Electrical properties of embryonic rat brainstem motoneurones in organotypic slice culture.

Organotypic co-cultures of embryonic E18-19 rat brainstem slices and tongue muscle maintained in vitro for more than 16 days were used to study the electrogenic properties of developing embryonic brainstem motoneurones derived from oculomotor, facial or hypoglossal nuclei. This preparation offers a unique opportunity to study the development of prenatal mammalian embryonic motoneurones. Our results show that embryonic rat brainstem motoneurones grown in organotypic culture with tongue muscle develop electrogenic membrane properties that can be compared with those described in newborn and adult animals in slice preparation. These motoneurones displayed a variety of sodium and calcium conductances. Two types of sodium conductances were present in all the recorded motoneurones. An Hodgkin-Huxley TTX-sensitive sodium conductance was involved in the spike potential, whereas a TTX-insensitive high threshold sodium conductance was uncovered when potassium and calcium currents were suppressed. Prominent calcium potentials contributed to the large delayed depolarizations following the spike potentials. High threshold calcium potentials were triggered with a slightly lower threshold than the sodium spike and contributed to the control of the pattern of discharge of the cell in response to slight shifts of membrane potentials. Low threshold calcium potentials were seen in 16% of the recorded motoneurones in hyperpolarized conditions. These calcium currents underlie the triggering of doublet spikes and rebound responses. Brainstem motoneurones in culture did not display differences that could be correlated with the origin of the motoneurone pool explanted and retained undifferentiated features suggesting that development of electrophysiological properties specific of each motoneurone pool are determined by presynaptic networks and target properties.