Plasticity of primary afferent acid phosphatase expression following rerouting of afferents from muscle to skin in the adult rat

Recent progress in histochemistry and cell biology

Studies published in Histochemistry and Cell Biology in the year 2011 represent once more a manifest of established and newly sophisticated techniques being exploited to put tissue- and cell type-specific molecules into a functional context. The review is therefore the Histochemistry and Cell Biology's yearly intention to provide interested readers appropriate summaries of investigations touching the areas of tissue biology, developmental biology, the biology of the immune system, stem cell research, the biology of subcellular compartments, in order to put the message of such studies into natural scientific-/human- and also pathological-relevant correlations.

Phenotype of distinct primary sensory afferent subpopulations and caspase-3 expression following axotomy

Specific sensory neuronal subpopulations show contrasting responses to peripheral nerve injury, as shown by the axotomy-induced death of many cutaneous sensory neurons whilst muscular sensory afferents survive an identical insult. We used a novel combination of retrograde neuronal tracing with immunohistochemistry and laser microdissection techniques, in order to describe the neurochemistry of medial gastrocnemius (muscular sensory afferents) and sural (cutaneous sensory afferents) branches of the rat sciatic nerve and relate this to the pro-apoptotic caspase-3 gene expression following nerve transection. Our results demonstrated distinctions in medial gastrocnemius and sural neuron populations with the most striking difference in the respective proportions of isolectin B4 (IB4) staining neurons (3.7 V 32.8%). The mean neuronal area of the medial gastrocnemius (MG) neurons was larger than that of the sural (SUR) neurons (1,070.8 V 646.2 μm²) and each phenotypic group was significantly smaller in sural neurons than in MG neurons. At 1 week post-axotomy, MG neurons markedly downregulated caspase-3, whilst SUR neurons upregulated caspase-3 gene expression; this may be attributable to the differing IB4-positive composition of the subpopulations. These findings provide further clarification in the understanding of two distinct neuronal populations used increasingly in nerve injury models.

Prostatic acid phosphatase is an ectonucleotidase and suppresses pain by generating adenosine

Thiamine monophosphatase (TMPase, also known as fluoride-resistant acid phosphatase) is a classic histochemical marker of small-diameter dorsal root ganglia neurons. The molecular identity of TMPase is currently unknown. We found that TMPase is identical to the transmembrane isoform of prostatic acid phosphatase (PAP), an enzyme with unknown molecular and physiological functions. We then found that PAP knockout mice have normal acute pain sensitivity but enhanced sensitivity in chronic inflammatory and neuropathic pain models. In gain-of-function studies, intraspinal injection of PAP protein has potent antinociceptive, antihyperalgesic, and antiallodynic effects that last longer than the opioid analgesic morphine. PAP suppresses pain by functioning as an ecto-5'-nucleotidase. Specifically, PAP dephosphorylates extracellular adenosine monophosphate (AMP) to adenosine and activates A1-adenosine receptors in dorsal spinal cord. Our studies reveal molecular and physiological functions for PAP in purine nucleotide metabolism and nociception and suggest a novel use for PAP in the treatment of chronic pain.

Altered primary afferent anatomy and reduced thermal sensitivity in mice lacking galectin-1

The transmission of nociceptive information occurs along non-myelinated, or thinly myelinated, primary afferent axons. These axons are generally classified as peptidergic (CGRP-expressing) or non-peptidergic (IB4-binding), although there is a sub-population that is both CGRP-positive and IB4-binding. During neuronal development and following injury, trophic factors and their respective receptors regulate their survival and repair. Recent reports also show that the carbohydrate-binding protein galectin-1 (Gal1), which is expressed by nociceptive primary afferent neurons during development and into adulthood, is involved in axonal pathfinding and regeneration. Here we characterize anatomical differences in dorsal root ganglia (DRG) of Gal1 homozygous null mutant mice (Gal1(-/-)), as well as behavioural differences in tests of nociception. Gal1(-/-) mice have a significantly reduced proportion of IB4-binding DRG neurons, an increased proportion of NF200-immunoreactive DRG neurons, increased depth of central terminals of IB4-binding and CGRP-immunoreactive axons in the dorsal horn, and a reduced number of Fos-positive second order neurons following thermal (cold or hot) stimulation. While there is no difference in the total number of axons in the dorsal root of Gal1(-/-) mice, there are an increased number of myelinated axons, suggesting that in the absence of Gal1, neurons that are normally destined to become IB4-binding instead become NF200-expressing. In addition, mice lacking Gal1 have a decreased sensitivity to noxious thermal stimuli. We conclude that Gal1 is involved in nociceptive neuronal development and that the lack of this protein results in anatomical and functional deficits in adulthood.

Dorsal Horn Plasticity Following Re-routeing of Peripheral Nerves: Evidence for Tissue-Specific Neurotrophic Influences from the Periphery

Some properties of primary sensory neurons change when they reinnervate new peripheral targets (McMahon et al., Neuroscience, 33, 67 - 75, 1989). We ask here if such influences can extend to the central connectivity of sensory neurons. In adult rats the nerve to the gastrocnemius muscle (GN) and the cutaneous sural nerve (SN) were self- and cross-anastomosed on left- and right-hand sides, respectively, so that they regenerated to either appropriate or inappropriate targets. Ten to 14 weeks later, the distribution and strength of spinal connections of the SN and GN were determined. The unmyelinated afferents in the GN innervating skin increased their connectivity to 286% of that seen for the GN innervating muscle (P < 0.005), and came to resemble normal cutaneous afferents. However, for the SN there was no significant difference between appropriately and inappropriately regenerated nerves by this measure. The ability of myelinated fibres to produce inhibitions and facilitations in dorsal horn cells was also assessed. The intact or self-anastomosed SN produced predominantly inhibitory effects, whilst the GN produced predominantly facilitatory effects. After the SN had regenerated to muscle its central effects became predominantly facilitatory, whilst those of the GN innervating skin became inhibitory. These changes were statistically significant. In conclusion, we have found that major changes in the physiology of central connections in the dorsal horn may occur following peripheral reinnervation of foreign targets. The changes that were seen were appropriate to the new target, and could not easily be explained by non-specific changes due to axotomy, or changes in A-fibre-mediated inhibitions. We suggest that these effects might arise because of trophic influences arising in and specific to different peripheral targets.

GDNF rescues nonpeptidergic unmyelinated primary afferents in streptozotocin-treated diabetic mice

Sensory deficits induced by diabetes commonly affect small unmyelinated peptidergic and nonpeptidergic sensory neurons. The peptidergic population responds to nerve growth factor (NGF), while the nonpeptidergic DRG neurons postnatally switch their dependency from NGF to glial cell line-derived neurotrophic factor (GDNF). Recent studies have demonstrated that deficient NGF support of peptidergic nociceptors is involved in problems with small-fiber diabetic neuropathy. To determine if nonpeptidergic GDNF-responsive neurons are similarly affected by hyperglycemia, diabetes was induced in mice using streptozotocin (STZ). Four weeks following diabetes induction, staining of axon terminals of nonpeptidergic unmyelinated neurons labeled with the isolectin IB4 or enzyme activity for thiamine monophosphatase (TMP) was reduced in lamina IIi of the lumbar dorsal horn, particularly in the medial region which receives distal sciatic afferents. In contrast, NGF-responsive CGRP-immunoreactive (ir) axons showed no or only a slight decrease in spinal terminations. Insulin treatment in diabetic mice failed to improve deficits in IB4/TMP central afferents. To test whether GDNF or NGF could restore spinal deficits in nonpeptidergic afferents, STZ-treated mice were treated intrathecally for 2 weeks with NGF or GDNF. NGF administration enhanced CGRP-ir staining but failed to improve IB4/TMP projections. GDNF treatment had no effect on CGRP-ir projections but restored TMP labeling in lamina IIi. Our results demonstrate that nonpeptidergic unmyelinated sensory neurons are vulnerable to diabetes and that GDNF administration can selectively reverse deficits caused by diabetes in the IB4/TMP subpopulation.

On the problem of lamination in the superficial dorsal horn of mammals: a reappraisal of the substantia gelatinosa in postnatal life

Although it is one of the most distinctive and earliest recognized features in the spinal cord, the substantia gelatinosa (SG) remains among the most enigmatic of central nervous system regions. The present neuroanatomical studies employed transganglionic transport of horseradish peroxidase conjugates of choleragenoid (B-HRP) and the B4 isolectin of Bandeiraea simplicifolia (IB4-HRP) on opposite sides to compare the projection patterns of myelinated and unmyelinated cutaneous primary afferents, respectively, within the superficial dorsal horn of the spinal cord in postnatal mice, from shortly after birth to adulthood. Putative unmyelinated afferents labeled with IB4-HRP gave rise to a dense sheet of terminal-like labeling restricted to the outer half of the SG. In contrast, myelinated inputs labeled with B-HRP gave rise to a similarly dense sheet of terminal-like labeling that occupied the inner half of the SG. This adult organization, with two dense sheets of terminal labeling in the superficial dorsal horn, was clearly evident shortly after birth using these markers, prior to the emergence of the SG. Furthermore, the location of the SG proper varied considerably within the dorsoventral plane of the dorsal horn according to mediolateral and segmental locations, a finding that was also seen in comparative studies in rat and cat. These findings caution against equating the SG in particular, and the superficial dorsal horn in general, with nociceptive processing; at minimum, the SG subserves a clear duality of function, with only a thin portion of its outermost aspect devoted to pain.

Neuronal supernumerary and dendritic sprouting of the nucleus ambiguus after chronic alteration of peripheral targets in cats

Anatomic changes of neuronal profiles in response to chronic alteration of peripheral targets were investigated in the nucleus ambiguus (NA) of cats. Unilateral vagal-hypoglossal nerve anastomosis was performed by suturing the transected proximal stump of the vagus nerve to the transected distal stump of the hypoglossal nerve. After comparing horseradish peroxidase (HRP)-labeled neurons on the ipsilateral operated side of the NA with the contralateral unoperated NA and the NA following transection and reuniting to the vagus itself, a remarkable ramification and elongation of the dendritic trees was observed in the HRP-positive neurons on the ipsilateral NA. Quantitative analysis of neuronal profiles revealed that the number of the medium and large neurons on the ipsilateral NA was greater than the contralateral NA and the NA following autologous suturing of the vagus. Comparisons of variable dendritic lengths of the medium and large neurons on the ipsilateral NA revealed longer distances and more branches of the tertiary and perisomatic dendrites than those of the contralateral NA and the NA ipsilateral to autologous reunion. Our results suggest that remarkable sprouting and elongation of the dendritic trees as well as cell supernumerary occurred in the dominant NA motoneurons ipsilateral to the nerve anastomosis. In conclusion, there is a trophic influence in the tongue musculature, which was retrogradely transported to the NA neurons via the regenerating axons and caused the morphological changes in the NA in response to the rerouting of efferents from the vagus nerve to the hypoglossal nerve to innervate intimate tongue musculature.

The generation of neuronal heterogeneity in a rat sensory ganglion

Adult sensory neurons differ chemically, morphologically, and functionally, but the factors that generate their diversity remain unclear. For example, neuropeptides are generally found in small neurons, whereas abundant neurofilament is common in large neurons. Neurons containing the neuropeptides calcitonin gene-related peptide (CGRP) or substance P were quantified using immunohistochemistry in rat lumbar dorsal root ganglion (DRG) at times before and after sensory neurons contact central and peripheral targets in vivo. No neurons in the newly formed DRG expressed neuropeptide or neuropeptide mRNA, but neuropeptides were detectable about the time that axons connect with peripheral targets. To determine the requirement for target in neuropeptide regulation, embryonic DRG neurons were isolated at times before central and peripheral connections had formed, placed in culture, and immunocytochemically assayed for CGRP and substance P. Cultured neurons expressed neuropeptides with a time course and in proportions similar to those in vivo. Thus, some neurons in the embryonic DRG seem to be intrinsically specified to later express CGRP and substance P. The percentage of CGRP-immunoreactive neurons was not changed by cell density, non-neuronal cells, neurotrophins in addition to nerve growth factor (NGF), or antibody inactivation of neurotrophin-3 in the presence of NGF. To test the role of extrinsic cues on CGRP expression, DRG neurons were co-cultured with potential target tissues. Co-culture with a rat epidermal or smooth muscle cell line increased the proportion of CGRP-containing neurons, whereas primary skeletal muscle and 3T3 cells had no effects. Thus, multiple appropriate sensory neuron phenotypes arise in a regulated fashion in cultured neurons isolated before target connections have formed, and some candidate target tissues can modulate that intrinsic expression pattern.

Influence of peripheral targets on the expression of calcitonin gene-related peptide immunoreactivity in rat cranial motoneurones

Calcitonin gene-related peptide-like immunoreactivity (CGRP-ir) is displayed by motoneurons that innervate striated muscle but is absent from preganglionic parasympathetic motoneurons. One hypothesis to explain this is that CGRP gene expression in motoneurons is, in part, dependent on influences from the innervated organ. To test this hypothesis, we cross-anastomosed the right hypoglossal and cervical vagal nerves of rats so that the vagal motoneurons grew to innervate the musculature of the tongue. Following a recovery period of 17 to 52 weeks, the distribution of CGRP-ir in the dorsal motor vagal nucleus was determined in both cross-anastomosed animals and self-anastomosed control animals. Successful reinnervation of the tongue musculature by vagal motoneurons was demonstrated by showing that electrical stimulation of the central vagus/peripheral hypoglossal nerve produced a twitch of the tongue muscles. Motoneurones of the dorsal motor vagal nucleus, which now innervated the tongue were found to express CGRP-ir, which was evident from the double labeling of neurons with both horseradish peroxidase and CGRP-ir. Motoneurones of the dorsal motor vagal nucleus contralateral to the cross-anastomosis remained CGRP negative. Similarly, motoneurons of the dorsal motor vagal nucleus in control animals where the vagus nerve was self-anastomosed remained CGRP negative, showing that an induction of CGRP expression is not a result of nerve section itself. We suggest that a signal from the striated muscle transported retrogradely via the motor axon regulates expression of CGRP-ir in motoneurons.

Peripheral target reinnervation following orthotopic grafting of fetal allogeneic and xenogeneic dorsal root ganglia

The sensory reinnervation of dermal papillae and epidermis of glabrous skin, interosseal Pacinian corpuscles, and muscle spindles of the soleus and extensor digitorum longus muscles has been examined 1, 3, and 8 months (allografts) or 3 and 5 weeks (xenografts) following orthotopic grafting of fetal allogeneic or xenogeneic (mouse) dorsal root ganglia (DRG) into ganglionectomized adult rats. Sensory axons in target tissues were identified immunohistochemically by monoclonal antibodies against growth-associated peptide (GAP-43), heavy neurofilament protein (RT-97), anti-mouse-specific membrane glycoprotein Thy-1.2, and polyclonal antibody to calcitonin gene-related peptide (CGRP). Absence of axonal marker staining in target structures of control animals 10 days or 3 months following ipsilateral enucleation of the L3-L6 DRG without grafting indicated an elimination of host normal (intact), regenerating, or collaterally sprouting nerve fibers. The consistent finding of immunolabeled axons ending free and in encapsulated structures in the target tissues of both allo- and xenografted rats indicates that grafted primary sensory neurons can survive and send axonal processes down the full length of the hind limb, to terminate in host target tissues. Axons of xenografted fetal mouse sensory neurons grow in adult rat hosts for distances of 4 cm or more, attaining lengths far greater than called for by their normal developmental programs.

Birth dates and survival after axotomy of neurochemically defined subsets of trigeminal ganglion cells

Trigeminal (V) ganglion cells with different neurochemical phenotypes or different birth dates are affected differently by neonatal axonal transection. The aim of the present study was to determine if V ganglion cell birth date and neurochemical phenotype were correlated and if these two variables could be related to responses to neonatal axonal transection. Immunocytochemistry, histochemistry, and [3H]thymidine labelling were used to determine the birth dates of V ganglion cells recognized by antibodies directed against neurofilament protein (NF), calcitonin gene-related peptide (CGRP), and substance P (SP) and those that bound the lectin Bandierea simplicifolia-I (BS-I). All V ganglion cells were born between embryonic days (E-) 9.5 and 14.5. All ganglion cells were born between E-9.5 and E-14.5. In a normalized population (percentages normalized to equal 100%), over 90% of NF-positive V ganglion cells were born between E-10.5 and E-12.5. The majority of CGRP-positive and SP-positive ganglion cells (> 90%) were generated from E-13.5 to E-14.5 and E-12.5 through E-14.5, respectively. Almost 85% of BS-I-positive ganglion cells were generated on E-12.5 through E-14.5. Previous results and additional data from this study indicated that NF- and BS-I-positive ganglion cells are proportionally more likely to be lost after neonatal axotomy and that SP-positive cells are more likely to remain. The percentage of CGRP-positive cells in the V ganglion was not significantly altered by neonatal infraorbital nerve transection. Overall, these findings do not indicate a strong relationship between cell birth date and the probability of survival after neonatal axonal damage for all V ganglion cell phenotypes.

Long-term age-related consequences of forelimb damage upon expression of primary afferent phenotypes in the cervical dorsal horn

Rats that sustained forelimb removal on either embryonic day 16 (E-16) or the day of birth (P-0), or transection of the brachial plexus in adulthood, had sections through the cervical dorsal horn stained for galanin, calcitonin gene-related peptide (CGRP), or the plant lectin Bandieria simplicifolia-I (BS-I) 35-50 days after these lesions. The results of these experiments demonstrated age-related differences in the effects of peripheral nerve damage upon the distributions of each of these three primary afferent markers in the dorsal horn. Damage to the brachial plexus in adulthood caused a significant increase in the density of galanin immunoreactivity in the medial portion of layers I and II and the appearance of galanin immunoreactivity in layers III and IV of the cervical dorsal horn. Such lesions resulted in significant reductions in the density of CGRP immunoreactivity in layers I and II and of BS-I binding in lamina II. Forelimb removal on the day of birth resulted in no significant change in the density of galanin immunoreactivity in layers I and II, but in the appearance of galanin-immunoreactive fibers in layers III-V. Neonatal forelimb removal resulted in no significant change in the density of CGRP immunoreactivity in layers I and II, but in a significant reduction in the density of BS-I binding in the medial portion of lamina II. Removal of the forelimb on E-16 caused a significant increase in the density of galanin immunoreactivity in layers III-V, but had no significant effect on the density or distribution of either CGRP immunoreactivity or BS-I binding in the cervical dorsal horn. These results suggest that peripheral nerve damage at all ages may cause an up-regulation of galanin in a wider distribution of ganglion cell types than was previously thought to be the case, and that there are different sensitive periods for lesion-induced, long-term changes in the innervation of the dorsal horn by CGRP- and BS-I-positive primary afferent axons.

Expression of calcium-binding proteins in the neurotrophin-3-dependent subpopulation of rat embryonic dorsal root ganglion cells in culture

In this study we have examined the calcium-binding protein expression in rat embryonic (E16) dorsal root ganglia (DRG) neurons in vitro in the presence of neurotrophin-3 (NT-3). A comparison was made with the expression of calcium-binding proteins in DRG subpopulations that depended in vitro on nerve growth factor (NGF) or brain-derived neurotrophic factor (BDNF). Our results show that NT-3 promotes the survival of a DRG subpopulation of which over 75% expresses parvalbumin (PV). The majority of these PV-positive NT-3-dependent DRG neurons were large 'type A' neurons. Expression of calbindin-D28k (CaBP) and calretinin (Calr) in the NT-3-dependent DRG population was seen in smaller fractions (between 12 and 17%) of the surviving DRG neurons and in both type A and B neurons. The preferential expression of PV in NT-3-dependent type A neurons is unique in comparison to the expression of PV and the other calcium-binding proteins in DRG neurons surviving in vitro in the presence of NGF or BDNF.

Neuronal phenotypes in mouse dorsal root ganglion cell cultures: enrichment of substance P and calbindin D-28k expressing neurons in a defined medium

The primary sensory neurons in mouse dorsal root ganglia consist of diversified subpopulations which express distinct phenotypic characteristics such as substance P or calbindin D-28k. To determine whether neuronal phenotypes are altered or not in in vitro cultures carried out in a defined synthetic medium, dissociated dorsal root ganglion cells from newborn mice were grown in the alpha-modified minimum essential medium either supplemented with 10% fetal calf serum or serum-free. About 80% of the neurons survived after 5 days of culture in both media, but only 35% or 65% were rescued after 12 days in serum-free or fetal calf serum supplemented medium, respectively. The neuronal subpopulations expressing substance P or calbindin D-28k displayed similar morphological properties in both media and a higher resistance to culture conditions than the whole neuronal cell population, especially in serum-free medium. It is therefore concluded that a defined synthetic medium offers reproducible conditions to culture dorsal root ganglion cells for at least 5 days, stimulates the expression of substance P and enriches preferentially neuronal phenotypes expressing substance P or calbindin D-28k, for a longer period of culture.

Non-neuronal cells inhibit catecholaminergic differentiation of primary sensory neurons: role of leukemia inhibitory factor

Although some sensory ganglion cells in mature animals are catecholaminergic, most mammalian sensory neurons that express the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH) do so only transiently during early gangliogenesis in vivo. The lack of TH expression at later stages appears to be due to modulation of this catecholaminergic potential. A previous study showed that the phenotype reappears, for example, when E16.5 and older sensory ganglia are dissociated in culture into single cells, suggesting that extracellular influences can modulate TH expression. Moreover, TH expression in dissociate cultures is cell-density dependent, as a four-fold increase in plating density led to a 30% decrease in the percentage of TH neurons. The present study demonstrates that inhibition of TH expression in high density cultures is mediated by ganglionic non-neuronal cells (NNC), as removal of NNC abolished density-dependent inhibition. Moreover, plating E16.5 trigeminal neurons at low density on top of NNC monolayers resulted in an 85% decrease in the percentage of TH neurons. Treatment of cultures with non-neuronal cell conditioned medium (NNC-CM) reproduced the effect of coculture with NNC, suggesting that diffusible factors from NNC were involved in the inhibition of TH. The inhibitory effect of NNC-CM was mimicked by treatment of dissociate cultures with ciliary neurotrophic factor (CNTF) and leukemia inhibitory factor (LIF). However, immunoprecipitation of NNC-CM with antibodies against LIF or CNTF showed that only anti-LIF antibodies were able partially to remove the TH inhibitory activity of NNC-CM. Therefore, LIF is one, but not the only, factor mediating NNC inhibition of TH expression in cultured sensory neurons. In summary, these data indicate that ganglionic NNC can regulate sensory transmitter phenotype in culture by inhibiting expression of specific molecular traits. The finding that LIF can partially account for the inhibitory effect of ganglionic NNC on TH expression suggests a novel role for this cytokine in regulating differentiation of catecholaminergic properties in sensory neurons.

In vitro and in vivo modulation of 5-hydroxytryptamine-, thyrotropin-releasing hormone- and calcitonin-gene related peptide-like immunoreactivities in adult rat sensory neurons

In a previous work we have shown that culturing adult rat dorsal root ganglia neurons modifies their neurotransmitter phenotype in such a way that cultured neurons synthesize transmitters that are not found in situ, while several other transmitters are expressed in a much higher percentage of neurons in culture than in situ [Schoenen J. et al. (1989) J. Neurosci. Res. 22, 473-487]. The aim of the present study was to investigate the origin and the nature of the relevant environmental signals that allow this plasticity to be expressed, focusing on three neurotransmitters: 5-hydroxytryptamine, thyrotropin-releasing hormone and calcitonin-gene related peptide. The main results can be summarized as follows: (1) culturing cells in fetal calf serum or on feeder layers of astrocytes, Schwann cells or fibroblasts partially inhibits the serotoninergic phenotype of dorsal root ganglia neurons; (2) in vivo disconnection of dorsal root ganglia from their spinal targets but not from their peripheral or supraspinal targets induces a significant increase of the percentage of 5-hydroxytryptamine- and thyrotropin-releasing hormone-positive neurons in disconnected ganglia; (3) growth factors such as ciliary neuronotrophic factor or basic fibroblast growth factor but not nerve growth factor repress 5-hydroxytryptamine and calcitonin gene-related peptide immunoreactivity in cultured sensory neurons. In conclusion, neurotransmitter gene expression of adult dorsal root ganglia neurons is controlled by complex influences. Our data suggest that thyrotropin-releasing hormone and 5-hydroxytryptamine gene expression are tonically repressed in vivo by factors originating from the spinal segmental level and that growth factors such as ciliary neurotrophic factor or basic fibroblast growth factor could be potential vectors of this repressing effect.

Transient expression of somatostatin peptide is a widespread feature of developing sensory and sympathetic neurons in the embryonic rat

Previous studies from this and other laboratories demonstrated that many embryonic sensory ganglion cells in the rat transiently express the catecholamine synthesizing enzyme tyrosine hydroxylase (TH), a trait not expressed by most mature sensory neurons. We, therefore, sought to determine whether transient expression was uniquely associated with catecholaminergic traits, or, alternatively, whether embryonic ganglion cells transiently expressed peptidergic properties as well. Of the four peptides examined (somatostatin [somatotropin release inhibiting factor] (SRIF), galanin (Gal), calcitonin gene-related peptide (CGRP), and substance P (SP)), only SRIF was found to be transiently expressed during early stages of sensory gangliogenesis. Surprisingly, SRIF immunoreactivity was observed in virtually all cranial and spinal sensory ganglion cells on embryonic day (E) 12.5. In addition to perikaryal labeling, intense SRIF immunoreactivity was also observed in the central and peripheral processes of E12.5 sensory neurons, suggesting the peptide may be released from nerve endings. The time course of SRIF appearance in cranial ganglion cells paralleled that previously described for TH, and double-labeling studies revealed extensive co-localization of these two phenotypes. By E16.5, however, the number of neurons expressing SRIF had diminished markedly, indicating that SRIF is only transiently expressed by most sensory neurons during early stages of ganglion development. An unexpected finding was that transient expression of SRIF is also a prominent feature of sympathetic ganglion cells; however, the temporal pattern of staining in the sympathetic and sensory ganglia differed substantially. Whereas virtually no SRIF staining was observed in E12.5 sympathetics, the vast majority of cells in the E16.5 superior cervical ganglion (SCG) were labeled. This contrasted sharply with the adult SCG, in which only low levels of SRIF expression were found. These findings demonstrate that SRIF peptide is transiently expressed at high levels in peripheral sensory and sympathetic neurons during embryogenesis. The time course and widespread distribution of SRIF expression indicates that the peptide may play a role in early stages of ganglion cell growth and development. Moreover, these data, in conjunction with previous studies demonstrating SRIF immunoreactivity in developing central neurons, suggest that transient expression of this peptide is a common property of diverse neuronal cell types.

Expression of substance P and preprotachykinin mRNA by primary sensory neurons in culture: regulation by factors present in peripheral and central target tissues

Primary sensory neurons display various neuronal phenotypes which may be influenced by factors present in central or peripheral targets. In the case of DRG cells expressing substance P (SP), the influence of peripheral or central targets was tested on the neuronal expression of this neuropeptide. DRG cells were cultured from chick embryo at E6 or E10 (before or after establishment of functional connections with targets). Preprotachykinin mRNA was visualized in DRG cell cultures by either Northern blot or in situ hybridization using an antisense labeled riboprobe, while the neuropeptide SP was detected by immunostaining with a monoclonal antibody. In DRG cell cultures from E10, only 60% of neurons expressed SP. In contrast, DRG cell cultures performed at E6 showed a significant hybridization signal and SP-like immunoreactivity in virtually all the neurons (98%). The addition of extracts from muscle, skin, brain or spinal cord to DRG cells cultured at E6 reduced by 20% the percentage of neurons which express preprotachykinin mRNA and SP-like immunoreactivity. Our results indicate that factors issued from targets inhibit SP-expression by a subset of primary sensory neurons and act on the transcriptional control of preprotachykinin gene.

Plasticity of sympathetic reflex organization following cross-union of inappropriate nerves in the adult cat

1. The present study has investigated the reflex organization of sympathetic neurones and its control of autonomic effector organs following nerve injury and repair. A well-defined population of vasoconstrictor neurones supplying blood vessels of the hairy skin was forced to innervate a territory that contained some appropriate, but mainly inappropriate autonomic effector organs. For this purpose the central stump of the cut sural nerve was sutured to the peripheral stump of the cut tibial nerve 11-12 months prior to the terminal experiment. 2. The activity of postganglionic sympathetic neurones was recorded from fine strands of the sural nerve proximal to the nerve lesion. Using a laser-Doppler device cutaneous blood flow was measured in the hairless skin of the hindpaw that was now reinnervated by the sural nerve. The results show a qualitative change of the reflex organization of sympathetic neurones following cross-union of these nerves. 3. Stimulation of arterial chemoreceptors by ventilating the animals with a hypoxic gas mixture (8% O2 in N2 for 3-8 min) increased the activity in twelve out of thirteen strands containing postganglionic sympathetic fibres. The increase of sympathetic activity contrasts with results from normal animals where systemic hypoxia causes a reflex decrease of activity in postganglionic fibres of the sural nerve. 4. Reflex changes of sympathetic activity were closely followed by corresponding changes of cutaneous blood flow. Systemic hypoxia produced vasoconstriction in operated animals in contrast to the vasodilatation observed in normal animals. 5. We conclude that the reflex organization of sympathetic neurones can change qualitatively following nerve lesion when sympathetic neurones regenerate and supply inappropriate target tissues. This long-lasting change reflects the plasticity of the autonomic nervous system and can produce a sustained abnormal control of reinnervated autonomic effector organs.

Substance P-like-immunoreactive sensory neurons in dorsal root ganglia of the chick embryo: ontogenesis and influence of peripheral targets

The expression of substance P (SP) was studied in sensory neurons of developing chick lumbosacral dorsal root ganglia (DRG) by using a mixture of periodic acid, lysine and paraformaldehyde as fixative and a monoclonal antibody for SP-like immunostaining. The first SP-like-immunoreactive DRG cells appeared first at E5, then rapidly increased in number to reach a peak (88% of ganglion cells) at E8, and finally declined (59% at E12, 51% after hatching). The fall of the SP-like-positive DRG cells resulted from two concomitant events affecting a subset of small B-neurons: a loss of neuronal SP-like immunoreactivity and cell death. After one hindlimb resection at an early (E6) or late (E12) stage of development (that is before or after establishment of peripheral connections), the DRG were examined 6 days later. In both cases, a drastic neuronal death occurred in the ispilateral DRG. However, the resection at E6 did not change the percentage of SP-like-positive neurons, while the resection at E12 severely reduced the proportion of SP-like-immunoreactive DRG cells (25%). In conclusion, connections established between DRG and peripheral target tissues not only promote the survival of sensory neurons, but also control the maintenance of SP-like-expression. Factors issued from innervated targets such as NGF would support the survival of SP-expressing DRG cells and enhance their SP content while other factors present in skeletal muscle or skin would hinder SP expression and therefore lower SP levels in a subset of primary sensory neurons.

Sprouting of peripherally regenerating primary sensory neurones in the adult central nervous system

We have studied the ability of primary afferent neurones to proliferate within the grey matter of the dorsal horn following the degeneration of other, nearby, afferent fibres. The peripheral branches of primary afferents have the capacity to regenerate successfully over long distances, and we have examined the possibility that when they are so doing, the neurones' status changes to facilitate greatly the sprouting of afferent fibres within the dorsal horn. "Spared root" preparations (rhizotomies of L3, L4, L6, S1, and the caudal half of L5, sparing the rostral half of the L5 dorsal root) were made in adult rats. In some animals (acute preparations) the distribution of the central terminals of the spared root was assessed by labelling the sciatic nerve with WGA-HRP at the time of the rhizotomies. In other animals (chronic preparations), symmetrical bilateral spared roots were made and the sciatic nerve on one side was concomitantly crushed to trigger regrowth of the peripheral branches of these axons. Eight to 10 weeks later the sciatic nerves on both sides were labelled with HRP-WGA. In the acute preparations the reaction product was found in a limited rostrocaudal and mediolateral region of the dorsal horn. In lamina II (the lamina of densest labelling) the labelled terminals occupied an average of 1.17 +/- 0.21 mm2. In chronic preparations, the area of labelled terminals on the side of the uncrushed sciatic nerve was 1.34 +/- 0.28 mm2 (not significantly different from acute animals). However, the labelled area on the side of the crushed sciatic nerve was significantly greater, averaging 2.17 +/- 0.14 mm2.(ABSTRACT TRUNCATED AT 250 WORDS)

Neonatal infraorbital nerve transection in the rat: comparison of effects on substance P immunoreactive primary afferents and those recognized by the lectin Bandierea simplicifolia-I

Retrograde tracing, immunocytochemical, and histochemical methods were used to determine the manner in which different classes of trigeminal (V) ganglion cells respond to transection of their axons during infancy. Retrograde tracing with true blue (TB), histochemistry using the plant lectin Bandieraea simplicifolia-I (BS-I), and immunocytochemistry using an antiserum directed against substance P (SP) were carried out in the V ganglion and V brainstem complex of normal adult rats. In the adult V ganglion, 11.9 +/- 1.9% of the cells that sent axons into the infraorbital nerve (ION) contained SP-like immunoreactivity (SPLI) and 26.9 +/- 3.6% bound the lectin BS-I. Only 2.7 +/- 1.6% of ION cells were labelled by both the SP antiserum and BS-I. Transection of the ION on the day of birth had very different effects upon primary afferent neurons containing SPLI and those labelled by BS-I. We have previously shown that such lesions result in a significant expansion of the portion of SpC innervated by primary afferents containing SPLI and we have also provided data consistent with the proposal that ganglion cells recognized by an antiserum directed against SP are more likely than other primary afferent neurons to survive neonatal axotomy. In the present study, combination of retrograde tracing with TB and lectin binding histochemistry showed that cells recognized by BS-I were selectively lost after neonatal ION transection. Only 14.2 +/- 4.4% of the ION ganglion cells that projected into this nerve at the time of the lesion and that survived neonatal axotomy were BS-I positive when the animals reached adulthood. Neonatal ION transection also resulted in a permanent reduction in the density of BS-I binding in SpC. Bandieraea simplicifolia-I binding in the brainstem ipsilateral to the damaged nerve was almost completely gone within 1 day of the nerve transection and recovered only partially by the time the rats were 2 months of age. In alternate sections tested with the SP antiserum, there was a slight reduction in the density of SPLI in the deafferented SpC on postnatal days 4 and 5, but this change never approached that observed for BS-I binding.

Changes in spinal cord reflexes after cross-anastomosis of cutaneous and muscle nerves in the adult rat

Evidence exists that the specification of afferent nerves and their central connections in the embryo may depend in part on influences from the peripheral target innervated. We have now investigated whether such peripheral determination persists in the adult rat using the unmyelinated afferent system of C fibres, which differ chemically in the adult depending on their target. We have previously shown that if the cutaneous sural nerve and the muscle gastrocnemius nerve are cross-anastomosed so that they grow to each other's target, the A fibres establish functional endings and the C fibres change their chemistry to that which is appropriate for the new target. Here we report that in normal adult rats, a short train of stimuli to the cutaneous sural nerve produced a brief facilitation of the flexion reflex, lasting on average only 5 min, whereas similar stimulation of the gastrocnemius-muscle nerve enhanced this reflex for an average of 54 min. In cross-anastomosed animals, stimulation of the gastrocnemius nerve (innervating skin) induced a brief potentiation of the flexion reflex, lasting on average only 3 min. By contrast, stimulation of sural nerve (innervating muscle) produced a potentiation of this reflex lasting 57 min. Thus the ability of adult afferent nerves to potentiate the flexion reflex depends on the target with which they make contact. We propose that tissue-specific factors influence some of the central actions of primary afferent neurons in the adult.

Quantitative analysis of peptide levels and neurogenic extravasation following regeneration of afferents to appropriate and inappropriate targets

We have studied quantitatively the levels of substance P and calcitonin gene-related peptide in nerves innervating skin and muscle of rats, and examined the effects of cross-anastomosing these nerves so that they regenerate to an inappropriate target. We have also compared the ability of nerves to induce neurogenic extravasation with their peptide content. Peptide was measured by radioimmunoassay in the proximal section of ligated peripheral nerves, and neurogenic oedema was measured by determination of Evans Blue extravasation induced by either systemic capsaicin treatment or topical mustard oil application. The levels of these peptides are higher in cutaneous nerves than muscle nerves. This cannot be explained by differences in the number of fibres in the nerves studied. The levels of peptides fall when cutaneous afferents reinnervate muscle, and rise when muscle afferents reinnervate skin. We suggest that these changes occur because of some tissue-specific trophic influence arising from the tissue innervated. The ability to produce extravasation in skin is highly correlated with the substance P and calcitonin gene-related peptide levels of its innervation, even when this occurs in inappropriate nerves which do not normally produce extravasation.

Differences in the chemical expression of rat primary afferent neurons which innervate skin, muscle or joint

The fluorescent dye Fast Blue was injected in anaesthetized rats into either skin, muscle or knee joint of the hindlimb. Following retrograde transport of the dye to lumbar dorsal root ganglia, the cell bodies of primary afferent neurons innervating these different target tissues were identified in ganglion sections by fluorescence microscopy. The sections were processed to demonstrate activity of the enzyme thiamine monophosphatase, or immunoreactivity to calcitonin gene-related peptide, substance P, or somatostatin, in Fast Blue labelled neurons. In all cases immunoreactivity to the antineurofilament antibody RT97 was used to classify dorsal root ganglion cells as being either small dark (RT97 negative, unmyelinated axons) or large light (RT97 positive, myelinated axons). The proportion of small dark cells labelled from each target decreased in the order: skin, muscle, joint. Thiamine monophosphatase and somatostatin were present only in small dark cells, while calcitonin gene-related peptide and substance P were found in both small dark and large light cells. In large light cells of all three targets, more contained calcitonin gene-related peptide than substance P. Among small dark cells, thiamine monophosphatase and somatostatin were found predominantly in skin afferents, while calcitonin gene-related peptide and substance P were more common in muscle and joint afferents. The chemical expression of primary afferents is therefore characteristic of the peripheral target they innervate. This could reflect either a maintained influence of the target on the afferents, or the factors which operate only during development.