Behavioral Effects Of Opioids Research Articles

Potentiation of morphine-induced antinociception and locomotion by citalopram is accompanied by anxiolytic-like effects

Morphine and related opioids are the mainstay of analgesic treatment, especially in patients suffering chronic pain. Besides their antinociceptive effects they may also exhibit anxiolytic-like properties that could contribute to pain relief. The pharmacological manipulation of the serotonergic system may not only modulate pain transmission and processing but also other behavioral effects of opioids. The present study aimed to analyze the effect of the concurrent treatment with citalopram, a selective serotonin reuptake inhibitor, on the antinociceptive, locomotor and anxiety-related effects induced by acute and subchronic administration of morphine in mice. Citalopram (15mg/kg) enhanced the acute antinociceptive effects of morphine when concurrently administered as evidenced by a two-fold increase in the ED50 for the antinociceptive effect of morphine in the hot-plate test. Chronic studies also revealed that concurrent citalopram treatment (15mg/kg) delayed the development of tolerance to the thermal antinociceptive effects of morphine. Additionally, morphine-induced hyperlocomotion was potentiated by citalopram as assessed in the open-field test and in the spontaneous activity recording in the home cage, a behavioral outcome to which tolerance or desensitization was not developed. Interestingly, chronic administration of both drugs promoted an anxiolytic effect as evidenced by the increased central activity in the open field test. Future investigations on this pharmacological interaction, such as the possible translational research in clinics, might have consequences in future strategies for the therapeutic management of pain.

Use and Misuse of Opioids in Chronic Pain.

The prescribing of opioid analgesics for pain management-particularly for management of chronic noncancer pain (CNCP)-has increased more than fourfold in the United States since the mid-1990s. Yet there is mounting evidence that opioids have only limited effectiveness in the management of CNCP, and the increased availability of prescribed opioids has contributed to upsurges in opioid-related addiction cases and overdose deaths. These concerns have led to critical revisiting and modification of prior pain management practices (e.g., guidelines from the Centers for Disease Control and Prevention), but the much-needed changes in clinical practice will be facilitated by a better understanding of the pharmacology and behavioral effects of opioids that underlie both their therapeutic effects (analgesia) and their adverse effects (addiction and overdose). With these goals in mind, this review first presents an overview of the contemporary problems associated with opioid management of CNCP and the related public health issues of opioid diversion, overdose, and addiction. It then discusses the pharmacology underlying the therapeutic and main adverse effects of opioids and its implications for clinical management of CNCP within the framework of recent clinical guidelines for prescribing opioids in the management of CNCP.

Effects of imidazoline I2 receptor agonists and morphine on schedule-controlled responding in rats

Accumulating evidence indicates that imidazoline I2 receptor agonists enhance the antinociceptive effects of opioids and therefore may be suitable for combination therapy with opioids for pain treatment. However, little is known of the effects of I2 receptor agonists on other behavioral effects of opioids. This study used schedule-controlled responding and dose-addition analyses to examine interactions between the μ opioid receptor agonist morphine and two imidazoline I2 receptor agonists, 2-BFI and BU224. In 8 rats responding under a fixed ratio 10 schedule of food presentation, morphine (3.2–17.8mg/kg), 2-BFI (3.2–17.8mg/kg), and BU224 (5.6–17.8mg/kg) each dose-dependently decreased responding. The addition of fixed proportions of 2-BFI or BU224 shifted the morphine dose–effect curves leftward. The interactions between morphine and 2-BFI or BU224 were infra-additive when the same proportions of morphine and I2 receptor agonists were mixed; however, the interaction between morphine and I2 receptor agonists was additive when the drugs were mixed at other proportions. These results provide quantitative evidence that I2 receptor agonists do not enhance the response rate-decreasing effect of morphine and suggest that the enhancement of morphine antinociception is selective. Together, these results further support the therapeutic potential of combining I2 receptor agonists and opioids for pain control.

Sex differences in analgesic, reinforcing, discriminative, and motoric effects of opioids.

This review summarizes evidence for sex differences in behavioral effects of opioids, primarily in rats. Whereas micro agonists have been found to be more potent and in some cases more efficacious in producing analgesia and sedation in males than females, females are more sensitive than males to reinforcing and locomotor stimulant effects of opioids. Sex differences in motoric effects of opioids may contribute to sex differences in other behavioral effects of opioids; for example, sex differences in rats' ability to discriminate morphine from saline can be attributed entirely to greater morphine-induced sedation in males. Chronic estradiol blunts females' sensitivity to morphine's analgesic and sedative effects, but enhances females' sensitivity to the reinforcing and locomotor stimulant effects of micro opioids. The neurobiological basis for sex differences in and estradiol modulation of behavioral effects of opioids includes brain opioid receptor density (greater in males and under low-estradiol conditions in females) and dopaminergic function (greater in females and under high-estradiol conditions). Given the significant and growing use of opioids by women, both medicinally and recreationally, understanding how female biology influences analgesic and other effects of opioids is crucial.

Alfentanil increases cortical dopamine D2/D3 receptor binding in healthy subjects

Animal studies have shown that opioids modulate the function of dopaminergic neurons. The effect of alfentanil on cortical and thalamic binding of the D2/D3 receptor ligand [ 11C]FLB 457 was evaluated in eight healthy subjects with positron emission tomography. The simplified reference tissue model was used to calculate tracer binding potential (BP) during a baseline condition and target-controlled infusion of alfentanil, and the results were analyzed using a comparison group not receiving opioid. Behavioral and analgesic effects of alfentanil were also evaluated. In the region-of-interest analysis, alfentanil increased the BP of [ 11C]FLB 457 in the medial frontal cortex ( P=0.028), dorsolateral prefrontal cortex ( P=0.027), superior temporal cortex ( P=0.028), and medial thalamus ( P=0.003). These results were confirmed in a voxel-based analysis, which further revealed an opioid-induced increase in [ 11C]FLB 457 BP in the anterior cingulate cortex ( P<0.001). Alfentanil induced euphoria ( P=0.03) and analgesia ( P=0.006). Cheerfulness ( r=0.982, P<0.001) and euphoria ( r=0.918, P=0.001) were associated with increased BP of [ 11C]FLB 457 in the left posterior cingulate cortex, but the analgesic effect of alfentanil did not correlate with changes in [ 11C]FLB 457 BP. The results of this study demonstrate opioid–dopamine interactions in frontal and temporal cortical regions and the thalamus in healthy subjects. Increased D2/D3 tracer binding during opioid infusion may reflect decreased synaptic dopamine levels. The association of the uplifting effect of alfentanil with increased D2/D3 binding in the posterior cingulate cortex suggests that cortical dopamine may be involved in the behavioral effects of opioids.

Heroin-associated environmental stimuli modulate the expression of inducible nitric oxide synthase in the rat.

Heroin (diacetylmorphine) administration has been shown to alter the induction of nitric oxide, a molecule known to play a critical role in the regulation of immune responses and resistance to infectious challenges. There is evidence that the physiological and behavioral effects of opioids can be conditioned to environmental stimuli associated with drug administration. The present study tests whether environmental stimuli paired with the administration of heroin alter the expression of inducible nitric oxide synthase (iNOS), one of the isoforms of the enzyme responsible for nitric oxide production. The experimental group of rats received three conditioning sessions each involving a subcutaneous injection of heroin (1.0 mg/kg) immediately upon being placed in a chamber that served as the conditioned stimulus. Following the conditioning sessions, the rats remained in their home cages for 6 days to recover from the effects of heroin per se. The rats then were re-exposed to the conditioning chamber in the absence of heroin and immediately following the session were injected subcutaneously with lipopolysaccharide (LPS) to induce iNOS expression. Six hours later, the expression of iNOS messenger ribonucleic acid (mRNA) and protein was determined in spleen, lung, and liver using real-time reverse transcriptase polymerase chain reaction (RT-PCR) and western blotting, respectively. Control procedures were used to determine whether the alterations in iNOS expression were due to conditioning processes. The results showed that exposure to the conditioned stimulus produced a pronounced reduction in the expression of iNOS mRNA and protein in spleen, lung, and liver tissue. Control procedures showed that this effect was due to conditioning processes. This study provides the first evidence that heroin-induced alteration of iNOS expression can be conditioned to environmental stimuli, and suggest that environmental stimuli associated with drug use may contribute to the alteration in susceptibility to infection.