Morphological analysis of the cerebellum and its efferent system in a basal actinopterygian fish, Polypterus senegalus

Volume X-Ray Micro-Computed Tomography Analysis of the Early Cephalized Central Nervous System in a Marine Flatworm, Stylochoplana pusilla

Platyhelminthes are a phylum of simple bilaterian invertebrates with prototypic body systems. Compared with non-bilaterians such as cnidarians, the bilaterians are likely to exhibit integrated free-moving behaviors, which require a concentrated nervous system "brain" rather than the distributed nervous system of radiatans. Marine flatworms have an early cephalized 'central' nervous system compared not only with non-bilaterians but also with parasitic flatworms or freshwater planarians. In this study, we used the marine flatworm Stylochoplana pusilla as an excellent model organism in Platyhelminthes because of the early cephalized central nervous system. Here, we investigated the three-dimensional structures of the flatworm central nervous system by the use of X-ray micro-computed tomography (micro-CT) in a synchrotron radiation facility. We found that the obtained tomographic images were sufficient to discriminate some characteristic structures of the nervous system, including nerve cords around the cephalic ganglion, mushroom body-like structures, and putative optic nerves forming an optic commissure-like structure. Through the micro-CT imaging, we could obtain undistorted serial section images, permitting us to visualize precise spatial relationships of neuronal subpopulations and nerve tracts. 3-D micro-CT is very effective in the volume analysis of the nervous system at the cellular level; the methodology is straightforward and could be applied to many other non-model organisms.

Distribution of gamma-aminobutyric acid immunoreactivity in the brain of the Siberian sturgeon (Acipenser baeri): Comparison with other fishes

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the central nervous system (CNS) of vertebrates. Immunohistochemical techniques with specific antibodies against GABA or against its synthesizing enzyme, glutamic acid decarboxylase (GAD) allowed characterizing GABAergic neurons and fibers in the CNS. However, studies on the CNS distribution of GABAergic neurons and fibers of bony fishes are scant and were done in teleost species. With the aim of understanding the early evolution of this system in bony vertebrates, we analyzed the distribution of GABA-immunoreactive (-ir) and GAD-ir neurons and fibers in the CNS of a basal ray-finned fish, the Siberian sturgeon (Chondrostei, Acipenseriformes), using immunohistochemical techniques. Our results revealed the presence and distribution of GABA/GAD-ir cells in different regions of the CNS such as olfactory bulbs, pallium and subpallium, hypothalamus, thalamus, pretectum, optic tectum, tegmentum, cerebellum, central grey, octavolateralis area, vagal lobe, rhombencephalic reticular areas, and the spinal cord. Abundant GABAergic innervation was observed in most brain regions, and GABAergic fibers were very abundant in the hypothalamic floor along the hypothalamo-hypophyseal tract and neurohypophysis. In addition, GABA-ir cerebrospinal fluid-contacting cells were observed in the alar and basal hypothalamus, saccus vasculosus, and spinal cord central canal. The distribution of GABAergic systems in the sturgeon brain shows numerous similarities to that observed in lampreys, but also to those of teleosts and tetrapods.

Diversity and evolution of serotonergic cells in taste buds of elasmobranchs and ancestral actinopterygian fish

A subset of gustatory cells are serotonin immunoreactive (ir) in the mammalian taste bud. In the taste bud of lamprey, elongated gustatory-like cells are also serotonin-ir. In contrast, flattened serotonin-ir cells are located only in the basal region of the taste buds in the teleosts and amphibians. These serotonin-ir cells are termed as basal cells. To evaluate the evolution and diversity of serotonergic cells in the taste bud of amniote animals, we explored the distribution and morphology of serotonin-ir cells in the taste buds of ancestral actinopterygian fish (spotted gar, sturgeon, Polypterus senegalus) and elasmobranch (stingray). In all examined animals, the taste buds contained serotonin-ir cells in their basal part. The number of serotonin-ir basal cells in each taste bud was different between these fish species. They were highest in the stingray and decreased in the order of the Polypterus, sturgeon, and gar. While serotonin immunoreactivity was observed only in the basal cells in the taste buds of the ancestral actinopterygian fish, some elongated cells were also serotonin-ir in addition to the basal cells in the stingray taste buds. mRNA of tryptophan hydroxylase 1 (tph1), a rate-limiting enzyme of the serotonin synthesis, is expressed in both the elongated and basal cells of stingray taste buds, indicating that these cells synthesize the serotonin by themselves. These results suggest that the serotonin-ir basal cells arose from the ancestor of the cartilaginous fish, and serotonin-ir cells in the elasmobranch taste bud exhibit an intermediate aspect between the lamprey and actinopterygian fish.

Development, circuitry, and function of the zebrafish cerebellum

The cerebellum represents a brain compartment that first appeared in gnathostomes (jawed vertebrates). Besides the addition of cell numbers, its development, cytoarchitecture, circuitry, physiology, and function have been highly conserved throughout avian and mammalian species. While cerebellar research in avian and mammals is extensive, systematic investigations on this brain compartment in zebrafish as a teleostian model organism started only about two decades ago, but has provided considerable insight into cerebellar development, physiology, and function since then. Zebrafish are genetically tractable with nearly transparent small-sized embryos, in which cerebellar development occurs within a few days. Therefore, genetic investigations accompanied with non-invasive high-resolution in vivo time-lapse imaging represents a powerful combination for interrogating the behavior and function of cerebellar cells in their complex native environment.

Histological and Molecular Characterization of the Inferior Olivary Nucleus and Climbing Fibers in the Goldfish, Carassius auratus

The cerebellum receives inputs via the climbing fibers originating from the inferior olivary nucleus in the ventral medulla. In mammals, the climbing fibers entwine and terminate onto both major and peripheral branches of dendrites of the Purkinje cells. In this study, the inferior olivary nucleus and climbing fiber in the goldfish were investigated with several histological techniques. By neural tracer application to the hemisphere of the cerebellum, labeled inferior olivary neurons were found in the ventral edge of the contralateral medulla. Kainate stimulated Co ^{+ +} uptake and gephyrin immunoreactivities were found in inferior olivary neurons, indicating, respectively, that they receive both excitatory (glutamatergic) and inhibitory (GABAergic or glycinergic) inputs. Inferior olivary neurons express vglut2.1 transcripts, suggesting they are glutamatergic. Around 85% of inferior olivary neurons were labeled with anti-calretinin antiserum. Calretinin immunoreactive (ir) climbing fiber terminal-like structures were distributed near the Purkinje cells and in the molecular layer. Double labeling immunofluorescence with anti-calretinin and zebrin II antisera revealed that the calretinin-ir climbing fibers run along and made synaptic-like contacts on the major dendrites of the zebrin II-ir Purkinje cells. In teleost fish, cerebellar efferent neurons, eurydendroid cells, also lie near the Purkinje cells and extend dendrites outward to intermingle with dendrites of the Purkinje cells within the molecular layer. Here we found no contacts between the climbing fiber terminals and the eurydendroid cell dendrites. These results support the idea that Purkinje cells, but not eurydendroid cells, receive strong inputs via the climbing fibers, similar to the mammalian situation.

Exceptional fossil preservation and evolution of the ray-finned fish brain

Brain anatomy provides key evidence for the relationships between ray-finned fishes¹, but two major limitations obscure our understanding of neuroanatomical evolution in this major vertebrate group. First, the deepest branching living lineages are separated from the group's common ancestor by hundreds of millions of years, with indications that aspects of their brain morphology-like other aspects of their anatomy^2,3-are specialized relative to primitive conditions. Second, there are no direct constraints on brain morphology in the earliest ray-finned fishes beyond the coarse picture provided by cranial endocasts: natural or virtual infillings of void spaces within the skull^4-8. Here we report brain and cranial nerve soft-tissue preservation in Coccocephalus wildi, an approximately 319-million-year-old ray-finned fish. This example of a well-preserved vertebrate brain provides a window into neural anatomy deep within ray-finned fish phylogeny. Coccocephalus indicates a more complicated pattern of brain evolution than suggested by living species alone, highlighting cladistian apomorphies¹ and providing temporal constraints on the origin of traits uniting all extant ray-finned fishes^1,9. Our findings, along with a growing set of studies in other animal groups^10-12, point to the importance of ancient soft tissue preservation in understanding the deep evolutionary assembly of major anatomical systems outside of the narrow subset of skeletal tissues^13-15.

Diversity of cellular physiology and morphology of Purkinje cells in the adult zebrafish cerebellum

This study was designed to explore the functional circuitry of the adult zebrafish cerebellum, focusing on its Purkinje cells and using whole-cell patch recordings and single cell labeling in slice preparations. Following physiological characterizations, the recorded single cells were labeled for morphological identification. It was found that the zebrafish Purkinje cells are surprisingly diverse. Based on their physiology and morphology, they can be classified into at least three subtypes: Type I, a narrow spike cell, which fires only narrow Na⁺ spikes (<3 ms in duration), and has a single primary dendrite with an arbor restricted to the distal molecular layer; Type II, a broad spike cell, which fires broad Ca²⁺ spikes (5-7 ms in duration) and has a primary dendrite with limited branching in the inner molecular layer and then further radiates throughout the molecular layer; and Type III, a very broad spike cell, which fires very broad Ca²⁺ spikes (≥10 ms in duration) and has a dense proximal dendritic arbor that is either restricted to the inner molecular layer (Type IIIa), or radiates throughout the entire molecular layer (Type IIIb). The graded paired-pulse facilitation of these Purkinje cells' responses to parallel fiber activations and the all-or-none, paired-pulse depression of climbing fiber activation are largely similar to those reported for mammals. The labeled axon terminals of these Purkinje cells end locally, as reported for larval zebrafish. The present study provides evidence that the corresponding functional circuitry and information processing differ from what has been well-established in the mammalian cerebellum.