285
Functional Neurobiology in Xenopus
cranial nerves (Lambert et al., 2008), to calcium-imaging of
identifed neurons (Gensberger et al., 2016). In the case of
neuronal activity during self-motion, appropriate functional
proxies include motion of the eyes and tail, which report
on the extent and pattern of sensorimotor transformations
(Lambert et al., 2020). The kinematics of eye motion can be
captured non-invasively (Figure 19.2D ) in such preparations
with infrared video-recordings. Extracellular recordings of
extraocular motor nerves (Figure 19.2E), calcium-imaging of corresponding neurons (Figure 19.2F), and motion
recording of tail oscillations (Figure 19.2G) contributed
toward the understanding of the spatio-temporal processing of self-motion. A particular advantage of in vitro methods in Xenopus is in the ability to acquire neuronal activity
during fctive behaviors, such as swimming (Figure 19.2I).
Fictive in vitro behaviors, which can be elicited experimentally at all developmental stages (Combes et al., 2004),
allow simultaneous recordings of the activity carried by, for
example, spinal or extraocular motor nerves (Figure 19.2G)
in the absence of muscle contractions.
In addition to discoveries of computational features of
cells and circuits (Gensberger et al., 2016; Dietrich et al.,
2017 ), in vitro approaches in Xenopus are utilized in studies with a focus on translative neurobiological concepts
with direct relevance for human pathologies (I Gusti Bagus
et al., 2019; Soupiadou et al., 2020). Such studies often make
use of targeted manipulations, either in the embryo (Gordy
et al., 2018) or tadpole (Lambert et al., 2009), and are followed by functional profling to gauge neuronal response
mechanisms, which are often the result of conserved plasticity processes. Embryonic addition of ectopic inner ears, for
example, challenged the classical understanding of developmental processes by incorporating additional sensory
signals from a novel source and served to identify features
which drive circuit formation in the brainstem (Gordy et al.,
2018). Such studies have therapeutic consequences for aging
populations or those with inner ear impairments. Similar
perturbations of inner ear sensory organs, such as targeted
lesions of the eighth cranial nerve or the entire ensemble
of inner ear endorgans in Xenopus tadpoles, revealed an
adaptive plasticity with considerable developmental consequences, which recapitulate features commonly observed in
patients with vestibular impairments (Branoner et al., 2016;
Lambert et al., 2009; Soupiadou et al., 2020). More clinically relevant approaches, such as drug application (I Gusti
Bagus et al., 2019), imitated biological principles resulting
from therapeutic targeting. Furthermore, in vitro Xenopus
preparations have helped to identify cellular substrates and
response patterns of neurons following galvanic vestibular
stimulation, a common diagnostic tool used in the clinic
(Gensberger et al., 2016). This latter fnding is particularly
relevant given the need to reclassify the outcome of clinical
tests with respect to the interpretation of underlying vestibular pathologies.
In vitro methodologies in Xenopus laevis offer additional
benefts beyond insights into functional neurobiology. In
vivo studies usually require institutional animal protocol
approvals for the use of Xenopus older than stage 46, a
developmental time point at which the intracellular yolk has
been consumed and self-feeding commences (Nieuwkoop
and Faber, 1994). In contrast, euthanasia prior to the isolation of the tissue to obtain an in vitro preparation renders the
legal requirements and compliance documentation considerably simpler and usually puts such preparations at the same
legal level as the generation of mammalian brain slice preparations. This is useful for researchers opting to study the
nervous system in older tadpoles and adults. In addition, longitudinal studies are approachable when using Xenopus in
vitro preparations, given the period of days that they remain
functionally viable in Ringer solution (Lambert et al., 2008).
Collectively, in vitro approaches employing isolated brain/
body Xenopus laevis preparations at any developmental
stage can be used to study neurobiological questions and
offer a wide variety of methods which can supplement in
vivo approaches. This approach is therefore benef cial for
a considerably larger number of members of the Xenopus
scientifc community. The experimental and administrative
simplicity renders normally invasive technical approaches
readily feasible and brings scientifc questions that are only
addressable in developmentally advanced stages into reach
for meaningful answers.
19.5. FUTURE DIRECTIONS
Xenopus as a model system has considerably advanced our
understanding of biological principles of the nervous system. As technical and theoretical innovations have ushered
in an era of neurobiological research with declining numbers of model systems, Xenopus continues to advance in
stride. This is largely due to its suitability for many f elds
of research rather than for a single, specialized application.
The comprehensive toolkit available in these animals, which
includes tractable genetic and molecular manipulations as
well as analytical techniques, allows a sizable depth of
neuronal assessments. Innovative next steps in neuroscientifc discoveries will continue to capitalize on these existing methods, particularly those which inf uence functional
characteristics in both developing and mature neuronal circuits. These approaches offer valuable insight into def ned
neuronal disease phenotypes, such as those which manifest
with computational impairments. Such cases often arise as
the result of congenital miswiring, after CNS injury, or due
to age-related deteriorations. The latter is highly relevant
in modern neurobiological practice given the increase in
age-related impairments in elderly populations. Continued
manipulations in this manner will advance our current understanding of basic biological principles of the nervous system
both during states of health and disease. More provocatively,
the use of Xenopus offers possible exploration into avenues
which are only just emerging. Neurobiological implications
of spacefight, a steadily evolving enterprise, are of particular interest due to the infuence of microgravity on sensory
signal processing and the retention of navigational skills.
Such research could beneft from Xenopus due to the variety
Functional Neurobiology in Xenopus
cranial nerves (Lambert et al., 2008), to calcium-imaging of
identifed neurons (Gensberger et al., 2016). In the case of
neuronal activity during self-motion, appropriate functional
proxies include motion of the eyes and tail, which report
on the extent and pattern of sensorimotor transformations
(Lambert et al., 2020). The kinematics of eye motion can be
captured non-invasively (Figure 19.2D ) in such preparations
with infrared video-recordings. Extracellular recordings of
extraocular motor nerves (Figure 19.2E), calcium-imaging of corresponding neurons (Figure 19.2F), and motion
recording of tail oscillations (Figure 19.2G) contributed
toward the understanding of the spatio-temporal processing of self-motion. A particular advantage of in vitro methods in Xenopus is in the ability to acquire neuronal activity
during fctive behaviors, such as swimming (Figure 19.2I).
Fictive in vitro behaviors, which can be elicited experimentally at all developmental stages (Combes et al., 2004),
allow simultaneous recordings of the activity carried by, for
example, spinal or extraocular motor nerves (Figure 19.2G)
in the absence of muscle contractions.
In addition to discoveries of computational features of
cells and circuits (Gensberger et al., 2016; Dietrich et al.,
2017 ), in vitro approaches in Xenopus are utilized in studies with a focus on translative neurobiological concepts
with direct relevance for human pathologies (I Gusti Bagus
et al., 2019; Soupiadou et al., 2020). Such studies often make
use of targeted manipulations, either in the embryo (Gordy
et al., 2018) or tadpole (Lambert et al., 2009), and are followed by functional profling to gauge neuronal response
mechanisms, which are often the result of conserved plasticity processes. Embryonic addition of ectopic inner ears, for
example, challenged the classical understanding of developmental processes by incorporating additional sensory
signals from a novel source and served to identify features
which drive circuit formation in the brainstem (Gordy et al.,
2018). Such studies have therapeutic consequences for aging
populations or those with inner ear impairments. Similar
perturbations of inner ear sensory organs, such as targeted
lesions of the eighth cranial nerve or the entire ensemble
of inner ear endorgans in Xenopus tadpoles, revealed an
adaptive plasticity with considerable developmental consequences, which recapitulate features commonly observed in
patients with vestibular impairments (Branoner et al., 2016;
Lambert et al., 2009; Soupiadou et al., 2020). More clinically relevant approaches, such as drug application (I Gusti
Bagus et al., 2019), imitated biological principles resulting
from therapeutic targeting. Furthermore, in vitro Xenopus
preparations have helped to identify cellular substrates and
response patterns of neurons following galvanic vestibular
stimulation, a common diagnostic tool used in the clinic
(Gensberger et al., 2016). This latter fnding is particularly
relevant given the need to reclassify the outcome of clinical
tests with respect to the interpretation of underlying vestibular pathologies.
In vitro methodologies in Xenopus laevis offer additional
benefts beyond insights into functional neurobiology. In
vivo studies usually require institutional animal protocol
approvals for the use of Xenopus older than stage 46, a
developmental time point at which the intracellular yolk has
been consumed and self-feeding commences (Nieuwkoop
and Faber, 1994). In contrast, euthanasia prior to the isolation of the tissue to obtain an in vitro preparation renders the
legal requirements and compliance documentation considerably simpler and usually puts such preparations at the same
legal level as the generation of mammalian brain slice preparations. This is useful for researchers opting to study the
nervous system in older tadpoles and adults. In addition, longitudinal studies are approachable when using Xenopus in
vitro preparations, given the period of days that they remain
functionally viable in Ringer solution (Lambert et al., 2008).
Collectively, in vitro approaches employing isolated brain/
body Xenopus laevis preparations at any developmental
stage can be used to study neurobiological questions and
offer a wide variety of methods which can supplement in
vivo approaches. This approach is therefore benef cial for
a considerably larger number of members of the Xenopus
scientifc community. The experimental and administrative
simplicity renders normally invasive technical approaches
readily feasible and brings scientifc questions that are only
addressable in developmentally advanced stages into reach
for meaningful answers.
19.5. FUTURE DIRECTIONS
Xenopus as a model system has considerably advanced our
understanding of biological principles of the nervous system. As technical and theoretical innovations have ushered
in an era of neurobiological research with declining numbers of model systems, Xenopus continues to advance in
stride. This is largely due to its suitability for many f elds
of research rather than for a single, specialized application.
The comprehensive toolkit available in these animals, which
includes tractable genetic and molecular manipulations as
well as analytical techniques, allows a sizable depth of
neuronal assessments. Innovative next steps in neuroscientifc discoveries will continue to capitalize on these existing methods, particularly those which inf uence functional
characteristics in both developing and mature neuronal circuits. These approaches offer valuable insight into def ned
neuronal disease phenotypes, such as those which manifest
with computational impairments. Such cases often arise as
the result of congenital miswiring, after CNS injury, or due
to age-related deteriorations. The latter is highly relevant
in modern neurobiological practice given the increase in
age-related impairments in elderly populations. Continued
manipulations in this manner will advance our current understanding of basic biological principles of the nervous system
both during states of health and disease. More provocatively,
the use of Xenopus offers possible exploration into avenues
which are only just emerging. Neurobiological implications
of spacefight, a steadily evolving enterprise, are of particular interest due to the infuence of microgravity on sensory
signal processing and the retention of navigational skills.
Such research could beneft from Xenopus due to the variety