Morphology and physiology of neurons in the rat perirhinal-lateral amygdala area

Neuronal structure-function relationships were studied in rat brain slices containing the perirhinal cortex (PR) and immediately adjacent lateral nucleus of the amygdala (ALa). Using video microscopy, whole-cell recordings were made from visually preselected neurons that were labeled with biocytin for subsequent anatomical reconstructions. Most cells were 1 of 4 primary neurophysiological types: fast-spiking (FS), regular-spiking (RS), late-spiking (LS), and burst-spiking (BS). Fast-spiking neurons (small somata) were found throughout PR; RS neurons (stellates and pyramids) were present from layer II/III through VI of PR; BS neurons (large pyramids with thick nonbifurcating apical dendrites) were found in layer Va of PR; and LS neurons (stellates, small pyramids, and cone cells) were encountered in layers II/III and VI of PR. One subpopulation of LS neurons (small pyramids) was found in layer II/III; another (cone cells) was found in clusters spanning layer VI through the lateral portion of ALa. Layer Va also contained large RS pyramidal neurons whose axons were seen traveling in the external capsule, but not entering the ALa. Conversely, the axons of large RS pyramidal neurons in layer Vb typically projected deep into the ALa. The four primary firing patterns were present in ALa, which also contained irregular-spiking, slow-charging, and single-spiking cells. Spontaneous synaptic currents differed markedly among cell types and layers. There was excellent agreement between somatic areas measured from video images of living neurons and somatic areas from the same neurons following fixation. Representative montages, which combined the cellular neuroanatomy and neurophysiology, suggested a circuit-level organization that helps elucidate information processing through the PR-ALa region.