Steady-state methadone blocks cocaine seeking and cocaine-induced gene expression alterations in the rat brain

Abstract

To elucidate the effects of steady-state methadone exposure on responding to cocaine conditioned stimuli and on cocaine-induced alterations in central opioid, hypocretin/orexin, and D2 receptor systems, male Sprague-Dawley rats received intravenous infusions of 1 mg/kg/inf cocaine paired with an audiovisual stimulus over three days of conditioning. Then, mini pumps releasing vehicle or 30 mg/kg/day methadone were implanted (SC), and lever pressing for the stimulus was assessed in the absence of cocaine and after a cocaine prime (20 mg/kg, IP). It was found that rats treated with vehicle, but not methadone, responded for the cocaine conditioned stimulus and displayed elevated mu-opioid receptor mRNA expression in the nucleus accumbens core and basolateral amygdala, reduced hypocretin/orexin mRNA in the lateral hypothalamus, and reduced D2 receptor mRNA in the caudate-putamen. This is the first demonstration that steady-state methadone administered after cocaine exposure blocks cocaine-induced behavioral and neural adaptations.

Figure 1

Panel A: mean (SEM) responses made on the active lever at each test of responding for the activation of an audiovisual compound stimulus before (i.e., Baseline; B) and after (Tests 1-5 Primes I & II) Pavlovian conditioning of this stimulus with IV infusions of cocaine (1 mg/kg/infusion). On Prime I session, all rats were injected with vehicle (V). On Prime II session, some rats (CVC, CMC and VVC) were injected with a 20 mg/kg cocaine prime (C). The * indicates a significant difference between VVV and CVC groups. The # indicates a significant difference between VVV and CVV groups. Panel B: mean (SEM) horizontal distance (cm) travelled by drug-naïve rats (VVV group) and cocaine conditioned/primed groups implanted with 30 mg/kg/day methadone-(CMC group) and vehicle-(CVC) filled mini-pumps. This test of locomotion activity was administered before the period of conditioning.

Figure 2

Mean (SEM) concentrations of different mRNAs in different regions of the rat brain. The horizontal line represents mean levels measured in drug-naïve rats (VVV group). The upper and lower dashed lines represent positive and negative SEMs. The * indicates a significant difference from VVV group. CMC group = conditioned with cocaine, implanted with methadone-filled mini-pumps; primed with cocaine. CVC group = conditioned with cocaine, implanted with vehicle-filled mini-pumps; primed with cocaine. CVV group = conditioned with cocaine, implanted with vehicle-filled mini-pumps; primed with vehicle. VVC group = conditioned with vehicle, implanted with vehicle-filled mini-pumps; primed with cocaine.

Figure 3

Mean (SEM) concentrations of different mRNAs in different regions of the rat brain. The horizontal line represents mean levels measured in drug-naïve rats (VVV group). The upper and lower dashed lines represent positive and negative SEMs. The * indicates a significant difference from VVV group. CMC group = conditioned with cocaine, implanted with methadone-filled mini-pumps; primed with cocaine. CVC group = conditioned with cocaine, implanted with vehicle-filled mini-pumps; primed with cocaine. CVV group = conditioned with cocaine, implanted with vehicle-filled mini-pumps; primed with vehicle. VVC group = conditioned with vehicle, implanted with vehicle-filled mini-pumps; primed with cocaine.

Figure 4

Mean (SEM) concentrations of different mRNAs in different regions of the rat brain. The horizontal line represents mean levels measured in drug-naïve rats (VVV group). The upper and lower dashed lines represent positive and negative SEMs. The * indicates a significant difference from VVV group. CMC group = conditioned with cocaine, implanted with methadone-filled mini-pumps; primed with cocaine. CVC group = conditioned with cocaine, implanted with vehicle-filled mini-pumps; primed with cocaine. CVV group = conditioned with cocaine, implanted with vehicle-filled mini-pumps; primed with vehicle. VVC group = conditioned with vehicle, implanted with vehicle-filled mini-pumps; primed with cocaine.

Figure 5

Mean (SEM) concentrations of different mRNAs in different regions of the rat brain. The horizontal line represents mean levels measured in drug-naïve rats (VVV group). The upper and lower dashed lines represent positive and negative SEMs. The * indicates a significant difference from VVV group. CMC group = conditioned with cocaine, implanted with methadone-filled mini-pumps; primed with cocaine. CVC group = conditioned with cocaine, implanted with vehicle-filled mini-pumps; primed with cocaine. CVV group = conditioned with cocaine, implanted with vehicle-filled mini-pumps; primed with vehicle. VVC group = conditioned with vehicle, implanted with vehicle-filled mini-pumps; primed with cocaine.