Membrane currents recorded from sexually dimorphic motoneurones of the bulbocavernosus muscle in neonatal rats.

1. The electrophysiological properties were compared between sexually dimorphic motoneurones in the spinal nucleus of the bulbocavernosus (SNB) and those innervating hindlimb muscles by the whole-cell recording technique in thin slices of the neonatal rat spinal cord. 2. The mean duration of action potentials in SNB motoneurones was significantly longer than that in hindlimb motoneurones. 3. The spike duration of motoneurones was inversely related to the magnitude of transient K+ currents (IA), and in this relation, there was a continuous gradation between SNB and hindlimb motoneurones. 4. The mean duration of spike after-hyperpolarization (AHP) in female SNB motoneurones was significantly longer than that in male SNB motoneurones. 5. In both SNB and hindlimb motoneurones, sustained, Ca(2+)-dependent K+ currents (IAHP) appear to be responsible for the generation of AHP. When IAHP was recorded as a tail current following an identical depolarizing pulse, the magnitude and time course of IAHP were relatively uniform, regardless of the type of motoneurone. 6. In both SNB and hindlimb motoneurones, voltage-gated Ca2+ currents showed an initial transient phase followed by a sustained phase. The mean magnitude of sustained Ca2+ currents was larger in female SNB motoneurones than in male SNB motoneurones, whereas the mean magnitude of transient Ca2+ currents showed no significant different between male and female SNB motoneurones. 7. It is concluded that SNB and hindlimb motoneurones cannot be classified into two distinct neurone types in terms of their electrophysiological properties. 8. It is suggested that the predominant occurrence of natural cell death in female SNB motoneurones during early development may be due in part to high densities of sustained Ca2+ channels in these neurones.