Intrahypothalamic injections of norepinephrine facilitate feline affective aggression via alpha 2-adrenoceptors

A Circuit Node that Integrates Convergent Input from Neuromodulatory and Social Behavior-Promoting Neurons to Control Aggression in Drosophila

Diffuse neuromodulatory systems such as norepinephrine (NE) control brain-wide states such as arousal, but whether they control complex social behaviors more specifically is not clear. Octopamine (OA), the insect homolog of NE, is known to promote both arousal and aggression. We have performed a systematic, unbiased screen to identify OA receptor-expressing neurons (OARNs) that control aggression in Drosophila. Our results uncover a tiny population of male-specific aSP2 neurons that mediate a specific influence of OA on aggression, independent of any effect on arousal. Unexpectedly, these neurons receive convergent input from OA neurons and P1 neurons, a population of FruM+ neurons that promotes male courtship behavior. Behavioral epistasis experiments suggest that aSP2 neurons may constitute an integration node at which OAergic neuromodulation can bias the output of P1 neurons to favor aggression over inter-male courtship. These results have potential implications for thinking about the role of related neuromodulatory systems in mammals.

GH response to intravenous clonidine challenge: absence of relationship with behavioral irritability, aggression, or impulsivity in human subjects

Previous study suggests a role for post-synaptic alpha(2)-noradrenergic receptor sensitivity in irritability and/or aggression and impulsivity. In this study, we conducted intravenous challenges with the alpha(2)-noradrenergic agonist, clonidine, to assess the relationship between measures of impulsive aggression and post-synaptic alpha(2)-noradrenergic receptor sensitivity in human subjects. Subjects included 38 individuals with personality disorder and 28 healthy volunteer controls. Measures included the Irritability score and the Total Assault score from the Buss-Durkee Hostility Inventory (BDHI), Aggression score from Life History of Aggression (LHA) assessment, and Impulsivity scores from the Barratt Impulsivity Scale (BIS-11) and Eysenck Personality Questionnaire-II (EPQ-II). The Log of Peak DeltaGH[CLON] response was used as the index of post-synaptic alpha(2)-noradrenergic receptor sensitivity. No significant correlations were found between the Log of Peak DeltaGH[CLON] response and any measure used in this study. Unlike a previous investigation, this study provides little support for a role of post-synaptic alpha(2)-noradrenergic receptor sensitivity in aggression in healthy or personality disordered subjects.

Role of Serotonin and Dopamine System Interactions in the Neurobiology of Impulsive Aggression and its Comorbidity with other Clinical Disorders

Impulsive aggression is characterized by an inability to regulate affect as well as aggressive impulses, and is highly comorbid with other mental disorders including depression, suicidal behavior, and substance abuse. In an effort to elucidate the neurobiological underpinnings of impulsive aggression and to help account for its connections with these other disorders, this paper reviews relevant biochemical, brain imaging, and genetic studies. The review suggests that dysfunctional interactions between serotonin and dopamine systems in the prefrontal cortex may be an important mechanism underlying the link between impulsive aggression and its comorbid disorders. Specifically, serotonin hypofunction may represent a biochemical trait that predisposes individuals to impulsive aggression, with dopamine hyperfunction contributing in an additive fashion to the serotonergic deficit. The current paper proposes a modified diathesis-stress model of impulsive aggression in which the underlying biological diathesis may be deficient serotonergic function in the ventral prefrontal cortex. This underlying disposition can be manifested behaviorally as impulsive aggression towards oneself and others, and as depression under precipitating life stressors. Substance abuse associated with impulsive aggression is understood in the context of dopamine dysregulation resulting from serotonergic deficiency. Also discussed are future research directions in the neurobiology of impulsive aggression and its comorbid disorders.

Potentiating role of interleukin-1beta (IL-1beta) and IL-1beta type 1 receptors in the medial hypothalamus in defensive rage behavior in the cat

Recently, this laboratory provided evidence that interleukin-1beta (IL-1beta), an immune and brain-derived cytokine, microinjected into the medial hypothalamus, potentiates defensive rage behavior in the cat elicited from the midbrain periaqueductal gray (PAG), and that such effects are blocked by a 5-HT2 receptor antagonist. Since this finding represents the first time that a brain cytokine has been shown to affect defensive rage behavior, the present study replicated and extended these findings by documenting the specific potentiating role played by IL-1beta Type 1 receptor (IL-1RI), and the anatomical relationship between IL-1beta and 5-HT2 receptors in the medial hypothalamus. IL-1beta (10 ng) microinjected into the medial hypothalamus induced two separate phases of facilitation, one at 60 min and another at 180 min, post-injection. In turn, these effects were blocked with pretreatment of the selective IL-1 Type I receptor antagonist (IL-1ra) (10 ng), demonstrating the selectivity of the effects of IL-1beta on medial hypothalamic neurons upon PAG-elicited defensive rage behavior. The next stage of the study utilized immunohistochemical methods to demonstrate that IL-1beta and 5-HT2 receptors were present on the same neurons within regions of the medial hypothalamus where IL-1beta and the IL-1beta receptor antagonists were administered. This provided anatomical evidence suggesting a relationship between IL-1RI and 5-HT2 receptors in the medial hypothalamus that is consistent with the previous pharmacological observations in our laboratory. The overall findings show that activation of IL-1RI in the medial hypothalamus potentiates defensive rage behavior in the cat and that these effects may also be linked to the presence of 5-HT2 receptors on the same groups of neurons in this region of hypothalamus.

Cytokine modulation of defensive rage behavior in the cat: role of GABAA and interleukin-2 receptors in the medial hypothalamus

Defensive rage behavior is a form of aggressive behavior occurring in nature in response to a threatening stimulus. It is also elicited by stimulation of the medial hypothalamus and midbrain periaqueductal gray (PAG) and mediated through specific neurotransmitter-receptor mechanisms within these regions. Since interleukin (IL)-2 modulates the release of neurotransmitters linked to aggression and rage, we sought to determine whether IL-2 microinjected into the medial hypothalamus would modulate defensive rage. Microinjections of relatively low doses of IL-2 into the medial hypothalamus significantly suppressed defensive rage elicited from the PAG in a dose-dependent manner and in the absence of signs of sickness behavior. Pre-treatment with an antibody directed against IL-2Ralpha or a GABA(A) receptor antagonist blocked IL-2's suppressive effects upon defensive rage. Since the suppression of defensive rage is also mediated by 5-HT(1) receptors in the medial hypothalamus, a 5-HT(1) antagonist was microinjected into this region as a pretreatment for IL-2; however, it did not block IL-2's suppressive effects. Immunocytochemical data provided anatomical support for these findings by revealing extensive labeling of IL-2Ralpha on neurons in the medial hypothalamus. IL-2 microinjected into the medial hypothalamus did not modulate predatory attack elicited from the lateral hypothalamus. In summary, we provide evidence for a novel role for IL-2 in the medial hypothalamus as a potent suppressor of defensive rage behavior. These effects are mediated through an IL-2-GABA(A) receptor mechanism.

Interleukin-1 beta in the hypothalamus potentiates feline defensive rage: role of serotonin-2 receptors

The neurochemistry of aggression and rage has largely focused on the roles played by neurotransmitters and their receptor mechanisms. In contrast, little attention has been given to the possible functions of other substances. Interleukin-1beta is an immune and brain-derived cytokine that is present in the hypothalamus. Functionally, interleukin-1 has been shown to induce the release of serotonin (5-HT), a neurotransmitter known to potently affect aggression and rage behavior. Thus, the goal of the present study was to test the hypothesis that interleukin-1beta in the medial hypothalamus could modulate defensive rage behavior in the cat. In the first experiment, electrical stimulation of sites in the medial hypothalamus from which defensive rage could be elicited and where microinjections of specific compounds were later placed, facilitated defensive rage elicited from the periaqueductal gray (PAG), thus demonstrating the functional relationship between these two regions. In the second experiment, microinjections of relatively low doses of interleukin-1beta into the medial hypothalamus potentiated defensive rage behavior elicited from the midbrain periaqueductal gray in a dose-related manner. In the third experiment, pretreatment with a selective 5-HT2 receptor antagonist, LY-53857, blocked the facilitating effects of interleukin-1beta upon defensive rage. These findings reveal for the first time that brain cytokines can dramatically alter aggressive behavior. In particular, interleukin-1beta in the medial hypothalamus potentiates defensive rage behavior elicited from the periaqueductal gray in the cat, and the potentiating effects of interleukin-1beta on this form of emotional behavior are mediated via a 5-HT2 receptor mechanism.

Norepinephrine function in personality disorder: plasma free MHPG correlates inversely with life history of aggression

OBJECTIVE

Clinical and pre-clinical data suggest the possibility of a facilitory role for norepinephrine (NE) in impulsive-aggressive behavior. While clinical studies have focused on putative central measures of NE activity, few studies have been published using peripheral measures. In this study, the relationship between plasma free NE metabolite, plasma free 3-methoxy-4-hydroxyphenylglycol (pMHPG), and impulsive aggression was explored in subjects with personality disorder.

METHODS

Subjects were 30 male subjects with personality disorder in whom basal plasma free MHPG concentrations were obtained. Aggression was assessed using the Life History of Aggression (LHA) assessment and the Buss-Durkee Aggression scales; impulsivity was assessed using the Barratt Impulsiveness and the Eysenck Personality Questionnaire Impulsivity scales.

RESULTS

A significant inverse correlation was found between LHA-Aggression and pMHPG in these subjects. Correlations with other behavioral measures were not statistically significant. pMHPG was significantly lower among subjects with borderline personality disorder but not significantly lower after controlling for LHA-Aggression scores.

CONCLUSION

These data suggest that pMHPG is inversely correlated with life history of aggression in subjects with personality disorder and that central and/or peripheral NE may play a role in modulating aggressive behavior in these subjects.

NK1 receptors in the medial hypothalamus potentiate defensive rage behavior elicited from the midbrain periaqueductal gray of the cat

Defensive rage in the cat occurs naturally in response to a threat and is also elicited by electrical or chemical stimulation over the rostro-caudal extent of the medial hypothalamus and dorsolateral aspect of the periaqueductal gray (PAG). This behavior is mediated over a descending projection from the hypothalamus to the midbrain PAG. The underlying hypothesis for the present study was that medial hypothalamic defensive rage neurons are excited in two ways: by NK(1) receptors and by an ascending input from the PAG. The first aspect of this hypothesis was tested by eliciting defensive rage by electrical stimulation of the PAG and then microinjecting a selective NK(1) agonist and antagonist into the hypothalamus. Microinjections of 16 or 12 nmol/0.25 microl of the NK(1) agonist, GR 73632, resulted in facilitation of defensive rage. These facilitatory effects were then blocked by pretreatment with the NK(1) antagonist, GR 82334. However, microinjections of GR 82334 alone had no effect. The second aspect of the hypothesis was tested by stimulating defensive rage sites in the PAG and using immunohistochemical methods to test for the presence of c-Fos in the hypothalamus. The results revealed the presence of c-Fos immunoreactivity in the medial but not lateral hypothalamus. Overall, the findings indicate that NK(1) receptors in the medial hypothalamus facilitate defensive rage elicited from PAG neurons whose axons project back to the medial hypothalamus. The likely ethological significance of the ascending input is that it allows for potentiation and prolongation of defensive rage in response to a threatening stimulus.

Chronic subordination stress in male tree shrews: replacement of testosterone affects behavior and central alpha(2)-adrenoceptors

Subordination stress induced by social defeat in male animals is known to inhibit gonadal functions and it has been discussed whether the resulting deficit in testosterone might play a role in subordination behavior. One of the major transmitter systems involved in regulation of behavior is the noradrenergic system. To analyze whether a testosterone replacement can alter subordination behavior and whether this might be related to changes in the brain noradrenergic system, we quantified alpha(2)-adrenoceptors (alpha(2)-ARs) in the central nervous system of male tree shrews. Animals were submitted to chronic subordination stress and received testosterone at the same time. Behavior was monitored during all phases of the experiment: the control period of 10 days, the period of social stress lasting 10 days when subordinates were confronted daily with a dominant male, and, subsequently, the stress and treatment period of 18 days when in parallel to the stress, animals received either injections of testosterone or vehicle. Brain alpha(2)-ARs were quantified by in vitro receptor autoradiography using the antagonist ligand (3)H-RX821002. Locomotor activity decreased significantly during the stress period and was not re-normalized by testosterone. In contrast, testosterone re-normalized scent marking behavior and autogrooming, parameters that had both been reduced due to the subordination stress. Vehicle injections improved none of these behaviors. In 8 of 10 brain regions that were analyzed, numbers of alpha(2)-adrenergic binding sites were increased in stressed animals that received vehicle injections, but a difference between testosterone and vehicle injected animals was only observed in five regions. These brain regions are all known to be involved in emotional behavior (anterior hypothalamus, medial nucleus of the amygdala, cingulate cortex) or autonomic regulation, respectively (solitary tract nucleus, dorsal motor nucleus of vagus). Therefore, our data show that testosterone influences behavior of male subordinates and modulates alpha(2)-AR expression in their brains. Androgen-mediated alterations in receptors occur in brain regions that are known to be involved in emotionality, e.g., in the anterior hypothalamus which regulates aggressive behavior. One can therefore conclude that alpha(2)-ARs contribute to neuronal functions that are responsible for subordination of stress behavior, and that testosterone-induced receptor changes are related to the partial restoration of normal behavior.

Brain structures and neurotransmitters regulating aggression in cats: implications for human aggression

1. Violence and aggression are major public health problems. 2. The authors have used techniques of electrical brain stimulation, anatomical-immunohistochemical techniques, and behavioral pharmacology to investigate the neural systems and circuits underlying aggressive behavior in the cat. 3. The medial hypothalamus and midbrain periaqueductal gray are the most important structures mediating defensive rage behavior, and the perifornical lateral hypothalamus clearly mediates predatory attack behavior. The hippocampus, amygdala, bed nucleus of the stria terminalis, septal area, cingulate gyrus, and prefrontal cortex project to these structures directly or indirectly and thus can modulate the intensity of attack and rage. 4. Evidence suggests that several neurotransmitters facilitate defensive rage within the PAG and medial hypothalamus, including glutamate, Substance P, and cholecystokinin, and that opioid peptides suppress it; these effects usually depend on the subtype of receptor that is activated. 5. A key recent discovery was a GABAergic projection that may underlie the often-observed reciprocally inhibitory relationship between these two forms of aggression. 6. Recently, Substance P has come under scrutiny as a possible key neurotransmitter involved in defensive rage, and the mechanism by which it plays a role in aggression and rage is under investigation. 7. It is hoped that this line of research will provide a better understanding of the neural mechanisms and substrates regulating aggression and rage and thus establish a rational basis for treatment of disorders associated with these forms of aggression.

Neuropharmacology of brain-stimulation-evoked aggression

Evidence is reviewed concerning the brain areas and neurotransmitters involved in aggressive behavior in the cat and rodent. In the cat, two distinct neural circuits involving the hypothalamus and PAG subserve two different kinds of aggression: defensive rage and predatory (quiet-biting) attack. The roles played by the neurotransmitters serotonin, GABA, glutamate, opioids, cholecystokinin, substance P, norepinephrine, dopamine, and acetylcholine in the modulation and expression of aggression are discussed. For the rat, a single area, largely coincident with the intermediate hypothalamic area, is crucial for the expression of attack; variations in the rat attack response in natural settings are due largely to environmental variables. Experimental evidence emphasizing the roles of serotonin and GABA in modulating hypothalamically evoked attack in the rat is discussed. It is concluded that significant progress has been made concerning our knowledge of the circuitry underlying the neural basis of aggression. Although new and important insights have been made concerning neurotransmitter regulation of aggressive behavior, wide gaps in our knowledge remain.

Catecholaminergic involvement in the control of aggression: hormones, the peripheral sympathetic, and central noradrenergic systems

Noradrenaline is involved in many different functions, which all are known to affect behaviour profoundly. In the present review we argue that noradrenaline affects aggression on three different levels: the hormonal level, the sympathetic autonomous nervous system, and the central nervous system (CNS), in different, but functionally synergistic ways. Part of these effects may arise in indirect ways that are by no means specific to aggressive behaviour, however, they are functionally relevant to it. Other effects may affect brain mechanisms specifically involved in aggression. Hormonal catecholamines (adrenaline and noradrenaline) appear to be involved in metabolic preparations for the prospective fight; the sympathetic system ensures appropriate cardiovascular reaction, while the CNS noradrenergic system prepares the animal for the prospective fight. Indirect CNS effects include: the shift of attention towards socially relevant stimuli; the enhancement of olfaction (a major source of information in rodents); the decrease in pain sensitivity; and the enhancement of memory (an aggressive encounter is very relevant for the future of the animal). Concerning more aggression-specific effects one may notice that a slight activation of the central noradrenergic system stimulates aggression, while a strong activation decreases fight readiness. This biphasic effect may allow the animal to engage or to avoid the conflict, depending on the strength of social challenge. A hypothesis is presented regarding the relevance of different adrenoceptors in controlling aggression. It appears that neurons bearing postsynaptic alpha2-adrenoceptors are responsible for the start and maintenance of aggression, while a situation-dependent fine-tuning is realised through neurons equipped with beta-adrenoceptors. The latter phenomenon may be dependent on a noradrenaline-induced corticosterone secretion. It appears that by activating very different mechanisms the systems working with adrenaline and/or noradrenaline prepare the animal in a very complex way to answer the demands imposed by, and to endure the effects caused by, fights. It is a challenge for future research to elucidate how precisely these mechanisms interact to contribute to functionally relevant and adaptive aggressive behaviour.

The effect of alpha 2 adrenoceptor blockers on aggressive behavior in mice: implications for the actions of adrenoceptor agents

The effects of three alpha 2 adrenoceptor blockers (idazoxan, yohimbine and CH-38083) on isolation-induced aggressive behavior was studied in male mice. The three drugs produced different behavioral profiles. Idazoxan reduced aggressiveness dose-dependently by decreasing the duration of offensive/aggressive interactions and increasing the duration of defensive behaviors. The other two drugs produced only parts of the dual action of idazoxan: yohimbine affected mainly defensive behaviors, while CH-38083 affected only the time spent with fighting. Saline injections per se also influenced behavior and, in contrast to alpha 2 adrenoceptor blockers, induced an increase in aggressiveness. These results are different from those previously obtained in rats, which show bell-shaped dose-response curves in response to alpha 2 adrenoceptor blockers (small doses increased, while large doses decreased aggression). It is postulated that the strong behavioral reaction of mice to the injection per se may mask the aggression-heightening effects of small doses of alpha 2 adrenoceptor blockers in this species. A theory is also presented regarding the complexity of adrenoceptor interactions when both pre-, and postsynaptic alpha 2 adrenoceptors are blocked.

Behavioral and autonomic responses to intermittent social stress: differential protection by clonidine and metoprolol

The present study investigated physiological and pharmacological characteristics of socially "stressed" animals. Specifically, we examined (1) to what degree autonomic and behavioral "stress" reactions during intermittent confrontations between an intruder male adult Long-Evans rat with an aggressive resident undergo habituation, and (2) to what extent the defeat-experienced animal can be protected against these "stress" reactions with clonidine or metoprolol, two adrenergic agents with clinical anxiolytic effects. We developed an acute social stress situation that consisted of initially placing an experimental rat as an intruder into the homecage of a resident while the resident was not present, thereafter permitting brief physical agonistic interactions with the reintroduced resident until the intruder was forced into a submissive supine posture and emitted ultrasonic vocalizations (USV), and eventually exposing the intruder to the resident's threats for one hour, while being shielded from potentially injurious attacks ("threat encounter"). Over the course of the initial 4-weekly threat encounters the acute tachycardia but not the hyperthermic stress responses decreased in magnitude. Following the first three threat encounters core temperature (Tc) was significantly elevated for at least 3 h. The Tc was already elevated when the repeatedly defeated intruder was confronted with the olfactory cues of the resident's cage. This conditioned "anticipatory" hyperthermia developed in the course of the first three confrontations and was paralleled by a decrease in exploratory and motor behavior and by an increase in defensive behaviors and in both types of USV emitted in the "low" (20-30 kHz) and the "high" (31-70 kHz) frequency range. Clonidine (0.01-0.1 mg/kg, IP), an alpha 2-adrenergic agonist and metoprolol, a beta-adrenergic blocker (1.0-10.0 mg/kg, IP), dose-dependently prevented the tachycardic response to stress. Only clonidine, but not metoprolol, also attenuated the rise in Tc during the 1-h agonistic interaction. Clonidine decreased those aspects of motor behavior (e.g. rearing, walking) that are of lesser "cost" for the individual but maintained high levels of defensive reactions and increased the duration of "low" USV. The high doses of clonidine (0.06, 0.1 mg/kg) attenuated the homeostatic regulation and sedated the intruder while exposed to threats during a social confrontation. The absence of attenuation of the high level of defensive behavior and the prolonged "low" USV suggest a stress intensification by the higher doses of clonidine. In conclusion, after the fourth encounter, the autonomic, behavioral and vocal response pattern prior to and during repeated weekly confrontations show no evidence for habituation for the following 6 weeks.(ABSTRACT TRUNCATED AT 400 WORDS)