279
Functional Neurobiology in Xenopus
beforehand (Giorgi and van der Loos, 1978). This method
was further ref ned by employing xenografts from different
Xenopus species and different sensory tissue origins (Koo
and Graziadei, 1995). Eye primordia in Xenopus borealis were successfully replaced with olfactory primordia of
Xenopus laevis, demonstrating that grafted olfactory tissue,
identifed by fuorescent chromosome staining, innervated
the brain at the level of the missing optic nerve (Koo and
Graziadei, 1995). This replacement approach was complemented by target-specifc ablations, which, in combination
with visualization of axonal pathways, revealed the extent of
Xenopus CNS plasticity for rewiring within brain regions.
Specifcally, removal of the otic placode caused an expansion of adjacent cranial nerve projections into areas of the
hindbrain that would normally receive inner ear afferent
input ( Fritzsch, 1990 ) ( Figure 19.1A 3 ).
Embryonic manipulation-based approaches to assess
neuronal circuit formation during Xenopus development
were paralleled by more descriptive approaches, excessively
exploiting horseradish-peroxidase placement and tracing
techniques. The vast amount of resultant anatomical connection schemes, such as cerebellar projections (Gonzalez
et al., 1984; van der Linden et al., 1990) (Figure 19.1A2) or
descending pathways to the spinal cord (ten Donkelaar et
al., 1981, 1991), formed the basis of the present knowledge
about connectivity and circuit roadmaps of the Xenopus
CNS (Nieuwenhuys et al., 1998). These anatomical descriptions guided corresponding physiological studies, thereby
complementing the understanding of how anatomically
defned neural circuits acquire and execute neuronal computations based on afferent and efferent connections, although
many of these electrophysiological studies were made on
ranid frogs (Llinás and Precht, 1976). The discovery of
neurophysiological principles particularly benef tted from
studies on the frog spinal cord (Eccles, 1944). This included
fundamental aspects of sensory signal processing as well
as the morphological arrangement of spinal motoneurons
( Frank and Westerfeld, 1982). Initiated by Eccles’ use of
frogs to study signal propagation and synaptic transmission
in spinal motoneurons utilizing ortho- and antidromic electrical stimulation of spinal roots (Eccles, 1944), the spinal
cord became for a long time a dominating model for studying motor control principles. Facilitated by their large cell
size, single-cell recordings characterized how motoneurons
interact with each other (Grinnell, 1966) and how musclespecifc functional neuronal populations innervate and
control muscle fbers (Frank and Westerfeld, 1982). This
ultimately led to formulating hypotheses about physiological principles of limb motion control by the spinal cord in
the framework of “force-f elds” (Giszter et al., 1993; Tresch
et al., 1999).
Meanwhile, physiological studies on Xenopus utilized the
specif c eco-physiology of this species, which in contrast to
most other frogs includes a retention of the mechanosensory
lateral line system in adults (Shelton, 1970). Following the
anatomical description of this sensory system throughout
development (Shelton, 1970; Winklbauer, 1989), subsequent
functional analyses characterized the properties of neuromast sensory afferents in response to mechanical stimuli
(Figure 19.1A4, top) (Kroese et al., 1978). The easy access
to this sensory system at all hierarchical neuronal levels,
in contrast to other aquatic anamniotes, made Xenopus an
excellent model to investigate the spatio-temporal resolution of sensory encoding of water motion and the subsequent neuronal representation and computation, as well as
the range and performance of induced motor responses such
as turning behavior (Elepfandt et al., 1985) (Figure 19.1A4,
bottom). However, the assessment of function by either orienting behaviors in adult Xenopus or of motoneuron activity during the ontogenetic implementation of spinal circuits
(Sillar et al., 1992) (Figure 19.1A5) was at that time limited
to qualitative descriptions without the possibility for a more
f ne-tuned differentiation.
In summary, Xenopus and Rana greatly contributed to
the understanding of how sensory systems are peripherally
structured, centrally represented, and involved in general
vertebrate behavioral repertoires (Figure 19.1A). Various
anuran species individually assisted in establishing the
comprehension of neuronal connectivity in the CNS with
growing relevance of Xenopus for investigating embryonic
development. In particular, grafting methods followed by
subsequent anatomical and functional assessments became
instrumental in determining these principles. However, the
lack of appropriate experimental tools often prevented quantifcation at cellular and sub-cellular levels, and due to limited recording techniques, behavioral analyses were largely
qualitative and categorical. Nonetheless, such detailed
analyses are required for a more profound understanding of
basic neurophysiological concepts and thus also for pathophysiological principles related to diseases.
19.3. PRESENT STATUS OF THE FIELD—
EXPERIMENTAL APPROACHES
Recent decades have witnessed a considerable advancement
in technical and analytical approaches in neuroscientif c
research. While many of these innovations were initiated in
individual model systems, such as the development of the
patch-clamp technique in Rana muscle fbers (Neher and
Sakmann, 1976), a transfer to other organisms and cell types
with respective species-specifc adaptations has nonetheless
expanded steadily (e.g. Liu et al., 2018). Such methods, however, are only as useful as they are applicable to a particular model system. In Xenopus, functional neurobiological
studies have profted considerably from characteristics that
permit the use of a wide variety of analytical techniques,
which allow a sizable depth of probing and experimental
manipulations (Figure 19.1B). In the following, these attributes will be featured to highlight the increasing ease with
which Xenopus is currently used to answer fundamental
neurobiological questions, driven to a considerable extent by
more modern methodologies and conceptually innovative
approaches.
Functional Neurobiology in Xenopus
beforehand (Giorgi and van der Loos, 1978). This method
was further ref ned by employing xenografts from different
Xenopus species and different sensory tissue origins (Koo
and Graziadei, 1995). Eye primordia in Xenopus borealis were successfully replaced with olfactory primordia of
Xenopus laevis, demonstrating that grafted olfactory tissue,
identifed by fuorescent chromosome staining, innervated
the brain at the level of the missing optic nerve (Koo and
Graziadei, 1995). This replacement approach was complemented by target-specifc ablations, which, in combination
with visualization of axonal pathways, revealed the extent of
Xenopus CNS plasticity for rewiring within brain regions.
Specifcally, removal of the otic placode caused an expansion of adjacent cranial nerve projections into areas of the
hindbrain that would normally receive inner ear afferent
input ( Fritzsch, 1990 ) ( Figure 19.1A 3 ).
Embryonic manipulation-based approaches to assess
neuronal circuit formation during Xenopus development
were paralleled by more descriptive approaches, excessively
exploiting horseradish-peroxidase placement and tracing
techniques. The vast amount of resultant anatomical connection schemes, such as cerebellar projections (Gonzalez
et al., 1984; van der Linden et al., 1990) (Figure 19.1A2) or
descending pathways to the spinal cord (ten Donkelaar et
al., 1981, 1991), formed the basis of the present knowledge
about connectivity and circuit roadmaps of the Xenopus
CNS (Nieuwenhuys et al., 1998). These anatomical descriptions guided corresponding physiological studies, thereby
complementing the understanding of how anatomically
defned neural circuits acquire and execute neuronal computations based on afferent and efferent connections, although
many of these electrophysiological studies were made on
ranid frogs (Llinás and Precht, 1976). The discovery of
neurophysiological principles particularly benef tted from
studies on the frog spinal cord (Eccles, 1944). This included
fundamental aspects of sensory signal processing as well
as the morphological arrangement of spinal motoneurons
( Frank and Westerfeld, 1982). Initiated by Eccles’ use of
frogs to study signal propagation and synaptic transmission
in spinal motoneurons utilizing ortho- and antidromic electrical stimulation of spinal roots (Eccles, 1944), the spinal
cord became for a long time a dominating model for studying motor control principles. Facilitated by their large cell
size, single-cell recordings characterized how motoneurons
interact with each other (Grinnell, 1966) and how musclespecifc functional neuronal populations innervate and
control muscle fbers (Frank and Westerfeld, 1982). This
ultimately led to formulating hypotheses about physiological principles of limb motion control by the spinal cord in
the framework of “force-f elds” (Giszter et al., 1993; Tresch
et al., 1999).
Meanwhile, physiological studies on Xenopus utilized the
specif c eco-physiology of this species, which in contrast to
most other frogs includes a retention of the mechanosensory
lateral line system in adults (Shelton, 1970). Following the
anatomical description of this sensory system throughout
development (Shelton, 1970; Winklbauer, 1989), subsequent
functional analyses characterized the properties of neuromast sensory afferents in response to mechanical stimuli
(Figure 19.1A4, top) (Kroese et al., 1978). The easy access
to this sensory system at all hierarchical neuronal levels,
in contrast to other aquatic anamniotes, made Xenopus an
excellent model to investigate the spatio-temporal resolution of sensory encoding of water motion and the subsequent neuronal representation and computation, as well as
the range and performance of induced motor responses such
as turning behavior (Elepfandt et al., 1985) (Figure 19.1A4,
bottom). However, the assessment of function by either orienting behaviors in adult Xenopus or of motoneuron activity during the ontogenetic implementation of spinal circuits
(Sillar et al., 1992) (Figure 19.1A5) was at that time limited
to qualitative descriptions without the possibility for a more
f ne-tuned differentiation.
In summary, Xenopus and Rana greatly contributed to
the understanding of how sensory systems are peripherally
structured, centrally represented, and involved in general
vertebrate behavioral repertoires (Figure 19.1A). Various
anuran species individually assisted in establishing the
comprehension of neuronal connectivity in the CNS with
growing relevance of Xenopus for investigating embryonic
development. In particular, grafting methods followed by
subsequent anatomical and functional assessments became
instrumental in determining these principles. However, the
lack of appropriate experimental tools often prevented quantifcation at cellular and sub-cellular levels, and due to limited recording techniques, behavioral analyses were largely
qualitative and categorical. Nonetheless, such detailed
analyses are required for a more profound understanding of
basic neurophysiological concepts and thus also for pathophysiological principles related to diseases.
19.3. PRESENT STATUS OF THE FIELD—
EXPERIMENTAL APPROACHES
Recent decades have witnessed a considerable advancement
in technical and analytical approaches in neuroscientif c
research. While many of these innovations were initiated in
individual model systems, such as the development of the
patch-clamp technique in Rana muscle fbers (Neher and
Sakmann, 1976), a transfer to other organisms and cell types
with respective species-specifc adaptations has nonetheless
expanded steadily (e.g. Liu et al., 2018). Such methods, however, are only as useful as they are applicable to a particular model system. In Xenopus, functional neurobiological
studies have profted considerably from characteristics that
permit the use of a wide variety of analytical techniques,
which allow a sizable depth of probing and experimental
manipulations (Figure 19.1B). In the following, these attributes will be featured to highlight the increasing ease with
which Xenopus is currently used to answer fundamental
neurobiological questions, driven to a considerable extent by
more modern methodologies and conceptually innovative
approaches.