The effect of lowering the 5-hydroxytryptamine content of the rat spinal cord on analgesia produced by morphine

Uses of fluoxetine in nociceptive pain management: A literature overview

Fluoxetine is one of the top ten prescribed antidepressants. Other therapeutic applications were approved for fluoxetine including, anxiety disorders, bulimia nervosa, and premature ejaculation. However, the role of fluoxetine in nociceptive pain management is still unclear. In this review, we discuss an overview of five possible roles of fluoxetine in pain management: intrinsic antinociceptive effect, enhancement of acute opioid analgesia, attenuation of tolerance development to opioid analgesia, attenuation of dependence development and abstinence syndrome, and attenuation of opioid induced hyperalgesia. Conflicting data were reported about fluoxetine intrinsic anti-nociceptive effect in preclinical and clinical studies except for inflammatory pain. Similar controversy was described in preclinical and clinical studies which explored the possible enhancement of opioid analgesia by fluoxetine co-administration. However, fluoxetine was found to have a promising effect on opioid tolerance and dependence in animal and human studies. Regarding opioid induced hyperalgesia, no studies examined fluoxetine effects in this regard. Our literature review revealed that, the most likely beneficial use of fluoxetine in nociceptive pain management is for alleviation of inflammatory pain and attenuation of opioid tolerance and dependence. Non-steroidal anti-inflammatory and corticosteroids carry many adverse effects and toxicities. Effective alleviation of opioid tolerance and dependence represents a huge health burden and growing unmet medical need. Moreover, most agents used to attenuate these phenomena are either experimental or poorly tolerable drugs which limit their transitional value. Fluoxetine offers an effective, safe, and tolerable alternative for management of both inflammatory pain and opioid tolerance and dependence presently available to clinicians.

Genetically expressed transneuronal tracer reveals direct and indirect serotonergic descending control circuits

Despite the evidence for a significant contribution of brainstem serotonergic (5HT) systems to the control of spinal cord "pain" transmission neurons, attention has turned recently to the influence of nonserotonergic neurons, including the facilitatory and inhibitory controls that originate from so-called "on" and "off" cells of the rostroventral medulla (RVM). Unclear, however, is the extent to which these latter circuits interact with or are influenced by the serotonergic cell groups. To address this question we selectively targeted expression of a transneuronal tracer, wheat germ agglutinin (WGA), in the 5HT neurons so as to study the interplay between the 5HT and non-5HT systems. In addition to confirming the direct medullary 5HT projection to the spinal cord we also observed large numbers of non-5HT neurons, in the medullary nucleus reticularis gigantocellularis and magnocellularis, that were WGA-immunoreactive, i.e., were transneuronally labeled from 5HT neurons. FluoroGold injections into the spinal cord established that these reticular neurons are not only postsynaptic to the 5HT neurons of the medulla, but that most are also at the origin of descending, bulbospinal pathways. By contrast, we found no evidence that neurons of the midbrain periaqueductal gray that project to the RVM are postsynaptic to midbrain or medullary 5HT neurons. Finally, we found very few examples of WGA-immunoreactive noradrenergic neurons, which suggests that there is considerable independence of the monoaminergic bulbospinal pathways. Our results indicate that 5HT neurons influence "pain" processing at the spinal cord level both directly and indirectly via feedforward connections with multiple non-5HT descending control pathways.

5-HT receptors involved in opioid-activated descending inhibition of spinal withdrawal reflexes in the decerebrated rabbit

The role of 5-HT(1B/1D), 5-HT(2) and 5-HT(3) receptors in mediating descending inhibition of spinal reflexes activated by application of fentanyl to the fourth ventricle has been studied in rabbits decerebrated under N(2)O/isoflurane anaesthesia. In the control state, intraventricular fentanyl (3-30 microg kg(-1)) depressed, to an equal extent, short- and long-latency reflexes in the medial gastrocnemius muscle nerve evoked by electrical stimulation of all sural nerve afferents. Inhibition of reflexes resulted from a decreased base line excitability in the reflex pathway accompanied by a reduction in the rate of temporal summation of responses. Fentanyl-induced suppression of short- and long-latency reflexes was significantly reduced after intrathecal administration of the selective 5-HT(2)-receptor antagonist ICI 170,809 (300 microg). The same dose of the selective 5-HT(1B/1D) blocker GR 127,935 reduced inhibition from intraventricular fentanyl only for long-latency reflexes (i.e. those parts of the response for which the afferent drive is provided mainly by Adelta and C-fibre afferents). The 5-HT(3) antagonist tropisetron (also 300 microg intrathecal) did not significantly alter the descending inhibition of reflexes evoked by fentanyl. Both GR 127,935 and tropisetron reduced temporal summation of reflexes per se, effects that were reversed by intraventricular fentanyl. These data suggest that the descending pathway(s) activated by intraventricular fentanyl liberate 5-HT in the spinal cord to inhibit withdrawal reflexes by acting at 5-HT(2) and 5-HT(1B/1D), but not 5-HT(3) receptors. 5-HT(1B/1D), and to a lesser extent 5-HT(3) receptors also appear to have a role in modulating temporal summation of reflexes evoked by repetitive stimuli.

The role of 5-HT(1A)-receptors in fentanyl-induced bulbospinal inhibition of a spinal withdrawal reflex in the rabbit

The sural to gastrocnemius withdrawal reflex is inhibited after injection of the OP(3) (micro)-receptor-selective opioid fentanyl into the fourth ventricle of decerebrated rabbits. This effect is abolished by complete section of the spinal cord but not by the selective alpha(2)-adrenoceptor antagonist RX 821002 (Clarke RW, Parry-Baggott C, Houghton AK, Ogilvie J. The involvement of bulbo-spinal pathways in fentanyl-induced inhibition of spinal withdrawal reflexes in the decerebrated rabbit. Pain 1998;78:197-207). We have now investigated the role of 5-HT(1A) receptors in mediating the descending inhibition activated by intraventricular fentanyl. In the control state, intraventricular fentanyl (3-30 microgram/kg) inhibited gastrocnemius reflex responses to a median of 34% of pre-drug levels. After intrathecal administration of the selective 5-HT(1A) receptor antagonist WAY-100635 (100 microgram), fentanyl reduced reflex responses to 83% of pre-fentanyl values, significantly less inhibition than in the control state. In a separate group of experiments, intravenous fentanyl (0.3-30 microgram/kg) depressed the sural-gastrocnemius reflex to 17% of pre-drug controls. This inhibition was not affected by intrathecal WAY-100635 (100 microgram), but combined administration of the 5-HT(1A) antagonist with RX 821002 (100 microgram) significantly reduced the effectiveness of i.v. fentanyl. After the highest dose reflexes were 37% of pre-fentanyl levels. These data show that the bulbospinal inhibition activated by fentanyl is mediated, at least in part, by activation of spinal 5-HT(1A) receptors. That blockade of these receptors failed to influence the inhibition induced by i.v. fentanyl might be taken to mean that the brain-stem action of fentanyl does not contribute significantly to the systemic actions of this opioid. A more probable explanation is that, in the preparation used in the present study, the bulbospinal and direct spinal actions of fentanyl occlude each other to produce an overall inhibition that is less than the sum of the two effects.

Effect of p-chlorophenylalanine and alpha-methyltyrosine on the antinociceptive effect of antidepressant drugs

The role of para-chlorophenylalanine and alpha-methyl-DL-p-tyrosine in the antinociceptive effects of the intracerebroventricular administration of the antidepressant drugs clomipramine, zimelidine, imipramine and maprotiline was studied using the acetic acid writhing test in mice. The results demonstrated an antinociceptive effect for all these antidepressants. Pretreatment with para-chlorophenylalanine significantly reduced the antinociception induced by the ED50's of imipramine and maprotiline, and did not modify the effects of zimelidine and clomipramine, pretreatment with alpha-methyl-tyrosine did not modify the antinociception induced by these drugs except maprotiline. Pretreatment with para-chlorophenylalanine plus alpha-methyltyrosine significantly reduced the antinociceptive effect of all the antidepressants tested. The main finding of the present study is that the association of para-chlorophenylalanine plus alpha-methyltyrosine reduced the antinociceptive action of all the antidepressants. This means that critical levels of both 5-HT and NA are responsible for mediating the antinociceptive effects of antidepressants on the writhing test in mice.

Serotonin modulation of pain responsiveness in the aged rat

This study was designed to examine differences in basal nociceptive responsiveness between young (3 months) and old (25 months) male Fischer-344 rats and also to evaluate the effects of methysergide and fluoxetine on this behavioral paradigm. The results indicate that the aged animals were less sensitive than young animals to pain responsiveness in simple nociceptive tests such as the tail-flick, hot plate (55 degrees C), and hind-paw pressure tests. In both groups of animals, this behavioral response followed a circadian rhythm, with peak of pain latency during the dark phase and trough occurring in the light phase. In all three analgesic tests, treatment with methysergide, which is a serotonin antagonist, resulted in hyperalgesia in both groups of animals within the first hour, followed by a return to basal response level after 2 h. Fluoxetine treatment resulted in a nonsignificant increase in nociceptive response at 30 min posttreatment which returned to the baseline by 1 h. Moreover, in both young and old animals morphine produced moderate analgesia in the hot plate and hind-paw tests, which was potentiated by simultaneous treatment with fluoxetine. This study shows that noxious response was reduced in the aged male Fischer-344 rats, and the data obtained provide evidence that the serotonergic system modulates pain sensitivity similarly in young and old animals.

Catecholaminergic input to spinally projecting serotonin neurons in the rostral ventromedial medulla oblongata of the rat

The midline of the rostral ventral medulla (RVM) is the portion in which many serotonin (5-HT) neurons of the nucleus raphe magnus and the rostral nucleus raphe pallidus are located and where dense catecholaminergic (CA) fibers are distributed. In this study, we investigated the connection between spinally projecting 5-HT neurons and CA fibers in the rat RVM by light and electron microscopic immunocytochemistry. First, light microscopic immunocytochemistry using a triple labeling method revealed that the 5-HT-immunoreactive (IR) neuron containing retrograde tracer from the cervical cord was intimately surrounded by tyrosine hydroxylase (TH)-IR fibers. Second, silver-gold intensified TH-IR axon terminals were found to make synaptic contacts with 5-HT-IR neuronal perikarya and dendrites by double labeling immunoelectron microscopy. These morphological findings suggest that spinally projecting 5-HT neurons, presumed to be involved in pain modulation or sympathetic autonomic control, are directly regulated by CA neurons at the level of the RVM.

Formalin pain is expressed in decerebrate rats but not attenuated by morphine

Subcutaneous injection of dilute formalin induces pain in humans and behaviors indicative of pain in animals. The formalin test, which is based on these observations, is now widely used as a model of pain produced by tissue injury, but the neural mechanisms of pain and analgesia in this test have not been identified. Rats with transections of the brain rostral or caudal to the pons show behavioral reactions to formalin similar to those of normal rats, although the temporary abatement of pain 10-15 min after formalin is absent in transected animals. Doses of morphine that suppress the behavioral response to formalin in normal rats are not antinociceptive in the formalin test in decerebrate rats although sedation, catalepsy and inhibition of the tail-flick reflex still occur. These results suggest that the response to formalin is organized in the brain stem but the antinociceptive effect of morphine in this test is mediated by the diencephalon or forebrain.

Effects of systemic morphine on diffuse noxious inhibitory controls: role of the periaqueductal grey

The effects of systemic morphine (1 mg/kg i.v.) on diffuse noxious inhibitory controls (DNIC) were studied in both sham-operated animals and those with quinolinic acid-induced lesions of the periaqueductal grey (PAG). DNIC acting on convergent neurones in the dorsal horn of the spinal cord were similar in the sham-operated and lesioned animals. However, following morphine injection, DNIC were blocked in the sham-operated but not in the PAG-lesioned animals. It is concluded that, although the PAG is not directly involved in the supraspinal loop subserving DNIC, it can modulate these controls. In addition, as naloxone reversed the effects of morphine in the control group but reduced DNIC in the PAG-lesioned animals, it is suggested that more than one opioidergic system is involved in DNIC.

Do opioids evoke the release of serotonin in the spinal cord? An in vivo microdialysis study of the regulation of extracellular serotonin in the rat

This study investigated the regulation of serotonin (5-HT) and its major metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the dorsal spinal cord of awake, freely moving rats, using microdialysis coupled to HPLC with electrochemical detection and tested the hypothesis that opioids exert their analgesic effect in part through the increased release of 5-HT in the dorsal horn. A dialysis tube was placed transversely at the L4 segment of the dorsal spinal cord and the basal concentration of 5-HT in the dialysate was characterized by infusion of a variety of substances through the dialysis probe: tetrodotoxin (TTX), KCl, imipramine, fluoxetine and amphetamine (AMPH). To evaluate the contribution of opioids, we also studied the effects of either systemic or intracerebroventricular (i.c.v.) injection of morphine or DAMGO. Extracellular concentrations of 5-HT and 5-HIAA were partially and reversibly reduced by TTX. In the presence of KCl, imipramine, fluoxetine or AMPH, 5-HT levels significantly increased. Under these conditions, extracellular 5-HIAA levels usually decreased. By contrast, the effects of opioids on 5-HT concentrations were highly variable. Low doses of morphine administered systemically increased 5-HT concentrations in only 3 of 6 rats. This was paralleled by a decrease in 5-HIAA. Higher doses of morphine, alone or in the presence of fluoxetine, did not change 5-HT concentrations. Intracerebroventricular injection of morphine or DAMGO increased the extracellular concentrations of 5-HT in only about one third of the animals. After intracerebroventricular opioid injection, extracellular concentrations of 5-HIAA either decreased by about 20% or did not change.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute tryptophan depletion blocks morphine analgesia in the cold-pressor test in humans

The effects of depletion of the serotonin precursor, L-tryptophan, on the threshold and tolerance to cold pressor pain, and the analgesic effect of morphine (10 mg intramuscularly), were tested in a double blind trial on human volunteers. Effects on mood were also assessed using the Profile of Mood States and the Addiction Research Center Inventory (ARCI) Scales. To deplete tryptophan, subjects were fed a tryptophan-deficient amino acid mixture 4.5 h before morphine was administered. Controls received the mixture with tryptophan, which is equivalent to a nutritionally balanced protein. The tryptophan-deficient meal reduced plasma tryptophan more than 70% but had no effect on threshold or tolerance to cold pressor pain. After morphine, tolerance to cold pressor pain increased in controls. Tryptophan depletion abolished this analgesic effect. Pain threshold was not altered by morphine. In subjects with normal tryptophan, the analgesic effect of morphine was predicted by the level of plasma morphine-6-glucuronide, but not by the level of morphine. Morphine increased scores on the LSD scale of the ARCI, but had no effect on other measures of mood. Tryptophan depletion also failed to alter mood in these subjects, who had unusually low depression scores before tryptophan depletion.

6-Hydroxydopamine lesions of the ventral tegmentum abolish D-amphetamine and morphine analgesia in the formalin test but not in the tail flick test

The effect of bilateral 6-hydroxdopamine (with desipramine) lesions of the ventral tegmental area-substantia nigra region on analgesia produced by morphine and D-amphetamine was examined with the formalin and tail flick tests in rats. These lesions depleted dopamine, but not noradrenaline, in the ventral and anterior striatum. Morphine and D-amphetamine produced analgesia in the formalin test in sham lesioned rats but not in lesioned rats. In contrast, morphine produced the same degree of analgesia in the tail flick test in sham lesioned and lesioned rats while D-amphetamine did not produce analgesia in this test. These data suggest that morphine and D-amphetamine analgesia in the formalin test involves dopamine, whereas morphine analgesia in the tail flick test does not.

Serotonin involvement in analgesia induced by transcranial electrostimulation

The experiments described here were intended to investigate whether serotonin (5HT) may be involved in analgesia induced by low current transcranial electrostimulation (TE). The TE stimulus is a 10 mu-ampere, 10 Hz, pulsed current transmitted via electrodes in the pinnae. Combinations of the following were given as intraperitoneal injections: 300 mg/kg p-chlorophenylalanine (pCPA) 48 hours before testing, 100 mg/kg 5-hydroxytryptophan (5HTP) 30 min before testing and the saline vehicle for these drugs. Rats were tested prior to and 30 minutes after TE or sham TE. Testing for analgesia consisted of putting progressively increasing pressure on the rat tail 1/4 inch from the tip with a pneumatically driven, right angle wedge. The amount of pressure at which the rat moved its tail was measured both before and after TE, or sham TE, and recorded as the difference in tolerated peak pressure (DTPP). TE produced analgesia as manifested by a 613 percent increase in DTPP compared with sham TE treatment values. Among TE treated rats, pretreatment with pCPA decreased DTPP 91.5 percent compared with saline control values, indicating 5HT involvement. 5HTP restored TE induced analgesia in pCPA treated rats to the level of saline treated control animals, confirming 5HT involvement.

Tryptophan-morphine interactions and postoperative pain

Patients undergoing abdominal surgery were infused with saline or the 5-hydroxytryptamine (5-HT) precursor tryptophan starting in the operating room and continuing for three hours in the recovery room. There was a nonsignificant trend for patients who received tryptophan to have higher pain scores. In the saline-treated patients, plasma tryptophan was below the range for normal healthy subjects, and there was a strong positive relationship between plasma tryptophan and morphine requirements. These data, taken together with animal data obtained using the formalin pain test, suggest that a 5-HT system in the brain can antagonize the dissociative state produced by morphine, which helps patients to tolerate pain. When plasma tryptophan falls below normal levels, brain 5-HT falls and morphine requirements are reduced. While tryptophan may potentiate spinal 5-HT function to decrease nociceptive afference in some circumstances, there may be clinical conditions in which the use of tryptophan is contraindicated.

Reduction in opioid and non-opioid forms of swim analgesia by 5-HT2 receptor antagonists

Acute exposure to continuous (CCWS) or intermittent (ICWS) cold-water swims elicits non-opioid and opioid forms of analgesia respectively. Intrathecal administration of methysergide blocks ICWS, but not CCWS analgesia. The present study evaluated the role of serotonin (5-HT) receptor subtypes in the mediation of CCWS and ICWS analgesia on the tail-flick and jump tests following administration of methysergide, a non-specific 5-HT antagonist and pirenpirone and ketanserin, two 5-HT2 receptor subtype antagonists. Systemic methysergide was more effective in reducing CCWS analgesia (50-58%, 0.1-1.0 mg/kg) than ICWS analgesia (21%, 5 mg/kg) on both pain tests. Systemic pirenpirone (0.04-0.2 mg/kg) and ketanserin (1-5 mg/kg) were also more effective in reducing CCWS analgesia (43-57%) on both tests than ICWS analgesia (pirenpirone: 0.4 mg/kg, 34%; ketanserin: 5 mg/kg, 21%) on the tail-flick test. Indeed, both 5-HT2 receptor antagonists potentiated ICWS analgesia on the jump test. While serotonin antagonist effects upon hypothermia could not account for CCWS analgesia effects, similar potentiations in ICWS analgesia and hypothermia were observed following pirenpirone and ketanserin. Finally, both 5-HT2 receptor antagonists differentially reduced CCWS hypothermia and potentiated ICWS hypothermia. These data suggest differential serotonergic modulation of the two forms of swim analgesia with opioid-mediated ICWS analgesia acting through spinal 5-HT1 receptors and non-opioid-mediated CCWS analgesia acting through supraspinal 5-HT2 receptors.

Intrathecal administration of 5,6-DHT or 5,7-DHT reduces morphine and substance P-antinociceptive activity in the rat

The administration of SP (15 and 50 ug) and morphine sulphate (10 ug) either into the IV ventricle or intrathecally produces an analgesic effect. This effect was attenuated when the rats received previously an intrathecal dose of 5,6-DHT or 5,7-DHT. The consequences of neurotoxin administration upon monoamine descending systems were evaluated by measuring 14C-5HT and 3H-NA synaptosomal uptake in different structures of the CNS. SP levels were also determined in the animals injected with the neurotoxins. Our results confirm the existence of a relationship injected with the neurotoxins. Our results confirm the existence of a relationship between the 5HT and SP descending systems, which control the nociceptive information at the level of the dorsal horn of the spinal cord.

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.

Attenuation of morphine analgesia by the S2 antagonists, pirenperone and ketanserin

The involvement of serotonin type-2 (S2) receptors in morphine-induced analgesia was assessed by challenging the effect of 10 mg/kg of morphine sulphate (IP) with the S2 receptor blockers, pirenperone and ketanserin. Tail-flick latencies were assessed at 0, 30, 60, 90 and 120 min after injections by measuring the time that it took each rat to remove its tail from a 52 degrees C water bath. Pirenperone, at 0.08, 0.16, and 0.24 mg/kg (SC) attenuated morphine-induced antinociception. In contrast, only the high 10 mg/kg (SC) dose of ketanserin attenuated the effect of morphine. Because pirenperone easily enters the central nervous system whereas ketanserin does not, these results indicate the involvement of central S2 receptors in morphine-induced antinociception. The 10 mg/kg dose of ketanserin, however, did not attenuate the antinociception produced by 100 mg/kg of ketamine. Thus, the antianalgesic effect of S2 receptor blockers may be specific to opioid-mediated analgesia.

Role of ascending and descending serotonergic pathways in the antinociceptive effect of baclofen

The role of ascending and descending serotonergic pathways in the antinociceptive effect of baclofen was examined by lesioning specific pathways with the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). Antinociception in rats was assessed using the tail flick and hot plate tests 3/4 and 10/11 days after lesioning and the placement of lesions verified by analysis of serotonin (5-HT) in brain and spinal cord. Lesions to the ventromedial tegmentum depleted 5-HT selectively in brain and inhibited the antinociceptive effect of baclofen in the tail flick test 3/4 but not 10/11 days after lesioning. Lesions to the nucleus raphe medianus produced a marked depletion of 5-HT in the hippocampus and produced the same effect on baclofen. Lesions to the nucleus raphe dorsalis were less selective, depleting 5-HT in a number of brain regions and in the spinal cord, and inhibited the antinociceptive effect of baclofen at the later but not the earlier time interval. Lesions to descending pathways by microinjection of 5,7-DHT into the ventral raphe or nucleus raphe magnus did not affect the action of baclofen significantly. Lesions to both ascending and descending pathways by intracerebroventricular 5,7-DHT increased the effect of baclofen. The hot plate test generally was less sensitive to these manipulations, although changes parallel to the tail flick test were observed in a number of instances. Both the destruction of 5-HT pathways and development of supersensitivity at 5-HT receptors may contribute to the interactions observed.

Differential effects of p-chlorophenylalanine on indoleamines in brainstem nuclei and spinal cord of rats. I. Biochemical and behavioral analysis

The involvement of endogenous serotonergic pathways in the mediation of antinociception has been indicated by electrophysiological, pharmacological and behavioral experiments. However, manipulation of the indole pathway, either by lesioning of raphe nuclei or drug intervention, often produces disparate results. In particular, serotonin (5-HT) synthesis inhibition with p-chlorophenylalanine (PCPA) has been reported to produce either hyperalgesia or analgesia, depending upon the type of pain measurement examined. In the present study, we sought to evaluate the effects of PCPA on (1) behavioral responses to noxious stimulation, and (2) levels of serotonin, tryptophan and 5-hydroxyindoleacetic acid (5-HIAA) in raphe nuclei (pallidus, obscurus, magnus and dorsalis) and spinal cord regions by HPLC with electrochemical detection. Treatment of rats with 400 or 600 mg/kg of PCPA for 3 consecutive days resulted in significant elevations in pain thresholds assessed by tail withdrawal from radiant heat as well as vocalization to electric shock of the tail. The effect of PCPA on vocalization threshold was particularly striking, for the majority of animals showed a nociceptive-specific attenuation of this response. Although the PCPA induced changes in indole content of the various raphe nuclei were not unequivocally dose-dependent, differential reductions of serotonin and 5-HIAA were clearly detected in the various raphe regions. Nuclei raphe pallidus and obscurus were depleted of 5-HT and 5-HIAA to the greatest extent, whereas levels detected in nuclei raphe magnus and dorsalis were reduced by 30-40% from control values. Metabolism of 5-HT and 5-HIAA appeared unaffected by PCPA in all regions examined except the dorsal portion of the spinal cord. These findings collectively suggest that the effects of PCPA are not uniform throughout the central nervous system and raise the possibility that discrepancies in the behavior literature may be attributed to drug-induced changes in some, but not all serotonergic pathways.

Enhancement of clonidine-induced analgesia by lesions induced with spinal and intracerebroventricular administration of 5,7-dihydroxytryptamine

The role of serotonin (5-HT) in analgesia induced by clonidine was examined by determining the effect of intraspinal (i.s.) and intracerebroventricular (i.c.v.) injections of 5,7-dihydroxytryptamine (5,7-DHT) on analgesia produced by clonidine in the tail-flick and hot plate tests. Depletion of amines was verified by high performance liquid chromatography analysis. Intraspinal injections of 5,7-DHT potentiated the action of clonidine in both tests for analgesia and caused depletion of 5-HT in the spinal cord. Intracerebroventricularly injected 5,7-DHT also increased the action of clonidine and depleted 5-HT in brain as well as in the spinal cord. In the groups given intracerebroventricular injections, there appeared to be a biphasic increase in the action of the clonidine. Significant hyperalgesia from pretreatment with neurotoxin was observed only on a limited number of occasions. The present results indicate that 5-HT mechanisms in the CNS are important mediators of the analgesic action of clonidine. Interactions between clonidine and 5-HT systems at both spinal and supraspinal sites are considered.

The effect of exposure to positive space charge on aversive responses to noxious stimuli in rats

Exposure of rats to positive atmospheric ions for four days or longer results in an increase in aversive response times (tail flick and hot-plate). While these effects are similar to those produced after opiate administration, they could not be inhibited by naloxone treatment. Prompt reversal of the positive ion analgesic effect, however was brought about by treatment with parachlorophenylalanine which lowers brain serotonin levels. The data suggest that the action of positive ions on the pain inhibitory system is dependent upon an intact serotonin pathway and that the effects on this system are mediated via central rather than peripheral neural mechanisms.

The effect of different lesions of the median raphe on morphine analgesia

The effect of different manipulations of the nucleus medianus raphe (MR) on morphine analgesia was investigated in rats using the tail-immersion test. Electrolytic lesions of this structure antagonized morphine analgesia, while injections of 5,7-dihydroxytryptamine (to destroy serotonergic neurons) or ibotenic acid (to destroy cell bodies) in the medianus raphe did not alter the effect of morphine. Injection of naloxone (0.5 and 0.1 micrograms) in the MR antagonized morphine analgesia. These results suggest the importance of this structure for morphine analgesia in this test, although the substrates within the nucleus that mediate this action are still unknown.

Effect of depletion of spinal cord norepinephrine on morphine-induced antinociception

We studied whether antinociception produced by injection of morphine into the nucleus reticularis paragigantocellularis (NRPG) or by superfusion onto the spinal cord involved norepinephrine (NE)-containing neurons that descend from brainstem into the spinal cord. Spinal cord NE concentrations were depleted with the neurotoxin, 6-hydroxydopamine, and antinociception was measured following morphine injection into NRPG or onto spinal cord. Depletion of cord NE by approximately 90% did not attenuate the antinociceptive effect of either 2 or 10 micrograms of morphine injected intrathecally. In contrast, the depletion did significantly attenuate the antinociceptive effect of 2.5 micrograms morphine injected bilaterally into the NRPG. These results suggest that NE-containing neurons descending from brainstem nuclei into the spinal cord are not important in the analgesia produced by injecting morphine directly onto the spinal cord but may be involved with analgesia produced by morphine injection into the NRPG.

Altered responsiveness to substance P and 5-hydroxytryptamine in cat dorsal horn neurons after 5-HT depletion with p-chlorophenylalanine

The responsiveness of functionally identified cat spinal dorsal horn neurons to iontophoretically applied substance P (SP) and 5-hydroxytryptamine (5-HT) has been investigated by means of extracellular recording after 5-HT depletion with p-chlorophenylalanine (p-CPA). In addition, the spinal levels of 5-HT, SP, cholecystokinin octapeptide, neurotensin, and vasoactive intestinal polypeptide have been measured in intact and p-CPA-pretreated cats. In the present study we have demonstrated an altered responsiveness of dorsal horn neurons to locally applied SP and 5-HT. We found in p-CPA-pretreated cats that the proportion of neurons responding with excitation to SP and 5-HT was significantly increased. At the same time, depression induced by 5-HT in the dorsal horn cells was virtually absent in p-CPA-pretreated animals. Our finding that spinal level of 5-HT was significantly decreased in p-CPA-treated animals is consistent with previous studies. No convincing alteration in the spinal levels of 4 analyzed peptides was found in p-CPA-treated animals. The present study has shown that pharmacological depletion of 5-HT has two major effects: (1) it increases significantly the proportion of dorsal horn neurons excited by SP and 5-HT; and (2) it is ineffective in inducing 5-HT supersensitivity. Further work is needed to explain mechanisms involved in these effects.

The effect of intracerebroventricular 5,7-dihydroxytryptamine on morphine analgesia is time-dependent

The analgesic effect of morphine in the tail immersion test was studied in rats three and ten days after intracerebroventricular 5,7-dihydroxytryptamine (5,7-DHT) given to selectively destroy serotonergic neurons. Morphine analgesia was reduced three but not ten days after the neurotoxin. Ten days after 5,7-DHT, the inhibiting effect of metergoline, a serotonin antagonist, on morphine analgesia was still present, suggesting that functional recovery of the serotonergic system may partly explain the different results.

Immunohistochemistry of synaptic input and functional characterizations of neurons near the spinal central canal

Neurons surrounding the central canal in sacral spinal segments were functionally characterized on the basis of somatic and/or visceral afferent input, then intracellularly marked with horseradish peroxidase (HRP). Tissue sections containing portions of HRP-stained neurons were subsequently immunohistochemically examined for the presence of contacts made by axonal enlargements containing vasoactive intestinal polypeptide (VIP), substance P (SP), somatostatin (SS), Leu-enkephalin (ENK), or serotonin (5-HT). ENK-and 5-HT-containing enlargements were found to contact all neurons examined. SP and SS terminals contacted fewer neurons, and were not associated with specific functional classes. On the other hand, VIP-containing fibers contacted only those neurons receiving visceral afferent input, thus supporting the contention that VIP is contained in a population of visceral afferent fibers projecting to the gray matter surrounding the central canal at sacral levels.

Functional and morphological features of neurons in the midline region of the caudal spinal cord of the cat

Neurons surrounding and dorsal to the central canal in caudal segments of the cat spinal cord were functionally and morphologically characterized. From electrophysiologically obtained responses these neurons were categorized into 3 functional groups based on excitation by somatic afferent stimulation. Eighteen of 36 units were activated by both low threshold and high threshold primary afferent inputs. Of the remaining 18 units, 9 responded only to innocuous intensities of stimulation and the other 9 were excited selectively by noxious peripheral stimulation. Neurons intracellularly marked with horseradish peroxidase formed a heterogeneous population with respect to perikaryal size, dendritic orientation and dendritic extent, and no evident correlations between functional categories and morphological features were observed in light microscopic analyses. Neurons immediately surrounding the central canal were functionally similar to neurons located in the dorsally adjacent gray matter.

Differential responses of serotonergic and non-serotonergic neurons in nucleus raphe magnus to systemic morphine in rats

Forty-eight raphe-spinal units in nucleus raphe magnus were identified by antidromic stimulation and further classified into serotonergic and non-serotonergic populations according to their conduction velocity and spontaneous discharge rate. Following morphine administration (5 mg/kg, i.p.) they showed different responses: excitatory, depressive or non-responsive. It was found that 6 of 7 non-responsive, 3 of 20 depression-responsive and only 1 of 21 excitation-responsive units proved to be serotonergic neurons, indicating that the involvement of serotonergic neurons in morphine analgesia is probably insignificant. It is suggested that the raphe-spinal fiber systems, both excitatory and inhibitory are originated mainly from the non-serotonergic neurons.

Topographic principles in the spinal projections of serotonergic and non-serotonergic brainstem neurons in the rat

The spinal projections from the raphe-associated brainstem areas containing serotonergic neurons were studied with aldehyde-induced fluorescence in combination with the retrograde fluorescent tracer True Blue in the rat. This technique makes it possible to determine simultaneously the projections of individual neurons and to detect whether serotonin is present in the same neurons. After tracer injections into the spinal cord retrogradely labeled serotonergic and non-serotonergic neurons were found in the medullary raphe nuclei and adjacent regions and to a lesser extent in association with the dorsal and median raphe nuclei in the mesencephalon. Large True Blue injections that covered one side of the spinal cord at mid-cervical level labeled about 60% of the ipsilaterally situated serotonergic neurons in the medullary raphe regions while the corresponding figure contralaterally was about 25%. On both sides a larger number of labeled non-serotonergic neurons were found; these were sometimes located dorsal to, but often intermingled with, the serotonergic cells. While the serotonergic projection from the mesencephalon could not be labeled from injections below cervical levels, the labeling in more caudal brainstem regions exhibited only minor variations depending on the rostrocaudal level of the spinal segment injected. Furthermore, quantitative data from injections at different levels indicate that the majority of the spinal-projecting neurons traverse most of the length of the cord. Summarizing the results obtained from small injections restricted to subregions of the cord we feel that it is possible to distinguish three fairly distinct pathways for spinal projections from the medullary raphe and adjacent regions: The dorsal pathway originates mainly from cells in the caudal pons and rostral medulla oblongata (rostral part of nucleus raphe magnus, nucleus raphe magnus proper, nucleus reticularis gigantocellularis pars alpha and nucleus paragigantocellularis). This pathway, which contains a large non-serotonergic component, descends through the dorsal part of the lateral funiculus and terminates mainly in the dorsal horn at all spinal cord levels. The intermediate pathway is largely serotonergic with its cell bodies located within the arcuate cell group (situated just ventral and lateral to the pyramids very close to the ventral surface of the brainstem) and in the nucleus raphe obscurus and pallidus and terminates in the intermediate grey at thoracolumbar and upper sacral levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Descending control of spinal nociceptive transmission. Actions produced on spinal multireceptive neurones from the nuclei locus coeruleus (LC) and raphe magnus (NRM)

The effects of electrical stimulation in the nuclei locus coeruleus (LC) and raphe magnus (NRM) were examined on the background and/or evoked discharge of neurones in the spinal dorsal horn of anaesthetized cats. These were qualitatively, and in most cases quantitatively similar, in their action on multireceptive neurones. In these neurones an inhibitory action on the discharge evoked by noxious cutaneous stimuli or by activation of A delta and C fibres was most prominent although in some neurones (22%) an initial excitation lasting up to 100 ms preceded the inhibition which could last up to 1 s. Excitation alone was observed in only 3% of multireceptive neurones. Electrical stimulation also produced an inhibitory action on the discharge of low threshold mechanoreceptive neurones (80%). In four of ten multireceptive neurones examined in detail, LC stimulation produced a selective inhibitory action on the discharge evoked by noxious cutaneous stimuli. In the remaining six multireceptive neurones it was partially selective against noxious as compared with non-noxious inputs. The inhibitory action was also more pronounced on the discharge evoked by activity in A delta and C fibres than fast conducting afferents. The inhibitory action evoked by electrical stimulation in LC on nociceptive transmission in the spinal cord is suggested to play a part in mediating analgesia from LC.

Potentiation of morphine antinociception by monoamine reuptake inhibitors in the rat spinal cord

Potentiation of the antinociceptive effects of morphine by the tricyclic antidepressants was assayed in awake restrained rats using the tail-flick test. Intrathecally administered amitriptyline, desipramine or sertraline at doses that had no effect themselves (25-30 micrograms) potentiated a subthreshold parenteral dose of morphine (0.5 mg/kg). The morphine potentiating effect of amitriptyline was prevented by prior administration of parachlorophenylalanine (PCPA). This effect of PCPA was not restored by 5-hydroxytryptophan (5-HTP) but was restored when the animals were left for 14 days to replete. The morphine potentiating effects of amitriptyline, desipramine and sertraline were blocked by intrathecal administration of low doses of the serotonin antagonist methysergide and the alpha-adrenergic antagonists yohimbine and phentolamine but not by the beta-adrenergic antagonist propranolol. The results are consistent with the hypothesis that the potentiation of morphine's antinociceptive effect by tricyclic antidepressants depends on activation of both spinopetal serotonin and adrenergic neurons.

Contrasting effects of acute vs. chronic tricyclic antidepressant treatment on central morphine analgesia

Antinociception following central opioid microinjection in rats was assessed weekly via a tail-flick procedure during chronic tricyclic antidepressant (TCA) treatment. (1) Daily TCA: Subcutaneous injections of desipramine (DMI), 30 mg/kg, chlorimipramine (CMI), 10 mg/kg, or saline, 1 ml/kg, were given daily for 22 days. Morphine sulfate (M), 5 micrograms, was microinjected into the ventrolateral periaqueductal gray (VLPAG) at 7 day intervals. On day 1, DMI or CMI enhanced M analgesia whereas saline did not. Augmentation of M disappeared by days 8 and 15 for CMI and DMI, respectively, and was replaced by attenuation which was still observed on day 22 for both TCAs. L-Tryptophan (LT), 100 mg/kg, i.p., on days 15 and 22 temporarily restored TCA enhancement of M. Fourteen days after cessation of all daily TCA treatments, enhancement of M by CMI was similar to that observed on day 1, whereas recovery of DMI-induced facilitation was incomplete. (2) Weekly TCA: Weekly treatment with DMI, CMI, or saline in the same doses as above had similar effects. M analgesia was enhanced by the TCAs but not saline on day 1; this facilitation was absent by day 15. Attenuation of M by DMI or CMI was evident on day 22; 2 weeks after cessation of all weekly TCA treatments, complete recovery of TCA-induced augmentation was observed. Loss of M facilitation during chronic daily or weekly TCA administration may be related to reduction of central opioid and/or 5-HT2 receptors.

Stress analgesia: effects of PCPA, yohimbine, and naloxone

Evidence suggests that morphine analgesia depends on the integrity of monoaminergic transmitter systems. Some forms of stress analgesia seem to be related to morphine analgesia, while others are not. To assess whether opioid and non-opioid stress analgesia differ in their reliance on monoamine systems, the effects of parachlorophenylalanine (PCPA) and yohimbine on analgesia produced by prolonged intermittent and brief continuous footshock were examined on the hotplate test. The interaction of adrenergic and endorphinergic activity with serotonergic mechanisms following these stressors was also investigated by testing the effects of yohimbine and naloxone on rats with prior PCPA treatment. Yohimbine alone significantly reduced baseline hotplate latencies, while PCPA and naloxone did not. The two stressors differed in the effects produced by both naloxone and PCPA. Naloxone significantly reversed stress analgesia in the prolonged stress condition, but not the brief stress condition. PCPA significantly enhanced the antinociceptive effect of brief continuous shock, while leaving the response to prolonged intermittent shock unaffected. In contrast, yohimbine blocked the analgesic effects of prolonged stress. For the brief stress condition, naloxone reversed the elevated thresholds elicited in PCPA treated rats. Naloxone also reversed stress analgesia for PCPA treated rats exposed to prolonged intermittent stress. Yohimbine lowered the responses of PCPA treated rats subjected to brief continuous shock. These results support an interactive model of stress analgesia dependent upon serotonergic, adrenergic, and endorphinergic transmitter systems.

Neurophysiology of pain and pain modulation

An endogenous central nervous system pain-modulating network, with links in the mid brain, medulla, and spinal cord, has recently been discovered. This system produces analgesia by interfering with afferent transmission of neural messages produced by intense stimuli. Although other neurotransmitters are involved, the analgesia produced by this system depends on the release of endogenous opioid substances, generically referred to as endorphins. The system is set in motion by clinically significant pain--such as that resulting from bony fractures or postoperative pain. The analgesia network monitors the pain and controls it at the level of the spinal cord. Complex psychologic factors play an important role in the variability of perceived pain, partly because of their ability to trigger this pain-suppressing system. For example, this system contributes to the analgesic potency of placebo administration and is also activated by stress. Knowledge of this analgesia system has greatly expanded our understanding of the mechanisms underlying pain management. Opiates, like morphine and meperidine, produce analgesia by mimicking the action of endorphins in the pain. Tricyclic drugs may produce analgesia by enhancing the nonendorphin links of the same system. Future research on this system will provide new insights and, consequently, new approaches to the management of pain.

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.

Reductions in pain thresholds and morphine analgesia following intracerebroventricular parachlorophenylalanine

The selective decreases in both basal and analgesic pain thresholds following systemic administration of parachlorophenylalanine (PCPA) has been attributed to the inhibition of tryptophan hydroxylase and subsequent depletion of brain serotonin. These effects only occur at high systemic doses which have other general debilitating effects. The present study examined the relationship between PCPA's nociceptive and serotonin-depleting effects following intracerebroventricular (ICV) administration. The first experiment determined that an ICV dose of 3 mg, but not 1 mg, of PCPA significantly decreased jump thresholds at 0.5, 48 and 120 hr after injection. These effects were not due to osmolarity shifts since hypertonic saline injections failed to alter thresholds. The second experiment demonstrated a time-dependent reduction of morphine (5 mg/kg) analgesia as a function of the interval between ICV PCPA and the systemic morphine injection. PCPA reduced morphine analgesia if it was administered 24 hr prior to the opiate and eliminated morphine analgesia if it was administered 48 hr prior to the opiate. Pretreatment with ICV PCPA either 0.5 or 72 hr prior to the opiate failed to alter morphine analgesia. The third and fourth experiments indicated that hippocampal and spinal levels of either serotonin or 5-hydroxyindoleacetic acid were not significantly affected by ICV PCPA pretreatment. These data indicate that the hyperalgesia and morphine analgesia impairments noted following ICV PCPA do not correspond with changes in serotonin from hippocampal or spinal tissue and such effects are discussed in terms of alternative modes of action.

Increase in 5-HT synthesis in the dorsal part of the spinal cord, induced by a nociceptive stimulus: blockade by morphine

The effects of a nociceptive peripheral stimulus and/or morphine upon endogenous tryptophan levels (TRP), specific activity of tryptophan (S.A. of TRP) and serotonin (5-HT) synthesis in the dorsal and ventral spinal cord, the brainstem and the forebrain were investigated in anaesthetized rats. Whereas endogenous TRP and S.A. of TRP were not found to be affected by any of the manipulations described below, 5-HT synthesis was markedly altered. The application of a prolonged and intense nociceptive electrical stimulus to the tail induced a rise in 5-HT synthesis which was dependent on the part of the CNS considered, with the dorsal cord being the most sensitive (25%), the ventral cord and the brainstem being effected to a lesser extent (14% and 16% respectively), and the forebrain not being affected significantly. By contrast, the application of a prolonged and innocuous electrical stimulus on the tail was not followed by any detectable changes in 5-HT synthesis. Morphine administration (1 mg/kg; i.v.) did not significantly alter 5-HT synthesis in the four CNS regions considered. Nevertheless, the same morphine dose did induce a highly significant (P less than 0.005) reduction in the increase in 5-HT synthesis induced by the nociceptive stimulus, both in the dorsal cord and in the brainstem. Such an effect was not seen in the ventral cord. The specificity of these morphine effects was demonstrated by their naloxone reversibility; on the other hand, naloxone alone failed to modify the stimulus-induced increase in 5-HT synthesis seen in the dorsal cord and the brainstem. The results, particularly those concerning the dorsal cord, are discussed with reference to pain mechanisms and morphine analgesia. They suggest that peripheral nociceptive messages induce an increased activity in some bulbo-spinal 5-HT pathways and that a low dose of morphine can counteract such an effect. It is proposed that exogenous opiates exert a complex regulation of bulbo-spinal 5-HT pathways. Functional significances of these processes are discussed.

Modification of alpha-adrenoceptor agonist antinociceptive activity by nisoxetine: a selective inhibitor of noradrenergic uptake

The alpha-adrenoceptor agonists, clonidine and xylazine, produced a dose-dependent antinociceptive effect in the mouse writhing assay as does morphine. Fluoxetine, a highly-specific inhibitor of serotonin uptake, enhanced the antinociceptive effect of morphine in this test but not that of clonidine or xylazine. In contrast, nisoxetine, a selective inhibitor of noradrenergic uptake, significantly potentiated the antinociceptive activity of morphine, clonidine, and xylazine. These findings strengthen the evidence for an involvement of a noradrenergic mechanism in the antinociceptive effects of alpha-adrenoceptor agonists.

Nutrition and the neurosurgical patient

There has been a rapid expansion of knowledge in the field of nutrition and metabolism with regard to the general surgical patient. However, only recently has there been greater appreciation of the benefits of adequate nutrition and appropriate metabolic care of the neurosurgical patient. In this review, the authors attempt to outline 1) the metabolic response to stress in general, and how it applies to the neurosurgical patient; 2) how best to provide adequate nutritional support for the neurosurgical patient; 3) the effects of nutrition on neurotransmitters; and 4) the effect of diet and nutrition on patients with malignant brain tumors.

Therapeutic effects of antidepressants in chronic pain

Depression and chronic pain syndromes are often associated. Over the last twenty years there has been a number of controlled and uncontrolled studies evaluating the efficacy of antidepressants in various pain problems. The administration of antidepressant medications in the management of chronic pain has become a commonly prescribed therapeutic modality in the treatment of this complex syndrome. This paper reviews the clinical studies in which antidepressants have been used to control chronic pain, summarizes the results of the clinical studies, and comments on the mechanism of action of antidepressants in chronic pain.

Methysergide and metergoline reduce morphine analgesia with no effect on the development of tolerance in rats

A single subcutaneous injection of 5 mg/kg metergoline or 10 mg/kg methysergide, two serotonin antagonists, or 1 mg/kg naloxone, significantly reduced the effect of a subcutaneous dose of 3 mg/kg morphine in the tail immersion test in rats. The same drugs and doses were administered concurrently with 10 mg/kg morphine twice daily for 3 days and nociceptive responses were measured 96 h later. Tolerance to the effect of 3 mg/kg morphine was comparable in animals which had received vehicle + morphine or serotonin antagonists + morphine, whereas naloxone completely prevented the development of tolerance. The results argue against a role of serotonin in the development of tolerance to the antinociceptive effect of morphine and suggest it may be possible to dissociate morphine analgesia from tolerance development, at least in the conditions used in the present study.

Anatomical and physiological studies of the gray matter surrounding the spinal cord central canal

Recent histochemical evidence suggests that neurons in the gray matter surrounding the central canal may play a role in nociception. We attempted to evaluate this possibility by studying the response properties and ascending projections of these cells in the rat. In the first series of experiments, the ascending projections of neurons around the central canal were studied by the method of retrograde transport of horseradish peroxidase (HRP). Predominantly contralateral labeling of neurons around the central canal appeared after HRP injections into the paramedian medullary or pontine reticular formation in intact or cerebellectomized animals. Far fewer cells were labeled by injections into the lateral medulla and/or pons. A small number of cells was retrogradely labeled by HRP injections into the vermal and intermediate regions of the cerebellum or the periaqueductal gray matter. Injections into other brainstem areas outside of the reticular formation also failed to label large numbers of neurons around the centra canal. In a second set of experiments, we recorded extracellular unitary activity from the lumbar enlargement in spinalized, decerebrate, unanesthetized rats. Thirteen units were functionally characterized and histologically localized within 300 micrometers of the central canal. All of the units identified responded exclusively to noxious stimuli applied within highly circumscribed receptive fields. Thus, neurons around the central canal contribute strongly to long ascending spinal cord projections. Physiologically, neurons found within this region are reminiscent of the noxious-specific cells in the outer most layers of the dorsal horn.