Neuroscience and Biobehavioral Reviews, Vol. 22, No. 5, pp.

623–633, 1998
q 1998 Elsevier Science Ltd
Pergamon Printed in Great Britain
0149-7634/98 $19.00 + .00

PII: S0149-7634(97)00058-4

Biological Bases of Anxiety

YAN CLEMENT* AND GEORGES CHAPOUTHIER

URA CNRS 1957, Université Paris VI, Hôpital Pitié-Salpêtrière, 91 boulevard de l’Hôpital, 75634 Paris, Cedex 13, France

CLEMENT, Y. AND G. CHAPOUTHIER. Biological bases of anxiety. NEUROSCI BIOBEHAV REV 22(5) 623–633.—Anxiety is a
complex psychological and behavioural trait, related to behavioural responses as different as seizures, memory, impulsivity or
aggression. The biological bases analysed in the present article involve neurotransmitter systems, specific regions in the brain and
genetic factors. The genetic approach is given special emphasis as it offers, in rodents, a promising field for acquiring knowledge on
biological factors modulating anxiety. q 1998 Elsevier Science Ltd. All rights reserved.

Anxiety Neurotransmitter systems Brain regions Genetic factors

INTRODUCTION disorder.To date animal models have been divided into

two main groups (164):
CLINICAL RESEARCH into psychiatric diseases recog-
nises not just one anxious syndrome but a number of syn- 1. models based on conflict-conditioned fear;
dromes involving several different biological substrates. 2. models using the spontaneous aversion (unconditioned
Anxiety occurs in response to various stressors that can be fear) of an animal to a new environment.As anxiety is an
physiological and/or environmental. Although anxiety is ubiquitous behavioural phenomenon, animal models are
considered to be a common emotional phenomenon in the used to study both normal and pathological anxiety. We
human population (17,148,161), it may be a cause of have therefore reviewed some of the most widely used
disturbance in daily life and may express a state of patho- models and the state of studies on the neurobiological
logical anxiety causing suffering in patients. The fourth basis of anxiety.
edition of the Diagnostic and Statistical Manual (DSM-IV)
of the American Psychiatric Association (58) describes
several syndromes: DIFFERENT ANIMAL MODELS

In conflict situations, a mild negative (and avoidable)

1. panic disorders (with or without agoraphobia); reinforcement (electric shock) is associated with the pro-
2. agoraphobia; curement of positive reinforcement (food and water). In the
3. social phobia; classical paradigm (67) a food reward obtained during non-
4. obsessive–compulsive disorder; conflict periods after pressing a lever can be associated during
5. post-traumatic stress disorder; conflict periods with an (avoidable) mild electric foot-shock.
6. acute stress disorder; and Conflict periods are indicated to the animals by a stimulus
7. generalised anxiety disorder. such as a light being turned on. During conflict periods, the
rate of lever pressing tends to be lower; this effect is
To gain a better understanding of the neurobiology of interpreted as anxiogenic as it is reduced by benzodiazepines
anxiety, neuroscientists have developed animal models to (BZ) (8,97,149) and increased by the BZ receptor inverse
reproduce some of the symptoms observed in humans agonist, ß-CCM increases it (134). Other paradigms (174)
(78,146). Animal models currently used for research into associate the electric shock with a water reward. The validity
anxiety need to satisfy two criteria: of these models is suggested by the consistent correlations
observed between ‘‘anti-conflict’’ BZ doses in rodents
and anxiolytic doses in humans (149,163). In conditioned
1. predictivity, i.e., manipulations known to influence the emotional responses, a stimulus informs an animal accus-
pathological state should have the same effects on the tomed to an operant type of conditioning of the impending
model; delivery of an unavoidable form of punishment. The behav-
2. syndrome selectivity, i.e., similarity between the iour of the animal thus tends to be repressed. In these tasks,
behaviour observed in animals and the human however, the effect of BZs is not as apparent (88,97).

* To whom requests for reprints should be addressed.

623
624 CLEMENT AND CHAPOUTHIER

The second group of models involves non-conditioned complex sub-unit composition may be responsible for
behavioural responses, usually based on novelty-induced behavioural diversity (for review, see Ref. (30)]).
variations in exploratory activity. The classical test is no In experimental animals, stressors and anxiogenic situa-
doubt the open-field situation, where an animal is placed tions modify the activity of ligands at BZ sites, as do the
for a set time in a new open (large) area. The area is divided ligands themselves (35). To date the neurobiological
into several subunits and the animal can move freely mechanisms involved in modifications of behavioural and
between these. Several behavioural parameters are thus physiological reactions to extrinsic stimuli have not
noted including a number of anxiety-related measures. It been discovered. Three main hypotheses are usually made
is widely believed that ‘‘anxious’’ animals will spend (109).
more time in the peripheral subunits and therefore less in
1. Stress and/or anxiety is/are caused by the secretion of
the central ones. Increased defecation and increased groom-
endogenous inverse agonists in the region of the BZ
ing can also be interpreted as anxiogenic activities. BZs tend
receptor, decreasing GABAergic tone and therefore
to reduce all of these anxiogenic measures (6,175). In the
causing a decrease in inhibitory pathway function.
two-compartment test, animals can move between a brightly
2. Changes occur with the BZ receptor and the activity of
lit (aversive) compartment and a dark (safe) compartment.
ligands is shifted towards the inverse agonist position in
An increase in the latency for entering the dark box, an
the spectrum. It may be hypothesised that patients
increase in the time spent in the lit box and an increase in the
with anxiety disorders have either differently constituted
number of transitions are all viewed as indices of reduced
BZ–GABA complexes, making them more vulnerable to
anxiety. This is generally the case with BZ-related com-
pounds (40,41) whereas ß-CCM, for example, has the stressors, or that stress or anxiety can induce such
changes.
opposite effect (12). In the elevated plus maze, rodents are
3. An endogenous agonist tone can be observed in normal
placed in a plus-shaped maze with two open and two
enclosed arms raised above the ground. More time spent individuals; this is diminished or less effective in the
presence of anxiety disorders. Many compounds have
in the open arms rather than the enclosed arms is said to
been proposed as putative endogenous candidates
reflect the rodents’ aversion to open (and elevated) spaces
(22,53,119,145). The inverse agonists, Diazepam bind-
(83,126,167). Anxiolytics tend to increase the time spent in
ing inhibitor (DBI) and an octadecapeptide, a product of
the open arms, whereas anxiogenic compounds tend to
its trypsic digestion were initially thought to be endo-
reduce it (102,126). However, caution must be exercised
genous (2,39). Unfortunately these products were arte-
when interpreting data expressed as a percentage, because
facts produced by biochemical extraction of brain tissue
of the very low basal level of open arm activity commonly
and there has been no clear demonstration of an increase
observed. In the social interaction test, the time spent on
in patients suffering from anxiety disorders. The search
active social interactions by pairs of alien male rats placed
in a neutral arena, is thought to indicate anxiogenic behav- for endogenous compounds with anxiogenic/anxiolytic
clinical effects which bind to the BZ site is being
iour. Interactions are higher in strong light/unfamiliar
pursued.
conditions than in a dim light/familiar conditions. BZs
tend to increase interactions, especially in the strong light/ Since it was discovered (159), the central BZ receptor
unfamiliar conditions, whereas anxiogenic compounds (cBZR) has attracted considerable interest because of the
produce the opposite effect (63,77). possible effect on the sedative, anticonvulsant and anxioly-
tic properties of BZs. In animal experiments, alterations to
the density (Bmax), affinity (Kd) and/or regional brain
PHARMACOLOGY AND THE NEUROTRANSMITTER SYSTEMS distribution of cBZR may be associated with anxiety
disorders (35,90,115,117)
The topic of anxiety continues to attract the interest of In humans ( 11C)flumazenil (Ro 15-1788) imidazo-
basic pharmacologists and clinical researchers. The follow-
benzodiazepine is a highly specific and selective
ing section reviews recent developments in the clinical
cBZR antagonist. This labelled ligand has been proposed
pharmacology of anxiolytic drugs and provides an update for clinical research studying cBZR in the human brain
on previous publications.
by means of positron emission tomography (1,9,143).
Flumazenil is either neutral or has weak anxiogenic
effects when administered in low doses to normal
GABA A-benzodiazepine
volunteers (59,87). In high doses it has partially agonist
The BZ site is an allosteric modulatory site on the effects (82).
GABA A pentamer. The stoichiometry of the GABA A recep- Numerous animal studies have shown clear evidence of
tor is not yet understood and the binding BZ site may be the potent anxiolytic effect of BZ agonists (55,80,95,150)
located on the a subunit. The prototypical full BZ agonists, and/or the anxiogenic action of BZ inverse agonists
chlordiazepoxyde and diazepam, have anxiolytic properties (23,32,129,134) as observed in various experimental
and a wide safety margin, especially when compared to procedures.
barbiturates. The effect of BZ site ligands can be influenced On the basis of many behavioural and pharmacological
both by ontogeny and phylogeny, so that rodent strains can studies, it is clear that the GABA A –BZ system is
be bred with diverse BZ sensitivity producing different involved in the pathophysiology of anxiety disorders.
behavioural responses. There is also increasing evidence Once specific and selective ligands for the different subunits
to show that the effects of these ligands differ according are accessible, there is no doubt that clinicians will
to the sub-unit composition of the particular GABA A recep- develop tools appropriate for patients suffering from such
tor, opening up the possibility that a different GABA A –BZ disorders.
BIOLOGICAL BASES OF ANXIETY 625

Serotoninergic neurotransmission (5-HT) pharmacological studies have reported that 5-HT 3 receptor
antagonists display anxiolytic properties (7,160).
Serotoninergic neurotransmission is a complex system of
At present, pharmacologists are waiting on a large
mechanisms involving pre- and post-synaptic events and
distinct 5-HT receptor subtypes. number of new types of drugs, such as 5-HT re-uptake
inhibitors which are alleged to have anxiolytic properties
Clinical and animal studies have produced increasing
(75,130) or more selective and specific ligands to 5-HT 1
evidence to suggest that serotonergic neurotransmission
or 5-HT 2 binding sites. The possible existence of links
may be involved in the etiology, expression and treatment
between other 5-HT subtypes (5-HT 4, 5-HT 5, 5-HT 6 and
of anxiety (24,71,73,156). Anxiety is usually associated
5-HT 7) and anxiety is yet to be proven. The 5-HT 4 subtype
with increased endogenous 5-HT, whereas anxiolysis
may prove to be a novel candidate for regulating anxiety
tends to be associated with decreased endogenous 5-HT.
(61).
To date, progress in pharmacology and molecular
biology have suggested that 5-HT receptors cover at
least 14 receptor subtypes. The 5-HT 1A, 5-HT 2 and 5-HT 3 Cholecystokinin system
subtypes play roles of differing importance in anxiety The role of Cholecystokinin (CCK) in anxiety was first
(84,128). observed in normal human volunteers (52). CCK is the most
Administration of partial/full agonist 5-HT 1A receptors common neuropeptide in the mammalian cerebral cortex
such as 8-OH-DPAT, buspirone, gepirone and ipsapirone and limbic system (42). Two molecular forms of CCK have
was tested on various behavioural models of anxiety been isolated (10) in the brain. The sulphated octapeptide
(54,74). In the light/dark exploratory model, there was fragment of cholecystokinin (CCK 8) is the form most
clear evidence that 5-HT 1A ligands had an anxiolytic commonly found. It has the same interactive affinity with
effect on mice (104), while the direct administration of a CCK A and CCK B receptors. The shorter fragment, such as
selective agonist into the hippocampus increased grooming the C-terminal tetrapeptide (CCK 4) shows greater selectiv-
and decreased social interaction (3). ity for CCK B than CCK A receptors. Specific CCK recog-
The postsynaptic 5-HT 1A subtype is located in limbic nition sites, divided into CCK A and CCK B subtypes, have
areas (neocortex, septum, hippocampus, amygdala) (124), been circumscribed and drugs that are selectively active on
with somatodendritic localisation (autoreceptor) in the raph these receptor subtypes have been synthesised (178).
é nuclei (173). To date, some experimental data suggest that Many pharmacological experiments in various animal
anxiolytic potency may be associated with presynaptic sites models have shown the putative role of CCKergic system
(94) whereas anxiogenic effects could be affected by post- in anxiety disorders (13,28,56). Systemic or intraventricular
synaptic receptors (3). It is more likely that the anxiolytic administration of a CCK agonist induced an anxiogenic-like
action of the 5-HT 1A agonists could be associated with both behavioural response (155,169). CCK B antagonists such as
pre- and post-synaptic receptors (affected by agonistic CI 988 and RB 211 produced anxiolytic effects in a
action on the pre-synaptic receptors) and an antagonistic marmoset used as a ‘‘human threat’’ model (86) or in the
effect on the post-synaptic receptors (141). black and white test in mice (for review, see Ref. (49)). In
The 5-HT 1B receptors are mainly found in the substantia humans, it was shown that CCK 4, a CCK agonist, induced
nigra, globus pallidus and striatum (124). The 5-HT 1B panic attacks (20,21). These effects were blocked by the
subtype is a presynaptic terminal autoreceptor (85). CCK B antagonist, L365,260 (19). The anxiolytic action of
Activation of these receptors may lead to an increase in CCKB antagonists has also been demonstrated in humans
anxiety and locomotion and a decrease in food intake, (139,176).
sexual activity and aggressive behaviour (16,144,147). The experimental data summarised above suggest that the
Although various 5-HT 1B agonists show anxiogenic-like CCK system is involved in the mechanisms of anxiety (par-
activity in animal models of anxiety, such as the elevated ticularly in panic disorders). The anatomical and pharmaco-
plus-maze test using mice (142), their involvement in the logical properties of the CCK B receptor make it a potential
anxiety process has not yet been clearly established, as we candidate for a neurobiological hypothesis on anxiety dis-
suggested in a recent study (33), particularly as 5-HT 1B orders. Nevertheless, the exact mechanisms are still unclear
receptors are also heteroreceptors and modulate the release because of interactions with BZs (18,56).
of other neurotransmitters (84).
The 5-HT 2A subtype is mainly located in the cortex,
Other systems
hippocampus, striatum, olfactory bulb and spinal cord
(123). The 5-HT 2C subtype is detected only in the choroid Aside from the three main neurotransmitter systems
plexus (123). Pharmacological studies have reported an (GABA A, 5-HT, CCK) responsible for the heterogeneity
anxiogenic effect of the non-selective agonist, mCPP for of symptoms in anxiety disorders, other neuromodulations
the 5-HT 2C, not only in animals but also in humans are possible and each constitutes a pharmacological target
(for review, see Ref. (47)). The DOI antagonist for the 5- for research to find more efficient therapeutic treatments.
HT 2A subtype is reported to induce an increase in ear- These pharmacological studies support the observation
scratching behaviour in mice (48) while ketanserin produces that anxiety is not determined by a single neurotransmitter
an anxiolytic-like profile in Vogel’s test (160). system, suggesting that a neurochemical disruption of
Although the 5-HT 3 receptor is found in limbic areas such homeostasis may lead to an expression of anxiety. Summa-
as the amygdala, the entorhinal cortex and, to a lesser extent, ries are presented here of a selection of the many putative
the hippocampus, the density of the 5-HT 3 receptor is quite systems.
low in the central nervous system (100). While the receptor The NMDA receptor is one of a subclass of receptors
mainly seems to be linked to anti-emetic properties (111), that bind excitatory amino acids. It is comprised of
626 CLEMENT AND CHAPOUTHIER

at least six different binding sites: an NMDA site, a traits—peripheral and central activities, leaning, rearing,
strychnine-insensitive glycine site, a phencyclidine (PCP) grooming and defecation—in the parent strains showed that
site, a magnesium site, a polyamine site and a zinc site. the ABP/Le strain was more reactive than the C57BL/6By
N-methyl-D-aspartate antagonists have been shown to strain. A further study segregating according to four
produce anticonvulsant (44) and muscle relaxant effects phenotypic markers in F 2 and backcross populations
(168). One animal study (14) and a pre-clinical evaluation strongly suggested that three autosomal regions were
(177) suggest that they may also have anxiolytic effects. involved in the control of anxiogenic behaviour: one in
Repeated administration of the NMDA antagonist CGP the chromosomal region containing the b locus on chromo-
37849 in Vogel and open-field procedures induced anxio- some 4, one in the chromosomal region containing the p
lytic effects (92). locus on chromosome 7 and one in the chromosomal region
Non-NMDA receptor antagonists, the AMPA (alpha-amino- containing the se locus on chromosome 9. Mice with the b
3-hydroxy-5-methyl-4-isoxazole-propionic acid)-selective and p loci in the homozygous form spent more time in the
receptor antagonist LY326325 and the AMPA/kainate- periphery of the open-field (31), a behavioural phenomenon
selective receptor antagonist NBQX have anxiogenic action often interpreted as a sign of anxiety (50,108). Mice carry-
in the plus-maze test (96). ing the se locus in the homozygous form spent more time
The adrenergic and dopaminergic systems have also been grooming (34), another behavioural phenomenon related to
shown to play a role in anxiety. The role of noradrenaline in anxiogenic processes (158).
anxiety is supported by a link between clinically effective This approach can be compared to the study by Mathis
anxiolytic compounds and the NA system. For example, a 2 et al. (112) using different mouse strains. Mathis et al.
adrenoceptor agonists produce anxiolytic effects in the plus- used Recombinant Inbred Strains, produced by crossing
maze and Vogel tests (37,93) and the a 2 adrenoceptor C57BL/6J and A/J mice for behavioural studies to estimate
antagonist, yohimbine, induces anxiogenic effects in both the number and location of the chromosomal loci
humans (27) and animals (150). responsible for ß-CCM-induced seizures and anxiety related
The dopaminergic system may also be involved in behavioural phenomena. Anxiety was measured in a light–
anxiety disorders, in particular those where dopaminergic dark exploratory model with or without diazepam treatment
activation involves both D1 and D2 receptors (153,154) and in an open-field situation. Of the many loci likely to be
involved in anxiogenic mechanisms, the highest probabil-
ities of linkage were found between the (anxiolytic)
REGIONS response to diazepam and loci on chromosomes 1 (Xmv-41)
and 10 (D10Mit2).
Since defensive behaviour in animals has some of the
All these data concur with other recent research studies
same components as human anxiety, it is often used to
(99,140) and lead to the conclusion that genetic factors and
test the role of the dorsal periacqueductal grey (DPAG) in chromosomal fragments containing putative loci modulate
the mid-brain in the mechanisms underlying anxiety. Elec-
anxiety-related behaviour. It is likely that many different
trical stimulation of the area in or near the lateral edge of the
loci are involved in the control of anxiogenic behaviour.
central grey produced strong emotional reactions in most
Their effects depend on their reciprocal epistatic effects and
patients (121). In animals, systemic injection of either BZ
on the genetic method used.
agonists reduced the effects of aversive DPAG stimulation
(29), while the administration of a 5-HT2 antagonist pro-
duced an increase (91). ANXIETY AS RELATED TO OTHER BEHAVIOURAL PROCESSES
The limbic system (amygdala, hippocampus) is another
Several studies (see also reviews in Refs (66,79)) provide
key site for anxiety, being involved in the integration of
evidence for a possible relationship between anxiety and
behavioural and physiological mechanisms in defensive other behavioural processes. Emphasis has been focused on
reactions. Micro-injections of BZs or 5-HT antagonists
seizures, memory processes and impulsivity.
into the amygdala produced anxiolytic effects in conflict
tests, while the injection of the 5-HT 1A agonist produced
an anxiogenic effect (72,51). Interestingly, a recent Anxiety and seizures
microdialysis study using a Vogel test observed an increase
As mentioned earlier (Section 3.1), BZs are both potent
in 5-HT in the dorsal hippocampus (113), highlighting the
anticonvulsants and anxiolytics. Conversely, in mice, many
involvement of the limbic system in the etiology of anxiety.
compounds known to have a convulsive effect at high doses,
such as ß-CCM (5–10 mg/kg i.p.), PTZ (90 mg/kg i.p.) or
picrotoxin (2.5 mg/kg i.p.) (133), when administered at
THE GENETIC BASES OF ANXIETY
moderate doses, also produce anxiogenic effects. For
Many animal studies using inbred strains have reported example, our observations have shown that ß-CCM at
strain differences in behaviour-related anxiety, suggesting 1 mg/kg i.p. is anxiogenic in mice both in a Geller–Seifter
that genetic factors could be associated with anxious phe- conflict situation (134) and in a light–dark preference task
notypes (99,112,125,162). A series of studies conducted (12). Similar effects were observed with other ß-carbolines
with mice attempted to analyse any possible control of and BZ partial agonist inverses in the plus-maze test
anxiogenic processes exercised by genetic loci. The study with rats (FG 7142, ß-CCE, ZK 132,556, ZK 90886, ro
observed reactivity to a new environment, using an open- 15-4513 (36))
field procedure. Two strains of mice were used: C57BL/6By The same applies to PTZ (25 mg/kg i.p.) or picrotoxin
and ABP/Le, the F 1 populations and the F 2 intercrosses and (0.85 mg/kg i.p.) in the Geller–Seifter test (171). All these
backcross segregating populations. Analysis of behavioural compounds exert their action through the GABA–BZ
BIOLOGICAL BASES OF ANXIETY 627

TABLE 1
Action on convulsions, anxiogenic effects and learning of molecules acting or not acting on the GABA–benzodiazepine receptor cpmplex in mice

Effect Drug (mg/kg)

Pentylenetetrazol Picrotoxin ß-CCM Strychnine

Convulsive effect 90 2.5 10 1.5

Anxiogenic effect 25 0.85 1 no effect
Learning/enhancing efect 10 0.3 0.2–0.3 no effect

receptor complex. However, strychnine, a convulsant acting together with observations recorded by Mathis et al. suggest
on glycine transmission (46), seems to be devoid of any that, despite occasional related actions between ß-CCM-
anxiogenic effect (170). induced seizures and anxiogenic processes, which can
Several studies parallel to our work have provided evi- probably be attributed to common underlying GABAergic
dence of anxiogenic effects of either convulsant or procon- mechanisms, most of the data do not tally with the hypoth-
vulsant ß-carbolines. In Rhesus monkeys, Ninan et al. (122) esis: ß-CCM-induced seizures may be independent of any
found that ethyl ß-carboline-3-carboxylate (ß-CCE) pro- anxiogenic processes.
duced a wide range of responses, such as an increase in
heart rate, blood pressure, plasma cortisol or catechola-
mines, which have been proposed as indications of Anxiety and memory processes
‘‘anxious’’ responses in other primate models of ‘‘anxiety’’. Numerous studies have reported effects of BZs on mem-
In rodents, File et al. (64) observed that ß-CCE significantly ory processing. Marked anterograde amnesia is observed in
reduced active social interaction. In conflict situations using humans, both in patients and healthy volunteers (see review
rats and water reinforcement, several ß-carbolines showed in Refs (45,101)), as well as in rodents (157). This phenom-
pro-conflict effects (38,129). enon is routinely used in surgery where BZs are adminis-
Most of these anxiogenic effects can be suppressed by the tered as premedication. BZs do not produce ‘‘retrograde
administration of flumazenil (for a review, see Ref. (127)).It amnesia’’ for events memorised, even shortly before BZ
has even been hypothesised (57) that ß-carboline FG 7142 administration (70). The anterograde impairment induced in
may have an anxiogenic effect. memory processing by BZs in humans clearly concerns the
Thus, both convulsant ß-carbolines at sub-convulsive acquisition of information, as assayed by a long term
doses and proconvulsant ß-carbolines have anxiogenic retention test. For example, when subjects learn lists of
effects. These effects can be generalised to other convulsive words or of geometrical patterns just after BZ administra-
ligands of the GABA-benzodiazepine receptor complex, tion (i.e., undergoing BZ treatment), recall of the lists in a
such as PTZ and picrotoxin, but not to other convulsants later test is strongly impaired; this impairment is usually
not acting on the GABA receptor complex, such as strych- correlated to the dose administered and to the route of
nine (see Table 1). Only convulsant agents acting on the administration (5,68,101) or to the difficulty of the task
GABA–BZ receptor complex appear to be anxiogenic (45). When acquisition impairment is assayed in short-term
(171). retention procedures, the observations differ according to
This could lead to the idea that seizures and anxiogenic the authors, with some reporting effects (25) and others
processes may be physiologically related. This hypothesis refuting them (69,103). Anterograde effects of BZs might be
was tested on a genetic level by Mathis et al. (112) in the partly linked to their sedative effect, although a specific
study previously quoted. Using the Recombinant Inbred action in disrupting consolidation cannot be precluded (45).
Strains, the authors also calculated the number and location State dependency has also been suggested by some authors
of the chromosomal loci responsible for ß-CCM-induced and rejected by others (45,101). Depending on the clinical
seizures. The highest probabilities of linkage for this trait use of the term, BZs continue to be called ‘‘amnesic’’ or
were found with a locus on chromosome 15 (D15Mit5). ‘‘amnestic’’ agents, the effect seems to be more an impair-
However, when compared with the anxiety results for the ment of learning (acquisition) than an effect on memory
strains mentioned earlier (see Section Section 5), the data (retention) itself and terms such as ‘‘acquisition impairing
suggest that, at least for this set of Recombinant Inbred agents’’ would certainly be more appropriate.
Strains, ß-CCM-induced seizures and anxiogenic processes While BZs impair memory processing, inverse agonists
are not genetically related. We undertook a study of this such as ß-CCM have the opposite—enhancing—effect. The
question on both genetic and biochemical levels, working in first demonstration was provided by our group studying
collaboration with the Launay laboratory, observing mouse mice in two learning situations (habituation to a new envir-
populations sensitive to (ABP/Le, ABPB6.ABPB6.F2 and onment and classical passive avoidance) and chicks in one
ABP.ABPB6.BC) or resistant (C57BL/6ByJ) to ß-CCM- learning situation (26,172). In mice, a 0.2–0.3 mg/kg dose
induced seizures. We analysed both anxiogenic sensitivity of ß-CCM administered before training improved learning
in an open field and ( 3H)flumazenil binding. The only result for the habituation to a new environment task as well as for
to tally with the hypothesis was a possible relation between the passive avoidance task, whereas BZ administration
ß-CCM-induced seizures, anxiogenic processes and a lowered learning performance. The effects of ß-CCM
Bmax decrease in ( 3H)flumazenil binding (32). Our results, were confirmed in an acquisition study on a completely
628 CLEMENT AND CHAPOUTHIER

different model: imprinting in chicks. As demonstrated by complex but on the 5-HT 1A receptor (135). This could lead
Konrad Lorenz (105,106), newly hatched chicks tend to to a different approach, studying the interactions between
follow a moving decoy introduced during an imprinting memory and anxiogenic processes on a physiological level.
session. When retested in the same situation 24 h later,
imprinted chicks, having memorised the characteristics of
Anxiety and impulsivity
the situation and of the decoy, follow the decoy for a longer
time than non-imprinted chicks. When the first test session Many convergent studies in the field of biological psy-
was conducted after an administration of ß-CCM (2.5 mg/kg), chiatry suggest that several behavioural disorders could be
retention performance was enhanced, whereas benzodiaze- subsumed into a cross-nosological dimension: impulsivity.
pine administration produced the opposite effect. In all these An animal model of impulsivity was devised by Thiébot
tasks, the effects of ß-CCM (and BZ) were obtained only et al. (165) on the basis of the waiting ability of rats.
when the drugs were administered before the training When given the choice in a T-maze of obtaining either a
session, suggesting an effect on acquisition rather than on large quantity of food following a waiting period or a small
retention. This again confirms that the term ‘‘impairing quantity of food without delay, impulsive rats tend to prefer
acquisition’’ would be more appropriate than ‘‘amnesic’’. the second alternative whereas non-impulsive animals
Other experiments made it possible to extend these findings prefer the first one. The pharmacological study of this
to a multiple-trial learning procedure in mice: brightness model led to slightly paradoxical results: while antidepres-
discrimination in a T-maze with negative (138) or positive sants such as fluoxetine enhance the ability of the animals to
(136) reinforcement. For this task, the BZ receptor antago- tolerate a longer waiting period, BZs tend to reduce this
nist flumazenil, known to reverse BZ-induced amnesia in ability (166). The paradox may be in appearance only, as
humans without inducing any intrinsic effect (107), is not improved impulse control by antidepressants, as suggested
totally innocuous but has an enhancing ß-CCM-like by the enhanced waiting capacity, could easily be related to
effect on learning (137), an effect also observed by Lal the therapeutic activity of these drugs on pathologies
et al. (98). In differently negatively reinforced task tests on (bulimia, obsessive–compulsive disorders) characterised
mice, ß-CCM was also found to have an enhancing effect on by a number of clinical features, including deficient impulse
learning in rats (4,114). The study by Mayo showed that control (132). BZs, however, are known to suppress inhibi-
local infusion of ß-CCM (3 mg/0.5 ml) into the nucleus tions and reduce impulsive behaviour. According to these
basalis magnocellularis of the rat enhances recognition results, a decrease in anxiogenic processes may also involve
performance in a two-trial recognition task. Other studies an increase in impulsivity.
(170,171) have shown that other ligands of the GABA
receptor complex such as PTZ or picrotoxine have the
Anxiety and aggressivity
same learning-enhancing action at low doses, whereas
strychnine has no such effects (Table 1). We can thus Aggressive behaviour (or antagonistic behaviour) has
conclude that only the ligands of the GABA–BZ receptor been extensively studied in rodents. Several elements of
complex, which are anxiogenic in moderate doses, can, in data provided evidence of strong involvement of GABAergic
low doses, enhance learning. mechanisms in the control of aggressive behaviour (116).
As mentioned by Belzung et al. (15), the relationship The GABA level was found to be lower in the olfactory
between anxiety and memory is complex, as neither bulbs of aggressive versus non-aggressive mouse strains
memory nor anxiety are unitary phenomena. After compar- (60), as well as in muricidal rats as opposed to non-
ing several situations producing anxiogenic responses in muricidal rats (118). Pharmacological reduction of
mice (11), Belzung et al. used the neophobic response in a GABAergic inhibition in rats induces an increase in
modified open-field as a paradigm for an anxiety ‘‘trait’’; muricidal behaviour (110). BZs, however, are known to
the response of the animals in a light/dark choice procedure have anti-aggressive effects (62,116). Social isolation,
was interpreted as ‘‘state anxiety’’. Memory as assessed in a which has been shown to reduce GABAergic activity
passive avoidance test was linked to ‘‘state’’, but not (110,152), also increases aggressive behaviour (43,81,151).
‘‘trait’’ anxiety. Further studies of this kind could produce It was thus interesting to conduct a direct study into the
a better understanding of the different memory types linked possible relationship between anxiety and aggressive behav-
to different types of anxiety. iour. A comparison was made with male mice from ten
Finally the problem of the possible physiological different inbred strains, using both a two-compartment test
relationship between anxiogenic and learning-enhancing (for estimating anxiogenic processes) and an aggression test
processes led to the same question as the relationship against a standard opponent. A strong negative correlation
between anxiety and seizures. An obvious relationship can was found (76) between the proportion of attacking males
be found for aversive learning tasks. In aversive situations it and the time spent in the lit compartment, suggesting that
could be assumed that the apparent effects of ß-carbolines the more attacking males are also more anxious. A reduction
on acquisition are due to changes in emotional reactivity to in GABAergic inhibition could thus lead to both a higher
the aversive stimuli used. This interpretation is not true, tendency to attack and to anxious behaviour.
however, in non-aversive learning situations (114,136).
Only systematic analyses of the possible relationship CONCLUSION
between anxiety and acquisition on genetic and biochemical
levels, similar to those undertaken for anxiety and seizures, Anxiety is a complex psychological and behavioural trait,
will be able to shed light on this important question. Recent possibly related to behavioural responses as different as
studies have been able to show an effect on the memory seizures, memory, impulsivity or aggression. Progress made
processing of molecules not acting on the GABA receptor in the neurobiology of anxiety has therefore encountered
BIOLOGICAL BASES OF ANXIETY 629

difficulties in identifying a single causal agent. As has been isolated or cloned. Anxiety being a complex quantitative
emphasised in the present paper, many neurotransmitter psychological trait, it is likely to be regulated by polygenes,
systems are involved in determining anxiety. Animal studies making it even harder to decipher the precise genetic
are an essential method for improving our knowledge of control. Because of the numerous Mendelian tools (inbred
anxiogenic processes and therefore the pharmacological strains, multiple marker strains etc.) offered by rodents for
treatment of anxiety disorders. Much is expected from the the analysis of genetic factors, it is this genetic approach,
knowledge of specific regions of the brain which may be involving modern molecular methods, which is likely to
involved in the physiology and biochemistry of specific produce new impacts on research into the biologically
anxiety syndromes. For this purpose, the discovery of determining factors causing anxiety.
genetic factors modulating anxiety also presents an interest-
ing tool as many chromosomal fragments in rodents are in
synteny with human chromosomal fragments (120). ACKNOWLEDGEMENT
Although there is strong evidence for the role of certain We wish to thank Ms Shan Benson for improving the
genetic factors (studies by the present authors and English text.
Refs (65,89,112,131)), putative genes have not yet been

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