Analysis of field potentials evoked in Ca1 by angular bundle stimulation in the rat.
Anatomical data and in vitro electrophysiological data suggest that entorhinal cortical efferents monosynaptically excite CA1 pyramidal cells. In vivo stimulation of the angular bundle (AB) evokes an extracellular waveform, suggestive of a population spike. Previously, this waveform has been claimed to be activation of the direct entorhinal cortex to CA1 pathway and a volume-conducted response from the dentate gyrus. This study sought to distinguish between these two possibilities using several techniques in urethane anesthetized rats. A comparison of waveforms evoked in the dentate gyrus (DG) and in CA1 by a single-shock test stimulus to the AB revealed that the early positive-to-negative phase of the waveform recorded in CA1 correlated in time and amplitude to the population spike recorded in the DG. When the AB test stimulus was preceded by a single shock to the AB or to the contralateral hilus, the amplitudes of the DG population spike and the early phase of the CA1 waveform decreased equally. Current source density analysis of the response to the AB test stimulus revealed that early current sinks localized only to the DG. Finally, focal application of the local anesthetic tetracaine into CA1 did not decrease the amplitude of the waveform evoked by the AB test stimulus. Tetracaine, however, greatly deceased the population spike evoked in CA1 by stimulating the contralateral CA3. Together these data indicate that the early positive-to-negative phase of the waveform in CA1 evoked by the AB test stimulus reflects events in the DG and not direct activation of CA1 by entorhinal cortical afferents.
Stringer
J L
JL
Department of Pharmacology, Baylor College of Medicine, Houston, TX 77030.
Colbert
C M
CM
D000818:Animals; D004558:Electric Stimulation; D004597:Electroshock; D005071:Evoked Potentials / Q000187:drug effects / Q000502:physiology*; D007839:Functional Laterality; D006624:Hippocampus / Q000502:physiology*; D008297:Male; D008845:Microinjections; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D011712:Pyramidal Tracts / Q000187:drug effects / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D013748:Tetracaine / Q000008:administration & dosage / Q000494:pharmacology
D013748:Tetracaine