Neonatal synapse elimination in the rat submandibular ganglion: Effect of retarded target growth.

We have studied synapse elimination in the submandibular ganglion of neonatal rats to determine the effects of retarded target growth on synaptic development. Neurons of this ganglion provide parasympathetic innervation to the submandibular and sublingual salivary glands. Ligating the main salivary ducts 2-4 days after birth at a point where nerve fibers were not damaged reduces gland weight by 55% during the 2nd wk after birth and 80% by adulthood. In control amimals, the average number of preganglionic inputs/neuron normally declines steadily during the first few weeks after birth, before stabilizing during the 5th wk at the control adult level. Between birth and adulthood, the number of ganglionic neurons increases by 150%. Ganglia from duct-ligated animals showed an acceleration in the process of synapse elimination. Input number in experimental ganglia reached the control adult level during 3rd wk after birth. This acceleration is confined solely to ganglia that innervate the underdeveloped glands. The loss of inputs was not further enhanced by prolonged target atrophy. Thus average input numbers to neurons of 5th wk or adult experimental ganglia were not different from age-matched control values. No differences from control values were seen in most cases for resting potentials, input resistances, or cell size. However, the increase in neuron number was retarded in experimental animals, and the number of synapses/neruonal profile was reduced in the adult animals. Thus subnormal target growth leads to an acceleration in the process of synaptic elimination in neonatal rats. This acceleration may be mediated by alterations in the level of trophic factors emanating from the target.