Differential expression of GDNF, BDNF, and NT-3 in the aging nigrostriatal system following a neurotoxic lesion

Protein levels for brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and glial cell line-derived neurotrophic factor (GDNF) were measured in the striatum and ventral midbrain of young and aged Brown Norway/F344 F1 (F344BNF(1)) hybrid rats following a unilateral 6-hydroxydopamine (6-OHDA) lesion of the nigrostriatal pathway. At 2 weeks post-lesion, protein levels of BDNF and GDNF were higher in the denervated striatum when compared to the intact striatum for young (4-5 months old) but not old (31-33 months old) rats. Interestingly, in old rats BDNF protein in the denervated striatum was significantly lower than that measured in the intact striatum. At the same time point BDNF protein levels in the ventral midbrain were higher on the lesioned versus intact side for both young and old rats while no significant side differences were detected for GDNF protein in the ventral midbrain of young or old rats. No significant differences in NT-3 protein levels were detected between the lesioned and intact sides for striatal or ventral midbrain regions in either young or old brain. While no significant age effects were detected for BDNF or NT-3 protein, young rats showed higher GDNF protein levels in both the striatum (lesioned or intact) and ventral midbrain (lesioned or intact) than old rats. These data show that two endogenous neurotrophic factors, BDNF and GDNF, are differentially affected by a 6-OHDA lesion in the aging nigrostriatal system with young brain showing a significant compensatory increase of these two factors in the denervated striatum while no compensatory increase is observed in aged brain.     

DSP-4, a noradrenergic neurotoxin, produces more severe biochemical and functional deficits in aged than young rats

The present study examines the effects of noradrenergic lesions (either DSP-4 i.p. or 6-hydroxydopamine (6-OHDA) into the dorsal noradrenergic bundle on biochemical (noradrenaline (NA), dopamine (DA), serotonin (5-HT) and choline acetyltransferase (ChAT) activity) and cortical EEG (quantitative EEG (qEEG) and high-voltage spindle (HVS) activity in young and aged rats. Near complete 6-OHDA NA lesions, but not partial DSP-4 NA lesions, increased HVS activity in young rats. DSP-4 and 6-OHDA lesions produced no significant changes in the 5-HT or DA levels or in the ChAT activity in young rats. In some of the aged rats, DSP-4 produced similar biochemical and HVS effects, as it induced in young rats. In the remainder of the aged rats, NA levels were greatly and 5-HT levels slightly decreased. DA levels and ChAT activity were unaltered in either set of aged rats. HVS activity was increased only in that group of aged rats with the greatly lowered NA content. These results suggest that: (1) some of the aged rats are more sensitive to DSP-4 treatment than young adult rats; and (2) NA depletions have to be complete to produce an increase in HVS activity in young and aged rats.     

Behavioural and biochemical effects of dopamine and noradrenaline depletion within the medial prefrontal cortex of the rat

The effects of 6-hydroxydopamine (6-OHDA) lesions of catecholamine terminals within the medial prefrontal cortex on spontaneous motor activity, dopamine (DA)-dependent stereotyped behaviour and subcortical dopamine turnover were investigated in the rat. Two types of lesions were examined, bilateral injection of 6-OHDA into the medial prefrontal cortex of untreated rats (6-OHDA alone), and bilateral injection of 6-OHDA into the medial prefrontal cortex of animals pretreated with the noradrenaline (NA) uptake blocking agent desmethylimipramine (6-OHDA/-DMI). Ten days after surgery the 6-OHDA lesions produced no significant change in spontaneous motor activity and had no overall effects on stereotyped behaviour induced by apomorphine or (+)-amphetamine. This lesion caused gross depletion of NA within the medial prefrontal cortex and curiously, elevated DA concentrations within this site. No changes in DA concentration were recorded within subcortical sites, although concentrations of DA metabolites within striatum and nucleus accumbens were reduced. In contrast, the 6-OHDA/DMI lesion of the medial prefrontal cortex significantly enhanced spontaneous motor activity and amphetamine-induced stereotyped behaviour. Apomorphine-induced stereotypy, on the other hand, was significantly reduced. Biochemically the lesion caused a large depletion of DA with relatively little loss of NA within the medial prefrontal cortex. In addition, from this and another study (ref. 33), increases in DA and its metabolite concentrations were measured in striatum and nucleus accumbens, together with an apparent increased in DA turnover within these subcortical sites. It is thus apparent that in the absence of a substantial portion of the DA innervation of the medial prefrontal cortex, with a largely intact NA innvervation, there is an increase in motor activity and amphetamine-induced stereotypy which may be related to functional changes in DA activity within subcortical telecephalic structures. Such a finding might suggest that DA within the frontal cortex has a behaviourally inhibitory role in the rat, although further work is required to substantiate this.     

Striatal trophic activity is reduced in the aged rat brain

Our previous studies demonstrated that the survival of a mesencephalic graft was reduced in aged animals suggesting an age-related decline in target-derived neurotrophic activity. We tested this hypothesis by examining dopamine (DA) and trophic activities from the striatum of intact or unilateral 6-hydroxydopamine (6-OHDA) lesioned rats of increasing age. Fisher 344 rats were 4, 12, 18, and 23 months old (m.o.) at sacrifice. Half the animals had received unilateral 6-OHDA lesions of the mesostriatal DA pathway 8 weeks earlier. Striatal tissue punches were analyzed for DA, homovanillic acid (HVA), and DA activity (HVA/DA) using HPLC. The remainder of the striatal tissue was homogenized to generate tissue extracts which were added to E14.5 ventral mesencephalic cultures to test trophic activity. In the non-lesioned animals, striatal DA was reduced and striatal DA activity was increased in the 18 and 23 m.o. animals relative to the 4 and 12 m.o. animals. Striatal trophic activity was inversely related to age. In the lesioned animals, striatal DA ipsilateral to 6-OHDA infusion was below detection limits while the contralateral striatum exhibited age-related changes in DA similar to those seen in the non-lesioned animals. In 4 m.o. lesioned rats, striatal trophic activity ipsilateral to 6-OHDA infusion was elevated by 26% relative to the contralateral side. The ipsi/contra-lateral differences in striatal trophic activity were reduced in 12 m.o. animals and absent in the 18 and 23 m.o. groups. These data suggest that advancing age is associated with a reduction in striatal DA as well as trophic activity. Moreover, the aged striatum loses its ability to biochemically and trophically compensate for DA reduction and therefore may represent a more challenging environment for the survival, growth, and function of a fetal graft.     

6-羟基多巴胺单侧纹状体注射对大鼠双侧多巴胺能神经元的影响

目的探讨帕金森病大鼠模型中6-羟基多巴胺（6-OHDA）单侧纹状体注射对双侧黑质纹状体多巴胺能神经元的影响。方法大鼠随机分成模型组和对照组，模型组自一侧纹状体注射6-OHDA，对照组注射PBS；用免疫组织化学方法分别检测大鼠双侧黑质和纹状体区酪氨酸羟化酶（TH）阳性细胞和纤维的表达；高效液相色谱检测双侧纹状体多巴胺（DA）及其代谢产物3，4-二羟基苯乙酸（DOPAC）和高香草酸（HVA）含量。结果模型组大鼠双侧（毁损侧与其对侧）黑质致密区TH阳性细胞数量均少于对照组（P〈0．01），模型组双侧纹状体区TH阳性纤维密度均低于对照组；模型组大鼠双侧纹状体区DA含量均低于对照组（P〈0．01）；双侧DOPAC和HVA含量也降低。结论6-羟多巴胺单侧纹状体注射制作的帕金森病大鼠模型的对侧黑质纹状体也有损伤。     

Age-dependent effects of 6-hydroxydopamine on locomotor activity in the rat

This experiment examined the effects on locomotor activity of intraventricular 6-hydroxydopamine (6-OHDA) administered to developing and adult rats. 6-OHDA was administered subsequent to parygline desmethylimipramine (DMI) treatmen(6-OHDA/DMI) at 3 and 6 days of age, 11 and 14 days of age, 20 and 23 days of age, or 46 and 48 days of age. Locomotor activity of vehicle-treated rats assessed in stabilimeter cages peaked between 14 and 16 days of age and subsequently declined to levels characteristic of the adult. Treatment with pargyline and 6-OHDA at 3 days of age, or 6-OHDA/DMI at 3 and 6 or 11 and 14 days of age, did not alter the early rise in locomotor activity but prevented the decline in activity normally seen during the third and fourth weeks of life. When tested as adults, locomotor activity was greater in rats that had been treated with 6-OHDA/DMI at 3 and 6 and at 11 and 14 days of age than in those that had been treated at 20 and 23 days of age. Treatment with 6-OHDA/DMI at 46 and 48 days of age was without significant effect on locomotor activity. 6-OHDA (with pargyline pretreatment) produced large decreases in NE content in telencephalon and diencephalon and in dopamine (DA) content in striatum. 6-OHDA/DMI also produced large decreases in DA content in striatum and, in some of the treatment groups, only small decreases in norepinephrine (NE) content in telencephalon, diencephalon, and brain stem. These data suggest that the maturation of neuronal systems utilizing dopamine as a neurotransmitter is essential for the suppression of locomotor activity normally seen during development. The data further suggest that dopamine depletion per se does not lead to increased locomotor activity, but rather it is the destruction of dopamine-containing fibers prior to the normal period of locomotor suppression that increases locomotor activity.     

Hyperactivity and hypoactivity produced by lesions to the mesolimbic dopamine system

Spontaneous locomotor activity and the locomotor response to amphetamine and apomorphine were studied in rats subjected to either radiofrequency (RF), 6-hydroxydopamine (6-OHDA) or both RF and 6-OHDA lesions of the mesolimbic dopamine (DA) system. Large 6-OHDA lesions of the ventral tegmental area (VTA) or of the nucleus accumbens (N.Acc.) produced hypo-activity in the open field, a complete blockade of the locomotor stimulating effects of D-amphetamine and a profound supersensitive response to apomorphine as measured by a significant increase in locomotor activity as compared to sham-operated animals. In contrast, smaller 6-OHDA lesions of the VTA produced significant increases in spontaneous daytime and nocturnal activity with the biggest effect occurring at the lowest dose. RF lesions to the VTA produced even greater hyperactivity which was blocked by the addition of a 6-OHDA lesion to the N.Acc. The rats with RF lesions to VTA alone that were spontaneously hyperactive remained hyperactive after injection of amphetamine, whereas apomorphine produced a significant decrease in this hyperactivity. In contrast, the rats with the combined RF lesion and N.Acc. 6-OHDA lesion showed a blockade of the locomotor stimulating effects of D-amphetamine and a potentiated response to apomorphine identical to that observed with a N.Acc. lesion alone. All lesion groups revealed massive depletion of DA in the N.Acc. and anterior striatum with significantly greater depletions in those groups showing hypoactivity and hypo-responsiveness to amphetamine. All groups except the N.Acc. 6-OHDA alone group showed significant depletions of DA in the posterior striatum. Thus, limited destruction of the mesolimbic DA system can produce hyperactivity, but more extensive destruction of this system in the region of the N.Acc. and anterior striatum can reverse this hyperactivity and produce a hypo-responsiveness to the locomotor stimulating effects of amphetamine. These results suggest an essential role for dopamine in the expression of spontaneous and stimulant-induced activity. Furthermore, the much larger increase in spontaneous activity in the RF-VTA lesion group as compared to the VTA-6-OHDA groups suggests the presence of an, as yet unidentified, powerful inhibitory influence to the mesolimbic DA system within the midbrain tegmentum.     

Normal and drug-induced locomotor behavior in aging: comparison to evoked DA release and tissue content in Fischer 344 rats

The consequences of aging on dopamine (DA) regulation within the nigrostriatal and mesolimbic systems were investigated with a combination of behavioral, in vivo electrochemical, and high-performance liquid chromatography measurements using 6-, 12-, 18- and 24-month old male Fischer 344 (F344) rats. Spontaneous locomotor testing demonstrated that aged (18- and 24-month) rats moved significantly less and at a slower speed than younger (6- and 12-month) animals. Additionally, systemic injection (intraperitoneal) of the DA uptake inhibitor, nomifensine, was significantly less efficacious in augmenting the locomotor activity of aged rats compared to the younger animals. Age-dependent alterations in the release capacity of DA neurons within the regions involved in movement were investigated using in vivo electrochemistry. These recordings indicated that both the magnitude and temporal dynamics of potassium (70 mM)-evoked DA overflow were affected by the aging process. Signal amplitudes recorded in the 24-month rats were 30–60% reduced in both the striatum and nucleus accumbens as compared to the young adult groups. In addition, the duration of the electrochemical DA signals recorded within the striatum of 24-month old rats was twice that in the younger animals (6- and 12-month). Whole tissue measurements of DA and DA metabolites suggest age-related deficits in locomotion and DA release were not related to decreases in the storage or synthesis of DA within the striatum, nucleus accumbens, substantia nigra, ventral tegmental area or medial prefrontal cortex. Taken together, these results indicate age-dependent deficits in movement are related to the dynamic properties of DA release and not static measures of DA content.     

L-DOPA modulation of corpus striatal dopamine and dihydroxyphenylacetic acid output from intact and 6-OHDA lesioned rats

Summary In the present report we examined the differences in in vitro dopamine (DA) and dihydroxyphenylacetic acid (DOPAC) efflux from the corpus striatum (CS) of intact versus 6-hydroxydopamine (6-OHDA) lesioned (in substantia nigra) male rats in response to different doses of two pulse infusions of L-dihydroxyphenylalanine (L-DOPA). In the first experiment, we tested the effects of two 20-min infusions of 5 uM L-DOPA. In the second experiment we repeated this protocol using 50 uM L-DOPA. There was an overall significantly greater output of DA for intact versus 6-OHDA lesioned rats for both doses. Moreover, in Experiment 1, the 5 uM L-DOPA produced a peak DA response to the second infusion which was significantly higher than that of the first infusion in the intact, but not lesioned rats. In Experiment 2, the 50 uM L-DOPA group showed no significant differences in DA output between the two infusions for both intact and lesioned rats. In contrast to DA responses, there were no overall significant differences in DOPAC output between intact and 6-OHDA lesioned rats for both doses. However, for both doses tested, the peak DOPAC output from the second infusion was significantly increased in lesioned, but not intact rats. These data demonstrate that L-DOPA evoked DA and DOPAC output are differentially modulated in intact and 6-OHDA lesioned striatum. The lesions of the striatal dopaminergic system may alter these responses through changes in intraneuronal storage and metabolism of DA following L-DOPA infusion.     

GM1 enhances dopaminergic markers in the brain of aged rats

A number of presynaptic markers are compromised in the dopaminergic neurons of aged Sprague-Dawley rats (22 months old) compared with young rats (3 months old). Indeed, in the striatum of the aged rats there is a diminished capacity to transport dopamine (DA), to bind the dopamine transporter (DAT) marker mazindol, to bind the vesicular monoamine transporter 2 (VMAT2) marker dihydrotetrabenazine, and to release DA under basal conditions or after induction by K(+) or amphetamine. Furthermore, the expression of DAT and VMAT2 mRNA in the midbrain is suppressed. GM1 ganglioside, 30 mg/kg ip daily, administered for 30 days, restores the afore-mentioned markers to values approaching those for young rats. Taken together with our published observations that GM1 partially restores tyrosine hydroxylase activity and DA metabolism in aged nigrostriatal and mesoaccumbal neurons and improves their morphology, our work suggests that GM1 might act as a dopaminergic neurotrophic factor in the aged brain and be a useful adjuvant for treating age-associated dopaminergic deficits.     

Glial cell line-derived neurotrophic factor (GDNF) gene delivery protects dopaminergic terminals from degeneration

Previously, we observed that injection of an adenoviral (Ad) vector expressing glial cell line-derived neurotrophic factor (GDNF) into the striatum, but not the substantia nigra (SN), prior to a partial 6-OHDA lesion protects dopaminergic (DA) neuronal function and prevents the development of behavioral impairment in the aged rat. This suggests that striatal injection of AdGDNF maintains nigrostriatal function either by protecting DA terminals or by stimulating axonal sprouting to the denervated striatum. To distinguish between these possible mechanisms, the present study examines the effect of GDNF gene delivery on molecular markers of DA terminals and neuronal sprouting in the aged (20 month) rat brain. AdGDNF or a control vector coding for beta-galactosidase (AdLacZ) was injected unilaterally into either the striatum or the SN. One week later, rats received a unilateral intrastriatal injection of 6-OHDA on the side of vector injection. Two weeks postlesion, rats injected with AdGDNF into either the striatum or the SN exhibited a reduction in the area of striatal denervation and increased binding of the DA transporter ligand [(125)I]IPCIT in the lesioned striatum compared to control animals. Furthermore, injections of AdGDNF into the striatum, but not the SN, increased levels of tyrosine hydroxylase mRNA in lesioned DA neurons in the SN and prevented the development of amphetamine-induced rotational asymmetry. In contrast, the level of T1 alpha-tubulin mRNA, a marker of neuronal sprouting, was not increased in lesioned DA neurons in the SN following injection of AdGDNF either into the striatum or into the SN. These results suggest that GDNF gene delivery prior to a partial lesion ameliorates damage caused by 6-OHDA in aged rats by inhibiting the degeneration of DA terminals rather than by inducing sprouting of nigrostriatal axons.     

Long-term effects of dopamine-depleting brain lesions on spontaneous activity of type II striatal neurons: Relation to behavioral recovery

The long-term effects of dopamine (DA)-depleting brain lesions on behavior and spontaneous activity of Type II striatal neurons were measured in rats after intraventricular injection of the neurotoxin 6-hydroxydopamine (6-OHDA). Spontaneous firing rates were increased relative to control values when recorded 4-8 days or 4-6 weeks postlesion in animals displaying aphagia, adipsia and akinesia. In contrast, spontaneous activity was not increased when recorded 4-6 weeks after the lesion in animals that had recovered from behavioral deficits. Other animals that had recovered from the effects of an earlier 6-OHDA treatment were given either a second injection of 6-OHDA or a systemic injection of haloperidol, a DA receptor antagonist. In both groups, discharge rates were elevated relative to control levels in association with a reinstatement of behavioral deficits. These results demonstrate that behavioral recovery after large DA-depleting brain lesions is associated with a return of spontaneous activity of striatal neurons to normal levels, and suggest that both behavioral and electrophysiological measures are dependent on the functioning of residual elements of the DA system.     

Substantia nigra 6-hydroxydopamine lesions alter dopaminergic synaptic markers in the nucleus basalis magnocellularis and striatum of rats

Recent findings have demonstrated the existence of dopaminergic (DA) markers in the nbM of the human brain and a reduction of these markers in both the nbM and the striatum of patients suffering from Alzheimer's disease (AD). To investigate the source of the DA synaptic markers found in the nbM, rats received unilateral 6-OHDA lesions of the substantia nigra pars compacta (SNc). The SNc lesions caused significant reductions in DA and DOPAC but not HVA in the nbM and the striatum; 3H-sulpiride binding to D2 receptors ipsilateral to the SNc lesion was significantly increased in the striatum (16%), consistent with denervation supersensitivity, but single-point analysis showed no significant changes in the nbM. These data suggest that the decreases in DA and 3H-spiperone binding levels observed in the nbM of AD patients may be due to partial destruction of DA nbM afferent projections from the brainstem.     

Age Related Effects of Levodopa on Rat Brain Striatal Acetylcholinesterase

Long-term treatment of Parkinson’s disease (PD) with levodopa is accompanied by dyskinesia. Alteration in striatal acetylcholine signaling is partly responsible, but the involved mechanisms have yet to be determined. This study aimed to compare repeated levodopa treatment on dopamine (DA) content and acetylcholinesterase (AChE) activity in the young and old rat brain striatum. Male Wistar rats (3 and 30 months old) were injected with a mixture of levodopa + carbidopa (10 + 1 mg/kg). Control animals received normal saline only. Rats were killed and brain striatum was homogenized and centrifuged at 4°C. AChE activity was assayed in the supernatant and DA was extracted from the homogenate and measured by high performance liquid chromatography with electrochemical detection. The levels of DA in young and aged rats were 53.1 ± 4.5 and 28.4 ± 3.1 nmol/ g wet weight tissue respectively. AChE activity in corresponding supernatant was 32.4 ± 2.7 and 58.1 ± 3.3 μmol/min/mg protein. A single dose of the drug mixture increased DA content and decreased AChE activity in both ages. When the drug mixture was injected daily for a period of 30 days, a lower DA content and a higher AChE activity were observed, though the changes were more pronounced in the aged animals. The result indicates that long-term treatment with levodopa + carbidopa in aged rat renders a dramatic rise in the striatal AChE, leading to imbalance ACh/DA levels in the striatum. It is concluded that AChE might be considered as a therapeutic target for combating levodopa-induced dyskinesia affecting PD patients.     

Dopaminergic neuronal function, anterior pituitary dopamine content, and serum concentrations of prolactin, luteinizing hormone and progesterone in the aged female rat

The serum concentrations of prolactin (PRL), progesterone and luteinizing hormone (LH), and the content and rate of synthesis of dopamine (DA) in selected brain regions were determined in young (3–6 months), intermediate (13–15 months) and aged (24–25 months) female Long-Evans rats. Young rats were examined on the days of diestrus 2 and estrus. Intermediate rats were divided into 2 groups, a group which was cycling regularly (examined on the day of estrus) and a group which exhibited constant estrus. Aged rats were divided into 3 groups one which cycled regularly (examined on day of estrus), one which exhibited constant estrus, and one which exhibited repetitive pseudopregnancies.Serum PRL was increased in all intermediate and aged rats when compared to values in young animals. Serum LH was increased and progesterone decreased in those intermediate and aged rats which exhibited constant estrous reproductive patterns. The DA content was generally decreased in the median eminence, posterior pituitary, striatum, nucleus accumbens and olfactory tubercle of all aged rats, while the rate of DA synthesis was decreased only in the median eminence of aged, non-cycling rats. This suggests that all DA neuronal systems except those in the tuberoinfundibular system are able to compensate for the age-related loss. Despite the apparent reduction of tuberinfundibular DA neuronal function the concentration of DA in the anterior pituitary, which is believed to represent amine released from the neurons, is dramatically increased in intermediate age rats in constant estrus, and in all groups of aged rats. The maintenance of high PRL secretion despite the elevated content of DA in the anterior pituitary suggests in age-related defect in the dynamics of DA in this gland; this defect may contribute to the loss of reproductive function in the aging rat.     

Global Depletion of Dopamine Using Intracerebroventricular 6-Hydroxydopamine Injection Disrupts Normal Circadian Wheel-Running Patterns and PERIOD2 Expression in the Rat Forebrain

Normal circadian rhythms of behavior are disrupted in disorders involving the dopamine (DA) system, such as Parkinson’s disease. We have reported previously using unilateral injections of the catecholamine toxin, 6-hydroxydopamine (6-OHDA), into the medial forebrain bundle that DA signaling regulates daily expression of the clock protein, PERIOD2 (PER2), in the dorsal striatum of the rat. In the present study, we made widespread lesions of DA fibers using large injections of 6-OHDA into the third ventricle to determine the involvement of DA in normal daily rhythms of wheel-running activity and PER2 patterns in the suprachiasmatic nucleus (SCN) and several regions of the limbic forebrain. Rats injected with 6-OHDA and housed in constant darkness were less active in the wheel and showed a disorganized pattern of activity in which wheel running was not confined to a specific phase over 24 h. The 6-OHDA injection had no effect on the daily PER2 pattern in the SCN, but blunted the normal rise in PER2 in the dorsal striatum. 6-OHDA also blunted PER2 expression in the periventricular nucleus of the hypothalamus, a region in which a daily PER2 pattern has not been previously reported in male rats, and in the oval nucleus of the bed nucleus of the stria terminalis, but not in the central nucleus of the amygdala. These results indicate that DA plays a prominent role in regulating circadian activity at both behavioral and molecular levels.     

Chronic effects of single intrastriatal injections of 6-hydroxydopamine or 1-methyl-4-phenylpyridinium studied by microdialysis in freely moving rats

Extracellular dopamine (DA) and its main cerebral metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), were measured by bilateral striatal microdialysis in rats at different times (2, 7, 15 and 60 days) after unilateral administration into the right striatum of 1-methyl-4-phenylpyridinium ion (MPP⁺) or 6-hydroxydopamine (6-OHDA). In both cases the decrease in extracellular dopamine did not exceed 40% of control values. The response of DOPAC and HVA depended on the treatment: MPP⁺ caused a marked acute decrease in the dopamine metabolites but allowed a progressive recovery that was very evident after 60 days; 6-OHDA caused a progressive decrease in the dopamine metabolites throughout the two months of the study. Tyrosine hydroxylase immunostaining revealed severe neuronal loss in substantia nigra two months after striatal administration of 6-OHDA, whereas no significant neuronal loss was found at the same time after MPP⁺ administration. A bilateral challenge infusion of MPP⁺ through the microdialysis probe was used to assess the dopaminergic capacity of both striata: at all the times studied there was a sharp depletion of DA on the non-lesioned side; both MPP⁺- and 6-OHDA-treated striata were unresponsive after a short time (2 days); after 2 months the response in MPP⁺-lesioned rats was similar on both sides, whereas 6-OHDA-lesioned striata were still unresponsive to MPP⁺. In rats, then, the effects of MPP⁺ could be partly reversed whereas the effects of 6-OHDA were not. These results suggest that neurotoxins causing striatal dopamine loss may act through different mechanisms, which could be significant for the etiopathogenic development of Parkinson's disease.     

Diminished survival of mesencephalic dopamine neurons grafted into aged hosts occurs during the immediate postgrafting interval

The survival rate of dopamine (DA) neurons in mesencephalic grafts to young adult rats is poor, estimated at 5-20%, and even poorer in grafts to the aged striatum. Grafted cells die in young adult rats during the first 4 days after implantation. The present study was undertaken to determine whether the decreased survival of DA neurons in grafts to aged rats is (1) due to additional cell death during the immediate postgrafting interval or (2) due to protracted cell loss during longer postgrafting intervals. We compared survival rates of tyrosine hydroxylase-immunoreactive (THir) neurons in cell suspension grafts to young adult (3 months) and aged (24 months) male Fischer 344 rats at 4 days and 2 weeks after transplantation. At 4 days after grafting, mesencephalic grafts within the aged rat striatum contain approximately 25% of the number of THir neurons in the same mesencephalic cell suspension grafted to young adult rats. This corroborates the decreased survival of grafted DA neurons we have demonstrated previously at 10 weeks postgrafting. THir neurons in grafts to the intact striatum possessed a significantly shorter "long axis" than their counterparts on the lesioned side. No significant differences in the number of apoptotic nuclear profiles or total alkaline phosphatase staining between mesencephalic grafts to young and aged rats were detectable at 4 days postgrafting. In summary, the present study indicates that the exaggerated cell death of grafted DA neurons that occurs following implantation to the aged striatum occurs during the immediate postgrafting interval, timing identical to that documented for young adult hosts.     

Specificity of dopaminergic neuronal degeneration induced by intracerebral injection of 6-hydroxydopamine in the nigrostriatal dopamine system.

The neurotoxic specificity of injections of 6-hydroxydopamine (6-OHDA) into areas containing either dopamine (DA) cell bodies (substantia nigra) or DA axon terminals (striatum) was studied. This selective effect was compared to the unspecific effects of copper sulfate (CuSO4) injection and electrocoagulation. One to two days after unilateral nigral injection of 2 mug of either 6-OHDA or CuSO4 into the nigra the volume of the unspecific lesions around the tip of the cannula was very similar. Only the 6-OHDA-induced lesions were associated with elective degeneration of the nigral DA neurons. Ten days after the administration of the same compounds the gliosis in the substantia nigra was much more extensive in CuSO4-than in 6-OHDA-treated rats; however, the reduction of DA concentrations in the ipsilateral striatum was only noticeable after 6-OHDA (-62%). A somewhat similar decrease of striatal DA levels (-52%) was observed after large electrocoagulation of the substantia nigra. Ten days after 6-OHDA (8mug) or electrolytic lesion of the striatum the Km for DA, serotonin and choline uptakes were similar in the striata of both sides, suggesting that the uptake process in the non-damaged neurons of the lesioned side was functionally normal. Following electrolytic lesion of the striatum, serotonin and choline Vmax values were decreased to about the same extent as the striatal reduction in weight and DA levels. When directly administered into the striatum 6-OHDA also produced a decline in DA concentration and Vmax but in contrast did not affect serotonin and choline uptake (Vmax), suggesting that the drug specifically destroyed dopaminergic neurons. The present data confirm that selective DA denervation can be achieved when appropriate amounts of the drug are injected into brain tissue in order to limit the unspecific lesion.     

In vivo microdialysis studies of age-related alterations in potassium-evoked overflow of dopamine in the dorsal striatum of Fischer 344 rats

Intracerebral microdialysis was used to measure basal levels and potassium (K(+))-stimulated overflow of dopamine (DA), homovanillic acid (HVA) and dihydroxyphenylacetic acid (DOPAC), in the dorsal striatum of young (6 months) and aged (24 months) Fischer 344 (F344) rats. Basal levels of HVA were lower in aged rats whereas basal DA and DOPAC did not differ significantly between the two groups. The administration of three low to moderate doses of K(+) (10, 25, and 50 mM) through the microdialysis probe for one collection period revealed differences between the two age groups of F344 rats. DA overflow increased in a dose-dependent manner in the young but not aged rats. Extracellular levels of DOPAC and HVA decreased during the K(+) stimulation and there was a significant difference in the changes in HVA produced by K(+) stimulation in the young vs aged animals. These data support the hypothesis that low to moderate doses of K(+) may be necessary to demonstrate age-related differences in K(+)-evoked DA overflow, since previous microdialysis studies using higher doses have not reported age-related differences in DA overflow.