Complex action potential waveform recorded from supraoptic and paraventricular neurones of the rat: evidence for sodium and calcium spike components at different membrane sites.

Magnocellular neurones of the supraoptic and paraventricular nuclei display a complex waveform when recorded extracellularly. The present work has examined the possible reasons for the complexity of this waveform by making extracellular recordings from antidromically identified neurones in the anaesthetized rat and extra- and intracellular recordings from similar neurones in the hypothalamic slice preparation, where external solutions can be changed. In extracellularly recorded units, action potentials had two positive going components. The first of these was abolished by tetrodotoxin. The second, slower component was abolished when external Ca++ concentration was lowered, but enhanced in magnitude and duration when Ba++ was placed in the external medium. The second component was sensitive to electrode movement and was not observed to the same degree when intracellular recordings were made. In light of these observations and the known morphology of these neurones, it is suggested that the magnocellular neurone action potential is comprised of a fast Na+-dependent component and a slower component dependent on Ca++ entry which may originate from a part of the cell other than the soma. The most likely site for this Ca++ component to occur is at the cell dendrite.