Synaptic mechanisms of inflammatory pain

Inflammation leads to low frequency discharges in nociceptive nerve fibres and to sensitization of nociceptive spinal dorsal horn neurons by hitherto unknown cellular mechanisms. The clinical consequences of which are abnormal sensitivity to pain (hyperalgesia) and significant impairment of quality of life. Recent evidence suggests that a small, well defined group of neurons in superficial spinal dorsal horn plays a key role for hyperalgesia. These neurons are located in superficial lamina I, express the receptor for substance P and project to the periaqueductal grey (PAG) or the parabrachial area (PB), two areas in the brain involved in emotional and vegetative dimensions of pain experience. We will explore aberrant processing (synaptic long-term potentiation, LTP) of sensory information at the junction between nociceptive nerve fibres and lamina I neurons which project to the PAG or PB. We will evaluate the roles of classical neurotransmitters (glutamate) and neuromodulators (substance P) and their receptors for induction and maintenance of LTP. The subsequent steps in signal transduction, i.e. the contribution of calcium ions as a universal 2nd messenger and calcium-dependent cellular enzymes to LTP will also be assessed. Modern in vitro technologies will be applied to a thick dorsal-root spinal cord slice preparation. The 2-photon laser-scanning microscopy will be used to assess activity-dependent changes in calcium ion gradients at the subcellular level. Whole-cell patch-clamp recordings will provide quantitative measures of signal flow between nociceptive nerve fibres and identified spinal lamina I projection neurons. In vivo recordings in superficial spinal dorsal horn of deeply anaesthetized rats will show, if inflammation-induced low frequency, highly irregular and asynchronous discharges in a subset of nociceptive nerve fibres trigger LTP in spinal cord. This is an issue of principal importance, as the role of LTP for any kind of behaviour, including pain, is still controversial. LTP was previously induced only by electrical high frequency nerve stimulation which leads to regular, synchronous discharges in all nerve fibres. We will test, if pathological conditions such as inflammation which cause hyperalgesia in pain patients also lead to spinal LTP in vivo. We will compare the known pharmacological profile of pain behaviour with that of sensitization at the (sub-) cellular level in spinal cord. We anticipate that this project will provide new insights into central mechanisms of abnormal pain sensitivity and may identify novel targets for the prevention and treatment of chronic pain states.

Information transfer between nerve cells takes places at special points of contact, the synapses. At synapses neuronal information may be modulated or stored. It is, for example, well known that at some synapses strong activity may cause a long-lasting increase in synaptic efficacy. This long-term potentiation of synaptic strength (LTP) is a much studied cellular model of synaptic plasticity. It is generally believed that LTP at synapses in the hippocampus, a brain region, is involved in learning and memory formation. In this FWF project we have identified two novel forms of LTP in pain pathways. Noxious stimuli are encoded by sensory C-fibres that make synaptic contacts with spinal dorsal horn neurons. These spinal neurons then relay pain-related information to the brain 3. LTP at synapses between C-fibres and spinal dorsal horn neurons projecting to the midbrain periaqueductal grey (PAG) can be induced by low level presynaptic activity and by natural noxious stimuli2. This finding was surprising, as induction of activity-dependent LTP at synapses in the brain requires high frequency discharges of presynaptic nerve fibres. This novel form of LTP in pain pathways might cause long-lasting pain amplification under conditions of inflammation, tissue damage or nerve injury long after the initial cause for pain may have disappeared. Opioids are the strongest drugs to combat pain. Paradoxically pain amplification may result from acute withdrawal from opioids (one of the symptoms of the opioid withdrawal syndrome, the so called "Cold Turkey"). In this FWF-sponsored project we studied potential mechanisms of the withdrawal syndrome and discovered a previously unrecognized form of LTP at C-fibre synapses. Unlike the above described form of LTP the withdrawal LTP does not require any activity in sensory C-fibres. It is rather induced upon abrupt withdrawal from opioids. When, however, the opioid was withdrawn slowly, no LTP was induced1. This novel opioid action may underlie hyperalgesia after opioid withdrawal and will now be studied in greater detail in the follow-up project recently granted by the FWF. Reprints may be downloaded in PDF-Format from our website: http://cbr.meduniwien.ac.at/research/publications/ 1. Drdla R, Gassner M, Gingl E, Sandkühler J: Induction of synaptic long-term potentiation after opioid withdrawal. Science 325:207-210, 2009 2. Ikeda H, Stark J, Fischer H, Wagner M, Drdla R, Jäger T, Sandkühler J: Synaptic amplifier of inflammatory pain in the spinal dorsal horn. Science 312:1659-1662, 2006 3. Sandkühler J: Models and mechanisms of hyperalgesia and allodynia. Physiol Rev 89:707-758, 2009