Brain Research, 572 (1992) 261-264 ~) 1992 Elsevier Science Publishers B.V. All rights reserved. 0006-8993/92/$05.00
261
BRES 25030
Chronic intracerebral infusions of vasopressin and vasopressin antagonist modulate social recognition in rat Rose-Marie Bluth6 and Robert Dantzer INRA-1NSERM U176, Rue Camille Saint-Saens, Bordeaux (France)
(Accepted 29 October 1991) Key words: Vasopressin; Social recognition; Male rat; Castration
To assess the role of androgen-dependent brain vasopressinergic transmission in the modulation of social recognition, castrated male rats which are deficient in vasopressin were implanted intracerebroventricularly with an Accurel collodion mini-device containing 10/~g vasopressin (AVP) whereas intact male rats were implanted with a similar device containing 50/tg of 1-deaminopeniciUamine2-O-methyltyrosine arginine vasopressin (dPTyr(Me)AVP), a specific antagonist of the vasopressor like receptors of vasopressin. Control rats in each experimental group were implanted with an Accurel device containing water. Castrated rats treated with AVP explored familiar juvenile conspecificsin the same manner as intact male rats. Conversely, intact male rats treated with dPTyr(Me)AVP explored familiar juveniles in the same manner as castrated male rats. These results confirm the role of androgen-dependent vasopressinergie neurotransmission in social recognition. Social recognition is an important component of life in social groups. In rodents, it is mainly based upon chemosensory cues 11. Rats exposed to conspecifics invariably display bouts of olfactory investigation before engaging in more specific behavioral patterns such as attack or mounting, depending on the nature of the social stimulus. The duration and intensity of this initial exploratory behavior depends on the degree of familiarity with the other animal. Unfamiliar conspecifics are explored longer and more intensely than familiar ones. These variations in duration of social exploration can therefore be used to assess the modalities and mechanisms of social recognition of conspecifics presented at different time intervals 11'13. The use of juveniles as social stimuli enables us to study social recognition in a relatively pure manner because of the lack of aggression or sexual behavior directed toward these social stimuli. In a previous series of experiments, we have observed that male rats recognized juveniles for only a limited amount of time, typically 30-60 rain. In contrast, castrated male rats and females were able to recognize juveniles for a longer interval, about 2-3 h, in spite of shorter durations of exploration on the occasion of their first exposure to the social stimulus ~'2. Furthermore, the chemosensory experience which forms the basis of this recognition appears to be modulated by vasopressinergic neurons in intact but not in castrated male rats since social recognition was blocked by an antagonist of the vasopressor receptors of vasopressin (AVP) in the former
but not in the later animals 2. The neural basis of such a phenomenon is likely to be represented by the sexually dimorphic vasopressinergic neurons which originate from the bed nucleus of the stria terminalis and project to the olfactory tubercle, the lateral habenular nucleus and the lateral septum 5'6. Castration leads to a dramatic reduction of immunoreactive AVP in the septal area 5'6 and AVP m R N A in the cell bodies of the vasopressinergic neurons located in the bed nucleus of the stria terminalis 1°. The functional involvement of this androgen-dependent vasopressinergic pathway in social recognition has been demonstrated by the observation that local injections of the vasopressor antagonist of vasopressin in the lateral septum blocked social recognition in intact male rats 4. In all these experiments, we used acute injections of vasopressin and its antagonist peptide. Brain levels of vasopressin can be altered over a long period by implanting into the lateral ventricle a small piece of microporous polypropylene tubing with a semi-permeable coat, loaded with the relevant peptide in solution. This device allows diffusion of the peptide at a relatively constant rate in the cerebro-spinal fluid (CSF) without altering CSF flow7'8. Using this technique, we show in the present paper that chronic central perfusion of AVP in castrated male rats make them behave like intact male rats in response to familiar juveniles whereas chronic administration of an AVP antagonist has the opposite effect in intact male rats.
Correspondence: R.M. Bluth6, INSERM U176, Rue Camille Saint-Sachs 33077 Bordeaux Cedex, France. Fax : (33) 56 98 90 29.
262 Wistar male rats (Charles River, France), 2 months of age, were used as subjects. They were housed individually in 30 × 45 x 19 cm transparent plastic cages with free access to food and water and maintained on a reverse dark-light cycle (lights on at 16.00). Castration was performed under ether anesthesia and testing occurred 5 months post-castration, at different times, before and after implantation of an Accurel mini-device. Onemonth-old laboratory-reared juvenile Wistar rats were used as social stimuli 13 All behavioral tests took place in the home cage of the test animals and were carried out during the dark phase of the cycle. Each test consisted of a 4-min exposure to a juvenile followed by a second exposure to the same juvenile at different intervals. The interval (interexposure interval, IEI) was 30 min on the 1st day and 120 min on the 2nd day. During the recognition test, social investigatory behavior was observed continuously to a criterion of neglect or up to a maximum of 4 min. The criterion of neglect was defined as 30 continuous s during which there was no investigation of the juvenile by the adult. Investigation consisted mainly of ano-genital sniffing. Observations were carried out with a video camera using an infrared light. A trained observer unaware of the treatment group of the experimental subject recorded the total duration of social investigation of the juvenile by using pre-set keys on the keyboard of an Apple lie computer. The observation procedure had been previously validated 2.
I~Before
implantation
For chronic intracerebral application of AVP and its antagonist peptide, Accurel mini-devices were used ~, Two groups of rats, intact (n = 11) and castrated (n = 13) male rats were implanted with an Accurel mini-device in the lateral ventricle. The Accurel/collodion minidevices were made of 5-mm pieces of Accurel polypropylene (P9/80/1; 0.95 mm o.d., 0.6 m m i.d.). The lumen was filled with 1/~1 AVP solution (10/~g AVP/bd apyrogenic water, AVP provided by Clin-Midy, Montpellier), 1/A solution of a specific antagonist of vasopressor-like receptors of AVP, 1-deaminopenicillamine 2-Omethyltyrosine arginine vasopressin (dPTyr(Me)AVP, provided by Dr. J. Rivier, San Diego, 50 pg/pl apyrogenic water 9) or 1 /A apyrogenic water. After having been filled up, each device was heat-sealed and dipcoated with a membrane of collodium. Doses of AVP and dPTyr(Me)AVP were selected on the basis of available data 7'8. In each group, half the animals received the peptide loaded device while the others received the water filled device. In intact male rats, the peptide loaded device contained dPTyr(Me)AVP whereas in castrated male rats, it contained AVP. After 3 weeks of postoperative recovery, rats were submitted to a social recognition test carried out in the same conditions as before surgery, i.e. with interexposure intervals of 30 min and 120 rain. One week separated the two behavioral tests. Durations of social investigation were analyzed by 3-way analysis of variance with repeated measurements, followed by post-hoc comparisons of individual group means using the least significant difference test. Durations of social investigation on the first exposure to the juvenile served as baseline values. They were
I - - I A f t e r implantation
IEI = 30 min
100
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
150
tO
F/21Before implantation
J_
.O'}
[--IA tier implantation r***
r+7 IE1=120 min v
10£ . . . . . . . . .
.... ............
I 50
CONTROl.. AAVP INTACT
C O N T R O l _ AVP
.E
CASTRATE
Fig. 1. Duration of social investigation, expressed as percent of baseline values in intact or castrated male rats implanted with a capsule containing 1-deaminopenicillamine 2-O-methyltyrosine arginine vasopressin (dPTyr(Me)AVP) (AAVP) or vasopressin (AVP). Control animals were implanted with a capsule containing water. The social recognition test was carried out before (dashed colums) and after (empty colums) implantation of the capsule. The interexposure interval was set to 30 min.
CONTROL AAVP INTACT
CONTROL AVP CASTRATE
Fig. 2. Duration of social investigation upon a 120 min interval test. Same legends as in Fig. 1. (+ P < 0.10, *** P < 0.001 for paired comparisons of individual group means).
263
intervals 1,2.
These results confirm and extend further the results of our previous studies on the role of sex differences in sensitivity of social recognition to vasopressin-like peptides 1'2. We had previously shown that acute administration of AVP was able to increase the duration of social recognition in both intact and castrated male rats. In contrast, the ability of acute injections of the antagonist peptide to block social recognition in intact male rats was lost after castration and it was absent in female rats. We interpreted these last findings as supporting the hypothesis that social recognition is controlled by a different neural system in castrates compared to intact male rats 2. A likely candidate is represented by androgen-dependent vasopressinergic neurons in the brain of rodents 5'6. Accurel collodion devices have been shown to deliver their peptide content at a constant rate over 3 weeks. In rats implanted with a 10/~g AVP-loaded device, AVP levels in the cerebrospinal fluid increased from 28 pg/ml to 175 pg/ml and remained constant over the whole observation period 7. Although AVP levels in the CSF are not altered following castration since most of its content comes from the suprachiasmatic nucleus, it is likely that these supranormal concentrations are able to influence the functioning of structures close to ventricles, such as the septum. Intact male rats which have functional AVP-containing neurons should behave as castrated animals when chronically exposed to a specific antagonist of vasopressor-like receptors of AVP, i.e. they should be able now to recognize a previously presented juvenile even after a 120 min interval since their first encounter with it. In contrast, exposing castrated male rats to chronic AVP should compensate for the insufficient levels of this peptide in androgen-dependent AVP containing neurons, i.e. AVPtreated castrates should now behave as intact males and therefore be unable to recognize juveniles after a 120 min IEI. Both these predictions were verified in the present experiment, giving further support to the postulated involvement of androgen-dependent vasopressinergic neurons in social recognition.
The important finding, however, in this experiment is that this characteristic was abrogated by chronic exposure to AVP or dPTyr(Me)AVP. AVP-treated castrated animals behaved as if they were no longer able to recognize the juvenile at the 120 min IEI, whereas dPTyr(Me)AVP-treated intact rats displayed just the opposite trend.
Many thanks are due to Dr. G.J. Boer (Brain Research Institute, Amsterdam) for demonstration of the manufacture and implantation of Accurel devices and for provision of Accurel tubing, and to Claire Beaudu who helped to test the rats. The vasopressin antagonist dPTyr(Me)AVP was generously supplied by Dr. Jean Rivier (The Salk Institute, San Diego).
higher in intact males (87 + 7 and 76 + 7 for IEI 30 and 120 rain, respectively) than in castrates (44 + 7 and 41 + 7 min) and this difference was highly significant (P < 0.001). After implantation of the Accurel device there was a significant decrease in baseline values and this decrease was more marked in castrated than in intact males (33% vs 13%, respectively, P < 0.05). Because of these variations, durations of social investigation on the second exposure test were expressed as percent of baseline values (Figs. 1 and 2). A 3-way A N O V A (castration x peptide x time) revealed a significant effect of the treatment factor (P < 0.05) and the treatment x time interaction when the IEI was set to 30 min (Fig. 1). This interaction was due to the tendency of rats implanted with the peptide-loaded device to display lower durations of investigation after surgery than before when compared to control rats. A similar 3-way A N O V A on duration of investigation, expressed as percent of baseline values, in rats exposed to a previously investigated juvenile at the 120 min IEI revealed that the castration factor and all interactions were significant (P < 0.01). In general, castrated males spent less time investigating juveniles than intact males, with the important exception of animals which had received the peptide-loaded device. Intact males treated with dPTyr(Me)AVP explored less after implantation than before (P < 0.10) whereas castrated males treated with AVP explored more (P < 0.001). As expected, intact and castrated male rats implanted with the water-filled device showed a shorter duration of exploration of the juvenile social stimulus when it was presented for the second time 30 min after the first presentation, whereas this decrease occurred only in castrates but not in intact males when the inter-exposure interval was extended to 120 min. These results have previously been interpreted to suggest that intact male rats recognize juvenile conspecifics only for a limited amount of time (30-60 min) whereas castrated male rats, like females, are able to recognize juveniles for longer
REFERENCES 1 Bluth6, R.M. and Dantzer, R., Social recognition does not involve vasopressinergic neurotransmission in female rats, Brain Research, 535 (1990) 301-304.
2 Bluth6, R.M., Schoenen, J. and Dantzer, R., Androgendependent vasopressinergic neurons are involved in social recognition in rats, Brain Research, 519 (1990) 150-157. 3 Boer, G.J., Van der Woude, T.P., Kruisbrink, J. and Van Heerikhuize, J., Successful ventricular application of the miniatur-
264
4
5
6
7
8
ized controlled delivery Accurel technique for sustained enhancement of cerebrospinal fluid peptide levels in the rat, J. Neurosci, Methods., 11 (1984) 281-289. Dantzer, R., Koob, G.F., Bluth6, R.M. and Le Moal, M., Septal vasopressin modulates social memory in male rats, Brain Research, 457 (1989) 143-147. De Vries, G.J., Buijs, R.M. and Sluiter, A.A., Gonadal hormone actions on the morphology of the vasopressinergic innervation of the adult rat brain, Brain Research, 298 (1984) 141145. De Vries, G.J., Buijs, R.M. and Swaab, D.E, Ontogeny of the vasopressinergic neurons of the suprachiasmatic nucleus and their extrahypothalamic projections in the rat brain. Presence of a sex difference in the lateral septum, Brain Research, 218 (1981) 67-78. Kruisbrink, J., Van der Woude, T.P., Boer, G.J. and Mirmiran, M., Manipulation of vasopressin level in the cerebrospinal fluid of rat by means of an easely interchangeable controlled-delivery Accurel mini-device, J. Neurosci. Methods, 17 (1986) 103-108. Kruisbrink, J., Mirmiran, M., Van der Woude, T.P. and Boer,
9
10
11
12
13
G.J., Effects of enhanced cerebrospinal fluid levels of vasopressin, vasopressin antagonist or vasoactive intestinal polypeptide on circadian sleep-wake rhythm in the rat, Brain Research, 419 (1987) 76-86. Manning, M. and Sawyer, W.H., Design and uses of selective agonist and antagonist analogs of the neuropeptides oxytocin and vasopressin, Trends Neurosci., 7 (1984) 6-9. Miller, M.A., Urban, J.H. and Dorsa, D.M., Steroid dependency of vasopressin neurons in the bed nucleus of the stria terminalis by in situ hybridization, Endocrinology, 125 (1989) 2335-2340. Sawyer, T.E, Hengehold, A.K. and Perez W.A., Chemosensory and hormonal mediation of social memory in male rats, Behav. Neurosci., 98 (1984) 908-913. Thor, D.H., Testosterone and persistence of social investigation in laboratory rats, J. Cornp. Physiol. Psychol., 94 (1980) 970976. Thor, D.H. and Holloway, W.R., Social memory of the male laboratory rat, J. Comp. Physiol. Psychol., 96 (1982) 10001006.
261
BRES 25030
Chronic intracerebral infusions of vasopressin and vasopressin antagonist modulate social recognition in rat Rose-Marie Bluth6 and Robert Dantzer INRA-1NSERM U176, Rue Camille Saint-Saens, Bordeaux (France)
(Accepted 29 October 1991) Key words: Vasopressin; Social recognition; Male rat; Castration
To assess the role of androgen-dependent brain vasopressinergic transmission in the modulation of social recognition, castrated male rats which are deficient in vasopressin were implanted intracerebroventricularly with an Accurel collodion mini-device containing 10/~g vasopressin (AVP) whereas intact male rats were implanted with a similar device containing 50/tg of 1-deaminopeniciUamine2-O-methyltyrosine arginine vasopressin (dPTyr(Me)AVP), a specific antagonist of the vasopressor like receptors of vasopressin. Control rats in each experimental group were implanted with an Accurel device containing water. Castrated rats treated with AVP explored familiar juvenile conspecificsin the same manner as intact male rats. Conversely, intact male rats treated with dPTyr(Me)AVP explored familiar juveniles in the same manner as castrated male rats. These results confirm the role of androgen-dependent vasopressinergie neurotransmission in social recognition. Social recognition is an important component of life in social groups. In rodents, it is mainly based upon chemosensory cues 11. Rats exposed to conspecifics invariably display bouts of olfactory investigation before engaging in more specific behavioral patterns such as attack or mounting, depending on the nature of the social stimulus. The duration and intensity of this initial exploratory behavior depends on the degree of familiarity with the other animal. Unfamiliar conspecifics are explored longer and more intensely than familiar ones. These variations in duration of social exploration can therefore be used to assess the modalities and mechanisms of social recognition of conspecifics presented at different time intervals 11'13. The use of juveniles as social stimuli enables us to study social recognition in a relatively pure manner because of the lack of aggression or sexual behavior directed toward these social stimuli. In a previous series of experiments, we have observed that male rats recognized juveniles for only a limited amount of time, typically 30-60 rain. In contrast, castrated male rats and females were able to recognize juveniles for a longer interval, about 2-3 h, in spite of shorter durations of exploration on the occasion of their first exposure to the social stimulus ~'2. Furthermore, the chemosensory experience which forms the basis of this recognition appears to be modulated by vasopressinergic neurons in intact but not in castrated male rats since social recognition was blocked by an antagonist of the vasopressor receptors of vasopressin (AVP) in the former
but not in the later animals 2. The neural basis of such a phenomenon is likely to be represented by the sexually dimorphic vasopressinergic neurons which originate from the bed nucleus of the stria terminalis and project to the olfactory tubercle, the lateral habenular nucleus and the lateral septum 5'6. Castration leads to a dramatic reduction of immunoreactive AVP in the septal area 5'6 and AVP m R N A in the cell bodies of the vasopressinergic neurons located in the bed nucleus of the stria terminalis 1°. The functional involvement of this androgen-dependent vasopressinergic pathway in social recognition has been demonstrated by the observation that local injections of the vasopressor antagonist of vasopressin in the lateral septum blocked social recognition in intact male rats 4. In all these experiments, we used acute injections of vasopressin and its antagonist peptide. Brain levels of vasopressin can be altered over a long period by implanting into the lateral ventricle a small piece of microporous polypropylene tubing with a semi-permeable coat, loaded with the relevant peptide in solution. This device allows diffusion of the peptide at a relatively constant rate in the cerebro-spinal fluid (CSF) without altering CSF flow7'8. Using this technique, we show in the present paper that chronic central perfusion of AVP in castrated male rats make them behave like intact male rats in response to familiar juveniles whereas chronic administration of an AVP antagonist has the opposite effect in intact male rats.
Correspondence: R.M. Bluth6, INSERM U176, Rue Camille Saint-Sachs 33077 Bordeaux Cedex, France. Fax : (33) 56 98 90 29.
262 Wistar male rats (Charles River, France), 2 months of age, were used as subjects. They were housed individually in 30 × 45 x 19 cm transparent plastic cages with free access to food and water and maintained on a reverse dark-light cycle (lights on at 16.00). Castration was performed under ether anesthesia and testing occurred 5 months post-castration, at different times, before and after implantation of an Accurel mini-device. Onemonth-old laboratory-reared juvenile Wistar rats were used as social stimuli 13 All behavioral tests took place in the home cage of the test animals and were carried out during the dark phase of the cycle. Each test consisted of a 4-min exposure to a juvenile followed by a second exposure to the same juvenile at different intervals. The interval (interexposure interval, IEI) was 30 min on the 1st day and 120 min on the 2nd day. During the recognition test, social investigatory behavior was observed continuously to a criterion of neglect or up to a maximum of 4 min. The criterion of neglect was defined as 30 continuous s during which there was no investigation of the juvenile by the adult. Investigation consisted mainly of ano-genital sniffing. Observations were carried out with a video camera using an infrared light. A trained observer unaware of the treatment group of the experimental subject recorded the total duration of social investigation of the juvenile by using pre-set keys on the keyboard of an Apple lie computer. The observation procedure had been previously validated 2.
I~Before
implantation
For chronic intracerebral application of AVP and its antagonist peptide, Accurel mini-devices were used ~, Two groups of rats, intact (n = 11) and castrated (n = 13) male rats were implanted with an Accurel mini-device in the lateral ventricle. The Accurel/collodion minidevices were made of 5-mm pieces of Accurel polypropylene (P9/80/1; 0.95 mm o.d., 0.6 m m i.d.). The lumen was filled with 1/~1 AVP solution (10/~g AVP/bd apyrogenic water, AVP provided by Clin-Midy, Montpellier), 1/A solution of a specific antagonist of vasopressor-like receptors of AVP, 1-deaminopenicillamine 2-Omethyltyrosine arginine vasopressin (dPTyr(Me)AVP, provided by Dr. J. Rivier, San Diego, 50 pg/pl apyrogenic water 9) or 1 /A apyrogenic water. After having been filled up, each device was heat-sealed and dipcoated with a membrane of collodium. Doses of AVP and dPTyr(Me)AVP were selected on the basis of available data 7'8. In each group, half the animals received the peptide loaded device while the others received the water filled device. In intact male rats, the peptide loaded device contained dPTyr(Me)AVP whereas in castrated male rats, it contained AVP. After 3 weeks of postoperative recovery, rats were submitted to a social recognition test carried out in the same conditions as before surgery, i.e. with interexposure intervals of 30 min and 120 rain. One week separated the two behavioral tests. Durations of social investigation were analyzed by 3-way analysis of variance with repeated measurements, followed by post-hoc comparisons of individual group means using the least significant difference test. Durations of social investigation on the first exposure to the juvenile served as baseline values. They were
I - - I A f t e r implantation
IEI = 30 min
100
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
150
tO
F/21Before implantation
J_
.O'}
[--IA tier implantation r***
r+7 IE1=120 min v
10£ . . . . . . . . .
.... ............
I 50
CONTROl.. AAVP INTACT
C O N T R O l _ AVP
.E
CASTRATE
Fig. 1. Duration of social investigation, expressed as percent of baseline values in intact or castrated male rats implanted with a capsule containing 1-deaminopenicillamine 2-O-methyltyrosine arginine vasopressin (dPTyr(Me)AVP) (AAVP) or vasopressin (AVP). Control animals were implanted with a capsule containing water. The social recognition test was carried out before (dashed colums) and after (empty colums) implantation of the capsule. The interexposure interval was set to 30 min.
CONTROL AAVP INTACT
CONTROL AVP CASTRATE
Fig. 2. Duration of social investigation upon a 120 min interval test. Same legends as in Fig. 1. (+ P < 0.10, *** P < 0.001 for paired comparisons of individual group means).
263
intervals 1,2.
These results confirm and extend further the results of our previous studies on the role of sex differences in sensitivity of social recognition to vasopressin-like peptides 1'2. We had previously shown that acute administration of AVP was able to increase the duration of social recognition in both intact and castrated male rats. In contrast, the ability of acute injections of the antagonist peptide to block social recognition in intact male rats was lost after castration and it was absent in female rats. We interpreted these last findings as supporting the hypothesis that social recognition is controlled by a different neural system in castrates compared to intact male rats 2. A likely candidate is represented by androgen-dependent vasopressinergic neurons in the brain of rodents 5'6. Accurel collodion devices have been shown to deliver their peptide content at a constant rate over 3 weeks. In rats implanted with a 10/~g AVP-loaded device, AVP levels in the cerebrospinal fluid increased from 28 pg/ml to 175 pg/ml and remained constant over the whole observation period 7. Although AVP levels in the CSF are not altered following castration since most of its content comes from the suprachiasmatic nucleus, it is likely that these supranormal concentrations are able to influence the functioning of structures close to ventricles, such as the septum. Intact male rats which have functional AVP-containing neurons should behave as castrated animals when chronically exposed to a specific antagonist of vasopressor-like receptors of AVP, i.e. they should be able now to recognize a previously presented juvenile even after a 120 min interval since their first encounter with it. In contrast, exposing castrated male rats to chronic AVP should compensate for the insufficient levels of this peptide in androgen-dependent AVP containing neurons, i.e. AVPtreated castrates should now behave as intact males and therefore be unable to recognize juveniles after a 120 min IEI. Both these predictions were verified in the present experiment, giving further support to the postulated involvement of androgen-dependent vasopressinergic neurons in social recognition.
The important finding, however, in this experiment is that this characteristic was abrogated by chronic exposure to AVP or dPTyr(Me)AVP. AVP-treated castrated animals behaved as if they were no longer able to recognize the juvenile at the 120 min IEI, whereas dPTyr(Me)AVP-treated intact rats displayed just the opposite trend.
Many thanks are due to Dr. G.J. Boer (Brain Research Institute, Amsterdam) for demonstration of the manufacture and implantation of Accurel devices and for provision of Accurel tubing, and to Claire Beaudu who helped to test the rats. The vasopressin antagonist dPTyr(Me)AVP was generously supplied by Dr. Jean Rivier (The Salk Institute, San Diego).
higher in intact males (87 + 7 and 76 + 7 for IEI 30 and 120 rain, respectively) than in castrates (44 + 7 and 41 + 7 min) and this difference was highly significant (P < 0.001). After implantation of the Accurel device there was a significant decrease in baseline values and this decrease was more marked in castrated than in intact males (33% vs 13%, respectively, P < 0.05). Because of these variations, durations of social investigation on the second exposure test were expressed as percent of baseline values (Figs. 1 and 2). A 3-way A N O V A (castration x peptide x time) revealed a significant effect of the treatment factor (P < 0.05) and the treatment x time interaction when the IEI was set to 30 min (Fig. 1). This interaction was due to the tendency of rats implanted with the peptide-loaded device to display lower durations of investigation after surgery than before when compared to control rats. A similar 3-way A N O V A on duration of investigation, expressed as percent of baseline values, in rats exposed to a previously investigated juvenile at the 120 min IEI revealed that the castration factor and all interactions were significant (P < 0.01). In general, castrated males spent less time investigating juveniles than intact males, with the important exception of animals which had received the peptide-loaded device. Intact males treated with dPTyr(Me)AVP explored less after implantation than before (P < 0.10) whereas castrated males treated with AVP explored more (P < 0.001). As expected, intact and castrated male rats implanted with the water-filled device showed a shorter duration of exploration of the juvenile social stimulus when it was presented for the second time 30 min after the first presentation, whereas this decrease occurred only in castrates but not in intact males when the inter-exposure interval was extended to 120 min. These results have previously been interpreted to suggest that intact male rats recognize juvenile conspecifics only for a limited amount of time (30-60 min) whereas castrated male rats, like females, are able to recognize juveniles for longer
REFERENCES 1 Bluth6, R.M. and Dantzer, R., Social recognition does not involve vasopressinergic neurotransmission in female rats, Brain Research, 535 (1990) 301-304.
2 Bluth6, R.M., Schoenen, J. and Dantzer, R., Androgendependent vasopressinergic neurons are involved in social recognition in rats, Brain Research, 519 (1990) 150-157. 3 Boer, G.J., Van der Woude, T.P., Kruisbrink, J. and Van Heerikhuize, J., Successful ventricular application of the miniatur-
264
4
5
6
7
8
ized controlled delivery Accurel technique for sustained enhancement of cerebrospinal fluid peptide levels in the rat, J. Neurosci, Methods., 11 (1984) 281-289. Dantzer, R., Koob, G.F., Bluth6, R.M. and Le Moal, M., Septal vasopressin modulates social memory in male rats, Brain Research, 457 (1989) 143-147. De Vries, G.J., Buijs, R.M. and Sluiter, A.A., Gonadal hormone actions on the morphology of the vasopressinergic innervation of the adult rat brain, Brain Research, 298 (1984) 141145. De Vries, G.J., Buijs, R.M. and Swaab, D.E, Ontogeny of the vasopressinergic neurons of the suprachiasmatic nucleus and their extrahypothalamic projections in the rat brain. Presence of a sex difference in the lateral septum, Brain Research, 218 (1981) 67-78. Kruisbrink, J., Van der Woude, T.P., Boer, G.J. and Mirmiran, M., Manipulation of vasopressin level in the cerebrospinal fluid of rat by means of an easely interchangeable controlled-delivery Accurel mini-device, J. Neurosci. Methods, 17 (1986) 103-108. Kruisbrink, J., Mirmiran, M., Van der Woude, T.P. and Boer,
9
10
11
12
13
G.J., Effects of enhanced cerebrospinal fluid levels of vasopressin, vasopressin antagonist or vasoactive intestinal polypeptide on circadian sleep-wake rhythm in the rat, Brain Research, 419 (1987) 76-86. Manning, M. and Sawyer, W.H., Design and uses of selective agonist and antagonist analogs of the neuropeptides oxytocin and vasopressin, Trends Neurosci., 7 (1984) 6-9. Miller, M.A., Urban, J.H. and Dorsa, D.M., Steroid dependency of vasopressin neurons in the bed nucleus of the stria terminalis by in situ hybridization, Endocrinology, 125 (1989) 2335-2340. Sawyer, T.E, Hengehold, A.K. and Perez W.A., Chemosensory and hormonal mediation of social memory in male rats, Behav. Neurosci., 98 (1984) 908-913. Thor, D.H., Testosterone and persistence of social investigation in laboratory rats, J. Cornp. Physiol. Psychol., 94 (1980) 970976. Thor, D.H. and Holloway, W.R., Social memory of the male laboratory rat, J. Comp. Physiol. Psychol., 96 (1982) 10001006.
