IONIC MECHANISMS INVOLVED IN THE SPONTANEOUS FIRING OF TEGMENTAL PEDUNCULOPONTINE NUCLEUS NEURONS OF THE RAT

We have previously defined three types of tegmental pedunculopontine n uclei neurons based on their electrophysiological characteristics: Typ e I neurons characterized by low-threshold Ca2+ spikes, Type II neuron s which displayed a transient outward current (A-current), and Type II I neurons having neither low-threshold spikes nor A-current [Kang: Y. and Kitai S. T. (1990) Brain Res. 535, 79-95]. In this report, ionic m echanisms underlying repetitive firing of Type I (n = 15) and Type II (n = 69) neurons were studied in in vitro slice preparations. Type I n eurons did not fire rhythmically but their spontaneous firing frequenc y ranged From 0 to 19.5 spikes/s (mean 9.7 spikes/s). The spontaneous firing of Type II neurons was rhythmic, with a mean frequency of 9.6 s pikes/s (range 3.5-16.0 spikes/s). Choline acetyltransferase immunohis tochemistry combined with biocytin labeling indicated that none of the Type I neurons were immunopositive to choline acetyltransferase, whil e 60% (42 of 69) of Type II neurons were immunopositive. There was no apparent difference in the electrophysiological membrane properties of immunopositive and immunonegative Type II neurons. At membrane potent ials subthreshold for Na+ spikes (- 50 mV), spontaneous membrane oscil lations (11.6 Hz) were observed: these underlie the spontaneous repeti tive firing of Type I neurons. The subthreshold membrane oscillation w as tetrodotoxin sensitive but was not affected by Ca2+-free medium. A similar tetrodotoxin-sensitive subthreshold membrane oscillation (10.5 Hz) was also observed in Type II neurons. However, in Type II neurons a membrane oscillation was also observed at higher membrane potential s (-50 mV). This high-threshold oscillation was insensitive to tetrodo toxin and Na+-free medium, but was eliminated in Ca2+-free conditions. The amplitude and frequency of the high-threshold oscillation was inc reased upon membrane depolarization. Ar the most prominent oscillatory level (around -40 mV), the high-threshold oscillation had a mean freq uency of 8.8 Hz. The high-threshold Ca2+ spike was triggered from the peak potential (- 35 to - 30 mV) of the high-threshold oscillation. Ap plication of tetraethylammonium chloride (<5 mM) increased the amplitu de of the high-threshold oscillation, while nifedipine greatly attenua ted the high-threshold oscillation without changing the shape of the h igh-threshold Ca2+ spike. Application of Cd2+ eliminated both the high -threshold oscillation and the high-threshold Ca2+ spike, and omega-co notoxin reduced the size of the high-threshold Ca2+ spike without affe cting the frequency of the high-threshold oscillation. Nickel did not have any effect on either the high-threshold oscillation or the high-t hreshold Ca2+ spike. These data suggest an involvement of N- and L-typ e Ca2+ channels in the generation of the high-threshold oscillation an d the high-threshold Ca2+ spike, respectively. The results indicate th at a persistent Na+ conductance plays a crucial role in the subthresho ld membrane oscillation, which underlies spontaneous repetitive firing in Type I neurons. On the other hand, in addition to a persistent Na conductance for subthreshold membrane oscillation, a voltage-dependen t Ca2+ conductance with Ca2+ dependent K+ conductance (for the high-th reshold oscillation) may be responsible for rhythmic firing of Type II neurons. (C) 1997 IBRO.