Receptors In Dorsal Horn Research Articles

α5GABAA receptors play a pronociceptive role and avoid the rate-dependent depression of the Hoffmann reflex in diabetic neuropathic pain and reduce primary afferent excitability.

Diabetic neuropathy is an incapacitating complication in diabetic patients. The cellular and molecular mechanisms involved in this pathology are poorly understood. Previous studies have suggested that the loss of spinal GABAergic inhibition participate in painful diabetic neuropathy. However, the role of extrasynaptic α5 subunit-containing GABAA (α5GABAA) receptors in this process is not known. The purpose of this study was to investigate the role of α5GABAA receptors in diabetes-induced tactile allodynia, loss of rate-dependent depression (RDD) of the Hoffmann reflex (HR), and modulation of primary afferent excitability. Intraperitoneal administration of streptozotocin induced tactile allodynia. Intrathecal injection of α5GABAA receptor inverse agonist, L-655,708, produced tactile allodynia in naive rats, whereas it reduced allodynia in diabetic rats. In healthy rats, electrical stimulation of the tibial nerve at 5 Hz induced RDD of the HR, although intrathecal treatment with L-655,708 (15 nmol) abolished RDD of the HR. Streptozotocin induced the loss of RDD of the HR, while intrathecal L-655,708 (15 nmol) restored RDD of the HR. L-655,708 (15 nmol) increased tonic excitability of the primary afferents without affecting the phasic excitability produced by the primary afferent depolarization. α5GABAA receptors were immunolocalized in superficial laminae of the dorsal horn and L4 to L6 dorsal root ganglion. Streptozotocin increased mean fluorescence intensity and percentage of neurons expressing α5GABAA receptors in dorsal horn and L4 to L6 dorsal root ganglia in 10-week diabetic rats. Our results suggest that spinal α5GABAA receptors modulate the HR, play an antinociceptive and pronociceptive role in healthy and diabetic rats, respectively, and are tonically active in primary afferents.

396 Downregulation of M4 but not M2 muscarinic receptors in dorsal horn after peripheral nerve injury relates to responsiveness to SCS

396 Downregulation of M4 but not M2 muscarinic receptors in dorsal horn after peripheral nerve injury relates to responsiveness to SCS

Stimulation of the lateral hypothalamus produces antinociception mediated by 5-HT 1A, 5-HT 1B and 5-HT 3 receptors in the rat spinal cord dorsal horn

The lateral hypothalamus is part of an efferent system that modifies pain at the spinal cord dorsal horn, but the mechanisms by which lateral hypothalamus-induced antinociception occur are not fully understood. Previous work has shown that antinociception produced from electrical stimulation of the lateral hypothalamus is mediated in part by spinally projecting 5-hydroxytryptamine (5-HT) neurons in the ventromedial medulla. To further examine the role of the lateral hypothalamus in antinociception, the cholinergic agonist carbamylcholine chloride (125nmol) was microinjected into the lateral hypothalamus of female Sprague–Dawley rats and nociceptive responses measured on the tail-flick and foot-withdrawal tests. Intrathecal injections of the selective 5-HT 1A, 5-HT 1B, 5-HT 3 receptor antagonists, WAY 100135, SB-224289, and tropisetron, respectively, and the non-specific antagonist methysergide, were given. Lateral hypothalamus stimulation with carbamylcholine chloride produced significant antinociception that was blocked by WAY 100135, tropisetron, and SB-224289 on both the tail-flick and foot-withdrawal tests. Methysergide was not different from controls on the tail flick test, but increased foot-withdrawal latencies compared with controls. These results suggest that the lateral hypothalamus modifies nociception in part by activating spinally projecting serotonin neurons that act at 5-HT 1A, 5-HT 1B, and 5-HT 3 receptors in the dorsal horn.

Distribution of antinociceptive adenosine a 1 receptors in the spinal cord dorsal horn, and relationship to primary afferents and neuronal subpopulations

Adenosine can reduce pain and allodynia in animals and man, probably via spinal adenosine A 1 receptors. In the present study, we investigate the distribution of the adenosine A 1 receptor in the rat spinal cord dorsal horn using immunohistochemistry, in situ hybridization, radioligand binding, and confocal microscopy. In the lumbar cord dorsal horn, dense immunoreactivity was seen in the inner part of lamina II. This was unaltered by dorsal root section or thoracic cord hemisection. Confocal microscopy of the dorsal horn revealed close anatomical relationships but no or only minor overlap between A 1 receptors and immunoreactivity for markers associated with primary afferent central endings: calcitonin gene-related peptide, or isolectin B4, or with neuronal subpopulations: μ-opioid receptor, neuronal nitric oxide synthase, met-enkephalin, parvalbumin, or protein kinase Cγ, or with glial cells: glial fibrillary acidic protein. A few adenosine A 1 receptor positive structures were double-labeled with α-amino-3-hydroxy-5-methyl-4-isoaxolepropionic acid glutamate receptor subunits 1 and 2/3. The results indicate that most of the adenosine A 1 receptors in the dorsal horn are located in inner lamina II postsynaptic neuronal cell bodies and processes whose functional and neurochemical identity is so far unknown. Many adenosine A 1 receptor positive structures are in close contact with isolectin B4 positive C-fiber primary afferents and/or postsynaptic structures containing components of importance for the modulation of nociceptive information.

Effects of NMDA and non-NMDA ionotropic glutamate receptor antagonists on the development and maintenance of hyperalgesia induced by repeated intramuscular injection of acidic saline

Two unilateral injections of pH 4.0 saline into the gastrocnemius muscle result in a bilateral decrease in mechanical withdrawal threshold after the second injection. This decrease is significant by 4 h and lasts through 1 week. The purpose of this study was to characterize the involvement of both N-methyl- d-aspartate (NMDA) and non-NMDA glutamate receptors in the spinal cord dorsal horn in the development and maintenance of mechanical hyperalgesia from repeated intramuscular injections of acidic saline. 2-amino-5-phosphonovaleric acid (AP5) (2–20 nmol, 10 μl, pH 7) or 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo[f]quinoxaline-7-sulfonamide (NBQX) (1–10 nmol, 10 μl, pH 8–9) was administered intrathecally to the lumbar spinal cord to block NMDA and non-NMDA ionotropic glutamate receptors in the dorsal horn, respectively. Drugs were administered at one of three different time points: (1) prior to the first intramuscular injection of pH 4.0 saline on Day 0, (2) prior to the second intramuscular injection of pH 4.0 saline on Day 5, and (3) 1 week after the second injection. Mechanical withdrawal thresholds were measured with von Frey filaments before, 4 h, and 24 h after injection 1 and before, 4 h, 24 h, and 1 week after injection 2. AP5 had no effect on mechanical withdrawal thresholds when administered prior to the first intramuscular injection of pH 4.0 saline. When AP5 was administered before the second intramuscular injection, the bilateral decrease in mechanical withdrawal thresholds was delayed for up to 24 h. Intrathecal administration of AP5 1 week after the second intramuscular injection of pH 4.0 saline produced a bilateral increase in mechanical withdrawal thresholds. Blockade of non-NMDA glutamate receptors in the spinal cord dorsal horn prior to either the first or second intramuscular injection of pH 4.0 saline had no effect on the development of mechanical hyperalgesia. However, spinal injection of NBQX 1 week after the second intramuscular injection of pH 4.0 saline resulted in an increase in mechanical withdrawal thresholds when compared to vehicle controls. These data suggest that both NMDA and non-NMDA glutamate receptors are involved in the maintenance of chronic, muscle-induced hyperalgesia.

Localization of glutamate receptors in dorsal horn of rat spinal cord.

Immunocytochemical localization of metabotropic glutamate receptors (mGluRs) and ionotropic glutamate receptors (NMDA-type: NMDAR1 and NMDAR2A-C; AMPA-type: GluR1-4) was performed on sections of rat dorsal horn. Immunoreactivity for mGluR1 alpha was detected in laminae I-III of the dorsal horn, whilst mGluR2/3 immunoreactivity was detected primarily in lamina III. Immunoreactivity for NMDAR1, GluR1, GluR2, GluR2/3, GluR4 and GluR5/6/7 was strongly localized in neuronal elements of laminae I-III. Immunoreactivity for NMDAR2B was localized in laminae I-III. No mGluR5, NMDAR2A and NMDAR2C immunoreactivity was detected. In addition, immunoreactivity for receptors was found to co-localize with immunoreactivity for glutamate in the dorsal horn. The present results indicate that glutamate receptors are differentially localized in neuronal elements of dorsal horn where receptor-neurotransmitter interaction takes place.

The effects of neurokinin receptor antagonists on mustard oil-evoked activation of rat dorsal horn neurons

Previous evidence indicated that brief nociceptive responses of neurons in laminae IV V of both rat and cat dorsal horn are more readily inhibited by antagonists at NK 2 rather than at NK 1 neurokinin receptors. Further support for a role of spinal NK 2 receptors in nociception has been provided from experiments assessing modulation of the nociceptive flexor reflex by tachykinins and activation of dorsal horn neurons by brief application of capsaicin to afferents. The present experiments were designed to compare the contribution of NK 1 and NK 2 receptors in dorsal horn to the sustained neuronal activity induced by peripheral application of the chemical algogen mustard oil (reported to be a selective activator of C afferents). In nearly all of the multireceptive laminae IV V neurons tested, a selective NK 2 receptor antagonist L 659,874 inhibited previously established mustard oil-induced activity. In contrast, two selective NK 1 receptor antagonists L 668,169 and GR 82334 were only rarely effective. These results further underline the apparent importance of NK 2 receptors in spinal nociceptive processing. NK 1 receptors do not appear to play a major role in the present experimental protocol, but they may of course do so under different circumstances.

Multiple opiate receptor sites on primary afferent fibres.

In addition to their actions at supraspinal sites, opiates can act directly at the spinal cord level to produce analgesia1–3. Opiate receptors and enkephalins are found in abundance in the dorsal horn of the spinal cord, in the region of termination of small-diameter primary afferents4–6. Furthermore, there is evidence that exogenously administered opiates or endogenous enkephalins act both on postsynaptic receptors in dorsal horn and presynaptically to block transmitter release from nociceptive primary afferent terminals7–10. The possiblity of these two distinct sites of opiate action in dorsal horn is of particular interest in view of recent evidence for the existence of multiple opiate receptors11–13. On the basis of differences in the rank orders of potency of opiate alkaloids and opoid peptides observed in different tissues, Lord et al.14 have proposed two categories of opiate receptor, μ and δ, and these can be differentiated in binding studies, provided that selective radioglands are used in the appropriate experimental conditions15–18. Here we have used the selective radioligands 3H-morphine (μ sites) and 3H-D-Ala2, D-Leu5-enkephalin (δ sites) to examine directly in rat the distribution and binding characteristics of opiate receptors on dorsal root and in various regions of the adjacent spinal cord. Primary afferent tissue (dorsal root) and dorsal horn were found to contain μ and δ opiate binding sites with a relatively high proportion of μ sites. Partial destruction of small-diameter primary afferents after cutting the sciatic nerve led to a significant reduction in both μ and δ binding sites on dorsal roots, suggesting that both types of opiate receptors may exist on small-diameter primary afferents.