Target-dependent modulation of neurotransmitter release in cultured Helix neurons involves adhesion molecules

The secretory capabilities of the serotonergic neuron C1 of cerebral gangli on of Helix pomatia were markedly reduced when it was cultured in contact w ith the wrong target neuron, C3. When the neuron B2, one of its physiologic al targets, was micromanipulated within the network made of intermingled ne urites originating from the axonal stumps of both C1 and C3 neurons, C1 inc reased the amount of the evoked transmitter release, which, after 30 min, r eached the level observed when cocultured with the appropriate target. The removal of the appropriate target brought C1 back to the low release condit ion. By imaging C1 neurites with a fluorescent dye, morphological changes i nvolving a local increase in the number of varicosities could be observed a s early as 30 min after contact with the appropriate target. Monoclonal ant ibody 4E8 against apCAM, a family of Aplysia adhesion molecules, recognizes apCAM-like molecules of the Helix central nervous system on immunocytochem istry and Western blot analysis. The contact with the appropriate target pr eviously incubated in a 4E8 solution, which did not interfere with its capa city to respond to serotonin, failed to increase the transmitter release of C1 cocultured in the presence of the wrong target, C3. These results sugge st that the apCAM-like antigens bound to the target membrane participate in the molecular processes responsible for the assembly of the "release machi nery" present in the functional presynaptic structure. (C) 2001 Wiley-Liss, Inc.