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The effects of chronic morphine administration on the development of long-term potentiation (LTP) were investigated at the Schaffer collateral-CA1 pyramidal cell synapses of the rat hippocampal slices using primed-bursts tetanic stimulation. Significant enhancement of orthodromic population spike (OPS) was found for all stimulus intensities after tetanic stimulation. OPS enhancement was greatest when tested with low to mid-range stimulus intensities (25, 50 and 100 µA). There was also significant decrease in OPS delay. These responses were similar in slices from both control and morphine dependent rats. At all delivered stimulus intensities, the amount of LTP of OPS in slices from dependent rats was larger than that of control slices. However, these differences in LTP of OPS were significant at low stimulus intensities. These findings suggest that chronic morphine administration had induced changes in CA1 neurocircuitry which modulated synaptic plasticity during high frequency stimulation and appeared as augmented LTP and also inhibition of LTP decay.

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  The effects of chronic morphine administration on the development of long-term potentiation (LTP) were investigated at the Schaffer collateral-CA1 pyramidal cell synapses of the rat hippocampal slices using primed-bursts tetanic stimulation. Significant enhancement of orthodromic population spike (OPS) was found for all stimulus intensities after tetanic stimulation. OPS enhancement was greatest when tested with low to mid-range stimulus intensities (25, 50 and 100 µ A). There was also significant decrease in OPS delay. These responses were similar in slices from both control and morphine dependent rats. At all delivered stimulus intensities, the amount of LTP of OPS in slices from dependent rats was larger than that of control slices. However, these differences in LTP of OPS were significant at low stimulus intensities. These findings suggest that chronic morphine administration had induced changes in CA1 neurocircuitry which modulated synaptic plasticity during high frequency stimulation and appeared as augmented LTP and also inhibition of LTP decay.

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  The involvement of NMDA receptors and voltage-dependent calcium channels in augmentation of long-term potentiation (LTP) was investigated at the Schaffer collateral CA1 pyramidal cell synapses in hippocampal slices of morphine dependent rats, using primed-burst tetanic simulation. The amplitude of the population spike and its delay were measured as indices of increase in postsynaptic excitability. D,L-APV and nifedipine were used as an NMDA receptor antagonist and a voltage-dependent calcium channel blocker, respectively. The amount of LTP of the orthodromic population spike (OPS) was higher in slices from dependent rats. Perfusion of slices from control and dependent rats with ACSF containing D,L-APV (25 µ M) and delivering tetanic simulation showed that D,L- APV completely blocked the LTP of OPS in slices from both control and dependent rats, while nifedipine (10 µ M) attenuated the amount of LTP of OPS in dependent slices and had no effect on controls. The results suggest that the enhanced LTP of OPS in the CA1 area of hippocampal slices from morphine-dependent rats is primarily induced by NMDA receptor activity, and the voltage-dependent calcium channels may also be partially involved in this phenomenon.

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  In this study, we investigated the effect of nicotinic receptor agonists and antagonists on the analgesic response to morphine in the formalin test. In experiments conducted in mice, nicotine produced an early dose-dependent analgesic effect. At a dose of 0.5 mg/kg, mecamylamine, a nicotinic receptor inhibitor, suppressed the analgesic effect induced by 0.1 mg/kg nicotine in both stages of the formalin test, while hexamethonium had no such effect. Atropine, a muscarinic receptor antagonist, reduced the nicotine response at doses of 5 and 10 mg/kg. Mecamylamine, hexamethonium and atropine had no effect on morphine-induced analgesia. Administered separately, the antagonists had no effect on the analgesic response either. High doses of mecamylamine lead to an increase of the pain response. We conclude that cholinergic and opioid receptors play a possible role in the analgesic effect induced by nicotine.

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  The involvement of nitric oxide (NO) in the reinforcing properties of opiate reward was studied by examining the effect of a NO precursor (L-arginine) and a NO synthase inhibitor (L-NAME) on the rate of intravenous self-administration of morphine. This experiment was investigated in male albino Sprague Dawley rats (300 ± 50 g). At doses of 5 and 10 mg/kg (i.p.), L-arginine decreased morphine self- administration significantly, while 20 and 60 mg/kg doses were not significantly effective. In addition, L-arginine per se induced self-administration behavior. At doses of 0.75, 1.25, 2.5, and 5 mg/kg (i.p.), L-NAME had no effect on self-administration, whereas injection of 5 mg/kg 60 minutes before the test did increase self-administration significantly. We may therefore conclude that NO is involved in the VTA-mesolimbic pathway, and it can be involved in the psychological dependence to morphine.

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  There is evidence that sweeteners such as sucrose and saccharin interact with endogenous opioid systems. Further research has shown that feeding different concentrations of sucrose and saccharin alter latency in the tail-flick test. In this study, the influence of a 12-day regimen of different sweetening agents, sucrose (32%), saccharin (0.08%) and aspartame (0.16%) on morphine-induced analgesia in the formalin test was investigated. Male albino mice (20-27 g) were used for the experiments. The animals were given 12 days to adapt to the dietary conditions. An initial subcutaneous injection of saline or morphine (1.5, 3, 6 or 9 mg/kg) was given 30 minutes before the observation period. Recording of the early phase began immediately and continued for 10 minutes. Recording of the late response began 20 minutes after injection and continued for 10 minutes. Sucrose and aspartame increased the analgesia of morphine in the early phase while saccharin had no effect. On the other hand saccharin and sucrose decreased the effect of morphine in the late phase while aspartame increased the effect of morphine-induced analgesia. In conclusion, the present data provide further evidence for an important role of dietary variables in determining the effects of exogenous opioids on pain sensitivity.

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The effects of caerulin, a CCK receptor agonist, and proglumide, a receptor antagonist, on hyperalgesia induced by sciatic nerve ligation, was studied in mice. Tolerance to the morphine response was obtained 3,7, 14,21 and 28 days after unilateral sciatic nerve ligation. Maximum hyperalgesia was found 14 days after nerve ligation. Caerulin increased morphine antinociception in nerve-ligated animals. The drug also induced antinociception in intact animals. Proglumide also induced antinociception in nerve-ligated mice. The antagonist increased the morphine response but did not alter the effect of caerulin. We conclude that the CCK receptor mechanism may have a role in hyperalgesia induced by nerve ligation

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  The involvement of nitric oxide (NO) in the reinforcing properties of opioids was studied by examining the effect of a NO precursor (L-arginine) and NO synthase inhibitor (L-nitro-amino-methyl-ester, L-NAME) on the conditioned place preference (CPP) induced by morphine. The experiment was performed on male albino Swiss- White mice (25 ± 5 g). L-arginine in doses of 100, 200 and 500 mg/kg (i.p.) decreased CPP significantly, while L-arginine induced CPP in doses of 200 and 500 mg/kg per se. L-NAME doses of 0.75, 1.25, 2.5, 5, 10, 20 and 300 mg/kg (i.p.)had no effect on CPP induced by morphine, but injection of L-NAME (5 mg/kg) significantly decreased the effect of L-arginine on morphine-induced CPP. Therefore it may be concluded that NO may be involved in the psychological dependence to morphine.

 

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  In this study, the effects of lead acetate on two types of pain (nociception and inflammation) induced by formalin and its interaction with the opioid system and morphine-induced analgesia were examined. Male albino mice weighing 22-27 g were used in the experiments. Morphine was administered subcutaneously 30 minutes before formalin injection. Lead acetate was intraperitoneally administered 90 minutes before any injection. Different doses of morphine induce antinociception in both phases of the formalin test. Lead acetate induced no does-dependent nociception in the early phase, but caused dose-dependent analgesia in the late phase. Pretreatment with lead acetate antagonized the effect of morphine in the early phase. On the other hand, the effect of lead acetate in the early phase was reduced by morphine and its effect was eliminated in the late phase. We conclude that lead acetate can modulate the pain response and interact with morphine-induced antinociception. Additional research is suggested to identify the mechanisms of these effects.

 

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  There is evidence indicating that adrenoceptor mechanisms may influence some of the behaviors in rat. However the role of adrenoceptor agents on sniffing has not been identified. In the present study, the influence of adrenoceptor agents on sniffing induced by apomorphine and amphetamine has been investigated. Male Albino rats, weighing 150-250 g were used for all experiments. The behavior was measured based on the method we used previously. Sniffing was scored every 15 sec according to the following scale: 0=absent, 1=sniffing. At each 60 min period, the sniffing score ± SEM of at least 9 rats was scored Score 0 indicates no sniffing, score 240 indicated maximum sniffing. Effects of adrenergic agonists and antagonists on sniffing induced by amphetamine and apomorphine have been tested in rats. Intraperitoneal (IP) administration of different doses of amphetamine (2-8 mg/kg) induced a dose-related sniffing. IP administration of apomorphine (0.5 mg/kg) also induced sniffing. Different doses of clonidine, phenylephrine and prazocin decreased sniffing induced by both amphetamine and apomorphine. Yohimbine reduced inhibitory influence of clonidine on amphetamine- and apomorphine-induced sniffing. However the response of phenylephrine was not altered by prazocin pretreatment. It is concluded that α₂ adrenoceptor stimulation is able to reduce sniffing induced by dopaminergic system.

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  Tolerance and dependence are two main problems that have limited morphine administration as an analgesic drug and they might be as a result of changes in the number and affinity of receptors, dysfunction of adenylate cyclase, impaired coupling between activated µ receptor and K⁺ channels, and changes in the K⁺ and Ca²⁺ channels. There are several reports concerning the role of some of these factors in the occurrence of tolerance and dependence. It has been suggested that K_ATP channels are involved in morphine-induced analgesia. In this study the effect of a constitutive K_ATP opening state by chronic administration of minoxidil (a K_ATP opener) on morphine tolerance and dependence was studied using tail-flick test. A single dose (5 mg/kg) of morphine, but not its chronic administration produced analgesia (p<0.001). In addition single and chronic (2 mg/kg) administration of minoxidil produced analgesia (p<0.0001 vs ethanol) and chronic co-administration of morphine and minoxidil did not reduce morphine tolerance, while It reduced jumping (p<0.01) and weight loss (p<0.05) as signs of dependence. Naloxone did not antagonize minoxidil analgesia. Morphine analgesia was reduced by glibenclamide 2 mg/kg (p<0.001). These results may suggest that co-administration of morphine and minoxidil is able to reduce some dependence signs of morphine. Since this treatment reduced the jumping and weight loss, but not the writhing sign, it is concluded that different mechanisms and sites of action are involved in the development of each of the dependence signs.

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In the present study the effect of intracerebroventricular (ICV) injection of cerulein, an agonist of CCK receptor and proglumide, a receptor antagonist for hyperalgesia induced by sciatic nerve ligation were investigated in mice. Subcutaneous administration of morphine caused anti-nociception in both intact and nerve-ligated mice. However, the response to opioids was lower in ligated mice as compared to the intact animals. Cerulein induced antinociception only in the nerve-Iigated animals. Combination of cerulein with morphine elicited higher response in both intact and ligated animals. However, the induced response was much prominent in ligated animals. Proglumide alone did not elicit any response in both groups. The antagonist decreased the response of cerulein in the non-ligated mice. In addition a low dose of proglumide in combination with cerulein induced antinociception in the ligated mice. It can be concluded that CCK receptor-related

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In this study the effect of adenosine and caffeine on spontaneous activity of paragigantocellularis (PGi) neurons was investigated. The spontaneous activity of PGi neurons was significantly decreased by microinjection of adenosine (10 nM, 0.5 µl) into PGi nucleus of both control and morphine-dependent rats. The decrease in firing rate of PGi neurons of morphine-dependent rats was greater than that of control. There was also significant enhancement of spontaneous activity of PGi neurons 8-15 min after caffeine administration (50 mg/kg i.p.) in both control and morphine-dependent rats. However, the effect of caffeine in morphine-dependent rats was higher than that of control. These data suggest that there is an increase in the sensitivity to chemicals, which interact with adenosine receptors in morphine- dependent rats. Nevertheless, considering a common second messenger system (cAMP) for adenosine (A₁) and opioid (µ) receptor, it is proposed that up-regulation and hypersensitivity of A₁ adenosine receptors are responsible for these results in morphine-dependent rats.

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Post-operative pain and its management remains one of the most important issues in the field of surgery and health care system. On the other hand, opioid drug abuse has a large prevalence in Iran. Formalin test has been used as a method for assessing pain and analgesia in rats. In the present study, the post-operative pain in morphine addicted rats was compared with that of non-addicted ones using formalin test. For this purpose, 26 rats were chosen and randomly divided into two groups. First group received morphine sulphate in their drinking water for 21 days. The other group received only tap water. On the day of their assessment, a longitudinal incision (1-cm) was made through skin fascia of their plantar aspect of one foot under ether anesthesia. One hour later, their pain was assessed with formalin test through intra-plantar subcutaneous injection of 0.05 ml of 2.5% formalin solution. Our results showed that acute pain (the first 5 minutes after injection) in both groups was not different significantly, but chronic pain (the next 15-45 minutes) was much intense in morphine-addicted rats.

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Recent studies have shown that NMDA receptors are involved in the tolerance and dependence to opioids. In addition, it has been reported that ketamine, dextromethorphan and amantadine have antagonistic activity at NMDA receptors. Therefore, this study was conducted to clarify the effect of these drugs on morphine withdrawal syndrome. Morphine dependence was induced by increasing doses of morphine (30, 45, 60, and 90 mg/kg, s.c.) administered at 2 h intervals. Dextromethorphan (12.5, 25, and 50 mg/kg), amantadine (50, 100, and 150 mg/kg), ketamine (50 mg/kg), and clonidine (0.2 mg/kg) were injected (i.p.) 90 minutes after the last injection of morphine. Furthermore, these drugs were administered orally 60 minutes after the last morphine injection. All animals received naloxone (5 mg/kg, i.p.) 2 h after the last morphine injection. Withdrawal syndrome (number of jumping, standing on feet. and diarrhea) was recorded during a 30-min period. Dextromethorphan and amantadine inhibited jumping, standing on feet and diarrhea in a dose-dependent manner. Ketamine (50 mg/kg) had a similar effect and clonidine produced an almost complete inhibition of withdrawal syndrome. Although these test drugs belong to different pharmacological classes, but they share NMDA receptor antagonistic effect and it may be the probable cause for inhibition of withdrawal syndrome.

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Various physiological parameters including level of sex steroids undergo alterations following chronic administration of morphine. In this study, the effect of chronic administration of morphine on phasic and tonic pain was studied in morphine-dependent male and female rats in the presence and absence of gonads using formalin test. In addition, for evaluation of differences in dependency, withdrawal signs were observed using naloxone hydrochloride. For dependency induction, morphine sulphate was administered in drinking water for a period of 30 days. The results showed that although chronic pain is significantly greater in female rats than male ones but this pain increases in male dependent rats and decreases in female dependent animals. Thus, no gender differences were found between male and female dependent rats. Furthermore, gonadectomy led to a significant decrease in chronic pain only in male dependent rats. Meanwhile, withdrawal signs were significantly greater in female dependent rats than male ones and gonadectomy did not influence these signs. It can be concluded that following morphine addiction, pain increases in male and decreases in female rats and morphine dependency is not affected by sex hormones.

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In this study, the effect of reversible inactivation of locus ceruleus (LC) on naloxone- induced withdrawal syndrome in paragigantocellular (PGi) neurons in morphine- dependent rats was investigated. For inactivation of LC, 1 µl of lidocaine (2%) was used and for induction of withdrawal syndrome, naloxone hydrochloride (2 mg/kg) was injected systemically. The results showed that in dependent group with intact LC, PGi single unit activity increased following naloxone administration, whereas in dependent group with inactivated LC, the observed increase was not significant.

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In the present study, the effects of excitation and inhibition of GABAA and GABAB receptor subtypes on the expression of morphine-induced conditioned place preference (CPP) were investigated. For this purpose, male Wistar rats (250-300 g) were used in the experiments. Five days after surgical cannulation in the ventral tegmental area (VTA), different doses of morphine were injected into the animals and conditioned biased method was applied. The results showed that subcutaneous injections of morphine sulfate (1-10 mg/kg) induced CPP in a dose-dependent manner. Furthermore, administration of GABAA receptor agonist, muscimol (6 ng/rat) into ventral tegmental area reduced the morphine effects. The drug at doses of 12 and 25 ng/rat enhanced morphine-induced CPP. Reduction of morphine-induced CPP was observed following injection of GABAA receptor antagonist, bicuculline (6, 12 and 25 ng/rat) into VTA. Injection of baclofen as GABAB receptor agonist (6 ng/rat) into VTA caused an increase in CPP induced by morphine, whereas the drug at doses of 12 and 25 ng/rat reduced the morphine effects. Injection of CGP38345 (a GABAB receptor antagonist) into the VTA at doses of 6, 12 and 25 ng/rat significantly reduced the CPP induced by morphine. It could be concluded that excitation and inhibition of GABAA and GABAB receptor subtypes in the VTA may have different effects on morphine-induced CPP.

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It has been reported that morphine tolerance does not develop in the presence of chronic pain. Therefore, this study was conducted to find out whether chronic inflammatory pain is able to eliminate or attenuate the developed tolerance to analgesic effect of morphine and also to investigate the role of lumbar spinal cord as a candidate site for this interaction. Tolerance was induced in adult male NMRI rats using daily injection of morphine at a dose of 20 mg/kg (i.p.) for 4 days, or using daily injection of morphine at a dose of 15 pg/rat (i.t.) for 7 days. Chronic inflammatory pain was induced using 50 µl of 5% formalin, injected into the hind paws. The antinociceptive effect of morphine at a dose of 10 mg/kg on day 5 (for i.p. treated rats) or morphine at a dose of 15 pg/rat on day 8 (for i.t. treared rats) were assessed using tail flick test. The results showed that those animals receiving saline (i.t. or i.p.) have potent analgesia (p<0.001), while animals treated with chronic morphine have only a weak analgesia for i.p. treatment (p<0.05). In addition, animals treated with both repeated morphine and 5% formalin (s.c.) into the hind paw showed potent analgesia (p<0.01). Meanwhile, the developed tolerance was reversed by chronic pain induction in the following days (p<0.01). It is concluded that chronic formalin-induced inflammatory pain, not only could prevent tolerance development, but also is able to reverse the developed tolerance to antinociceptive effect of morphine. Since in i.t.-treated animals, tolerance was induced in lumbar spinal cord level, it can be concluded that chronic formalin-induced inflammatory pain, as a stress (through HPA axis) or as a factor which directly exerts some modulations on pain transmission system, is able to prevent tolerance to analgesic effect of morphine through lumbar spinal cord.

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It has been shown that L-arginine like the abused drugs could release dopamine from meso-cortico-limbic system. The purpose of the present study was to compare the effect of L-arginine with morphine as a standard abuse drug in the induction of conditioned place preference (CPP) in rat. For this purpose, male Wistar rats (250-300 g) were used in the experiments. Five days after surgical cannulation in the ventral tegmental area (VTA), different doses of L-arginine, L-NAME or morphine were injected and conditioned biased method was preformed. The results showed that: 1. Intraperitoneal (i.p.) (0.1, 1 and 5 mg/kg) but not intra-VTA (i-VTA) (0.3, 0.1 and 3 µg/rat) injections of L-arginine induced CPP in a dose-dependent manner. Injection of 10 mg/kg (i.p.) and 2 µg/rat of morphine (i-VTA) also caused a significant CPP in rats. Both i.p. (0.1, 1 and 5 mg/kg) and i-VTA (0.3, 0.1 and µg/rat) injection of L-NAME did not produce any effect in the rats, but the drug at higher dose (5 mg/kg, i.p.) attenuated the CPP induced by L-arginine (5 mg/kg, i.p.) Administration of morphine (10 mg/kg, i.p.) on three consecutive days and, then with 3-day interval caused the occurrence of sensitization to morphine, so that injection of ineffective dose of morphine (0.5 mg/kg, i.p.) induced CPP in sensitized rats. However, when the procedure was repeated for L-arginine (5 mg/kg, i.p.), rats showed negative response to the ineffective dose of L-arginine (0.1 mg/kg, i.p.). Meanwhile, in rats that received L-arginine (5 mg/kg, i.p.) on training days, injection of the same dose L-arginine on the test day dramatically decreased the time spent in drug-paired side while injection of morphine (10 mg/kg, i.p.) on the test day to those rats which had received morphine (10 mg/kg, i.p.) on training days caused an increase in time spent in the drug-paired compartment. It could be concluded that i.p. injection of L-arginine causes significant CPP rats and this behavior was attenuated by nitric oxide synthase inhibitor, L-NAME. Furthermore, the site of action of the drug seems not to be VTA. On the other hand behaviors due to administration of L-arginine did not show some properties of abused drugs such as sensitization and state dependency.