Activity-independent synaptic long-term potentiation in nociceptive pathways

Discharges in nociceptive nerve fibres, e.g. in course of an inflammation, injury or surgery can induce long-term potentiation (LTP) at their synapses with spinal neurons. The activity- dependent form of LTP at synapses of the nociceptive system is considered a cellular mechanism of pain amplification at the site of the injury ("primary hyperalgesia"). Pain amplification can, however, also develop at body areas without any previous history of enhanced neuronal activity, for example in an area immediately surrounding a wound ("secondary hyperalgesia") or at corresponding areas of the contralateral site of the body. Likewise hyperalgesia may develop in the absence of any sensory stimulus after the withdrawal from opioids. These forms of hyperalgesia cannot be explained by activity- dependent types of synaptic plasticity. We now propose testing the hypothesis that in the spinal nociceptive system LTP can also be induced at inactive synapses. Recent studies demonstrate that noxious stimuli not only excite spinal dorsal horn neurons but also activate glial cells in the spinal dorsal horn. Opioids as well may activate glial cells. Glial cell activation may lead to the synthesis and the release of gliotransmitters including cytokines, chemokines and neurotrophic factors. We suggest that some of these molecules, such as interleukin 1ß and tumour necrosis factor diffuse into the spinal cord parenchyma to reach inactive synapses. This could then lead to activity-independent forms of LTP and to hyperalgesia at remote body areas. We plan to address this hypothesis by electrophysiological recordings in a spinal cord-dorsal root slice preparation. This in vitro preparation allows us to quantitatively assess synaptic strength in independent sensory pathways converging onto individual spinal dorsal horn neurons. In several previous studies we have extensively characterized activity-dependent forms of LTP in nociceptive pathways (1-3). We have further shown that selective activation of spinal microglia facilitates synaptic strength (4). We now plan to extend these findings by studying the impact of conditioning stimulation of one pathway on synaptic strength in the inactive pathway (heterosynaptic LTP). We plan to test if heterosynaptic LTP involves the activation of spinal glial cells and if so, if activation of glial cells is sufficient for the induction of LTP (gliogenic LTP). We will further explore if opioid-withdrawal LTP is another variety of gliogenic LTP. Modern electrophysiological and imaging technologies, including 2-photon laser-scanning microscopy are at our hands along with tools to detect gliotransmitters in tissues and in spinal superfusates such as amperometry, ELISA and cytokine assays. If our hypothesis should prove correct it may reveal novel targets for future prevention and therapies of some forms of chronic pain that are presently difficult to treat. 1. Science 299, 1237 (2003) 2. Science 312, 1659 (2006) 3. J. Neurosci. 33, 6540 (2013) 4. J. Neurosci. 35, 4552 (2015) 1

Acute pain fulfils essential functions for health and well-being. Acute pain has a warning function and triggers appropriate avoidance or protective behaviours. This allows preventing potentially harmful situations and leads to better and faster healing. Acute pain transforms into chronic pain when it outlasts its initial cause, thereby losing any beneficial effects. Pain then becomes a disease in its own right. It is generally accepted that acute pain triggers excitation of nociceptive C-fibres, which in turn activate neurons in the spinal cord, and at higher centres of the central nervous system (CNS). Intense or enduring acute pain may cause long-term synaptic plasticity in the CNS that may underlie the development and maintenance of chronic pain. In this FWF-funded project we discovered, that synaptic plasticity might, in stark contrast to the prevailing view, be triggered in the absence of any neuronal activity. We induced synaptic long-term potentiation in nociceptive C-fibres by selective activation of spinal glial cells and termed this novel form of neuronal plasticity "gliogenic long-term potentiation". We published these findings in the journal "Science". Gliogenic long-term potentiation was accompanied by long-lasting, enhanced pain sensitivity in behaving animals and may thus contribute to some forms of chronic pain. We next showed that acute withdrawal from the strongest painkillers, the opiates, lead to the activation of astrocytes, the most common glial cells in the grey matter of the CNS. Activation of astrocytes was both, necessary and sufficient for the induction of long-term potentiation at spinal C-fibre synapses. The mechanisms underlying the induction of activity-independent forms of long-term potentiation by withdrawal from opiates were fundamentally different in both sexes. Because of the paramount importance of spinal astrocytes in these processes, we extensively investigated and described this insufficiently studied cell population.