Sensory Somata Research Articles

Immunohistochemical phenotype of sensory neurons associated with sympathetic plexuses in the trigeminal ganglia of adult nerve growth factor transgenic mice.

Following peripheral nerve injury, postganglionic sympathetic axons sprout into the affected sensory ganglia and form perineuronal sympathetic plexuses with somata of sensory neurons. This sympathosensory coupling contributes to the onset and persistence of injury-induced chronic pain. We have documented the presence of similar sympathetic plexuses in the trigeminal ganglia of adult mice that ectopically overexpress nerve growth factor (NGF), in the absence of nerve injury. In this study, we sought to further define the phenotype(s) of these trigeminal sensory neurons having sympathetic plexuses in our transgenic mice. Using quantitative immunofluorescence staining analyses, we show that the invading sympathetic axons specifically target sensory somata immunopositive for several biomarkers: NGF high-affinity receptor tyrosine kinase A (trkA), calcitonin gene-related peptide (CGRP), neurofilament heavy chain (NFH), and P2X purinoceptor 3 (P2X3). Based on these phenotypic characteristics, the majority of the sensory somata surrounded by sympathetic plexuses are likely to be NGF-responsive nociceptors (i.e., trkA expressing) that are peptidergic (i.e., CGRP expressing), myelinated (i.e., NFH expressing), and ATP sensitive (i.e., P2X3 expressing). Our data also show that very few sympathetic plexuses surround sensory somata expressing other nociceptive (pain) biomarkers, including substance P and acid-sensing ion channel 3. No sympathetic plexuses are associated with sensory somata that display isolectin B4 binding. Though the cellular mechanisms that trigger the formation of sympathetic plexus (with and without nerve injury) remain unknown, our new observations yield an unexpected specificity with which invading sympathetic axons appear to target a precise subtype of nociceptors. This selectivity likely contributes to pain development and maintenance associated with sympathosensory coupling.

Hyperglycemia alters enzyme activity and cell number in spinal sensory ganglia

Peripheral sensory diabetic neuropathy is characterized by morphological, electrophysiological and neurochemical changes to a subpopulation of primary afferent neurons. Here, we utilized a transgenic mouse model of diabetes (OVE26) and age-matched controls to histologically examine the effect of chronic hyperglycemia on the activity or abundance of the enzymes acid phosphatase, cytochrome oxidase and NADPH-diaphorase in primary sensory neuron perikarya and the dorsal horn of the spinal cord. Quantitative densitometric characterization of enzyme reaction product revealed significant differences between diabetic, compared to control, animals for all three enzymes. Levels of acid phosphatase reaction product were found to be significantly reduced in both small diameter primary sensory somata and the dorsal horn. Cytochrome oxidase activity was found to be significantly lower in small primary sensory somata while NADPH-diaphorase labeling was found to be significantly higher in small primary sensory somata and significantly lower in the dorsal horn. In addition to these observed biochemical changes, ratiometric analysis of the number of small versus large diameter primary sensory perikarya in diabetic and control animals demonstrated a quantifiable decrease in the number of small diameter cells in the spinal ganglia of diabetic mice. These results suggest that the OVE26 model of diabetes mellitus produces an identifiable disturbance in specific metabolic pathways of select cells in the sensory nervous system and that this dysfunction may reflect the progression of a demonstrated cell loss.

Regeneration of unmyelinated and myelinated sensory nerve fibres studied by a retrograde tracer method

Regeneration of myelinated and unmyelinated sensory nerve fibres after a crush lesion of the rat sciatic nerve was investigated by means of retrograde labelling. The advantage of this method is that the degree of regeneration is estimated on the basis of sensory somata rather than the number of axons. Axonal counts do not reflect the number of regenerated neurons because of axonal branching and because myelinated axons form unmyelinated sprouts. Two days to 10 weeks after crushing, the distal sural or peroneal nerves were cut and exposed to fluoro-dextran. Large and small dorsal root ganglion cells that had been labelled, i.e., that had regenerated axons towards or beyond the injection site, were counted in serial sections. Large and small neurons with presumably myelinated and unmyelinated axons, respectively, were classified by immunostaining for neurofilaments. The axonal growth rate was 3.7 mm/day with no obvious differences between myelinated and unmyelinated axons. This contrasted with previous claims of two to three times faster regeneration rates of unmyelinated as compared to myelinated fibres. The initial delay was 0.55 days. Fewer small neurons were labelled relative to large neurons after crush and regeneration than in controls, indicating that regeneration of small neurons was less complete than that of large ones. This contrasted with the fact that unmyelinated axons in the regenerated sural nerve after 74 days were only slightly reduced.

Absence of the p75 Neurotrophin Receptor Alters the Pattern of Sympathosensory Sprouting in the Trigeminal Ganglia of Mice Overexpressing Nerve Growth Factor

Sympathetic axons invade the trigeminal ganglia of mice overexpressing nerve growth factor (NGF) (NGF/p75(+/+) mice) and surround sensory neurons having intense NGF immunolabeling; the growth of these axons appears to be directional and specific (). In this investigation, we provide new insight into the neurochemical features and receptor requirements of this sympathosensory sprouting. Using double-antigen immunohistochemistry, we demonstrate that virtually all (98%) trigeminal neurons that exhibit a sympathetic plexus are trk tyrosine kinase receptor (trkA)-positive. In addition, the majority (86%) of those neurons enveloped by sympathetic fibers is also calcitonin gene-related peptide (CGRP)-positive; a smaller number of plexuses (14%) surrounded other somata lacking this neuropeptide. Our results show that sympathosensory interactions form primarily between noradrenergic sympathetic efferents and the trkA/CGRP-expressing sensory somata. To assess the contribution of the p75 neurotrophin receptor (p75(NTR)) in sympathosensory sprouting, a hybrid strain of mice was used that overexpresses NGF but lacks p75(NTR) expression (NGF/p75(-/-) mice). The trigeminal ganglia of NGF/p75(-/-) mice, like those of NGF/p75(+/+) mice, have increased levels of NGF protein and display a concomitant ingrowth of sympathetic axons. In contrast to the precise pattern of sprouting seen in the ganglia of NGF/p75(+/+) mice, sympathetic axons course randomly throughout the ganglionic neuropil of NGF/p75(-/-) mice, forming few perineuronal plexuses. Our results indicate that p75(NTR) is not required to initiate or sustain the growth of sympathetic axons into the NGF-rich trigeminal ganglia but rather plays a role in regulating the directional patterns of axon growth.

Neurotrophic factors are required by mature sympathetic neurons for survival, transmission and connectivity.

1. Two neuronal growth factors, nerve growth factor (NGF) and neurotrophin 3 (NT3), have been studied for their action on the developing and mature sympathetic nervous system. 2. Antibodies to each factor have proved useful as reagents for the detection and quantification of NGF and NT3. They have also proved valuable in uncovering the functional roles of each factor by their ability to neutralize the endogenous molecules. 3. Nerve growth factor acts on postnatal neurons to control neurotransmission, connectivity and survival. Like NGF, NT3 is synthesized by effector tissues and is retrogradely transported by post-ganglionic neurons to prevent cell death. However, the two factors have been shown to have quite distinct functions in mature neurons, indicating the existence of different signalling pathways. This differential action extends to secondary influences on satellite glia. 4. Pathological consequences result from excessive growth factor synthesis leading, in the hypertensive rat, to hyperinnervation and elevated blood pressure. Satellite glial cell synthesis of the factors and their receptors following peripheral nerve damage appears to be responsible for the establishment of inappropriate neuronal connections between sympathetic nerve terminals and sensory somata. 5. It is concluded that these potent factors control, by both coincident and independent mechanisms, sympathetic neuronal function throughout the life of the animal.

REVIEW ■ : Pain Mechanisms

Over the last few years, a new synthesis has emerged concerning the neural mechanisms of acute and chronic pain. This new model deals far more successfully than do classical models with the peculiarities of chronic pain syndromes seen in the clinic. As in earlier models, Aδ- and C-nociceptive afferents detect the initial noxious event. In addition, however, this input is now known to rapidly trigger a central hyperexcitability state ("central sensitization") that amplifies sensory signals subsequently entering the CNS along other afferent fiber types. As a result, in the presence of central sensitization, pain sensation is evoked by Aβ touch input, as well as by Aδ- and C-nociceptor input. Tenderness in subacute (e.g., inflammatory) pain, for example, is due to both peripherally sensitized nociceptors and centrally amplified, low-threshold input. The new synthesis also stresses the common ground between the subacute pain of injured tissue and the chronic pain that sometimes develops after nerve injury (i.e., neuropathic pain). In the event of neuropathic pain, the affected afferent axons and their sensory cell somata in the associated dorsal root ganglia (DRGs) become hyperexcitable to applied stimuli. Some even fire spontaneously. Hyperexcitability of the afferent neuron apparently results from specific changes in the regulation of membrane channel and receptor proteins. The resulting ectopic discharge (ectopia) contributes a direct neuropathic pain signal. In addition, neuropathic ectopia sets up and maintains a central sensitization state that amplifies ongoing pain and is responsible for pain on weak stimulation of adjacent areas of skin and deep tissues with residual innervation. The discovery that normal and ectopic Aβ touch input, as well as Aδ- and C-nociceptor input, contributes to subacute and chronic pathophysiological pain states opens previously unanticipated avenues for clinical pain control. NEUROSCIENTIST 2:233-244, 1996

Replacement of an inherited stretch receptor by a newly evolved stretch receptor in hippid sand crabs

Primary sensory neurons that are motoneuron-like in morphology and often nonspiking (transmit afferent signals as graded depolarizations) characterize an unusual type of stretch receptor in decapod crustaceans. Nonspiking and spiking receptors occur in similar positions at homologous joints in different species and have been presumed to be homologous, the spiking one considered "primitive". To better understand the evolutionary origin of these stretch receptors and why some are nonspiking, we examined the spiking telson-uropod stretch receptors in the spiny sand crab Blepharipoda occidentalis (Albuneidae) and the squat lobster Munida quadrispina (Galatheidae) and compared them with the nonspking telson-uropod stretch receptor of the mole sand crab Emerita analoga (Hippidae). The position, morphology and responses to stretch of the sensory neurons, and the ultrastructure of the elastic strand portion of the receptor are similar in M. quadrispina and B. occidentalis, except that in B. occidentalis the receptor muscles are substantially smaller and the extracellular matrix of the elastic receptor strand is both more extensive and more organized, reminiscent of the ultrastructure of E. analoga's nonspiking receptor. We conclude that the spiking telson-uropod stretch receptors of albuneids and galatheids are homologous. The differences in the ultrastructure of their receptor strands imply that the efficiency of coupling receptor length change to deformation of the dendritic termini increases in the order M. quadrispina < B. occidentalis < E. analoga. The spiking and nonspiking telson-uropod stretch receptors differ anatomically in three major respects that appear to preclude their homology. (1) The receptor strands are on opposite sides of a conserved muscle. (2) The sensory somata are in different regions of the sixth abdominal ganglion: a lateral cluster of somata for the spiking sensory neurons and two medial clusters, one anterior, one posterior, for the nonspiking sensory neurons. (3) The neuropil projections of the sensory neurons are different. We conclude that the hippid's nonspiking telson-uropod stretch receptor evolved de novo and not by modification of the ancestral anomuran telson-uropod stretch receptor (which Hippidae have lost).

Organ of Bellonci of an Antarctic crustacean, the marine isopod Glyptonotus antarcticus.

The paired organ of Bellonci protrudes from the optic lobe of the giant Antarctic isopod, Glyptonotus antarcticus. It is linked to the cortex by a broad peduncle. No connection to the cuticle or "sensory pore organ" was found. A cluster of sensory-like cells forms two outer ciliary segments branching into numerous microvilli with microtubules. The putative sensory somata are irregular in shape and contain a very high density of glycogen granules. The two outer segments sprout from two pits of the soma in different directions, forming a right angle. Glial cells wrap around the sensory cells and also delimit lacunae into which bundles of microvilli project. These lacunae contain electron-dense granules of small size and with species-specific patterns. Lacunae and dense granules show features typical of a degeneration process in the sensory cells. This general morphology corresponds to the unilobular type of organ of Bellonci, known in other isopods; it differs from the plurilobular type with onion bodies found in other Crustacea.

5-HT 1 receptor agonists reduce the Ca + component of sensory neuron action potentials

Serotonin (5-HT) agonists, selective for 5-HT 1, 5-HT 2 or 5-HT 3 receptor subtypes, were tested for their ability to mimic 5-HT in narrowing the tetraethylammonium-induced calcium-dependent plateau of action potentials recorded from frog sensory neuroons. 5-Carboxyamidotryptamine, 5-HT, α-methyl 5-HT and 5-methoxy-tryptamine possessed full agonist activity, with EC 50s of 19 nM, 210 nM, 3.7 μM, and 1.7 μM, respectively. 2-Methyl 5-HT was inactive. This agonist profile indicates that the calcium-dependent plateau in these sensory somata is modulated by a 5-HT 1-like receptor.

Sympathetic and afferent somata projecting in hindlimb nerves and the anatomical organization of the lumbar sympathetic nervous system of the rat.

The anatomy of the sympathetic pathways from the spinal cord to the lumbar sympathetic trunk and the inferior mesenteric ganglion was studied systematically in the rat. Details of the arrangements of white and gray rami communicantes, sympathetic trunk ganglia, the intermesenteric nerve, and the lumbar splanchnic nerves are summarized. A modified nomenclature for the segmental ganglia of the paravertebral sympathetic chain is proposed. Cell bodies of sensory and sympathetic axons projecting to the skin and skeletal muscle of the rat hindlimb were labeled retrogradely with horseradish peroxidase (HRP) in order to study numbers, segmental distribution, and location of the somata of these neurons quantitatively. HRP was applied to the nerves supplying skeletal muscle (gastrocnemius-soleus, GS), hairy skin (sural, SU; saphenous, SA) and to a mixed nerve (tibial, TI). All sensory somata and 96.4% of the sympathetic cell bodies were located ipsilaterally. Sensory somata were commonly restricted to two adjacent dorsal root ganglia (usually L3-4 for SA; L4-5 for GS, TI; L5-6 for SU). Although the sympathetic somata were more widely distributed rostrocaudally (four to six segments), their maximum was always located one or two segments more cranially than the sensory outflow, i.e., corresponding to the rami communicantes grisei. From the data, it is estimated that 420 sympathetic and 530 afferent neurons project into GS, 590 and 3,610 into SU, 920 and 3,750 into SA, and 1,070 and 5,760 into TI. These absolute neuron numbers are compared with electron microscopic fiber counts from the literature.

Ultrastructure of aesthetasc innervation and external morphology of the lateral antennule setae of the spiny lobster Panulirus interruptus (Randall)

Six types of setae and one type of cuticular depression were examined on the lateral antennule of the spiny lobster Panulirus interruptus using scanning electron microscopy. The organization and ultrastructure of the innervation of the most numerous setal type, the aesthetasc, were investigated using light- and transmission electron microscopy. Each aesthetasc is innervated by approximately 300 bipolar neurons whose sensory dendrites penetrate the hair and extend toward the tip, and whose axons project towards the central nervous system. The neuronal somata and two types of glia form a cluster within the antennular lumen. The inner sheath-cell somata encircle the dendritic tract distal to the sensory somata. These cells appear to extend distal processes which wrap the dendritic tract to the base of the aesthetasc. Elongate outer sheath cells are interposed between the glia-wrapped dendritic tract and the hypodermis which underlies the antennule cuticle. A continuous investment of neural lamella separates the hypodermis, the entire cluster of somata, and sensillar nerve from the antennule lumen. The organization of the neuronal somata and their association with outer and inner sheath cells in this marine species appear similar to those of crustaceans from freshwater and terrestrial habitats.

Motor, sympathetic and sensory innervation of rat skeletal muscles

This study reports on the location, number and size of motor, sympathetic and sensory neurons innervating the following muscles of rat: quadriceps femoris (QF), tibialis anterior (TA), extensor digitorum longus (EDL), peroneus longus (PL), gastrocnemius medius (GM) and soleus (SOL). Cells were labelled by application of horseradish peroxidase (HRP) to transected muscle nerves. Counts of neurons were compared with counts of myelinated (MF) and unmyelinated (UMF) fibers in normal, deafferented and chemically sympathectomized nerves. The topographical arrangement of spinal motor nuclei resembled that reported previously in other mammals and birds. Sensory somata were aggregated without precise somatotopic organization, preferentially in one of the lumbar dorsal root ganglia at a segmental level corresponding to that of the motor innervation. Because lumbar sympathetic ganglia were often poorly circumscribed, the segmental position of sympathetic ganglion cells could not be localized with certainty. Sensory and sympathetic somata demonstrated a unimodal size-frequency distribution, while QF, TA and PL motoneurons could be subdivided according to size in alpha and gamma cells. For all muscles except unsuccessfully deafferented QF, counts of motor fibers after deafferentation correlated closely with counts of labelled motoneurons. Similarly, estimates of sympathetic axons, averaging 30,7% of the UMF, in most instances exceeded only marginally the ganglion cell population. In contrast, the number of peripheral afferent fibers outnumbered markedly that of sensory cell bodies, with an average of 2.8 axons per ganglion cell.

On the fate of sympathetic and sensory neurons projecting into a neuroma of the superficial peroneal nerve in the cat.

Cell bodies of sympathetic and sensory axons projecting via the superficial peroneal (SP) nerve supplying hairy skin of the distal hindlimb have been labeled retrogradely with horseradish peroxidase (HRP) on both sides of three cats in which the left SP nerve had been cut and ligated about 5 months previously. Three SP nerves from unoperated cats have also been studied. The location, size, and numbers of labeled somata have been determined from serial sections of lumbosacral dorsal root and sympathetic chain ganglia after standard histochemical processing. The numbers of myelinated fibers in each nerve have also been counted. The segmental distributions of both sympathetic and sensory cell bodies were identical bilaterally in each operated animal, but the number of labeled neurons was reduced on the lesioned side. There were only about 31% of sympathetic and about 51% of sensory somata relative to the numbers on the contralateral side. The average total number of neurons labeled from SP nerves in unoperated animals was about 8% higher than on the control side of operated animals. The average number of myelinated fibers in the neuromatized nerves was not reduced with respect to that in the contralateral nerve and both of these were not significantly different from the number in unoperated animals. The dimensions of samples of labeled sympathetic and sensory somata were reduced on the side with the neuroma, both in comparison with the contralateral side and with unlabeled neurons at the same levels. The mean cross-sectional area of profiles of sympathetic ganglion cells was 76% of the control; of sensory ganglion cells, 65% of the control. Assuming that HRP labeling was not impaired, we conclude that large numbers of neurons with unmyelinated axons had degenerated in the neuromatized cutaneous nerves.

The cell bodies of origin of sympathetic and sensory axons in some skin and muscle nerves of the cat hindlimb.

Cell bodies of sensory and sympathetic axons projecting to skin and skeletal muscle of the cat hindlimb have been labeled retrogradely with horseradish peroxidase (HRP) in order to study location, size, and numbers of the somata of these neurons. HRP was applied to the freshly transected axons of nerves supplying hairy skin (superficial peroneal, SP; sural, Su), hairy and hairless skin of the paw (medial plantar, MP), or skeletal muscle (gastrocnemius-soleus, GS). Serial sections of lumbosacral dorsal root and sympathetic ganglia were studied after standard histochemical processing. Additionally, the numbers of myelinated fibers in the same nerves were determined. All sensory somata and 99.4% of sympathetic cell bodies were located ipsilaterally. Sensory somata were commonly restricted to two adjacent dorsal root ganglia (usually L6-7 for SP, MP; L7-S1 for Su, GS). Although sympathetic somata were more widely distributed rostrocaudally, their maximum frequency always occurred in the segmental ganglia immediately rostral to the sensory outflows, i.e., corresponding to rami communicantes grisei. Dimensions of sympathetic somata varied little between populations projecting to different tissues and were unimodally distributed. The size distributions of sensory somata were characterized by a peak between 10 and 20 microns radius, similar to sympathetic somata, and a varying smaller number of cells ranging up to 60 microns radius. Each nerve had a characteristic distribution profile of afferent somata. A population of very small cells was only present in GS, while the largest sensory somata in GS and MP were bigger than those in SP and Su. Numerical analysis of the data disclosed the characteristic composition of both myelinated and unmyelinated fibers in each nerve studied.

Current excitation threshold in sensory neurons of leech central nervous system.

1. Sensory cells in segmental ganglia of the leech (Hirudo medicinalis L.), N (noxious), P (pressure), and T (touch) cells, were stimulated by linearly rising currents. The electrical response to intracellular stimulation with prolonged square-wave currents, the shape of the action potential, and the degree of repetitive activity were also examined. 2. The thresholds for first production of an action potential by linearly rising currents (expressed as multiples of rheobase, I/IO), plotted as a function of the action-potential latency, provide a measure of accomodation. These T-L curves show that the P and T cells accommodate more rapidly than the N cells. 3. An index of accommodation, the accommodation coefficient (I/Io)s/2, was defined as the current required to elicit a first action potential 0.5 s after the beginning of a linearly rising stimulus current. These accommodation coefficients also reflect the property that P and T cells accommodate more rapidly, on the average, than N cells. 4. Threshold depolarization is higher with linearly rising currents than with square-wave currents. 5. Comparison of the accommodation rates of these sensory somata with the published data for other somata and axons, on the basis of accommodation coefficient, minimal current gradient, and/or threshold-latency curve, shows that a) N cells, at the slow end of the leech-soma accommodation range, accommodate at about the same rate as most vertebrate neurons; and b) some vertebrate neurons have accomodation rates as high as or higher than those of the T and P cells. The accommodation rates of most vertebrate neurons are related in a characteristic way to undershoot shape, repetitive-firing properties, and adaptation. Comparable relationships are found in the leech if the N and T cells alone are considered, but the P cells (with extremely rapid accomodation) depart notably from this pattern--they adapt slowly, fire repetitively in response to a maintained stimulus, and have an undershoot of intermediate amplitude and duration.

Degeneration and Regeneration in Crustacean Neuromuscular Systems

Crayfish motor neurons seem to repair damage to peripheral axons by selective fusion of outgrowing proximal stumps with severed distal processes that can survive morphologically and physiologically intact for over 200 days. Survival of isolated motor and CNS giant axons is associated with much hypertrophy of their glial sheath. The severed stumps of peripheral sensory neurons often degenerate within 21 days and their glial sheath does not hypertrophy. Denervation and immobilization produce relatively little change in the morphology and physiology of the opener muscle, whereas tenotomy produces much atrophy within 30-60 days. Crayfish motor and CNS giant neurons show no capability for regenerating ablated cell bodies, whereas peripheral sensory somata regenerate after limb autotomy. An entire opener muscle can be replaced after limb autotomy but the organism shows little or no ability to redifferentiate an entire muscle in the absence of body part regeneration. However, a few opener muscle fibers can be regenerated if the bulk of the muscle mass remains intact. The significance of all these findings are interpreted with respect to the developmental capabilities and environmental adaptations of the crayfish together with the evolution of regenerative abilities in anthropods and vertebrates.