282
Xenopus
animals of all age groups, including post-metamorphic froglets (Combes et al., 2004; De Vidts et al., 2019). This further
presents as a method to identify differences in kinematic
profles between Xenopus stages (Hänzi and Straka, 2017),
allowing inferences across developmental periods.
The computational ability of cells and neuronal circuits,
while often approximated by behavior, are more precisely
investigated by electrophysiological recordings ( Figure
19.1B 3 ). Xenopus species are suitable for such studies due to
the accessibility of the CNS. Patch-clamping and sharp electrode intracellular recordings can be performed at various
levels, depending on the feature of interest. The resolution
of patch-clamping (Figure 19.1B3) can infer ion channel current dynamics and synaptic properties and thus yield insight
into the integrative capacity of cell membranes (Engert et
al., 2002; Pratt and Aizenman, 2007). Extracellular singleor multi-unit recordings, while lacking this specif city, nonetheless approximate neuronal activity and are experimentally
more rapid, effectively increasing the number of recorded
cells. In Xenopus research, such techniques were extensively
used to characterize neuronal response characteristics in the
midbrain to a range of stimuli following mechanosensory
lateral line stimulation (Behrend et al., 2006) or to measure
receptive felds in the optic tectum (Gaze et al., 1963). On
a broader level, low-impedance electrodes record the f eld
potential produced by larger numbers of neurons, providing
the activity status across an entire group of neurons (Bibikov
and Elepfandt, 2005). Collectively, these methods are ideally suited for linking a particular behavior to the underlying
neuronal activity. For example, neuronal correlates of avoidance behavior in Xenopus were approximated by examining receptive felds of tectal neurons following visual scene
motion (Dong et al., 2009). After quantifying receptive f eld
size, a correlation of the strength of the avoidance response
with sharper receptive felds was discovered. Chemical and
physical manipulations of the tectum or training of the visual
system further explored the dependency of this behavior on
receptive feld properties. While certain electrode recordings
are still unparalleled in their level of detail on the electrical
signature of neurons (see also subsequently), non-invasive
alternatives, such as calcium-imaging, have now all but
replaced extracellular recordings. For these optical methods, Xenopus tadpoles excel due to their transparency and
small size and have been used in determining computations
across entire brain regions. Calcium-imaging of cellular
activity is particularly compelling in its capacity to assess
larger brain regions (Podgorski et al., 2012), an ability that
is often constrained by size and transparency in mammalian
models. In line with such optical measurements, time-lapse
imaging of neurons and their associated neurites are readily
accomplished in Xenopus ( Figure 19.1B 4 ), and are suitable
for imaging of, for example, dendritic growth of tectal cells
in real time (Munz et al., 2014). While most physiological
studies focus predominantly on visual pathways (e.g. Liu
et al., 2018), investigations of other systems have been just
as illuminating, including motion-sensitive circuits during
locomotion (Lambert et al., 2020) and mechanosensation
(Behrend et al., 2006). Given the conserved nature of these
sensory and motor control systems, emerging conclusions
fnd themselves applicable to other vertebrates.
While embryos and tadpoles represent the stages most
commonly used with modern technical applications, postmetamorphic stages are just as suited to a variety of functional studies. A particularly useful and unique experimental
feature of Xenopus is the gradual change in body plan during
metamorphosis. Xenopus possess most advantages present
also in zebrafsh, such as transparency of the brain and body,
small numbers of neurons, and almost unlimited accessibility for experimentation. However, Xenopus has additional
benefts provided by the metamorphic transition into a quadrupedal vertebrate, with comparable motor control principles as present in mammalian species. This duality in
lifestyle makes Xenopus ideally suited for studies aiming at
the identifcation of functional features that undergo modif -
cation during metamorphosis, such as spinal motor patterns
(Combes et al., 2004), which provides standing precedence
for the plasticity of spinal network function in vertebrates.
19.4. FUNCTIONAL NEUROBIOLOGY
USING IN VITRO PREPARATIONS
Disorders of the nervous system often manifest as functional
impairments, which can involve a variety of brain regions
and modalities ( Raichle, 2015). While an in vivo approach
is usually necessary for systemic assessment of biomedical
questions, it is often associated with considerable experimental constraints. Aspects such as level and type of anesthesia,
bleeding, accessibility for surgery, and unexpected movements
form considerable challenges that can render neurobiological
studies on intact animals extremely diffcult, if not impossible. In vitro models, such as slice preparations (e.g. Götz
et al., 2021) or organotypic cell cultures (e.g. Koehler et al.,
2017) have historically been employed as suitable alternatives
to circumvent these constraints. However, while extremely
benefcial for the discovery of many cellular, subcellular, and
molecular aspects of brain function, questions concerning
systemic neurobiological principles or even behavioral consequences of CNS manipulations or impairments remain out of
reach with in vitro approaches such as these.
While the benefts of experimental accessibility limit the
range of addressable scientifc questions for most in vitro
vertebrate models, amphibians form a remarkable exception.
In particular, Xenopus species are able to almost entirely
bypass such in vitro restrictions. This is related to the fact
that larvae and adults of Xenopus allow the generation of
isolated, semi-intact in vitro preparations with various levels
of surgical reductions of tissues ( Straka and Simmers, 2012).
Preparations, such as isolated whole heads, maintained in
simple frog Ringer solution remain viable for several days
and provide access to all CNS circuits between the olfactory bulb and the caudal end of the spinal cord. This plain
visibility and accessibility of the CNS in isolated Xenopus
preparations (Figure 19.2) represent unique advantages that
allow unimpaired, μm-precise impalements of specif c brain
Xenopus
animals of all age groups, including post-metamorphic froglets (Combes et al., 2004; De Vidts et al., 2019). This further
presents as a method to identify differences in kinematic
profles between Xenopus stages (Hänzi and Straka, 2017),
allowing inferences across developmental periods.
The computational ability of cells and neuronal circuits,
while often approximated by behavior, are more precisely
investigated by electrophysiological recordings ( Figure
19.1B 3 ). Xenopus species are suitable for such studies due to
the accessibility of the CNS. Patch-clamping and sharp electrode intracellular recordings can be performed at various
levels, depending on the feature of interest. The resolution
of patch-clamping (Figure 19.1B3) can infer ion channel current dynamics and synaptic properties and thus yield insight
into the integrative capacity of cell membranes (Engert et
al., 2002; Pratt and Aizenman, 2007). Extracellular singleor multi-unit recordings, while lacking this specif city, nonetheless approximate neuronal activity and are experimentally
more rapid, effectively increasing the number of recorded
cells. In Xenopus research, such techniques were extensively
used to characterize neuronal response characteristics in the
midbrain to a range of stimuli following mechanosensory
lateral line stimulation (Behrend et al., 2006) or to measure
receptive felds in the optic tectum (Gaze et al., 1963). On
a broader level, low-impedance electrodes record the f eld
potential produced by larger numbers of neurons, providing
the activity status across an entire group of neurons (Bibikov
and Elepfandt, 2005). Collectively, these methods are ideally suited for linking a particular behavior to the underlying
neuronal activity. For example, neuronal correlates of avoidance behavior in Xenopus were approximated by examining receptive felds of tectal neurons following visual scene
motion (Dong et al., 2009). After quantifying receptive f eld
size, a correlation of the strength of the avoidance response
with sharper receptive felds was discovered. Chemical and
physical manipulations of the tectum or training of the visual
system further explored the dependency of this behavior on
receptive feld properties. While certain electrode recordings
are still unparalleled in their level of detail on the electrical
signature of neurons (see also subsequently), non-invasive
alternatives, such as calcium-imaging, have now all but
replaced extracellular recordings. For these optical methods, Xenopus tadpoles excel due to their transparency and
small size and have been used in determining computations
across entire brain regions. Calcium-imaging of cellular
activity is particularly compelling in its capacity to assess
larger brain regions (Podgorski et al., 2012), an ability that
is often constrained by size and transparency in mammalian
models. In line with such optical measurements, time-lapse
imaging of neurons and their associated neurites are readily
accomplished in Xenopus ( Figure 19.1B 4 ), and are suitable
for imaging of, for example, dendritic growth of tectal cells
in real time (Munz et al., 2014). While most physiological
studies focus predominantly on visual pathways (e.g. Liu
et al., 2018), investigations of other systems have been just
as illuminating, including motion-sensitive circuits during
locomotion (Lambert et al., 2020) and mechanosensation
(Behrend et al., 2006). Given the conserved nature of these
sensory and motor control systems, emerging conclusions
fnd themselves applicable to other vertebrates.
While embryos and tadpoles represent the stages most
commonly used with modern technical applications, postmetamorphic stages are just as suited to a variety of functional studies. A particularly useful and unique experimental
feature of Xenopus is the gradual change in body plan during
metamorphosis. Xenopus possess most advantages present
also in zebrafsh, such as transparency of the brain and body,
small numbers of neurons, and almost unlimited accessibility for experimentation. However, Xenopus has additional
benefts provided by the metamorphic transition into a quadrupedal vertebrate, with comparable motor control principles as present in mammalian species. This duality in
lifestyle makes Xenopus ideally suited for studies aiming at
the identifcation of functional features that undergo modif -
cation during metamorphosis, such as spinal motor patterns
(Combes et al., 2004), which provides standing precedence
for the plasticity of spinal network function in vertebrates.
19.4. FUNCTIONAL NEUROBIOLOGY
USING IN VITRO PREPARATIONS
Disorders of the nervous system often manifest as functional
impairments, which can involve a variety of brain regions
and modalities ( Raichle, 2015). While an in vivo approach
is usually necessary for systemic assessment of biomedical
questions, it is often associated with considerable experimental constraints. Aspects such as level and type of anesthesia,
bleeding, accessibility for surgery, and unexpected movements
form considerable challenges that can render neurobiological
studies on intact animals extremely diffcult, if not impossible. In vitro models, such as slice preparations (e.g. Götz
et al., 2021) or organotypic cell cultures (e.g. Koehler et al.,
2017) have historically been employed as suitable alternatives
to circumvent these constraints. However, while extremely
benefcial for the discovery of many cellular, subcellular, and
molecular aspects of brain function, questions concerning
systemic neurobiological principles or even behavioral consequences of CNS manipulations or impairments remain out of
reach with in vitro approaches such as these.
While the benefts of experimental accessibility limit the
range of addressable scientifc questions for most in vitro
vertebrate models, amphibians form a remarkable exception.
In particular, Xenopus species are able to almost entirely
bypass such in vitro restrictions. This is related to the fact
that larvae and adults of Xenopus allow the generation of
isolated, semi-intact in vitro preparations with various levels
of surgical reductions of tissues ( Straka and Simmers, 2012).
Preparations, such as isolated whole heads, maintained in
simple frog Ringer solution remain viable for several days
and provide access to all CNS circuits between the olfactory bulb and the caudal end of the spinal cord. This plain
visibility and accessibility of the CNS in isolated Xenopus
preparations (Figure 19.2) represent unique advantages that
allow unimpaired, μm-precise impalements of specif c brain