The ventromedial hypothalamus and the control of avoidance behavior and aggression: fear hypothesis versus response-suppression theory of limbic system function

Neural Circuits Underlying Rodent Sociality: A Comparative Approach

All mammals begin life in social groups, but for some species, social relationships persist and develop throughout the course of an individual's life. Research in multiple rodent species provides evidence of relatively conserved circuitry underlying social behaviors and processes such as social recognition and memory, social reward, and social approach/avoidance. Species exhibiting different complex social behaviors and social systems (such as social monogamy or familiarity preferences) can be characterized in part by when and how they display specific social behaviors. Prairie and meadow voles are closely related species that exhibit similarly selective peer preferences but different mating systems, aiding direct comparison of the mechanisms underlying affiliative behavior. This chapter draws on research in voles as well as other rodents to explore the mechanisms involved in individual social behavior processes, as well as specific complex social patterns. Contrasts between vole species exemplify how the laboratory study of diverse species improves our understanding of the mechanisms underlying social behavior. We identify several additional rodent species whose interesting social structures and available ecological and behavioral field data make them good candidates for study. New techniques and integration across laboratory and field settings will provide exciting opportunities for future mechanistic work in non-model species.

Fezf1 is a novel regulator of female sex behavior in mice

Female sexual behavior is a complex process regulated by multiple brain circuits and influenced by sex steroid hormones acting in the brain. Several regions in the hypothalamus have been implicated in the regulation of female sexual behavior although a complete circuitry involved in female sexual behavior is not understood. Fez family zinc finger 1 (Fezf1) gene is a brain specific gene that has been mostly studied in the context of olfactory development, although in a recent study, FEZF1 has been identified as one of the genes responsible for the development of Kallman syndrome. In the present study, we utilized shRNA approach to downregulate Fezf1 in the ventromedial nucleus of the hypothalamus (VMN) with the aim to explore the role of this gene. Adult female mice were stereotaxically injected with lentiviral vectors encoding shRNA against Fezf1 gene. Mice injected with shRNA against Fezf1 had significantly reduced female sexual behavior, presumably due to the downregulation of estrogen receptor alpha (ERα), as the number of ERα-immunoreactive cells in the VMN of Fezf1 mice was significantly lower in comparison to controls. However, no effect on body weight or physical activity was observed in mice with downregulated Fezf1, suggesting that the role of Fezf1 in the VMN is limited to the regulation of sexual behavior.

SIGNIFICANCE STATEMENT

Fezf1 gene has been identified in the present study as a regulator of female sexual behavior in mice. Regulation of the female sexual behavior could be through the regulation of estrogen receptor alpha expression in the ventromedial nucleus of the hypothalamus, as the expression of this receptor was reduced in mice with downregulated Fezf1. As expression of Fezf1 is very specific in the brain, this gene could present a potential target for the development of novel drugs regulating hypoactive sexual desire disorder in women, if similar function of FEZF1 will be confirmed in humans.

Ventromedial hypothalamic neurons control a defensive emotion state

Defensive behaviors reflect underlying emotion states, such as fear. The hypothalamus plays a role in such behaviors, but prevailing textbook views depict it as an effector of upstream emotion centers, such as the amygdala, rather than as an emotion center itself. We used optogenetic manipulations to probe the function of a specific hypothalamic cell type that mediates innate defensive responses. These neurons are sufficient to drive multiple defensive actions, and required for defensive behaviors in diverse contexts. The behavioral consequences of activating these neurons, moreover, exhibit properties characteristic of emotion states in general, including scalability, (negative) valence, generalization and persistence. Importantly, these neurons can also condition learned defensive behavior, further refuting long-standing claims that the hypothalamus is unable to support emotional learning and therefore is not an emotion center. These data indicate that the hypothalamus plays an integral role to instantiate emotion states, and is not simply a passive effector of upstream emotion centers.

DOI:http://dx.doi.org/10.7554/eLife.06633.001

Animals have evolved a large number of ‘defensive behaviors’ to deal with the threat of predators. Examples include reptiles camouflaging themselves to avoid discovery, fish and birds swarming to confuse predators, insects releasing toxic chemicals, and humans readying themselves to fight or flee.

In mammals, defensive behaviors are thought to be mediated by a region of the brain called the amygdala. This structure, which is known as the brain's ‘emotion center’, receives and processes information from the senses about impending threats. It then sends instructions on how to deal with these threats to other regions of the brain including the hypothalamus, which pass them on to the brain regions that control the behavioral, endocrine and involuntary responses of the mammal.

For many years it has been thought that the role of the hypothalamus is to serve simply as a relay for emotion states encoded in the amygdala, rather than as an emotion center itself. However, Kunwar et al. have now challenged this assumption with the aid of a technique called optogenetics, in which light is used to activate specific populations of genetically labeled neurons. When light was used to directly activate neurons within the ventromedial hypothalamus in awake mice, the animals instantly froze and/or fled, just as they would when faced with a predator. Given that the optical stimulation had completely bypassed the amygdala, this suggested that the hypothalamus must be capable of generating this defensive response without any input from the amygdala.

The freezing and fleeing responses resembled the responses to a predator in a number of key ways. Mice chose to avoid areas of their cage in which they had received the stimulation, suggesting that—like a predator—these areas induced an unpleasant emotional state, perhaps akin to anxiety or fear. Freezing and fleeing persisted for several seconds after the stimulation had stopped, just as freezing and fleeing responses to predators do not immediately cease after the threat has gone. And finally, destroying the neurons targeted by the stimulation made mice less likely to avoid one of their main predators, the rat. It also made the animals less anxious.

Overall the results suggest that the hypothalamus may be more than simply a relay for the amygdala, and that ‘amygdala-centric’ views of emotion processing may need to be re-visited.

DOI:http://dx.doi.org/10.7554/eLife.06633.002

Relations between peripheral and brain serotonin measures and behavioural responses in a novelty test in pigs

Pigs differ in their behavioural responses towards environmental challenges. Individual variation in maladaptive responses such as tail biting, may partly originate from underlying biological characteristics related to (emotional) reactivity to challenges and serotonergic system functioning. Assessing relations between behavioural responses and brain and blood serotonin parameters may help in understanding susceptibility to the development of maladaptive responses. The objective of the current study was, therefore, to assess the relationship between the pigs' serotonergic parameters measured in both blood and brain, and the behaviour of pigs during a novelty test. Pigs (n=31) were subjected to a novelty test at 11weeks of age, consisting of 5-min novel environment exposure after which a novel object (a bucket) was introduced for 5min. Whole blood serotonin, platelet serotonin level, and platelet serotonin uptake were determined at 13weeks of age. Levels of serotonin, its metabolite and serotonin turnover were determined at 19weeks of age in the frontal cortex, hypothalamus and hippocampus. The behaviour of the pigs was different during exposure to a novel object compared to the novel environment only, with more fear-related behaviours exhibited during novel object exposure. Platelet serotonin level and brain serotonergic parameters in the hippocampus were interrelated. Notably, the time spent exploring the test arena was significantly correlated with both platelet serotonin level and right hippocampal serotonin activity (turnover and concentration). In conclusion, the existence of an underlying biological trait - possibly fearfulness - may be involved in the pig's behavioural responses toward environmental challenges, and this is also reflected in serotonergic parameters.

Context fear learning specifically activates distinct populations of neurons in amygdala and hypothalamus

The identity and distribution of neurons that are involved in any learning or memory event is not known. In previous studies, we identified a discrete population of neurons in the lateral amygdala that show learning-specific activation of a c-fos-regulated transgene following context fear conditioning. Here, we have extended these studies to look throughout the amygdala for learning-specific activation. We identified two further neuronal populations, in the amygdalo-striatal transition area and medial amygdala, that show learning-specific activation. We also identified a population of hypothalamic neurons that show strong learning-specific activation. In addition, we asked whether these neurons are activated following recall of fear-conditioning memory. None of the populations of neurons we identified showed significant memory-recall-related activation. These findings suggest that a series of discrete populations of neurons are involved in fear learning in amygdala and hypothalamus. The lack of reactivation during memory recall suggests that these neurons either do not undergo substantial change following recall, or that c-fos is not involved in any such activation and change.

The medial hypothalamic defensive circuit and 2,5-dihydro-2,4,5-trimethylthiazoline (TMT) induced fear: comparison of electrolytic and neurotoxic lesions

The neural circuits for unconditioned fear to predator odors (e.g., cat fur odor, trimethylthiazoline, TMT) are not well delineated. A putative neural circuit for predator odor fear, the medial hypothalamic defensive circuit (MHDC), consisting of the anterior hypothalamic (AHN), ventromedial hypothalamic (VMH) and dorsal premammillary nuclei (PMd), has been proposed. Electrolytic and ibotenic acid lesions of the PMd have been shown to reduce unconditioned fear in rats presented with either a cat or cat odor. Whether the PMd, AHN and VMH are involved in unconditioned fear to another predator odor derived from fox feces, 2,5-dihydro-2,4,5-trimethylthiazoline (TMT), has not been explored. The present study compared the effects of electrolytic and neurotoxic lesions of MHDC nuclei in rats on unconditioned fear to TMT and shock-induced contextually conditioned fear, as measured by freezing. Electrolytic lesions of the PMd did not reduce TMT-induced freezing, but diminished post-shock and shock-induced contextually conditioned freezing, suggesting a role for the PMd in contextually conditioned fear. In contrast, electrolytic lesions of the AHN and VMH reduced freezing to TMT while not affecting conditioned fear. However, neither NMDA lesions of the AHN nor ibotenic acid lesions of the VMH reduced freezing in shock-induced conditioned or TMT-induced unconditioned fear paradigms. The data suggest that fibers passing through the AHN and VMH, and not cells in the MHDC, mediate unconditioned freezing to the predator odor TMT.

Aggressive behaviors in adult SF-1 knockout mice that are not exposed to gonadal steroids during development

Sex hormones are a major factor responsible for the development of sex differences. Steroidogenic factor 1 (SF-1) is a key regulator of gonadal and adrenal development, and SF-1 knockout mice (SF-1 KO) are born without gonads and adrenal glands. Consequently, these mice are not exposed to gonadal sex steroids. SF-1 KO pups die shortly after birth due to adrenal deficiency. In the present study, SF-1 KO mice were rescued by neonatal corticosteroid injections followed by adrenal transplantations on day 7-8 postnatally. Control mice received corticosteroid injections and were gonadectomized prior to puberty. Mice were observed interacting with ovariectomized hormone primed females and gonad-intact males. In the absence of sex steroid replacement, adult SF-1 KO mice were significantly more aggressive than control mice in tests with stimulus females. After testosterone treatment, control males displayed significantly more aggression towards male intruders than control female mice, or male and female SF-1 KO mice, suggesting a developmental role of gonadal hormones in the expression of aggressive behavior and affirming SF-1 KO mice as a behavioral model to investigate affects of fetal gonad deficiency.

Alterations in muscarinic cholinergic receptor densities induced by thiamine deficiency: autoradiographic detection of changes in high- and low-affinity agonist binding

Animals fed a diet deficient in thiamine or treated with a drug preventing the utilization of thiamine (thiamine antagonist) exhibited alterations in ligand binding to muscarinic receptors in several brain regions. Using quantitative techniques of receptor autoradiography, an increase in muscarinic receptor binding was demonstrated in such regions as the corpus callosum, lamina VI of the parietal cortex, caudate-putamen, ventral nucleus of the thalamus, stratum lacunosum moleculare and stratum oriens of the hippocampus, and the hilus of the area dentata. As a result of thiamine deficiency, this increase in muscarinic receptor populations was primarily due to an increase in the binding of the low-affinity agonist site. In the same experiment, a decrease in muscarinic receptor binding was found in the ventromedial region of the hypothalamus. Thiamine deficiency thus causes an up-regulation of muscarinic receptor binding in several regions of rat brain while causing a down-regulation of these same receptors in other brain areas.

A comparison of prey eating by spontaneous mouse killing rats and rats with lateral septal, medial accumbens, or medial hypothalamic lesions

Rats with lesions of the medial hypothalamus and spontaneous mouse killing rats were tested for mouse and rat pup killing in their living cages 1 and 3 days postoperatively. The lesioned and spontaneous killers did not differ significantly in amount of prey eaten within 10 min following a mouse kill on either Day 1 or 3 postoperatively. Both groups ate significantly more of the prey than did sham-lesioned rats that were presented with a freshly killed mouse. When 4 hr was allowed for eating following a kill, rats with lesions of the lateral septum, medial accumbens, and medial hypothalamus each ate significantly more than spontaneous mouse killing rats. The greater prey eating by the lesioned animals is probably not the result of the prey being a highly palatable food since rats with medial hypothalamic lesions but not those with medial accumbens or septal lesions showed enhanced consumption of a sweetened lab chow over a 4 hr period. The quantitative similarity in the prey eating by spontaneous and lesion-induced mouse killers in the period immediately following the kill serves to further establish a relationship between these two kinds of killing. The greater eating that occurs in lesioned animals when a longer time is allowed for eating is consistent with other observations of excesses in the killing behavior of lesioned animals relative to spontaneous killers.

The inhibitory modulation of agonistic behavior in the rat brain: a review

Neural regions which exercise an inhibitory influence on agonistic behavior are identified by the enhancement of agonistic behavior that follows their removal. The specific kinds of agonistic behaviors altered by each region are then examined. Increased reactivity to the experimenter and enhanced shock-induced fighting are produced by lesions of the region ventral to the anterior septum, the lateral septum, the medial hypothalamus, and the dorsal and median raphe nuclei. It is argued that the increased reactivity and shock-induced fighting correspond to an enhancement of defensive behavior. Mouse killing is induced by lesions of the anterior olfactory nucleus, the region ventral to the anterior septum, the lateral septum, the medial hypothalamus, the dorsal and median raphe nuclei, and the medial amygdala. Because the lesion-induced mouse killing is similar to that emitted by spontaneous mouse killers, it is argued that these regions normally exert an inhibitory control over predatory killing. The available evidence on social attack behavior has not convincingly identified regions exerting an inhibitory control over this dimension of behavior. Our conclusion is that separate brain systems exert an inhibitory control over defensive behavior, predatory killing, and social attack behavior. To a substantial extent, the regions modulating these behaviors appear to act independently of one another. The only neurotransmitter that is clearly active in these inhibitory systems is serotonin, and has only been directly implicated in the control of mouse killing by neurons originating in the dorsal and median raphe nuclei.

Differential effects of medial, central and basolateral amygdaloid lesions on four models of experimentally-induced aggression in rats

To clarify whether various nuclei of the amygdaloid complex play different roles in aggressive behavior including muricide, 4 types of aggression were experimentally induced in rats. These include olfactory bulbectomy (OB rats), midbrain raphe lesions (Raphe rats), administration of delta 9-tetrahydrocannabinol (THC rats) and long-term isolation (Iso rats). Rats which exhibited muricide following these treatments were subjected to bilateral lesions of either the medial (AME), central (ACE) or basolateral (ABL) amygdaloid nuclei. Both muricide and hyperemotionality in the OB rat were markedly inhibited by AME lesions. Those of the Iso and THC rats were moderately inhibited. However, in the Raphe rat, aggressive behavior was not inhibited by AME lesions. Furthermore, ACE or ABL lesions caused no significant changes in all 4 models of aggression. These results suggest that the AME plays a facilitatory role in aggression of OB, Iso and THC rats, but aggression in Raphe rat is independent of amygdaloid activity.

[Effects of hypothalamus and globus pallidus lesions and of apomorphine injections into the globus pallidus, caudate nucleus, substantia nigra and septum on the aggressive behavior induced by apomorphine treatment of rats (author's transl)]

Intraspecific apomorphine-induced aggressive behavior in the rat was not affected following electrolytic lesions of the ventromedial hypothalamus. Some inhibition of the aggressive behavior was found after lateral lesions and an almost total suppression after destruction of globus pallidus. These results, as well as those following localized injections of apomorphine into the septum, substantia nigra, caudate nucleus, and globus pallidus suggested that the latter anatomical region may be the major site of the action of apomorphine in the behavior studies. The role of acetylcholine is discussed.