Postnatal development of signal generation in auditory thalamic neurons

Using whole cell recording techniques, we distinguished immature from mature stages of development in auditory thalamic neurons of rats at ages P5 to P21. We compared voltage responses to injected currents and firing patterns of neurons in ventral partition of medial geniculate body (MGBv) in slices. Resting potential, input resistance and membrane time constant diminished to mature values between P5 and P14. Responses of young neurons to hyperpolarizing pulses showed delayed inward rectification; after P13, this was obscured by a rapid onset of another inward rectifier. All neurons possessed tetrodotoxin (TTX)-sensitive, depolarization-activated rectification, implying persistent Na+-current involvement. Despite a slightly higher voltage threshold for spiking, the current threshold was lower in younger neurons. Young neurons fired a short latency spike with afterhyperpolarization whereas older neurons exhibited a slow ramplike depolarization before tonic firing. Large currents caused continuous firing in all neurons. Before day P13, a high threshold Ca2+ spike (HTS) often was appended to action potentials. The low threshold Ca2+-spike (LTS) was too small in amplitude to evoke action potentials before P11 but produced a single spike at P12 and P13 and burst firing with HTS after P13. MGBv neurons have mature properties after P14, relevant for reactions to sound and the oscillations of slow-wave sleep.