Chemical and electrotonic connections between Aplysia neurons in primary culture.

Dissociated Aplysia neurons will regenerate neurites and form functional connections in primary cell cultures. The specificity of intercellular connectivity in these cultures was investigated by coculturing neurosecretory bag cells with neurons dissociated from the buccal ganglion. It was found that bag-bag and buccal-buccal electrotonic synapses form with high frequency, consistent with previous findings in pure bag and buccal cultures. There is specificity in the formation of these connections, since no bag-buccal electrotonic synapses were observed. Chemical interactions, on the other hand, are present between bag and buccal neurons. In a buccal-bag pair, injection of sufficient depolarizing current into the buccal cell to elicit a train of action potentials leads to a slow hyperpolarizing response in the bag cell. The bag cell hyperpolarization is accompanied by an increase in the cell's input conductance. This connection appears to be unidirectional, produces a voltage shift in the bag cell which is opposite in sign to that in the buccal cell, and is blocked by the removal of Ca2+ from the extracellular medium, indicating that it is mediated by a chemical neurotransmitter. The selective formation of electrotonic synapses in these mixed bag-buccal cultures, together with the presence of chemically mediated interactions, make this system particularly useful for investigating the establishment of intercellular connectivity.