title: Ionic mechanisms involved in the spontaneous firing of tegmental pedunculopontine nucleus neurons of the rat.
abstract: We have previously defined three types of tegmental pedunculopontine nuclei neurons based on their electrophysiological characteristics: Type I neurons characterized by low-threshold Ca2+ spikes, Type II neurons which displayed a transient outward current (A-current), and Type III 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 mechanisms underlying repetitive firing of Type I (n=15) and Type II (n=69) neurons were studied in in vitro slice preparations. Type I neurons did not fire rhythmically but their spontaneous firing frequency 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 spikes/s (range 3.5-16.0 spikes/s). Choline acetyltransferase immunohistochemistry combined with biocytin labeling indicated that none of the Type I neurons were immunopositive to choline acetyltransferase, while 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 potentials subthreshold for Na+ spikes (-50 mV), spontaneous membrane oscillations (11.6 Hz) were observed: these underlie the spontaneous repetitive firing of Type I neurons. The subthreshold membrane oscillation was 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 potentials (-50 mV). This high-threshold oscillation was insensitive to tetrodotoxin and Na+-free medium, but was eliminated in Ca2+-free conditions. The amplitude and frequency of the high-threshold oscillation was increased upon membrane depolarization. At the most prominent oscillatory level (around -40 mV), the high-threshold oscillation had a mean frequency of 8.8 Hz. The high-threshold Ca2+ spike was triggered from the peak potential (-35 to -30mV) of the high-threshold oscillation. Application of tetraethylammonium chloride (< 5 mM) increased the amplitude of the high-threshold oscillation, while nifedipine greatly attenuated the high-threshold oscillation without changing the shape of the high-threshold Ca2+ spike. Application of Cd2+ eliminated both the high-threshold oscillation and the high-threshold Ca2+ spike, and omega-conotoxin reduced the size of the high-threshold Ca2+ spike without affecting the frequency of the high-threshold oscillation. Nickel did not have any effect on either the high-threshold oscillation or the high-threshold Ca2+ spike. These data suggest an involvement of N- and L-type Ca2+ channels in the generation of the high-threshold oscillation and the high-threshold Ca2+ spike, respectively. The results indicate that a persistent Na+ conductance plays a crucial role in the subthreshold 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-dependent Ca2+ conductance with Ca2+-dependent K+ conductance (for the high-threshold oscillation) may be responsible for rhythmic firing of Type II neurons.
authors.affiliation: Department of Anatomy and Neurobiology, The University of Tennessee, College of Medicine, Memphis 38163, USA.
authors.forename: K
authors.initials: K
authors.lastname: Takakusaki
authors.forename: S T
authors.initials: ST
authors.lastname: Kitai
mesh_terms: D000818:Animals; D001369:Axons / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002795:Choline O-Acetyltransferase / Q000378:metabolism; D004594:Electrophysiology; D007150:Immunohistochemistry; D066298:In Vitro Techniques; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D008636:Mesencephalon / Q000166:cytology / Q000201:enzymology / Q000502:physiology*; D009474:Neurons / Q000502:physiology*; D010275:Parasympathetic Nervous System / Q000166:cytology / Q000201:enzymology / Q000502:physiology; D018408:Patch-Clamp Techniques; D011149:Pons / Q000166:cytology / Q000201:enzymology / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D015222:Sodium Channels / Q000187:drug effects / Q000378:metabolism; D013681:Tegmentum Mesencephali / Q000166:cytology / Q000201:enzymology / Q000502:physiology*
chemical_list: D002121:Calcium Channel Blockers; D015222:Sodium Channels; D002795:Choline O-Acetyltransferase; D002118:Calcium