Ionic basis for the electro-responsiveness and oscillatory properties of guinea-pig thalamic neurones in vitro.
The ionic requirements for electro-responsiveness in thalamic neurones were studied using in vitro slice preparations of the guinea-pig diencephalon. Analysis of the current-voltage relationship in these neurones revealed delayed and anomalous rectification. Substitution of Na+ with choline in the bath or addition of tetrodotoxin (TTX) abolished the fast spikes and the plateau potentials, described in the accompanying paper. Ca2+ conductance blockage with Co2+, Cd2+ or Mn2+, or replacement of Ca2+ by Mg2+ abolished the low-threshold spikes (l.t.s.). Substitution with Ba2+ did not significantly increase the duration of the l.t.s., suggesting that under normal conditions the falling phase of this response is brought about by inactivation of the Ca2+ conductance. The after-hyperpolarization (a.h.p.) following fast spikes was markedly reduced in amplitude and duration by bath application of Cd2+, Co2+ or Mn2+, indicating that a large component of this response is generated by a Ca2+-dependent K+ conductance (gK[Ca]). Following hyperpolarizing current pulses, the membrane potential showed a delayed return to base line. This delay is produced by a transient K+ conductance as it can be modified by changing the drive force for K+. Presumptive intra-dendritic recording demonstrated high-threshold Ca2+ spikes (h.t.s.s.) which activate a gK[Ca]. Such h.t.s.s. were also seen at the somatic level when K+ conductance was blocked with 4-aminopyridine. It is proposed that the intrinsic biophysical properties of thalamic neurones allow them to serve as relay systems and as single cell oscillators at two distinct frequencies, 9-10 and 5-6 Hz. These frequencies coincide with the alpha and theta rhythms of the e.e.g. and, in the latter case, with the frequency of Parkinson's tremor.
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D001464:Barium / Q000494:pharmacology; D002118:Calcium / Q000502:physiology; D002412:Cations / Q000494:pharmacology*; D003712:Dendrites / Q000502:physiology; D004553:Electric Conductivity; D004594:Electrophysiology; D006168:Guinea Pigs; D066298:In Vitro Techniques; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D012964:Sodium / Q000502:physiology; D013779:Tetrodotoxin / Q000494:pharmacology; D013787:Thalamic Nuclei / Q000187:drug effects / Q000502:physiology*
D002412:Cations; D001464:Barium; D013779:Tetrodotoxin; D012964:Sodium; D002118:Calcium