Effects of electrolytic and 6-hydroxydopamine lesions of the lateral hypothalamus on rotation evoked by electrical stimulation of the substantia nigra in rats

Contralateral rotation evoked by electrical stimulation of the left substantia nigra was studied in rats before and after electrolytic or 6-hydroxydopamine (6-OHDA) lesions of the lateral hypothalamus. Electrolytic lesions (2 mA DC, 15 sec) which produced mean ipsilateral striatal dopamine depletion of 58% significantly reduced the rotation at 2 h to 14 days postlesion. 6-OHDA (8 μg in 4 μl) which produced mean ipsilateral striatal dopamine depletion of 93% significantly increased the rotation at 3 to 14 days postlesion. Haloperidol 0.1 and 0.5 mg/kg i.p. partially reduced rotation in both control and lesioned rats in a dose-related manner. Control and lesioned rats showed no sognificant differences in haloperidol sensitivity. If stimulus induced rotation were mediated by activation of dopaminergic neurons, one would have expected lesion effects in the present experiments to parallel those on rotation caused by pharmacologically evoked release of dopamine. The lesion effects we obtained on stimulus induced rotation, however, parallel those on rotation evoked by the predominantly directly acting dopamine agonist, apomorphine, rather than those on rotation evoked by the indirect (presynaptic) action of amphetamine. We suggest that contralateral rotation evoked by electrical stimulation of the substantia nigra may reflect direct activation of neurons postsynaptic to the dopaminergic nigrostriatal neurons.     

Enhanced survival of cultured dopamine neurons by treatment with soluble extracts from chemically deafferentiated striatum of adult rat brain

A soluble fraction was extracted from a chemically deafferentiated striatum of adult Wistar rats after unilateral lesioning of the nigrostriatal pathway by 6-hydroxydopamine (6-OHDA) injection. The soluble extract from the lesioned side enhanced the survival of cultured mesencephalic dopamine (DA) neurons of 14-day-old rat embryos as evidenced by quantitative counting of tyrosine hydroxylase-like immunoreactive cells. The neurotrophic activity of this striatal extract for DA neurons was highest 14 days after 6-OHDA injection and became negligible in 28 days. The extract showed no promoting effects on cultured γ-aminobutyric acid (GABA)-containing mesencephalic neurons. These observations indicate that the striatum of adult rats may initiate de novo synthesis of trophic substance(s) for DA neurons but not for GABA neurons when subjected to nigral dopaminergic deafferentiation.     

Intrastriatal Dopamine Infusion Reverses Compensatory Increases in D2-Dopamine Receptors in the 6-OHDA Lesioned Rat

Direct infusion of dopamine into the corpus striatum has been proposed as a potential approach for the treatment of Parkinson's disease. The present study examined the effect of intrastriatal dopamine infusion on D₂-dopamine receptors in the 6-hydroxydopamine (6-OHDA) lesioned rat brain. The completeness of the 6-OHDA-induced nigrostriatal injury was confirmed using [³H]-mazindol autoradiography and apomorphine-induced behaviour. Intrastriatal infusion of three different dopamine doses significantly reduced the apomorphine-induced behaviour. [³H]-spiperone autoradiography performed one day after the termination of dopamine infusion into the striatum revealed a dramatic reduction of D₂-dopamine receptor binding. The mean ± SEM percent reduction of D₂receptor binding in the affected areas of the striatum was 28.8±1.0% for 4.74 μg dopamine/h infusion rate, 35.0 ± 1.6% for 9.48 μg dopamine/h infusion rate and 33.3 ± 5.0% for 14.22 μg dopamine/h infusion rate when compared to the unlesioned side. Infusion of vehicle alone did not have any effect. The present results support the concept that intrastriatal dopamine infusion may be a useful therapeutic approach for the treatment of Parkinson's disease.     

Delayed behavioral response to antidepressant drugs following selective damage to the hippocampal noradrenergic innervation in rats

The present study was undertaken to investigate in rats the role of the noradrenergic innervation of the hippocampus in the reversal by antidepressant drugs of escape failures caused by previous exposure to insecapable shocks (learned helplessnes design). Rats were either sham-operated or given a bilateral infusion of 6-hydroxydopamine (6-OHDA, 8.1 μg of free base in 0.8 μl saline containing 0.02% ascorbic acid) into the dorsal hippocampus. Three weeks later, sham lesioned rats were exposed (experimental rats) or not exposed (control rats) to 60 randomized, inescapable footschocks (0.8 mA, 15 s duration) and 48, h later, experimental and control rats were subjected to daily consecutive shuttle-box sessions (30 trials/day; intertrial INTERVAL = 30 s). Separate groups of experimental animals were given twice daily injection (i.p.) of clomipramine (total daily dose: 16 or 32 mg/kg), desipramine (24 mg/kg), imipramine (32 mg/kg), nialamide (16 mg/kg) or saline. After behavioral testing, animals were sacrificed and [³H]noradrenaline uptake was assayed in synaptosomal preparations from the hippocampus, the cerebral cortex and the septum, [³H]serotonin uptake being assessed in hippocampal synaptosomes. We found that hippocampal 6-OHDA infusion resulted in a mean 70% reduction in local [³H]noradrenaline uptake and a marked delay in the response to antidepressants (reversal of escape failures over consecutive shuttle-box sessions). The 6-OHDA infusion was found not to alter hippocampal [³H]serotonin uptake but to decrease (30%) [³H]noradrenaline uptake in the cerebral cortex. These data clearly suggest that the noradrenergic innervation of the hippocampus and perhaps the cerebral cortex might be a crucial neuronal target in the ‘antidepressant’ action of imipramine-like drugs and monoamine oxidase inhibitors in animals.     

Dopamine released from mesencephalic transplants restores modulation of striatal acetylcholine release after neonatal 6-hydroxydopamine: An in vitro analysis

After chemical lesions which destroy the nigrostriatal dopamine pathway, transplants rich in dopamine neurons innervate the striatum and, with appropriate stimulation, drive host motor behaviors normally mediated by dopamine. We wished to determine whether dopamine released from the transplant also reinstated dopaminergic inhibition of striatal acetylcholine release. Three-day-old rat pups received bilateral intraventricular injections of 6-hydroxydopamine. Three days later cell suspensions prepared from embryonic ventral mesencephalon were injected unilaterally into the striatum. Tail pinch and amphetamine were able to elicit contralateral turning in many of these animals. Only those animals which rotated ≥5 turns/min were included for further analysis. Subsequent assays indicated that 6-hydroxydopamine had depleted striatal dopamine to 4% of control and that the transplant had increased dopamine levels to 11% of control. Superfused striatal slices were stimulated (8 Hz, 1 min) and then exposed to amphetamine (10 μM, 3 min). The slice released dopamine, as measured by HPLC, and acetylcholine, as measured by tritium efflux after preincubation with [³H]choline. Moreover, the release of acetylcholine was inhibited by endogenous dopamine as indicated by the ability of sulpiride (1 μM) to increase tritium efflux. Striatal slices prepared from lesioned animals showed a reduction in dopamine overflow in response to both electrical stimulation (0.6% of control) and amphetamine (1% of control), and a decrease in the ability of sulpiride to increase electrically evoked acetylcholine overflow (12% of control). Transplantation partially restored the dopaminergic response to electrical stimulation (21% of control), and amphetamine (15% of control) and fully restored the sulpiride-induced increase in acetylcholine overflow (98% of control). We conclude that transplanted dopaminergic neurons are able to release dopamine and to restore inhibitory control over the release of striatal acetylcholine.     

Bilateral changes in striatal dopamine metabolism after unilateral intracarotid and intrastriatal administration of apomorphine

Following cannulation of the common carotid artery of female Sprague-Dawley rats, 3 μCi (10 μg) of [³H]apomorphine were infused. At various time intervals, drug concentrations were determined in the right and left striata, anterior forebrains, posterior forebrains and cerebella. One minute following intracarotid infusion of apomorphine, approximately a 65-fold right/left difference in apomorphine concentrations was attained in all forebrain structures, and this difference steadily diminished with time as a result of declining drug levels in the infused hemisphere. The concentrations of dopamine and its metabolites (DOPAC, HVA and 3-MT) were quantified by gas chromatography-mass spectrometry in the right and left striata at 5 and 15 min after unilateral intracarotid infusion of 1 μg apomorphine. At both time intervals and regardless of the side infused, the metabolites of dopamine increased ipsilateral to the side of infusion. Moreover, 3-MT levels were significantly decreased in the contralateral striatum. After direct intrastriatal injection of either 0.1 or 1.0 μg apomorphine into the right striatum, the levels of dopamine metabolites were again increased in the ipsilateral striatum. 3-MT levels were also decreased significantly in the left striatum. In contrast to the effects observed after systemic administration of apomorphine, these results demonstrate that dopamine release in the striatum is increased by selectively delivering higher concentrations of apomorphine to the nerve terminals of the nigrostriatal neurons. The effects of unilateral apomorphine on dopamine metabolism in the contralateral striatum are most likely the effect of interhemispheric communication.     

Striatal D1 dopamine receptor distribution following chemical lesion of the nigrostriatal pathway

The morphochemical and biochemical distribution of the adenylate cyclase-linked dopamine receptor, or D1 subpopulation, has been examined in the rat striatum following a chemical lesion of the dopaminergic nigrostriatal pathway. The cellular pattern of D1 dopaminergic binding was assessed using in vitro autoradiographic localization of [3H]SCH 23390 or [125I]SCH 23982, selective D1 receptor antagonists, 1 week following unilateral infusion of the neurotoxin 6-hydroxydopamine (6-OHDA) into the substantia nigra. The specific association of the D1 binding sites with cyclic AMP-immunoreactive striatal neurons was abolished after lesion of the dopaminergic nigral afferents. The morphochemical disruption of the caudate D1 dopamine binding sites in relation to cyclic AMP-positive elements, a large proportion of which are striatonigral neurons, probably contributes to the dysfunctions in this subpopulation of dopamine receptor after depletion of the catecholamine neurotransmitter.     

Acutel-DOPA pretreatment potentiates 6-hydroxydopamine-induced toxic effects on nigro-striatal dopamine neurons in mice

We have studied the effect of various agents on the decreases in striatal levels of dopamine (DA) and its metabolites which were observed 14 days after an intracerebroventricular (i.c.v.) administration of 50 μg 6-hydroxydopamine (6-OHDA) to mice. A pretreatment of mice with either a tyrosine hydroxylase inhibitor (α-methyl-p-tyrosine), a D₂ receptor agonist (bromocriptine) or antagonist (haloperidol), or a vesicular uptake inhibitor (tetrabenazine) did not modify the 6-OHDA-induced decreases in DA and metabolites, indicating that DA synthesis, vesicular storage and neuronal firing rates are not mainly involved in the 6-OHDA-induced toxicity on the DA neurons. Conversely, a pretreatment withl-DOPA + benserazide potentiated the 6-OHDA-induced decreases in striatal levels of DA, homovanillic acid and 3-metho:xy-tyramine. This effect was not due to an increased 6-OHDA uptake via the neuronal carrier since a pretreatment withl-DOPA + benserazide, performed 1-1.5 h before sacrifice, decreased the apparent affinity of the uptake, an effect which disappeared when considering the total DA concentration present in incubation medium ([³H]DA and cold released DA). In conclusion, potentiation of the 6-OHDA neurotoxicity by 1.-DOPA rises again the important problem of the safety of the latter drug in therapeutics.     

Dopamine receptor-modulated [S]GTPγS binding in striatum of 6-hydroxydopamine-lesioned rats

The role of dopamine receptor–G protein coupling in the development of striatal dopamine receptor supersensitivity was studied in rats with a 6-hydroxydopamine (6-OHDA)-induced unilateral lesion of the nigrostriatal pathway. This coupling was assessed by the measurement of dopamine agonist-induced guanosine 5′-O-(γ[³⁵S]thio)triphosphate ([³⁵S]GTPγS) binding in striatal membranes, at different periods of time (1–5 weeks) following the microinjection of the neurotoxin. From the first to the fifth week following the lesion, basal and dopamine-stimulated [³⁵S]GTPγS-specific binding were found to be enhanced in the denervated striata as compared to their control counterpart. D₂ dopamine receptors were clearly demonstrated to be involved in this supersensitivity, as assessed by measuring N-propylnorapomorphine (NPA)-, quinpirole- and bromocriptine-induced [³⁵S]GTPγS-specific binding. The involvement of D₁ dopamine receptors was indirectly studied by the combination of dopamine with a saturating concentration of the selective and potent D₂ antagonist domperidone. In these conditions, the remaining response to dopamine was also found to be significantly increased following the lesion. These results are consistent with the hypothesis that, in addition to D₂ dopamine receptor upregulation, modulation of dopamine receptor–G protein interaction is involved in the hypersensitivity accompanying striatal dopamine depletion.     

Effects of pergolide treatment on in vivo hydroxyl free radical formation during infusion of 6-hydroxydopamine in rat striatum

This study focused on the early neurochemical events involved in 6-hydroxydopamine (6-OHDA) neurotoxicity and the putative neuroprotective effects of pergolide. 6-OHDA in 0.1% ascorbic acid/saline was delivered into rat striatum by means of microdialysis and 2,3-dihydroxybenzoic acid (2,3-DHBA) was measured as an index of hydroxyl free radical formation using salicylate trapping. Infusion of 6-OHDA (2–20 mM) via the dialysis probe for 15 min was associated with an immediate and striking increase in the extracellular levels of 2,3-DHBA and dopamine, and this effect was dose-dependent. An infusion of 10 mM 6-OHDA, equivalent to a direct injection of 4 μg free base, resulted in dopamine overflow with a maximum approx. 200-fold above the baseline. This massive overflow of toxic amounts of dopamine, much greater than expected of reuptake inhibition, seems to be the earliest response of nigrostriatal neurones to 6-OHDA. In rats treated with pergolide mesylate (7 days 0.5 mg/kg/day, i.p.), the average amount of 2,3-DHBA associated with 6-OHDA striatal infusion was significantly smaller than that in controls. This suggests that pergolide treatment leads to an increased ability of striatal tissue to quench hydroxyl radical formation in vivo.     

Axonal transport of proteins and glycoproteins in the rat nigro-striatal pathway and the effects of 6-hydroxydopamine

Following stereotaxic injection of [35S]methionine into the substantia nigra of adult rats, there was rapid local incorporation of radioactivity into acid-insoluble material. Incorporation peaked by 4 h and then decreased. In contrast, acid-precipitable radioactivity in the corpus striatum (the major projection site of the substantia nigra) rose markedly between 1 and 8 h followed by a plateau period and another even more marked increase between 24 h and 6 days. Experiments involving injection of [3H]fucose gave similar results except that most of the acid-precipitable radioactivity in the striatum appeared in an early wave. In each case radioactivity in the contralateral striatum was less than 11% of that on the ipsilateral side. Stereotaxic injection of colchicine (20 microgram) into the nigrostriatal pathway (within the median forebrain bundle) blocked transport of [35S]protein and [3H]glycoprotein by 90% and 50%, respectively. In animals treated with 6-hydroxydopamine (6-OHDA; treated neonatally or as adults) the accumulation of striatal [35S]protein was reduced to 7 to 26% of control levels; striatal [3H]glycoprotein was also reduced, but not as much (29% to 73% of control). In control experiments, [3H]DOPA wa injected into the substantia nigra, and [3H]dopamine was measured in corpus striatum; 6-OHDA treatment reduced the amounts of striatal [3H]dopamine recovered to 3% of control values. The failure of colchicine or 6-OHDA to block transport of incorporated fucose as effectively as the transport of incorporated methionine is possible due to greater diffusion of fucose away from the injection site to non-dopaminergic nuclei projecting to the striatum. The molecular weight distribution of radioactive proteins at the substantia nigra and corpus striatum was analyzed by polyacrylamide gel electrophoresis. For both [35S]methionine and [3H]fucose, the gel electrophoretic pattern of radioactive proteins in the injection site (substantia nigra) was complex and did not change greatly between 2 h and 6 days. At the projection site (striatum) the electrophoretic distribution pattern was initially different from that of the substantia nigra, and changed markedly over the course of several days. In 6-OHDA-treated animals (treated neonatally or as adults), the bulk of proteins transported in nigro-striatal non-dopaminergic neurons appears to be very similar to that transported in the intact pathway in control rats. However, in striata of 6-OHDA-treated animals, a consistent reduction in striatal 35S- and 3H-radioactivitiy was observed in proteins with molecular weight from about 67,000 to 77,000. Assuming that the 6-OHDA treatment did not substantially affect the non-dopaminergic neurons, we interpret this to mean that some of the proteins in this molecular weight range are transported primarily by dopaminergic neurons.     

Chronic L-DOPA administration is not toxic to the remaining dopaminergic nigrostriatal neurons, but instead may promote their functional recovery, in rats with partial 6-OHDA or FeCl(3) nigrostriatal lesions.

In this study, we have examined the effects of chronic L-3,4-dihydroxyphenylalanine (L-DOPA) administration on the remaining dopaminergic neurons in rats with 6-hydroxydopamine (6-OHDA) or buffered FeCl(3) partial lesions to the nigrostriatal tract. L-DOPA administration increased the turnover of dopamine in the striatum. L-DOPA administration for 1 week produced an increase in the level of striatal RTI-121 binding, a specific marker for dopamine uptake sites on the dopaminergic nerve terminals in the striatum. However, longer periods of L-DOPA treatment decreased the level of RTI-121 binding in the striatum. In the partial 6-OHDA lesion model, L-DOPA treatment had a time-dependent effect on the number of neurons demonstrating a dopaminergic phenotype i.e., neurons that are tyrosine hyrdoxylase (TH)-immunopositive, on the lesioned side of the brain. In the first few weeks of treatment, L-DOPA decreased the number of TH-positive neurons but with long-term treatment, i.e., 24 weeks, L-DOPA increased the number of neurons demonstrating a dopaminergic phenotype. Even in the buffered FeCl(3) infusion model, where the levels of iron were increased, L-DOPA treatment did not have any detrimental effects on the number of TH-positive neurons on the lesioned side of the brain. Consequently, chronic L-DOPA treatment does not have any detrimental effects to the remaining dopaminergic neurons in rats with partial lesions to the nigrostriatal tract; indeed in the 6-OHDA lesion model, long-term L-DOPA may increase the number of neurons, demonstrating a dopaminergic phenotype.     

Dopaminergic inhibition of striatal GABA release after 6-hydroxydopamine

We have examined the regulation of striatal GABA release by endogenous dopamine in rats with partial degeneration of dopamine-containing neurons. 6-Hydroxydopamine was administered into the lateral ventricles or medial forebrain bundle. Either 3 days or 3 weeks later, slices of neostriatum were prepared, preloaded with [³H]GABA, and superfused in order to measure [³H]GABA overflow in response to electrical stimulation (8 Hz). The loss of dopaminergic terminals was estimated by measuring tissue levels of dopamine. The impact of endogenous dopamine on [³H]GABA was evaluated by measuring the ability of sulpiride, a D₂ dopamine receptor antagonist, to increase the depolarization-induced [³H]GABA overflow. In non-treated or vehicle-pretreated rat neostriatum, sulpiride (10 μM) increased the depolarization-induced [³H]GABA overflow to 193% of control. Three days after lesioning, the stimulatory effect of sulpiride on [³H]GABA overflow was identical to that seen in control rats so long as the loss of tissue dopamine did not exceed 60%, although with larger lesions the sulpiride-induced response was reduced. Three weeks after lesioning, however, the stimulatory effect of sulpiride on electrically evoked [³H]GABA overflow remained at the level seen in control tissue even in cases where tissue dopamine was reduced to 13% of normal. In contrast, no sulpiride-induced increase in [³H]GABA overflow was detected 3 weeks after nearly complete lesions which reduced tissue dopamine to 2% of normal. These data suggest that short- and long-term compensatory changes maintain dopaminergic control over GABAergic projection neurons and interneurons until the loss of dopamine innervation is almost complete.     

NAIP protects the nigrostriatal dopamine pathway in an intrastriatal 6-OHDA rat model of Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder of the basal ganglia, associated with the inappropriate death of dopaminergic neurons of the substantia nigra pars compacta (SNc). Here, we show that adenovirally mediated expression of neuronal apoptosis inhibitor protein (NAIP) ameliorates the loss of nigrostriatal function following intrastriatal 6-OHDA administration by attenuating the death of dopamine neurons and dopaminergic fibres in the striatum. In addition, we also addressed the role of the cysteine protease caspase-3 activity in this adult 6-OHDA model, because a role for caspases has been implicated in the loss of dopamine neurons in PD, and because NAIP is also a reputed inhibitor of caspase-3. Although caspase-3-like proteolysis was induced in the SNc dopamine neurons of juvenile rats lesioned with 6-OHDA and in adult rats following axotomy of the medial forebrain bundle, caspase-3 is not induced in the dopamine neurons of adult 6-OHDA-lesioned animals. Taken together, these results suggest that therapeutic strategies based on NAIP may have potential value for the treatment of PD.     

Influence of prelimbic and sensorimotor cortices on striatal neurons in the rat: electrophysiological evidence for converging inputs and the effects of 6-OHDA-induced degeneration of the substantia nigra

These studies were designed to investigate whether there are convergent prelimbic and sensorimotor cortical inputs onto striatal neurons in the rat and whether dopaminergic (DA) nigrostriatal fibers regulate these inputs. The influence of the nigrostriatal DA system was assessed in rats with either small or large 6-hydroxydopamine-induced lesions of the substantia nigra. In normal rats 39 out of 74 neurons (52.7%) were excited by stimulation of both the prelimbic and the sensorimotor cortex. No marked change in corticostriatal transmission was evident in rats with small 6-OHDA-induced lesions (defined as 10–35% decrease in [³H]DA uptake in striatal synaptosomes). In rats with large lesions (75–85% decrease in striatal [³H]DA uptake), however, a complete rearrangement of the corticostriatal transmission occurred. This was evident in a decrease of thresholds to obtain cortical responses, by modifications of the discharge properties of striatal neurons receiving input from cortices and by an increase in the number of neurons responding to cortical stimulation. In addition, a significantly higher percentage of striatal neurons responded to stimulation of both prelimbic and sensorimotor cortices in rats with large lesions than in rats with small lesions or in control rats. This data suggests that: (1) no functional separation of prelimbic and sensorimotor cortical inputs occurs in the rat striatum, (2) the nigrostriatal DA system exerts a focusing effect on these inputs, (3) the striatum is actively involved in the integrative processing of descending cortical information.     

Low-affinity sulfonylurea binding sites reside on neuronal cell bodies in the brain

The antidiabetic sulfonylurea drugs bind to sites associated with an ATP-sensitive potassium (K_atp) channel on cell bodies and terminals of neurons which increase their firing rates or transmitter release when glucose concentrations rise or sulfonylureas are present. High-affinity sulfonylurea binding sites are concentrated in areas such as the substantia nigra (SN) where glucose and sulfonylureas increase transmitter release from GABA neurons. But there is a paucity of high-affinity sites in areas such as the hypothalamic ventromedial nucleus (VMN) where many neurons increase their activity when glucose rises. Here we assessed both high- and low-affinity sulfonylurea binding autoradiographically with 20 nM [³H]glyburide in the presence or absence of Gpp(NH)p. Neurotoxin lesions with 6-hydroxydopamine (6-OHDA), 5,7-dihydroxytryptamine (5,7-DHT) and ibotenic acid were used to elucidate the cellular location of the two sites in the VMN, SN and locus coeruleus (LC). In the VMN, 25% of the sites were of low affinity. Neither 6-OHDA nor 5,7-DHT affected [³H]glyburide binding, while ibotenic acid reduced the number of VMN neurons and abolished low-affinity without changing high-affinity binding. In cell-attached patches of isolated VMN neurons, both 10 mM glucose and 100 μM glyburide decreased the open probability of the K_atp channel suggesting that the low-affinity binding site resides on these neurons. In the SN pars reticulata, ibotenic acid reduced the number of neurons and high-affinity [³H]glyburide binding was decreased by 20%, while 6-OHDA had no effect. In the SN pars compacta, both 6-OHDA and ibotenic acid destroyed endogenous dopamine neurons and selectively ablated low-affinity binding. In the LC, 6-OHDA destroyed norepinephrine neurons and abolished low-affinity binding. These data suggest that low-affinity sulfonylurea binding sites reside on cell bodies of VMN, SN dopamine and LC norepinephrine neuron cell bodies and that high-affinity sites may be on axon terminals of GABA neurons in the SN.     

Striatal membrane-bound and soluble catechol-O-methyl-transferase after selective neuronal lesions in the rat

Summary Activities of the two forms of catechol-O-methyltransferase (COMT), viz. the soluble (S-COMT) and the membrane-bound (MB-COMT), have been studied in the rat striatum to characterize their localization in relation to the nigrostriatal dopaminergic neurons. Selective unilateral nigrostriatal dopaminergic lesions were produced by an intranigral injection of 6-hydroxydopamine (6-OHDA; 8g/site). 6-OHDA caused an extensive lesion of the dopaminergic neurons as revealed by non-detectable concentrations of dopamine in the striata of the lesioned sites. In spite of that neither S-COMT nor MB-COMT activities were altered in comparison with the intact control striata. The intrastriatal injection of kainic acid significantly increased S-COMT activity but to some extent decreased MB-COMT activity. Kainic acid did not alter the striatal concentration of dopamine.These results suggest that both S-COMT and MB-COMT reside postsynaptically the nigrostriatal dopaminergic neurons. S-COMT seems to be found mainly in striatal glial cells, whereas striatal MB-COMT might be located both in postsynaptic neuronal and extraneuronal cells.     

Forskolin attenuates the evoked release of [H]GABA from striatal neurons in primary culture

The actions of the diterpene forskolin, and cyclic AMP analogues, on the evoked release of [³H]GABA (γ-aminobutyric acid) was examined in intact striatal neurons in primary culture, generated from the fetal mouse brain. Exposure of striatal neurons to forskolin (100 μM) resulted in a 40–55% attenuation of [³H]GABA release evoked by either KCl (30 mM) or veratrine (2 μg/ml), while baseline levels of release were unaffected. The dose-dependence for forskolin in striatal neurons. Exposure of striatal neurons to membrane-permeable identical to the dose-dependent elevation of cyclic AMP levels by forskolin in striatal neurons. Exposure of striatal neurons to membrane-permeable analogues of cyclic AMP, such as p-chlorophenylthio cyclic AMP (0.5 mM) and dibutyryl cyclic AMP (1 mM), resulted in a 25 and 26% attenuation of [³H]GABA release, respectively; dibutyryl cyclic GMP (1 mM) was without effect. The similarity between the actions of forskolin and the cyclic AMP analogues suggests that, in striatal neurons in primary culture, the elevation of cyclic AMP levels results in the attenuation of the evoked release of [³H]GABA. The greater effectiveness of forskolin, compared to the cyclic AMP analogues, may be related to the recently reported, additional direct actions of forskolin on neuronal membrane ion channels.     

The relationship between loss of dopamine nerve terminals, striatal [H]spiroperidol binding and rotational behavior in unilaterally 6-hydroxydopamine-lesioned rats

The relationship between the destruction of dopamine-containing nerve terminals, specific binding of [³H]spiroperidol and contralateral rotation in response tol-DOPA, was studied in rats with unilateral lesions of the nigrostriatal dopamine (DA) system, induced by intracerebral injections of the neurotoxin 6-hydroxydopamine (6-OHDA). Animals with significant rotational behavior in response tol-DOPA had both a greater amount of specific binding of [³H]spiroperidol in the lesioned striatum compared to the non-lesioned striatum, and at least 90% destruction of DA terminals in the lesioned striatum (less than 10% of control uptake). The non-rotators tol-DOPA had considerably less destruction of DA terminals and no significant increase in specific binding on the lesioned side. The data from this study suggest that beforel-DOPA is effective as an inducer of contralateral rotational behavior, there must be both unilateral damage to the DA terminals greater than 90%, and increased specific binding.     

Non-dopaminergic nigrostriatal pathway

The nigrostriatal projection was studied with a retrograde tracing method (Evans blue, EB) combined with a technique for dopamine histofluorescence. The study, realized in control rats and in animals with 6-hydroxydopamine-induced lesions of the dopaminergic pathway, yielded the following results.

(1) In 3 control rats injected with 0.2 μl of a 10% solution of EB in thecenter of the caudate-putamen 1 mm anterior to the globus pallidus, 96% of all substantia nigra neurons retrogradely labelled with the dye contained dopamine fluorescence. The remaining ones (average 350 per brain) were devoid of dopamine fluorescence and predominantly found in the posterior 75% of the substantia nuigra. These last cells were confined to the upper-half of the pars reticulata.

(2) In a series of 6 animals, the cytotoxic agent 6-hydroxydopamine was injected in various locations in the vicinity of either the substantia nigra ir the nigrostriatal tract 12–15 days prior to the injections of 0.2 μl of EB in the same striatal locations as in the controls. Despite a reduction of up to 85% in the number of dopaminergic cell bodies, the substantia nigra of these rats contained the same average number of EB-labelled neurons devoid of dopamine fluorescence.

(3) Eight rats received smaller injections (0.1 μl) of EB in various striatal sites and in tqo further cases such injections were placed in the globus pallidus to determine more accurately the anatomical location of the dopamine-negative nigral neurons retrogradely labelled with the dye. Following the striatal injections, these cells were found mostly in the upper-half of the pars reticulata and were arranged in longitudinally oriented clusters whose mediolateral location depended on the striatal injection site.

Following the pallidal injections, retrogradely labelled neruons devoid of dopamine fluorescence were found in greater numbers and were located in all areas of the pars reticulata. The possibility of retrograde labelling of some nigrothalamic neurons was not entirely ruled out in these two cases.

(4) Finally 6 rats received 0.1 μl injections of EB in various parts of the parietal cortex. In these cases the substantia nigra did not contain any EB-positivedopamine-negative neurons.

These results are interpreted as evidence in support of the existence of a topographically organized non-dopaminergic nigrostriatal projection.