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Joseph R Fetcho

Publications and source records attributed to Joseph R Fetcho.

15 recordsLinked to original sources
PubMed2007

The utility of zebrafish for studies of the comparative biology of motor systems.

Although zebrafish are best known as a model for studies of development, there is now a growing role for the model in studies of the functional organization of the nervous system, including studies of a variety of sensory systems, central processing, and motor output. The zebrafish has much to offer for such work because of the unique combination of genetics, optical methods, and physiology it allows. Here I illustrate, using three examples, the broad range of avenues along which zebrafish can inform us about motor systems. The examples include efforts to understand the functional organization and evolution of spinal interneurons, the role of mutants in informing us about motor dysfunction and human disease, and the ability to use the special features of zebrafish to explore strategies to restore function after injury. The most important aspects of these studies are evident only when they are placed in a comparative context, so they serve to highlight the power of zebrafish in studies of the comparative biology of motor control.

Animals↗
PubMed2007

Grading movement strength by changes in firing intensity versus recruitment of spinal interneurons.

Animals can produce movements of widely varying speed and strength by changing the recruitment of motoneurons according to the well-known size principle. Much less is known about patterns of recruitment in the spinal interneurons that control motoneurons because of the difficulties of monitoring activity simultaneously in multiple interneurons of an identified class. Here we use electrophysiology in combination with in vivo calcium imaging of groups of identified excitatory spinal interneurons in larval zebrafish to explore how they are recruited during different forms of the escape response that fish use to avoid predators. Our evidence indicates that escape movements are graded largely by differences in the level of activity within an active pool of interneurons rather than by the recruitment of an inactive subset.

Action Potentials↗
PubMed2006

A genetic screen identifies genes essential for development of myelinated axons in zebrafish.

The myelin sheath insulates axons in the vertebrate nervous system, allowing rapid propagation of action potentials via saltatory conduction. Specialized glial cells, termed Schwann cells in the PNS and oligodendrocytes in the CNS, wrap axons to form myelin, a compacted, multilayered sheath comprising specific proteins and lipids. Disruption of myelinated axons causes human diseases, including multiple sclerosis and Charcot-Marie-Tooth peripheral neuropathies. Despite the progress in identifying human disease genes and other mutations disrupting glial development and myelination, many important unanswered questions remain about the mechanisms that coordinate the development of myelinated axons. To address these questions, we began a genetic dissection of myelination in zebrafish. Here we report a genetic screen that identified 13 mutations, which define 10 genes, disrupting the development of myelinated axons. We present the initial characterization of seven of these mutations, defining six different genes, along with additional characterization of mutations that we have described previously. The different mutations affect the PNS, the CNS, or both, and phenotypic analyses indicate that the genes affect a wide range of steps in glial development, from fate specification through terminal differentiation. The analysis of these mutations will advance our understanding of myelination, and the mutants will serve as models of human diseases of myelin.

Animals↗
PubMed2005

Early development of functional spatial maps in the zebrafish olfactory bulb.

In the adult olfactory bulb (OB), particular chemical classes of odorants preferentially activate glomeruli within loosely defined regions, resulting in a coarse and fractured "chemotopic" map. In zebrafish, amino acids and bile acids predominantly stimulate glomeruli in the lateral and medial OB, respectively. We studied the development of these spatial response maps in zebrafish. At 3 d postfertilization (dpf), the OB contained protoglomerular structures that became refined and more numerous during subsequent days. In a transgenic zebrafish line expressing the Ca2+ indicator protein inverse pericam, mainly in mitral cells, odor responses in the OB were first detected at 2.5-3 dpf. Already at this stage, amino acids and bile acids evoked activity predominantly in the lateral and medial OB, respectively. Two-photon Ca2+ imaging using a synthetic indicator was used to reconstruct activity patterns at higher resolution in three dimensions. Responses to amino acids and bile acids were detected predominantly in the lateral and medial OB, respectively, with little overlap. Between 2.5 and 6 dpf, the number of odor-responsive units increased, but the overall spatial organization of activity persisted. Hence, a coarse spatial organization of functional activity maps is established very early during OB development when glomeruli are not yet differentiated. This spatial organization is maintained during development and growth of neuronal circuits and may have important functions for odor processing in larvae, for the differentiation of glomeruli, and for the refinement of activity maps at later developmental stages.

Animals↗
PubMed2005

Fictive swimming motor patterns in wild type and mutant larval zebrafish.

Larval zebrafish provide a unique model for investigating the mechanisms involved in generating rhythmic patterns of behavior, such as swimming, due to the array of techniques available including genetics, optical imaging, and conventional electrophysiology. Because electrophysiological and imaging studies of rhythmic motor behaviors in paralyzed preparations depend on the ability to monitor the central motor pattern, we developed a fictive preparation in which the activity of axial motor neurons was monitored using extracellular recordings from peripheral nerves. We examined spontaneous and light induced fictive motor patterns in wild type and mutant larval zebrafish (4-6 days post-fertilization) paralyzed with curare. All spontaneous and light-induced preparations produced alternation of motor activity from side-to-side (mean contralateral phase = 50.7 +/- 7.0%; mean burst frequency = 35.6 +/- 4.7 Hz) and a progression of activity from head-to-tail (mean ipsilateral rostrocaudal delay = 0.8 +/- 0.5 ms per segment), consistent with lateral undulation and forward propulsion during swimming, respectively. The basic properties of the motor pattern were similar in spontaneous and light-induced swimming. This fictive preparation can be used in combination with conventional electrophysiological and imaging methods to investigate normal circuit function as well as to elucidate functional deficits in mutant lines. Toward this end, we show that two accordion class mutants, accordion and bandoneon, have alternating activity on opposite sides of the body, contradicting the hypothesis that their deficit results from the absence of the reciprocal glycinergic inhibition that is typically found in the spinal cord of swimming vertebrates.

Action Potentials↗
PubMed2004

Distribution of prospective glutamatergic, glycinergic, and GABAergic neurons in embryonic and larval zebrafish.

Zebrafish are an excellent model for studies of the functional organization of neuronal circuits, but little is known regarding the transmitter phenotypes of the neurons in their nervous system. We examined the distribution in spinal cord and hindbrain of neurons expressing markers of transmitter phenotype, including the vesicular glutamate transporter (VGLUT) genes for glutamatergic neurons, the neuronal glycine transporter (GLYT2) for glycinergic neurons, and glutamic acid decarboxylase (GAD65/67) for GABAergic neurons. All three markers were expressed in a large domain in the dorsal two-thirds of spinal cord, with additional, more ventral expression domains for VGLUT2 and GAD/GABA. In the large dorsal domain, dual in situ staining showed that GLYT2-positive cells were intermingled with VGLUT2 cells, with no dual-stained neurons. Many of the neurons in the dorsal expression domain that were positive for GABA markers at embryonic stages were also positive for GLYT2, suggesting that the cells might use both GABA and glycine, at least early in their development. The intermingling of neurons expressing inhibitory and excitatory markers in spinal cord contrasted markedly with the organization in hindbrain, where neurons expressing a particular marker were clustered together to form stripes that were visible running from rostral to caudal in horizontal sections and from dorsomedial to ventrolateral in cross sections. Dual labeling showed that the stripes of neurons labeled with one transmitter marker alternated with stripes of cells labeled for the other transmitter phenotypes. The differences in the distribution of excitatory and inhibitory neurons in spinal cord versus hindbrain may be tied to differences in their patterns of development and functional organization.

Amino Acid Transport Systems, Neutral↗
PubMed2004

Neurotransmitter properties of spinal interneurons in embryonic and larval zebrafish.

Many classes of spinal interneurons in zebrafish have been described based on morphology, but their transmitter phenotypes are largely unknown. Here we combine back-filling or genetic labeling of spinal interneurons with in situ staining for markers of neurotransmitter phenotypes, including the vesicular glutamate transporter (VGLUT) genes for glutamatergic neurons, the neuronal glycine transporter (GLYT2) for glycinergic neurons, and glutamic acid decarboxylase (GAD) for GABAergic neurons. Neurons positive for VGLUT include the commissural CoPA, MCoD, UCoD, and some of the CoSA neurons. The CiD interneurons, which have ipsilateral descending axons, were also VGLUT-positive, as were the ventrally located VeMe interneurons, whose descending axonal trajectory has not been clearly revealed. Cells positive for GLYT2 include the commissural CoLAs as well as some of the CoBL and CoSA neurons. The CiA cells were the only GLYT2-positive cells with an ipsilateral axon. Cells staining for GAD included, most notably, the dorsal longitudinal ascending (DoLA) and KA interneurons. Our approach allowed us to define the likely transmitter phenotypes of most of the known classes of spinal interneurons. These data provide a foundation for understanding the functional organization of the spinal networks in zebrafish.

Amino Acid Transport Systems, Neutral↗
PubMed2004

Ontogeny and innervation patterns of dopaminergic, noradrenergic, and serotonergic neurons in larval zebrafish.

We report the development of aminergic neurons from 0-10 days postfertilization (dpf) in zebrafish (Danio rerio). This study was prompted by the lack of information regarding patterns of spinal aminergic innervation at early stages, when the fish are accessible to optical, genetic, and electrophysiological approaches toward understanding neural circuit function. Our findings suggest that aminergic populations with descending processes are among the first to appear during development. Descending aminergic fibers, revealed by antibodies to tyrosine hydroxylase (TH) and serotonin (5-hydroxytryptamine; 5-HT), innervate primarily the ventral (TH, 5-HT), but also the dorsal (5-HT) aspects of the spinal cord by 4 dpf, with the extent of innervation not changing markedly up to 10 dpf. By tracking the spatiotemporal expression of TH, 5-HT, and dopamine beta hydroxylase reactivity, we determined that these fibers likely originate from neurons in the posterior tuberculum (dopamine), the raphe region (5-HT) and, possibly, the locus coeruleus (noradrenaline). In addition, spinal neurons positive for 5-HT emerge between 1-2 dpf, with processes that appeared to descend along the ventrolateral cord for only 1-2 muscle segments. Their overall morphology distinguished these cells from previously described "VeMe" (ventromedial) interneurons, which are also located ventromedially, but have long, multisegmental descending processes. We confirmed the distinction between spinal serotonergic and VeMe interneurons using fish genetically labeled with green fluorescent protein. Our results suggest that the major aminergic systems described in adults are in place shortly after hatching, at a time when zebrafish are accessible to a battery of techniques to test neuronal function during behavior.

Aging↗
PubMed2004

Relationship of tyrosine hydroxylase and serotonin immunoreactivity to sensorimotor circuitry in larval zebrafish.

Our previous study tracked the ontogeny of aminergic systems in zebrafish (Danio rerio). Here we use tyrosine hydroxylase (TH) and serotonin (5-hydroxytryptamine; 5-HT) immunoreactivity, in conjunction with retrograde and genetic labeling techniques, to provide a more refined examination of the potential synaptic contacts of aminergic systems. Our focus was on different levels of the sensorimotor circuit for escape, from sensory inputs, through identified descending pathways, to motor output. We observed 5-HT reactivity in close proximity to the collaterals of the Rohon-Beard sensory neurons in spinal cord. In the brainstem we found TH and 5-HT reactivity closely apposed to the dendritic processes of the nucleus of the medial longitudinal fascicle (nMLF), in addition to the ventral dendrites of the Mauthner neuron and its serial homologs MiD2cm and MiD3cm. Only TH reactivity was observed near the lateral dendrites of the Mauthner cell. TH and 5-HT reactivity were also positioned near the outputs of reticulospinal cells in spinal cord. Finally, both TH and 5-HT reactivity were detected close to the dendritic processes of primary and secondary spinal motor neurons. We also confirmed, using dual TH and 5-HT staining and retrograde labeling, that the sources of spinal aminergic reactivity include the posterior tuberculum (dopamine) and inferior raphe region (5-HT). Our data indicate that aminergic systems may interact at all levels of the sensorimotor pathways involved in escape. The identification of some of these likely sites of aminergic action will allow for directed studies of their functional roles using the powerful combination of techniques available in zebrafish.

Afferent Pathways↗
PubMed2004

Cyclic AMP-induced repair of zebrafish spinal circuits.

Neurons in the human central nervous system (CNS) are unable to regenerate, as a result of both an inhibitory environment and their inherent inability to regrow. In contrast, the CNS environment in fish is permissive for growth, yet some neurons still cannot regenerate. Fish thus offer an opportunity to study molecules that might surmount the intrinsic limitations they share with mammals, without the complication of an inhibitory environment. We show by in vivo imaging in zebrafish that post-injury application of cyclic adenosine monophosphate can transform severed CNS neurons into ones that regenerate and restore function, thus overcoming intrinsic limitations to regeneration in a vertebrate.

Animals↗
PubMed2004

Engrailed-1 expression marks a primitive class of inhibitory spinal interneuron.

Studies in chicks and mice have suggested that transcription factors mark functional subtypes of interneurons in the developing spinal cord. We used genetic, morphological, and physiological studies to test this proposed association in zebrafish. We found that Engrailed-1 expression uniquely marks a class of ascending interneurons, called circumferential ascending (CiA) interneurons, with ipsilateral axonal projections in both motor and sensory regions of spinal cord. These cells express the glycine transporter 2 gene and are the only known ipsilateral interneurons positive for this marker of inhibitory transmission. Patch recordings show that the CiA neurons are rhythmically active during swimming. Pairwise recordings from the CiA interneurons and postsynaptic cells reveal that the Engrailed-1 neurons produce monosynaptic, strychnine-sensitive inhibition of dorsal sensory interneurons and also inhibit more ventral neurons, including motoneurons and descending interneurons. We conclude that Engrailed-1 expression marks a class of inhibitory interneuron that seems to provide all of the ipsilateral glycinergic inhibition in the spinal cord of embryonic and larval fish. Individual Engrailed-1-positive cells are multifunctional, playing roles in both sensory gating and motor pattern generation. This primitive cell type may have given rise to several, more specialized glycinergic inhibitory interneurons in birds and mammals. Our data support the view that the subdivision of spinal cord into different regions by transcription factors defines a primitive functional organization of spinal interneurons that formed a developmental and evolutionary foundation on which more complex systems were built.

Amino Acid Transport Systems, Neutral↗
PubMed2004

Genes and photons: new avenues into the neuronal basis of behavior.

A convergence of advances in optical methods and a better understanding of the genetics of development promise to revolutionize the study of neuronal circuits and their links to behavior. One of the great challenges in systems neurobiology has been to monitor and perturb activity in populations of identified neurons in vivo. Recent work has begun to achieve this goal through a combination of modern imaging methods with genetic labeling and perturbation.

Animals↗
PubMed2003

Mutations in deadly seven/notch1a reveal developmental plasticity in the escape response circuit.

The relatively simple neural circuit driving the escape response in zebrafish offers an excellent opportunity to study properties of neural circuit formation. The hindbrain Mauthner cell is an essential component of this circuit. Mutations in the zebrafish deadly seven/notch1a (des) gene result in supernumerary Mauthner cells. We addressed whether and how these extra cells are incorporated into the escape-response circuit. Calcium imaging revealed that all Mauthner cells in desb420 mutants were active during an elicited escape response. However, the kinematic performance of the escape response in mutant larvae was very similar to wild-type fish. Analysis of the relationship between Mauthner axon collaterals and spinal neurons revealed that there was a decrease in the number of axon collaterals per Mauthner axon in mutant larvae compared with wild-type larvae, indicative of a decrease in the number of synapses formed with target spinal neurons. Moreover, we show that Mauthner axons projecting on the same side of the nervous system have primarily nonoverlapping collaterals. These data support the hypothesis that excess Mauthner cells are incorporated into the escape-response circuit, but they divide their target territory to maintain a normal response, thus demonstrating plasticity in the formation of the escape-response circuit. Such plasticity may be key to the evolution of the startle responses in mammals, which use larger populations of neurons in circuits similar to those in the fish escape response.

Animals↗
PubMed2003

Imaging neuronal activity during zebrafish behavior with a genetically encoded calcium indicator.

Genetically encoded calcium indicators, such as cameleon, have offered the promise of noninvasively monitoring activity of neurons, but no one has demonstrated whether these indicators can report calcium transients in neurons of behaving vertebrates. We show that cameleon can be expressed at high levels in sensory and spinal cord neurons in zebrafish by using neural-specific promoters in both transient expression experiments and in a stable transgenic line. Using standard confocal microscopy, calcium transients in identified motoneurons and spinal interneurons could be detected during escape behaviors produced by a touch on the head of the fish. Small movements of the restrained fish during the behavior did not represent a major problem for analyzing the calcium responses because of the ratiometric nature of cameleon. We conclude that cameleon can be used to noninvasively study the activity of neurons in an intact, behaving vertebrate. The ability to introduce an indicator genetically allows for studies of the functional roles of local interneurons that cannot easily be monitored with other approaches. Transgenic lines such as the one we generated can also be crossed into mutant lines of fish to study both structural and functional consequences of the mutations.

Animals↗