Functional circuitry of a unique cerebellar specialization: the valvula cerebelli of a mormyrid fish

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.

Purkinje cells located in the adult zebrafish valvula cerebelli exhibit variable functional responses

Purkinje cells are critically involved in processing the cerebellar functions by shaping and coordinating commands that they receive. Here, we demonstrate experimentally that in the adult zebrafish valvular part of the cerebellum, the Purkinje cells exhibited variable firing and functional responses and allowed the categorization into three firing classes. Compared with the Purkinje cells in the corpus cerebelli, the valvular Purkinje cells receive weak and occasional input from the inferior olive and are not active during locomotion. Together, our findings expand further the regional functional differences of the Purkinje cell population and expose their non-locomotor functionality.

The Mormyrid Optic Tectum Is a Topographic Interface for Active Electrolocation and Visual Sensing

The African weakly electric fish Gnathonemus petersii is capable of cross-modal object recognition using its electric sense or vision. Thus, object features stored in the brain are accessible by multiple senses, either through connections between unisensory brain regions or because of multimodal representations in multisensory areas. Primary electrosensory information is processed in the medullary electrosensory lateral line lobe, which projects topographically to the lateral nucleus of the torus semicircularis (NL). Visual information reaches the optic tectum (TeO), which projects to various other brain regions. We investigated the neuroanatomical connections of these two major midbrain visual and electrosensory brain areas, focusing on the topographical relationship of interconnections between the two structures. Thus, the neural tracer DiI was injected systematically into different tectal quadrants, as well as into the NL. Tectal tracer injections revealed topographically organized retrograde and anterograde label in the NL. Rostral and caudal tectal regions were interconnected with rostral and caudal areas of the NL, respectively. However, dorsal and ventral tectal regions were represented in a roughly inverted fashion in NL, as dorsal tectal injections labeled ventral areas in NL and vice versa. In addition, tracer injections into TeO or NL revealed extensive inputs to both structures from ipsilateral (NL also contralateral) efferent basal cells in the valvula cerebelli; the NL furthermore projected back to the valvula. Additional tectal and NL connections were largely confirmatory to earlier studies. For example, the TeO received ipsilateral inputs from the central zone of the dorsal telencephalon, torus longitudinalis, nucleus isthmi, various tegmental, thalamic and pretectal nuclei, as well as other nuclei of the torus semicircularis. Also, the TeO projected to the dorsal preglomerular and dorsal posterior thalamic nuclei as well as to nuclei in the torus semicircularis and nucleus isthmi. Beyond the clear topographical relationship of NL and TeO interconnections established here, the known neurosensory upstream circuitry was used to suggest a model of how a defined spot in the peripheral sensory world comes to be represented in a common associated neural locus both in the NL and the TeO, thereby providing the neural substrate for cross-modal object recognition.

Quantitative Classification of Cerebellar Foliation in Cartilaginous Fishes (Class: Chondrichthyes) Using Three-Dimensional Shape Analysis and Its Implications for Evolutionary Biology

A true cerebellum appeared at the onset of the chondrichthyan (sharks, batoids, and chimaerids) radiation and is known to be essential for executing fast, accurate, and efficient movement. In addition to a high degree of variation in size, the corpus cerebellum in this group has a high degree of variation in convolution (or foliation) and symmetry, which ranges from a smooth cerebellar surface to deep, branched convexities and folds, although the functional significance of this trait is unclear. As variation in the degree of foliation similarly exists throughout vertebrate evolution, it becomes critical to understand this evolutionary process in a wide variety of species. However, current methods are either qualitative and lack numerical rigor or they are restricted to two dimensions. In this paper, a recently developed method for the characterization of shapes embedded within noisy, three-dimensional data called spherical wave decomposition (SWD) is applied to the problem of characterizing cerebellar foliation in cartilaginous fishes. The SWD method provides a quantitative characterization of shapes in terms of well-defined mathematical functions. An additional feature of the SWD method is the construction of a statistical criterion for the optimal fit, which represents the most parsimonious choice of parameters that fits to the data without overfitting to background noise. We propose that this optimal fit can replace a previously described qualitative visual foliation index (VFI) in cartilaginous fishes with a quantitative analog, i.e. the cerebellar foliation index (CFI). The capability of the SWD method is demonstrated in a series of volumetric images of brains from different chondrichthyan species that span the range of foliation gradings currently described for this group. The CFI is consistent with the qualitative grading provided by the VFI, delivers a robust measure of cerebellar foliation, and can provide a quantitative basis for brain shape characterization across taxa.

The energetic cost of vision and the evolution of eyeless Mexican cavefish

One hypothesis for the reduction of vision in cave animals, such as the eyeless Mexican cavefish, is the high energetic cost of neural tissue and low food availability in subterranean habitats. However, data on relative brain and eye mass in this species or on any measure of the energetic cost of neural tissue are not available, making it difficult to evaluate the "expensive tissue hypothesis." We show that the eyes and optic tectum represent significant metabolic costs in the eyed phenotype. The cost of vision was calculated to be 15% of resting metabolism for a 1-g fish, decreasing to 5% in an 8.5-g fish as relative eye and brain size declined during growth. Our results demonstrate that the loss of the visual system in the cave phenotype substantially lowered the amount of energy expended on expensive neural tissue during diversification into subterranean rivers, in particular for juvenile fish.

Synaptic dynamics and long-term plasticity at synapses of Purkinje cells onto neighboring Purkinje cells of a mormyrid fish: a dual cell recording study

The input synapses of cerebellar Purkinje cells (PCs) have been extensively studied and much has been learned about their dynamics, plasticity and functionality. In contrast there is limited information available about PC output synapses. This study uses dual cell recording methods to investigate synaptic dynamics and plasticity at individual PC synapses onto neighboring PCs in in vitro preparations of the mormyrid cerebellum. This synaptic connectivity may be strong or weak. For strong connections, inhibitory postsynaptic potentials (IPSPs) or currents (IPSCs) are synchronized with the action potentials of the presynaptic cell. For weak connections, however, the pre- and postsynaptic potentials are no longer synchronized, and presynaptic burst firing at intraburst rates of ∼50 Hz or higher is required to reliably induce the postsynaptic inhibition. A depression of this postsynaptic inhibition was observed for both types of connectivity following repeated presynaptic bursts, which was subsequently largely reversed following pairings of the presynaptic burst-induced IPSPs/IPSCs with evoked burst firing of the postsynaptic cell. Moreover, the original postsynaptic depression was found to be either augmented or reversed depending on the temporal order of each pair of additional pre- and postsynaptic cell activations, hence demonstrating a reversible and spike timing-dependent plasticity (STDP) at this synapse.