Receptive properties of sacral primary afferent neurons supplying the colon

1. Conscious perception of noxious and innocuous distension of the colon as well as the reflex control of anal continence and defecation largely depend on an intact sacral primary afferent innervation. Here we have studied the functional properties of these visceral primary afferent neurons in the dorsal root S2 in 17 cats. Single fibers projecting into the pelvic nerve were identified electrically and studied with innocuous and noxious mechanical stimulation of colon and anal canal. 2. A total of 59 units responding to one of these stimuli were investigated and they could be separated into two subpopulations of afferents. Thirty-six fibers were reproducibly excited by distension of the colon, but not by mechanical stimulation of the anal canal. They were thin myelinated or unmyelinated fibers with a median conduction velocity of 3.2 m/s. The remaining 23 units had receptive fields in the mucosa of the anal canal and responded readily to an innocuous proximodistal shearing stimulus, but not to distension stimuli applied to the same area. All, but two of these afferents were thin myelinated with a median conduction velocity of 7.7 m/s, which was significantly different from the conduction velocity of afferent neurons responding to distension of the colon. 3. Units responding to distension of the colon had thresholds in the innocuous range of the intracolonic pressure. Receptors that were activated only by noxious intraluminal pressure were absent. On the basis of their response to supramaximal isotonic distension, colonic afferents could be subclassified as phasic (n = 17) or tonic (n = 19) units. Phasic afferents were only transiently excited during filling or emptying of the colon, whereas tonic afferents discharged throughout the distension. The two populations had also significantly different median conduction velocities of 8.0 (n = 16) and 1.7 (n = 15) m/s, respectively. 4. Stimulation response functions were evaluated for 12 tonic afferents. All units encoded an increase of intracolonic pressure by the intensity of their discharge frequency. Increases of intracolonic pressure produced significantly higher discharge frequencies from unmyelinated than from thin myelinated afferents. 5. In three animals the percentage of unmyelinated fibers responding to mechanical stimulation of colon and anal canal was determined. Out of 213 electrically identified unmyelinated units projecting into the pelvic nerve, only 11 (5.2%) were excited. Thus, acute innocuous and noxious mechanical stimuli of the large intestine do not appear to be the adequate stimulus for the large majority of unmyelinated pelvic afferents. 6. In conclusion, distension of the colon and mechanical stimulation of the anal canal activates distinct populations of primary afferent neurons.(ABSTRACT TRUNCATED AT 400 WORDS)     

骶髓后连合核接受盆内脏伤害性信息传入的形态学证明

为阐明投射至骶髓后连合核的盆内脏初级传入中是否含有传递伤害性信息成分，本研究综合运用特异性标记初级传入Ｃ纤维的ＢＳＩ－Ｂ４－ＨＲＰ跨节追踪技术，神经干局部涂抹Ｃ纤维毒素ＣａＰｓａｉｃｉｎ并结合ＳＰ免疫组化方法，研究了猫投射至骶髓后连合核的盆神经初级传入纤维中是否含有传递伤害性刺激的成分；同时观察了秋水仙素处理的骶２后根节内ＢＳＩ－Ｂ４标记的初级传入神经元与ＳＰ免疫阳性神经元的关系。结果如下：（１）向盆神经注入ＢＳＩ－Ｂ４－ＨＲＰ，骶１～３后根节内出现平均直径３４μｍ的标记细胞，后连合核内出现密集的标记终末，电镜下证明通过Ｌｉｓｓａｕｅｒ氏束进入脊髓内的标记纤维均为无髓纤维；（２）对盆神经进行局部Ｃａｐｓａｉｃｉｎ处理，引起后连合核内的ＳＰ免疫阳性纤维和终末明显减少；（３）骶２后根节内ＢＳＩ－Ｂ４－ＦＩＴＣ标记细胞有１７％同时呈ＳＰ免疫阳性；（４）骶２后根节内ＢＳＩ－Ｂ４－ＨＲＰ标记的盆内脏初级传人神经元的３９％同时呈ＳＰ免疫阳性。本研究结果在形态学上证实了骶髓后连合核接受盆腔内脏伤害性信息的传入，它可能是中继和整合盆内脏伤害性信息的低级中枢。     

High density of small nerve fibres in the functional layer of the endometrium in women with endometriosis

BACKGROUND: Endometriosis is a common gynaecological disease and is frequently associated with recurrent and serious pelvic pain such as dysmenorrhoea and dyspareunia, but the mechanisms by which these symptoms are generated are not well understood. METHODS: Histological sections of endometrial tissue were prepared from endometrial curettings and hysterectomies performed on women with endometriosis (n=25 and n=10, respectively) and without endometriosis (n=47 and n=35, respectively). These were stained immunohistochemically for the highly specific polyclonal rabbit anti-protein gene product 9.5 (PGP9.5) and monoclonal mouse anti-neurofilament protein (NF) to demonstrate both myelinated and unmyelinated nerve fibres. RESULTS: Small nerve fibres were identified throughout the basal and functional layers of the endometrium in all endometriosis patients, but were not seen in the functional layer of the endometrium in any of the women without endometriosis (P<0.001). NF-immunoreactive nerve fibres were present in the basal layer in all endometriosis patients but not in non-endometriosis patients, with one exception (P<0.001). CONCLUSIONS: Small nerve fibres detected in the functional layer in all women with endometriosis may have important implications for understanding the generation of pain in these patients. The presence of nerve fibres in an endometrial biopsy may be a novel surrogate marker of clinical endometriosis.     

Localization of TRPV1 and P2X3 in unmyelinated and myelinated vagal afferents in the rat

The vagus nerve is dominated by afferent fibers that convey sensory information from the viscera to the brain. Most vagal afferents are unmyelinated, slow-conducting C-fibers, while a smaller portion are myelinated, fast-conducting A-fibers. Vagal afferents terminate in the nucleus tractus solitarius (NTS) in the dorsal brainstem and regulate autonomic and respiratory reflexes, as well as ascending pathways throughout the brain. Vagal afferents form glutamatergic excitatory synapses with postsynaptic NTS neurons that are modulated by a variety of channels. The organization of vagal afferents with regard to fiber type and channels is not well understood. In the present study, we used tract tracing methods to identify distinct populations of vagal afferents to determine if key channels are selectively localized to specific groups of afferent fibers. Vagal afferents were labeled with isolectin B4 (IB4) or cholera toxin B (CTb) to detect unmyelinated and myelinated afferents, respectively. We find that TRPV1 channels are preferentially found in unmyelinated vagal afferents identified with IB4, with almost half of all IB4 fibers showing co-localization with TRPV1. These results agree with prior electrophysiological findings. In contrast, we found that the ATP-sensitive channel P2X3 is found in a subset of both myelinated and unmyelinated vagal afferent fibers. Specifically, 18% of IB4 and 23% of CTb afferents contained P2X3. The majority of CTb-ir vagal afferents contained neither channel. Since neither channel was found in all vagal afferents, there are likely further degrees of heterogeneity in the modulation of vagal afferent sensory input to the NTS beyond fiber type.     

Nerve fibres in peritoneal endometriosis

BACKGROUND: Endometriosis is a gynaecological disease that can be associated with severe pelvic pain; however, the mechanisms by which pain is generated remain unknown. METHODS: Peritoneal endometriotic lesions and normal peritoneum were prepared from women with and without endometriosis (n = 40 and 36, respectively). Specimens were also prepared from endosalpingiosis lesions (n = 9). These sections were stained immunohistochemically with antibodies against protein gene product 9.5, neurofilament (NF), nerve growth factor (NGF), NGF receptor p75 (NGFRp75), substance P (SP), calcitonin gene-related peptide (CGRP), acetylcholine (ACh) and tyrosine hydroxylase (TH) to demonstrate myelinated, unmyelinated, sensory, cholinergic and adrenergic nerve fibres. RESULTS: There were significantly more nerve fibres identified in peritoneal endometriotic lesions than in normal peritoneum (P < 0.001) or endosalpingiosis lesions (P < 0.001). These nerve fibres were SP, CGRP, ACh or TH immunoreactive. Many of these markers were co-localized. There was an intense NGF immunoreactivity near endometriotic glands, and NGFRp75 immunoreactive nerve fibres were present near endometriotic glands and blood vessels in the peritoneal endometriotic lesions. CONCLUSIONS: Peritoneal endometriotic lesions were innervated by sensory Adelta, sensory C, cholinergic and adrenergic nerve fibres. These nerve fibres may play an important role in the mechanisms of pain generation in this condition.     

Transganglionic transport of choleragenoid by capsaicin-sensitive C-fibre afferents to the substantia gelatinosa of the spinal dorsal horn after peripheral nerve section

Choleratoxin B subunit-binding thick myelinated, A-fibre and unmyelinated, capsaicin-sensitive nociceptive C-fibre primary afferent fibres terminate in a strict topographic and somatotopic manner in the spinal cord dorsal horn. Injection of choleratoxin B subunit-horseradish peroxidase conjugate into injured but not intact peripheral nerves produced transganglionic labelling of primary afferents not only in the deeper layers (Rexed's laminae III-IV), but also in the substantia gelatinosa (Rexed's laminae II) of the spinal dorsal horn. This was interpreted in terms of a sprouting response of the Abeta-myelinated afferents and suggested a contribution to the pathogenesis of neuropathic pain [Nature 355 (1992) 75; J Comp Neurol 360 (1995) 121]. By utilising the selective neurotoxic effect of capsaicin, we examined the role of C-fibre sensory ganglion neurons in the mechanism of this phenomenon. Elimination of these particular, capsaicin-sensitive C-fibre afferents by prior intrathecal or systemic capsaicin treatment inhibited transganglionic labelling by the choleratoxin B subunit-horseradish peroxidase conjugate of the substantia gelatinosa evoked by chronic sciatic nerve section. More importantly, prior perineural capsaicin treatment of the transected nerve proximal to the anticipated site of injection of choleragenoid 12 hours later prevented the labelling of the substantia gelatinosa, but not that of the deeper layers. Electron microscopic examination of the dorsal roots revealed no significant difference in the proportion of labelled myelinated fibres relating to the intact (54.4+/-5.5%) and the transected (62.4+/-5.4%) sciatic nerves. In contrast, the proportion of labelled unmyelinated dorsal root axons relating to the transected, but not the intact nerves showed a significant, six-fold increase after sciatic nerve transection (intact: 4.9+/-1.3%; transected: 35+/-6.7%).These observations indicate that peripheral nerve lesion-induced transganglionic labelling of the substantia gelatinosa by choleratoxin B subunit-horseradish peroxidase may be primarily accounted for by the uptake and transganglionic transport of choleragenoid by injured capsaicin-sensitive C-fibre afferents rather than a sprouting response of A-fibre afferents. The present findings suggest an essential role of capsaicin-sensitive primary sensory neurons in lesion-induced spinal neuroplastic changes and provide further support for C-fibre nociceptor neurons being promising targets for the development of new strategies in pain management.     

Bladder and cutaneous sensory neurons of the rat express different functional P2X receptors

The expression and functional responses of P2X receptors in bladder and cutaneous sensory neurons of adult rats and mice have been studied using immunohistochemistry and patch clamp techniques. Cell bodies of bladder pelvic afferents were identified in L6 and S1 dorsal root ganglia (DRG), following Fast Blue injection into the muscle wall of the urinary bladder. Similarly, cutaneous sensory neurons were identified in L3 and L4 DRG, following Fast Blue injection into the saphenous nerve innervating the skin. Bladder sensory neurons contained only weak to moderate P2X(3)-immunoreactivity (IR), in contrast to strong P2X(3)-IR observed in a sub-population of cutaneous afferents. Whole-cell patch-clamp recordings revealed that approximately 90% of bladder afferent neurons responded to alpha beta-methylene ATP (alpha beta meATP) and ATP (30 microM) with persistent currents, which were inhibited by 2',3'-O-trinitrophenyl-ATP (TNP-ATP) (0.3 microM) to 6.4+/-1.9% and 8.0+/-2.6% of control, respectively (n=8). The remaining bladder sensory neurons demonstrated biphasic, transient or no response to P2X agonists. In contrast, only 24% of cutaneous afferent neurons gave persistent currents to alpha beta meATP (30 microM), with 66% of cells giving transient or biphasic currents and the remaining 10% being non-responsive. Our results suggest that, in contrast to DRG neurons in general, bladder sensory neurons projecting via pelvic nerves express predominantly P2X(2/3) heteromeric receptors, which are likely to mediate the important roles of ATP as a signaling molecule of urinary bladder filling and nociception.     

骶髓后连合核内躯体初级传入粗纤维的证明及其意义探讨

近 2 0年来的研究表明 ,骶髓后连合核是接受和中继盆腔脏器初级传入信号的重要内脏感觉核团。通过盆神经传入脊髓的盆腔脏器初级传入神经纤维除一部分投射于中间带外侧核区参与排尿反射活动的调控外 ,其余部分基本上都投射于骶髓后连合核。本文作者等又曾发现躯体神经 (坐骨神经、阴部神经 )的初级传入粗纤维有一部分经后索投射于后连合核 ,推测这些躯体初级传入粗纤维可能与内脏传入细纤维汇聚于后连合核神经元并在此进行机能的整合、产生新的神经效应。但是投射于后连合区的躯体初级传入粗纤维 ,必须在电镜下直接证实。为此 ,本研究通过透射电镜及 HRP标记电镜技术探索了阴部神经躯体初级传入粗纤维在后连合核区的存在 ,并用 Philips CM-10 0电镜附件 Measuring装置测量了这种纤维的轴突直径和髓鞘厚度、通过计算求得了它们之间的比例 ( A/M值 ) ,藉此取得了科学地辨认并确证粗纤维的根据     

Substance P- and calcitonin gene-related peptide immunoreactivity in primary afferent neurons of the cat's knee joint.

The distribution of substance P and calcitonin gene-related peptide was determined in primary afferent neurons of the medial and posterior articular nerve of the cat's knee joint. Perikarya of articular afferents were visualized by retrograde labelling with the fluorescent dye Fast Blue which was applied at the transected end of the peripheral nerves. Substance P was found in about 17% of labelled medial articular afferents and in about 16% of labelled posterior articular afferents, respectively, whereas calcitonin gene-related peptide was present in about 35 and 32% of the medial and posterior articular nerve cells, respectively. Taking into account that these neuropeptides are known to be co-localized, probably not more than one-third of the joint afferents contain substance P and/or calcitonin gene-related peptide. Quantification of cell diameters revealed that substance P was found only in small- or intermediate-sized perikarya (less than 50 microns) indicating that this peptide is predominantly found in unmyelinated neurons. Calcitonin gene-related peptide was present mainly in small- and intermediate- but also in some large-sized neurons (greater than 50 microns) providing evidence that this peptide is found in unmyelinated and to a lesser extent in myelinated neurons. This is consistent with previous studies that show that substance P and calcitonin gene-related peptide are present primarily in unmyelinated and thinly myelinated primary afferents. When the portion of substance P-positive neurons of the medial articular nerve is compared to the number of articular afferents displaying a nociceptive function as determined in earlier electrophysiological studies, it can be calculated that at most 30% of the nociceptive-specific articular afferents contain this neuropeptide.(ABSTRACT TRUNCATED AT 250 WORDS)     

Unmyelinated afferents constitute a second system coding tactile stimuli of the human hairy skin.

Impulses were recorded from unmyelinated afferents innervating the forearm skin of human subjects using the technique of microneurography. Units responding to innocuous skin deformation were selected. The sample (n = 38) was split into low-threshold units (n = 27) and high-threshold units (n = 11) on the basis of three distinctive features, i.e., thresholds to skin deformation, size of response to innocuous skin deformation, and differential response to sharp and blunt stimuli. The low-threshold units provisionally were denoted tactile afferents on the basis of their response properties, which strongly suggest that they are coding some feature of tactile stimuli. They exhibited, in many respects, similar functional properties as described for low-threshold C-mechanoreceptive units in other mammals. However, a delayed acceleration, not previously demonstrated, was observed in response to long-lasting innocuous indentations. It was concluded that human hairy skin is innervated by a system of highly sensitive mechanoreceptive units with unmyelinated afferents akin to the system previously described in other mammals. The confirmation that the system is present in the forearm skin and not only in the face area where it first was identified suggests a largely general distribution although there are indications that the tactile C afferents may be lacking in the very distal parts of the limbs. The functional role of the system remains to be assessed although physiological properties of the sense organs invite to speculations that the slow tactile system might have closer relations to limbic functions than to cognitive and motor functions.     

Role of primary afferent nerves in allodynia caused by diabetic neuropathy in rats

Both myelinated and unmyelinated afferents are implicated in transmitting diabetic neuropathic pain. Although unmyelinated afferents are generally considered to play a significant role in diabetic neuropathic pain, pathological changes in diabetic neuropathy occur mostly in myelinated A-fibers. In the present study, we first examined the role of capsaicin-sensitive C-fibers in the development of allodynia induced by diabetic neuropathy. We then studied the functional changes of afferent nerves pertinent to diabetic neuropathic pain. Diabetes was induced in rats by i.p. streptozotocin. To deplete capsaicin-sensitive C-fibers, rats were treated with i.p. resiniferatoxin (300 microg/kg). Mechanical and thermal sensitivities were measured using von Frey filaments and a radiant heat stimulus. Single-unit activity of afferents was recorded from the tibial nerve. Tactile allodynia, but not thermal hyperalgesia, developed in diabetic rats. Resiniferatoxin treatment did not alter significantly the degree and time course of allodynia. Post-treatment with resiniferatoxin also failed to attenuate allodynia in diabetic rats. The electrophysiological recordings revealed ectopic discharges and a higher spontaneous activity mainly in Adelta- and Abeta-fiber afferents in diabetic rats regardless of resiniferatoxin treatment. Furthermore, these afferent fibers had a lower threshold for activation and augmented responses to mechanical stimuli. Thus, our study suggests that capsaicin-sensitive C-fiber afferents are not required in the development of allodynia in this rat model of diabetes. Our electrophysiological data provide substantial new evidence that the abnormal sensory input from Adelta- and Abeta-fiber afferents may play an important role in diabetic neuropathic pain.     

Distribution of sensory properties among axotomized cutaneous C-fibres in adult rats

A few hours after peripheral axons of cutaneous afferent neurons have been transected, some of their novel endings become excitable by physical or chemical stimuli. It has been assumed that these axon endings preferentially respond to those stimuli which have excited their previous receptive endings. We studied the prevalence of sensory properties among 784 unmyelinated sural nerve fibres which had been axotomized 2-24 h before, by applying mechanical and thermal forces to the nerve lesion site. Among 126 responding C-fibres, the majority was unimodal, responding exclusively to mechanical (31%), cold (23%) or heat (18%) stimuli. The remainder were either mechano-heat sensitive (10%), mechano-cold sensitive (6%) or heat and cold sensitive (12%). The distribution of sensory properties among acutely axotomized sural nerve C-fibres is therefore largely similar to the recently published distribution of receptor types among intact sural nerve C-fibre afferents. Thus, the hypothesis that responses of axotomized afferent fibres reflect their original receptive properties is corroborated. Knowledge of underlying transduction mechanisms may lead to specific pharmacological tools for suppression of ectopic discharges in unmyelinated axotomized afferents, which probably contribute to neuropathic pain states.     

大鼠额前皮质的丘脑传入纤维联系

实验用ＨＲＰ法研究了２７只大鼠额前皮质的丘脑传入纤维联系。观察结果发现：丘脑背内侧核向额前皮质的投射有局部定位关系，核团的内侧部和中间投射到额前皮质的外侧部，而周边部投射到额前皮质的内侧部；丘脑腹后内侧核小细胞部向额前皮质各区有投射；丘脑至额前皮质的投射为双侧起源，以同侧为主；丘脑与额前皮质之间为往返投射，室旁核，带旁核，胶状核是以接受投射为主，其他核团以发出投射为主。本实验对解释牵涉痛和内脏与躯     

Neural mechanisms of pelvic organ cross-sensitization

Clinical observations of viscerovisceral referred pain in patients with gastrointestinal and genitourinary disorders suggest an overlap of neurohumoral mechanisms underlying both bowel and urinary bladder dysfunctions. Close proximity of visceral organs within the abdominal cavity complicates identification of the exact source of chronic pelvic pain, where it originates, and how it relocates with time. Cross-sensitization among pelvic structures may contribute to chronic pelvic pain of unknown etiology and involves convergent neural pathways of noxious stimulus transmission from two or more organs. Convergence of sensory information from discrete pelvic structures occurs at different levels of nervous system hierarchy including dorsal root ganglia, the spinal cord and the brain. The cell bodies of sensory neurons projecting to the colon, urinary bladder and male/female reproductive organs express a wide range of membrane receptors and synthesize many neurotransmitters and regulatory peptides. These substances are released from nerve terminals following enhanced neuronal excitability and may lead to the occurrence of neurogenic inflammation in the pelvis. Multiple factors including inflammation, nerve injury, ischemia, peripheral hyperalgesia, metabolic disorders and other pathological conditions dramatically alter the function of directly affected pelvic structures as well as organs located next to a damaged domain. Defining precise mechanisms of viscerovisceral cross-sensitization would have implications for the development of effective pharmacological therapies for the treatment of functional disorders with chronic pelvic pain such as irritable bowel syndrome and painful bladder syndrome. The complexity of overlapping neural pathways and possible mechanisms underlying pelvic organ crosstalk are analyzed in this review at both systemic and cellular levels.     

Fifth lumbar spinal nerve injury causes neurochemical changes in corresponding as well as adjacent spinal segments: A possible mechanism underlying neuropathic pain

Previous investigations of the anatomical basis of the neuropathic-like manifestations in the spinal nerve ligation animal model have shown that the central terminations of the unmyelinated primary afferents of L5 spinal nerve are not restricted to the corresponding L5 spinal segment, and rather extend to two spinal segments rostrally and one segment caudally where they intermingle with primary afferents of the adjacent L4 spinal nerve. The aim of the present study was to investigate the neurochemical changes in the dorsal horn of the spinal cord and DRGs after L5 nerve injury in rats. In the first experiment, the right L5 nerve was ligated and sectioned for 14 days, and isolectin B4 (IB4, a tracer for unmyelinated primary afferents) was injected into the left L5 nerve. The results showed that the vasoactive intestinal peptide (VIP) was up-regulated in laminae I–II of L3–L6 spinal segments on the right side in exactly the same areas where IB4 labelled terminals were revealed on the left side. In the second experiment, L5 was ligated and sectioned and the spinal cord and DRGs were stained immunocytochemically with antibodies raised against various peptides known to be involved in pain transmission and hyperalgesia. The results showed that L5 nerve lesion caused down-regulation of substance P, calcitonin-gene related peptide and IB4 binding and up-regulation of neuropeptide Y and neurokinin-1 receptor in the dorsal horn of L4 and L5 spinal segments. Similar neurochemical changes were observed only in the corresponding L5 DRG with minimal effects observed in L3, L4 and L6 DRGs. Although, L5 nerve injury caused an up-regulation in NPY, no change in SP and CGRP immunoreactivity was observed in ipsilateral garcile nucleus. These neuroplastic changes in the dorsal horn of the spinal cord, in the adjacent uninjured territories of the central terminations of the adjacent uninjured nerves, might explain the mechanism of hyperalgesia after peripheral nerve injury.     

Intraspinal sprouting of unmyelinated pelvic afferents after complete spinal cord injury is correlated with autonomic dysreflexia induced by visceral pain

Autonomic dysreflexia is a potentially life-threatening hypertensive syndrome following high thoracic (T) spinal cord injury (SCI). It is commonly triggered by noxious pelvic stimuli below the injury site that correlates with increased sprouting of primary afferent C-fibers into the lumbosacral (L/S) spinal cord. We have recently demonstrated that injury-induced plasticity of (L/S) propriospinal neurons, which relay pelvic visceral sensations to thoracolumbar sympathetic preganglionic neurons, is also correlated with the development of this syndrome. To determine the phenotype of pelvic afferent fiber sprouts after SCI, cholera toxin subunit beta (CTb) was injected into the distal colon 2 weeks post–T4 transection/sham to label colonic visceral afferents. After 1 week of transport, the (L/S) spinal cords were cryosectioned and immunohistochemically stained for CTb, the nociceptive-specific marker calcitonin gene-related peptide (CGRP), and the myelinated fiber marker RT97. Quantitative analysis showed that the density of CGRP⁺ afferent fibers was significantly increased in the L/S dorsal horns of T4-transected versus sham rats, whereas RT97⁺ afferent fiber density showed no change. Importantly, CTb-labeled pelvic afferent fibers were co-localized with CGRP⁺ fibers, but not with RT97⁺ fibers. These results suggest that the sprouting of unmyelinated nociceptive pelvic afferents following high thoracic SCI, but not myelinated fibers, contributes to hypertensive autonomic dysreflexia induced by pelvic visceral pain.     

Tachykinins mediate changes in spinal reflexes after activation of unmyelinated muscle afferents in the rat.

The effect of intrathecally applied tachykinin antagonist D-NicLys1, 3-Pal3, D-Cl2Phe5, Asn6, D-Trp7.9, Nle11-substance P, spantide II, on the long-term increase of spinal cord excitability after activation of unmyelinated muscle afferents was studied in decerebrate, spinalized, unanaesthetized rats. A conditioning stimulus train (1 Hz, 20 s) that activated unmyelinated fibres in the gastrocnemius muscle nerve facilitated the flexor reflex for about 1 h, which was strongly blocked by pretreatment with spantide II (3 micrograms). The present results indicate that the facilitation of the flexor reflex by conditioning stimulation of a muscle nerve is mediated by tachykinins and possibly other neuropeptides which may be released from the central terminals of these unmyelinated afferents.     

Chemical mediators enhance the excitability of unmyelinated sensory axons in normal and injured peripheral nerve of the rat

Ectopic excitation of nociceptive axons by chemical mediators may contribute to symptoms in neuropathic pain. In this study, we have measured the excitability of unmyelinated rat C-fiber axons in isolated segments of sural nerves under different experimental conditions. (1) We demonstrate in normal rats that several mediators including ATP, serotonin (5-HT), 1-(3-chlorophenyl)biguanide (5-HT3 receptor agonist), norepinephrine, acetylcholine and capsaicin alter electrophysiological parameters of C-fibers which indicate an increase of axonal excitability. Other mediators such as histamine, glutamate, prostaglandin E(2) and the cytokines tumor necrosis factor alpha, interleukin-1beta and interleukin-6 did not produce such effects. (2) The effects of several mediators were tested after peripheral nerve injury (partial ligation or spared nerve injury). Sural nerves from such animals did not show significant changes when compared with controls. (3) We tested whether the effects of chemical mediators on axonal excitability are due to actions on the sensory C-fiber afferents or the postganglionic sympathetic efferents. In order to distinguish these effects, we performed surgical sympathectomy of the lumbar sympathetic chain, including the L3, L4 and L5 ganglia. Sympathectomy did not markedly influence the effects of mediators on axonal excitability (except that the norepinephrine effect was significantly diminished). In conclusion, our data suggest a constitutive rather than inducible expression of axonal receptors for some chemical mediators on the axonal membrane of unmyelinated fibers. Most of the changes in axonal excitability take place in sensory C-fiber afferents rather than in postganglionic sympathetic efferents. Thus, it is possible that certain immune and glial cell mediators released in or around the nerve following injury or inflammation influence the excitability of intact nociceptive fibers. This mechanism could contribute to ectopic excitation of axons in neuropathic pain.     

Number, distribution and neuropeptide content of rat knee joint afferents

Retrograde tracing with Fluoro-Gold was used to identify the complete population of knee joint afferents in the lumbar dorsal root ganglia of adult female Wistar rats. There was an average of 581±31 (mean± S.D. ) afferents supplying each joint. These were found distributed from L1 to L5 with the great majority localised in the L3 and L4 ganglia. Electron microscopy of the posterior articular nerve of the knee revealed an average of 103±15 (mean± S.D. ) myelinated and 513±39 unmyelinated axonal profiles. Since about 50–60% of the unmyelinated profiles would be expected to be sympathetic efferents, these numbers are consistent with the numbers of afferents found by Fluoro-Gold retrograde tracing and suggest that the posterior articular nerve contains about 50% of the total number of knee joint afferents in the rat. Immunohistochemistry revealed that an average of 10% of identified joint afferents expressed substance P-like immunoreactivity and that 33% expressed calcitonin gene-related peptide-like immunoreactivity.     

The representation of some visceral afferents in the anterior lobe of the cerebellum

1. Evoked single unit discharges have been recorded from the right side of the anterior lobe of the cerebellum following distension of the gall-bladder, mechanical stimulation of abdominal viscera and electrical stimulation of the right splanchnic nerve.2. The majority of the evoked units were located in the posterior part of the culmen, lateral to the paravermian vein. Unit depths ranged from 0.2 to 5.0 mm below the pial surface.3. Units responding to distension of the gall-bladder or mechanical stimulation of the viscera were not affected by other peripheral stimuli.4. Responses to gall-bladder stimulation were either excitatory or inhibitory. Responses to mechanical stimulation of the viscera were mainly inhibitory.5. Units evoked by electrical stimulation of the right splanchnic nerve fell into two groups: (1) high threshold. The majority of units responded to activation of the Adelta group of splanchnic afferents with a latency of 20-25 msec, and a high probability that the unit would respond to successive stimuli over long periods of time. (2) Low threshold. Some units were evoked by excitation of the Abeta group of splanchnic afferents only. They had a shorter latency and a lower probability of response, which was not increased by exciting Adelta fibres.6. It is suggested that the two groups of splanchnic afferent fibres project to the anterior lobe of the cerebellum as two separate functional systems.