0022-3956/92 $5 00 + 0” 5 ,992 Pergamon Prex Lfd

J psychrat Res Vol 26, No 4. pp 449-460, 1992 Printed ,n Great Brmm

BUILDING

A BRIDGE BETWEEN MENTAL

Instttute

AND

ILLNESS

ERMINIO Ftdra-Georgetown

NEUROBIOLOGY

COSTA

for the Neurosctences, Georgetown DC 20007. U S A

Umverstty

Medtcal

School,

Washmgton,

Summary-GABA (y ammo butyrtc acid) 1s the most abundant and tmportant mhrbttory transmrtter m mammahan CNS It counterbalances the glutamate medrated neuronal excttatron Abnormahttes of the mteractron of these two transmitters mrght change the mechanisms of neuronal group selection that according to Edelman [NeurulDarwrrmm Baste Books, New York1 play a role m medratmg several bram functtons mcludmg cogmtron processes Indeed Imbalances m GABAergrc functions were shown to ehctt psychoses They can be obtained by admmtstratton of drugs that affect synthesis, metabohsm and uptake of GABA and thereby cause a perststent strmulatton of GABA, receptors or perhaps by genettc abnormahttes m DNA transcrrptron, pre-mRNA sphcmg, mRNA translation and posttranslatton modrfrcatrons of GABA, receptor subumts The complexmes m the regulatron of GABA, receptor subumt structure, syntheses, assembly and the brain location of spectftc mRNA encoding for these subumts are mvesttgated with m srtu mRMA hybrtdtzatton specific for subumts of GABA, receptors The role of the vartabrhty resulting from the complextttes m the regulatton of GABA, receptor allostertc modulatton by drugs and putative endogenous allosterrc modulators of GABA actton at GABA, receptors IS discussed Thus dtscusston gtves relevance to the posstbthty that genetrc abnormahttes in the expressron of protems parttcrpatmg m GABAergtc function are to be constdered as a possible target of the genettc defects operative m psychoses In hne with this thmkmg, tt 1s suggested that partral allostertc modulators (partial agonists) of GABA, receptors and the phosphothtoate or methylphosphonate analogs antrsense to specrftc mRNA ohgonucleottdes that mediate the expression of genettc mformatton concerning GABA, and glutamate receptor subumts may become valuable tools m psychtatrrc research Perhaps m the future these studies might generate new ideas useful m the therapy of genettcally determmed psychratrrc illness

Introductton ASSUMPTION underlying the vtews expressed m this paper 1s that an unknown abnormality m brain function is the basis of psychopathology; perhaps, this abnormality 1s expressed m the neocortex and causes rrregulartttes m the phystologrcal process of neuronal group selection that was proposed to be operative m cognitive function (Edelman, 1987). Smce the neuronal typology, mtrtcactes of neocortrcal lammar orgamzatton and the molecular processes that function as an anatomical substrate for the variable ctrcmtry associations operative m neuronal group selection have yet to be described, let alone understood, tt 1s not surprtsmg that we have dtfftculttes m relating either neocorttcal function to cogmtton physiology or neocorttcal malfunction to cognition pathology. In the absence of some understanding of this neocorttcal physrology rt 1s not easy to select experimental strategies leading to an elucrdatton of the molecular and anatomical nature of the neocortrcal dysfunctions that may have a crucial role m the etiology of psychoses. An important aspect of brain function IS its capability to be structurally and functronally THE

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plastic and thereby resilient to injury. Perhaps, psychopathology 1s nurtured by abnormahtres m the regulatory mechanisms of synaptrc vartabrhty that are available m the neocortex and are used by humans to cope cogmttvely wtth envrronmental changes This rmphes that various forms of psychopathology may relate etrologlcally to a dysfunctron of neuronal mechanisms that activate the transcription of genetic programs operatrve m the modulations of synaptic transmrsslon that underlie neocortrcal neuronal plastrcny A functional aspect of such plasticity operates m the theory of neural Darwmrsm proposed by Edelman (1987) to explain the functional orgamzatron of cortical actrvrty Numerous fast acting hgand-gated channels that are the receptors for ammo acid neurotransmrtters operate m the neocortex Among these are y-ammobutyrrc acid (GABA,) (Olsen & Tobm, 1990) or glutamate receptors (Hememann et al , 1991) which might be relevant m the neuronal group selection operatrve m neocortrcal function In these heteroohgomertc membrane proteins, the appropriate subunit assembly 1s obtained by selecting subunit combmatrons from a genetically determined menu of drverstty (Hememann et al , 1991, Olsen & Tobm, 1990). In psychopathology, genetrcal aberrations may exrst m the structure of one or more subunits causing abnormahttes m the GABA and glutamate structure and/or subunit stolchlometry Each of such aberrations or a combmatlon of them may lead to a disruption of harmony m neuronal plastrclty mechanisms important m regulating neuronal group selection m neocortex The fast acting receptors for GABA and glutamate are hgandgated channels that operate m about 60% of mammalian bram synapses mcludmg neocortex GABA, receptors are encoded by 15 genes (Olsen & Tobm, 1990) whrch express an equal number of receptor subunits that are assembled m heteroohgomerlc structures (probably pentamerrc) to form various subtypes of GABA-gated amomc channels Very little 1s known about the processes operative m subumt selection and assembly to form different GABA, receptor subtypes Current evidence indicates a role of NMDA selective glutamate receptor function m changmg GABA, receptor subunit expresston durmg dtfferenttatron of neurons m culture (Memo, Bovohn, Costa, & Grayson, 1991) Should thts mechanism also be operattve m mature brain the regulation of GABA, function might depend m some measure from signal transduction operated by NMDA selective glutamate receptors Also these glutamate receptors are heteroohgomerrc complexes mvolvmg a slgmftcant number of genes (Hememann et al , 1991) Smce GABA signaling at GABA, receptors causes neuronal actrvrty mhtbrtron while glutamate slgnalmg at NMDA selecttve glutamate receptors causes neuronal activity excrtatlon, one mrght envrsron that an eqmhbrrum between glutamaterglc and GABAergrc synaptic strength plays a role m the regulation of neocortrcal cogmtrve functton Present evidence suggests that since functional assoclatton and drsassocratlon of neuronal groups m neocortrcal layers may change wtth some relatron to GABA and glutamate synaptic strength, a bridge might be built between psychopathologrcal abnormahtres m neocortrcal function and the functional coordmatron of the genetrc programs encoding GABA and glutamate receptor subunits Neurochemistry

and Psychopathology

Under the influence of Kraepehn, there has evolved a two-ply categorrzatton of psychoses dementia precox (schlzophrema (SCH)) and the mamc-depressive illness (MDI). As a general

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451

gmdehne this classification has survived 100 years of controversy and still permeates psychiatric thinking. However, the efficacy of procedures used to treat SCH, MD1 and affective psychoses (AP) does not allow the making of specific recommendattons or exclusions with regard to the molecular mechanisms operative m the etiology of SCH, MD1 and AP. For instance, pentylenetetrazol (Meduna & Friedman, 1939) and electrical convulsive therapres (for history of ECT see Endler, 1988) were used with success m the treatment of SCH and AP. The mechanism of action of ECT is not known but it may involve down regulation of GABAergic tone due to mhrbrtron of GABA synthesis and release. Moreover, a modification of GABA, and glutamate receptor assembly might be possrble. The treatment of MD1 and AP with antidepressants and that of SCH with anttpsychotics was mitrated m the 1950s by serendipity. This was followed by massive research programs directed to increase our understanding of the molecular mechamcs medratmg drug efficacy m psychoses. Now it is widely believed that antipsychotrcs may act by mhrbrtmg dopammergic receptors while antidepressants may affect the function of NE (norepmephrme) and 5HT (serotonm) mediated synaptic transmission m a manner secondary to the mhrbrtron of then reuptake. Undoubtedly, there IS some truth m this belief but not m the sense that an alteration of monoamme function is the cause of psychoses In fact, at present, It 1s not credible that SCH is due to an excess of DA signal transaction m accumbens and/or frontal cortex. It is equally important to stress that metabolic studies have never proved consistently that MD1 or AP etrologrcally are tied to abnormalities of synthesis, metabolism, storage or release of NE and 5HT. It is most important to note that the onset of antipsychotics or antidepressants beneficial effects occurs with a time lag of 7 to 15 days Since the onset of monoamine uptake mhibuion by antidepressants or the inhrbrtion of DA receptors by antrpsychotrcs 1s prompt, it appears probable that their therapeutic action derives from an effect secondary to drug mduced reequrhbratron of an altered neuronal activity m areas innervated by monoammergic neuronal systems. Very probably, drugs trigger multiple transsynapttc mechanisms m yet undefined neuronal cncuns that are the result of drug induced persistent modification of monoammergrc synaptic functions Neuronal monoammergic pathways should be viewed as important neuronal systems which by hnkmg distant bran-r structures harmonize and integrate neuronal activity of these brain structures. Drug induced changes of the coordmatron mediated by these neuronal proJections can trigger modifications similar m nature, albert less intense, to those caused by convulsant therapy. Actually, several reports indicate that effective antipsychotrc and antidepressant drugs exhibit preconvulsant or convulsant actions. Moreover, it has not been difficult to demonstrate that the long term action of antrdepressants and antipsychotics 1s associated with a modrfrcatron of either GABAergic or glutamatergrc tone m the neuronal areas innervated by long axon monoammergrc neurons. Hence, rt could be surmized that genetically induced alterations m the expression of GABA or glutamate receptors might be associated with malfunction of bran-r neural actrvrties related to the regulation of cogmtrve processes GABA

Receptors:

Structure

and Function

There are two major classes of GABA receptors* A and B GABA, receptors are heteroohgomeric structures that form an array of structurally different GABA-gated amomc

channels (Schofield et al , 1987), however, we still do not know the subunit stolchlometry and composition of various GABA, receptor subtypes (Olsen & Tobln, 1990); nevertheless, several lines of independent mvestlgatlons clearly show that there 1s a high degree of structural mlcroheterogenelty of GABA, receptors expressed m various brain nuclei (Table 1) GABA, receptors (WoJcik & Holopamen, 1992) represent a class of metabotroprc receptors which, via second messengers operate m the modulation of Ca*+ Table 1 ExpressIon Rat Brarn

Level of GABA,

Receptor

Subunrt mRNA

m Varrous Regions of

Bram regions Subumts

ffi ‘y? a3 Q4

Qh ;:

133 72 6

Striatum

Hlppocampus

Cortex

Thalamus

Cerebellum

‘;

xxx xxx 0 xxx 0

xxx x xx

xxx x

xxx

xx 0 0 0 Y

xx

Y

0

0

x xxx

xx xxx

YY

xx

xx xx

YX

xx

xx';

xxx

YXY

xx

xx

xxx

xxx

xxx

ii

YX9

;, 0

Y

; 0 xxx

Data from D Grayson, L Marher, and N Rebaudengo, unpublished data

and K+ voltage dependent channels Thus GABA, receptors express a high degree of functional heterogeneity by generating second messengers that act on a variety of voltage dependent channels. Thus, via GABA, receptors, GABA can also modulate voltage dependent channels We do not know whether such heterogeneity might be enhanced by an mtrmslc structural heterogeneity of GABA, receptors because their structure IS not yet known. Both GABA, and GABA, receptors are present m various bram structures with a relatively balanced representation of both types of GABA receptors m each brain structure However, due to the functional polymorphism of GABA, receptor targets such a balance may not reflect a functional balance Turmng back to the GABA, receptors, it IS important to keep m mmd that their structure includes four major classes of subunits (a, /3, y, 6) which have among each other a homology of less than 40% (Olsen & Tobm, 1990). Each class has a number of subclasses each coded by a different gene (Table 1) Assuming that GABA gated channels are pentamerlc proteins, the theoretical number of structurally diverse GABA, receptors probably exceeds 4 x 10’. In addition, the subumt dlverslty can be increased m number by alternative splicing (Whiting, McKenan, & Iversen, 1990) Several mvestlgators (Costa, 1991; Pula et al , 1990, 1991) have studied the susceptlblhty of structural variants of GABA gated channels to GABA, neurosterolds (Pula et al , 1990) or pentobarbltal gatmg and to the positive allosterlc modulation of GABA response by benzodlazepmes, lmldazopyrldmes or other drugs endowed with positive modulatory actlvlty on the GABA efficacy m gatmg the Cl channels The mam findings

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of these studies (Pma et al., 1991; Siegel, Baur, Trube, Mohler, & Malherbe, 1990; Malherbe et al., 1990) can be summarized as follows. homomerrc receptors weakly respond to GABA, and they are also less responsive to pentobarbital gatmg and vtrtually irresponsive to the allosterrc modulation by benzodtazepmes and their congeners; receptors mcludmg two different subunits have variable responsrvenesses to CABA and pentobarbrtal gatmg and are virtually uresponsrve to allosterrc modulation by drugs with the exception of the yZ-&, confrguratron, where GABA efficacy can be modulated by drazepam; GABA, receptors including three or more different subunits are responsrve to the three challenges provided they mclude the yZ and one (Ysubunit. In contrast, the presence m a GABA, receptor of a y1 subunit either drastically reduces or abolishes the response to the allosteric modulatron by drugs. In addrtron, between the two confrguratron of the yZ subunit (long and short) generated by alternative splicing the short form always reduces the ethanol modulatron of GABA action and perhaps m part also that of benzodrazepmes (Wafford et al , 1991). Since the short configuration of the y2 subunit lacks a phosphorylatron consensus m the mtraneuronal subumt domain one can infer that phosphorylatron of the yZ receptor subunit might participate m the ethanol and probably benzodrazepme As is shown m Table 1, m the modulatron of GABA responses at GABA, receptors brain there are definite regional patterns that characterize the expression of various mRNAs encoding GABA, receptor subunits (Mohler et al., 1991, Pershon, Malharbe, & Richards, 1991). The (Yesubunit appears to be specific for the cerebellum, whereas the q subunit that is abundant m the hippocampus, is present m smaller amounts m the cortex but it is absent m the strratum, thalamus and cerebellum (Table 1j. The yZ and 6 subunits are expressed m every bram structure studied (Table 1) The (Yesubunit 1s present m moderate amounts m the cortex and m limited amounts m the thalamus but is absent m the other structures we have investigated (Table 1). Thus, m the strratum, the expression of q, &, yz and 6 predominates; m the hrppocampus CX~,cy2, a4, P,, Pz, P,, y2 and 6 are present; aI, 2, cy3, yz and 6 predominate m the cortex; the thalamus expresses CX,,(Y,, p2, p3, y2 ff39 P and 6, and the cerebellum expresses (Y], (.ys,p2, p3, y2 and 6 (see Table 1) Collectrvely these data Indicate that the varrabrhty of GABA, receptor structure should be viewed as a factor that determines the synaptic strength m GABA transduction at GABA, receptors located m different brain regrons. Thus, it is clear that rf GABA plays a role m the neuronal group selection of neocortex and If such selection contributes to cogmtrve function, cogmtrve disorders might be associated with alterations of GABA, receptor subunit assembly and/or subunit structure. Structural

Configuration

of GABA, Receptors and the Allosterrc Action of Drugs

Modulatory

The view that alterations of GABAergic synaptic strength might be important m ehcrtmg psychoses is supported by reports that persistent paroxysmal actrvation of brain GABA, receptors by muscimol and THIP can produce psychotomrmetrc responses m humans; also, when blockers of the natural catabolism of GABA are administered m doses that increase brain GABA content they can produce psychotomrmetrc effects m selected mdrvrduals (Hoehn-Sarrc, 1983; Theobald et al , 1968). Fmally, GABA uptake mhrbrtors can ehcn SCH-

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hke symptomatology (Sedman et al , 1990) Moreover, the benzodrazepmes that posmvely modulate GABA action at GABA, receptors can produce exacerbatron of psychoses m schizophrenics (Meldrum, 1982) and can ehcrt dlsorrentatron and confuston m elderly mdrvlduals or m a restrrcted number of otherwise normal mdlvrduals (Hall & Zlsook, 1981) Particularly alprezolam can exacerbate the symptoms of SCH m patients receiving neuroleptics (Dretch & Jennings, 1988; Dixon, Welder, Frances, 8c Sweeney, 1991) Trtazolam has been reported to impan memory and to produce psychoses, for this reason the drug has recently been withdrawn from the market m several countries The question arises, are these untoward affects of benzodrazepmes related to a preferential action on selected GABA, receptor subtypes? Indeed, a preferentral action of various benzodlazepmes on certain GABA, receptor subtypes can be shown to occur (Table 2) (Pma et al., 1990; Siegel et al., 1990) In tumoral human kidney cell lines transfected with human cDNAs encoding for varrous GABA, Table 2 Drug Induced Posrtwe and Negative Structural Confrguratrons of GABA, % Variation GABA, subumt

receptor assembly

Modulatron Receptors

of CABA-gated

on GABA-activated

Cl-

C1

Induced

ZOLP

Channels

rn Various

by

DZ

CLZ

ALP

0 0

0 0

0 0

0 0

0 0

0 0

+ 150 + 280 + 400 + 100 +70 +70 + 100 + 50

+ 130 + 220 + 300 +60 +60 t30 + 40 +42

+ 320 + 300 +210 + 10 +40 +5 -25 + 15

+ 230 +210 + 280 + 15 +45 -5 +5 + 10

- 50 -50 -75 -40 +70 + 29 +2 + 18

-4s -43 - 38 -70 +50 + 85 -4s -2

DMCM

/3CCM

BRT 0 0

+95 + 50 + 130 +45 + 50 -

Anxlolytic drugs DZ = Dlazepam, CLZ = Clonazepam, ALP = Alptdem, ZOLP = Zolpldem, BRT = Bretazend Anxlogemc drugs /3-CCM = methyl @carbohne-3-carboxylate, DMCM = methyl-6, 7-dlmethoxy-4ethyl $3 carbohne-3-carboxylate The concentrations tested (lO-sM) were per se mactlve on the GABA-gated Cl- channels The recombinant expression of GABA, receptors was performed via cDNA transfectlon In the 293 tumor kidney cell hne (Ptua et al , 1990)

receptor subunits, dlazepam (DZ) 1s more active on receptors obtained following transfectron with (Y?, /3, and y2 than on those obtained with (Y,, /3], y2 cDNA transfectron. Also, the P, and y2 receptors appear to be preferentrally modulated by DZ (Table 2) Several a53 allostertc modulators of GABA action are less active on receptors including y, subunits Actually these subunits can not only reduce the efficacy of clonazepam (CLZ), alprdem (ALP) and zolpldem (ZOLP) but they can change the directron of modulatron of selected positive and negative allosterrc modulators (Table 2) When this selectrvtty of drug action for specific GABA, receptor confrguratlons 1s seen m relationship wrth the spectfrc bram

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drstrtbutron of mRNAs encoding for various GABA, receptor subumts, one can appreciate the possrbthty that drugs can preferentrally enhance the synaptic strength of GABAergrc mhtbttton m neurons of selected brain structures For instance, DZ will unbalance the eqmhbrmm of GABAergrc transmission m cortex by preferentially facrhtatmg those receptors including the oj receptor subunits VISa VISthe GABAA receptors, mcludmg the (Y, receptor subunit. Since the quantities of mRNA encoding for yz receptor subunits present m primary cerebellar cultures 1s at odds wrth the pentamerrc stotchrometry proposed for GABA, receptors, two questions arise: 1s the presence of mRNA encoding for a given GABA, receptor subunit an index of the permanent expression of that subunit? Or shall we assume that the mRNA encoding for various subunits are stored m neurons ready to be alternatively expressed m response to specific mcommg strmuh’J Answers to these questions are not available yet However, studies of cultures of morphologrcally identical cerebellar granule cells mdtcate that each neuron contains multiple mRNAs encodmg for different a, 0, and y receptor subunits (Bovohn, Santa, Memo, Costa, & Grayson, 1992). Does this negate the argument that all the neurons m a culture that 1s morphologtcally homogeneous express a similar pattern of GABA, receptor subtypes? Shall we then conclude that a uniform neuronal morphology may include a structural diversity of GABA, receptors operative m cell to cell signaling among morphologrcally Identical neurons? We do not have yet precise answers to these questions The data of Table 2 also show that bretazeml (BRT) falls to modulate, wtth some preferenttahty, any of the receptor assembhes tested. In the presence of yz subunits, the modulatory efficacy of BRT 1s I% to % of the maximal response obtained with the maximal effrcacrous modulator for that GABA, receptor subtype. This particular behavior of BRT 1s consrstent with the described partial efficacy of this posmve allostertc modulator (Haefely, 1990) In contrast, the profiles of all the other allostertc modulators listed m Table 2 appear to reflect a preferential action of these drugs on selected GABA, receptor subtypes Benzodtazodrazepmes with Partial Posrttve Modulatory Molecular Mechanisms and Clmtcal Expectations

Profile.

From the data presented m Table 2 tt appears that there 1s a structural-dependent GABA, receptor suscepttbthty to the positive allostertc modulatton of various benzodtazepmes This means that allosterrc modulators will change the GABAergrc tone of various bram areas according to their preferenttahty profile and the abundance of certain GABA, receptor subtypes expressed m that structure. Thus, the action of drugs that act as allosterrc modulators of GABA, receptors can be drstmgutshed into two main categories. (a) preferential modulators of selected GABA, receptors, and (b) partial modulators devoid of GABA, receptor structure preferenttahty It 1s interesting to note that GABA, receptor preferenttahty of a given drug might be a factor generating the onset of benzodtazepmes untoward effects, mcludmg memory deficit and psychotic reactions In contrast, the benzodtazepmes that are the partral modulators of GABA, receptors will uniformly increase, by a moderate extent, the responsiveness to GABA of every GABA, receptor subtype. Moreover, since these drugs have a high affmtty for the modulatory center and a low intrmstc activity they may protect the modulatory center from

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the modulatron by endogenous hgands (Costa & Gurdottl, 1991; Olasmaa et al., 1990). In fact, it 1s important to keep m mmd that there are two classes of endogenous allosterrc modulators. (a) peptrdes (DBI and its processing products) (Costa & Gmdottt, 1990), and (b) nonpeptldlc hgands which have some affinity for antisera raised agamst benzodtazepmes (Olasmaa et al , 1990). The brain content of these endogenous hgands appears to change m neuropsychtatrtc disorders (Olasmaa et al., 1989; Costa & Gmdottl, 1991) Hence, while the level of modulation of GABA efficacy by preferential allostertc modulators must take mto account the mtrmslc activity of endogenous modulators because they are additive to the action of benzodlazepmes with preferential action on certain GABA, receptor subtypes, the partial modulators, by vtrtue of then high affmrty and low efficacy on every GABA, receptor subtype, harmomously establish a moderate increase m the level of GABA efficacy. And, m fact, partial modulators can reduce or abohsh the efficacy of other allostertc preferential modulators that are co-administered (Pma, G., unpublished) Finally, partial agonists modulators, because of then low mtrmstc activity, increase GABAergrc tone below the maxrmal cethng of GABA efficacy and thereby allow the expression of preferential phystologtcal changes m the transduction of GABA that IS released by nerve Impulses. These considerations suggest some possible lines of mvesttgatton mto whether the psychotomimetic action exerted by allosterlc modulators is limited to those with preferential actton on GABA, receptor subtypes For instance, since trrazolam causes amnesta and psychosis and appears to be a highly efftcactous postttve modulator of several GABA, receptors, one wonders whether thus action ~111 create an imbalance of mhlbnory synaptic strength stmrlar to this of GABA uptake mhlbrtors which are known to have produced impressive psychotomlmetrc side effects (Sedman et al., 1990). There are several reasons why at the present trme benzodtazepmes are under severe scrutmy. The malor reason is the development of tolerance and dependence to these drugs BRT and other partial allostertc modulators fall to cause tolerance and dependence. Actually when BRT 1s given to animals tolerant to benzodtazepmes endowed with preferential modulatory capacrty, tt possesses the usual efficacy In addition partial modulators fall to cause sedation or ataxta (Haefely, 1990). Finally, because of the nature of then activity partial modulators possess a stronger action as antrproconfhct than as antrconfhct drugs (Gmstt, Gmdettt, Costa, & Gutdottl, 1991). The latter possrbthty suggests that BRT and its congeners may be Indicated m the treatment of various forms of affective drsorders m which panic attacks and phobias are the prevailing symptomatology Moreover, should an increase of GABAerglc tone due to excessive release of GABA be the cause of AP, partral allosterlc modulators may help because they can reduce the increase of GABAergtc synaptic strength due to excessive allosterrc modulatton of GABA, receptors by endogenous hgands They can stabilize the GABAergtc synaptic strength at a lower level than that determined by the high content of endogenous modulators and yet still allow the expression of the physrologlcal response to variable amounts of GABA released by nerve impulse Regulation Since the structure

of GABA,

of the 5’ upstream

Receptor region

Subtype

Expression

of genes encoding

for GABAA receptor

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subumts 1s not yet extensively studied, the regulation of GABA, receptor subtype transcription 1s not well understood at present (Buckle et al , 1989). Moreover, as mentioned earlier, the pre-mRNA alternative sphcmg is another structural varrant to be considered m the regulation of the efficacy of quanta of GABA released by nerve impulses. We do not know whether alternative sphcmg is regulated functionally, we only know that rt is regulated developmentally. Finally, also the assembly of the subunits which 1s the final step m the GABA, receptor biosynthesis, mrght be also functionally regulated. If rt is, we have a potential target for a new generation of drugs acting to modify the storchtometry of native GABA, receptor subtypes. Two lines of mvestrgatron indicate that: (a) GABA, receptor subunits can be developmentally regulated (Bovolm et al , 1992); and (b) In vztro expression of GABA, receptors can be changed by strmulatton or blockade of NMDA selecttve glutamate receptors (Memo et al , 1991). These fmdmgs can be summarized as follows (1) In rat cerebellum, the content of the mRNA encoding for the (Y] subunit increases postnatally from day 1 to day 21; m contrast, the content of mRNA encoding for the CQ subunit postnatally decreases continuously from day 1 to day 21. In rat cerebellum the content of the yZ subunit at birth is about that of the adult. At birth the only molecular form of yz subumt mRNA present 1s the y2 S (the form that 1s spliced); the y2 L begins to appear at 7 days after brrth and increases contmuously until 21 days when tt represents about 37% of the total yz subunit present m the cerebellum. In the adult rat, the cerebellar content of the yS and yL subumts 1s about equal. This suggests that sphcmg may be regulated by an mhrbttory process that increases m efficacy wrth brain development This suggests that regulatton of y2 subunit sphcmg increases wtth the maturation of GABA, receptor function. (2) Exposure of primary cultures of rat cerebellar granule cells to antagonists of the NMDA selective glutamate receptor reduces the granule cell steady state levels of various mRNAs encoding for various GABA, receptor subunits. Granule cells are glutamatergtc and require a depolarizing concentratton of K (25 mM) for optimal development and survival. When the neuronal drfferentratton rate IS retarded by the lowering of the medmm K content to 12.5 mM, the expression of the GABA, receptor subumts 1s modified. Moreover, a persistent stimulation of the glutamate receptors of the granule cells growing m 12 5 mM K with NMDA changes the expression of GABA, receptor subumts, thus, the neurotrophic effect of glutamate might be expressed m granule cells developmg In vztro m lowered K concentrations via a modulation of the expression of mRNA for GABA receptor subunits. Conclusions Now turning back to the role of GABAergtc transmission m the etrology of psychosis, we can conclude that there are a number of processes that can be altered m the harmonious functions that regulate GABA, receptor assembly and subunit synthesis. These can be genetic, developmental, and in the mature bram may be related to brain function Any of these changes can be the cause of the abnormalities m the GABAergtc modulation of neuronal group selection m the neocortex and if a change occurring m this process 1s of some

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relevance it might reverberate as a defect m cogmtlve function. As mentioned earlier, m humans, psychotomlmetlc effects can be elicited with InJections of inhibitors of GABA degradation or mhlbltors of GABA uptake or musclmol These responses are highly suggestive that an increase of GABAerglc tone may trigger the onset of abnormahtles m cogmtlve processes In addition symptoms very similar to SCH can be elicited by treatment with noncompetltlve receptor blockers of NMDA selective glutamate receptors and with mhlbltors of GABA reuptake In v&o, the treatment of cerebellar granule cells with NMDA receptor blockers changes the expression of selected mRNA encoding for GABA, receptor subumts We do not know whether this action can be obtained in vlvo but should this be the case, it would become of relevance to study whether a receptor-mediated alteration m the expression of the genetic code for GABA, receptor subunits can partlclpate m the etiology of psychiatric disorders It 1s at present lmposslble to relate the symptoms of SCH, MD1 and AP to a dlsruptlon of the mechanisms operative m neuronal group selection m neocortex but, collectively, all the observations mentioned above encourage the contmuatlon of experiments directed to evaluate the role that abnormahtles m GABAerglc synaptic strength may have m the disruption of thought processes. Perhaps slmllar abnormahtles of GABAerglc tone at selected GABA, receptor subtypes can be obtamed without drug admmlstratlon, m conJunctlon with genetically related alterations m GABA, receptor subunit assembly or subunit structure, moreover, abnormahtles m the alternative splicing of yz receptors must also be considered as a target of the genetic abnormahtles operative m psychoses An mterestmg new horizon 1s opening with the mtroductlon of synthetic antisense phosphorothlonate and methylphosphonate ohgonucleotlde analogues (Cohen, 1989) which can be engineered to block translation of mRMA encoding for specific brain protems lncludmg a specific mhlbltlon of mRNAs for GABA, receptor subunits. One might postulate that m psychosis, a genetically determined high expression of a mRNA for a speclflc GABA, receptor subunit might be operative. This could be brought to normal levels by InJections of stable antisense ohgonucleotldes that may cross the blood brain barrier In addition, modlflcatlon of y2 receptor subunits alternatlve sphcmg might be obtained with drugs intended to reduce or counteract abnormahtles of S/L yZ subunit ratios Unfortunately, phosphorothloate derivatives of ohgodeoxynucleotldes do not readily reach the brain In contrast, methylphosphonates have probably a physical chemical structure that allows a moderate crossmg of the blood-brain barrier. These drugs are presently being prepared and studied. They not only might become possible future drugs for treatment of SCH, MD1 and AP but most important may help to establish genetically derived ammal models of various forms of psychiatric Illnesses that are m line to reproduce experimentally the genetlcally derived forms of human psychoses References Bovohn, P , Santa, M R , Memo, M , Costa, E , & Grayson, D R (1992) Dlstmct developmental patterns of expresslon of the rat oil, 015, y2S and y2L GABA, receptor subumt mRNAs rn vwo and m vlfro Journal of Neurochemistry, 59, 62-72 Buckle, V .I , FuJlta, N , Ryder-Cook, A S , Derry, J M J , Bernard, P J , Lebo, R V , Schofield, P R , Seeburg, P H , Bateson, A N , Darhson, M G , & Barnard, E A (1989) Chromosomal locatlon of GABA, receptor subumt genes RelatIonshIp to human genetlc disease Neuron, 3, 647-654 Cohen, J S (1989) Deslgmng antrsense ohgonucleotldes as pharmaceutical agents TIPS, 10, 435-437

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Costa, E (1991) The allosterlc modulation of GABA, receptors Seventeen years of research Neuropsychophannacology, 4, 225-235 Costa, E , & Cmdottl, A (1991) Dlazepam bmdmg mhlbltor (DBI) a peptlde with multiple bIologIca actlons Life Screnee, 49, 325-344 Dletch, J T , & Jenmngs, R K (1988) Aggressive dyscontrol m patients treated with benzodlazepmes Journal of Clrmcal Psychology, 49, 184-188 Dixon, L , Welder, P J , Frances, A J , & Sweeney, J (1991) Alprazolam Intolerance m stable schlzophremc outpatlents Psychopharmacologla Brrtannra, 25, 213-214 Edelman, G M (1987) Neural Darwmwn Basic Books, New York Endler, N S (1988) The orlgms of electroconvulsant therapy (ECT) Convulsrve Therapy, 4, 5-23 Gmstl, P , Gmdettl, G , Costa, E , & Gmdottl, A (1991) The preferential antagonism of pentylenetetrazole proconfllct responses dlfferentlates a class of anxlolytlc benzodlazepmes with potential ant]-pamc actlon Journal of Pharmacology and Expenmental Therapeutrcs, 257, 1062-1068 Haefely, W E (1990) Novel anxlolytlcs that act as partial agomsts at benzodlazepme receptors TIPS, 11, 452-456 Hall, R C , & Zlsook (1981) Perlodlcal reactlons to benzodlazepmes Brrtrsh Journal of Clrmcal Psychopharmacotherapy, 11, 995-1045 Hememann, S , Bettler, B , Boulter, J , Denerls, E , Gasic, G , Hartley, M , Hollmann, M , Hughes, T E , O’Shee-Greenfield, A , & Rogers, S (1991) The glutamate receptors genes, structure, and expresslon Neurotransmltter regulations of gene expresslon In E Costa & T H Joh (Eds ), Frdra Research Found&on Symposium Serres (Vol 7) (pp 143-165) Hoehn-Sarlc, R (1983) Effect of THIP on chrome anxiety Psychopharmacotherapy, 80, 338-391 Malherbe, P , Siegel, E , Baur, R , Persohn, E , Richards, J G , & Mohler, H (1990) Functional characteristics and sites of gene expresslon of the 01,. p,, y2-lsoform of the rat GABA, receptor Journal of Neuroscrence, 10, 2330-2338 Meduna, L , & FrIedman, E (1939) The convulsion lrrltatlve therapy of psychoses Journal of the Amencan Medrcal Assocratron, 112, 501-509 Meldrum, B (1982) GABA, and acute psychoses Psychology and Medrcme, 12, l-5 Memo, M , Bovolm, P , Costa, E , & Grayson, D R (1991) Regulation of y-ammobutyrlc acid--A receptor subumt expression by activation of N-Methyl-o-Asparate-Selective glutamate receptor Molecular Pharmacology, 39, 599-603 Mohler, H , Malherbe, P , Richards, J G , Persohn, E , Benke, D , Barth, M , Rhyner, T , & Siegel, E (1991) GABA, receptor gene expresslon and regulation In E Costa & T H Joh (Eds ), Neurotransmrtter regulation of gene-expression Rdra Research Foundation Symposrum Serres (Vol 7) (pp 11 l-124) Olasmaa. M . Gmdottl. A , Costa. E , Rothstem, J D , Weber, R J , & Paul, S M (1989) Endogenous benzodlazepmes m hepa& encephalopathy Lancet, 4, 491-492 Olasmaa, M , Rothstem, J D , Gmdottl, A , Weber, R J , Paul, S M , Spector, S , Zeneroh, M L , Baraldl, M , & Costa, E (1990) Endogenous benzodlazepme receptor hgands m human and ammal hepatlc encephalopathy Journal of Neurochematry, 55, 2015-2023 Olsen, R W , & Tobm, A J (1990) Molecular biology of GABAA receptors Faseb Journal, 4, 1469-1480 Persohn, E , Malherbe, P , & Richards, J G (1991) In situ hybrldrzatlon hlstochemlstry reveals a diversity of GABA, receptor subunit mRNAs m neurons of the rat spmal cord and dorsal root gangha Neuroscrence, 42, 497-507 Pma, G , San& M R , Vlcml, S , Pntchett, D B , Purdy, S M , Seeburg, P H , &Costa, E (1990) Neurosterolds act on recombmant human GABA, receptor Neuron, 4, 759-765 Pma, G , Vlcml, S , Seeburg, P H , & Costa, E (1991) Influence of recombmant y-ammobutync acid-A receptor subunit composition on the actlon of allosterlc modulators of y-ammobutyrlc acid-gated Cl- currents Molecular Pharmacology, 36, 691-696 Schofield, P R , Darhson, M G , FuJlta, N , Burt, D R , Stephenson, F A , Rodriguez, H , Rhee, L M , Ramachandron, J , Reale, V , Glencorse, I A , Seeburg, P H , & Barnard, E A (1987) Sequence and functional expression of the GABA, receptor shows a hgand-gated receptor super famdy Nature, 328, 221-227 Sedman, A J , Gimlet, G P , Sayed, A J , & Posvar, E L (1990) lmtlal human safety and tolerance study of a GABA uptake mhlbitor, C1966 Potential role of GABA as a mediator m the pathogenesls of schlzophrema and enemla Drug Developmental Research, 21, 235-242 Slegel, E , Baur, R , Trube, G , Mohler, H , & Malherbe, P (1990) The effect of subumt compositlon of rat bram GABA, receptors on channel function Neuron, 5, 703-711 Theobald, W , Buck, 0 , Kumar, H A , Krupp, P Stenger, E , & Helmann, H (1968) Pharmakologlsche und experimental psychologlsche mtersuchungunmlt 2 mhaltsoffern des fhagnepllzes Alhermrt Forsch 18, 31 l-315

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E COYTA

Wafford, K A , Burnett, D M , Leldenhelmer, N 3 , Burt, D R , Wang, J B , KO~UJI, P , Dun Waddle, T V Harris, A , & Slkela, J M (1991) Ethanol sensltlwty of the GABA, receptor expressed m xenopus, oocytes reqmres 8 amino acids contamed m the yz C subumt Neuron, 7, 27-33 Whltmg, P , McKenan, R M , & Iversen, L L (1990) Another mechamsm for creating dlverslty m y-ammobutyrtc type A receptors RNA splrcmg directs expresslon of two forms of yz subunit, one of which contams a protem kmase C phosphorylatlon site Proceedrngs of the Natronal Academy ofScIences of the U S A , 87,9966-9970 WoJcIk, W , & Holopamen, I (1992) Role of central GABA, receptors in physiology and pathology Neuropsychopharmacology, 6, 20 l-2 I4

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