Analgesic effect of delta 9-tetrahydrocannabinol in the dog

Current review of hemp-based medicines in dogs

Medical use of Cannabis (or hemp) began thousands of years ago. In the 20th century, mechanisms of action were demonstrated with the discovery of its active substances, the phytocannabinoids, and its pharmacological targets, the endocannabinoid system. This system is composed of receptors, endogenous substances, and enzymes, and it participates in the modulation of physiological mechanisms in several species, including dogs. Studies indicate that changes in this system may contribute to the genesis of some diseases. Therefore, the use of substances that act on its components may help in the treatment of these diseases. The main phytocannabinoids described are ^Δ9- tetrahydrocannabinol (THC) and cannabidiol (CBD). In humans, the benefits of using CBD in several diseases have been demonstrated. The popularization of this type of treatment has also reached veterinary medicine, which on one hand was related to an increase in adverse event records, but on the other also allowed reports of anecdotal evidences of its effectiveness and safety in animals. Clinical studies published so far indicate that the use of CBD in dogs can be safe at given doses and can contribute to osteoarthritis and idiopathic epilepsy treatments. Clinical and pre-clinical studies and case reports were reviewed in this report to identify the main characteristics of hemp-based therapies in dogs, including its pharmacokinetics, pharmacodynamics, safety, and efficacy in the treatment of diseases.

The impact of state cannabis legislation, county-level socioeconomic and dog-level characteristics on reported cannabis poisonings of companion dogs in the USA (2009-2014)

With current trends in cannabis legalization, large efforts are being made to understand the effects of less restricted legislation on human consumption, health, and abuse of these products. Little is known about the effects of cannabis legalization and increased cannabis use on vulnerable populations, such as dogs. The objective of this study was to examine the effects of different state-level cannabis legislation, county-level socioeconomic factors, and dog-level characteristics on dog cannabis poisoning reports to an animal poison control center (APCC). Data were obtained concerning reports of dog poisoning events, county characteristics, and state cannabis legislation from the American Society for the Prevention of Cruelty to Animals’ (ASPCA) APCC, the US Census Bureau, and various public policy-oriented and government websites, respectively. A multilevel logistic regression model with random intercepts for county and state was fitted to investigate the associations between the odds of a call to the APCC being related to a dog being poisoned by a cannabis product and the following types of variables: dog characteristics, county-level socioeconomic characteristics, and the type of state-level cannabis legislation. There were significantly higher odds of a call being related to cannabis in states with lower penalties for cannabis use and possession. The odds of these calls were higher in counties with higher income variability, higher percentage of urban population, and among smaller, male, and intact dogs. These calls increased throughout the study period (2009–2014). Reporting of cannabis poisonings were more likely to come from veterinarians than dog owners. Reported dog poisonings due to cannabis appear to be influenced by dog-level and community-level factors. This study may increase awareness to the public, public health, and veterinary communities of the effects of recreational drug use on dog populations. This study highlights the need to educate dog owners about safeguarding cannabis products from vulnerable populations.

Influence of central nicotinic receptors on arachidonylcyclopropylamide (ACPA)-induced antinociception in mice

The nicotinic cholinergic receptors have been reported to be involved in several actions of cannabinoids (e.g., bradycardia, hypothermia). However, the influence of central cholinergic system on cannabinoids antinociceptive effect has not been reported. This study investigated the possible part played by nicotinic cholinergic modulator drugs on the antinociceptive effect of central administration of arachidonylcyclopropylamide (ACPA) in mice. The antinociceptive effects of intracerebroventricular (i.c.v.) administration of ACPA using the formalin test have been studied in mice. The effects of nicotine or mecamylamine (a nicotinic cholinergic antagonist) on ACPA analgesia are also studied. i.c.v. administration of ACPA (0.004-1 microg/mice) induced antinociceptive effect in mice. i.c.v. administration of nicotine (0.1 or 0.5 microg/mice) or mecamylamine (2 microg/mice) potentiated or antagonized ACPA antinociceptive effects, respectively. It is concluded that ACPA-induced analgesia is influenced by central nicotinic cholinergic activity.

On the pharmacological properties of Delta9-tetrahydrocannabinol (THC)

Cannabis is one of the first plants used as medicine, and the notion that it has potentially valuable therapeutic properties is a matter of current debate. The isolation of its main constituent, Delta9-tetrahydrocannabinol (THC), and the discovery of the endocannabinoid system (cannabinoid receptors CB1 and CB2 and their endogenous ligands) made possible studies concerning the pharmacological activity of cannabinoids. This paper reviews some of the most-important findings in the field of THC pharmacology. Clinical trials, anecdotal reports, and experiments employing animal models strongly support the idea that THC and its derivatives exhibit a wide variety of therapeutic applications. However, the psychotropic effects observed in laboratory animals and the adverse reactions reported during human trials, as well as the risk of tolerance development and potential dependence, limit the application of THC in therapy. Nowadays, researchers focus on other therapeutic strategies by which the endocannabinoid system might be modulated to clinical advantage (inhibitor or activator of endocannabinoid biosynthesis, cellular uptake, or metabolism). However, emerging evidence highlights the beneficial effects of the whole cannabis extract over those observed with single components, indicating cannabis-based medicines as new perspective to revisit the pharmacology of this plant.

Participation of the opioid system in cannabinoid-induced antinociception and emotional-like responses

Several anatomical, biochemical and pharmacological evidence support the existence of bidirectional interactions between cannabinoid and opioid systems. The present review is focused on the participation of the endogenous opioid system in the antinociceptive and emotional-like responses induced by cannabinoids, and the development of tolerance to cannabinoid pharmacological effects. Cannabinoid and opioid agonists produce antinociception by acting on similar structures within the central nervous system, and a peripheral mechanism has been also proposed for both compounds. Pharmacological studies have suggested that the endogenous opioid system could be involved in cannabinoid antinociception and the development of cannabinoid tolerance. Recent studies using knockout mice have also demonstrated the role of the opioid system in cannabinoid antinociception and tolerance, although some discrepancies with the previous pharmacological results have been reported when using knockout mice. On the other hand, cannabinoid administration can induce anxiolytic-like responses that are mediated at least in part by an endogenous opioid activity on micro- and delta-opioid receptors.

Rapid tolerance to Delta(9)-tetrahydrocannabinol and cross-tolerance between ethanol and Delta(9)-tetrahydrocannabinol in mice

Motor incoordination in the rota-rod test was used to assess the development of rapid tolerance to Delta(9)-tetrahydrocannabinol and rapid cross-tolerance between ethanol and Delta(9)-tetrahydrocannabinol in mice. Further, the influence of the cannabinoid receptor antagonist SR 141716A (N-(piperidin-1-yl)-5-(4-chlorophenyl)-4-methyl-1H-pyrazole-3-carboxyamide) on the motor impairment induced by both drugs was examined. Mice were injected on day 1 with equipotent doses of Delta(9)-tetrahydrocannabinol (28 mg/kg, i.p.) and ethanol (2.25 g/kg, i.p.) and tested at 30, 60 and 90 min after the injections. On day 2, control groups received ethanol or Delta(9)-tetrahydrocannabinol, some groups received the same treatment as the day before, while the remaining groups switched the treatment. All groups were tested to evaluate tolerance. The development of rapid tolerance to Delta(9)-tetrahydrocannabinol was observed and pretreatment with ethanol resulted in rapid cross-tolerance to Delta(9)-tetrahydrocannabinol. SR 141716A (2 mg/kg, i.p.) failed to block the development of rapid tolerance to both drugs, ethanol and Delta(9)-tetrahydrocannabinol. These results suggest that Delta(9)-tetrahydrocannabinol, similarly to ethanol, can induce rapid tolerance to motor incoordination in mice. They also support the use of the 2-day protocol as an effective procedure to reduce the length of drug exposure necessary to induce tolerance.

Cannabinoid receptors and pain

Mammalian tissues contain at least two types of cannabinoid receptor, CB(1) and CB(2), both coupled to G proteins. CB(1) receptors are expressed mainly by neurones of the central and peripheral nervous system whereas CB(2) receptors occur centrally and peripherally in certain non-neuronal tissues, particularly in immune cells. The existence of endogenous ligands for cannabinoid receptors has also been demonstrated. The discovery of this 'endocannabinoid system' has prompted the development of a range of novel cannabinoid receptor agonists and antagonists, including several that show marked selectivity for CB(1) or CB(2) receptors. It has also been paralleled by a renewed interest in cannabinoid-induced antinociception. This review summarizes current knowledge about the ability of cannabinoids to produce antinociception in animal models of acute pain as well as about the ability of these drugs to suppress signs of tonic pain induced in animals by nerve damage or by the injection of an inflammatory agent. Particular attention is paid to the types of pain against which cannabinoids may be effective, the distribution pattern of cannabinoid receptors in central and peripheral pain pathways and the part that these receptors play in cannabinoid-induced antinociception. The possibility that antinociception can be mediated by cannabinoid receptors other than CB(1) and CB(2) receptors, for example CB(2)-like receptors, is also discussed as is the evidence firstly that one endogenous cannabinoid, anandamide, produces antinociception through mechanisms that differ from those of other types of cannabinoid, for example by acting on vanilloid receptors, and secondly that the endocannabinoid system has physiological and/or pathophysiological roles in the modulation of pain.

Time course for the induction and maintenance of tolerance to Delta(9)-tetrahydrocannabinol in mice

The time course for the development of tolerance to delta-9-tetrahydrocannabinol (Delta(9)-THC) was studied in an effort to determine the role that length of dosing may have in the onset and maintenance of tolerance. Mice were chronically treated with either vehicle or 10 mg/kg of Delta(9)-THC subcutaneously twice a day. The mice were tested 24 h after the last injection for tolerance as assessed by the production of antinociception and suppression of spontaneous activity. Tolerance was first observed after three injections of Delta(9)-THC (1.5 days) resulting in a 7-fold and 23-fold decrease in potency for the measures of antinociception and hypoactivity, respectively. Seven injections of Delta(9)-THC (3.5 days of dosing) resulted in a 12-fold and 36-fold decrease in potency, respectively, while 13 injections of Delta(9)-THC (6.5 days of dosing) produced a 6.2-fold and 9.8-fold degree of tolerance. The time course for the recovery from Delta(9)-THC-induced tolerance was also determined with a separate group of animals. Mice were dosed for 6.5 days with 10 mg/kg of Delta(9)-THC and were not tested until 4.5, 7.5, and 11.5 days after cessation of drug treatment. After 4.5 days without drug treatment the mice exhibited a 7.5-fold and 2.3-fold degree of tolerance as measured by antinociception and hypoactivity, respectively. After 7. 5 days without drug treatment a 3.4-fold degree of tolerance remained for the measure of antinociception, while no tolerance was detected for the measure of hypoactivity. No tolerance was observed for the measure of antinociception after 11.5 days without drug treatment. This time course indicates that the mechanisms responsible for either the production or maintenance of tolerance differ between the measures of antinociception and suppression of spontaneous activity.

Cannabinoid transmission and pain perception

The use of cannabis for the management of a wide range of painful disorders has been well documented in case reports throughout history. However, clinical evaluations of cannabis and its psychoactive constituent THC have not led to a consensus regarding their analgesic effectiveness. On the other hand, THC and its synthetic derivatives have been shown to be effective in most animal models of pain. These antinociceptive effects are mediated through cannabinoid receptors in the brain that in turn appear to interact with noradrenergic and kappa opioid systems in the spinal cord to modulate the perception of painful stimuli. The endogenous ligand, anandamide, is also an effective antinociceptive agent. The extent to which the endogenous cannabinoid system is involved in the modulation of pain is currently unknown.

Behavioral effects of levonantradol and nantradol in the rhesus monkey

In rhesus monkeys, acute administration of levonantradol and nantradol produced signs of CNS depression, including ataxia with body sag, pupil dilation, ptosis, dozing, and reduced responsivity to external stimuli. Neither compound suppressed the morphine withdrawal syndrome; however, both alleviated the chronic abdominal contraction associated with withdrawal. The directly observable effects of these compounds were not antagonized by naloxone. When levonantradol was administered every 6 hours, marked tolerance developed to both the effects of levonantradol and nabilone and THC. No signs of withdrawal were observed when levonantradol injections were abruptly discontinued. When substituted in lieu of codeine under an intravenous drug self-administration procedure, neither levonantradol nor nantradol maintained responding at rates higher than those maintained by their vehicle. Finally, the discriminative effects of levonantradol were not equivalent to those of the narcotics ethylketazocine or etorphine.

A comparison of THC, nantradol, nabilone, and morphine in the chronic spinal dog

Morphine and delta 9-tetrahydrocannabinol (THC) have been shown to have certain pharmacologic characteristics in common. Among these are antinociception, hypothermia, and the suppression of precipitated abstinence in morphine-dependent rats. In the present study the effects of morphine were compared with the effects of THC and two synthetic cannabinoids, nantradol and nabilone, in both nondependent and morphine-dependent chronic spinal dogs. Single doses of THC, nantradol, and nabilone depressed the flexor and skin twitch reflexes and had a calming effect after intravenous infusion. These effects are similar to those of morphine. Morphine, nantradol, and nabilone, but not THC, depressed rectal temperature. Unlike morphine, however, the cannabinoids produced mydriasis and an increased startle response, and these effects were not antagonized by naltrexone. THC, nantradol, and nabilone suppressed withdrawal abstinence in 40-hour and maximally abstinent morphine-dependent chronic spinal dogs. The results suggest that THC, nantradol, and nabilone share some properties with morphine since they increased the latency of the skin twitch reflex and suppressed withdrawal abstinence. It is doubtful, however, that these actions of the cannabinoids are mediated through opioid receptors since they were not antagonized by naltrexone.

Delta 9-Tetrahydrocannabinol, ethanol, and amphetamine as discriminative stimuli-generalization tests with other drugs

Three groups of rats (A, B, C) were trained in a T-maze discriminate between drug-and control solution-induced internal discriminative stimuli. The drugs used to induce discriminative stimuli were: delta 9-THC, 5.0 mg/kg (Group A); ethanol, 1.2 g/kg (Group B), and amphetamine, 1.0 mg/kg(Group C). After discrimination acquisition several drugs were tested for generalization in each group. Group A was tested with delta 8-THC, CBD, CBN, ethanol, pentobarbital,chlorpromazine, amphetamine, and apomorphine; only delta8-THC and CBN induced delta9-THC-like responses. Group B was tested with delta 9-THC, delta 8-THC, CBD, CBN, pentobarbital, and amphetamine; pentobarbital induced ethanol-like response. Group C was tested with delta 9-THC, apomorphine, and ethanol; delta 9-THC and apomorphine elicited amphetamine-like responses.

Comparative analgesic activity of various naturally occurring cannabinoids in mice and rats

The analgesic effectiveness of delta-9-tetrahydrocannabinol (THC), a crude marihuana extract (CME), cannabinol (CBN), cannabidiol (CBD), morphine SO-4 and aspirin following oral administration was directly compared in mice using the acetic-induced writhing and hot plate tests and the Randall-Selitto paw pressure test in rats. THC and morphine were equipotent in all tests except that morphine was significantly more potent in elevating pain threshold in the uninflamed rat hind paw. In terms of THC content, CME was nearly equipotent in the hot plate and Randall-Selitto tests, but was 3 times more potent in the acetic acid writhing test. On the other hand, CBN, like aspirin, was only effective in reducing writhing frequency in mice (3 times more potent than aspirin) and raising pain threshold of the inflamed hind paw of the rat (equipotent with aspirin). CBD did not display a significantly analgesic effect in any of the test systems used. The results of this investigation seem to suggest that both THC and CME possess narcotic-like analgesic activity similar to morphine, while CBN appears to be a non-narcotic type analgesic like aspirin.