Ellen A. Walker

Cannabidiol inhibits paclitaxel-induced neuropathic pain through 5-HT1A receptors without diminishing nervous system function or chemotherapy efficacy

Paclitaxel (PAC) is associated with chemotherapy‐induced neuropathic pain (CIPN) that can lead to the cessation of treatment in cancer patients even in the absence of alternate therapies. We previously reported that chronic administration of the non‐psychoactive cannabinoid cannabidiol (CBD) prevents PAC‐induced mechanical and thermal sensitivity in mice. Hence, we sought to determine receptor mechanisms by which CBD inhibits CIPN and whether CBD negatively effects nervous system function or chemotherapy efficacy.

Effects of a Cannabinoid1 Receptor Antagonist and Serotonin2C Receptor Agonist Alone and in Combination on Motivation for Palatable Food: A Dose-Addition Analysis Study in Mice

The cannabinoid and serotonin systems modulate feeding behavior in humans and laboratory animals. The present study assessed whether a cannabinoid (CB)1 receptor antagonist and a serotonin (5-HT)2C receptor agonist alone and in combination attenuate motivation for the liquid nutritional drink Ensure as measured by a progressive ratio (PR) schedule of reinforcement in male C57BL/6 mice. Pretreatment (15 min i.p.) with either the CB1 receptor antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboximide hydrochloride (SR141716) (SR; Rimonabant or Acomplia) or the 5-HT2C receptor agonist m-chlorophenylpiperazine (mCPP) dose-dependently decreased the maximum ratio completed under the PR schedule (break point) in mice. ED25 values for SR and mCPP to decrease break point were determined, and the relative potency of each drug alone was quantified. Fixed dose-ratio pairs of SR/mCPP based on their relative potency were then administered. Dose-addition analysis comparing the experimentally determined potency for SR/mCPP combinations with their predicted additive potency revealed that SR/mCPP combinations in 1:1 and 2:1 ratios based on relative potency produced significant synergistic attenuation of break point for Ensure. The ED25 values for decreasing break point were consistently lower than ED25 values for decreasing response rate, and synergistic effects of SR/mCPP combinations on break point were seen independent of synergistic effects on response rate. These results indicate that cannabinoid CB1 and serotonin 5-HT2C receptors are involved in motivated feeding behavior in mice and that these compounds can synergistically modulate motivation for palatable food with the synergy dependent upon the ratio of SR/mCPP in the combination.

Effects of acute oral naltrexone on the subjective and physiological effects of oral D-amphetamine and smoked cocaine in cocaine abusers.

Despite the prevalent worldwide abuse of stimulants, such as amphetamines and cocaine, no medications are currently approved for treating this serious public health problem. Both preclinical and clinical studies suggest that the opioid antagonist naltrexone (NTX) is effective in reducing the abuse liability of amphetamine, raising the question of whether similar positive findings would be obtained for cocaine. The purpose of this study was to evaluate the ability of oral NTX to alter the cardiovascular and subjective effects of D-amphetamine (D-AMPH) and cocaine (COC). Non-treatment-seeking COC users (N=12) completed this 3-week inpatient, randomized, crossover study. Participants received 0, 12.5, or 50 mg oral NTX 60 min before active or placebo stimulant administration during 10 separate laboratory sessions. Oral AMPH (0, 10, and 20 mg; or all placebo) was administered in ascending order within a laboratory session using a 60-min interdose interval. Smoked COC (0, 12.5, 25, and 50 mg; or all placebo) was administered in ascending order within a laboratory session using a 14-min interdose interval. Active COC and AMPH produced dose-related increases in cardiovascular function that were of comparable magnitude. In contrast, COC, but not AMPH, produced dose-related increases in several subjective measures of positive drug effect (eg, high, liking, and willingness to pay for the drug). NTX did not alter the cardiovascular effects of AMPH or COC. NTX also did not alter positive subjective ratings after COC administration, but it did significantly reduce ratings of craving for COC and tobacco during COC sessions. These results show that (1) oral AMPH produces minimal abuse-related subjective responses in COC smokers, and (2) NTX reduces craving for COC and tobacco during COC sessions. Future studies should continue to evaluate NTX as a potential anti-craving medication for COC dependence.

Cannabidiol Prevents the Development of Cold and Mechanical Allodynia in Paclitaxel-Treated Female C57Bl6 Mice

The taxane chemotherapeutic paclitaxel frequently produces peripheral neuropathy in humans. Rodent models to investigate mechanisms and treatments are largely restricted to male rats, whereas female mouse studies are lacking. We characterized a range of paclitaxel doses on cold and mechanical allodynia in male and female C57Bl/6 mice. Because the nonpsychoactive phytocannabinoid cannabidiol attenuates other forms of neuropathic pain, we assessed its effect on paclitaxel-induced allodynia. Paclitaxel produced allodynia that was largely dose independent and more robust in female mice, and this effect was prevented by treatment with cannabidiol. Our preliminary findings therefore indicate that cannabidiol may prevent the development of paclitaxel-induced allodynia in mice and therefore be effective at preventing dose-limiting paclitaxel-induced peripheral neuropathy in humans.

The analgesic efficacy of fentanyl: Relationship to tolerance and μ-opioid receptor regulation

This study determined if fentanyl analgesic efficacy predicts the magnitude of tolerance and mu-opioid receptor regulation. To estimate efficacy, mice were injected i.p. with saline or clocinnamox (CCAM), an irreversible mu-opioid receptor antagonist, (0.32-25.6 mg/kg) and 24 h later fentanyl cumulative dose-response studies were conducted. CCAM dose dependently shifted the fentanyl dose-response function to the right. The apparent efficacy (tau) of fentanyl, based on the operational model of agonism, was estimated as 58, indicating that fentanyl is a high analgesic efficacy agonist. Next, mice were infused with fentanyl (1, 2 or 4 mg/kg/day) for 7 days. Controls were implanted with placebo pellets. At the end of 7 days, morphine cumulative dose-response studies or mu-opioid receptor saturation binding studies were conducted. Fentanyl infusions dose dependently decreased morphine potency with the highest fentanyl dose reducing morphine potency by approximately 6 fold. Chronic infusion with fentanyl (4 mg/kg/day) significantly reduced mu-opioid receptor density by 28% without altering affinity, whereas lower infusion doses had no effect. Taken together, the present results strengthen the proposal that opioid analgesic efficacy predicts mu-opioid receptor regulation and the magnitude of tolerance.

Effect of Cannabinoid CB1 Receptor Antagonist SR141714A and CB1 Receptor Knockout on Cue-Induced Reinstatement of Ensure ® and Corn-Oil Seeking in Mice

The cannabinoid CB1 receptor antagonist SR141716A decreases cue-induced reinstatement of sucrose and drug seeking in rats. Reinstatement behavior is not well characterized in C57Bl/6 mice, including CB1 receptor knockout mice generated on a C57Bl/6 background. In the present study, male C57Bl/6, CB1 knockout (CB1 KO), and wild-type littermate (WT) mice were trained to respond for the sweet reinforcer Ensure® or corn oil. Responding was maintained on a fixed ratio 1 (FR1) schedule of reinforcement for 10 days, and then extinguished by the removal of the reinforcer and associated cues. Subsequently, the effect of either pretreatment with SR141716A or CB1 receptor knockout on cue-induced reinstatement of Ensure® or corn-oil seeking was assessed. Both 1.0 and 3.0 mg/kg SR141716A decreased reinstatement of Ensure® seeking in C57Bl/6 mice. A tenfold higher dose of SR141716A (10.0 mg/kg) was required to attenuate reinstatement behavior in C57Bl/6 mice responding for corn oil, suggesting that CB1 receptors may be selectively involved in the neurobiology underlying reinstatement of responding for some food reinforcers but not others. Whereas CB1 receptor antagonism selectively attenuated reinstatement of responding for Ensure®, genetic deletion of the CB1 receptor produced only a trend in decreasing reinstatement of Ensure® seeking, and did not attenuate reinstatement of corn-oil seeking. Baseline differences in levels of operant responding were also observed in WT vs CB1 KO mice maintained by Ensure® and corn oil. This and other possible reasons for the observed discrepancy between pharmacological blockade vs genetic invalidation of the CB1 receptor on reinstatement of Ensure® seeking are discussed.

Beta-lactam antibiotic decreases acquisition of and motivation to respond for cocaine, but not sweet food, in C57Bl/6 mice.

No medication is approved to treat cocaine addiction, but mounting evidence suggests that glutamate-directed approaches may reduce cocaine dependence and relapse. We tested the hypotheses that the glutamate transporter subtype 1 activator, ceftriaxone, disrupts acquisition of cocaine self-administration, motivation to self-administer cocaine, and conditioned place preference in mice. Repeated ceftriaxone (200 mg/kg) reduced the ability of mice to acquire cocaine and the motivation to self-administer cocaine after successful acquisition without affecting acquisition of or motivation for sweet food. Repeated ceftriaxone had no effect on cocaine-conditioned place preference. These results suggest that a &bgr;-lactam antibiotic reduces the direct reinforcing strength of cocaine without producing nonspecific deficits in conditioned learning processes.

Hydromorphone efficacy and treatment protocol impact on tolerance and μ-opioid receptor regulation

This study examined the antinociceptive (analgesic) efficacy of hydromorphone and hydromorphone-induced tolerance and regulation of mu-opioid receptor density. Initially s.c. hydromorphones time of peak analgesic (tail-flick) effect (45 min) and ED50 using standard and cumulative dosing protocols (0.22 mg/kg, 0.37 mg/kg, respectively) were determined. The apparent analgesic efficacy (tau) of hydromorphone was then estimated using the operational model of agonism and the irreversible mu-opioid receptor antagonist clocinnamox. Mice were injected with clocinnamox (0.32-25.6 mg/kg, i.p.) and 24 h later, the analgesic potency of hydromorphone was determined. The tau value for hydromorphone was 35, which suggested that hydromorphone is a lower analgesic efficacy opioid agonist. To examine hydromorphone-induced tolerance, mice were continuously infused s.c. with hydromorphone (2.1-31.5 mg/kg/day) for 7 days and then morphine cumulative dose response studies were performed. Other groups of mice were injected with hydromorphone (2.2-22 mg/kg/day) once, or intermittently every 24 h for 7 days. Twenty-four hours after the last injection, mice were tested using morphine cumulative dosing studies. There was more tolerance with infusion treatments compared to intermittent treatment. When compared to higher analgesic efficacy opioids, hydromorphone infusions induced substantially more tolerance. Finally, the effect of chronic infusion (31.5 mg/kg/day) and 7 day intermittent (22 mg/kg/day) hydromorphone treatment on spinal cord mu-opioid receptor density was determined. Hydromorphone did not produce any change in mu-opioid receptor density following either treatment. These results support suggestions that analgesic efficacy is correlated with tolerance magnitude and regulation of mu-opioid receptors when opioid agonists are continuously administered. Taken together, these studies indicate that analgesic efficacy and treatment protocol are important in determining tolerance and regulation of mu-opioid receptors.

Opioid antagonists differ according to negative intrinsic efficacy in a mouse model of acute dependence

1 The purpose of the present study is to compare the capacity of opioid antagonists to elicit withdrawal jumping in mice following two acute pretreatment doses of the opioid agonist morphine. Antagonists that precipitate vigorous withdrawal jumping across both morphine treatment doses are hypothesized to be strong inverse agonists at the μ‐opioid receptor, whereas antagonists that elicit withdrawal jumping in mice treated with the high but not the low dose of morphine are hypothesized to be weak inverse agonists. 2 Male, Swiss‐Webster mice (15–30 g) were acutely treated with 56 or 180 mg kg−1 morphine 4 h prior to injection with naloxone, naltrexone, diprenorphine, nalorphine, or naloxonazine. Vertical jumping, paw tremors, and weight loss were recorded. Naloxone, naltrexone, and diprenorphine produced withdrawal jumping after 56 and 180 mg kg−1morphine pretreatment. Nalorphine and naloxonazine produced moderate withdrawal jumping after 180 mg kg−1 morphine pretreatment, but failed to elicit significant withdrawal jumping after 56 mg kg−1 morphine pretreatment. Nalorphine and naloxonazine blocked the withdrawal jumping produced by naloxone. All antagonists produced paw tremors and weight loss although these effects were generally not dose‐dependent. 3 Taken together, these findings reveal a rank order of negative intrinsic efficacy for these opioid antagonists as follows: naloxone=naltrexonediprenorphine>nalorphine=naloxonazine. Furthermore, the observation that nalorphine and naloxonazine blocked the naloxone‐induced withdrawal jumping provides additional evidence that nalorphine and naloxonazine are weaker inverse agonists than naloxone.

Selective and nonselective serotonin antagonists block the aversive stimulus properties of MK212 and m-chlorophenylpiperazine (mCPP) in mice

Serotonin(2C) (5-HT(2C)) receptors have been implicated to treat mood disorders such as depression and anxiety. In the present study, the capacities of two 5-HT(2C) agonists, MK212 and mCPP, to produce conditioned taste aversions in mice were evaluated. On two training days, Swiss-Webster male mice (19-34g) were trained to associate the flavor of a novel solution with the injection of various doses of MK212 or mCPP. On two alternate training days, mice were trained to associate a different flavored solution with an injection of saline. For testing, both flavored solutions were presented simultaneously and an avoidance of the MK212 or mCPP-paired solution indicated conditioned taste aversion. Robust conditioned taste aversions were observed to solutions paired with 1.0 or 10mg/kg MK212 or mCPP. Acquisition of conditioned taste aversions was blocked by nonselective serotonin antagonists cyproheptadine, bromo-LSD, metergoline, methysergide and mianserin. Selective 5-HT(2B/2C) antagonist SB206,553 blocked both MK212- and mCPP-induced conditioned taste aversion although selective 5-HT(2B/2C) antagonist SB200,646 only blocked mCPP-induced conditioned taste aversion. In a single-bottle procedure, MK212, bromo-LSD, and mianserin failed to alter acquisition rate of a LiCl-induced conditioned taste aversion. Taken together, these data indicate that the serotonin agonists MK212 and mCPP produce conditioned taste aversion and that these effects are mediated predominantly through 5-HT(2C) receptors.