Electrophysiological properties of neurones with CGRP-like immunoreactivity in rat dorsal root ganglia

Confocal microscopic analysis reveals sprouting of primary afferent fibres in rat dorsal horn after spinal cord injury

Following high thoracic spinal cord transection (SCT) in rats, abnormal changes in arterial pressure in response to sensory stimulation (autonomic dysreflexia) are correlated with changes in neural circuitry in the injured spinal cord. Anterograde transport of wheat germ agglutinin conjugated to Texas Red (WGATR) and confocal microscopy were used to characterize the increased arbourization of Adelta and Abeta fibre populations in laminae III-V of the dorsal horn. In cord-injured animals, significantly greater areas of WGATR-labeled fibres were found in the deeper laminae of the dorsal horn than in control rats. This increased area likely reflects sprouting of the Adelta, Abeta, and possibly C fibre populations. The time course of sprouting matches the onset of autonomic dysreflexia, indicating a possible functional correlation between the two phenomena.

Maturation of cutaneous sensory neurons from normal and NGF-overexpressing mice

In the rodent, cutaneous sensory neurons mature over the first two postnatal weeks, both in terms of their electrical properties and their responses to mechanical stimulation of the skin. To examine the coincidence of these events, intracellular recordings were made from neurons in the dorsal root ganglion (DRG) in an in vitro spinal cord, DRG, and skin preparation from mice between the ages of postnatal day 0 and 5 (P0-P5). We also examined mice in which nerve growth factor (NGF) is overexpressed in the skin. NGF has been shown to be involved in a number of aspects of sensory neuron development and function. Therefore we ask here whether excess target-derived NGF will alter the normal course of development, either of somal membrane properties, physiological response properties, or neuropeptide content. In wild-type mice, somal action potentials (APs) were heterogeneous, with some having simple, uninflected falling phases and some displaying an inflection or break on the falling limb. The proportion of neurons lacking an inflection increased with increasing age, as did mean conduction velocity. A variety of rapidly and slowly adapting responses could be obtained by gently probing the skin; however, due to relatively low thresholds and firing frequencies, as well as lack of mature peripheral receptors such as hairs, it was not possible to place afferents into the same categories as in the adult. No correlation was seen between the presence or absence of an inflection on the somal AP (a marker for high-threshold mechanoreceptors in adult animals) and either peripheral threshold or calcitonin-gene related peptide (CGRP) content. Small differences in the duration and amplitude of the somal AP were seen in the NGF-overexpressing mice that disappeared by P3-P5. Excess target-derived NGF did not alter physiological response properties or the types of neurons containing CGRP. The changes that did occur, including a loss of the normal relationship between AP duration and conduction velocity, and a decrease in mean conduction velocity in the inflected population, might best be explained by an increase in the relative proportions of myelinated nociceptors. Of greatest interest was the finding that in both NGF overexpressers and wild-type mice, the correlation between mechanical threshold and presence or absence of an inflection on the somal spike is not apparent by P5.

Changes in somatic action potential shape in guinea-pig nociceptive primary afferent neurones during inflammation in vivo

We have examined whether there are changes during inflammation in the membrane properties of nociceptive primary afferent neurones in the guinea-pig that might contribute to hyperalgesia. Inflammation was induced by intradermal injections of complete Freund's adjuvant (CFA) in the left leg. Intracellular voltage recordings were made from the somata of ipsilateral L6 and S1 dorsal root ganglion neurones in anaesthetised untreated guinea-pigs at 2 or 4 days after CFA treatment. 2. Units were classified as C, Adelta or Aalpha/beta on the basis of their dorsal root conduction velocities (CVs). Units with receptive fields on the left leg were characterized as nociceptive, low- threshold mechanoreceptive (LTM) or unresponsive according to their responses to mechanical and thermal stimuli. The shapes of their somatic action potentials (APs) evoked by dorsal root stimulation were recorded. 3. Comparisons of data from nociceptive neurones recorded in CFA treated animals after 2 and 4 days with data from CFA untreated (control) animals showed the following significant changes: in C-fibre nociceptors, decreased AP duration at base, AP rise time and AP fall time, and increased maximum rates of AP rise and fall with no change in afterhyperpolarization measured to 80 % recovery (AHP80); in Adelta-fibre nociceptors, decreased AP duration at base, AP fall time and a reduction in AHP80; and in Aalpha/beta-fibre nociceptors, a decreased AHP80 but no change in AP duration. Apart from a more negative membrane potential and AHP depth below 0 mV in Aalpha/beta nociceptors at 4 days compared with 2 days post-CFA, none of the above variables differed significantly between units recorded 2 or 4 days after CFA. Therefore the two groups were pooled and called CFA2 + 4d. 4. The reduction in AP duration in C-fibre nociceptors was apparent both in high threshold mechanoreceptor and polymodal nociceptors and also in units with either cutaneous or subcutaneous receptive fields. 5. No significant changes in AP duration at base or AHP80 were seen 2 or 4 days after CFA compared with control in either LTM or unresponsive neurones, although some of the latter may have become classified as nociceptors after CFA treatment. 6. The alterations in membrane properties of nociceptors should permit higher discharge frequencies, thus contributing to inflammatory hyperalgesia. They suggest active changes in the expression or activation of cation channels during peripheral inflammation.

Neurotrophins, nociceptors, and pain

It is now well established that neurotrophins play a crucial role in the development of the nervous system. However, there is increasing evidence that the function of neurotrophins persists throughout adulthood. The broad scope of neurotrophin action is well documented in the case of nerve growth factor (NGF) and its effect on nociceptors and nociception. Here, we review the evidence for these multiple roles for NGF. Two manipulations influencing NGF levels are discussed in detail. The first involves the use of transgenic mice that overexpress or underexpress neurotrophins. A second strategy involves administration of NGF or its antibody in vivo to increase or decrease its level. During prenatal development, NGF is required for survival of nociceptors. In the early postnatal period, NGF is required for expression of the appropriate nociceptor phenotype. In adults, NGF acts as an important intermediate in inflammatory pain, contributing to both peripheral and central sensitization. The sensitization of peripheral nociceptors can be very rapid and can involve non-neural cells such as mast cells, neutrophils, fibroblasts, and macrophages. Recent evidence indicates that other neurotrophins also play key supporting roles in the development of nociceptors (e.g., NT-3) and in inflammatory pain (e.g., BDNF, NT-4/5). Furthermore, molecules from other superfamilies (e.g., GDNF) also are required to assure survival of certain classes of nociceptors. The diverse effects of neurotrophins on nociceptive processing emphasize their broad importance in the development and function of the nervous system.

Neutralization of endogenous NGF prevents the sensitization of nociceptors supplying inflamed skin

Evidence suggests that nerve growth factor (NGF) is an important mediator in inflammatory pain states: NGF levels increase in inflamed tissue, and neutralization of endogenous NGF prevents the hyperalgesia which normally develops during inflammation of the skin. Here we asked whether NGF contributes to sensitization of primary afferent nociceptors, which are an important component of pain and hyperalgesia in inflamed tissue. An in vitro skin nerve preparation of the rat was used to directly record the receptive properties of thin myelinated (Adelta) and unmyelinated (C) nociceptors innervating normal hairy skin, carrageenan-inflamed skin and carrageenan-inflamed skin where endogenous NGF had been neutralized by application of a trkA-IgG (tyrosine kinase Aimmunoglobulin G) fusion molecule. Following carrageenan inflammation, there was a marked increase in the proportion of nociceptors which displayed ongoing activity (50% of nociceptors developed spontaneous activity compared to 4% of nociceptors innervating normal uninflamed skin), and this was reflected in a significant increase in the average ongoing discharge activity. Spontaneously active fibres were sensitized to heat and displayed a more than twofold increase in their discharge to a standard noxious heat stimulus. Furthermore, the number of nociceptors responding to the algesic mediator bradykinin increased significantly from 28% to 58%. By contrast, the mechanical threshold of nociceptive afferents did not change during inflammation. When the NGF-neutralizing molecule trkA-IgG was coadministered with carrageenan at the onset of the inflammation, primary afferent nociceptors did not sensitize and displayed essentially normal response properties, although the inflammation as evidenced by tissue oedema developed normally. We therefore conclude that NGF is a crucial component for the sensitization of primary afferent nociceptors associated with tissue inflammation.

Comprehensive immunofluorescence and lectin binding analysis of intervibrissal fur innervation in the mystacial pad of the rat

The innervation of the intervibrissal fur in the mystacial pad of the rat and mouse was examined by immunofluorescence with a wide variety of antibodies for neuronal related structural proteins, enzymes, and peptides as well as for lectin binding histofluorescence with Griffonia simplicifolia (GSA). Anti-protein gene product 9.5 (PGP) immunofluorescence labeled all sets of axons and endings. The innervation in the upper dermis and epidermis was distributed through a four tiered dermal plexus. From deep to superficial, the second tier was the source of all apparent myelinated mechanoreceptors, the third tier of nearly all the peptidergic and GSA binding innervation, and the fourth tier of nonpeptidergic GSA negative innervation (peptide-/GSA-). Three types of mechanoreceptors-Merkel, transverse lanceolate, and longitudinal lanceolate endings-innervated guard hair follicles. All had similar labeling characteristics for 160 kDa and 200 kDa neurofilament subunits, peripherin, carbonic anhydrase, synaptophysin, and S100. Palisades of longitudinal lanceolate endings were part of piloneural complexes along circumferentially oriented sets of transverse lanceolate endings, peptidergic free nerve endings (FNEs), and peptide-/GSA- FNEs. The longitudinal lanceolate endings were the only mechanoreceptors in the mystacial pad that had detectable calcitonin gene-related peptide. The epidermis contained four types of unmyelinated endings: simple free nerve endings (FNEs), penicillate endings, cluster endings and bush endings. Only the simple FNEs were clearly peptidergic. Virtually all others were peptide-/ GSA-. Each bush ending was actually an intermingled cluster of endings formed by several unmyelinated axons and occasionally an Adelta axon. In contrast to the other unmyelinated innervation to the epidermis, bush endings labeled with an antibody against the Schwann cell protein S100. The necks and mouths of follicles, as well as superficial vasculature, were innervated by a mixture of unmyelinated peptidergic and/or GSA labeled sensory and sympathetic axons. Small presumptive sweat glands were innervated by three sets of peptidergic axons of which one was immunoreactive for somatostatin. Potential functions of the various sets of innervation are discussed.

Nerve growth factor receptor TrkA is down-regulated during postnatal development by a subset of dorsal root ganglion neurons

Nerve growth factor (NGF), signaling through its receptor tyrosine kinase, TrkA, is required for the survival of all small and many intermediate-sized murine dorsal root ganglion (DRG) neurons during development, accounting for 80% of the total DRG population. Surprisingly, NGF/TrkA-dependent neurons include a large population that does not express TrkA in adult mice (Silos-Santiago et al., 1995). This finding suggests two hypotheses: Neurons lacking TrkA in the adult may express TrkA during development, or they may be maintained through a paracrine mechanism by TrkA-expressing neurons. To determine whether TrkA is expressed transiently by DRG neurons that lack the receptor in adulthood, we examined the distribution of TrkA protein during development. We show here that TrkA expression is strikingly developmentally regulated. Eighty percent of DRG neurons expressed TrkA during embryogenesis and early postnatal life, whereas only 43% expressed TrkA at postnatal day (P) 21. Because the period of TrkA down-regulation corresponds with a critical period during which nociceptive phenotype can be altered by NGF (see Lewin and Mendell [1993] Trends Neurosci. 16:353-359), we examined whether NGF modulates the down-regulation of TrkA. Surprisingly, neither NGF deprivation nor augmentation altered the extent of TrkA down-regulation. Our results demonstrate a novel form of regulation of neurotrophin receptor expression that occurs late in development. All DRG neurons that require NGF for survival express TrkA during embryogenesis, and many continue to express TrkA during a postnatal period when neuronal phenotype is regulated by NGF. The subsequent down-regulation of TrkA is likely to be importantly related to functional distinctions among nociceptive neurons in maturity.