Evaluation of the Reinforcing Strength of Phendimetrazine Using a Progressive-Ratio Schedule of Reinforcement in Rhesus Monkeys

Selective dopamine D3 receptor partial agonist (±)VK4-40 reduces the reinforcing strength of d-amphetamine but not cocaine in rhesus monkeys responding under a progressive-ratio schedule of reinforcement

BACKGROUND

Although countless studies have aimed to identify and test novel therapeutics for stimulant misuse, there are still no FDA-approved pharmacotherapies for stimulant use disorders. One potential treatment target is the dopamine D3 receptor (D3R) and studies in rodents have suggested that the novel D3R partial agonist (±)VK4-40 may be effective at decreasing cocaine self-administration. However, no previous studies have examined the efficacy of (±)VK4-40 in reducing cocaine self-administration in nonhuman primates nor the generality of effects by examining self-administration of other stimulants using a within-subjects design.

METHODS

Experiment 1 examined how acute treatment with (±)VK4-40 (1.7-3.0mg/kg, i.v.) influenced cocaine and d-amphetamine self-administration in three rhesus monkeys responding under a progressive-ratio (PR) schedule of reinforcement. In Experiment 2, the reinforcing effects of (±)VK4-40 were evaluated under a PR schedule and compared to cocaine and d-amphetamine.

RESULTS

When given as a pretreatment, (±)VK4-40 significantly reduced the reinforcing strength of d-amphetamine but not cocaine. In general, the effects of (±)VK4-40 on d-amphetamine responding were parallel downward shifts in the self-administration dose-response curve. When (±)VK4-40 was substituted for cocaine, it functioned as a reinforcer in 2 of 3 monkeys. However, the reinforcing strength of (±)VK4-40 was significantly lower than cocaine and d-amphetamine, suggesting lower potential for misuse.

CONCLUSIONS

Overall, these findings support further exploration of D3R partial agonists, including (±)VK4-40, for treatment of d-amphetamine misuse or potentially in combination with d-amphetamine for treatment of cocaine use disorder.

Behavioral and neuronal extracellular vesicle biomarkers associated with nicotine's enhancement of the reinforcing strength of cocaine in female and male monkeys

While the majority of people with cocaine use disorders (CUD) also co-use tobacco/nicotine, most preclinical cocaine research does not include nicotine. The present study examined nicotine and cocaine co-use under several conditions of intravenous drug self-administration in monkeys, as well as potential peripheral biomarkers associated with co-use. In Experiment 1, male rhesus monkeys (N = 3) self-administered cocaine (0.001-0.1 mg/kg/injection) alone and with nicotine (0.01-0.03 mg/kg/injection) under a progressive-ratio schedule of reinforcement. When nicotine was added to cocaine, there was a significant leftward/upward shift in the number of injections received. In Experiment 2, socially housed female and male cynomolgus monkeys (N = 14) self-administered cocaine under a concurrent drug-vs-food choice schedule of reinforcement. Adding nicotine to the cocaine solution shifted the cocaine dose-response curves to the left, with more robust shifts noted in the female animals. There was no evidence of social rank differences. To assess reinforcing strength, delays were added to the presentation of drug; the co-use of nicotine and cocaine required significantly longer delays to decrease drug choice, compared with cocaine alone. Blood samples obtained post-session were used to analyze concentrations of neuronally derived small extracellular vesicles (NDE); significant differences in NDE profile were observed for kappa-opioid receptors when nicotine and cocaine were co-used compared with each drug alone and controls. These results suggest that drug interactions involving the co-use of nicotine and cocaine are not simply changing potency, but rather resulting in changes in reinforcing strength that should be utilized to better understand the neuropharmacology of CUD and the evaluation of potential treatments.

A comparison of the reinforcing strength of cocaethylene and cocaine in monkeys responding under progressive-ratio and concurrent choice schedules of reinforcement

BACKGROUND

Individuals who use cocaine have high rates of co-morbid alcohol use and when ethanol and cocaine are administered concurrently, the metabolite cocaethylene is formed. Cocaethylene is equipotent to cocaine in blocking dopamine reuptake and substitutes for cocaine in drug discrimination studies. However, no previous work has directly compared the reinforcing strength of cocaine to cocaethylene.

METHODS

In Experiment 1, three individually-housed adult male rhesus macaques self-administer cocaine under a progressive-ratio (PR) schedule of reinforcement, during daily 4-hr sessions. Under this schedule, the primary dependent variable is the number of injections received, or the break point (BP). Saline, cocaine (0.001-0.3mg/kg/injection) and cocaethylene (0.0003-0.1mg/kg/injection) dose-response curves were determined. In Experiment 2, two female cynomolgus and one rhesus macaque responded under a concurrent schedule of drug (cocaine or cocaethylene) vs. 1.0-g banana-flavored food pellets, during daily 1-hr sessions.

RESULTS

Both cocaine and cocaethylene functioned as reinforcers under the PR and concurrent choice schedules of reinforcement. Under the PR schedule, peak BPs were not significantly different, nor were ED50 values on the ascending limb, suggesting that cocaethylene has equal reinforcing strength and potency to cocaine. Under the concurrent drug-food choice procedure, cocaethylene was also equally potent to cocaine.

CONCLUSIONS

Under two schedules of reinforcement designed to assess reinforcing strength, cocaethylene and cocaine were equipotent and of equal reinforcing strength. Because cocaethylene has a longer duration of action, it is important for studies designed to evaluate treatments for cocaine use to also consider the effects of these interventions on cocaethylene.

Chronic phenmetrazine treatment promotes D2 dopaminergic and α2-adrenergic receptor desensitization and alters phosphorylation of signaling proteins and local cerebral glucose metabolism in the rat brain

Phenmetrazine (PHEN) is a putative treatment for cocaine and psychostimulant recidivism; however, neurochemical changes underlying its activity have not been fully elucidated. We sought to characterize brain homeostatic adaptations to chronic PHEN, specifically on functional brain activity (local cerebral glucose utilization), G-Protein Coupled Receptor-stimulated G-protein activation, and phosphorylation of ERK1/2Thr202/Tyr204, GSK3βTyr216, and DARPP-32Thr34. Male Sprague-Dawley rats were implanted with sub-cutaneous minipumps delivering either saline (vehicle), acute (2-day) or chronic (14-day) low dose (25 mg/kg/day) or high dose (50 mg/kg/day) PHEN. Acute administration of high dose PHEN increased local cerebral glucose utilization measured by 2-[14C]-deoxyglucose uptake in basal ganglia and motor-related regions of the rat brain. However, chronically treated animals developed tolerance to these effects. To identify the neurochemical changes associated with PHEN's activity, we performed [35S]GTPγS binding assays on unfixed and immunohistochemistry on fixed coronal brain sections. Chronic PHEN treatment dose-dependently attenuated D2 dopamine and α2-adrenergic, but not 5-HT1A, receptor-mediated G-protein activation. Two distinct patterns of effects on pERK1/2 and pDARPP-32 were observed: 1) chronic low dose PHEN decreased pERK1/2, and also significantly increased pDARPP-32 levels in some regions; 2) acute and chronic PHEN increased pERK1/2, but chronic high dose PHEN treatment tended to decrease pDARPP-32. Chronic low dose, but not high dose, PHEN significantly reduced pGSK3β levels in several regions. Our study provides definitive evidence that extended length PHEN dosage schedules elicit distinct modes of neuronal acclimatization in cellular signaling. These pharmacodynamic modifications should be considered in drug development for chronic use.