Segregation of On and Off Bipolar Cell Axonal Arbors in the Absence of Retinal Ganglion Cells

Abstract

Retinal cells that respond selectively to light onset or offset are segregated into On and Off pathways. Here, we describe the development of cone bipolar cells whose axonal arbors at maturity synapse onto ganglion cell dendrites confined to On and Off strata of the inner plexiform layer (IPL). In particular, we sought to determine whether the formation of this segregated pattern is dependent on the presence of ganglion cells. Developing bipolar cells were visualized using an antibody against recoverin, the calcium binding protein that labels On and Off cone bipolar cells in the adult rat retina. Recoverin-positive cells were apparent in the ventricular zone on the day of birth [postnatal day 0 (P0)], before bipolar cells begin to migrate to the inner nuclear layer. Two distinct strata were first apparent in the IPL at P8, with the Off pathway maturing earlier than the On pathway. There was no indication of exuberant bipolar cell projections. Throughout development, there were also a small number of recoverin-positive cells of unknown origin in the ganglion cell layer. To assess whether the formation of On and Off cone bipolar cell projections is dependent on the presence of ganglion cells, these target neurons were eliminated by unilateral section of the optic nerve. This was done on the day of birth, resulting in a total loss of ganglion cells 5-6 d before bipolar cell axons innervate the IPL. In retinas with optic nerve sections, On and Off cone bipolar cells were present, albeit at a lower than normal density, and the axonal arbors of these interneurons were organized into two distinct strata. This indicates that ganglion cells are not essential for the formation of segregated On and Off bipolar cell inputs. These results lend support to the hypothesis that specific ingrowth patterns of bipolar cell terminal arbors could regulate the formation of stratified retinal ganglion cell dendrites.

Recruitment of the Rod Pathway by Cones in the Absence of Rods

“…The pattern and level of stratification of the axonal arbors of bipolar cells in the inner plexiform layer, which reflect their functional diversity, are not influenced by major changes in retinal architecture: the neonatal elimination of ganglion cells, which are their major target (Gunhan-Agar et al, 2000), the selective removal of cholinergic amacrine cells (Gunhan et al, 2002), the formation of supernumerary neurons in the inner nuclear layer (Strettoi and Volpini, 2002), or an imbalance in afferent activity (Bodnarenko et al, 1995), do affect the number of bipolar cells but not the connectivity in the ipl. Our data demonstrate that the lamination of rod bipolar cells, and probably the wiring of their axons, are not affected by the absence of their normal presynaptic partners.…”

Unique Functional Properties of On and Off Pathways in the Developing Mammalian Retina

“…In line with this idea, recent studies have shown that the axon terminals of On-and Off-cone bipolar cells form their stratified projection patterns in a remarkably precise manner, without any initial period of intermingling (Miller et al, 1999;Günhan-Agar et al, 2000). Moreover, electrophysiological recordings have reported that very few ganglion cells in the developing cat retina respond with On-Off discharges to flashing spots of light at a time when multistratified cells would be expected to be plentiful (Dubin et al, 1986;Tootle, 1993).…”

Transient requirement for ganglion cells during assembly of retinal synaptic layers

“…The severe IPL phenotype seen in lak mutants contrasts with the relatively mild effects of GC ablation by optic nerve transection (Günhan-Agar et al, 2000;Williams et al, 2001). In the latter studies, GCs form but are experimentally killed in newborn rats or ferrets.…”

“…Furthermore, it is unclear whether AC and BC axonal terminals are sculpted on a scaffold initially laid down by GC dendrites or are otherwise dependent on GC-derived cues. Günhan-Agar et al (Günhan-Agar et al, 2000) and Williams et al (Williams et al, 2001) showed that, in rats and ferrets, ACs and BCs develop and maintain ON and OFF stratification patterns after experimentally induced degeneration of GCs.…”