Laure Pain

Fentanyl enhancement of carrageenan-induced long-lasting hyperalgesia in rats: Prevention by the N-methyl-d-aspartate receptor antagonist ketamine

Background Tissue damage may produce hyperalgesia, allodynia, and persistent pain. The authors recently reported that fentanyl elicits analgesia but also activates N-methyl-d-aspartate–dependent pain facilitatory processes opposing analgesia. In nonsuffering rats, this leads to a long-lasting enhancement in pain sensitivity. The current study assessed whether fentanyl could amplify carrageenan-induced hyperalgesia. Methods First, rats were injected once with carrageenan in a hind paw, with fentanyl (60 or 100 &mgr;g/kg each given four times at 15-min intervals [4 × 60 or 4 ×100]) or saline. Second, rats were injected with carrageenan twice without fentanyl (7-day interval), with the second injection either in the previously injected paw or in the other paw. Third, rats were injected twice with carrageenan in the same hind paw: the first ketamine injection was given (10 mg/kg each given three times at 5-h intervals) with or without fentanyl (4 × 60 &mgr;g/kg), and second injection was given without ketamine or fentanyl. The consequences of treatments on long-term hyperalgesia were examined by the paw-pressure vocalization test. Results The long-lasting hyperalgesia induced by the first carrageenan injection was dose-dependently enhanced in both duration and magnitude in 4 × 60 or 4 × 100 &mgr;g/kg fentanyl-treated rats: 5 or 10 days, respectively, as compared with 2 days in saline-treated rats. Hyperalgesia observed in the hind paw contralateral to the first carrageenan injection was enhanced in fentanyl-treated rats. The second carrageenan injection, performed in any hind paw, induced an exaggerated hyperalgesia, especially in fentanyl-treated rats. Pretreatment with ketamine totally prevented the carrageenan- and fentanyl-induced enhancement of the long-lasting hyperalgesia. Conclusion Central sensitization in inflammatory pain states is reinforced by an opiate treatment, which could be prevented by N-methyl-d-aspartate receptors blockade.

Effect of propofol on affective state as assessed by place conditioning paradigm in rats.

Background Whether propofol produces a pleasant affective state remains unclear from clinical studies. In the current study, the effect on affective state of subanesthetic and anesthetic doses of propofol was assessed at a preclinical level with rats in a place conditioning paradigm. Propofol was compared with methohexital. Methods In the place conditioning paradigm, propofol-induced effect was repeatedly paired with one of two distinguishable compartments of the apparatus, whereas the vehicle-induced effect was repeatedly paired with the other compartment. During a subsequent free-choice test, a preference for the drug-paired compartment over the vehicle-paired compartment would be indicative of pleasant state induced by the drug. For all experiments, the conditioning session lasted 8 days and consisted of four pairings of the drug with one compartment and four pairings of the equivalent volume of vehicle with the other compartment. In experiment 1A, four groups of rats were designated according to the dose of propofol that they received intraperitoneally: 0, 30, 60, or 90 mg/kg. In experiment 1B, the same procedure was used with subanesthetic doses of intraperitoneal methohexital: 0, 10, 20, or 30 mg/kg. In experiment 2, the rats were conditioned during the recovery period from short-term anesthesia. For one group, anesthesia was induced by propofol (100 mg/kg) whereas for the other group, anesthesia was induced by an equivalent anesthetic dose of methohexital (40 mg/kg). Results In experiment 1A, the 30-mg/kg, 60-mg/kg, and 90-mg/kg groups showed a place preference for the drug-paired compartment, but only the group conditioned with 60 mg/kg propofol significantly differed from the 0-mg/kg group. In experiment 1B, the groups conditioned with methohexital showed no place preference for the drug-paired compartment. In experiment 2, the rats showed a place preference for the compartment in which they recovered from propofol-induced anesthesia but no place preference for the compartment in which they recovered from methohexital-induced anesthesia. Conclusions Propofol, but not methohexital, induced a pleasant affective state in rats at subanesthetic doses as well as during recovery from an anesthetic dose.

Reciprocal Relationships between General (Propofol) Anesthesia and Circadian Time in Rats

Several common postdischarge symptoms, such as sleep disorders, headache, drowsiness or general malaise, evoke disturbances of circadian rhythms due to jet lag (ie crossing time zones) or shift work rotation. Considering that general anesthesia is associated with numerous effects on the central nervous system, we hypothesized that it may also act on the circadian timing system. We first determined the effects of the circadian timing on general anesthesia. We observed that identical doses of propofol showed marked circadian fluctuations in duration of effects, with a peak at the middle of the resting period (ie 7u2009h after lights on). Then, we examined the effects of general anesthesia on circadian timing, by analysing stable free-running circadian rhythms (ie in constant environmental conditions), an experimental approach used widely in circadian biology. Free-running rats were housed in constant darkness and temperature to assess possible phase-shifting effects of propofol anesthesia according to the time of the day. When administered around (±2u2009h) the daily rest/activity transition point, a 30-min propofol anesthesia induced a 1-h phase advance in the free-running rest-activity rhythm, while anesthesia had no significant resetting effect at other times of the day. Anesthesia-induced hypothermia was not correlated with the phase-shifting effects of propofol anesthesia. From our results, anesthesia itself can reset circadian timing, and acts as a synchronizing cue for the circadian clock.

Involvement of the basal cholinergic forebrain in the mediation of general (propofol) anesthesia.

Background:Recent studies have pointed out the involvement of the basal forebrain &ggr;-aminobutyric acid–mediated system in mediating the effects of general anesthesia. In this study, the authors asked whether the basal forebrain cholinergic system is also involved in mediating the effects of general anesthetics such as propofol. Methods:Cholinergic lesions were produced by administration of the selective immunotoxin 192 immunoglobulin G–saporin into the lateral ventricles, the medial septum, or the nucleus basalis magnocellularis. The anesthetic potency of propofol was determined using an anesthetic score with a crossover counterbalanced design. Animals were given intraperitoneal propofol (25 or 50 mg/kg) repeatedly every 15 min to set up a subanesthetic (low-dose) or anesthetic (high-dose) state. The anesthetic score was assessed for each cumulative dose. Control of the cholinergic depletion was performed using histochemical acetylcholinesterase staining on brain slices. Results:A shift from a subanesthetic state to an anesthetic state was observed mainly in the rats with the immunotoxin injected into the lateral ventricles or the medial septum and vertical diagonal band of Broca, compared with controls. In those rats, the density of acetylcholinesterase reaction products was normal in the striatum and the thalamus, but reduced in the cortex and the hippocampus. Conclusion:The anesthetic potency of propofol was increased in all rats with hippocampal lesions, whatever the injection sites, compared with controls. These results demonstrate that a cholinergic dysfunction in the basal forebrain potentiates the anesthetic effects of propofol.

Effect of Midazolam on Propofol-induced Positive Affective State Assessed by Place Conditioning in Rats

Background: The effect of either midazolam or the combination of midazolam and propofol on the affective state was assessed in rats at subanesthetic doses and at recovery from anesthesia. Methods: The putative drug(s)‐induced affective states were repeatedly paired with one of two distinguishable compartments of an experimental cage, whereas the vehicle(s)‐induced effect was repeatedly paired with the other compartment. During a subsequent choice test for one compartment over the other, the rats preference for the drug(s)‐paired compartment over the vehicle(s)‐paired compartment is indicative of a pleasant state induced by the drug(s). In experiment 1, rats were conditioned with different doses of midazolam either at subanesthetic states or at recovery from anesthesia. In experiment 2, groups of rats were conditioned with different combinations of midazolam and propofol either at subanesthetic states or at recovery from anesthesia induced jointly by midazolam (10 mg/kg) and propofol (60 mg/kg). Experiment 3 was conducted in the same way as experiment 2, except that midazolam was paired with both compartments. In addition, these groups were tested not only in an undrugged state but also in a drugged (with midazolam) state. Results: In experiment 1, rats exhibited a place preference for the environment previously associated with midazolam, at subanesthetic and anesthetic doses. Experiment 2 showed that a propofol‐induced place preference was found to be dose‐dependently suppressed by midazolam. Experiment 3 replicated the findings of experiment 2 and extended them to the mechanism by which midazolam blocked a propofol‐induced place preference. Conclusions: Midazolam administered before propofol blocked the expression of a propofol‐induced pleasant state.

Circadian Disruption of Body Core Temperature and Rest–Activity Rhythms after General (Propofol) Anesthesia in Rats

Background:General anesthesia is commonly associated with sleep disorders, fatigue, drowsiness, and mood alterations in patients. The authors examined whether general (propofol) anesthesia can impact the circadian temporal structure by disturbing circadian rest–activity and body temperature rhythms under normal light–dark conditions (light–dark 12:12 h) in rats. Methods:A group of rats was anesthetized with propofol, and another was injected with 10% Intralipid, which was used as a control lipidic solution. The authors examined six groups of rats according to the Zeitgeber time of intraperitoneal administration (ZT6, ZT10, ZT16) and the substance injected (propofol or Intralipid). Results:On the day after anesthesia, propofol induced a significant 60- to 80-min phase advance of both rest–activity and body temperature rhythms. A significant 45- to 60-min phase advance of body temperature and a significant 20-min phase advance of rest–activity were still observed on the second day after anesthesia. The amplitudes of both rest–activity and body temperature rhythms were decreased on the first and second days after anesthesia. The 24-h mean rest–activity rhythm was decreased on the day after anesthesia, whereas the 24-h mean body temperature rhythm was not modified. Conclusion:The results demonstrate the disturbing effects of propofol anesthesia on the circadian time structure in rats under normal light conditions.

Effect of a nonsedative dose of propofol on memory for aversively loaded information in rats.

Background The effects of propofol on memory for aversive information are not well determined. The authors evaluated the effects of a minimal nonsedative dose of propofol or midazolam on memory in rats, using an apparatus composed of two compartments: a large bright anxiogenic one and a small dark neutral one. Methods Groups of rat received propofol (9 mg/kg, intraperitoneally) or midazolam (3 mg/kg). Anxiety was assessed in rats placed in the anxiogenic compartment as the time before the animals entered the neutral compartment. Memory for an aversive event was assessed in rats placed in the anxiogenic compartment as the time to enter the neutral one where they previously experienced foot shocks (fear conditioning). To assess the memory for a nonaversive event, rats were placed in the neutral compartment with no shocks (preexposure). The following day, rats were placed in it and they experienced foot shocks. As a result of the preexposure, rats exhibit less fear to enter it. Results Propofol and midazolam increased the time to enter the neutral compartment. Propofol or midazolam was given to rats before experiencing foot shocks in the neutral compartment. When later tested, the time to enter it was decreased. Propofol or midazolam was given to rats before the preexposure to the neutral compartment. When later tested, the latency to enter it was not modified by the preexposure. Conclusions Propofol and midazolam impaired memory for aversive and for nonaversive experiences at equianxiolytic doses that do not produce locomotor impairment in rats.

Effect of nonsedative doses of propofol on an innate anxiogenic situation in rats

BACKGROUNDnThe effect of propofol on anxiety has not been well studied. In humans, such investigations are confused by the fact that sedation often coexists with anxiolysis. Therefore, the authors evaluated the effects of minimal sedation with propofol in rats placed in an innate anxiogenic situation, the elevated plus-shaped maze.nnnMETHODSnIn experiment 1, spontaneous locomotor activity was determined in rats as a measure of sedative effect induced by propofol (0-9 mg/kg administered intraperitoneally). In experiment 2, groups of rats received propofol (0-9 mg/kg) or diazepam (0-2 mg/kg) and then were placed on a plus-shaped maze elevated above the ground that was composed of two opposite closed arms and two opposite open arms. On an initial exposure to the maze, undrugged rats avoid the open arms, with the number of entries into and time spent within the open arms constituting approximately 20% of their total activity. This reflects normal anxiety in a rodent for any elevated open platform.nnnRESULTSnIn experiment 1, 0-9 mg/kg propofol did not alter spontaneous activity in rats. In experiment 2, propofol and diazepam significantly increased the number of entries into and the time spent within the open arms. Propofol at a dose of 9 mg/kg significantly increased the rats level of exploration of the open arms to about 50% of all exploratory activity, and a similar observation was made with 2 mg/kg diazepam.nnnCONCLUSIONSnIn a standard animal model, propofol has anxiolytic properties at doses that do not produce sedation.

Propofol anesthesia significantly alters plasma blood levels of melatonin in rats.

Background:General anesthesia combined with surgery has been shown to decrease the nocturnal peak of melatonin in patients. However, the role of anesthesia itself on melatonin secretion remains unknown. We previously showed that anesthesia induced by propofol modifies the circadian time structure in both rats and humans and phase advances the circadian rest-activity rhythm in rats. In this study, we examined the secretion of melatonin during 24 h after a 30-min propofol anesthesia in rats. Methods:Rats were exposed to 12-h light/12-h dark alteration conditions and anesthetized with propofol (120 mg/kg intraperitoneally) around their peak of melatonin secretion (Zeitgeber time 16). Trunk blood samples were collected at seven subsequent Zeitgeber times to assess the effects of propofol on circadian melatonin secretion. Results:Propofol modifies the peripheral melatonin by significantly decreasing its concentration (∼22-28%) during the immediate 3 h after the wake up from anesthesia and then significantly increasing melatonin secretion 20 h after anesthesia (∼38%). Cosinor analysis suggests that propofol induces a phase advance of the circadian secretion of peripheral melatonin. Conclusions:The results demonstrate the disturbing effects of propofol anesthesia on the circadian rhythm of plasma melatonin in rats under normal light conditions. These results parallel the desynchronization of the circadian rhythms of locomotor activity and temperature previously observed after propofol anesthesia.

Cholinergic effects on fear conditioning II: nicotinic and muscarinic modulations of atropine-induced disruption of the degraded contingency effect

RationaleIn a companion study (Carnicella et al., 2005), we showed that the muscarinic antagonist atropine, when administered after extensive training during both conditioning and testing, affected neither cued nor contextual fear memories when both of them did not compete for the control of the overt behaviour. In contrast, atropine altered the degraded contingency effect (DCE), that is, the processes by which contextual fear memory competes with the cued one for the control of the conditioned response. Atropine-induced disruption of the DCE was fully reversed by the administration of the anticholinesterase inhibitor physostigmine, which suggests a direct cholinergic implication.ObjectiveThe present series of experiments was conducted in order to define more precisely the involvement of the cholinergic system in such an effect.MethodsOxotremorine (0.0, 0.0075, 0.015, or 0.03xa0mg/kg), pilocarpine (0.0, 0.3, 1, or 3xa0mg/kg), xanomeline (0.0, 2.5, 5.0, 10.0 or 20.0xa0mg/kg) and nicotine (0.0, 0.1, 0.2, or 0.4xa0mg/kg) were tested for reversal of the atropine-induced alteration of the DCE.ResultsOxotremorine and pilocarpine did not reverse the atropine-induced alteration of the DCE. In contrast, xanomeline and nicotine reversed the effect of atropine on the DCE.ConclusionThe present series of experiments reveals complex pharmacological interactions within the cholinergic system when cued and contextual fear memories interact. Results are discussed in this connection and with regard to the relation between the properties of cholinergic agonists and their therapeutic values.