Developmental Brain Research, 8 (1983) 311-320 3 1 1 Elsevier Functional Elimination of Afferent Pathways and Decreased Safety Factor During Postembryonic Development of Cockroach Giant Interneurons M. E. SPIRA and Y. YAROM Department of Neurobiologv, Life Sciences Institute, Hebrew Universi(v(Givat Ram), Jerusalem (Israel) (Accepted December 14th, 1982) Key words: safety factor - postembryonic development - interneurons - cockroach The giant interneurons (GIN) from the cockroach CNS undergo two major physiological changes during the postembryonic developmental period: (A) a marked decrease in the number of afferent pathways innervating the GIN at the metathoracic gan- glion (Ts); and (B) a gradual decrease in the safety factor for impulse propagation along the intraganglionic segment in T3. In 100% of the experiments (n ~ 100) performed on GIN from early developmental stages, spontaneous postsynaptic potentials (SPSPs) were recorded; in adults, on the other hand, SPSPs have been recorded in only 34% of the experiments (n -- 74), Evoked synaptic potentials can be elicited in nymphal stages by stimulation of 8 nerves ofT3, the contralateral connectives, ipsi- and con- tralateral nerve roots 2, 3, 5, and by stimulation of adjacent GINs. In adult, PSPs can be evoked by stimulation of adjacent GINs, and contralateral thoracic connectives, but not from nerves 2, 3 and 5. The functional disappearance of synaptic inputs to the GINs does not reflect a general phenomenon of reduction in synaptic transmission efficacy. In previous studies it was demonstrated that high frequency stimulation of adult GIN leads to blockage of impulse propa- gation in T3. In nymphal stages, the safety factor for propagation of impulses along T3 is higher. The reduction in safety factor appears gradually during the postembryonic developmental period. From analysis of the mechanisms underlying the elimination of functional afferent pathways and the appearance of low safety factor (see consecutive paper by Yarom and Spira43) it is concluded that the functional elimination of afferents is a consequence of decreased transmission efficacy, while the appearance of low safety regions for impulse propagation is a consequence of mor- phological changes of the GIN segment within ganglion T3. INTRODUCTION The development of the central and peripher- al nervous systems of both vertebrates and in- vertebrates is not completed during embryoge- nesis (for review see ref. 2). Changes in geometry of soma, dendrites and axons 1.6.7-16.26,30,3L36-41, a s well as changes in membrane properties and connectivity~s.23,37, have been observed in post- natal stages. Structural modifications of a neu- ron with or without changes in its membrane properties may alter its integrative and func- tional properties. As a consequence of such changes, behavioral patterns may be altered. Another common feature in the development of the peripheral and central nervous systems in vertebrates is a reduction in the number of axons that synapse with a given target cell during em- bryonic and postembryonic development (for review, see Purves and Lichtman32). For exam- pie, the number of axons innervating a single muscle cell decrease during the first few weeks after birth of mammals and a one-on-one pat- tern of innervation is established 32.3-~. A similar phenomenon was documented to occur in au- tonomic ganglia 2°-22, Purkinje cells ~L and in the visual c o r t e x 1732 34. In the present work, we examine the changes in properties and pattern of innervation of iden- tifiable interneurons during postembryonic de- velopment. The ventral giant interneurons (GINs) of the cockroach (Periplaneta ameri- cana) were selected for this study, since they can be morphologically identified throughout the various postembryonic developmental stages and are large enough throughout these stages to allow for intracellular recording and stimula- tion. The giant interneurons of the cockroach Peri- planeta americana originate in the last abdomi- 0165-3806/83/$03.00 (~ 1983 Elsevier Science Publishers B.V.