Competence to form electrical connections is restricted to growing neurites in the snail, Helisoma

Identified neurons of the snail, Helisoma, undergo extensive remodeling in response to axotomy, including the formation of specific sets of novel electrical connections. This communication addresses the question of why, under the conditions employed, some neurons readily form new connections with a single "test" neurons, whereas others do not. The present experiments are a test of the hypothesis that, for these adult neurons, competence to form electrical connections is restricted to pairs of neurons with interacting regions of active outgrowth. Morphological observations demonstrated profuse overlapping outgrowth from neurons which formed electrical connections, whereas neurons which did not connect displayed no simultaneous new outgrowth, although there could be regions of physical overlap or proximity. The causal relationship between growth and the ability to form new connections was tested more directly by two means: (1) Previously nonconnecting neurons were recruited into the connectivity pattern by axotomy-induced growth. (2) Previously connecting neurons did not connect when they were not induced to grow. Thus, growth or lack of growth is an effective discriminator for determining specific sets of interconnected neurons.