterase, but not nonspecific cholinesterases; changes also were observed in both caudate nuclei and the contralateral substantia nigra. The local evoked release of acetylcholinesterase and of total protein differed in the extent to which they were calcium-dependent. Control studies suggest that release of these compounds, both spontaneous and evoked, is related, at least in part, to neuronal activity. The significance of the neuronal release of proteins is discussed.

Contractile activity and prostacyclin generation in isolated coronary arteries from diabetic dogs

The present study was aimed at determining the generation of "prostacyclin (PGI2)-like-material" in coronary arteries from normal and diabetic (pancreatectomized) dogs as well as the contractile responses to prostacyclin of preparations from normal, diabetic and insulin-treated diabetic animals. PGI2 produced a dose-dependent relaxation of coronary arteries from normal dogs. In contrast, those from diabetic animals were not related; indeed, at low concentrations PGI2 failed to evoke any effect but at higher ones it induced a distinct contraction. In arteries from diabetic animals treated with insulin, PGI2 induced a biphasic contractile effect, which lay between that of normal controls and untreated diabetics. In addition the basal generation of "PGI2-like-material" by coronary arteries was significantly higher in the diabetic (141 +/- 0.2 pg/mg, mean +/- SEM) than in normal dogs (59 +/- 0.2 pg/mg). The present experiments demonstrate that the generation of "PGI2-like substance" is significantly increased in coronary arteries from diabetic dogs, but the same vessels are unable to respond to added authentic PGI2 with relaxation; on the contrary they react with a distinct positive contractile response.

Partial lesions of the dopaminergic nigrostriatal system in rat brain: biochemical characterization

Various doses of 6-hydroxydopamine injected into the rat substantia nigra produced partial, dose-dependent lesions of the dopaminergic nigrostriatal tract. The resulting reduction in striatal dopamine concentrations and tyrosine hydroxylase activities tended to be proportional, allowing these measurements to serve as indices for lesion severity in any particular animal. Lesions destroying two-thirds or more of the nigrostriatal neurons accelerated dopamine's synthesis in, and release from, surviving neurons, as indicated by increased striatal levels of the dopamine metabolites dihydroxyphenylacetic acid and homovanillic acid. Formation of these metabolites was also enhanced in dendrites of dopaminergic neurons in the substantia nigra. Supersensitivity of striatal postsynaptic receptors, as judged by induction of rotational behavior after apomorphine or L-DOPA administration, occurred when 90% or more of the nigrostriatal neurons had been destroyed. In contrast, rotational behavior could be induced by amphetamine in animals with only 50% of these neurons destroyed.

Tyrosine administration increases striatal dopamine release in rats with partial nigrostriatal lesions

Partial, unilateral nigrostriatal lesions of varying severity were produced in rats by injecting graded doses of 6-hydroxydopamine into the substantia nigra. Formation of the dopamine metabolites dihydroxyphenylacetic acid and homovanillic acid in each surviving nigrostriatal neuron (estimated by the ratios of dihydroxyphenylacetic acid to dopamine and homovanillic acid to dopamine in the striatum) increased significantly when dopamine concentrations in striata containing lesions had been reduced to 25% or less of control values, but remained unchanged in rats with less severe lesions. These findings suggest that, in rats with severe damage of nigrostriatal dopaminergic neurons, surviving neurons increase their firing rates and accelerate dopamine synthesis and release. In rats that had lesions and enhanced striatal dopamine release, but not in rats with lesser lesions (i.e., which reduced ipsilateral dopamine concentrations by less than 75%), administration of tyrosine (250 mg/kg) caused further significant increases in formation of dihydroxyphenylacetic acid and homovanillic acid. These findings provide further evidence that tyrosine availability can enhance dopamine synthesis in and release from nigrostriatal neurons if the firing rates of these neurons are accelerated.

Effects of isoprenaline on functional capillary density in the subendocardial and subepicardial layer of the rat myocardium

The functional capillary density in subepicardial and subendocardial layers of rat heart was measured during rest and during isoprenaline-induced (5.0 microgram X kg-1 X min-1, i.v. over 3 minutes) cardiac stimulation. For determination of the number of perfused capillaries, a fluorescent dye (thioflavine S) was infused into the left atrium; 1, 3, 5 and 10 sec, respectively, after starting dye application, hearts were excised and rapidly cooled down to -50 degrees C. In histological sections capillaries which had been perfused during the dye infusion could be identified and counted. An increase in the number of stained vessels was found in both layers of the myocardium when the time of dye exposure was prolonged. Under these conditions the rise was much smaller in isoprenaline-treated animals, this effect being most marked in the subendocardial layer (3560 +/- 199 cap./mm2, control group; 2190 +/- 30 cap./mm2, isoprenaline-treated group; dye exposure 10 sec). Isoprenaline - at the dose used - induced an increase in total blood flow (3.7 +/- 0.6 ml X min-1 X g-1, control group; 6.8 +/- 0.7 ml X min-1 X g-1, isoprenaline-treated group), however, with a relatively less pronounced increase in the subendocardial blood flow (subendocardial/subepicardial flows: 1.08 +/- 0.13, control group; 0.66 +/- 0.01, isoprenaline-treated group). These results favour the view that isoprenaline-induced relative reduction in the subendocardial blood flow is due to disturbance of perfusion pressure and extravascular compression rather than to exhaustion of the myocardial capillary reserve.

Increased prostacyclin release from perfused hearts of acutely diabetic rats

The release of prostacyclin and PGE2 from the isolated perfused hearts of acutely diabetic (streptozotocin 100 mg/kg) rats was studied and compared with hearts from control animals. Prostacyclin and PGE2 were measured by a differential bioassay technique. No basal release of either prostaglandin ws detected. However, after addition of arachidonic acid, a dose dependent release of prostacyclin and PGE2 was noted. Prostacyclin was identified as the major prostaglandin. Release of prostacyclin and PGE2 from acutely diabetic rat hearts was increased 2-3 times compared to control hearts. No release of prostaglandin endoperoxides was observed in either group of hearts.

Intranigral GABA antagonists produce dopamine-independent biting in rats

Behavioural effects following bilateral intranigral administration of GABA antagonists have been investigated. Bicuculline methiodide (BMI), picrotoxin and isopropylbicyclophosphate all induced biting behaviour, teeth-chattering and chewing. Sub-threshold doses for biting induced locomotor activity and sniffing. The strongest response was observed after injection into the caudal pars reticulata, whereas weaker effects were seen after injection into the rostral pars reticulata or the pars compacta. The biting induced by intranigral BMI was not antagonized by prior catecholamine depletion with reserpine plus alpha-methyl-p-tyrosine or by dopamine receptor blockade with haloperido. Concomitant intranigral injection of the GABA agonists muscimol and THIP, however, completely antagonized biting. Systemic GABAergic drugs also antagonized the BMI-induced biting: the benzodiazepine, diazepam and the GABA transaminase inhibitor, gamma-acetylenic GABE, were most effective, whereas muscimol was only partially effective and THIP was without effect. It is suggested that this animal model may be used for the evaluation of antidyskinetic drugs.

Symmetric bilateral changes in dopamine release from the caudate nuclei of the cat induced by unilateral nigral application of glycine and GABA-related compounds

The release of [3H]DA synthesized from [3H]tyrosine was estimated in the two caudate nuclei (CN) during the unilateral nigral application of glycine and GABA-related compounds in 'encéphale isolé' cats using push-pull cannulae. Glycine (10(-5) M) reduced the release of [3H]DA in both CN and these effects were antagonized by strychnine (10(-5) M). A decrease in [3H]DA release was also seen in both CN during the unilateral nigral application of diazepam (10(-5) M). In contrast, muscimol (10(-6) M) and GABA (10(-5) M) stimulated [3H]DA release on both sides. The effect of GABA was blocked by picrotoxin (10(-5) M). Picrotoxin alone stimulated the release of [3H]DA in the ipsilateral CN and was without effect in the contralateral side. Bicuculline (10(-5) M) stimulated [3H]DA release only in the contralateral CN. A symmetric increase in [3H]DA release in both CN was also observed during the unilateral nigral application of potassium (30 mM). A model involving a facilitatory polysynaptic pathway originating from the substantia nigra (SN) and acting presynaptically on ther terminals of the contralateral DA neurons is proposed to explain the changes in [3H]DA release induced in the contralateral CN in these various situations. The results are discussed taking into account previous data on the reciprocal control of the two dopaminergic pathways induced by the unilateral nigral application of dopaminergic drugs.

Alterations in neurotransmitter activity after acute and chronic ethanol treatment: studies of transmitter interactions

Acute and chronic ethanol treatment has multiple effects on the neurotransmitter systems in the nigrostriatal complex. A single dose of ethanol increases striatal dopamine release at low doses, but depresses it at high doses. In ethanol-dependent rats, dopamine release is accelerated during intoxication, but is reduced during a withdrawal syndrome. Concomitantly, high-affinity choline uptake, an index of cholinergic activity, is elevated at times when dopamine release is depressed. Changes in dopaminergic or cholinergic receptor activity do not induce or result from these effects. Neither has a role for GABA or substance P yet been implicated. The data suggest that interactions between at least two trasmitters in the caudate nucleus may occur after acute and chronic ethanol treatment.

Effects of the unilateral nigral application of baclofen on dopamine release in the two caudate nuclei of the cat

"Encéphale isolé" cats were implanted with three push-pull cannulae. The two caudate nuclei (CN) were superfused continuously with 3H-tyrosine to measure the release of 3H-DA in serially collected fractions. Three hours after the beginning of the experiment, d,l-baclofen, d- or l-baclofen or gamma-hydroxybaclofen were introduced for 15 min in the medium used to superfuse the left substantia nigra. d,l-Baclofen (10(-6) M) stimulated 3H-DA release in the ipsilateral as well as in the contralateral CN but the effect was more pronounced in the ipsilateral CN. Similar effects were observed with l-baclofen. d-Baclofen and gamma-hydroxybaclofen were inactive. The results obtained are discussed in the light of effects previously observed during the nigral application of gamma-aminobutyric acid (GABA) and related compounds.

Amphetamine-induced circling behavior in rats: effects of unilateral microinjections of GABA and GABA-related drugs into substantia nigra

In these experiments, the GABA control upon dopaminergic nigrostriatal neurons has been investigated using circling behavior in the rat. Chronically cannulated rats were given D-amphetamine (2 mg/kg i.p.) 45 min before unilateral microinjection (0.2 microliter/2 min) into the substantia nigra (SN) of GABA, muscimol, chlordiazepoxide (CDP) or bicuculline. Circling behavior was continuously recorded for 165 min using an automated rotometer. (1) Non-microinjected control rats exhibit a 'spontaneous' circling behavior after amphetamine. (2) When applied to the SN contralateral to the preferential side of the 'spontaneous' rotations, saline enhances contraversive circling; GABA (5 X 10(-5)M), CDP (5 X 10(-5)M) and muscimol (5 x 10(-7)M, 5 X 10(-8)M) counteract this effect and induce (except CDP) light ipsiversive rotations; GABA (10(-2)M) and muscimol (5 x 10(-5)M) further enhance contraversive turning. (3) When applied to the SN ipsilateral to the preferential side of the 'spontaneous' rotations, saline has no marked effect on the ipsiversive circling behavior but induces weak contraversive turning; GABA (5 X 10(-5)M), CDP (5 X 10(-5)M) and muscimol (5 x 10(-8)M) enhance the ipsiversive rotations; GABA (10(-2)M) and muscimol (5 x 10(-5)M) transiently decrease the ipsiversive circling; bicuculline (5 x 10(-5)M) induces a vigorous contraversive turning associated with a transient inhibition of the ipsiversive rotations. These results suggest that the activity of the nigral neurons is presumably stimulated by the microinjection itself, by bicuculline and, to a lesser extent, by high concentrations of GABA and muscimol, and inhibited by low concentrations of GABA, muscimol and CDP. These findings could further support the hypotheses of a GABAergic inhibitory control upon DA nigrostriatal pathways and of a GABA-like activity of CDP.

The action of the dihydro derivatives of prostacyclin--(6R)-PGI1 and (6S)-PGI1 on the heart and the coronary vasculature

The action of the dihydro prostacyclins, (6R)-PGI1 and (6S)-PGI1, was studied on the isolated guinea pig heart and bovine coronary artery strips. PGE2 and PGI2 were used as standards. In the isolated guinea pig heart (6S)-PGI1 decreased the coronary perfusion pressure (CPP), myocardial force of contraction (MFC) and oxygen consumption (QO2). (6R)-PGI1 did not produce a significant change in these parameters. The ED50 (50% o maximum coronary dilation) was approximately 20 times higher for (6S)-PGI1 than for PGI2 or PGE2. Treatment of the hearts with reserpine + tyramine abolished the (6S)-PGI1-induced decrease in MFC but not the decrease in the CPP. The same pattern of responses was seen with PGE2. Bovine coronary artery strips were contracted by both (6S)-PGI1 and (6R)-PGI1, the ED50 (50% of maximum increase in tension) being 5 and 10 times higher than that for PGE2. The (6S)-PGI1-induced contraction was preceeded by a small relaxation, which, however, was much less than that seen after PGI2. It is concluded that the hydration of the 5,6 double bound in the PGI2 molecule results in an almost complete loss of PGI2-like activity and generates PGE-like activity. The same biological activity of both dihydro prostacyclins in the isolated coronary artery strip but not in the intact coronary vascular bed leads to suggest that the sites of action in these systems are different.

Prostacyclin (PGI2) decreases the cyclic AMP level in coronary arteries

The effects of prostacyclin (PGI2) on vascular tension and cAMP content were measured in isolated bovine coronary artery strips. 3 nM PGI2 did not alter the tension but diminished the cAMP content by 56% of the control level (P less than 0.005). 30 and 300 nM PGI2 diminished the tension and further reduced the cAMP content, which amounted to only 5% of the control at 300 nM PGI2. These results are in contrast to the increase in cAMP level by PGI2 in blood platelets and might indicate a different mechanism of action of PGI2 in platelets and vascular tissue.

Contralateral turning evoked by the intranigral microinjection of muscimol and other GABA agonists

Contraversive turning was evoked by the microinjection of GABAergic agents into the substantia nigra (SN) of the rat. Muscimol, the most potent GABA agonist, evoked contralateral turning when injected into the SN in doses of 0.005, 0.05, 0.5 and 5 microgram, whereas 0.5 microgram of muscimol applied at extranigral sites produced no turning. A shorter lived contraversive turning response was evoked by the intranigral micro-injection of imidazole acetic acid (10 or 50 microgram), ethanolamine-O-sulphate (25 or 50 microgram), or GABA (50 microgram). No increase in GABA-induced turning was produced by local pretreatment with pipecolic acid (5 microgram). When injected into the SN, neither picrotoxin, in doses of 0.1, 0.5, 1.0 or 2.0 microgram, nor bicuculline methiodide (Bm), in doses of 0.1 or 0.2 microgram, elicited a significant amount of turning. Picrotoxin, however, partially blocked the turning evoked by the intranigral injection of muscimol, both via the i.p. and intranigral routes of administration whereas Bm did not. In addition, haloperidol (1 mg/kg i.p.) antagonized the muscimol-induced turning. Hence, we feel GABA mimetic substances injected within the SN might evoke contralateral turning via activation of a heretofore undescribed neural system arising from the SN or by activating the ipsilateral dopaminergic neurons projecting from the SN.

Release of dopamine in both caudate nuclei and both substantia nigrae in response to unilateral stimulation of cerebellar nuclei in the cat

The effects of unilateral focal electrical stimulation of the deep cerebellar nuclei on the activity of the nigrostriatal dopaminergic neurons on both sides of the brain were examined in halothane anaesthetized cats. For this purpose, push-pull cannulae were inserted into both caudate nuclei and both substantia nigrae, and the release of [3H] dopamine ([3H]DA) continuously formed from [3,5-3H]L-tyrosine was estimated in superfusates. The unilateral electrical stimulation of the right cerebellar dentate nucleus induced a long-lasting increase in the release of [3H]DA in the left caudate nucleus and a simultaneous decrease in the release of [3H]transmitter in the right caudate nucleus. These changes were associated with opposite fluctuations in the release of [3H]DA from the corresponding substantia nigrae. Thus, the electrical stimulation of the right dentate nucleus induced a pronounced decrease in the release of the [3H]-amine in the [3H]transmitter in the corresponding substantia nigra, whereas the activity of the contralateral substantia nigra, whereas the release in the ipsilateral substantia nigra was simultaneously increased. In contrast, the unilateral electrical stimulation of the right cerebellar fastigial nucleus resulted only in an increased release of [3H]DA in the ipsilateral (right) caudate nucleus, associated with a decreased release of the [3H]transmitter in the corresponding substantia nigra, whereas the activity of the contralateral (left) dopaminergic system was not significantly affected. These results support a direct functional interaction between the cerebellum and the basal ganglia. They also suggest that the release of DA from dopaminergic axonal terminals is inversely correlated to the extent of the transmitter release from dendrites.

Inhibition of dopamine release in the cat caudate nucleus by nigral application of glycine

"Encéphale isolé" cats were implanted with two push-pull cannulae, one in the left caudate nucleus and the other in the ipsilateral substantia nigra. L-3,5(-3)H-Tyrosine was introduced continuously into the caudate nucleus to study the release of 3H-DA. Glycine (10(-5)M) added to the superfusing medium of the push-pull cannula inserted into the substantia nigra reduced (25%) the spontaneous release of 3H-DA. Conversely, strychnine (10(-5)M) slightly stimulated the 3H-transmitter release. The inhibiting effect of glycine (10(-5)M) on 3H-DA release was no longer seen in the presence of strychnine (10(-5) M). The results support the hypothesis of a tonic glycinergic inhibitory control of the activity of the nigrostriatal dopaminergic neurons.