GABA mediated excitatory responses on Aplysia neurons

γ-Aminobutyric acid (GABA), a known inhibitory neurotransmitter in mammals, can elicit two different types of excitatory response in the nervous system of the marine mollusc, Aplysia. These responses are depolarizing when GABA is applied ionophoretically, and result from either an increase in membrane conductance to Na⁺ or a decrease in conductance to K⁺. In addition, GABA on other neurons causes an inhibitory response similar to that commonly found in other preparations. Although not all neurons have GABA receptors, identified single cells consistently have the same type of response. These observations suggest the possibility that GABA may function in at least some preparations as an excitatory neurotransmitter in addition to its documented inhibitory function.