Nicotine prevents striatal dopamine loss produced by 6-hydroxydopamine lesion in the substantia...

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Brain Research 888 (2001) 336–342 www.elsevier.com / locate / bres Interactive report Nicotine prevents striatal dopamine loss produced by 1 6-hydroxydopamine lesion in the substantia nigra * Gustavo Costa, J.A. Abin-Carriquiry, Federico Dajas ´ ´ ´ Division Neuroquımica, Instituto de Investigaciones Biologicas Clemente Estable, Avda. Italia 3318, 11 600 Montevideo, Uruguay Accepted 23 October 2000 Abstract While the work of several groups has shown the neuroprotective effects of nicotine in vitro, evidences for the same effects in vivo are controversial, mainly regarding neuroprotection in experimental models of Parkinson’s disease. In this context, we investigated the capability of various systemic administration schedules of nicotine to prevent the loss of striatal dopamine levels produced by partial or extensive 6-hydroxydopamine (6-OHDA) lesion of rat substantia nigra (SN). Eight days after 6- and 10-mg injections of 6-OHDA in the SN there was a significant decrease of dopamine concentrations in the corpus striatum (CS) and a concomitant increase in dopamine turnover. While 10 mg 6-OHDA produced an almost complete depletion of dopamine in the SN, 6 mg decreased dopamine levels by 50%. Subcutaneous nicotine (1 mg/kg) administered 4 h before and 20, 44 and 68 h after 6 mg 6-OHDA, prevented significantly the striatal dopamine loss. Administered only 18 or 4 h before or only 20, 44 and 68 h after, nicotine failed to counteract the loss of dopamine or the increase in dopamine turnover observed in the CS. Nicotine also failed to prevent significantly the decrease of striatal dopamine levels produced by the 10-mg 6-OHDA intranigral dose. Chlorisondamine, a long-lasting nicotinic acetylcholine receptor antagonist, reverted significantly the nicotinic protective effects on dopamine concentrations. These results are showing that putative neuroprotective effects of nicotine in vivo depend on an acute intermittent administration schedule and on the extent of the brain lesion. 2001 Elsevier Science B.V. All rights reserved. Theme: Disorders of the nervous system Topic: Degenerative disease: Parkinson’s Keywords: Nicotine; Dopamine; Corpus striatum; Substantia nigra; Nicotinic acetylcholine receptor; 6-Hydroxydopamine; Chlorisondamine 1. Introduction that nicotine could have a positive effect in several neuronal processes like attention, memory processing, Although initially introduced in Europe with tobacco pain, or neuroprotection [9]. The identification of more plants for medicinal purposes, nicotine has become, than 10 genes codifying the different sub-units of the through tobacco addiction, a harmful presence in our nAChR gave a structural basis for the search of specific modern societies. The physiological effects of nicotine are subunit combinations with beneficial effects in the CNS varied, affecting the peripheral (cardiovascular, neuro- [9]. muscular) and the central nervous system (CNS) [18], and Several reports have shown that there is a negative are based on its ability to mimic the actions of acetyl- correlation between smoking and the appearance of Parkin- choline at the nicotinic acetylcholine receptors (nAChR). son’s disease. These epidemiological findings gave support The deleterious effects of tobacco use have precluded a to the hypothesis of a neuroprotective role of nicotine comprehensive study of the effects of nicotine in the CNS. [12,36] and prompted the study of the effects of nicotine In recent decades, however, several studies demonstrated stimulation in several models of neurotoxicity. At present, numerous studies have confirmed the protection conferred 1 by nicotine to neuronal cultures against toxic insults like Published on the World Wide Web on 1 December 2000. excitotoxins [1,11,22,31,35,42] or b-amyloid toxicity [29]. *Corresponding author. Tel. / fax: 1598-2-487-2603. E-mail address: [email protected] (F. Dajas). Blockade of the neuroprotection effects by nAChR antago- 0006-8993 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0006-8993(00)03087-0

Transcript of Nicotine prevents striatal dopamine loss produced by 6-hydroxydopamine lesion in the substantia...

Brain Research 888 (2001) 336–342www.elsevier.com/ locate /bres

Interactive report

Nicotine prevents striatal dopamine loss produced by16-hydroxydopamine lesion in the substantia nigra

*Gustavo Costa, J.A. Abin-Carriquiry, Federico Dajas´ ´ ´Division Neuroquımica, Instituto de Investigaciones Biologicas Clemente Estable, Avda. Italia 3318, 11 600 Montevideo, Uruguay

Accepted 23 October 2000

Abstract

While the work of several groups has shown the neuroprotective effects of nicotine in vitro, evidences for the same effects in vivo arecontroversial, mainly regarding neuroprotection in experimental models of Parkinson’s disease. In this context, we investigated thecapability of various systemic administration schedules of nicotine to prevent the loss of striatal dopamine levels produced by partial orextensive 6-hydroxydopamine (6-OHDA) lesion of rat substantia nigra (SN). Eight days after 6- and 10-mg injections of 6-OHDA in theSN there was a significant decrease of dopamine concentrations in the corpus striatum (CS) and a concomitant increase in dopamineturnover. While 10 mg 6-OHDA produced an almost complete depletion of dopamine in the SN, 6 mg decreased dopamine levels by 50%.Subcutaneous nicotine (1 mg/kg) administered 4 h before and 20, 44 and 68 h after 6 mg 6-OHDA, prevented significantly the striataldopamine loss. Administered only 18 or 4 h before or only 20, 44 and 68 h after, nicotine failed to counteract the loss of dopamine or theincrease in dopamine turnover observed in the CS. Nicotine also failed to prevent significantly the decrease of striatal dopamine levelsproduced by the 10-mg 6-OHDA intranigral dose. Chlorisondamine, a long-lasting nicotinic acetylcholine receptor antagonist, revertedsignificantly the nicotinic protective effects on dopamine concentrations. These results are showing that putative neuroprotective effects ofnicotine in vivo depend on an acute intermittent administration schedule and on the extent of the brain lesion. 2001 Elsevier ScienceB.V. All rights reserved.

Theme: Disorders of the nervous system

Topic: Degenerative disease: Parkinson’s

Keywords: Nicotine; Dopamine; Corpus striatum; Substantia nigra; Nicotinic acetylcholine receptor; 6-Hydroxydopamine; Chlorisondamine

1. Introduction that nicotine could have a positive effect in severalneuronal processes like attention, memory processing,

Although initially introduced in Europe with tobacco pain, or neuroprotection [9]. The identification of moreplants for medicinal purposes, nicotine has become, than 10 genes codifying the different sub-units of thethrough tobacco addiction, a harmful presence in our nAChR gave a structural basis for the search of specificmodern societies. The physiological effects of nicotine are subunit combinations with beneficial effects in the CNSvaried, affecting the peripheral (cardiovascular, neuro- [9].muscular) and the central nervous system (CNS) [18], and Several reports have shown that there is a negativeare based on its ability to mimic the actions of acetyl- correlation between smoking and the appearance of Parkin-choline at the nicotinic acetylcholine receptors (nAChR). son’s disease. These epidemiological findings gave supportThe deleterious effects of tobacco use have precluded a to the hypothesis of a neuroprotective role of nicotinecomprehensive study of the effects of nicotine in the CNS. [12,36] and prompted the study of the effects of nicotineIn recent decades, however, several studies demonstrated stimulation in several models of neurotoxicity. At present,

numerous studies have confirmed the protection conferred1 by nicotine to neuronal cultures against toxic insults likePublished on the World Wide Web on 1 December 2000.

excitotoxins [1,11,22,31,35,42] or b-amyloid toxicity [29].*Corresponding author. Tel. / fax: 1598-2-487-2603.E-mail address: [email protected] (F. Dajas). Blockade of the neuroprotection effects by nAChR antago-

0006-8993/01/$ – see front matter 2001 Elsevier Science B.V. All rights reserved.PI I : S0006-8993( 00 )03087-0

G. Costa et al. / Brain Research 888 (2001) 336 –342 337

nists provided evidence for the specific mediation by in aCSF containing 0.2% ascorbic acid. Nicotine and CHLdifferent subtypes of nAChR [29,31,43]. were dissolved in saline.

In spite of this in vitro evidence, in vivo studies are stillcontroversial when regarding the neuroprotective prop- 2.3. Intranigral injection of 6-OHDAerties of nicotine treatments. While continuous nicotineinfusion has been demonstrated to protect against neuronal Animals were anaesthetized with halothane (Fluothane,loss produced by dopamine pathways hemitransection Zeneca) and placed in a D. Kopf stereotaxic frame.[25,26], the striatal depletion of dopamine after injection of Through a skull hole, the needle (0.022 mm o.d., 0.0136-hydroxydopamine (6-OHDA) in the substantia nigra mm i.d.) of a Hamilton syringe (5 ml), attached to a(SN) was unaltered by the same treatment [10]. 1-Methyl- micro-injection unit (D. Kopf), was lowered to the SN.4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) systemic ap- Coordinates (H, 24.8; L, 22.2; V, 27.2) were determinedplication in vivo resulted in a significant decrease of from bregma, according to the atlas of Paxinos and Watsonstriatal dopamine content that was not prevented by [37].nicotine in some studies [21], while another group showed A total of 2.0 ml of a 6-OHDA solution (3 and 5 mg/mlthat nicotine had protective effects against diethyldithio- for the 6- and 10-mg doses) was injected for 4 min and thecarbamate enhancement of MPTP lesions [34]. Moreover, needle was slowly withdrawn, allowing the drug to diffusein the MPTP model of experimental parkinsonism, nicotine for another minute. Body temperature was maintained athas even shown an enhancement of the neurotoxicity 378C using a temperature control system.[5,19].

Beyond these discrepancies, it is likely that the great 2.4. Experimental groups and nicotine administrationvariety of experimental conditions reported, differing in schedulethe schedule and method of nicotine administration, may inpart explain the differences observed. Thus, reports using 2.4.1. Partial lesion (6 mg 6-OHDA in the SN)chronic treatments did not show effects in vivo [21,27], Five groups of rats (n58) injected with 6-OHDA (6 mg)while the acute intermittent administration appeared to in the right SN, received 1 mg/kg nicotine subcutaneouslyshow protective effects in the models reported [27,34]. according to the following protocols: (1) nicotine 18 h

In this context, and as a contribution to the characteriza- before 6-OHDA; (2) nicotine 4 h before 6-OHDA; (3)tion of the role played by nAChR in Parkinson’s disease, nicotine 4 h before, and 20, 44 and 68 h after 6-OHDA; (4)we studied the putative neuroprotective effects of nicotine nicotine 20, 44, 68 h after 6-OHDA; (5) similar adminis-in the 6-OHDA model of experimental parkinsonism, tration schedule to (3) plus CHL 10 mg/kg s.c. 30 minassessing whether the timing and schedule of nicotine before the first application of nicotine.administration as well as the extent of the lesion are factors Each of the five experimental groups had its own controlthat could effectively influence the neuroprotection profile. group of 6-OHDA plus saline instead of nicotine.

For control of 6-OHDA vehicle, six rats were injectedwith similar volumes of aCFS with 0.2% ascorbic acid.

2. Materials and methods2.4.2. Extensive lesion (10 mg 6-OHDA in the SN)

Animals injected with 10 mg 6-OHDA received nicotine2.1. Animals

or saline treatment according to the administrationschedule (3).

Experiments were carried out using male Sprague–Daw-ley rats (230–260 g). Animals had access to food and

2.5. Neurochemical analysiswater ad libitum, and were housed in groups of six in atemperature controlled environment on a 12-h light /dark

For neurochemical analysis rats were decapitated 8 dayscycle.

after 6-OHDA injection, brains rapidly removed and theleft and right SN and corpus striatum (CS) dissected out

2.2. Drugs and reagents and kept at 2708C. Next day tissue samples were weighed,sonicated in perchloric acid 0.1 M (200 and 1000 ml for

Chemicals for HPLC analysis, artificial cerebrospinal the SN and CS, respectively) and centrifuged (15 0003g)fluid (aCSF) and saline were purchased from Baker for 15 min at 48C. Then, samples were injected into an(Phillipsburg, PA, USA). Dopamine (hydrochloride), 3,4- HPLC system (PM-80 BAS, USA) equipped with a C-18dihydroxyphenylacetic acid (DOPAC), 6-OHDA, (2)-nico- column (5-mm particles, 220 mm34.6 mm; BAS, USA)tine (tartrate) and L-ascorbic acid were obtained from and a electrochemical detector (LC-4C BAS) with oxida-Sigma (St. Louis, MO, USA). Chlorisondamine (CHL) was tion potential at 10.75 V (glassy working carbon electrodedonated by Novartis Pharmaceuticals (NJ, USA). 6-OHDA versus an Ag/AgCl reference electrode). The mobile phasesolution for intranigral injection was prepared dissolving it was composed of citric acid (0.15 M), sodium octyl

338 G. Costa et al. / Brain Research 888 (2001) 336 –342

sulphate (0.6 mM), 4% acetonitrile and 1.6% tetrahydro-furan at pH 3.0; with a flow rate of 1.0 ml /min.

2.6. Statistical analysis

Dopamine is presented as percent of right versus lefttissue levels, considering the left side (untreated) as 100%.In each experimental group comparison were made againstcontrols as defined in Section 2.4. For turnover analysis, ineach experimental group, the ratio DOPAC/DA of theipsilateral (injected) side was compared with its control(contralateral side). Means comparison was performed

Fig. 1. Striatal dopamine assessed 8 days after the injection of 6-OHDAusing independent t-test (two-tailed). Statistical signifi-(6 mg) expressed as percent (mean1S.D.) of right versus left CS.cance was chosen at P,0.05.Subcutaneous administration of nicotine 4 h before, and 20, 44 and 68 hafter 6-OHDA injection counteracted significantly the dopamine decrease(6-OHDA1nicotine versus 6-OHDA1saline; *P,0,05). The same ad-ministration schedule of nicotine did not prevent the dopamine decrease3. Resultsobserved with 6-OHDA when CHL was injected 30 min before. For eachgroup, n58. Control striatal concentration of dopamine (contralateral

3.1. 6-OHDA side): saline 17 65162832; nicotine 15 81661996; CHL1saline15 96361175; CHL1nicotine 16 85861248 (values are expressed asng/mg of wet tissue weight).Intranigral injections of 6 and 10 mg 6-OHDA, produced

a significant decrease of dopamine tissue levels in the rightCS and SN (ipsilateral, treated side) when compared withthe left, non-treated, one (contralateral) (Table 1). Asshown in Table 1, the 10-mg dose produced an effect in thedopamine concentrations higher than the 6-mg dose. Con-trol rats injected in the SN with 2 ml of aCSF (in 0.2%ascorbic acid), did not show any difference in dopaminelevels between right and left CS (right, 17 58361733; left,17 65162832 ng/mg of wet tissue weight). Dopamineturnover, expressed as the ratio DOPAC/DA, was sig-nificantly increased in the ipsilateral CS of 10 mg 6-OHDAtreated rats (Fig. 4). In 6 mg 6-OHDA treated DOPAC/DA Fig. 2. Striatal dopamine assessed 8 days after the injection of 6-OHDAshowed a marked tendency to increase with border line (10 mg) expressed as percent (mean1S.D.) of right versus left CS.

Subcutaneous administration of nicotine 4 h before, and 20, 44 and 68 hstatistical significance (P50.06).after 6-OHDA failed to counteract significantly the dopamine decrease.For each group, n58. Control striatal concentration of dopamine (con-

3.2. 6-OHDA1Nicotine tralateral side): saline 16 29262511; nicotine 16 05461851 (valuesexpressed as ng/mg of wet tissue weight).

3.2.1. Partial lesion (6 mg 6-OHDA in the SN)In experimental group 3, nicotine treatment (4 h before, ratio in the contralateral CS and SN (data shown in the

and 20, 44 and 68 h after 6-OHDA injection), prevented respective figure legend).significantly the 6-OHDA effects on striatal dopamine In experimental groups 1, 2 and 4, nicotine treatmenttissue levels (Fig. 1), but not in the SN (Fig. 3). Nicotine only before (18 or 4 h); or only after (20, 44 and 68 h)also reverted the tendency of 6-OHDA treated to show an 6-OHDA injection, failed to have any effect on striatalincreased striatal dopamine turnover (Fig. 4). Nicotine by dopamine and DOPAC/DA ratio (Figs. 5 and 6). Nicotineitself did not change dopamine levels and DOPAC/DA by itself did not change dopamine levels and DOPAC/DA

Table 1Dopamine levels as ng/g of wet tissue weight6S.D. Ipsilateral dopamine level compared with contralateral one for each group

6-OHDA, 6 mg 6-OHDA, 10 mg

Contralateral Ipsilateral Contralateral Ipsilateral

Corpus striatum 15 5256530 477563.790* 16 29162.511 4586465*Substantia nigra 10556319 4976300* 9106207 1656214*

* P,0.05. For each group, n58.

G. Costa et al. / Brain Research 888 (2001) 336 –342 339

Fig. 6. Bars represent the mean1S.D. of dopamine striatum turnover,Fig. 3. Nigral dopamine assessed 8 days after the injection of 6-OHDAexpressed as DOPAC/DA ratio. Dopamine and DOPAC levels were(6 or 10 mg) expressed as percent (mean1S.D.) of right versus left SN.assessed 8 days after injection of 6-OHDA in the right SN. DifferentSubcutaneous administration of nicotine 4 h before, and 20, 44 and 68 hprotocols of nicotine subcutaneous administration were applied. P1, 18 hafter 6-OHDA failed to counteract significantly the dopamine decrease.before 6-OHDA; P2, 4 h before 6-OHDA; P3, 4 h before and 20, 44 andFor each group n58. Control nigral concentration of dopamine (contrala-68 h after 6-OHDA; P4, 20, 44 and 68 h after 6-OHDA. Ipsilateralteral side): 6 mg 6-OHDA saline 10556319 and nicotine 9616342; 10 mgturnover compared with contralateral one for each group, *P,0.05. For6-OHDA saline 9106207 and nicotine 7896285 (values are expressed aseach group, n58.ng/mg of wet tissue weight).

ratio in the contralateral CS and SN (data shown in therespective figure legend).

3.2.2. Extensive lesion (10 mg 6-OHDA in the SN)When a dose of 10 mg 6-OHDA was used, nicotine

treatment according to the administration schedule 3,neither restored significantly striatal dopamine levels (Fig.2) nor dopamine turnover (Fig. 4). Nicotine by itself didnot change dopamine levels and DOPAC/DA ratio in thecontralateral CS and SN (data shown in the respectivefigure legend).

Fig. 4. Bars represent the mean1S.D. of dopamine striatum turnover, 3.3. 6-OHDA1nicotine1CHLexpressed as DOPAC/DA ratio. Dopamine and DOPAC levels wereassessed 8 days after injection of 6-OHDA in the right SN. Ipsilateral

When the nicotinic antagonist CHL was administeredturnover compared with contralateral one for each group, *P,0.05. Foreach group, n58. before nicotine and 6 mg 6-OHDA (experimental group 5),

nicotine did not revert the striatal dopamine loss inducedby 6-OHDA (Fig. 1). Dopamine and DOPAC/DA ratioboth in ipsilateral and contralateral side did not changeafter CHL (data shown in the respective figure legend).

4. Discussion

The injection of 6-OHDA into the rat SN leads to aprogressive and massive death of dopaminergic nerve cellsand to a corresponding depletion of dopamine in the CS[3,13,28]. It is recognized that 6-OHDA insults involve at

Fig. 5. Striatal dopamine assessed 8 days after the injection of 6-OHDA least two mechanisms of cell death: free radical production(6 mg) expressed as percent (mean1S.D.) of right versus left CS.

and/or reactivity of quinone products [13,23,30,32]. InDifferent protocols of subcutaneous nicotine administration were applied.agreement with these previous reports [3,13,39], the in-P1, 18 h before 6-OHDA; P2, 4 h before 6-OHDA; P3, 4 h before and 20,

44 and 68 h after 6-OHDA; P4, 20,44 and 68 h after 6-OHDA. Each tranigral injection of 6-OHDA produced a significantgroup compared with it respective saline, *P,0.05. For each group, n58. decrease of dopamine levels in the ipsilateral CS, aControl striatal concentration of dopamine (contralateral side): P1 saline reduction that can be directly correlated with a neuronal17 98661450; nicotine 18 1196414; P2 saline 17 72263321; nicotine

loss in the SN pars compacta [3,8]. The 6-OHDA treat-18 52161788; P3 saline 15 5256530; nicotine 15 81661996; P4 salinements also produced an increase in the DOPAC/DA ratio,14 40461621; nicotine 12 64961486 (values are expressed as ng/mg of

wet tissue weight). evidencing an accelerated dopamine turnover that likely

340 G. Costa et al. / Brain Research 888 (2001) 336 –342

reflects an increase in the activity of the remaining the nigral lesion. Although not yet proved, the desensitiza-dopaminergic terminals. This increase in dopamine turn- tion of nAChR has been postulated as the basis for the lackover has been recognized as a functional response to the of effects of continuous and/or chronic nicotine treatmentloss of dopaminergic terminals after neuronal death in the [25]. As it is known, nicotine administration to ratsSN, to maintain the normal levels of extracellular dopa- produces an increase in nAChR population in the CNS,mine [3,39]. which appears to be a result of nAChR receptor desensiti-

The decrease of dopamine concentrations in the SN and zation [20,42]. Desensitization has also been utilized as anCS produced by 6 mg 6-OHDA was utilized as the milder explanation for the protective effects of nicotine at thetoxic stimulus for the assessment of nicotine protection. level of postsynaptic receptors in the SN, in view that itThe 10-mg dose produced a more pronounced decrease of would decrease the activation tone of neurons, probablydopamine concentrations in the SN and the CS and was contributing to their survival after 6-OHDA lesion [25].therefore used to test nicotine properties against a more However, desensitization cannot explain the protectiontoxic insult. Altogether, the utilization of two different given by nicotine, when considering that nAChR blockadedoses of 6-OHDA, with marked differences in the extent of with CHL, not only failed to be protective per se, but alsotoxicity achieved, allowed us to assess the importance of significantly prevented the protection observed after nico-the lesion extent in the nicotine protective effects. tine treatment. Accordingly, our results suggest that the

Nicotine treatment prevented the striatal dopamine loss functional activation of nAChR is deemed necessary forafter the 6-mg 6-OHDA injection when administered 4 h the achievement of nicotine protection.before and 20, 44 and 68 h after the toxin. The nAChR The intracellular mechanisms by which nicotine exertsantagonist CHL, administered 30 min before this adminis- its effects are not yet fully understood. In vitro, sometration schedule, almost completely reverted the nicotine studies have shown that actions of nicotine are dependentprotection after 6-OHDA. Previously, only Maggio et al. upon the entry of calcium to the cell [14,15], but not much[34] had studied the blockade of nicotine protection by the progress has been observed regarding the intracellularantagonist mecamylamine, demonstrating its dependence cascades activated by increased cytoplasmic calcium. Inon the activation of nAChR. CHL is a biquaternary amine, vivo, nicotine or nicotine agonist treatments, have beenclassically utilized as a nicotinic antagonist at autonomic shown to produce regional increases in trophic factors, likeganglia, which produces a remarkably persistent blockade the basic fibroblast growth factor (FGF-2) [6,7]. In thisof central nicotinic responses [16,17,38]. Nicotine protec- regard, it has been shown that the glial cell line-derivedtion has been linked to different subtypes of nAChR, growth factor (GNDF), can protect SN neurons againstmainly the homopentameric a7 and the a4b2 [29,31,43]. It 6-OHDA neurotoxicity [40,41,44]. Other trophic factorsis not clear on which nAChR subtype CHL exerts its have been shown to be protective in models of ischaemia,action, though evidence for a rather broad specificity, excitotoxicity and hypoxia [2,4,24]. In agreement with theexcluding the a7 subtypes, has been described [38]. lack of protection after continuous nicotine treatmentAccordingly, the reversion of nicotine action in our results mentioned above, the growth factor increase obtained withcannot be safely linked to any sub-population of nAChR acute nicotine treatment is not seen after chronic continu-subtypes. ous infusion [10,27]. Moreover, the induction of FGF-2

When nicotine was injected only 18 or 4 h before or expression peaked 4 h after an injection of nicotine andonly 20, 44 and 68 h after, it did not prevent the striatal returned to normal levels within 24 or 48 h [6,7]. Conse-dopamine loss. The dissimilar effects observed after di- quently, our protective administration schedule of nicotineverse nicotine treatments suggests that the schedule and would coincide with the peak production of growth factors,means of nicotine administration are crucial in the achieve- suggesting that they could ultimately be the active protec-ment of nicotine protection. In a previous study [10], tive substances.nicotine infusion in a chronic and continuous manner, While nicotine prevented the effect of the 6-mg injectionfailed to protect dopamine depletion in the CS after 6- of 6-OHDA, it failed to achieve the same protection whenOHDA lesion in the SN. In addition,several studies the higher dose of 6-OHDA (10 mg) was injected. More-assessing nicotine protection in MPTP-induced experimen- over, the most significant recovery on dopamine levelstal parkinsonism reported that chronic continuous nicotine after partial lesions, is only seen in the CS, and not in thedid not prevented the dopamine loss, and even worsened it SN. These results are indicating that during mild toxic[5,10,21,27]. In contrast, acute intermittent application insults a high percentage of the neuronal population would[27,34] appeared to protect SN neurons. Concurring with still be responsive to the protective actions of nicotinethese latter reports, our results are showing that an acute stimulation, a situation that is probably not occurring whenintermittent administration schedule is meaningful for higher doses of 6-OHDA are tried. During the progressionrestoring dopamine concentrations and turnover in an of degenerative diseases like Parkinson’s, neuronal lossexperimental model of Parkinson’s disease. In addition, has a progressive development, most probably resemblingand besides being intermittent, our data suggest that the the effects of a milder dose of 6-OHDA.nicotine stimulus should be applied before and also after Previous reports have shown that 6-OHDA oxidative

G. Costa et al. / Brain Research 888 (2001) 336 –342 341

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