PharmacologyBiochemistryand Behavior,Vol.43, pp. 807-814. 1992
0091-3057/92$5.00 + .00
Copyright©1992PergamonPress Ltd.
Printedin the U.S.A. All rightsreserved.
Effects of Central Administration of
Kynurenic Acid on Spontaneous
Locomotor Activity in the Kindled Rat:
A Multivariate Approach Using the
Automated Digiscan Monitoring System
Z O E D E N N I S O N , *l K L A U S - P E T E R O S S E N K O P P I " $ A N D D O N A L D P. C A I N t
*Department o f Psychology, M o u n t Allison University, Sackville, N e w Brunswick, Canada, EOA 3C0
Departments o f ~Psychology and SPharmacology and Toxicology,
University o f Western Ontario, London, Ontario, Canada N6A 5C2
Received 22 A p r i l 1991
DENNISON, Z., K. P. OSSENKOPP AND D. P. CAIN. Effects ofcentraladministration of kynurenic acid on sponta-
neous locomotor activity in the kindled rat: A multivariate approach using the automated Digiscan monitoring system.
PHARMACOL BIOCHEM BEHAV 43(3) 807-814, 1992.- Changes in spontaneous motor activity in kindled hooded rats
were measured following intracerebroventricular administration of three doses of kynurenic acid (65, 39, and 6.5 ~g, dissolved
in 3.3 #1 isotonic saline). Behavior was measured in the automated Digiscan system on every third day during 13 days of drug
administration to assess initial behavioral impairment and the development of tolerance. Activity data were collected beginning 5 min after drug administration for six consecutive 5-min samples. The results revealed a suppressive effect of central
administration of kynurenic acid on the pattern of spontaneous locomotor activity and showed the development of behavioral
tolerance. Initially, the degree of suppression was dose related, but as tolerance developed group differences were minimized.
Most measures returned to predrug levels by day 13 except vertical movement, which remained suppressed in the 65-~g group
throughout testing. This measure may have been more sensitive to the subtle and long-lasting motor impairments resulting
from kynurenic acid.
Kynurenic acid
Digiscan monitoring system
Behavioral impairment
Drug tolerance
Multivariate assessment
Excitatory amino acid antagonist
Locomotor activity
Rats
ANTAGONISTS of excitatory amino acids (EAAs) have been
suggested as potentially useful anticonvulsants. EAA antagonists are specific in varying degrees to the main subtypes of
EAA receptors. These receptors have been divided in three
general subtypes, named for the agonists that preferentially
activate them; kainate preferring, quisqualate preferring, and
NMDA preferring (30,31). Ongoing research is dividing the
non-NMDA subtypes into more precise categorizations that
more accurately reflect the action of agonists and antagonists
at these receptors (1,15,27).
Use of these compounds as anticonvulsants in animal research has revealed a variety of behavioral effects that must
be considered before clinical use is considered. The primary
feature of these effects is discoordination, ataxia, and overall
suppression of behavior. Swiss and DBA/2 mice exhibited
Kindling
sedation, reduced or impaired locomotor activity, and ataxia
when NMDA antagonists were administered (ICV) or IP (5).
Meldrum et al. (17) gave 2-amino-5-phosphonovaleric acid
(APV), a potent and highly selective NMDA antagonist (30),
intravenously in the baboon Papio papio and reported signs
of sedation, including drooping eyelids and loss of muscle
tone. Koek et al. (13) reported catalepsy after pigeons received
APV ICV. Gilbert and Mack (10) reported that the NMDA
antagonist MK-801 produced an atoxic state. Studies in our
laboratory indicated that ICV administration of either APV
or kynurenic acid results in ataxia and overall suppression of
behavior in rats (3,6).
Kynurenic acid is an endogenous tryptophan metabolite
that acts as a postsynaptic EAA receptor blocker (9). Perkins
and Stone (23) reported that kynurenic acid blocked EAA
To whom requests for reprints should be addressed.
807
808
DENNISON, OSSENKOPP AND CAIN
transmission at synapses using quinolinic acid, NMDA, and
quisqualic acid with no preferential antagonism. However,
Ganong et al. (9) reported that at low doses kynurenic acid
appeared to act preferentially on the NMDA receptor.
We have reported that both APV (3) and kynurenic acid
(6) significantly retard the development of kindled seizures,
an experimental model o f temporal lobe epilepsy (11,25), in
the rat. This article consists of a quantitative analysis of the
changes in spontaneous motor activity in rats used in our
previous report of the effect of kynurenic acid on amygdala
kindling (6). Spontaneous activity was assessed in a multivariate fashion using the Digiscan Animal Activity Monitor.
METHOD
Subjects
Male hooded rats of the Royal Victoria strain, weighing
between 250-450 g at the time o f surgery, were housed individually in stainless steel cages with wire mesh bottoms. The
cages were kept in racks in a colony room, which was on a 12
L : 1 2 D cycle, with the light on from 0800-2000 h. Food
(Purina lab pellets) and water were available freely, and the
colony room was maintained at 22 _+ 1 *C. Animals were
placed on trays in groups and allowed to interact freely for
periods of several hours both previous to surgery and throughout the experiment.
Apparatus
The spontaneous behavior of rats was monitored using
four Digiscan Animal Activity Monitors (Omnitech Electronics Inc., Columbus, OH; model RXYZCM-16) [cf. (26)]. Each
monitor consisted of a clear Plexiglas box, measuring 40 x
40 × 30.5 cm, with infrared monitoring sensors mounted every 2.54 cm along the perimeter, for a total of 16 beams per
side. These were 4.5 cm above the floor. In addition, a second
bank of 16 sensors on two sides were located 15 cm above the
floor of the box to tabulate vertical activity. Data were collected and analyzed by a Digiscan Analyzer (Omnitech Model
DCM), which in turn transmitted the data to an Apple II +
computer for storage. The Plexiglas boxes were thoroughly
washed between each session to remove any odor cues.
Surgery and Drug Administration
Rats were implanted with bilateral cannulae aimed at the
dorsal surface of the lateral ventricles. Bilateral electrodes
aimed at the basolateral amygdala were implanted at the same
time using standard stereotaxic techniques (6). Four groups of
animals (HI, MED, LOW, SAL) received 65, 39, 6.5, 0 #g
kynurenic acid dissolved in 3.3/~1 isotonic saline, respectively.
All solutions were adjusted to a p H of 7.4. The solution was
infused over 10 s using a Sage infusion pump (Sage Instruments, divis, of Onon Research Inc., Cambridge, MA). The
injection cannula was left in place for 30 s after infusion to
allow for diffusion o f the drug or saline. The drug was administered every other day, followed 40 min later by electrical
stimulation in a standard kindling procedure (6). Movement
data were collected between 1200 and 1500 h, under normal
room light levels, in the period between drug injection and
electrical stimulation, beginning 5 min after drug injection.
Measurement of Spontaneous Activity
The behavioral measures were defined as follows: horizontal activity (HA), the total beam interruptions for the lower
bank of sensors; number of horizontal movements (NHM),
the number of continuous-beam interruptions, with a minimum l-s cessation separating movements; total distance traveled (TD), the number of inches travelled; average speed (AS),
the average distance per given time interval; movement time
(MT), the number of seconds the animal was in motion during
the given time sample; and vertical activity (VA), the total
number of beam interruptions for the upper bank of infrared
beams.
Animals were habituated in the activity monitor for two
sessions prior to experimental manipulation [cf. (19)]. Data
were collected for the two habituation sessions (Hab I and 2),
the first drug day (DD1), and every third drug day thereafter
(DD4, DD7, DDI0, DD13). For each test session, data were
summated six times at 5-min intervals and then totaled and
averaged.
Forty minutes following infusion, each animal was electrically stimulated and both convulsive behavior and length of
afterdischarge (AD) recorded. The kindling procedure
consisted of administering a unilateral low-intensity electrical stimulation daily. This stimulus initially evokes little
or no change in behavior or electrographic activity. During the course of the process, the stimulation begins to evoke
convulsive behavior and complex epileptiform electrographic activity. The process is terminated when the animal has
a fully generalized convulsion (6). Following testing, animals were deeply anesthetized and perfused with formalin saline, after which brains were removed, frozen, and
sectioned. The placement of electrodes and cannulae was
verified (22). Following verification, there were 10 animals in
the HI group, 8 in the MED, 8 in the LOW, and 10 in the
SAL group.
Data Analysis
The data collected by the activity monitor were analyzed in
two ways. First, to determine the overall effect of the drug
and reveal the development of tolerance the pattern of behavior change across days was analyzed by collapsing the six
samples per day to obtain an average 5-rain sample per day
(20,21). These data were then examined with a group × day
design repeated-measures multivariate analysis of variance
(ANOVA) for all six measurements. The group factor had
four levels (HI, MED, LOW, and SAL) and the day factor
had five levels (DD1, DD4, DD7, DD10, and DD13). Comparisons between all four groups on DD1 and DDI3 were carried
out subsequently.
Second, the pattern of behavior across samples (within
days) was examined with a group × day × sample design repeated-measures ANOVA for each of four measures: HA,
VA, TD, and NHM. The sample factor represented the six
bins of data collected within each test session at five minute
intervals, and the day factor represented the two habituation
sessions plus the five drug days. Separate ANOVAs were performed for the first habituation session and selected drug days
for the H A and VA variables.
RESULTS
Control Group Activity Pattern
Before examination of spontaneous activity after administration of kynurenic acid, the nature of normal movement
patterns in the vehicle control group was examined.
KYNURENIC A C I D AND SPONTANEOUS ACTIVITY
809
Across days. The pattern of baseline locomotion seen over
the course of the experiment was demonstrated by the data
from the SAL group. There were only small fluctuations in
activity rate for the duration of the study (Figs. 1A-D) and
the pattern was relatively stable, indicating that changes seen
in the experimental groups were attributable to drug manipulations.
Across samples. A basic pattern that was relatively consistent across measures was observed in the habituation phase.
Animals showed an initial high level of activity at the first
sample that declined quickly with habituation and remained
relatively level for the second half of the session (Figs. 2A and
3A). The only measure that differed from this basic pattern
was NHM, which did not show the typical initially high peak
of activity on the first sample (not shown). This suggests that
while animals spend more time in motion and travel further
at the beginning of a session they do so by increasing the
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Across days. A repeated-measures multivariate analysis of
variance (MANOVA) was carried out, with the four groups
(group) being compared across the five drug sessions (day) on
all six measures (HA, VA, NHM, TD, AS, and MT). The six
5-min data summaries collected on each test day were collapsed into one average session for the across-days analyses.
The multivariate tests of these data were significant (group
x days, p < 0.012; group, p < 0.0001; days, p < 0.037).
For each of the six variables, the group main effect was significant (p < 0.0001), as was the days main effect (p < 0.03).
The group x days interaction was significant for MT, NHM,
and H A (p < 0.01).
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FIG. 1. Spontaneous behavior in an average 5-min sample over 2 days of habituation and 13 days of drug administration. (A) Horizontal
activity. (B) Average speed. (C) Vertical activity. (D) Number of horizontal movements. Error bars are SEM.
810
DENNISON, OSSENKOPP AN D CAIN
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drug day 13. Error bars are SEM.
Following the multivariate analysis, Newman-Keuls comparisons were carried out between pairs of means from the
four groups on DDI and DD13. As performance on the HA,
MT, and TD measures appeared almost identical when plotted, only the statistical comparisons carried out on the HA,
VA, AS, and N H M measures are discussed.
Horizontalactivity. On DD1, the group effect was significant,
F(3, 32) = 8.02, p < 0.0004. HI and MED were both significantly different from LOW and SAL (p < 0.05) but not from
each other. LOW and SAL were not significantly different.
In Fig. 1A, it can be seen that for DD1 and DD4 the HI and
MED groups followed a similar pattern, as did LOW and
SAL. By DD7, MED was more similar to LOW and SAL. By
DDI3, the group effect was no longer significant.
Average speed of movement. On D D I , the group effect was
not significant (p < 0.06), although there was a trend for HI
to be significantly different from SAL. The group factor was
also not significant on DDI3 (p = 0.28). Despite the failure
of the group differences to achieve significance on DD1, visual
inspection of Fig. 2B suggests some dose-related separations
between the drug groups.
Vertical activity. This measure provides the best demonstration of a dose-related separation between groups (Fig. IC).
On DD1, the group effect was significant, F(3, 32) = 12.17,
p < 0.0001. HI was significantly different from LOW and
SAL (p < 0.05) but not from MED. Both MED and LOW
were significantly different from SAL (p < 0.05). On DDI3,
the group effect was no longer significant but inspection of
Fig. 1C suggests a weak residual dose-related drug effect.
Number of horizontal movements. On DD1, the group effect
was significant, F(3, 32) = 11.81, p < 0.0001. HI, MED,
and LOW were all significantly different from SAL (p <
0.05). HI was also significantly different from MED and
LOW (p < 0.05). On DDI3, the group effect was still significant, F(3, 32) = 3.98, p < 0.016. HI was significantly different from both MED and LOW (p < 0.05). The elevated performance of the LOW group on DD7 seems due primarily to
extraordinarily high performance of two animals in this group
(Fig. 1D). As this is not evident in the other measures, it
means number of movements may have increased but actual
time spent in movement did not.
Across samples. The pattern of motor activity was examined with a repeated-measures A N O V A for variations across
samples within a test session on four measures; HA, TD,
NHM, and VA. Across-day analyses were not carried out on
MT and AS, as the pattern of variation over samples and days
KYNURENIC ACID AND SPONTANEOUS ACTIVITY
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FIG. 3. Vertical activity across samples. (A) During the first habituation session. (B) During drug day 1. (C) During drug day 7. (D) During
drug day 13. Error bars are SEM.
appeared identical for MT and HA and for AS and NHM
when these measures were plotted. Further analyses were carried out for HA and VA on selected days.
Horizontal activity. All three main effects were significant
[group, F(3, 31) = 3.46, p < 0.028; day, F(6, 26) = 2.57,
p < 0.043; sample, F(5, 27) = 54.89, p < 0.0001]. Of the
possible interactions, group × day, F(18, 74.02) = 2.37, p
< 0.005, and group x sample, F(15, 74.94)) = 1.95, p <
0.031, were significant, but day × sample and group × day
x sample were not, indicating that groups changed differentially over days and over samples. Separate ANOVAs were
done on the data from each of the days shown in Fig. 2. In
the first habituation session, the pattern of movement seen
across samples in Fig. 2A was consistent for all groups.
Groups did not differ significantly, and there was no group
x sample interactions. On DDI (Fig. 2B), groups differed
significantly (p < 0.0001), and the group x sample interaction was significant (p < 0.002). On DD10 (Fig. 2C), the
drug effect was reduced and the group × sample interaction
was no longer significant (p < 0.087). By DD13 (Fig. 2D),
tolerance to the effects had appeared and the original pattern
seen in Fig. 2A was restored and the group x sample interaction was not significant (p < 0.161). The sample main effect
was significant for all days (p < 0.0001).
Vertical activity. All three main effects were significant
[group, F(3, 31) = 6.21, p < 0.002; day, F(6, 26) = 3.12,
p < 0.019; sample, F(5, 27) = 42.07, p < 0.0001]. Of the
possible interactions, group x day, F(18, 74.02) = 3.13, p
< 0.0001, and group x sample, F(15, 74.94)) = 1.90, p <
0.036, were significant, but day x sample and group x day
x sample were not. This indicates that groups differed significantly over days and over samples. Separate ANOVAs were
carried out on the data from the 4 days shown in Fig. 3. The
predrug pattern showed the characteristic habituation curve
(Fig. 3A), and the group x sample interaction was not significant. On DD1 (Fig. 3B), groups HI and MED were completely
suppressed, whereas the LOW group was halfway between the
SAL group and the other two drug groups. The group ×
sample interaction was significant (p < 0.0001). By DD7
(Fig. 3C), the MED group had begun to recover and the HI
group was still suppressed. Paradoxically, the group x sample interaction was not significant (p < 0.516) on DD7 al-
812
DENNISON, OSSENKOPP AND CAIN
though it was significant on DDI3 (p < 0.027). The lack of
significance on DD7 may be due to the high variance and
inflated performance of the LOW group. By DDI3 (Fig. 3D),
the MED group had reached the SAL group level but the
HI group was still somewhat lower. Note that as tolerance
developed the original baseline pattern of locomotor activity
was reestablished with the exception of the HI group, which
continued to exhibit suppression of movement. In the acrossdays analysis of VA, comparisons for DDI3 did not show a
statistical difference. The across-days analysis was based upon
a daily average 5-rain sample, whereas the across-samples
analysis was based upon consecutive time bins. These data did
reveal significant differences, as indicated by the significant
group × sample interaction (p < 0.027) and group main effect (p < 0.05).
Total distance travelled. (Data are not shown.) Both the day
main effect, F(6, 26) --- 4.93,p < 0.002, and the sample main
effect, F(5, 27) = 79.68, p < 0.0001, were significant. Group
differences did not achieve significance, but a trend for differences was observed (p < 0.10). Of the possible interactions,
the group x day, F(18, 74.02)) = 2.91, p < 0.001, and the
group x sample, F(15, 74.94)) = 1.96, p < 0.03, were significant, but the day × sample and the group x day x sample were not. This indicates that groups differed significantly
over days and over samples. The first habituation session
showed the standard habituation pattern over the six withinday samples. The initial administration of the drug (DDI)
produced a general flattening of activity for the entire session
for all three drug groups, showing little dose separation. By
DD7, the normal activity pattern was reestablished, but the
HI group still showed reduced distance traveled. By DDI3,
both normal patterns and normal levels of distance were
present.
Number o f horizontal movements. (Data are not shown.) All
three main effects were significant [group, F(3, 31) = 40.76,
p < 0.0001; day, F(6, 26) = 10.79,p < 0.0001; sample, F(5,
27) = 30.01, p < 0.0001]. All possible interactions were significant [group x day, F(18, 74.02) = 8.95, p < 0.0001;
group x sample, F(15, 74.94) = 5.2, p < 0.0001; day x
sample, F(2, 30) = 32.0, p < 0.031; group x day × sample, F(6.87, 90) = 3.69, p < 0.038]. This measure differed
from the other measures in terms of patterns across sessions
and days. The characteristic within-session habituation curve
was absent, and behavior declined gradually over the session
in a linear fashion. Initial drug administration (DDI) resulted
in suppression of number of movements in a dose-related pattern, but by DDI3 tolerance had developed and the pattern
more closely resembled predrug behavior.
DISCUSSION
The results indicate a suppressive effect of central administration of kynurenic acid on the pattern of spontaneous locomotor activity and show development of behavioral tolerance
to this drug. This effect followed a dose-response pattern
initially, and with repeated administration suppression was
reduced and behavioral tolerance demonstrated as activity
level increased toward predrug baseline. After 13 administrations of the drug, the HI and MED groups no longer differed
from the SAL group on almost all measures. The exception
was vertical movement, where the HI group remained suppressed in comparison with the SAL group. Perhaps vertical
movements (rearing) required more coordination, balance,
and muscular effort than any of the other movement measures
and therefore may have reflected a residual loss of coordina-
tion or strength still present in the HI group upon the 13th
drug administration that was not detected by the other measures. Tolerance to this effect might have developed if kynurenic acid had been administered for a longer time period. The
vertical movement measure was also more sensitive in terms
of dose-related separations between the various groups, showing a separation between the LOW and SAL groups that was
not as apparent on other measures.
When the levels of activity within a session were examined,
drug administration initially reduced levels of activity and flattened the curve that was characteristic of predrug and control
group performance. As tolerance developed, this predrug pattern of movement reappeared before the level of activity
reached control levels.
Interpretation of these results must take into consideration
that all groups underwent kindling from DD1-DDI3. The effect of the kindling process on spontaneous behavior is unclear. Behavior changes have been noted in animals that have
had several fully generalized convulsions (4,7,24). These findings are not applicable in our study, as our animals did not
have a fully generalized convulsion during the behavioral testing and displayed only minimal convulsive behavior during
the initial behavioral testing (DDI, DD4), where the greatest
impairments were seen. The data in Fig. 1 reveal that over the
course of testing animals return to prekindling (Hab 1 and
Hab 2) rates of activity, presumably due to the development
of tolerance to the drug effects. This suggests that the changes
in spontaneous behavior can be attributed primarily to the
action of kynurenic acid.
The degree of tolerance seen in this study contrasts with
the report by Boast et al. (2), who found only a slight tolerance
to the impairments induced by CGS 19755, a competitive
NMDA receptor blocker, in mice. These differences may be
due to the specificity of CGS 19755 as an NMDA receptor
blocker and possible species differences.
One interesting behavioral effect noted after kynurenic
acid administration was rats' ability to react quickly in response to certain stimuli. While no quantitative measures were
obtained, it was noticed that animals that responded in an
irritable manner to handling were quite capable of making
coordinated and competent aggressive movements toward the
hand of the experimenter when startled. Future assessments
of the motor side effects of EAA antagonists should include
efforts to measure the effect of arousal on motor suppression.
The underlying physiological mechanisms responsible for
the changes seen in behavior are unknown at this time. One
common cause for suppression of behavior as a result of administration of a drug is sickness or malaise, which could
result in decreased levels or changes in pattern of motor activity as a secondary effect. However, in the present study animals remained healthy throughout the course of the experiment. No changes were noticed in grooming or food or water
intake, and animals were even observed to gnaw food pellets
following kynurenic acid administration.
Paradoxically, some researchers reported increases in behavioral output following administration of an EAA antagonist. Liebman et al. (14) gave 2-amino-7-phosphonoheptanioc
acid (AP7) ICV to rats and measured their behavior in the
Digiscan apparatus. Samples were taken at 20-min intervals,
and they noted that behavioral activation occurred for some
measures after approximately 20-40 min. These differences in
observed effects might have been a function of time course
differences except we did not see any behavioral activation
within our 30-min measurement period following drug administration. Interestingly, Boast et al. did not observe behavioral
KYNURENIC ACID AND SPONTANEOUS ACTIVITY
activation in their measure of rearing behavior, and this measure showed longer-lasting impairment in our subjects. Differences might also be due to the differential actions of kynurenic
acid and AP7 at EAA receptors. This lends support to the
idea that different brain sites may be involved in different
behavioral effects seen after drug administration.
EAA antagonists have been suggested as possibly providing
new treatments for human epilepsy (16). While this suggestion
has merit, based upon the similarities between kindling and
human partial epilepsy (29) and the effect of EAA antagonists
on kindling, there are some drawbacks to these compounds
that must be resolved before clinical administration can be
recommended. Greater lipophilicity is needed to allow compounds to cross the blood-brain barrier and avoid the intracerebral administration used experimentally (16). In addition,
much more information about the possible motoric and other
side effects of these drugs is necessary.
A possible method of circumventing the side effects seen
with ICV administration of these compounds was suggested
by Frenk et al. (8), who administered several kinds of EAA
antagonists subdurally onto cortex. They found that this route
of administration still resulted in suppression of epileptiform
activity generated by strychnine, morphine, and picrotoxin,
but did not produce obvious behavioral side effects. Unfortunately, no quantitative measures of behavior were taken. It is
possible that a more detailed quantitative analysis of behavior,
such as in an activity monitor, might reveal more subtle druginduced changes.
A second concern is the actual effectiveness of these compounds in established epileptic conditions. While it seems clear
that EAA antagonists can retard the development of seizure
activity, their effect on fully kindled seizures seems to be not
as strong nor as clearly demonstrated (3,6). Although we
found (6) that kynurenic acid administration did retard the
rate of kindling in these rats, a measure of the rate of develop-
813
ment of epileptiform activity, it had only a weak effect on the
expression of seizures in fully kindled rats (where the epileptic
state was already established).
Another consideration concerning the role of EAA antagonists as anticonvulsants is whether the retardation on rate of
kindling is related to the degree of motor impairment. The
differing degrees of tolerance, with some motor impairment
seen while kindling was progressing, suggest that distinct neuronal mechanisms are involved in the mediation of these effects. In addition, the ability of animals, as previously discussed, to move quickly in arousing situations seems to
suggest that if the seizure activity had been present it would
have been expressed. Alternately, the small degree of change
in AD rates lends support to the idea that convulsive behavior
was suppressed while electrographic AD remained unchanged.
However, it should be noted that duration is only one way in
which AD can be assessed, and there are other measures, such
as the amplitude of AD in the contralateral hemisphere, that
seem more closely connected with degree of seizure activity
(28). Meldrum and Chapman (16) suggested that further receptor subtype selectivity may result in a compound that does
not block the NMDA receptors involved in motor output but
still acts as an anticonvulsant.
Antagonists of EAAs have also been used to demonstrate
effects on learning and memory independent of the effect on
behavior (18). In a recent review of this area, Keith and Rudy
(12) suggested that the motor impairments produced by these
compounds and their anxiolytic properties may be primarily
responsible for the observed effects on learning tasks. Our
results support the idea that any conclusions about the effect
of EAA antagonists on learning can only be made with knowledge about the general behavioral effects of these compounds.
Rats that appear unaffected or fully tolerant to the effects of
administration of these antagonists may in fact be impaired
on more sensitive measures, such as rearing.
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