Neonatal 6-hydroxydopamine treatment: noradrenaline levels and in vitro 3Hcatecholamine synthesis in discrete brain regions of adult rats

Endogenous noradrenaline levels are elevated in medulla oblongata, mesencephalon, pons and thalamus of adult rats which had been treated with 6-hydroxydopamine on days 1, 2, 8 and 15 after birth. Levels in spinal cord, cerebellum, hippocampus/amygdala and cortex are depressed, whereas no significant changes are observed in striatum, hypothalamus and medulla spinalis. The rate at which medulla oblongata synthesizes tritiated noradrenaline and dopamine from tritiated tyrosine $\text{in}$$\text{vitro}$ is markedly enhanced. No effect was apparent on catecholamine synthesis in hypothalamus. Tritiated noradrenaline synthesis, but not tritiated dopamine synthesis, in the cortex is depressed. These results support the view that neonatal 6-hydroxydopamine treatment causes a degeneration of noradrenaline nerve terminals in the cortex and induces an increase in noradrenaline terminals in the medulla oblongata.     

CHANGES IN CENTRAL NORADRENALINE NEURONSADMINISTRATION AFTER SYSTEMIC 6-HYDROXYDOPAMINE ADMINISTRATION

—Intravenous injection of a large dose of 6-hydroxydopamine (100 mg/kg) to adult rats caused a significant and long-lasting reduction (about 30 per cent) of the in oirro uptake of [³H]NA in the cerebral cortex and spinal cord, while no changes were seen in the hypothalamus. The endogenous NA in whole brain was similarly reduced (about 20 per cent). Fluorescence histochemistry revealed catecholamine accumulations which are degenerative signs, induced by 6-hydroxydopamine, in axons of the dorsal NA bundle innervating the cerebral cortex. It is concluded that the blood–brain barrier in adult rats is not completely protective with respect to the neurotoxic action of systemically injected 6-hydroxydopamine, which can produce degeneration of a significant number of NA nerve terminals in the cerebral cortex and spinal cord. Previous studies have shown that 6-hydroxydopamine caused a permanent and selective degeneration of a large number of central NA nerve terminals when injected systemically up to 1 week after birth, due to an incompletely developed blood-brain barrier. This barrier for 6-hydroxydopamine develops between the 7th and 9th day after birth (Sachs , 1973). In the present study 6-hydroxydopamine was found to cause a small transient reduction in [³H]NA uptake in cerebral cortex of rats between 9 and 28 days of age, while in older rats the damage produced by 6-hydroxydopamine was long-lasting. Thus, the NA nerves ascending to the cerebral cortex seem to possess a regenerative capacity to a 6-hydroxydopamine-induced degeneration up to about 28 days postnatally, but which later disappears or is markedly retarded.     

Axoplasmic transport of dopamine in nigro-striatal neurons

The possibility that dopamine is transported in the nigro-striatal system was investigated by the stereotaxic injection of labelled tyrosine or l -DOPA into the substantia nigra of tranylcypromine-pretreated rats. At various intervals thereafter (2-48 h), significant quantities of labelled material were recovered from the ipsilateral substantia nigra, globus pallidus and caudate-putamen, The activity in the substantia nigra consisted of DOPA, dopamine, methoxytyramine, acid metabolites and other unidentified metabolites. In the caudate-putamen, however, nearly all of the activity (85 per cent) was recovered in the dopamine fraction, the remainder being distributed among some of the metabolites. No DOPA was recovered from the caudate-putamen. On the basis of time-course studies after the injection of [¹⁴C]DOPA into the substantia nigra, we calculated the transport rate of dopamine in the nigro-striatal bundle to be 0.8 mm/h. Electrolytic lesions of the nigrostriatal bundle at the level of the lateral hypothalamus, pretreatment with 6-hydroxydopamine, or injections of [¹⁴C]DOPA dorsal to the substantia nigra each produced profound reductions in the amount of activity subsequently recovered from the caudate-putamen. These data suggest that the activity recovered from the caudate-putamen after injections of [¹⁴C]DOPA into the or substantia nigra reflected axonal transport rather than other processes such as diffusion or transport via the circulation. Pretreatment with the DOPA decarboxy-lase inhibitor, R_o 4-4602, significantly reduced the amount of activity recovered in the caudate-putamen, an indication that decarboxylation of DOPA to dopamine was a prerequisite for transport. Pretreatment with reserpine also severely reduced the transport of dopamine in the nigro-striatal bundle, an observation suggesting that dopamine was transported by binding to the amine storage granules. There was no evidence of retrograde transport of dopamine in the nigrostriatal bundle. Injections of larger than tracer quantities of labelled tyrosine into the substantia nigra did not produce the degree of transport of dopamine that was obtained after injections of DOPA, a result suggesting that the amine storage granules may not normally be filled during axonal transport.     

In vivo characterization of somatodendritic dopamine release in the substantia nigra of 6-hydroxydopamine-lesioned rats

We investigated the effect of an injection of 6-hydroxydopamine (6-OHDA) into the rat medial forebrain bundle (MFB) on the degeneration and the function of the dopaminergic cell bodies in the substantia nigra (SN) 3 and 5 weeks after lesioning. After injection of 6-OHDA into the MFB a complete loss of dopamine content was apparent in the striatum 3 weeks after lesioning. In the SN the amount of tyrosine hydroxylase-immunoreactive dopamine cells decreased gradually, with a near-complete lesion (> 90%) obtained only after 5 weeks, indicating that neurodegeneration of the nigral cells was still ongoing when total dopamine denervation of the striatum had already been achieved. Baseline dialysate and extracellular dopamine levels in the SN, as determined by in vivo microdialysis, were not altered by the lesion. A combination of compensatory changes of the remaining neurones and dopamine originating from the ventral tegmental area may maintain extracellular dopamine at near-normal levels. In both intact and lesioned rats, the somatodendritic release was about 60% tetrodotoxin (TTX) dependent. Possibly two pools contribute to the basal dopamine levels in the SN: a fast sodium channel-dependent portion and a TTX-insensitive one originating from diffusion of dopamine. Amphetamine-evoked dopamine release and release after injection of the selective dopamine reuptake blocker GBR 12909 were attenuated after a near-complete denervation of the SN (5 weeks after lesioning). So, despite a 90% dopamine cell loss in the SN 5 weeks after an MFB lesion, extracellular dopamine levels in the SN are kept at near-normal levels. However, the response to a pharmacological challenge is severely disrupted.     

DEVELOPMENT OF THE BLOOD-BRAIN BARRIER FOR 6-HYDROXYDOPAMINE

The postnatal development of the blood-brain barrier for the neurotoxic action of 6-hydroxydopamine on central noradrenaline neurons has been investigated by recording the in vitro uptake of [³H]noradrenaline in slices from cerebral cortex, hypothalamus and spinal cord in rats treated with large doses of 6-hydroxydopamine at different ages. The [³H]noradranaline uptake was permanently and markedly reduced in all regions when the animals were treated at birth, certainly related to degeneration of noradrenaline neurons, caused by 6-OH-DA. In the cerebral cortex and hypothalamus an efficient protection against the effects of 6-OH-DA on [³H]noradrenaline uptake developed postnatally, while in the spinal cord this protection was never seen to become complete. The results obtained indicate a rapid formation of a blood-brain barrier for 6-OH-DA in the cerebral cortex between the 7th and 9th day after birth. In the hypothalamus the development of this barrier seemed to have a more gradual time-course, but appeared to be fully developed already at day 5 postnatally. Also in the spinal cord the barrier developed more gradually from birth to the adult age. It was observed, however, that both in the cerebral cortex and in the spinal cord, the blood-brain barrier developed, could not completely protect the central noradrenaline neurons from the neurotoxic actions of large doses of 6-OH-DA administered systemically to adult rats. Furthermore, the results obtained support the view that 6-OH-DA does not seem to apparently affect the outgrowth of remaining NA neurons which have not been destroyed by the 6-OH-DA treatment.     

Specific Noradrenergic Neurones destroyed by 6-Hydroxydopamine Injection into Newborn Rats

SINCE 6-hydroxydopamine does not cross the blood-brain barrier in adult animals¹, it has to be given by intracerebral injection to produce specific degeneration of catecholamine-containing nerve endings in the central nervous system^2,3. The resulting permanent depletion of noradrenaline and dopamine in all regions of the brain makes it difficult to determine the neuronal tracts and brain regions involved in a physiological or behavioural disfunction after 6-hydroxydopamine treatment⁴. Localization of nerve destruction can be improved by direct intracerebral injection of 6-hydroxydopamine at specific neuronal sites⁵ by pretreatment with drugs that affect catecholamine uptake⁶ or by varying the dosage of 6-hydroxydopamine⁷. These procedures are, however, technically difficult.     

The effect of cyclo (Leu-Gly) on chemical denervation supersensitivity of dopamine receptors induced by intracerebroventricular injection of 6-hydroxydopamine in mice

Chemical denervation supersensitivity of postsynaptic dopamine receptors was induced in mice by intracerebroventricular injection of 6-hydroxydopamine. Fourteen days after the 6-hydroxydopamine injection, mice exhibited greater spontaneous locomotor activity and hypothermic response when challenged intraperitoneally with apomorphine. Whole brain levels of dopamine were reduced by 80%. Daily subcutaneous administration of cyclo (Leu-Gly) (50 μg/mouse/day) for 14 days inhibited the development of dopamine receptor supersensitivity induced by 6-hydroxydopamine as evidenced by the blockade of an apomorphine induced locomotor and hypothermic effect. Cyclo (Leu-Gly) did not alter the depletion of brain dopamine induced by 6-hydroxydopamine. These data suggest that cyclo (Leu-Gly) can block the development of dopamine receptor supersensitivity induced by 6-hydroxydopamine without protecting the neurons from dopamine depletion.     

Selective Depletion of Dopamine, Octopamine, and 5-Hydroxytryptamine in the Nervous Tissue of the Cockroach (Periplaneta americana)

High-performance liquid chromatography with electrochemical detection was used to measure the concentrations of 3,4-dihydroxyphenylethylamine (dopamine), 5-hydroxytryptamine (5-HT), p-hydroxyphenylethanolamine (octopamine), alpha-methyl-p-tyrosine, and tryptophan in the cerebral ganglia of cockroaches (Periplaneta americana) after peripheral administration of alpha-methyl-p-tyrosine and alpha-methyltryptophan. In addition, the levels of dopamine, 5-HT, octopamine, alpha-methyl-p-tyrosine, and tryptophan were determined after injection of alpha-methyl-p-tyrosine, 6-hydroxydopamine, or 5,7-dihydroxytryptamine directly into the cerebral ganglia by means of microinjection needles. Peripheral administration of alpha-methyl-p-tyrosine (400-1,600 micrograms/insect) caused a reduction in dopamine and 5-HT concentrations in cockroach cerebral ganglia, although the reduction in dopamine concentrations was more pronounced. Peripheral injections of alpha-methyl-p-tyrosine also reduced octopamine levels in the cerebral ganglia. Peripheral injection of alpha-methyltryptophan (400-1,600 micrograms/insect) caused a marked reduction in 5-HT and tryptophan concentrations in cockroach cerebral ganglia without altering dopamine or octopamine concentrations. Central injections of alpha-methyl-p-tyrosine (80 micrograms/insect) reduced dopamine concentrations in the cerebral ganglia. However, neither 6-hydroxydopamine (20 micrograms/insect) nor 5,7-dihydroxytryptamine (20 micrograms/insect) caused reductions in amine levels when applied near or directly into the cerebral ganglia. The results suggest that specific lesions of aminergic neurons in insects by either 6-hydroxydopamine or 5,7-dihydroxytryptamine are impractical. The specific, long-lasting depletion of 5-HT by alpha-methyltryptophan suggests that this chemical may be useful in elucidating the functions of 5-HT in insects.     

Abnormal dorsal light response in teleost fish after intraocular injection of 6-hydroxydopamine

Following unilateral intraocular injection with 6-hydroxydopamine to the right eye, Midas cichlids displayed an acute abnormal dorsal light response, in favour of the right eye. This condition was reversed immediately by dopamine or dopamine receptor agonists delivered by intraocular injection but this recovery was not sustained, Retinal dopamine production reappeared gradually from day 30 post-6-hydroxydopamine treatment, and coincided with the fish beginning to adopt a normal dorsal light reaction. Dopamine production recovered completely at day 45 post-6-hydroxydopamine injection and coincided with complete recovery of the normal dorsal light reaction. These observations suggest a role for intraocular dopamine in maintenance of the visual part of the normal dorsal light reaction.     

Changes in the development of central noradrenaline neurones following neonatal administration of 6-hydroxydopamine

Abstract— The effects of the neurotoxic compound 6-hydroxydopamine on central noradrenaline (NA) neurones have been investigated in the adult rat after systemic administration of the drug at birth. This treatment produced a permanent and selective reduction in endogenous noradrenaline, [³H]noradrenaline uptake in vitro and the number of histochemically demonstrable noradrenaline nerve terminals in the forebrain, certainly related to neuroneal degeneration. The fluorescence morphology of the noradrenaline perikarya in the locus coeruleus was not notably affected. In the pons-medulla region, the 6-hydroxydopamine treatment led to an almost two-fold increase in endogenous noradrenaline with a similar increase in [³H]noradrenaline uptake and formation of ³H-catecholamines from [³H]tyrosine. Fluorescence histochemistry revealed an increased number of noradrenaline nerve terminals which in addition showed an increased fluorescence intensity. Subcellular distribution studies of endogenous noradrenaline in pons—medulla disclosed the highest relative noradrenaline increase in the microsomal fraction after 6-hydroxydopamine at birth. Sucrose gradient centrifugations disclosed that the pons-medulla synaptosomes from 6-OH-DA treated rats sedimented at a higher sucrose concentration than those from untreated controls. It is concluded that treatment of neonate rats with 6-hydroxydopamine produces a selective degeneration of noradrenaline nerve terminals in the forebrain, especially in the cerebral cortex, whereas in the pons-medulla this treatment leads to an increased intraneuronal noradrenaline concentration due to accumulation of noradrenaline in collateral systems not affected by 6-hydroxydopamine and probably also to an increased outgrowth of noradrenaline nerve terminals.     

Brain Amine Concentrations after Monoamine Oxidase Inhibitor Administration

The concentrations of 5-hydroxytryptamine (5HT), noradrenaline, and dopamine were estimated post mortem in brain stem, hypothalamus, and caudate nucleus in 33 patients who had been treated with isocarboxazid, clorgyline, or tranylcypromine and 11 controls. Similar and highly significant increases in 5HT and noradrenaline concentration occurred with all three drugs. The distribution was unimodal, but about a quarter of the patients showed only a small increase in brain amines. Tranylcypromine seemed to have a significantly greater effect on dopamine in caudate nucleus and hypothalamus compared with isocarboxazid and clorgyline. In the doses used chlorpromazine did not reduce the amine concentrations. Four patients with Parkinson''s syndrome had low concentrations of dopamine in caudate nucleus in spite of monoamine oxidase inhibitor administration.     

Effect of 6-hydroxydopamine on the electrical characteristics of snail neurons in long-term sensitization

Studies of the influence of neurotoxin 6-hydroxydopamine selectively destroying the catecholamine terminals on long-term sensitization, and the role of dopamine in manifestation of characteristics of a membrane of identified neurons during elaboration of plasticity, were fulfilled. Injection of saline was used as the control. It is shown that preliminary injection of 6-hydroxydopamine reduces duration of long-term sensitization, but does not block it completely. It was shown that injection of 6-hydroxydopamine prevents diminishing of membrane and threshold potentials in withdrawal interneurons during formation of long-term sensitization. The experiments demonstrate that shift of electrical characteristics of withdrawal interneurons caused by injection of neurotoxin 6-hydroxydopamine to both naive snails and sensitized snails, statys during at least 10 days.     

Wlds-mediated protection of dopaminergic fibers in an animal model of Parkinson disease

Parkinson disease (PD) is characterized by the progressive degeneration of substantia nigra dopaminergic neurons projecting to the striatum. Since the deficit in striatal dopamine is the main cause of PD symptoms, it appears critical to preserve axon terminals. Significant axon protection from peripheral nerve Wallerian degeneration is observed in Wlds mice, a phenotype conferred by a spontaneous dominant mutation. To assess any Wlds-mediated rescue of dopamine fibers in a PD model, the nigrostriatal pathway of Wlds mice was lesioned with 6-hydroxydopamine (6-OHDA), a catecholaminergic neurotoxin. Following 6-OHDA injection in the medial forebrain bundle, Wlds mice showed remarkable dopamine fiber protection in the striatum. Drug-induced rotational behavior confirmed the nigrostriatal fiber ability to release dopamine, although revealing an abnormal neurotransmitter control presumably due to disrupted axonal transport. Following 6-OHDA injection in the midstriatum, only a protection trend was observed. Strikingly, no protection of Wlds nigral dopaminergic cell bodies was obtained following either nigrostriatal lesion. Besides showing subtle differences in the degeneration process between subcellular compartments, the reported Wlds-mediated protection of the dopamine axon terminals in an animal model of PD may lead to the understanding of mechanisms underlying axon loss and to the development of new therapeutic approaches.     

EFFECTS OF 6-HYDROXYDOPAMINE ON CATECHOLAMINE CONTAINING NEURONES IN THE RAT BRAIN

After the intraventricular injection of 6-hydroxydopamine (6-OHDA), there was a long lasting reduction in the brain concentrations of noradrenaline (NA) and dopamine (DA). The brain concentration of NA was affected by lower doses of 6-OHDA than were required to deplete DA. A high dose of 6-OHDA which depleted the brain of NA and DA by 81 per cent and 66 per cent respectively, had no significant effect on brain concentrations of 5-hydroxytryptamine (5-HT) or γ-aminobutyric acid (GABA). The fall in catecholamines was accompanied by a long lasting reduction in the activities of tyrosine hydroxylase and DOPA decarboxylase in the hypothalamus and striatum, areas in the brain which are rich in catecholamine containing nerve endings. There was, however, no consistent effect on catechol-O-methyl transferase or monamine oxidase activity in these brain regions. The initial accumulation of [³H]NA into slices of the hypothalamus and striatum was markedly reduced 22–30 days after 6-OHDA treatment. These results are consistent with the evidence in the peripheral sympathetic nervous system that 6-OHDA causes a selective destruction of adrenergic nerve endings and suggest that this compound may have a similar destructive effect on catecholamine neurones in the CNS.     

Distribution of intrinsic nerve cell bodies and axons which take up aromatic amines and their precursors in the small intestine of the guinea-pig

Summary The sites of uptake, decarboxylation and retention of 1-dopa and the uptake and retention of dopamine and 6-hydroxytryptamine in the small intestine of the guinea-pig have been localised histochemically with a fluorescence technique for arylethylamines. In segments of ileum from untreated guinea-pigs only noradrenergic axons are fluorescent; these axons were eliminated by surgical denervation (crushing nerves running to the intestine through the mesentery) or by chemical denervation with 6-hydroxydopamine. In denervated segments of ileum, cell bodies and processes of intrinsic neurons become fluorescent after the injection of 1-dopa, dopamine or 6-hydroxytryptamine and the inhibition of monoamine oxidase, as do cells of Brunner's glands and Paneth cells. About 11% of the nerve cell bodies in the submucous plexus and 0.4% of those in the myenteric plexus become fluorescent. Varicose intrinsic axons which take up amines are found amongst the nerve cell bodies of the myenteric and submucous plexuses. They also ramify in the principal connections of the plexuses, in the tertiary strands of the myenteric plexus, in the deep muscular plexus and contribute sparse supplies of axons to arterioles in the submucosa and to the lamina propria of the mucosa. The axons are resistant to the degenerative actions of 6-hydroxydopamine.It is suggested that the intrinsic amine handling axons are more likely to utilise an indolamine related to 5-hydroxytryptamine than they are to utilise a catecholamine as a neurotransmitter.     

Dopamine independent rotational response to unilateral intranigral injection of serotonin

Unilateral injections of 5-hydroxytryptamine (5-HT) into the pars reticulata of the substantia nigra of rats pretreated with a monoamine oxidase inhibitor induced a strong and long-lasting contralateral circling behaviour which was selectively increased as a function of time after degeneration of central 5-HT neurons with 5,7 dihydroxytryptamine. Rotations were not abolished after 6-hydroxydopamine lesion of nigrostriatal dopamine (DA) neurons, or after striatal kainic acid lesions, but were on the contrary increased. It is concluded that the contralateral circling response to intranigral 5-HT injection is caused by a specific stimulation of certain post-synaptic nigral 5-HT receptors susceptible to the development of denervation supersensitivity but does not require the participation of nigrostriatal DA neurons.     

Parkinson's disease: studies with an animal model

Parkinson' disease has been associated with degeneration of dopamine-containing neurons of the nigrostriatal bundle. Many neurological features of Parkinsonism can be produced in rats by selective destruction of central dopaminergic neurons using the neurotoxin 6-hydroxydopamine. In this review we discuss two aspects of Parkinson's disease that have been investigated in these animals. First, we consider why near-total degeneration of nigrostriatal bundle neurons is required before neurological symptoms emerge. It appears that the loss of dopaminergic neurons is accompanied by an exponential increase in the ratio of tyrosine hydroxylase activity to dopamine content. Thus, after the brain lesions there may be a compensatory increase in the capacity of residual dopaminergic neurons to synthesize and release transmitter. Second, we consider why stress produces severe neurological deficits in patients who are only mildly impaired otherwise. It appears that a variety of stressors produce an abrupt but transient increase in dopaminergic activity in the striatum of intact animals and that this increase is markedly attenuated by 6-hydroxydopamine treatment. Thus, stress-induced akinesia in animals with dopamine-depleting brain lesions and in Parkinsonian patients may result from the impaired ability of residual neurons to respond approximately to such stimuli.     

Effects of Dopaminergic Transmission Interruption on the D2 Receptor Isoforms in Various Cerebral Tissues

We examined the effects of an interruption of dopamine neurotransmission, by either dopamine receptor blockade or degeneration of dopamine neurons by 6-hydroxydopamine, on the levels of D2 receptor mRNAs. In addition, we evaluated by the polymerase chain reaction (PCR) the relative abundance of the two D2 receptor isoform mRNAs generated by alternative splicing. Daily injections of 4 mg/kg of haloperidol to rats elicited in striatum a rapid and progressive increase in D2 receptor mRNA levels, which reached 70% after a 15-day treatment. By contrast, there was no apparent change in D2 receptor mRNA levels in cerebral cortex and pons-medulla, in spite of an increased density of D2 receptor in the former tissue. Using the PCR with primers flanking the alternative exon, we observed that the relative proportion of the shorter receptor isoform (D2S) mRNA was slightly but significantly enhanced in cerebral cortex (17%) and pons-medulla (18%) after a 15-day haloperidol treatment. Unilateral degeneration of dopamine neurons induced by local injection of 6-hydroxydopamine resulted in a marked decrease in levels of total D2 receptor mRNAs in substantia nigra (-79%) and ventral tegmental (-63%) area, two cell body areas. In the substantia nigra, the longer isoform (D2L) mRNA was significantly more decreased in content than the D2S isoform mRNA, so that there was a large enhancement in the relative abundance of the latter (81%).(ABSTRACT TRUNCATED AT 250 WORDS)     

6-AMINODOPAMINE-INDUCED DEGENERATION OF CATECHOLAMINE NEURONS

the effects of 6-aminodopamine on central and peripheral catecholamine neurons using fluorescence histochemical and isotope techniques have been investigated. Systematic administration of 6-aminodopamine (20 mg/kg intraveneously) produced a rapid (within 1 h) and long-lasting depletion of endogenous noradrenaline in adrenergic nerves of mouse atrium and iris with a concomitant loss of [³H]noradrenaline uptake. The effects were dosedependent. Accumulations of noradrenaline in non-terminal axons were observed histochemically, indicating that 6-aminodopamine induces neuronal damage. Desipramine completely blocked the 6-aminodopamine induced noradrenaline depletion and reduction in [³H]noradrenaline uptake, indicating that 6-aminodopamine has to be taken up by the axonal ‘membrane pump’ to produce its effects. Themonoamine oxidase inhibitor, nialamide, potentiated the effect of 6-aminodopamine on [³H]noradrenaline uptake. 6-Aminodopamine did not affect the cell bodies of the adrenergic neurons and there was a reappearance of adrenergic nerves and recovery of [³H]noradrenaline uptake. 6-Aminodopamine does not seem to pass the blood-brain barrier after systemic injection. Intraventricular injection of 6-aminodopamine in rats led to a considerable reduction in endogenous whole brain noradrenaline and [³H]noradrenaline uptake in slices from cerebral cortex and hypothalamus. Similar, but less pronounced effects were observed on dopamine neurons in the caudate nucleus. Histochemically, pronounced accumulations of transmitter were observed in the axons of the catecholamine neurons. The results obtained favour the view that 6-aminodopamine is able to produce an acute and selective degeneration of catecholamine neurons similar to that seen after the neurotoxicagent, 6-hydroxydopamine. Both compounds seemed to be approximately equally potent in their neurotoxicity, although 6-aminodopamine seemed to be more generally toxic.     

Functional reinnervation of the neostriatum in the adult rat by use of intraparenchymal grafting of dissociated cell suspensions from the substantia nigra

Summary Dissociated cell suspensions were prepared from the substantia nigra of 15–17 day-old rat embryos and grafted via an intraparenchymal injection into the depth of the neostriatum of adult recipient rats. The survival and fibre outgrowth of the dopamine-containing neurones in the implants were studied by fluorescence histochemistry, and the functional capacity of the grafts was monitored by repeated testing of the amphetamine-induced turning behaviour of the implanted rats.Before transplantation the target neostriatum of the recipient rats was denervated of its normal dopaminergic innervation by an injection of 6-hydroxydopamine into the ipsilateral nigrostriatal dopamine pathway. The completeness of the denervation was ascertained by measurement of the intensity of the amphetamine-induced turning response. After injection of the dissociated cells large numbers of dopamine-containing neurones were found in clusters at the site of injection as well as scattered in the apparently intact neostriatal tissue up to a distance of about 0.5 mm from the site of injection. Extensive dopamine-containing fibre networks had developed around the implant. These newly formed fibres, which were most abundant around the cell clusters at the injection site, extended in a loose network into large areas of the initially denervated caudate-putamen. In all animals with surviving dopamine neurones the amphetamine-induced turning response was reduced, and in the most extensively reinnervated cases even reversed, within 3–5 weeks after transplantation. This strongly suggests that the implanted dopamine neurones are capable of restoring dopaminergic neurotransmission in the denervated neostriatum, probably via reinnervation of the denervated neostriatal tissue.The use of dissociated brain tissue preparations thus permits reliable intraparenchymal grafting of neurones to plausibly any desired site within the central nervous system, and should open entirely new possibilities for investigation of neuronal growth dynamics and functional reconstruction of damaged brain circuits, perhaps even in brains of larger mammals.