Background Gabapentin recruits descending inhibition to produce analgesia after nerve injury, but whether this is a local action in the brainstem is unknown. The authors hypothesized that gabapentin activates noradrenergic neurons in the locus coeruleus (LC) by a local action. Methods Male rats underwent L5-L6 spinal nerve ligation (SNL) and received drugs by intra-LC or systemic routes for behavior testing, immunohistochemistry in the LC, and microdialysis in the spinal dorsal horn. In other studies, brainstem slices from normal and SNL animals were used for immunohistochemistry. Results SNL increased phosphorylated cyclic adenosine monophosphate response element binding protein (pCREB)-expressing nuclei bilaterally in the LC, and increased noradrenaline release in the spinal dorsal horn. Gabapentin, whether in isolated brainstem slices or in conscious or anesthetized animals, increased pCREB-expressing nuclei in the LC. The net increase in pCREB expression by gabapentin did not differ between normal and SNL conditions. This gabapentin-induced pCREB activation in LC neurons was abolished by an AMPA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Intra-LC-injected gabapentin reduced hypersensitivity in SNL rats in a dose-dependent manner. Both intra-LC coadministration of CNQX and intrathecal administration of the α2-adrenoceptor antagonist idazoxan blocked antihypersensitivity by intra-LC gabapentin. Intravenous gabapentin induced noradrenaline release in the spinal dorsal horn. The net amount of noradrenaline release by gabapentin is larger in SNL rats compared with the normal condition, although the percentage increases from the baseline were the same. Conclusions These results suggest that gabapentin acts directly in the brainstem via a glutamate-dependent mechanism to stimulate descending inhibition to produce antihypersensitivity after peripheral nerve injury.

These data suggest that gabapentin activates the descending noradrenergic system and induces spinal norepinephrine release, which produces analgesia via spinal alpha2-adrenoceptor stimulation, followed by activation of G protein-coupled inwardly rectifying potassium channels. The authors' clinical data suggest that gabapentin activates the descending noradrenergic system after preoperative oral administration at the time of surgery. These data support a central mechanism of oral gabapentin to reduce postoperative pain and suggest that this effect could be magnified by treatments that augment the effect of norepinephrine release.

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Many treatments for neuropathic pain activate or augment norepinephrine release in the spinal cord, yet these treatments are less effective against acute nociceptive stimuli. We previously showed in mice that peripheral nerve injury results in sprouting of spinal noradrenergic fibers, possibly reflecting the substrate for this shift in drug efficacy. Here, we tested whether such sprouting also occurs in rats after nerve injury and examined one signal for such sprouting. Ligation of L5 and L6 spinal nerves unilaterally in rats resulted in hypersensitivity to tactile stimulation of the ipsilateral paw, and sprouting of noradrenergic fibers in the dorsal horn of the lumbar spinal cord. Brain derived nerve growth factor (BDNF) content increased in L4-L6 dorsal root ganglia ipsilateral to injury and in lumbar spinal cord following nerve injury, and intrathecal infusion of BDNF antiserum prevented spinal noradrenergic sprouting. This treatment also prevented the increased analgesic efficacy of intrathecal clonidine observed after nerve injury. Intraspinal injection of BDNF in non-injured rats mimicked the sprouting of spinal noradrenergic fibers seen after nerve injury. These results suggest that increased BDNF synthesis and release drives spinal noradrenergic sprouting following nerve injury, and that this sprouting may paradoxically increase the capacity for analgesia in the setting of neuropathic pain from drugs which utilize or mimic the noradrenergic pathway.

These results suggest that increased extracellular glutamate in the LC consequent to down-regulation of GLT-1 contributes to LC dysfunction and impaired pain-evoked endogenous analgesia after nerve injury.

Fast conducting myelinated high threshold mechanoreceptors (AHTMR) are largely thought to transmit acute nociception from the periphery. However, their roles in normal withdrawal and in nerve injury induced hyperalgesia are less well accepted. Modulation of this subpopulation of peripheral neurons would help define their roles in withdrawal behaviors. The optically active proton pump, ArchT, was placed in an AAV8 viral vector with the CAG promoter and was administered by intrathecal injection resulting in expression in myelinated neurons. Optical inhibition of peripheral neurons at the soma and transcutaneously was possible in the neurons expressing ArchT, but not in neurons from control animals. Receptive field characteristics and electrophysiology determined that inhibition was neuronal subtype specific with only AHTMR neurons being inhibited. One week following nerve injury the AHTMR are hyperexcitable, but can still be inhibited at the soma and transcutaneously. Withdrawal thresholds to mechanical stimuli in normal and in hyperalgesic nerve injured animals were also increased by transcutaneous light to the affected hindpaw. This suggests that AHTMR neurons play a role not only in threshold related withdrawal behavior in the normal animal, but also in sensitized states after nerve injury. This is the first time this subpopulation of neurons has been reversibly modulated to test their contribution to withdrawal related behaviors before and after nerve injury. This technique may prove useful to define the role of selective neuronal populations in different pain states.

Background Physical injury, including surgery, can result in chronic pain; yet chronic pain following childbirth, including cesarean delivery in women, is rare. The mechanisms involved in this protection by pregnancy or delivery have not been explored. Methods We examined the effect of pregnancy and delivery on hypersensitivity to mechanical stimuli of the rat hindpaw induced by peripheral nerve injury (spinal nerve ligation) and after intrathecal oxytocin, atosiban and naloxone. Additionally, oxytocin concentration in lumbar spinal cerebrospinal fluid was determined. Results Spinal nerve ligation performed at mid-pregnancy resulted in similar hypersensitivity to nonpregnant controls, but hypersensitivity partially resolved beginning after delivery. Removal of pups after delivery prevented this partial resolution. Cerebrospinal fluid concentrations of oxytocin were greater in normal postpartum rats prior to weaning. To examine the effect of injury at the time of delivery rather than during pregnancy, spinal nerve ligation was performed within 24 h of delivery. This resulted in acute hypersensitivity that partially resolved over the next 2–3 weeks. Weaning of pups resulted only in a temporary return of hypersensitivity. Intrathecal oxytocin effectively reversed the hypersensitivity following separation of the pups. Postpartum resolution of hypersensitivity was transiently abolished by intrathecal injection of the oxytocin receptor antagonist, atosiban. Conclusions These results suggest that the postpartum period rather than pregnancy protects against chronic hypersensitivity from peripheral nerve injury and that this protection may reflect sustained oxytocin signaling in the central nervous system during this period.

Introduction Spinal α2-adrenoceptor stimulation produces analgesia in neuropathic pain states, and this effect in animals is blocked by inhibitors of brain derived neurotrophic factor (BDNF) function. In rats, α2-adrenoceptor stimulation normally inhibits acetylcholine release, but excites release after nerve injury. We examined the roles of BDNF and excitatory Gs protein in this change. Methods Male rats underwent L5-L6 spinal nerve ligation (SNL) and their lumbar spinal dorsal horns with or without spinal BDNF infusion were used for either synaptosome preparation for acetylcholine release or immunostaining for choline acetyltransferase. Results SNL did not alter spontaneous release from synaptosomes or choline acetyltransferase-immunoreactivity in the spinal dorsal horn, but reduced KCl-evoked acetylcholine release. Dexmedetomidine inhibited KCl-evoked acetylcholine release in synaptosomes from normal rats, but excited KCl-evoked release in synaptosomes from SNL rats, and both effects were blocked by the α2-adrenoceptor antagonist, idazoxan. Spinal infusion of an antibody to BDNF reduced choline acetyltransferase-immunoreactivity in the spinal dorsal horn in both normal and SNL rats, and abolished facilitation of KCl-evoked acetylcholine release by dexmedetomidine in SNL rats. Dexmedetomidine’s facilitation of acetylcholine release was also blocked by inhibitors of Gs function. Discussion The increased reliance of spinal α2-adrenoceptors on cholinergic stimulation to cause analgesia after nerve injury reflects in part a shift from direct inhibition to direct excitation of spinal cholinergic neurons. Our results suggest this shift relies on an interaction with Gs proteins and BDNF.