Action of narcotic analgesics and antagonists on spinal units responding to natural stimulation in the cat.

Morphine and morphine-related agents were applied by microiontophoresis in the lumbar spinal cord of spinal cats to single units classified on the basis of their responses to natural cutaneous or proprioceptive stimulation. Opiate application had a current-dependent depressant effect on the ongoing activities of about one-third of the units tested. This effect was observed in laminae I and IV--VI, but only with units responding to noxious cutaneous stimuli: the nociceptive responses were themselves depressed. Excitatory and inhibitory responses to glutamate and gamma-aminobutyric acid, respectively, were also depressed. Intravenous administration of the opiates at doses reported to produce analgesia in the cat also depressed only units responding to noxious cutaneous stimuli, including their nociceptive responses. This depression could be reversed by either the iontophoretic application (100 nA) or the intravenous administration (0.1--0.8 mg/kg) of naloxone. These results are interpreted as further evidence that the analgesic effects of opiates are at least partly due to an action at the spinal level.     

Induction of glycerol phosphate dehydrogenase gene expression during seizure and analgesia

Using mRNA differential display, we found that the gene for NAD(+)-dependent glycerol phosphate dehydrogenase (GPDH; EC 1.1.1.8) is induced in rat brain following seizure activity. Northern blot and in situ hybridization analysis confirmed the differential display results; they also showed, in a separate model of neuronal activation, that after thermal noxious stimulation of the hind-paws, a similar increase in GPDH mRNA occurs in the areas of somatotopic projection in the lumbar spinal cord. Surprisingly, administration of analgesic doses of morphine or the nonsteroidal antiinflammatory drugs aspirin, metamizol (dipyrone), and indomethacin also increased GPDH mRNA levels in rat spinal cord. The opioid receptor antagonist naloxone completely blocked morphine induction of GPDH but had no effect on GPDH induction by noxious heat stimulation or metamizol treatment, implicating different mechanisms of GPDH induction. Nevertheless, in all cases, induction of the GPDH gene requires adrenal steroids and new protein synthesis, as the induction was blocked in adrenalectomized rats and by cycloheximide treatment, respectively. Our results suggest that the induction of the GPDH gene upon peripheral noxious stimulation is related to the endogenous response to pain as it is mimicked by exogenously applied analgesic drugs.     

Role of endogenous opiates and extracellular K+ accumulation in the inhibition of frog spinal reflexes by electrical skin stimulation

Electrical skin stimulation of the hind limb (10-100 Hz, 30 s-5 min) at the intensity which leads only to the excitation of low threshold afferents depressed (for 1-30 min) the flexor reflex evoked in spinal frogs by nociceptive stimuli. The inhibition, which lasted for longer than 5 min was blocked by naloxone. Short-term poststimulation effects were associated with an increase of extracellular K+ concentration (delta [K]e) and were not blocked by naloxone. Enkephalins or morphine applied to the spinal cord surface increased the threshold for flexor reflexes while naloxone decrease their threshold. The stimulation was followed by short-term hyperpolarization of primary afferents (PAH; 1-5 min) and by depression of dorsal root potentials (DPRs) which had a similar time course to the delta [K]e, and were not blocked by naloxone. This period was frequently followed by longlasting PAH and enhancement of DRPs (5-30 min), which were abolished by naloxone. Superfusion of the isolated spinal cord with opioids produced PAH and enhanced DRPs evoked by nociceptive stimuli, while naloxone or increase of [K] in Ringer solution depolarized primary afferents and depressed DRPs. It is suggested that the antinociceptive effects of electrical stimulation of low threshold cutaneous afferents in spinal frogs involves at least two mechanisms. The short-term effect may result from delta [K]e, especially at high stimulus strength and is equally effective against noxious and non-noxious stimuli. The longlasting effects selectively affecting nociceptive transmission appear to be produced by endogenous opioids.     

中脑导水管周围灰质在侧脑室注入微量吗啡或纳洛酮所引起的镇痛或拮抗镇痛中的作用

本工作进一步探索中脑导水管周围灰质(PAG)在吗啡镇痛与纳洛酮拮抗吗啡镇痛中的作用。实验在清醒受限制的大鼠上进行,以电刺激鼠尾出现的甩尾和嘶叫为痛反应指标。结果表明:(1)侧脑室注射微量纳洛酮后,可使电刺激 PAG 或注射微量吗啡于 PAG 所引起的镇痛效应受到明显拮抗;(2)损毀 PAG 或注射微量纳洛酮于 PAG 后,可使由侧脑室注入微量吗啡所引起的镇痛效应显著减弱。由此可见 PAG 既是侧脑室注射吗啡镇痛作用的重要中枢部位,又是侧脑室注射纳洛酮拮抗吗啡镇痛的重要中枢部位。     

In vivo catechol activity in the rat locus coeruleus following different nociceptive stimuli and naloxone.

The nucleus locus coeruleus (LC) has been implicated in the processing of spinal reflexes following noxious stimuli. It has been demonstrated that noxious stimuli activate LC neuronal firing, but little is known about the neurochemical changes that might occur following such activation. To determine the effects of different noxious stimuli on LC neuronal activity, anaesthetized rats were exposed to mechanical (tail pinch), thermal (55 degrees C water), and chemical (5% Formalin injected in the hind paw) stimuli; the catechol oxidation current (CA.OC), an index of noradrenergic neuronal activity, in the locus coeruleus was monitored using differential normal pulse voltammetry. In addition, the effect of the opioid antagonist naloxone on the CA.OC in the LC was examined. Exposure to both mechanical and chemical stimuli significantly increased CA.OC indicating an increase in LC noradrenergic neuronal activity, while the thermal stimulus had no effect. Treatment with naloxone (1 mg/kg i.v.) had no effect on CA.OC in the LC. The results show a differential responsiveness of LC noradrenergic neurons to different modes of noxious stimuli and fail to demonstrate a tonic opioid regulation of these neurons in the anaesthetized rat.     

Reversal by naloxone of adaptation of rats to noxious stimuli.

Bradykinin, a potent pain-producing substance, injected into the common iliac artery of the rat produced flexor reflexes of the hind-limb. The bradykinin-induced response was reduced following the repetition of such noxious stimuli in some male Sprague-Dawley rats classified as the noxious-adaptable group, while the reduction of bradykinin-response did not occur in the majority of the same strain rats in the noxious-non-adaptable group. Naloxone in doses of 0.1 – 2.0 mg/kg restored the flexor reflexes to the noxious stimulation, in a dose dependent manner, in the former group; ED_{5 0} value being 0.61 mg/kg. On the other hand, naloxone in doses up to 5.0 mg/kg produced no hyperalgesic effect in the latter group. Thus, genetic variation in endogenous opioid systems has to be considered.     

Restraint stress enhances morphine-induced analgesia in the rat without changing apparent affinity of receptor

Antagonism of morphine analgesia (tail-flick assay) by naloxone was assessed quantitatively by in vivo "apparent" pA2 determination in unstressed rats and in rats subjected to restraint stress. Restrained rats had a higher baseline tail-flick latency than did unstressed (unrestrained) animals, and were more sensitive to the analgesic effect of morphine, as reflected in lower morphine ED50s. There was no significant difference between apparent pA2 values of unstressed and restrained rats using pA2 regression line analysis. This suggests that while stress enhances the analgesic effect of morphine, it does not appreciably alter opiate receptor affinity for naloxone under the conditions of this study.     

Activation of pontine and bulbar reticulo-spinal neurons in the cat by somatosensory stimuli of different modalities

The response pattern of reticulo-spinal (RS) neurons in two reticulo-spinal structures (n. reticularus pontis caudalis and n. reticularis gigantocellularis) to both electrical (somatic) nerve stimulation and natural mechanical innocuous (tapping with varying force) and noxious (pinch and prick) stimulation were investigated in chloralose-anesthetized cats. Bulbar and pontine neurons were found to vary considerably in their sensory characteristics: of the former 43% were activated only by high-threshold electrical nerve stimulation and noxious stimuli, while the remainder responded to innocuous stimuli as well. In the case of pontine neurons 81% produced a response to stimulation of low-threshold nerve fibers, and to innocuous as well as noxious stimuli. A relationship was found between the sensory characteristics of reticulo-spinal neurons and their axon conductance velocities. Various aspects and the likely functional significance of specialization in brainstem neurons of the pontine and bulbar reticular formation come under discussion.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 18, No. 4, pp. 461–469, July–August, 1986.     

Descending influences of periaqueductal gray matter and somatosensory cerebral cortex on neurones in trigeminal brain stem nuclei.

Single relay (to thalamus) and nonrelay neurones that responded to innocuous and/or noxious oral-facial stimuli were located in trigeminal brain stem nuclei oralis and caudalis. The responses of the cells and the digastric muscle to these stimuli were tested with conditioning stimulation of the periaqueductal gray matter (PGM) and somatosensory cerebral cortex in cats. A greater suppression of nociceptive responses with PGM stimulation was noted, and this effect may contribute to the profound analgesic action that has been reported to occur with PGM stimulation.     

Vascular effects of substance P change synaptic responsiveness of cat dorsal horn neurons

The effects of systemically administered substance P were examined on the responses induced by noxious (radiant heat) and non-noxious (air jets) peripheral stimuli in dorsal horn neurons of the feline spinal cord. Substance P produced a significant fall in arterial blood pressure and selectively enhanced responses to noxious heat stimulation. Other vasoactive substances administered systemically caused either selective increases or decreases in noxious heat induced responses which coincided with decreases or increases in systemic arterial blood pressure respectively. It was concluded that the selective neuronal effects of substance P were secondary to changes in vascular perfusion in the area stimulated by the radiant heat source.     

电刺激家兔大脑皮层感觉区对丘脑束旁核痛敏单位放电活动的影响

本工作目的在于探讨大脑皮层与丘脑束旁核痛觉信息活动之间的关系。用玻璃微电极记录了60只清醒麻痹状态下家兔束旁核痛敏单位的诱发放电活动。按其对外周伤害性刺激的反应型式而分成两类,分别称为痛兴奋单位和痛抑制单位。观察到在电刺激大脑皮层感觉区时,多数痛兴奋单位呈抑制性改变,并且有时施予皮层的每一单个刺激均能立即遏止一阵痛放电,少数呈易化性改变;痛抑制单位则对伤害性刺激呈现抑制反应解除,而施予皮层的每一单个刺激均可使之先有一阵短暂的放电,然后出现抑制的现象。本组实验观察过程,完整的总例数共32个单位,包括25个痛兴奋单位(18个呈抑制改变,7个呈易化改变)和7个痛抑制单位(全部呈易化改变)。实验结果表明,大脑皮层感觉区传出系统对机体的痛觉信息传递活动具有下行调制作用。由于皮层刺激对束旁核放电的影响是逐渐发展起耒的,因此考虑在上述过程中可能有体液因素的参与。     

Dynamic baroreflex control of blood pressure: influence of the heart vs. peripheral resistance

The aim in the present experiments was to assess the dynamic baroreflex control of blood pressure, to develop an accurate mathematical model that represented this relationship, and to assess the role of dynamic changes in heart rate and stroke volume in giving rise to components of this response. Patterned electrical stimulation [pseudo-random binary sequence (PRBS)] was applied to the aortic depressor nerve (ADN) to produce changes in blood pressure under open-loop conditions in anesthetized rabbits. The stimulus provided constant power over the frequency range 0-0.5 Hz and revealed that the composite systems represented by the central nervous system, sympathetic activity, and vascular resistance responded as a second-order low-pass filter (corner frequency approximately 0.047 Hz) with a time delay (1.01 s). The gain between ADN and mean arterial pressure was reasonably constant before the corner frequency and then decreased with increasing frequency of stimulus. Although the heart rate was altered in response to the PRBS stimuli, we found that removal of the heart's ability to contribute to blood pressure variability by vagotomy and beta(1)-receptor blockade did not significantly alter the frequency response. We conclude that the contribution of the heart to the dynamic regulation of blood pressure is negligible in the rabbit. The consequences of this finding are examined with respect to low-frequency oscillations in blood pressure.     

NMDA and strychnine diversely modulate spinal dorsal horn noxious responsiveness in normal rats: potential significance to sensory disorders in neuropathic pain

Changes in neuronal excitability due to increase in excitatory transmitters and/or removal of local inhibition underlie central neuron sensitization and altered responsiveness related to painful sensory disorders. To distinguish the contribution of each of the two mechanisms, they have been mimicked separately in intact rats, by iontophoretically applying excitatory (NMDA) and disinhibitory (the glycine antagonist strychnine) substances during dorsal horn neuron recording. Wide dynamic range (WDR) neurons were extracellularly recorded at the L5-L6 lumbar level in anesthetized and paralyzed rats and an analysis was made, before and during the substance application, of the characteristics of the response to noxious stimuli applied to areas supplied by the ipsilateral sciatic nerve and the contralateral sciatic and saphenous nerves ("inappropriate" areas). The results show that the neuronal response properties were modified differently during the NMDA-induced hyperexcitability and strychnine-induced release of inhibition. Both manipulations brought about the unmasking of responses to previously ineffective, noxious stimuli applied to the contralateral sciatic and saphenous nerve areas, and the enhancement of the responses to noxious stimulation of the ipsilateral sciatic nerve area. However, it was only during the increased excitation induced by NMDA that the neurons exhibited hyperresponsiveness, with long-lasting afterdischarge, to noxious stimulation of the ipsi- and contralateral areas. Such response features resemble those described in sensitized neurons in neuropathic rats and associated with behavioral signs of hyperalgesia. This suggests, by inference, a crucial contribution of the NMDA-induced increased excitability to the expression of neuronal sensitization related to this painful sensory disorder.     

NMDA and strychnine diversely modulate spinal dorsal horn noxious responsiveness in normal rats: potential significance to sensory disorders in neuropathic pain

Changes in neuronal excitability due to increase in excitatory transmitters and/or removal of local inhibition underlie central neuron sensitization and altered responsiveness related to painful sensory disorders. To distinguish the contribution of each of the two mechanisms, they have been mimicked separately in intact rats, by iontophoretically applying excitatory (NMDA) and disinhibitory (the glycine antagonist strychnine) substances during dorsal horn neuron recording. Wide dynamic range (WDR) neurons were extracellularly recorded at the L5-L6 lumbar level in anesthetized and paralyzed rats and an analysis was made, before and during the substance application, of the characteristics of the response to noxious stimuli applied to areas supplied by the ipsilateral sciatic nerve and the contralateral sciatic and saphenous nerves ("inappropriate" areas). The results show that the neuronal response properties were modified differently during the NMDA-induced hyperexcitability and strychnine-induced release of inhibition. Both manipulations brought about the unmasking of responses to previously ineffective, noxious stimuli applied to the contralateral sciatic and saphenous nerve areas, and the enhancement of the responses to noxious stimulation of the ipsilateral sciatic nerve area. However, it was only during the increased excitation induced by NMDA that the neurons exhibited hyperresponsiveness, with long-lasting afterdischarge, to noxious stimulation of the ipsi- and contralateral areas. Such response features resemble those described in sensitized neurons in neuropathic rats and associated with behavioral signs of hyperalgesia. This suggests, by inference, a crucial contribution of the NMDA-induced increased excitability to the expression of neuronal sensitization related to this painful sensory disorder.     

Effects of naloxone and fentanyl in acutely decerebrated dogs

Cardiovascular, respiratory and analgesic effects of fentanyl and naloxone were studied in normotensive acutely decerebrated dogs. Naloxone (1 mg/kg, i.v.) increased skin twitch reflex latency, mean blood pressure, pulse pressure, respiratory rate and minute volume. Fentanyl (50 μg/kg, i.v.) decreased heart rate and blood pressure while the animals were artificially ventilated. The skin twitch reflex latency was not significantly altered. Nine minutes later, naloxone (1 mg/kg, i.v.) was administered and the fentanyl-induced cardiovascular depression was reversed above the control level. The skin twitch reflex latency remained unchanged. These findings give further evidence that the endogenous opioid system plays an important role in the brainstem control of circulation and respiration. The mechanism of the anomalous analgesic response of the acutely decerebrated dog requires further investigation.     

Further characterization of nociception-related and arterial pressure-related neuronal responses in the nucleus reticularis gigantocellularis of the rat

The present study was undertaken to further characterize the nucleus reticularis gigantocellularis (NRGC) of the medulla oblongata in the central processing of nociceptive and cardiovascular signals, and its modulation by metenkephalin. In Sprague-Dawley rats anesthetized with pentobarbital sodium, we found that all 125 spontaneously active NRGC neurons that responded to noxious stimuli (tail clamp) also exhibited arterial pressure-relatedness. Forty neurons additionally manifested cardiac periodicity that persisted even during nociceptive responses. While maintaining their cardiovascular responsive characteristics, the nociception-related NRGC neuronal activity was blocked, naloxone-reversibly (0.5 mg/kg, i.v.), by morphine (5 mg/kg, i.v.). Microiontophoretically applied met-enkephalin suppressed the responsiveness of NRGC neurons to individually delivered tail clamp or transient hypertension induced by phenylephrine (5 µg/kg, i.v.). Interestingly, in NRGC neurons that manifested both nociception and arterial pressure relatedness, the preferential reduction in the response to noxious stimuli upon simultaneous elevation in systemic arterial pressure was reversed to one that favored nociception in the presence of met-enkephalin. All actions of met-enkephalin were discernibly blocked by the opioid receptor antagonist, naloxone. Our results suggest that individual NRGC neurons may participate in the processing of both nociceptive and cardiovascular information, or in the coordination of the necessary circulatory supports during nociception. In addition, neuropeptides such as met-enkephalin may exert differential modulation on neuronal responsiveness according to the prevailing physiologic status of the animal. They also showed that NRGC may be a central integrator for pain and cardiovascular-related functions.     

强电针穴位对背角神经元镇痛效应广泛性的中枢机制

实验用雄性大鼠,玻璃微电极细胞外记录Ｔ１２－Ｌ１脊髓背角会聚神经元对后爪伤害性刺激的反应,观察到低强度（２Ｖ）电针作用于与痛源接近的“足三里”穴对背角神经元的伤害性反应有明显的抑制作用,而远隔穴位“下关”穴则无效。而当采用超过Ｃ类纤维阈值１８Ｖ电针时,则远隔穴位“下关”也有明显的镇痛作用。表现为强电针穴位镇痛作用的广泛性。而损毁ＮＲＭ后,强电针（１８Ｖ）远节段“下关”穴的镇痛作用消失,而近节段“足     

Isoflurane anesthesia blunts cerebral responses to noxious and innocuous stimuli: a fMRI study

We used functional magnetic resonance imaging to determine how isoflurane affected cerebral neuronal activation resulting from noxious and innocuous stimuli. Five male volunteers were subjected to mild electrical shock and tactile stimuli applied to the hand. During low (0.7%) and moderate (1.3%) isoflurane anesthesia the stimuli were repeated and a supramaximal electrical shock was also applied. Tactile stimulation activated bilateral SI and SII, but resulted in no significant activation at low or moderate anesthesia. Electrical shock activated contralateral SI and bilateral SII; low anesthesia completely abolished this response. The supramaximal stimulus activated the caudate nucleus and bilateral thalamus at low anesthesia; these responses were diminished at moderate anesthesia. Isoflurane anesthesia blunts cerebral responses to somatosensory stimuli, and the absence of cortical activation during supramaximal stimulation suggests that noxious-induced movement is generated in lower CNS structures.     

Dynamic Processing of Nociception in Cortical Network in Conscious Rats: A Laser-evoked Field Potential Study

(1) Field potential study in conscious rats provides a convenient and effective animal model for pain mechanism and pharmacological research. However, the spatial-temporal character of nociception processing in cortex revealed by field potential technique in conscious rats remains unclear. (2) In the present study, multi-channel field potentials evoked by noxious laser stimulation applied to the hind paw of conscious rats were recorded through 12 chronically implanted skull electrodes. Independent component analysis (ICA) was used to remove possible artifacts and to extract the specific nociception-related component. (3) Two fast sharp responses and one slow blunt response were evoked by noxious laser stimulation. Systemic morphine (5 mg/kg, i.p.) preferentially attenuated the amplitude of the slow blunt response while had no significant effect on the first two sharp responses. ICA revealed that those responses came from activities of contralateral anterior parietal area, medial frontal area and posterior parietal area. A movement artifact was also detected in this study. Partial directed coherence (PDC) analysis showed that there were changes of information flows from medial frontal and posterior parietal area to anterior parietal area after noxious laser stimulation. (4) Characterization of the spatio-temporal responses to noxious laser stimulation may be a valuable model for the study of pain mechanisms and for the assessment of analgesia.     

Environmental crowding modifies responding to noxious stimuli and the effects of mu- and kappa-agonists

The effects of social crowding in rats on nociception and analgesic actions of mu- and kappa-opioid agonists agonist qualitatively different stimuli were examined. Crowding produced higher control thresholds for electroshock and paw pinch pressure but not for noxious heat. Naloxone did not effect shock thresholds in crowded or noncrowded rats but it produced hyperalgesia in both groups when pressure and heat were the stimuli. Crowding reduced the antinociceptive potency against pressure of kappa- and mu-agonists but the opposite effect was seen with heat. A comparison of the ratios of potencies against pressure and heat indicates that crowding differentially modified kappa- and mu-receptor systems.