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

Fixed-Ratio Schedule of Intravenous THC Injection in Squirrel Monkeys  During daily one-hour sessions, every 10 th 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

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 N = 4 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 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 A 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