Intracellular recordings were made from the lateral nucleus of the amygdala in tissue slices cut from rat brain and maintained in vitro. 2. Three types of neurones were distinguished according to the after-potential that followed an action potential. Type 1 cells (44%, n = 225) had depolarizing after-potentials, resulting from a calcium-dependent chloride conductance. Type 2 cells (48%) had long-lasting (> 250 ms) hyperpolarizing after-potentials and type 3 cells (8%) had shorter hyperpolarizing after-potentials. The average resting potentials of the three cell types were -78, -69 and -62 mV respectively. Intracellular labelling with biocytin showed that type 1 cells were pyramidal neurones; type 2 and type 3 cells were non-pyramidal. 3. Experiments with receptor antagonists identified synaptic potentials mediated by excitatory amino acids and by GABA (acting at GABAA receptors) in all three cell types. A longer duration inhibitory synaptic potential resulting from activation of GABAB receptors was present in type 1 (pyramidal) and type 2 cells. 4. Cholecystokinin (100 nM to 1 microM) depolarized type 2 and type 3 cells but had no effect on type 1 (pyramidal) cells. Baclofen (1-3 microM) hyperpolarized type 1 and type 2, but not type 3 cells. [Met5]enkephalin (1-10 microM) hyperpolarized only type 2 cells. 5. It is concluded that the lateral nucleus of the amygdala contains pyramidal neurones and two types of non-pyramidal neurone; these can be differentiated by membrane properties, synaptic inputs and sensitivities to transmitters.
