Naloxone blocks long-term depression of excitatory transmission in rat CA1 hippocampus in vitro.
In rat hippocampal slices, high intensity tetanic stimulation (two 1 s trains of 100 Hz separated by 20 s, 3-5X intensity of the test stimulus) of the Schaffer collateral-commissural (SCC) fibers induced a long-term depression (LTD) of the negative field excitatory postsynaptic potentials (fEPSP) in stratum radiatum of the CA1 region. The initial slope of the fEPSP, evoked by a single test shock applied to the SCC fibers, was depressed for a period longer than 40 min following such high intensity tetanic stimulation to this fiber system. However, the same tetanic stimulation delivered at low (test) intensity induced long-term potentiation (LTP) of the fEPSPs. Thus, similar patterns of stimulation can induce either LTP or LTD, depending on whether low- or high-intensity tetanic stimuli are delivered. The LTD induced by high strength tetanic stimulation was clearly blocked by the opioid antagonist naloxone (1 microM); however, the N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonopentanoate (AP5; 50 microM) had no effect on the LTD. Our data suggest that the strong stimulation used for LTD induction may have activated other afferent fiber systems and/or local interneurons in addition to SCC fibers, such as the enkephalin-containing terminals of the perforant path (PP) projecting to the stratum lacunosum moleculare or opioid peptide-containing interneurons. Thus, the resulting release of endogenous opioid peptides could play a role in the cellular mechanisms involved in some forms of long-term synaptic depression.
Francesconi
W
W
Department of Physiology and Biochemistry G. Moruzzi, University of Pisa, Italy.
Berton
F
F
Demuro
A
A
Madamba
S G
SG
Siggins
G R
GR
D015763:2-Amino-5-phosphonovalerate / Q000494:pharmacology; D000818:Animals; D001931:Brain Mapping; D004558:Electric Stimulation; D005071:Evoked Potentials / Q000187:drug effects*; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D006624:Hippocampus / Q000187:drug effects / Q000502:physiology*; D066298:In Vitro Techniques; D008297:Male; D009270:Naloxone / Q000494:pharmacology*; D009412:Nerve Fibers / Q000187:drug effects / Q000502:physiology; D009473:Neuronal Plasticity / Q000187:drug effects*; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley
D018691:Excitatory Amino Acid Antagonists; D009270:Naloxone; D015763:2-Amino-5-phosphonovalerate