We have studied the effects of curare on responses resulting from iontophoretic 
application of several putative neurotransmitters onto Aplysia neurons. These 
neurons have specific receptors for acetylcholine (ACh), dopamine, octopamine, 
phenylethanolamine, histamine, gamma-aminobutyric acid (GABA), aspartic acid, 
and glutamic acid. Each of these substances may on different specific neurons 
elicit at least three types of response, caused by a fast depolarizing Na+, a 
fast hyperpolarizing Cl-, or a slow hyperpolarizing K+ conductance increase. All 
responses resulting from either Na+ or Cl- conductance increases, irrespective 
of which putative transmitter activated the response, were sensitive to curare. 
Most were totally blocked by less than or equal to 10-4 M curare. GABA responses 
were less sensitive and were often only depressed by 10-3 M curare. K+ 
conductance responses, irrespective of the transmitter, were not curare 
sensitive. These results are consistent with a model of receptor organization in 
which one neurotransmitter receptor may be associated with any of at least three 
ionophores, mediating conductance increase responses to Na+, Cl-, and K+, 
respectively. In Aplysia nervous tissue, curare appears not to be a specific 
antagonist for the nicotinic ACh receptor, but rather to be a specific blocking 
agent for a class of receptor-activated Na+ and Cl- responses.