Biomedical subjects

S M Carlton

Publications and source records attributed to S M Carlton.

At least 37 records · Page 2Linked to original sources

PubMed1998

Evidence for the interaction of glutamate and NK1 receptors in the periphery.

It is known that Substance P (SP) enhances glutamate- and N-methyl-D-aspartate (NMDA)-induced activity in spinal cord dorsal horn neurons and that this enhancement is important in the generation of wind-up and central sensitization. It is now known that SP and glutamate receptors are present on sensory axons in rat glabrous skin. This raises the issue as to whether SP and glutamate interact in the periphery. Using the tail skin in rats, the present study demonstrates 1) that unmyelinated axons at the dermal-epidermal junction immunostain for antibodies directed against NMDA, non-NMDA or SP (NK1) receptors; 2) that glutamate injected into the tail skin results in dose-dependent nociceptive behaviors interpreted as mechanical hyperalgesia, mechanical allodynia and thermal hyperalgesia, which are blocked following co-injection with glutamate antagonists; 3) that peripheral injection of SP potentiates glutamate-induced nociceptive behaviors in that the co-injection of SP+glutamate results in a significantly longer duration of behavioral responses compared to the responses seen following injection of either substance alone. These data provide support for the hypothesis that primary afferent neurons might well be subject to similar mechanisms that result in wind-up or central sensitization of spinal cord neurons.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMed1998

Intraplantar injection of dextrorphan, ketamine or memantine attenuates formalin-induced behaviors.

The possible prophylactic effects of local injection of NMDA receptor antagonists that are currently used in humans was investigated in the present study. Intraplantar pretreatment with either 5 mM dextrorphan (DEX), 10 mM memantine (MEM) or 10 mM ketamine (KET) significantly attenuated formalin-induced lifting and licking behaviors, however flinching behavior was not effected. Control experiments indicated that these drug actions could be attributed to local and not systemic effects of the antagonists. We hypothesize that these actions result from blocking NMDA receptors present on unmyelinated sensory axons in the skin. These data suggest that peripheral NMDA receptors contribute to nociceptor activation and can be manipulated to reduce pain of peripheral origin. Since DEX, MEM and KET are currently used in humans and considered clinically safe, they have potential therapeutic value in the treatment of physiologic or pathologic pain states which are induced or maintained by peripheral nociceptor activity. Topical or local application would avoid the side effects that can accompany systemic or intrathecal injection of NMDA antagonists.

Animals↗

PubMed1998

Plasticity in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunits in the rat dorsal horn following deafferentation.

Immunostaining for GluR1 and GluR2/3 subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor is prominent in laminae I and II of the normal dorsal horn, with much less staining in deeper laminae. Three days after dorsal rhizotomy, image analysis demonstrates a significant decrease in staining density for both antibodies in the superficial dorsal horn. By contrast, staining density is significantly increased in laminae III, IV and the reticulated region of lamina V for GluR1 only. This increase suggests that AMPA receptors contribute to deafferentation or radicular pain by at least two mechanisms: (1) up-regulation of GluR1 subunits of the AMPA receptor in deeper laminae, and (2) enhanced Ca2+ permeability of dorsal horn neurons because AMPA receptors lacking the GluR2 subunit are Ca2+ permeable. Glutamate activation of these receptors could initiate or potentiate second messenger cascades that could well contribute to neuronal changes documented in the dorsal horn of deafferented animals.

Animals↗

PubMed1998

Ultrastructural analysis of NMDA, AMPA, and kainate receptors on unmyelinated and myelinated axons in the periphery.

The present study determines the proportions of unmyelinated cutaneous axons at the dermal-epidermal junction in glabrous skin and of myelinated and unmyelinated axons in the sural and medial plantar nerves that immunostain for subunits of the ionotropic glutamate receptors. Approximately 20% of the unmyelinated cutaneous axon profiles at the dermal-epidermal junction immunostain for either N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), or kainate receptor subunits. These findings are consistent with previous observations that NMDA and non-NMDA antagonists ameliorate nociceptive behaviors that result from noxious peripheral stimulation. In the sural nerve, where the large majority of myelinated fibers are sensory, approximately half of the myelinated axon profiles immunostain for the NMDA receptor 1 (R1) subunit, 28% immunostain for the glutamate receptor 1 (GluR1) AMPA subunit, and 11% for the GluR5,6,7 kainate subunits. Even higher proportions immunostain for these receptors in the medial plantar nerve, a mixed sensory and motor nerve. In the sural nerve, 20% of the unmyelinated axon profiles immunostain for NMDAR1 and only 7% label for GluR1 or GluR5,6,7. Because the sural nerve innervates hairy skin, these data suggest that glutamate will activate a higher proportion of unmyelinated axons in glabrous skin than in hairy skin. Measurements of fiber diameters indicate that all sizes of myelinated axon profiles, including Adelta and Abeta, are positively labeled for the ionotropic receptors. The presence of glutamate receptors on large-diameter myelinated axons suggests that these mechanosensitive receptors, presumably transducing touch and pressure, may also respond to local glutamate and thus be chemosensitive.

Animals↗

PubMed1998

Attenuation of formalin-induced nociceptive behaviors following local peripheral injection of gabapentin.

Gabapentin (GP) has been shown to have antihyperalgesic properties and the site of drug action is reported to be the central nervous system. The goal of the present study was to determine whether GP also has a peripheral site of action. Rats received intraplantar 20-microl injections of 6, 60 or 600 microg GP + 2% formalin, 300 or 600 microg S-(+)-3-isobutylgaba + 2% formalin, 600 microg R-(-)-3-isobutylgaba + 2% formalin or formalin alone. The two lower doses of GP significantly reduced flinching and lifting/licking behavior during phase 2; however, phase 1 behaviors were unaffected, 600 microg GP significantly reduced these nociceptive behaviors during both phases. 600 microg S-(+)-3-isobutylgaba also reduced formalin-induced nociceptive behaviors; however, 600 microg of the isomer R-(-)-3-isobutylgaba had no effect. The antihyperalgesic effect of GP (1) was not due to a systemic effect since animals injected with 600 microg GP in one hindpaw and 2% formalin into the contralateral hindpaw developed nociceptive behaviors which were no different than those seen in animals injected with formalin alone; (2) was not due to a local anesthetic effect since needle sticks within the drug-injected region evoked paw withdrawal behavior which was not different from pre-drug levels; (3) was blocked by 20 microl D-serine but not by L-serine. Although the mechanism of action of GP has yet to be elucidated, these results indicate that GP has a peripheral site of action and thus may offer a novel therapeutic agent for topical or local treatment of pain of peripheral origin.

Acetates↗

PubMed1998

Glutamate receptors on postganglionic sympathetic axons.

The present study demonstrates that approximately 36% of postganglionic sympathetic axons in gray rami express receptors for the N-methyl-D-aspartate receptor 1 subunit of the N-methyl-D-aspartate receptor and 10% express the glutamate receptor 1 subunit of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor. If these receptors are active, glutamate released from primary afferent terminals could activate these receptors resulting in the release of noradrenaline and other substances from postganglionic sympathetic neurons. This interaction would constitute a non-synaptic, sensory-sympathetic, peripheral reflex that might be important in local vascular control and in pain states that have a sympathetic component.

Animals↗

PubMed1998

Memantine attenuates responses of spinothalamic tract cells to cutaneous stimulation in neuropathic monkeys.

Several lines of evidence indicate that N-methyl-D-aspartate (NMDA) receptors play an important role in nociception in general and in pathological pain in particular. It has been previously demonstrated in behavioral studies that NMDA receptor antagonists attenuate pathological pain in humans and nociceptive behaviors in animals. In the present study, we investigated the effect of the NMDA receptor antagonist memantine (MEM) on the responses of spinothalamic tract (STT) cells in normal and neuropathic monkeys. Memantine was delivered into the spinal cord through a microdialysis fiber acutely implanted into the dorsal horn. Responses of STT cells to peripheral stimulation within their receptive fields were recorded before and after MEM infusion. In normal animals (n = 7), 10 mm MEM did not affect STT cell (n = 7) baseline activity or responses to mechanical stimuli (brush, press or pinch). In neuropathic animals (n = 6), 1.0, 3.0, 10.0 and 100 mm MEM did not affect baseline activity of STT cells (n = 7); however, in a dose-dependent fashion, it significantly reduced responses of these cells to all cutaneous stimuli. The data suggest that MEM can have a direct effect on STT cells, blocking NMDA receptors known to be present on this cell population and, furthermore, may be a therapeutic agent for chronic pain.

Journal Article↗

PubMed1997

The reorganization of mu opioid receptors in the rat dorsal horn following peripheral axotomy.

The mu opioid receptor is concentrated in laminae I and II (LI and LII, respectively) of the normal rat dorsal horn. Fourteen days after transection of the L4-L6 segmental peripheral nerves, image analysis demonstrates a 49, 34 and 17% decrease in mu opioid staining density in the medial, middle and lateral thirds of the superficial dorsal horn, respectively, when comparing the operated to the unoperated side. Intralaminar analysis demonstrates that the greatest change in density occurs in LI and LII outer, compared to LII inner. By 31 days post-surgery, staining has returned to normal with side to side differences no longer present. These results imply that mu opioid ligands such as morphine might be less effective in ameliorating pain 2 weeks after a peripheral nerve lesion than they are in the normal condition, but that this effectiveness should return as the receptors are restored to their normal levels. Thus, the time following a lesion may be an important variable in assessing the effectiveness of mu opioid ligands in alleviating neuropathic pain. Furthermore, this study shows that the organization of opioid receptors in the superficial dorsal horn is malleable and could lead to changes in drug efficacy.

Animals↗

PubMed1997

Opioid receptors on peripheral sensory axons.

Opioid receptors have been demonstrated by light microscopic techniques in fine cutaneous nerves in naive animals. The present study extends these findings by showing that 29 and 38% of unmyelinated cutaneous sensory axons can be immunostained for mu- or delta-opioid receptors respectively. Local cutaneous injection of DAMGO, a mu-opioid ligand, ameliorates the nociceptive behaviors caused by local cutaneous injection of glutamate, a purely nociceptive chemical stimulus showing that the mu-receptors are functional. By contrast the delta-opioid ligand [2-D-penicillamine, 5-D-penicillamine]enkephalin (DPDPE) had no effect on these behaviors. These findings indicate a wider function for opioid receptors in naive animals than previously envisioned.

Animals↗

PubMed1997

Immunohistochemical localization of 5-HT2A receptors in peripheral sensory axons in rat glabrous skin.

Serotonin (5-hydroxytryptamine, 5-HT) is a well known inflammatory mediator and algesic substance. It has been hypothesized that 5-HT can have a direct action on peripheral sensory axons, but there has been no anatomical demonstration of 5-HT receptors on peripheral primary afferent processes. The present study shows that 32% of unmyelinated axons at the dermal-epidermal junction are immunohistochemically stained with antibodies directed against the 5-HT2A receptor providing anatomical evidence that 5-HT can have a direct effect on sensory fibers in the skin. Furthermore, encapsulated nerve endings in Pacinian corpuscles also contain reaction product following immunostaining for 5-HT2A receptors, indicating that large myelinated axons can be activated by endogenous serotonin. These data suggest that peripherally acting 5-HT2A antagonists may be effective in reducing pain of peripheral origin.

Animals↗

PubMed1997

Dextrorphan attenuates responses of spinothalamic tract cells in normal and nerve-injured monkeys.

Spinal cord N-methyl-D-aspartate (NMDA) receptors play an important role in the transmission of acute and chronic pain. The present study investigated the ability of dextrorphan (DEX), a metabolite of dextromethorphan and a clinically safe NMDA antagonist, to attenuate the responses of nociceptive spinothalamic tract (STT) neurons in anesthetized monkeys. The STT cells were recorded extracellularly in the lumbosacral enlargement and were identified by antidromic activation from the ventral posterior lateral thalamic nucleus. DEX administered through a microdialysis fiber inserted into the dorsal horn inhibited the responses of STT cells in normal animals to noxious pinch and heat stimuli. In monkeys made neuropathic by tight ligation of the L7 or S1 spinal nerve, DEX significantly attenuated the responses of STT cells to noxious pinch and heat, as well as to innocuous brushing, pressure and von Frey filament stimuli. These findings strongly suggest that DEX should be considered a potentially useful therapeutic agent for the treatment of neuropathic pain in humans.

Animals↗

PubMed1997

Time course of degenerative and regenerative changes in the dorsal horn in a rat model of peripheral neuropathy.

The time course of histochemical changes in the dorsal horn of rats subjected to an experimental peripheral neuropathy has been examined. Qualitative and quantitative analyses of the changes in dorsal horn staining were made for soybean agglutinin (SBA)-binding glycoconjugates, the soluble lectins RL-14.5 and RL-29, the growth-associated protein (GAP)-43, and the neuropeptides substance P (SP) and calcitonin gene-related peptide (CGRP). These analyses were made at various time points after chronic constriction of the sciatic nerve. Quantitative analysis indicated that staining density increased in the normal territories stained for SBA-binding glycoconjugates, RL-14.5, RL-29, and GAP-43 on the neuropathic side compared with the control side. In addition, there was an extension of the territories stained for SBA-binding glycoconjugates and RL-29 ipsilateral to the injury. The peak increases occurred at 14 or 28 days, followed by a decrease toward control levels by 70 days. In contrast, the staining density for SP in the ipsilateral dorsal horn decreased at 3 and 5 days and reached a peak decrease at 14 days. Then, the staining for SP returned toward control values. The staining for CGRP was unchanged at all time points examined. Dorsal rhizotomies ipsilateral to the nerve injury in neuropathic rats indicated that the increases in staining were attributable to changes in primary afferent neurons. These data suggest that peripheral neuropathy causes complex degenerative and regenerative changes in the central branches of primary afferent neurons. The associated synaptic reorganization may contribute to the sensory abnormalities that accompany peripheral neuropathy.

Animals↗

PubMed1997

Peripheral NMDA and non-NMDA glutamate receptors contribute to nociceptive behaviors in the rat formalin test.

The present study demonstrates that local cutaneous administration of either the N-methyl-D-aspartate (NMDA) glutamate receptor antagonist MK-801 or the non-NMDA glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) significantly attenuates formalin-induced nociceptive behaviors. Specifically, pretreatment with either drug reduced the magnitude and time course of lifting and licking behavior in the late phase of formalin pain; however, flinching behavior was not affected. In contrast, post-treatment of formalin pain with either antagonist did not affect lifting and licking behavior, although flinching behavior was mildly attenuated. We hypothesize that these actions result from blocking of peripheral glutamate receptors located on unmyelinated axons at the dermal-epidermal junction. These data suggest that peripheral glutamate receptors on cutaneous axons can be manipulated to reduce certain aspects of pain of peripheral origin. This route of administration offers the advantage of avoiding the side effects of systemic administration.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMed1997

Immunohistochemical localization of enkephalin in peripheral sensory axons in the rat.

Dorsal root ganglia (DRG) contain measurable amounts of met-enkephalin (ENK), and a significant number of DRG cells contain mRNA for the manufacture of ENK. Yet almost no DRG cells are immunostained for ENK and dorsal rhizotomy does not diminish ENK staining in the dorsal horn. A hypothesis which would explain these seemingly discrepant results is the phenomenon of differential transport, where DRG cells making ENK rapidly transport the peptide only to their peripheral sensory axons. Evidence consistent with this hypothesis would be the demonstration of ENK-containing peripheral sensory axons. The present study demonstrates that approximately 17% of peripheral cutaneous axons label for ENK. The presence of a significant number of ENK-containing axons suggests an endogenous neural source of opiate ligand in the periphery and, in addition to ENK-containing inflammatory cells, this neural source may be functionally important in responses to physiologic as well as inflammatory pain.

Animals↗

PubMed1997

Receptor localization in the mammalian dorsal horn and primary afferent neurons.

The dorsal horn of the spinal cord is a primary receiving area for somatosensory input and contains high concentrations of a large variety of receptors. These receptors tend to congregate in lamina II, which is a major receiving center for fine, presumably nociceptive, somatosensory input. There are rapid reorganizations of many of these receptors in response to various stimuli or pathological situations. These receptor localizations in the normal and their changes after various pertubations modify present concepts about the wiring diagram of the nervous system. Accordingly, the present work reviews the receptor localizations and relates them to classic organizational patterns in the mammalian dorsal horn.

Animals↗

PubMed1996

Localization and activation of substance P receptors in unmyelinated axons of rat glabrous skin.

Immunohistochemical staining for the substance P (neurokinin INK1) receptor labels 32% of the unmyelinated axons in the glabrous skin of the rat hindpaw. This is the first demonstration of substance P (SP) receptors associated with the membranes of primary afferent fibers. Injection of SP into the subcutaneous tissue of the third hind toe results in behavioral changes interpreted as mechanical hyperalgesia and mechanical allodynia. These nocifensive behaviors can be blocked by the NK1 antagonist CP99,994-1. The presence of peripheral axons immunolabelled for NK1 receptors and the demonstration that exogenous peripheral SP causes nocifensive behaviors would seem to indicate that SP can have a direct effect on sensory afferents with activation of these receptors resulting in fine afferent firing and thus the pain-related behaviors. Additionally, the presence of round, clear vesicles in some of the SP receptor-labelled axons suggests the presence of autoreceptors since it is known that many primary sensory afferents contain SP.

Animals↗

PubMed1996

Distribution of glycine-immunoreactive profiles in the monkey spinal cord: a light microscopic and ultrastructural study.

The present study analyzed the relationships of glycine (GLY)-immunoreactive (-IR) and unlabeled profiles in the primate spinal cord. Light microscopic analysis demonstrated GLY-IR profiles in laminae III-VII, with fewer labeled profiles in laminae I, II, VIII, IX and X. The dorsal part of the lateral funiculus and the dorsal funiculus contained few labeled axons, in contrast to all other areas of white matter, which were heavily labeled. At the electron microscopic level, GLY-IR terminals in monkeys contained mainly round, with occasional pleomorphic, clear vesicles; however, F-type GLY-IR terminals synapsing on motoneurons contained pleomorphic vesicles. This seems to be an important species difference because vesicles in GLY-IR terminals in rat and cat are predominantly oval or elliptical. GLY-IR terminals synapsed on unlabeled as well as GLY-IR cell bodies and dendrites. This is morphological evidence that GLY may be both an inhibitor (GLY-IR terminals synapse on and presumably inhibit non-GLY cell bodies and dendrites) and a disinhibitor (GLY-IR terminals synapse on and presumably inhibit other GLY elements) of spinal activity. Also noteworthy was the conspicuous absence of axoaxonic interactions involving GLY-IR terminals. A related finding was that GLY profiles were always postsynaptic, never presynaptic, to glomerular primary afferent terminals. The functional implications would seem to be that primary afferent input can activate the spinal GLY system but that there is little GLY presynaptic control of afferent input in monkeys. This is in contrast to rats and cats, in which axoaxonic interactions involving GLY-IR terminals have been observed and where it is common to find GLY-IR terminals presynaptic to glomerular primary afferent terminals.

Afferent Pathways↗

PubMed1996

Sprouting of A beta fibers into lamina II of the rat dorsal horn in peripheral neuropathy.

Cholera toxin beta-subunit conjugated to horseradish peroxidase was used to label the large myelinated (A beta) fiber input to the dorsal horn in a model of peripheral neuropathy induced by tight ligation of the L5 and L6 spinal nerves. Following induction of neuropathy, A beta fibers were present in lamina II of the ipsilateral dorsal horn, a region normally devoid of A beta input. This reorganization of large fiber input to the superficial dorsal horn provides some anatomical basis for sensory changes found in this model of neuropathic pain.

Afferent Pathways↗