Abstract This study examines the effects of the selective serotonin reuptake inhibitor (SSRI), fluoxetine (PROZAC), on the ontogeny of spontaneous swimming activity (SSA) in developing zebrafish. The ...development of zebrafish motor behavior consists of four sequential locomotor patterns that develop over 1–5 days post fertilization (dpf), with the final pattern, SSA, established at 4–5 dpf. In stage specific experiments, larvae were exposed to 4.6 μM fluoxetine for 24 h periods beginning at 24 h post fertilization (hpf) and extending through 5 dpf. From 1–3 dpf, there was no effect on SSA or earlier stages of motor development, i.e., spontaneous coiling, evoked coiling and burst swimming. Fluoxetine exposure at 3 dpf for 24 h resulted in a transient decrease in SSA through 7 dpf with a complete recovery by 8 dpf. Larvae exposed to 4.6 μM fluoxetine for 24 h on 4 or 5 dpf showed a significant decrease in SSA by day 6 with no recovery through 14 dpf. Although SSA was significantly affected 24 h after fluoxetine exposure, there was little or no effect on pectoral fin movement. These results demonstrate both a stage specific and a long term effect of 4.6 μM fluoxetine exposure in 4 and 5 dpf larvae. Reverse transcriptase polymerase chain reaction (RT-PCR) was performed to determine the relative levels of a serotonin transporter protein (SERT) transcript and the serotonin 1A (5-HT1A ) receptor transcript in developing embryos/larvae over 1–6 dpf. Both transcripts were present at 24 hpf with the relative concentration of SERT transcript showing no change over the developmental time range. The relative concentration of the 5-HT1A receptor transcript, however, showed a two-tiered pattern of concentration. RT-PCR was also used to detect potential changes in the SERT and 5-HT1A receptor transcripts in 6 dpf larvae after a 24 h exposure to 4.6 μM fluoxetine on 5 dpf. Three separate regions of the CNS were individually analyzed, two defined brain regions and spinal cord. The two brain regions showed no effect on transcript levels subsequent to fluoxetine exposure, however, the spinal cord showed a significant decrease in both transcripts. These results suggest a correlation between decreased concentration of SERT and 5-HT1A receptor transcripts in spinal cord and decreased SSA subsequent to fluoxetine exposure.
Abstract In this study, p -chlorophenylalanine (pCPA), an inhibitor of tryptophan hydroxylase (the rate limiting enzyme of serotonin synthesis), was used to reduce serotonin (5HT) levels during early ...development in zebrafish embryos. One day old dechorionated embryos were treated with 25 μM pCPA for 24 h and subsequently rescued. Immunohistological studies using a 5HT antibody confirmed that 5HT neurons in the brain and spinal cord were depleted of transmitter by 2 days post fertilization (dpf). Twenty four hours after pCPA exposure embryos were unable to burst swim and were nearly paralyzed. Movement began to improve at 4 dpf, and by 7 dpf, larvae exhibited swimming activity. Rescued larvae continued to grow in rostrocaudal length over 5 days post-rescue, but their length was always 16–21% below controls. Surprisingly, both groups displayed the same number of myotomes. To examine whether hypertonicity of myotomes in treated embryos played a role in their shorter rostrocaudal lengths, 1 dpf embryos were exposed to a combination of 25 μM pCPA and 0.6 mM of the sodium channel blocker ethyl 3-aminobenzoate methanesulfonate (MS-222). After a 24 hour exposure, the embryos exhibited the same rostrocaudal length as control embryos suggesting that myotome hypertonicity plays a major role in the decreased axial length of the treated larvae. In addition, pCPA treated 2 dpf embryos exhibited abnormal notochordal morphology that persisted throughout recovery. Reverse transcriptase polymerase chain reaction (RT-PCR) was performed to determine the relative levels of the serotonin 1A receptor (5HT1A ) transcript and the serotonin transporter (SERT) transcript in the brain and spinal cord of control and treated embryos. Transcripts were present in both brain and spinal cord as early as 1 dpf and reached maximal concentrations by 3 dpf. Embryos treated with pCPA demonstrated a decrease in the concentration of 5HT1A transcript in both brain and spinal cord. While SERT transcript levels remained unaffected in brain, they were decreased in spinal cord. Five days subsequent to pCPA rescue, 5HT1A transcript concentrations remained decreased in brain while SERT transcript levels were elevated in both regions. These findings suggest that reduction of 5HT during early zebrafish development may have an adverse effect on body length, notochordal morphology, locomotor behavior, and serotonin message-related expression.
Light and electron microscopic degeneration studies were used to examine the telencephalotectal pathway in goldfish. Both techniques showed that each telencephalic lobe sent bilateral projections to ...several tectal laminae. Degenerating synaptic terminals and fibers were observed in the major retinal projection lamina as well as in other tectal laminae. The terminals contained round to oval synaptic vesicles, asymmetric synapses and contacted relatively small postsynaptic profiles.
Retinal terminal profiles in the goldfish optic tectum were identified electron microscopically after (1) labeling with horseradish peroxidase and (2) in the early stages of degeneration in ...short‐term eye enucleates. All labeled terminals shared certain common morphological characteristics which were identical to those of a population of terminals in normal tecta. Terminals of this type disappeared 30 days after enucleation of the contralateral eye. Retinal terminal presynaptic profiles were characterized by (1) round and oval synaptic vesicles; (2) mitochondria with irregular, randomly oriented cristae, large intracristal spaces, dilated membrane spaces, and primarily light matrices; (3) a wide range in profile area, 0.06–6.82 μm2; (4) large numbers of synaptic vesicles per profile area 168± 33 synaptic vesicles per μm2; (5) asymmetric synapses; and (6) multiple synaptic contacts (1.46 ± 0.73 per terminal profile). The postsynaptic elements included both dendritic and, less commonly, pleomorphic vesicle‐containing profiles. The majority of postsynaptic dendritic profiles were small (0.01–0.40 μm2). Serial synaptic contacts were occasionally seen. The combination of vesicular and mitochondrial morphology (1 and 2 above) was necessary and sufficient to establish the retinal origin of a terminal, but use of such criteria would underestimate the number of retinotectal terminals by omitting those which did not have a mitochondrion in the plane of section. The number of such terminals was calculated from independent measurements, and the total number of retinal terminal profiles per area of neuropil was estimated.
In this study, large areas of goldfish telencephalon were ablated including rostral nucleus preopticus periventriculare (rNPP), and degenerating axons were traced by a modified Fink and Heimer ...procedure. The lesioning procedure ablated large regions of area dorsalis telencephali pars medialis, centralis, and dorsolateral complex; and completely removed area ventralis telencephali pars dorsalis, ventralis, and lateralis. In addition, the supracommissural nucleus and rNPP were lesioned specifically because both nuclei have been thought to be involved in courtship behavior and endocrine control of reproduction. This investigation demonstrated extensive fiber projections from telencephalic nuclei and/or rNPP to the hypothalamus. Lesioned telencephalon and/or rNPP projected bilaterally to nucleus preopticus and the suprachiasmatic nucleus and unilaterally to the following tuberal nuclei: nucleus anterior tuberis, and the lateral hypothalamic nucleus. A much larger fiber projection to the inferior lobe nuclei was also observed with a large contralateral as well as ipsilateral input.
When one tectum of an adult goldfish is removed, the severed retinal fibers regenerate ipsilaterally into the remaining tectal lobe. Initially fibers from the two eyes overlap in the tectum but ...EM-HRP data suggest that few mature retinal synapses are formed between the ipsilateral eye and tectum at this time. At longer time periods, when some fibers appear to segregate into eye-specific termination bands, our data suggest that a significant number of synapses from the ipsilateral eye are present. These findings have important implications for how eye-specific termination bands are formed in doubly innervated tecta.
Light and electron microscopic degeneration studies were used to examine the telencephalotectal pathway in goldfish. Both techniques showed that each telencephalic lobe sent bilateral projections to ...several tectal laminae. Degenerating synaptic terminals and fibers were observed in the major retinal projection lamina as well as in other tectal laminae. The terminals contained round to oval synaptic vesicles, asymmetric synapses and contacted relatively small postsynaptic profiles.
This study examined the effect of optic tract ablation on retinotectal fiber regeneration in goldfish. Approximately two-thirds of the left optic tract was removed, and, at various times post lesion ...(10-75 days), the course of regenerating retinotectal fibers was traced using horseradish peroxidase. In all experimental animals, axons were observed regenerating through the visual pathway but at the brachia most of the fibers were channeled through the ventral brachium. We present evidence that fibers in the ventral brachium originated from ganglion cells in all regions of retina and that these fibers grew almost exclusively into ventral half tectum even though some of these fibers would normally synapse in dorsal half tectum. These observations suggest that optic tract ablation does not prevent retinal fiber regeneration but results in aberrant growth through the brachia and significant inhibition of exploratory fiber growth within the tectum.
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