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Postnatal Development of Membrane Properties and δ Oscillations in Thalamocortical Neurons of the Cat Dorsal Lateral Geniculate Nucleus

The development of membrane properties, firing patterns, and δ oscillations in neurons of the cat dorsal lateral geniculate nucleus (dLGN) was investigated in vitro during the first 7 postnatal weeks. Compared with adult neurons, the resting membrane potential was more depolarized at postnatal days 1–9 (P1–P9), the input resistance was higher at P1–P7, and action potentials had a higher threshold and a smaller amplitude at P1–P3 and a longer duration at P1–P9. At P1–P3 trains longer than 200 msec were rarely observed, and trains with more than three action potentials were only present in 41% of the neurons, whereas at P1–P7 the normalized slope of the instantaneous frequencies at the first five interspike intervals was smaller than in the adult. A long-lasting (up to 6 sec) afterhyperpolarization followed a short train of action potentials in 88 and 30% of neurons at P1–P3 and P30–P32, respectively, but it was rarely observed in the adult. The low-threshold Ca2+potential could evoke a burst of action potentials since P1. However, at P1–P7 the number of action potentials per burst was smaller (range, one to five), and at P1–P9 their maximum instantaneous frequency was lower (<190 Hz) than in the adult (range, six to eight, and 344 Hz, respectively). No δ oscillations were observed until P17, and their frequency (0.36 Hz) was lower than that in the adult (1.8 Hz). The percentage of neurons displaying δ oscillations and their frequency reached adult values by the end of the seventh postnatal week, i.e., well after the maturation of the membrane properties and firing patterns (second postnatal week). In conclusion, the maturation of the electrophysiological properties of thalamocortical neurons in the cat dLGN is completed later than the retinogeniculate axon segregation (Shatz CJ, 1983), and the immaturity of the oscillatory, and not of the burst-firing, activity is a limiting factor in the development of δ waves (Jouvet-Mounier et al., 1970).