Poor rearing conditions and social stress in pigs: repeated social challenge and the effect on behavioural and physiological responses to stressors.

Effects of rearing condition on behavioural and physiological reaction to social confrontations and to social and non-social stressors were studied in female pigs. The pigs were reared under either poor (the standard farrowing crate) or enriched (group of free-ranging sows with piglets) conditions. At the age of 14-17 weeks, the pigs were exposed to a series of social confrontations where an intruder was introduced into the home pen of a resident. The results show the presence of a clear difference in terms of aggressive behaviour between residents and intruders from enriched but not from poor rearing conditions. Furthermore pigs reared under poor conditions inflicted more wounds on each other. We suggest that this reflects a difficulty in establishing a dominance relationship in poorly reared pigs, caused by impaired development of social skills in these pigs. Subsequently, reaction to novel object, non-social and social stress was measured in adult age, showing that the effects of rearing conditions are long-lasting, and give rise to differences in reaction to challenges in that pigs from enriched rearing conditions showed more avoidance behaviour than pigs from poor rearing conditions.

Rearing piglets in a poor environment: developmental aspects of social stress in pigs.

Effects of rearing conditions on the development of social relationships were investigated in female pigs. For that purpose, one group of piglets was reared in a poor environment (i.e., a commercially used indoor 4.2-m2 standard farrowing crate) and the other in an enriched environment (i.e., 0.5-ha outdoor pasture with half-open farrowing crates). After weaning, all piglets were housed in pairs under standard conditions. Dominant and subordinate individuals were distinguished within pairs and social relationships between members of a pair were studied in adulthood. The results show that pairs reared in a poor environment behave more aggressively. The subordinates of these pairs also develop symptoms indicative of chronic social stress exposure: i.e., they show a delayed onset of puberty, a smaller daily weight gain, and elevated basal cortisol levels that are also higher 5 h after an acute restraint stressor (tethering stress). No such deviations were found in subordinates reared in an enriched environment. It is concluded that rearing piglets in a poor environment may facilitate the development of social stress in adult (subordinate) pigs. Two underlying mechanisms may be proposed: a) deteriorated social skills lead to increased social stress, or b) a failure to cope with stressors in general, and the stress of being subordinate in particular, occurs in animals reared under poor conditions.

SDN-POA volume, sexual behavior, and partner preference of male rats affected by perinatal treatment with ATD.

The present study investigated 1) the importance of the aromatization process during the perinatal period for the development of the sexually dimorphic nucleus in the preoptic area of the hypothalamus (SDN-POA) of male rats, and 2) the relationship between SDN-POA volume and parameters of masculinization in male rats that were treated perinatally with the aromatase-inhibitor ATD. Males were treated with ATD either prenatally or pre- and neonatally, or with the vehicle. Masculine sexual behavior and partner preference were investigated in adulthood. Thereafter, animals were sacrificed and SDN-POA volume was measured. The SDN-POA volume was reduced in both the prenatally and the pre- and neonatally treated group, with a larger reduction in the latter than in the former group. Combined pre- and neonatal ATD treatment resulted in reduced frequency of mounts, intromissions, and ejaculations, as well as a reduced preference for a female over a male. The SDN-POA size was significantly and positively correlated with frequency of masculine sexual behavior, as well as preference for a female over a male.

Proceptive and receptive aspects of oestrus behaviour in gilts.

Proceptive and receptive aspects of behaviour were studied during the oestrus cycle of the female pig. These aspects were studied in a T-maze with one or two stimulus compartments that did or did not contain a teaser boar. Receptive behaviour was observed during several consecutive days and was quantified in front of the boar by using the back pressure test (BPT). Three aspects of proceptive behaviour, i.e. the tendency to 'search the boar', 'stay close to the boar' and 'to present' in front of the boar peaked in the middle of the receptive period, and in most gilts on the day preceding the last receptive day (Experiment 1). In contrast, preference for a particular boar, another aspects of proceptive behaviour, was only expressed during the days preceding and following the receptive period (Experiment 2). The results are discussed in terms of the possible functional implications of proceptive and receptive behaviour in relation to fertilization.

Lesions of the suprachiasmatic nucleus do not disturb sexual orientation of the adult male rat.

An enlarged suprachiasmatic nucleus (SCN) has been found earlier in a group of homosexual men, as compared to heterosexual controls. In order to assess a possible relationship between the SCN and sexual orientation, the present study was undertaken to investigate whether the rat SCN might play a role with respect to the expression of sexual orientation. Sexual orientation was measured in partner preference tests as the percentage of time spent in the vicinity of sexually active male and female incentives, that were separated from the experimental animal by a wire mesh. The results show that established patterns of sexual orientation towards female incentives by the adult male rat are not influenced by SCN-lesions. Disturbances in circadian rhythmicity of activity in SCN-lesioned animals did not affect this conclusion.

Cerebral glucose utilization during conditioned sexual arousal.

Local cerebral glucose utilization was investigated in male rats during conditioned sexual arousal. Increased glucose utilization was found in three amygdaloid nuclei after exposure to a stimulus associated with exposure to a sexually active female. No changes were observed in areas known to be of crucial importance for the expression of consummatory aspects of sexual behavior. These results corroborate and extend previous results showing a dissociation between the expression of appetitive and consummatory aspects of sexual behavior at a neural level.

Changes in male copulatory behavior after sexual exciting stimuli: effects of medial amygdala lesions.

A paradigm was developed to investigate how precoital sexual arousal affects parameters of sexual behavior in male rats. Estrous females in a wire mesh cage were used to induce sexual arousal before the sexual interaction test. In control procedures, males were presented in a wire mesh cage or else there was no stimuli at all. The results indicate that ejaculation latency is consistently reduced after preexposure to a female, but not after preexposure to a male, showing that the effect is specific for precoital sexual arousal. Other parameters were affected by precoital sexual arousal in some, but not in all experiments. Reductions in intromission latency moreover, were observed after both preexposure to a male and preexposure to a female, indicating that general social excitement affects this parameter. Preexposure to females for 10 minutes or 3 hours produced similar results. It was subsequently found that medial amygdala-lesioned (AME) animals differed from sham-lesioned (SHAM) controls with respect to their reaction to precoital sexual arousal. The results show that AME-lesioned animals, in contrast to SHAM-animals, do not show reduced ejaculation latencies after preexposure to an estrous female. The results are in line with the idea that AME-lesioned animals are deficient in the assimilation of information on sexual exciting stimuli.

Conditioned place preference induced by sexual interaction in female rats.

The present study was undertaken to establish whether the conditioned place preference paradigm can be utilized to investigate and elucidate the neuroendocrine basis of the appetitive elements of female sexual behavior. Females were exposed to a male with which copulation occurred in a distinctive compartment of the place preference apparatus and did not receive an incentive in the alternative compartment. After six pairings to each compartment a place preference test was conducted. Both estradiol benzoate and estradiol benzoate plus progesterone treated, ovariectomized females showed a preference for the compartment associated with sexual interaction. A second group of estradiol plus progesterone treated females was exposed to a male with which copulation occurred in one compartment of the place preference apparatus and to a sexually active, but caged, male in the other. The females tended to prefer the compartment paired with the caged male. After noncontingent intromissions, immediately preceding an additional test, the females showed a place preference for the compartment paired with sexual interaction. The presented observations indicate the potential use of the place preference procedure in studying opposing motivational processes associated with the unconditioned sequence of responses that characterize the species-specific pattern of sexual behavior.

Lesions of the SDN-POA inhibit sexual behavior of male Wistar rats.

Discrete bilateral lesions in the SDN-POA of sexually naive adult male rats were found to decrease the number of animals ejaculating and/or to increase latencies to the first mount, intromission and ejaculation. The deleterious effects of the lesions disappeared after 4 tests for sexual behavior but were reinstated when the males were tested under suboptimal conditions, i.e., when they were tested with a marginally receptive female or when they had only limited access to the stimulus female. It was subsequently shown that males with a bilaterally lesioned SDN-POA still showed an increase in plasma testosterone. LH and prolactin levels in response to sexual stimulation. Effects of the lesions on scent marking were not found. Together with previous data indicating that SDN-POA-lesions disrupt masculine sexual behavior in females, these data are taken as evidence that the SDN-POA plays a role in the regulation of masculine sexual behavior. The data further suggest that previously reported negative results of SDN-POA-lesions on masculine sexual behavior in male rats might be attributed to the use of sexually experienced instead of sexually inexperienced animals.

Sexual behavior and sexual orientation of the female rat after hormonal treatment during various stages of development.

The amount of circulating sex steroids during Postnatal Days 30-90 was varied in normally developed and in androgenized female rats. The influence of these manipulations on sexual behavior and sexual orientation was investigated. Normally developed or neonatally androgenized females were ovariectomized and implanted with estradiol through Postnatal Days 30-90 or sham-implanted. The remaining subjects were left intact during that period. The hormonal condition during Postnatal Days 30-90 significantly affected the behavior of normally developed females, but affected the behavior of neonatally androgenized females only to minor extent. Estrogen implants in normally developed females enhanced masculine sexual responses and induced a female-directed sexual orientation. Feminine sexual responses were unaffected by this treatment. Sham-implanted, normally developed females showed a male-directed sexual orientation and fewer masculine sexual responses than subjects which were left intact during Postnatal Days 30-90. Neonatal androgen treatment in general resulted in elevated levels of masculine Neonatal androgen treatment in general resulted in elevated levels of masculine sexual responses, inhibited feminine sexual behavior, and facilitated a female-directed sexual orientation.

Sexual experience and preference for males or females in the female rat.

The influence of sexual experience on preference behavior was investigated in adult female rats. In the first experiment, preference behavior for sexually active males versus estrous females was investigated in female rats who were experienced with mounting behavior. Preference of these mount experienced females was compared to preference of females who were naive in this respect. Mount experience with estrous females induced a female-directed preference in ovariectomized female rats, irrespective of whether the females were treated with OIL or with testosterone-propionate (500 micrograms TP, injected once 48 h prior to testing). Sexually naive, OIL-treated females did not show a preference for males or females, but TP induced a preference for a male. Individual differences in mount frequency were not correlated with preference behavior. In the second experiment, the influence of experience with feminine sexual responses on preference behavior was investigated. OIL-treated and TP-treated females oriented equally toward males and females when they had been given the opportunity to copulate with males prior to the preference tests. The results strongly suggest that prior sexual experience is an important determinant of preference behavior. The female's behavior during sexual interactions was, however, not predictive of later preference.

Attractivity of male and female rats which are hormonally manipulated during early development and in adulthood.

Attractivity is one aspect of female sexuality relevant for the understanding of male-female sexual interactions. In a previous study, it was shown that intact males were equally attracted to early androgenized, gonadally intact females as to normally developed, estrous females. The present study was designed to investigate in what way hormones given in adulthood might influence attractivity of early androgenized females in adulthood. Specifically, we compared the attractivity of neonatally androgenized females (NeoTP) to the attractivity of normally developed females (NeoOIL), neonatally castrated males (NeoCASTR), and neonatally sham-castrated males (NeoSHAM) when different groups received either OIL, estradiol benzoate (EB) or testosterone propionate (TP) in adulthood. The male's preference to stay in the vicinity of one incentive in favor of the other was taken as an index of attractivity. The results show that, under the present hormonal conditions, NeoTP-females are generally less attractive than NeoOIL-females, more attractive than NeoSHAM-males, and equally attractive as NeoCASTR-males. TP-treated androgenized females were found to be equally attractive as TP-treated NeoSHAM-males. It is concluded that, relative to normally developed females, androgenized females become less attractive when the endogenous secretion of sex steroids is artifically controlled by gonadectomy and/or by administration of fixed amounts of sex steroids.

Effects of testosterone, estrogen, and dihydrotestosterone upon aggressive and sexual behavior of female rats.

Groups of female TMD rats were treated either with estradiol benzoate (EB), dihydrotestosterone propionate (DHTP), testosterone propionate (TP), EB + DHTP (EB/DHTP), or with oil. These groups of females were tested for social aggression and for masculine and feminine sexual behavior. In addition, patterns of masculine and feminine sexual responses during the aggressive encounters, were investigated. TP-treated females of the same strain were used as opponents in the tests for aggression. In accordance with previous results, EB did not activate aggression whereas TP treatment resulted in a significant increase in aggression in females. Aggressive responses were activated by adding DHTP to EB, up to levels equal to those activated by TP. Sexual responses were observed in the tests for aggression as well as in tests for sexual behavior. The results indicated that feminine and masculine sexual responses were affected significantly by hormonal treatment. Mounting behavior in the test for aggression was activated by TP and by EB/DHTP. Lordosis and proceptive responses were inhibited in these groups as compared to EB-treated females, both in tests for aggression and in tests for sexual behavior. The results are consistent with the idea that dihydrotestosterone inhibits feminine and activates masculine sexual activity. The results also indicate that EB and DHTP synergistically activate aggression.

Sex-specific interactions between aggressive and sexual behavior in the rat: effects of testosterone and progesterone.

The influence of progesterone on sexual and aggressive behaviors during aggressive encounters was investigated in pairs of TP-treated male and female rats. Gonadectomized females, chronically injected with testosterone propionate (TP), showed low but consistent levels of feminine sexual behavior which alternated with aggression. Progesterone when given in addition to TP facilitated receptive and proceptive behaviors, but reduced levels of aggression. In TP-treated males, levels of aggression were the same as observed in TP-treated females. However, TP-treated males seldomly showed sexual behavior during aggressive encounters and additional treatment with progesterone did not affect their behavior. After the aggression tests, animals were tested in a social preference test in which an ovariectomized female cage mate and the opponent from the aggressive encounter served as incentives. Positive correlations between levels of aggression and social preference for an opponent were found in both sexes, although correlations only reached statistical significance when progesterone was given in addition to TP. These correlations were found in both sexes, despite the fact that group analysis revealed pronounced sex differences in social preference: males preferred to spend their time near ovariectomized female cage mates, whereas females divided their time equally among female cage mates and opponents.

Variable mounting levels in the female rat: the influence of experience and acute effects of testosterone.

Ovariectomized female rats were tested for mounting behaviour along 4 different procedures in order to investigate which factors may contribute to the variability of this behaviour observed in different experiments. Mounting behaviour was facilitated by repeated testing (group A). Estradiol-benzoate, when given for a short period of time (group B), induced a dramatic and long-lasting increase in mounting in females which were repeatedly tested according to the procedure of group A. Prolonged adaptation to the test environment (group C) did not facilitate mounting in repeatedly tested females. The 3 procedures used did not reduce inter-individual variability. Highly significant, positive, inter-test correlation coefficients indicated that relative individual differences in mounting remained highly stable throughout testing. Facilitatory effects of testosterone propionate (TP, 500 micrograms, injected once 48 h prior to testing) on mounting behaviour appeared to stimulate existing behavioural tendencies, rather than activating new patterns of behaviour: TP facilitated mounting in groups A and B, but not in group C. Neither did TP facilitate mounting in a group which had been left sexually naive (group D) until the time of TP-injection. It is concluded that highly stable inter-individual differences in mounting behaviour are present in the adult female rat. Differences in sensitivity for gonadal hormones are not likely to underly these individual differences.

Acute effects of sex steroids on lordosis behaviour of the female rat.

In the present series of experiments, dose-response and time-response relationships between single injections of sex steroids and the display of lordosis behaviour in the female rat, were investigated. When injected 48 h prior to testing, increasing doses of estradiol benzoate (EB) or testosterone propionate (TP) resulted in increasing levels of lordosis behaviour. TP was much less effective than EB in facilitating lordosis behaviour in the female rat. The optimum time interval between hormonal treatment and testing was 48-72 h for TP, but 48 h for EB. Single injections of dihydrotestosterone propionate (DHTP), also injected 48 h prior to testing, inhibited lordosis behaviour in EB-primed female rats. DHTP-inhibition of lordosis behaviour in EB-primed female rats was most effective when given prior to and least effective when given after EB-treatment. Progesterone, injected 4 h prior to testing, facilitated lordosis in EB-, TP- or EB + DHTP-treated female rats.

Androgens are specifically implicated in female rat sexual motivation. The influence of methyltrienelone (R1881) on sexual orientation.

The present experiment was designed to investigate whether androgens are specifically involved in the induction of a male-directed orientation in adult female rats. Ovariectomized female rats were either treated with the non-aromatizable androgen methyltrienelone (R1881), with testosterone propionate (TP), with estradiol benzoate (EB) or with an equal volume of the solvent. Sexual orientation of these females towards either sexually active males or estrous females was then investigated and related to levels of receptive and mounting behavior. Compared to the solvent-treated females, females treated with R1881, TP or EB spent more time near sexually active males. Mounting behavior was stimulated in the R1881- and TP-treated females, but EB-treated females mounted as often as females treated with the solvent only. Lordosis behavior was only observed in TP-treated or EB-treated females. Mount frequency of the females of the different treatment groups was positively correlated with time spent near males. These correlations reached statistical significance in the TP-treated and EB-treated females. In the TP-treated females, the lordosis quotient was negatively correlated with time spent near males. The results of the present experiment suggest that androgens need not be converted into estrogens in order to facilitate a male-directed orientation in ovariectomized female rats.

On the involvement of progesterone in sexually rewarded choice behavior of the female rat.

The present experiment was designed to investigate the role of Progesterone in the regulation of two aspects of the female's appetitive sexual behavior. Ovariectomized females were tested for partner preference using either a test in which sexual interaction was not possible ("sexual orientation") or a test which included the possibility of sexual interaction ("sexually rewarded choice behavior"). The females were either primed with OIL, Estradiol benzoate (EB) or with Testosterone propionate (TP). Additional groups of females which were primed with EB or TP were treated with Progesterone (P) four hours prior to testing. The results indicate that females which are either treated with EB or with TP, show a male-directed orientation, while OIL-treated females do not. Sexually rewarded choice behavior however, was not affected by hormonal treatment with EB or TP. Additional treatment with P did not further increase the female's male-directed sexual orientation. Sexually rewarded choices for males, in contrast, were drastically increased. EB + P- or TP + P-treated females also showed a significant increase in heterosexual interactions with males in the goal box. The present results support the hypothesis that P plays a role in the female's appetitive sexual behavior, only when the possibility for interaction with a male is present.

The influence of estrogen, testosterone and progesterone on partner preference, receptivity and proceptivity.

The influence of Estradiol benzoate (EB), Testosterone propionate (TP) and Progesterone (P) on the female's partner preference for sexually active males was investigated and compared to levels of receptive and proceptive behaviors observed in a tethered male test situation. Doses of EB (1 micrograms), TP (500 micrograms) and P were selected on the basis of previous investigations indicating that female rats treated with these dosages will show comparable levels of lordosis behavior. The results indicate that TP stimulates partner preference for sexually active males over estrous females in ovariectomized female rats. Females treated with EB tended to prefer the company of sexually active males more than Oil-treated females and less and TP-treated females. However, preference behavior of EB-treated females was not significantly different from that of Oil- or TP-treated females. Additional treatment with P (100 micrograms) did not influence partner preference of Oil-, EB- or TP-treated females. In the tethered male tests, P stimulated proceptivity of EB- or TP-treated females and receptivity of EB-treated females. Significant differences in proceptive and receptive behaviors between EB- or TP-treated were not found. Although facilitation of receptive and in particular, proceptive behaviors were found to be generally accompanied by an increased partner preference for males, it is concluded that gonadal hormones are differentially affecting aspects of female rat sexuality: Relative to the activation of receptive behavior, TP was found to be more effective (than EB) to increase preference for a male; P (given to EB- or TP-treated females) was found to stimulate receptive and proceptive behaviors considerably, while being ineffective to stimulate preference for a male.