Author:
COOK P. MILTON,PRUSKY GLEN,RAMOA ARY S.
Abstract
During early mammalian development, inputs from
the two retinas intermix within the lateral geniculate
nucleus (LGN), then segregate during the first postnatal
week into layers that receive input from a single retina.
Functionally, the LGN also changes markedly during the
first postnatal month; early geniculate responses to retinal
input are mainly excitatory, then inhibitory circuits mature
within the LGN. These remarkable changes in form and function
of the retinogeniculate pathway occur at a time when patterned
visual activity is not present, but retinal ganglion cells
already manifest spontaneous action potential activity.
To examine the role of early retinal activity in these
critical developmental processes, we placed the slow release
polymer Elvax embedded with tetrodotoxin (TTX) into the
vitreous chamber of one or both eyes of neonatal ferrets.
Animals receiving monocular injection of TTX had the other
eye treated with Elvax containing control citrate buffer.
Intraocular injection of horseradish peroxidase was made
at the end of the period of TTX treatment to reveal the
retinal terminals in the LGN. Chronic monocular or binocular
blockade of retinal activity during the first postnatal
week did not prevent eye-specific segregation, although
it made the boundaries between layers less distinct. Retinal
terminals ended preferentially in the appropriate layer,
but a large number of terminals were also present in the
inappropriate layer. Further segregation was achieved during
the second postnatal week of activity blockade, when most
retinal terminals ended preferentially in the appropriate
geniculate layer and sharper layer boundaries were present.
However, a small but significant number of terminals still
extended into the inappropriate layer. Together, these
findings indicate that monocular as well as binocular blockade
of retinal activity resulted in some anomalous retinogeniculate
projections and delayed eye-specific patterning, but segregation
was largely intact at the end of the second postnatal week.
We also report here that intraocular tetrodotoxin had a
marked effect on the maturation of intrinsic geniculate
circuits prior to eye opening. Whole-cell patch-clamp recordings
in the LGN slice preparation revealed that activity blockade
prevented the maturation of the slow, but not the fast,
hyperpolarizing potential of LGN neurons during the first
postnatal month and up to P38, the oldest age studied.
In conclusion, these results indicate that spontaneous
retinal activity modulates the time course of binocular
segregation but does not alone account for the segregation
of retinogeniculate terminals. However, early retinal activity
plays an important role in developing the intrinsic circuitry
of the LGN.
Publisher
Cambridge University Press (CUP)
Subject
Sensory Systems,Physiology
Cited by
56 articles.
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