Orexin receptor type-1 antagonist SB-334867 inhibits the development of morphine analgesic tolerance in rats

Herein the effect of orexin receptor type-1 antagonist SB-334867 on the development of tolerance to analgesic effects of morphine was studied in rats. To incite tolerance, morphine sulfate was injected intraperitoneally (i.p., 10mg/kg) once a day for 7 days. The tail flick test was used to evaluate antinociceptive effects of the morphine. A selective OxR1 receptor antagonist, SB-334867, was microinjected (i.c.v.) into the right cerebral ventricle (10 μg/10 μl) immediately before each morphine injection. Repeated morphine application resulted in tolerance to morphine analgesic effects as a decreasing trend during 7 days. Also, repeated administration of SB-334867 (i.c.v.) alone was without significant effect on the nociception as compared to control. Microinjection of SB-334867 prior to each morphine injection inhibited the development of tolerance, so that the analgesic effects of morphine were significantly higher in SB-334867 plus morphine treated rats than that of vehicle plus morphine treated ones on days 4-7. It is concluded that orexin receptor type-1 might be involved in the development of tolerance to morphine analgesic effects.     

Analgesic efficacy of orally administered buprenorphine in rats: methodologic considerations

Buprenorphine has been widely recommended for treatment of pain in rodents. We have previously documented that the recommended postoperative oral dose of buprenorphine in male Long-Evans rats, 0.5 mg/kg, is not as effective as the recommended parenteral dose of buprenorphine (0.05 mg/kg, s.c.) as an analgesic. In the series of experiments reported here, we compared: the analgesic effect of buprenorphine when prepared in two ways in the laboratory with that of a commercially available injectable solution of buprenorphine; the analgesic effect of buprenorphine in Long-Evans rats with that in Sprague-Dawley rats; and Long-Evans and Sprague-Dawley rats for development of pica, a commonly reported side effect of buprenorphine. We followed the pica experiment with assessment of the effectiveness of buprenorphine in establishing a conditioned flavor aversion. The results indicated that method of preparation did not result in any significant differences in the efficacy of injected buprenorphine. Strain of rat was not associated with a significant difference in the efficacy of buprenorphine. However, a significant strain difference was found in development of pica. Buprenorphine treatment was effective in inducing a conditioned flavor aversion. We concluded that the recommended oral dose of buprenorphine (0.5 mg/kg) is ineffective as an analgesic, and that this was not the result of method of preparation of the buprenorphine or strain of rat used. Furthermore, we concluded that buprenorphine treatment may induce gastrointestinal distress in both strains tested. The results reaffirm our previous conclusion that oral administration of buprenorphine at 0.5 mg/kg, despite the general recommendation, is not a reasonable treatment for postsurgical pain in rats.     

Maternal hyperglycemia leads to gender-dependent deficits in learning and memory in offspring

Pregnancy in the diabetic woman has long been associated with an increased risk of congenital malformation in the offspring. However, little is known about the effects of maternal diabetes on development of the central nervous system. To begin to gain an understanding of this problem, diabetes was induced in adult female Sprague-Dawley rats by injection with streptozotocin. Only animals with serum glucose levels greater than 200 mg/dl were used. Diabetic and control females were bred, and all newborn pups were cross-fostered to nondiabetic mothers. At 60 days of age, pups were tested in an elevated plus-maze to assess differences in emotionality and anxiety. There were no significant differences between offspring of diabetic dams and controls on this measure. All pups were then housed individually, put on food restriction, and maintained at 85% of their ad libitum weight. They were then trained in a Lashley III maze, which assesses learning and retention capability. The female offspring of diabetic dams performed poorer than controls, a finding that was supported by inhibitory avoidance data from a separate group of animals. All animals were then trained in a radial-arm maze. Results failed to find differences between experimental and control animals. It was concluded that the diabetic intrauterine environment has gender-specific effects on central nervous system development.     

Assessment of the postoperative discomfort of intra-auricularly hypophysectomized rats

Rats subjected to hypophysectomy make up one of the largest groups of experimental animals in Europe, since there is a legal demand for batch testing of industrially produced growth hormones. To describe the clinical performance of rats having undergone hypophysectomy, animals were examined postoperatively by monitoring behaviour, body temperature and food intake. Behavioural changes were observed in rats that had only been anaesthetized, as well as in sham-operated rats, while no behavioural deviations could be shown in hypophysectomized rats. On the first day after surgery all rats had declining body temperature and food intake; and this change was not reversed by treatment with carprofen, buprenorphine or oxytetracycline. The mortality rate in rats treated with buprenorphine was increased, as was the mortality rate in rats hypophysectomized when weighing more than 100 g. As there seemed to be no differences whether methohexital or a combination of fentanyl, fluanison and midazolam was used, the latter anaesthesia is recommended due to its analgesic potential. For post-surgical analgesic treatment, carprofen is recommended rather than buprenorphine. At best, the use of hypophysectomized rats should be replaced in industrial batch testing by an existing in vitro method.     

Gender-based changes in cognition and emotionality in a new rat model of epilepsy

Summary. Epilepsy research relies heavily on animal models that mimic some, or all, of the clinical symptoms observed. We have previously described a new developmental rat model of epilepsy that demonstrates both behavioural seizures and changes in hippocampal morphology. In the current study we investigated whether these rats also show changes in cognitive performance as measured using the Morris water maze task, and emotionality as measured using the Elevated plus maze task. In the water maze, significant differences between male and female rats were found in several performance variables regardless of treatment. In addition, female but not male rats, treated neonatally with domoic acid had significant impairments in learning new platform locations in the water maze. In the elevated plus maze, a significant proportion of female rats spent more time in the open arm of the maze following prior exposure to the maze whereas this effect was not seen in male rats. We conclude that perinatal treatment with low doses of domoic acid results in significant gender-based changes in cognition and emotionality in adult rats.     

Analgesic effect of the electromagnetic resonant frequencies derived from the NMR spectrum of morphine

Exposure to various types of electromagnetic fields (EMFs) affects pain specificity (nociception) and pain inhibition (analgesia). Previous study of ours has shown that exposure to the resonant spectra derived from biologically active substances' NMR may induce to live targets the same effects as the substances themselves. The purpose of this study is to investigate the potential analgesic effect of the resonant EMFs derived from the NMR spectrum of morphine. Twenty five Wistar rats were divided into five groups: control group; intraperitoneal administration of morphine 10 mg/kg body wt; exposure of rats to resonant EMFs of morphine; exposure of rats to randomly selected non resonant EMFs; and intraperitoneal administration of naloxone and simultaneous exposure of rats to the resonant EMFs of morphine. Tail Flick and Hot Plate tests were performed for estimation of the latency time. Results showed that rats exposed to NMR spectrum of morphine induced a significant increase in latency time at time points (p < 0.05), while exposure to the non resonant random EMFs exerted no effects. Additionally, naloxone administration inhibited the analgesic effects of the NMR spectrum of morphine. Our results indicate that exposure of rats to the resonant EMFs derived from the NMR spectrum of morphine may exert on animals similar analgesic effects to morphine itself.     

Differential modulation of nociceptive responses to mu and kappa opioid receptor directed drugs by blood glucose in experimentally induced diabetes rats

The study has evaluated the effect of diabetes associated hyperglycaemia on nociception and antinociception induced by morphine, buprenorphine and pentazocine in female albino rats. Rats were allocated into 3 groups of 20 each--group I consisted of control having normal blood glucose levels (BGLs), group II consisted of streptozotocin-induced diabetics (STZ-D) having hyperglycaemia and group III consisted of diabetic rats controlled with insulin treatment. Immediately before and 15, 30 min, 1, 2 and 3 hr after injection with test drugs, rats were subjected to a thermal noxious stimulus using tail withdrawal from hot water and tail-flick latencies (TFL) so generated were recorded. Similarly, before and 30, 45 min and 1 hr after injection with drugs rats were subjected to abdominal writhing with hypertonic saline and number of writhes were counted per 90 sec. In STZ-D animals (BGLs 317.95 +/- 3.8 mg/dl) a decreased TFL with an increase in the number of writhes compared to control and diabetes controlled with insulin treatment was observed. Percent maximum possible effect of morphine (5 mg/kg, s.c.) and buprenorphine (2 mg/kg, s.c.) was significantly lower when compared to control as well as STZ-D controlled with insulin treatment groups. Similarly percent protection from writhing of morphine (0.05 mg/kg, s.c.) and buprenorphine (0.01 mg/kg, s.c.) was significantly less in comparison to control and STZ-D controlled with insulin treatment group. However, percent maximum possible effect of pentazocine (20 mg/kg, s.c.) and percent protection from writhing of pentazocine (1 mg/kg, s.c.) was significantly high in STZ-D rats when compared to control and STZ-D rats controlled with insulin treatment groups. The results suggest that both mu and kappa--opioid receptors may be modulated by blood glucose levels possibly involving cellular energetics mediated change in potassium (KATP) channels in females rats, albeit differentially.     

Are repeated doses of buprenorphine detrimental to postoperative recovery after laparotomy in rats?

Buprenorphine is a widely used analgesic for relief of postoperative pain in rats. The effect of repeated doses of buprenorphine throughout the postoperative pain and stress response is unknown. This investigation tested the hypotheses that (a) daily analgesic doses of buprenorphine for 7 d ameliorate the stress response after laparotomy in rats and (b) preoperative buprenorphine better ameliorates the response than do peri- and postoperative administration. Postoperative effects on body weight, daily food and water consumption, and daily fecal and urinary outputs were monitored in groups of rats treated for 7 d with analgesic doses of buprenorphine initiated at different time points relative to the time of laparotomy. Analgesic doses of buprenorphine had no effect on the study parameters in healthy unoperated rats. Daily injection of buprenorphine delayed the time at which the preoperative body weight was restored without decreasing the postoperative changes in daily food consumption, water intake, and fecal and urinary outputs in the operated rats. The effects of daily analgesic doses of buprenorphine for 7 d on body weight, daily food, and water consumption, and fecal and urinary outputs were minimal and less statistically significant than the changes caused by surgery itself. However, this dosing regimen seems to delay the restoration of body weight after abdominal surgery in rats.     

Anxiety level during morphine abstinence correlates with the status of nitrergic system in the rat hippocampus

Opiate addiction is accompanied by long-term structural and functional changes in brain regions persisting during abstinence, this status being an experimental model of the aberrant neuroplasticity. Nitric oxide is known to be involved in mechanisms of psychopathological events during opiate abstinence. In this study, indices of a nitregic system (nitric synthase activity--NOS, nitrites and nitrates concentration--NOx-) were measured in the rat brain region during morphine abstinence. Prior to this, the rats were tested for anxiety in an elevated plus maze. NOS activity increased in hippocampus 3 days after morphine withdrawal, while NOx--6 days after withdrawal. No changes of the nitrergic system could be revealed in other brain regions under study. Six days (but not 3 days) after morphine withdrawal, rats visited the open arms of the plus maze more frequently and spent more time in these arms as compared with respective controls. The data suggest that nitrergic system changes in the hippocampus may be involved in molecular mechanisms of behavioural alteration during morphine abstinence in rats.     

Static magnetic field exposure affects behavior and learning in rats

The present work investigated the behavioral effects of a moderate exposure (1 h per day for 5 consecutive days) to a static magnetic field (SMF, 128 mT) in male rats. SMF effects were evaluated in two sets of control and SMF-exposed rats. One set of animals was used for evaluation of SMF potential effects on emotional behaviors in the elevated plus maze and in the open field. The other set of animals was tested for learning and memory abilities in different procedures of the Morris water maze task. We found no significant difference between control and SMF-exposed rats in anxiety tests. However, the ratio of open arms time in the plus maze was reduced by half in SMF-exposed rats. In the Morris water maze, SMF-exposed rats were partially impaired during the initial learning task as well as in the retention task at one week. We conclude that static magnetic field exposure altered emotional behaviors in the plus maze and led to cognitive impairments, or at least to substantial attention disorders, in the Morris water maze.     

Relationship between three intensity levels of Toxocara canis larvae in the brain and effects on exploration, anxiety, learning and memory in the murine host

Outbred LACA mice were administered low (100 ova), medium (1000 ova), high (3000 ova) and trickle (4x250 ova) doses of Toxocara canis ova and the effect of infection was examined with respect to the number of larvae recovered from the brain and their behaviour. Recovery of larvae from the brain was generally low with the % recovery expressed in terms of the total dose administered being highest for the 3000 dose (6.1%) and 1000 dose (6%), followed by the 100 (4.4%) and trickle (3.5%) doses. The variation in larval recoveries was large between individual mice receiving similar doses. The level of infection in the brain was lower in mice receiving a multiple as opposed to an equivalent single dose of ova. Mice were then divided into three larval intensity groupings based upon the number of larvae recovered from their brain. The ranges for the groups were as follows: low intensity group, 0-15 larvae; moderate intensity group, 27-55 larvae; high intensity group, 66-557 larvae. Three behavioural tests were carried out on control and infected mice. Exploration and response to novelty was examined using a 'T' maze and learning was investigated by means of a water-finding task. Anxiety was measured using an elevated plus maze apparatus. Infected mice were less explorative and less responsive to novelty in the 'T' maze and this was particularly pronounced for the heavily infected mice. In the elevated plus maze, infected mice displayed reduced levels of anxiety to aversive and exposed areas of the maze, particularly in the case of the moderate and high intensity mice. There was evidence for impaired learning ability in the water task apparatus for moderate and high intensity mice. In general, the effects of infection on behaviour were more pronounced in the moderate and high intensity groups compared to the low intensity group.     

The effect of opiates and opiate antagonists on heat latency response in the parasitic nematode Ascaris suum

The effects of the opiates morphine and morphine-6-glucuronide (M6G), the mu opioid receptor specific antagonist D-Phe-Cys-Tyr-D-Trp-Om-Thr-Pen-Thr-NH(2) (CTOP), and the general opiate antagonist naloxone on the latency of response to thermal stimulation were determined in the parasitic nematode Ascaris suum. Thermal detection and avoidance behaviors of the worms were evaluated with a tail flick analgesia meter using a modification of a technique employed for nociception experiments in rodents. Morphine and M6G were shown to have a dose dependent analgesic effect on A. suum's latency of response to heat with morphine being the most potent. The analgesic effect of morphine was reversed by naloxone but not CTOP. Neither naloxone nor CTOP was able to block the analgesia of M6G. CTOP but not naloxone had significant analgesic effects on its own. These findings are generally consistent with previous results on the effects of opiates and nitric oxide release from A. suum tissue. Apparently these nematodes possess opioid receptors that effect nociception.     

Wild ginseng attenuates anxiety- and depression-like behaviors during morphine withdrawal

The purpose of this study was to evaluate whether wild ginseng (WG) administration could attenuate anxiety- and depression-like behaviors and expression of corticotrophin-releasing factor (CRF) and neuropeptide Y (NPY) following withdrawal from repeated morphine administration in rats. Male rats were administered daily doses of WG (50, 100, or 200 mg/kg, i.p.) for 5 days, 30 min before morphine injection (40 mg/kg, s.c). The anxiety- and depression-like behavioral responses were measured 72 h after the last morphine injection using an elevated plus maze (EPM) and forced swimming test (FST), respectively. Changes in hypothalamic CRF and NPY expressions were also examined by analyzing their immunoreactivities in the hypothalamus. Daily administration of WG significantly reduced anxiety-and depression-like behavior, and elicited the suppression of CRF expression and the stimulation of NPY expression in the hypothalamus. Our results demonstrated that WG extract might be effective at inhibiting the anxiety and depression responses due to morphine withdrawal by possibly modulating the hypothalamus CRF and NPY systems. Furthermore, these findings imply that WG extract can be used for developing new medication to cure or alleviate morphine withdrawal symptoms and to prevent relapses of morphine use.     

The effects of buprenorphine on behaviour in the ACI and BN rat inbred strains

Buprenorphine is a partial mu, kappa agonist that has been shown to influence spontaneous behaviour in animals. Previously, we have demonstrated significant differences in the analgesic response to buprenorphine between the August Copenhagen Irish (ACI)/SegHsd and the Brown Norway (BN)/RijHsd inbred rat strains. The purpose of this study was to determine whether these strains also differed in their behavioural response to buprenorphine in order to provide an additional parameter for the genetic analysis and localization of genes involved in this response. Male and female rats of both strains were used (n = 6/strain/sex) for this study. Each rat was subjected, respectively, to three treatment regimens at 15:00 h: (A) unchallenged; (B) intravenous saline; (C) intravenous buprenorphine (0.05 mg/kg) according to a crossover design. The relative duration (s/h) of locomotion, grooming, drinking and eating behaviour was subsequently determined from 15:30 to 07:00 h using the automatic registration system, Laboratory Animal Behaviour Registration and Analysis System(trade mark). Significant strain differences were observed in unchallenged behaviour between the ACI and the BN rats. ACI rats, but not BN rats, responded to buprenorphine treatment with decreased levels of locomotion, drinking and eating behaviour. The same treatment resulted in an increased grooming behaviour in both strains. Slight but significant sex differences were observed for locomotion and eating in the analysis of variance procedure, but did not reach the level of statistical significance in the multiple comparison procedure. The results of this study emphasize the possibility that strain-specific effects must be taken into account when using behavioural parameters for the assessment of the analgesic effects of buprenorphine in rats.     

Effect of negative air ions on morphine-induced changes in the latency of the tail-flick reflex

Adult male Long-Evans rats were exposed to negatively charged air ions at high concentrations (7 x 10(5)/cm3) for six days. Sham-exposed rats were treated identically except that the source of ions was not activated. At the end of the exposure, the latency of the tail-flick reflex was measured in each rat before and 30 and 60 min after an injection of morphine sulphate. The tail-flick reflex was initiated by thermal stimulation. Two heat settings were employed, the lower considered to impart a submaximal and the higher a maximal thermal stimulus. Three morphine doses were tested: 0.5, 1.0, and 1.5 mg/kg. Statistically significant differences between ion-exposed and sham-exposed rats were observed in tail-flick latencies 30 min after the administration of the two lower doses, but not after the highest dose of morphine sulphate. These differences were found at both intensities of thermal stimulation. Tail-flick latencies measured in each group prior to morphine injection were not affected by negative-ion exposure. The data indicate that exposure of rats to negative air ions tends to inhibit the action of morphine on the latency of the tail-flick reflex at morphine doses below 1.0 mg/kg.     

Adverse effects on growth rates in rats caused by buprenorphine administration

As a routine postoperative treatment, a single dose of buprenorphine was given to rats at a dose of 0.05 mg/kg subcutaneously. However, some rats developed abnormal secretions around the nose and mouth and some animals died 3-5 days after surgery and analgesic treatment. At autopsy a yellow fibrous mass was found in the stomach and intestines. Observations of animals given buprenorphine revealed an abnormal ingestion of bedding material. This caused a disturbance to normal digestion, with gastric distension, weight loss or decreased growth rate, constipation and occasionally death. In this study rats were monitored for 6 days following surgery and analgesic treatment. A comparison of growth rates was made between rats given saline and buprenorphine or nalbuphine and between animals kept on bedding or grid floors for the first 24h after treatment. Of the animals held on bedding, the buprenorphine-treated animals did not lose weight as the other animals did, but had on the other hand a decreased growth rate during the measuring period of 6 days after surgery. When denied access to bedding for the first 24 h after surgery, rats given saline or nalbuphine had a reduced weight gain over the first 24 h, similar to the groups held on bedding. Rats held on grid floors and given buprenorphine continued to gain weight for the first 24 h. From day 3, there was no significant difference between the groups, which all gained weight.     

Haemodynamic effects of buprenorphine after heart surgery.

The effect of buprenorphine on the cardiovascular system was examined in 11 patients during the period of reduced cardiac reserve after open-heart surgery. Within 10 minutes of giving the full analgesic dose (5 microgram/kg) intravenously the mean heart rate had fallen significantly by six beats/min. Although in two patients the mean arterial pressure fell by 24 mm Hg, there was no overall change in mean arterial pressure, cardiac output, or peripheral resistance. In a further six patients buprenorphine was used successfully as the sole analgesic after open-heart surgery. Buprenorphine appears to be safer than morphine for use in patients with reduced cardiac reserve and is of similar analgesic efficacy.     

Evidence That Intermittent,Excessive Sugar Intake Causes Endogenous Opioid Dependence

Objective: The goal was to determine whether withdrawal from sugar can cause signs of opioid dependence. Because palatable food stimulates neural systems that are implicated in drug addiction, it was hypothesized that intermittent, excessive sugar intake might create dependency, as indicated by withdrawal signs. Research Methods and Procedures: Male rats were food‐deprived for 12 hours daily, including 4 hours in the early dark, and then offered highly palatable 25% glucose in addition to chow for the next 12 hours. Withdrawal was induced by naloxone or food deprivation. Withdrawal signs were measured by observation, ultrasonic recordings, elevated plus maze tests, and in vivo microdialysis. Results: Naloxone (20 mg/kg intraperitoneally) caused somatic signs, such as teeth chattering, forepaw tremor, and head shakes. Food deprivation for 24 hours caused spontaneous withdrawal signs, such as teeth chattering. Naloxone (3 mg/kg subcutaneously) caused reduced time on the exposed arm of an elevated plus maze, where again significant teeth chattering was recorded. The plus maze anxiety effect was replicated with four control groups for comparison. Accumbens microdialysis revealed that naloxone (10 and 20 mg/kg intraperitoneally) decreased extracellular dopamine (DA), while dose‐dependently increasing acetylcholine (ACh). The naloxone‐induced DA/ACh imbalance was replicated with 10% sucrose and 3 mg/kg naloxone subcutaneously. Discussion: Repeated, excessive intake of sugar created a state in which an opioid antagonist caused behavioral and neurochemical signs of opioid withdrawal. The indices of anxiety and DA/ACh imbalance were qualitatively similar to withdrawal from morphine or nicotine, suggesting that the rats had become sugar‐dependent.     

Analgesic efficacy of orally administered buprenorphine in rats

The analgesic effect of orally administered buprenorphine was compared with that induced by a standard therapeutic injected dose (0.05 mg/kg of body weight, s.c.) in male Long-Evans rats. Analgesia was assessed by measuring pain threshold, using the hot-water tail-flick assay before and after administration of buprenorphine. The results suggest that a commonly used formula for oral buprenorphine in flavored gelatin, at a dose of 0.5 mg/kg, does not increase pain threshold in rats. Instead, oral buprenorphine doses of 5 and 10 mg/kg were necessary to induce significant increases in pain threshold. However, these doses had to be administered by orogastric infusion because the rats would not voluntarily eat flavored gelatin containing this much buprenorphine. The depth of analgesia induced by these infused doses was comparable to that induced by the clinically effective s.c. treatment (0.05 mg/kg).     

Potentiation of analgesia and reversal of tolerance to morphine by calcium channel blockers

Effect of four calcium channel blockers (CCBs) belonging to different chemical classes, alone and in combination with morphine was investigated on two models of pain sensitivity, i.e. formalin and tail flick tests in mice. All the studied CCBs, i.e. diltiazem, flunarizine, nimodipine and verapamil inhibited formalin-induced pain responses; however, with verapamil, though there was a trend towards a reduction of paw-licking response to formalin, it was not found to be statistically significant. In contrast, none of the CCBs affected the tail flick latency at any of the doses studied. Morphine, a mu-receptor agonist exerted a significant analgesic effect in formalin as well in tail flick tests. Pretreatment with all CCBs significantly enhanced the analgesic effect of morphine in both tests of nociception. Further, concomitant administration of one of the CCBs, diltiazem with morphine prevented the development of tolerance to the latter. However, combination of diltiazem with morphine, like morphine alone was found to be ineffective in morphine tolerant animals. Results, thus, show that CCBs produced an analgesic effect of their own in formalin-induced tonic pain and potentiated the analgesic activity of morphine. They also modulated opioid-induced tolerance.