Morphology and distribution of electrophysiologically defined classes of pyramidal and nonpyramidal neurons in rat ventral subiculum in vitro.

Intracellular electrophysiological recordings were made from 210 ventral subicular neurons in rat brain slices. Recordings were classified as burst-firing or nonburst-firing. Eighteen burst-firing neurons were filled with Neurobiotin, and all had pyramidal morphology. Nine of these recordings were made from intrinsically burst-firing (IB) cell bodies, and nine were made from burst-firing dendrites (BD). Twelve nonburst-firing neurons were also filled with Neurobiotin. Eight were regular spiking (RS) and had pyramidal morphology, four were fast spiking (FS) and nonpyramidal. Additional electrophysiological parameters distinguished IB from BD, RS from FS, and RS from IB recordings. The distribution of IB and RS neurons was examined by using 180 recordings. Information from the first series of experiments was used to distinguish between somatic and dendritic recordings. The deep-superficial axis (alveus-hippocampal fissure) was divided into four equal rows. RS neurons accounted for 12%, 28%, 58%, and 50% of presumed somatic recordings in successively more superficial rows. The proximal-distal (CA1-perforant path) axis was divided into five equal columns. RS cells accounted for 52% of presumed somatic impalements in the central column compared with 16% in the most proximal and 10% in the most distal columns. Thus, two electrophysiological classes of pyramidal neuron were localized to particular regions of the ventral subiculum. In the light of existing knowledge of the topography of subicular inputs and outputs, our results are consistent with the hypothesis that the ratio of RS to IB pyramidal neurons will be different in different transhippocampal circuits.