Development of Theta Rhythmicity in Entorhinal Stellate Cells of the Juvenile Rat

Author:

Burton Brian G.,Economo Michael N.,Lee G. Jenny,White John A.

Abstract

Mature stellate cells of the rat medial entorhinal cortex (EC), layer II, exhibit subthreshold membrane potential oscillations (MPOs) at theta frequencies (4–12 Hz) in vitro. We find that MPOs appear between postnatal days 14 (P14) and 18 (P18) but show little further change by day 28+ (P28–P32). To identify the factors responsible, we examined the electrical responses of developing stellate cells, paying attention to two currents thought necessary for mature oscillation: the h current Ih, which provides the slow rectification required for resonance; and a persistent sodium current INaP, which provides amplification of resonance. Responses to injected current revealed that P14 cells were often nonresonant with a relatively high resistance. Densities of Ih and INaP both rose by about 50% from P14 to P18. However, Ih levels fell to intermediate values by P28+. Given the nonrobust trend in Ih expression and a previously demonstrated potency of even low levels of Ih to sustain oscillation, we propose that resonance and MPOs are limited at P14 more by low levels of INaP than of Ih. The relative importance of INaP for the development of MPOs is supported by simulations of a conductance-based model, which also suggest that general shunt conductance may influence the precise age when MPOs appear. In addition to our physiological study, we analyzed spine densities at P14, P18, and P28+ and found a vigorous synaptogenesis across the whole period. Our data predict that functions that rely on theta rhythmicity in the hippocampal network are limited until at least P18.

Publisher

American Physiological Society

Subject

Physiology,General Neuroscience

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