The neurotransmitter synthesized by a given class of neurones is subject to modification and, indeed, a qualitative switchover in transmitter biochemistry recently has been demonstrated (Furshpan, POtter & Landis, 1980; Walicke, Campenot & Patterson, 1977). In conjunction with the specification of transmitter biosynthesis that becomes established in a given neurone, a complementary specification of appropriate receptor production is required in any cell functionally post-synaptic to that neurone. An additional requirement of peculiar force in the nervous system has to do with the spatial organization of the receptors in the surface membrane of the post-synaptic cell once the receptors are synthesized. Inappropriately distributed receptors are useless receptors. The perfect registration of a variety of types of presynaptic release sites with high post-synaptic concentrations of appropriate receptors constitutes one of the outstanding features of nervous-system organization that must be accounted for. We report some experiments directed toward understanding the cell biology of regulation of receptor distribution over the surface membrane of muscle cells. Functional synaptic connexions are formed quite early in development and the stability and maturation of synaptic networks is contingent on a number of factors. One interesting contingency is that related to the functional activity of developing networks. Do only those networks survive and mature which are activated by stimuli impinging from the environment? (Wiesel & Hubel, 1963). Put more simple, are action potentials and synaptic activity essential for neuronal maturation? We address this question in cell culture systems from the mammalian central nervous system.

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