Andrew N. Bubak

Heightened serotonin influences contest outcome and enhances expression of high-intensity aggressive behaviors.

The outcome of behavioral interactions between organisms can have significant fitness implications. Therefore, it is of great theoretical and practical importance to understand the mechanisms that modify different agonistic behaviors. Changes in central monoamines, such as serotonin (5-HT), contribute to modifying the expression of aggressive encounters in both vertebrates and invertebrates. In several invertebrate groups, neural 5-HT has been linked to heightened aggression and conflict escalation. The male stalk-eyed fly (Teleopsis dalmanni) competes with conspecifics daily over access to resources such as food and mates. Because encounters escalate in a stereotypical manner, stalk-eyed flies provide an excellent model system to study behavioral syndromes. We hypothesized that noninvasive, pharmacological augmentation of brain 5-HT by administration of the precursor, 5-hydroxytryptophan (5-HTP), would increase stereotypic behavioral escalation and the probability of winning a conflict over food. Size-matched male 5-HTP-treated and untreated flies were placed in a forced-fight paradigm and their aggressive behaviors scored. Individuals with higher brain 5-HT levels had a markedly higher probability of winning the contests, displayed greater levels of high-intensity aggressive behaviors and fewer retreats. Pretreatment with 5-HTP did not significantly alter octopamine or tyramine, suggesting that central 5-HT may modulate aggression in these organisms and play a role in determining reproductive success and resource attainment.

Whole brain monoamine detection and manipulation in a stalk-eyed fly

Understanding the physiological mechanisms that influence conflict resolution is of great importance because the outcome of contests over limited resources such as mates, territories, and food has significant fitness consequences. Male stalk-eyed flies (Teleopsis dalmanni) compete over territory and mates and provide an excellent model system to study aggression. To investigate potential effects of serotonin (5-HT) on aggressive behavior in these flies, we developed a dissection and sample preparation method sufficiently sensitive to measure monoamine concentrations from whole brain samples of small insects. This new method allows the detection of monoamines from a single fly brain using high performance liquid chromatography with electrochemical detection. The method allows for the detection and quantification of octopamine (OA), 5-hydroxytryptophan (5-HTP), dopamine (DA), 5-hydroxyindoleacetic acid (5-HIAA), tyramine (TA), and serotonin (5-HT) and provides a means for assessing changes in stalk-eyed fly brain monoamine concentrations in response to drug administration in food media. We successfully elevated 5-HT levels approximately 8-fold that of control levels in stalk-eyed fly brains by oral administration of the 5-HT precursor 5-HTP. Furthermore, in size-matched competitions for a food resource, flies that had elevated 5-HT in response to 5-HTP pretreatment exhibited a high probability of winning the contests. These results suggest that 5-HT enhances aggression in the stalk-eyed fly and highlight the potential of our method for testing putative roles of monoamines in modulating self and rival assessment in conflict resolution.

David vs. Goliath: Serotonin modulates opponent perception between smaller and larger rivals.

During agonistic encounters, the perception of a larger opponent through morphological signaling typically suppresses aggression in the smaller individual, preventing contest intensity escalation. However, non-morphological factors such as central serotonin (5-HT) activity can influence individual aggression, potentially altering contest intensity despite initial size discrepancies. When male stalk-eyed flies (Teleopsis dalmanni) fight, contest escalation is directly proportional to similarity in body size, with escalation being lower in size-mismatched contests. We have shown that both high-intensity aggression and the probability of winning are increased in males with pharmacologically elevated 5-HT relative to size-matched non-treated opponents. Here, we hypothesized that, in size-mismatched contests, increasing brain 5-HT in the smaller opponent could similarly increase aggression and counteract the low contest intensity normally driven by size discrepancy. Size-mismatched male pairs (greater than 5% difference in eyestalk length) engaged in a forced fight paradigm, with the smaller fly either untreated or with pharmacologically elevated 5-HT levels. The expression of high-intensity aggressive behaviors was significantly increased in smaller treated opponents, but the probability of winning was not altered. This suggests that while elevated serotonergic activity can increase aggression and intensity despite perception of a larger opponent, this is not sufficient to overcome size biases with respect to contest outcome. However, the fact that larger opponents continued to win against smaller treated flies was not simply a function of size. Instead, untreated larger males adjusted their fighting strategy to match the increased aggression of their smaller treated opponent, suggesting contextual flexibility in behavior based on individual opponent assessment.

Neuromodulation of Nestmate Recognition Decisions by Pavement Ants

Ant colonies are distributed systems that are regulated in a non-hierarchical manner. Without a central authority, individuals inform their decisions by comparing information in local cues to a set of inherent behavioral rules. Individual behavioral decisions collectively change colony behavior and lead to self-organization capable of solving complex problems such as the decision to engage in aggressive societal conflicts with neighbors. Despite the relevance to colony fitness, the mechanisms that drive individual decisions leading to cooperative behavior are not well understood. Here we show how sensory information, both tactile and chemical, and social context—isolation, nestmate interaction, or fighting non-nestmates—affects brain monoamine levels in pavement ants (Tetramorium caespitum). Our results provide evidence that changes in octopamine and serotonin in the brains of individuals are sufficient to alter the decision by pavement ants to be aggressive towards non-nestmate ants whereas increased brain levels of dopamine correlate to physical fighting. We propose a model in which the changes in brain states of many workers collectively lead to the self-organization of societal aggression between neighboring colonies of pavement ants.

The organization of societal conflicts by pavement ants Tetramorium caespitum: an agent-based model of amine mediated decision making

Abstract Ant colonies self-organize to solve complex problems despite the simplicity of an individual ant’s brain. Pavement ant Tetramorium caespitum colonies must solve the problem of defending the territory that they patrol in search of energetically rich forage. When members of 2 colonies randomly interact at the territory boundary a decision to fight occurs when: 1) there is a mismatch in nestmate recognition cues and 2) each ant has a recent history of high interaction rates with nestmate ants. Instead of fighting, some ants will decide to recruit more workers from the nest to the fighting location, and in this way a positive feedback mediates the development of colony wide wars. In ants, the monoamines serotonin (5-HT) and octopamine (OA) modulate many behaviors associated with colony organization and in particular behaviors associated with nestmate recognition and aggression. In this article, we develop and explore an agent-based model that conceptualizes how individual changes in brain concentrations of 5-HT and OA, paired with a simple threshold-based decision rule, can lead to the development of colony wide warfare. Model simulations do lead to the development of warfare with 91% of ants fighting at the end of 1 h. When conducting a sensitivity analysis, we determined that uncertainty in monoamine concentration signal decay influences the behavior of the model more than uncertainty in the decision-making rule or density. We conclude that pavement ant behavior is consistent with the detection of interaction rate through a single timed interval rather than integration of multiple interactions.

A potential compensatory role for endogenous striatal tyrosine hydroxylase-positive neurons in a nonhuman primate model of Parkinson's disease.

The possibility of enhancing endogenous brain repair following neurological disorders, such as Parkinsons disease (PD), is of considerable recent interest. One such mechanism may exist in the striatum as an upregulated population of tyrosine hydroxylase (TH)-immunoreactive neurons that appear after 1-methyl-4-phenyl-1,2,3,6-tetra-hydropyridine (MPTP) lesions in nonhuman primates as well as in humans with PD. An intriguing possibility is that these endogenous neurons reflect a compensatory mechanism to mitigate the loss of striatal DA due to progressive destruction of the nigrostriatal pathway. The possibility of enhancing the number and function of this population is attractive; however, it is crucial to gain further information about these cells in order to comprehend more fully their possible therapeutic potential. The current research was designed to investigate the fate of this endogenous population in African green monkeys rendered parkinsonian by MPTP lesions. Specifically, we assessed changes in the numbers of striatal neurons expressing TH at differing stages of the toxin-induced behavioral disability and discovered a close relationship with symptom severity and striatal DA neuron numbers. Increased numbers of striatal TH-positive neurons were associated with MPTP treatment that produced parkinsonian symptoms compared to numbers of these neurons in MPTP-treated asymptomatic animals and untreated controls. Expression of striatal DA neurons peaked at the manifestation of symptoms in mild/moderate animals and remained stable in animals that were severely parkinsonian. Furthermore, in severely debilitated animals that improved after fetal dopaminergic grafts, we discovered a return to control levels of the endogenous population. Taken together, our results further support the concept that this population of DA neurons responds to variations in striatal DA tone and may serve as a compensatory mechanism to restore striatal DA levels in the context of significant depletion. Artificially manipulating this endogenous population could prove beneficial for PD treatment, especially for individuals in early disease stages.

Assessment strategies and fighting patterns in animal contests: a role for serotonin?

Abstract Accurate assessment of the probability of success in an aggressive confrontation with a conspecific is critical to the survival and fitness of the individuals. Various game theory models have examined these assessment strategies under the assumption that contests should favor the animal with the greater resource-holding potential (RHP), body size typically being the proxy. Mutual assessment asserts that an individual can assess their own RHP relative to their opponent, allowing the inferior animal the chance to flee before incurring unnecessary costs. The model of self-determined persistence, however, assumes that an individual will fight to a set personal threshold, independent of their opponent’s RHP. Both models have been repeatedly tested using size as a proxy for RHP, with neither receiving unambiguous support. Here we present both morphological and neurophysiological data from size-matched and mismatched stalk-eyed fly fights. We discovered differing fighting strategies between winners and losers. Winners readily escalated encounters to higher intensity and physical contact and engaged in less low-intensity, posturing behaviors compared with losers. Although these fighting strategies were largely independent of size, they were associated with elevated levels of 5-HT. Understanding the neurophysiological factors responsible for mediating the motivational state of opponents could help resolve the inconsistencies seen in current game theory models. Therefore, we contend that current studies using only size as a proxy for RHP may be inadequate in determining the intricacies of fighting ability and that future studies investigating assessment strategies and contest outcome should include neurophysiological data.

Sex Differences in Aggression: Differential Roles of 5-HT2, Neuropeptide F and Tachykinin

Despite the conserved function of aggression across taxa in obtaining critical resources such as food and mates, serotonin’s (5-HT) modulatory role on aggressive behavior appears to be largely inhibitory for vertebrates but stimulatory for invertebrates. However, critical gaps exist in our knowledge of invertebrates that need to be addressed before definitively stating opposing roles for 5-HT and aggression. Specifically, the role of 5-HT receptor subtypes are largely unknown, as is the potential interactive role of 5-HT with other neurochemical systems known to play a critical role in aggression. Similarly, the influence of these systems in driving sex differences in aggressive behavior of invertebrates is not well understood. Here, we investigated these questions by employing complementary approaches in a novel invertebrate model of aggression, the stalk-eyed fly. A combination of altered social conditions, pharmacological manipulation and 5-HT2 receptor knockdown by siRNA revealed an inhibitory role of this receptor subtype on aggression. Additionally, we provide evidence for 5-HT2’s involvement in regulating neuropeptide F activity, a suspected inhibitor of aggression. However, this function appears to be stage-specific, altering only the initiation stage of aggressive conflicts. Alternatively, pharmacologically increasing systemic concentrations of 5-HT significantly elevated the expression of the neuropeptide tachykinin, which did not affect contest initiation but instead promoted escalation via production of high intensity aggressive behaviors. Notably, these effects were limited solely to males, with female aggression and neuropeptide expression remaining unaltered by any manipulation that affected 5-HT. Together, these results demonstrate a more nuanced role for 5-HT in modulating aggression in invertebrates, revealing an important interactive role with neuropeptides that is more reminiscent of vertebrates. The sex-differences described here also provide valuable insight into the evolutionary contexts of this complex behavior. Significance Statement Serotonin’s (5-HT) modulatory role in aggression is generally reported as inhibitory in vertebrates but stimulatory in invertebrates. Using a novel invertebrate model system, we provide evidence of common pathways of aggression at the 5-HT receptor subtype level as well as 5-HT’s interactive role with other neurochemical systems namely neuropeptide F and tachykinin. Additionally, we found that these effects were sex-dependent as well as stage-dependent affecting either the initiation or escalation stage of an aggressive contest. Our results reveal the impressive level of conservation with respect to neurochemical mechanisms among species as diverse as vertebrates and invertebrates, and highlights the need to consider multiple factors when determining potential taxonomic differences in how 5-HT mediates aggression.

Varicella Zoster Virus Vasculopathy

Varicella zoster virus (VZV) is a ubiquitous, exclusively human alphaherpesvirus that produces varicella then becomes latent in ganglionic neurons. In elderly and immunocompromised individuals, VZV reactivates and typically produces herpes zoster. Studies of patients with VZV vasculopathy have identified key clinical, imaging, and laboratory features to assist in diagnosis and treatment. Complementary studies have further expanded the spectrum of VZV vasculopathy to include the extracranial circulation and identified mechanisms contributing to its pathogenesis. Given our increasing aging population and recognition that VZV reactivation manifesting as zoster is a risk factor for stroke and myocardial infarction, recognition of VZV as a potential cause of vascular disease with or without associated zoster rash is essential to decrease associated morbidity and mortality because VZV vasculopathy can be treated with antiviral therapy.

Varicella Zoster Virus Induces Nuclear Translocation of the Neurokinin-1 Receptor, Promoting Lamellipodia Formation and Viral Spread in Spinal Astrocytes

Background Varicella zoster virus (VZV) can present as a myelopathy with spinal astrocyte infection. Recent studies support a role for the neurokinin-1 receptor (NK-1R) in virus infections, as well as for cytoskeletal alterations that may promote viral spread. Thus, we examined the role of NK-1R in VZV-infected primary human spinal astrocytes (HA-sps) to shed light on the pathogenesis of VZV myelopathy. Methods Mock- and VZV-infected HA-sps were examined for substance P (subP) production, NK-1R localization, morphological changes, and viral spread in the presence or absence of the NK-1R antagonists aprepitant and rolapitant. Results VZV infection of HA-sps induced nuclear localization of full-length and truncated NK-1R in the absence of the endogenous ligand, subP, and was associated with extensive lamellipodia formation and viral spread that was inhibited by NK-1R antagonists. Conclusions We have identified a novel, subP-independent, proviral function of nuclear NK-1R associated with lamellipodia formation and viral spread that is distinct from subP-induced NK-1R cell membrane/cytoplasmic localization without lamellipodia formation. These results suggest that binding of a putative viral ligand to NK-1R produces a dramatically different NK-1R downstream effect than binding of subP. Finally, the NK-1R antagonists aprepitant and rolapitant provide promising alternatives to nucleoside analogs in treating VZV infections, including myelopathy.