Ž .Brain Research 773 1997 203–207

Short communication

Repeated cocaine and stress increase dopamine clearance in the rat medialprefrontal cortex

Susan M. Meiergerd a,1, James O. Schenk a, Barbara A. Sorg b,)

a Department of Chemistry, Washington State UniÕersity, Pullman, WA 99164-4630, USAb Department of ComparatiÕe Anatomy, Pharmacology and Physiology, Washington State UniÕersity, Pullman, WA 99164-6520, USA

Accepted 22 July 1997

Abstract

Ž .The effects of repeated footshock stress or cocaine on the kinetics of dopamine clearance in the medial prefrontal cortex mPFC wereŽ . Ž .measured by rotating disk electrode voltammetry RDEV . Five groups of rats were used: animals were either naive non-handled ,

Ž . Ž .pre-treated with five daily saline 1 mlrkg i.p. or cocaine 15 mgrkg i.p. injections, or pre-treated with five daily 20-min sessions ofŽ .sham shock or footshock 0.05 mAr200 msrs . Dopamine clearance was measured after a 1-week withdrawal period. No difference in

K values was present among the treatment groups, with the mean K value at f0.5 mM for all groups. However, V values werem m max

f50% higher in daily sham shock-, footshock- and cocaine-pre-treated animals compared to naive rats. The increased ability to removedopamine in these animals suggests that altered dopamine clearance may serve an adaptive mechanism in the mPFC. q 1997 ElsevierScience B.V.

Keywords: Cocaine; Dopamine transporter; Norepinephrine transporter; Prefrontal cortex; Stress

Ž .The medial prefrontal cortex mPFC is defined as thecortical region receiving projections from the medial dor-

w xsal thalamus 28 . This region also receives dopaminergicŽ .projections mesocortical from the ventral tegmental area

Ž w x.VTA 22 . Mesocortical dopamine neurons are differentŽfrom other mesotelencephalic projections e.g. mesolim-

.bic in several respects. They have a higher turnover andw xrelease of dopamine 4,16 , lack synthesis- and impulse-

w xmodulating autoreceptors 5,11 , possess an increasedw xbursting activity 4 , and are highly sensitive to the effects

w xof stress 1,40,42 .A further difference between mesocortical and

mesolimbic dopaminergic projections recently explored inthis laboratory is the response of mesoprefrontal dopamineneurons to administration of repeated cocaine or stressw x40,41 . Repeated exposure to psychostimulants or stressproduces the phenomenon of behavioral sensitization,which is the progressive increase in psychostimulant-in-

) Corresponding author. Program in Neuroscience, Department ofVCAPP, Washington State University, Pullman, WA 99164-6520, USA.

Ž .Fax: q1 509 335 4650; E-mail: barbsorg@vetmed.wsu.edu1 Present address: S.M. Meiergerd, MS M888 NMT-6, Los Alamos

National Laboratory, Los Alamos, NM 87544, USA.

duced locomotor activity in rodents. Several laboratorieshave found that extracellular dopamine levels in the nu-cleus accumbens are enhanced in rats exhibiting behavioralsensitization, and the increase is believed to partially con-

w xtribute to the sensitized locomotor response 25,33,39 . Incontrast to the augmentation in extracellular dopaminelevels measured in the nucleus accumbens after sensitiza-tion to stress or psychostimulants, extracellular dopamine

w xlevels in the mPFC were found to be diminished 40,41 .Thus, dopamine in the mPFC demonstrates tolerance ratherthan sensitization.

This tolerance is found when either subsequent foot-w xshock stress is applied 40 or after systemic injection of

w xcocaine 41 . However, complicating this issue is the ob-servation that local dopamine re-uptake blockade via infu-sion of cocaine into the mPFC through the microdialysisprobe produces an increase in the level of dopamine in

w xdaily cocaine-pre-treated rats 41 . Since basal levels ofextracellular dopamine are not altered in cocaine-pre-

w xtreated rats 41 , it was of interest to determine whetherthere was an increase in the ability to remove dopaminefrom the synaptic cleft. Rats administered either no treat-

Ž .ment naive or daily saline or cocaine were examined. Inaddition, it was of interest to measure dopamine clearancein animals given daily sham shock or footshock stress

0006-8993r97r$17.00 q 1997 Elsevier Science B.V. All rights reserved.Ž .PII S0006-8993 97 00926-8

( )S.M. Meiergerd et al.rBrain Research 773 1997 203–207204

Fig. 1. The velocity of dopamine uptake in the mPFC from naive, dailyŽ .saline- and daily cocaine-pre-treated rats upper panel and naive, daily

Žsham shock- and daily footshock-pre-treated rats lower panel; the same.naive group is shown in both panels . Values were measured after a

1-week withdrawal period. V and K values were obtained frommax m

Eadie-Hofstee analyses and the lines are described by the best fit of thew xdata to the Michaelis-Menten expression. DA , concentration ofo

dopamine on the outside.

since daily footshock also produced tolerance to a subse-w xquent cocaine injection 40 . Rotating disk electrode

Ž .voltammetry RDEV was used to measure the kinetics ofŽ w x.dopamine clearance in the mPFC for review, see 31 .

All experiments were conducted according to the Na-tional Institutes of Health Guide for the Care and Use ofLaboratory Animals. Male Sprague-Dawley rats weighing

Ž .250–300 g were group housed 3–4rcage in a tempera-ture- and humidity-controlled environment with access toad libitum food and water. Animals were maintained on a12-h lightrdark schedule, with lights on at 07:00 h.

Ž .Five groups of rats were used: naive non-handled , ratsŽ .pre-treated with five daily saline 1 mlrkg i.p. or cocaine

Ž .15 mgrkg i.p. injections, or rats pre-treated with fiveŽdaily 20-min sessions of sham shock or footshock 0.05

.mAr200 msrs . One week after daily pre-treatment wasdiscontinued, rats were decapitated and brains rapidly re-moved. The mPFC, which consisted of all medial pre-

w xfrontal regions anterior to q2.7 mm from bregma 32Žprelimbic, dorsal anterior cingulate and the most medial

.portions of the medial precentral region was dissected andweighed. The tissue was chopped and placed into 500 mlphysiological buffer as described by Meiergerd and Schenkw x31 and maintained at 378C in a temperature-controlledchamber. The tissue was disrupted by repetitive pipettingand washed by the repeated addition and removal of 250ml fresh buffer 7= . The rotating disk electrode waslowered into the chamber and rotated at 2000 rpm, and apotential of 450 mV relative to a AgrAgCl referenceelectrode was applied. The tissue was incubated for 20 minuntil a steady baseline was reached. After the incubationperiod, increasing concentrations of dopamine were addedŽ .in mM: 0.10, 0.15, 0.25, 0.50, 1.00 . After each addition,the disappearance of dopamine was monitored. The initialrate of disappearance of dopamine was estimated as de-

w x Ž .scribed previously 31 . The velocity y of clearance wasexpressed as pmolrsrg wet weight. Values for the Km

and V were estimated by Eadie-Hofstee analyses usingmax

the commercially available software, Prism Graph PadŽ .2.0 for linear regressions. The errors are expressed as"S.E. of regression.

The top panel in Fig. 1 shows the results from velocitymeasurements from naive, daily saline- and daily cocaine-pre-treated rats, while the bottom panel shows these mea-surements from naive, daily sham- and daily footshock-pre-treated animals. Table 1 lists the resulting K andm

V values obtained from transformation of the data frommax

all five groups using Eadie-Hofstee analyses. K valuesm

were not different among the treatment groups, with amean K of 0.52 mM for all groups. However, Vm max

values were significantly higher in rats given daily co-caine, sham shock and footshock stress compared to naiveanimals, with an approximate increase of 53%. Dailysaline-pre-treated rats showed a non-significant trend to-ward an increase in V compared to naive animals.max

These data suggest that repeated cocaine, footshockŽstress or the handling control group for footshock sham

.shock enhances the removal of released dopamine fromthe synaptic cleft in the mPFC. A few studies have exam-ined the effects of prior cocaine treatment on dopamine

w xtransporter ligand binding in the PFC 20,21 . Hitri et al.

Table 1K and V values for dopamine clearance in the mPFC a

m max

Ž . ŽDaily K mM V pmolrsrgm max.pre-treatment wet weight

Naive 0.47"0.13 31.7"4.1Saline 0.53"0.13 39.4"5.0

bCocaine 0.47"0.10 48.6"5.1bSham shock 0.53"0.12 48.0"5.2bFootshock 0.59"0.12 50.6"5.5

a All values represent mean"S.E. of regression, ns4–6rgroup.b P -0.05, compared to naive rats.

( )S.M. Meiergerd et al.rBrain Research 773 1997 203–207 205

w x21 recently reported a decrease in the binding of GBR12935 in the prefrontal cortex after intermittent cocaineand a 7-day withdrawal. However, since this ligand alsobinds to a site other than the dopamine transporter, espe-

w xcially in the PFC 2,17 , it is not known if binding to thew xdopamine transporter is decreased. Masserano et al. 30

w3 xhave demonstrated a decrease in the V for H dopa-max

mine uptake in the mPFC after repeated daily cocaine.However, this study employed pargyline in the incubationbuffer, which reduces dopamine uptake in the mPFC by

Ž . w x50–70% in vitro unpublished results and in vivo 29 .One explanation for the increased V in repeated shammax

shock-, footshock- and cocaine-treated rats is an increasein the number of dopamine andror norepinephrine trans-porters. The increase in V and concurrent absence of anmax

effect on K suggests that these treatments also alter them

binding of dopamine to the transporter.The increased V in cocaine-pre-treated rats may helpmax

to explain our previous finding that daily cocaine pre-treat-ment does not lead to alterations in basal levels of extracel-lular dopamine in the mPFC but produces enhanced levelsof dopamine when challenged with a local infusion of

w xcocaine into the mPFC 41 . Therefore, in the mPFC, dailycocaine produces both an increase in the ability to releasedopamine coupled with an increased ability to removedopamine from the synaptic cleft. In order to observe atolerance effect of extracellular dopamine levels after sys-temic cocaine challenge, alterations probably occur ineither the mesoprefrontal cell-firing rate or pre-synapticcontrol in the mPFC by terminals containing other neuro-

w x w xtransmitters such as norepinephrine 9,18 , serotonin 3,10w xor glutamate 15,24 .

It should be pointed out that repeated cocaine producessensitization, rather than tolerance, of nucleus accumbensdopamine levels in response to a cocaine challenge, yetdopamine uptake is increased in both the nucleus accum-

Žw x .bens and the mPFC 35 present study . However, cocaineblocks )95% of dopamine uptake into the nucleus ac-

w xcumbens 36 while only partially preventing dopamineŽw x .uptake into the mPFC 19,23 unpublished results . There-

fore, systemic cocaine challenge in cocaine sensitized ratsproduces an augmentation in nucleus accumbens dopaminelevels essentially independent of re-uptake, while in themPFC, extracellular dopamine levels are still partiallydependent on clearance by the transporter because ofcocaine’s limited effect in this brain area. Thus, a cocainechallenge could conceivably alter the relative ratio of

Ždopamine release : re-uptake such that a decreased toler-.ance response is observed in the mPFC of cocaine-pre-

treated rats.Although an increase in the V for dopamine clear-max

ance was observed in cocaine-, sham shock- and foot-shock-pre-treated rats, it remains unknown if this changecan be attributed to the dopamine transporter. Dopamineserves as a better substrate for the norepinephrine trans-

w xporter, which is also present in abundance in the mPFC 6 .

Therefore, the increased V may be partially or whollymax

ascribed to an increased removal by norepinephrine trans-porter-containing terminals. A second possibility is thatchanges occur in a previously uncharacterized transporterwhich removes dopamine from the synapse in the mPFC.

w xPrevious studies 19,23 and our own recent preliminaryŽunpublished results with RDEV indicate that cocaine up

.to 100 mM blocks dopamine clearance by only f50%.This is in striking contrast to the )95% cocaine-inducedblockade observed in the striatum or nucleus accumbensw x19,23,36 . However, evidence to explain the relative in-sensitivity of dopamine clearance to cocaine has not beenprovided, and only a single gene encoding the dopamine

w xtransporter has thus far been identified 27,38 . A differ-ence in the post-translational modification or associationwith other proteins may confer partial insensitivity tococaine. However, studies using antibody to the dopamine

w125 xtransporter and the cross-linker, I DEEP, to measurethe glycosylation status of the dopamine transporter revealno qualitative differences between the striatum and mPFCw x43 . Therefore, characteristics of the dopamine transporterare similar, at least when restricted to examining the

w125 xspecific protein which binds to I DEEP. An alternativeexplanation is that a different transporter insensitive tococaine is partially responsible for re-uptake of dopaminein the mPFC, leaving open the possibility that an alterna-tive transport process is modified after repeated cocaine,sham shock andror footshock pre-treatment.

The elevated V values for dopamine clearance inmax

cocaine, footshock and sham shock groups suggests thatthe increases may be due to adaptive changes in the mPFCto stressful stimuli in general. Evidence points to a role formPFC dopamine in the acquisition of a coping responsew x12,13 , and the degree of handling has been associated

w xwith changes in dopamine levels in the mPFC 14 . Anincrease in V values may be related to the intensity ofmax

stress, since minimal handling for saline injections pro-duced only a non-significant trend toward increased activ-ity. Also, the more extensive handling of sham shock ratsŽremoval from the home cage, placement into the foot-shock apparatus and subsequent replacement into the home

.cage produced an increase in V similar to that of dailymax

footshock. The present finding that daily injection of co-caine, but not saline, induced a significant enhancement inV suggests that cocaine may serve as a stressor or that itmax

produces direct effects via its pharmacological action ondopamine uptake inhibition.

Repeated cocaine and stress treatment in rodents pro-vide animal models for cocaine-induced psychosis and

w xschizophrenia 34 . The ratio of dopamine release to re-up-take in the mPFC may determine the ability of stimuli suchas drugs of abuse and stressful life events to impactpost-synaptic events, altering subcortical regions via the

w xinfluence of mPFC dopamine on excitatory output 7,8,37 .Examination of cocaine- and stress-induced changes indopamine clearance rates and extracellular dopamine lev-

( )S.M. Meiergerd et al.rBrain Research 773 1997 203–207206

els should provide enhanced understanding of altered pre-frontal cortical function in idiopathic and drug-induced

w xpsychosis 26 .

Acknowledgements

This work was supported by USPHS Grants DA-07384,Ža Research Career Development Award K02 DA-00184 to

. Ž .J.O.S. and DA-08212 to B.A.S. .

References

w x1 E.D. Abercrombie, K.A. Keefe, D.S. DiFrischia, M.J. Zigmond,Differential effect of stress on in vivo dopamine release in striatum,nucleus accumbens, and medial frontal cortex, J. Neurochem. 52Ž .1989 1655–1658.

w x w3 x2 P. Allard, M. Danielsson, K. Papworth, J.O. Marcusson, H GBR-12935 binding to human cerebral cortex is not to dopamine uptake

Ž .sites, J. Neurochem. 62 1994 338–341.w x3 C.R. Ashby Jr., Y. Minabe, E. Edwards, R.Y. Wang, 5-HT -like3

receptors in the rat medial prefrontal cortex: an electrophysiologicalŽ .study, Brain Res. 550 1991 181–191.

w x4 M.J. Bannon, R.H. Roth, Pharmacology of mesocortical dopamineŽ .neurons, Pharmacol. Rev. 35 1983 53–68.

w x5 M.J. Bannon, R.L. Michaud, R.H. Roth, Mesocortical dopamineneurons: lack of autoreceptors modulating dopamine synthesis, Mol.

Ž .Pharmacol. 19 1981 270–275.w x6 E.L. Barker, R.D. Blakely, Norepinephrine and serotonin trans-

porters: molecular targets of antidepressant drugs, in: F.E. Bloom,Ž .D.J. Kupfer Eds. , Psychopharmacology: The Fourth Generation of

Progress, Raven, New York, NY, 1995, pp. 321–333.w x7 H.W. Berendse, Y. Galis-de Graaf, H.J. Groenewegen, Topographi-

cal organization and relationship with ventral striatal compartmentsof prefrontal corticostriatal projections in the rat, J. Comp. Neurol.

Ž .316 1992 314–347.w x8 C.J. Carter, C.J. Pycock, Behavioral and biochemical effects of

dopamine and noradrenaline depletion within the medial prefrontalŽ .cortex, Brain Res. 192 1980 163–169.

w x9 W.A. Cass, G.A. Gerhardt, In vivo assessment of dopamine uptakein rat medial prefrontal cortex: comparison with dorsal striatum and

Ž .nucleus accumbens, J. Neurochem. 65 1995 201–207.w x10 J. Chen, W. Predes, H.M. Van Praag, J.H. Lowinson, E.L. Gardner,

Presynaptic dopamine release is enhanced by 5-HT receptor activa-3

tion in medial prefrontal cortex of freely moving rats, Synapse 10Ž .1992 264–266.

w x11 L.A. Chiodo, M.J. Bannon, A.A. Grace, R.H. Roth, B.S. Bunney,Evidence for the absence of impulse-regulating somatodendritic andsynthesis-modulating nerve terminal autoreceptors on subpopula-

Ž .tions of mesocortical dopamine neurons, Neuroscience 12 19841–16.

w x12 M. D’Angio, A. Serrano, P. Driscoll, B. Scatton, Stressful environ-mental stimuli increase extracellular DOPAC levels in the prefrontal

Ž .cortex of hypoemotional Roman high avoidance but not hyperemo-Ž .tional Roman low-avoidance rats. An in vivo voltammetric study,

Ž .Brain Res. 451 1988 237–247.w x13 A.Y. Deutch, C.D. Young, A model of the stress-induced activation

of prefrontal cortical dopamine systems, in: M.J. Friedman, D.S.Ž .Charney, A.Y. Deutch Eds. , Neurobiological and Clinical Conse-

quences of Stress, Lippincott-Raven, Philadelphia, PA, 1995, pp.163–175.

w x14 M.G. Feenstra, M.H. Botterblom, J.F. Van Uum, Novelty-inducedincrease in dopamine release in the rat prefrontal cortex in vivo:

Ž .inhibition by diazepam, Neurosci. Lett. 189 1995 81–84.

w x15 M.G. Feenstra, W. Van der Weij, M.H. Botterblom, Concentration-dependent dual action of locally applied N-methyl-D-aspartate onextracellular dopamine in the rat prefrontal cortex in vivo, Neurosci.

Ž .Lett. 201 1995 175–178.w x16 P.A. Garris, R.M. Wightman, Different kinetics govern dopaminer-

gic transmission in the amygdala, prefrontal cortex, and striatum: anŽ .in vivo voltammetric study, J. Neurosci. 14 1994 442–450.

w x w3 x17 I. Gordon, R. Weizman, M. Rehavi, H GBR 12935 labels mainlythe piperazine acceptor site in the rat prefrontal cortex, Brain Res.

Ž .674 1995 205–210.w x18 P.J. Gresch, A.F. Sved, M.J. Zigmond, J.M. Finlay, Local influence

of endogenous norepinephrine on extracellular dopamine in ratŽ .medial prefrontal cortex, J. Neurochem. 65 1995 111–116.

w x19 M.G. Hadfield, E.A. Nugent, Cocaine: comparative effect ondopamine uptake in extrapyramidal and limbic systems, Biochem.

Ž .Pharmacol. 32 1983 744–746.w x20 A. Hitri, R.J. Wyatt, Regional differences in rat brain dopamine

transporter binding: function of time after chronic administration,Ž .Clin. Neuropharmacol. 16 1993 526–539.

w x21 A. Hitri, K.Y. Little, E.H. Ellinwood Jr., Effect of cocaine ondopamine transporter receptors depends on routes of chronic cocaine

Ž .administration, Neuropsychopharmacology 14 1996 205–210.w x22 T. Hokfelt, K. Fuxe, O. Johansson, Ljungdahl, Pharmaco-¨

histochemical evidence of the existence of dopamine nerve terminalsŽ .in the limbic cortex, Eur. J. Pharmacol. 25 1974 108–112.

w x23 S. Izenwasser, L.L. Werling, B.M. Cox, Comparison of the effectsof cocaine and other inhibitors of dopamine uptake in rat striatum,nucleus accumbens, olfactory tubercle, and medial prefrontal cortex,

Ž .Brain Res. 520 1990 303–309.w x24 H.P. Jedema, B. Moghaddam, Glutamatergic control of dopamine

release during stress in the rat prefrontal cortex, J. Neurochem. 63Ž .1994 785–788.

w x25 P.W. Kalivas, P. Duffy, The effect of acute and daily cocainetreatment on extracellular dopamine in the nucleus accumbens,

Ž .Synapse 5 1990 48–58.w x26 P.W. Kalivas, B.A. Sorg, Animal models of psychosis reveal in-

volvement of hippocampal-corticostriatal-mesencephalic circuitry, in:Ž .R.J. Beninger, T. Palomo, T. Archer Eds. , Dopamine Disease

States, CYM, Madrid, 1996, pp. 463–475.w x27 J.E. Kilty, D. Lorang, S.G. Amara, Cloning and expression of a

Ž .cocaine-sensitive rat dopamine transporter, Science 254 1991 578–579.

w x28 J.E. Krettek, J.L. Price, The cortical projections of the mediodorsalnucleus and adjacent thalamic nuclei in the rat, J. Comp. Neurol.

Ž .171 1977 157–192.w x29 S.R. Long, An in vivo voltammetric characterization of the uptake of

catecholamines in the medial prefrontal cortex and striatum of therat. PhD Thesis, Washington State University, 1997.

w x30 J.M. Masserano, D. Venable, R.J. Wyatt, Effects of chronic cocainew xadministration on 3H dopamine uptake in the nucleus accumbens,

striatum and frontal cortex of rats, J. Pharmacol. Exp. Ther. 270Ž .1994 133–141.

w x31 S.M. Meiergerd, J.O. Schenk, Measurement of the time-resolvedkinetics of biogenic amine release and transporter activity by rotat-ing disk electrode voltammetry in vitro, in: A. Boulton, G. Baker,

Ž .R.N. Adams Eds. , Neuromethods, vol. 27: Voltammetric Methodsin Brain Systems, Humana, NJ, 1995, pp. 305–337.

w x32 G. Paxinos, C. Watson, The Rat Brain in Stereotaxic Coordinates,Plenum, New York, NY, 1986.

w x33 H.O. Pettit, H.-T. Pan, L.H. Parsons, J.B. Justice Jr., Extracellularconcentrations of cocaine and dopamine are enhanced during chronic

Ž .cocaine administration, J. Neurochem. 55 1990 798–804.w x34 R.M. Post, S.R.B. Weiss, Sensitization and kindling: implications

for the evolution of psychiatric symptomatology, in: P.W. Kalivas,Ž .C.D. Barnes Eds. , Sensitization in the Nervous System, Telford,

NJ, 1988, pp. 257–292.w x35 S.L. Povlock, J.O. Schenk, Effects of withdrawal from repeated

( )S.M. Meiergerd et al.rBrain Research 773 1997 203–207 207

treatment with cocaine on dopamine transporter in the n. accumbens:transport kinetics and relationship between binding sites for cocaine

Ž .and mazindol, Soc. Neurosci. Abstr. 21 1995 375.w x36 S.L. Povlock, J.O. Schenk, A multisubstrate kinetic mechanism of

dopamine transport in the nucleus accumbens and its inhibition byŽ .cocaine, J. Neurochem. in press .

w x37 S.R. Sesack, A.Y. Deutch, R.H. Roth, B.S. Bunney, Topographicalorganization of the efferent projections of the medial prefrontalcortex in rat: an anterograde tract-tracing study with Phaseolus

Ž .Õulgaris leucoagglutinin, J. Comp. Neurol. 290 1989 213–242.w x38 S. Shimada, S. Kitayama, C.-L. Lin, A. Patel, E. Nanthakumar, P.

Gregor, M. Kuhar, G. Uhl, Cloning and expression of a cocaine-sen-sitive dopamine transporter complementary DNA, Science 254Ž .1991 576–578.

w x39 B.A. Sorg, P.W. Kalivas, Effects of cocaine and footshock stress on

extracellular dopamine levels in the ventral striatum, Brain Res. 559Ž .1991 29–36.

w x40 B.A. Sorg, P.W. Kalivas, Effects of cocaine and footshock stress onextracellular dopamine levels in the medial prefrontal cortex, Neuro-

Ž .science 5 1993 695–703.w x41 B.A. Sorg, D.L. Davidson, P.W. Kalivas, B.M. Prasad, Repeated

daily cocaine alters subsequent cocaine-induced increase of extracel-lular dopamine in the medial prefrontal cortex, J. Pharmacol. Exp.

Ž .Ther. 281 1997 54–61.w x42 A.M. Thierry, J.P. Tassin, G. Blanc, J. Glowinski, Selective activa-

Ž .tion of the mesocortical DA system by stress, Nature 263 1976242–244.

w x43 R.A. Vaughan, V.L. Brown, M.T. McCoy, M.J. Kuhar, Species- andbrain region-specific dopamine transporters: immunological and

Ž .glycosylation characteristics, J. Neurochem. 66 1996 2146–2152.