DEVELOPMENTAL SYNAPTIC DEPRESSION UNDERLYING REORGANIZATION OF VISCERAL REFLEX PATHWAYS IN THE SPINAL-CORD

During development, neuronal connectivity has a remarkable plasticity. Synaptic refinement in the spinal autonomic nucleus might be involved in the elimination of primitive segmental reflexes and the emergence of mature spinobulbospinal reflexes, which occurs a few weeks after bi rth. To address this possibility, we examined the postnatal changes of segmental excitatory synaptic transmission by applying the whole-cell recording technique to parasympathetic preganglionic neurons in slice preparations of the rat lumbosacral spinal cord. The mean magnitude o f unitary excitatory synaptic currents evoked in preganglionic neurons by stimulation of single interneurons remained unchanged during the f irst two postnatal weeks but was reduced by 50% during the third postn atal week. This reduction in synaptic efficacy was associated with a d ecrease in the amount of transmitter release from interneurons. Moreov er, this developmental depression of segmental synaptic transmission w as prevented by spinal cord transection at the thoracic level on postn atal day 14. Thus, developmental modification of excitatory synapses o n preganglionic neurons appears to be attributable to competition betw een segmental interneuronal and descending bulbospinal inputs, which r esults in the developmental reorganization of parasympathetic excretor y reflex pathways.