AMER. ZOOL., 24:775-782 (1984)
Central Nervous System Changes Related to the Reduction of Visual
Input in a Naturally Blind Fish (Astyanax hubbsi)1
THEODORE J. VONEIDA AND STEPHEN E. FISH
Neurobtotogy Program, Northeastern Ohio Universities College of Medicine,
Rootstown, Ohio 44272
SYNOPSIS. Tectal anatomy and physiology of the blind cave characin, Astyanax hubbsi,
have been compared with that of its sighted ancestor Astyanax mexicanus (the river fish)
and with goldfish. Normal and experimental neuroanatomic methods have revealed that,
with the exception of a greatly reduced retinotectal projection, connectivity and structure
of cave fish tecta are similar to those described in sighted species. It appears that the
rudimentary retinotectal input is nonfunctional, since no tectal evoked responses could
be elicited with electrical or visual stimulation of the optic cyst, and all attempts to visually
condition cave fish were unsuccessful.
Attempts have also been made to record somatosensory, auditory and lateral line activity
in the tecta of the blind and sighted fish. A sparse somatic representation was found in
the deeper portion of the sighted fish tecta which contrasts with a dense, well-organized
one in the cave fish. No tectal responses were found to auditory or lateral line stimuli.
CNS plasticity is discussed in relation to studies of fish, amphibians, reptiles, birds and
mammals, in which a reduction of sensory input by any one of a number of means has
resulted in alterations of structure and function.
INTRODUCTION
The blind cave fish, Astyanax hubbsi (formerly A. jordani), was discovered by C. B.
Jordan in caves of the Sierra de El Abra
region of Mexico, and was first described
by Hubbs and Innes in 1936. The river
fish, Astyanax mexicanus, is ancestral to A
hubbsi (Mitchell et ai, 1977), and is native
to the Rio Tampaon, which drains into the
valley where the above caves are found. A.
Mexicanus presently ranges into the southern United States, where we obtained our
breeding stock. Our blind fish were originally obtained from tropical fish suppliers,
though we now maintain a breeding colony
for both groups.
The availability of the blind fish offers a
unique opportunity for the study of alterations in brain structure and function
resulting from the relatively recent natural
loss of a single sensory system. Other sensory systems appear to have remained
intact. Indeed, behavioral studies suggest
that in some cases these fish are superior
to sighted forms. Thus Weiss and Martini
(1970) and Gertychowa (1970) have shown
that the blind fish outperforms visual fish
in tasks depending on lateral line input;
Breder and Rasquin (1943) and Humbach
(1960) have reported that its chemical sense
is much better developed than that of
sighted fish. In addition, Popper (1970) has
demonstrated that auditory capacity in the
blind fish is equal to that of A mexicanus,
and John (1957) has described its capability
to respond to somatic cues.
STUDIES OF THE VISUAL SYSTEM
IN THE CAVE FISH
Eye development
It has been known since the studies of
Cahn (1958) that optic cup and lens abnormalities are present throughout embryonic
development in the cave fish. A degenerate
eye remnant, called the "optic cyst," is visible as a surface structure at larval day 1,
but by larval day 4 an overgrowth of the
cyst by the optic lip is grossly apparent (Zille
et ai, 1983). This overgrowth continues
during the next week of development, until
the optic cyst is completely covered by adipose, connective and epithelial tissues, and
is no longer visible as a surface structure.
' From the Symposium on Evolution of Neural Sys- An early stage (second larval day) of optic
tems in the Vertebrates: Functional-Anatomical Approaches lip overgrowth is shown in Figure 1A. A
presented at the Annual Meeting of the American
number of studies have been carried out
Society of Zoologists, 27-30 December 1982, at
on
the genetic control of eye and lens
Louisville, Kentucky.
775
776
T . J . VONEIDA AND S. E. FlSH
FIG. 1. Head region of A. hubbsi (left) and A. mexicanus (right) at 2 days posthatching. The optic cyst is
beginning to recede in A. hubbsi, with concurrent overgrowth of the optic lip (arrow). A. mexicanus eyes are
well-developed at this stage. Scanning EM x 78.
development and their relation to body
pigmentation in these fishes (Sadoglu,
1956, 1957, 1975; Pfeiffer, 1967; Wilkens,
1971). There appear to be separate genes
for at least retinal and lens development,
and Sadoglu (1975) suggests that separate
genetic controls act in the development of
various other parts of the eye as well.
and Gaffney, 1981), and Ictalurus (Prasada
Rao and Sharma, 1982). Its significance
remains unclear. Retinotectal fibers in the
blind fish are entirely contralateral, penetrating the medial third of the tectum. Similar findings have been reported by Halpern (1973) in her studies of retinal
projections in the blind snake Typhlops. The
fact that retinotectal connections have
Retinal projections: Anatomy and function
remained in these specialized forms, in spite
In an effort to clarify the question of of their apparent blindness, suggests the
retinofugal connections in the blind fish, possibility that the tectum may have
Voneida and Sligar (1976), using a modi- retained a potential for processing visual
fied Nauta-Gygax (1954) technique, traced information. The question remains, if a
degenerated optic nerve fibers following functional eye were made available, would
removal of the optic cyst. We found both retinal axons follow the existing tectal
retinotectal and retinodiencephalic (hypo- pathways, and would synaptic contacts be
thalamic and pretectal) connections in the established? Finally, would the tectum
blind cave form, though as might be prove capable of processing information
expected, these were greatly reduced as mediated over this "newly acquired" syscompared to those found in the river fish. tem?
We also found a small ipsilateral retinotecThe presence of sparse but consistent
tal projection in the river fish, which has retinotectal connections in the blind fish
been described as well in Petrotnyzon led us to ask whether these connections are
(Northcutt and Przybylski, 1973), Serra- capable of mediating information from the
salmus (Ebbesson and Ito, 1980), Pohpterus optic cyst. The studies of Breder and Ras(Reperant et ai, 1981), Caras.uu* (Springer quin (1947) have shown that variabilities
CNS REMODELING IN A BLIND FISH
exist among these animals in their degree
of visual function. Our own findings indicate they are unresponsive to visual cues.
We have been unsuccessful in our attempts
to condition adult blind fish to respond to
visual stimuli in a shock avoidance situation
(unpublished data), whereas river and
goldfish readily learned the problem.
We have also studied the cave fish retinotectal connections electrophysiologically. To do this we have applied both light
and electrical stimuli to the optic cyst and
made extracellular recordings in the tectum. Again, river fish and goldfish served
as controls. We were unable to elicit either
slow wave potentials or single unit responses
in the cave fish tectum. It appears that, if
any information is conveyed to the tectum
from the optic cyst, it is so weak as to be
difficult to detect eletrophysiologically. As
might be expected, visual response properties and topography similar to those
reported in numerous earlier studies (e.g.,
Meyer, 1977) were observed in the river
fish and goldfish.
Tectal function: Anatomic and
physiologic studies
The above considerations (a degenerate
eye and loss of visual function) lead directly
into the question of tectal function in the
blind fish. It is possible, of course, that the
loss of visual input has resulted simply in
a greatly reduced level of tectal function
in these animals. Another possibility is that
it has increased its processing capability for
other, non-visual modalities and/or that
the tectum has assumed new functions altogether. Whatever the case, the question
lends itself to experimental analyses, some
of which will be described below.
Relatively little work has been done on
tectal structure of connections in A. hubbsi.
Sligar and Voneida's study (1976) demonstrates a rather widely distributed pattern of tectal efferents in this form, with
connections to ipsilateral nucleus isthmi,
torus longitudinalis, torus semicircularis,
dorsal thalamus, pretectum, contralateral
tectum and medullary reticular nuclei
(bilaterally). In addition, we have found
relatively large numbers of pyramidal, fusiform and large pyriform cells in the blind
777
fish tectum (unpublished data; Fig. 2). The
pyramidal cells have long, sparsely
branched apical dendrites extending dorsally from the periventricular layer into the
superficial cell and fiber layer; the large
pyriform cells are most abundant in deeper
parts of the superficial cell/fiber layer,
while fusiform cells lie slightly deeper, in
the more superficial parts of the central
cell zone. The latter two cell types have
short, highly branched apical dendrites and
short, sparsely branched basal dendrites.
These three principal tectal cell types have
been described in numerous cytoarchitectural analyses of the tectum, dating from
the early work of Cajal (1911). The fact
that they have been retained in a blind form
may be of some significance, and opens the
question of their role in tectal information
processing in this animal. Schroeder and
Vanegas(1977), for example, have pointed
out that these cells are also found in relatively large numbers in two siluroid fishes
(Bagrus and Ictalurus), both of which are
known to have restricted visual input. They
have suggested that these cell types, particularly the pyradmidal cell, may be significantly involved in non-visual inputs.
Our own investigations of tectal function
in the cave fish include electrophysiological tests for somatosensory, auditory and
lateral line evoked responses in the tecta
of cave, river and goldfish. In these studies,
we used standard single and multiple unit
extracellular recording techniques. The
fish were anesthetized using MS 222 (1 part
to 10,000-20,000 parts water perfused
over the gills) or urethane (3 g/kg IP) and
their gills were perfused with artificially
aerated water. The only evoked activity that
we were able to record in the cave fish
tectum was that generated by somatosensory stimulation. Rubbing or tapping the
side of the fish with small brushes or glass
probes evoked multiple unit activity as soon
as the electrode tip entered the tectum
contralateral to the stimulation. Somatically driven single units were isolated in all
depths of the tectum but they tended to be
found with increasing frequency as the
electrode penetrated to the deeper tectal
layers, as shown in Figure 3.
Somatotopy was clearly evident, since
778
T . J. VONEIDA AND S. E. FlSH
779
CNS REMODELING IN A BLIND FISH
ANTERIOR
POSTERIOR
100
10
15
20
25
30
NUMBER OF UNITS
Fic. 3. Numbers of somatosensory units in the tectum of A. hubbsi are plotted according to depth in the
tectum. Notice that units are recorded more frequently in deeper layers.
successive tectal penetrations from rostral
to caudal or medial to lateral yielded corresponding rostro-caudal and dorso-ventral receptive fields, respectively, on the
body surface. A composite map developed
from histological reconstruction of several
cave fish tecta is shown in Figure 4.
In electrode penetrations normal to the
tectal surface, successively recorded units
yielded overlapping receptive fields which
tended to decrease in size with depth below
the surface. Receptive fields on the head
tended to be smaller than on the rest of
the body (Fig. 5), and this is reflected in
the large area of the tectum devoted to the
head. This latter organization (a direct
relationship between central representation and peripheral input) is typical of sensory topography in the vertebrate central
nervous system.
Somatosensory single units were recorded only in the deepest layers of the river
fish and goldfish optic tecta. There was a
suggestion of the organization found in the
cave fish but this was difficult to determine
since these units were only rarely encountered. It seems, then, that there has been
an increase in somatic input to the blind
fish tectum which parallels the decrease in
MEDIAL
FIG. 4. A modified dorsal surface view of the left
tectum of A. hubbsi is depicted. The electrophysiologically plotted somatosensory map is somewhat distorted as the tectum is a hemispherical structure and
the ventral lateral aspect would normally be out of
sight from this view.
visual input. It still remains to be determined, however, where this somatic input
arises.
Extravisual sensory modalities have been
found in the midbrain tectum of every animal studied. A somatosensory representation is one of the most common findings
in such diverse animals as, for example, the
chicken (Cotter, 1976), hamster (Chalupa
and Rhoades, 1977), axolotyl (Gruberg and
Harris, 1981), and iguana (Gaither and
Stein, 1979). Our finding of somatic activity in the tectum of the three fish we have
studied is, therefore, not surprising. The
usual organization of the midbrain tectum
is a visual input to the superficial layers,
with deeper layers receiving other modalities. Typically the sensory modalities have
a well-organized topographic representation relative to the surface of the tectum
and they are in spatial register. Thus cells
with visual receptive fields directly in front
of the animal near the horizontal and vertical meridians would be locatedjust above
somatosensory cells with receptive fields on
the surface of the body of the animal near
Fic. 2. Characteristic cell types in the optic tectum of A. hubbsi: A. Pyramidal cell (arrow) in the periventricular
layer, with its apical dendrite extending dorsally into the superficial cell/fiber layer. B. Large pyriform cells
in the deeper part of the superficial cell/fiber layer (arrows); note also the (fusiform?) cell (open arrow) lying
somewhat more ventrally.
780
T . J . VONEIDA AND S. E. FlSH
FIG. 5. Four representative somatosensory receptive fields of tectal units (cross-hatched areas) are shown
plotted on the right body surface of A. hubbsi. Notice that the receptive fields on the head are smaller than
those plotted more posteriorly (compare with the large head area on the tectal margin Fig. 4) and that one
of them is on the skin overlying the optic cyst.
its nose. Likewise neurons with temporal
visual receptive fields would be located near
ones with somatosensory receptive fields
on the hindquarters of the animal.
Although the somatic representation we
found in the tectum of the visual fish was
too sparse to determine if there was a match
with the retinotopy, it is interesting that
the cave fish somatotopy (Fig. 4) is in good
register with goldfish visuotopy (e.g., Meyer,
1977). This same plan has been found to
obtain for other sensory modalities as well.
Thus, auditory localization in the cat (Gordon, 1973) and infrared responsive neurons in the rattlesnake (Hartlinee/ ah, 1978)
are organized topographically, such that
their receptive fields correspond spatially
with visual receptive fields. It has been suggested that this polymodal input to the tectum plays a role in orienting behaviors
which can be initiated by the separate
modalities. It is very likely that fish use
lateral line information to orient to their
environment, and an input from this system to the tectum has been identified in
the longnose gar (Northcutt, 1982). We
were somewhat surprised, therefore, that
we were totally unsuccessful in the blind
fish in identifying tectal units which are
responsive to auditory or lateral line stimuli (low frequency tones and agitation of
the water). These same stimuli, however,
invariably elicited evoked responses in the
torus semicircularis. Our investigation of
lateral line involvement in the cave fish tectum is continuing.
It appears that a primary role of the midbrain tectum in the blind cave fish is that
of processing somatosensory information,
and it may prove that this change is an
adaptation to cave life. It has been proposed that the optic tectum plays a major
role in orienting activities in visually
responsive animals. It is possible that this
orienting role in the blind fish is still
mediated by the tectum, but that somatic
input has now become the principal guiding modality. Consistent with this view are
our observations of cave fish placed in a
new environment (unpublished; see also
John, 1957; Gertychowa, 1970). Initially
the fish make much physical contact with
their surroundings by bumping into
objects, but within a relatively short time
CNS REMODELING IN A BLIND FISH
they appear to negotiate obstacles as if
they are able to see them. It is as if they
have established a spatial map of their surroundings based on the use of somatic
input.
CENTRAL CHANGES RESULTING FROM A
REDUCTION OF SENSORY INPUT: A
COMPARISON WITH OTHER MODELS
It is interesting to compare the above
observations of peripheral and central
changes in a naturally evolved system with
those which have resulted from experimentally induced changes. When normally
sighted hamsters (Rhoades, 1980) are enucleated at an early age, their tecta change
during subsequent development. The normal somatosensory representation is
expanded dorsally to fill the entire tectum,
resulting in an organization much like that
found in the cave fish. A similar finding
has been made in eyeless mutant salamanders (Gruberg and Harris, 1981), and we
have recently observed the same change in
mutant anophthalmic mice (preliminary
electrophysiological results). Is the cave fish
then simply an evolutionarily enucleated
river fish? It seems unlikely that the differences between cave and river fish tecta
are due entirely to the developmental consequences of a reduction of visual input.
The expansion of somatic representation
could be explained by mechanisms such as
axonal sprouting (Liu and Chambers,
1958), though the somatosensory representation in the sighted fish would seem to
be too sparse to account for the major reorganization we have documented in the cave
fish. We have attempted to shed some light
on this question by removing one or both
of the eyes of young river fish so that we
might compare their brains, as adults, with
those of cave fish.2 These data remain to
be analyzed. Will the tectum contralateral
2
An incidental finding of some interest relates to
the very high mortality rate (>99%) observed in these
animals. Eyed animals which live in the dark may be
subject to a greatly increased risk of infection as a
result of damage to these anterior "protrusions" during collisions with the cave walls. This could presumably lead to a strong selective pressure against the
retention of eyes.
781
to eye removal undergo structural and
physiological changes which are more typical of tectal structure and function in the
blind fish? In other words, will it become
a "somatosensory" tectum? Conversely, is
it possible to modify the blind fish tectum
from a "somatosensory" to a "visual" tectum by transplanting a river fish eye to the
blind animal? Answers to these questions
could provide further clues to an understanding of central nervous plasticity,
whether it results from experimental
manipulations, genetic mutations, or naturally occurring adaptations, as observed
in a blind cave animal such as Astyanax
hubbsi.
ACKNOWLEDGMENTS
The authors would like to extend special
thanks to Mr. and Mrs. James White, local
fish hobbyists, for their expert assistance
in the maintenance and breeding of our
experimental animals. This work was supported by NIH grants MH-07051, NS
18369 and NS 06136.
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