The 5-HT2 receptor antagonist, MDL 28,133A, disrupts the serotonergic-dopaminergic interaction mediating the neurochemical effects of 3,4-methylenedioxymethamphetamine

Tz receptor antagonist, 28,133 serotonergic-dopaminergic interaction mediating t effects of 3,4-methylenedioxymethamphetamine

ica

Christopher J. Schmidt, Christine K. Black, Vicki L. Taylor, Gina M. Fadayel, Teresa M. Humphrcys, Thaddeus R. Nieduzak and Stephen M. Sorensen Ahriorr hfuwil

Ilw

f
hs~irrtrt: Ciruimufi,

OH. USA

Rcccivcd 20 March IYY2. revised MS received IO June lYY2. acccptcd 2.1 June 1002

The sclcctivc S-HT, rcccptor antagonist MDL 28,133A dose dcpcndcntly blocked the long-term deficits in rat hrain S-HT concentrations produced hy the suhstitutcd amphclaminc analoguc 3,4-mcthylcncdioxymclhamphctaminc (MDMA). This protcctivc cffcct of MDL 28.133A could hc aholishcd by coadministration of the dopaminc precursor, L-dihydroxyphcnylalaninc (L-DOPA). slowing

ElcclrophysicllogicaI of A0

interaction

dopamincrgic

of MDL

MDMA-induced

28.133A

of dopaminc

scrotoncrgic

hypothesis that S-HT,

synthesis

of MDL

Consistent

synthesis

on MDMA-induced

rcccptors m-c pcrmissivc

of terminal

for the stimulation

to block the MDMA-induced

Both sets of cxpcriments suggest an with this explanation, MDL 28,133A antagonized the

S-HT rclcasc did not rcducc the firing inhibition

with rr-methyl-p-lyrosinc

dopaminc synthesis

by an incrcasc in dapaminc cell firing

cffcct on the sensitivity

28,133A

administration.

of dopaminc synthesis

The

This

is not due simply to the removal

and autorcccptor

dopaminc autorcccptors.

rate of

(wMPT).

activation. MDL

results

;1rc consistent

2X.133A with the

necessary to support MDMA-induced

cfflux. MDMA

(3,4-mcthylcncdioxymcthamphct;lminc):

AmphcWninc;

1. Introduction

mine (MDMA),

to laboratory animals results

lcctivc and pcrsistcnt dccrcasc in central (S-hydroxytryptaminc, S-HT) concentrations

in a sescrotonin (Schmidt

et al., 10x6; Stone et al., lY86). Other neurochcmical markers of scrotoncrgic function such as S-HT uptake and tryptophan hydroxylase activity show corrcsponding reductions (Schmidt, 1087). The loss of 5-HT uptake sites as measured by ligand binding (Battaglia ct al., 1987) and the associated argyrophilia (Commins cl al., lY87) support the suggestion that these long-term ncurochcmical

changes

rcflcct

the

scrotoncrgic nerve terminals. A number of the investigations rcsponsiblc for this loss of S-HT

(‘orrcspondcucc

S-HT,

rcccptors; Dopaminc neurons

focusscd on the role of dopaminc as a key mediator of

Administration of a single high dose of the amphctaminc analoguc, 3,4-mcthylcncdioxymcthamphcta-

Institulc.

the ability

in vivo. MDMA-induced

rcccptor antagoms!f

input followed

was also shown to bc without transmitter

that

to L-DOPA

as asscsscd by dopaminc: dcplction following

indicates thut the cffcct of S-HT, of an inhibitory

dcmonstratcd

was also scnsitivo

at the level of dopaminc synthesis.

stimulation

dopamincrgic ncurnns

cxpcrimcnls neurons

to: C.J. Schmidl.

21 IO E. Ci;llhr;lith

Rod,

degcncration

of

into the mechanism ncrvc terminals

M;wion

Mcrrcll

C’inciunuti.

Ott

IIow

have

Rcscarch

45215. USA.

this process. MDMA has been shown to release dopaminc as well as S-HT through the same Ca’+-indcpcndcnt, carrier-mcdialcd process characterized for amphetamine (Schmidt et al., IYt(7; Hekmatpanah and Pcroutka, 1990). Intcrfcrcncc with this dopaminc cfflux by blockade of the dopaminc uptake carrier, chemical lesioning of the mcscncephalic dopamine systems or inhibition of dopaminc synthesis all prcvcnt the scrotoncrgic dcgcncration produced by MDMA (Stone ct al., 108X; Schmidt et al., 1YYOa). S-HT release was also implicated in this process by the observation that coadministration of sclcctivc S-HT, receptor antagonists would prcvcnt MDMA-induced scrotoncrgic deficits (Nash ct al., 1900; Schmidt and Kchnc, 1990; Schmidt ct al., 1990a.l~). Further invcsligation indicated that the protective effect of the S-HT, rcccptor antagonists arc also ultimately mcdiatcd by the dopaminc system. Interactions bctwccn these two monoaminergic ncurotransmitter systems have been dcscribcd in numerous studies. In gcncral, thcsc studits suggest that the scrotoncrgic system cxcrts an in-

Yissue ~~~n~~~tr~~ti~~nsof S-HT, DOPA and dopaminc were determined by high performance liquid &l~r~~~~at~~grilphy with el~~Fr~~b~~rni~aldetectj~~n- Samples were h~~rn~~geuj~ed in f1.J M rno~o~hi~~r~~~eti~acid containing 0.1 mM EDTA at pH 3.0. After cenFrifugation at 3°C (.15 min. 3OOflOXX) the samples were iujeeted onto a 250 x 4 mm, 5 pm Lichro~phe~ 100 RP-IX cartridge column (EM Separations, ~ibb~t~wn, NJ) and eluted with a mobife phase containing (mM): NaH -7PO, JStf,EDTA 0.10 and PIC-J37 5 mM tW~t~rs, Milford, MA) at pH Xtl. The methanol concentration was varied from f to 5% ta modify s~p~~nti~~~. A Couloehem SlOOA electrochemical detector (ESA Inc., Bedford, MA) set to a potential of -t-O.4 V was used for dctcction. ~~?rnp~~n~nts were quantified by comparison to a curve generated frcrm externat ~tnndards. S-HT ~~~n~ntrations in the hipp~~carn~us and crtrtcx were corrected for re~~~ve~ by ~rnpar~so~ to an internal standard (N-acetyfserotoninL Recovery in the striafaf samples was greater than 90% and did not require correction. Neurochemical results were analyzed far statistical s~g~if~~~~~~e using the Mi~rostat-~~ program (Eeosoft, Inc, indianapolk, IN). Pallowing an ANOVA, differenc~s between groups were dcFermiued using: the least significant difference test with a lcvcl of P < 0.05 being accepted as signifi~anF.

MaJc Sp~agu~-~~~~w~ey rats ~~~~~-~~~~ gl were mainFailed on a JZ-h light and dark cycle and aJfowrzdfree access to food and water. ttFng-term neur~~hem~~a~ studies were c~?ad~~tedonly after animals had been on site for 1 W&L The timing of individ~a~ sxperiments, ~~~~t~sof drug adrninist~~~t~~?n and doses are provided in the Results section. At the termination of each experiment the animals were killed by decapitation and their brains were rapidly dkscctcd on ice. Samples of cortex. bipp~~ca~lp~sand ~FriaFumwcrc removed and iately frozen on dry ice. Tissue samples were sF~?r~~aF - 7WC until asliaycd,

Rats were au~sFh~ti~ed with chloral b~drat~ t4flfJ mg/kg, i.p.1 and then secured in a stercotaxic frame
through a high impedance (200 Hz-4

amplifier,

kHz and monitored

band pass filtered

corkx

with a digitizing oscillo-

scope (Tektronix. Inc., Beaverton, OR). The firing rate was recorded, analyzed, and stored utilizing the data acquisition program, Brainwave Discovery (Brainwavc Systems, Broomfield, CO). After observing and recording a baseline firing rate for at least 10 min, drugs were administered as described in the Results. Data from individual neurons were normalized to basal firing rates so that the averages could be expressed as a percentage

of the control

firing

rate.

Statistical evaluations were then made using Student’s t-test with paired observations. The level of significancc was set at P < 0.05.

Hippocampus

Drugs for all studic:, were administered the salt with

the exception

of MDMA,

in saline as cu-mcthyl-p-

tyrosine ((u-MPT) and L-DOPA which were administered as the free b;se. Saline was used as the vehicle control. MDL 28,133A rcquircd sonication for l-2 min to insure solubility. MDMA-HCI was provided by the National Institute on Drug Abuse MDL 28,133A tl(4_::uorophcny!?~2_[4114_ .L, . mrth:lnc~;lrlfcln:lmido-Dhcn?ll). .._ . .._._-.._ carbonyll- I -piperidinyl]-cthanonc hydrochloride) was synthesized at the Marion

Merrell

Sfi-idtum

Dow Research Insti-

tute (Cincinnati, OH). Apomorphine, L-DOPA methyl cstcr, NSD 1015. (Y-MPT methyl cstcr wcrc purchased from Sigma Chemical Co. (St. Lor:i.,, MO) and halopcridol was purchased from Research Biochcmicals (Natrick, MA). Carbidopa was the gcncrous gift of Merck, Sharp and Dohmc Research Laboratories

f Rahway, NJ).

3. Results

3. I. Rc~r~crsalof‘ MDMA-induced

5-HT

deficits

by MDL

28,133A

Fig. I. Antagonism S-IIT

2X.133A.

MDMA

simultaneously

Figure

1 shows the effect of MDL

28.133A

on the

MDMA-induced reduction in forebrain S-HT concentrations I week after drug administration. The .?(I mg/kg S.C. dose of I’V1DMA rcduccd S-HT in the cortex

ol’ MDMA-induced

conwntr;llions

prcxhuxd

hy

Ihc

(30 me/kg)

5-I{T,

fordwin

;Int;lgonisi.

MDL

and the ;mlagoniht were’ dminislrrtd

hy the B.C. route

significant dcplctions

dcl’icils in rcgiwul

sclcctivc

I wtxk

in S-IIT

rcgicms (P -C 0.01). All values rcprescnl 0.05. il.01. rcsptxtivcly.

prior

IO killing.

concentrations

ihr mwn t S.E.M.

comparrd

MDMA

in dl rhrcc t*.*

* P -.

10 MDMA).

and hippocampus by greater than tWX with a characteristically smaller effect in the striatum (approximately 40% reduction).

Coadministration

tor antagonist, MDL 5-HT concentrations

of the S-HTI

rcccp-

28,lIUA reversed the reduction in measured in all three regions of

the forebrain. Significant antagonism of the MDMA effect was observed with doses of MDL 28,1.73A as low as 0.1 mg/kg and a complctc reversal of the MDMA cffcct was obscrvcd in all regions.

The cffcct of MDMA

(30 mg/kg

s.c.1 on cortical,

hippocampal and striatal S-HT concentrations 1 week after drug administration is shown again in fig. 2. As already

dcmonstratcd.

coadministration

of

MDL

28,133A (I mg/kg s.c.) with MDMA significantly attenuated the transmitter deficits (P < 0.05). although a complctc rcvcrsal of MDMA’s cffcct was not O~SCIVC~

mg/kg with

was sclccted for thcsc studies to be consistent

the high doses

cal studies. This dramatic MDL

used in the long-term neurochemidose of MDMA produced a rapid and

slowing of the cell firing rate. The effect of

2X,I33A

prior to MDMA the antagonist

(I

mg/kg

i.v.) administration

20 min

is shown in panel B. The prcscncc of completely prevented the effect of

MDMA on cell firing but did not block the effect of the direct dopaniine reccptur antagonist, apomorphine (25 pg/kg.

The c’ffuct of MDL 3% l33A on the reduction in A9 cell firing rates following MDMA administration was examined as an additional endpoint for MDMA-induced dopaminr release. Figure 3 contains firing rate hist~~gr~~nlsfor three A9 d~)p~~min~rgic neurons exposed to three different treatment rcgimcns. Basal firing rates wcrc routinely recorded for 20 min prior to the administration of any drugs. Panel A shows the cffcct of MDMA f IS mg,Jkp i.v.1 on A9 firing. A high dose of IS

i.v.1. In panel C, I,-DOPA

was eoadministercd

with the 5-HT,

30

prctruatmcnt

min

mg/kg

following

i.p.1. Administration

completely

(IO0 mg/kg

with

carbidopa

of the dopamine

restored the cffcct of MDMA

in the prescncc of the S-HT,

i.v.1

receptor antagonist (25

precursor

on cull firing

rcccptor antagonist.

Figure 4 summarizes the data from the electrophysiologioal experiments. The IS mg/kg dose of MDMA reduced A9 firing to 25% of the controf rate. This cffcct was significantly attcnuatcd in animals pretreated with MDL 2X,l33A fP < 0.01) such that rates dechncd to only 93% of the control values following MDMA administration. MDL 28,l.S.fA alone did produce a small clcvation in the basal firing rate although this did not reach the levei of statistical significance in thcsc experiments. precursor rcstorcd

Administration of the dopamine the ability of MDMA to slow cell

firing even in the prescncc of the S-HT, antagonist (P < 0.01). Intcrcstingly, L-DOPA

rcccptor itself did

not alter the basal firing rate of the cells nor did it affect the extent of slowing produced by MDMA.

The ability of L-DOPA administration to prevent the rcvcrsitl of MDMA’s neurochcmical and clcctrophysiological cffccts by MDL 2X,l33A suggests a requiri-mcnt for S-HT1 rcccptors in the activation of dopamino synthesis and relcasc. To cxaminc this qucslion more directly, MDMA-stinlul~lt~d

the effect pf MDL 28,133A on the ~i~~urnul~itil~n of DOPA ws detcr-

mined in rats following

dccarboxylasc

inhibition.

NSD

IO15 ( 100 mg/kg i.p.) was administered 30 min prior to killing. Administration of MDMA (20 mg/kg s.c.) 30 min prior to the NSD 1015 produced a small but significant increase in DOPA arcumulation as shown in fig. 5. Prctrcatmcnt with MDL 28,133A 311min prior to MDMA blctckcd this slimulati~~n with a U-shaped dose-response c:nvc. The results shown in fig. 5 are the mean results from two expcrimcnts in which csscntially identical results wcrc obscrvcd. The lowest dose of MDL 2&133A tcstcd, fO.OI mg/kg), completely prcvcntcd the MDMA-induced stimulation of dopaminc synthesis. This cffcct was lost at intcrmcdiatc doses of It.1 and 0.3 but was observed again at the highest dose

( I mg/kg)

of the ~~ntag~~nist.

5 Hz

Control

I

MDMA

B,i

J

Control

Apomorphine

MDMA

MDL 28,133A

c .dd

Y,,

Control

U.U.,D*.....~.*.

.,.I

a

>I

MDMA

L-DOPA

MDLT28,133A I%. 3. D;lta from individual expcrimcnts showing the effect af MD- . 28.133A with w wilhout L-DOPA OR the MDMA-induced slowing of 3 single AU dopamine neuron. Panel (A) is P strip chart recording of the firing rate of a single neuron treated with MDMA (IS mg/kg i.v.). Panel (B) illustrates the trring rate of J sin& ncurcm ~xpr~cf IO MDL 2H.13.7A (0.2 mg/kg i.v.) 20 min prior 10 MDMA. Panel (0 is a cell pretrc:rted with both hllDL 2X.lMA and L-DOPA (IO0 mg/kg i.v.1 before the MDMA challenge. Carhidqw ;Z5 mg/kg i.p.f uw it(iminisi~r~d .?I!min prior tct L-DOPA.

All values rcprcsem the mcuns+S.E.M,

3.5. MDMA-induced chattges in dopatttitte mttow the presence of cu-MPT

in

To ci.mine the role of MDMA-induced 5-HT release in th,, a:ontrol of dopamine turnover, the effect of MDMA on the rate of dopamine depletion following the administration of a-MPT was determined. Preliminary experiments indicated that a 2 h treatment period with MPT

(200 mg/kg

_.

_~~

i.p.) reduced striatal dopamine

Conlrol

--.-F~

_

..

a MDMA . .._ .~

-.

--

t

flu 60 40

As shown in fig. 6, pharaa-

.i

Jowl

I20 loo

by 4%50%.

cological treatments modifying the firing rate of the neurons can alter this rate of dopamine depletion. The increase in dopaminergic cell tiring produced by haloperidol (0.1 mg/kg SC.) rcsufts in a significant reduction in striataf dopamine c~?ncentrations compared to that observed with MPT alone (P < G.05). MDMA (20 mg/kg s.c.1 administered simultaneously with MPT and again at I h had no zffect on the rate of striatal dopamine depletion. The combination of MDL 2X,l33A and MDMA was similarly without

I

t

140

concentratinns

i

-*

i *

1

1 20 i \ I_

! Vehicle

MDL Zd33A

I.-tx-md

L-WIPA MDL &133A

Fig. 4. Mean data illustrating the cffcct of MDL ?X,l33A with (IT without L-DOPA on the MDMA-induced slowing of A0 dop;tmine neurons. Control bars rcprcscnt prutrcutmcnl with either no drug (vehicle). MDL %.l33A (II.2 mg/kg i.v.) or MDL 28.l.W with L-DOPA (100 mg/kg i.v.). The effect cif L-DOPA alone is also siii?iin. MDMA-!aheted bars reprcscnt trc;rtment with MDMA (IS m&kg i.v.) 20 min after the ~ppropri~Itc control c~ndiii~ln. Ail bars represent the means f S.E.M. uf between five and eight neurons as n perccntagc of their control. (** P < 0.01 compared to vchiclc control: ” P < 0.01 compared to MDMA alone.)

Sdinc

MVMA

0.01 ____

0.03

I,. I

-.

._---_-__--_+

pluvMDL

0.1

I .A

2X.13.1A

Fig. 5. Antagonism of MDMA-stimulated dopaminr synthesis in viva hy MDL 2X.1.731\ as ussessedby striatal DWA accumulation. MDL 2X.1.13A was udministcrcd S.C. 20 min prior to MDMA (?I) mg/kp, s,c.f. NSD 101.5f100 mg/kg i.p.1 was ~id~~ini~tcrcd.70 later min and AIt mluc~ rq~~nt thr all rats were killed 1 h affcr MIjtiA. me;ms+S.E.M. (” P < 0.05 compared to saline: ‘.” P < lW5. Wl. rcspcctivcly. compared to MAMA alone.)

prtxeded

NSD I015 (IUO mg/kg i.p.1 by 30, 10 and 5 min. rcspcctivcly. Killing was 30 min following NSD IOU administr~~tic~n.Although significant inhibition of DOPA accumulation was ohscrved only at the high dose of apomorphinc. MDL 28,133A did not appear to modify the cffcct of the agonist at either dose nor did the antagonist a!onc affect the accumulation of DOPA produced by GBL (table 1).

4. Discussion

In this study we have made a detai!zd examination of the cffccts of S-HT, rcccptor blockade on the ncurochcmical effects of MDMA. Thcsc cxpcrimcnts wcrc performed using the novel S-HT, receptor antagonist, MDL 28,133A (Carr ct al.. 1991). Consistent with our previous results (Schmidt and Kchne, I99Ob and those of Nash (1990), hlockadc of S-HT, rcccptors with MDL 28. I33A antagonized the long-term scrotoncrgic deficits produced by MDMA. MDL 28,133A dose dcpcndcntly rcverscd the depletions produced 1 week after 30 mg/kg of MDMA at doses as low as 0.1 mg/kg. The potency of this cffcct and the specificity of MDL 2X,133A in~I~tes such protcdion is mccliated by hlockadc of S-HT, rcccptors and not some other nonspecific cffcct of the drug. WC have also shown this effect of S-MT, receptor antagonists to be stereoselcctivc using the optical isumcrs of the S-HT2 antagonist, MDL 11,939, furthcl supporting this claim (Schmidt ct al., 19911. The postulated role of dopaminc rcleasc in the long-term ncur~~chcmical cffccts of MDMA and ihc ability of S-HT1 rcccptor ;intag~)nists to prcvcnt thcsc cffccts suggest that an interaction bctwccn S-HT, rcccptors and the dopamincrgic system may hc an important component of the long-term ncurochcmical cffccts of MDMA. The rcvcrsal of the protcctivc cffccts of MDL 2X.133A and other S-HT, rcccptor rcccptor antagonists by I.-DOPA (Schmidt et al., 1991) stlggests that this interaction with the d~)p~lmincrgic systrm may occur at the lcvcl of transmitter synthesis. Intcrfcrcncc with dopaminc synthesis is consistent with protection against the long-term cffccts of MDMA as MPT has hccn shown to bc protcctivc in this model (Stone ct al., 19X8; Schmidt ct al.. I99Oa). Howcvcr, since the role of dopaminc in the scrotoncrgic deficits produced by MDMA is itself hyp~)thctic~il, the cffcct of MDMA on the slowing of A9 d~~pamincrgic neurons was examined as an additional indicator of MDMA-induced dopaminc rclcasc. MDMA reduces the firing rate of A9 dopamincrgic neurons by a mechanism scnsitivc to synthesis inhibition with MPT (Kclland et al., 1989). This is similar to results observed with amphctaniine (Bunncy and Aghajanian, 1975) and ~uggcsts that dopaminc r&cased from

a

newfy

synthesized pool is responsibfe for this inThus, the effect of 5=HT, receptor antagonists in this system also suggests an alteration of drug-induced dopamine release. Several recent studies have reported that selective 5=HTz receptor antagonists can afso block ampheta~jne-induced stowing of dopami~~ cell firing rates fGofdstein et al., 198% Sorer&en et al., f9921. fn the AIO region, the effect of amphetamine on ceff firing could be prevented by pretreatment with either ~itaflser~n or MDL 28,133A and completely reinstated by the administration of L-DOPA (Sorensen et al., 1992). These results and those described here indicate the mechanism of action of the S-NT, receptor antagonists is similar in both mesnfimbic (Al01 and nigrostriatal (A9) dopaminergic pathways. Both the ~~urochemical and eicctrophysioi~~i~~f results are compatible with an effect of 5=HT, receptor antagonists on dopamine synthesis. Furthermore, this explanation is consistent with the reported ability of PCPA pretreatment or i.c.v, 5,7-d~hydro~t~ptamin~ to reduce the potency of MAMA for decreasing nigrostriatal cell firing rates ~Keffand et al., 1989). We have reported a simifar effect of 5=HT d~pfet~on with PCPA on the amphetamine-induced slowing of A10 dopamini-r&c ncurens (Sorensen et al., 1992). The remaining experiments described in this study further tested the validity of this hypothesis and considered aft~rnative explanations for observed effects of the S-NT, receptor antagonists. Nash et al. (19901 previously reported that the 5-HT, antagonist, ketanseriff~ can bf~k MDMA-stimuf~ted DQPA ~e~umuIation as well as the tong-term 5=HT depletions. As predicted, MDL 28,133A also antagonized the in viw increase in dopaminc synthesis occurring in response to MDMA, Although a W-shaped dose-response curve was observed, the results surest that over time, the net effect of the antagonist woulr! ‘le a reduction in the size of the newly synthesized ~GO$of dopami~e= Since MDMA releases transmitter from the newIy synthesized pool, an immediate conscqucnce of such an cffeet would be a reduction in MDMA-induced dopamine release. in support of this, microdialysis studies with k~~nseriu have shown that the ~~ntagonist dots partially reduce the refease of dopamine produced by MDMA in viva (Nash, 19902 The s~rot~~ncrg~c input to the midbrain do~ami~~r= gic systems is befieved to be largely inhjbito~ with respect to the firing rate (Fibiger and Miller, 1977). Blockade of this input by the S-HT, receptor antagonist rita~serin has been shown to increase the firing rate of both A9 and AfO neurons fUgedo et al., 19901. Tfte small stimufato~ effect of MDL 28,f33A ordered here is in agreement with these results. Aithou~h the effect of MDL 28,133A on basaf firing rates was smaff, the S-NT releasing action of MDMA (Johnson et af., hibitory effect.

1986 Schmidt et al, 19871 would significantly increase input to the midbrain and could therefyy increase the relative effect of a S-HTz receptor antagonist. A potential outcome of the ability of MDL 28,133~ to maintain a near nor-mat firing rate in the presence of Ml3MA is that the amount of dopamiue release occurring in the striatum would be greater than that pro= duced by MDMA alone. AIth~u~h this is inconsist: it with the effect of S-HT2 receptor antagonists on the long-term effect of MDMA, the resultant increase in striataf autoreceptor activation could produce the observed inhibition of M~MA=stimufated dopamine synthesis. TO determine if a 5=HTz receptor antagonist did increase MDMA-induced dopamine reiease in viva or if MDMA-induced 5=HT refease did directfy inhibit dopam~ne cefl firing, the rate of dopamine turnover was examined neurochemicalfy by measuring dopamine concentrations following MPT administration as described by And& et al. (1967, 1971). In the absence of’ synthesis_ the rate of dopamine depfetion is a function of the rate of neuronaf activity as demonstrated by the ability of hafoperiduf to accelerate the depletion of transmitter. Amphetamine-fake reteasing agents woufd be expected to have Little effect in this modef due to the lack of a newly synthesized and hence readify releasable pool of transmitter (Miller and Shore. 1982). However, in contrast to amphetamine, MDMA is a very potent releaser of 5-HT as well as dopamine (Johnson et al., 1986; Schmidt et al., 19871 hence any effect of 5=HT on dopamine turnover should be apparent. Yet thcrc was no indi~~tjon that the administration of even two doses of MDMA could modify the rate of dopamine depletion in the MPT model nor did MDL 28,133A product any further effect. These neurochemieaf results demonstrate that the changes observed in the efe~trophys~~~ogj~al experjments with MDMA or MDMA pfur MDL 28,133A do not occur in the presence of MPT, or afte~ati~efy~ in the absence of conrjnuous dopamine synthesis. Therefore. if MDMA-induced 5=HT release does modify the firing rate of dopaminergic neurons ir must do so through an initial effect an dopamine synth& with the suhsequcnt enhanccmcnt of transmitter rcfcasc. It follows that the S-HT, receptor antagonist effect On firing rate is secondary to an alteration in synthesis as weft. lt is wrw:h noting that tbc :thifity c~f MDMA to reverse the effect of h~f~pe~~d~~f OII do~~iine dcpietion jn the MPT model is similar to that observed for amphetamine by Miller and Short (iBX2). This suggests that fike amphetamjne, MDMA may cause some rcdistribution of the intrancuronaf poof of transmitter. HOW this may relate to the other effects t>f MAMA described here is not clear. The ability of 5=HT, receptor antagl~nists to negate the effects of MDMA or arnpb~t~lrnjn~ on dopamint: SerotOnergk

Pttxinos, G. and C. Watson. tY86. The Rat &in in Steretrtaxic Coordinates. 2nd cdn. (Academic Press. New York). Schmidt. Cd.. 19X7. Neuroloxicity of the psychedelic ;~mphetaminc. methytrnedioxymrthamphetamine, J. Pharmacot. Exp. Thrr. 240. I. Schmidt. C.J. and J.H. Kdme, 19ctO.Neurotoxicily of MDMA: Neurochemical dfects, in: The N~ur~~ph~rrnnc;)lo~y of Scratonin. Ann. N.Y. Acad. Sci. hO0, 615. Schmidt, C.J.. L. Wu and W. Lovenherg. tc)Nh. M~thyt~n~dioxymethomphet;tmine. a polcntidty neurotoxic ampheraminc anatogue. Eur. J. Pharmacot. 114. 175. Schmidt. C.J., J.A. Lrvin anJ W. Lovrnhcrg. IYX?, In vitro and in viva neurochcmical effects d mcthylen~dioxymelh;Imphetaminc on the striatal monoaminergic systems in the rd h&n. Biochem. Phatmacot. 36, 747. Schmidt. C.J., C.K. Black and V.L. Taylor. i~,YOa.Anragonism of thr n~urn~~lxici~y due lo a single ~l~rninis~r~li~~nof n~~lt~yt~n~dictxymrttl~mphctantine, Eur. J. Pharmacctl. IHI. 5% Schmidt, C.J.. G.M. Ahhate. C.K. Btrtck anal V.L. Taylor, tstY@h. Selective S-ttydroxytrypt;~mine_ receptor antagonists protect against Ihe neurotoxicily of n~ctt~ytcnedioxymethamphctamine in rots, J. Pharmacd. Exp. Ther. 2.55. 47X.

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