During postnatal development, profound changes take place in the excitability of nerve cells, including modification in the distribution and properties of receptor‐operated channels and changes in the density and nature of voltage‐gated channels. We studied here the firing properties of abducens motoneurons (aMns) in transverse brainstem slices from postnatal day (P) 1–13 rats. Recordings were made from aMNs in the whole‐cell configuration of the patch‐clamp technique. Two main types of aMn could be distinguished according to their firing profile during prolonged depolarizations. Both types were identified as aMns by their fluorescence following retrograde labelling with the lipophilic carbocyanine DiI in the rectus lateralis muscle. The first type (BaMns) exhibited a burst of action potentials (APs) followed by an adaptation of discharge and were encountered in ∼70 % of aMns. Their discharge profile resembled that of adult aMns and was encountered in all aMns after P9. BaMns exhibited a hyperpolarization‐induced rebound potential that was blocked by low concentrations of Ni2+ or by Ca2+‐free external solution. This current had the properties of the T‐type current. Action potentials of BaMns showed a complex afterhyperpolarization (AHP). An inward rectification was evidenced following hyperpolarization and was blocked by external application of caesium or ZD7288, indicating the presence of the hyperpolarization‐activated cationic current (IH). Blocking the IH current almost doubled the input resistance of BaMns. The second class of aMns (DaMns) displayed a delayed excitation that was mediated by A‐type K+ currents and was observed only between P4 and P9. DaMns exhibited immature characteristics: an action potential with a simple AHP, a linear current‐voltage relation and a large input resistance. The number of aMns remained unchanged when both types were present (P5–P6) and later in development when only BaMns were encountered (P19), suggesting that DaMns mature into BaMns during postnatal development. We conclude that aMns display profound reorganization in their intrinsic excitability during postnatal development.

-Pestiviruses have been isolated from live sheep pox Tunisian vaccines. Vaccination with these vaccines caused outbreaks of Border Disease in Tunisia. In order to study more precisely the pathogenicity of these isolates, three groups of eight four month old lambs from a pestivirus-free flock were infected by the intratracheal route with a French strain (AV) and two Tunisian isolates (SN3G and Lot21). Clinical, hematological, immunological and virological parameters were evaluated. The three groups developed mild fever and leucopaenia by day 3 to 6 post infection (pi). The differences in the weight curves were not significant. Viruses were isolated from the peripheral blood buffy coat cells by day 4 to 9 pi. Antibodies were present on day 16 pi following infection by the French strain and on day 21 pi with the Tunisian isolates. The results demonstrated that SN3G and Lot21 are almost similar to the French strain used as the reference strain. In field conditions, they could induce economical losses in naive flocks, alone or in association with other pathogens. (33) 492943701; e-mail: p.russo@sophia.afssa.fr 8 agneaux de 4 mois issus d'un troupeau indemne de pestivirus ont été infectés par voie intratrachéale avec une souche française de référence (AV) et 2 isolats tunisiens (SN3G et Lot21). Les données cliniques, hématologiques, immunologiques et virologiques ont été suivies. Les 3 groupes ont développé une légère hyperthermie et une leucopénie entre le 3 e et le 6 e jour post inoculation (pi). Les courbes de poids ne montraient pas de différence significative. Des pestivirus ont été isolés à partir des cellules blanches du sang périphérique entre le 4 e et le 9 e jour pi. Des anticorps ont été mis en évi-dence 16 jours pi avec la souche française et 21 jours pi avec les isolats tunisiens. L'ensemble des ré-sultats a montré que les 2 isolats tunisiens ont un pouvoir pathogène presque similaire à celui de la souche française; ils pourraient provoquer, dans les conditions du terrain, des pertes économiques, seuls ou en association avec d'autres agents pathogènes.Pestivirus / ovin / vaccins anti-clavelée / pouvoir pathogène