Spinal Nociceptive Flexor Reflex Research Articles

Learned control over spinal nociception reduces supraspinal nociception as quantified by late somatosensory evoked potentials.

We have recently shown that subjects can learn to use cognitive-emotional strategies to suppress their spinal nociceptive flexor reflex (RIII reflex) under visual RIII feedback and proposed that this reflects learned activation of descending pain inhibition. Here, we investigated whether learned RIII suppression also affects supraspinal nociception and whether previous relaxation training increases success. Subjects were trained over 3 sessions to reduce their RIII size by self-selected cognitive-emotional strategies. Two groups received true RIII feedback (with or without previous relaxation training) and a sham group received false feedback (15 subjects per group). RIII reflexes, late somatosensory evoked potentials (SEPs), and F-waves were recorded and pain intensity ratings collected. Both true feedback groups achieved significant (P < 0.01) but similar RIII suppression (to 79% ± 21% and 70% ± 17% of control). Somatosensory evoked potential amplitude (100-150 milliseconds after stimulation) was reduced in parallel with the RIII size (r = 0.57, P < 0.01). In the sham group, neither RIII size nor SEP amplitude was significantly reduced during feedback training. Pain intensity was significantly reduced in all 3 groups and also correlated with RIII reduction (r = 0.44, P < 0.01). F-wave parameters were not affected during RIII suppression. The present results show that learned RIII suppression also affects supraspinal nociception as quantified by SEPs, although effects on pain ratings were less clear. Lower motor neuron excitability as quantified by F-waves was not affected. Previous relaxation training did not significantly improve RIII feedback training success.

Analgesic effects of dyspnoea: “Air hunger” does not inhibit the spinal nociception reflex in humans

Dyspnoea has distinct sensory modalities, including air hunger and the sensation of excessive breathing “work/effort”. Both have analgesic properties. In the case of work/effort, spinal mechanisms have been documented (inhibitory effect on the spinal nociceptive flexor reflex, RIII). This mechanism involves C-fibres. As C-fibres are unlikely to play a major role in air hunger, we hypothesised that inducing this type of dyspnoea would not result in RIII inhibition. Eight healthy volunteers were exposed to a hypercapnic hyperoxic gas mixture (5% CO2 and 95% O2) and asked to voluntarily fight the corresponding ventilatory reflex response by reducing tidal volume below its spontaneous level. Ventilatory variables and dyspnoea intensity (ordinal scale) were measured. Electromyography of the biceps femoris was used to record the amplitude of RIII in response to painful electrical sural nerve stimulation. Air hunger failed to inhibit the RIII reflex. We conclude that the mechanisms of air hunger induced analgesia do not include a spinal contribution and are therefore mostly central.

Effects of intrathecal nocistatin on the flexor reflex and its interaction with orphanin FQ nociceptin.

We studied the effects of intrathecal (i.t.) nocistatin, a peptide identified from the precursor of orphanin FQ/nociceptin (OFQ) on the spinal nociceptive flexor reflex in decerebrate, spinalized, unanesthetized rats and its interaction with i.t. OFQ. Nocistatin induced a moderate, non-dose-dependent facilitation of the flexor reflex without producing reflex depression whereas i.t. OFQ induced a biphasic dose-dependent facilitatory and inhibitory effect. The facilitatory effect of low dose (0.55 pmol) OFQ was significantly increased by nocistatin. On the other hand, the duration, but not magnitude, of reflex depression induced by a high (550 pmol) dose of OFQ was significantly shortened by 5.5 nmol nocistatin. Thus, nocistatin interacts with OFQ in a complex fashion, increasing excitation and reducing inhibition. No evidence was obtained for an antinociceptive effect of nocistatin in rat spinal cord.

Interferon-gamma receptors are expressed at synapses in the rat superficial dorsal horn and lateral spinal nucleus.

Interferon-gamma can facilitate the spinal nociceptive flexor reflex and may elicit neuropathic pain-related behavior in rats and mice. Immunoreactivity for the interferon-gamma receptor (IFN-gamma R) occurs in the superficial layers of the dorsal horn and the lateral spinal nucleus in the rat and mouse spinal cord, as well as in subsets of neurons in the dorsal root ganglia. The aim of the present study was to examine the cellular localization and origin of the IFN-gamma R in the spinal cord. As viewed by confocal microscopy, the immunopositivity for the IFN-gamma R was co-localized with that of the presynaptic marker synaptophysin and with neuronal nitric oxide synthase in the lateral spinal nucleus, whereas only a minor overlap with these molecules was observed in laminae I and II of the dorsal horn. There was no co-localization of the IFN-gamma R with markers for astrocytes and microglial cells. Ultrastructurally, the IFN-gamma R was found predominantly in axon terminals in the lateral spinal nucleus, but at postsynaptic sites in dendrites in laminae I and II. The IFN-gamma R expressed in neurons in dorsal root ganglia was transported in axons both centrally and peripherally. Hemisection of the spinal cord caused no reduction in immunolabelling of the IFN-gamma R in the dorsal horn or the lateral spinal nucleus. Since rhizotomy does not affect the immunolabelling in the lateral spinal nucleus, our observation indicates that the presynaptic receptors in this nucleus are derived from intrinsic neurons. The localization of the IFN-gamma R in the spinal cord differed from that of the AMPA glutamate receptor subunits 2 and 3 and the substance P receptor (NK1). Our results, showing localization of IFN-gamma R to pre- and postsynaptic sites in the dorsal horn and lateral spinal nucleus indicate that IFN-gamma can modulate nociception at the spinal cord level.

The effect of naturally occurring fragments of galanin message-associated peptide on spinal cord excitability in rats

Galanin message-associated peptide (GMAP), a 60 amino acid fragment of galanin precursor protein, is present in dorsal root ganglion cells and upon intrathecal (i.t.) administration influences the spinal nociceptive flexor reflex in rat in a complex manner. The present study assessed the effects on spinal cord excitability of N-terminal fragment GMAP (1–30) and C-terminal fragments GMAP (34–60) or GMAP (35–60), which were formed from GMAP following enzymatic degradation. The effect of the fragments was compared with the effects of the complete peptide sequence. The GMAP fragments slightly facilitated the flexor reflex and dose-dependently blocked hyperexcitability following C-fiber conditioning stimulation. The potency of the blocking effect of GMAP(1–30) was comparable to GMAP(1–60) and was one order of magnitude higher than the potency of the C-terminal fragments. The results indicated that both naturally formed N- and C-terminal fragments of GMAP are pharmacologically active and produce effects which are similar to the full peptide sequence.

Fragments of galanin message-associated peptide (GMAP) modulate the spinal flexor reflex in rat

We have previously reported that galanin message-associated peptide (GMAP), a fragment of galanin precursor protein, is present in dorsal root ganglion cells and upon intrathecal (i.t.) administration influences the spinal nociceptive flexor reflex in a complex manner in the rat. GMAP elicited a moderate facilitation of the flexor reflex, but when administered prior to conditioning stimulation of C-afferents, it dose dependently blocked spinal cord hyperexcitability. The present study examined the effects of four fragments of GMAP-(1-60), GMAP-(1–12), GMAP-(10–24), GMAP-(25–44) and GMAP-(37–60), on the flexor reflex and compared them with the effects of the complete peptide sequence. All four GMAP fragments facilitated the flexor reflex. However, this effect was dose-dependent only for GMAP-(1–12) and the effect of GMAP-(1–12) was stronger than GMAP-(1–60). In contrast, only GMAP-(25–44) dose dependently blocked the facilitation of the flexor reflex induced by the C-fiber conditioning stimulation. The potency of the blocking effect of GMAP-(25–44) was one order of magnitude lower than that of GMAP-(1–60). The results indicated that two fragments of GMAP are pharmacologically active and produce effects which are similar to the full sequence. It is possible that the complex effect of GMAP may be mediated by different subtypes of GMAP receptors which recognize different portions of the GMAP sequence.

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.

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 hyperexcitability 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 μg i.t. GMAP did not influence spinal cord blood flow whereas 10 μg 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. The present results could thus represent another example where different peptides arising from the same peptide precursor produce differential pharmacological actions.

The effects of intrathecal neuropeptide Y on the spinal nociceptive flexor reflex in rats with intact sciatic nerves and after peripheral axotomy

We examined the effects of intrathecally administered neuropeptide Y on the spinal nociceptive flexor reflex in decerebrate, spinalized, unanesthetized rats with intact sciatic nerves, or 11–39 days after unilateral transection of the sciatic nerve. In rats with intact sciatic nerve, intrathecal neuropeptide Y at low doses (10 and 100ng) caused a brief facilitation of the flexor reflex. At a dose of 300 ng, the effect of neuropeptide Y on the flexor reflex was biphasic i.e. a brief facilitation followed by slight depression. At higher doses (1 and 10 μg), the effect of neuropeptide Y was mainly inhibitory, causing substantial and usually prolonged depression of the flexor reflex magnitude. The reflex depression caused by intrathecal neuropeptide Y was not reversed by the opioid antagonist naloxone or theα 2 adrenoceptor antagonist atipamezole. Intrathecal neuropeptide Y at doses up to 1 and 10μg had no effect on reflex facilitation caused by conditioning stimulation of C-fibers, intrathecal substance P or neurokinin A. Topical application of neuropeptide Y (1 μg/μl) failed to influence the monosynaptic reflex in normal rats. Eleven to 16 days after peripheral axotomy, the initial excitation of the flexor reflex to intrathecal neuropeptide Y was significantly enhanced in axotomized compared with normal rats. However, the depressive effect of neuropeptide Y on the flexor reflex was unchanged. Neuropeptide Y did not influence the monosynaptic reflex in axotomized rats at this period. In experiments performed on rats in which the sciatic nerve had been transected 31–39 days previously, the facilitatory effect of neuropeptide Y on the flexor reflex remained enhanced compared with normal rats. Furthermore, the inhibitory effect of neuropeptide Y also increased as 100 ng intrathecal neuropeptide Y was able to produce reflex depression in a similar fashion as 300 ng neuropeptide Y normally and the reflex depression caused by 1 μg neuropeptide Y was stronger and longer lasting than in normal rats. Intrathecal neuropeptide Y (100 ng-10 μg) in rats with intact sciatic nerves caused a moderate decrease in spinal cord dorsal surface blood flow as measured with a laser Doppler flowmeter. This effect of neuropeptide Y was unchanged in axotomized rats. The present results support previous observations that spinal application of neuropeptide Y in normal rats caused antinociception. As the depressive effect of neuropeptide Y is independent of spinal opioid andα 2-adrenergic systems, it may be mediated by its own receptors. The biphasic effect of neuropeptide Y may reflect its action on different subtypes of neuropeptide Y receptors. Experiments with neuropeptide Y receptor antagonists are required to further define the role of endogenous neuropeptide Y in nociception normally and after peripheral axotomy.

Intrathecal interferon-γ facilitates the spinal nociceptive flexor reflex in the rat

The effect of intrathecal (i.t.) injection of the cytokine interferon-γ (IFN-γ) on the spinal nociceptive flexor reflex was examined in decerebrate, spinalized, unanesthetized rats. IFN-γ elicited an initial intense, brief facilitation of the flexor reflex followed by a sustained reflex facilitation lasting 40 ± 5 min (range 20–65 min). The initial and prolonged reflex facilitations by IFN-γ were partially and totally blocked, respectively, by i.t. pretreatment with nitro- l-arginine-ester, an inhibitor of nitric oxide synthase, at doses which did not influence spinal cord blood flow. Spinal application of IFN-γ produced powerful and prolonged facilitation of the flexor reflex, possibly reflecting a hyperalgesic action of this cytokine. The facilitatory effect of IFN-γ was mediated, at least in part, by the activation of the l-arginine-nitric oxide pathway. Thus, IFN-γ released in the CNS may participate in eliciting pain and hyperalgesia in infectious or neuroinflammatory diseases where there is increased production of this cytokine.

Intrathecal oxytocin facilitates the spinal nociceptive flexor reflex in the rat.

We examined the effects of intrathecal oxytocin over a wide dose range (10 ng to 10 micrograms) on the spinal nociceptive flexor reflex in decerebrate, spinalized, unanaesthetized rats. Oxytocin facilitated the flexor reflex at all doses. The duration, but not magnitude, of facilitation was dose-related. No reflex depression was observed with intrathecal oxytocin at any dose. It is suggested that activation of spinal oxytocin receptors excites the spinal cord and therefore intrathecal oxytocin is unlikely to be an analgesic agent.

Neither cholecystokinin nor galanin modulate intrathecal clonidine-induced depression of the nociceptive flexor reflex in the rat

Previous studies have established that the antinociceptive effect of morphine was subjected to peptidergic modulation at spinal level. Intrathecal (i.t.) galanin (GAL) potentiated morphine-induced analgesia, whereas i.t. cholecystokinin (CCK) antagonized morphine's hypoalgesic effect. In the present study, we examined the possible interaction between GAL, CCK and clonidine, an α 2 adrenoceptor agonist and potent spinal antinociceptive agent, in the spinal nociceptive flexor reflex. I.t. clonidine dose-dependently depressed the flexor reflex similarly to i.t. morphine However, unlike morphine, the reflex depressive effect of i.t. clonidine was neither potentiated by i.t. GAL nor blocked by i.t. CCK. The present results suggested that the analgesia elicited by activation of spinal α 2 adrenoceptors is not subjected to the modulatory effect of CCK and GAL and therefore may be mediated through different mechanisms than opioids.

The depressive effect of intrathecal clonidine on the spinal flexor reflex is enhanced after sciatic nerve section in rats

The effect of intrathecal (i.t.) α 2-adrenoceptor agonist, clonidine, on the spinal nociceptive flexor reflex was studied in decerebrate, spinalized, unanesthetized rats with intact sciatic nerves or in rats in which the sciatic nerve had been ipsilaterally sectioned. In rats with intact nerves i.t. clonidine caused a dose-dependent biphasic effect on flexor reflex excitability. At low dose (10 ng) the effect of clonidine was purely facilitatory, whereas with 50–100 ng clonidine the initial facilitation was often followed by reflex depression. Long-lasting, strong reflex depression was observed after i.t. injection of high doses of clonidine (1 and 10 μg). Four to 18 days after sciatic nerve section, the depressive effect of clonidine on the flexor reflex was dramatically enhanced. Depression was frequently observed already with doses of 5 and 10 ng, and maximal depression was reached at 100 ng and 1 μg in axotomized rats. The facilitatory effect of low doses of clonidine on the reflex was also observed, although somewhat less frequently than in normals. The depressive effect of clonidine on the flexor reflex was reversed by the selective α 2-receptor antagonist, atipamezole (20 μg i.t.), in rats with both intact and sectioned sciatic nerves. The present results revealed an increased sensitivity and effectiveness of the depression of spinal reflex mechanisms by i.t. clonidine after sciatic nerve section, which is opposite to the decreased sensitivity to i.t. morphine after axotomy that we observed previously. Since the flexor reflex is evoked by the activation of afférents that may be the source of neuropathic pain after nerve lesion, the results support the usefulness of i.t. clonidine in treating pain associated with peripheral nerve injury.

Intrathecal CP-96,345 blocks reflex facilitation induced in rats by substance P and C-fiber-conditioning stimulation

We have examined the effects of intrathecally (i.t.) administered CP-96,345, a non-peptide NK 1 receptor ligand, on the spinal nociceptive flexor reflex and on the facilitation of this reflex evoked by i.t. substance P (SP), neurokinin A (NKA) and electrical conditioning stimulation of cutaneous C-afferents. CP-96,345 i.t. at 24 pmol-2.4 nmol had no significant effect on flexor reflex excitability. At the highest dose tested (24 nmol), CP-96,345 caused a brief facilitation of the flexor reflex, which was similar to the effect of the vehicle used at this drug concentration. CP-96,345 did not depress the flexor reflex at any dose. In rats with chronically implanted i.t. catheters, CP-96,345 at 24 nmol caused neither motor impairment nor morphological damage to the spinal cord. Pretreatment with CP-96,345 dose dependently and similarly antagonized facilitation of the flexor reflex induced by 7 pmol i.t. SP or by a 20-s, 1-Hz conditioning stimulus train applied to cutaneous C-fibers in the sural nerve innervation area. The vehicle had no effect. The antagonistic effect of CP-96,345 on the SP- and C-fiber reflex facilitation induced by conditioning stimulation became maximal only 20–30 min after the i.t. injection and lasted 3–4 h at the highest dose. CP-96,345 did not significantly block the facilitatory effect of 7 pmol i.t. NKA on the flexor reflex. These results demonstrate that CP-96,345 is a potent, long-lasting and selective antagonist of SP in rat spinal cord. Furthermore, facilitation of the flexor reflex (central sensitization) induced by conditioning stimulation of cutaneous C-afferents is mediated by NK 1 tachykinin receptors, but the NK 1 receptor may not be involved in the transmission of the flexor reflex. CP-96,345 is thus useful in experimental studies of the role of SP in the central nervous system.

Intrathecal neurokinin A facilitates the spinal nociceptive flexor reflex evoked by thermal and mechanical stimuli and synergistically interacts with substance P.

The neuropeptide neurokinin A was injected intrathecally and its effect on the spinal nociceptive flexor reflex was examined. The reflex, which was evoked by electrical, thermal or mechanical stimulation of the foot and was recorded from the ipsilateral hamstring muscles, was substantially facilitated by 7 pmol intrathecally injected neurokinin A. The facilitatory effect of neurokinin A to thermal stimulation was, however, significantly stronger than to electrical or mechanical stimuli. Furthermore, co-administration of neurokinin A with substance P induced a significant synergistic facilitation of the reflex. It is suggested that neurokinin A, like substance P, may be released in association with activation of polymodal C-nociceptors.

The threshold for the depressive effect of intrathecal morphine on the spinal nociceptive flexor reflex is increased during autotomy after sciatic nerve section in rats

The effect of intrathecal (i.t.) morphine on the spinal nociceptive flexor reflex in doses ranging between 10 ng and 10 μg was studied in decerebrate, spinalized, unanesthetized rats with intact sciatic nerves or in rats in which the sciatic nerve had been unilaterally sectioned. In rats with intact nerves the initial effect of i.t. morphine on the flexor reflex was a brief facilitation followed by depression. The threshold dose of morphine for reflex depression was 100 ng. In animals which did not develop autotomy after nerve section or in which autotomy had ceased for several days prior to the acute experiments i.t. morphine had a similar depressive effect on the flexor reflex as in animals with intact nerves. However, in rats which were autotomizing at the time of the acute experiment, the threshold dose of the depressive effect of morphine was increased 3–5 fold. With higher doses of morphine (1–3 μg), similar depression of the reflex was found in all groups. The present results revealed a decreased sensitivity of spinal reflex mechanisms to low, but not high, doses of morphine after sciatic nerve section accompanied by autotomy. Nerve section per se did not alter opioid sensitivity. Thus, decreased effectiveness of morphine in this model for neuropathic pain may be partially due to a desensitization to the analgesic action of opioids in the spinal cord. Since after sciatic nerve section there is a differential sensitivity to the antinociceptive effect of i.t. morphine between autotomizing and non-autotomizing rats, it is further suggested that autotomy after peripheral nerve section in rats is a useful model for the study of neuropathic pain.

Studies on the effect of systemic PD134308 (CAM 958) in spinal reflex and pain models with special reference to interaction with morphine and intrathecal galanin

The effect of intravenous (i.v.) PD134308, which is a CCK-B antagonist, morphine and intrathecal (i.t.) galanin (GAL) on the excitability of the spinal nociceptive flexor reflex and in the hot plate test was examined in rats. PD134308 (1 mg/kg, i.v.) caused a weak, naloxone-reversible depression of the flexor reflex and moderate antinociception in the hot plate test. PD134308 significantly potentiated the antinociceptive effect of morphone, as well as its depressive effect on the flexor reflex. PD134308 and i.t. GAL synergistically depressed the flexor reflex, which was reversed by naloxone. Finally, the magnitude and duration of the depression of the flexor reflex by morphine was synergistically increased by coadministering i.v. PD134308 and i.t. GAL. The results demonstrate that a CCK antagonist directed to the central CCK-B receptor potentiates the analgesic effects of opioid and non-opioid drugs at spinal level in the rat, thus supporting the notion that CCK in the CNS may be an endogenous, physiological opioid antagonist.

On the role of NK-2 tachykinin receptors in the mediation of spinal reflex excitability in the rat

The effects of intrathecal administration of neurokinin A, substance P and [Tyr 5, d-Trp, 6,8,9 Arg 10]neurokinin A-(4–10) (Men 10207), a specific NK-2 receptor antagonist, on the spinal nociceptive flexor reflex were studied in decerebrate, spinalized, unanesthetized rats. Intrathecal neurokinin A and substance P facilitate the flexor reflex in a similar manner. The reflex facilitation to intrathecal neurokinin A, but not substance P, is dose-dependently blocked by pretreatment with Men 10207. The NK-2 receptor antagonist by itself facilitates the flexor reflex with a potency about 10 times less than that of neurokinin A, indicating a partial agonistic property. Reversible depression of the flexor reflex, which is not due to nonspecific spinal blockade, is observed after 700 pmol Men 10207. Further increasing the dose of Men 10207 to 7 nmol totally and irreversibly blocks the flexor reflex. Conditioning stimulation of peripheral nerves at 1 Hz for 20 s at an intensity that activates unmyelinated (C) fibers facilitates the ipsilateral flexor reflex. The duration of the facilitation after conditioning stimulation to the cutaneous sural nerve is several minutes and about 1 h after conditioning stimulation of the gastrocnemius muscle nerves. Pretreatment with Men 10207 (70–700 pmol) has no effect on facilitation by the sural nerve conditioning stimulation, but effectively blocks the long-term reflex facilitation to the gastrocnemius nerve stimulation. The present results indicate a distinct role for NK-2 tachykinin receptors in mediation of spinal reflex excitability in the rat. Neurokinin A may be involved in the long-term increase of spinal reflex excitability after activation of unmyelinated fibers innervating muscle.

Depression off the flexor reflex by systemic morphine increases in chronically spinalized rats

The effect of i.V. morphine on the spinal nociceptive flexor reflex was examined in decerebrate, unanesthetized rats in which the spinal cord was acutely or chronically transected. In acutely spinalized rats i.v. morphine dose dependently depressed the flexor reflex with an ED 50 of 1.4 mg/kg. In rats in which the spinal cord was transected 1–3 days prior to the electrophysiological experiments, i.v. morphine had a similar depressive effect ( ED 50 =1.1 mg/kg). However, 4–6 days after spinalization there was a 4-fold increase in morphine's depressive effect ( ED 50 = 0.35 mg/kg). The sensitivity to morphine was further increased 7–10 days post-spinalization with ED 50 of 0.17 mg/kg. This supersensitivity was maintained up to 30–60 days. The reflex depression caused by morphine in both acutely and chronically spinalized rats could be reversed by naloxone. Morphine i.v. did not influence the monosynaptic reflex in either acutely or chronically spinalized rats. The present results support the clinical finding that patients with spinal lesions have an increased sensitivity to the antinociceptive and reflex depressive effects of systemic and intrathecal morphine.

On the Role of Galanin, Substance P and Other Neuropeptides in Primary Sensory Neurons of the Rat: Studies on Spinal Reflex Excitability and Peripheral Axotomy.

The interaction of intrathecally (i.t.) applied galanin (GAL) with substance P (SP), calcitonin gene-related peptide (CGRP), vasoactive intestinal polypeptide (VIP), somatostatin (SOM) and C-fibre conditioning stimulation (CS) with regard to their effects on the spinal nociceptive flexor reflex was studied in decerebrate, spinalized, unanaesthetized rats with intact or sectioned sciatic nerves. SP, CGRP, VIP and SOM applied onto the surface of lumbar spinal cord or a brief CS train (1 Hz, 20 s) to the sural nerve facilitated the flexor reflex for several minutes in animals with intact or sectioned nerves. Pretreatment with GAL, which by itself had a biphasic effect on the flexor reflex in a dose-dependent manner, antagonized the reflex facilitation induced by sural CS before and after sciatic nerve section. SP-induced facilitation of the flexor reflex was antagonized by GAL in rats with intact sciatic nerves, but not after nerve section. In contrast, VIP-induced reflex facilitation was antagonized by GAL only after sectioning of the sciatic nerve. GAL was effective in antagonizing the facilitatory effect of CGRP under both situations, but had no effect on SOM-induced facilitation. A parallel immunohistochemical study revealed that after sciatic nerve section GAL-like immunoreactivity (LI) and VIP-LI are increased in the dorsal root ganglia and that these two peptides coexist in many cells. The present results indicate that GAL antagonizes the excitatory effect of some neuropeptides which exist in the spinal cord. This antagonism could explain the inhibitory effect of GAL on C-fibre CS-induced facilitation of the flexor reflex, which is presumably due to the release of some of these neuropeptides from the terminals of primary afferents. Furthermore, the interaction between GAL and other neuropeptides is altered by sciatic nerve section, paralleling changes in the levels of these neuropeptides in primary afferents and their pattern of coexistence after nerve section. It is proposed that SP and CGRP are important mediators of the spinal flexor reflex in intact rats. However, after axotomy VIP may replace SP in this capacity, paralleling the decrease in SP and marked increase in VIP levels. In general the study provides further support for involvement of peptides in sensory function.