title: Intracellular and extracellular in vivo recording of different response modes for relay cells of the cat's lateral geniculate nucleus.
abstract: Prior studies of thalamic neurons have demonstrated that they exhibit at least two response modes: a relay mode and a burst mode. During the relay mode, sensory information is faithfully relayed to cortex; during the burst mode, which is caused by a voltage-dependent Ca2+ conductance, this relay of sensory information is interrupted. We began in vivo studies of these response modes in neurons from the lateral geniculate nucleus of anesthetized, paralyzed cats. Each of the 9 X and 10 Y cells we recorded intracellularly displayed voltage-dependent, low threshold spikes that were presumably the Ca2+ spikes described from in vitro recording. These spikes were triangular in waveform and typically had 2-7 fast action potentials (interspike intervals of 1.2-4 ms) riding its crest. Furthermore, the cell's membrane had to be hyperpolarized to de-inactivate the low threshold spike before a depolarization could then activate it. We could activate these low threshold spikes in Y cells from EPSPs, whether spontaneous or evoked from activation of the optic chiasm. However, in only one of the X cells could we activate low threshold spikes from chiasm shock; in the remainder, we could activate low threshold spikes only via depolarizing current pulses, possibly because the EPSPs of these X cells were too small to activate these spikes. We also used extracellular recording to study spontaneous activity and responses to chiasm shock from 114 geniculate neurons and, as a control, 57 optic tract axons. We concentrated on periods of bursty responsiveness signifying the burst mode. We define a burst as 2-7 action potentials with interspike intervals less than or equal to 4 ms, and the bursts are separated by greater than 100 ms; from our intracellular recording, we know that such bursts signify low threshold spikes. We found that, during extracellular recording, 20 of the 39 X cells and each of the 75 Y cells displayed evidence of the burst response mode, although burst periods were rare in X cells. Electrical activation of the optic chiasm greatly enhanced the burstiness of Y cells for periods of 500 ms or more. We also electrically stimulated the parabrachial region of the midbrain, which provides a mostly cholinergic innervation to the lateral geniculate nucleus. Although parabrachial activation by itself had no detectable effect on Y cell response modes, prior parabrachial activation prevented the enhanced burstiness caused by chiasm stimulation. This parabrachial effect lasted for roughly 500 ms after stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)
authors.affiliation: Department of Neurobiology, State University of New York, Stony Brook 11794-5230.
authors.forename: F S
authors.initials: FS
authors.lastname: Lo
authors.forename: S M
authors.initials: SM
authors.lastname: Lu
authors.forename: S M
authors.initials: SM
authors.lastname: Sherman
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000758:Anesthesia; D000818:Animals; D001369:Axons / Q000502:physiology; D002415:Cats; D004558:Electric Stimulation; D004566:Electrodes; D005071:Evoked Potentials / Q000502:physiology; D005829:Geniculate Bodies / Q000166:cytology* / Q000502:physiology; D008564:Membrane Potentials / Q000502:physiology; D008636:Mesencephalon / Q000166:cytology / Q000502:physiology; D009474:Neurons / Q000502:physiology*; D009897:Optic Chiasm / Q000502:physiology; D013569:Synapses / Q000502:physiology