Chemical modulation of ephaptic activation of CA3 hippocampal pyramids.
In rats under urethane anaesthesia, antidromic population spikes were evoked in CA3 pyramidal layer by fimbrial/commissural stimulation at a very low frequency (approximately 0.5 Hz). Submaximal population spikes--between 20 and 90% of maximum--were enhanced by 8-38% by applications of acetylcholine and bicuculline, or by medial septal stimulation. Noradrenaline had a less pronounced and regular facilitatory action, whereas gamma-aminobutyrate and glutamate only depressed population spikes. Maximal enhancement by acetylcholine or bicuculline was observed when the antidromic population spike was initially at 38-53% of maximum amplitude. A simple explanation of these results is that acetylcholine and bicuculline, by raising their excitability, facilitate the excitation of non-invaded pyramidal cells by antidromic field potentials. They are fully in keeping with previous intracellular observations on ephaptic interactions between CA3 neurons, and provide a further illustration, in situ, of the importance of increased excitability and disinhibition--whether caused by drugs or synaptic action--in promoting synchronized excitation by ephaptic currents.
Dalkara
T
T
Krnjević
K
K
Ropert
N
N
Yim
C Y
CY
D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D001640:Bicuculline / Q000494:pharmacology*; D004558:Electric Stimulation; D005071:Evoked Potentials / Q000187:drug effects; D005971:Glutamates / Q000494:pharmacology; D018698:Glutamic Acid; D006624:Hippocampus / Q000187:drug effects* / Q000502:physiology; D008297:Male; D009474:Neurons / Q000187:drug effects* / Q000502:physiology; D018377:Neurotransmitter Agents / Q000494:pharmacology*; D009638:Norepinephrine / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
D005971:Glutamates; D018377:Neurotransmitter Agents; D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid; D000109:Acetylcholine; D009638:Norepinephrine; D001640:Bicuculline