Brain Research, 535 (1990) 301-304 Elsevier
301
BRES 16124
Social recognition does not involve vasopressinergic neurotransmission in female rats Rose-Marie Bluth6 and Robert Dantzer INRA-INSERM, Psychobiologie des Comportements adaptatifs, Bordeaux (France) (Accepted 26 June 1990) Key words: Vasopressin; Social recognition; Sex difference; Response persistence; Rat
Social recognition is the ability to recognize a previously investigated conspecific. This phenomenon has been shown to be modulated by androgen-dependent vasopressinergic transmission in intact but not in castrated male rats. The dependence of social recognition on vasopressinergic transmission was studied in female rats. In comparison to intact males, females showed less persistence in investigating juvenile conspecifics and held social memories for longer intervals. Social recognition was enhanced by peripheral injections of vasopressin (6 ~g/kg) in both sexes. However, in contrast to what had been observed in males, social recognition in females was insensitive to the blocking effects of a vasopressor antagonist of vasopressin, dPTyr(Me)AVP (30/~g/kg, s.c.). These results suggest that social recognition is not mediated by vasopressinergic transmission in female rats. INTRODUCTION The ability to recognize conspecifics is an important requirement of life in social groups. Social recognition is expressed at different levels of complexity, from the distinction between familiar versus novel conspecifics to the classification of individuals in distinct categories (e.g., males vs females, sexually receptive females vs nonreceptive ones, dominant animals vs subdominant ones). Specialized information processing systems have developed for this purpose. They are based on different sensory modalities depending on species, physiological stage and familiarity with social stimuli. In rodents, social recognition is mainly based upon chemosensory cues. When exposed to a conspecific, animals invariably engage in bouts of olfactory investigation of the stimulus animal. Since novel animals are investigated longer than familiar ones, the difference between investigation times of the same stimulus animal presented at two different intervals provides a convenient index of the memory for this particular stimulus. For example, adult male rats confronted with a sexually immature rat (in order to minimize instances of aggression and sexual behavior) display a dramatic reduction of investigatory behavior upon re-exposure to the same juvenile when this exposure takes place up to 30 min after the first one, but not when it is delayed by an interval of 2 h 17. This reduced tendency to investigate the stimulus
animal does not occur when the second exposure is to a different juvenile 17. Social recognition has been shown to be based on olfaction since it is blocked by olfactory bulbectomy 7. In addition, the temporary presentation of chemosensory stimuli (soiled bedding or urine) from a particular juvenile can replace that juvenile in the formation of social memory t5. Juveniles are not recognized on the basis of the olfactory cues they leave in the test cage since removing the adult rat out of its home cage during the interval between successive tests does not alter the duration of memory 14. The possibility that rats are actually able to form olfactory memories of juveniles is supported by the results of experiments showing that the duration of such memories is decreased by retroactive interference procedures and increased by retroactive potentiation procedures 5. In a previous series of experiments, we have demonstrated that the olfactory memory adult male rats form of juvenile conspecifics is enhanced by peripheral or central administration of vasopressin and blocked by injection of an antagonist of the vasopressor receptors of vasopressin 5'6. The involvement of vasopressin in this form of memory is dependent on circulating levels of androgens since social memory in castrated male rats is insensitive to the blocking effects of the vasopressor antagonist of vasopressin 3. This is not due to the effect of castration on the basic pharmacological properties of the vasopressin antagonist since the antagonist peptide is still able to
Correspondence: R. Dantzer, INSERM, Rue Camille Saint-Sachs, 33077 Bordeaux Cedex, France. 0006-8993/90/$03.50 (~ 1990 Elsevier Science Publishers B.V. (Biomedical Division)
302 block the facilitating effects of vasopressin on social m e m o r y in castrated male rats. In addition, testosterone replacement fully restores sensitivity of social memory to blockade of vasopressinergic transmission3. These results point to the role of a n d r o g e n - d e p e n d e n t vasopressinergic neurotransmission in the processing of socially relevant chemosensory cues in male rats. The neural basis of such a p h e n o m e n o n is likely to be represented by the sexually dimorphic vasopressinergic neurons which originate from the bed nucleus of the stria terminalis (BNST) and project to the olfactory tubercle, the lateral habenular nucleus and the lateral septum s'9. The n u m b e r of vasopressin-like immunoreactive neurons in the BNST and their projections to the lateral septum is greater in intact males than in castrates and in females. This difference is due to the regulation of the expression
adjacent room, recorded the total duration of active investigation of the juvenile by the adult animal (mainly in the form of ano-genital sniffing), by using pre-set keys on the keyboard of an Apple lie computer. On the recognition test, social investigation was recorded to a criterion of neglect or up to a maximum of 5 rain. The criterion of neglect was defined as 30 continuous seconds during which there was no investigation of the juvenile by the test animal. This observation procedure had been previously validated 5'r. To test the role of vasopressin in social recognition, test animals were injected subcutaneously (2 ml/kg) with arginine vasopressin (AVP, Clin-Midy, Montpellier) or an antagonist of vasopressor receptors of vasopressin, l-deaminopenicillamine 2-O-methyltyrosine arginine vasopressin (dPTyr(Me)AVP) 11 (Dr. Jean Rivier, Salk Institute, San Diego), immediately after the first exposure to the juvenile. This vasopressin antagonist has been shown to cross the blood-brain barrier and act centrally ~°. Physiological saline was injected to placebo-treated animals. Durations of social investigation were analyzed by 2- or 3-way analysis of variance with repeated measurements.
of the vasopressin gene in neurons of the BNST by circulating androgens 12,13. The functional involvement of
RESULTS
this sex-dependent vasopressinergic pathway in social
Female rats recognize juveniles for longer intervals than do male rats
recognition abilities of adult males has been demonstrated by the observation that local injections of a vasopressor antagonist of vasopressin in the lateral septum blocked social recognition in a m a n n e r similar to the effect observed after peripheral injection of a more lipophilic antagonist6.
To assess sex differences in the ability to recognize juvenile conspecifics, 8 male rats were compared to 9 female rats. Recognition tests with the same juvenile as the one presented on the first exposure were carried out at different intervals, from 5 to 180 min. Males were
If the sexually dimorphic vasopressin innervation of the brain is involved in social recognition, then the ability of
tested at 5, 30 and 120 min whereas females were tested at 30, 120 and 180 rain. These different intervals were
female rats to recognize juvenile conspecifics should differ from that of males in not being critically d e p e n d e n t on vasopressinergic neurotransmission. We report here that it is indeed the case. Female rats exposed to a juvenile conspecific form a memory of this particular
selected on the basis of preliminary investigations, in order to retain for each sex two intervals short enough for allowing social recognition and one interval too long for that. In both sexes, a second exposure to a different juvenile at a 30-min interval was used as a control condition for lack of recognition.
juvenile for a longer duration than do males. In contrast to males, however, this social memory is insensitive to the blocking effects of a vasopressor antagonist of vasopressin.
MATERIALS AND METHODS
Animals Sprague-Dawley male and female rats (Iffa Credo, France), 3-4 months of age, were used as subjects. They were housed individually in 30 x 45 x 19 cm transparent plastic cages with free access to food and water and maintained on a reverse dark-light cycle (light off from 04.00 to 16.00 h). One-month-old laboratory-reared juvenile Wistar rats of both sexes were used as social stimuli.
Apparatus and procedure All behavioral tests took place in the home cage of the test animals and were carried out during the dark phase of the cycle. Within a particular session, an experimental subject was tested twice, with a variable time interval between tests, depending on the particular experiment. The initial test consisted of a 5-min exposure to a juvenile conspecific. The second test was a recognition test. Test animals were presented either with the same or a different juvenile. Focal observation of the adult animal was carried out with a video camera using an infrared light. A trained observer, sitting in an
O n the first exposure, females spent less time investigating the juveniles than males (58.9 ___ 4.22 vs 87.3 + 4.76 s, F1,45 = 9.66, P < 0.01). O n the second exposure, durations of investigation of the same juvenile were significantly decreased in comparison to investigation of a different juvenile, at 30 and 120 min intervals in females and at 5 and 30 min intervals in males (Fig. 1) (F3,24 = 4.87, P < 0.01 and F3,2t = 17.9, P < 0.01, respectively). These results indicate that female rats spend less time investigating juvenile conspecifics than male rats but that the memory they form of a particular juvenile is longer lasting.
Vasopressin facilitates social recognition in female rats To assess the influence of vasopressin on the duration of social recognition, female rats (n = 8) were injected with vasopressin at a dose (6 pg/kg) that had previously been shown to enhance social recognition in both intact and castrated male rats 3'5. Second exposure was either to the same juvenile or to a different one and took place at
303
100 J
MALE
FEMALE SecorlO exposure to . S a r n e juvenile ~ Different juvenile
~ 100.~. .....
I00 ~,~
Second exposure to
if) Z
i
Same juvenile 30 min later
-- 75 L~ _ ~
"'7 ..... ,"~" ....... " ......... 50
,
*------a
~
5°
z ~0
W > Z
5C
Z
_o 25
25 a
C
5
30
120
30
/
30
120 180
30
0
INTER--EXPOSURE INTERVAL (MIN)
Fig. 1. Duration of social memory in male and female rats exposed to juvenile conspecifics. For each sex, the horizontal dotted line represents the mean duration of social investigation on the first exposure to the juvenile and the columns the duration of investigation of the same juvenile or a different one on the recognition test run at different intervals, as indicated on the horizontal axis, after the first session. (*P < 0.05; **P < 0.01 in comparison to mean duration of investigation of a different juvenile or the same juvenile at a different interval.)
an interval of 180 min following the first exposure. Durations of investigation on the second exposure test (Fig. 2) were submitted to a 2-way analysis of variance with repeated m e a s u r e m e n t s (same or different juvenile x saline or AVP). The juvenile factor and the juvenile × t r e a t m e n t interaction were both significant (F1,7 = 27.4, P < 0.001 and FL7 = 14.9, P < 0.001, respectively). Post hoc comparisons of individual group means revealed that female rats treated with AVP spent less time investigating familiar juveniles than new juveniles (P < 0.001). This decrease in social investigation was not due to a toxic effect of the treatment since investigation of a new juvenile was not affected by the AVP treatment. These results indicate that peripheral injections of
SAL
AAVP
Fig. 3. Lack of effect of a vasopressor antagonist of vasopressin, dPTyr(Me)AVP, on social recognition in female rats. The horizontal dotted line represents the mean duration of social investigation on the first exposure to the juvenile (baseline) and the columns the duration of investigation of the same juvenile on the recognition test run 30 rain later (***P < 0.001 in comparison to baseline).
AVP facilitate social recognition in females in the same way as previously observed in intact male rats and in castrates 3,5.
Female rats are insensitive to the blocking effects of dPTyr(Me)AVP To assess the role of e n d o g e n o u s vasopressin in their ability to memorize the olfactory characteristics of juveniles, female rats (n = 9) were treated with the vasopressor antagonist of vasopressin, dPTyr(Me)AVP, at a dose (30/zg/kg) which has b e e n shown to be able to block the effects of peripherally injected A V P 3'5. Animals were used as their own control and were injected immediately after the first exposure to the juvenile with saline or dPTyr(Me)AVP, in a randomized order. The second exposure test to the same juvenile took place 30 min later, at a time at which female rats are still able to recognize the juvenile.
o_
Z
Durations of investigation on the second exposure test (Fig. 3) were submitted to a one-way A N O V A with repeated measurements. The t r e a t m e n t factor was not significant (F1, 8 = 0.75).
~, 5c _z
These results indicate that a dose of d P T y r ( M e ) A V P that blocks social recognition in male rats 3'5 has no effect on social recognition in female rats.
lOO o 60
~
Second exposure to Same juvenile T
i, 0
DISCUSSION rr 0
SAL
AVP SAL
AVP
Fig. 2. Facilitatory effects of AVP on social recognition in female rats. The horizontal dotted line represents the mean duration of social investigation on the first exposure to the juvenile and the columns the duration of investigation of the same juvenile or a different one on the recognition test run 3 h later. (***P < 0.001 in comparison to saline-treated animals.)
The present set of studies show that female rats differ from males both in the duration of social recognition and its sensitivity to blockade of vasopressor-like receptors of vasopressin. Female rats differ from males in their reduced persistence to investigate juveniles. Although they display a strong interest for this social stimulus, their investigatory
304 behavior is short-lived. This sex difference is mediated by circulating androgens since androgenized females treated with testosterone as adults investigated a novel juvenile conspecific as long as did males and longer than did castrated males, ovariectomized females, or intact females 3A6As. It is of interest to note that the persistence effects of testosterone are much more marked in social than in non-social situations 1"2'~6"1s. The mechanisms of these effects are not yet understood and their neural
The sole administration of an agonist does not allow to distinguish between what is pharmacological and what is physiological in the observed effects. Administration of receptor antagonists or antibodies to the putative transmitter under investigation is better from this perspective 1°. In the present case, the fact that a dose of the vasopressor antagonist, dPTyr(Me)AVP, that is able to block the effects of exogenously administered AVP in both intact and castrated male rats 3"5, has no effect on
targets have not been identified.
social recognition in females leads to the conclusion that
In spite of their reduced persistence to investigate juveniles, females recognize juveniles for longer durations than do intact male rats. This difference could be interpreted to suggest that juveniles represent more
social recognition is not d e p e n d e n t on vasopressinergic neurotransmission in this sex. From a neuroanatomical perspective, it is tempting to relate these results to the
important social stimuli for females than for intact male rats. However, this interpretation still needs to be tested by assessing in a systematic way the duration of social recognition for various social stimuli differing by their biological importance. Peripherally injected vasopressin enhances the duration of social recognition in females in a similar way than in intact males and in castrates. Peripheral injections of vasopressin are known to act in a non-specific way, by enhancing arousal 1°. The exact target neural structures for this effect have not yet been identified. They are likely to be the same in females and in intact male rats, since females do not differ from male rats in their response to the so-called ' m e m o r y - e n h a n c i n g ' effects of vasopressin peptides 4.
REFERENCES 1 Andrew, R.J., Increased persistence of attention produced by testosterone, and its implication for the study of sexual behaviour. In J.B. Hutchinson (Ed.), Biological Determinants of Sexual Behaviour, Wiley, New York, 1978, pp. 255-275. 2 Archer, J., Testosterone and persistence in mice, Anirn. Behav., 25 (1977) 479-488. 3 Bluth6, R.M., Schoenen, J. and Dantzer, R., Androgendependent vasopressinergic neurons are involved in social recognition in rats, Brain Research, 519 (1990) 150-157. 4 Broekkamp, C.L.E., Gower, A.J. and Van Delft, A.M.L., Vasopressin-peptides, physostigmine and pramiracetam on latent learning in the rat, Br. J. Pharmacol., 89 (1986) 728P. 5 Dantzer, R., Bluth6, R.M., Koob, G.E and Le Moal, M., Modulation of social memory in the male rat by neurohypophyseal peptides, Psychopharrnacology, 91 (1987) 363-368. 6 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. 7 Dantzer, R., Tazi A. and Bluth6, R.M., Cerebral lateralization of olfactory mediated affective processes in rats, Behav Brain Res., in press. 8 De Vries, G.J., Buijs, R.M. and Sluiter, A.A., Gonadal hormone actions on the morphology of the vasopressinergicinnervation of the adult rat brain, Brain Research, 298 (1984) 141-145. 9 De Vries, G.J., Buijs, R.M. and Swaab, D.F., 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)
sexual dimorphism that characterizes vasopressinergic innervation of the BNST and the lateral septum. The n u m b e r of AVP neurons in the BNST and the density of immunoreactive AVP in these cells and their projections to the lateral septum is much greater in males than in females s'9. This sexual dimorphism results from the modulation by testosterone and/or its metabolites of expression of the AVP gene by n e u r o n s in the BNST 12A3. In functional terms, processing of social olfactory stimuli could involve neural structures that are similar in females and in males, but with different transmitters in each sex group. Alternatively, it might be speculated that females use a different n e u r o n a l system to process and encode recognition of juveniles than do male rats. Further studies are necessary to determine which possibility is the right one. 67-78. 10 Koob, G.E, Lebrun, C., Bluth6, R.M., Dantzer, R. and Le Moal, M., Role of neuropeptides in learning versus performance: focuson vasopressin, Brain Res. Bull., 23 (1989) 359-364. 11 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. 12 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. 13 Miller, M.A., Vician, L., Clifton, D.K. and Dorsa, D.M., Sex differences in vasopressin neurons in the bed nucleus of the stria terminalis by in situ hybridization, Peptides, 10 (1989) 615-619. 14 P6rio, A, Terranova J.P., Worms P., Biuth6 R.M., Dantzer R. and Biziere K., Specific modulation of social memory in rats by cholinomimetic and nootropic drugs by benzodiazepine inverse agonists, but not by psychostimulants, Psychopharmacology, 97 (1989) 262-268. 15 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. 16 Thor, D.H., Testosterone and persistence of social investigation in laboratory rats, J. Comp. Physiol. Psychol., 94 (1980) 970-976. 17 Thor, D.H. and Holloway, W.R., Social memory of the male laboratory rat, J. Comp. Physiol. Psychol., 96 (1982) 1000--1006. 18 Thor, D.H., Wainwright, K.L. and Holloway, W.R., Persistence of attention to a novel conspecific: some developmental variables in laboratory rats, Dev. Psychobiol., 15 (1982) 1-8.
301
BRES 16124
Social recognition does not involve vasopressinergic neurotransmission in female rats Rose-Marie Bluth6 and Robert Dantzer INRA-INSERM, Psychobiologie des Comportements adaptatifs, Bordeaux (France) (Accepted 26 June 1990) Key words: Vasopressin; Social recognition; Sex difference; Response persistence; Rat
Social recognition is the ability to recognize a previously investigated conspecific. This phenomenon has been shown to be modulated by androgen-dependent vasopressinergic transmission in intact but not in castrated male rats. The dependence of social recognition on vasopressinergic transmission was studied in female rats. In comparison to intact males, females showed less persistence in investigating juvenile conspecifics and held social memories for longer intervals. Social recognition was enhanced by peripheral injections of vasopressin (6 ~g/kg) in both sexes. However, in contrast to what had been observed in males, social recognition in females was insensitive to the blocking effects of a vasopressor antagonist of vasopressin, dPTyr(Me)AVP (30/~g/kg, s.c.). These results suggest that social recognition is not mediated by vasopressinergic transmission in female rats. INTRODUCTION The ability to recognize conspecifics is an important requirement of life in social groups. Social recognition is expressed at different levels of complexity, from the distinction between familiar versus novel conspecifics to the classification of individuals in distinct categories (e.g., males vs females, sexually receptive females vs nonreceptive ones, dominant animals vs subdominant ones). Specialized information processing systems have developed for this purpose. They are based on different sensory modalities depending on species, physiological stage and familiarity with social stimuli. In rodents, social recognition is mainly based upon chemosensory cues. When exposed to a conspecific, animals invariably engage in bouts of olfactory investigation of the stimulus animal. Since novel animals are investigated longer than familiar ones, the difference between investigation times of the same stimulus animal presented at two different intervals provides a convenient index of the memory for this particular stimulus. For example, adult male rats confronted with a sexually immature rat (in order to minimize instances of aggression and sexual behavior) display a dramatic reduction of investigatory behavior upon re-exposure to the same juvenile when this exposure takes place up to 30 min after the first one, but not when it is delayed by an interval of 2 h 17. This reduced tendency to investigate the stimulus
animal does not occur when the second exposure is to a different juvenile 17. Social recognition has been shown to be based on olfaction since it is blocked by olfactory bulbectomy 7. In addition, the temporary presentation of chemosensory stimuli (soiled bedding or urine) from a particular juvenile can replace that juvenile in the formation of social memory t5. Juveniles are not recognized on the basis of the olfactory cues they leave in the test cage since removing the adult rat out of its home cage during the interval between successive tests does not alter the duration of memory 14. The possibility that rats are actually able to form olfactory memories of juveniles is supported by the results of experiments showing that the duration of such memories is decreased by retroactive interference procedures and increased by retroactive potentiation procedures 5. In a previous series of experiments, we have demonstrated that the olfactory memory adult male rats form of juvenile conspecifics is enhanced by peripheral or central administration of vasopressin and blocked by injection of an antagonist of the vasopressor receptors of vasopressin 5'6. The involvement of vasopressin in this form of memory is dependent on circulating levels of androgens since social memory in castrated male rats is insensitive to the blocking effects of the vasopressor antagonist of vasopressin 3. This is not due to the effect of castration on the basic pharmacological properties of the vasopressin antagonist since the antagonist peptide is still able to
Correspondence: R. Dantzer, INSERM, Rue Camille Saint-Sachs, 33077 Bordeaux Cedex, France. 0006-8993/90/$03.50 (~ 1990 Elsevier Science Publishers B.V. (Biomedical Division)
302 block the facilitating effects of vasopressin on social m e m o r y in castrated male rats. In addition, testosterone replacement fully restores sensitivity of social memory to blockade of vasopressinergic transmission3. These results point to the role of a n d r o g e n - d e p e n d e n t vasopressinergic neurotransmission in the processing of socially relevant chemosensory cues in male rats. The neural basis of such a p h e n o m e n o n is likely to be represented by the sexually dimorphic vasopressinergic neurons which originate from the bed nucleus of the stria terminalis (BNST) and project to the olfactory tubercle, the lateral habenular nucleus and the lateral septum s'9. The n u m b e r of vasopressin-like immunoreactive neurons in the BNST and their projections to the lateral septum is greater in intact males than in castrates and in females. This difference is due to the regulation of the expression
adjacent room, recorded the total duration of active investigation of the juvenile by the adult animal (mainly in the form of ano-genital sniffing), by using pre-set keys on the keyboard of an Apple lie computer. On the recognition test, social investigation was recorded to a criterion of neglect or up to a maximum of 5 rain. The criterion of neglect was defined as 30 continuous seconds during which there was no investigation of the juvenile by the test animal. This observation procedure had been previously validated 5'r. To test the role of vasopressin in social recognition, test animals were injected subcutaneously (2 ml/kg) with arginine vasopressin (AVP, Clin-Midy, Montpellier) or an antagonist of vasopressor receptors of vasopressin, l-deaminopenicillamine 2-O-methyltyrosine arginine vasopressin (dPTyr(Me)AVP) 11 (Dr. Jean Rivier, Salk Institute, San Diego), immediately after the first exposure to the juvenile. This vasopressin antagonist has been shown to cross the blood-brain barrier and act centrally ~°. Physiological saline was injected to placebo-treated animals. Durations of social investigation were analyzed by 2- or 3-way analysis of variance with repeated measurements.
of the vasopressin gene in neurons of the BNST by circulating androgens 12,13. The functional involvement of
RESULTS
this sex-dependent vasopressinergic pathway in social
Female rats recognize juveniles for longer intervals than do male rats
recognition abilities of adult males has been demonstrated by the observation that local injections of a vasopressor antagonist of vasopressin in the lateral septum blocked social recognition in a m a n n e r similar to the effect observed after peripheral injection of a more lipophilic antagonist6.
To assess sex differences in the ability to recognize juvenile conspecifics, 8 male rats were compared to 9 female rats. Recognition tests with the same juvenile as the one presented on the first exposure were carried out at different intervals, from 5 to 180 min. Males were
If the sexually dimorphic vasopressin innervation of the brain is involved in social recognition, then the ability of
tested at 5, 30 and 120 min whereas females were tested at 30, 120 and 180 rain. These different intervals were
female rats to recognize juvenile conspecifics should differ from that of males in not being critically d e p e n d e n t on vasopressinergic neurotransmission. We report here that it is indeed the case. Female rats exposed to a juvenile conspecific form a memory of this particular
selected on the basis of preliminary investigations, in order to retain for each sex two intervals short enough for allowing social recognition and one interval too long for that. In both sexes, a second exposure to a different juvenile at a 30-min interval was used as a control condition for lack of recognition.
juvenile for a longer duration than do males. In contrast to males, however, this social memory is insensitive to the blocking effects of a vasopressor antagonist of vasopressin.
MATERIALS AND METHODS
Animals Sprague-Dawley male and female rats (Iffa Credo, France), 3-4 months of age, were used as subjects. They were housed individually in 30 x 45 x 19 cm transparent plastic cages with free access to food and water and maintained on a reverse dark-light cycle (light off from 04.00 to 16.00 h). One-month-old laboratory-reared juvenile Wistar rats of both sexes were used as social stimuli.
Apparatus and procedure All behavioral tests took place in the home cage of the test animals and were carried out during the dark phase of the cycle. Within a particular session, an experimental subject was tested twice, with a variable time interval between tests, depending on the particular experiment. The initial test consisted of a 5-min exposure to a juvenile conspecific. The second test was a recognition test. Test animals were presented either with the same or a different juvenile. Focal observation of the adult animal was carried out with a video camera using an infrared light. A trained observer, sitting in an
O n the first exposure, females spent less time investigating the juveniles than males (58.9 ___ 4.22 vs 87.3 + 4.76 s, F1,45 = 9.66, P < 0.01). O n the second exposure, durations of investigation of the same juvenile were significantly decreased in comparison to investigation of a different juvenile, at 30 and 120 min intervals in females and at 5 and 30 min intervals in males (Fig. 1) (F3,24 = 4.87, P < 0.01 and F3,2t = 17.9, P < 0.01, respectively). These results indicate that female rats spend less time investigating juvenile conspecifics than male rats but that the memory they form of a particular juvenile is longer lasting.
Vasopressin facilitates social recognition in female rats To assess the influence of vasopressin on the duration of social recognition, female rats (n = 8) were injected with vasopressin at a dose (6 pg/kg) that had previously been shown to enhance social recognition in both intact and castrated male rats 3'5. Second exposure was either to the same juvenile or to a different one and took place at
303
100 J
MALE
FEMALE SecorlO exposure to . S a r n e juvenile ~ Different juvenile
~ 100.~. .....
I00 ~,~
Second exposure to
if) Z
i
Same juvenile 30 min later
-- 75 L~ _ ~
"'7 ..... ,"~" ....... " ......... 50
,
*------a
~
5°
z ~0
W > Z
5C
Z
_o 25
25 a
C
5
30
120
30
/
30
120 180
30
0
INTER--EXPOSURE INTERVAL (MIN)
Fig. 1. Duration of social memory in male and female rats exposed to juvenile conspecifics. For each sex, the horizontal dotted line represents the mean duration of social investigation on the first exposure to the juvenile and the columns the duration of investigation of the same juvenile or a different one on the recognition test run at different intervals, as indicated on the horizontal axis, after the first session. (*P < 0.05; **P < 0.01 in comparison to mean duration of investigation of a different juvenile or the same juvenile at a different interval.)
an interval of 180 min following the first exposure. Durations of investigation on the second exposure test (Fig. 2) were submitted to a 2-way analysis of variance with repeated m e a s u r e m e n t s (same or different juvenile x saline or AVP). The juvenile factor and the juvenile × t r e a t m e n t interaction were both significant (F1,7 = 27.4, P < 0.001 and FL7 = 14.9, P < 0.001, respectively). Post hoc comparisons of individual group means revealed that female rats treated with AVP spent less time investigating familiar juveniles than new juveniles (P < 0.001). This decrease in social investigation was not due to a toxic effect of the treatment since investigation of a new juvenile was not affected by the AVP treatment. These results indicate that peripheral injections of
SAL
AAVP
Fig. 3. Lack of effect of a vasopressor antagonist of vasopressin, dPTyr(Me)AVP, on social recognition in female rats. The horizontal dotted line represents the mean duration of social investigation on the first exposure to the juvenile (baseline) and the columns the duration of investigation of the same juvenile on the recognition test run 30 rain later (***P < 0.001 in comparison to baseline).
AVP facilitate social recognition in females in the same way as previously observed in intact male rats and in castrates 3,5.
Female rats are insensitive to the blocking effects of dPTyr(Me)AVP To assess the role of e n d o g e n o u s vasopressin in their ability to memorize the olfactory characteristics of juveniles, female rats (n = 9) were treated with the vasopressor antagonist of vasopressin, dPTyr(Me)AVP, at a dose (30/zg/kg) which has b e e n shown to be able to block the effects of peripherally injected A V P 3'5. Animals were used as their own control and were injected immediately after the first exposure to the juvenile with saline or dPTyr(Me)AVP, in a randomized order. The second exposure test to the same juvenile took place 30 min later, at a time at which female rats are still able to recognize the juvenile.
o_
Z
Durations of investigation on the second exposure test (Fig. 3) were submitted to a one-way A N O V A with repeated measurements. The t r e a t m e n t factor was not significant (F1, 8 = 0.75).
~, 5c _z
These results indicate that a dose of d P T y r ( M e ) A V P that blocks social recognition in male rats 3'5 has no effect on social recognition in female rats.
lOO o 60
~
Second exposure to Same juvenile T
i, 0
DISCUSSION rr 0
SAL
AVP SAL
AVP
Fig. 2. Facilitatory effects of AVP on social recognition in female rats. The horizontal dotted line represents the mean duration of social investigation on the first exposure to the juvenile and the columns the duration of investigation of the same juvenile or a different one on the recognition test run 3 h later. (***P < 0.001 in comparison to saline-treated animals.)
The present set of studies show that female rats differ from males both in the duration of social recognition and its sensitivity to blockade of vasopressor-like receptors of vasopressin. Female rats differ from males in their reduced persistence to investigate juveniles. Although they display a strong interest for this social stimulus, their investigatory
304 behavior is short-lived. This sex difference is mediated by circulating androgens since androgenized females treated with testosterone as adults investigated a novel juvenile conspecific as long as did males and longer than did castrated males, ovariectomized females, or intact females 3A6As. It is of interest to note that the persistence effects of testosterone are much more marked in social than in non-social situations 1"2'~6"1s. The mechanisms of these effects are not yet understood and their neural
The sole administration of an agonist does not allow to distinguish between what is pharmacological and what is physiological in the observed effects. Administration of receptor antagonists or antibodies to the putative transmitter under investigation is better from this perspective 1°. In the present case, the fact that a dose of the vasopressor antagonist, dPTyr(Me)AVP, that is able to block the effects of exogenously administered AVP in both intact and castrated male rats 3"5, has no effect on
targets have not been identified.
social recognition in females leads to the conclusion that
In spite of their reduced persistence to investigate juveniles, females recognize juveniles for longer durations than do intact male rats. This difference could be interpreted to suggest that juveniles represent more
social recognition is not d e p e n d e n t on vasopressinergic neurotransmission in this sex. From a neuroanatomical perspective, it is tempting to relate these results to the
important social stimuli for females than for intact male rats. However, this interpretation still needs to be tested by assessing in a systematic way the duration of social recognition for various social stimuli differing by their biological importance. Peripherally injected vasopressin enhances the duration of social recognition in females in a similar way than in intact males and in castrates. Peripheral injections of vasopressin are known to act in a non-specific way, by enhancing arousal 1°. The exact target neural structures for this effect have not yet been identified. They are likely to be the same in females and in intact male rats, since females do not differ from male rats in their response to the so-called ' m e m o r y - e n h a n c i n g ' effects of vasopressin peptides 4.
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sexual dimorphism that characterizes vasopressinergic innervation of the BNST and the lateral septum. The n u m b e r of AVP neurons in the BNST and the density of immunoreactive AVP in these cells and their projections to the lateral septum is much greater in males than in females s'9. This sexual dimorphism results from the modulation by testosterone and/or its metabolites of expression of the AVP gene by n e u r o n s in the BNST 12A3. In functional terms, processing of social olfactory stimuli could involve neural structures that are similar in females and in males, but with different transmitters in each sex group. Alternatively, it might be speculated that females use a different n e u r o n a l system to process and encode recognition of juveniles than do male rats. Further studies are necessary to determine which possibility is the right one. 67-78. 10 Koob, G.E, Lebrun, C., Bluth6, R.M., Dantzer, R. and Le Moal, M., Role of neuropeptides in learning versus performance: focuson vasopressin, Brain Res. Bull., 23 (1989) 359-364. 11 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. 12 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. 13 Miller, M.A., Vician, L., Clifton, D.K. and Dorsa, D.M., Sex differences in vasopressin neurons in the bed nucleus of the stria terminalis by in situ hybridization, Peptides, 10 (1989) 615-619. 14 P6rio, A, Terranova J.P., Worms P., Biuth6 R.M., Dantzer R. and Biziere K., Specific modulation of social memory in rats by cholinomimetic and nootropic drugs by benzodiazepine inverse agonists, but not by psychostimulants, Psychopharmacology, 97 (1989) 262-268. 15 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. 16 Thor, D.H., Testosterone and persistence of social investigation in laboratory rats, J. Comp. Physiol. Psychol., 94 (1980) 970-976. 17 Thor, D.H. and Holloway, W.R., Social memory of the male laboratory rat, J. Comp. Physiol. Psychol., 96 (1982) 1000--1006. 18 Thor, D.H., Wainwright, K.L. and Holloway, W.R., Persistence of attention to a novel conspecific: some developmental variables in laboratory rats, Dev. Psychobiol., 15 (1982) 1-8.
