Involvement of opioid and dopaminergic systems in isolation-induced pinning and social grooming of young rats

Neuropharmacology Vol. 28, No. 4, pp. 41 I-418, Printed in Great Britain. All rights reserved

1989 Copyright

0

0028-3908/89 53.00 + 0.00 1989 Pergamon Press plc

INVOLVEMENT OF OPIOID AND DOPAMINERGIC SYSTEMS IN ISOLATION-INDUCED PINNING AND SOCIAL GROOMING OF YOUNG RATS R. J. M. NIESINK*and J. M. VAN REE’ ‘Rudolf Magnus Institute for Pharmacology, Medical Faculty, University of Utrecht, Vondellaan 6, 3521 GD Utrecht, The Netherlands and ‘Department of Natural Sciences, Open University (RNI), Heerlen, The Netherlands (Accepted

24 November

1988)

Summary-Pinning, as a measure for play, and social grooming were simultaneously studied in juvenile rats. Short-term social isolation increased both behavioural responses. This increase was attenuated by the opioid antagonist naltrexone, whilst the opiate, morphine, and the opioid peptide fiendorphin, increased the responses. Pinning was more sensitive to the effects of naltrexone, whilst /?-endorphin stimulated particularly social grooming. Small doses of the dopaminergic drug, apomorphine, decreased both pinning and grooming behaviour of the short-term isolated rats. Some of the effects were partially antagonized by the dopamine antagonist haloperidol, and the neurolepticum-like peptide, desenkephaliny-endorphin (DE-y-E). A small dose of haloperidol and DE-y-E stimulated social grooming in particular, whilst a larger dose of haloperidol decreased pinning and social grooming. It is concluded that both opioid and dopaminergic systems are implicated in the increase of pinning and social grooming induced by short-term social isolation. The differences in sensitivity of pinning and social grooming for opioid and dopaminergic drugs and peptides are discussed in relation to possible differences in the neural systems underlying both social activities. Key wor&-_B-endorphin, social isolation.

desenkephalin-y-endorphin,

Several studies have been performed to investigate the importance of social behaviour in youth for the development of later social and other aspects of behaviour. A typical, distinct category of behaviour, only performed by juvenile animals and easily recognized by human observers, is called “rough and tumble play” or, in brief, play. Play in young rats consists of vigorous wrestling and chasing among peers (Humphreys and Einon, 1981; Poole and Fish, 1975). Play with peers has been shown to occur in the majority of mammals (Beckoff, 1972). Ethologists suppose that play has an important functional significance and this would imply that deprivation of play may have important consequences (Taylor, 1980; Rosenzweig, 1971). A behavioural element that is performed by very young animals, but which is still present in adulthood, is social grooming or allogrooming. This behaviour can be found in a variety of species, from rodents to primates. Social grooming can be distinguished from other social acts, and the frequencies and duration of bouts can easily be measured. These behavioural responses, play in juvenile animals and social grooming in adult animals, seem to have at least two things in common. First, short-term social deprivation increases the intensity of play as well as of social grooming (Panksepp and Beatty, 1980; Niesink and Van Ree, 1982). Second, at the level of neurochemical control of both behavioural

apomorphine, haloperidol, play, social grooming,

responses, opioid systems seem to be involved (Panksepp, Siviy and Normansell, 1984; Van Ree and Niesink, 1983). Morphine reliably increases the frequency of play behaviour, while the antagonist naloxone has the opposite effect (Panksepp et al., 1984; Beatty and Costello, 1982). In previous studies it was shown that /I-endorphin increased social grooming in adult rats (Van Ree and Niesink, 1983; Niesink and Van Ree, 1984). Social play is one of the first non-mother directed, voluntary social activities to appear in life. Simultaneous observation of play-behaviour and social grooming in juvenile rats might be of great value in the study of drugs and other chemicals on social behaviour. However, before such a procedure can be used as a predictive model for drug induced deviations in social behaviour, information is needed about the neurochemical and other physiological processes underlying social grooming and play behaviour. The effect of isolation, at different times, on play behaviour and social grooming in young rats was investigated. Subsequently the involvement of the opioid and the dopaminergic systems in these behavioural responses were analysed. For these experiments, /I-endorphin and naltrexone were selected to investigate the implication of the opioid systems. Apomorphine and haloperidol were used to study dopaminergic involvement. One dose of morphine

411

R. J. M. NIESINKand J. M.

412

that has been proved to be very effective in play behaviour (Panksepp et al., 1984) was used as a control for the effect of opiates. The neuroleptic-like neuropeptide, desenkephalin-y -endorphin (DE-y-E; Van Ree, Innemee, Louwerens, Kahn and De Wied, 1982; Van Ree and De Wied, 1982), in a dose that had been proved to be effective in other behavioural tests, was used to compare the behavioural effect of haloperidol. METHODS Animals

Male Wistar rats bred from the laboratory stock, aged 28 days, weighing between 50 and 6Og, served as subjects. Immediately after sibbling and prior to experimentation, the rats were housed in plastic cages first in groups of 5 rats per cage and, during the hours of isolation, housed singly. Translucent home cages of 26 x 40 cm and 13 x 22 cm, respectively, were used. The animals rooms were temperature and light controlled (25 f 1°C; lights on 5.00 a.m., off 7.00 p.m.). Standard food and tap water were available ad libitum. Animals were used only once.

VAN

REE

Drugs and peptides

The following drugs and peptides were used: Morphine (Morphine HCl, O.P.G., Utrecht, The Netherlands) 1mg/kg (s.c.); Naltrexone (DuPont, Delaware, U.S.A.) 0.01, 0.03, 0.1, 0.3 and 1 mg/kg body weight (s.c.); Apomorhine (O.P.G., Utrecht) 10, 25, 35, 60 and lOOpg/kg body weight (s.c.); Haloperidol (O.P.G.; Utrecht) 5 and 25 pg/kg (s.c.). 8-Endorphin (/I-E) (Organon International B.V., Oss, the Netherlands) 0.1, 1, 10 and lOOpg/kg body weight (s.c.). Desenkephalin-y-endorphin (DE-y-E, /3E-(6.17)) (Grganon International B.V., Oss, the Netherlands) 50 pg (s.c.). Analysis of the data

Group means + SEM were calculated. For statistical analysis of two group means, two-tailed Student’s t-tests were used. Data of experiments in which more than two groups were compared were analysed by one way analysis of variance (ANOVA) testing, followed by Newman-Keuls tests. Interaction effects were analysed by a two-way analysis of variance procedure. RESULTS

Apparatus

Testing was carried out in an acrylic plastic test cage (1 x w = 35 x 35 cm) placed in a sound attenuating outer chamber. The floor of the test cage was covered with approximately 2 cm of wood shavings. Illumination was from a 25-W red light bulb mounted above the test cage. The experiments were scored through an online video display in an adjacent room. Procedure

Two rats of equal weight were placed in the test cage for 15 min. The dependent measures of pinning and grooming in each pair of animals were scored using digital counters. The behaviour of the individual rats was not registered. During this testing period the following parameters were scored: -Frequency, duration and latency of pinning; pinning was defined as one of the animals lying with its dorsal-lateral surface to the floor of the testing cage with the other animal standing above it (Panksepp et al., 1984; Siegel and Jensen, 1986); the latency was the time until one of the animals performed pinning behaviour. -Frequency and time of social grooming; social grooming was defined as a rat licking and chewing the fur of the conspecific with its forepaws on the back or neck of the other rat (Niesink and Van Ree, 1982). In a few of the experiments pinning behaviour was the only parameter scored. Treatment with peptide (/?-endorphin, DE-y-E) and/or drug (morphine, naltrexone, haloperidol) was investigated by injecting both animals of a pair, 60 min before the test, with the same peptide or drug in the nape of the neck. Apomorphine was given at the onset of the interaction test.

Effect of isolation

The effect of short-term isolation on the induction of pinning and social grooming was investigated by housing the rats individually for 0, 1, la, 2, 3f, 4, 6, 24 and 48 hr prior to the experiment (grooming was measured in the 0, li, 3f and 24 hr isolated animals only). Time-dependent increases in the frequency and duration of pinning (F = 17.96; df= 8,79; P < 0.01 and F = 14.89; df = 8, 79; P < 0.01 respectively) and social grooming (F = 4.12; df= 3,43; P < 0.01 and F = 6.78; df = 3,43; P < 0.01 respectively) were found (Fig. 1). The increase in pinning was maximal after 24 hr of individual housing, while a maximal increase in social grooming was present after 3f hr of isolation. The isolation procedure also increased the duration of bouts of grooming (F = 3.33; df = 3,43; P < 0.02), but not the duration of bouts of pinning. Simultaneously with the increase in frequency of pinning, the latency for pinning behaviour decreased (From 704 + 87 set in non isolated, to 142 + 16 set in 48 hr isolated animals; F = 11.2; df = 8,75; P < 0.01). Morphine

Table 1 shows the effects of treatment with morphine (1 mg/kg s.c., injected 1 hr prior to testing) in animals isolated for 3f hr. A significant increase in the frequency of pinning (P < 0.01) and frequency of grooming (P < 0.05) was found. Morphine neither significantly increased the duration of pinning nor the duration of grooming. Naltrexone (Table 1)

Subcutaneous treatment with naltrexone, in a dose of 0.03-l mg/kg body weight, doserange

Involvement of opioid and dopaminergic systems 100

A:PINNING

413

00

&SOCIAL GROOMING

r

40

30

20

0

50

0

10

HouiGf isoIa&

Hours of isolation

Fig. 1. The influence of different periods of isolation on the frequency (H) and duration (*) of pinning (A) and grooming (B) in 28-day-old rats. Rats were tested in pairs for 15 min.

dependently decreased the frequency and duration of pinning in animals isolated for 3f hr; this decrease in pinning was observed with a small dose of 0.1 mg/kg. A 1 mg/kg dose in 1: and 24-hr-isolated animals also decreased pinning behaviour (data not shown). This large dose also decreased grooming behaviour in animals isolated for 3f hr, but not in animals isolated for 1: and 24 hr. Smaller doses did not significantly affect grooming behaviour (Table 1). Treatment with j?-endorphin The results of different doses of /?-endorphin on pmning behaviour in socially housed rats (= 0 hr) or

individually housed for Ii, 3f or 24 hr are given in Figures 2A and B. Two-way analysis of variance revealed significant interactions between treatment and isolation-time for the frequency of pinning (I; = 4.36; df = 12, 188; P < 0.001) and duration (t: = 4.74; df = 12, 188; P < 0.001). An increase in frequency of pinning was found with a 10 pg/kg dose (I’ < 0.05) of j-endorphin and a decrease with a

Table

100 pggikg dose (P < 0.01) in the animals isolated for 3f hr. fl-Endorphin lOOpg/kg, also decreased the duration of pinning in animals isolated for 3f hr (P < O.Ol), whilst doses of 1 and 10 pg/kg increased the duration of pinning in those animals (P c 0.05 and P < 0.01). Slight increases in the duration of pinning were found in the 1 (P < 0.05) and 100 pg/kg (P < 0.05) groups of 24-hr-isolated animals. The effects of /I-endorphin on social grooming are shown in Figures 2C and D. A two way ANOVA revealed a significant interaction between treatment and isolation-time for the frequencies of grooming, F =2.14; df = 12, 188; P ~0.02) , but not for the time of grooming. Dose-dependent increases in the frequency and duration of social grooming were found in the animals isolated for 14, 3f and 24-hr. The duration of grooming was increased by 28-55%, with the largest dose tested. b-Endorphin did not significantly affect the duration of bouts of pinning or grooming (data not shown).

1. Effect of treatment with morphine and naltrexone on pinning and grooming behaviour Pinning

(n) Treatment A: Morphine (7) saline (8) (1 mg/kg-‘) B: Naltrexone (22) saline (6) (0.01 mg kg-‘) (6) (0.03 mg kg- ’ ) (12) (0.1 mg kg-‘) (12)(0.3mgkg-‘) (@(I mgkg-‘) F-WllE

Grooming

_

Frequency (frequency 15 min-‘)

Duration (set ISmin-‘)

Bout duration (se@

Frequency (frequency ISmin-‘)

Duration (s= ISmin-‘)

23.3 + 3.7 41.5 f 3.5tt

49.1 f 7.1 63.3 f 6.5

2.2 * 0.2 1.6 f 0.2

55.7 f 5.7 74.8i6.1t

80.9k11.8 86.5 i 13.8

1.4 f0.1 1.1 _+O.l

21.0+ 1.8 24.3 + 3.2 21.3 k 3.3 14.0 f I.911 13.6 _+3.311 8.8 f 1.6Jj

34.0 + 3. I 37.0 + 6.8 34.9 f 6.8 27.0 k 4. I 25.6 f 5.1 15.5 ZL2.611

1.6+0.1 1.5 f 0.2 1.6kO.2 1.9 +0.1 1.8kO.l 1.8 k 0.2

42.7 + 37.6 5 37.4 f 33.9 f 34.5 f 28.0 +

96.4 +- 6.4 74.4 + 4.3 89.3 f 4.5 84.9 & 7.2 88.2 + 7.8 59.3 k6.11

2.3 f 0.2 2.0 i 0.1 2.5 +O.l 2.6 f 0.3 2.6 + 0.2 2.1 f 0.2

F = 4.24 P < 0.01

F = 2.76

P < 0.05

“S.

3.5 2.5 1.5 I .9 2.7 I.91

F = 4.00

F = 3.37

P co.01

P < 0.01

Bout duration (W

n.s.

Animals were isolated for 3$ hr prior to testing; injections were given subcutaneously 60 min before testing. (n) = pairs of animals tested. Student’s T-test (A) or ANOVA tests (B: F-values) and subsequent Newman-Keuls tests (J/11) were performed tf or ii= P < 0.01; t or J= P < 0.05.

R. J. M.

414 A:

NIESINK and J. M. VAN REE

/lPINNl%

-

-

10

1

100

O-Endorphin ~/kg

h3.16t4-42) ~~002 F:lZlb-73) p
c:

SOClALGRKtlINi

.id 'C co 8 !

F.2.01 K-42)p
10

$00

O-Endorphin pg/kg

‘T .Gm E

s g.0

x z 00

0

01

1

F:3.26 (r-42) ~~0.02 F.12.03W3lp
Fig. 2. Influence of b-endorphin on the frequency (A) and time (B) of pinning and the frequency (C) and time (D) of grooming in socially-housed animals (0, 0) and in animals isolated for 1: 0, 3f H and 24 hr n . Pairs of rats were tested for 15 min, 1 hr after subcutaneous injection with placebo or graded doses of /f-endorphin. The mean frequency and duration of pinning and social grooming per treatment group is plotted against the dose of p-endorphin @g/kg). Vertical bars indicate SEM. (n) = number of pairs tested. ANOVA tests (F-values) and Newman-Keuls tests (t, 1) were performed. Arrows indicate significant different from corresponding placebo treated, controls. 7 or I= P < 0.05; tt or 11 = P < 0.01.

Haloperidol and DE-y-E Subcutaneous treatment with apomorphine, given at the onset of the interaction test, dose-dependently decreased the frequency and duration of pinning in animals isolated for 34 hr (Table 2). There was a significant effect after 60 and 100 pg/kg of apomorphine. The latency for pinning hehaviour increased simultaneously with larger doses of apomorphine (data not shown).

Table 3 shows the effects of pretreatment with haloperidol (5 and 25 pg/kg) or DE-y-E on the

pinning and grooming of apomorphine-treated rats. Apomorphine dose-dependently decreased pinning behaviour and social grooming. The smallest doses of apomorphine that were effective were 60pg/kg in pinning behaviour and 35 pg/kg in grooming behaviour, respectively. Interactions between pretreatment

Involvement

of opioid

Table 2. Effect of treatment with apomorphine on pinning behaviour in rats isolated for 3: hr Pinning

(n) Treatment

Frequency (frequency ISmin-‘)

Duration (set 15 min-‘)

Bout duration (=)

Apomorphine (10) saline i6)(lOpgkg-‘) (.6)(25pgkg-‘) 16)(35pgkg-‘) (l3)W~gkg~‘) t6)(lOOpgkg-‘)

18.5 + 3.0 20.2 + 4.9 17.1 + 4.6 14.2 + 2.1 5.7 & I.511 5.5 f I.411

28.1 f 4.4 31.7 f7.1 32. I + 7.0 24.5 k 3.9 10.8 k 2.711 8.7+3.111

1.5 f0.1 1.5 _+O.l 2.0 i 0.2 1.7 f 0.1 2.0 i 0.21 1.4+0.2

F = 5.28

F = 5.49 P < 0.01

F-Value

P <

0.01

F = 2.57 P < 0.05

Animals were isolated for 3f hr prior to testing; injections were given subcutaneously 5min before testing. ANOVA tests (F-values) and Newman-Keuls tests (1) were performed. t = P < 0.05; 11 = P < 0.01.

(==0,5 and 25 pg/kg haloperidol) and treatment (= 0, 35, 60 and lOOpg/kg apomorphine) were significant for the frequency and duration of pinning (F = 4.32; c$”= 6,91 and F = 7.24; df = 6,91) and the frequency and duration of grooming (F = 6.41; df = 6,91 and F = 6.04; df = 6,91). The pretreatment with haloperidol significantly affected the frequency and duration of pinning in the saline-treated animals (F = 10.88; df = 2,35 and F = 5.49; df = 2,35). Haloperidol decreased the frequency and duration

and dopaminergic

systems

415

of pinning (P < 0.01) in the 25 pgg/kg dose, but not in the 5 pg/kg dose. Haloperidol in the smaller dose of 5 pg/kg did not prevent the decrease of frequency and duration of pinning induced by treatment with apomorphine. The decrease in pinning behaviour induced by 25 pg/kg haloperidol was normalized by a 35 pggikg dose of apomorphine. The frequency and duration of pinning in animals treated with 100 pg/kg apomorphine were significantly increased after pretreatment with 25pgg/kg haloperidol. The pretreatment with haloperidol also significantly affected the frequency and duration of grooming in the salinetreated rats (F = 22.88; df = 2,35 and F = 23.65; df = 2,35). The small dose of haloperidol (5 pg/kg body weight) significantly increased the frequency and duration of social grooming whilst the largest dose caused a slight, but significant decrease of grooming frequency. Haloperidol (5 and 25pg/kg) partially prevented the decrease in the frequency and duration of social grooming induced by apomorphine (see Table 3). Desenkephalin-y-endorphin significantly increased the frequency of pinning (P < 0.05) and the frequency (P < 0.01) and duration of grooming (P < 0.01) in saline-treated animals (Table 3). The frequency and duration of pinning in apomorphinetreated animals in DE-y-E-pretreated animals, were not different from those in saline-pretreated animals. Besides a little increase in the frequency and duration

Table 3. Interaction between haloperidol and desenkephalin-y-endorphin on the effects induced by apomorphine on pinning and social grooming Haloperidol (ualkn)

Apomorphine (NE/kg)

Saline Saline Saline Saline

35 60 100

(n)

:‘3,’ (13) (15)

Pinning frequency 22.7 f 19.0 + 7.8 f 2. I f

1.3 3.7 2.0** 0.6**

F = 42.0 P < 0.01

Saline 35 60 100

(6) (6) (5) (6)

18.7 i 17.3 f 6.4 f 5.0 *

3.8 4.6 4.6’ 1.4’

F = 4.0 P < 0.05

25 25 25 25

Saline 35 60 100

(5) (5) (5) (5)

7.4 k 24.2 k 13.4 It 7.6 f

I .8Jl 4.9’. 5.3 2.277

F=4.l P < 0.05

DE-Y-E

~.-,

Apomorphine (unlka)

50 50

Saline 35

(6) (6)

50

60

(6)

50

100

(6)

”

II

Pinning duration 28.7 + I .8 27.2 _+5.4 12.0 f 3.0.’ 3.3 f I .o** F = 30.2 P < 0.01 24.8 i 5.2 23.7 + 7.1 10.2 f 8.2 6.8_f2.1’ F = 2.9 “.S. 13.2 + 3.511 64.3 * 9.9**tt 22.2 + 8.3 l1.6*3.5T F = 12.7 P < 0.01

Grooming frequency

Grooming duration

35.1 f 25.4 + 19.6 + 14.7 f

69.2 If: 2.9 52.0 f 4.4’ 46.2 f 3.4.’ 33.5 f 2.5..

2.3 3.0’ 1.3** 1.3.’

F = 59.0 P < 0.01

46.5 i 27.5 + 22.0 f 28.3 f

2.177 2.0.’ 4.8** 4.7+*tT

F = 26.6 P < 0.01

121.2 * 60.7 5 53.0 f 54.7 *

9.2tt 4.0** 5.6*’ 7.3**t

F = 12.3

F = 23.2

P < 0.01

P
26.8 + I.711 21.8k4.5 28.6 f 1.7tT 22.8 f 2.01

71.8 f 9.1 47.5 ZL9.5 63.0 f 2.317 51.8 + 2.4tf F = 2.7

F=l.4

n.s.

n.s.

li

29.3 f 21.5 It 14.3 f 1.8 +

2.81 3.6 3.3.. 1.2..

F = 12.5 P
36.0 + 31.0 f 19.7 * 2.7 +

3.6 5.5 4.2. 2.0’.

F = 9.9 P < 0.01

49.2 i 29.3 f 28 0 f 18:3 f

4.3tt 2.4,. 3.4**7 4.0’.

F = 12.8 P < 0.01

105.2 f 65.2 + 69.2 f 43.5 f

9.3Tt 7.2*’ 7.2**t 9.3*.

F = 9.5 P
Effect of pretreatment with haloperidol or desenkephalin-y-endorphin (DE-y-E) on the apomorphine-induced decrease in pinning and social grooming. Animals were isolated for 3 hr prior to experimentation. (n) = pairs of animals tested. Interaction effects have been analysed by two-way analysis of variance. ANOVA tests (F-values) and subsequent Newman-Kc& tests (7, 1) have been performed. Arrows indicate different from saline-pretreated, identically-treated rats. *Indicates different from identically-pretreated, saline-treated rats. t, lP c 0.05; tt, 11 and l*P c 0.01.

416

R. J.

M.

NIESINKand

of grooming after pretreatment with DE-y-E in rats that received 60 pg/kg apomorphine, DE-y-E did not significantly prevent the effects of apomorphine on social grooming. DISCUSSION

Short-term social isolation amplifies the frequency of pinning in juvenile rats (Panksepp et al., 1984; Panksepp and Beatty, 1980; Beatty, Dodge, Dodge, White and Panksepp, 1982; this study). In addition, an increase in social grooming activity was observed. These data resemble the findings of previous studies with adult rats in which an increase in social grooming was shown after short-term isolation (Niesink and Van Ree, 1982; 1987). Although both the pinning and grooming of young rats are facilitated by short-term social isolation, notable differences can be distinguished. Thus, the maximum effect, of the short-term isolation procedure on social grooming was reached much earlier than that of pinning (Figs 1A and B). Moreover, the duration of bouts of grooming was also increased by the isolation procedure, in contrast to the duration of bouts of pinning. Although pinning behaviour is sufficiently clear-cut as a behavioural act that inexperienced observers can easily be trained to score, procedural differences between and in laboratories, however, may account for the marked differences in the absolute levels of pinning observed in control animals between and within the different reports. Apart from social housing condition, the basal levels of play behaviour may be influenced by other methodological differences before or during the test procedure. Factors that have been implicated are condition of housing (e.g. size of cage: Siegel and Jensen, 1986), strain and sex differences, hormonal condition (Thor and Holloway, 1984), time of day that testing was performed, light condition in the test cage (Beatty and Costello, 1982), course of time since weanling (Janus, 1987) and the time since maternal separation (Niesink and Van Ree, unpublished observations). When comparing the absolute levels of different studies, the methodological facts mentioned should be taken into account. The increase in pinning and grooming after 3: hr of isolation did not yet reach a ceiling value, thus these effects still could be modulated (attenuated as well as inhibited). Therefore, 3f hr was used as the standard isolation time for testing the effects of drugs and peptides on pinning and social grooming. In experiments in which social deprivation was used as an experimental variable, the changes in physiological and behavioural parameters have been related to changes in the “basal state” of endogenous opioid systems (Panksepp, 1980; Schenk, Britt, Atalay and Charleson, 1982). In the present study, morphine significantly increased pinning and social grooming behaviour in 3$hr-isolated rats, confirming the results of Panksepp, Jalowiec, DeEskenazi and

J. M.

VAN REE

Bishop (1985). As has been reported for naloxone, a dose-dependent decrease in the frequency and duration of pinning by treatment with naltrexone (0.1-l mg/kg) was observed. The duration of bouts of pinning was not affected. Naltrexone also decreased the frequency and duration of social grooming, but for this effect more naltrexone was needed as compared to the effect on pinning. From these data it can be concluded that the endogenous opioid system is involved in the expression of social play, as measured by pinning and by social grooming in juvenile rats. Opioid-antagonists may inhibit the increased motivation (drive) for social contact in juvenile animals, but do not seem to affect the satiation effects of these social activities, because durations of bouts were not affected. The opioid agonist, morphine, had an opposite effect; it may thus enhance the increased motivation for social contact. Exogenous administration of the endogenous opioid b-endorphin stimulates the more intimate social activities, i.p. contact behaviour in adult rats (Van Ree and Niesink, 1983; Niesink and Van Ree, 1984). These increases in social contacts are mediated by opiate receptor systems, since the effects were completely antagonized by pretreatment with naltrexone. The present study shows that in the animals isolated for 3f hr, a dose of 10 pg/kg of b-endorphin slightly increased the frequency of pinning but a larger dose decreased the frequency of pinning in these animals. The total time spent pinning was stimulated in the 1 and 10 pg/kg dose per animal, but the duration of bouts was not significantly affected. P-Endorphin dose-dependently increased frequency and duration of social grooming. The effect of fl-endorphin was only observed in the short-term isolated young animals. In adult animals the effect on social grooming is also observed in group-housed animals, albeit only when the largest dose was given (Niesink and Van Ree, 1984). The results indicate that social grooming was more sensitive to systemic administration of /?-endorphin than pinning. Since the opposite was found for naltrexone, the endogenous fi-endorphin systems may be involved in the expression and/or realization of social grooming and other endogenous opioid systems might be implicated in the expression of pinning behaviour. It is not known whether the decrease in pinning produced by 100 pg/kg p-endorphin reflects any meaningful change or that this was simply related to the fact that, in the presence of so much social grooming, there is hardly any time for other types of (social) activities. A possible rewarding effect of play has been investigated in different test systems like homing (Panksepp and DeEskenazi, 1980) and the T-maze (Humphreys and Einon, 1981; Van Ree and Van Schaik, unpublished). Opioid receptor systems, as well as dopaminergic systems, have been implicated in several behavioural systems in which reward may be experienced, for instance electrical intracranial self stimulation (Van Wolfswinkel, Seifert and Van Ree,

Involvement of opioid and dopaminergic systems 1988). For this reason, dopamine (DA) might be a possible candidate for a CNS substrate, involved in play behaviour. In small doses [O.l mg/kg and less (Van Ree et al., 1982; Stahle and Ungerstedt, 1983)], apomorphine appears to preferentially affect presynaptic DA receptors, reducing the release of DA and suppressing spontaneous behaviour, while in large doses (0.2mg/kg and more; Stahle and Ungerstedt, 1983) t.he drug produces stereotyped behavioural patterns, presumably as a result of activation of postsynaptical receptors. According to the doses used in the present study (0.035Xt. 100 mg/kg), apomorphine would probably have affected presynaptic DA-receptors. Thus, both pinning and social grooming may be suppressed by stimulation of DA autoreceptors. Haloperidol decreased the frequency and duration of pinning. This intrinsic effect was counteracted by apomorphine, although apomorphine itself also decreased the frequency and duration of pinning. Similar paradoxical drug-drug interactions have been observed in other psychopharmacological studies (cf. Niesink and Van Ree, 1983). A second effect of haloperidol was its antagonizing effect on the dec:rease in pinning caused by apomorphine. Although it seems likely to assume that dopaminergic systems are involved in the decreased pinning caused by haloperidol or apomorphine, the present results do not allow any definite conclusions in this respect to be drawn. Contrary to haloperidol, DE-y-E significantly enhanced pinning behaviour. Desenkephalin-y endorphin did not antagonize the effect of apomorphine on pinning behaviour and the increasing effect of DE-y-E on pinning was not prevented by apomorphine. Thus, there is no reason to assume a dopaminergic involvement in the increase in pinning caused by DE-y-E. Haloperidol, in small doses, stimulated the frequency and duration of social grooming. Furthermore, haloperidol partially antagonized the effect of small doses of apomorphine on social grooming. These effects might be due to a blocking effect of haloperidol on presynapticallylocated dopamine receptors. A similar explanation has been suggested for the effect of haloperidol on exploration (Stahle and Ungerstedt, 1983). The effects of DE-y-E on social grooming were identical with the effects of the small dose of haloperidol. It has been suggested that DE-y-E acts as a dopamine antagonist, selectively on certain dopamine receptor systems which are stimulated by small doses of apomorphine and which may be located presynaptically (Van Ree et al., 1982). Thus, the described effects of apomorphine, haloperidol and DE-y-E on social grooming may be mediated by presynaptically located dopamine receptor systems and suggest that dopamine systems are implicated in social grooming. Although striking similarities between pinning and grooming exist, the present study has shown several differences. Both pinning and grooming were inNP ZE/4--G

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creased by social isolation, but ceiling levels were reached after different periods of isolation. Both behavioural responses were attenuated by morphine and inhibited by naltrexone, but pinning seemed to be more sensitive to the effect of naltrexone. On the other hand, social grooming was more sensitive to the effect of /.I-endorphin. Although different opioid systems may be concerned in pinning and grooming behaviour, it is also possible that anatomically different systems of the brain are involved. From binding studies and social interaction experiments there is evidence that endogenous opioid systems in the amygdala are of special importance in social behaviour (Panksepp and Bishop, 1981; File and Rodgers, 1979). On the basis of lesion experiments, Siviy and Panksepp (1985) suggested a possible involvement of opioid mechanisms in the dorsomedial thalamus and the parafascicular region of the thalamus in the juvenile play of rats. Pinning and social grooming were both affected by apomorphine, but these effects were differentially sensitive to the effect of the DAantagonist haloperidol and the neuropeptide DE-y-E. Thus, DA-ergic systems may be involved in both sociobehavioural activities. These systems may be different with respect to the subtypes of DA receptors or to the neuroanatomical structures involved. In conclusion, the simultaneous measurement of pinning and social grooming is a sensitive procedure for measuring social behaviour in juvenile rats. Behavioural changes due to psychopharmacological agents are apparent in small groups of animals. Although dopaminergic and endogenous opioid systems are involved in the realization and/or expression of both pinning and social grooming, differences e.g. in dose-effect relationships suggest differential mechanisms underlying both social activities. Therefore, this behavioural procedure seems to be promising for the screening of the effects of drugs in juvenile rats and for studying mechanisms in the central nervous system, underlying social behaviour. Acknowledgements-The

excellent technical assistance of Karien Janssen-Schwiebbe and Emilie Burbach-Bloemarts is gratefully acknowledged. The authors wish to thank Ingrid Van Schaik for carrying out part of the experiments. REFERENCES

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