Electrophysiological characteristics of neurons in neocortical expiant cultures

We examined the electrophysiological and morphological properties of neocortical neurons maintained in expiant cultures prepared from the parietal cortex of newborn Sprague-Dawley rats. After 3-6 weeks in vitro, cultures showed regional differences in cellular density reminiscent of cortical layering, and an abundance of axonal processes. Pyramidal-shaped neurons with spinous dendrites were the dominant elements revealed by Lucifer yellow injections. Intracellular recordings revealed that many electrophysiological properties of neurons in the explants resembled those of neocortical neurons in vivo and in slice preparations. In response to depolarizing current injection, neurons in the expiants showed the same three patterns of repetitive firing described in neocortical slices, as well as a similar array of responses. Spontaneous synaptic potentials were recorded from all neurons and complex PSPs were evoked in response to focal extracellular stimulation. GABAa receptors mediated a significant component of the evoked responses. Fifteen of sixty neurons generated action potentials that arose spontaneously from resting potentials. Neurons in many slices generated large, prolonged depolarizing potentials that reflected coordinated synaptic activity within the expiants. These results underscore the usefulness of the neocortical explant as a valuable model for studying aspects of the behavior of circuits of cortical neurons.