SPECIFICITY OF IDENTIFIED CENTRAL SYNAPSES IN THE EMBRYONIC COCKROACH- APPROPRIATE CONNECTIONS FORM BEFORE THE ONSET OF SPONTANEOUS AFFERENT ACTIVITY

The mechanisms by which neurons recognize the appropriate postsynaptic cells remain largely unknown. A useful approach to this problem is to use a system with a few identifiable neurons that form highly specifi c synaptic connections. We studied the development of synapses between two identified cereal sensory afferents and two giant interneurons (G Is) in the embryonic cockroach Periplaneta americana. By 46% of embryo nic development, the axons of the filiform hair sensory neurons have e ntered the terminal ganglionic neuropil and grow alongside the GI prim ary dendrites, although they do not form synapses. From 50% of develop ment, the GI dendrites grow outward from the center of the neuropil to contact the presynaptic axons and their branches. The sensory neurons begin to spike at 52% of development,and, from 55% of development, th ese action potentials evoked excitatory postsynaptic potentials in the GIs. Synaptic contacts were first seen at this time. The pattern of s ynaptic connections was highly specific from the outset; GI2 had stron g input from the medial (M) afferent and had almost negligible input f rom the lateral (L) afferent, whereas GI3 had input from both. This sp ecificity was present before bursts of spontaneous activity began in t he sensory neurons at 59% of development; GI2 filopodia selectively fo rmed synaptic contacts with the M axon rather than the L axon. The few contacts made by GI2 with the L axon had a normal morphology but fewe r presynaptic densities. Filopodial insertions were not involved in se lective synapse formation. In this system, highly specific synaptic re cognition appears to be activity independent. (C) 1996 Wiley-Liss, Inc .