Refinement of the dopaminergic system of anuran amphibians based on connectivity with habenula, basal ganglia, limbic system, pallium, and spinal cord

Medial and orbital frontal cortex in decision-making and flexible behavior

The medial frontal cortex and adjacent orbitofrontal cortex have been the focus of investigations of decision-making, behavioral flexibility, and social behavior. We review studies conducted in humans, macaques, and rodents and argue that several regions with different functional roles can be identified in the dorsal anterior cingulate cortex, perigenual anterior cingulate cortex, anterior medial frontal cortex, ventromedial prefrontal cortex, and medial and lateral parts of the orbitofrontal cortex. There is increasing evidence that the manner in which these areas represent the value of the environment and specific choices is different from subcortical brain regions and more complex than previously thought. Although activity in some regions reflects distributions of reward and opportunities across the environment, in other cases, activity reflects the structural relationships between features of the environment that animals can use to infer what decision to take even if they have not encountered identical opportunities in the past.

The Dopaminergic Control of Movement-Evolutionary Considerations

Dopamine is likely the most studied modulatory neurotransmitter, in great part due to characteristic motor deficits in Parkinson's disease that arise after the degeneration of the dopaminergic neurons in the substantia nigra pars compacta (SNc). The SNc, together with the ventral tegmental area (VTA), play a key role modulating motor responses through the basal ganglia. In contrast to the large amount of existing literature addressing the mammalian dopaminergic system, comparatively little is known in other vertebrate groups. However, in the last several years, numerous studies have been carried out in basal vertebrates, allowing a better understanding of the evolution of the dopaminergic system, especially the SNc/VTA. We provide an overview of existing research in basal vertebrates, mainly focusing on lampreys, belonging to the oldest group of extant vertebrates. The lamprey dopaminergic system and its role in modulating motor responses have been characterized in significant detail, both anatomically and functionally, providing the basis for understanding the evolution of the SNc/VTA in vertebrates. When considered alongside results from other early vertebrates, data in lampreys show that the key role of the SNc/VTA dopaminergic neurons modulating motor responses through the basal ganglia was already well developed early in vertebrate evolution.

Amphibian behavioral diversity offers insights into evolutionary neurobiology

Recent studies have served to emphasize the unique placement of amphibians, composed of more than 8000 species, in the evolution of the brain. We provide an overview of the three amphibian orders and their respective ecologies, behaviors, and brain anatomy. Studies have probed the origins of independently evolved parental care strategies in frogs and the biophysical principles driving species-specific differences in courtship vocalization patterns. Amphibians are also important models for studying the central control of movement, especially in the context of the vertebrate origin of limb-based locomotion. By highlighting the versatility of amphibians, we hope to see a further adoption of anurans, urodeles, and gymnophionans as model systems for the evolution and neural basis of behavior across vertebrates.

5-HT neurons of the medullary raphe contribute to respiratory control in toads

Air-breathing vertebrates undergo respiratory adjustments when faced with disturbances in the gas composition of the environment. In mammals, the medullary raphe nuclei are involved in the neuronal pathway that mediates the ventilatory responses to hypoxia and hypercarbia. We investigate whether the serotoninergic neurons of the medullary raphe nuclei of toads (Rhinella diptycha) play a functional role in respiratory control during resting conditions (room air), hypercarbia (5% CO₂), and hypoxia (5% O₂). The raphe nuclei were located and identified based on the location of the serotoninergic neurons in the brainstem. We then lesioned the medullary raphe (raphe pallidus, obscurus and magnus) with anti-SERT-SAP and measured ventilation in both control and lesioned groups and we observed that serotonin (5-HT) specific chemical lesions of the medullary raphe caused reduced respiratory responses to both hypercarbia and hypoxia. In summary, we report that the serotoninergic neurons of the medullary raphe of the cururu toad Rhinella diptycha participate in the chemoreflex responses during hypercarbia and hypoxia, but not during resting conditions. This current evidence in anurans, together with the available data in mammals, brings insights to the evolution of brain sites, such as the medullary raphe, involved in the ventilatory chemoreflex in vertebrates.

Input of sensory, limbic, basal ganglia and pallial/cortical information into the ventral/lateral habenula: Functional principles in anuran amphibians

The habenula - a phylogenetically old brain structure present in all vertebrates - is involved in pain processing, reproductive behaviors, sleep-wake cycles, stress responses, reward, and learning. We performed intra- and extracellular recordings of ventral habenula (VHb) neurons in the isolated brain of anurans and revealed similar cell and response properties to those reported for the lateral habenula of mammals. We identified tonic regular, tonic irregular, rhythmic firing, and silent VHb neurons. Transitions between these firing patterns were observed during spontaneous activity. Electrical stimulation of various brain areas demonstrated VHb input of auditory, optic, limbic, basal ganglia, and pallial information. This resulted in three different response behaviors in VHb neurons: excitation, inhibition, or alternating facilitation and suppression of neuronal activity. Spontaneously changing activity patterns were observed to modulate, reset, or suppress the response behavior of VHb neurons, indicating a gating mechanism. This could be a network status or context dependent selection mechanism for which information are transmitted to task relevant brain areas (i.e., sensory system, limbic system, basal ganglia). Furthermore, alternating facilitation and suppression sequences upon auditory nerve stimulation correlated positively fictive motor activities recorded via the compound potential of the vagal nerve. Stimulation of the auditory nerve or the habenula led to facilitation, suppression, or alternating facilitation and suppression of neuronal activity in putative dopaminergic neurons. Due to complex habenula feedback loops with basal ganglia, limbic, and sensory systems, the habenula involvement in a variety of functions might therefore be explained by a modulatory effect on a task-relevant input stream.

Dynamics of perineuronal nets over amphibian metamorphosis

Extracellular matrix materials known as perineuronal nets (PNNs) have been shown to have remarkable consequences for the maturation of neural circuits and stabilization of behavior. It has been proposed that, due to the possibly long-lived biochemical nature of their components, PNNs may be an important substrate by which long-term memories are stored in the central nervous system. However, little empirical evidence exists that shows that PNNs are themselves stable once established. Thus, the question of their temporal dynamics remains unresolved. We leverage the dramatic morphological and behavioral transformations that occur during amphibian metamorphosis to show that PNNs can be highly dynamic in nature. We used established lectin histochemistry to show that PNNs undergo drastic reconstruction during the metamorphic transition. Pre-metamorphic tadpoles have abundant lectin-labeled pericellular material, which we interpret to be PNNs, surrounding neurons throughout the central nervous system. During the metamorphic transition, these structures degrade, and begin to reform in the months following metamorphosis. We show that PNN sizes and staining intensity further change over metamorphosis, suggesting compositional rearrangement. We found PNNs in brain regions with putative homology to regions in mammals with known PNN function, and in other shared regions where PNN function is unknown. Our results suggest that PNNs are susceptible to remodeling by endogenous mechanisms during development. Interpreting the roles of PNNs in circuit maturation and stability requires understanding their temporal relationship with the neurons and synapses they surround. Our work provides further impetus to investigate this relationship in tandem with developmental and behavioral studies.