Baclofen: 10 years on

400 metaholited to GABA or acted directly as a GABA-mimetic was of secondaty importance. /3-phenyl-y-aminoburyric acid was the original product of their lahours and because this substance proved capable of inhibiting spinal reflexes after oral adminis~ati~ other structurally related compoundswere synthesized. More active substances were obtained when substituents were introduced in the pheny: ;lucleusand lisl& by the observation of Kmjevic and cotta~~t~~~ that it mimiek~ synaptic in particular when the substituent was a halogen in the para position. Thus, & inhibition in the cerebral torte%. Evidence p-chloropbenyl GAB A (baclofen) emerged for a similar role in other areas followed. as a possible GABA-mimetic which could E’&hemical studies performed by numernus groups, before and after this. strongly be administeredorally. Administration of this substance in supported a synaptic function. Funher evidence was obtained with the demonstration mammals reduces skeletal muscle tone that the alkaloid bic~~l~ ~tagoni~ (l-5 mg kg’ p-o.) and inhibits spinal both the action of GABA and synaptic mono- and m&synaptic reflex activity Inhibition which was believed to be medi- (0.14 mg kg-’ i.v.) although ntonosynap ated by this amino acid’. The action of tic transmissionis more readily blocked. It GABA results in a selective increase in ailso reduces rigidity and the tonic stretch reflex in decerebrate animals at doseslower membrane conductanceto Cl The importance of GABA prompted than I mg kg’ i.v. In addition, when bacBeins and Keberle and FaigleS to design an iofen is applied by microiont~ho~tic techniquesit inhibits neuronal activity both analogue which unlike GABA itself co&d within the mammalian spinal cord and at readily gain access to the brain. By t&ning the atomic groups of functional impor- higher centres. Yet, &spite this similarity tance whilst inkling a Iipophilic sub. with GABA, the possibility that it acts at stituent it was hoped that the substance the same bicucullinesensitive receptors is doubtful at least in moderate doses. The would penetrate the brain and act at GABA effect of baclofen is receptors. Whether the substance was ~uron~-~~s~nt

IBaclofenr10 mars on Serendipity associated with astute clinical ob~rvation has played a major role in the discovLTyof centrally active dmgs such as the otiginal antipsych&cs and antidepoS ants. This is perhaps not sutprisiig when ooe considers &at only relatively recently has there been much information available about possible transmitter systems of the brain. With the possible exception of the neurotransmitter geneni anaeslhcliCS. mech454n.5 appear to form the focal point ofattack by drugs. But even with adequate *n&edge of a transmitter 5ystem one wonders if some drugs would have ever h!8ldL?SipdW&hlkilJUlO%llaaionilI

mind. W~~~~~~n~i~ witbtheknowfedgewenowhaveaboutthe enzkaphim and enkephalin5? Amon.g fk neurotransmirterr of the ail brain there is no doubt that the amino acid. y-aminobutyric acid (GABA) is impnt%mtin inhibitmy mechanisms~. A syn;rpic role for GABA wac firmly estab.

Itihibition of gangl~ dcprlmti

binding

evoked faH]nor.

nl6l”se va.5

tms

adlKnaline

defemis

ctualet

rekay: in

!wifch

(msHCl+

Ca’*

hindiing

* isopvacmej

atria I EDm

= il.Spwj

t ffGs--0.3pMj

I

1

I

oc5e-l.Zpu)

(I&-3.1

p%j

(!cmyO.mpMj

1 (r~=o.~~Mj

s4

NA

I

1.15

0.62

I

NA

NA

0.93

1.10

0.62

0.22

NA

NA

co.0 I

co.03

OLWl

WI01

N.4

NA

0.12

0.50

0.W

U.O.5

NA

NA

0.03

0.50

O.MM

0.002

%A

NA

0.20

0.50

0.05

0.03

NA

NA

NA

NA

0.W

N.4

‘j.30

0.W

u.03

0.06

007

0.03

0.25

0.09

-pO.o1

0.017

0.W

co.01

om3 NA NA 0.10 O.CHM 0.013

NA NA

I‘IP.5. -October

IV82

-WI

not antagonizedby bicuculline in dosessuflicient IO block the action of GABA. Only at very high-dose levels of baclofen is there any suggestion of a bicucullinescnsitive depressant action’. In many other isolated invertebrate and vertebrate systems including the crayfish stretch receptor, lobster muscle ftbre and mammtalian sympathetic and dorsal root ganglia, in which GABAreceptor activation produces an increase in membrane conductance to CI. baclofen clearly does not mimic the action of GABA. In common with other neurotransmitter systems. the advent of binding :echniques using radiolabelled ligands has facilitated the study of the GABA recognition sites(s)

within the mammalian brain”. GAB.4 analogues which are known to mimic the inhibitory action of GABA in Intact WC ternswill displace (“HIGABA from binding siteson brain synaptic membranes (Table I - left hand columns). In line with this, numerousstudieshave shown that baclofen does not displace [‘HJGABA from membranes incubated in Nab-free triscitrate buffer. the so!ution routinely used for GABA receptor assay. Thus. the effects of baclofen could not be attributed to a GABA-like effect. Even so. the compound was introduced into medicine in the 1970s for the treatment of spasticity of spinal and cerebral origin. At the time little was known about its mechan

In the searchfor the me2hanihmof ;Ictlon of ha&fen a variety of trrn5mittrr s~‘,remn have been implicated. Its phenylethykrminc moiety has prompted tts suggestion Ihat adrenergic mechanisms may be inko1ve.j but evidence to supportthis is lacking. Bn_ lofen increasesbrain levels of .%HT and ins metabolite S-hydroxyindoleacetic acid as weil as thal of dopamine ?‘!.uh. a link unh their role in neural n~hanismz has &en suggested although an anti-dopamme action has also been proposedbecausehatlolen can potentiate neuroleptic-InducrJ catalepsy. In fact. a beneticial effect m xhizophrenia has been reponed but with out confirmation by other groups. In the fmg spinal cord ha&fen reduces acetylcholine-mediated tran\rnlssmn hut II seems unlikely that this could account for its mechanism of action due to the wlativr imensitivrty. Jr. intriguing possibility-for the role of haclofen has been supportedbl Otsuka and his group!‘. They have shoun that low cot+ centrdtions

of baclofen

can

inhilw

the

depolanzing xtion of substanceP m \pmal moroneuronesoi the newborn tat. l_‘,~fonunatrly. this has not been >ubstantiar,sdb? others in different brain regions or uhing peripheral tissue. In addition. the sfrcific binding uf t”H]substance P to rat bram synaptosomalmembranes 15unaffrcid h? haclofen”‘. Thus. an! interfrrence I\ lth the action of subhtanceP is unlikeI\ to resu!t from an inimction uuh the substance P recognition site assuming that [ W]~uh stance P labels all suhstanlYP rctzept~)n. It i* gene&l\ agreed that the mn\r prt+ able site of a&n a ithin the ginal

for haclofen. cenainl) cord. is on pre>>nanttc

terminals. DAvidoff and SPXS” ht pOSti tit+Wh&5i

o elec. stimulation : 2,Hz 0.5 msecs. 1OV for A

n

10 ‘GABA l0~~1Baclofen I

;

1

mm

313sets before and during sttmulation

A

I

Time (hours)

i

I

iii&h~

i&i~X

!!W

pm. It4ClW

of excitafon neurotr~nsmitur from Itrmlnals in thtts&al cord. SVKXZ then nur~~en~~ audtot-z,have supported this vieu using biochemic:das well as clecrro,ph~s~ol~~g~t ai techniqws. An actlon a~ non-chohnsrgIc. possibl! dnin+acid releastng. tsmuna\c 1, faboured by most. If this is so then uhat ia the nature t>fthe site on the nene tem:inala at \\ hich bacl+ fen acts’! Of course. one poss~h~l~~~ ih 1hat II acts at sites normall! ,tuaulatzd h> G.\BX uhich are responsible for mediating presynaptic inhibition. Houe\rr. Da\ &ii andSemi’ con&&d that ‘hacloien Joe> not function by simulatmp the &ecIs of GABA’ at such synapses. In fact. CUIW t’t d. I* su_gge ,t that It ‘might reduce the r&raw of GAB.4 at depolarizing gabaergic a~% axonic s) nspses upon 1a affr*xnt termins [ions

402 ed’” i5 the existence of a pofnilation of CABA ~ceptors in the mammai~an p3@teral and central nervous qstems ~hiih are phafmacok$cally distinct from the ‘classical’ bicuculline-sensitive siles associatedwith Cl- chazmels. These note1 GABAa sites, as we have designatedthem” lo contrast with bi~ulii~ ensit% GABA& sites. appe;u lo be located on pre-

synaptic terminals at kziit in the pcriphety. They do not seem to be linked to Cl channels but pnssibly to Cai’ channelsl”” and are activated by relatively few GABA analogue~ (Table 1 - right band columns). Even ml&mnl is only weakpi active whereas it is aiways more actlve than GABA at GABAn sites. Comercal 1. baclofen is stereospccif~ally aclive at GABAs sites. T~I: (-) isomer is as potent a. GABA .md it is this isomer which isresporlsible for

the rruronal deprcelsant activity of brzlofen within the CM. Activa& of GABAH site* reduces the evoked release of neum namm&er from sympatheticand, 111 a lesser exrcnl, parasympathetic nerve terminals. This inhibitory action is illuslrarcd in ‘Fig. I in whtch GABA and baclofen are &own to dccreav the eftlux of ~3HJnoritclrenaline

from rat-Mated atria evoked by trammural stimulation. Neither r:he action 01’GABA nor that of bacJofeo was prevented by hicuculhne metbobromidc. Maximal activation of CABAn sites &es not abolish the release of trancmmer Only a pattial (up to

ihe sannzble binding of !‘:H)haclofen is increased to ~~oxirnatcl~ 25% of totai bindtng equivalent to that obtained in KrebsHcnseleit solution. The optimal concentration of CaCIz is between 2 and 5 InM equivalent to the extracellular 50% i reduction occurs. We have &served physiological Ca*’ concentration. Above a similar reductionof K--evokedn:leaseof catecholamines and 5-HT from brain this. saturirble binding decreases. The addidices”. T&is effect could a!xl be attributed tion of 2.5 m&t Cat3 to tricHCl also to an activation of GABAe ~itcs. but increases rotal [“H)GABA binding to membranc fractions. This increase is ~Juesolely whetherthis site can be Jinked with the eleclo a caturablc component which can only be trnph?rsiologicaland therapeutic effects of displaced by GABAe site ligands such as baclqfen remains to be seen. Nevertheless. an zs%iation with GABA is clear and baclofen. Neither this extra portion of bound pM)GABA nor the the saturable porwould si#ppM tl!P ori@lal idea= i&at bat* tion of [‘:H]baclofen binding is influenced lofen is a GABA-mimetic. Fur&r evi. derice for this sterns from experiments sigfli~c~tly by ~~~ulli~, In the presence showing that GABAa sites can be readily of the GABA analogue isoguvacine which V by radiolabelled Jim bind- is ac!ive at GABAe sites, tite Ca”ing te&niques using [“H)baclofen or Lependent ponion of [3H]GABA binding r.cmains. This has formed the basis of an (‘H~ABA~~. Under approprinte CO&Iion5 tise tuc3 ligands bind tS?the same ~ysay system for GABAe sites using site. To demonstrate t&s ir is cm&l that f ‘H]GABA in concert witlt parallel assays t.:FingL’Hfbacktfen. divalent cationsCat. or big=- arc psent at p+&&iJ concentrations in the incuba Gross re@onal distribution studies have tinn medium. Thus, ir; tt%cirratc buffer. .odic;rted thz presence of GABAs sites Ihe ~illm IlCKmally used I0 detect iatur‘+itJti~the spinal cord and higher centres”‘. aMe (*~~IGABA binding. ‘neither. in pa&ular, the cerebellum appears to be ~~GABA nor [“H)baclofen exhibits any 41 endowed altJrou@ the number of ~~GA~~~~~t~~~sy~ GAB& sites in pellets prepared from the r&cS=l&amS * YresumabJy any ;avaiJabJe u&&z cerebellum aunt the GABAs diva&t ion in the mentbranc p&t would hae populatii. The results of recent ame bc &&ted by the pre%.%ceof citrate. In radiographiiaJ experiments performed with tri&CJ a &I sammble fraction ,of bound Graham Wilkin, David Will and Alan PHjbaclofen fapprox. .T’%,of the tutal) can ‘Hudson have. however, sltown that ~~andonadding2.5m~CaCh GABAn sites are confined tn the-molecular

layer of the cerebeEIum uhereas GABAn sites predominate in the granule cell IayeP. Quantitative comparison between GABAa and GABA* sites is therefore not really appropriate since they appear to have different locations. Whether the location of GI~BAAsites accords with proposals for the sire of action of baclofen. as determined by o&r techniques, remains to be seen. Another crucial question which has yet to be answered concerns the possibility of an endogenous input. Are GAB& sites innervated? Is baclofen mimicking endogen ously releaserl GABA? If GABAs sites are located presynaptically in, for example. the cerebellum, thendirect inne~ationmay not be feasible since there is no morphological evidence for axtFaxonic synapses on terminal boutom in this region. Possibly, GABA released from recogniwl rerminals might ‘washovei on to the terminal receptors. Alternatively, GAB.46 sites may be located on conventions post-synaptic elements. Hopcfuliy. future experiments will answer thL problem. Tl& abundance of GABAn sites at supraspinal levels would suggest that if the action of haclofen is mediated via this receptortben it should not be restrictedto a spinal locus of action. This is certainly the case. Numerous effects uf baclofen can be attributed to a supraspinal action. These include hypertension, a decrease in growth horr:Ione secretion. an increase in plasma prolactin and antinociception although this latl +r effect bas also been connected with a

spinal locus of ac*‘l,u. It ha\ hccn pwul;~~ed

I XI of

that the analgesic from GABAII the analgesia GAB.4

hach>tcn rcsuhs

site ar&ation”. afforded

Moretner.

by musing brain xxi

levels with aminrwxyacetic

rodents is not prevented suggesting

an

in

hy hicuculline.

invoiivenient

of a ?,ite. Cutting and

bicuculline-insensitive

Jordan” have proposed that in line u ith a presynaptic locus of action the analgesia produced

by haclofen t~sults from a reduc-

tion in transmitter

release fmm

affcrent

neurnnes in the spinal cord. This mechaw ism may be similar to that proposed k; morphine Unlike

analgesia morphine,

at the spinal however.

level.

haclofcn

analgesia is not prevented by nrJoxone. To speculate further. this cotnparison with morphine might be related IO the effective use of Ihaclofen in the treatment of the morphine abstinence syndrome. During chronic treatment with morphine it could be assumed that a cessation in sensory trans mit!er release will occur due to the pm longed action on presynaptic terminals. This may lead to post-synaptic supersensitivity.

On aithdra\nal

of the morphine.

the normal release of transmitter resumes but can now cause excessive post-synaptic stimulation due to the change in receptor sensittvity. Abstinence symptoms result from this excessive acttvation. If bvclofen reduces transmiticr release from the hame nerve

terminals

as morphine

then

this

would be expected to decrease the PSIsynaptic activity. This hypothesis is rep resented diagrammatically in Fig. 2. It might bc expected that withdraual

from

chronic baclofen treatment could itself produce an abstinence syndrome. Houever. this seems unlikely since haclofen only produces a partial reduction in trans mitter release” unlike morphine which can abolish it. Thus. baclofen would not pruduce

any

receptor

superwnsitivity

therefore no rebound phenomenon occur on H ithdra% al. In partial support

of

and would

this hypothesis

recent tentative autoradiographic ohserwtions obtained in collah>ration u ith Graham Wilkin, Imperial C~ollege, London. U.K.

indicate thal GAB.4e

sites are con-

lined IO 3 limited region of the dorsal horn of the rat spinal cord. The possibilit)

that

thew hmding sites are on the same temlinals as opiate receptors is an rxcitsn_c pro+ pet’. In conclusion, the genenl consensus ia that the major site of action of ha&fen is on, presynaptic terminals where it depres.ses the release

of

excitatory.

probably

non-

chnlinergic. transmitter. The long-kindlco belief that baclofen is a GABA-mimetic may well be true. However. it still remaim to be seen whether the therapeutic actions

Gravity and Ming organisms ih witro August0 Cogoli and Alexander Tschopp