Barium-induced bistability in rat ocular motoneurones in vitro

Suppression of visually and memory-guided saccades induced by electrical stimulation of the monkey frontal eye field. II. Suppression of bilateral saccades

To understand the neural mechanism of fixation, we investigated effects of electrical stimulation of the frontal eye field (FEF) and its vicinity on visually guided (Vsacs) and memory-guided saccades (Msacs) in trained monkeys and found that there were two types of suppression induced by the electrical stimulation: suppression of ipsilateral saccades and suppression of bilateral saccades. In this report, we characterized the properties of the suppression of bilateral Vsacs and Msacs. Stimulation of the bilateral suppression sites suppressed the initiation of both Vsacs and Msacs in all directions during and approximately 50 ms after stimulation but did not affect the vector of these saccades. The suppression was stronger for ipsiversive larger saccades and contraversive smaller saccades, and saccades with initial eye positions shifted more in the saccadic direction. The most effective stimulation timing for the suppression of ipsilateral and contralateral Vsacs was approximately 40-50 ms before saccade onset, indicating that the suppression occurred most likely in the superior colliculus and/or the paramedian pontine reticular formation. Suppression sites of bilateral saccades were located in the prearcuate gyrus facing the inferior arcuate sulcus where stimulation induced suppression at < or =40 microA but usually did not evoke any saccades at 80 microA and were different from those of ipsilateral saccades where stimulation evoked saccades at < or =50 microA. The bilateral suppression sites contained fixation neurons. The results suggest that fixation neurons in the bilateral suppression area of the FEF may play roles in maintaining fixation by suppressing saccades in all directions.

Differential expression of voltage-activated calcium channels in III and XII motoneurones during development in the rat

To further our understanding of the role that voltage-activated Ca2+ channels play in the development, physiology and pathophysiology of motoneurones (MNs), we used whole-cell patch-clamp recording to compare voltage-activated Ca2+ currents in oculomotor (III) and hypoglossal (XII) MNs of neonatal [postnatal day (P)1-5] and juvenile (P14-19) rats. In contrast to III MNs that innervate extraocular muscles, XII MNs that innervate tongue muscles mature more rapidly, fire bursts of low frequency action potentials and are vulnerable to degeneration in amyotrophic lateral sclerosis. In neonates, low voltage-activated (LVA) Ca2+ current densities are similar in XII and III MNs but high voltage-activated (HVA) Ca2+ current densities are twofold higher in XII MNs. The HVA Ca2+ channel antagonists (nimodipine and nifedipine for L-type, omega-agatoxin-TK for P/Q-type and omega-conotoxin-GVIA for N-type) revealed that, while N- and P/Q-type HVA Ca2+ channels are present in both MN pools, a 3.5-fold greater P/Q-type Ca2+ current in XII MNs accounts for their greater HVA Ca2+ currents. Developmentally, LVA and HVA Ca2+ current densities decrease in III MNs but remain unchanged in XII MNs. Thus, the differences between these MN pools increase developmentally so that, in juveniles, the LVA Ca2+ current density is twofold greater and the HVA Ca2+ current density is threefold greater in XII compared with III MNs. We propose that this differential expression of LVA and HVA Ca2+ channels in XII and III MNs during development contributes to their distinct physiology and may also be a factor contributing to the greater susceptibility of XII MNs to degeneration as seen in amyotrophic lateral sclerosis.

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.