Connections of the bullfrog striatum: afferent organization

Does a Vertebrate Morphotype of Pallial Subdivisions Really Exist?

BACKGROUND

Comparative neuroanatomists have long sought to determine which part of the pallium in nonmammals is homologous to the mammalian neocortex. A number of similar connectivity patterns across species have led to the idea that the basic organization of the vertebrate brain is relatively conserved; thus, efforts of the last decades have been focused on determining a vertebrate "morphotype" - a model comprising the characteristics believed to have been present in the last common ancestor of all vertebrates.

SUMMARY

The endeavor to determine the vertebrate morphotype has been riddled with controversies due to the extensive morphological diversity of the pallium among vertebrate taxa. Nonetheless, most proposed scenarios of pallial homology are variants of a common theme where the vertebrate pallium is subdivided into subdivisions homologous to the hippocampus, neocortex, piriform cortex, and amygdala, in a one-to-one manner. We review the rationales of major propositions of pallial homology and identify the source of the discrepancies behind different hypotheses. We consider that a source of discrepancies is the prevailing assumption that there is a single "morphotype of the pallial subdivisions" throughout vertebrates. Instead, pallial subdivisions present in different taxa probably evolved independently in each lineage.

KEY MESSAGES

We encounter discrepancies when we search for a single morphotype of subdivisions across vertebrates. These discrepancies can be resolved by considering that several subdivisions within the pallium were established after the divergence of the different lineages. The differences of pallial organization are especially remarkable between actinopterygians (including teleost fishes) and other vertebrates. Thus, the prevailing notion of a simple one-to-one homology between the mammalian and teleost pallia needs to be reconsidered.

Non-thalamic origin of zebrafish sensory nuclei implies convergent evolution of visual pathways in amniotes and teleosts

Ascending visual projections similar to the mammalian thalamocortical pathway are found in a wide range of vertebrate species, but their homology is debated. To get better insights into their evolutionary origin, we examined the developmental origin of a thalamic-like sensory structure of teleosts, the preglomerular complex (PG), focusing on the visual projection neurons. Similarly to the tectofugal thalamic nuclei in amniotes, the lateral nucleus of PG receives tectal information and projects to the pallium. However, our cell lineage study in zebrafish reveals that the majority of PG cells are derived from the midbrain, unlike the amniote thalamus. We also demonstrate that the PG projection neurons develop gradually until late juvenile stages. Our data suggest that teleost PG, as a whole, is not homologous to the amniote thalamus. Thus, the thalamocortical-like projections evolved from a non-forebrain cell population, which indicates a surprising degree of variation in the vertebrate sensory systems.

Amphibian thalamic nuclear organization during larval development and in the adult frog Xenopus laevis: Genoarchitecture and hodological analysis

The early patterning of the thalamus during embryonic development defines rostral and caudal progenitor domains, which are conserved from fishes to mammals. However, the subsequent developmental mechanisms that lead to the adult thalamic configuration have only been investigated for mammals and other amniotes. In this study, we have analyzed in the anuran amphibian Xenopus laevis (an anamniote vertebrate), through larval and postmetamorphic development, the progressive regional expression of specific markers for the rostral (GABA, GAD67, Lhx1, and Nkx2.2) and caudal (Gbx2, VGlut2, Lhx2, Lhx9, and Sox2) domains. In addition, the regional distributions at different developmental stages of other markers such as calcium binding proteins and neuropeptides, helped the identification of thalamic nuclei. It was observed that the two embryonic domains were progressively specified and compartmentalized during premetamorphosis, and cell subpopulations characterized by particular gene expression combinations were located in periventricular, intermediate and superficial strata. During prometamorphosis, three dorsoventral tiers formed from the caudal domain and most pronuclei were defined, which were modified into the definitive nuclear configuration through the metamorphic climax. Mixed cell populations originated from the rostral and caudal domains constitute most of the final nuclei and allowed us to propose additional subdivisions in the adult thalamus, whose main afferent and efferent connections were assessed by tracing techniques under in vitro conditions. This study corroborates shared features of early gene expression patterns in the thalamus between Xenopus and mouse, however, the dynamic changes in gene expression observed at later stages in the amphibian support mechanisms different from those of mammals.

The Effects of Melatonin on Brain Arginine Vasotocin: Relationship with Sex and Seasonal Differences in Melatonin Receptor Type 1 in Green Treefrogs (Hyla cinerea)

The neuroendocrine mechanisms by which animals synchronise their physiological state with environmental cues are vital to timing life-history events appropriately. One important endocrine transducer of environmental cues in vertebrates is the pineal hormone melatonin, the secretion of which is directly sensitive to photoperiod and temperature. Melatonin modulates arginine vasotocin (AVT)-immunoreactive (-IR) cell number in the brain of green treefrogs (Hyla cinerea) during the summer breeding season, and this modulation is sexually dimorphic. In the present study, we investigated whether the influence of melatonin on vasotocin varies seasonally. We show that treatment of nonreproductive male green treefrogs with melatonin-filled silastic implants for 4 weeks during the winter does not alter vasotocin-IR cell number in any brain region (i.e. nucleus accumbens, amygdala, preoptic area, suprachiasmatic nucleus or ventral hypothalamus). Taken together, these results suggest that the influence of melatonin on AVT is associated with sex and seasonal variation in melatonin receptor expression. We tested this hypothesis by using immunohistochemistry to characterise the distribution of melatonin receptor type 1 (MT1, also known as Mel1a) in the brain of reproductive and nonreproductive male and female frogs. We quantified MT1-IR cell number in regions known to contain AVT cell populations. Reproductive males had significantly more MT1-IR cells than nonreproductive males in all brain regions, including the combined nucleus accumbens, diagonal band of Broca and septum, striatum, amygdala, combined preoptic area and suprachiasmatic nucleus, as well as the ventral hypothalamus. In the accumbens region, where the effect of melatonin on AVT is known to be sexually dimorphic, males had significantly more MT1-IR cells than females during the summer breeding season. Based on these findings, we suggest that MT1 plays a role in mediating the interactions between melatonin and vasotocin that regulate seasonal and sexually dimorphic changes in sociosexual behaviour.

Organization of afferents to the striatopallidal systems in the fire-bellied toad Bombina orientalis

The cerebral hemispheres of amphibians display paired dorsal and ventral striatum (commonly referred to as striatum proper and nucleus accumbens, respectively). Each striatal region is proposed to be closely associated with a pallidal structure located caudal to it to form a striatopallidal system. In the present study, afferents to the dorsal and ventral striatopallidal systems of the fire-bellied toad (Bombina orientalis) were investigated using the neuronal tracer biocytin. A quantitative analysis of the topographical distribution of afferent neurons from the thalamus and posterior tubercle/ventral tegmentum was emphasised. The main results show that inputs to the two striatopallidal systems originate from distinct dorsal thalamic nuclei, with dorsal and ventral striatopallidal afferent neurons favouring strongly the lateral/central and anterior thalamic nuclei, respectively. However, afferent neuron distribution in the dorsal thalamus does not differ in the rostrocaudal axis of the brain. Afferent neurons from the posterior tubercle and ventral tegmentum, on the other hand, are organised topographically along the rostrocaudal axis. About 85 % of afferent neurons to the dorsal striatopallidal system are located rostrally in the posterior tubercle, while 75 % of afferent neurons to the ventral striatopallidal system are found more caudally in the ventral tegmentum. This difference is statistically significant and confirms the presence of distinct mesostriatal pathways in an amphibian. These findings demonstrate that an amphibian brain displays striatopallidal systems integrating parallel streams of sensory information potentially under the influence of distinct ascending mesostriatal pathways.

Neocortical arealization: evolution, mechanisms, and open questions

The mammalian neocortex is a structure with no equals in the vertebrates and is the seat of the highest cerebral functions, such as thoughts and consciousness. It is radially organized into six layers and tangentially subdivided into functional areas deputed to the elaboration of sensory information, association between different stimuli, and selection and triggering of voluntary movements. The process subdividing the neocortical field into several functional areas is called "arealization". Each area has its own cytoarchitecture, connectivity, and peculiar functions. In the last century, several neuroscientists have investigated areal structure and the mechanisms that have led during evolution to the rising of the neocortex and its organization. The extreme conservation in the positioning and wiring of neocortical areas among different mammalian families suggests a conserved genetic program orchestrating neocortical patterning. However, the impressive plasticity of the neocortex, which is able to rewire and reorganize areal structures and connectivity after impairments of sensory pathways, argues for a more complex scenario. Indeed, even if genetics and molecular biology helped in identifying several genes involved in the arealization process, the logic underlying the neocortical bauplan is still beyond our comprehension. In this review, we will introduce the present knowledge and hypotheses on the ontogenesis and evolution of neocortical areas. Then, we will focus our attention on some open issues, which are still unresolved, and discuss some recent studies that might open new directions to be explored in the next few years.

Oculomotor learning revisited: a model of reinforcement learning in the basal ganglia incorporating an efference copy of motor actions

In its simplest formulation, reinforcement learning is based on the idea that if an action taken in a particular context is followed by a favorable outcome, then, in the same context, the tendency to produce that action should be strengthened, or reinforced. While reinforcement learning forms the basis of many current theories of basal ganglia (BG) function, these models do not incorporate distinct computational roles for signals that convey context, and those that convey what action an animal takes. Recent experiments in the songbird suggest that vocal-related BG circuitry receives two functionally distinct excitatory inputs. One input is from a cortical region that carries context information about the current “time” in the motor sequence. The other is an efference copy of motor commands from a separate cortical brain region that generates vocal variability during learning. Based on these findings, I propose here a general model of vertebrate BG function that combines context information with a distinct motor efference copy signal. The signals are integrated by a learning rule in which efference copy inputs gate the potentiation of context inputs (but not efference copy inputs) onto medium spiny neurons in response to a rewarded action. The hypothesis is described in terms of a circuit that implements the learning of visually guided saccades. The model makes testable predictions about the anatomical and functional properties of hypothesized context and efference copy inputs to the striatum from both thalamic and cortical sources.

Characterization of the bed nucleus of the stria terminalis in the forebrain of anuran amphibians

Major common features have been reported for the organization of the basal telencephalon in amniotes, and most characteristics were thought to be acquired in the transition from anamniotes to amniotes. However, gene expression, neurochemical, and hodological data obtained for the basal ganglia and septal and amygdaloid complexes in amphibians (anamniotic tetrapods) have strengthened the idea of a conserved organization in tetrapods. A poorly characterized region in the forebrain of amniotes has been the bed nucleus of the stria terminalis (BST), but numerous recent investigations have characterized it as a member of the extended amygdala. Our study analyzes the main features of the BST in anuran amphibians to establish putative homologies with amniotes. Gene expression patterns during development identified the anuran BST as a subpallial, nonstriatal territory. The BST shows Nkx2.1 and Lhx7 expression and contains an Islet1-positive cell subpopulation derived from the lateral ganglionic eminence. Immunohistochemistry for diverse peptides and neurotransmitters revealed that the distinct chemoarchitecture of the BST is strongly conserved among tetrapods. In vitro tracing techniques with dextran amines revealed important connections between the BST and the central and medial amygdala, septal territories, medial pallium, preoptic area, lateral hypothalamus, thalamus, and prethalamus. The BST receives dopaminergic projections from the ventral tegmental area and is connected with the laterodorsal tegmental nucleus and the rostral raphe in the brainstem. All these data suggest that the anuran BST shares many features with its counterpart in amniotes and belongs to a basal continuum, likely controlling similar reflexes, reponses, and behaviors in tetrapods.

The vertebrate mesolimbic reward system and social behavior network: a comparative synthesis

All animals evaluate the salience of external stimuli and integrate them with internal physiological information into adaptive behavior. Natural and sexual selection impinge on these processes, yet our understanding of behavioral decision-making mechanisms and their evolution is still very limited. Insights from mammals indicate that two neural circuits are of crucial importance in this context: the social behavior network and the mesolimbic reward system. Here we review evidence from neurochemical, tract-tracing, developmental, and functional lesion/stimulation studies that delineates homology relationships for most of the nodes of these two circuits across the five major vertebrate lineages: mammals, birds, reptiles, amphibians, and teleost fish. We provide for the first time a comprehensive comparative analysis of the two neural circuits and conclude that they were already present in early vertebrates. We also propose that these circuits form a larger social decision-making (SDM) network that regulates adaptive behavior. Our synthesis thus provides an important foundation for understanding the evolution of the neural mechanisms underlying reward processing and behavioral regulation.

Development of tectal connectivity across metamorphosis in the bullfrog (Rana catesbeiana)

In the bullfrog (Rana catesbeiana), the process of metamorphosis culminates in the appearance of new visual and visuomotor behaviors reflective of the emergence of binocular vision and visually-guided prey capture behaviors as the animal transitions to life on land. Using several different neuroanatomical tracers, we examined the substrates that may underlie these behavioral changes by tracing the afferent and efferent connectivity of the midbrain optic tectum across metamorphic development. Intratectal, tectotoral, tectotegmental, tectobulbar, and tecto-thalamic tracts exhibit similar trajectories of neurobiotin fiber label across the developmental span from early larval tadpoles to adults. Developmental variability was apparent primarily in intensity and distribution of cell and puncta label in target nuclei. Combined injections of cholera toxin subunit β and Phaseolus vulgaris leucoagglutinin consistently label cell bodies, puncta, or fiber segments bilaterally in midbrain targets including the pretectal gray, laminar nucleus of the torus semicircularis, and the nucleus of the medial longitudinal fasciculus. Developmentally stable label was observed bilaterally in medullary targets including the medial vestibular nucleus, lateral vestibular nucleus, and reticular gray, and in forebrain targets including the posterior and ventromedial nuclei of the thalamus. The nucleus isthmi, cerebellum, lateral line nuclei, medial septum, ventral striatum, and medial pallium show more developmentally variable patterns of connectivity. Our results suggest that even during larval development, the optic tectum contains substrates for integration of visual with auditory, vestibular, and somatosensory cues, as well as for guidance of motivated behaviors.

Neuroethology of releasing mechanisms: Prey-catching in toads

“Sign stimuli” elicit specific patterns of behavior when an organism's motivation is appropriate. In the toad, visually released prey-catching involves orienting toward the prey, approaching, fixating, and snapping. For these action patterns to be selected and released, the prey must be recognized and localized in space. Toads discriminate prey from nonprey by certain spatiotemporal stimulus features. The stimulus-response relations are mediated by innate releasing mechanisms (RMs) with recognition properties partly modifiable by experience. Striato-pretecto-tectal connectivity determines the RM's recognition and localization properties, whereas medialpallio-thalamo-tectal circuitry makes the system sensitive to changes in internal state and to prior history of exposure to stimuli. RMs encode the diverse stimulus conditions referring to the same prey object through different combinations of “specialized” tectal neurons, involving cells selectively tuned to prey features. The prey-selective neurons express the outcome of information processing in functional units consisting of interconnected cells. Excitatory and inhibitory interactions among feature-sensitive tectal and pretectal neurons specify the perceptual operations involved in distinguishing the prey from its background, selecting its features, and discriminating it from predators. Other connections indicate stimulus location. The results of these analyses are transmitted by specialized neurons projecting from the tectum to bulbar/spinal motor systems, providing a sensorimotor interface. Specific combinations of such projective neurons – mediating feature- and space-related messages – form “command releasing systems” that activate corresponding motor pattern generators for appropriate prey-catching action patterns.

Evolution and ontogeny of neural circuits

Recent studies on neural pathways in a broad spectrum of vertebrates suggest that, in addition to migration and an increase in the number of certain select neurons, a significant aspect of neural evolution is a “parcellation” (segregation-isolation) process that involves the loss of selected connections by the new aggregates. A similar process occurs during ontogenetic development. These findings suggest that in many neuronal systems axons do not invade unknown territories during evolutionary or ontogenetic development but follow in their ancestors' paths to their ancestral targets; if the connection is later lost, it reflects the specialization of the circuitry.

The pattern of interspecific variability suggests (1) that overlap of circuits is a more common feature in primitive (generalized) than in specialized brain organizations and (2) that most projections, such as the retinal, thalamotelencephalic, corticotectal, and tectal efferent ones, were bilateral in the primitive condition. Specialization of these systems in some vertebrate groups has involved the selective loss of connections, resulting in greater isolation of functions. The parcellation process may also play an important role in cell diversification.

The parcellation process as described here is thought to be one of several underlying mechanisms of evolutionary and ontogenetic differentiation.