Induction of thermal hyperalgesia and synaptic long-term potentiation in the spinal cord lamina I by TNF-α and IL-1ß is mediated by glial cells.

Long-term potentiation (LTP) of synaptic strength in nociceptive pathways is a cellular model of hyperalgesia. The emerging literature suggests a role for cytokines released by spinal glial cells for both LTP and hyperalgesia. However, the underlying mechanisms are still not fully understood. In rat lumbar spinal cord slices, we now demonstrate that conditioning high-frequency stimulation of primary afferents activated spinal microglia within <30 min and spinal astrocytes within ~2 s. Activation of spinal glia was indispensible for LTP induction at C-fiber synapses with spinal lamina I neurons. The cytokines interleukin-1ß (IL-1ß) and tumor necrosis factor-α (TNF-α), which are both released by activated glial cells, were individually sufficient and necessary for LTP induction via redundant pathways. They differentially amplified 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)-propanoic acid receptor-mediated and N-methyl-D-aspartic acid receptor-mediated synaptic currents in lamina I neurons. Unexpectedly, the synaptic effects by IL-1ß and TNF-α were not mediated directly via activation of neuronal cytokine receptors, but rather, indirectly via IL-1 receptors and TNF receptors being expressed on glial cells in superficial spinal dorsal horn. Bath application of IL-1ß or TNF-α led to the release profiles of pro-inflammatory and anti-inflammatory cytokines, chemokines, and growth factors, which overlapped only partially. Heat hyperalgesia induced by spinal application of either IL-1ß or TNF-α in naive animals also required activation of spinal glial cells. These results reveal a novel, decisive role of spinal glial cells for the synaptic effects of IL-1ß and TNF-α and for some forms of hyperalgesia.

Direct excitation of spinal GABAergic interneurons by noradrenaline.

Endogenous pain control is, in part, mediated by descending inhibition of spinal nociception via spinal release of noradrenaline. Antinociception by activation of descending noradrenergic fibres has partially been attributed to the direct inhibition of nociceptive spinal neurons. Here, we tested the alternative hypothesis: the direct excitation of inhibitory spinal interneurons by noradrenaline. Transverse lumbar spinal cord slices were obtained from adult mice expressing enhanced green fluorescent protein (EGFP) in GABAergic neurons under control of the GAD67 promoter. Recordings were made from a total of 113 EGFP-expressing neurons and non-EGFP-expressing neurons in spinal laminae II and III with the perforated patch-clamp technique. In lamina II, where mainly nociceptive afferents terminate, noradrenaline (20 microM) depolarised significantly more EGFP-labelled (41%) than non-EGFP-labelled GABAergic neurons (4%). In contrast, noradrenaline hyperpolarised significantly more non-EGFP-labelled (46%) than EGFP-labelled GABAergic neurons (20%). In lamina III, where low threshold afferents terminate, EGFP-labelled neurons were never depolarised but either hyperpolarised (25%) or not affected (75%) by noradrenaline. Depolarisations of EGFP-labelled lamina II neurons were mimicked by the alpha(1)-adrenoceptor agonist phenylephrine (10-20 microM) and abolished by the alpha(1)-adrenoceptor antagonist prazosin (2 microM). Hyperpolarisations of EGFP- and non-EGFP-labelled neurons were abolished by the alpha(2)-adrenoceptor antagonist yohimbine (2 microM). These results show that noradrenaline directly excites inhibitory (GABAergic) lamina II interneurons in addition to its inhibitory effect on (putatively excitatory) interneurons in superficial spinal dorsal horn. Both effects of noradrenaline constitute a synergism in descending inhibition of nociceptive information in the spinal dorsal horn.

Induction of synaptic long-term potentiation after opioid withdrawal.

mu-Opioid receptor (MOR) agonists represent the gold standard for the treatment of severe pain but may paradoxically also enhance pain sensitivity, that is, lead to opioid-induced hyperalgesia (OIH). We show that abrupt withdrawal from MOR agonists induces long-term potentiation (LTP) at the first synapse in pain pathways. Induction of opioid withdrawal LTP requires postsynaptic activation of heterotrimeric guanine nucleotide-binding proteins and N-methyl-d-aspartate receptors and a rise of postsynaptic calcium concentrations. In contrast, the acute depression by opioids is induced presynaptically at these synapses. Withdrawal LTP can be prevented by tapered withdrawal and shares pharmacology and signal transduction pathways with OIH. These findings provide a previously unrecognized target to selectively combat pro-nociceptive effects of opioids without compromising opioid analgesia.