The effects of morphine and delta-9-tetrahydrocannabinol on motor activity in rats

Opioid Analgesia and Opioid-Induced Adverse Effects: A Review

Opioids are widely used as therapeutic agents against moderate to severe acute and chronic pain. Still, these classes of analgesic drugs have many potential limitations as they induce analgesic tolerance, addiction and numerous behavioural adverse effects that often result in patient non-compliance. As opium and opioids have been traditionally used as painkillers, the exact mechanisms of their adverse reactions over repeated use are multifactorial and not fully understood. Older adults suffer from cancer and non-cancer chronic pain more than younger adults, due to the physiological changes related to ageing and their reduced metabolic capabilities and thus show an increased number of adverse reactions to opioid drugs. All clinically used opioids are μ-opioid receptor agonists, and the major adverse effects are directly or potentially connected to this receptor. Multifunctional opioid ligands or peripherally restricted opioids may elicit fewer adverse effects, as shown in preclinical studies, but these results need reproducibility from further extensive clinical trials. The current review aims to overview various mechanisms involved in the adverse effects induced by opioids, to provide a better understanding of the underlying pathophysiology and, ultimately, to help develop an effective therapeutic strategy to better manage pain.

Profiling the Effects of Repetitive Morphine Administration on Motor Behavior in Rats

Efficient repetitive clinical use of morphine is limited by its numerous side effects, whereas analgesic tolerance necessitates subsequent increases in morphine dose to achieve adequate levels of analgesia. While many studies focused on analgesic tolerance, the effect of morphine dosing on non-analgesic effects has been overlooked. This study aimed to characterize morphine-induced behavior and the development and progression of morphine-induced behavioral tolerance. Adult male Sprague-Dawley rats were repetitively treated with subcutaneous morphine for 14 days in two dose groups (A: 5 mg/kg/day (b.i.d.) → 10 mg/kg/day; B: 10 mg/kg/day (b.i.d.) → 20 mg/kg/day). Motor behavior was assessed daily (distance traveled, speed, moving time, rearing, rotation) in an open-field arena, before and 30 min post-injections. Antinociception was measured using tail-flick and hot-plate assays. All measured parameters were highly suppressed in both dosing groups on the first treatment day, followed by a gradual manifestation of behavioral tolerance as the treatment progressed. Animals in the high-dose group showed increased locomotor activity after 10 days of morphine treatment. This excitatory phase converted to an inhibition of behavior when a higher morphine dose was introduced. We suggest that the excitatory locomotor effects of repetitive high-dose morphine exposure represent a signature of its behavioral and antinociceptive tolerance.

Cannabis use, abuse, and withdrawal: Cannabinergic mechanisms, clinical, and preclinical findings

Cannabis sativa is the most widely used illicit drug in the world. Its main psychoactive component is delta‐9‐tetrahydrocannabinol (THC), one of over 100 phytocannabinoid compounds produced by the cannabis plant. THC is the primary compound that drives cannabis abuse potential and is also used and prescribed medically for therapeutic qualities. Despite its therapeutic potential, a significant subpopulation of frequent cannabis or THC users will develop a drug use syndrome termed cannabis use disorder. Individuals suffering from cannabis use disorder exhibit many of the hallmarks of classical addictions including cravings, tolerance, and withdrawal symptoms. Currently, there are no efficacious treatments for cannabis use disorder or withdrawal symptoms. This makes both clinical and preclinical research on the neurobiological mechanisms of these syndromes ever more pertinent. Indeed, basic research using animal models has provided valuable evidence of the neural molecular and cellular actions of cannabis that mediate its behavioral effects. One of the main components being central action on the cannabinoid type‐one receptor and downstream intracellular signaling related to the endogenous cannabinoid system. Back‐translational studies have provided insight linking preclinical basic and behavioral biology research to better understand symptoms observed at the clinical level. This narrative review aims to summarize major research elucidating the molecular, cellular, and behavioral manifestations of cannabis/THC use that play a role in cannabis use disorder and withdrawal.

A genetic reduction in the serotonin transporter differentially influences MDMA and heroin induced behaviours

BACKGROUND

Despite ongoing study and research to better understand drug addiction, it continues to be a heavy burden. Only a small percentage of individuals who take drugs of abuse go on to develop addiction. However, there is growing evidence to suggest that a reduction in the serotonin transporter may play an important role for those that transition to compulsive drug taking. Studies have demonstrated that reduced serotonin transporter function potentiates self-administration of psychostimulant drugs ("ecstasy," MDMA; cocaine); however, additional research revealed no differences between genotypes when the opioid heroin was self-administered. These results suggest that a reduction in the serotonin transporter may confer susceptibility to the development of addiction to some classes of drugs but not others. Importantly, the mechanism underlying facilitated psychostimulant self-administration is currently unknown.

METHODS

Therefore, to continue investigating the relationship between compromised serotonergic function and different classes of drugs, a series of experiments was conducted investigating locomotor activity (LMA) and conditioned taste aversion (CTA) in the serotonin transporter knockout (SERT KO) rat model.

RESULTS

MDMA-induced hyperactivity was reduced, while MDMA-induced CTA was enhanced, in SERT KO rats. However, there were no genotype differences in heroin-induced behaviours.

CONCLUSIONS

These results reinforce the idea that a reduction in the serotonin transporter drives differential effects between disparate classes of drugs of abuse.

Placebo-induced analgesia in an operant pain model in rats

Analgesia is particularly susceptible to placebo responses. Recent studies in humans have provided important insights into the neurobiology underlying placebo-induced analgesia. However, human studies provide incomplete mechanistic explanations of placebo analgesia because of limited capacity to use cellular, molecular, and genetic manipulations. To address this shortcoming, this article describes the development of a rat model of conditioned analgesia in an operant pain assay. Specifically, rats were conditioned to associate a placebo manipulation with the analgesic effect of 1mg/kg morphine (subcutaneously) on facial thermal pain. We found that conditioned (placebo) responding bore 3 of the hallmarks of placebo-induced analgesia: (1) strong interanimal variability in the response, (2) suppression by the opiate antagonist naloxone (5mg/kg subcutaneously), and (3) a positive predictive relationship between the unconditioned analgesic effect and the conditioned (placebo) effect. Because of the operant nature of the assay and the use of only a mild noxious thermal stimulus, we suggest that these results provide evidence of placebo-induced analgesia in a preclinical model that utilizes an affective behavioral end point. This finding may provide opportunities for invasive preclinical studies allowing greater understanding of placebo-induced analgesia, thus paving the way for avenues to harness its benefits.

Signal transduction via cannabinoid receptors

The endocannabinoids anandamide and 2-arachidonoylglycerol are lipid mediators that signal via CB(1) and CB(2) cannabinoid receptors and Gi/o-proteins to inhibit adenylyl cyclase and stimulate mitogen-activated protein kinase. In the brain, CB(1) receptors interact with opioid receptors in close proximity, and these receptors may share G-proteins and effector systems. In the striatum, CB(1) receptors function in coordination with D(1) and D(2) dopamine receptors, and combined stimulation of CB(1)-D(2) receptor heteromeric complexes promotes a unique interaction to stimulate cAMP production. CB(1) receptors also trigger growth factor receptor signaling cascades in cells by engaging in cross-talk or interreceptor signal transmission with the receptor tyrosine kinase (RTK) family. Mechanisms for CB(1) receptor-RTK transactivation can include stimulation of signal transduction pathways regulated by second messengers such as phospholipase C, metalloprotease cleavage of membrane-bound precursor proteins such as epidermal growth factor which activate RTKs, RTK autophosphorylation, and recruitment of non-receptor tyrosine kinases. CB(1) and CB(2) receptors are expressed in peripheral tissues including liver and adipose tissue, and are induced in pathological conditions. Novel signal transduction resulting from endocannabinoid regulation of AMP-regulated kinase and peroxisome proliferator-activated receptors have been discovered from studies of hepatocytes and adipocytes. It can be predicted that drug discovery of the future will be based upon these novel signal transduction mechanisms for endocannabinoid mediators.

Chronic treatment with Delta(9)-tetrahydrocannabinol enhances the locomotor response to amphetamine and heroin. Implications for vulnerability to drug addiction

Cannabis sativa preparations are some of the most widely used illicit recreational drugs. In addition to their direct addictive potential, cannabinoids may influence the sensitivity to other drugs. The aim of the present study was to determine if a cross-sensitization between Delta(9)-tetrahydrocannabinol (Delta(9)-THC) and other drugs (amphetamine and heroin) could be demonstrated. We examined the effects of a chronic treatment with Delta(9)-THC (0.6, 3 and 15mg/kg, ip) on the locomotor response to amphetamine (1mg/kg, ip) and heroin (1mg/kg, ip). Chronic treatment with Delta(9)-THC resulted in tolerance to the initial hypothermic and anorexic effects. Pre-treatment with Delta(9)-THC increased the locomotor responses to amphetamine and heroin. This cross-sensitization was time-dependent as it was observed three days after the last injection of Delta(9)-THC for amphetamine, and a relatively long time after the end of chronic treatment (41 days) for heroin. Moreover, the enhanced response to amphetamine or heroin was noted in some individuals only: the high-responder rats (HR). These animals have previously been shown to be vulnerable to drug taking behaviors. It is hypothesised that repeated use of Cannabis derivates may facilitate progression to the consumption of other illicit drugs in vulnerable individuals.

Naltrexone does not block the subjective effects of oral Delta(9)-tetrahydrocannabinol in humans

Delta(9)-tetrahydrocannabinol (THC) and opioids have many common effects. In addition, some THC effects in laboratory animals can be blocked or attenuated by opioid antagonists. This suggests that opioid systems mediate or modulate some THC effects. To determine whether opioid systems mediate THC effects in humans, the effects of the opioid antagonist naltrexone on subjective responses to THC were examined in 14 marijuana users. Subjects participated in a double-blinded, cross-over design in which each subject received all combinations of naltrexone (0 or 50 mg) and THC (0, 7.5, or 15 mg). THC increased heart rate and self-reported drug effects, such as euphoria and marijuana-like effects, and decreased psychomotor performance. Naltrexone increased heart rate and decreased self-reported measures of vigor and hunger but did not alter any of the effects of THC. These results suggest that the subjective, physiological, and behavioral effects of THC in humans are not mediated through opioid systems.

Physiological and behavioural effects of the endogenous cannabinoid, arachidonylethanolamide (anandamide), in the rat

1. Arachidonylethanolamide (AEA; anandamide) has been isolated from mammalian brain and found to bind to, and is thought to be, an endogenous ligand for the cannabinoid receptor. In order to understand better its behavioural and physiological properties, we have examined its acute effects in unanaesthetized freely behaving rats. 2. Intravenous AEA caused dose-related decreases in locomotor behaviour, a pronounced hyperreflexia, and a moderate antinociceptive state. At doses between 3 and 30 mg kg-1, a dose-dependent hypothermia and profound, time-dependent cardiovascular changes were also observed. 3. An immediate bradycardia exceeding 50% was seen within 10-15 s of administration and lasted up to 11 min following the highest dose of the drug. In contrast, the change in mean arterial pressure was biphasic: an immediate 20% decrease in mean arterial pressure followed by a significant increase in blood pressure that lasted about 13 min after the highest dose. 4. These data demonstrate that AEA in the unanaesthetized rat exerts behavioural and physiological effects generally similar to those seen following natural cannabinoids and synthetic cannabimimetic agents and suggests a role for AEA in regulation of various physiological processes.

Naloxone reduces social locomotor activity in rats

Naloxone, a nonspecific opioid antagonist, has been found to decrease the activity and social behavior of rats tested in pairs but the effects on individual locomotor activity have been equivocal. In the present study, groups of male Long-Evans hooded rats received naloxone (1 or 4 mg/kg, IP) or vehicle alone (isotonic saline) 30 min prior to testing sessions. Individual locomotor activity was measured in two activity boxes (41-cm3) equipped with two infrared photobeams using daily 30-min testing sessions for 5 consecutive days. Following a 1-week washout period (no testing), activity and social attraction (paired distance and contact) were examined in pairs of rats from each group using daily 15-min testing sessions for 4 consecutive days. Locomotor activity and its habituation were not significantly affected by naloxone in rats tested individually. However, both doses of naloxone significantly reduced paired locomotor activity compared to the control group. Measures of social attraction were not significantly affected by naloxone. The present findings suggest that naloxone does not produce nonspecific depressant effects on activity but rather may antagonize opioid release in situational contexts of high arousal (e.g., social activity) with consequent reduction of activity.

Effect of acute intravenous administration of delta-9-tetrahydrocannabinol on the episodic secretion of immunoassayable growth hormone in the rat

Blood samples from unrestrained, unanesthetized, male rats (300-350 g) were obtained every 15 min. for 9 consecutive hrs. (1000-1900 h). Each rat received, intravenously, a vehicle injection (controls) or a 2.0 mg/kg dose of delta-9-tetrahydrocannabinol (THC) at 1300 h to determine the effect of this drug on the spontaneous episodic secretion of plasma immunoassayable rat growth hormone (rGH). Acute administration of THC suppressed the secretion of rGH, as is evident from mean plasma level (p less than .01), peak height (p less than .02), and integrated peak amplitude (p less than .02) analyses. Episodic secretion was inhibited in all animals (n = 7) receiving THC. Although further investigation is needed to define clearly the physiological mechanisms involved in this response, these data indicate that THC can inhibit the hypothalamic-pituitary control of normal episodic growth hormone secretion in the rat.

An assessment of the role of opioid receptors in the response to cannabimimetic drugs

Cannabimimetic drugs have been shown to inhibit adenylate cyclase activity in N18TG2 neuroblastoma cells. This investigation examines the possible role of opioid receptors in the cannabimimetic response. Opioid receptors of the delta subtype were found on N18TG2 membranes using [3H]D-Ala2-D-Leu5-enkephalin. No mu or kappa receptors were detected using selective ligands for these sites. The delta binding affinity and capacity were unaltered by cannabimimetic drugs. To test if cannabimimetic drugs may modulate opioid effector mechanisms, cyclic AMP metabolism was determined in intact cells and in membranes. N18TG2 adenylate cyclase was inhibited by the cannabimimetic drugs delta 9-tetrahydrocannabinol and desacetyllevonantradol, and by the opioid agents morphine, etorphine, and D-Ala2-Met5-enkephalinamide. The opioid inhibition was reversed by naloxone and naltrexone; however, the cannabimimetic response was unaffected. Both cannabimimetic and opioid drugs decreased cyclic AMP accumulation in intact cells, but opioid antagonists blocked the response only to the latter. Thus, cannabimimetic effects are observed even though opioid receptors are blocked by antagonist drugs. The interaction between desacetyllevonantradol and etorphine was neither synergistic nor additive at maximal concentrations, suggesting that these two drugs operate via the same effector mechanism. Other neuronal cell lines having an opioid response were also examined. The cannabimimetic inhibition of cyclic AMP accumulation in NG108-15 neuroblastoma X glioma cells was not as great as the response in N18TG2. N4TG1 neuroblastoma cells did not respond to cannabimimetic drugs under any conditions tested. Thus, the cannabimimetic inhibition of adenylate cyclase is not universally observed, and the efficacy of the cannabimimetic response does not correlate with the efficacy of the opioid response.

Cannabis and delta-9-tetrahydrocannabinol. Effects on elements of social behaviour in mice

Behavioural effects of cannabis and delta-9-tetrahydrocannabinol (THC) in male mice encountering solvent-treated males, and of cannabis in males encountering control females, were examined by ethological procedures. Effects on elements and element-groups within behavioural categories were investigated, and effects of cannabis on the response sequence to the behaviour of male partners examined. Effects were detected on elements that previously had been grouped within categories. Flight elements "flag, evade, retreat and flee" and immobility were increased both by cannabis (50 and 100 mg/kg in male-male encounters and 25 mg/kg in male-female encounters) and by THC (5 mg/kg), whereas "scan", "wash and self-groom" and some elements of social investigation were reduced. Bout length and duration of "explore" were increased by THC. In encounters with females, cannabis decreased sexual behaviour. Sequence analysis showed that cannabis, within the limitations of the comparatively small size of sample, to have no significant effect upon normal behavioural responses at 50 and 100 mg/kg, apart from enhancement of the flight response to aggression and some elements of social investigation by partners. These more detailed analyses revealed effects that would not be detectable by the conventional approach.