Quantitative histochemical measurement of GABA-transaminase: method for evaluation of intracerebral lesions produced by excitotoxic agents

GABA-transaminase (GABA-T) activity of fresh frozen coronal sections through rat striatum was evaluated 4-6 weeks after intrastriatal application of kainic or ibotenic acid. 16 micrograms sections were processed for GABA-T histochemistry and were evaluated quantitatively by computerized densitometry using image analysis. Alternate sections (200 micrograms) were assayed for GABA-T activity in vitro. Gross examination of sections stained for GABA-T revealed obvious lack of staining in the vicinity of lesions produced by either kainate (0.5-1.0 micrograms) or ibotenate (10-20 micrograms); the extent of each lesion was clearly delineated by the stain. Quantitative analysis of stained sections revealed that the lesioned tissue contained 80-90% less GABA-T activity than control tissue. This loss of GABA-T was in agreement with values obtained in adjacent sections assayed in vitro. Similar studies in substantia nigra clearly and quantitatively demonstrated damage induced by ibotenate or kainate in this nucleus as well as in tissue in the overlying reticular formation. Moreover, two compartments of GABA-T were discriminated in substantia nigra: one associated with neural perikarya, which was sensitive to kainic and ibotenic acids (80% of total GABA-T), and a second associated with afferent terminals arising from forebrain projections (20%). Thus, after destruction of neurons with kainic or ibotenic acid, GABA-T activity is largely eliminated. Under these conditions, it appears that glia contribute relatively little to the GABA-T activity measured either histochemically or by direct chemical assay in homogenates.     

Substantia nigra and motor control in the rat: effect of intranigral α-kainate and γ-d-glutamylaminomethylsulphonate on motility

Bilateral microinjections of an excitatory amino acid, α-kainate (KA), 5–50 ng, into the substantia nigra pars reticulata (SNR) result in an increase in the muscle tone and catalepsy in rats. The preferential KA/quisqualate antagonist, γ-d-glutamylaminomethylsulphonate (γ-D-GAMS), 10 μg, blocks the actions of KA, 25 ng, when coadministered into the SNR. The chemical lesion of the caudate-putamen with 6-hydroxydopamine (6-OHDA) does not affect either increases in the muscle tone or catalepsy produced by KA, 25 ng, from the SNR. The lesion of the caudate-putamen with ibotenate moderately enhances the effect of KA, 25 ng, on the muscle tone. Microinjections of KA, 25 ng, into the substantia nigra pars compacta (SNC) do not increase the muscle tone and lead to significantly less pronounced catalepsy relative to the observed following the injections of KA into the SNR. Unilateral microinjections of KA, 10–50 ng, into the SNR elicit ipsilateral turning in rats in a dose- and time-dependent manner. Unilateral application of γ-d-GAMS, 1–10 μg, into the SNR produces contralateral turning. The turning evoked by KA, 25 ng, or γ-D-GAMS, 10 μg, is affected neither by 6-OHDA nor by ibotenate lesion of the caudate-putamen. These results demonstrate that excitatory neurotransmission in the substantia nigra participates in the regulation of the muscle tone and posture in rats.     

Role of dorsal mesencephalic reticular formation and deep layers of superior colliculus as out-put stations for turning behaviour elicited from the substantia nigra pars reticulata

In order to investigate the role of dorsal mesencephalic reticular formation (MRF) and deep layers of superior colliculus (DLSC) as out-put areas for non-dopamine mediated behavioural functions arising in the substantia nigra, discrete unilateral kainate-lesions as well as sham-lesions were placed in the MRF and DLSC. Ten days later kainate (0.75 μg) was microinjected into the substantia nigra pars reticulata. Lesions of MRF as well as lesions of DLSC reduced the contralateral turning induced by kainate lesions of pars reticulata ipsilateral to the MRF or DLSc lesions. The lesions of MRF were more effective than lesions of DLSC. Lesions of MRF or DLSC on the side contralateral to intranigral kainate were ineffective. The results indicate that the MRF-DLSC complex is an essential area for the expression of motor behaviour elicited from the substantia nigra.     

Lesions of substantia nigra by kainic acid: Effects on apomorphine-induced stereotyped behaviour

Fifteen days after bilateral lesions of the substantia nigra by local infusion of kainic acid (0.75 μg) or after intranigral injection of vehicle, rats were administered 0.1, 0.25, 1.0 and 2.5 mg/kg s.c. of apomorphine and the stereotyped items (locomotion, sniffing and gnawing) were recorded on an event-recorder and motility was measured by a photocell apparatus. After low doses of apomorphine (0.1, 0.2 mg/kg), rats lesioned in the substantia nigra with kainic acid showed a degree of stimulation of motility and of sniffing similar to controls; on the other hand, in rats lesioned with kainic acid in the nigra, a dramatic reduction of gnawing and its replacement by sniffing was observed after administration of higher doses of apomorphine (1.0, 2.5 mg/kg). Bilateral infusion of kainic acid (0.75 μg) into the reticular information, 2.0 mm dorsal to the substantia nigra, had no effect on apomorphine-induced stereotyped behaviour.These results are in agreement with the concept that the substantia nigra, through non-DA pars reticulata neurons, mediates motor and behavioural syndromes of striatal origin.     

Role of substantia nigra pars reticulata neurons in the expression of neuroleptic-induced catalepsy

Bilateral kainate-induced lesions of the substantia nigra prevented or dramatically reduced the catalepsy produced by haloperidol. In contrast, infusion of 1 or 4 μg 6-OHDA in the medial forebrain bundle, which decreased striatal DA by 30% and 80% respectively, failed to affect or actually potentiated haloperidol catalepsy. Since intranigral kainate, in contrast to 6-OHDA, destroys pars reticulata neurons it appears that these neurons are essential for the expression of haloperidol-catalepsy.     

Stimulatory effects of centrally injected kainate and N-methyl- -aspartate on gastric acid secretion in anesthetized rats

The effects of N-methyl- -aspartate (NMDA), kainate and (±)-α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), ionotropic glutamate agonists, on gastric acid secretion were investigated in the continuously perfused stomach of anesthetized rats. The lateral ventricular (LV) injection of kainate (0.01–1 μg) or NMDA (0.3–3 μg) dose-dependently stimulated gastric acid secretion. AMPA (3–10 μg) also stimulated gastric acid secretion but the effect was very weak. Repeated injections of kainate (0.1 μg) or NMDA (1 μg), at least twice, stimulated gastric acid secretion to a similar degree. The effect of kainate (0.1 μg) was blocked by the kainate receptor antagonists, 6-cyano-7-nitroquinoxaline-2,3-dione disodium (3 μg, LV) and -γ-glutamylaminomethanesulfonic acid (30 μg, LV), but not by NMDA receptor antagonists. The effect of NMDA (10 μg) was blocked by (±)-3-(2-carboxypiperazin-4-yl)-1-propylphosphonic acid (10 μg, LV), a competitive NMDA receptor antagonist, and (+)-5-methyl-10,11-dihydro-5H-dibenzocyclo-hepten-5,10-imine hydrogen maleate (10 μg, LV), a non-competitive NMDA receptor antagonist, but not by kainate receptor antagonists. Moreover, the gastric acid secretion stimulated by kainate and NMDA were completely blocked by systemic atropine injection (1 mg/kg, i.v.) and vagotomy. These findings suggest that kainate and NMDA receptor mechanisms are independently involved in the central nervous system to control gastric acid secretion through vagus cholinergic activation.     

Kainate-induced lesion in the optic tectum: dependency upon optic nerve afferents or glutamate

Kainic acid is known to induce characteristic lesions in neurons receiving an intact input with presumed glutamate-mediated neurotransmission. There are indications for glutamate as a transmitter of retinal afferent terminals in the pigeon optic tectum. After tectal injection of kainic acid (0.5–2.0 μg in 0.5 μl) the optic tectum was studied by light and electron microscopy and the following changes were observed: (a) within 1–48 h important neuropil vacuolization predominantly in lower part of layer 5. Such vacuoles were sometimes postsynaptic to identified retinal afferent terminals: (b) within 1 h to 21 days progressive neuronal cell loss throughout the tectal layers. These toxic effects were not observed 2–12 weeks after contralateral retinal ablation but could partially be restored by combined glutamate (0.2 mg) and kainate injection. Thus in the pigeon tectum, kainic acid neurotoxicity is dependent upon an intact retinal input, a finding consistent with a special role for glutamate — possibly as a transmitter — in retinal terminals.     

Effect of intracerebroventricular administration of β-/Tyr/-melanotropin-/9–18/ on rotational behavior induced by substantia nigra lesion in rats

β-/Tyr⁹/melanotropin-/9–18/ administered intracerebroventricularly /icv./ in doses of 1 μ/rat or 10 μg/rat had no influence on the substantia nigra lesion-induced turning activity of rats, however, it was able to potentiate apomorphine-induced contralateral turning. In a dose of 20 μg/rat /icv./ the peptide alone induced clear contralateral turning. The results suggest that at lower doses the peptide facilitates apomorphine-induced dopamine receptor stimulation, while in a higher dose the peptide alone can stimulate dopamine receptor activity.     

Picolinic acid modulates kainic acid-evoked glutamate release from the striatum in vitro

Since picolinic acid, a tryptophan metabolite yielded by the kynurenine pathway, selectively attenuates quinolinic and kainic acid excitotoxicity that is dependent on the presence of a glutamatergic afferent input, it was hypothesized that this agent may inhibit the presynaptic release of glutamate. Using superfused rat striatal slices, this study examined the potential of picolinic acid, and related pyridine monocarboxylic acids, to modify kainic acid-induced glutamate release. Kainic acid (0.25, 0.5 and 1.0 mM) stimulated the release of glutamate, an effect which was calcium dependent and was attenuated in the presence of the kainate/AMPA receptor antagonist, 6,7-dinitroquinoxalene-2,3-dione (500 μM). Picolinic acid significantly decreased glutamic acid release evoked by exposure of striatal slices to 1 mM kainate in the presence of calcium. The inhibitory action of picolinic acid on kainate-induced release was also shared by nicotinic and isonicotinic acid. In the absence of external calcium, kainic acid-induced glutamate release was significantly reduced by approximately 65%. Under this condition, picolinic acid (100 μM) failed to influence kainic acid-induced release. Picolinic acid (100 μM) itself increased glutamate release by 35% over basal release. While the ability of picolinic acid to inhibit excitotoxin-induced release supports the notion that it may act presynaptically to modify excitotoxicity, lack of structural specificity in its action tends to cast doubt on this mechanism of action.     

Specific alterations in local cerebral glucose utilization following striatal lesions

Regional cerebral glucose utilization was measured in conscious, lightly restrained rats, using the 2-deoxyglucose autoradiographic technique, 10 days after the unilateral injection of kainic acid into the striatum. The stereotactic infusion of kainic acid (2 μg in 2 μl of mock CSF) resulted in lesions localized to the caudate nucleus with no involvement of surrounding brain areas, such as septal nucleus and nucleus accumbens. Only mechanical damage around the needle tract was observed in CSF injected control animals. Local cerebral glucose use was most markedly affected ipsilateral to the infusion site in areas which normally receive input from the caudate nucleus. In globus pallidus and substantia nigra pars reticulata, increases in glucose use of 82% and 74%, respectively, were measured when compared with CSF injected controls. However, significant increases were also measured in contralateral pallidus and substantia nigra reticulata (16% and 20%, respectively). Of the brain structures examined, significant unilateral increases from control were observed in ipsilateral habenula (23%) and ventrolateral thalamus (13%), and contralateral substantia nigra pars compacta (14%) and sensory-motor cortex (15%). However, the side-to-side difference in response from control was not large. Symmetrical, bilateral increases in glucose use were found in the nucleus accumbens (15%), ventral tegmental area (24%), and red nucleus (17%). The only area in which the measured rate of glucose use was decreased was the ipsilateral caudate nucleus. However, these changes were invariably associated with histologically definable tissue damage. Caution must therefore be exercised in the interpretation of this result and that from other areas where damage was apparent.The increases of functional activity, as measured by glucose utilization within certain regions in the absence of cellular damage, provide an insight into the mechanisms by which overt motor behavior returns to normal a short time after the removal of striatal interneurons and efferent perikarya by the neurotoxic action of kainic acid. Of particular interest are the responses observed contralateral to the affected striato-nigral system in view of the proposed functional interaction between the two sides of the brain in the absence of direct neuronal pathways.     

Excitatory Amino Acid-Induced Degeneration of Dendrites of Catecholamine Neurons in Rat Substantia Nigra

We have recently established a rat substantia nigra (SN) slice preparation in which a sensitive index of excitatory amino acid (EAA) toxicity was degeneration of the dendritic arbor of catecholamine neurons labelled by immunostaining for tyrosine hydroxylase (TH). The present study examined the pharmacological characteristics of EAA-induced neurotoxicity. Rats were anesthetised by halothane inhalation and killed, the brain was rapidly removed, and 400-μm-thick SN slices cut in the horizontal plane on a vibratome. Slices were incubated in saline buffer at 35°C for 15 min to 6 h in the presence or absence or absence of kainic acid (KA) orN-methyl- -aspartate (NMDA) in concentrations ranging from 10 to 500 μM. The slices were then fixed and resectioned into 40-μm sections that were coplanar with the parent slice. Dopaminergic SN neurons were labeled using antibody to tyrosine hydroxylase (TH) coupled to diaminobenzidine. A feature of the immunostaining was that it labeled not only the cell body but also the prolific dendritic arborization of SN neurons. Dendritic damage was quantified by counting the proportion of neurons with intact dendrites after treatment with EAA. KA and NMDA caused loss of dendrites that was prevented by CNQX (20 μM) and MK-801 (20 μM), respectively, indicating that activation of either NMDA or non-NMDA receptors produces neurotoxicity. EAA-induced dendritic damage was observed within 2 h of treatment with a low concentration (10 μM) of KA and within 15 min if the concentration was increased to 500 μM. Thus the loss of dendrites occurs rapidly and precedes disintegration of the cell bodies. Furthermore, brief (15 min) exposure to EAA initiated damage in the dendrites which progressed after the EAA was removed from its receptor. The observations are consistent with the postulated role of EAAs in neurodegenerative diseases. Labeling the dendritic arbor provides a sensitive approach to investigating the cellular mechanisms of neurodegeneration of catecholamine neurons.     

Changes in multiunit activity of nigral neurons induced by cholinergic and dopaminergic stimulation of the caudate nucleus

The effects of stimulation of the caudate cholinergic and dopaminergic receptors on multiunit activity in the ipsilateral substantia nigra were studied in cats locally anesthetized, paralyzed and artificially respired. Cholinergic stimulation by intracaudate micro-injections of 10 μg of carbachol diminished multiunit activity by 36% in the ventral substantia nigra (SN) and increased activity by 48% in the dorsal SN. This effect was abolished after electrolytic lesion of the ipsilateral striatonigral pathway. Opposite responses were observed following intracaudate administration of 20 μg of dopamine or 20 μg ofd-amphetamine. Multiunit activity in the ventral SN increased by 36% and 34%, respectively, while the activity in the dorsal SN was reduced by 56% and 53%, respectively. Similar results were obtained in response to systemic administration of d-amphetamine.Extracaudate microinjections of carbachol and dopamine left multiunit activity in the SN unaffected.In conclusion, our results indicate an opposite action of caudate cholinergic and dopaminergic receptors on multiunit activity in the SN of the cat.     

Effect of morphine on dynorphin B and GABA release in the basal ganglia of rats

In vivo microdialysis was used to study the effects of systemic, as well as intracerebral administration of morphine and naloxone on dynorphin B release in neostriatum and substantia nigra of rats. The release of dopamine (DA), γ-aminobutyric acid (GABA), glutamate (Glu) and aspartate (Asp) was also investigated. Systemic injection of morphine (1 mg/kg s.c.) induced long-lasting increases in extracellular dynorphin B and GABA levels in the substantia nigra, whereas DA, Glu and Asp levels, measured in the same region, were not significantly affected. No effect on striatal neurotransmitter levels was observed following systemic morphine administration. Local perfusion of the substantia nigra with morphine (100 μM) through the microdialysis probe also increased nigral dynorphin B and GABA levels. Perfusion of the neostriatum with morphine (100 μM) significantly increased GABA and dynorphin B levels in the ipsilateral substantia nigra, but no effect was observed locally. Naloxone blocked the effect of systemic morphine administration on nigral dynorphin B and GABA release, already at a dose of 0.2 mg/kg s.c. Naloxone alone, given either systemically (0.2–4 mg/kg s.c.) or intracerebrally (1–100 μM), did not affect dynorphin B or amino acid levels, either in neostriatum or in substantia nigra. However, naloxone produced a concentration-dependent increase in DA levels. The present results indicate that systemic morphine administration stimulates the release of dynorphin B in the substantia nigra, probably by activating the μ-subtype of opioid receptor, since the effect of morphine on nigral dynorphin B and GABA was antagonized by a low dose of naloxone. The increase in extracellular DA levels produced by high concentrations of naloxone, both in neostriatum and substantia nigra, indicates a disinhibitory effect of this drug on DA release, probably via a non-μ subtype of opioid receptors located on nigro-striatal DA neurones.     

Neuronal activity of the substantia nigra (pars compacta) after injection of kainic acid into the caudate nucleus

In rats the caudate nucleus (Cd) of one side was injected with 2.5 μg kainic acid (KA) in 1 μl saline and that of the other side with saline of the same volume. Three to sixty days later, neuronal activities of the substantia nigra were examined by stimulating the caudate nucleus on each side. Two major types of activity were distinguished: pure inhibition and antidromic excitation followed by inhibition. With 65 substantia nigra units sampled from the side of saline injection, the frequency of incidence for the pure inhibition was 58.5% and that for the antidromic excitation followed by inhibition was 21.5%. In five units (7.7%), caudate stimulation remained ineffective. A total of 97 substantia nigra units was recorded from the side of KA injection and characterized as follows: (i) 42.3% had no effect of caudate stimulation, this being much higher than the control. Pure inhibition was encountered at a reduced frequency (29.9%) and its duration was markedly shortened. These findings are interpreted to mean that GABAergic strionigral inhibitory neurons were killed or damaged by locally injected KA. (ii) Antidromic excitation was seen as frequently as in the control side (19.6%). Inhibition following it was not reduced in duration. The antidromic inhibition of substantia nigra neurons from the caudate nuclei was suggested to be due to dopaminergic self-inhibition as proposed by Groves and his co-workers.     

Stress-induced increase in extracellular dopamine in striatum: role of glutamatergic action via N-methyl- -aspartate receptors in substantia nigra

There is considerable support for an influence of excitatory amino acids released from corticofugal neurons on dopaminergic activity in the basal ganglia. However, the relative importance of cortico-striatal and cortico-mesencephalic projections remains unclear, particularly with respect to the nigro-neostriatal pathway. We have therefore examined the influence of endogenous excitatory amino acids in substantia nigra on stress-induced dopaminergic activity in neostriatum. Microdialysis probes were implanted unilaterally into substantia nigra and ipsilateral neostriatum, and dopamine release in neostriatum was monitored by measuring changes in extracellular dopamine. In separate animals, neostriatal dopamine synthesis was assessed by measuring extracellular DOPA in the presence of 3-hydroxylbenzylhydrazine (NSD-1015; 100 μM), an inhibitor of aromatic amino acid decarboxylase. Thirty minutes of intermittent foot shock increased both dopamine release (+41%) and synthesis (+37%) in neostriatum. Infusion of 2-amino-5-phosphonovalerate (APV; 100 μM), an inhibitor of N-methyl- -aspartate (NMDA) receptors, into substantia nigra greatly attenuated the stress-induced increase in neostriatal dopamine release, while having no effect on the apparent increase in stress-induced dopamine synthesis. These data suggest that excitatory amino acids such as glutamate act on NMDA receptors in substantia nigra to increase striatal dopamine release produced by exposure to stress, but that the increase in dopamine synthesis is mediated through a separate mechanism.     

GABA-transaminase activity in rat and human brain: regional,age and sex-related differences

Summary The activity of 4-aminobutyrate:2-oxoglutarate transaminase (GABA-T) has been investigated in the rat and human brain. Both rat and human brain GABA-T retained its full activity for at least 2 months and with a loss of less than 10% after 6 months when frozen at –20 C as tissue parts. There was a loss of activity of mouse brain GABA-T of about 15% per 24 hours postmortem. In the rat brain, GABA-T activity varied from low values in cortex and hippocampus to high in brain stem and cerebellum. There was a significant increase of GABA-T acitivity with age from 1 to 6 weeks and a significant reduction of the activity with age thereafter. Male rats had significant higher activity than female rats.In the human brain, GABA-T activities were measured in twelve regions of autopsied brains from 10 adult control subjects. No difference was found between the activities in the left and right sides. There is considerable variation in enzyme activity across the brain, with low activities in e.g. pons and medulla oblongata and high activities in e.g. caudatus, substantia nigra and hypothalamus. The acitivity of the enzyme is significantly different both between brain regions and between individuals.     

GDNF improves dopamine function in the substantia nigra but not the putamen of unilateral MPTP-lesioned rhesus monkeys

Microdialysis measurements of dopamine (DA) and DA metabolites were carried out in the putamen and substantia nigra of unilateral 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned rhesus monkeys that received intraventricular injections of vehicle or glial-derived neurotrophic factor (GDNF, 300 μg) 3 weeks prior to the microdialysis studies. Following behavioral measures in the MPTP-lesioned monkeys, they were anesthetized with isoflurane and placed in a stereotaxic apparatus. Magnetic resonance imaging (MRI)-guided sterile stereotaxic procedures were used for implantations of the microdialysis probes. Basal extracellular levels of DA and the DA metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), were found to be decreased by >95% in the right putamen of the MPTP-lesioned monkeys as compared to normal animals. In contrast, basal DA levels were not significantly decreased, and DOPAC and HVA levels were decreased by only 65% and 30%, respectively, in the MPTP-lesioned substantia nigra. Significant reductions in -amphetamine-evoked DA release were also observed in the MPTP-lesioned substantia nigra and putamen of the monkeys as compared to normal animals. A single intraventricular administration of GDNF into one group of MPTP-lesioned monkeys elicited improvements in the parkinsonian symptoms in these animals at 2–3 weeks post-administration. In addition, -amphetamine-evoked overflow of DA was significantly increased in the substantia nigra but not the putamen of MPTP-lesioned monkeys that had received GDNF. Moreover, post-mortem brain tissue studies showed increases in whole tissue levels of DA and DA metabolite levels primarily within the substantia nigra in MPTP-lesioned monkeys that had received GDNF. Taken together, these data support that single ventricular infusions of GDNF produce improvements in motoric behavior in MPTP-lesioned monkeys that correlate with increases in DA neuronal function that are localized to the substantia nigra and not the putamen.     

Neurotoxic Effects of Kainic Acid on Substantia Nigra Neurons in Rat Brain Slices

Excitatory amino acids (EAAs) have been implicated as mediators of cell death in neurodegenerative diseases involving catecholamine neurons. Few studies, however, have examined the toxic effects of EAAs on identified catecholamine neuronsin vitro.We have investigated the neurotoxic effects of kainic acid in a rat brain substantia nigra (SN) slice preparation. Rats (60–80 g) were anesthetised with halothane and killed by cervical dislocation. SN slices, 300 μm thick, were incubated at 35°C in a modified Krebs solution in the presence or absence of kainic acid and then fixed and processed for either immunohistochemistry (IHC) or electron microscopy (EM). In IHC experiments, SN neurons were labeled using antibody to tyrosine hydroxylase (TH) coupled to diaminobenzidine. In control slices, the antibody labeled not only the cell body but also the prolific dendritic arbor of SN neurons. Treatment with 50 μMkainic acid for 15 min or 2 h resulted in loss of TH staining and apparent fragmentation of the dendrites. EM provided ultrastructural evidence for kainic acid-induced degeneration of the dendritic arbor of SN neurons. Typically, the dendritic membrane was broken, or diffuse and collapsed. Ultrastructural damage, including clumping and marginalization of chromatin and vacuolation of the cytoplasm, was also observed in cell bodies. Damage to the dendritic arbor may occur early in the neurotoxic events leading to cell death, preceding the loss of the cell body. Our observations are consistent with the postulated role of EAAs as mediators of catecholamine neuron death.     

In vivo labelling and axonal transport of monoamine oxidase in the rat basal ganglia using radioactive pargyline

Summary The enzyme monoamine oxidase was labelled in the rat striatum or substantia nigra with locally injected radioactive pargyline. The binding was prevented by a pretreatment with non-radioactive pargyline, or with a combination of clorgyline and deprenyl. Most of the MAO labelled with³H-pargyline was of the B-type, but also some MAO-A was labelled, as shown in rats pretreated with clorgyline or deprenyl separately.Seven days after the injection of (³H)-pargyline into the striatum a significant labelling was observed in the substantia nigra. This labelling was clorgyline sensitive, indicating type A MAO, and was not present when striatal neurons were destroyed with kainic acid. Labelling of the striatum following³H-pargyline injection into the substantia nigra was also less in kainate intoxicated striata. Damage of nigral dopamine neurons with 6-hydroxydopamine did not influence the distribution of the label.Thus by using³H-pargyline, specific labelling and axonal transport of type A MAO in striatal neurons projecting to the substantia nigra was demonstrated.     

Long-term local and distal increases in tryptophan hydroxylase activity following intracerebral kainic acid injections in the rat

The administration of kainic acid (1–2 μg) into the right striatum of adult rats resulted in a marked local increase in tryptophan hydroxylase activity (+ 54–106%). This change was significant as soon as on the second day after the treatment and persisted for at least 12 days. In addition, long-lasting elevations of tryptophan hydroxylase activity were also observed in the anterior raphe area, septum and ipsilateral hippocampus and cerebral cortex. In contrast, the intrahippocampal injection of kainic acid (1 μg) induced a long-term increase in tryptophan hydroxylase activity only in the injected structure. In all cases, the changes in tryptophan hydroxylase activity were associated with significant increases in the V_max of the enzyme with no alteration of its apparent affinities for tryptophan and the pterin cofactor. Studies of the sensitivity of tryptophan hydroxylase from control and from kainic acid-treated rats to in vitro activating conditions (Ca⁺-dependent phosphorylation, partial trypsinization, exposure to sodium dodecyl sulfate) suggest that the intrastriatal injection of the neurotoxin induced a long-lasting activation of the enzyme.These findings indicate that intracerebral injections of kainic acid may be a valuable approach to explore further the mechanisms controlling tryptophan hydroxylase activity in vivo.