Mice Lacking Specific Nicotinic Acetylcholine Receptor Subunits Exhibit Dramatically Altered Spontaneous Activity Patterns and Reveal a Limited Role for Retinal Waves in Forming ON and OFF Circuits in the Inner Retina

Abstract

Before phototransduction, spontaneous activity in the developing mammalian retina is required for the appropriate patterning of retinothalamic connections, and there is growing evidence that this activity influences the development of circuits within the retina itself. We demonstrate here that the neural substrate that generates waves in the mouse retina develops through three distinct stages. First, between embryonic day 16 and birth [postnatal day 0 (P0)], we observed both large, propagating waves inhibited by nicotinic acetylcholine receptor (nAChR) antagonists and small clusters of cells displaying nonpropagating, correlated calcium increases that were independent of nAChR activation. Second, between P0 and P11, we observed only larger propagating waves that were abolished by toxins specific to ␣3 and ␤2 subunit-containing nAChRs. Third, between P11 and P14 (eye opening) we observed propagating activity that was abolished by ionotropic glutamate receptor antagonists. The time course of this developmental shift was dramatically altered in retinas from mice lacking the ␤2 nAChR subunit or the ␤2 and ␤4 subunits.These retinas exhibited a novel circuit at P0, no spontaneous correlated activity between P1 and P8, and the premature induction at P8 of an ionotropic glutamate receptor-based circuit. Retinas from postnatal mice lacking the ␣3 nAChR subunit exhibited spontaneous, correlated activity patterns that were similar to those observed in embryonic wild-type mice. In ␣3Ϫ/Ϫ and ␤2Ϫ/Ϫ mice, the development and distribution of cholinergic neurons and processes and the density of retinal ganglion cells (RGCs) and the gross segregation of their dendrites into ON and OFF sublaminae were normal. However, the refinement of individual RGC dendrites is delayed. These results indicate that retinal waves mediated by nAChRs are involved in, but not required for, the development of neural circuits that define the ON and OFF sublamina of the inner plexiform layer.

The role of nAChR‐mediated spontaneous retinal activity in visual system development

“…Changes in nAChR signaling can also alter the development of wave-mediating circuitry as revealed in studies with transgenic mice (Bansal et al, 2000). Mice lacking the ␣3 subunit of nAChR continue to support nAChR-based retinal activity, but also support non-nAChR-based activity that may be an extension of an embryonic wave-generating mechanism.…”

“…This activity is initially mediated by cholinergic circuits and then by glutamatergic circuits (Wong, 1999;Zhou and Zhao, 1999;Bansal et al, 2000;Zhou and Zhao, 2000). Several experimental results indicate that waves mediated by cholinergic circuits require the activation of nAChRs.…”

“…Second, the periodic calcium increases, action potentials, and compound postsynaptic currents associated with waves can all be blocked by a variety of nAChRs antagonists [ Fig. 2(B)] (Feller et al, 1996;Penn et al, 1998;Sernagor and Grzywacz, 1999;Bansal et al, 2000;Zhou and Zhao, 2000). Third, simultaneous recordings from rabbit starburst amacrine cells and RGCs show that the cholinergic starburst cells that synapse onto RGCs undergo spontaneous depolarizations that are correlated with the depolarizations of neighboring RGCs (Zhou, 1998) These recordings also show that, like RGCs, starburst amacrine cells themselves are excited via synaptic input during waves.…”

“…High-affinity antagonists selective for nAChRs containing particular subunits or subunit combinations have been used to identify the functional receptors mediating retinal waves in ferrets (Penn et al, 1998) and mice (Bansal et al, 2000) [ Fig. 2(B)].…”

“…Based on this pharmacology result, Bansal et al (2000) studied the spontaneous retinal activity in transgenic mice lacking either the ␣3, ␤4, or ␤2 nAChR subunits (Xu et al, 1999a(Xu et al, , 1999b. They found that ␣3Ϫ/Ϫ and ␤4Ϫ/Ϫ mice have retinal waves.…”