Pergamon
`
`Neuroscience and Biobehavioral Reviews, Vol. 18, No.2, pp. 291-304, 1994
`Copyright c 1994 Elsevier Science Ltd
`Printed in the USA. All rights reserved
`0149-7634/94$6.00 + .00
`
`0149-7634(93)E0003-5
`
`Gammahydroxybutyrate:
`An Overview of the Pros and Cons
`for it Being a Neurotransmitter
`And/Or a Useful Therapeutic Agent
`
`CHRISTOPHER D. CASH
`
`Centre de Neurochimie, 5 rue Blaise Pascal, Strasbourg 67084 France
`
`Received 31 May 1993
`
`CASH, C. D. Gammahydroxybutyrate: an overview of the pros and cons for it being a neurotransmitter and/or a useful
`therapeutic agent. NEUROSCI BIOBEHA V REV 18(2) 291-304, 1994.- Gamma-hydroxybutyrate (GHB) is a catabolite in
`brain of -y-aminobutyrate (GABA) and is also found in nonneuronal tissues. It is present in the brain at about one thousandth
`of the concentration of its parent compound. High affinity and specific uptake, and energy dependent transport systems for
`GHB have been described in brain in addition to a class of high affinity binding sites, functional at a rather unphysiologically
`low pH.
`Administration of large doses of GHB to animals and man leads to sedation, and at the highest doses, anaesthesia. These
`effects are prominent when GHB brain levels are over one hundred-fold the endogenous levels. In some animals, GHB
`administration also induces an electroencephalographic and behavioural changes resembling that of human petit mal epilepsy.
`GHB has been used in man as an anaesthetic adjuvant. GHB lowers cerebral energy requirements and may play a neuroprotec(cid:173)
`tive role. Administered GHB profoundly effects the cerebral dopaminergic system by a mechanism which remains to be
`unravelled. GHB has been tested with success on alcoholic patients where it attenuates the withdrawal syndrome. It is
`indicated here that in this situation, it may owe its effect by acting as a pro-drug of the neurotransmitter GABA into which it
`can be transformed. As administration of GHB, a GABA8 receptor agonist and a natural opioid peptide all elicit similar
`abnormal EEG phenomena, it may be suggested that they are acting via a common pathway. The petit mal epileptic effects of
`GHB might be ascribed to its direct, or indirect agonist properties after transformation to a pool of GABA at the GABA8
`receptor or via interactions at its own binding sites linked to a similar series of biochemical events. Some anticonvulsant
`drugs, the opiate antagonist naloxone and a synthetic structural GHB analogue antagonise certain behavioural effects of
`GHB administration. It is postulated that GHB exerts some of its effects via transformation to GABA pools, and that
`substances which inhibit this process antagonise its effects by blocking GABA formation. GHB has been proposed as a
`neurotransmitter, although straightforward evidence for this role is lacking. Evidence for and against GHB, as a neurotrans(cid:173)
`mitter, is reviewed here together with a discussion of its potential as a therapeutically useful drug.
`
`Gammahydroxybutyrate
`
`Neurotransmitter
`
`Therapeutic agent
`
`GABA receptors
`
`INTRODUCTION
`
`GABA is now generally accepted as the major inhibitory
`neurotransmitter of the central nervous system (CNS). It is
`derived principally from glutamate (Glu) which itself is an
`excitatory neurotransmitter and interacts with several receptor
`subclasses which in turn provoke various neurophysiological
`events.
`Endogenous GHB which appears to be formed from
`GABA is present in brain tissue at micromolar concentrations
`
`whereas its potential precursor amino acids are found in milli(cid:173)
`molar concentrations.
`Unlike GABA, Glu, or other neurotransmitters, GHB is
`able to traverse the blood-brain barrier after peripheral ad(cid:173)
`ministration, and in high doses, it induces behavioural re(cid:173)
`sponses including sedation and eventually anaesthesia accom(cid:173)
`panied in some animal species with an BEG response which is
`similar to that seen in human petit mal epilepsy.
`GHB is also found in nonneural tissues; indeed the kidney
`contains more than 10 times the concentration found in brain
`
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`292
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`CASH
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`tissue whereas heart and muscle display 5-times greater levels
`(134). However, very little is known about the biochemical
`role for GHB outside of the brain.
`In 1964, Laborit presented a strong case for the use of
`GHB in anaesthesia and in psychiatry due to its low toxicity
`and lack of side effects (103). Thirteen years later, Snead re(cid:173)
`viewed the evidence that endogenous GHB might be involved
`in the modulation of dopaminergic neuronal activity, sleep
`regulation and in some pathologic states (170). Another 10 yr
`elapsed before Vayer et al. proposed that GHB may play a
`neurotransmitter role (200).This belief has recently been rein(cid:173)
`forced by Tunnicliff (189). Mamelak has proposed that GHB
`is a major endogenous inhibitor of energy metabolism and
`thus may play a protective role when energy supplies are lim(cid:173)
`ited in both neuronal and peripheral tissues (122).
`In this review, my concern will center around the published
`data to provide the reader with a clearer view as to whether or
`not endogenous GHB plays a physiological role, possibly as
`a neurotransmitter and/or in pathology. In addition, I will
`examine its further potential as a therapeutic agent. With this
`in mind, I have taken into account various mechanisms where
`a role of GHB has been implicated or proposed. An effort
`has been made here to deal with the existing controversies
`pertaining to the mode of action of GHB.
`
`THE IN VITRO BIOCHEMISTRY OF GHB
`It is widely accepted that a neurotransmitter fulflls certain
`biochemical criteria, namely those of high affinity and specific
`binding, uptake and release. From such a standpoint, the ma(cid:173)
`jor biochemical data of GHB in the brain are presented below
`and are grouped into two categories, firstly those that, in the
`author's view, support its role as a neurotransmitter, and sec(cid:173)
`ondly, results of considerable physiological relevance that do
`not commit or omit a role for GHB as a neurotransmitter.
`
`PRO-NEUROTRANSMITTER DATA
`An apparently specific enzyme for GHB biosynthesis from
`GABA via succinic semialdehyde (SSA) has been described in
`both rat (150) and human brain (30) and this molecule is re(cid:173)
`leased from preloaded brain slices under depolarising condi(cid:173)
`tions, a process that requires Ca2+ and is blocked by tetrodo(cid:173)
`toxin (118). In brain tissue culture, there exist GHB binding
`sites which are exclusive to neurones as opposed to glia (87).
`Subcellular fractionation of rat brain indicates that this bind(cid:173)
`ing is principally localised in the synaptosomal fraction (120).
`GHB is unevenly distributed in the rat brain, and its con(cid:173)
`centrations do not closely follow that of its proposed bio(cid:173)
`synthetic enzyme: "specific" succinic semialdehyde reductase
`(197). However, endogenous GHB is found at the highest con(cid:173)
`centrations first in the cytosol, and, second, in the synapto(cid:173)
`somal fraction prepared from rat brain. Its addition to the
`homogenising medium before tissue fractionation results in its
`enrichment in synaptosomes (166).
`
`DATA WHICH ARE NOT POSITIVELY SUPPORTIVE OF
`A NEUROTRANSMITTER ROLE
`Binding sites for GHB exist both in rat (12), and human
`(179) brain membranes. In rat brain, high and low affinity
`components of these binding sites are characterised. The high
`affinity binding has a Kd of 95 nM at the pH optimum of S.S
`with no affinity for GABA or baclofen (12). This could qual(cid:173)
`ify GHB as a neurotransmitter if the pH optimum for binding
`were in the neutral physiological range. In addition, there ex(cid:173)
`ists a Na +-dependent uptake system operating at a Km value
`
`of about 50 p.M which is 20 times higher than the endogenous
`brain concentration of GHB. Moreover, GABA competes
`with this transport system (13). In striatal slices, a Km value of
`700 p.M has been reported (77).
`The enzymes that are able to catabolise GHB are not spe(cid:173)
`cific and their Km values are much higher than the endogenous
`GHB concentration (93). In cortical brain tissue slices, 0.5 ttM
`GHB stimulates the rate of 0 2 consumption in the presence of
`glucose. In the absence of glucose, GHB is unable to support
`respiration (187). Thus, its catabolism does not seem to in(cid:173)
`volve a single enzymatic step but is rather linked to a general
`energy requiring metabolic process.
`GHB levels are higher in postmortem human brain taken
`from individuals with Huntingdon's disease (3), and they are
`increased in an animal model of this disease induced by striatal
`injection of the neurotoxin, kainic acid (4). These data suggest
`disposal of GHB in the neurones because this disease is char(cid:173)
`acterised by extensive striatal neuronal loss (99).
`Application of 1 mM GHB to cultured neurones from the
`rat causes hyperpolarization which is reversed by the GABA
`receptor antagonist, bicuculline (88). In guinea-pig pars com(cid:173)
`pacta neurones, derived from the substantia nigra, GHB at
`a minimum concentration of 100 p.M hyperpolarises the cell
`membrane and facilitates Ca2+ conductance (75). At this same
`concentration, GHB antagonises the depolarisation-induced
`release of newly synthesised dopamine from rat brain striatal
`slices (26). Thus, GHB at a concentration that might not be
`attainable in the brain under physiological conditions can in(cid:173)
`teract directly or indirectly with the gabaergic and dopaminer(cid:173)
`gic systems.
`A selective though weak affinity of GHB for GABA8 re(cid:173)
`ceptors has recently been reported (IC50 = 150 ttM) (14), al(cid:173)
`though previous data showed that GHB cannot compete with
`GABA at this site (168). Activation of the GABA8 receptor
`by GHB, in vitro, hyperpolarize& hippocampal neurones (212)
`and depresses synaptic potentials in hippocampal slices (211).
`In addition, GHB binds to a population of sites that show
`high affinity for trans--y-hydroxycrontonic acid (THCA) (81),
`a substance that has been identified in both renal (135) and
`brain tissues (196). The possible significance of this interaction
`has not been defined, although THCA may be a catabolite
`ofGHB.
`Hippocampal slices incubated with about 0.5 mM GHB
`result in a significant increase in both intracellular cyclic GMP
`and inositol phosphates levels (199). Because a major route
`for cyclic GMP synthesis is via activation of soluble guanylyl
`cyclase by nitric oxide formation generated by agonist induced
`calcium entry through the N-methyl o-aspartate category of
`glutamate receptors (24,60,61), and as nitric oxide can stimu(cid:173)
`late phosphoinositol hydrolysis (163), it would seem that in
`this pathway GHB exhibits a neuroexcitatory role.
`These data taken alone certainly are not of much conse(cid:173)
`quence in delineating GHB as a possible neurotransmitter in
`the CNS. Nevertheless, although most established neurotrans(cid:173)
`mitters bind optimally to their receptors at a neutral pH, the
`binding of 1,3,4,5-tetrakisphosphate to its cerebellar receptor
`has been found to be optimal at pH 5.0 (46). Moreover, GHB
`is a simple molecule which possesses only an alcohol and a
`carboxyl as functional groups. This latter may be required to
`exist in the protonated form for recognition and binding.
`
`BIOCHEMICAL CORRELATIONS OF GHB ADMINISTRATION
`
`GHB is a psychoactive drug that manifests its effects when
`administered intravenously or even taken orally and as such
`
`

`
`GAMMA-HYDROXYBUTYRATE: NEUROTRANSMITTER AND/OR THERAPEUTIC AGENT
`
`293
`
`differs fundamentally from established neurotransmitters that
`do not normally pass the blood-brain barrier. Indeed, it may
`well be present in the food chain, for example, the presence
`of 'Y-butyrolactone (GBL), its lactone precursor, has been de(cid:173)
`tected in wine (128). Because GBL is often employed as a
`pro-drug that is efficient, indeed with a higher availability
`(110) than the active compound GHB (146) into which it can
`be rapidly converted (148) by a blood-born 'Y-lactonase (54),
`it will be specified which substance is administered. None of
`the following data can thus be considered as directly indicating
`or negating a neurotransmitter role for GHB because the doses
`employed other than that required for the pyretic effect re(cid:173)
`ferred to below would be expected to induce brain levels of
`some two orders of magnitude higher than the normal endoge(cid:173)
`nous concentration.
`
`PHARMACOKINETIC CONSIDERATIONS
`
`Intravenous injection of about 3.5 mmol /Kg GHB (an
`anaesthetic dose) to the cat resulted in a brain GHB levels of
`about 0.7 14M and about double this value in the blood (147).
`A similar result was obtained in dogs, and with a rapid out(cid:173)
`flow of GHB from the brain to the cerebrospinal fluid (159).
`Thus, GHB passes readily from the bloodstream to the brain
`and gives rise to a concentration of over 100 times its normal
`endogenous level but does not appear to be actively taken up
`or retained by brain.
`
`INTERACTIONS WITH THE DOPAMINERGIC,
`SEROTONERGIC AND GABAERGIC SYSTEMS
`
`Brain dopamine levels are increased by GHB administra(cid:173)
`tion. In rats, these levels were doubled 1 h after IV injection
`of 20 mmol GHB (62). Lower doses also increase these levels
`but are without effect on the serotonergic system (131). Mice
`perfused with 7.5 mmol/Kg GBL intraperitonealy, displayed
`increased immunostaining for tyrosine hydroxylase in the stri(cid:173)
`ata (76). Subsequent to GHB administration, the kinetic con(cid:173)
`stants of tyrosine hydroxylase are unchanged, but the de(cid:173)
`creased affinity that it displays for its pteridine cofactor after
`treatment with the neuroleptic drug, haloperidol, is antagon(cid:173)
`ised by GHB pretreatment (214). However, in awake rats,
`subanaesthetic doses of GHB stimulate the firing rate of nigral
`dopaminergic neurones, whereas high doses suppress the fir(cid:173)
`ing (44). In in vivo microdialysis experiments, low doses of
`GHB inhibit dopamine release whereas high doses strongly
`increase it (78). Administration of the GABA8 receptor ago(cid:173)
`nist, baclofen, induces a similar dopaminergic response to that
`of GHB administration (38). In adolescent rats, a high dose
`of GHB increases the rate of synthesis and degradation of
`serotonin whereas in neonatal animals it had no effect (82).
`However, chronic, high dose administration of GBL to mice
`appears to downregulate the gabarergic system (63).
`When GHB is microiontophoretically applied to cortical
`neurones of the rabbit, it depresses their firing, an effect that
`is blocked by the GABAA receptor antagonist, biccuculline
`(100), although prior treatment of rats with high doses of
`GHB failed to modify the ex-vivo characteristics of this recep(cid:173)
`tor (157).
`
`INTERACTIONS WITH
`THE NITRINERGIC/CYCLIC GMP SYSTEM
`
`Injection of GHB in rats at a dose of 10 mmol!Kg induces
`a sustained hypertensive effect (90). The cyclic GMP content
`of rat brain hippocampus was approximately doubled after
`
`administration of 5 mmol/Kg GHB (199), whereas the cerebel(cid:173)
`lar levels of cyclic GMP are considerably reduced (14).
`
`INTERACTIONS WITH THE OPIOID SYSTEM
`Administration to rats of an anaesthetic dose of GHB in(cid:173)
`creases the brain opioid peptide content (107). As opiate an(cid:173)
`tagonists effect the behavioural attributes of GHB administra(cid:173)
`tion, this subject is further discussed below.
`
`METABOLIC EFFECTS
`Profound effects of GHB on energy metabolism are ob(cid:173)
`served. Injection of 5 mmol/Kg GHB in the rat appears to
`specifically stimulate the pentose phosphate shunt pathway
`(187). A similar dose of GHB or GBL profoundly diminishes
`the rate of cerebral glucose utilisation (65,116,210). Whereas
`high doses of GHB lowered the body temperature (104), a low
`dose, (0.1 mmol!Kg) induced a pyretic effect (92).
`
`HYPOTHETICAL EXPLANATIONS FOR THE BIOCHEMICAL
`IN VITRO AND IN VIVO EFFECTS
`It is well known that GHB can inhibit the depolarisation(cid:173)
`induced release of newly synthesised dopamine from brain
`tissue (26) and this, in part, might explain the increased brain
`levels after its administration. Such an effect may be due to
`its inhibitory action on the cell body of dopamine-releasing
`neurones (2) acting possibly through GHB-uptake by these
`cells. It might, on the other hand, be a result of GABA8
`receptor activation because these effects are mimicked by
`baclofen (38). In the latter case, it could be either due to a
`direct action of GHB on this receptor (14), or, after its meta(cid:173)
`bolic transformation to an appropriate pool of neuroactive
`GABA. However, the GABA8 receptor is coupled to a GTP(cid:173)
`binding protein (84), an effector site that is shared by at least
`one other neurotransmitter (5). Thus GHB might be acting at
`its specific binding sites causing a similar cascade of 0-
`protein-mediated events. This finds support because previous
`studies have shown that GTP inhibits GHB binding (164),
`although, a contribution by, or synergy between both GABA8
`receptors and GHB binding sites cannot be ruled out. The
`increased opioid peptide levels induced by GHB (107) might
`also be a factor involved in the stimulation of dopamine syn(cid:173)
`thesis (15).
`The serotonin-increasing effects of high doses of GHB are
`more likely due to its interaction with its own binding sites
`because GHB-binding sites increase postnatally (12) whereas
`this serotonergic response is absent at birth (82).
`Reversible hypertension can be induced by inhibition of
`nitric oxide synthesis (59). GHB reduces the cyclic GMP con(cid:173)
`tent of the cerebellum, a brain region where GHB binding
`sites are almost absent (79). The principal activator of soluble
`guanylyl cyclase is now thought to be nitric oxide (24). If this
`phenomenon is applicable to peripheral tissues where GHB
`binding sites are also absent, it could be postulated that GHB
`decreases nitric oxide synthesis by an as yet unknown nonneu(cid:173)
`ronal mechanism.
`The increase in the cyclic GMP level observed in the hippo(cid:173)
`campus after GHB treatment is more probably due to a neu(cid:173)
`ronal stimulation of nitric oxide synthesis via GHB binding
`sites which are abundant in this region (79). Although nitric
`oxide synthase-containing neurones are not very prevalent in
`this tissue, a majority of these neurones also contain GABA
`(193). The biochemical basis of this phenomenon remains to
`be elucidated but behavioural data indicate that a ORB(cid:173)
`mediated event is associated with the activation of the N-
`
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`294
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`CASH
`
`methyl-D-aspartate category of glutamate receptors (9). Acti(cid:173)
`vation of these receptors in brain tissue opens a Ca2+ channel
`(50). Ca2+ is a requirement both for the synthesis of neuronal
`nitric oxide (97) and for agonist binding to the GABA8 recep(cid:173)
`tor (83). Thus there is, at least in part, a tenuous biochemical
`relationship for the cyclic nucleotide and gabaergic events.
`The mechanism by which high doses of GHB increase brain
`opioid peptide content cannot be facilely explained as a ga(cid:173)
`baergic process because the GABA8 agonist, baclofen potenti(cid:173)
`ates the K+ -evoked release of methionine-enkephalin from rat
`brain striatal slices (151), whereas the GABAA agonist, musci(cid:173)
`mol decreases this release (136). Because administration of the
`general opioid antagonist, naloxone, overcomes many of the
`biochemical (37,198), and behavioural (174) effects of GHB
`administration, the effects of GHB must nevertheless be inter(cid:173)
`related with the endogenous opioid system.
`Naloxone antagonises the enkephalin-stimulated increase
`in rat brain dopamine synthesis (15). In addition, it blocks the
`increase in dopamine accumulation and release, induced by
`application of GHB to the rat brain by in vivo microdialysis
`(78) Thus it would appear that an effect on the opioid system
`is a prerequisite for the dopaminergic effects. Dopamine re(cid:173)
`ceptor stimulation elevates depolarisation-induced enkephalin
`release (114), whereas GHB administration increases brain
`opioid content (107). This may be attributed to the increase in
`dopamine synthesis in the striatum induced by GHB adminis(cid:173)
`tration (62) and inhibition of its release (26). As naloxone
`blocks, in particular, the EEG effects of GHB administration
`(vide infra), it might be expected that GHB induces enkapha(cid:173)
`lin release by an as yet unknown mechanism, perhaps through
`interactions with its own binding sites. Another possibility is
`that GHB is transformed to a pool(s) of GABA which then
`modulate enkephalin release (17,18,161), If this is true, then
`gabaergic activation preceeds both the opioid and dopaminer(cid:173)
`gic events.
`The decrease in energy metabolism caused by administra(cid:173)
`tion of GHB could be related to an increased availability of
`reduced NADPH due to its oxidation catalised by aldehyde
`reductase working as a dehydrogenase (95). However, this
`hypothesis is discordant with the data that indicates that the
`pentose phosphate shunt pathway is stimulated by GHB ad(cid:173)
`ministration (187), because this metabolic route requires oxi(cid:173)
`dised NADP+ for its functioning. It could however, explain
`the lowering of body temperature induced by high doses of
`GHB, although the mechanism by which naloxone overcomes
`some of the GHB-induced metabolic effecs (37) is open to
`speculation. The pyretic effect attained after administration of
`small doses of GHB, is of interest and suggests rather a neu(cid:173)
`ronal mechanism of action, possibly via interaction with its
`high affinity binding site.
`These results tempt me to hypothesise that administered
`GHB is, at least in part, a precursor for neuronally active
`GABA pools which in turn are capable of interacting with
`both GABAA and GABA8 receptors. Much of the biochemical
`data and behavioural correlates can be rationalised by this
`hypothesis.
`
`DRUGS AND PATHOLOGIC FACTORS WHICH AFFECT THE
`ENDOGENOUS GHB SYSTEM
`Acute administration of ethanol, decreases the GHB con(cid:173)
`tent of rat striatum, (141) whereas morphine increases its lev(cid:173)
`els in certain brain regions (158).
`Valproate is the most well-known agent that acutely in(cid:173)
`creases cerebral GHB levels (175). This is interesting insofar
`
`as this drug antagonises some of the effects of GHB admin(cid:173)
`istration which will be the subject of a later section. The
`mechanism of this increase might be due to inhibition of the
`mitochondrial enzyme, succinic semialdehyde dehydrogenase
`(SSADH). In the rat brain, valproate is a competitive inhibitor
`of SSADH vis a vis SSA with a Km value of about 0.8 ILM.
`(the author, unpublished results). The SSA level in brain tissue
`has not been well documented and its concentration of about
`0.1 !LM as once reported, seems rather low (127). Competitive
`inhibition of SSADH might be expected to increase the SSA
`level to a point where reduction of this catabolite to GHB is
`favoured, because the specific SSA reductase, which is not
`inhibited by valproate, (150) appears at least, in vitro, to
`be responsible for SSA reduction to GHB (149). However, re(cid:173)
`cent evidence indicates that an accumulation of GHB would
`not otherwise affect the functioning of the GABA shunt path(cid:173)
`way(25).
`The nonspecific cytosolic aldehyde reductase with a high
`Km which has been proposed as the enzyme that metabolises
`GHB to GABA, in vivo (94) is inhibited by valproate (29,95),
`thus indicating another potential mechanism for elevating the
`endogenous GHB level. In addition, the mitochondrial mono(cid:173)
`carboxylic fatty acid oxidising system for which GHB is a
`substrate in vitro, is also inhibited by valproate (48). Nonethe(cid:173)
`less, both of these possible metabolic systems require in vitro
`far higher GHB concentrations than the normal endogenous
`level, so their significance with regard to the endogenous GHB
`system is dubious. Valproate may thus block the formation of
`the GABA pool(s) derived from the administration of GHB.
`A mentally and physically retarded patient excreted sub(cid:173)
`stantial amounts of GHB and SSA in the urine (89). A number
`of such cases are now reported, and it is recognised as a rare
`genetic disease which is caused by a severe lack of SSADH
`resulting in a considerable build-up of GHB in the serum
`(137,144). This is most probably produced through the action
`of specific SSA reductase on the surfeit of the available SSA.
`The accompanying pathology might be due to the excessive
`tissue levels of GHB, its reactive aldehyde precursor, SSA, or,
`GABA metabolically derived from SSA which cannot enter
`the Krebs cycle. The GABA shunt has been estimated to ac(cid:173)
`count for about 100/o of the energy flux through the Krebs
`cycle in brain tissue (7), thus the energy balance could evi(cid:173)
`dently be locally perturbed and this could well be a major
`factor in the symptomology of the disease.
`Post-mortem cerebral tissue from Huntingdon's patients of
`which the disease is also characterised by neurological disor(cid:173)
`ders (22), contain higher than normal levels of GHB (3) and
`there is a striatal deficiency of succinate dehydrogenase (185),
`that is, an enzyme deficiency analogous to that observed in
`the genetic metabolic disorder referred to above. It could be
`suggested that a common factor is involved in the aetiology
`of both diseases. Whether GHB accumulation, and possibly
`its biotransformation to GABA, is a cause or a symptom of
`these diseases remains to be elucidated, although valproate
`has been used with some success to treat some of the symp(cid:173)
`tomology of the metabolic disorder (144). Valproate might
`block the formation of the GHB-derived GABA pool(s).
`However, there is a trend towards an inverse relationship be(cid:173)
`tween the GABA and GHB levels in Huntingdon's desease (3),
`and in an animal model of this disease induced by administra(cid:173)
`tion of kainic acid (4). As the Krebs cycle should be partially
`blocked by the lack of succinate dehydrogenase, the biosyn(cid:173)
`thesis of the metabolically active GABA pool might also be
`inhibited.
`
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`GAMMA-HYDROXYBUTYRA TE: NEUROTRANSMITTER AND/OR THERAPEUTIC AGENT
`
`295
`
`The data presented in this section do not in the author's
`opinion present either positive or negative evidence for a neu(cid:173)
`rotransmitter role for GHB in the CNS. However, they do
`suggest that malfunctioning of its metabolic pathways may
`provoke neurotoxic sequelae.
`
`BEHAVIOURAL AND EEG EFFECTS OF GHB OR
`GBL ADMINISTRATION
`The most striking effect is the induction of sedation, sleep
`and eventually anaesthesia in various animals and man. (Re(cid:173)
`viewed by Laborit in ref. 102) The major questions in this
`domain are to what extent this sleep resembles the natural
`form and how does it compare with that induced by other
`hypnotics such as the barbiturates, benzodiazepines, and even
`ethanol. To begin with, the dose of GHB required to induce
`sleep is considerably greater than that of currently used hyp(cid:173)
`notic drugs but is far smaller than a hypnotic dose of ethanol.
`However, in this model, GHB and ethanol display synergy
`(128) which may indicate that either they act on a common
`system or that ethanol induces an increase in neuroactive
`GHB. Such an increase has been documented in the liver
`(143), although ethanol actually reduces the striatal GHB
`levels (141).
`In rats, the 24h pattern of GBL-induced sleep parallels the
`normal pattern and is similar to that induced by barbiturates
`(183). In a human clinical trial, a dose of about 4 mmol /Kg
`GHB induced sleep which was indistinguishable from natural
`sleep as determined by behavioural and electroencephal(cid:173)
`graphic criteria (123). In another trial, GHB administration
`proved to be an excellent hypnotic with few side effects (85).
`No BEG abnormalities other than those observed with the
`normal sleep processes are observed (128). However, in cats
`(208), rats (124), rabbits (155), and monkeys (168), anaesthetic
`doses of GHB or GBL induced a depressed behavioural state
`which has been referred to in the rat, as catalepsy (80), and is
`accompanied by BEG recordings which resemble nonconvul(cid:173)
`sive epilepsy. Convulsions have been provoked in the cat by
`tactile stimuli after GHB administration (209). Thus it was
`suggested that GHB might play a role in the aetiology of
`absence epilepsy in man (67) despite the huge discrepancies
`between the normal brain concentrations of this compound
`and those induced in animals after administration of pro(cid:173)
`epileptic doses. However, in at least one experimental animal
`model, GHB exerts anticonvulsant activity (138), and it atten(cid:173)
`uates convulsions induced by the administration of a GABA
`synthesis inhibitor (191). Nevertheless the pro-absence epilep(cid:173)
`tic effects in several species have been proposed and used to
`serve as animal models of human petit mal epilepsy (167,171),
`a subject that will be dwelt with in more detail in the next
`section.
`
`A RESUME AND COMPARISONS OF
`THE GENETIC, GHB-INDUCED AND
`GABA8 RECEPTOR-MEDIATED ABSENCE EPILEPSIES
`It may be stressed at the outset that the rat model of genetic
`absence epilepsy (125,202), is exclusively an animal model,
`but that the BEG and behavioural attributes resemble those
`observed in human petit mal epilepsy particularly in children.
`Many data suggest that activation of GABA8 receptors is im(cid:173)
`plicated in this phenomenon (86,91,113,165,184), and that a
`GTP-binding protein is involved (164). Administration of ago(cid:173)
`nists at the GABA8 site are in some cases proconvulsant (36)
`whereas antagonists are anticonvulsant (86). Indeed, this re(cid:173)
`ceptor has been proposed to play a role in the generation and
`
`control of generalised absence epilepsy (113). The petit mal
`epileptic symptoms induced by peripheral administration of
`GHB (reviewed in ref. 171) is not applicable to humans (129).
`In the genetic form of this epilepsy, GHB administration exac(cid:173)
`erbates the symptoms (43,112) as do general gabamimetic
`drugs including the GABA8 receptor agonist, baclofen (203),
`whereas a GABA8 receptor antagonist suppresses the symp(cid:173)
`toms (113). Similarly, in the GHB model, local application of
`the GABA8 receptor agonist, baclofen increase the spontane(cid:173)
`ous epileptic events whereas an antagonist of this receptor
`Intracerebroventricularly applied
`them (168).
`suppresses
`GABA itself and peripheral administration of an agonist,
`muscimol of the GABAA·receptor binding sites, prolong the
`duration of seizures exclusively in young animals (172). As
`muscimol may be metabolised, it is not certain that it is act(cid:173)
`ing exclusively as a GABAA receptor agonist in this scenario.
`Thus, the genetic and GHB-induced models of absence epi(cid:173)
`lepsy are similar and share certain biochemical correlations,
`one of which may be the result of a common GTP-binding
`protein mechanism (164).
`The biochemical mechanism by which GHB exerts its be(cid:173)
`havioural effects is not known, but interactions with the bind(cid:173)
`ing sites referred to earlier and with the GABA8 receptor are
`of possible consequence.
`
`FACTORS THAT MODULATE THE BEHAVIOURAL AND
`EEG EFFECTS OF GHB/GBL ADMINISTRATION:
`POSSIBLE MODE OF ACTION
`The first drugs described that antagonise the BEG effects
`of GHB administration are some anticonvulsant& (66,169)
`which include valproate that has proven efficacious in the
`treatment of human petit mal epilepsy. Moreover, it sup(cid:173)
`presses seizures in the rat genetic absence model referred to
`above (125). This drug will be the mainstay of this part of
`the discussion as considerable biochemical data are available
`concerning its possible mechanisms of action. In this context,
`valproate inhibits the aldehyde reductase (29,207) which is
`believed to be responsible for the conversion of GHB to
`GABA (94,201). In addition, it inhibits the depolarisation(cid:173)
`induced release of GHB from rat brain hippocampal slices
`(195). Thus it would be expected to reduce the GABA pool
`derived both from endogenous GHB and that derived from
`its administration. In this context it should be noted that val(cid:173)
`proate suppresses the visual evoked potentials facilitated by
`the GABA transaminase inhibitor, -y-vinyl GABA (133) and
`as such, in this case at least, it could be considered to be acting
`as a