The distribution of substance P receptor (NK1)-like immunoreactive neurons in the newborn and adult human spinal cord

cAMP signaling through protein kinase A and Epac2 induces substance P release in the rat spinal cord

Using neurokinin 1 receptor (NK1R) internalization to measure of substance P release in rat spinal cord slices, we found that it was induced by the adenylyl cyclase (AC) activator forskolin, by the protein kinase A (PKA) activators 6-Bnz-cAMP and 8-Br-cAMP, and by the activator of exchange protein activated by cAMP (Epac) 8-pCPT-2-O-Me-cAMP (CPTOMe-cAMP). Conversely, AC and PKA inhibitors decreased substance P release induced by electrical stimulation of the dorsal root. Therefore, the cAMP signaling pathway mediates substance P release in the dorsal horn. The effects of forskolin and 6-Bnz-cAMP were not additive with NMDA-induced substance P release and were decreased by the NMDA receptor blocker MK-801. In cultured dorsal horn neurons, forskolin increased NMDA-induced Ca²⁺ entry and the phosphorylation of the NR1 and NR2B subunits of the NMDA receptor. Therefore, cAMP-induced substance P release is mediated by the activating phosphorylation by PKA of NMDA receptors. Voltage-gated Ca²⁺ channels, but not by TRPV1 or TRPA1, also contributed to cAMP-induced substance P release. Activation of PKA was required for the effects of forskolin and the three cAMP analogs. Epac2 contributed to the effects of forskolin and CPTOMe-cAMP, signaling through a Raf - mitogen-activated protein kinase pathway to activate Ca²⁺ channels. Epac1 inhibitors induced NK1R internalization independently of substance P release. In rats with latent sensitization to pain, the effect of 6-Bnz-cAMP was unchanged, whereas the effect of forskolin was decreased due to the loss of the stimulatory effect of Epac2. Hence, substance P release induced by cAMP decreases during pain hypersensitivity.

Neurokinin 1 receptor activation in the rat spinal cord maintains latent sensitization, a model of inflammatory and neuropathic chronic pain

Latent sensitization is a model of chronic pain in which a persistent state of pain hypersensitivity is suppressed by opioid receptors, as evidenced by the ability of opioid antagonists to induce a period of mechanical allodynia. Our objective was to determine if substance P and its neurokinin 1 receptor (NK1R) mediate the maintenance of latent sensitization. Latent sensitization was induced by injecting rats in the hindpaw with complete Freund's adjuvant (CFA), or by tibial spared nerve injury (SNI). When responses to von Frey filaments returned to baseline (day 28), the rats were injected intrathecally with saline or the NK1R antagonist RP67580, followed 15 min later by intrathecal naltrexone. In both pain models, the saline-injected rats developed allodynia for 2 h after naltrexone, but not the RP67580-injected rats. Saline or RP67580 were injected daily for two more days. Five days later (day 35), naltrexone was injected intrathecally. Again, the saline-injected rats, but not the RP67580-injected rats, developed allodynia in response to naltrexone. To determine if there is sustained activation of NK1Rs during latent sensitization, NK1R internalization was measured in lamina I neurons in rats injected in the paw with saline or CFA, and then injected intrathecally with saline or naltrexone on day 28. The rats injected with CFA had a small amount of NK1R internalization that was significantly higher than in the saline-injected rats. Naltrexone increased NK1R internalization in the CFA-injected rats but nor in the saline-injected rats. Therefore, sustained activation of NK1Rs maintains pain hypersensitivity during latent sensitization.

Intrathecal substance P-saporin in the dog: distribution, safety, and spinal neurokinin-1 receptor ablation

BACKGROUND

Neurokinin-1 receptors (NK1-rs) located on superficial dorsal horn neurons are essential for integration of nociceptive input. Intrathecal injection of substance P-saporin (SP-SAP) leads to local loss of spinal NK1-r (+) neurons suggesting its potential as a therapeutic agent for chronic pain. The authors determined, in a canine model, effects of lumbar intrathecal SP-SAP.

METHODS

Distribution of SP-SAP and Saporin was determined in plasma, lumbar cerebrospinal fluid, and tissue. Safety of intrathecal SP-SAP was determined in four groups (six dogs each) administered 0 (0.9% saline), 1.5, 15, or 150 µg SP-SAP through lumbar intrathecal catheters. Behavioral, physiologic, and biochemical variables were assessed. Spinal tissues were collected at 7 and approximately 90 days, or earlier if significant morbidity developed, and analyzed for NK1-r (+) neuron loss and histopathology.

RESULTS

SP-SAP and Saporin were detectable in lumbar cerebrospinal fluid for up to 4 and 24 h, respectively. Animals receiving intrathecal saline, 1.5, or 15 µg of SP-SAP showed no persistent neurologic deficits. Three animals receiving 150 µg of SP-SAP developed pelvic limb paraparesis and were euthanized prematurely. Immunohistochemistry and in situ hybridization cell counts confirmed a significant reduction in NK1-r (+) in superficial dorsal horn neurons from lumbar spinal cord after intrathecal administration of 15 and 150 µg of SP-SAP. A significant loss of NK1-r neurons in the lumbar ventral horn occurred only with 150-µg SP-SAP.

CONCLUSION

Intrathecal 15-µg SP-SAP reduced dorsal, but not ventral, NK1-r (+) neurons at the spinal level of delivery with minimal side effects, whereas 150-µg SP-SAP resulted in motor neuron toxicity.

Neurokinin-1 receptor immunoreactive neuronal elements in the superficial dorsal horn of the chicken spinal cord: with special reference to their relationship with the tachykinin-containing central axon terminals in synaptic glomeruli

Synaptic glomeruli that involve tachykinin-containing primary afferent central terminals are numerous in lamina II of the chicken spinal cord. Therefore, a certain amount of noxious information is likely to be modulated in these structures in chickens. In this study, we used immunohistochemistry with confocal and electron microscopy to investigate whether neurokinin-1 receptor (NK-1R)-expressing neuronal elements are in contact with the central primary afferent terminals in synaptic glomeruli of the chicken spinal cord. We also investigated which neuronal elements (axon terminals, dendrites, cell bodies) and which neurons in the spinal cord possess NK-1R, and are possibly influenced by tachykinin in the glomeruli. By confocal microscopy, NK-1R immunoreactivities were seen in a variety of neuronal cell bodies, their dendrites and smaller fibers of unknown origin. Some of the NK-1R immunoreactive profiles also expressed GABA immunoreactivities. A close association was observed between the NK-1R-immunoreactive neurons and tachykinin-immunoreactive axonal varicosities. By electron microscopy, NK-1R immunoreactivity was seen in cell bodies, conventional dendrites and vesicle-containing dendrites in laminae I and II. Among these elements, dendrites and vesicle-containing dendrites made contact with tachykinin-containing central terminals in the synaptic glomeruli. These results indicate that tachykinin-containing central terminals in the chicken spinal cord can modulate second-order neuronal elements in the synaptic glomeruli.

Postnatal changes in the spatial distributions of substance P and neurokinin-1 receptor in the trigeminal subnucleus caudalis of mice

Nociceptive afferent signals from the orofacial area are transmitted to the trigeminal subnucleus caudalis (Vc) through the release of glutamate and/or substance P (SP). Although nociceptive transmission and/or modulating mechanisms are known to develop during the postnatal period, the specific developmental changes in nociception and/or modulation remain unclear. The present study examined postnatal changes in the spatial relationship between SP and its receptor, the NK1 receptor (NK1R), in the mouse Vc by immunohistochemistry and quantitative analysis. The medulla was removed from C57BL/6N mice (1, 2, 4, and 8 weeks of age) after perfusion and fixation, and cut horizontally at a thickness of 40 mum. The relative densities of SP- and NK1R-immunoreactive areas and their changes with age were assessed statistically. One- and 2-week-old mice showed relatively high densities of SP-positive structures in the marginal layer (Mar) and the deep part of the magnocellular layer (Mag). The SP distribution in the superficial Vc remained unchanged, but the density in the deep Mag gradually decreased with age, resulting in a complete loss after postnatal week 4. The NK1R-immunoreactivity exhibited a similar distribution pattern to that of SP, but the pattern remained unchanged during the postnatal period. Double-immunofluorescence staining for SP and NK1R demonstrated only moderate direct contact of SP-positive structures with NK1R in the superficial area. These separate distributions and the postnatal changes in SP and NK1R suggest the possibility of another nociceptive afferent transmission mechanism, that is, volume transmission, in the Vc other than synapse-mediated transmission.

A tachykinin NK1 receptor antagonist attenuates the 4β-phorbol-12-myristate-13-acetate-induced nociceptive behaviour in the rat

Antinociceptive effect of a tachykinin NK1 receptor antagonist ezlopitant [(2S,3S-cis)-2-(diphenylmethyl)-N-{(2-methoxy, 5-isopropylphenyl)methyl}-1-azabicyclo[2.2.2]octan-3-amine] was investigated in the 4beta-phorbol-12-myristate-13-acetate (PMA)-induced nociceptive test in the rat. Intraplantar injection of PMA-induced paw-licking and flinching behaviour lasted up to 120 min and was accompanied by inflammatory reactions, such as swelling and invasion of granulocytes. Pretreatment with resiniferatoxin [200 microg/kg, subcutaneous (s.c.)] blocked the PMA-induced nociceptive behaviour, suggesting that vanilloid VR1 receptor-expressing primary sensory neurons play a major role in this response. Subcutaneous pretreatment with ezlopitant (0.3-30 mg/kg) and morphine (0.3-6 mg/kg) caused a dose-dependent inhibition of the behaviour. Ezlopitant (3-30 mg/kg) given subcutaneously after PMA injection also significantly attenuated the behavioural response. When administered intrathecally, ezlopitant and a nonselective glutamate receptor antagonist MK-801 had an inhibitory effect, whereas CJ-12,191, an inactive isomer of ezlopitant, was unaffected. These results suggest that spinal tachykinin NK1 receptors contribute to processing of ongoing pain associated with peripheral inflammation.

Calcitonin gene-related peptide-receptor component protein expression in the uterine cervix, lumbosacral spinal cord, and dorsal root ganglia

The neuropeptide calcitonin gene-related peptide (CGRP) may play a role in neurogenic inflammation, tissue remodeling of the uterine cervix, promoting vasodilation, parturition, and processing of sensory information in the spinal cord. CGRP-immunoreactive nerves of the cervix and spinal cord have been studied but cellular identification of the CGRP receptor has received little attention. CGRP-receptor component protein (CGRP-RCP) is a small protein associated with the CGRP receptor; thus, immunostaining for the CGRP-RCP can be used to identify sites of the CGRP receptor. We determined sites of CGRP-RCP immunoreactivity relative to the presence of CGRP-ir nerve fibers in the female rat uterine cervix, spinal cord, and dorsal root ganglia. CGRP-RCP immunoreactivity was expressed in the dorsal horn of the spinal cord, venules of the uterine cervix, and perikarya of sensory neurons in dorsal root ganglia. CGRP-immunoreactive fibers were adjacent to CGRP-RCP-immunoreactive vessels in the cervix and among CGRP-RCP-immunoreactive structures in the dorsal horn of the spinal cord. This suggests CGRP-RCP is associated with structures innervated by CGRP nerves and these interactions may be changed in tissues in response to an appropriate stimulus.

Neurotransmitters and neuromodulators during early human development

BACKGROUND

Neurotransmitters such as monoamines appear in the embryo before the neurones are differentiated. They may have other functions than neurotransmission during embryogenesis such as differentiation and neuronal growth. For example, serotonin may act as a morphogen. A number of neuropeptides are expressed during ontogenesis, but their function has been difficult to establish. Maybe some of them remain as evolutionary residues. Fast-switching neurotransmitters like the excitatory amino acids and the more ionotropic receptors dominate in the human brain, but appear probably later during evolution as well as during ontogeny.

METHODS

The distribution of catecholamines during development has been analysed with a fluorescence method, while most of the other neurotransmitters have been mapped with immunohistochemical methods. The classical method to determine the physiological role of a neurotransmitter or modulator is to study the physiological effect of its antagonist, blocking the endogenous activity. By transgenic technique, the genes encoding for enzymes involved in the synthesis of neurotransmitters can be knocked-out.

MAJOR FINDINGS

Pharmacological blocking of endogenous activity has, for example, demonstrated that adenosine suppresses fetal respiration. Knocking out the dopamine beta-hydroxylase gene results in fetal death, suggesting that noradrenaline is essential for survival. Some neurotransmitters change their effect during embryogenesis, e.g. GABA which is excitatory in the embryo, but inhibitory after birth due to a switch from a high to low chloride content in the nerve cells. It is possible that this is of importance for the wiring of neuronal network in early life. NMDA receptors dominate in the foetus, while kainate and AMPA receptors appear later. At birth, there is a surge of neurotransmitters such as catecholamines, which may be of importance for the neonatal adaptation.

CONCLUSIONS

Neurotransmitters and modulators are not only important for the neural trafficking in the embryo, but also for the development of the neuronal circuits. Prenatal or neonatal stress (hypoxia), as well as various drugs, may disturb the wiring and cause long-term behavioural effects (fetal and neonatal programming).