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Department of Physiology and Biophysics, University of Washington School of Medicine, Seattle 98195, USA.
1. The repetitive firing properties of neonatal and adult rat hypoglossal motoneurones (HMs) were investigated in a brainstem slice preparation. Neonatal HMs could be classified into two main groups: (1) neurones with a decrementing or adapting firing pattern (type D); exhibiting an early and a late phase; and (2) neurones with an incrementing or accelerating firing pattern (type I). 2. The pattern of repetitive firing changed markedly during development. While most HMs recorded from young rats (< postnatal day (P) 4) were type D, the majority of HMs recorded during the second postnatal week were type I. In adults (> P21), nearly all HMs had a decrementing firing pattern, characterized by a brief period of adaptation and high steady-state firing rates. 3. The calcium-dependent after-hyperpolarization (AHP) was shortest in type I neonatal HMs, and decreased in amplitude during trains of action potentials (APs). In type D neurones, these same trains caused a slight enhancement of AHP amplitude. In adult HMs, with a decrementing firing pattern, trains of APs also caused summation of the AHP. 4. Type D neonatal HMs showed a progressive prolongation of the AP during repetitive firing. In contrast, type I neonatal HMs had almost no change in AP duration. In adult HMs the AP was short and experienced only a modest increase in duration during fast repetitive firing. 5. The function relating steady-state firing frequency to injected current (f-I curve) was linear. The mean steady-state f-I slope was significantly higher in neonates than in adults (approximately 30 vs. approximately 20 Hz nA-1), and was weakly correlated with input resistance. The f-I slope was negatively correlated with AHP duration in neonatal HMs only. In addition, for a given AHP duration the slope was higher in neonatal HMs. 6. Two threshold behaviours were observed among neonatal HMs: (a) a progressive rhythmic firing threshold, and (b) a sudden transition from subthreshold to regular repetitive firing. Current threshold for repetitive firing was strongly correlated with cell input conductance. Type I neonatal HMs had higher minimal steady firing rates (fmin) than type D HMs. In neonates, fmin was strongly correlated with AHP duration. Adult HMs showed a weaker correlation between these two parameters, and fmin was higher than predicted by AHP duration. 7. In summary, HMs responded to depolarizing current pulses with different firing patterns during postnatal development.(ABSTRACT TRUNCATED AT 400 WORDS)
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