Comparative neuroendocrinology: A call for more study of reptiles!

Temporal Dynamics of Plasma Catecholamines, Metabolic and Immune Markers, and the Corticosterone:DHEA Ratio in Farmed Crocodiles before and after an Acute Stressor

Commercial crocodilian farms face significant economic and livestock losses attributed to stress, which may be linked to their adopted husbandry practices. The development of appropriate and modernized husbandry guidelines, particularly those focused on stress mitigation, is impeded by the limited understanding of the crocodilian stress response. Fifteen grower Nile crocodiles were subjected to simulated acute transport stress, with blood samples collected at various intervals post-stress. Plasma levels of corticosterone (CORT), dehydroepiandrosterone (DHEA), adrenaline, and noradrenaline were determined using high-performance liquid chromatography. Glucose and lactate were measured using portable meters and the heterophil-to-lymphocyte ratio (HLR) was determined via differential leucocyte counts. Significant differences were elicited after the stressor, with acute fluctuations observed in the fast-acting catecholamines (adrenaline and noradrenaline) when compared to the baseline. Downstream effects of these catecholamines and CORT appear to be associated with a persistent increase in plasma glucose and HLR. Lactate also showed acute fluctuations over time but returned to the baseline by the final measurement. DHEA, which is used in a ratio with CORT, showed fluctuations over time with an inverted release pattern to the catecholamines. The study highlights the temporal dynamics of physiological markers under acute stress, contributing to our understanding of crocodilian stress and potentially informing improved farming practices for conservation and sustainable management.

Comparative MRI analysis of the forebrain of three sauropsida models

The study of the brain by magnetic resonance imaging (MRI) allows to obtain detailed anatomical images, useful to describe specific encephalic structures and to analyze possible variabilities. It is widely used in clinical practice and is becoming increasingly used in veterinary medicine, even in exotic animals; however, despite its potential, its use in comparative neuroanatomy studies is still incipient. It is a technology that in recent years has significantly improved anatomical resolution, together with the fact that it is non-invasive and allows for systematic comparative analysis. All this makes it particularly interesting and useful in evolutionary neuroscience studies, since it allows for the analysis and comparison of brains of rare or otherwise inaccessible species. In the present study, we have analyzed the prosencephalon of three representative sauropsid species, the turtle Trachemys scripta (order Testudine), the lizard Pogona vitticeps (order Squamata) and the snake Python regius (order Squamata) by MRI. In addition, we used MRI sections to analyze the total brain volume and ventricular system of these species, employing volumetric and chemometric analyses together. The raw MRI data of the sauropsida models analyzed in the present study are available for viewing and downloading and have allowed us to produce an atlas of the forebrain of each of the species analyzed, with the main brain regions. In addition, our volumetric data showed that the three groups presented clear differences in terms of total and ventricular brain volumes, particularly the turtles, which in all cases presented distinctive characteristics compared to the lizards and snakes.

Neural control of reproduction in reptiles

Reptiles display considerable diversity in reproductive behavior, making them great models to study the neuroendocrine control of reproductive behavior. Many reptile species are seasonally breeding, such that they become reproductively active during their breeding season and regress to a nonreproductive state during their nonbreeding season, with this transition often prompted by environmental cues. In this review, we will focus on summarizing the neural and neuroendocrine mechanisms controlling reproductive behavior. Three major areas of the brain are involved in reproductive behavior: the preoptic area (POA), amygdala, and ventromedial hypothalamus (VMH). The POA and VMH are sexually dimorphic areas, regulating behaviors in males and females respectively, and all three areas display seasonal plasticity. Lesions to these areas disrupt the onset and maintenance of reproductive behaviors, but the exact roles of these regions vary between sexes and species. Different hormones influence these regions to elicit seasonal transitions. Circulating testosterone (T) and estradiol (E2) peak during the breeding season and their influence on reproduction is well-documented across vertebrates. The conversion of T into E2 and 5α-dihydrotestosterone can also affect behavior. Melatonin and corticosterone have generally inhibitory effects on reproductive behavior, while serotonin and other neurohormones seem to stimulate it. In general, there is relatively little information on the neuroendocrine control of reproduction in reptiles compared to other vertebrate groups. This review highlights areas that should be considered for future areas of research.

Aggressive but not reproductive boldness in male green anole lizards correlates with baseline vasopressin activity

Across species, individuals within a population differ in their level of boldness in social encounters with conspecifics. This boldness phenotype is often stable across both time and social context (e.g., reproductive versus agonistic encounters). Various neural and hormonal mechanisms have been suggested as underlying these stable phenotypic differences, which are often also described as syndromes, personalities, and coping styles. Most studies examining the neuroendocrine mechanisms associated with boldness examine subjects after they have engaged in a social interaction, whereas baseline neural activity that may predispose behavioral variation is understudied. The present study tests the hypotheses that physical characteristics, steroid hormone levels, and baseline variation in Ile³-vasopressin (VP, a.k.a., Arg⁸-vasotocin) signaling predispose boldness during social encounters. Boldness in agonistic and reproductive contexts was extensively quantified in male green anole lizards (Anolis carolinensis), an established research organism for social behavior research that provides a crucial comparison group to investigations of birds and mammals. We found high stability of boldness across time, and between agonistic and reproductive contexts. Next, immunofluorescence was used to colocalize VP neurons with phosphorylated ribosomal protein S6 (pS6), a proxy marker of neural activity. Vasopressin-pS6 colocalization within the paraventricular and supraoptic nuclei of the hypothalamus was inversely correlated with boldness of aggressive behaviors, but not of reproductive behaviors. Our findings suggest that baseline vasopressin release, rather than solely context-dependent release, plays a role in predisposing individuals toward stable levels of displayed aggression toward conspecifics by inhibiting behavioral output in these contexts.

Mechanisms, development, and comparative perspectives on experience-dependent plasticity in social behavior

Revealing the mechanisms underlying experience-dependent plasticity is a hallmark of behavioral neuroscience. While the study of social behavior has focused primarily on the neuroendocrine and neural control of social behaviors, the plasticity of these innate behaviors has received relatively less attention. Here, we review studies on mating-dependent changes to social behavior and neural circuitry across mammals, birds, and reptiles. We provide an overview of species similarities and differences in the effects of mating experiences on motivational and performative aspects of sexual behaviors, on sensory processing and preferences, and on the experience-dependent consolidation of sexual behavior. We also discuss recent insights into the neural mechanisms of and developmental influences on mating-dependent changes and outline promising approaches to investigate evolutionary parallels and divergences in experience-dependent plasticity.

Social boldness correlates with brain gene expression in male green anoles

Within populations, some individuals tend to exhibit a bold or shy social behavior phenotype relative to the mean. The neural underpinnings of these differing phenotypes - also described as syndromes, personalities, and coping styles - is an area of ongoing investigation. Although a social decision-making network has been described across vertebrate taxa, most studies examining activity within this network do so in relation to exhibited differences in behavioral expression. Our study instead focuses on constitutive gene expression in bold and shy individuals by isolating baseline gene expression profiles that influence social boldness predisposition, rather than those reflecting the results of social interaction and behavioral execution. We performed this study on male green anole lizards (Anolis carolinensis), an established model organism for behavioral research, which provides a crucial comparison group to investigations of birds and mammals. After identifying subjects as bold or shy through repeated reproductive and agonistic behavior testing, we used RNA sequencing to compare gene expression profiles between these groups within various forebrain, midbrain, and hindbrain regions. The ventromedial hypothalamus had the largest group differences in gene expression, with bold males having increased expression of neuroendocrine and neurotransmitter receptor and calcium channel genes compared to shy males. Conversely, shy males express more integrin alpha-10 in the majority of examined regions. There were no significant group differences in physiology or hormone levels. Our results highlight the ventromedial hypothalamus as an important center of behavioral differences across individuals and provide novel candidates for investigations into the regulation of individual variation in social behavior phenotype.

Effects of air pollution exposure on social behavior: a synthesis and call for research

BACKGROUND

There is a growing literature from both epidemiologic and experimental animal studies suggesting that exposure to air pollution can lead to neurodevelopmental and neuropsychiatric disorders. Here, we suggest that effects of air pollutant exposure on the brain may be even broader, with the potential to affect social decision-making in general.

METHODS

We discuss how the neurobiological substrates of social behavior are vulnerable to air pollution, then briefly present studies that examine the effects of air pollutant exposure on social behavior-related outcomes.

RESULTS

Few experimental studies have investigated the effects of air pollution on social behavior and those that have focus on standard laboratory tests in rodent model systems. Nonetheless, there is sufficient evidence to support a critical need for more research.

CONCLUSION

For future research, we suggest a comparative approach that utilizes diverse model systems to probe the effects of air pollution on a wider range of social behaviors, brain regions, and neurochemical pathways.

Brain transcriptomic responses of Yarrow's spiny lizard, Sceloporus jarrovii, to conspecific visual or chemical signals

Species with multimodal communication integrate information from social cues in different modalities into behavioral responses that are mediated by changes in gene expression in the brain. Differences in patterns of gene expression between signal modalities may shed light on the neuromolecular mechanisms underlying multisensory processing. Here, we use RNA-Seq to analyze brain transcriptome responses to either chemical or visual social signals in a territorial lizard with multimodal communication. Using an intruder challenge paradigm, we exposed 18 wild-caught, adult, male Sceloporus jarrovii to either male conspecific scents (femoral gland secretions placed on a small pebble), the species-specific push-up display (a programmed robotic model), or a control (an unscented pebble). We conducted differential expression analysis with both a de novo S. jarrovii transcriptome assembly and the reference genome of a closely related species, Sceloporus undulatus. Despite some inter-individual variation, we found significant differences in gene expression in the brain across signal modalities and the control in both analyses. The most notable differences occurred between chemical and visual stimulus treatments, closely followed by visual stimulus versus the control. Altered expression profiles could explain documented aggression differences in the immediate behavioral response to conspecific signals from different sensory modalities. Shared differentially expressed genes between visually- or chemically-stimulated males are involved in neural activity and neurodevelopment and several other differentially expressed genes in stimulus-challenged males are involved in conserved signal-transduction pathways associated with the social stress response, aggression and the response to territory intruders across vertebrates.

Decision-making in a social world: Integrating cognitive ecology and social neuroscience

Understanding animal decision-making involves simultaneously dissecting and reconstructing processes across levels of biological organization, such as behavior, physiology, and brain function, as well as considering the environment in which decisions are made. Over the past few decades, foundational breakthroughs originating from a variety of model systems and disciplines have painted an increasingly comprehensive picture of how individuals sense information, process it, and subsequently modify behavior or states. Still, our understanding of decision-making in social contexts is far from complete and requires integrating novel approaches and perspectives. The fields of social neuroscience and cognitive ecology have approached social decision-making from orthogonal perspectives. The integration of these perspectives (and fields) is critical in developing comprehensive and testable theories of the brain.

Differences in cautiousness between mainland and island Podarcis siculus populations are paralleled by differences in brain noradrenaline/adrenaline concentrations

Adaptive behavior is shaped by the type and intensity of selection pressures coming from the environment, such as predation risk and resource availability, and can be modulated by individual's neuroendocrine profile involving steroid hormones and the brain-stem monoaminergic circuits projecting to forebrain structures. Boldness when faced with a predator and exploration/activity when confronted with a new environment reflect the degree of cautiousness and/or "risk-taking" of an individual. In this study we have explored to which extent two populations of Podarcis siculus occupying different ecological niches: mainland (ML) and an islet (ISL) differ in the level of cautiousness and whether these differences are paralleled by differences in their monoaminergic profiles. Boldness was tested in the field as antipredator behavior, while novel space and object explorations were tested in a laboratory setting in an open field apparatus. Finally, serotonin, dopamine, noradrenaline (NA) and adrenaline (ADR) concentrations were measured in whole brain samples by ELISA. Lizards from ML population spent significantly more time hiding after a predator encounter in the field, displayed lower intensity of novel space exploration in a laboratory setting, and contained significantly higher whole-brain concentrations of NA and ADR than their ISL counterparts. Parallelism between the level of risk-taking behavior and concentrations of neurotransmitters mediating alertness and reaction to stress suggests that the differing environmental factors on ML and ISL may have shaped the degree of cautiousness in the residing lizard populations by affecting the activity of NA/ADR neural circuits.

Influence of biological sex on social behavior, individual recogntion, and non-associative learning in the adult gray short-tailed opossum (Monodelphis domestica)

Social behavior is critical for relationship formation and is influenced by myriad environmental and individual factors. Basic and preclinical research typically relies on rodent models to identify the mechanisms that underlie behavior; however, it is important to use non-rodent models as well. A major objective of the present study was to test the hypothesis that biological sex and social experience modulate the expression of social behavior in the adult gray short-tailed opossum (Monodelphis domestica), a non-traditional model. We also investigated the non-associative learning abilities of these animals. Following a period of social isolation, animals of both sexes were paired with a non-familiar, same-sex partner for 10 min on three different occasions, with 24-hour inter-trial intervals. We are the first research group to find significant sex differences in submissive and nonsocial behaviors in Monodelphis. Females displayed significantly higher durations of nonsocial behavior that increased over trials. Males were more aggressive; their latencies to the first attack and submissive behavior decreased over trials whereas these latencies increased for females; males' duration of submissive behavior increased over trials whereas it decreased for females. A different group of subjects habituated in response to repeated presentations to neutral odors and dishabituated in response to novel odors. In addition, both males and females demonstrated the ability to form social memories in a standard individual (social) recognition test. Our results contribute to the characterization of this marsupial species, an important first step in developing it as a model of complex social behaviors.

Neural activity in the social decision-making network of the brown anole during reproductive and agonistic encounters

Animals have evolved flexible strategies that allow them to evaluate and respond to their social environment by integrating the salience of external stimuli with internal physiological cues into adaptive behavioral responses. A highly conserved social decision-making network (SDMN), consisting of interconnected social behavior and mesolimbic reward networks, has been proposed to underlie such adaptive behaviors across all vertebrates, although our understanding of this system in reptiles is very limited. Here we measure neural activation across the SDMN and associated regions in the male brown anole (Anolis sagrei), within both reproductive and agonistic contexts, by quantifying the expression density of the immediate early gene product Fos. We then relate this neural activity measure to social context, behavioral expression, and activation (as measured by colocalization with Fos) of different phenotypes of 'source' node neurons that produce neurotransmitters and neuropeptides known to modulate SDMN 'target' node activity. Our results demonstrate that measures of neural activation across the SDMN network are generally independent of specific behavioral output, although Fos induction in a few select nodes of the social behavior network component of the SDMN does vary with social environment and behavioral output. Under control conditions, the mesolimbic reward nodes of the SDMN actually correlate little with the social behavior nodes, but the interconnectivity of these SDMN components increases dramatically within a reproductive context. When relating behavioral output to specific source node activation profiles, we found that catecholaminergic activation is associated with the frequency and intensity of reproductive behavior output, as well as with aggression intensity. Finally, in terms of the effects of source node activation on SDMN activity, we found that Ile⁸-oxytocin (mesotocin) populations correlate positively, while Ile³-vasopressin (vasotocin), catecholamine, and serotonin populations correlate negatively with SDMN activity. Taken together, our findings present evidence for a highly dynamic SDMN in reptiles that is responsive to salient cues in a social context-dependent manner.