Interaction of pentobarbital and morphine in the tail-flick test performed on rats: synergism at the spinal and antagonism at the supraspinal level

Pharmacologic Modulation of Noxious Stimulus-evoked Brain Activation in Cynomolgus Macaques Observed with Functional Neuroimaging

Maintaining effective analgesia during invasive procedures performed under general anesthesia is important for minimizing postoperative complications and ensuring satisfactory patient wellbeing and recovery. While patients under deep sedation may demonstrate an apparent lack of response to noxious stimulation, areas of the brain related to pain perception may still be activated. Thus, these patients may still experience pain during invasive procedures. The current study used anesthetized or sedated cynomolgus macaques and functional magnetic resonance imaging (fMRI) to assess the activation of the parts of the brain involved in pain perception during the application of peripheral noxious stimuli. Noxious pressure applied to the foot resulted in the bilateral activation of secondary somatosensory cortex (SII) and insular cortex (Ins), which are both involved in pain perception, in macaques under either propofol or pentobarbital sedation. No activation of SII/Ins was observed in macaques treated with either isoflurane or a combination of medetomidine, midazolam, and butorphanol. No movement or other reflexes were observed in response to noxious pressure during stimulation under anesthesia or sedation. The current findings show that despite the lack of visible behavioral symptoms of pain during anesthesia or sedation, brain activation suggests the presence of pain depending on the anesthetic agent used. These data suggest that fMRI could be used to noninvasively assess pain and to confirm the analgesic efficacy of currently used anesthetics. By assessing analgesic efficacy, researchers may refine their experiments, and design protocols that improve analgesia under anesthesia.

Potentiation of morphine antinociception by pentobarbital in female vs. male rats

It has been shown previously that female rats are more sensitive than males to barbiturate anesthesia, whereas males may be more sensitive than females to opioid antinociception. The aim of the present study was to determine whether enhancement of morphine antinociception by pentobarbital, previously demonstrated in male animals and humans, occurs similarly in females. Pentobarbital (50 mg/kg i.p.) produced longer-lasting anesthetic effects (loss of muscle tone, righting reflex) in gonadally intact female rats than in males, but greater antinociceptive effects in males at some time points post-injection. There were no significant sex differences in morphine-induced anesthesia or antinociception; however, 50 mg/kg pentobarbital produced greater leftward shifts in the morphine antinociceptive dose-effect curve in gonadally intact females than males, whether pentobarbital was administered 30 vs. 120 min before morphine (times at which there were no sex differences vs. sex differences, respectively, in pentobarbital's effects when administered alone). Dose-addition analysis confirmed that pentobarbital enhancement of morphine antinociception was supra-additive in both sexes; morphine also significantly enhanced pentobarbital-induced anesthesia in both sexes. In gonadectomized males, testosterone did not significantly alter pentobarbital enhancement of morphine antinociception; in contrast, in gonadectomized females, estradiol significantly attenuated the drug interaction. Estradiol did not significantly alter the effects of pentobarbital alone or morphine alone, indicating that the attenuation of the pentobarbital's potentiation of morphine antinociception in estradiol-treated rats is specific to the drug interaction. These results suggest that barbiturate potentiation of opioid antinociception may be greater in females - particularly those in low ovarian hormone states - than in males.

Effects of temperature and volume on intraperitoneal saline-induced changes in blood pressure, nociception, and neural activity in the rostroventral medulla

ON and OFF cells of the rostral ventromedial medulla are hypothesized to modulate nociception with ON cells facilitating pain and OFF cells inhibiting pain. The current study analyzed the effects of intraperitoneal saline at different volumes and temperatures on nociception (tail flick reflex), blood pressure, and the activity of ON and OFF cells in lightly anesthetized rats. At large volumes (20 cc/kg), room temperature saline excited 7/11 ON cells and inhibited 10/12 OFF cells for 2-5 min. In contrast, large volumes (20 cc/kg) of body temperature saline (37 degrees C) excited only 1/10 ON cells and inhibited only 3/13 OFF cells, and small volumes (1-2 cc/kg) of room temperature saline excited only 3/10 ON cells and inhibited only 4/11 OFF cells. Tail flick latency increased following saline administration at large volumes with a significant effect of time, but not temperature. The excitation of ON cells and inhibition of OFF cells indicate that cold intraperitoneal saline could be painful and the increase in tail flick latency may indicate a diffuse noxious inhibitory control. It is also possible that the changes in ON and OFF cell activity caused a hyperalgesia that was masked by a simultaneous hypoalgesia that was mediated independent of the ON and OFF cells. Because intraperitoneal saline may produce pain or hyperalgesia, care should be used when saline is used experimentally or clinically.

Effect of analgesics on audible and ultrasonic pain-induced vocalization in the rat

Brief electrical pulses applied to the rat tail elicit a complex vocal response which includes audible (peeps, chatters) and ultrasonic (USV) components. Aspirin and amitriptyline had no effect on the vocal responses. Morphine showed a dose-dependent and naloxone reversible antinociceptive effect on the 1st and 2nd audible peeps by decreasing their intensity (evaluated by their envelopes which correspond to the outer bounds of the soundwave amplitude plotted as a function of time), with ED50 values of 1.96 mg/kg and 0.36 mg/kg i.v. respectively. Paracetamol significantly reduced only the intensity of the second peep at the dose of 200 mg/kg iv. Chatter intensity was decreased by doses of 1 and 3 mg/kg i.v. of morphine which would suggest an effect on emotional components of pain. The intensity of USV was affected by morphine injection although the variations observed were non-significant. These data clearly implicate a specific role for the opioid analgesics in modifying the vocal pain related behaviors.

Analgesia and abuse potential: an accidental association or a common substrate?

The fact that centrally acting analgesics have abuse potential commensurate with their analgesic activity raises the question of whether these effects are related. The abuse potential of drugs depends on their ability to produce reinforcing effects, which are mediated by a neural system that includes the ventral tegmental dopamine cells and their connections with the ventral striatum. Morphine and amphetamine are both powerful analgesics and have high abuse potential. Their analgesic and reinforcing effects are mediated by similar receptors, similar sites of action, and overlapping neural substrates. These coincidences suggest that reinforcers may produce analgesia by transforming the aversive affective state evoked by pain into a more positive affective state. The implications of this hypothesis and its relation to other known mechanisms of analgesia are discussed. The hypothesis predicts that drugs with reinforcing effects should produce analgesia. A survey of drugs acting through 21 classes of receptors reveals that in 13 classes there is evidence for both analgesic and reinforcing effects that are approximately equipotent. The GABA(A) agonists were found to be the only drugs with confirmed abuse potential that lack analgesic activity. The interpretation of this and several other anomalous cases is discussed.

An innovative cold tail-flick test: the cold ethanol tail-flick test

An innovative antinociceptive test, the cold ethanol tail-flick test (CET), was developed for evaluating the actions of opioid analgesics. To select an optimal operation temperature range for the CET, temperatures from -5 degrees C to -30 degrees C were screened. After screening, temperatures ranging between -20 degrees C and -30 degrees C were both strong and effective enough to act as a noxious cold stimulus. In the following study, -20 degrees C was selected as the cold stimulus for the CET. The sensitivity and specificity of this test were challenged by opioid analgesics: an agonist (morphine) and two agonist-antagonists (buprenorphine and nalbuphine), two tranquilizers (droperidol and diazepam), and four nonopioid analgesics (acetaminophen, aspirin, indomethacin, and ketoprofen). The sensitivity of the CET was also compared with the assays using heat (radiant heat and hot water). The AD50 values determined by the CET for morphine, buprenorphine, and nalbuphine were 0.16 mg/kg, 0.22 micrograms/kg, and 0.19 mg/kg, respectively. Naloxone, an opioid antagonist, blocked the antinociceptive effects of these opioids which were determined by the CET. Furthermore, the tranquilizers and nonopioid analgesics did not show any activity in the CET. Our results show that not only can the CET assess the antinociceptive activity of both opioid agonist and mixed agonist-antagonist, it also possess the characteristics of sensitivity, specificity, simplicity, and reproducibility.

Pentobarbital, diazepam, and ethanol abolish the interphase diminution of pain in the formalin test: evidence for pain modulation by GABAA receptors

There are two phases to the behavioral response to injection of formalin. After an initial vigorous response, a period of reduced pain occurs 10 to 15 min after formalin, followed by reemergence of pain-related behaviors. These phases are believed to represent acute chemical stimulation of afferent neurons followed by injury-related inflammatory pain. Pentobarbital (10, 15, or 25 mg/kg), diazepam (0.5, 1.5, or 5.0 mg/kg), or ethanol (0.5, 1.0, or 1.5 g/kg) attenuated the diminution of pain between the two phases, so that pain was continuous throughout 60 min of testing, but had no effect on pain scores during the peaks of either phase. The effects of pentobarbital and diazepam were blocked by picrotoxin (2.5 mg/kg), which itself had no effect. Ro 15-1788 also blocked the effect of diazepam. Picrotoxin did not effectively antagonize the effect of ethanol. A high dose of picrotoxin (5.0 mg/kg) caused seizures in some rats and also eliminated the interphase depression of pain. The results suggest that the biphasic time course of formalin pain is produced by a central antinociceptive mechanism that is inhibited by GABAA receptors.

A comparison of anesthetic agents and their effects on the response properties of the peripheral auditory system

Anesthetic agents were compared in order to identify the most appropriate agent for use during surgery and electrophysiological recordings in the auditory system of the tokay gecko (Gekko gecko). Each agent was first screened for anesthetic and analgesic properties and, if found satisfactory, it was subsequently tested in electrophysiological recordings in the auditory nerve. The following anesthetic agents fulfilled our criteria and were selected for further screening: sodium pentobarbital (60 mg/kg); sodium pentobarbital (30 mg/kg) and oxymorphone (1 mg/kg); 3.2% isoflurane; ketamine (440 mg/kg) and oxymorphone (1 mg/kg). These agents were subsequently compared on the basis of their effect on standard response properties of auditory nerve fibers. Our results verified that different anesthetic agents can have significant effects on most of the parameters commonly used in describing the basic response properties of the auditory system in vertebrates. We therefore conclude from this study that the selection of an appropriate experimental protocol is critical and must take into consideration the effects of anesthesia on auditory responsiveness. In the tokay gecko, we recommend 3.2% isoflurane for general surgical procedures; and for electrophysiological recordings in the eighth nerve we recommend barbiturate anesthesia of appropriate dosage in combination if possible with an opioid agent to provide additional analgesic action.

Pentobarbital attenuates antinociception induced by i.c.v. morphine but not beta-endorphin in the mouse

The effects of pentobarbital anesthesia (45 mg/kg i.p.) on the inhibition of the tail-flick response induced by beta-endorphin and morphine injected intracerebroventricularly (i.c.v.) and intrathecally (i.t.) were studied in male ICR mice. Pentobarbital anesthesia attenuated the inhibition of the tail-flick response induced by morphine but not beta-endorphin given i.c.v. However, the tail-flick inhibition induced by morphine given i.t. was not attenuated by pentobarbital. beta-Endorphin-(1-27) (3 micrograms) given i.c.v. or naloxone (2 micrograms) given i.t. blocked inhibition of the tail-flick response induced by morphine given i.c.v. only in pentobarbital-anesthetized mice but not in conscious mice. beta-Funaltrexamine (beta-FNA, 2.5 micrograms) given i.c.v. or yohimbine (2 micrograms) and methysergide (2 micrograms) injected i.t. blocked the morphine (i.c.v.)-induced inhibition of the tail-flick response in conscious mice but not in pentobarbital-anesthetized mice. The results indicate that pentobarbital attenuates the morphine-induced inhibition of the tail-flick response by inhibiting descending noradrenergic and serotonergic pathways and uncovers a descending opioid system. The tail-flick inhibition induced by supraspinal morphine is mediated by stimulation of mu-opioid receptors in conscious mice and epsilon-opioid receptors in pentobarbital-anesthetized mice. The epsilon-opioid receptor-mediated descending system activated by supraspinally injected beta-endorphin is not attenuated by pentobarbital anesthesia.

Functional activity mapping of the rat brainstem during formalin-induced noxious stimulation

Functional activity changes in 35 selected structures of the rat brainstem elicited by subcutaneous formalin injection in a forepaw were investigated by the [14C]2-deoxyglucose method in unanesthetized, freely moving animals. Experiments were initiated 2 min ("early" group) or 60 min ("late" group) after the injection. Treatment induced a significant increase of [14C]2-deoxyglucose uptake relative to controls in 17 structures of the "early" group, including portions of the bulbar, pontine and mesencephalic reticular formation, nucleus raphe magnus, median and dorsal raphe nuclei, the ventrolateral and dorsal subdivisions of the periaqueductal gray matter, deep layers of the superior colliculus and the anterior pretectal nucleus. Most changes were bilateral, with the exception of the increases observed in the nucleus reticularis paragigantocellularis and the lateral parabrachial area, which were contralateral, and the one in the mesencephalic reticular formation, which was ipsilateral to the injected paw. In pentobarbital-anesthetized rats a significant difference in metabolic activity values between formalin- and saline-injected animals was only detected at the medullary level. In the "late" unanesthetized formalin group functional activity levels were higher than controls in four structures, including the lateral reticular and paragigantocellular nuclei, contralaterally, and nucleus cuneiformis and ventrolateral periaqueductal gray matter, bilaterally. No between-groups difference was observed in visual or auditory structures. These results provide evidence for activation of several brainstem regions, which are conceivably involved in different sensory, motivational or motor circuits, during the initial phase of formalin-evoked noxious stimulation in unanesthetized animals. Functional changes blunted over time as did pain-related behavior integrated at the supraspinal level, but they persisted in some brainstem regions for which involvement in endogenous antinociceptive systems have been suggested. The mechanisms underlying these time-related changes need to be clarified.

Barbiturate-induced analgesia: permissive role of a GABAA agonist

Three doses (0.025, 0.25 and 2.5 mg) of the short-acting barbiturate, pentothal, were injected intrathecally at the lumbar level of the spinal cord of female rats and did not produce analgesia in either the tail-flick latency to radiant heat (TFL) or vocalization-threshold-to-tail-shock (VTTS) tests. However, when the high dose of pentothal (2.5 mg) was given in combination with a nonanalgesia producing dose of the GABAA agonist muscimol (1 microgram), a significant and prolonged analgesia was produced in both the VTTS and TFL tests, lasting up to one hour postinjection. Intrathecal injection of the intermediate dose of pentothal (0.25 mg) in combination with 1 microgram muscimol also produced significant analgesia in the TFL but not the VTTS test. We suggest that barbiturates may act on spinal nociceptive pathways to reduce pain thresholds only when sufficient GABAergic activity is present.

The general anesthesia induced by various drugs differentially affects analgesia and its variability

Responses to noxious stimuli in awake animals are not totally consensual but are influenced by environmental factors. We considered the possibility that the influence of the environment could be reduced by induction of general anesthesia. We, therefore, compared responses to nociceptive thermal stimuli by measuring tail flick latency, a spinal reflex, in anesthetized and awake mice. All anesthetics tested decreased the intraindividual variability in the measurement of response, suggesting that environmental factors may account for much of this variability in the awake mouse. Mice treated with pentobarbital showed a graded response to increasing levels of heat but were unresponsive to either morphine or naloxone. In mice anesthetized with pentobarbital, increases in latencies occurred only at very deep levels of anesthesia, while urethane nociceptive effect of ketamine was reversed by morphine. Thus, the various anesthetics could show differential effects towards opiate action. The decrease in statistical variability, the differential effects of general anesthetics on tail flick latency, and the distinctive effects of the different anesthetics on opiate action suggest that the anesthetized animal may be a useful tool in the study of nociception.

Different endogenous analgesia systems are activated by noxious stimulation of different body regions

Noxious pinch of the neck and the base of the tail can produce equipotent analgesia as measured by the tail flick method. However, noxious stimulation of the neck can suppress pain responsiveness both at the site of stimulation and at sites remote from the stimulated area while noxious stimulation of the tail produces analgesia only at sites remote from the stimulated area. Thus, neck pinched animals are immobile and completely unresponsive to the noxious pinch whereas pinch to the base of the tail, which results in tail flick suppression, causes vocalization and well organized biting behavior directed at the pinched area. The analgesia elicited by noxious stimulation applied to both body regions is eliminated by spinalization, the administration of intermediate doses of barbiturates (30 and 45 mg/kg) and transection at the midcollicular, but not more rostral, brain level. Concurrent with the elimination of the analgesic effect of noxious pinch on tail flick is the emergence of responses to noxious neck pinch with vocalization and intense motor reactions now elicited by noxious stimulation of the nape of the neck. These results indicate that different analgesic systems are activated by noxious tail and neck pinch both requiring the integrity of mesencephalic structures for their normal function. Furthermore, these systems can be distinguished by their ability to produce recurrent, inhibitory, supraspinal effects on nociceptive information originating at different body regions.

Depression by morphine and the non-opioid analgesic agents, metamizol (dipyrone), lysine acetylsalicylate, and paracetamol, of activity in rat thalamus neurones evoked by electrical stimulation of nociceptive afferents

Pyrazolone and salicylic acid derivatives and the aniline derivative, paracetamol, are often classified as peripherally acting analgesic agents, while morphine is a centrally acting analgesic agent. Since indications exist that the non-opioid analgesic agents can also produce central effects, experiments were carried out on rats under urethane anaesthesia in which activity was recorded from single neurones in the dorsomedial part of the ventral nucleus (VDM) of the thalamus that was elicited by supramaximal electrical stimulation of nociceptive afferents in the sural nerve. In addition, activity was recorded in ascending axons of the spinal cord which was evoked by electrical stimulation of nociceptive afferents in the sural nerve. The substances studied were morphine, the pyrazolone derivatives, metamizol (dipyrone) and aminophenazone ('Pyramidon'), lysine acetylsalicylate, and paracetamol. All drugs were found to depress dose-dependently evoked activity in VDM neurones after intravenous (i.v.) injection. The ED50 of morphine in depressing evoked activity in VDM neurones is 0.05 mg/kg. Morphine also dose-dependently reduced activity in ascending axons of the spinal cord, the ED50 being 1.7 mg/kg. The ED50 of metamizol in depressing evoked activity in VDM neurones is 120 mg/kg, and that of aminophenazone is 22.7 mg/kg. The 2 ED50 values differ significantly. It has been found previously that metamizol increased nociceptive activity in some ascending axons and aminophenazone increased this activity in all ascending axons tested. The ED50 of lysine acetylsalicylate in depressing evoked activity in VDM neurones is 74 mg/kg. The drug did not reduce nociceptive activity in ascending axons of the spinal cord. The ED50 of paracetamol in depressing evoked activity in VDM neurones is 19.0 mg/kg. Paracetamol did not depress nociceptive activity in ascending axons of the spinal cord at a dose as high as 150 mg/kg administered by intraperitoneal injection. Naloxone (0.2 mg/kg i.v.) abolished the depressant effects of morphine but failed to reduce those of the non-opioid analgesic agents even at a high dose (1 mg/kg i.v.). Unlike morphine, the non-opioid analgesic agents did not completely block evoked activity in VDM neurones but only partially blocked their activation. The results suggest that the non-opioid analgesic agents tested can produce a central analgesic effect which, however, is weaker than that of morphine.

Pentobarbital, in subanesthetic doses, depresses spinal transmission of nociceptive information but does not affect stimulation-produced descending inhibition in the cat

The present study evaluates the effect of systemic pentobarbital on the spinal transmission of nociceptive information and on stimulation-produced descending inhibition in the deeply anesthetized, paralyzed cat. Single neuronal responses to noxious skin heating were recorded extracellulary in the lumbar dorsal horn and found to be depressed by pentobarbital at subanesthetic doses (4.0, 8.0, 17.0 and 24.5 mg/kg) in a dose-dependent manner. At 0.5 and 1.5 mg/kg, depression by pentobarbital was positively correlated with the depth of the recording site in the spinal cord (laminae IV-VI), i.e., neurons in deeper laminae (V-VI) were attenuated, while neurons in lamina IV were unaffected. At all doses tested, pentobarbital failed to affect stimulation-produced descending inhibition from either the midbrain periaqueductal gray or the medullary nucleus raphe magnus. The present data furnish evidence for the antinociceptive potency of pentobarbital, they do not support the view that a 'partial pharmacological spinal cord transection' would attenuate stimulation-produced descending inhibition of nociceptive dorsal horn neurons.