Nociceptive Primary Sensory Neurons Research Articles

Neuroanatomical effects of capsaicin on the primary afferent neurons.

Studies by N. JANCSO and his associates in the 1970's established that capsaicin in paprika exerts selective damage on nociceptive primary sensory neurons. The physiological and pharmacological aspects of capsaicin's effect have been repeatedly reviewed, but no report seems available concerning the neuroanatomical changes caused by capsaicin. This paper first reviews the neuroanatomical aspect of the lesion caused by capsaicin. Special attention is paid to quantitative estimations made by our group and others on the loss of dorsal root ganglion (DRG) cells, dorsal root nerve fibers, the saphenous nerve, chorda tympani nerve, and pulp nerves after neonatal treatment with capsaicin. The degenerating process of DRG cells induced by capsaicin is discussed with respect to necrosis and apoptosis. The capsaicin receptors found recently are concisely introduced with reference to their action. A discrepancy between a marked loss of dorsal root C-fibers and an unexpectedly intact response to noxious heat in mice treated with capsaicin at neonate is discussed, and attension is given to nerves sprouting from capsaicin-resistant DRG cells in the superficial dorsal horn. In addition, the architecture of the synapses between the central endings of the capsaicin-sensitive primary afferent neurons and the intrinsic inhibitory interneurons is described and its possible significance considered in terms of the transmission of nociceptive information.

Inhibition of rapid heat responses in nociceptive primary sensory neurons of rats by vanilloid receptor antagonists.

Recent studies demonstrated that heat-sensitive nociceptive primary sensory neurons respond to the vanilloid receptor (VR) agonist capsaicin, and the first cloned VR is a heat-sensitive ion channel. Therefore we studied to what extent heat-evoked currents in nociceptive dorsal root ganglion (DRG) neurons can be attributed to the activation of native vanilloid receptors. Heat-evoked currents were investigated in 89 neurons acutely dissociated from adult rat DRGs as models for their own terminals using the whole cell patch-clamp technique. Locally applied heated extracellular solution (effective temperature approximately 53 degrees C) rapidly activated reversible and reproducible inward currents in 80% (62/80) of small neurons (< or = 32.5 microm), but in none of nine large neurons (P < 0.001, chi(2) test). Heat and capsaicin sensitivity were significantly coexpressed in this subpopulation of small DRG neurons (P < 0.001, chi(2) test). Heat-evoked currents were accompanied by an increase of membrane conductance (320 +/- 115%; mean +/- SE, n = 7), had a reversal potential of 5 +/- 2 mV (n = 5), which did not differ from that of capsaicin-induced currents in the same neurons (4 +/- 3 mV), and were carried at least by Na(+) and Ca(2+) (pCa(2+) > pNa(+)). These observations are consistent with the opening of temperature-operated nonselective cation channels. The duration of action potentials was significantly higher in heat-sensitive (10-90% decay time: 4.45 +/- 0.39 ms, n = 12) compared with heat-insensitive neurons (2.18 +/- 0.19 ms, n = 6; P < 0.005, Student's t-test), due to an inflection in the repolarizing phase. This property as well as capsaicin sensitivity and small cell size are characteristics of nociceptive DRG neurons. When coadministered with heat stimuli, the competitive VR antagonist capsazepine (1 microM to 1 mM) significantly reduced heat-evoked currents in a dose-dependent manner (IC(50) 13 microM, Hill slope -0.58, maximum effect 75%). Preincubation for 12-15 s shifted the IC(50) by approximately 0.5 log(10) units to an estimated IC(50) of approximately 4 microM. The noncompetitive VR antagonist ruthenium red (5 microM) significantly reduced heat-evoked currents by 33 +/- 6%. The effects of both VR antagonists were rapidly reversible. Our results provide evidence for a specific activation of native VRs in nociceptive primary sensory neurons by noxious heat. The major proportion of the rapid heat-evoked currents can be attributed to the activation of these temperature-operated channels, and noxious heat may be the signal detected by VRs under physiological conditions.

Endomorphins: novel endogenous mu-opiate receptor agonists in regions of high mu-opiate receptor density.

Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2, EM-1) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2, EM-2) are peptides recently isolated from brain that show the highest affinity and selectivity for the mu (morphine) opiate receptor of all the known endogenous opioids. The endomorphins have potent analgesic and gastrointestinal effects. At the cellular level, they activate G-proteins (35S-GTP gamma-S binding) and inhibit calcium currents. Support for their role as endogenous ligands for the mu-opiate receptor includes their localization by radioimmunoassay and immunocytochemistry in central nervous system regions of high mu receptor density. Intense EM-2 immunoreactivity is present in the terminal regions of primary afferent neurons in the dorsal horn of the spinal cord and in the medulla near high densities of mu receptors. Chemical (capsaicin) and surgical (rhizotomy) disruption of nociceptive primary afferent neurons depletes the immunoreactivity, implicating the primary afferents as the source of EM-2. Thus, EM-2 is well-positioned to serve as an endogenous modulator of pain in its earliest stages of perception. In contrast to EM-2, which is more prevalent in the spinal cord and lower brainstem, EM-1 is more widely and densely distributed throughout the brain than EM-2. The distribution is consistent with a role for the peptides in the modulation of diverse functions, including autonomic, neuroendocrine, and reward functions as well as modulation of responses to pain and stress.

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

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

Muscarinic M2-receptors in rat thoracic dorsal root ganglia

The occurrence and distribution of the muscarinic M2-receptor subtype (M2R) was investigated in rat thoracic dorsal root ganglia (DRG). Messenger RNA for M2R was demonstrated by RT-PCR in total RNA from DRG. Immunoreactivity to M2R-protein was localized to 26% of sensory neurons, the majority of them (85%) belonging to the size class of 25–40 μm in diameter. Double-labeling (immuno)histochemistry revealed that all M2R-immunoreactive neurons bind the lectin, I-B4, whereas they are generally devoid of substance P-immunoreactivity. These data show the presence of M2R on a subpopulation of presumably nociceptive primary afferent neurons, thereby extending previous pharmacological and electrophysiological studies that indicated a role of M2R and/or M4R in inhibition of calcium channel currents in rat sensory neurons (Wanke, E., Bianchi, L., Mantegazza, M., Guatteo, E., Macinelli, E. and Ferroni, A., Muscarinic regulation of Ca 2+ currents in rat sensory neurons: channel and receptor types, dose-response relationships and cross-talk pathways. Eur. J. Neurosci., 6 (1994) 381–391).

Dynamic mechanical allodynia in humans is not mediated by a central presynaptic interaction of A β-mechanoreceptive and nociceptive C-afferents

Recently, Cervero and Laird (NeuroReport, 7 (1996) 526–528; Pain, 68 (1996) 13–23) proposed a new pathophysiological mechanism of dynamic mechanical allodynia in skin. Using the capsaicin pain model in humans, they showed that light mechanical stimulation within an area of secondary mechanical allodynia induces vasodilatation measured by laser-Doppler flowmetry. They suggested that the low-threshold A β-mechanoreceptive fibres depolarize the central terminals of nociceptive primary afferent neurons via interneurons. Consequently, the vasodilatation is produced by impulses conducted antidromically in nociceptive C-axons. The allodynia was proposed to result from depolarization of central terminals of primary afferent neurons with C-fibres with activation of nociceptive dorsal horn neurons. In order to extend these findings, we used the same experimental approach but additionally stimulated the A β-fibres electrically to evoke secondary allodynia during simultaneous monitoring skin blood flow. Twenty microlitres of a 0.5% capsaicin solution was injected intradermally into the dorsal forearm. Skin sites that demonstrated dynamic mechanical allodynia but were not located within the area of primary hyperalgesia and flare were investigated. Ten mm away from a laser-Doppler probe, dynamic mechanical allodynia was induced for 1 min (1) by moving a cotton swab and (2) by electrically stimulating the afferent nerve endings transdermally. Increasing stimulus intensities were applied (0.3–4 mA, 40 Hz, pulse duration 0.2 ms). After intracutaneous injection of capsaicin, light mechanical stimulation elicited a burning painful sensation (numeric analogue scale (NAS) 1.5–3) and concomitant movement artefacts at the laser signal. Antidromic vasodilatation was never observed. In this area of dynamic allodynia, electrical stimulation at stimulus intensities that were not painful before capsaicin injection (A β-stimulation) was now able to elicit a burning painful sensation (NAS 1.5–3). No change in blood flow was detected. When the stimulus intensities were increased reaching levels that were also painful before capsaicin treatment (C-fibre stimulation), an increase in blood flow could be induced showing the time course of an axon reflex vasodilatation. In conclusion, electrical stimulation of A β-fibres in allodynic skin does not induce antidromic vasodilatation. Consequently, interaction of A β-mechanoreceptive fibres and nociceptive C-fibres at a presynaptic level is unlikely to produce antidromically conducted impulses and therefore cannot explain the pathophysiology of mechanical allodynia. Alternatively, it is much more likely that under pathophysiological conditions, activity in A β-fibres may activate nociceptive second-order neurons, i.e. in the spinal cord.

Attenuation of experimental pruritus and mechanically evoked dysesthesiae in an area of cutaneous allodynia

We investigated the effects of tactile allodynia on the itch and mechanically evoked dysesthesiae produced by an intradermal injection of histamine in human volunteers. After an intradermal injection of capsaicin into the volar surface of one forearm, there developed an area of tactile allodynia to stroking and hyperalgesia to pricking the skin. Histamine was then injected simultaneously into the area of allodynia (experimental arm) and into the opposite forearm (control arm). Magnitude estimates of itch were obtained every 15 s for 5 min, and the areas of cutaneous hyperalgesia (pricking-evoked pain), alloknesis (stroking-evoked itch), hyperknesis (pricking-evoked itch) and wheal and flare were measured. The areas of wheal and flare were not significantly different on the two arms. The magnitude of itch and the areas of hyperknesis and alloknesis developed normally on the control arm but were absent or greatly reduced on the experimental arm. Thus, both the itch and the alloknesis and hyperknesis normally induced by histamine were absent or greatly reduced when histamine was injected in an area of capsaicin-induced allodynia. These results are compatible with the hypothesis that activity in capsaicin-sensitive, nociceptive primary afferent neurons evokes a central neuronal inhibitory process that prevents or reduces the itch and mechanically evoked dysesthesiae normally produced by an intradermal injection of histamine.

The Expression of P2X3Purinoreceptors in Sensory Neurons: Effects of Axotomy and Glial-Derived Neurotrophic Factor

We have studied the distribution and regulation of the P2X3receptor (a ligand-gated ion channel activated by ATP) in adult dorsal root ganglion (DRG) neurons using a polyclonal antibody. P2X3receptor immunoreactivity was normally present in about 35% of L4/5 DRG neurons, virtually all small in diameter. In the dorsal horn, P2X3receptor expression was restricted to the terminals of sensory neurons terminating in lamina IIinner. P2X3receptors were expressed in approximately equal numbers of sensory neurons projecting to skin and viscera but in very few of those innervating skeletal muscle. P2X3receptors were found mostly in sensory neurons that bind the lectin IB4. After sciatic nerve axotomy, P2X3receptor expression dropped by more than 50% in L4/5 DRG. Glial cell line-derived neurotrophic factor (GDNF), delivered intrathecally, completely reversed axotomy-induced down-regulation of the P2X3receptor. We conclude that P2X3receptors are normally expressed in nociceptive primary sensory neurons, predominantly the nonpeptidergic nociceptors. P2X3receptors are down-regulated following peripheral nerve injury and their expression can be regulated by GDNF.

Distribution of P2X 3 receptors in the rat trigeminal ganglion after inferior alveolar nerve injury

The ATP-gated cation channel receptor P2X 3 is associated with nociceptive primary sensory neurons. We have, using immunohistochemistry, examined the expression of P2X 3 in rat trigeminal ganglia 4–22 days after ligation/section or chronic constriction of the mandibular inferior alveolar nerve. In the normal trigeminal ganglion the anti-P2X 3 receptor antibody labeled 37–58% of all neurons. Double labeling demonstrated that about 70–95% of the small neurons that bind the isolectin I-B4 displayed P2X 3-immunoreactivity, and that about 40% of larger RT97-positive nerve cells were P2X 3 receptor-immunoreactive. At 4 and 10 days after inferior alveolar nerve injury, the proportion of P2X 3-immunoreactive neurons had increased to about 65% (range 52–78%). Examinations at the injury sites showed an intense P2X 3 receptor-immunoreactivity in nerve endings. At longer survival stages the proportion of P2X 3 receptor-positive sensory neurons had returned to control values. These results show that the P2X 3 receptor is transiently upregulated and anterogradely transported in trigeminal primary sensory neurons after nerve injury. Since the receptor is accumulated in injured nerve endings, it may be associated with abnormal impulse propagation from these sites.

Nociceptin/orphanin FQ-induced nociceptive responses through substance P release from peripheral nerve endings in mice.

We have studied the in vivo signaling mechanisms involved in nociceptin/orphanin FQ (Noci)-induced pain responses by using a flexor-reflex paradigm. Noci was 10,000 times more potent than substance P (SP) in eliciting flexor responses after intraplantar injection into the hind limb of mice, but the action of Noci seems to be mediated by SP. Mice pretreated with an NK1 tachykinin receptor antagonist or capsaicin, or mice with a targeted disruption of the tachykinin 1 gene no longer respond to Noci. The action of Noci appears to be mediated by the Noci receptor, a pertussis toxin-sensitive G protein-coupled receptor that stimulates inositol trisphosphate receptor and Ca2+ influx. These findings suggest that Noci indirectly stimulates nerve endings of nociceptive primary afferent neurons through a local SP release.

Hypoalgesia in mice with a targeted deletion of the tachykinin 1 gene.

The tachykinin neuropeptides, substance P and substance K, are produced in nociceptive primary sensory neurons and in many brain regions involved in pain signaling. However, the precise role and importance of these neuropeptides in pain responses has been debated. We now show that mice that cannot produce these peptides display no significant pain responses following formalin injection and have an increased pain threshold in the hotplate test. On the other hand, the mutant mice react normally in the tail flick assay and acetic acid-induced writhing tests. These results demonstrate that substance P and/or substance K have essential functions in specific responses to pain.

Recent developments in the pathophysiology of acute pain

Tissue damage induces a state of hyperalgesia. Traditionally, sensory physiology was elucidated by examining responses to ephemeral transient stimuli. However, in order to study acute clinical pain, it is necessary to re-examine nociceptive processing using models which include a significant hyperalgesic component. Primary hyperalgesia results from sensitisation of primary afferent nociceptive neurons by various components of the ‘inflammatory soup’, in a complex interaction between the immune and nervous systems. It is now possible to identify key molecules which participate in this process and exploitation of these may result in the development of novel peripherally acting analgesics: Nerve growth factor: In the developing animal this target-derived growth factor is vital to the survival of the small diameter neurons associated with nociception. However, in the developed animal there is now considerable evidence that this neurotrophin takes on a different role in that it occupies a pivotal position in the development of hyperalgesia. A ligand trap molecule which sequesters nerve growth factor attenuates inflammatory hyperalgesia in several animal models. Bradykinin: Although the potent algogenic effects of bradykinin have been appreciated for some time, the recent discovery of bradykinin receptor sub-types has renewed interest in this molecule. By the use of selective antagonists at the B1 and B2 receptors it has been ascertained that they play disparate roles in nociception and hyperalgesia, thus giving scope for further development. Cannabinoids: Although a component of cannabinoid-induced analgesia is mediated through central (CB1) receptors, the discovery of a second immune cell associated cannabinoid receptor (CB2) has allowed further investigation of the cannabinoids. It appears that putative endogenous CB2 ligands are active during inflammation and attenuate inflammatory hyperalgesia. Exploitation of CB2 receptor agonists may allow the analgesic actions of cannabinoids to be divorced from their CNS side effects.

Characterization of iCGRP release from adrenal capsule primary afferent neurons

The rat adrenal capsule and cortex are innervated by primary afferent neurons that contain neuropeptides, including calcitonin gene-related peptide (CGRP; 1). Furthermore, CGRP has been implicated in modulating adrenal cortical function (2–5). These results suggest that factors influencing the release of CGRP from these fibers may represent an additional regulatory system for modulation of adrenal cortical function in vivo. In the present study, we used an in vitro superfusion technique to pharmacologically characterize the release of immunoreactive CGRP (iCGRP) from adrenal capsular sensory fibers. These studies employed capsaicin, the pungent principle of hot peppers, which selectively activates a subset of nociceptive primary afferent neurons (6). In addition, the effects of capsaicin on aldosterone biosynthesis by adrenal capsules were assessed.Briefly, adrenal glands were quickly removed from male, Sprague-Dawley rats (250-275g) and decapsulated. The adrenal capsules were then placed into superfusion chambers and perfused with buffer. After a period of baseline collection, tissue was treated with either high K+ (50 mM) buffer, which results in non-specific activation of neurons, or capsaicin (100 μM), followed by a washout period. Superfusates were collected and assayed for iCGRP via RIA. In some experiments, superfusates were also assayed for aldosterone via RIA.

Inhibition of bradykinin-induced plasma extravasation produced by noxious cutaneous and visceral stimuli and its modulation by vagal activity.

Intrathecally applied nicotine reduces bradykinin-induced plasma extravasation (BK-induced PE) in the rat knee joint. This depression is mediated by the hypothalamo-pituitary-adrenal (HPA) axis and is enhanced by interruption of impulse traffic in afferents of the abdominal vagus nerve. Like intrathecal nicotine, electrical stimulation of unmyelinated cutaneous fibers also depresses BK-induced PE, which is also dependent on an intact HPA axis. In this study, we investigated whether the inhibitory effect of intrathecal nicotine can be mimicked by noxious stimulation of skin and of viscera. Furthermore we determined whether this depression is potentiated after subdiaphragmatic vagotomy. Stimulation of visceral afferents in the peritoneum, by intraperitoneal capsaicin injection, dose-dependently decreased BK-induced PE. The capsaicin dose-response function was shifted by 1.5-2 orders of magnitude to the left after vagotomy. Stimulation of visceral afferents in the urinary bladder by capsaicin also dose-dependently reduced BK-induced PE, which similarly was potentiated after vagotomy. Transcutaneous stimulation of unmyelinated nociceptive afferents from the plantar skin of the paw depressed BK-induced PE. This depression had a threshold of approximately 0.25 Hz and was maximal at a stimulation frequency of approximately 1 Hz. After subdiaphragmatic vagotomy, the stimulus response function shifted to the left and the inhibition was significantly larger than in control, in the range of 0.125-1 Hz stimulation. These results show that noxious stimulation of skin and viscera depressed BK-induced PE and that such depression was potentiated after subdiaphragmatic vagotomy in a manner similar to that of intrathecally applied nicotine. Based on these observations, we hypothesize that intrathecal nicotine depresses BK-induced PE by exciting spinal nociceptive neurons or the central projections of nociceptive primary afferent neurons.

The Role of Calcium in the Desensitization of Capsaicin Responses in Rat Dorsal Root Ganglion Neurons

Capsaicin (Cap) is a pungent extract of the Capsicum pepper family, which activates nociceptive primary sensory neurons. Inward current and membrane potential responses of cultured neonatal rat dorsal root ganglion neurons to capsaicin were examined using whole-cell and perforated patch recording methods. The responses exhibited strong desensitization operationally classified as acute (diminished response during constant Cap exposure) and tachyphylaxis (diminished response to successive applications of Cap). Both acute desensitization and tachyphylaxis were greatly diminished by reductions in external Ca2+ concentration. Furthermore, chelation of intracellular Ca2+ by addition of either EGTA or bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid to the patch pipette attenuated both forms of desensitization even in normal Ca2+. Release of intracellular Ca2+ by caffeine triggered acute desensitization in the absence of extracellular Ca2+, and barium was found to effectively substitute for calcium in supporting desensitization. Cap activated inward current at an ED50 of 728 nM, exhibiting cooperativity (Hill coefficient, 2.2); however, both forms of desensitization were only weakly dependent on [Cap], suggesting a dissociation between activation of Cap-sensitive channels and desensitization. Removal of ATP and GTP from the intracellular solutions resulted in nearly complete tachyphylaxis even with intracellular Ca2+ buffered to low levels, whereas changes in nucleotide levels did not significantly alter the acute form of desensitization. These data suggest a key role for intracellular Ca2+ in desensitization of Cap responses, perhaps through Ca2+-dependent dephosphorylation at a locus that normally sustains Cap responsiveness via ATP-dependent phosphorylation. It also seems that the signaling mechanisms underlying the two forms of desensitization are not identical in detail.

Fluoride-Resistant Acid Phosphatase (FRAP)-Positive Primary Afferent Central Terminals in the Mouse Substantia Gelatinosa.

The ultrastructure of the central terminals of nociceptive primary afferent (Cl terminals) neurons in the mouse substantia gelatinosa was studied by the cytochemical method using fluoride-resistant acid phosphatase (FRAP). Small roundish, slender, sinuous and large scalloped terminals showed FRAP reactivity. The FRAP reaction was seen as electron dense granular deposits between spherical synaptic vesicles and around the membrane of the synaptic vesicles. Especially, dark, small sinuous and large scalloped terminals showed intense FRAP reactivity. From the previous studies, these terminals were in accord with the capsaicin-sensitive Cl terminals. Furthermore, larger, roundish terminals with less packed spherical synaptic vesicles, many mitochondria and a few dense cored synaptic vesicles showed FRAP reactivity. Our recent results that Cll terminals are also sensitive to capsaicin support the FRAP activity of the Cll terminals in the present study. Thus, various types of FRAP-positive Cl terminals and some Cll terminals are considered to be central endings of the capsaicin-sensitive nociceptive primary afferents.

Differential ability of human cutaneous nociceptors to signal mechanical pain and to produce vasodilatation

We investigated the ability of human nociceptive primary afferent neurons to encode mechanical pain and to produce vasodilatation. Pain was induced by shooting a light metal cylinder (0.3 g) at different velocities (6-18 m/sec) perpendicularly against the hairy skin of the hand. When single impact stimuli were applied, monotonically increasing stimulus-response functions were obtained in 10 psychophysical experiments using magnitude estimation techniques. In 35 microneurographic experiments nine unmyelinated afferents were recorded from the superficial radial nerve. All units responded readily to impact stimulation even at stimulus intensities that were not rated as painful. However, there was a close linear correlation between the number of action potentials evoked from the nociceptors and the psychophysical magnitude estimates of the perceived sensation or the stimulus intensity. This was also reflected by a corresponding increase of neurogenic vasodilatation. While two thin myelinated afferents displayed qualitatively similar responses 12 low-threshold mechanosensitive afferents (4 rapidly adapting, 5 slowly adapting type 1, 3 slowly adapting type II) failed to encode the intensity of the applied impact force and often became desensitized. This indicates that the total number of action potentials is the determinant of the magnitude of mechanical pain and the associated vasodilatation following single brief stimuli. By contrast, the close correlation between nociceptor activity and sensation changed when trains of mechanical impact stimuli (five stimuli of constant intensity, intratrain frequency of 1/32 to 2 Hz) were applied. Magnitude estimates of pain intensity were frequency dependent and stimuli with short interstimulus intervals were perceived as more painful than those delivered with long intervals. However, the total number of action potentials evoked from C-fibers was higher at longer interstimulus intervals than shorter intervals, thus yielding a negative correlation between the magnitude estimates of the perceived painful sensation and the number of action potentials elicited from nociceptive afferents. This suggests that temporal summation of the nociceptive discharge at central neurons becomes increasingly more important for the sensory discriminative experience of pain evoked by repetitive stimulation. We conclude that human nociceptive C-fibers signal brief noxious mechanical stimuli by the total number of action potentials evoked during a short period of time. However, with repetitive stimulation the total number of action potentials evoked from nociceptors is less important for evoking pain and temporal summation of the nociceptive primary afferent discharge becomes the crucial factor for signaling the magnitude of sensation.

Subpopulations of “nocifensor neurons” contributing to pain and allodynia, itch and alloknesis

A model is proposed to explain the origins of two qualitatively different dysesthesias evoked by lightly stroking the skin surrounding a local cutaneous injury produced by different chemicals, one that evokes pain and another that elicits itch. Gently stroking the skin elicits pain (allodynia) after a pain-producing injection of capsaicin, but elicits itch (alloknesis) after an itch-producing injection of histamine. It is suggested that all primary sensory neurons that act to protect the body be called “nocifensor neurons” and that while some evoke sensations or reflexes that signal the occurrence of noxious events in peripheral tissue (“nociceptors”), others alter the properties of neuronal or non-neuronal tissue in the periphery or the central nervous system (“noceffectors”). The model proposes that pain and itch, produced, for example, by capsaicin and histamine, are evoked by activity in different populations of nociceptive primary afferent neurons that project onto two different types of spinothalamic tract cells in the dorsal horn. Allodynia and alloknesis are produced by other, different populations of noceffective primary afferents that act to produce a prolonged enhancement of activity in two different populations of interneurons in the dorsal horn. These interneurons receive a convergent input from low-threshold cutaneous mechanoreceptive afferents and project correspondingly onto the separate types of spinothalamic tract cells mediating pain or itch.

Levels of dynorphin peptides, substance P and CGRP in the spinal cord after subchronic administration of morphine in the rat

Rats were rendered dependent on morphine by repeated injections of morphine, in increasing doses for 14 days and sacrificed. Levels of peptides in the dorsal spinal cord and dorsal root ganglia were analyzed in rats decapitated 2 hr, 24 hr (acute abstinent) or 7 days (late abstinent) respectively, after the last injection of drug. Dynorphin A was significantly decreased in rats abstinent for 24 hr, while dynorphin B remained unaffected. Substance P and CGRP, both putative transmitters in nociceptive primary afferent neurones, and partly existing together in the same neurone, were affected differently. Significantly less substance P but unchanged levels of CGRP were detected in rats abstinent for 24 hr, while on the other hand, CGRP but not levels of substance P, were increased 2 hr after the final injection. In dorsal root ganglia, levels of substance P were lower at 2 hr, while levels of CGRP were unaffected. In late (7 days) abstinence, no effect of opiate on any peptide was detected.

Capsaicin causes prolonged inhibition of voltage-activated calcium currents in adult rat dorsal root ganglion neurons in culture

The effect of capsaicin on voltage-activated calcium currents was investigated in voltageclamped somata of cultured adult rat dorsal root ganglion neurons. About half the neurons studied were sensitive to capsaicin, which induced an inward current at negative membrane potentials accompanied by an increase in membrane conductance. In the sensitive neurons capsaicin inhibited voltage-activated calcium current to an extent that depended on the size and duration of the capsaicin-induced inward current. Calcium channels were protected from the long-lasting inhibitory action of capsaicin by substituting extracellular Ca with Mg or Ba when capsaicin was applied, which suggests that the inhibition was dependent on the presence of extracellular Ca. Substituting Ca with Co did not prevent the prolonged block of calcium channels. It is concluded that the inhibition of voltage-activated calcium currents by capsaicin is secondary to increased intracellular Ca levels due to calcium entry through capsaicin-activated cation-specific ion channels in the plasma membrane. Long-lasting inhibition of voltage-activated calcium channels may contribute to the mechanism of the analgesic and anti-inflammatory effects of capsaicin through inhibition of neurotransmitter release from central and peripheral terminals of primary afferent nociceptive neurons.