Photoreceptors in the ferret's retina have been shown
to project transiently to the inner plexiform layer (IPL) prior
to their differentiation of an outer segment. On postnatal day
15 (P-15), when this projection achieves maximal density, the
photoreceptors projecting into the IPL extend primarily to one
of two depths, coincident with the processes of cholinergic
amacrine cells. The present study has used an excitotoxic approach
employing subcutaneous injections of l-glutamate to
ablate these cholinergic amacrine cells on P-7, in order to
see whether their elimination alters this targeting of
photoreceptor terminals within the IPL. The near-complete
elimination of cholinergic amacrine cells at P-15 was confirmed,
although the population of retinal ganglion cells was also
affected, being depleted by roughly 50%. The rod
opsin-immunopositive terminals in such treated ferrets no longer
showed a stratified distribution, being found throughout the
depth of the IPL, as well as extending into the ganglion cell
layer. This effect should not be due to the partial loss of
retinal ganglion cells, however, since optic nerve transection
at P-2, which eliminates the ganglion cells entirely while leaving
the cholinergic amacrine cell population intact, was shown not
to affect the stratification pattern of the photoreceptors within
the IPL. These results strongly suggest that the targeting of
the photoreceptor terminals to discrete strata within the IPL
is dependent upon the cholinergic amacrine cell processes.
Relative contributions of bipolar cell and amacrine cell inputs to light responses of ON, OFF and ON–OFF retinal ganglion cells
