Full text of DOI:10.1038/sj.bjp.0703794

British Journal of Pharmacology (2001) 132, 1 ± 4
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SPECIAL REPORT
Pregabalin may represent a novel class of anxiolytic agents with a
broad spectrum of activity
1
1Mark J. Field, Ryszard J. Oles & *,1Lakhbir Singh
1Department of Biology, P®zer Global Research and Development, Cambridge Laboratories, Forvie Site, Robinson Way,
Cambridge, CB2 2QB
The present study examines the e€ect of pregabalin (previously S-Isobutylgaba and CI-1008) in two
distinct rat models of anxiety. Pregabalin binds with high anity and selectivity to the a2d subunit of
voltage dependent calcium channels (VDCC). Its corresponding R-enantiomer (R-isobutylgaba) is
approximately 10 fold weaker. Pregabalin dose-dependently induced anxiolytic-like e€ects in both
the rat con¯ict test and elevated X-maze with respective minimum e€ective doses (MED) of 3 and
10 mg kg71. In contrast, R-isobutylgaba only showed activity at the highest dose of 100 mg kg71 in
the con¯ict test. These data indicate that pregabalin may possess clinical utility as a novel anxiolytic
agent and demonstrates the importance of the a2d subunit of VDCC in the mediation of anxiety
related behaviours.
British Journal of Pharmacology (2001) 132, 1 ± 4
Keywords: Anxiety; con¯ict test; elevated X-maze; rat; R-isobutylgaba; a2d subunit of voltage-dependent calcium channels
Abbreviations: CCK, cholecystokinin; CDP, chlordiazepoxide; CNS, central nervous system; MED, minimum e€ective dose;
TOA, time on open arms; TEOA, time on ends of open arms; TE, total entries; VDCC, voltage-dependent
calcium channels
Introduction For the last four decades the pharmacological
treatment of generalized anxiety has been dominated by
benzodiazepines. However, the use of these compounds is
limited due to their side-e€ects. They induce sedation and
interact with other CNS depressants (Woods et al., 1992).
Major problems with benzodiazepines are dependence and
withdrawal symptoms which have hindered their long term
use (Lader, 1999; Woods et al., 1992). Thus, a great deal of
research has been dedicated towards the discovery of novel
anxiolytic agents with an improved side-e€ect pro®le.
Pregabalin (previously S-isobutylgaba and CI-1008) is
structurally related to the inhibitory neurotransmitter GABA.
However, radioligand binding studies have shown that
pregabalin has negligible anity for GABA and many other
receptors but binds with high anity and selectivity to a
novel binding site in the CNS, recently identi®ed as the a2d
subunit of voltage-dependent calcium channels (VDCC) (Gee
et al., 1996). The corresponding R-enantiomer (R-isobutylga-
ba) of pregabalin has a 10 fold lower anity for the a2d site
(Taylor et al., 1993). The present study examines and
compares the e€ects of pregabalin and R-isobutylgaba in
animal models of anxiety.
X-maze A standard elevated X-maze apparatus was used in
these experiments (Singh et al., 1991). Animals were placed
onto the centre of the X-maze facing one of the open arms.
The time (TOA) and % time (%TOA) spent on open arms
and time spent in the end half sections of the open arms
(TEOA) together with total arm entries (TE) were measured
in a 5 min test period.
Rat conflict test These methods have been described before
(Singh et al., 1991). Brie¯y, rats were trained to press levers
for food reward in operant chambers. The schedule consisted
of alternations of four 4-min unpunished periods on variable
interval of 30 s signalled by chamber lights on and three 3-
min punished periods on ®xed ratio 5 (by foot shock
concomitant to food delivery) signalled by chamber lights
o€. The degree of foot shock was adjusted for each rat to
obtain approximately 80 ± 90% suppression of responding in
comparison with unpunished responding. Rats received saline
vehicle on training days.
Drugs Drugs used were pregabalin and R-isobutylgaba
(P®zer Global Research and Development, Ann Arbor
Laboratories, MI, U.S.A.) and chlordiazepoxide (Sigma,
Poole, U.K.). All compounds were dissolved in 0.9% w/v71
NaCl (normal saline) and administered s.c. in a volume of
1 ml kg71, except the top dose of 100 mg kg71 pregabalin
and R-isobutylgaba which were administered in a volume of
3 ml kg71. The pretreatment times, route of administration
and doses were taken from previous studies (Taylor et al.,
1993; Singh et al., 1996).
Methods Animals Male Hooded Lister rats (200 ± 250 g)
were obtained from Interfauna Universal (Huntingdon,
U.K.). Animals were housed in groups of six under a
12 h light/dark cycle (lights on at 0700 h) with food and
water ad libitum, except rats used in the con¯ict
procedure were maintained at 80% of their free feeding
body weight.
Results Rat elevated X-maze The s.c. administration of
pregabalin (3 ± 30 mg kg71) 40 min before test, showed an
anxiolytic-like action in the elevated X-maze as indicated by
*Author for correspondence; E-mail: lakhbir.singh@p®zer.com

2
M.J. Field et al
Special Report
an increase in time and per cent time spent on the open arms
and time spent in the end half sections of the open arms with
a MED of 10 mg kg71 (Figure 1a ± c). The administration of
chlordiazepoxide (3 mg kg71) produced a similar anxiolytic
action on the X-maze to that of pregabalin (Figure 1a ± c).
Neither compound had any adverse e€ect on the total
number of arm entries indicating a lack of sedative action at
the doses used (Figure 1d).
possesses weaker anity for the a2d site only had a
modest e€ect in the con¯ict model at the highest dose
examined. These results indicate the importance of the a2d
subunit of VDCC in mediating the anxiolytic-like action of
pregabalin. This is consistent with previous studies
demonstrating activity for pregabalin and limited activity
of R-isobutylgaba in both epilepsy and pain models (Field
et al., 1997; 1999; Partridge et al., 1998; Taylor et al.,
1993). It has been shown that a2d binding sites are present
throughout the CNS, including the limbic system (Hill et
al., 1993). This area has high concentrations of benzodia-
zepine receptors and is known to play a major role in the
control of emotional behaviours.
It is known that the a2d subunit is common to all VDCC
(Hofmann et al., 1994; Isom et al., 1994) and it is also clear
that these channels play an important role in neurotransmis-
sion (e.g. control of neurotransmitter release). It seems
unlikely that pregabalin interacts with all VDCC as an
unselective blockade of Ca2+ channels would lead to severe
side e€ects. Animal studies examining pregabalin in models
of nociception indicate that it has a good side e€ect pro®le
(Field et al., 1997). Recently, it has been reported there are
at least three subtypes of a2d subunits (Klugbauer et al.,
1999) and pregabalin may interact speci®cally with one of
these.
Rat conflict test The s.c. administration of pregabalin
40 min before test, dose-dependently (1 ± 100 mg kg71
)
increased lever pressing in the punished period with a MED
of 3 mg kg71 which produced a 48.1% increase (Figure 2a).
A maximal e€ect (304.9% increase) was observed with
30 mg kg71 (Figure 2a). Unpunished responding was only
signi®cantly reduced at the highest dose of 100 mg kg71
(Figure 2a). R-isobutylgaba failed to a€ect punished
responding except at the highest dose of 100 mg kg71 which
produced a 76.4% increase (Figure 2b).
Discussion The results presented here show that pregaba-
lin possesses anxiolytic-like activity in both ethological and
con¯ict models of anxiety. Thus, it showed good
anxiolytic-like e€ects in the rat elevated X-maze and the
rat con¯ict test. The R-isomer of pregabalin, which
Figure 1 E€ect of pregabalin in the rat elevated X-maze. Pregabalin (3.30 mg kg72) or CDP (chlordiazepoxide, 3 mg kg71 s.c.)
was administered s.c. 40 min before test. The time (a: TOA) and per cent time spent (b: % TOA) on open arms and time spent in
the end half sections of the open arms (c: TEOA) together with total arm entries (d: TE) were measured. The duration of each test
was 5 min. Results are shown as the mean (vertical bars represent +s.e.mean) of 7 ± 10 animals per group. **Signi®cantly di€erent
from vehicle treated controls P50.01 (ANOVA followed by Dunnett's t-test).
British Journal of Pharmacology vol 132 (1)

M.J. Field et al
Special Report
3
In the present study the magnitude of the anxiolytic
e€ects of pregabalin was similar to those of the benzodia-
zepine CDP in both models (Singh et al., 1996). Most non-
benzodiazepine ligands that induce potent anxiolytic-like
e€ects in the rat elevated X-maze show much weaker activity
in shock-induced con¯ict tests. For example both CCKB and
5-HT3 receptor antagonists have been shown to possess
anxiolytic like action in the ethological models but are only
weakly active or inactive in con¯ict tests (Costall et al.,
1990; Singh et al., 1991). To date, it is unclear as to which
preclinical models are more predictive of clinical utility.
However, it is interesting to note that some non-benzodia-
zepine compounds with good activity in ethological models
and poor activity in con¯ict models failed to make a
signi®cant impact on the anxiolytic market (Rodgers &
Johnson, 1995). For example, CCK compounds have been
unsuccessful to date in a number of clinical trials despite the
positive ethological preclinical data (Griebel, 1999). This
may suggest that activity in con¯ict models is most desirable
in preclinical anxiolytic drug candidates. Further studies are
required to determine the dependence liability of pregabalin.
However, it should be noted that gabapentin, a related
compound which also selectively binds to the a2d subunit of
VDCC, is anxiolytic in animals (Singh et al., 1996) and has
been in clinical use as an antiepileptic agent for some years
now with a very clean side e€ect pro®le. Thus, it has been
shown to be safe and well tolerated with the main side
e€ects being somnolence/dizziness and no reports of any
abuse liability (Crawford, 1996; Walker & Patsalos, 1996).
In conclusion, results of the present study suggest that
pregabalin is a novel anxiolytic agent and demonstrate for
the ®rst time the importance of VDCC in anxiety related
behaviours.
Figure 2 E€ect of (a) pregabalin and (b) R-isobutylgaba in the rat
con¯ict test. Pregabalin or R-isobutylgaba was administered s.c.
40 min before test. The results are expressed as mean per cent
increase or decrease of lever pressing of at least six animals per group
on test day compared with mean performances obtained the two
previous days following vehicle administration. Signi®cantly di€erent
from previous control days *P50.05, **P50.01, ***P50.001
(paired student t-test).
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(Received September 21, 2000
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