Differential effects of ethanol on feline rage and predatory attack behavior: an underlying neural mechanism

Alcohol and violence: neuropeptidergic modulation of monoamine systems

Neurobiological processes underlying the epidemiologically established link between alcohol and several types of social, aggressive, and violent behavior remain poorly understood. Acute low doses of alcohol, as well as withdrawal from long-term alcohol use, may lead to escalated aggressive behavior in a subset of individuals. An urgent task will be to disentangle the host of interacting genetic and environmental risk factors in individuals who are predisposed to engage in escalated aggressive behavior. The modulation of 5-hydroxytryptamine impulse flow by gamma-aminobutyric acid (GABA) and glutamate, acting via distinct ionotropic and metabotropic receptor subtypes in the dorsal raphe nucleus during alcohol consumption, is of critical significance in the suppression and escalation of aggressive behavior. In anticipation and reaction to aggressive behavior, neuropeptides such as corticotropin-releasing factor, neuropeptide Y, opioid peptides, and vasopressin interact with monoamines, GABA, and glutamate to attenuate and amplify aggressive behavior in alcohol-consuming individuals. These neuromodulators represent novel molecular targets for intervention that await clinical validation. Intermittent episodes of brief social defeat during aggressive confrontations are sufficient to cause long-lasting neuroadaptations that can lead to the escalation of alcohol consumption.

NMDA receptor antagonism: escalation of aggressive behavior in alcohol-drinking mice

RATIONALE

Memantine is a potential treatment for alcoholic patients, yet few studies investigate the effect of concurrent treatment with memantine and ethanol on aggression. We evaluated aggressive behavior following ethanol consumption and treatment with glutamatergic drugs to characterize interactions between these compounds.

OBJECTIVE

This study aimed to use rodent models of aggression to examine interactions between glutamatergic compounds and ethanol.

MATERIALS AND METHODS

Once male CFW mice reliably self-administered 1 g/kg ethanol or water, they were assessed for aggression in resident-intruder confrontations. Alternatively, aggression was evaluated following a social-instigation procedure. Animals were then injected with memantine, ketamine, neramexane, MTEP, or LY379268 before aggressive confrontations. Effects of the pharmacological manipulations on salient aggressive and non-aggressive behaviors were analyzed.

RESULTS

Moderate doses of memantine, neramexane, and MTEP interacted with ethanol to increase the frequency of attack bites while ketamine did not. The highest dose of LY379268, an mGluR(2/3) agonist, reduced both aggressive and non-aggressive behaviors after water and ethanol self-administration. Attack bites increased with social instigation and decreased with administration of high doses of MTEP and LY379268. Memantine and MTEP both reduced attack bite frequency in the instigation condition without reducing locomotor behavior.

CONCLUSIONS

Memantine and neramexane interacted with ethanol to heighten aggression. The binding characteristics of these compounds allow for 'partial trapping' by which some NMDARs are unblocked between depolarizations. We propose that this feature may contribute to the differential aggression-heightening interactions between these compounds and ethanol. MTEP also interacted with ethanol to escalate aggression, possibly through inhibition of mGluR(5) modulation of NMDARs.

A model linking biology, behavior and psychiatric diagnoses in perpetrators of domestic violence

Research indicates that perpetrators of domestic violence have abnormalities in central serotonin and testosterone metabolism, an increased sensitivity to anxiogenic stimuli, and an impaired neuro-connection between their cortex and the amygdala. Clinical evaluations show that perpetrators of domestic violence also have a distinguishing set of behaviors and diagnoses related to anxiety, depression, intermittent explosive disorder, and borderline personality disorder. In this paper we propose a model to understand how the biological abnormalities can potentially explain the behaviors and diagnoses exhibited by the perpetrators. Changes in the perpetrator's neurotransmitters lead to a heightened sensitivity to environmental stimuli, anxiety, and conditioned fear. Lack of cortical input to the amygdala impairs the perpetrator's ability to extinguish anxiety and/or conditioned fear and gives rise to either innate behaviors (e.g., fight, flight, and shut down) or learned fear avoidant behaviors designed to avoid anxiety (e.g., alcohol consumption, self-injurious acts, and obsessive behaviors). Linking conditioned fear and fear avoidance to the behaviors and psychiatric diagnoses will serve to change the way the medical community perceives and treats perpetrators of domestic violence.

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.

Lead exposure potentiates predatory attack behavior in the cat

Epidemiologic studies have demonstrated that environmental lead exposure is associated with aggressive behavior in children; however, numerous confounding variables limit the ability of these studies to establish a causal relationship. The study of aggressive behavior using a validated animal model was used to test the hypothesis that there is a causal relationship between lead exposure and aggression in the absence of confounding variables. We studied the effects of lead exposure on a feline model of aggression: predatory (quiet biting) attack of an anesthetized rat. Five cats were stimulated with a precisely controlled electrical current via electrodes inserted into the lateral hypothalamus. The response measure was the predatory attack threshold current (i.e., the current required to elicit an attack response on 50% of the trials). Blocks of trials were administered in which predatory attack threshold currents were measured three times a week for a total of 6-10 weeks, including before, during, and after lead exposure. Lead was incorporated into cat food "treats" at doses of 50-150 mg/kg/day. Two of the five cats received a second period of lead exposure. Blood lead concentrations were measured twice a week and were <1, 21-77, and <20 micro g/dL prior to, during, and after lead exposure, respectively. The predatory attack threshold decreased significantly during initial lead exposure in three of five cats and increased after the cessation of lead exposure in four of the five cats (P<0.01). The predatory attack thresholds and blood lead concentrations for each cat were inversely correlated (r=-0.35 to -0.74). A random-effects mixed model demonstrated a significant (P=0.0019) negative association between threshold current and blood lead concentration. The data of this study demonstrate that lead exposure enhances predatory aggression in the cat and provide experimental support for a causal relationship between lead exposure and aggressive behavior in humans.

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.

Increased alcohol intake and behavioral disinhibition in rats with ventral striatal neuron loss

A previous study of ours reported excessive alcohol intake, enhanced defensive aggressiveness (hyperreactivity towards the experimenter), impulsive behavior, and reduced cortical serotonin levels in rats following extensive basal forebrain axon-sparing lesions involving the septal area and the ventral striatum. This constellation of signs resembles that seen clinically in "Dionysian" alcoholics. The present investigation aimed at examining the effect of ibotenic acid lesions restricted to the septal area or the ventral striatum on this behavioral profile. Experiment 1 indicated that medium-sized lesions (induced by infusing 0.35 microl ibotenic acid in each hemisphere) encompassing the septal area or the ventral striatum elicited a qualitatively similar behavioral profile. Both lesion types markedly enhanced the intake of 6% ethanol, and both groups were significantly more hyperreactive towards the experimenter. A brief doorbell signal elicited significantly more fleeing in rats with basal forebrain lesions, and licking from an electrified waterspout in the punished drinking test caused lesser suppression of locomotor activity than normal. Both groups also showed significant deficits in food hoarding. Histological examination revealed that the posterior portion of the ventral striatal lesion typically overlapped with the anterior portion of the septal lesion. Experiment 2 avoided this neuropathological overlap, and examined groups bearing small discrete lesions (induced by infusing 0.15 microl ibotenic acid in each hemisphere) restricted to either the accumbens part of the ventral striatum or the dorsal septal area. Lesions to the nucleus accumbens were associated with an increase in home-cage alcohol drinking, no hyperreactivity towards the experimenter, potentiation of fleeing at the expense of freezing in response to a sudden auditory signal, and disinhibited behavior in the punished drinking test with increased punished responding and reduced behavioral suppression. Rats with small septal lesions showed a weak enhancement of defensive aggression, but no other behavioral alterations. Our results suggest that ventral striatal neuron loss gives rise to excessive alcohol drinking and enhanced impulsivity.

Behavioral disinhibition following basal forebrain excitotoxin lesions: alcohol consumption, defensive aggression, impulsivity and serotonin levels

Research on alcoholism have identified a subgroup in which the drinking problem is associated with high rates of violence, an impulsive disposition and signs of reduced serotonin functioning in the brain. The present study reports that male Wistar rats with ibotenic acid-induced (5 micrograms/0.5 microliter) neuron loss in the basal forebrain (ventral striatum, septal area and adjacent structures) showed behavioral and neurochemical signs not unlike this subtype of alcoholics. Thus, rats with this lesion exhibited excessive 6% alcohol drinking in a two-bottle choice test and showed augmentation of certain defensive behaviors, including defensive aggression and increased activity-during signal. In the punished drinking test, a passive avoidance task which taps psychological mechanisms underlying impulsivity [56], experimental rats were not different from sham-operated controls with regard to the number of punished licks, but punishment evoked less disruption of ongoing behavior in subjects with basal forebrain damage. The virtual absence of food hoarding in the face of normal feeding may constitute yet another sign of increased impulsivity, indicating as it does a diminished influence of future rewards on behavior. As expected, in view of ibotenic acid's selectivity for neuronal perikarya, the concentrations of dopamine, serotonin and norepinephrine were normal in the lesioned area. However, the levels of serotonin and norepinephrine in the cortex were reduced. A separate experiment examined the extent to which serotonin depletion alone reproduced the behavioral profile induced by basal forebrain neuron loss. However, measures of alcohol consumption, defensive behavior and impulsivity were not different from controls in rats given intracerebroventricular 5,7-DHT (150 micrograms/20 microliters), except for a modest increase in defensive 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.

GABA receptor mediated suppression of defensive rage behavior elicited from the medial hypothalamus of the cat: role of the lateral hypothalamus

Recently, our laboratory has demonstrated that predatory attack behavior in the cat, elicited by electrical stimulation of the lateral hypothalamus, is suppressed following activation of the region of the medial hypothalamus from which defensive rage behavior is elicited [Han, Y., Shaikh, M.B., Siegel, A., Medical amygdaloid suppression of predatory attack behavior in the cat: II. Role of a GABAergic pathway from the medial to the lateral hypothalamus, Brain Res., 716 (1996) 72-83.]. The mechanism for this suppression is a direct GABAergic projection from the medial to lateral hypothalamus. The present study tested the hypothesis that the inhibitory relationship between these two regions of hypothalamus is reciprocal, namely, that a GABAergic neuron, which also projects from the lateral to medial hypothalamus, serves to suppress defensive rage elicited from the medial hypothalamus. Monopolar stimulating electrodes were implanted into lateral hypothalamic sites from which predatory attack behavior was elicited. In addition, cannula-electrodes were implanted into the medial hypothalamus for elicitation of defensive rage behavior and for microinjections of GABA compounds. Initially, in the absence of drug administration, the effects of dual stimulation of the lateral and medial hypothalamus upon response latencies were compared with those following single stimulation of the medial hypothalamus alone. Dual stimulation significantly (p<0.01) suppressed defensive rage behavior elicited from the medial hypothalamus. Then, administration of the GABAA receptor antagonist, bicuculline (10-60 pmol), into medial hypothalamic sites from which defensive rage was elicited blocked the suppressive effects of lateral hypothalamic stimulation. The GABAA receptor agonist, muscimol (0.3-30 pmol), microinjected into the medial hypothalamus, suppressed defensive rage elicited by single stimulation of the medial hypothalamus in a dose dependent manner. These suppressive effects of muscimol upon defensive rage were blocked following pretreatment with bicuculline (60 pmol). Administration of muscimol into adjoining regions of the lateral hypothalamus had no effect upon defensive rage, indicating its site specificity. Bicuculline (60 pmol) delivery into the medial hypothalamus had no effect upon defensive rage, suggesting the, presence of a phasic rather than tonic mechanism. A combination of immunocytochemical and retro grade tracing procedures were then employed to determine the origin of the putative GABAergic pathway projecting to the medial hypothalamus. In this experiment, the retrograde tracer, Fluoro-Gold (8%, 0.5 microl), was microinjected through a cannula-electrode in the medial hypothalamus from which defensive rage had been elicited. Following survival periods of 5-6 days, cats were perfused with 4% paraformaldehyde and brain tissue was processed for immunocytochemical staining of GABA neurons. Retrogradely labeled, immunopositively labeled, as well as Fluoro-Gold and GABA labeled cells, were identified in the lateral hypothalamus. Each type of neuron was distributed over wide regions of the lateral hypothalamus, extending from the area immediately caudal to the optic chiasm to the level of the posterior hypothalamus. Together, the behavioral pharmacological and anatomical data provide evidence of a direct inhibitory projection from the lateral to medial hypothalamus whose functions are mediated by GABAA receptors. When coupled with our previous findings, these results reveal the presence of reciprocal GABAergic inhibitory pathways between the medial and lateral hypothalamus. The findings suggest that functions associated with either the lateral or medial hypothalamus, but not both, can be activated at a given time.

Neurotransmitters regulating defensive rage behavior in the cat

This review summarizes recent findings of our laboratory that have been directed at: (1) identifying the neural circuits underlying the expression and modulation of defensive rage behavior in the cat and the neurotransmitters associated with these pathways; and (2) determining which components of the circuitry are affected by alcohol administration and which significantly alter the rage mechanism. The experiments described herein incorporated a number of converging methods, which include brain stimulation, behavioral pharmacology, immunocytochemistry, retrograde tract tracing and receptor binding. For behavioral pharmacological studies, monopolar electrodes and cannula-electrodes were implanted into selected regions along the limbic-midbrain axis for electrical stimulation and local microinfusion of drugs. The findings demonstrated: (1) a direct pathway from the anterior medial hypothalamus to the dorsal periaqueductal gray (PAG) over which this response is mediated. This pathway utilizes excitatory amino acids that act upon NMDA receptors within the midbrain PAG; (2) that the region of the dorsal PAG, from which defensive rage could be elicited, receives other inputs from the basal amygdala that facilitate this response by acting upon NMDA receptors; (3) a pathway from the medial amygdala to the medial hypothalamus that also facilitates defensive rage and whose functions are mediated by substance P receptors within the medial hypothalamus; (4) that the PAG also receives enkephalinergic inputs from the central nucleus of amygdala, which act upon mu receptors, and which powerfully suppress defensive rage; and (5) that recent findings reveal that ethanol administration facilitates defensive rage by virtue of its interactions with the medial hypothalamus, its descending projection to the PAG, and possibly with NMDA receptors within this pathway.