Modelling sexual violence in male rats: the sexual aggression test (SxAT)

Abstract

Sexual assault and rape are crimes that impact victims worldwide. Although the psychosocial and eco-evolutionary factors associated with this antisocial behavior have repeatedly been studied, the underlying neurobiological mechanisms are still largely unknown. Here, we established a novel paradigm to provoke and subsequently assess sexual aggression (SxA) in adult male Wistar rats: the sexual aggression test (SxAT). Briefly, male Wistar rats are sexually aroused by a receptive female, which is exchanged by a non-receptive female immediately after the first intromission. This protocol elicits forced mounting and aggressive behavior toward the non-receptive female to different degrees, which can be scored. In a series of experiments we have shown that SxA behavior is a relatively stable trait in rats and correlates positively with sexual motivation. Rats with innate abnormal anxiety and aggressive behavior also show abnormal SxA behavior. In addition, central infusion of oxytocin moderately inhibits aggressive behavior, but increases forced mounting. Finally, we identified the agranular insular cortex to be specifically activated by SxA, however, inhibition of this region did not significantly alter behavior in the SxAT. Altogether, the SxAT is a paradigm that can be readily implemented in behavioral laboratories as a valuable tool to find answers regarding the biological mechanisms underlying SxA in humans, as well as social decision-making in general.

Fig. 1. Behavioral profile of adult male Wistar rats in sexual aggression tests (SxAT).

A Latencies to copulate (CL) with a receptive female, (B) frequency of forced mounts (FM), and (C) frequency of aggressive behavior (AB) of intact (white circles n = 69) and i.c.v. cannulated (black circles, n = 57) males toward a non-receptive female during the SxAT performed on 3 consecutive days. D Frequency of forced mounts (FM) and aggressive behavior (AB) of males against females with a regular, a passive, or an aggressive response type (all three SxATs combined). All data are means ± S.E.M. E Percentage of trials where males ecountered females displaying regular, passive or aggressive responses to the male forced mounts n = 378. F Scatter plot of FM frequencies displayed during SxAT-1 vs. SxAT-3. G Scatter plot of AB frequencies during SxAT-1 vs. SxAT-3. H Scatter plot of FM frequencies (averaged over SxAT-1, SxAT-2 and SxAT-3) vs. AB frequencies (averaged over SxAT-1, SxAT-2 and SxAT-3). I scatter plots of the CL vs. FM (dark-gray diamonds) or AB (gray circles) averaged over SxAT-1, SxAT-2 and SxAT-3).

Fig. 2. Behavioral profile of adult male HAB (dark-gray circles and bars), LAB (light-gray circles and bars) and NAB (white circles and bars) rats in three consecutive daily sexual aggression tests (SxAT-1/2/3).

A Latency to copulate (CL) with a receptive female rat in SxAT-1 and SxAT-3 (note that SxAT-2 did not have an arousal phase). B Frequency of forced mounts (FM), and (C): Frequency of aggressive behaviors (AB) toward a non-receptive female in SxAT-1/2/3. D CL, FM, AB, and the proportion of lateral threats and attacks in AB (LTA%), averaged over SxAT1/2/3. Data are presented as mean ± s.e.m. *p < 0.05, **p < 0.01, ***p < 0.001 vs. NAB. ^##p < 0.01 HAB vs. LAB.

Fig. 3. Effects of synthetic oxytocin or vasopressin, and of ethanol on sexual aggression in adult male Wistar rats.

Effects of synthetic OXT and AVP, and of ethanol on sexual aggression in adult male Wistar rats. Sexual aggression was calculated as the percentage of time displaying forced mounting (FM) and aggressive behavior (AB) in a 10-min sexual aggression test (SxAT) following (A) i.c.v. infusion with OXT (1 or 100 ng/5 μl Ringer), B i.c.v. infusion with AVP (0.1 or 1 ng/5 μl Ringer), or C i.p. injection with ethanol (0.5 or 1.5 mg/kg ethanol), or vehicle (VEH, Ringer or saline). Data are mean + s.e.m. *p < 0.05 vs. VEH.

Fig. 4. Neuronal activity in selected brain regions in response to sexual aggression.

Neuronal activity was indicated by the number of Fos-IR neurons in response to 10 min of rest (CONTROL) or of exposure to the sexual aggression test (SxAT, purple bars), consensual mating (CMT, blue bars) or resident-intruder test (RIT, red bars). Brain areas are grouped according to activation patterns: A regions activated in all conditions, B regions activated exclusively by CMT, C regions activated only during the SxAT, and D regions activated only during the RIT. Data are presented as mean + s.e.m.; *p < 0.05 vs. CONTROL. Scatter plots of the number of consensual mounts (CM) in the CMT (E), or of aggressive behavior (F) in the SxAT vs. the number of Fos-IR in the aAIC, pAIC, LSD, and AHA. PRL pre-limbic cortex, IL infra-limbic cortex, aCg1 anterior cingulate cortex 1, pCg1 and pCg2 posterior cingulate cortex 1 and 2, respectively, aAIC and pAIC anterior and posterior agranular insular cortex, respectively, aAcC and pAcC anterior and posterior nucleus accumbens core, respectively, aAcS and pAcS anterior and posterior nucleus accumbens shell, BLA basolateral amygdala, CeA central amygdala, MeApd posterodorsal medial amygdala, LSD dorsal lateral septum, LSV ventral lateral septum, AH anterior hypothalamic attack area, LHA lateral hypothalamic attack area, LHb lateral habenula, MPOA medial preoptic area, VMHvl ventrolateral ventromedial hypothalamus.

Fig. 5. Effects of pharmacological inhibition of the posterior agranular insular cortex (pAIC) on sexual aggression (SxA) indicated by the percentage time displaying forced mounting (FM), and on general aggressive behavior (AB) and threat.

Adult male Wistar rats were bilaterally infused with a cocktail of 20 ng/μl muscimol and 200 ng/μl baclofen or vehicle into the pAIC 10 prior to exposure to the SxAT. Data are presented as mean + s.e.m.; *p < 0.05.