Recent studies have demonstrated that an acute, low dose of ketamine, a non-competitive NMDA receptor antagonist, provides rapid and sustained antidepressant effects in patients with major depressive disorder. Studies in rodents have shown that the antidepressant properties of ketamine are due to an increase in dendritic spine density in the cortex. Our goal was to determine whether these effects are specific to ketamine and whether they are dependent on dose, drug regimen and brain region. We observed that the effects of ketamine on spine density were dependent on dose and drug regimen and were also brain region specific. In addition, MK-801, another NMDA receptor antagonist, did not demonstrate the same effects on spine density as ketamine. Furthermore, genetic NMDA receptor hypofunction significantly reduced spine density. Our studies demonstrate that while acute ketamine treatment leads to an increase in cortical spine density, chronic administration has opposite and potentially detrimental effects.

Recent studies have demonstrated that an acute, low dose of ketamine, a non-competitive NMDA receptor antagonist, provides rapid and sustained antidepressant effects in patients with major depressive disorder. Studies in rodents have shown that the antidepressant properties of ketamine are due to an increase in dendritic spine density in the cortex. Our goal was to determine whether these effects are specific to ketamine and whether they are dependent on dose, drug regimen and brain region. We observed that the effects of ketamine on spine density were dependent on dose and drug regimen and were also brain region specific. In addition, MK-801, another NMDA receptor antagonist, did not demonstrate the same effects on spine density as ketamine. Furthermore, genetic NMDA receptor hypofunction significantly reduced spine density. Our studies demonstrate that while acute ketamine treatment leads to an increase in cortical spine density, chronic administration has opposite and potentially detrimental effects.

The hypocretin/orexin (HCRT) system is associated with compulsive stimulant drug use, involving both HCRT-receptor 1 (-R1) and HCRT-receptor 2 (-R2). Few studies, however, have examined the role of HCRT-R2 or combined HCRT-R1/2 on compulsive methamphetamine (METH) taking behavior. In this study, we examined the effects of HCRT-R1, -R2, and -R1/2 antagonists on compulsive METH self-administration, as modeled by escalated intake in adult male Wistar rats allowed extended access to METH. Three cohorts of rats were allowed either short (1h; ShA; n=7-10/cohort) or long (6h; LgA; n=7-9/cohort) access to METH intravenous self-administration for 14 sessions (fixed ratio 1 schedule). Each cohort was then systemically administered a single- or dual-HCRT-R antagonist 30 min prior to METH self-administration testing: cohort 1, selective HCRT-R1 antagonist (RTIOX-276; RTI-R1; 0, 10, and 20 mg/kg); cohort 2, selective HCRT-R2 antagonist (JNJ-10397049; JNJ-R2; 0, 10, and 20 mg/kg); and cohort 3, dual HCRT-R1/2 antagonist (Suvorexant; SUV-R1/2; 0, 30, and 60 mg/kg). RTI-R1 elicited a dose-dependent reduction in METH intake in LgA, but not ShA, in the first hour. Administration of JNJ-R2 had no effect on METH intake in the first hour in neither ShA nor LgA rats, but reduced METH intake during the full 6 h session at the lowest dose. SUV-R1/2 administration had no effect on METH intake in ShA rats, but showed significant attenuation of METH-taking at the highest dose in both the first hour and full 6h session for LgA rats. Locomotor activity was significantly reduced following RTI-R1 and SUV-R1/2 in ShA rats only. To further explore the role that HCRT plays in METH dependence after a period of abstinence, we used a shRNA-encoding adeno-associated viral vector (AAV) to silence Hcrt in a separate cohort of previously-escalated METH-dependent rats. Following an initial escalation phase, and prior to a 3-week period of drug abstinence, rats were injected with either a control scramble-RNA AAV (AAV-Scram; n= 4) or a Hcrt-knockdown AAV (AAV-HCRT-KD; n= 5). AAV-Scram rats showed a significant decrease in METH self-administration post-abstinence, and a subsequent increase in METH-taking following a re-escalation period. In contrast, AAV-HCRT-KD rats showed a significant attenuation of METH self-administration following the re-escalation period. Combined, these results suggest HCRT neurotransmission at both HCRT-R1 and -R2 may contribute to compulsive METH-taking behavior.