Chris M. Markham

Lesions of structures showing FOS expression to cat presentation: effects on responsivity to a Cat, Cat odor, and nonpredator threat.

Exposure of rats to a cat elicits Fos activity in a number of brain areas or structures. Based on hodological relationships of these, Canteras has proposed a medial hypothalamic defense system, with input from several forebrain sites. Both electrolytic and neurotoxic lesions of the dorsal premammillary nucleus, which shows the strongest Fos response to cat exposure, produce striking decrements in a number of defensive behaviors to a cat or to cat odor stimuli, but do not have a major effect on either postshock freezing, or responsivity to the odor of a female in estrus. Neurotoxic lesions of the medial amygdala produce decrements in defensiveness to predator stimuli, particularly odor stimuli, that are consistent with a view of this structure as involved with allomonal cues. While dorsal hippocampal lesions had little effect on responsivity to predator stimuli, neurotoxic lesions of the ventral hippocampus reduced freezing and enhanced a variety of nondefensive behaviors to both cat odor and footshock, with similar reductions in defensiveness during context conditioning tests for cat odor, cat exposure and footshock. These results support the view that the dorsal premammillary nucleus is strongly and selectively involved in control of responsivity to predator stimuli. Structures with important input into the medial hypothalamic defense system appear also to be functionally involved with antipredator defensive behaviors, and these lesion studies may suggest specific hypotheses as to the particular defense functions of different areas.

The rat exposure test: a model of mouse defensive behaviors.

In order to facilitate behavioral, and potentially pharmacological, analyses of risk assessment behaviors in mice, a rat exposure test (RET) was devised and evaluated. This test provides a home chamber connected via a tunnel to a rat (predator) exposure area. Familiar substrate is provided to permit burying, and mouse subjects are habituated to the apparatus prior to exposure to an amphetamine-activated rat. In comparison to toy-rat-exposed controls, rat-exposed BALB/c mice showed significantly more risk assessment [stretch attend posture (SAP) and stretch approach], freezing, and avoidance (time in the home chamber), and less time in contact with the wire mesh screen between itself and the threat stimulus. When BALB/c, C57BL/6, CD-1, and Swiss-Webster mice were compared in this test, the two inbred strains (BALB/c and C57BL/6) tended to show more extreme values of particular defensive behaviors, compared to the two outbred strains (Swiss-Webster and CD-1). C57BL/6 mice showed more avoidance and higher levels of SAP, freezing, and burying than BALB/c and more than one or both outbred strains as well. BALB/c mice showed little defensive burying, both in comparison to toy-exposed controls (Experiment 1), and in comparison to the three other strains in Experiment 2. These findings are somewhat at variance with characterizations of anxiety in C57BL/6 and BALB/c mice, based on tests utilizing novel areas and noxious stimuli, suggesting strain differences in defensiveness to such stimuli, compared to antipredator defense levels. Nonetheless, with the exception of burying in BALB/c mice, all strains showed all defensive behaviors measured to the rat stimulus. In particular, SAP levels were substantial in all strains tested, suggesting the usefulness of this test in assessment of the role of risk assessment in defense.

AVP V1b selective antagonist SSR149415 blocks aggressive behaviors in hamsters

Arginine vasopressin (AVP) has been implicated in a variety of physiological and behavioral responses to stress. Synthesis of receptor-selective AVP agonist and antagonist compounds allows differential analysis of the specific roles of particular receptor subtypes with respect to these responses. Here, effects of the recently synthesized AVP V1b selective antagonist, SSR149415, were examined for offensive aggression in male Syrian hamsters, using a resident-intruder paradigm. Oral administration of vehicle or 1, 10, or 30 mg/kg of SSR149415 to resident hamsters was followed by evaluation of a range of aggression-related measures of residents confronted by intruders. The 10 and 30 mg/kg doses significantly reduced the duration of offensive sideways and chase behaviors, and the 30 mg/kg dose also reduced chase frequency. The 10 and 30 mg/kg dose also significantly reduced frequency and duration of olfactory investigation and duration of flank marking. These findings suggest a link between activity of the V1b receptor and the modulation of offensive aggression. These findings agree with previous research on V1b receptor effects in suggesting that antagonism of this receptor may be useful in modulating a range of emotional responses to highly stressful or threatening conditions.

Dorsal premammillary nucleus differentially modulates defensive behaviors induced by different threat stimuli in rats

Lesions of the dorsal premammillary nucleus (PMd) have been reported to produce dramatic reductions in responsivity of rats to a live cat. Such lesions provide a means of analyzing the potentially differential neural systems involved in different defensive behaviors, and the relationship between these systems and concepts such as anxiety. Rats with bilateral electrolytic lesions of the PMd were run in an elevated plus maze (EPM), exposed first to cat odor and then to a live cat, and assessed for postshock freezing and locomotion. PMd lesions produced a dramatic reduction in freezing, avoidance, and stretch attend to the cat odor stimulus, and reduction in freezing, with greater activity, and enhanced stretch approach to cat exposure. However, PMd lesions had minimal effects in the EPM, and postshock freezing scores were unchanged. These results confirm earlier findings of reduced defensiveness of PMd-lesioned rats to a cat, extending the pattern of reduced defensiveness to cat odor stimuli as well, but also suggest that such lesions have few effects on nonolfactory threat stimuli.

Development of defensive behavior and conditioning to cat odor in the rat

Laboratory rats show a range of defensive behaviors, including freezing, avoidance, and risk assessment upon exposure to cat odor, an unconditioned but highly effective threat stimulus. This study examined defensive behaviors, and the rapid conditioning to context plus cue, of these behaviors, in 18-, 26-, and 38-day-old male and female rats exposed to cat odor. Rats were placed individually in a runway with a cloth covered (control or saturated with cat fur/skin odor) block for a 10-min trial. On the following day, a similar trial involved an odorless block. On the odor exposure day, rats of all ages showed less contact with the odor block than with the control block. The 26- and 38-day-old rats, but not the 18-day-old rats, also showed locomotor suppression, more avoidance of the area where the odor block was located, and more risk assessment than no-odor controls. On a test of conditioned behavior 24 h following exposure, 26- and 38-day-old rats exhibited defensive behavior including avoidance and reduction of locomotion while 18-day-old pups did not.

Modulation of predatory odor processing following lesions to the dorsal premammillary nucleus

Previous studies have shown that electrolytic lesions of the dorsal premammillary nucleus (PMd) produce robust reductions in responsivity of rats to the presence of a live predator as well as to its odor, suggesting a critical role for the PMd in the modulation of defense. The present study investigated whether disruptions in defensive responding were specific to predators or if they may indicate a more general deficit in responding to pheromonal odors. Sexually naive male rats with bilateral ibotenic acid lesions of the PMd were exposed to the odor of a female rat in estrus as well as to the presence of cat odor, and, a live cat. PMd lesions produced a dramatic reduction in freezing and avoidance to the cat odor; and, reductions in freezing, enhanced activity and risk assessment to cat exposure. However, PMd lesions produced no changes in response to the presentation of the female odorant. These results confirm earlier findings of attenuation in defensiveness following electrolytic PMd lesions while extending these findings to suggest that the reduced defensiveness occurs specifically in response to predatory odors.

Sexual and aggressive interactions in a visible burrow system with provisioned burrows.

The visible burrow system (VBS) is a habitat providing burrows and an open area for mixed-set rat colonies. Provisioning of food and water in the burrows makes it unnecessary for potentially defensive animals to leave the burrows to eat/drink on the surface, and enables evaluation of new types of agonistic interactions that may emerge when this necessity is removed. In such colonies, subordinate males showed high magnitude tunnel guarding behavior, occupying a tunnel opening onto the surface and confronting the dominant. Dominants, in response, made lunges into the tunnels, but quickly retreated without gaining entry, apparently stopped by contact with the defenders vibrissae. Dominants also made and continued to make lateral attacks to the wall adjacent to the tunnels guarded by subordinates, although these were useless in terms of affording contact with the subordinate. Dominant-female agonistic interactions were more frequent than those of dominants and subordinates. These were largely initiated by the male, and involved female defensive behavior. Nonetheless, females, unlike subordinates, failed to show tunnel guarding and continued to utilize the surface freely. They also spent more time in the vicinity of the dominant over days of colony formation. This apparent paradox may reflect that females were seldom wounded, and that the initial site of male contact with females was the females anogenital area, findings suggesting that interactions of males and females often reflect male sexual advances, countered by female defenses that effectively protect nonestrus females from mounting and copulation.

Effects of the CRF1 antagonist SSR125543A on aggressive behaviors in hamsters

Corticotropin-releasing factor (CRF) and its receptor subtypes have been implicated in endocrine and behavioral responsivity to stress and emotion, including fear, anxiety, and aggression. SSR125543A is a new nonpeptide selective antagonist at the CRF1 receptor that has been shown to produce an anxiolytic-like effect in a number of animal models of anxiety. The present study investigated effects of an oral dose of 10, or 30 mg/kg of SSR125543A on aggressive behaviors of resident male Syrian hamsters toward male intruders. The high dose (30 mg/kg) of the CRF1 receptor antagonist produced a higher latency to bite and lower lateral attack frequencies and chase durations, indicating a reduction in aggression toward intruders in resident hamsters. The same dose of SSR125543A also enhanced frequency and duration of olfactory investigation, indicating that neither avoidance of the opponent nor deficiency in social activity is responsible for the reduction in aggression seen in these animals.

Infralimbic D1 receptor agonist effects on spontaneous novelty exploration and anxiety-like defensive responding in CD-1 mice.

Mesocortical dopamine (DA) terminals in the ventromedial prefrontal cortex (vmPFC) integrate cognitive/emotional processing functions underlying adaptive and appropriate behavioral responding to stressful environmental events. Results from several studies have also shown that stressor-enhanced prefrontal DA activation exerts detrimental effects on cognitive performance. However, questions have arisen as to whether stressor-enhanced vmPFC DA transmission exerts direct control over conditioned or unconditioned responses to threatening events, or whether enhanced prefrontal DA transmission gates cognitive processing to facilitate adaptive responding in threatening situations. We have previously shown that infralimbic (IL) vmPFC dopamine D2 agonist and antagonist drug infusions reduced anxiety-like responding in the elevated plus-maze (EPM) and disrupted spontaneous exploration in the Y-maze in CD-1 mice. In the present study, the effects of IL vmPFC infusions of the specific D1 receptor agonist, SKF-81297, in CD-1 mice were evaluated on spontaneous exploration in the Y-maze, anxiety-like responding in a 2-trial elevated plus-maze procedure, and anti-predator defensive responding in the Mouse Defense Test Battery (MDTB). SKF-81297 infusions disrupted spontaneous alternation performance along with potentiated repetitive 2-arm responding in the Y-maze. In the elevated plus-maze, pre-trial 1 IL SKF-81297 infusions reduced anxiety-like responding (enhanced open arm entries and time ratio, unprotected stretch attends and head dips), and reduced closed arm time ratio and protected risk assessment activity (protected stretch attends). In trial 2, 24h later (no drug infusions), open arm entries, open time ratio, and unprotected head dips remained enhanced relative to trial 2 vehicle controls. In the MDTB, avoidance distance was enhanced in the approach test; risk assessment (approach) was enhanced in the closed alley test; and defensive threat (upright postures) was enhanced in the forced contact test. Results are discussed with respect to possible influences of IL vmPFC DA receptors on cognitively mediated responding to differing levels of threat in mice.

Physical environment modulates the behavioral responses induced by chemical stimulation of dorsal periaqueductal gray in mice

In order to investigate the relationship between behaviors elicited by chemical stimulation of the dorsal periaqueductal gray (dorsal PAG) and spontaneous defensive behaviors to a predator, the excitatory amino acid D,L-homocysteic acid (5 nmol in 0.1 micro l), was infused into the dorsal PAG and behavioral responses of mice were evaluated in two different situations, a rectangular novel chamber or the Mouse Defense Test Battery (MDTB) apparatus. During a 1-min period following drug infusion, more jumps were made in the chamber than in the MDTB runway but running time and distance traveled were significantly higher in the runway. Animals were subsequently tested using the standard MDTB procedure (anti-predator avoidance, chase and defensive threat/attack). No drug effects on these measures were significant. In a further test in the MDTB apparatus, the pathway of the mouse during peak locomotion response was blocked 3 times by the predator stimulus (anesthetized rat) to determine if the mouse would avoid contact. Ninety percent of D,L-homocysteic treated animals made direct contact with the stimulus (rat), indicating that D,L-homocysteic-induced running is not guided by relevant (here, threat) stimuli. These results indicate that running as opposed to jumping is the primary response in mice injected with D,L-homocysteic into the dorsal PAG when the environment enables flight. However, the lack of responsivity to the predator during peak locomotion suggests that D,L-homocysteic-stimulation into the dorsal PAG does not induce normal antipredator flight.