Androgen effects on responsiveness to aggression and stress-related odors of male mice

Sickness-related odor communication signals as determinants of social behavior in rat: a role for inflammatory processes

Infected animals are avoided by conspecifics, suggesting that the inflammatory cascade may play a significant role in odor communication. Injection of male rats with the bacterial mimetic, lipopolysaccharide (LPS, 100 microg/kg, i.p.), decreased investigation through a wire-mesh partition between healthy male partners. This avoidance response was observed in adult males in response to soiled bedding collected from sick rats, regardless of whether LPS was injected peripherally (100 microg/kg, i.p.) or centrally (0.25 or 2.5 microg, icv). The release of sickness-related odor cues was dose-dependently blocked by icv infusion of the anti-inflammatory cytokine, interleukin-10 (IL-10; 20 or 200 ng), and reproduced by icv infusion of pro-inflammatory cytokine, IL-1beta (5 or 50 ng). Subcutaneous pretreatment with either estradiol benzoate (20 microg/kg) or testosterone propionate (50 or 500 microg/kg) to adult males that were administered LPS inhibited release of aversive odor cues, but these hormones alone did not influence odor properties. Importantly, the avoidance response to sickness-related odor was not associated with changes in plasma corticosterone, testosterone, or IL-6 levels of odor donors. However, plasma IL-1beta concentrations of sick animals was in fact predictive of aversive responses in conspecifics, suggesting that the inflammatory cascade, but not plasma steroid hormones, is likely to mediate aversive properties in odor that functions to signal illness state to conspecifics.

Does elevated testosterone result in increased exposure and transmission of parasites?

Male-biased infection is a common phenomenon in vertebrate-parasite systems and male-biased transmission has been experimentally demonstrated. One mechanism that is hypothesized to create male-biased transmission is the immuno-suppressive effect of testosterone because it increases susceptibility to infection. Testosterone also influences host behaviour and, consequently, may increase exposure to parasites. To test how testosterone could increase exposure and transmission, we undertook a longitudinal mark-recapture study where we experimentally elevated testosterone levels in wild male rodents. Individuals in control populations reduced the average number of contacts over the treatment period, while populations with experimentally elevated testosterone levels maintained the number of contacts between hosts. As a result, the transmission potential was higher in testosterone treated populations compared to controls. Our results indicated that males with high-testosterone levels alter the population-level contacts, producing different social networks and increasing transmission potential compared to those where testosterone is at background levels.

Modulatory role of testosterone in alarm pheromone release by male rats

An alarm pheromone released from stressed conspecifics evokes behavioral and autonomic responses in rats. We have previously reported that male Wistar rats show behavioral changes including increased sniffing, walking and rearing, and decreased resting as well as exaggerated response of body temperature to a novel environment [known as stress-induced hyperthermia (SIH)] when they are exposed to an alarm pheromone released from other male rats receiving foot shocks. The purpose of the present study was to examine the role of testosterone in the production and release of the alarm pheromone using these behavioral and autonomic responses in recipient rats. Three groups of alarm pheromone donors were presented, namely, intact males, castrated males, and testosterone-implanted castrated males. The effects of the alarm pheromone on the autonomic responses did not differ among the three groups, regardless of the donor's steroidal milieu, whereas behavioral responses were altered by castrating the donor males and the effects were restored by testosterone implantation. These results suggest that the alarm pheromone released from stressed male rats can be classified into at least two categories according to the androgen dependency of their production and/or release.

Neurohypophyseal peptides and social recognition in rats

An encounter between rats results in bouts of social investigation consisting mainly of sniffing, nosing, following and grooming. The assessment of social recognition is based on the tendency of rodents to investigate unfamiliar conspecifics more intensely, than familiar ones. In the laboratory an immature conspecific is normally used as the social stimulus because the use of juveniles eliminates possible sexual and/or aggressive behaviors of the rat whose memory is assessed. When a juvenile is presented for the first time, it is intensely investigated. A second presentation shortly after the first one elicits less attention. This is not due to satiation or fatigue, since the presentation of a novel juvenile triggers the full sequence of investigation. Social recognition is defined as a specific decrease in social investigation during the second encounter of the same individual. This form of memory is short lasting (< 40 min) and based on the olfactory characteristics of the stimulus animal. Social memory is prolonged by repeated exposure to the stimulus juvenile rat and is impaired by retroactively interfering stimuli. It can be facilitated by vasopressin and derivatives as well as by several other memory facilitating compounds, and, depending on the dose, attenuated or facilitated by oxytocin and derivatives. Ethologically oriented memory tests, that are based on olfactory characteristics of the information to-be-remembered, have an advantage over 'classical' ones: they estimate behavioral patterns which are important to an animal and not only to the investigator. Social memory paradigms can reveal information about memory processes in animals that is relevant for memory deficits in humans.

Presence of cues from stressed conspecifics increases reactivity to aversive events in cattle: evidence for the existence of alarm substances in urine

In gregarious species like cattle, the presence of partners often affects fear-related reactions. The first experiment investigated whether behavioral and physiological responses to stress depend on the emotional state of the partner. Aubrac heifers were presented with food in a novel environment. Compared to heifers in the presence of a companion that had been previously habituated to the environment without receiving shocks, those in the presence of a companion animal that had previously received electric shocks in that environment had a stronger increase of cortisol response (11.2+/-1.1 vs. 7.4+/-0.9 ng/mL), showed a significantly longer latency to feed (60.1+/-12.3 vs. 17.8+/-5.9 s), and fed more slowly (54.5+/-11.4 vs. 110+/-7.3 s). After repeated exposure to the test conditions, when heifers of both treatments fed rapidly after entrance, response to an unexpected air blast from the feeding bucket was measured. Heifers in the presence of a stressed companion showed an increased latency to feed again compared to those with a nonstressed companion (44.5+/-5.1 vs. 22.8+/-4.3 s). Stressed companions urinated during tests; therefore the second experiment investigated whether heifers respond differently to urine collected from stressed and nonstressed conspecifics. In a first test, heifers were presented with food on a grid in a bucket in a novel environment. They had a longer latency to feed when the bucket contained urine from stressed rather than from nonstressed conspecifics underneath the grid (128.1+/-9.6 vs. 108.2+/-4.9 s). In a second test, heifers were presented with a novel object in a familiar environment. Heifers showed a longer latency to explore the object when it had been sprayed with urine from stressed compared to urine from nonstressed conspecifics (215.2+/-45.0 vs. 25.8+/-8.6 s). The results show that heifers perceive the state of increased stress of conspecifics and become more fearful as a result. They further show that this perception is at least partly mediated by olfactory cues in the urine.

Androgen-dependent vasopressinergic neurons are involved in social recognition in rats

Socal recognition of juvenile conspecifics by adult male rats has been shown to be modulated by vasopressin. Because part of the extrahypothalamic vasopressinergic innervation of the brain is androgen-dependent, the present experiments were undertaken to assess possible interactions between androgens and vasopressin in social recognition. Castrated male rats displayed a temporary disruption of social recognition when they were tested 1 week after surgery. There was no impairment, however, when they were tested every other day following surgery. The peripheral injection of a vasopressor antagonist of vasopressin, dPTyr(Me)AVP (30 micrograms/kg) impaired social recognition in normal male rats but was ineffective in castrated ones. This was not due to an effect of castration on the basic pharmacological properties of dPTyr(Me)AVP since the antagonist peptide was still able to block the facilitating effects of vasopressin on social recognition in castrated male rats. Implantation of a testosterone-filled capsule in castrated male rats restored sensitivity of social recognition to the action of the vasopressin antagonist. These results suggest that androgen-dependent vasopressinergic neurons are physiologically involved in the modulation of social recognition in male rats.

Odor-guided behavior in mammals

The odor-guided behaviors selected for presentation in this paper encompass the major areas of animal behavior, and illustrate the important principal that complex relations exist between odor-guided behaviors, hormonal state, and experiential factors. Clearly, experiences with odors at several life stages results in profound influences upon later behaviors, including those related to eating, mating, fighting, and nesting. Interestingly, only brief social encounters are needed in rats to induce such phenomena as ultrasonic calling to conspecific estrous females or their odors, preferences for estrous over non-estrous odors, and the short-term modification of feeding behaviors. Although the mechanisms behind these intriguing phenomena are poorly understood, it is noteworthy that rats can learn relatively complex concepts on the basis of odors, rivaling even the ability of our own species to learn analogous tasks by visual cues. Despite the fact that close relationships can be demonstrated between odor-guided behaviors and variables such as endocrine state and sexual experience, caution is warranted in assuming that simple causal relations exist between such variables. In normally cycling women, for example, the correlation between olfactory sensitivity and plasma levels of estradiol during the menstrual cycle is relatively high; however, attenuation of the cyclical estradiol fluctuations by oral contraceptives does not eliminate the olfactory fluctuations, suggesting the relation is not causal. In house mice, social experience can override hormonal factors in their odor-guided urine marking and submissive behaviors. Thus, even though androgen titer usually correlates with such measures, a mouse made subordinant in a social encounter will not exhibit scent marking even when its circulating testosterone is maintained at a high level by a silastic implant. Further reason for caution comes from studies that suggest olfactory input influences the endocrine systems of sexually experienced and sexually inexperienced animals in different ways. For example, in sexually experienced male rats, anosmia decreases testosterone and estradiol levels and increases corticosterone levels, whereas in sexually inexperienced ones it has no significant influence on the levels of these steroids. Taken together, such observations suggest that the causal bases of a number of the odor-guided behaviors described in this paper are complex.(ABSTRACT TRUNCATED AT 400 WORDS)