Assembly and disassembly of a retinal cholinergic network

Abstract

In the few weeks prior to the onset of vision, the retina undergoes a dramatic transformation. Neurons migrate into position and target appropriate synaptic partners to assemble the circuits that mediate vision. During this period of development, the retina is not silent but rather assembles and disassembles a series of transient circuits that use distinct mechanisms to generate spontaneous correlated activity called retinal waves. During the first postnatal week, this transient circuit is comprised of reciprocal cholinergic connections between starburst amacrine cells. A few days before the eyes open, these cholinergic connections are eliminated as the glutamatergic circuits involved in processing visual information are formed. Here, we discuss the assembly and disassembly of this transient cholinergic network and the role it plays in various aspects of retinal development.

Figure 1. Timeline showing major developmental events in mouse retina

Starburst cells become postmitotic between E11 and E17. ChAT expression is first seen at E17 and increases during the first week after birth. The IPL first appears at E17 and SAC processes form two discrete bands by P3. Conventional synapses first appear between amacrine and ganglion cells in the IPL at P3. Retinal waves transition through three stages during development: gap-junction, cholinergic, and glutamatergic. Bipolar cells first innervate the IPL at P7. Eyes open around P12. Ganglion cell dendrites stratify into lamina for an extended time beginning around P9 and continuing through the first month. The action of GABA switches from depolarizing to hyperpolarizing around P8. SACs are initially responsive to ACh action on nAChRs, but lose this responsiveness with the corresponding transition to glutamatergic waves. References:(1) (Voinescu et al., 2009) (2) (Kim et al., 2000) (3) (Hinds & Hinds, 1983) (4) (Fisher, 1979) (5) (Bansal et al., 2000) (6) (Johnson et al., 2003) (7) (Xu & Tian, 2004) (8) (Barkis et al., 2010) (9) (Zheng et al., 2004) (approximate age from rabbit data)

Figure 2. Properties of cholinergic waves are generated by the SAC network

A. Spatial and temporal properties of waves assessed with calcium imaging. TOP: Low power calcium imaging of Fura-2AM labeled neonatal ferret retinas show decreases in fluorescence following calcium increases associated with waves. BOTTOM: Spatial propagation of 30 waves during four consecutive minutes. Each panel shows the propagation of waves that occurred during a one-minute period. Cholinergic retinal waves initiate at random locations and propagate over finite regions. Waves occur at intervals between waves of around one minute for a given region (for example see asterisk). Adapted from (Feller et al., 1996). B. Cell-autonomous spontaneous depolarization of SACs may initiate retinal waves. SACs (red regions and traces) from perinatal rabbit retinas loaded with Fura-2AM show spontaneous depolarizations in the presence of antagonists to ionotropic and metabotropic glutamate, GABA, glycine, and ACh receptors, whereas retinal ganglion cells do not (blue regions and traces). Adapted from (Zheng et al., 2006). C. The inter-wave refractory period may be due to a slow after-hyperpolarization in SACs. Current clamp recording from a SAC in rabbit retina showing wave evoked and spontaneous (asterisk) depolarizations followed by slow after-hyperpolarizations. Adapted from (Zheng et al., 2006). D. Waves may propagate via reciprocal connections between SACs. Voltage clamp recordings from pairs of neighboring SACs show both fast GABAergic postsynaptic currents and slow cholinergic postsynaptic currents. Top: Schematic of voltage command of presynaptic SAC (−70mV to 0mV). Bottom: Evoked PSCs at positive and negative holding potentials to isolate cholinergic and GABAergic currents, respectively. Adapted from (Zheng et al., 2004).

Figure 3. Wave circuits transition through check-points

A. Schematics of changing circuits that mediate waves. Left: Prior to birth waves are thought to propagate via gap-junctions between ganglion cells. Middle: Postnatal day 1–10, waves are propagated via SAC release of acetylcholine onto other SACs (black box). Acetylcholine also depolarizes ganglion cells. During this period of development, the gap-junction signaling between ganglion cells is reduced (red box). Right: P10–P15 bipolar cells release glutamate to propagate waves in a mechanism that is thought to involve spillover of glutamate to excite neighboring bipolar cells (black box). Cholinergic signaling between SACs is reduced (red box). Adapted from (Blankenship & Feller, 2010). B. Summary timeline of how genetic disruption of cholinergic or glutamatergic waves result in an extended action of the previous wave generating circuit In wild type mice gap-junction mediated waves (gray) are followed by cholinergic waves (blue) starting at P0, then glutamatergic waves (green) at P10. In mice lacking the Beta2 subunit of the nicotinic acetylcholine receptor, gap-junction mediated waves persist until ~P8. In mice lacking vesicular glutamate transporter VGLUT1, cholinergic waves persist through the second postnatal week.