Reduced activity during development delays the normal rearrangement of synapses in the rabbit ciliary ganglion

The number of preganglionic axons that innervate individual ciliary ganglion cells has been investigated after cutting one or both optic nerves in new-born rabbits. In agreement with previous work (Johnson & Purves, 1981), ciliary ganglion cells in normal rabbits lose, on average, about one-half of their inputs from preganglionic axons during the first month of post-natal life. Although severing one optic nerve at birth abolished the subsequent appearance of the direct pupillary light response on that side, ciliary ganglion cells of normal neonatal rabbits are driven synaptically in the absence of direct retinal illumination when tested in vivo. This tonic activity persisted for at least 3 months after interruption of the optic nerve at birth. Thus neonatal optic nerve section reduced, but did not eliminate, synaptic activation of ciliary ganglion cells. Optic nerve section at birth delayed the elimination of synaptic connexions in the ipsilateral but not the contralateral ciliary ganglion. Through the first 4 post-natal weeks ciliary ganglion cells on the operated side had, on average, more than the normal number of inputs from preganglionic axons. By 8 weeks of age, however, the normal number of connexions was established in spite of the unilateral visual deprivation. The elimination of connexions in ganglia on the unoperated side followed a normal time course. The effect of bilateral optic nerve section on synaptic rearrangement was the same as the ipsilateral effect after cutting only one optic nerve. It is concluded that the rate of synaptic rearrangement in this ganglion is slowed by a chronic reduction of synaptic activity.