TETANIC STIMULATION AND CYCLIC ADENOSINE-MONOPHOSPHATE REGULATE SEGREGATION OF PRESYNAPTIC INPUTS ON A COMMON POSTSYNAPTIC TARGET NEURON IN-VITRO

Previous studies indicated that Aplysia sensory neurons (SNs) compete when reestablishing synapses with a motor cell target (L7) in vitro. T he competition is characterized by a cell number-dependent decrease in the efficacy of each connection, an increase in the elimination of SN varicosities, a reduction in the formation of new SN varicosities, an d the segregation of varicosities of each SN to restricted portions of the target axons. The changes do not require spike activity, since bo th the SNs and L7 do not fire spontaneously. Here, we examined whether adding activity to SNs during the early stages of synapse formation w ith stimuli known to evoke facilitatory responses in stable SN-L7 conn ections-tetanic stimulation or increase in intracellular cyclic adenos ine monophosphate (cAMP)-would modulate the intrinsic segregatory proc ess. Tetanic stimulation to one SN increased synapse efficacy and the number of varicosities of the stimulated SNs while reducing the functi onal changes by the nonstimulated SNs in the same cultures. An increas e in the stability of preexisting varicosities contributed to the over all increase in varicosities evoked by tetanus. The functional changes evoked by tetanus were not expressed when the same tetanic stimulatio n was also given to the other SN, or when L7 was hyperpolarized during the tetanus to the SN. Raising cAMP levels in one SN increased synaps e efficacy and the rate of new varicosity formation by the injected SN s without affecting the development of the connections formed by the n oninjected SNs. These results suggest that different forms of presynap tic and postsynaptic activities in neurons can regulate specific aspec ts of the competitive process associated with the fine-tuning of conne ctions formed by converging presynaptic inputs. (C) 1996 John Wiley & Sons, Inc.