Masayuki Yoshida

Central circuits controlling locomotion in young frog tadpoles.

Abstract: The young Xenopus tadpole is a very simple vertebrate that can swim. We have examined its behavior and neuroanatomy, and used immobilized tadpoles to study the initiation, production, coordination, and termination of the swimming motor pattern. We will outline the sensory pathways that control swimming behavior and the mainly spinal circuits that produce the underlying motor output. Our recent work has analyzed the glycinergic, glutamatergic, cholinergic, and electrotonic synaptic input to spinal neurons during swimming. This has led us to study the nonlinear summation of excitatory synaptic inputs to small neurons. We then analyzed the different components of excitation during swimming to ask which components control frequency, and to map the longitudinal distribution of the components along the spinal cord. The central axonal projection patterns of spinal interneurons and motoneurons have been defined in order to try to account for the longitudinal distribution of synaptic drive during swimming.

Comparison of behavioral responses to a novel environment between three teleosts, bluegill Lepomis macrochirus, crucian carp Carassius langsdorfii, and goldfish Carassius auratus

To investigate the emotional reactivity of fish in a novel environment, the swimway test was developed. The swimway apparatus consists of a shaded start chamber and an open, illuminated swimway. Fish were first introduced and habituated to the start chamber. A door partitioning the start chamber from the swimway was then opened, and behavioral responses of the fish in the apparatus were measured. By using the swimway test, behavioral responses to a novel environment of bluegill Lepomis macrochirus, crucian carp Carassius langsdorfii, and goldfish Carassius auratus were quantified and compared. The emotional reactivities in blue gill were found to be the lowest and crucian carp the highest, indicating bluegill are relatively active or ‘bold’, and that the crucian carp are relatively passive or ‘shy’, in a novel environment. It is suggested that the swimway test is applicable to examning inter-species differences in relative emotional reactivity or boldness in a simplified novel situation.

Defining classes of spinal interneuron and their axonal projections in hatchling Xenopus laevis tadpoles

Neurobiotin was injected into individual spinal interneurons in the Xenopus tadpole to discern their anatomical features and complete axonal projection patterns. Four classes of interneuron are described, with names defining their primary axon projection: Dorsolateral ascending and commissural interneurons are predominantly multipolar cells with somata and dendrites exclusively in the dorsal half of the spinal cord. Ascending interneurons have unipolar somata located in the dorsal half, but their main dendrites are located in the ventral half of the spinal cord. Descending interneurons show bigger variance in their anatomy, but the majority are unipolar, and they all have a descending primary axon. Dorsolateral commissural interneurons are clearly defined using established criteria, but the others are not, so cluster analysis was used. Clear discriminations can be made, and criteria are established to characterize the three classes of interneuron with ipsilateral axonal projections. With identifying criteria established, the distribution and axonal projection patterns of the four classes of interneuron are described. By using data from γ‐aminobutyric acid immunocytochemistry, the distribution of the population of ascending interneurons is defined. Together with the results from the axonal projection data, this allows the ascending interneuron axon distribution along the spinal cord to be estimated. By making simple assumptions and using existing information about the soma distributions of the other interneurons, estimates of their axon distributions are made. The possible functional roles of the four interneuron classes are discussed. J. Comp. Neurol. 441:248–265, 2001.

Cerebellar efferent neurons in teleost fish

In tetrapods, cerebellar efferent systems are mainly mediated via the cerebellar nuclei. In teleosts, the cerebellum lacks cerebellar nuclei. Instead, the cerebellar efferent neurons, termed eurydendroid cells, are arrayed within and below the ganglionic layer. Tracer injections outside of the cerebellum, which retrogradely label eurydendroid cells demonstrate that most eurydendroid cells possess two or more primary dendrites which extend broadly into the molecular layer. Some eurydendroid cells mostly situated in caudal portions of the cerebellum have only one primary dendrite. The eurydendroid cells receive inputs from the Purkinje cells and parallel fibers, but apparently do not receive inputs from the climbing fibers. Eurydendroid cells of the corpus cerebelli and medial valvula project to many brain regions, from the diencephalon to the caudal medulla. A few eurydendroid cells in the valvula project directly to the telencephalon. About half of the eurydendroid cells are aspartate immunopositive. Anti-GABA and anti-zebrin II antibodies that are known as markers for the Purkinje cells in mammals also recognize the Purkinje cells in the teleost cerebellum, but do not recognize the eurydendroid cells. These results suggest that the eurydendroid cells receive GABAergic inputs from the Purkinje cells. This relationship between the eurydendroid and Purkinje cells is similar to that between the cerebellar nuclei and Purkinje cells in mammals. The eurydendroid cells of teleost have both dissimilar as well as similar features compared to neurons of the cerebellar nuclei in tetrapods.

A novel cardio-excitatory peptide isolated from the atria of the african giant snail, Achatina fulica

An undecapeptide which potentiates the beat of the ventricle in the African giant snail, Achatina fulica Ferussac, was purified from the atria of the snail. Its primary structure was determined to be H-Ser-Gly-Gln-Ser-Trp-Arg-Pro-Gln-Gly-Arg-Phe-NH2. This peptide was found to have excitatory actions not only on the ventricle but also on the penis retractor muscle, the buccal muscle and the identified neurons controlling the buccal muscle movement of Achatina.

Involvement of the cerebellum in classical fear conditioning in goldfish

To investigate the emotional role of the cerebellum of fish, we conducted experiments examining effects of cerebellar manipulations on fear-related classical heart rate conditioning in goldfish. We performed total ablation of the corpus cerebelli to examine the effect of irreversible effects. We also performed localized cooling of the corpus cerebelli, in place of the ablation, for reversible inactivation of the cerebellar function. Both the cardiac arousal response to the first presentation of the conditioned stimulus and the cardiac reflex to the aversive unconditioned stimulus were not impaired by the ablation or cooling of the corpus cerebelli. On the other hand, inactivation of cerebellar function severely impaired the acquisition of a conditioned cardiac response in the fear-related conditioning. In addition, localized cooling of the corpus cerebelli reversibly suppressed the expression of established conditioned response. We suggest that the cerebellum of fish is not only being a motor coordination center but also is involved in emotional learning.

Purification of achatin-I from the atria of the African giant snail, Achatina fulica, and its possible function

Achatin-I previously purified from the ganglia of the African giant snail Achatina fulica was isolated from the atria of this snail. Achatin-I appeared to enhance the cardiac activity in two ways; centrally this peptide increased impulse frequency and produced spike broadening of the identified heart excitatory neuron, PON, and peripherally it enhanced amplitude and frequency of the heart beat. Achatin-I showed excitatory actions not only on the heart but on several other muscles.

Morphology and immunohistochemistry of efferent neurons of the goldfish corpus cerebelli

In teleosts, cerebellar efferent neurons, known as eurydendroid cells, are dispersed within the cerebellar cortex rather than coalescing into deep cerebellar nuclei. To clarify their morphology, eurydendroid cells were labeled retrogradely by biotinylated dextran amine injection into the base of the corpus cerebelli. Labeling allowed the cells to be classified into three types—fusiform, polygonal, and monopolar—depending on their somal shapes and numbers of primary dendrites. The fusiform and polygonal type cells were distributed not only in the Purkinje cell layer but also in the molecular and granule cell layers. The monopolar type cells were distributed predominantly in the Purkinje cell layer of the ventrocaudal portion of the corpus cerebelli. These results suggest that there are some functional differences between these eurydendroid cell types. The eurydendroid cells were double‐labeled by retrograde labeling and immunohistochemistry using specific antibodies against GABA, aspartate, and zebrin II. No GABA‐like immunoreactivity was detected in the retrogradely labeled eurydendroid cells. About half of retrogradely labeled cells were immunoreactive to the anti‐aspartate antibody, suggesting that some eurydendroid cells utilize aspartate as a neurotransmitter. Zebrin II reacts with cerebellar Purkinje cells but left all retrogradely labeled neurons nonreactive, although some of these were surrounded by immunopositive fibers. This relationship between the eurydendroid and Purkinje cells is similar to that between the deep cerebellar nuclei and Purkinje cells in mammals. J. Comp. Neurol. 487:300–311, 2005.

Efferent Connections of the Cerebellum of the Goldfish, Carassius auratus

Efferent fiber connections of the corpus and valvula cerebelli in the goldfish, Carassius auratus, were studied using an anterograde neural fiber tracing technique. Efferent targets of the corpus cerebelli are the posterior parvocellular preoptic nucleus, the ventromedial and ventrolateral thalamic nucleus, dorsal posterior thalamic nucleus, periventricular nucleus of posterior tuberculum, dorsal periventricular pretectal nucleus, inferior lobe, optic tectum, torus semicircularis, nucleus of the medial longitudinal fascicle, nucleus ruber, dorsal tegmental nucleus, nucleus lateralis valvulae, reticular formation, torus longitudinalis, and the medial and lateral lobe of the valvula cerebelli. Projections to the posterior parvocellular preoptic nucleus and the periventricular nucleus of posterior tuberculum are not reported in previous studies. Efferent targets of the medial lobe of the valvula cerebelli are similar to that of the corpus cerebelli except for lacking a projection to the inferior lobe and torus longitudinalis, but showing one to the corpus cerebelli. On the other hand, the lateral lobe of the valvula cerebelli projects only to the dorsal zone of the periventricular hypothalamus, the diffuse nucleus of the inferior lobe, corpus mamillare, vagal lobe and the corpus cerebelli. There are topographical projections from the lateral valvula to the inferior lobe. These results suggest that the function of the corpus and medial lobe of the valvula cerebelli include not only motor control but also functions similar to the mammalian higher cerebellum. This study also suggests that there are obvious functional divisions between the medial and lateral lobes of the valvula cerebelli.

Axon Projections of Reciprocal Inhibitory Interneurons in the Spinal Cord of Young Xenopus Tadpoles and Implications for the Pattern of Inhibition During Swimming and Struggling

We have examined the morphology and longitudinal axon projections of a population of spinal commissural interneurons in young Xenopus tadpoles. We aimed to define how the distribution of axons of the whole population constrains the longitudinal distribution of the inhibition they mediate. Forty‐three neurons at different positions were filled intracellularly with biocytin and processed with avidin‐conjugated horseradish peroxidase. Soma size did not vary longitudinally and only one ipsilateral axon was found. Contralateral axons ascended, descended, or usually branched to do both. Total axon length and the extent of dendritic arborisation decreased caudally. The distributions of ascending and descending axon lengths were different; there were more long ascending (mean 737 ± standard deviation 365 μm) than long descending (447 ± 431 μm) axons. We used the axon length distribution data with existing data on the distribution of commissural interneuron somata to calculate the overall longitudinal density of these inhibitory axons. Axon numbers showed a clear rostrocaudal gradient. Axon length distributions were then incorporated into a simple spatiotemporal model of the forms of inhibition during swimming and struggling motor patterns. The model predicts that the peak of inhibition on each cycle will decrease from head to tail in both motor patterns, a feature already confirmed physiologically for swimming. It also supports a previous proposal that ascending inhibition during struggling shortens cycle period by shortening rostral motor bursts, whereas descending inhibition could delay subsequent burst onset. J. Comp. Neurol. 400:504–518, 1998.