POSTNATAL-DEVELOPMENT OF TERMINALS AND SYNAPSES IN LAMINAE-I AND LAMINAE-II OF THE RAT MEDULLARY DORSAL HORN

To better understand developing orofacial nociceptive circuits and to provide a baseline for evaluating injury-induced plasticity, the ultra structure of the superficial laminae in the rat medullary dorsal horn was examined at birth and at postnatal days 1, 4, 17, and 90. Quantita tive features of terminals and synapses were studied with stereologica l methods. In laminae I and II: I) Axon terminal density increased sig nificantly from birth to day 4 and again from day 4 to day 90. 2) The density of degenerating profiles increased significantly from birth to day 1 and from birth to day 4 and then decreased from day 4 to day 90 . 3) Degenerating profiles were most dense on day 1 and declined stead ily thereafter; by day 90, such profiles were rare. 4) Cavitation was by far the most common form of degeneration seen at early postnatal ag es. 5) Growth cone-like profiles were most dense at birth and declined steadily during the first 2 postnatal weeks; by day 90, such profiles were absent. 6) Terminals with flat synaptic vesicles were rarely see n before day 90, when they accounted for 7% of the terminal population . 7) The density of synapses increased continuously from birth until d ay 90. These data suggest that, as in the spinal cord, medullary dorsa l horn circuits are very immature at birth. Adult-like quantitative fe atures are not attained until after day 17. Moreover, whereas degenera ting profiles are prevalent during early postnatal development, and th ey have features that resemble naturally occurring degeneration, the t otal numbers of terminals and synapses continue to increase dramatical ly and gradually during a protracted postnatal period (to postnatal da y 17). (C) 1997 Wiley-Liss, Inc.