Spinal NK1 receptors contribute to the increased excitability of the nociceptive flexor reflex during persistent peripheral inflammation

Hyperalgesia is a characteristic of inflammation and is mediated, in part, by an increase in the excitability of spinal neurons. Although substance P does not appear to mediate fast synaptic events that underlie nociception in the spinal cord, it may contribute to the hyperalgesia and increased excitability of spinal neurons during inflammation induced by complete Freund's adjuvant. We examined the role of endogenous substance P in changes in the excitability of spinal neurons during adjuvant-induced, peripheral inflammation by determining the effect of a selective NK1 receptor antagonist (RP67580) on the nociceptive flexor reflex in adult rats. Experiments were conducted 2 or 3 days after injection of adjuvant. Animals exhibited moderate thermal hyperalgesia at this time. The flexor reflex was evoked by electrical stimulation of the sural nerve and was recorded in the ipsilateral hamstring muscles. The flexor reflex ipsilateral to the inflamed hindpaw was enhanced approximately two-fold compared to the flexor reflex evoked in untreated animals as determined by the number of potentials and the duration of the reflex. The enhanced reflex in adjuvant-treated animals was most likely due to an increase in the excitability of spinal interneurons because short-latency activity in the hamstring muscles did not differ between untreated animals and adjuvant-treated animals following electrical stimulation of the L5 dorsal root or the nerve innervating the muscle with a stimulus that was 1.3-1.5 times the threshold for excitation of A-fibers. Intrathecal administration of RP67580 (2.3 and 6.8 nmol) attenuated the flexor reflex evoked in adjuvant-treated animals, but had no effect in untreated animals. Intravenous or intraplantar injection of RP67580 (6.8 nmol) did not affect the flexor reflex in adjuvant-treated animals indicating a spinal action of the drug following intrathecal administration. RP68651, the enantiomer of RP67580, was without effect at doses up to 6.8 nmol, indicating that the effects of comparable doses of RP67580 were due to an action of the drug at NK1 receptors. However, intrathecal administration of 23 nmol of both drugs attenuated the reflex in adjuvant-treated and control animals indicating that effects of RP67580 at this dose were not mediated entirely by its action at NK1 receptors. Overall, these data suggest that endogenous substance P has a role in the increased excitability of spinal interneurons observed during persistent inflammation and support the hypothesis that substance P released in the spinal cord contributes to the hyperalgesia that accompanies adjuvant-induced persistent, peripheral inflammation.

N-methyl-D-aspartate receptor antagonists potentiate morphine's antinociceptive effect in the rat

The interaction between morphine and three antagonists of the N-methyl-D-aspartate (NMDA) receptor. MK-801 (non-competitive channel blocker), dextromethorphan (clinically available non-competitive antagonist) and CGS19755 (competitive receptor antagonist), was examined in rats with the hot plate test. The NMDA antagonists were administered intraperitoneally and none of them caused antinociception at doses that did not produce motor deficits (0.1 mg kg-1 MK-801, 30 mg kg-1 dextromethorphan and 5 mg kg-1 CGS19755). However, pretreatment with the NMDA antagonists at these doses 30 min prior to subcutaneous injection of 5 mg kg-1 morphine significantly potentiated the antinociceptive effect of morphine, with strongest effect observed with dextromethorphan. It is suggested that blockade of NMDA receptors enhances the antinociceptive effect of morphine and NMDA antagonists may improve the analgesic efficacy of morphine in the clinic.

Effects of intrathecal vs. systemic clonidine in treating chronic allodynia-like response in spinally injured rats

A chronic pain-like response to innocuous mechanical stimuli (allodynia) was observed in rats after severe spinal cord ischemia, which resembled some painful conditions observed in spinally injured patients. The present studies examined the effects of clonidine, an alpha 2-adrenoceptor agonist, on this allodynia-like response. Intrathecal (i.t.) clonidine dose-dependently relieved allodynia and doses up to 10 micrograms did not induce motor deficits or sedation, but slightly increased systemic blood pressure. The anti-allodynic effect of i.t. clonidine was reversed by the selective alpha 2-adrenoceptor antagonist atipamezole. In contrast, 50 and 100 micrograms/kg intraperitoneal (i.p.) clonidine did not relieve the chronic allodynia, although the higher dose induced some motor deficits and sedation. Allodynic behavior was abolished after 200 micrograms/kg, i.p. clonidine, which, however, caused strong sedative and motor impairment. The present data suggested that spinal, but not systemic, alpha 2-adrenoceptor agonists may have therapeutic value in treating mechanical allodynia in patients with neuropathic pain of spinal origin.

Nociceptin or antinociceptin: potent spinal antinociceptive effect of orphanin FQ/nociceptin in the rat

A heptadecapeptide (orphanin FQ or nociceptin) was recently identified as an endogenous ligand for the orphan opioid-like receptor. Here we report that intrathecal orphanin FQ produces dose-dependent depression of a spinal nociceptive flexor reflex in the rat. Furthermore, administration of orphanin FQ in rats with intrathecal catheters produced behavioural antinociception in the tail flick test with no signs of sedation or motor impairment. The reflex depressive effect of orphanin FQ was not reversed by antagonists of opioidergic, alpha 2-adrenergic and GABA-A receptors. Thus, orphanin FQ may suppress nociceptive input at the spinal level through an novel mechanism. Orphanin FQ or agonists of its receptor may represent novel analgesics for pain conditions which are not responsive to existing pharmacological therapy.

Disposition and analgesic effects of systemic morphine, morphine-6-glucuronide and normorphine in rat

Morphine, morphine-6-glucuronide and normorphine were administered to male Sprague-Dawley-rats. Analgesic effect was estimated with the hot plate and spinal nociceptive reflex depression. After intraperitoneal administration the molar potency ratio of morphine-6-glucuronide/morphine was 1.7 estimated by the paw lick latency on the hot plate utilizing a linked pharmacokinetic-pharmacodynamic model. The potency ratio of morphine-6-glucuronide/morphine utilizing the spinal nociceptive reflex depression after intravenous administration was estimated to be within the earlier reported range of 1-4 after systemic administration of the drugs. In contrast to what is seen in man virtually no morphine-6-glucuronide was formed in Sprague-Dawley rats after administration of morphine, much lower levels of morphine-3-glucuronide were also seen. The molar AUC ratio of morphine-3-glucuronide/morphine was 1.8 +/- 0.5 and the corresponding ratio for normorphine/morphine was 0.2 +/- 0.06. After intraperitoneal administration of morphine, morphine-6-glucuronide and normorphine mean systemic clearance values of 413 +/- 95, 50 +/- 11 and 187 +/- 54 ml.min.kg-1 respectively were observed. Varea was 9.0 +/- 2.1, 0.8 +/- 0.2 and 4.9 +/- 1.4 L.kg-1 respectively. The slow absorption of morphine-6-glucuronide was illustrated by the mean Tmax-value of the 16 min. as compared with 9 min. for morphine and 10 min. for normorphine. It was possible to fit pharmacokinetic and pharmacodynamic data of behavioural analgesic effect of both morphine and morphine-6-glucuronide to a parametric model linking the sigmoid Emax model to standard pharmacokinetic equations.

Intrathecal pituitary adenylate cyclase activating polypeptide facilitates the spinal nociceptive flexor reflex in the rat

We have examined the effects of intrathecal (i.t.) pituitary adenylate cyclase activating polypeptide on the spinal nociceptive flexor reflex in decerebrate, spinalized, unanaesthetized rats. The flexor reflex was elicited by electrical stimulation applied subcutaneously to the sural nerve innervation area and recorded as electromyogram activity from ipsilateral hamstring muscles. Pituitary adenylate cyclase activating polypeptide(l-27) was administered over a wide dose range (10 ng to 10 mu g) and elicited a dose-dependent facilitation of the flexor reflex and did not depress the reflex at any dose. Furthermore, pituitary adenylate cyclase activating polypeptide did not inhibit the facilitation of the flexor reflex induced by repetitive stimulation of C-fibres. It is concluded that pituitary adenylate cyclase activating polypeptide had an excitatory effect on spinal cord function which may indicate a role for this peptide in nociceptive transmission and modulation. Moreover, in contrast to previous studies, we found no evidence suggesting that pituitary adenylate cyclase activating polypeptide exerts antinociceptive action at spinal level.

Capsaicin-sensitive afferents mediate chronic cold, but not mechanical, allodynia-like behavior in spinally injured rats

Pain-like responses to cold or innocuous mechanical stimuli were observed chronically in rats after spinal cord ischemia. These resembled the symptoms of mechanical allodynia and cold hyperalgesia that are frequently observed in spinally injured patients. We evaluated the involvement of capsaicin-sensitive afferents in mediating these responses. A single subcutaneous injection of resiniferatoxin (RTX), an ultrapotent capsaicin analogue, produced hypoalgesia to noxious heat stimulus and normalized the enhanced response to cold stimulus. In contrast, the mechanical allodynia-like response was not influenced by RTX. Thus, the enhanced response to cold, but not light touch, is mediated by capsaicin-sensitive afferents. Capsaicin and related compounds may have therapeutic potential in treating neuropathic pain elicited by some, but not all, modalities of stimulation.

L-740,093, a new antagonist of the CCK-B receptor, potentiates the antinociceptive effect of morphine: electrophysiological and behavioural studies

The interaction between morphine and L740,093, a newly developed highly potent antagonist of cholecystokinin-B receptors, was examined in electrophysiological and behavioural studies. Intravenous L740,093 at 0.01 mg/kg had no effect on its own, but significantly potentiated the depressive effect of 1 mg/kg morphine on the nociceptive flexor reflex in decerebrate, spinalized rats. Similarly, subcutaneous L740, 093 at 0.03 mg/kg significantly prolonged the duration of antinociception induced by 10 mg/kg morphine in the rat hotplate test. A bell shaped dose-response curve was noted in the behavioural studies with respect to the interaction between L740,094 and morphine. L740,093, which has excellent CNS penetration, may represent a new tool in studying the involvement of the endogenous cholecystokinin system in the modulation of opioid analgesia and in exploring novel analgesics.

Dextromethorphan potentiates morphine antinociception, but does not reverse tolerance in rats

The N-methyl-D-aspartate (NMDA) and cholecystokinin (CCK)-B receptors may have a role in the development and reversal of tolerance to morphine. In morphine-tolerant rats, addition of the CCK-B receptors antagonist CI 988 or the NMDA receptor blocker dextromethorphan enhanced the antinociceptive effect of morphine on the hot plate test. However, combined administration of CI 988 and dextromethorphan did not further potentiate the antinociceptive effect of morphine in tolerant rats. Dextromethorphan by itself had no effect in tolerant rats. In drug-naive rats, dextromethorphan by itself had no antinociceptive effect, but when combined with morphine or morphine and CI 988, it significantly potentiated the magnitude and duration of the effect of morphine. Thus, unlike the reversal of tolerance with CI 988 at doses that did not potentiate the effect of morphine, the antinociception observed with the NMDA antagonist in the presence of morphine in tolerant rats may not represent the reversal of tolerance, but may instead reflect the potentiation of morphine's analgesic effect by dextromethorphan.

Calcitonin gene-related peptide (8-37) does not antagonize calcitonin gene-related peptide in rat spinal cord

We have examined the effects of intrathecal (i.t.) human calcitonin gene-related peptide (hCGRP) and its C-terminal fragment hCGRP(8-37), a proposed CGRP antagonist, on the flexor reflex in decerebrate, spinalized, unanesthetized rats. I.t. hCGRP at 26 pmol caused a moderate facilitation of the reflex which was not antagonized by hCGRP(8-37) at doses ranging from 26 pmol to 5.2 nmol. Furthermore, hCGRP(8-37) by itself facilitated the reflex, with no signs of inhibition. It is concluded that the spinal CGRP receptor mediating the spinal facilitatory effect of hCGRP is not antagonized by hCGRP(8-37). Thus, it is unlikely that hCGRP(8-37) can be useful as a spinal analgesic.

Effects of peripheral axotomy on neuropeptides and nitric oxide synthase in dorsal root ganglia and spinal cord of the guinea pig: an immunohistochemical study

The effect of axotomy (3, 10 and 21 days) on the expression of some neuronal markers was analysed in dorsal root ganglia and spinal cord of guinea-pigs using immunohistochemistry. Three weeks following injury, substance P-like immunoreactivity (-LI) was slightly reduced in the DRGs of the ipsilateral side, whereas a marked increase in neuropeptide Y(NPY)-LI could be detected ipsilaterally and a smaller increase contralaterally. NPY-LI was mainly expressed in small, but also some medium-sized and large neuron profiles after axotomy. Galanin-LI showed a moderate bilateral increase. No significant changes could be observed in DRGs for calcitonin gene-related peptide (CGRP)-, vasoactive intestinal polypeptide-, peptide histidine isoleucine- or nitric oxide synthase-LIs. In the ventral horn CGRP-LI was slightly increased bilaterally in motoneurons, most pronounced on the injured side. Autotomy behaviour was seen in seven of the nine animals in the twenty-one day group. The present results demonstrate that also in guinea-pigs several peptides undergo distinct changes in their expression after peripheral nerve injury. However, in contrast to rats and monkeys, galanin-LI is only moderately increased in guinea-pigs. Neuropeptide Y showed a dramatic increase mainly in small neurons, in contrast to the upregulation in large neurons in the rat. Thus, distinct species differences exist with regard to the cellular response to nerve injury.

Differential effects of pre-treatment with intrathecal or intravenous morphine on the prevention of spinal cord hyperexcitability following sciatic nerve section in the rat

The effect of intrathecal (i.t.) and intravenous (i.v.) morphine on spinal hyperexcitability following unilateral section of the sciatic nerve was studied in decerebrate, spinalized, unanesthetized rats. Sciatic nerve section evoked a biphasic, prolonged hyperexcitability of the flexor reflex. Either i.v. (0.2, 1 or 10 mg center dot kg-1) or i.t. (3 or 10 mu g) morphine was administered prior to sciatic nerve section. All doses of morphine significantly depressed the baseline flexor reflex and abolished the less intense prolonged second component of reflex hyperexcitability. One and 10, but not 0.2, mg center dot kg-1 i.v. morphine significantly depressed the first phase of spinal cord sensitization. However, both 3 mu g and 10 mu g i.t. morphine were significantly more effective than i.v. morphine in suppressing spinal cord hyperexcitability. The present results suggest that moderate doses of i.t. morphine decrease spinal hyperexcitability following nerve transection more than even extremely large i.v. doses. The poorer effect of i.v. morphine on preventing spinal hyperexcitability may be due to low spinal concentration after systemic administration.

Increased levels of GMAP, VIP and nitric oxide synthase, and their mRNAs, in lumbar dorsal root ganglia of the rat following systemic resiniferatoxin treatment

Using in situ hybridization, the expression of mRNA encoding galanin, vasoactive intestinal polypeptide (VIP), neuropeptide Y (NPY), and nitric oxide synthase (NOS), respectively, was studied in lumbar dorsal root ganglia of rats given a single s.c. dose of 300 micrograms kg-1 resiniferatoxin (RTX), an ultrapotent capsaicin analogue. In control animals, 10% of the DRG neurones were positive for galanin mRNA, whereas no message for VIP, NPY or NOS could be detected. One week after RTX treatment, a markedly increased number (approximately 30%) of the neurones expressed galanin mRNA. Simultaneously, VIP and NOS mRNA became detectable in 6-8% of the neurones. The number of galanin-positive neurones declined after 2 weeks and returned to control levels by 8 weeks. The increase in number of VIP-, or NOS-positive neurones persisted up to 4 weeks after RTX treatment and declined thereafter. Also, there was a small increase in NPY mRNA-positive neurones. In parallel immunohistochemical experiments, similar increases were observed for galanin message-associated protein (GMAP)-, VIP- and NOS-like immunoreactivities. Our findings suggest that RTX can cause changes (messenger plasticity) in galanin, VIP and NOS expression in capsaicin-sensitive sensory neurones of the rat, similar to those described following axotomy.

Effects of intrathecal local anesthetics on spinal excitability and on the development of autotomy

The effect of 500 micrograms intrathecal (i.t.) lidocaine and tocainide injected prior to unilateral sciatic nerve section on the development of autotomy behavior was examined in rats. Neither drug inhibited autotomy compared with saline controls. The effect of lidocaine and tocainide on the flexor reflex in decerebrate, spinalized rats was also examined. Both drugs at 500 micrograms i.t. totally blocked the flexor reflex and hyperexcitability following acute nerve section, with a longer duration of action with tocainide. Recovery from the drugs was followed by a prolonged period of reflex hyperexcitability, even without axotomy. Lower dose lidocaine (100 micrograms) briefly blocked the reflex and recovery was followed by reflex hyperexcitability. Very low dose i.t. lidocaine (1 and 10 micrograms) did not block the flexor reflex, but only induced a prolonged increase in reflex magnitude. The possible mechanisms underlying these observations are discussed. The results suggest that i.t. local anesthetic pretreatment alone may not be beneficial for the prevention of the development of postoperative or neuropathic pain.

Effect of administration of high dose intrathecal clonidine or morphine prior to sciatic nerve section on c-Fos expression in rat lumbar spinal cord

The effects of moderate and high intrathecal doses of clonidine, an alpha 2 adrenoceptor agonist, or a high dose of morphine on sciatic nerve section-induced expression of c-Fos-like immunoreactivity was studied in laminae I and II of the dorsal horn and laminae VIII and IX of the ventral horn of rat lumbar spinal cord. c-Fos-like immunoreactivity was examined by immunohistochemistry in normal rats (group 1), rats implanted with an intrathecal catheter with its tip on the lumbar spinal cord (group 2), injected with 10 micrograms (group 3) or 50 micrograms (group 4) clonidine intrathecally 3 h before being killed. In other groups, saline, 10 or 50 micrograms clonidine or 30 micrograms morphine was injected 1 h before unilateral nerve section, and the expression of c-Fos-like immunoreactivity was examined 2 h after axotomy. Few labeled neurons were found in normal controls. The intrathecal catheter itself caused a significant increase in bilateral c-Fos-like immunoreactivity in spinal dorsal and ventral horn compared to normals. The level of c-Fos-like immunoreactivity after 10 or 50 micrograms intrathecal clonidine was similar as in the intrathecal catheter group. Sciatic nerve section caused a significant ipsilateral increase in c-Fos-like immunoreactivity in the dorsal horn compared to the intact side in rats injected with saline. Pretreatment with 10 or 10 micrograms clonidine did not reduce sciatic nerve section-induced expression of c-Fos-like immunoreactivity, but instead caused a significant bilateral increase in c-Fos-like immunoreactivity.(ABSTRACT TRUNCATED AT 250 WORDS)

New high affinity peptide antagonists to the spinal galanin receptor

1. The role of endogenous galanin in somatosensory processing has been studied with galanin receptor antagonists. The new galanin receptor ligands C7, M32, M38 and M40 bind with high affinity (Kd in nanomolar range) to spinal cord galanin receptors and possess oxidative stability as compared to earlier generations of peptide ligands. These peptides have been examined in the spinal flexor reflex model where exogenous galanin exhibited biphasic excitatory and inhibitory effects. 2. Intrathecal administration of C7 [galanin(1-13)-spantide] and M32 [galanin (1-13)-neuropeptide Y(25-36) amide] blocked facilitation of the nociceptive flexor reflex induced by 30 pmol intrathecal galanin in decerebrate, spinalized rats in a dose-dependent manner, thus behaving as antagonists of the galanin receptor. In contrast, M38[galanin(1-13)-(Ala-Leu)3-Ala amide] and M40 [galanin(1-13)-Pro-Pro-(Ala-Leu)2-Ala amide], exhibited only weak antagonism at high doses in this model. Moreover, lower doses of M40 potentiated galanin-induced reflex facilitation. C7 was neurotoxic at high doses in the rat spinal cord. 3. M32 and C7 were potent antagonists of galanin receptors in rat spinal cord, in correlation with their in vitro binding characteristics. In contrast, M38 and M40, despite their high in vitro affinity, exhibited only very weak antagonism. Moreover, M40 may also behave as a partial agonist. 4. Previous studies have shown that the galanin receptor may be heterogeneous. The discrepancy between in vitro binding and in vivo antagonistic potency of M38 and M40 may also suggest the presence of different galanin receptor subtypes within the rat spinal cord. However, other explanations for the discrepancy, such as differences in metabolic stability, diffusion rates and penetration to the site of action are also possible.

Central and peripheral expression of galanin in response to inflammation

Using in situ hybridization, immunohistochemistry and receptor binding methodology, the galanin messenger RNA levels, galanin binding and galanin-like immunoreactivity were examined in rats injected with carrageenan into the left hindpaw. Three days after injection, a distinct increase (63%) in galanin messenger RNA-positive neurons was observed in the medial laminae I and II of the ipsilateral dorsal horn (lumbar 4 and 5) as compared to the contralateral side. However, no alteration was found in galanin binding and galanin-like immunoreactivity in the dorsal horn. In dorsal root ganglia (lumbar 5), inflammation induced a significant decrease in galanin messenger RNA (39%) and galanin peptide (47%) on the ipsilateral side. Galanin binding was not detected in dorsal root ganglia, neither on the inflammatory nor on the control side. Increased levels of galanin-like immunoreactivity and galanin messenger RNA were seen in cells in the inflamed dermis and epidermis, especially in stratum granulosum. Most of the galanin-immunoreactive cells contained ED1-like immunoreactivity, a marker for macrophages. A strong galanin binding was seen in the inflamed dermis. Such binding sites may be targets for galanin released from local cells in inflamed dermis. Taken together, our results suggest that both neuronal and non-neuronal galanin or a galanin-like peptide is involved in the response to inflammation.

Subcutaneous carrageenan, but not formalin, increases the excitability of the nociceptive flexor reflex in the rat

The effects of subcutaneous (s.c.) administration of formalin or carrageenan to the rat hindpaw on the ipsilateral nociceptive flexor reflex was examined in decerebrate, spinalized, unanesthetized rats in order to assess the development of spinal hyperexcitability following peripheral inflammation. Carrageenan induced a weak and brief EMG discharge at the time of injection, followed by a gradual and delayed increase in flexor reflex magnitude, becoming significant 120 min postinjection. In addition, carrageenan induced a decrease in response threshold to peripheral mechanical stimulation as early as 30 min postinjection. Formalin induced an immediate and intense motor discharge which usually decreased after 5-10 min and was followed by a second phase of relatively weak motor discharge lasting for 20-70 min. However, the effect of s.c. formalin on the excitability of the flexor reflex was variable with either facilitation or inhibition. The flexor reflex was significantly depressed after formalin from 90 min post-injection. It is suggested that carrageenan induced prolonged spinal sensitization to nociceptive input, possibly through the induction of peripheral inflammation. In contrast, although formalin generated afferent input and evoked motor discharges, it did not induce significant spinal hyperexcitability. Thus, carrageenan, but not formalin, is useful to study central sensitization following peripheral chemical stimulation and/or inflammation.

The effects of intrathecal galanin message-associated peptide (GMAP) on the flexor reflex in rats

We have examined the effects of intrathecal (i.t.) galanin message-associated peptide (GMAP), the C-terminal flanking peptide in the galanin (GAL) precursor protein, which is produced in equimolar quantities with galanin and which is upregulated upon axotomy, on the spinal nociceptive flexor reflex in decerebrate, spinalized, unanesthetized rats. I.t. GMAP elicited a moderate facilitation of the flexor reflex. No depression of baseline flexor reflex was observed with any dose of GMAP. The facilitation of the flexor reflex induced by conditioning stimulation (CS) of cutaneous C-afferents was dose-dependently blocked by GMAP. The reflex facilitatory effect of exogenously applied substance P (SP), one of the endogenous modulators of reflex hyperexicitability following C-fiber CS, was only blocked by GMAP at a relatively high dose. I.t. GMAP did not antagonize the reflex facilitatory effect of vasoactive intestinal peptide and did not potentiate the reflex depressive effect of i.t. morphine or clonidine. Finally, 1 micrograms i.t. GMAP did not influence spinal cord blood flow whereas 10 micrograms GMAP induced a transient decrease in spinal cord blood flow in some experiments. The ability of GMAP to block the increase in spinal cord excitability following repetitive C-fiber stimulation may be through a presynaptic action. Although some of the effects of GMAP were similar to galanin, distinct differences were found, particularly in interaction with other excitatory and inhibitory agents. It is possible that GMAP exerts its action in the spinal cord through its own specific receptor. GMAP may act similarly to GAL in some, but not all pharmacological functions.(ABSTRACT TRUNCATED AT 250 WORDS)

Peripheral axotomy increases the expression of galanin message-associated peptide (GMAP) in dorsal root ganglion cells and alters the effects of intrathecal GMAP on the flexor reflex in the rat

We have previously reported that galanin message-associated peptide (GMAP), a fragment of galanin precursor protein, occurs in a limited number of dorsal root ganglion (DRG) cells in rats with intact sciatic nerves. In the present study, the localization of GMAP in dorsal root ganglia, dorsal roots and dorsal horn was analyzed immunohistochemically and compared between rats with intact and sectioned sciatic nerves. Furthermore, the effects of intrathecal (i.t.) GMAP on the flexor reflex in rats with intact and sectioned nerves were examined. In rats with intact sciatic nerves, i.t. GMAP elicited a moderate facilitation of the flexor reflex. The facilitation of the flexor reflex induced by conditioning stimulation (CS) of cutaneous C-fibers was strongly blocked by GMAP. GMAP also selectively antagonized the reflex facilitatory effect of i.t. substance P (SP), but not i.t. vasoactive intestinal peptide (VIP). Unilateral sciatic nerve section induced an upregulation of GMAP in the ipsilateral dorsal root ganglia 2 weeks after axotomy. The effect of GMAP on the baseline reflex was similar in normal and axotomized rats, but the blocking effect of GMAP on C-fiber CS-induced facilitation was significantly reduced after axotomy. GMAP did not antagonize the reflex facilitatory effect of SP after axotomy, whereas an antagonism on VIP-induced facilitation was observed. The possible role of GMAP in spinal transmission and comparison with the effects of galanin are discussed.

Differential influence of nerve growth factor on neuropeptide expression in vivo: a novel role in peptide suppression in adult sensory neurons

In this study the actions of NGF in regulating peptide expression were examined in vivo in adult rat primary sensory neurons. The hypothesis that NGF might tonically inhibit expression of some peptides was tested specifically. In situ hybridization and immunohistochemistry were used to detect presence or absence of alpha-CGRP, beta-CGRP, SP, SOM, VIP, CCK, NPY, and GAL as well as their mRNAs. In neurons in normal lumbar DRG alpha-CGRP, beta-CGRP, SP, and SOM are abundantly and heterogeneously expressed whereas few neurons have detectable VIP, CCK, NPY, or GAL. Two weeks following sciatic nerve transection, concentrations of alpha-CGRP, beta-CGRP, SP, and SOM plus their mRNAs have decreased to background in all but a few neurons. In contrast, VIP, CCK, NPY, and GAL are now synthesized in many neurons. Delayed intrathecal infusion of NGF (125 ng/microliter/hr) for 7 d, starting 2 weeks after injury counteracted the decrease in expression of alpha-CGRP, beta-CGRP and SP expression, but not SOM. This lack of influence of NGF on SOM is consistent with the absence of high-affinity NGF receptors and trk mRNA in SOM-positive neurons. Delayed infusion of NGF also reduced the number of neurons expressing VIP, CCK, NPY, and GAL after injury by approximately one-half in each subpopulation. Therefore, we suggest that NGF suppresses expression of these four peptides but only if the neurons also have NGF receptors. The results show that NGF can regulate peptide expression differentially and may also be part of the signal that allows reversion to normal of responses to injury as axons regenerate.

The effects of pretreatment with tachykinin antagonists and galanin on the development of spinal cord hyperexcitability following sciatic nerve section in the rat

The effects of acute section of the sciatic nerve on the excitability of the flexor reflex was examined in decerebrate, spinalized, unanaesthetized rats. In control experiments without drugs, the excitability of the flexor reflex was dramatically increased in two phases following axotomy. An early intense, brief reflex hyperexcitability was followed by a less intense, prolonged period of facilitation. The selective NK1 tachykinin receptor antagonist CP-96,345 injected intrathecally at lower (1.2-2.4 nmol) and higher (12 nmol) doses blocked both components of spinal sensitization. The selective NK2 tachykinin receptor antagonist Men 10376 at a dose of 2.4 nmol also reduced both response components, as did the same dose of the inhibitory neuropeptide galanin. Thus, antagonists of excitatory neuropeptides released during and after nerve section, such as substance P and neurokinin A, can block the spinal response to peripheral nerve injury. Furthermore, the inhibitory neuropeptide galanin also reduced spinal cord sensitization.