The response of the 5-hydroxyindole oxidation current to noxious stimuli in the spinal cord of anesthetized rats: modification by morphine.

The effects of cutaneous noxious heating and of systemic morphine on serotonergic activity in the spinal cord were examined in anesthetized rats. An oxidation current of 5-hydroxyindole signal was seen at 280-300 mV with differential normal pulse voltammetry. Noxious heat stimuli produced a mean signal increase over control values of 15.5 +/- 3.4% at 52 degrees C, and 7.2 +/- 5.5% at 45 degrees C. These increases lasted for 5-10 min. Non-noxious stimuli (37 degrees C) did not affect the 5-hydroxyindole signal. Morphine (0.5, 2.0 and 5.0 mg/kg, i.p.) in the absence of cutaneous stimulation did not change the signal significantly. Systemic morphine alone did not significantly modify the 5-hydroxytryptamine (5-HT) metabolism, as observed in in vivo voltammetry, in the spinal cord of anesthetized rat. However, a low dose of morphine (0.5 mg/kg, i.p.) attenuated the increase in the signal modified by noxious stimuli, and high doses (2.0 or 5.0 mg/kg, i.p.) enhanced it. Both effects of morphine were antagonized by naloxone (0.5 mg/kg, i.v.). It is likely that morphine with noxious stimuli modify the sensitivity of serotonergic descending inhibitory system. It is concluded that noxious heating of the skin increases the 5-HT metabolism in the spinal cord of anesthetized rats and that systemic administration of morphine modulates this 5-HT metabolism.     

In vivo electrochemical detection of 5-hydroxyindole within the trigeminal nucleus caudalis of freely moving rats: the effect of morphine

The trigeminal nucleus caudalis is considered the equivalent of the orofacial nociceptive system of the dorsal horn of the spinal cord. At the level of this trigeminal area (i.e. medullary dorsal horn) the transmission of noxious inputs is strongly modulated by a descending, serotonergic system mainly originating from the nucleus raphe magnus (NRM). The present study in freely moving animals reports the effect of morphine on the 5-hydroxyindole oxidation current recorded in the medullary dorsal horn. Complementary data from recordings in spinal dorsal horn in acutely anesthetized rats are also presented. A current recorded at 270–290 mV (peak ‘3’), characteristic of 5-hydroxyindoleacetic acid (5-HIAA), was measured with treated multi-fiber carbon electrodes, using differential pulse voltammetry (DPV) or differential normal pulse voltammetry (DNPV). In control rats, the amplitude of the peak remained constant for many hours. Morphine (10 mg/kg i.p.) caused a significant increase which plateaued between 35 and 80 min (mean increase: 127 ± 5 % of control values); recovery was complete by about 3 h. Simultaneous injection of naloxone (1 mg/kg i.p.) totally abolished the effect of morphine. By contrast, morphine was without effect on peak 3 recorded in the spinal dorsal horn of chloral hydrate (450 mg/kg i.p.) anesthetized rats. It is concluded that in non-anesthetized freely moving animals morphine clearly increases the metabolism of serotonin (5-HT) in the medullary dorsal horn. This finding confirms previous neurochemical data showing an increased synthesis or release of 5-HT in the spinal cord after systemic morphine or its microinjection into either the periaqueductal gray matter or the NRM, and underlines the value of in vivo electrochemistry in monitoring changes in 5-HT metabolism directly and continuously during various physiological and pharmacological procedures.     

Effects of tianeptine on 5-hydroxyindoles and on the morphine-induced increase in 5-HT metabolism at the medullary dorsal horn level as measured by in vivo voltammetry in freely moving rats

The present study, by the use of in vivo electrochemical detection of 5-hydroxyindole (peak ‘3’) in the bulbo spinal serotonergic system at the medullary dorsal horn (MDH) level, investigated the effects of the new tricyclic antidepressant (TCA) tianeptine, which has been shown to be a specific serotonin (5-HT) uptake enhancer. It was found that acutely administered tianeptine (10 mg/kg, i.p.) induced a marked significant increase in peak 3 within the dorsal horn, an in vivo observation which is in accordance with the biochemical properties of tianeptine as studied in forebrain structures. In addition, the effect of tianeptine on the morphine-induced increase in 5-HT metabolism was investigated, by comparison with the previous data obtained with the specific 5-HT uptake inhibitor femoxetine in the MDH. It was shown that tianeptine can display additive effect with morphine (10 mg/kg, i.p.) on 5-HT metabolism at the MDH level. These results are discussed in relation to the effects of classical TCAs and the particular properties of tianeptine.     

In vivo electrochemical evidence that the tricyclic antidepressant femoxetine potentiates the morphine-induced increase in 5-HT metabolism in the medullary dorsal horn of freely moving rats

Acute administration of tricyclic antidepressants (TCAs) is known to potentiate morphine antinociception. At the medullary dorsal horn (MDH) level systemic morphine has been shown to increase serotonin (5-HT) metabolism as measured by in vivo electrochemistry in freely moving rats. Using similar electrochemical detection of 5-hydroxyindole (peak '3') within the MDH, the present study investigated the effect of the specific 5-HT uptake inhibitor femoxetine on peak 3 and the effects of this TCA on changes in 5-HT metabolism induced by morphine. Acutely administered femoxetine (40 mg/kg i.p.) (i) induced a small but significant increase in peak 3 and (ii) strongly potentiated the effect of morphine (10 mg/kg i.p.) on 5-HT metabolism, this potentiation being opiate specific since simultaneous injection of naloxone (1 mg/kg i.p.) abolished the effect of morphine. These findings provide an in vivo neurochemical basis for the potentiation of morphine antinociception by TCAs. They further emphasize the importance of 5-HT bulbospinal descending pathways in morphine antinociception.     

Role of the nucleus raphe magnus in opiate analgesia as studied by the microinjection technique in the rat

The analgesic effects of morphine (5 μg, 0.2 μl) microinjected into the nucleus raphémagnus (NRM) and the surrounding reticular formation of the rat were tested using vocalization after electric shock to the tail as the test for analgesia. Only sites in the NRM produced powerful analgesic effects, strongest analgesia being equivalent to 3 mg/kg i.v. morphine. The analgesia produced by the microinjection was reversed by systemic naloxone. Pretreatment with systemic cinanserin, a blocker of serotonergic receptors, led to a pronounced diminution of the analgesic effects of the morphine. The effects of microinjection of naloxone (5 μg 0.2 μl) were studied for their effect on analgesia produced by systemic morphine. The analgesia following 3 mg/kg i.v. morphine was diminished by the microinjection of naloxone but the naloxone almost completely reversed the analgesic effects of 1.5 mg/kg i.v. morphine. These results further substantiate the role of the NRM in analgesic mechanisms.     

The effect of morphine on the potassium evoked release of tritiated 5-hydroxytryptamine from spinal cord slices in the rat

The effect of morphine on the potassium (40 mM) evoked release of exogenous [3H]5-HT from slices of the dorsal spinal cord of the rat was studied. The effects of in vitro applied morphine on the slices were compared to those produced by systemic morphine applied to the animals before preparation of the slices. The in vitro application of morphine (10(-6) to 10(-5) M) did not affect the release of [3H]5-HT. By contrast, it was observed that the potassium evoked release of [3H]5-HT from the slices of the spinal cord of rats which had received 10 mg/kg s.c. of morphine 30 min beforehand was significantly increased. The effect of systemic morphine was dose-dependent (in the range of 1.5-10 mg/kg s.c.) and could be blocked by prior administration of naloxone (1 mg/kg i.m.) 2 min before the morphine. The acute administration of 10 mg/kg s.c. of morphine, which did not induce analgesia in rats rendered tolerant to morphine, did not modify the [3H]5-HT release. Higher doses of morphine, which have been shown to restore analgesia in these rats, induced an increase in the release which was significant for a dose of 100 mg/kg s.c. These results demonstrating a specific and dose-dependent increase in the potassium evoked release of [3H]5-HT from spinal dorsal cord slices after systemic administration of morphine, emphasize the role of serotonergic systems in such analgesia. The lack of effect of the drug directly applied in vitro favours a supraspinal site of action of the drug and is in good agreement with recent results in the literature.     

Voltammetric study of 8-OH-DPAT effect on the raphe-trigeminal 5-HT system.

The effect of i.p. administration of the selective 5-HT1A agonist 8-hydroxy-2-(di-N-propylamino) tetralin (8-OH-DPAT) (100 micrograms kg-1) has been investigated by in vivo 5-hydroxyindole electrochemical (peak 3) detection in the nucleus raphe magnus (NRM) and medullary dorsal horn (MDH) of acute anaesthetized and unanaesthetized freely moving rats. 8-OH-DPAT induced a small but significant decrease in peak 3 in the NRM and MDH of anaesthetized rats. In freely moving animals, a similar small effect was observed at both NRM and MDH levels. With reference to similar in vivo studies demonstrating differential responsiveness of ascending serotonergic systems to 8-OH-DPAT, it is concluded that the serotonergic NRM-dorsal horn system is slightly affected by this 5-HT1A agonist.     

Development of tolerance to the antinociceptive effect of systemic morphine at the lumbar spinal cord level: a c-Fos study in the rat

The development of tolerance to the antinociceptive effects of morphine was investigated in rats using carrageenin-induced spinal c-Fos expression. We took advantage of this technique to especially study, at the cellular level, in freely moving animals, the development of tolerance based on the visualization of dorsal horn spinal cord neurons which play a major role in nociceptive processes. Two hours after intraplantar injection of carrageenin (6 mg/150 μl of saline), c-Fos-like immunoreactivity (FLI) was observed predominantly in the superficial and deep laminae of the dorsal horn in segments L4 and L5 of the spinal cord. In naive rats, acute intravenous morphine (3 mg/kg, i.v.) reduced the number of superficial and deep FLI neurons; 49% and 59% reduction respectively (p<0.0001 for both). In morphine-pretreated rats (daily administration of subcutaneous morphine: 1, 3, 5, 10, 20 or 40 mg/kg once a day for 4 days), antinociceptive tolerance tested on day 5 by acute morphine (3 mg/kg, i.v.) was manifest in those groups pretreated with the highest doses of morphine (10, 20 or 40 mg/kg). From regression analysis, it appeared that tolerance to the antinociceptive effect of morphine developed progressively as a function of the chronic morphine dose used on neurons involved in spinal nociceptive processes (superficial and deep dorsal horn neurons). Similarly, in rats pretreated with 10 mg/kg of morphine over 1, 2, 3 or 4 days, tolerance progressively developed, for both spinal neuronal populations, as a function of the duration of the pretreatment. These results are discussed in the context of the several possible sites of action of morphine.     

Involvement of 5-hydroxytryptamine-containing neurons in antinociception produced by injection of morphine into nucleus raphe magnus or onto spinal cord

We studied whether antinociception produced by injection of morphine into the nucleus raphe magnus (NRM) or superfusion onto the spinal cord involved serotonergic neurons that descend from brainstem to spinal cord. Involvement of 5-hydroxytryptamine (5-HT)-containing neurons was determined by correlating morphine-induced analgesia with an increase in turnover of 5-HT and by determining if depletion of cord 5-HT with the neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT) could attenuate the antinociceptive effects of morphine. When injected directly into the NRM, 10 micrograms of morphine produced profound analgesia as measured by the paw-pressure technique, and significantly increased the turnover of 5-HT in both posterior medulla and spinal cord. Depletion of cord 5-HT to less than 10% of control concentrations attenuated the antinociceptive effect of morphine injected into the NRM. When various concentrations of morphine (1, 10 or 50 micrograms) were injected directly into the spinal subarachnoid space, a dose-dependent analgesia was observed. No change in 5-HT turnover in spinal cord was observed with any dose of morphine superfused onto the cord. In addition, depletion of cord 5-HT with 5,7-DHT did not alter the analgesic response to either 1 or 10 micrograms of intrathecal morphine. These results suggest that although 5-HT-containing neurons descending from brainstem into spinal cord are involved with analgesia produced by morphine injection into the NRM, they are not involved in the analgesia induced by applying morphine directly to the cord.     

Morphine analgesia and newly synthesized 5-hydroxytryptamine in the dorsal and the ventral halves of the spinal cord of the rat

In the rat, morphine (5 mg/kg, s.c.) induced an increase in 5-hydroxytryptamine (5-HT) synthesis in the spinal cord. These effects appeared with a shorter latency and are much more marked in the dorsal half than in the ventral half. Although the increase in the dorsal half was slightly delayed by comparison with the onset of the analgesic effect the maxima for both phenomena were simultaneous. The role of the bulbo-spinal serotonergic system in morphine analgesia is discussed.     

Supraspinal and spinal mediation of naloxone-induced morphine withdrawal in rats

Unanesthetized rats, made physically dependent over 5 days by chronic intra-arterial infusion of increasing concentrations of morphine (35-100 mg/kg/day) underwent withdrawal by naloxone (6 micrograms) injection into either the lateral ventricle (i.c.v.), fourth ventricle (V4), intrathecal subarachnoid space (i.t.), or intra-arterially (i.a.) and were evaluated for cardiovascular and behavioral signs of precipitated abstinence. Naloxone i.c.v. produced a significantly greater increase in the magnitude and duration of withdrawal hypertension than did V4 injection. Naloxone i.t. produced a distinctively different, persistent, pressor response as compared to i.c.v., V4 or i.a. routes of administration, although no quantitative differences in behavioral signs of withdrawal were observed. Morphine-dependent, spinal transected (C1) animals generated an augmented pressor response to i.c.v. or i.t. naloxone. This pressor response was accompanied by a significant reduction in core temperature (0.50-0.79 degrees C). Both the naloxone-induced pressor and hypothermic responses were abolished by ganglionic (hexamethonium, 100 mg/kg, i.a.) or peripheral alpha-adrenergic (phentolamine 4 mg/kg, i.a.) blockade. The hypertensive and hypothermic effects of naloxone also were prevented in transected dependent rats by prior spinal pithing. We conclude that in morphine-dependent rats: supraspinal sites rostral to the V4 mediate a more intense naloxone-induced pressor response than caudal regions; cardiovascular and behavioral signs of withdrawal can be precipitated via the spinal cord of intact animals; and the production of withdrawal hypertension and hypothermia in spinal transected morphine-dependent rats indicates that these abstinence signs can be mediated through neuronal pathways within the spinal cord.     

N-nitro-L-arginine reverses L-arginine induced changes in morphine antinociception and distribution of morphine in brain regions and spinal cord of the mouse

Twice daily injections of L-arginine (200 mg/kg, i.p.), a precursor for nitric oxide (NO), for 4 days decreased morphine antinociception in male Swiss–Webster mice. Chronic treatment with L-arginine also produced significant decreases in morphine levels in midbrain, pons and medulla, hippocampus, corpus striatum and spinal cord of mice following an injection of morphine(10 mg/kg, s.c.) in comparison to vehicle-injected mice. N^G-nitro-L-arginine(L-NNA), an inhibitor of NO synthase(NOS),(5 mg/kg, i.p.) given prior to each injection of L-arginine reversed the effects of the latter on morphine antinociception and decreases in morphine levels in brain regions and spinal cord. Chronic injections of L-NNA alone did not modify either morphine antinociception or morphine distribution in brain regions and spinal cord of mice. These results suggest that decreases in morphine antinociception by chronic treatment with L-arginine is related to the decreases in the entry of morphine in the central sites. The reversal of L-arginine-induced effects by L-NNA suggests that NO-NOS system may be playing a critical role in the regulation of blood-brain barrier to morphine.     

Relationship of analgesia induced by centrally injected calcitonin to the CNS serotonergic system

The role of the serotonergic system in the antinociceptive effect of centrally administered salmon calcitonin (sCT) was studied in rats. The animals were given sCT either intracerebroventricularly (i.c.v.) or intrathecally (i.t.). I.c.v. administration of sCT (2,5 micrograms/rat) to animals depleted in CNS serotonin (5-HT) either by treatment with 25 mg/kg desmethylimipramine (DMI) i.p. plus 100 micrograms/rat 5,7 dihydroxytryptamine (5,7 DHT) i.c.v., ten days before or by 150 mg/kg p-chlorophenylalanine (pCPA) i.p., 72 and 24 h before, still significantly increased the hot-plate latencies comparable to those of non-depleted animals. The same result was obtained when the 5-HT receptors were blocked with metergoline. The i.t. administration of sCT (2 micrograms/rat) to animals with spinal cord 5-HT depleted by treatment with DMI plus 5,7 DHT, i.t., delayed but did not abolish the antinociceptive activity of i.t. injected sCT, which was of the same intensity as in non depleted animals. When 5,7 DHT was administered alone, either i.c.v. or i.t., without protection of the catecholaminergic neurons so that noradrenaline (NA) was greatly reduced, the antinociceptive effect of sCT was completely abolished even when NA had been depleted only in the spinal cord. We conclude that it is the catecholaminergic system, not the serotonergic, that plays a fundamental role in the anti-nociceptive effect of centrally administered sCT.     

In vivo evidence for serotonin 5-HT2C receptor-mediated long-lasting excitability of lumbar spinal reflex and its functional interaction with 5-HT1A receptor in the mammalian spinal cord

The purpose of the present study was to investigate the 5-HT(2C) receptor-mediated effects on the spinal monosynaptic mass reflex activities and also its functional interactions with 5-HT(1A) receptors in anesthetized, acutely spinalized mammalian adult spinal cord in vivo. Intravenous administration of (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI) (0.1 mg/kg), an agonist of 5-HT(2A/2C) receptors, significantly increased the excitability of spinal motoneurons as reflected by an increase in the spinal monosynaptic mass reflex amplitude to 150-200% of the control. 5-HT(2A/2C) receptor-induced motoneuron excitability was slow, persistent and long-lasting for more than 2h that was significantly inhibited by 5-HT(2C) receptor specific antagonist SB 242084 administered 10 min prior to DOI. Simultaneous administration of DOI (0.1 mg/kg, i.v.) along with (+/-)-8-hydroxy dipropylaminotetraline hydrobromide (8-OH-DPAT) (0.1 mg/kg, i.v.) completely inhibited DOI-induced spinal monosynaptic mass reflex facilitation. In another separate study, administration of 8-OH-DPAT (0.1 mg/kg, i.v.) at the maximum response of DOI also inhibited the motoneuron's excitability; however, the inhibition lasted only for a period of 40-60 min after administration of 8-OH-DPAT, after which the spinal monosynaptic mass reflex amplitude reached its maximum level. These findings suggest that the 5-HT(2C) receptor is primarily involved in the mediation of the long-lasting excitability of spinal motoneurons and possibly interacts with its functional counterpart, 5-HT(1A) receptors in the mammalian adult spinal cord.     

Effects of opioid receptor antagonists on the effects of i.v. morphine on carrageenin evoked c-Fos expression in the superficial dorsal horn of the rat spinal cord

This study performed in freely moving rats evaluated the ability of specific opioid receptor antagonists to reverse the inhibitory effects of morphine on carrageenin-induced c-Fos expression in the spinal cord. Our study focused on the superficial dorsal horn (laminae I-II), which is the main termination site of nociceptive primary afferent fibers and is rich in opioid receptors. In order to replicate clinical routes of administration, all agents were administered intravenously (i.v.). As previously demonstrated, pre-administered i.v. morphine (3 mg/kg) produced a marked decrease (58+/-5%) in the number of Fos-LI neurones measured at 2 h after intraplantar (i.pl.) carrageenin (6 mg/150 microl) and yet was without influence on peripheral oedema. This decrease in c-Fos expression was completely blocked by combined administration of morphine with the mu-opioid receptor antagonist, [D-Phe-Cys-Tyr-D-Orn-Thr-Pen-Thr-NH2] (CTOP-1+1 mg/kg). Naltrindole (NTI-1+1 mg/kg), a delta-opioid receptor antagonist partially blocked the effects of systemic morphine, so that the inhibitory effects of morphine after NTI injection are now 40+/-4%. However, this effect of NTI was weak since the depressive effects of morphine were still highly significant (p<0.001). In contrast, nor-binaltorphimine (nor-BNI-1+1 mg/kg), a kappa-opioid receptor antagonist, had no significant effect on the effects of morphine. These results indicate the major contribution of mu-opioid receptors to the antinociceptive effects of systemic morphine at the level of the superficial dorsal horn. The observed effect of NTI is not necessarily related to a direct action of morphine on delta-opioid receptors and some possible actions of this antagonist are discussed.     

Chronic intrathecal morphine administration produces homologous mu receptor/G-protein desensitization specifically in spinal cord

Previous studies have shown that chronic i.v. treatment with morphine or heroin decreased mu opioid receptor activation of G-proteins in specific brain regions. The present study examined the effect of intrathecal (i.t.) morphine administration on receptor/G-protein coupling in the spinal cord. In spinal cord membranes, [35S]GTP gamma S binding was stimulated by agonists of several G-protein-coupled receptors, including mu opioid (DAMGO), delta opioid (DPDPE), GABA(B) (baclofen), cannabinoid CB(1) (WIN 55,212-2), muscarinic cholinergic (carbachol) and adenosine A(1) (PIA). [35S]GTP gamma S autoradiography revealed that most of this agonist activation of G-proteins was localized to laminae I and II of dorsal horn. To determine the effects of chronic morphine on these receptor activities, rats were treated for 7 days with 0.11 mg/kg/day i.t. morphine, and receptor activation of G-proteins was determined by [35S]GTP gamma S autoradiography of brain and spinal cord. In spinal cord sections, chronic morphine treatment decreased DAMGO-stimulated [35S]GTP gamma S binding in laminae I and II at all levels of spinal cord examined. There were no effects of morphine treatment on [35S]GTP gamma S stimulation in spinal cord by other receptor systems examined (Adenosine A(1) and GABA(B)), and no significant effects of chronic i.t. morphine treatment were observed in brain sections. These data show that homologous desensitization of mu receptor/G-protein coupling occurs specifically in spinal cord following chronic morphine administration.     

Differential effects of spinal 5-HT1A receptor activation and 5-HT2A/2C receptor desensitization by chronic haloperidol

The effects of 7- and 21-day haloperidol treatment on the spinal serotonergic system were examined in vivo in acutely spinalized adult rats. Intravenous administration of a selective 5-HT(2A/2C) receptor agonist, (+/-)-2,5-Dimethoxy-4-iodoamphetamine hydrochloride (0.1 mg/kg) significantly increased the excitability of spinal motoneurones as reflected by increased monosynaptic mass reflex amplitude. This was significantly reduced in rats treated with haloperidol (1 mg/kg/day, i.p.) for 7 and 21 days. Administration of a 5-HT(1A/7) receptor agonist, (+/-)-8-Hydroxy dipropylaminotetraline hydrobromide (0.1 mg/kg, i.v.) significantly inhibited the monosynaptic mass reflex. This inhibition was greatly prolonged in haloperidol treated animals. These results demonstrate that the effects of haloperidol on the activation and desensitization of 5-HT(1A) and 5-HT(2A/2C) receptors respectively, may be mediated via intracellular mechanisms shared by these receptors with dopamine D(2) receptors in the mammalian spinal cord. The above serotonergic mechanisms may be partly responsible for haloperidol-induced extrapyramidal motor dysfunction.     

Diphenylbarbituric acid: its effects on neuromuscular and spinal cord function in the cat

In studies of neuromuscular function in the cat soleus muscle, diphenylbarbituric acid (DPB; 10-80 mg/kg i.v.) depressed both the degree and the duration of posttetanic potentiation (PTP) in a dose-dependent manner. At doses of greater than 20 mg/kg, twitch strength was increased in both indirectly stimulated and directly stimulated chronically denervated preparations, indicating a direct effect of DPB on muscle. In the spinal cord, DPB (10-60 mg/kg) depressed both monosynaptic (2N) and polysynaptic discharges, with flexor and extensor reflexes being similarly affected. In addition, DPB was not selective for isolated 2N as compared with posttetanically potentiated monosynaptic responses. Thus, in its actions on neuromuscular function DPB resembles phenytoin; in the spinal cord the drug resembles phenobarbital. The data suggest that the capacity of a drug to curb high-frequency repetitive discharges is more important than curbing recruitment as exhibited by PTP of the 2N reflex.     

Effect of subcutaneous administration of the chemical algogen formalin, on 5-HT metabolism in the nucleus raphe magnus and the medullary dorsal horn: a voltammetric study in freely moving rats

The effect of subcutaneous administration of the chemical algogen formalin, on serotonin (5-HT) metabolism in the nucleus raphe magnus (NRM) and the medullary dorsal horn (MDH) has been investigated using in vivo 5-hydroxyindole electrochemical (peak ‘3’) detection with treated, multi-carbon fiber electrodes and differential pulse, or normal pulse, voltammetry in freely moving rats. The subcutaneous (s.c.) injection of 50 μl of 10% formalin in the left forepaw was followed, at the NRM level, by a significant increase in the voltammograms as compared to controls (50 μl of saline 0.9% s.c. in left forepaw) for about 70 min after the injection, before a return to control values. At the MDH level, the formalin injection induced no significant effect on peak 3, as compared to controls, during the first 70 min. After that, the voltammograms significantly increased and remained above controls for up to 180 min. Thus, the time-courses of NRM and MDH effects appear markedly different. These findings suggest that, depending on the anatomical level (NRM or MDH) and/or the period of observation, one can measure differences in the time-course of the increase in 5-HT metabolism in the NRM-dorsal horn serotonergic system by tonic noxious stimuli, such as the formalin test.     

The effect of glutamate antagonists on c-fos expression induced in spinal neurons by irritation of the lower urinary tract

Chemical irritation of the lower urinary tract (LUT) of the rat increases the expression of c-fos in neurons in the dorsal horn, dorsal commissure and intermediolateral region of the spinal cord. The role of glutamatergic synapses in this response was examined using two glutamate receptor antagonists, MK-801 (an NMDA antagonist) and CNQX (an AMPA antagonist). In rats with an intact spinal cord, MK-801 (3.5 mg/kg, i.v.) administered 15 min before bladder irritation decreased (50–60%) the number of c-fos-positive cells in all regions of the cord. A smaller dose of MK-801 (0.8 mg/kg, i.v.) was ineffective. In spinal transected rats (4–7 days prior to the experiment) MK-801 (3.5 mg/kg, i.v.) decreased c-fos expression only in the medial dorsal horn. CNQX (1.2 mg/kg, i.v.) was ineffective in both preparations. These results indicate that activation of NMDA receptors at glutamate synapses in the central nervous system may play a role in the processing of nociceptive input from the LUT and may also be involved in reflex pathways mediating micturition.