Patterns of spontaneous activity and morphology of interneuron types in organotypic cortex and thalamus-cortex cultures

The physiological and morphological properties of interneurons in infragran ular layers of rat visual cortex have been studied in organotypic cortex mo nocultures and thalamus-cortex co-cultures using intracellular recordings a nd biocytin injections. Cultures were prepared at the day of birth and main tained for up to 20 weeks. Twenty-nine interneurons of different types were characterized, in addition to 170 pyramidal neurons. The cultures develope d a considerable degree of synaptically driven "spontaneous" bioelectric ac tivity without epileptiform activity. Interneurons in cortex monocultures a nd thalamus-cortex co-cultures had the same physiological and morphological properties, and also pyramidal cell properties were not different in the t wo culture conditions. All interneurons and the majority of pyramidal cells displayed synaptically driven action potentials. The physiological group o f fast-spiking interneurons included large basket cells, columnar basket ce lls (two cells with an arcade axon) and horizontally bitufted cells. The ph ysiological group of slow-spiking interneurons included Martinotti cells an d a "long-axon" cell. Analyses of the temporal patterns of activity reveale d that fast-spiking interneurons have higher rates of spontaneous activity than slow-spiking interneurons and pyramidal cells. Furthermore, fast-spiki ng interneurons fired spontaneous bursts of action potentials in the gamma frequency range. We conclude from these findings that physiological and morphological proper ties of interneurons in organotypic mono- and co-cultures match those of in terneurons characterized in vivo or in acute slice preparations, and they m aintain in long-term cultures a well-balanced state of excitation and inhib ition. This suggests that cortex-intrinsic or cell-autonomous mechanisms ar e sufficient for the expression of cell type-specific electrophysiological properties in the absence of afferents or sensory input. (C) 1999 IBRO. Pub lished by Elsevier Science Ltd.