# Presentation: 154735

## Cross-Talk Between Cannabinoid and Opioid Systems

- Steven R. Goldberg, Ph.D.

- Preclinical Pharmacology Section
- Behavioral Neuroscience Research Branch
- Intramural Research Program
- National Institute on Drug Abuse
- National Institutes of Health, DHHS
- Baltimore, Maryland, USA

## Cross-Talk Between Cannabinoid and Opioid Systems

- There is increasing experimental evidence from animal studies for reciprocal functional interactions between endogenous brain cannabinoid and opioid systems in the regulation of many physiological and behavioral processes
- Major bi-directional interactions have been found between cannabinoid and opioid systems in drug reinforcement/reward processes

## Recent Research at NIDA on Opioid-Cannabinoid Interactions 

- Reinforcement of drug-taking and drug-seeking behavior by THC and its modulation by opioid agonists and antagonists
- Modulation of subjective responses to THC by opioid agonists and antagonists, as measured by a drug-discrimination  procedure
- THC induced changes in extracellular levels of endogenous opioids in brain areas associated with reinforcing effects of cannabinoids and opioids

## Slide 4

## Fixed-Ratio Schedule of Intravenous THC Injection in Squirrel Monkeys 

- During daily one-hour sessions, every 10th lever press by the monkey produced an intravenous injection of 4 μg/kg of THC followed by a 60-second timeout
- When responding was stable, vehicle and different doses of THC were studied by substitution for 5 consecutive sessions

## THC self-administration in drug-naïve squirrel monkeys(means ± S.E.M.; n = 3)

## Fixed-Ratio Schedule of Anandamide Injection in Squirrel Monkeys

- Previous studies suggest that endogenous cannabinoid systems play a role in brain reward/reinforcement processes
-  We have studied the effectiveness of the endogenous cannabinoid anandamide as a reinforcer of intravenous drug-taking behavior in squirrel monkeys

## Self-administration of the endogenous cannabinoid anandamide in squirrel monkeys (means ± S.E.M.; n = 4)

## SUMMARY I

- Our studies provide the first clear demonstrations that THC, the main psychoactive ingredient in marijuana, and anandamide, an  endogenous cannabinoid, can serve as effective reinforcers of drug-taking behavior
- These findings provide direct evidence for involvement of endogenous cannabinoid systems in brain reward processes

## Opioid-Cannabinoid InteractionsModulation of the reinforcing effects of THC in squirrel monkeys

## Fixed-Ratio Schedule of THC or cocaine InjectionOpioid Antagonist Pretreatment (n = 4)

## Fixed-Ratio Schedule of THC InjectionOpioid Antagonist  Pretreatment (n = 4)

## Second-Order Schedule of Intravenous THC Injection in Squirrel Monkeys 

- Provides a measure of drug-seeking behavior in the absence of the direct pharmacological effects of the drug
- Every 10th lever press produces a brief 2-second flash of a light (a 10-response fixed-ratio schedule; FR 10)
- The first 10-response fixed-ratio completed after a 30-minute interval of time elapses produces both the light and an intravenous injection of THC and ends the daily session

## Slide 14

## Second-Order Schedule of THC Injection (n = 5)

## Second-Order Schedule of THC Injection Cannabinoid CB1 antagonist treatment (n = 4)

## Second-Order Schedule of THC InjectionOpioid Antagonist Pretreatment (n = 3)

## Reinstatement of Extinguished THC-Seeking Behavior 

- Reinstatement of Extinguished THC-Seeking Behavior

- N = 4

- N = 3

- N = 3

## SUMMARY II

- THC’s reinforcing effects under both fixed-ratio and second-order schedules are reduced by treatment with an opioid antagonist
- Extinguished THC-seeking behavior can be reinstated by a pre-session priming injection of the opioid agonist morphine

## Opioid-Cannabinoid InteractionsModulation of the subjective response to THC in rats 

- The ability of drugs of abuse such as cannabis to produce effects that can be clearly discriminated is central to reinforcement of drug-taking behavior
- Drug-discrimination procedures provide subjective reports of the degree to which effects of administered doses of test compounds are like or unlike those of a training dose of THC  previously received

## Drug Discrimination in Rats

- Two-lever choice schedule of food delivery
- Rats learn to press one lever for food on days they receive a pre-session injection of 3 mg/kg THC and the other lever on days they receive a pre-session injection of vehicle

## Morphine potentiates and naloxone reduces THC discrimination

## Injection of the mu1 selective opioid antagonist naloxonazine directly into the VTA prevents both THC- and heroin-induced dopamine elevations in the shell of the nucleus accumbens 

## β-Endorphin Levels in the VTA and Nucleus Accumbens Shell After i.p. Injection of THC

- Rats implanted unilaterally with microdialysis probes attached to fluid swivels for free movement
- 24 hours later, microdialysis samples taken at 30-min intervals for 3 hours before and after i.p. injection of 3 mg/kg THC (the training dose in discrimination studies)

## THC increases extracellular levels of β-endorphin in the nucleus accumbens shell and in the VTA

## Direct injection of β-Endorphin Into the  VTA or Nucleus Accumbens Shell 

- Rats were implanted bilaterally with guide cannulae in the VTA or in the shell of the nucleus accumbens
- Micro injections of different doses of beta-endorphin were given alone or 15 min before i.p. injection of a low threshold dose of THC

## Direct injection of β-endorphin into the VTA dose-dependently potentiates THC discrimination

## Direct injection of β-endorphin into the nucleus accumbens shell does not potentiate THC discrimination

## SUMMARY III

- These results suggest that the subjective response to THC, a behavioral effect critically related to its abuse liability, is regulated by THC-induced elevations in extracellular levels of beta-endorphin in the VTA
- Augmented levels of β-endorphins in the VTA are probably responsible for THC-induced dopamine elevations in the shell of the nucleus accumbens

## Opioid-Cannabinoid Interactions Are BidirectionalThe reinforcing effects of opioids can be modulated by cannabinoid agonist or antagonist treatment 

## THC potentiates and SR-141716 reduces the reinforcing efficacy of Heroin under a progressive-ratio schedule of intravenous drug injection

## SUMMARY IV

- These results suggest that the endocannabinoid system is involved in the reinforcing effects of heroin
- It is possible that heroin self-injections increase levels of endogenous cannabinoids in brain areas involved in reward processes, in a fashion similar to that demonstrated for THC and β-endorphin

## CONCLUSIONS 

- THC’s reinforcing and subjective effects are reduced by an opioid antagonist and reinstated or potentiated by an opioid agonist
- THC increases extracellular levels of β-endorphin in the VTA and the shell of the nucleus accumbens
- Beta-endorphin injected directly into the VTA, but not the nucleus accumbens shell, potentiates the discrimination of low doses of THC and this is reversed by naloxone
- Augmented levels of β-endorphins in the VTA may be responsible for previous findings by others of THC-induced dopamine elevations in the nucleus accumbens
- These studies provide a novel mechanism that could explain a wide range of observed opioid-cannabinoid interactions, including the alterations in the reinforcing effects of cannabinoids by opioid agonists and antagonists I reviewed today

## Acknowledgements

- Marcello Solinas, Ph.D.
- Zuzana Justinova, M.D.
- Gianluigi Tanda, Ph.D.
- Patrik Munzar, M.D.
- Abraham Zangen, Ph.D.
- Godfrey H. Redhi

## Intracranial Self-Administration of THCinto the VTA or Nucleus Accumbens Shell

- Each rat implanted unilaterally with a 26-gauge cannula directed at either the posterior VTA, 1.5mm dorsal to the  posterior VTA,  the anterior VTA, the accumbens shell or the accumbens core.
- Rats given access to two levers; each press on one “active” lever produced a microinjection while presses on other ”inactive lever” did not.
- Rats learned to press the active lever regularly within the first 3-hour session, without “shaping” or “priming”.
- Self-administration behavior extinguished when vehicle was substituted for THC and was reinstated when THC was again available.
- Zangen A, Solinas M, Ikemoto S, Goldberg SR, Wise RA

## Intracranial self-administration of THC

**into the VTA and into the nucleus accumbens**

## Abuse-Related Interactions Between Cannabinoid and Opioid Systems

- Opioid antagonists can precipitate withdrawal signs in THC dependent animals
- The cannabinoid antagonist SR-141716A can precipitate withdrawal signs in opiate dependent animals
- Opioid antagonists block THC induced dopamine elevations in the nucleus accumbens
- Opioid antagonists block the effects of THC on intracranial self-stimulation
- Mu-receptor deficient mice do not develop THC-induced conditioned place preferences