Submitted on March 31, 2008
Revised on September 26, 2008
Accepted on September 29, 2008
Development of theta rhythmicity in entorhinal stellate cells of the juvenile rat
Brian Burton, Michael N. Economo, G. Jenny Lee1, and John A. White2*
1 Boston University
2 University of Utah
* To whom correspondence should be addressed. E-mail: john.white{at}utah.edu.
Mature stellate cells of the rat medial entorhinal cortex (EC), layer II, exhibit subthreshold membrane potential oscillations (MPO) 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+ (P28-32). 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 non-resonant with a relatively high resistance. Densities of Ih and INaP, both rose by approximately 50% from P14 to P18. However, Ih levels fell to intermediate values by P28+. Given the non-robust 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 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.
