SYNAPSIN-LIKE MOLECULES IN APLYSIA-PUNCTATA AND HELIX-POMATIA - IDENTIFICATION AND DISTRIBUTION IN THE NERVOUS-SYSTEM AND DURING THE FORMATION OF SYNAPTIC CONTACTS IN-VITRO

The distribution and biochemical features of the synapsin-like peptide s recognized in Aplysia and Helix by various antibodies directed again st mammalian synapsins were studied. The peptides can be extracted at low pH and are digested by collagenase; further, they can be phosphory lated by both protein kinase A and Ca2+/calmodulin-dependent protein k inase II. In the ganglia of both snails, they are associated with the soma of most neurons and with the neuropil; punctate immunostaining is present along the neurites. Using cocultures of a Helix serotoninergi c neuron and of its target cell, we analysed the redistribution of the synapsin-like peptides during the formation of active synaptic contac ts. When the presynaptic neuron is plated in isolation, both synapsin and serotonin immunoreactivities are restricted to the distal axonal s egments and to the growth cones; in the presence of the target, the fo rmation of a chemical connection is accompanied by redistribution of t he synapsin and serotonin immunoreactivities that concentrate in highl y fluorescent round spots scattered along the newly grown neurites loc ated close to the target cell. Almost every spot that is stained for s erotonin is also positive for synapsin. In the presynaptic cell plated alone, the number of these varicosity-like structures is substantiall y stable throughout the whole period; by contrast, when the presynapti c cell synapses the target, their number increases progressively paral lel to the increase in the mean amplitude of cumulative excitatory pos tsynaptic potentials recorded at the same times. The data indicate tha t mollusc synapsin-like peptides to some extent resemble their mammali an homologues, although they are not exclusively localized in nerve te rminals and their expression strongly correlates with the formation of active synaptic contacts.