Buspirone increases social investigation in pair-housed male mice; comparison with the effects of chlordiazepoxide

Neuropharmacology Vol. 32, No. 5, pp. 429--437, 1993 Printed in Great Britain. All rightsreserved

0028-3908/93 $6.00+ 0.00 Copyright © 1993PergamonPress Ltd

BUSPIRONE INCREASES SOCIAL INVESTIGATION IN PAIR-HOUSED MALE MICE; COMPARISON WITH THE EFFECTS OF CHLORDIAZEPOXIDE B. GAo* and M. G. CUTLER Department of Biological Sciences, Glasgow Caledonian University, Cowcaddens Road, Glasgow G4 0BA, U.K. (Accepted 29 January 1993) Snmmm'y--Effects of buspirone (1, 5 and 10 mg/kg, i.p.) on the behaviour of adult male CDI mice have been compared with those of chlordiazepoxide (I, 4 and 8 mg/kg, i.p.). Commencing at 30 rain after injection, the behaviour of each mouse was examined by ethological procedures during 5 min social encounters with an untreated partner in the animal's home cage and in the more aversive situation of an unfamiliar neutral cage. In both test environments, buspirone at 1 and 5 mg/kg and chlordiazepoxide (CDP) at 1 and 4 mg/kg increased social investigation and some of its constituent elements, while decreasing non-social activity and the element, "explore" (and for CDP, of "scanning" also). In both test environments, the increase of social investigation by buspirone and CDP was less marked at 10 and 8 mg/kg, respectively. For CDP, although not for buspirone, this effect was related to dose-dependent increases of immobility coupled with reductions of exploratory non-social activity and scanning below those occurring at the intermediate dose level. Buspirone at 5 mg/kg increased social investigation to a greater extent in the home cage (P < 0.01) than in the unfamiliar neutral cage (P < 0.05), whereas CDP was approximately equipotent in the two test situations. In the neutral cage, buspirone at all dose levels showed an additional effect of increasing the time spent by the mice in digging, whereas chlordiazepoxide dose-dependently increased aggression. These results indicate anxiolytic activity by both compounds after acute administration, and identify certain differences in the profile of their other effects on social behaviour. Key words---buspirone, chlordiazepoxide, mouse, social behaviour, anxiolytic.

Psychopharmacological properties of the benzodiazepines form the basis for many of the paradigms currently employed to assess the anxiolytic activity of drugs (Lal and Emmett-Oglesby, 1983). However, these procedures may not be appropriate for detecting the anxiolytic effects of the more recently developed compounds (Lader, 1989; Sanger, Perrault, Morel, Joly and Zivkovic, 1991), which include drugs that act directly on receptors for 5-HT (Taylor, 1990; Thiebot, 1986; Traber and Glaser, 1987). Thus, buspirone, whose major effect is at the 5-HT~A receptor site (Carli, Prontera and Samanin, 1989), has been found to be as effective clinically as the benzodiazepines in controlling anxiety (Goa and Ward, 1986), and yet to give variable results in many of the classical tests in rodents that are predictive of anxiolytic potential (Chopin and Briley, 1987). These include conflict paradigms (Gardner, 1986; Schefke, Fontana and Commissaris, 1989), the social interaction test (Dunn, Corbett and Fielding, 1989; File, 1985) and the elevated plus maze (Kostowski, Plasznik and Stephanski, 1989; Moser, 1989). Nonetheless, buspirone has been found to show consistent anxiolytic-like activity in the two-compart-

*To whom correspondence should be addressed.

ment exploratory test in mice (Costall, Kelly, Naylor and Onaivi, 1988), in assessments of fear potentiated startle in rats (Mansbach and Geyer, 1988) and in conflict paradigms using the pigeon (Barrett, Witkin, Mansbach, Skolnick and Weissman, 1986). There is a considerable amount of experimental and clinical data to suggest that buspirone will show significant anxiolytic effects only when given by repeated administration (e.g. Schefke et al., 1989). Nonetheless, from some preclinical tests there is evidence that buspirone can induce anxiolytic-like activity in animals after a single injection (e.g. Lee and Rodgers, 1991). The present ethopharmacological study compares effects of acutely administered buspirone on the social behaviour of pair-housed male mice with those of the benzodiazepine receptor agonist, chlordiazepoxide. Many workers have demonstrated that these ethological procedures provide a useful insight into the dose-related effects of drugs on the naturally occurring social behaviour of laboratory rodents (Dixon, Fisch and McAUister, 1990; Krsiak, Sulcova, Donat, Tomasikova, Diohozkova, Kosar and Masek, 1984; Miczek, 1987). Using these methods, we found in previous subchronic experiments that buspirone increased social investigation and reduced flight behaviour in female mice when encountering male partners (Cutler, 1991), 429

430

B. GAO and M. G. CUTLER

and that its effects on the behaviour of pair-housed male mice during social encounters were similar to the effects of chlordiazepoxide (Gao and Cutler, 1992a). However, buspirone given subchronically at the same dose levels failed to induce anxiolytic-like effects in group-housed male mice when these were tested as intruders in the resident-intruder paradigm (Cutler, 1991). Thus, the social background of the animals and the situation of behavioural testing are important factors in enabling detection of the anxiolytic activity of this compound. The present series of experiments have been designed to obtain quantitative dose-related information on the behavioural effects in male mice of acutely administered buspirone and chlordiazepoxide under similar test procedures to those employed in the previous subchronic studies in pair-housed males. METHODS

Animals Adult male CD1 mice (Bantin & Kingman, Hull), weighing 35-45 g at the time of experimentation, were used. All animals were pair-housed for 10-14 days prior to behavioural observations. Mice were given an ad libitum supply of pelleted stock cubes (RM3 expanded diet, SDS, Wetham, Essex) and water. All animals were maintained at a temperature of 21 + 2°C under a 24 hr cycle of reversed lighting, with darkness from 6.00 to 18.00hr, and bright fluorescent lighting for the remaining period. Immediately prior to the experiments, the cages of animals were randomly distributed between 8 treatment groups. These comprised 6 groups containing 16 animals each to receive drug treatment and 2 control groups containing 20 animals each.

Experimental procedures Buspirone hydrochloride (Sigma) was given by single intraperitoneal (i.p.) injection to mice of 3 groups at 1, 5 and 10 mg/kg, respectively. In a parallel experiment, chlordiazepoxide hydrochloride (Sigma) was administered i.p. to mice of a further 3 groups at 1, 4 and 8 mg/kg, respectively. The drugs were dissolved in physiological saline prior to injection. Physiological saline was given at an equivalent volume i.p. to mice of the 2 control groups. In order to assess effects of the drugs on reactivity of the animals to different environments, behavioural responsiveness of the mice was examined in two different test situations, as in previous studies in this laboratory (Gao and Cutler, 1992a, b,c, 1993). Behaviour was firstly examined in the resident-intruder paradigm, commencing at 30 min after injection. A group-housed untreated unfamiliar mouse was introduced to replace one mouse from the original pair in the home cage ( 3 3 x 15x 13cm) and behaviour during the ensuing social encounter was recorded for a 5 rain period. Both mice from the pair were then immediately transferred to an unfamiliar large trans-

parent cage (60 × 25 x 25cm; light intensity 38 cd/m2), as employed in the social interaction test of File (1980), and their social behaviour during the subsequent 5 rain was recorded in the new environment. Preliminary tests had demonstrated that behaviour of the mice did not change significantly during an initial 10 min period of social interactions with the same partner, although habituation occurred subsequently (Donald, Cutler, Moore and Bradley, 1986). Videotape recordings were made of the behaviour occurring during these social encounters. The acts and postures shown by each mouse were recorded as a spoken commentary by tape recorder using the ethological profile described by Gao and Cutler (1992a). This is derived from the ethogram outlined by Dixon et al. (1990). Data from the spoken commentaries were transcribed from audiotape onto a floppy disc through keyboard input for analysis by computer of the frequency and duration of each behavioural element and of the five major behavioural categories of non-social activity, immobility, social investigation, aggression and flight. For comparability with the results quoted in our previous ethopharmacoiogical studies (Cutler, 1990, 1991; Cutler and Dixon, 1988; Cutler and Piper, 1990; Gao and Cutler, 1992a, b,c, 1993), data were recorded as the mean values and standard error of the means for each group. The significance of the differences between mean values from the drug-treated and control groups was estimated by the non-parametric Mann-Whitney U-test and the Kruskal-Wallis oneway analysis of variance (ANOVA). Changes having a two-tailed probability of 0.05 or less were deemed to be statistically significant. RESULTS

Effects of buspirone in the home cage Effects of buspirone on the categories of behaviour are illustrated in Fig. 1. Table 1 shows the significant effects on individual behavioural elements. From Fig. 1 it can be seen that buspirone at 1 and 5 mg/kg significantly increased social investigation in the home cage, while reducing non-social activity. These effects were most marked at 5 mg/kg. Table I shows that at both dose levels, the enhancement of social investigation was associated with significant increases in frequency and duration of the specific elements "attend" and "investigate" and with reductions in the frequency and duration of "explore". At 5 mg/kg, frequency and duration of the specific social element "nose" was also significantly increased, and frequency of the non-social element "dig" was reduced. At 10 mg/kg, buspirone failed to significantly alter occurrence of the categories of behaviour, although it did have significant effects on specific elements, increasing the frequency of "investigate" and the frequency and duration of "attend", while reducing the frequency and duration of "explore".

Buspirone and chlordiazepoxide on mouse behaviour

431

300

¢J

0

[]

Controls

200 Buspirone treated:

=

eo

1O0

[]

1 mg/kg

[]

5 mg/kg

[]

10 mg/kg

* P
O.

L

his

IMM

SOC

Category

AGG of

FL

behaviour

Fig. 1. Effects of buspirone on the duration of behaviour in the home cage (NS = non-social activity; IMM = immobility; SOC = social investigation; AGG = aggression; FL = flight).

Effects of buspirone in the neutral cage Effects of buspirone on behaviour in the neutral cage can be seen from Fig. 2, which illustrates effects on behavioural categories, and from Table 2, which shows significant effects on behavioural elements. Figure 2 shows that it significantly increased social investigation at 1 and 5 mg/kg. This effect was associated with an increase, at 1 mg/kg, in the frequency and duration of the specific element "investigate" and, at 5 mg/kg, with an increase in the frequency and duration of "attend" and "nose" and in the duration of "investigate" (Table 2). This enhancement of social investigation by buspirone at 1 and 5 mg/kg was associated with a

significant decrease in the time spent in non-social activity and in the frequency and duration of "exploring". Buspirone at all doses tested, significantly increased time spent by the mice in "digging" of the sawdust substrate, although the frequency of "digging" was not significantly altered. At 10 mg/kg, buspirone had no significant effects on the duration of categories of behaviour shown by mice in the neutral cage, although it did significantly increase duration of the specific social element "nose" and decrease duration of the non-social element "explore". Overall, buspirone had less marked effects on behaviour of the animals in the neutral cage, when given at lOmg/kg, than when administered at the lower dose levels. At all dose levels, buspirone

Table 1. Effects of buspirone on the frequency and duration of individual behavioural elements in male mice during encounters in their home cage; findings of statistical significance Buspirone Group n

Controls 20

1 mg/kg 16

5 mg/kg 16

I 0 mg/kg 16

Behavioural elements Mean J~'equency ( ± SEM)/observation Non-social activity Explore Dig Social investigation Attend Investigate Nose

54.5 ± 3.0 6.8 + 2.0

47. I ± 2.6* 4.8 ± 2.0

38.6 ± 2.6** 1.5 ± 0.8*

43.4 ± 9.0* 3.8 ± 1.3

12.2± 1.0 16.1 + 1.4 1.9 ± 0.4

14.8± 1.2" 22.1 _+ 1.9" 2.4 ± 0.5

17.1 ± 0.8"* 24.3 ± 1,5"* 2.8 _+ 0.4*

15,2± 1,0* 20,6 ± 1.7" 1,9 + 0.4

Mean duration, sec ( ± SEM)/observation Non-social activity Explore Dig Social investigation Attend Investigate Nose

97.5 _+ 7.8 9.5±4.8

52,5 ± 5.2** 15.5 ± 6.8

49.0 ± 5.3** 3.1 _+ 1.8

63. I ± 5.4** ll.0 ± 4.3

6.6 ± 0.8 44.5 +_ 6.7 2.7+0.8

8.8 _+ 0.7* 75.8 ± 10.2" 3.2 ± 0.7

10.4 _+ 0.6** 81.3 ± 8,6** 4.2±0.7*

9.3 _+ 0.8** 61.7 ± 7.5 3.1 ± 0 . 7

*P < 0.05, **P < 0.01, between drug-treated and control mice by the Mann-Whitney U- and Kruskal-Wallis tests.

B. GAOand M. G. CUTLER

432

300

[] 0 0 m

200

Buspirone treated:

o

a

Controls

100

[]

1 mg/kg

[]

5 mg/kg

I~i

lo r.g/kg

* P
0 N8

IMM

800

Category

~ of

FL

behaviour

Fig. 2. Effects of buspirone on the duration of behaviour in the neutral cage (NS = non-social activity; IMM = immobility; SOC = social investigation; AGG = aggression; FL = flight). showed no sedative activity and was without significant influence on aggression or flight in either of the test situations.

Effects of chlordiazepoxide in the home cage Effects on the categories of behaviour are shown in Fig. 3 and significant effects on behavioural elements can be seen from Table 3. Chlordiazepoxide (CDP) significantly increased the duration of social investigation at 1 and 4 mg/kg. At 8 mg/kg, CDP increased the frequency of social investigation (mean _ SEM; controls 43.5 _ 2.8; treated 52.4 _ 2.6; P < 0.05), but had no significant effects on its duration. The increase of social investigation by CDP was associated with a reduction in the duration of nonsocial activity and of the specific element "explore" at

all dose levels and by a decrease in the frequency of "exploring" and "scanning" at 1 and 4mg/kg. Sedative effects were apparent at 4 and 8 mg/kg, as evidenced by increase in the duration of immobility and reduction in the time spent in "scanning". The increase of social investigation by CDP at 1 and 4 mg/kg was associated with significant increases in frequency and duration of the specific elements, "attend", "investigate" and "nose", while CDP at 1 mg/kg also increased the frequency and duration of "follow" and the frequency of "sniff". Although the enhancement of social investigation by CDP at 8mg/kg was less marked, there were significant increases in the frequency and duration of "attend", in the frequency of "investigate" and in the duration of "nose".

Table 2. Effects of buspirone on the frequency and duration of individual behavioural elements in male mice during encounters in an unfamilar neutral cage: findings of statistical significance Buspirone Group n

Controls 20

1 mg/kg 16

5 mg/kg 16

10 mg/kg 16

Behavioural elements Mean .fi'equent y ( ± SEM)/obsert, ation Non-social activity Explore Dig Social investigation Attend Investigate Nose

73.7 ± 2.6 18.0_+2.5

64.6+_2.9* 22.9+2.7

62.1 ± 2 . 7 " 20.3_+2.0

66.4±3,2 20.1 +2.6

7.8_+ 1.0 6.8 + 0,7 1.4_+0.2

8.6_+0.1 10.3 ± 1.2' 1.9 ± 0.4

11.3± 1.1" 8.9 + 1.1 3.1 +0.5*

7.8_+0,8 7.4 _+0,9 2.3 ± 0,4

Mean duration, see (+ SEM)/obsert~ation Non-social activity Explore Dig Social investigation Attend Investigate Nose

172.7 + 5.2 20.7 ± 5.2 4.5 ± 0.7 13.6 + 7.3 1.4 + 0.3

121.5 + 6.2** 52.4 -+ 8.8** 5.5 + 0.7 23,8 _+ 3.7* 2.8 ± 0,6

122.3 -+ 5.8** 46.2 -+ 5.4**

133.0 + 5.7** 42.9 ± 6.7*

7.0 -+ 1.1" 20.3 + 2.5* 3.9 _+0.7*

4.6 +- 0.7 14.0 ± 2.2 2.6 ___0.5*

*P < 0.05, **P < 0.01, between drug-treated and control mice by the Mann-Whitney U- and KruskaI-Wallis tests.

Buspirone and chlordiazepoxide on mouse behaviour 30O

0 0 W

200

t.

433

[]

Controls

Chlordiazepoxide treated:

0

.a

100

[]

1 m~g

[]

4 mg/kg

[]

8 mg/kg

* P<0.05

0 NS

IMM Category

SOC of

AGG

FL

behavlour

Fig. 3. Effects of chlordiazepoxide on the duration of behaviour in the home cage (NS = non-social activity; IMM = immobility; SOC = social investigation; AGG = aggression; FL = flight).

Effects of chlordiazepoxide in the neutral cage Effects of CDP on the categories of behaviour in the neutral cage are shown in Fig. 4 and significant effects on the constituent elements can be seen from Table 4. Figure 4 shows that CDP significantly increased aggressive behaviour at 8 mg/kg and social investigation at all dose levels. These effects were associated with a significant dose-dependent reduction of non-social activity. Immobility was significantly increased in duration by CDP at 4 and 8 mg/kg. The treatment with CDP had no significant effect on flight behaviour.

The increase of social investigation by CDP at 1 and 4mg/kg was associated with a significant enhancement in the frequency and duration of the specific elements "attend", "investigate" and "nose" and, at 4 mg/kg, by an increase in the duration of "sniff". Enhancement of social investigation by CDP was less marked at 8 mg/kg, with the only significant effect on individual elements being an increase in the duration of "attend". "Exploration" and "scanning" were significantly reduced in frequency and duration by CDP at 4 and 8 mg/kg, and "exploration" was reduced in duration by CDP at I mg/kg. Treatment with CDP did not

Table 3. Effects of chlordiazepoxide on the frequency and duration of individual behavioural elements in male mice during encounters in their home cage; findings of statistical significance Chlordiazepoxide Group n

Controls 20

I mg/kg 16

4 mg/kg 16

8 mg/kg 16

Behavioural elements Mean frequent T (+_ SEM)/obsert,ation Non-social activity Explore Scan Social investigation Attend Investigate Nose Sniff Follow

49.9+2.5 41.5 _+ 7.4

38.1 _+2.8* 34.6 _+ 2.9*

37.8_+4,1" 31.7 _+ 3.7*

40.9+4.7 31.6 _+ 4.9

11.4_+0.7 16.1 _+ 1.4 4.4 _+ 0.6 7.5_+0.9 3.2 +_ 0.7

21.3_+ 1.7"* 22.1 _+ 1.9" 7.7 _+ 0.9* 13.8_+2.3" 5.6 +_ 0.9*

18.5_+ 1.4"* 24.3 +__1.5"* 6.6 _+ 0.8* 10.5+_2.1 5.3 _+ 1.4

18.2+ 1,8"* 20.6 _+ 1,7" 4.8 _+ 0,5 8.4+_ 1,4 3.6 +- 0.8

Mean duration, sec (+_ SEM)/observation Non-social activity Explore Scan Social investigation Attend Investigate Nose Sniff Follow

87.0 + 6.7 105.8 + 6.1

45.7 + 3.3** 89. I -+ I 1.3

55.0 + 7.7* 73.4 +_ 1.8"

57.0 +- 6.1" 67.2 + I 1.0"

6.0_+0.4 50.0 _+ 5.5 6.8-+ 1,0 17.7 + 2.4 3.4 + 0.8

15.0+2.2"* 80.9 + 7.5* 12.6-+ 1.7" 31.8 _ 6.0 6.2 +_+_1.2"

11.6_+ 1.3"* 64,5 + 6.7** 12.2_+ 1.6" 27,0 _+ 5.3 6.2 _+ 1.6

12.8+3.1"* 47. I +- 3,7 9 . 9 + 1.3" 22.2 _+ 4.1 4.0 _+ 1.0

*P < 0.05, **P < 0.01, between drug-treated and control mice by the Mann-Whitney U- and KruskaI-Wallis tests.

434

B. GAo and M. G.

CUTLER

300

eo

"~ ¢3

[]

200

Controls

Chlordiazepoxide treated:

[] [] []

100

1 mg/kg 4 mg/kg 8 mg/kg

* P
0 INS

IMM

SOC

Category

of

AC43

R.

behaviour

Fig. 4. Effects of chlordiazepoxide on the duration of behaviour in the neutral cage (NS = non-social activity; IMM = immobility; SOC = social investigation; AGG = aggression; FL = flight).

significantly influence the occurrence of "digging" in the neutral cage.

statistical significance. This effect arose from some increase of flight by partners in this group, related to an increase of aggression shown by mice treated with chlordiazepoxide at 8 mg/kg.

Behaviour of the partner animals As shown in Table 5, the only significant differences in behaviour between partners to the drugtreated mice and partners to the controls was an enhancement of social investigation among partners to most groups of the drug-treated animals, which was in some cases coupled with a reduction of non-social activity. Additionally, in the neutral cage, partners to mice treated with the highest dose of chlordiazepoxide showed an increase in the duration of "other behaviours" which was close to reaching

DISCUSSION

The present studies have shown that buspirone, when acutely administered, acted like chlordiazepoxide to increase social investigation by male mice in encounters with unfamiliar partners in an unfamiliar neutral cage as well as in their home cage. These effects, indicative of anxiolytic activity (Chopin and Briley, 1987), were sufficiently marked to increase

Table 4. Effects of chlordiazepoxide on the frequency and duration of individual behavioural elements in male mice during encounters in an unfamiliar neutral cage; findings of statistical significance Chlordiazepoxide Group n

Controls 20

1 mg/kg 16

4 mg/kg 16

8 mg/kg 16

Behavioural elements Mean frequency ( + SEM)/obsert,ation Non-social activity Explore Scan Social investigation Attend Investigate Nose Sniff

76.4 _+ 3.6 47.5 _+ 2.6 8.5_+0.9 5.8 _+ 1.1 3.1 _+ 0.5 3.1 ___0.4

65.4 _+ 4.0 46. I _+ 3.2

54.3 _+ 3.9** 33.4 _+ 3.4*

49.1 _+ 6.4* 27.6 _+ 4.3**

14.9_+ 1.3"* 10.7 _+ 2.0* 6.4 _+ 1.0" 5.1 _+ 1.4

13.8_+ 1.6" 11.6 _+ 1.7" 5.4 _+ 0.9* 5.0_+ 1.4

11.4_+ 1.5 6.1 + 0.9 3.4 _+ 0.6 2.3_+0.9

Mean duration, set" ( _+SEM)/obsert,ation Non-social activity Explore Scan Social investigation Attend Investigate Nose Sniff

177.4+4.7 58.5 _+ 3.3

117.9_+7.5'* 67.2 _+ 5.2

121.6_+7.3"* 42.9 _+ 9.2*

111.6_+8.8"* 31.5 _+ 5.0**

4.7_+0.7 9.7_+2.1 3.9 _+ 3.3 4.1 __+0.9

I1.1 _+ 1.5"* 23.1_+4.9" 8.4 _+ 1.4" 9.2 _+ 2.7

I1.1 _+2.4** 22.0_+2.7** 7.5 _+ 1.3" 9.4 _+ 2.5*

8.8_+ 1.7" 11.2_+1.8 5.6 + 1.0 4.1 _+ 1.5

*P < 0.05, **P < 0.01, between drug-treated and control mice by the Mann-Whitney U- and Kruskal-Wallis tests.

Buspirone and ehlordiazepoxide on mouse behaviour

435

Table 5. Behaviourof the partners to mice of each treatment group Partners to Group Dose n

Drug-treated mice Controls 20

low dose 16

medium dose 16

high dose 16

Behavioural category Mean duration, sec ( ± SEM)/observation I. Buspirone study (A) In the home cage Non-social activity Social investigation Other (B) In the neutral cage Non-social activity Social investigation Other

265.4 + 4.1 17.1 + 2.2 19.0 + 4.6

235.1 + 8.3** 36.0 + 5.3** 29.3 + 8.2

239.6 + 9.1"* 36.7 ± 3.5** 24.6 + 7.8

250.9 + 5.2* 36.4 + 3.8 14.1 + 4.7

272.7 ± 4.0 15.4 + 1.7 13.0 + 3.8

252.8 + 7.1 * 24.9 _ 4.3* 21.1 + 4.7

245.7 + 10.5" 25.6 + 2.7* 27.1 + 9.5

265.6 + 6.5 20.3 + 3.0 14.4 + 4.1

243.5 ± 10.0 29.1 + 3.4 29.2 ± 9.2

217.2 + 11.5" 49.4 ± 6.3** 33.9 ± 7.0

232.5 + 12.3 37.9 ± 3.6* 31.5 ± 6.8

237.9 + 9.6 38.6 ± 3.2* 24.1 ± 6.5

261.8 ± 6.6 15.1 ± 1.9 24.4 _+ 3.1

239.8 ± 13.5 33.1 +_ 8.6** 23. I ± 4.9

236.7 ± 10.8" 26.6 ± 4.1"* 36.3 +_ 8.1

236.8 ± 14.3 21.8 ± 1.9" 40.9 ± 12.5

2. Chlordiazepoxide study (A) In the home cage Non-social activity Social investigation Other (B) In the neutral cage Non-social activity Social investigation Other

*P < 0.05, **P < 0.01, by the Mann-Whitney U- and Kruskal-Wallis tests between partners to the drug-treated mice and partners to the controls.

social investigation by their untreated partners during the dyadic encounters. The enhancement of social investigation by buspirone, was most pronounced at 1 and 5 mg/kg and less evident at the higher dose of 10 mg/kg. Anxiolytic activity of buspirone relates in the main to its significant affinity for 5-HT~A receptors (Eison and Temple, 1986). At the higher dose of 10mg/kg, buspirone is known to cause profound changes to other transmitter systems including dopamine and acetylcholine (Carl± et al., 1989), and this may underlie its less marked anxiolytic-like effects at 10 mg/kg in the present studies. The increase of social investigation by chlordiazepoxide was most pronounced at 1 mg/kg in the present experiments and less marked at the greater dose levels of 4 and 8 mg/kg, which induced a dose-related increase of immobility in both test situations. Such sedative actions are one of the unwanted side-effects of anxiolytic benzodiazepines (Greenblatt and Shader, 1974; Kalynchuk and Beck, 1992). Buspirone did not induce the adverse effect of increased immobility in the present experiments. Acutely administered benzodiazepines have long been known, not only to increase immobility, but also to reduce non-social exploratory activity and scanning by rodents in both non-social and social situations (e.g. File, 1980; File and Pellow, 1987). This reduction of non-social activity is often interpreted as a sedative action. In our studies, both chlordiazepox±de and buspirone reduced exploration while chlordiazepoxide also induced a dose-dependent decrease of scanning. However, these reductions of non-social activity during social encounters may not necessarily arise from sedative effects, but could be due to an increased proportion of behavioural activity being directed towards social investigation. Thus, for bus-

pirone which was without other evidence of sedative activity, further studies are needed to examine its effects at high dose levels in a non-social situation. From studies in rats, Guy and Gardner (1985) have suggested that buspirone can exert non-specific sedative effects at high doses. Buspirone at all dose levels in the present acute studies increased digging of the sawdust in the unfamiliar neutral cage. It showed similar effects in our previous subchronic experiments (Gao and Cutler, 1992a). However, chlordiazepoxide increased digging in the neutral cage only after subchronic administration (Gao and Cutler, 1992a), an effect which also was observed in mice treated with ritanserin (Gao and Cutler, 1993). The reason for these differences is not known. It has been proposed that the increase of digging may represent enhanced reactivity by the drug-treated mice to normal non-aversive environmental stimuli such as odours from the unfamiliar sawdust (Gao and Cutler, 1992a). Chlordiazepoxide induced a dose-related increase of aggression in the neutral cage in the present acute studies. This effect was not seen when the drug had been given subchronically (Gao and Cutler. 1992a) and did not occur in mice which had been treated with buspirone. There is evidence that the pro-aggressive actions of benzodiazepines may be due to effects unrelated to interactions at the benzodiazepine-receptor site (Mos and Olivier, 1987; Mos, Olivier and van der Poel, 1987) and thus may be independent of their anxiolytic effects. Furthermore, benzodiazepines are known to reduce aggressive behaviour in several other test situations (Miczek, 1987). The enhancement of social investigation in the neutral cage by both buspirone and chlordiazepoxide, as well as the increase of aggression in the neutral

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B. GAO and M. G. CUTLER

cage by chlordiazepoxide in o u r experiments, were more m a r k e d after acute t h a n after s u b c h r o n i c treatm e n t o f the a n i m a l s ( G a o a n d Cutler, 1992a). However, the most noticeable difference between the results of the acute a n d s u b c h r o n i c studies was the complete absence o f sedative effects from chlordiazepoxide after its s u b c h r o n i c administration. The d e v e l o p m e n t o f tolerance to the sedative effects o f benzodiazepines has been a m p l y described by m a n y workers (e.g. File, 1980; Owen a n d Tyrer, 1983). Finally, the present results suggest t h a t b u s p i r o n e m i g h t induce anxiolytic effects after acute administration. T h e experiments o f D u n n et al. (1989) a n d Lee a n d Rodgers (1991) indicate a similar conclusion Nonetheless, there are several reports o f a n enigmatic profile o f effects from b u s p i r o n e on b e h a v i o u r [e.g. in the social interaction test a n elevated plus maze (Critchley a n d Handley, 1987; Pellow, J o h n s t o n a n d File, 1987; Moser, 1989)]. It appears, overall, t h a t the previous housing a n d social circumstances o f the animals as well as the situation o f testing, are crucial to the detection of the anxiolytic activity by drugs of this type. F u r t h e r studies are needed on the relationship between the effectiveness o f b u s p i r o n e therapy in h u m a n s a n d social circumstance o f the patients.

Acknowledgements--We wish to express our thanks to Mrs V. Graham and Mr B. Leiper for technical assistance.

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