The development of cranial nerve and visceral afferents to the nucleus of the solitary tract in the rat

We have used carbocyanine dye tracing techniques to examine the distributio n of afferents from the facial, trigeminal and vagal nerves to the nucleus of the solitary tract (NST) in the developing rat (E13 to P13). Crystals of DiI (1, 1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) were placed (unilaterally) into the facial or trigeminal ganglia, or into t he cervical vagus nerve, and the sections examined with a laser scanning co nfocal microscope. Inputs from some peripheral structures (tongue, aortic a rch, right atrium and lung) to the NST were also analyzed to provide inform ation on the distribution of organ-specific afferents. No afferents were la beled following DiI placement in the above sites at E13, At E14, a few axon s from the geniculate ganglion of the facial nerve were present in the NST anlage, but these were restricted to the area adjacent to the solitary trac t. These axons began to invade the medial NST at E15. By E17, facial affere nt axons had become widespread throughout rostral NST and from E19 the dist ribution of DiI labeling displayed a morphologically mature pattern. DiI-la beled afferent axons from the trigeminal nerve first emerged into the NST a nlage at E14, initially coursing medially to penetrate the ventricular zone . Between E15 and E17, axonal density increased markedly but after E17 beca me progressively confined to the lateral NST. Axons from the vagus nerve fi rst appeared in the caudal NST as early as E14 and coursed directly into th e proliferative zone of the alar plate at all rostrocaudal levels by E15. F rom E19 through postnatal life, the distribution of vagal afferent axons wa s essentially stable with particularly dense label in the caudal NST. Crani al nerve afferents to the NST appear to be distributed to appropriate sites from the beginning of ingrowth, with the exception of trigeminal afferents , where some small initial exuberance was found. The terminal fields derive d from selected peripheral organs such as lung, right atrium, aortic arch a nd tongue were also predominantly distributed to appropriate subnuclei from the beginning of ingrowth into the NST, although organ-specific afferent f ields appeared to develop dense arbors somewhat later than did individual c ranial nerves. Electron microscopy was used to examine regional synapse dev elopment in the rat NST. There was some delay between the ingrowth of affer ents to the NST (E15) and the first appearance of synaptic thickenings. The earliest synapses were simple (usually) symmetrical membrane thickenings ( from E17) and vesicles did not appear until E19. High synaptic density with in the C subnucleus appeared during early postnatal life. Synaptic glomerul i, which are a characteristic feature of afferent input to the adult NST, h ad not developed by birth, indicating that the pre- and perinatal function of the NST must be mediated through simpler, single, axodendritic inputs to NST neurons.