Long-term potentiation at C-fibre synapses by low-level presynaptic activity in vivo.
Inflammation, trauma or nerve injury trigger low-level activity in C-fibres and may cause long-lasting hyperalgesia. Long-term potentiation (LTP) at synapses of primary afferent C-fibres is considered to underlie some forms of hyperalgesia. In previous studies, high- but not low-frequency conditioning stimulation of C-fibres has, however, been used to induce LTP in pain pathways. Recently we could show that also conditioning low-frequency stimulation (LFS) at C-fibre intensity induces LTP in vitro as well as in the intact animal, i.e. with tonic descending inhibition fully active. In the slice preparation, this form of LTP requires a rise in postsynaptic Ca2+-concentration and activation of Ca2+-dependent signalling pathways. Here, we investigated the signalling mechanisms underlying this novel form of LTP in vivo. We found that the signal transduction pathways causing LFS-induced LTP in vivo include activation of neurokinin 1 and N-methyl-D-aspartate receptors, rise of [Ca2+]i from intracellular stores and via T-type voltage-dependent Ca2+ channels, activation of phospholipase C, protein kinase C and Ca2+-calmodulin dependent kinase II. These pathways match those leading to hyperalgesia in behaving animals and humans. We thus propose that LTP induced by low-level activity in C-fibres may underlie some forms of hyperalgesia.
Drdla
Ruth
R
Department of Neurophysiology, Center for Brain Research, Medical University of Vienna, Vienna, Austria. ruth.drdla@meduniwien.ac.at
Sandkühler
Jürgen
J
D000818:Animals; D020013:Calcium Signaling; D004558:Electric Stimulation; D006930:Hyperalgesia / Q000209:etiology; D017774:Long-Term Potentiation / Q000502:physiology*; D036421:Nerve Fibers, Unmyelinated / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D015398:Signal Transduction; D013569:Synapses