Correlated electrophysiology and morphology of the ependyma in rat hypothalamus

The ependyma lines the ventricular system of the vertebrate brain and spinal cord. Although its embryology and morphology have been studied extensively, little is known of its physiological properties, particularly in mammals. Tanycytes are modified ependymal cells that are found predominantly lining the floor of the third ventricle, overlying the median eminence. Their processes accompany and enwrap neuroendocrine axons that course from hypothalamic nuclei to terminals in the median eminence, but the significance of this interaction is not yet understood. Intracellular recording and injection techniques were used to study ependymal cells and tanycytes of the rat in the hypothalamic slice preparation after differentiating their respective regions morphologically. With extracellular [K+] = 6.24 mM, the mean membrane potential (+/- SD) for common ependyma was -79.9 +/- 1.40 mV and for tanycytes, -79.5 +/- 1.77 mV. Input resistances (Rin) were very low (much less than 1 M omega). Single-cell injection of Lucifer yellow revealed dye coupling among 2-70 ependymal cells and 5-48 tanycytes. In both freeze-fractured replicas and thin sections, large numbers of gap junctions were found between adjacent ependymal cells and between adjacent tanycytes. The observations of numerous gap junctions, extensive dye coupling and low input resistance demonstrated that both populations are strongly coupled networks. Perhaps for this reason, attempts to uncouple these cells using sodium propionate or CO2 were unsuccessful. Electrical stimulation of the arcuate nucleus did not elicit any detectable synaptic response in impaled tanycytes, so that the functional significance of synaptoid contacts between neuroendocrine neurons and the postsynaptic tanycytes is not yet apparent. Ependymal cells and tanycytes demonstrated a near-Nernstian response to changes in extracellular [K+] between 3 and 20 mM. This finding, as well as their high negative resting potential, low Rin, extensive coupling and absence of spontaneous electrical excitability demonstrate that ependymal cells possess numerous glial characteristics and may therefore have similar functions. In the hypothalamus, ependyma probably take up K+ released from adjacent endocrine neurons and shunt it to the ventricular space.