Tail-flick Reflex Research Articles

Neuropharmacological characterization of SR 140333, a non peptide antagonist of NK1 receptors

SR 140333 (1-[2-[3-(3,4-dichlorophenyl)-1-(3-isopropoxyphenylacetyl) piperidin-3-yl]ethyl]-4-phenyl-1-azonia-bicyclo[2.2.2]octane , chloride), a potent non peptide ligand of the substance P (SP) NK1 receptor subtype with high affinity for NK1 receptors from both rat cortical membranes and human IM9 cells (Ki = 0.02 nM and 0.01 nM, respectively) was studied in vivo on various effects induced by NK1 agonists in rats and mice. SR 140333 given intraperitoneally (i.p.) in mice antagonized dose-dependently and in a stereoselective manner the scratching responses induced by intracerebroventricular SP and septide (ID50 = 0.73 and 0.08 mg/kg, respectively) and the turning behavior elicited by intrastriatal SP and septide (ID50 = 0.07 and 0.06 mg/kg, respectively). This compound had little effect on the scratching responses and the turning behavior elicited by [Sar9, Met(O2)11]-SP. When SR 140333 was coadministered with the peptide agonist, the compound reduced the scratching responses elicited by SP, [Sar9, Met(O2)11]-SP and septide injected intrathecally (i.t.) in mice (ID50 = 72.0, 64.3 and 52.5 ng i.t., respectively). SR 140333 antagonized the salivation induced by SP, [Sar9, Met(O2)11]-SP and septide in rats (ID50 = 0.13, 0.18 and 0.09 mg/kg i.p., respectively). SR 140333 abolished the facilitation of the tail-flick reflex induced by noxious heat in rats (total reversal at 0.06 mg/kg, i.p.). This compound was also found to inhibit the turning behavior induced by intrastriatal apomorphine in mice (ID50 = 0.1 mg/kg, i.p.). In conclusion, these results indicate that SR 140333 behaves as a potent, selective and centrally active NK1 receptor antagonist.

Quantitative assessment of nocifensive behavioral responses and the underlying neuronal circuitry

This paper reviews several recently developed animal models that allow a quantitative assessment of the magnitude of nocifensive behavioral responses across a range of noxious stimulus intensities. Models discussed in detail include: (a) the rodent tail flick reflex, and a modification that allows measurement of tail flick magnitude, (b) rat hindlimb flexion withdrawal reflex elicited by noxious thermal stimulation of the paw, and (c) a learned operant response (nose bar press) evoked by noxious thermal stimulation of the rat's tail. These models are discussed in terms of their advantages over previous methods measuring response threshold, their fulfillment of criteria for ideal pain assessment models, and the neuronal circuitry underlying the behavioral response.

Characterization of the role of spinal n-methyl- d-aspartate receptors in thermal nociception in the rat

The effects of N-methyl- d-aspartate (100 amol–1 nmol) on the nociceptive tail-flick reflex were studied in awake rats. Lesser doses of N-methyl- d-aspartate (100 amol–10 pmol) administered intrathecally to the lumbar spinal cord produced a dose-dependent facilitation of the tail-flick reflex (maximum at 0.5–1 min). The greatest dose tested (1 nmol) inhibited the tail-flick reflex (maximum at 2–5 min) and produced a caudally directed scratching and biting behavior accompanied by vocalizations. Intrathecal pretreatment with the N-methyl- d-aspartate receptor antagonist, d-2-amino-5-phosphonovaleric acid (1 fmol–1 pmol), which produced no change in baseline tail-flick latency, blocked all N-methyl- d-aspartate produced effects in a dose-dependent manner (100 fmol d-2-amino-5-phosphonovaleric acid produced maximum blockage for about 40 min). The magnitude and duration of N-methyl- d-aspartate-produced biphasic effects on tail-flick latency were similar in awake and lightly pentobarbital-anesthetized rats, but caudally directed biting and scratching behavior was not produced in lightly anesthetized rats. Reversible spinalization at T 8–T 10 in lightly anesthetized rats (produced by cold-block) completely abolished inhibition of the tail-flick reflex produced by 1 nmol N-methyl- d-aspartate whereas facilitation produced by 10 pmol N-methyl- d-aspartate remained unchanged, indicating that N-methyl- d-aspartate-produced facilitation is a local, segmental effect and that N-methyl- d-aspartate-produced inhibition requires a supraspinal loop. To examine the nature of the supraspinal loop, potential contributions of descending noradrenergic and serotonergic systems were studied. Intrathecal pretreatment with 100 nmol phentolamine completely blocked N-methyl- d-aspartate-produced inhibition of the tail-flick reflex, while N-methyl- d-aspartate-produced facilitation and caudally directed biting and scratching behavior remained unchanged. Intrathecal pretreatment with 50 nmol methysergide reversed the inhibitory effect of 1 nmol N-methyl- d-aspartate, resulting in a potent and prolonged facilitation which could be blocked by d-2-amino-5-phosphonovaleric acid. (1 pmol). Intrathecal pretreatment with an alternate substrate for nitric oxide synthase, N G-nitro- l-arginine methyl ester (100 nmol), completely blocked N-methyl- d-aspartate-produced facilitation of the tail-flick reflex, whereas N-methyl- d-aspartate-produced inhibition and caudally directed biting and scratching behavior were unaffected. In summary, these data suggest that N-methyl- d-aspartate-produced facilitation of the tail-flick reflex is a local, segmental phenomenon, but inhibition of the tail-flick reflex and caudally directed scratching and biting behavior are produced by activation of ascending pathways which, in case of the tail-flick reflex, engages descending inhibitory pathways to produce inhibition of the tail-flick reflex. Taken together, these data suggest a multiplicity of effects of N-methyl- d-aspartate receptor activation in spinal nociceptive processing.

Interleukin-1 mediates the behavioral hyperalgesia produced by lithium chloride and endotoxin

The sickness-inducing agents lithium chloride (LiCl) and lipopolysacharide (LPS) produce a long-lasting facilitation of the nociceptive tailflick reflex. Many of the behavioral and physiological changes produced by illness are mediated by interleukin-1 (IL-1) released from monocytes stimulated by the pathogenic substance. Monocytes also produce an IL-1 receptor antagonist (IL-1 ra) which has been sequenced and cloned. The present experiments report that IL-1 can itself produce hyperalgesia as assessed by tailflick to radiant heat, and that recombinant IL-1 ra blocks the hyperalgesia produced by LiCl and LPS.

Rat tail flick reflex: magnitude measurement of stimulus-response function, suppression by morphine and habituation.

1. To quantitatively investigate a nocifensive behavioral response, we developed a method to measure the magnitude of the rat's tail flick reflex and its modulation. A radial array of force transducers measured forces of tail flicks (in rostral, horizontal, and vertical planes) elicited by graded noxious radiant thermal stimulation of the conscious rat's tail, from which the overall movement vector was calculated. 2. The rostrally directed component of tail flicks was always larger than dorsal or horizontal components; the latter was usually in a preferred (left or right) direction regardless of which side of the tail was heated. Tail flick force vectors increased from 40 to 46-52 degrees C and then leveled off. Stimulus-response functions were reproducible within and across rats and were fitted by second-order polynomial functions, whose correlation coefficients were similar when the left or right side of the tail was stimulated in separate sessions (r2 = 0.408 and 0.451, respectively). The inverse latency of tail flicks also increased with temperature in a manner fitted by a second-order polynomial (r2 = 0.707, 0.553 for left and right side, respectively). 3. Systemic administration of morphine (1 or 2 mg/kg ip) usually suppressed tail flicks in an all-or-none manner; i.e., flicks at all stimulus temperatures were either totally abolished (n = 7) or unaffected (n = 5) after morphine. In three rats, 1 mg/kg morphine suppressed tail flick magnitude subtotally, reducing the slope of the linear portion of the stimulus-response function. Morphine effects were reversed by the opiate antagonist naloxone. 4. Tail flick magnitude decreased over repeated trials of 44 degrees C heat stimuli delivered to one tail site, recovered after a 15-min rest period, and decremented more quickly with subsequent stimulus repetition. The decrement was less at long (2 or 4 min) than at short (1 min) interstimulus intervals, and high (50 degrees C) than at low (44 degrees C) stimulus intensities. The reflex decrement transferred to a nearby stimulus site in some rats, and was "dishabituated" after a noxious tail pinch. These observations are consistent with habituation of the tail flick reflex. 5. This method, therefore, provides a quantitative and reproducible measure of tail flick reflex magnitude that is sensitive to morphine. The underlying neural circuitry of the tail flick reflex is discussed in relation to limb withdrawal reflexes.

Intrathecal coadministration of serotonin and morphine differentially modulates the tail-flick reflex of intact and spinal rats

In a previous study, we found that the antinociceptive effect of IT-administered morphine on the tail-flick (TF) reflex of rats was potentiated within 1 day after spinal transection. This suggested that the analgesic effect of spinal morphine in the intact animal was tonically suppressed, presumably by the release of a transmitter(s) from descending supraspinal pathway(s), and that the potency of IT morphine was increased because these inputs were removed by spinalization. Because spinally projecting serotonin [5-hydroxytryptamine (5-HT)] fibers are known to be involved in modulating nociception at this site, the present studies examined the possibility that 5-HT might be the proposed “antiopiate” at the spinal cord. Separate groups of intact and spinal rats were pretested on the TF and then injected IT with either morphine (intact: 0.25–5.0 μg, spinal: 0.0312–0.5 μg) or 5-HT (1–200 μg), or combinations of these two agents, in a single solution. All rats were then retested 15 min later and the difference in latency was used to compare the effect of these treatments. The results confirmed that the antinociceptive effect of IT morphine was significantly increased by spinalization, whereas the antinociceptive effect of 5-HT was essentially abolished. In intact rats, morphine-induced analgesia was potentiated by a low (10 μg) dose of 5-HT but not by higher doses. However, in the spinal rat morphine-induced antinociception was antagonized by the same (10 μg) dose. The data suggest that IT 5-HT promotes antinociception in intact rats but acts pro-nociceptively in spinal rats. It is speculated that this difference may be due to an intervening mechanism(s) in the interaction between spinal morphine and serotonin that requires supraspinal mediation.

Is skin temperature a significant confounding factor in the tail-flick test?

I n their focus article, Tjelsen and Hole argue that the latency of the rodent tail-flick reflex depends on tail temperature. Thus, drugs or 0ther manipulations that raise tail skin temperature Could reduce the latency of the tail-flick reflex, and this might be misinterpreted as hyperalgesia. Conversely, an experimentally induced reduction in tail temperature could increase the tail-flick latency, and this might be misinterpreted as analgesia. Although the focus article serves to alert researchers to a potentially serious shortcoming of this widely used experimental pain assay, it also provides solutions to the problem. The main objective of this commentary is to critically examine the contention that variability in tail skin temperature is of sufficient import to warrant the methodological changes suggested by the authors. The rodent's tail temperature is usually at or close to the ambient temperature over a normal range (21°-26°C). Periodically, spontaneous vasodilation of the ventral (and to a lesser extent lateral) tail arteries may occur, resulting in an abrupt increase in tail temperature toward core temperature, followed by an exponential decline over a period of several minutes. 9 This allows heat dissipation from the taill an important thermoregulatory mechanism in rodents. Such vasodilation occurred very infrequently at ambient temperatures below 29°C and was continuous at temperatures above 33°C. In most laboratories with an ambient temperature range of 21°-24°C, we can therefore assume that the tail temperature will normally be at or near ambient. One might then expect that certain experimental manipulations would cause

Vagal afferent stimulation-produced effects on nociception in capsaicin-treated rats

1. The effects of electrical stimulation of cervical vagal afferent fibers on the nociceptive tail-flick (TF) reflex and responses of spinal dorsal horn neurons to noxious cutaneous stimulation were studied in adult rats treated as neonates with either capsaicin or vehicle. 2. Vagal afferent stimulation (VAS) produced biphasic, intensity-dependent effects on the TF reflex in vehicle-treated and untreated control rats. The TF reflex was facilitated in both groups of rats at lesser intensities of VAS (2.5-50 microA) and fully inhibited at greater intensities of VAS (50-100 microA). In contrast, biphasic effects of VAS on the TF reflex generally were not produced in rats treated as neonates with capsaicin. Facilitation of the TF reflex was produced in these rats by lesser intensities of VAS as well as by typically "inhibitory" intensities of VAS; the TF reflex was never inhibited in 6/12 rats, even at the greatest intensity of VAS tested (1,000 microA). When the TF reflex was inhibited by VAS in capsaicin-treated rats, the intensities required were significantly greater than those required in vehicle-treated or untreated rats. 3. In electrophysiological experiments, 77 neurons were recorded in the lumbar spinal dorsal horn of pentobarbital sodium-anesthetized, paralyzed rats treated as neonates with either vehicle or capsaicin. The neurons had receptive fields on the glabrous skin of the plantar surface of the ipsilateral hind foot, and all responded to mechanical stimuli of both nonnoxious and noxious intensities; 16/77 neurons also responded to noxious thermal stimulation. In vehicle-treated rats, nociceptive responses of 50% of 30 units studied were biphasically modulated by VAS, 33% were only inhibited, and 17% were only facilitated by VAS at the intensities tested (5-500 microA). In capsaicin-treated rats, nociceptive responses of 32% of 47 units studied were biphasically modulated by VAS, 15% were only inhibited, and 34% were only facilitated by VAS at the intensities tested (5-500 microA). In addition, nociceptive responses of neurons facilitated at lesser intensities of VAS and not affected at greater intensities of VAS were observed in capsaicin-treated rats (19% of the 47-unit sample). Overall, the proportion of the neuronal sample inhibited by VAS was less, and the proportion of the sample facilitated by VAS was greater in capsaicin-treated rats compared with vehicle-treated rats. 4. The efficacy of the capsaicin treatment was evaluated immunocytochemically.(ABSTRACT TRUNCATED AT 400 WORDS)

Lipopolysaccharide induces fever and decreases tail flick latency in awake rats

In humans, somatic aches and pains are frequently reported during fever. To determine whether exogenous pyrogen administration increases nociceptive responsiveness in rats, the tail flick reflex evoked by noxious heat was compared in animals that received an intraperitoneal injection of exogenous pyrogen (lipopolysaccharide) or vehicle (saline). There were no differences in skin temperature between lipopolysaccharide- and vehicle-injected rats. Awake rats injected with lipopolysaccharide exhibited an increase in colonic temperature and a decrease in tail flick latency when compared to vehicle-injected rats. Neural events that trigger fever may therefore also modulate nociceptive responsiveness.

Inhibition of the tail flick reflex following microinjection of morphine into the amygdala.

Recent evidence indicates that the amygdala plays a critical role in the activation of brain stem antinociceptive systems during stress. In the present experiment, bilateral microinjection of morphine sulfate (10 micrograms) into the amygdala of pentobarbital-anesthetized rats resulted in a time-dependent elevation in latency of the tail flick reflex evoked by radiant heat. The most effective sites within the amygdala were in or immediately adjacent to the basolateral nucleus. The relative amplitude of the tail flick reflex did not differ as a function of repeated testing or morphine treatment. These results suggest that important forebrain inputs which normally activate endogenous antinociceptive systems in behaving animals may be manipulated and studied in detail using the anesthetized rat.

Dynorphin A (1–13) potentiates dynorphin A (1–17) on loss of the tail-flick reflex after intrathecal injection in the rat

Dynorphin A (1–13) or dynorphin A (1–17) administered intrathecally to rats induces a dose-dependent loss of the tail-flick reflex and a reversible hindlimb paralysis. Although their potency is comparable, dynorphin A (1–17) is less efficacious in producing loss of the tail-flick reflex. These data suggest that dynorphin A (1–17) acts as a partial agonist or exerts its neurotoxic effect in the presence of an endogenous non-competitive inhibitor. To determine whether these substances act at similar sites we performed isobolographic analysis. This analysis revealed that dose-additivity exists for paralysis whereas deviation from dose-additivity exists for loss of the tail-flick reflex.

The use of specific opioid agonists and antagonists to delineate the vagally mediated antinociceptive and cardiovascular effects of intravenous morphine

Intravenous (i.v.) administration of morphine produces a dose-dependent inhibition of the tail-flick (TF) reflex, depressor response, and bradycardia in the rat. Some of these effects depend on interactions of i.v. morphine with peripheral opioid receptors and the integrity of cervical vagal afferents. The present studies used the relatively specific μ, δ, and κ opioid receptor agonists (DAGO, DPDPE or U-50,488H) and the relatively specific μ, δ, and κ opioid receptor antagonists (ß-FNA, naloxonazine, naltrindole or nor-BNI) in either intact rats or rats with bilateral cervical vagotomy (CVAG) to delineate the vagal afferent/opioid-mediated components of these effects. I.v. administration of DAGO in intact rats produced a dose-dependent inhibition of the TF reflex, depressor response, and bradycardia virtually identical to those produced by i.v. morphine. All of these effects of either i.v. DAGO or i.v. morphine were significantly attenuated by either bilateral CVAG or pre-treatment with theμ 2 opioid receptor antagonist ß-FNA. Pre-treatment with theμ 1 opioid receptor antagonist naloxonazine affected i.v. DAGO-induced inhibition of the TF reflex and bradycardia, but had no significant effects on i.v. morphine-produced responses. I.v. administration of DPDPE produced a dose-dependent pressor response, but had no marked effects on the either the TF reflex or heart rate (HR). The pressor response was unaffected by either bilateral CVAG or pre-treatment with naltrindole, naloxone, hexamethonium, or bertylium. i.v. administration of U-50, 488H produced a depressor response and bradycardia, but had no significant effect on the TF reflex. The depressor response and bradycardia produced by i.v. U-50,488H were unaffected by bilateral CVAG, but could be antagonized by pre-treatment with either nor-BNI or naloxone. These studies suggest that the vagal afferent-mediated antinociceptive and cardiovascular effects of i.v. morphine are primarily mediated by interactions with low affinityμ 2 opioid receptors.

Simultaneous multi- and single-unit recordings in the rostral ventromedial medulla of ketamine-anaesthetized rats, and the cross-correlogram analysis of their interactions.

In anaesthetized rats, neurons located in the rostral ventromedial medulla (RVM) are classified by their differential responses to noxious stimulation. "On" cells are excited by noxious stimulation, "off" cells are inhibited and "neutral" cells are unaffected. Past research has primarily investigated the activity of these cells in lightly barbiturate anaesthetized animals while inducing a tail flick reflex. We recorded neuronal activity in the RVM of deeply ketamine-xylazine anaesthetized rats using a novel method of recording single- and multiunit activity simultaneously with one electrode. Under this anaesthetic paradigm we were able to record RVM cells which responded to noxious tail heating in the same manner as on and off cells without inducing a tail flick. A cross-correlogram analysis was done between the two levels of activity. It demonstrated that the RVM is a highly coordinated system in both the spontaneous period and the noxious evoked activity period. Through this technique it was also shown that both on and off cells fire simultaneously in the ketamine-xylazine paradigm.

Activity of nociceptive modulatory neurons in the rostral ventromedial medulla associated with volume expansion-induced antinociception

A wide range of environmental stimuli have been shown to induce antinociception. Investigation of the brain regions contributing to environmentally induced antinociception (EIA) has focused primarily on the effect of disrupting neural transmission at various CNS sites. In contrast, the present study analyzed changes in the activity of nociceptive modulatory neurons in the rostral ventromedial medulla (RVM) following induction of antinociception by a physiological stimulus, volume expansion. Previous research indicates that 2 classes of RVM neuron, the on- and off-cells, facilitate and inhibit nociception, respectively. In the present study, volume expansion induced by intravenous administration of Ficoll inhibited the tail-flick reflex in 17 of 21 rats and simultaneously increased the firing rate of off-cells and decreased that of on-cells. Changes in cell activity occurred only in those rats in which antinociception was produced. Lidocaine inactivation of the RVM did not block antinociception produced by Ficoll infusion. These findings demonstrate that, although antinociception produced by physiological activation of vagal afferents is associated with an increase in off-cell and a decrease in on-cell activity, these changes are not required for volume expansion-induced antinociception.

Electrical stimulation of the subdiaphragmatic vagus in rats: inhibition of heat-evoked responses of spinal dorsal horn neurons and central substrates mediating inhibition of the nociceptive tail flick reflex

Electrical stimulation of the subdiaphragmatic branch of the vagus nerve (SDVAS) inhibits the nociceptive tail flick (TF) reflex. The present experiments examined (1) the central substrates mediating SDVAS-produced inhibition of the TF reflex and (2) the effects of SDVAS on either background or noxious heat-evoked responses of spinal dorsal horn neurons. Microinjections of ibotenic acid in the ipsilateral nucleus tractus solitarius (NTS), nucleus raphe magnus (NRM), or bilateral locus coeruleus/subcoeruleus (LC/SC) significantly increased the intensity of SDVAS required to inhibit the TF reflex in lightly anesthetized rats. In studies of class-2 spinal dorsal horn neurons, SDVAS produced significant intensity-dependent inhibition of noxious heat-evoked responses of 17 25 (68%) units, facilitation of 4 25 (16%) units, and no effect on 4 25 (16%) units. In studies of class-3 spinal dorsal horn neurons, SDVAS produced significant intensity-dependent inhibition of noxious heat-evoked responses of 8 9 (89%) units. Noxious heat-evoked responses of 1 9 (11%) unit were facilitated by SDVAS. In general, the background activity of either class-2 or class-3 units was not significantly affected by SDVAS. SDVAS produced a significant rightward, parallel shift in the stimulus response function (SRF) of class-2 neurons to noxious, graded heat stimuli ranging from 40 to 52°C, while SDVAS produced a significant increase in the threshold and a significant reduction in the slope of the SRF of class-3 neurons. These data indicate that SDVAS generally inhibits noxious heat-evoked responses of lumbosacral spinal dorsal horn neurons in the rat, but does not significantly affect background activity of the same neurons. Furthermore, the inhibition of the TF reflex produced by SDVAS depends on central relays in the ipsilateral NTS, NRM, and bilateral LC/SC.

Noradrenergic modulation of noxious heat-evoked fos-like immunoreactivity in the dorsal horn of the rat sacral spinal cord.

The tail-flick withdrawal reflex commonly is used to study spinal nociceptive mechanisms; noradrenergic agonists administered intrathecally inhibit the tail-flick reflex in a dose-dependent manner. The objectives of the present study were: (1) to use fos-like immunoreactivity as a marker for neuronal activity to examine the population of neurons in the spinal cord dorsal horn that are engaged by activation of nociceptive tail afferents, and (2) to determine whether fos-like immunoreactivity can be modulated by intrathecally administered alpha adrenoceptor agonists. Neurons demonstrating heat-evoked fos-like immunoreactivity were identified bilaterally in the sacral spinal cord in superficial and deep dorsal horn laminae. Heat-evoked fos-like immunoreactivity was inhibited dose-dependently by intrathecal norepinephrine (NE). The inhibition was attenuated significantly by: (1) phentolamine (PHEN), a nonselective alpha adrenoceptor antagonist; (2) yohimbine (YOH), an alpha-2 adrenoceptor antagonist; and (3) prazosin (PRAZ), an alpha-1 adrenoceptor antagonist. Thus, both spinal alpha-1 and alpha-2 adrenoceptors mediate the inhibition of heat-evoked fos-like immunoreactivity produced by intrathecal NE. ST-91, an alpha-2 adrenoceptor agonist, also inhibited significantly the expression of fos-like immunoreactivity; the inhibition was antagonized by YOH. In the absence of noxious heat, intrathecal NE dose-dependently evoked the expression of fos-like immunoreactivity in the superficial dorsal horn, which was antagonized by PHEN and PRAZ, but not by YOH, suggesting that the effect is mediated by spinal alpha-1 adrenoceptors.

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, catalapsy 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.

Opioid receptors mediating antinociception from beta-endorphin and morphine in the periaqueductal gray.

beta-Endorphin and morphine produce an increase in the latency of the tail-flick reflex when administered into the PAG of awake rats. The antinociceptive effect of both opioid agonists was blocked by the sequential local injection of either CTP (D-Phe-Cys-Tyr-D-Trp-Lys-Thr-Pen-Thr-NH2), a selective mu opioid receptor antagonist, naltrexone, or beta-endorphin (1-27), a putative epsilon opioid receptor antagonist, with minimal selectivity. When either CTP or naltrexone was used as the antagonist, the dose-inhibition curves generated for beta-endorphin and morphine were not parallel, suggesting the involvement of separate and distinct receptors. Also, synergism occurred when a dose of morphine producing submaximum antinociception was administered simultaneously with either a submaximal or ineffective dose of beta-endorphin. Inhibition of the antinociceptive response to beta-endorphin by mu antagonists and the non-selective antagonism of both beta-endorphin and morphine by beta-endorphin (1-27) suggested that epsilon opioid receptors were not involved. Additionally, a mu/delta opioid receptor complex was not involved, since ICI 174,864 (Allyl2-Tyr-Aib-Aib-Phe-Leu-OH), a selective delta opioid receptor antagonist, did not alter the response to beta-endorphin. Thus, although additional characterization is required, beta-endorphin and morphine appear to act (at least in part) through different opioid receptors, demonstrable using selected mu opioid receptor antagonists.

Intravenous morphine-induced activation of vagal afferents: peripheral, spinal, and CNS substrates mediating inhibition of spinal nociception and cardiovascular responses

1. Intravenous administration of 1.0 mg/kg of morphine produces inhibition of the nociceptive tail-flick (TF) reflex, hypotension, and bradycardia in the pentobarbital-anesthetized rat. The present experiments examined peripheral, spinal, and supraspinal relays for inhibition of the TF reflex and cardiovascular responses produced by morphine (1.0 mg/kg iv) in the pentobarbital-anesthetized rat using 1) bilateral cervical vagotomy, 2) spinal cold block or mechanical lesions of the dorsolateral funiculi (DLFs), or 3) nonselective local anesthesia or soma-selective lesions of specific CNS regions. Intravenous morphine-induced inhibition of responses of unidentified, ascending, and spinothalamic tract (STT) lumbosacral spinal dorsal horn neurons to noxious heating of the hindpaw were also examined in intact and bilateral cervical vagotomized rats. 2. Bilateral cervical vagotomy significantly attenuated inhibition of the TF reflex and bradycardia produced by intravenous administration of morphine. Bilateral cervical vagogtomy changed the normal depressor response produced by morphine into a sustained pressor response. Inhibition of the TF reflex in intact rats was not due to changes in tail temperature. 3. Spinal cold block significantly attenuated inhibition of the TF reflex, the depressor response, and the bradycardia produced by intravenous administration of morphine. However, bilateral mechanical transections of the DLFs failed to significantly affect either inhibition of the TF reflex or cardiovascular responses produced by this dose of intravenous morphine. 4. Microinjection of either lidocaine or ibotenic acid into the nuclei tracti solitarii (NTS), rostromedial medulla (RMM), or ventrolateral pontine tegmentum (VLPT) attenuated morphine-induced inhibition of the TF reflex. Similar microinjections into either the periaqueductal gray (PAG) or the dorsolateral pons (DLP) failed to affect morphine-induced inhibition of the TF reflex. 5. Microinjection of either lidocaine or ibotenic acid into the NTS, RMM, VLPT, DLP, or rostral ventrolateral medulla (RVLM) attenuated the depressor response produced by morphine, although baseline arterial blood pressure (ABP) was affected by ibotenic acid microinjections in the DLP. In all these cases, the microinjections failed to reveal a sustained pressor response as was observed with bilateral cervical vagotomy. Similar microinjections into the PAG failed to affect the depressor response produced by morphine. 6. The lidocaine and ibotenic acid microinjection treatments also showed that the bradycardic response produced by morphine depends on the integrity of the NTS, RMM, RVLM, and possibly the DLP, but not the PAG or VLPT.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of a cutaneous nociceptive reflex by a noxious visceral stimulus is mediated by spinal cholinergic and descending serotonergic systems in the rat

The present study examined the spinal pathway and receptors that mediate nocigenic inhibition of the tail-flick (TF) reflex produced by conditioning colorectal distension (CRD). Conditioning CRD (80 mmHg; 30 s) inhibited the TF reflex in all rats studied ( n = 29). In 19 rats where intensity-dependent effects of CRD were studied, conditioning CRD in 7 rats facilitated the TF reflex at lesser, non-noxious intensities (mean 7.9 ± 2.1 mmHg) and inhibited the TF reflex at greater, noxious intensities (40–100 mmHg); conditioning CRD at all intensities tested only inhibited the TF reflex in the other 12 rats. Inhibition of the TF reflex produced by 30 s CRD was short-lasting, repeatable and graded with the intensity of CRD. The mean threshold of CRD for inhibition of the TF reflex to cut off (10 s) was 61.4 ± 3.3 mmHg ( n = 29). Intrathecal pretreatment with atropine or methysergide significantly attenuated the inhibitory effect of CRD on the TF reflex; the effects were time- and dose-related. Intrathecal pretreatment with mecamylamine, phentolamine or naloxone was without effect. Intrathecal administration of physostigmine, an acetylcholinesterase inhibitory, significantly reduced that threshold intensity of conditioning CRD necessary to inhibit the TF reflex to cut off (mean 36.0 ± 4.0 mmHg; n = 5). Bilateral transections of the spinal dorsolateral funiculi (DLF) did not affect the inhibitory effect of CRD in 4/7 rats and attenuated the inhibitory effect of CRD in the other 3 rats. The antagonistic effect of methysergide on CRD-produced inhibition of the TF reflex was abolished following the DLF transections, while scopolamine retained its efficacy in rats with bilateral DLF transections. These findings provide evidence for involvement of spinal cholinergic interneurons as well as a descending serotoninergic pathway traveling in the DLF in CRD-produced inhibition of the TF reflex.