Dietary tryptophan modulation and aggressive behavior in mice

The effect of dietary niacin deficiency on reproduction of European brown hares: An experimental study

Large plots of maize in agricultural landscapes are associated with reduced reproductive output of females and impaired population development of free-living populations of European brown hares (Lepus europaeus, Pallas 1778). We investigated in captive brown hares experimentally whether these effects could result from an undersupply of niacin due to a suspected maize-biased diet in such areas. We repeatedly mated adult female hares, fed ad libitum either with a niacin deficient pellet mainly consisting of maize plant parts, or with the same pellet enriched with niacin to meet physiological requirements. We measured effects of the experimental feeding on body mass of females, reproductive output, growth and survival of leverets. We found significantly higher body mass of females fed the control pellet, and faster body mass gain of their leverets until standardized weaning. We found no significant difference between niacin deficient and control diet groups in reproductive output of females and survival rates of leverets. Our results show that even a diet severely depauperate of niacin affects reproductive success of female brown hares only slightly, presumably because of sufficient conversion of tryptophan to niacin, or of additional supply of niacin by caecal microorganism.

Supplementation in vitamin B3 counteracts the negative effects of tryptophan deficiencies in bumble bees

Increasing evidence highlights the importance of diet content in nine essential amino acids for bee physiological and behavioural performance. However, the 10th essential amino acid, tryptophan, has been overlooked as its experimental measurement requires a specific hydrolysis. Tryptophan is the precursor of serotonin and vitamin B3, which together modulate cognitive and metabolic functions in most animals. Here, we investigated how tryptophan deficiencies influence the behaviour and survival of bumble bees (Bombus terrestris). Tryptophan-deficient diets led to a moderate increase in food intake, aggressiveness and mortality compared with the control diet. Vitamin B3 supplementation in tryptophan-deficient diets tended to buffer these effects by significantly improving survival and reducing aggressiveness. Considering that the pollens of major crops and common plants, such as corn and dandelion, are deficient in tryptophan, these effects could have a strong impact on bumble bee populations and their pollination service. Our results suggest planting tryptophan and B3 rich species next to tryptophan-deficient crops could support wild bee populations.

Maize monoculture causes niacin deficiency in free-living European brown hares and impairs local population development

Maize (Zea mays) is the most produced crop worldwide and the second most important bio-energy plant. Huge maize monoculture is considered a threat to biodiversity in agricultural landscapes and may also contribute to the decline of European brown hares (Lepus europaeus, Pallas 1778). Indeed, the intensification of agriculture has been identified as one of the main factors responsible for the decline of brown hare populations. A reason why large maize cultures can be particularly detrimental to animals consuming this plant is its poor nutritional value with respect to niacin. In this study, we investigated the effects of the proportion of area under maize crops on liver concentrations of niacin in free-living hares, on the reproductive output of does (females), and on the development of local populations, at nine study sites in Lower Austria. Hare numbers were estimated from spotlight counts in spring and autumn. Liver samples and uteri were obtained from hares shot in the same areas during regular autumn hunts. Number of offspring born to an individual female during the preceding reproductive period was determined by counting placental scars. Our results show a significant negative effect of the area under maize crops on liver concentrations of niacin of does and on their reproductive output. Further, we found a significant negative effect of the area under maize on the development of a population. Altogether, our findings indicate that high proportions of the area under maize crops contribute to the decline of brown hares by reduced fecundity of does and impaired development of local populations.

Targeted Manipulation of Brain Serotonin: RNAi-Mediated Knockdown of Tryptophan Hydroxylase 2 in Rats

Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the biosynthesis of the biogenic monoamine serotonin (5-hydroxytryptamine, 5-HT). Two existing TPH isoforms are responsible for the generation of two distinct serotonergic systems in vertebrates. TPH1, predominantly expressed in the gastrointestinal tract and pineal gland, mediates 5-HT biosynthesis in non-neuronal tissues, while TPH2, mainly found in the raphe nuclei of the brain stem, is accountable for the production of 5-HT in the brain. Neuronal 5-HT is a key regulator of mood and behavior and its deficiency has been implicated in a variety of neuropsychiatric disorders, e.g., depression and anxiety. To gain further insights into the complexity of central 5-HT modulations of physiological and pathophysiological processes, a new transgenic rat model, allowing an inducible gene knockdown of Tph2, was established based on doxycycline-inducible shRNA-expression. Biochemical phenotyping revealed a functional knockdown of Tph2 mRNA expression following oral doxycycline administration, with subsequent reductions in the corresponding levels of TPH2 enzyme expression and activity. Transgenic rats showed also significantly decreased tissue levels of 5-HT and its degradation product 5-Hydroxyindoleacetic acid (5-HIAA) in the raphe nuclei, hippocampus, hypothalamus, and cortex, while peripheral 5-HT concentrations in the blood remained unchanged. In summary, this novel transgenic rat model allows inducible manipulation of 5-HT biosynthesis specifically in the brain and may help to elucidate the role of 5-HT in the pathophysiology of affective disorders.

Animal violence demystified

Violence has been observed in humans and animals alike, indicating its evolutionary/biological significance. However, violence in animals has often been confounded with functional forms of aggressive behavior. Currently, violence in animals is identified primarily as either a quantitative behavior (an escalated, pathological and abnormal form of aggression characterized primarily by short attack latencies, and prolonged and frequent harm-oriented conflict behaviors) or a qualitative one (characterized by attack bites aimed at vulnerable parts of the opponent's body and context independent attacks regardless of the environment or the sex and type of the opponent). Identification of an operational definition for violence thus not only helps in understanding its potential differences from adaptive forms of aggression but also in the selection of appropriate animal models for both. We address this issue theoretically by drawing parallels from research on aggression and appeasement in humans and other animals. We also provide empirical evidences for violence in mice selected for high aggression by comparing our findings with other currently available potentially violent rodent models. The following violence-specific features namely (1) Display of low levels of pre-escalatory/ritualistic behaviors. (2) Immediate and escalated offense durations with low withdrawal rates despite the opponent's submissive supine and crouching/defeat postures. (3) Context independent indiscriminate attacks aimed at familiar/unfamiliar females, anaesthetized males and opponents and in neutral environments. (4) Orientation of attack-bites toward vulnerable body parts of the opponent resulting in severe wounding. (5) Low prefrontal serotonin (5-HT) levels upon repeated aggression. (6) Low basal heart rates and hyporesponsive hypothalamus-pituitary-adrenocortical (HPA) axis were identified uniquely in the short attack latency (SAL) mice suggesting a qualitative difference between violence and adaptive aggression in animals.

The effect of increased serotonergic neurotransmission on aggression: a critical meta-analytical review of preclinical studies

RATIONALE

The role of serotonin (5-HT) on aggression has been extensively studied; nonetheless, the role of this neurotransmitter in aggression is still inconclusive.

OBJECTIVES

The current meta-analytical review investigated the role of increased 5-HT neurotransmission in aggression.

METHODS

Preclinical studies using serotonin reuptake inhibitors, 5-hydroxytryptophan, L-tryptophan, or serotonin (5-HT) to increase 5-HT levels were included in this meta-analysis. An overall effect of serotonin on aggression was calculated, and the role of several moderator variables was analyzed.

RESULTS

A total of 218 effect sizes revealed that increased 5-HT had an overall significant inhibitory effect on aggression (r = 0.3). The results showed that increased 5-HT had the strongest inhibitory effect on aggression when (1) a specific strain or species (e.g., Long Evans) was used; (2) aggression was offensive or predatory and/or induced by administration of 5,7-dihydroxytryptamine or p-chlorophenylalanine; (3) zimelidine, sertraline, L-tryptophan, citalopram, or 5-HT were used to increase 5-HT; (4) treatment was acute; (5) long chronic treatment durations were used; and (6) time between last injection and behavior testing was within 8 h before or after peak plasma concentration of drug. In contrast, the results revealed that increased-5-HT-facilitated aggression could be predicted when (1) Wistar rats, (2) social isolation or stress to induce aggression, and/or (3) animals treated for less than 3 weeks were used.

CONCLUSIONS

Although 5-HT has an overall inhibitory effect on aggression, the animal's genetic background, drug, treatment time, aggression inducing paradigm, and aggression type are critical variables that influence and modify this effect.

Disruption of the vasopressin 1b receptor gene impairs the attack component of aggressive behavior in mice

Vasopressin affects behavior via its two brain receptors, the vasopressin 1a and vasopressin 1b receptors (Avpr1b). Recent work from our laboratory has shown that disruption of the Avpr1b gene reduces intermale aggression and reduces social motivation. Here, we further characterized the aggressive phenotype in Avpr1b -/- (knockout) mice. We tested maternal aggression and predatory behavior. We also analyzed the extent to which food deprivation and competition over food increases intermale aggression. We quantified defensive behavior in Avpr1b -/- mice and later tested offensive aggression in these same mice. Our results show that attack behavior toward a conspecific is consistently reduced in Avpr1b -/- mice. Predatory behavior is normal, suggesting that the deficit is not because of a global inability to detect and attack stimuli. Food deprivation, competition for food and previous experience increase aggression in both Avpr1b +/+ and -/- mice. However, in these circumstances, the level of aggression seen in knockout mice is still less than that observed in wild-type mice. Defensive avoidance behaviors, such as boxing and fleeing, are largely intact in knockout mice. Avpr1b -/- mice do not display as many 'retaliatory' attacks as the Avpr1b +/+ mice. Interestingly, when territorial aggression was measured following the defensive behavior testing, Avpr1b -/- mice typically show less initial aggressive behavior than wild-type mice, but do show a significant increase in aggression with repeated testing. These studies confirm that deficits in aggression in Avpr1b -/- mice are limited to aggressive behavior involving the attack of a conspecific. We hypothesize that Avpr1b plays an important role in the central processing that couples the detection and perception of social cues (which appears normal) with the appropriate behavioral response.

Conceptual foundations of the UCSD Statin Study: a randomized controlled trial assessing the impact of statins on cognition, behavior, and biochemistry

BACKGROUND

Statin cholesterol-lowering drugs are among the most prescribed drugs in the United States. Their cardiac benefits are substantial and well supported. However, there has been persistent controversy regarding possible favorable or adverse effects of statins or of cholesterol reduction on cognition, mood, and behavior (including aggressive or violent behavior).

METHODS

The literature pertaining to the relationship of cholesterol or statins to several noncardiac domains was reviewed, including the link between statins (or cholesterol) and cognition, aggression, and serotonin.

RESULTS

There are reasons to think both favorable and adverse effects of statins and low cholesterol on cognition may pertain; the balance of these factors requires further elucidation. A substantial body of literature links low cholesterol level to aggressive behavior; statin randomized trials have not supported a connection, but they have not been designed to address this issue. A limited number of reports suggest a connection between reduced cholesterol level and reduced serotonin level, but more information is needed with serotonin measures that are practical for clinical use. Whether lipophilic and hydrophilic statins differ in their impact should be assessed.

CONCLUSION

There is a strong need for randomized controlled trial data to more clearly establish the impact of hydrophilic and lipophilic statins on cognition, aggression, and serotonin, as well as on other measures relevant to risks and quality-of-life impact in noncardiac domains.

The effect of raising or lowering tryptophan levels on aggression in vervet monkeys

Social groups of vervet monkeys (Cercopithecus aethiops) were given amino acid mixtures that were tryptophan-free (T-), nutritionally balanced (B), or contained excess tryptophan (T+). The T- mixture caused a marked decrease in plasma tryptophan and the T+ mixture a large increase. Behavioral observations were made on the animals after administration of the amino acid mixtures both during spontaneous activity and while the (fasted) animals were competing for food newly placed in the feeder. The only effect of the biochemical manipulations on spontaneous aggression was an increase in aggression of the male animals with the T- mixture. During competition for the food the T- mixture increased and the T+ mixture decreased aggression in the males, while the T+ mixture decreased aggression in females. These data indicate that brain 5-hydroxytryptamine can influence aggression in a primate and suggest that altered tryptophan levels can influence aggression more reliably at higher levels of arousal.

Stimulant-like effects of magnesium on aggression in mice

The effects of magnesium excesses resulting from daily injections of magnesium chloride (MgCl2) were examined on offensive behavior in a resident-intruder situation. Male mice tested 5 min post-injection of 15 mg/kg or 30 mg/kg exhibited a significantly greater number of attacks and threats than saline controls; while in mice injected with 125 mg/kg threat and attack behaviors were decreased. No tolerance developed to this decrease which persisted throughout the 15 day injection period. Tolerance to the aggression enhancing effects developed in the 30 mg/kg group which returned to normal by Day 4 and in the 15 mg/kg group which returned to normal by Day 15. Two weeks following the last injection, all groups performed equally. When mice were tested prior to daily injections on Days 4, 8, and 15 in a second experiment, there were no MgCl2 dose differences in threat or attack behavior, thus there was no cumulative effect of MgCl2 on behavior. These data and our previous data showing that MgCl2 deficiencies reduce offensive aggression suggest an inverted U-shape function to magnesium's influence on behavior. Since aggression has been linked to the neurotransmitters dopamine, norepinephrine, and serotonin, and magnesium has been shown to be an important cofactor in the activity of these neurotransmitters, it is possible that the effects seen here are related to changes in one or more of these systems.

The effects of isolation, grouping and aggressive interactions on indole- and catecholamine levels and apparent turnover in the hypothalamus and midbrain of the male golden hamster

Although the golden hamster is widely used for studies on aggression, there have been no previous investigations of the role of biogenic amines in this species. Like the mouse, isolated male hamsters are highly aggressive towards a conspecific while grouped males are relatively unaggressive. Isolated males exhibited no differences in levels of midbrain or hypothalamic serotonin (5HT), 5-hydroxyindole acetic acid (5HIAA) or norepinephrine (NE) compared with grouped males. Nor were there differences in the percentage changes in 5HT or 5HIAA following monoamine oxidase inhibition by pargyline, nor in the percentage fall in hypothalamic NE after tyrosine hydroxylase inhibition by alpha-methyl-p-tyrosine (AMPT); midbrain NE in isolates (unlike grouped males) did not decline after AMPT administration. The percentage changes in 5HT after pargyline administration were unaffected by fighting, but the expected percentage fall in hypothalamic 5HIAA did not occur in males (either grouped or isolated) taking part in an aggressive interaction; this may be an indication of increased 5HT turnover. The percentage change in NE after AMPT administration was unaffected by fighting. Pargyline administration (70 mg/kg IP) reduced the aggressive response of isolated male hamsters to an intruder while AMPT (200 mg/kg IP) had no significant effect.

Interaction of dietary tryptophan and social isolation on territorial aggression, motor activity, and neurochemistry in mice

This study examined the interaction of dietary tryptophan (TRP) and differential housing on territorial-induced aggression, locomotor activity, and monoamine neurochemistry in mice. Groups of male CF-1 mice were singly-housed or group-housed and administered a semisynthetic basal diet supplemented with TRP (0.25-1.0%). Behavioral measures were taken at various intervals up to 2 weeks after dietary administration was instituted. Separate groups of mice were given the same experimental treatment and sacrificed for whole brain determination of the monoamines and their metabolites. Isolated mice were consistently more aggressive than grouped animals, suggesting that territorial-induced aggression is synergistic with intermale aggression based on social isolation. The combination of isolation and 0.50% TRP was particularly effective in producing increases in aggression that reached maximal levels after 10 days of diet administration. However, motor activity of singly-housed mice was unaffected by TRP, while that of grouped mice was decreased after 5 days of 0.50% TRP. By day 14 of administration behavioral changes tended to return to baseline levels. Neurochemical measures indicated increased DA and 5-HT turnover in isolated mice, with the 5-HT system most affected by dietary TRP. Because housing conditions were a prominent factor in the aggression and neurochemistry, the results suggest the involvement of both transmitter systems in this behavior. However, there were no changes in monoamine turnover that could account for the development of behavioral tolerance.

Neural systems and the inhibitory modulation of agonistic behavior: a comparison of mammalian species

The olfactory bulb, lateral septum, medial accumbens, medial hypothalamus, dorsal and median raphe, and amygdala are known from experiments in rats to participate in the inhibitory modulation of defensiveness and predation but not social aggression. The present paper surveys the influence of these structures in the inhibitory control of these same dimensions of agonistic behavior in other species. The existing evidence suggests that lesions in the lateral septum, medial accumbens, medial hypothalamus, or the dorsal and median raphe (or PCPA-induced depletion or serotonin) induce hyperreactivity to the experimenter in mice, rats, cats, dogs, and humans in every instance where they have been tested with one exception. The exception is that lesions in the medial hypothalamus of mice do not induce heightened reactivity. The same lesions do not cause this dramatic increase in reactivity to the experimenter in gerbils, hamsters, guinea pigs, or rabbits but do heighten some other species typical patterns of defensiveness such as alarm calls and avoidance of contact with conspecifics. Lesions in these same areas in monkeys have not been observed to heighten defensive behaviors. Predatory killing or killing of young conspecifics has been observed in hamsters, mice, rats, and cats in every instance where they have been examined following lesions of the olfactory bulbs, lateral septum, medial accumbens, medial hypothalamus, or the dorsal and median raphe nuclei (or PCPA-induced depletion of serotonin). Social aggression has been decreased with these same lesions in each case where they have been examined except for septal lesions in hamsters which have been reported to heighten social aggression. Across species, the consistency with which lesions of the olfactory region, lateral septum, medial accumbens, medial hypothalamus, and dorsal and median raphe nuclei alter defensiveness and predation but not social aggression supports the inference that neural systems exist which subserve the inhibitory modulation of these dimensions of behavior. Finally, the evidence that the disruption of functioning within these structures in humans results in increased agonistic responses to environmental stimuli serves to further establish the important role of this neural circuitry in the normal inhibitory modulation of agonistic behavior in humans.

Habituation of aggression in mice: pharmacological evidence of catecholaminergic and serotonergic mediation

A pharmacological study was undertaken to evaluate the role of catecholamines and serotonin in the mediation of aggression. The dose-effect relationship of d-amphetamine, apomorphine, methysergide, lisuride, and the time-dependent effects of 10 mg/kg p-chloroamphetamine (PCA) were determined on aggressive behavior in a habituation paradigm. The frequency of attacks and sideways threats exponentially declined in ten consecutive confrontations between resident and intruder mice, and recovered when a new intruder was introduced. d-Amphetamine (0.1-5.0 mg/kg) and apomorphine (0.03-1.0 mg/kg), administered IP 5 min before the first confrontation, attenuated the usual decline of attack and threat in subsequent trials. PCA enhanced attack, sideways threat, and locomotion at day 1 after injection. Methysergide (1, 3, 10 mg/kg) and lisuride (0.03, 0.3, 1.0 mg/kg) suppressed all activity, including locomotion and aggression. The data support the hypothesis that catecholamines mediate the activation of aggression, but do not indicate an inhibitory role for serotonin. Aggression in the resident-intruder paradigm appears to require intact serotonin activity.