Partial cloning and expression of mRNA coding choline acetyltransferase in the spinal cord of the goldfish, Carassius auratus

Neuroanatomical organization of the cholinergic system in the central nervous system of a basal actinopterygian fish, the senegal bichir Polypterus senegalus

Polypterid bony fishes are believed to be basal to other living ray-finned fishes, and their brain organization is therefore critical in providing information as to primitive neural characters that existed in the earliest ray-finned fishes. The cholinergic system has been characterized in more advanced ray-finned fishes, but not in polypterids. In order to establish which cholinergic neural centers characterized the earliest ray-finned fishes, the distribution of choline acetyltransferase (ChAT) is described in Polypterus and compared with the distribution of this molecule in other ray-finned fishes. Cell groups immunoreactive for ChAT were observed in the hypothalamus, the habenula, the optic tectum, the isthmus, the cranial motor nuclei, and the spinal motor column. Cholinergic fibers were observed in both the telencephalic pallium and the subpallium, in the thalamus and pretectum, in the optic tectum and torus semicircularis, in the mesencephalic tegmentum, in the cerebellar crest, in the solitary nucleus, and in the dorsal column nuclei. Comparison of the data within a segmental neuromeric context indicates that the cholinergic system in polypterid fishes is generally similar to that in other ray-finned fishes, but cholinergic-positive neurons in the pallium and subpallium, and in the thalamus and cerebellum, of teleosts appear to have evolved following the separation of polypterids and other ray-finned fishes.

The Met receptor tyrosine kinase prevents zebrafish primary motoneurons from expressing an incorrect neurotransmitter

BACKGROUND

Expression of correct neurotransmitters is crucial for normal nervous system function. How neurotransmitter expression is regulated is not well-understood; however, previous studies provide evidence that both environmental signals and intrinsic differentiation programs are involved. One environmental signal known to regulate neurotransmitter expression in vertebrate motoneurons is Hepatocyte growth factor, which acts through the Met receptor tyrosine kinase and also affects other aspects of motoneuron differentiation, including axonal extension. Here we test the role of Met in development of motoneurons in embryonic zebrafish.

RESULTS

We found that met is expressed in all early developing, individually identified primary motoneurons and in at least some later developing secondary motoneurons. We used morpholino antisense oligonucleotides to knock down Met function and found that Met has distinct roles in primary and secondary motoneurons. Most secondary motoneurons were absent from met morpholino-injected embryos, suggesting that Met is required for their formation. We used chemical inhibitors to test several downstream pathways activated by Met and found that secondary motoneuron development may depend on the p38 and/or Akt pathways. In contrast, primary motoneurons were present in met morpholino-injected embryos. However, a significant fraction of them had truncated axons. Surprisingly, some CaPs in met morpholino antisense oligonucleotide (MO)-injected embryos developed a hybrid morphology in which they had both a peripheral axon innervating muscle and an interneuron-like axon within the spinal cord. In addition, in met MO-injected embryos primary motoneurons co-expressed mRNA encoding Choline acetyltransferase, the synthetic enzyme for their normal neurotransmitter, acetylcholine, and mRNA encoding Glutamate decarboxylase 1, the synthetic enzyme for GABA, a neurotransmitter never normally found in these motoneurons, but found in several types of interneurons. Our inhibitor studies suggest that Met function in primary motoneurons may be mediated through the MEK1/2 pathway.

CONCLUSION

We provide evidence that Met is necessary for normal development of zebrafish primary and secondary motoneurons. Despite their many similarities, our results show that these two motoneuron subtypes have different requirements for Met function during development, and raise the possibility that Met may act through different intracellular signaling cascades in primary and secondary motoneurons. Surprisingly, although met is not expressed in primary motoneurons until many hours after they have extended axons to and innervated their muscle targets, Met knockdown causes some of these cells to develop a hybrid phenotype in which they co-expressed motoneuron and interneuron neurotransmitters and have both peripheral and central axons.

Study of the spinal cords of the sturgeonAcipenser schrenckii, garLepisosteus oculatus, and goldfishCarassius auratus by morphological, immunohistochemical, and biochemical approaches

Little is known about the spinal cords of phylogenetically ancient actinopterygeans. The spinal cords of the chondrostean Acipenser schrenckii (Amur sturgeon), holostean Lepisosteus oculatus (spotted gar), and teleost Carassius auratus (goldfish) were, therefore, analyzed by immunohistochemistry, electron microscopy and two-dimensional gel electrophoresis. Morphology showed numerous similarities between sturgeons and gars. In both, a dorsal column between the two dorsal horns was lacking, giving the grey matter an inverted Y-shape. In goldfish, a small dorsal column was seen, the grey matter occupied a larger area, neuronal density was much higher, and a ventral commissure was apparent, which was absent in sturgeons and gars. In the white matter of sturgeons and gars, small caliber axons predominated, whereas larger axons were frequent in goldfish. Choline acetyltransferase immunoreactive neurons were prevalent in the ventral horns of all three fish, mainly in motoneurons, but stained fibers were only found in sturgeons and gars. gamma-aminobutyric acid positive cells were seen in both the ventral and the dorsal horns of all three fish. Distribution of serotonin (5-HT) and tyrosine hydroxylase (TH) immunoreaction was similar in sturgeons and gars, being located in both the ventral and the dorsal horns. In goldfish, 5-HT label was confined to the ventral horn and TH label was mainly observed in a cell group located ventromedially. Two-dimensional gel electrophoresis showed a gradual increase in protein number from sturgeons to gars to goldfish. In conclusion, the spinal cords of sturgeons and gars share many morphological and chemical features, distinguishing them from the goldfish spinal cord.

Central Mechanisms Underlying Fish Swimming

Although the basic swimming rhythm is created by central pattern generators (CPGs) located in each spinal segment, command signals from the brain should be indispensable for the activation of CPGs to initiate swimming. We hypothesized that the nucleus of medial longitudinal fascicles (Nflm) is the midbrain locomotor region driving swimming rhythms in teleosts. To test this hypothesis, we recorded neuronal activities from Nflm neurons in swimming carp and analyzed the cytoarchitecture of the nucleus. We identified two types of Nflm neurons exhibiting electric activities closely related to swimming rhythms. Remarkably, tonic neurons that continued firing during swimming were found. The Nflm and neighboring oculomotor nucleus contain about 600 neurons in total, and among them as many as 500 were labeled retrogradely by an intraspinal tracer implantation and 400 neurons showed glutamatergic immunoreactivity. They are the most likely candidates for the descending neurons as the origin of driving signals that initiate swimming. Double-labeling experiments demonstrated direct connections of Nflm neurons to spinal neurons consisting of the CPG. These data imply that most Nflm neurons possibly exert an excitatory drive to the spinal CPGs through the descending axons with excitatory transmitter(s), probably glutamate. Furthermore, we confirmed that the caudal part of Nflm and the rostral part of the oculomotor nucleus overlap rostrocaudally by approximately 200 mum. In connection with the control of swimming by the brain, we carried out experiments to clarify the efferent system of the cerebellum of the goldfish. Cerebellar efferent fibers terminated in most brain regions except for the telencephalon. Importantly, the cerebellum projected also to the Nflm, suggesting the involvement of this brain region in the control of swimming. We have also determined that in the carp so-called eurydendroid cells are cerebellar efferent neurons.