The morphology and electrophysiological properties of neurons in the nucleus accumbens were studied using intracellular recording techniques in rat brain slices maintained in vitro. Neurons were subdivided according to their location in the shell or core region of the nucleus accumbens. Most of the cells in both regions had small to medium-sized (15.8 +/- 2.8 microns) somata with densely spinous dendrites, somewhat similar to the striatal medium spiny neuron. However, minor morphological differences between neurons from accumbens core and shell regions were found, such as fewer primary dendrites in shell neurons than in the core (3.8 +/- 0.8 vs. 4.4 +/- 1.0) and the spatial organization of their dendritic trees. In general, the passive membrane properties of neurons in each region were similar. However, shell neurons appeared to be less excitable in nature, as suggested by (1) a faster time constant, (2) the absence of TTX-insensitive events resembling low-threshold spikes, and (3) the lower probability of evoking spikes in shell neurons by stimulation of amygdaloid or cortical afferents in comparison to the responses of core neurons to cortical afferent stimulation. In most nucleus accumbens neurons the action potentials evoked by membrane depolarization were preceded by a slow Ca(2+)-dependent depolarization and showed firing-frequency adaptation. Following TTX administration, all-or-none spike-like events resembling high-threshold calcium spikes were observed in both regions. In summary, except for minor differences, most of the properties of core and shell neurons are similar, supporting their characterization as subdivisions of a single structure. Therefore, differences in the functional properties of these neuronal populations are likely to be due to their distinct connectivity patterns.