Presynaptic activity and Ca2+ entry are required for the maintenance of NMDA receptor-independent LTP at visual cortical excitatory synapses.
We have shown that some neural activity is required for the maintenance of long-term potentiation (LTP) at visual cortical inhibitory synapses. We tested whether this was also the case in N-methyl-d-aspartate (NMDA) receptor-independent LTP of excitatory connections in layer 2/3 cells of developing rat visual cortex. This LTP occurred after 2-Hz stimulation was applied for 15 min and always persisted for several hours while test stimulation was continued at 0.1 Hz. When test stimulation was stopped for 1 h after LTP induction, only one-third of the LTP instances disappeared, but most did disappear under a pharmacological suppression of spontaneous firing, indicating that LTP maintenance requires either evoked or spontaneous activities. LTP was totally abolished by a temporary blockade of action potentials with lidocaine or the removal of extracellular Ca(2+) after LTP induction, but it persisted under a voltage clamp of postsynaptic cells or after a temporary blockade of postsynaptic activity with the glutamate receptor antagonist kynurenate, suggesting that LTP maintenance requires presynaptic, but not postsynaptic, firing and Ca(2+) entry. More than one-half of the LTP instances were abolished after a pharmacological blockade of P-type Ca(2+) channels, whereas it persisted after either L-type or Ni(2+)-sensitive Ca(2+) channel blockades. These results show that the maintenance of NMDA receptor-independent excitatory LTP requires presynaptic firing and Ca(2+) channel activation as inhibitory LTP, although the necessary level of firing and Ca(2+) entry seems lower for the former than the latter and the Ca(2+) channel types involved are only partly the same.
Liu
Hong Nian
HN
Dept. of Visual Neuroscience, Research Institute of Environmental Medicine, Nagoya University, Furo-Cho, Chikusa-ku, Nagoya 464-8601, Japan.
Kurotani
Tohru
T
Ren
Ming
M
Yamada
Kazumasa
K
Yoshimura
Yumiko
Y
Komatsu
Yukio
Y
D000779:Anesthetics, Local / Q000494:pharmacology; D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000378:metabolism*; D020865:Calcium Channels, P-Type / Q000378:metabolism; D004558:Electric Stimulation; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects; D008012:Lidocaine / Q000494:pharmacology; D017774:Long-Term Potentiation / Q000187:drug effects / Q000502:physiology*; D017729:Presynaptic Terminals / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D016194:Receptors, N-Methyl-D-Aspartate / Q000502:physiology*; D013569:Synapses / Q000502:physiology*; D013779:Tetrodotoxin / Q000494:pharmacology; D014793:Visual Cortex / Q000502:physiology*
D000779:Anesthetics, Local; D002121:Calcium Channel Blockers; D015220:Calcium Channels; D020865:Calcium Channels, P-Type; D016194:Receptors, N-Methyl-D-Aspartate; D013779:Tetrodotoxin; D008012:Lidocaine