Medullary pain facilitating neurons mediate allodynia in headache-related pain.

OBJECTIVE: To develop and validate a model of cutaneous allodynia triggered by dural inflammation for pain associated with headaches. To explore neural mechanisms underlying cephalic and extracephalic allodynia. METHODS: Inflammatory mediators (IM) were applied to the dura of unanesthetized rats via previously implanted cannulas, and sensory thresholds of the face and hind-paws were characterized. RESULTS: IM elicited robust facial and hind-paw allodynia, which peaked within 3 hours. These effects were reminiscent of cutaneous allodynia seen in patients with migraine or other primary headache conditions, and were reversed by agents used clinically in the treatment of migraine, including sumatriptan, naproxen, and a calcitonin gene-related peptide antagonist. Consistent with clinical observations, the allodynia was unaffected by a neurokinin-1 antagonist. Having established facial and hind-paw allodynia as a useful animal surrogate of headache-associated allodynia, we next showed that blocking pain-facilitating processes in the rostral ventromedial medulla (RVM) interfered with its expression. Bupivacaine, destruction of putative pain-facilitating neurons, or block of cholecystokinin receptors prevented or significantly attenuated IM-induced allodynia. Electrophysiological studies confirmed activation of pain-facilitating RVM "on" cells and transient suppression of RVM "off" cells after IM. INTERPRETATION: Facial and hind-paw allodynia associated with dural stimulation is a useful surrogate of pain associated with primary headache including migraine and may be exploited mechanistically for development of novel therapeutic strategies for headache pain. The data also demonstrate the requirement for activation of descending facilitation from the RVM for the expression of cranial and extracranial cutaneous allodynia, and are consistent with a brainstem generator of allodynia associated with headache disorders.

Nerve injury-induced tactile allodynia is present in the absence of FOS labeling in retrogradely labeled post-synaptic dorsal column neurons.

The dorsal column pathway consists of direct projections from primary afferents and of ascending fibers of the post-synaptic dorsal column (PSDC) cells. This pathway mediates touch but may also mediate allodynia after nerve injury. The role of PSDC neurons in nerve injury-induced mechanical allodynia is unknown. Repetitive gentle, tactile stimulus or noxious pinch was applied to the ipsilateral hindpaw of rats with spinal nerve ligation (SNL) or sham surgery that had previously received tetramethylrhodamine dextran in the ipsilateral n. gracilis. Both touch and noxious stimuli produced marked increases in FOS expression in other cells throughout all laminae of the ipsilateral dorsal horn after nerve injury. However, virtually none of the identified PSDC cells expressed FOS immunofluorescence in response to repetitive touch or pinch in either the nerve-injured or sham groups. In contrast, labeled PSDC cells expressed FOS in response to ureter ligation and labeled spinothalamic tract (STT) cells expressed FOS in response to noxious pinch. Identified PSDC neurons from either sham-operated or SNL rats did not express immunoreactivity to substance P, CGRP, NPY, PKCY, MOR, the NK1 and the NPY-Y1 receptor. Retrogradely labeled DRG cells of nerve injured rats were large diameter neurons, which expressed NPY, but no detectable CGRP or substance P. Spinal nerve injury sensitizes neurons in the spinal dorsal horn to repetitive light touch but PSDC neurons apparently do not participate in touch-evoked allodynia. Sensitization of these non-PSDC neurons may result in activation of projections integral to the spinal/supraspinal processing of enhanced pain states and of descending facilitation, thus priming the central nervous system to interpret tactile stimuli as being aversive.

Spinal NK-1 receptor expressing neurons mediate opioid-induced hyperalgesia and antinociceptive tolerance via activation of descending pathways.

Opioids can induce hyperalgesia in humans and in animals. Mechanisms of opiate-induced hyperalgesia and possibly of spinal antinociceptive tolerance may be linked to pronociceptive adaptations occurring at multiple levels of the nervous system including activation of descending facilitatory influences from the brainstem, spinal neuroplasticity, and changes in primary afferent fibers. Here, the role of NK-1 receptor expressing cells in the spinal dorsal horn in morphine-induced hyperalgesia and spinal antinociceptive tolerance was assessed by ablating these cells with intrathecal injection of SP-saporin (SP-SAP). Ablation of NK-1 receptor expressing cells prevented (a) morphine-induced thermal and mechanical hypersensitivity, (b) increased touch-evoked spinal FOS expression, (c) upregulation of spinal dynorphin content and (d) the rightward displacement of the spinal morphine antinociceptive dose-response curve (i.e., tolerance). Morphine-induced hyperalgesia and antinociceptive tolerance were also blocked by spinal administration of ondansetron, a serotonergic receptor antagonist. Thus, NK-1 receptor expressing neurons play a critical role in sustained morphine-induced neuroplastic changes which underlie spinal excitability reflected as thermal and tactile hypersensitivity to peripheral stimuli, and to reduced antinociceptive actions of spinal morphine (i.e., antinociceptive tolerance). Ablation of these cells likely eliminates the ascending limb of a spinal-bulbospinal loop that engages descending facilitation and elicits subsequent spinal neuroplasticity. The data may provide a basis for understanding mechanisms of prolonged pain which can occur in the absence of tissue injury.

Retrograde analyses of spinothalamic projections in the macaque monkey: input to posterolateral thalamus.

The distribution of retrogradely labeled spinothalamic tract (STT) neurons was analyzed in macaque monkeys following variously sized, physiologically guided pressure or iontophoretic injections of cholera toxin subunit B (CTb) in order to determine whether different STT termination sites receive input selectively from different sets of STT cells. This report focuses on posterolateral thalamus, where prior anterograde tracing observations identified the posterior part of the ventromedial nucleus (VMpo) as the major projection target of lamina I STT neurons. Large injections in posterolateral thalamus labeled predominantly STT cells in lamina I throughout the spinal cord. In cases with medium-sized or small injections centered in VMpo, almost all labeled STT cells ( approximately 90%) were lamina I neurons. Small injections revealed a posteroanterior (foot to hand) somatotopographic organization consistent with that observed in prior anterograde tracing work; injections in posterior VMpo labeled primarily lumbosacral lamina I cells, whereas injections placed more anteriorly in VMpo labeled primarily cervical lamina I cells. These findings support the concept that VMpo is a primate lamina I spinothalamocortical relay nucleus important for pain, temperature, itch, muscle ache, sensual touch, and other interoceptive feelings from the body, and they provide strong evidence for the general hypothesis that the STT consists of several functionally and anatomically differentiable components.

Selective mediation of nerve injury-induced tactile hypersensitivity by neuropeptide Y.

Prevention of nerve injury-induced tactile, but not thermal, hypersensitivity is achieved by ipsilateral lesions of the dorsal columns or lidocaine microinjection into the nucleus gracilis (n. gracilis). These and other data support the possibility that tactile hyperresponsiveness after nerve injury may be selectively mediated by a low-threshold myelinated fiber pathway to the n. gracilis. Here we identify a transmitter that might selectively mediate such injury-induced tactile hypersensitivity. Neuropeptide Y (NPY), normally not detected in the dorsal root ganglion (DRG) or in the n. gracilis of rats, became markedly upregulated at both sites and in the spinal cord after spinal nerve injury. Injury-induced NPY-IR occurred predominately in large-diameter DRG cells, and the NPY-IR in the n. gracilis was blocked by dorsal rhizotomy or dorsal column lesion. NPY microinjection into the n. gracilis of uninjured rats elicited reversible tactile, but not thermal, hypersensitivity only in the ipsilateral hindpaw. Administration of anti-NPY antiserum, but not control serum or preabsorbed serum, into the n. gracilis ipsilateral to nerve injury reversed tactile, but not thermal, hypersensitivity. Similarly, microinjection of the NPY antagonists NPY(18-36) and (R)-N-[[4-(aminocarbonylaminomethyl)-phenyl]methyl]-N2-(diphenylacetyl)-argininamide trifluoroacetate, into the n. gracilis ipsilateral to the injury reversed tactile, but not thermal, hypersensitivity. Antagonist administration into the contralateral n. gracilis had no effect on injury-induced hypersensitivity. These data suggest the selective mediation of nerve injury-induced tactile hypersensitivity by upregulated NPY via large fiber input to n. gracilis. Selective reversal of injury-induced tactile allodynia by NPY receptor antagonists would have significant implications for human neuropathic conditions.