Messenger plasticity in primary sensory neurons following axotomy and its functional implications

Ultrastructural studies on peptides in the dorsal horn of the rat spinal cord--III. Effects of peripheral axotomy with special reference to galanin

In this study co-localization of galanin- with calcitonin gene-related peptide (CGRP)-like immunoreactivity was examined in dorsal root ganglion neurons 14 days after sciatic nerve cut using a laser scanning confocal microscope. CGRP- and galanin-like immunoreactivities were also analysed in the dorsal horn of the spinal cord of these animals with immunofluorescence microscopy. The ultrastructural changes in galanin-immunoreactive, presumably primary afferent terminals in the superficial dorsal horn, were studied as well as the relationship between galanin-, substance P- and CGRP-like immunoreactivities in primary afferent terminals. Local galanin-positive neurons in lamina II were also analysed after peripheral axotomy. Under the confocal microscope, CGRP-like immunoreactivity was located in the perinuclear region, probably the Golgi complex, and in dot-like structures, probably representing large dense-core vesicles, in normal dorsal root ganglion neurons. However, after peripheral axotomy CGRP was mainly detected in dot-like structures. Only a slight decrease in percentage of CGRP neurons in dorsal root ganglion was seen after axotomy, and about 84% of the galanin-positive neurons contained CGRP. The field of galanin-positive nerve fibres in the superficial lumbar (L)4 and L5 dorsal horn expanded and the intensity of staining for CGRP was reduced in these regions 14 days after sciatic nerve cut. Using pre-embedding immunoelectron microscopy, several morphological changes were observed in galanin-positive terminals in laminae I and II ipsilateral to the lesion. Most importantly, the most frequently occurring type of galanin-positive terminals (type 1) showed distinct changes with a granular matrix, many immunoreactive, peripherally located large dense-core vesicles, empty large vesicles and synaptic vesicles which were displaced from the presynaptic zone. Other galanin-positive terminals underwent even more pronounced morphological changes, including extensive vesiculolysis, also of large dense-core vesicles, filamentous degeneration or formation of axonal labyrinths. An increased number of galanin-positive nerve terminals was observed in lamina III of the ipsilateral dorsal horn after axotomy. They did not form glomeruli and contained few large dense-core vesicles. Post-embedding immunocytochemistry combined with quantitative analysis revealed that significant changes occurred in a proportion of terminals also with regard to peptide content in large dense-core vesicles after axotomy. Thus, the percentage of galanin-positive large dense-core vesicles increased in several cases and that of substance P- and CGRP-immunoreactive ones decreased.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructural studies on peptides in the dorsal horn of the rat spinal cord--IV. Effects of peripheral axotomy with special reference to neuropeptide Y and vasoactive intestinal polypeptide/peptide histidine isoleucine

Using immunofluorescence histochemistry and pre- and post-embedding immunoelectron microscopy the rat lumbar dorsal horn was analysed in normal rats and 14 days after unilateral transection of the sciatic nerve. A marked increase in neuropeptide Y-like immunoreactivity was observed in the ipsilateral, superficial dorsal horn, especially in laminae III and IV, of the lumbar 4-5 spinal cord segments after peripheral axotomy. In the ipsilateral lamina II two types of neuropeptide Y-immunoreactive, presumably primary afferent terminals could be identified at the ultrastructural level. The first type contained many large dense-core vesicles (100-155 nm in diameter), whereas a second, more common type had only a few and smaller large dense-core vesicles (80-100 nm in diameter), plus synaptic vesicles of varying diameter (50-85 nm), large empty vesicles and tubular structures. Only occasionally were neuropeptide Y-positive terminals in lamina II involved in the formation of axonal labyrinths. In the ipsilateral lamina III, the number of neuropeptide Y-positive nerve terminals markedly increased after axotomy, with a moderate increase in lamina IV. These neuropeptide Y-positive terminals were morphologically similar to the second type of neuropeptide Y-positive terminal in lamina II, i.e. contained many synaptic vesicles (45-50 nm in diameter), a few small large dense-core vesicles (80-100 nm in diameter), electron-dense granular matrix and a few tubular structures. Fusion of synaptic vesicles with the plasma membrane was often observed at these synapses. These terminals frequently formed glomeruli but were not involved in axonal labyrinths. With regard to local neurons, neuropeptide Y-like immunoreactivity was observed in many dendrite-like profiles mostly making synaptic contacts with neuropeptide Y-negative dendrites and only rarely contacting the central terminal of the glomeruli. Neuropeptide Y-positive nerve endings were mainly seen in lamina I and the outer third of lamina II. After peripheral axotomy the number of vasoactive intestinal polypeptide/peptide histidine isoleucine immunoreactive terminals was increased in laminae I and II. They contained many large dense-core vesicles (100-120 nm in diameter), and some of them were positive for vasoactive intestinal polypeptide/peptide histidine isoleucine. Morphologically, the terminals were characterized by a granular matrix, tubular structures, empty vesicles, reduction in synaptic vesicles and absence of postsynaptic densities. Vasoactive intestinal polypeptide/peptide histidine isoleucine-like immunoreactivities were often found in association with labyrinth formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Plasticity in spinal nociception after peripheral nerve section: reduced effectiveness of the NMDA receptor antagonist MK-801 in blocking wind-up and central sensitization of the flexor reflex

We have examined and compared the effects of systemically applied MK-801, an NMDA receptor/channel blocker, on the wind-up and facilitation of the flexor reflex during and after conditioning stimulation (CS) of C-afferents in rats with intact sciatic nerves or 13-16 days after axotomy. In rats with intact sciatic nerves, intravenous MK-801 (0.5 mg/kg) partially reduced wind-up and totally blocked reflex facilitation following C-fiber CS to the sural nerve. In contrast, 13-16 days after unilateral section of the sciatic nerve, the same dose of MK-801 failed to reduce the wind-up and reflex facilitation following C-fiber CS to the axotomized sural nerve, although the duration of reflex facilitation was significantly shortened. These findings indicate that the involvement of NMDA receptors in mediating activity-dependent spinal hyperexcitability is substantially reduced after peripheral nerve section, possibly reflecting a reduced release of glutamate by primary sensory afferents.

Galanin antisense oligonucleotides reduce galanin levels in dorsal root ganglia and induce autotomy in rats after axotomy

Antisense (AS) oligonucleotides (ONs) to galanin (GAL) were applied to the proximal end of a transected sciatic nerve, allowing their cellular uptake and transport into injured axons. GAL expression in dorsal root ganglia and self-mutilation behavior (autotomy) were then studied. AS-ONs with phosphorothioate or allyl modifications significantly suppressed the axotomy-induced increase in GAL levels, as demonstrated by immunohistochemistry and exaggerated autotomy behavior, whereas no significant effect on GAL mRNA levels could be demonstrated with in situ hybridization. Allyl-ONs were more effective than phosphorothioate-ONs. An AS-ON with three base mismatches did not induce any of the above effects. These results support the view that the inhibition of axotomy-induced GAL up-regulation is related to autotomy.

Galanin receptor binding sites in adult rat spinal cord respond differentially to neonatal capsaicin, dorsal rhizotomy and peripheral axotomy

The discrete distribution and possible changes in specific [125I]galanin binding sites were evaluated in the rat spinal cord following neonatal capsaicin treatment, dorsal rhizotomy and sciatic nerve section. The highest density of [125I]galanin binding sites in the normal rat spinal cord was particularly evident in the superficial layers of the dorsal horn whereas moderate to low amounts of labelling were associated with the deeper dorsal horn, areas around the central canal and the ventral horn. Capsaicin-treated rats, compared to littermate controls, showed a significant bilateral increase in [125]galanin binding in the superficial laminae of the dorsal horn. Similarly, unilateral dorsal rhizotomy evoked a significant increase in the density of [125l]galanin binding sites in the superficial dorsal horn ipsilateral to surgery. Section of the sciatic nerve, on the other hand, induced a significant depletion in [125l]galanin binding in laminae I and II of the ipsilateral dorsal horn. These results, in parallel to those reported for galanin immunoreactivity under similar conditions, suggest that [125I]galanin binding sites are preferentially located postsynaptically to the primary afferent fibre terminals in the dorsal horn of the spinal cord. Thus it seems that galanin, at the level of the dorsal spinal cord, regulates the processing of nociceptive information by acting on its own class of specific receptors located postsynaptically to primary sensory terminals.

Neuronal differentiation in cultures of murine neural crest. II. Development of capsaicin-sensitive neurons

A subset of neurons differentiating in cultures of mammalian neural crest cells express a sensitivity to the excitotoxin, capsaicin. The properties of the capsaicin-sensitive neurons in the neural crest cultures are similar to those reported for authentic sensory neurons. For example, the application of capsaicin results in a large, rapid depolarization of capsaicin-sensitive neurons. Capsaicin-sensitive neurons were also detected by the influx of cobalt ions. The development of capsaicin sensitivity in neural crest cultures was dependent on time in culture and on the presence of NGF. A major difference between the crest-derived capsaicin-sensitive neurons and authentic sensory neurons was the simultaneous expression of immunoreactivity for ChAT/TH/CGRP by the former neurons. These experiments indicate that a specific physiological property of authentic sensory neurons is expressed by neurons differentiating from the neural crest in vitro.

Visualization by [3H]resiniferatoxin autoradiography of capsaicin-sensitive neurons in the rat, pig and man

[3H]Resiniferatoxin autoradiography revealed high densities of binding sites in rat dorsal root ganglia as well as in the superficial dorsal horn of the spinal cord, known to contain the cell bodies and central terminals, respectively, of capsaicin-sensitive, sensory neurons. This binding was fully displaced by non-radioactive resiniferatoxin and was absent following administration of high, neurotoxic doses of capsaicin. The binding thus has the characteristics expected for the vanilloid (capsaicin) receptor. High density, specific resiniferatoxin binding was also observed in pig spinal cord and dorsal root ganglia. Finally, similar high density binding was detected in the dorsal horn of human spinal cord obtained post-mortem. We conclude that [3H]resiniferatoxin autoradiography may afford a novel neurochemical tool to identify capsaicin-sensitive neurons in the central as well as in the peripheral nervous system, to explore the ontogeny of these neurons, and to detect changes in vanilloid (capsaicin) receptor expression under pathophysiological conditions.

Superfluous neurotransmitters?

In recent years, studies have suggested that the complexity of eukaryotic gene regulation, with its recurring and interacting motifs of cis and trans-acting regulatory elements, might result in superfluous gene expression. This conclusion is supported by a variety of experimental results that suggest that non-adaptive gene expression might be common. However, with few exceptions, the practical ramifications of unnecessary gene expression for cell biologists have not been addressed directly; this is particularly true for peptidergic neurophysiology, a field that might be plagued more than most with the consequences of this phenomenon. In this article, Chauncey W. Bowers discusses the superfluous expression of neuropeptides in the nervous system in the context of gene regulation extrapolated from studies in Drosophila.

The pathophysiology of chronic pain--increased sensitivity to low threshold A beta-fibre inputs

Chronic pain is characterized by abnormal sensitivity, which is due to the generation of pain in response to the activation of the low-threshold mechanoreceptive A beta fibres that normally generate innocuous sensations. Three different processes in the spinal cord can account for this dramatic alteration in sensory processing in the somatosensory system: increased excitability, decreased inhibition and structural reorganization. All have been shown to occur and each may contribute separately or together to the wide range of chronic inflammatory and neuropathic pain disorders. The unravelling of the cellular mechanisms involved both offers the potential for developing novel therapeutic strategies, which reduce functional synaptic plasticity and prevent central atrophic and regenerative responses in injured neurones, and illustrates the capacity of the adult nervous system for maladaptive modification.

Upregulation of nitric oxide synthase and galanin message-associated peptide in hypothalamic magnocellular neurons after hypophysectomy. Immunohistochemical and in situ hybridization studies

The expression of several bioactive molecules in magnocellular hypothalamic neurons is modified when the axons of these cells are transected. In this study we have evaluated by means of immunocytochemistry and in situ hybridization the effect of hypophysectomy on the expression of nitric oxide synthase (NOS)- and of galanin message-associated peptide (GMAP)-like immunoreactivities (-LIs) as well as on their respective mRNAs in hypothalamic magnocellular neurosecretory neurons. The results show a transient increase in NOS- and GMAP-LIs in magnocellular neurons of both the paraventricular and supraoptic nuclei when compared to normal animals. The maximal increase in staining was observed between 5 and 7 days, and by 14 days NOS-LI was back to normal levels, whereas strong GMAP-LI could still be detected in a few cells. A similar picture was observed for the NOS and GMAP mRNAs. The functional significance of the present findings is unclear, but they indicate a possible role of nitric oxide and GMAP in neurosecretory neurons after injury.

Pharmacology of chronic pain

Chronic pain, which is associated with prolonged tissue damage or injuries to the peripheral or central nervous system, results from a number of complex changes in nociceptive pathways. These include alterations of cell phenotype and changes in the expression of proteins such as receptors, transmitters and ion channels, as well as modifications of neural structure, for example, cell loss, nerve regeneration and synaptic reorganizations. The resultant increase in neural excitability can be reduced with receptor-selective drugs that block peripheral or central chemical mediators or that control ectopic activity or cellular phenotype changes. In this article, Andy Dray, Laszlo Urban and Anthony Dickenson focus on some current mechanistic aspects of chronic pain imposed by inflammation and peripheral neuropathy, and review in particular the molecular changes involving the pharmacology of nociceptive pathways since these have important implications for the management of pain.