REPETITIVE FIRING PROPERTIES OF DEVELOPING RAT BRAIN-STEM MOTONEURONS

1. The repetitive firing properties of neonatal and adult rat hypoglos sal motoneurones (HMs) were investigated in a brainstem slice preparat ion. Neonatal HMs could be classified into two main groups: (1) neuron es 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 fir ing changed markedly during development. While most HMs recorded from young rats (<postnatal day (P) 4) were type D, the majority of HMs rec orded during the second postnatal week were type I. In adults (>P21), nearly all HMs had a decrementing firing pattern, characterized by a b rief period of adaptation and high steady state firing rates. 3. The c alcium-dependent after-hyperpolarization (AHP) was shortest in type I neonatal HMs, and decreased in amplitude during trains of action poten tials (APs). In type D neurones, these same trains caused a slight enh ancement of AHP amplitude. In adult HMs, with a decrementing firing pa ttern, trains of APs also caused summation of the AHP. 4. Type D neona tal HMs showed a progressive prolongation of the AP during repetitive firing. In contrast, type I neonatal HMs had almost no change in AP du ration. In adult HMs the AP was short and experienced only a modest in crease in duration during fast repetitive firing. 5. The function rela ting steady-state firing frequency to injected current (f-I curve) was linear. The mean steady-state f-I slope was significantly higher in n eonates than in adults (similar to 30 vs. similar to 20 He nA(-1)), an d was weakly correlated with input resistance. The f-I slope was negat ively correlated with AHP duration in neonatal HMs only In addition, f or a given AHP duration the slope was higher in neonatal HMs. 6. Two t hreshold behaviours were observed among neonatal HMs: (a) a progressiv e rhythmic firing threshold, and (b) a sudden transition from subthres hold to regular repetitive firing. Current threshold for repetitive fi ring was strongly correlated with cell input conductance. Type I neona tal HMs had higher minimal steady firing rates (f(min)) than type D HM s. In neonates, f(min) was strongly correlated with AHP duration. Adul t HMs showed a weaker correlation between these two parameters, and f( min) was higher than predicted by AHP duration. 7. In summary, HMs res ponded to depolarizing current pulses with different firing patterns d uring postnatal. development. Changes in intrinsic membrane properties , including the calcium-dependent AHP conductance, play a role in the repetitive firing patterns displayed by HMs and therefore may influenc e how these motoneurones transform synaptic input into spike output.