Journal of Neuroscience, Vol 14, 4359-4367, Copyright © 1994 by Society for Neuroscience
Opioid receptor activation is one factor underlying the frequency dependence of mossy fiber LTP induction
BE Derrick and JL Martinez Jr
Department of Psychology, University of California, Berkeley 94720.
The contribution of high-frequency synaptic activity to the induction of
long-term potentiation (LTP) in the opioid peptide-containing mossy fiber
projection was investigated in vivo in anesthetized rats. Because
high-frequency mossy fiber activity is essential for both the release of
opioid peptides and the induction of mossy fiber LTP, we investigated
whether the activation of opioid receptors underlies the requirement of
sustained high-frequency mossy fiber activity for LTP induction. Mossy
fiber responses were found to have a distinct threshold for the number of
100 Hz pulses necessary to induce LTP, with bursts of 25-30 pulses being
the minimum for LTP induction. Application of 1 nmol of the mu-opioid
receptor agonist DAMGO to the CA3 region potentiated mossy fiber responses,
but, unlike for mossy fiber LTP, this potentiation could be reversed by
mu-opioid receptor antagonist CTOP. Stimulation of the mossy fibers with
either a single burst of 15 pulses at 100 Hz or application of 100 pmol of
DAMGO was ineffective in potentiating mossy fiber responses. However,
delivery of a 15 pulse burst 10 min following DAMGO application was
effective in potentiating mossy fiber responses. This potentiation was not
reversed by CTOP and it occluded stimulation-induced LTP, suggesting that
brief bursts delivered in the presence of DAMGO had induced mossy fiber
LTP. The release of opioid peptides and the resulting activation of
mu-opioid receptors is suggested as one factor that underlies the
requirement of sustained high-frequency stimulation for the induction of
mossy fiber LTP.(ABSTRACT TRUNCATED AT 250 WORDS)