Intracellular Mechanisms Associated with Cocaine Induced Conditioned Place Preference

Adviser: Shirzad Jenab, Ph.D. The development and maintenance of cocaine addiction and relapse after abstinence depend heavily on learned associations between environmental stimuli and the rewarding properties of the drug. Cocaine and other abused drugs activate intracellular signaling cascades throughout the mesocorticolimbic reward circuitry, leading to functional changes in brain structure. Therefore, the aim of this dissertation was to investigate the intracellular responses associated with the acquisition and expression of a cocaine-context association. ERK (extracellular regulated kinase), CREB (cAMP responsive element binding protein), FosB and DFosB protein expression were of particular interest due to their involvement in cocaine reward and in the structural synaptic plasticity underlying learning and memory. The conditioned place preference (CPP) paradigm, which employs a simple Pavlovian conditioning procedure to establish an association between a drug-paired environment and the drug’s rewarding effects, was used to further study the role of these signaling pathways in cocaine-associated contextual memories. We attempted to block the acquisition of cocaine CPP using the NMDA receptor antagonist MK-801. Pretreatment of MK-801 (0.25 mg/kg. i.p.), 30 minutes prior to cocaine administration during conditioning, blocked the acquisition of cocaine CPP. Additionally, MK-801 prior to cocaine during conditioning prevented the increase in NAc pERK levels observed following the CPP test (drug-free) in rats treated with cocaine only. The present study investigated intracellular responses after cocaine-induced conditioned place preference (CPP) in response to two doses of cocaine (5mg/kg, or 20mg/kg, i.p.) after reexposure to an environment previously associated with cocaine or a control environment. We found that CPP behavior was expressed only after conditioning with the higher dose of cocaine and locomotor responses in rats after re-exposure to the cocaine-chamber were greater than in rats re-exposed to the saline-paired chamber. Locomotor responses during the CPP test and re-exposure session were correlated to CPP scores after conditioning with either cocaine dose. Nucleus Accumbens (NAc) phosphorylated-ERK (pERK) levels were increased after re-exposure to the cocaine-paired, but not the saline-paired environment 24 hours after the CPP test, regardless of whether or not CPP behavior was expressed. Caudate Putamen (CPu) pERK and FosB protein levels increased after re-exposure to the cocaine chamber only after conditioning with the higher cocaine dose. Conversely, the higher cocaine dose, independent of environment, resulted in increased NAc FosB, ΔFosB and phosphorylated CREB (pCREB) protein levels compared to those conditioned with 5mg/kg cocaine. Our results suggest that NAc ERK phosphorylation may be involved with retrieving the contextual information of a cocaine-association, without the expression of the behavior. Additionally, we show different patterns of intracellular responses in the NAc and CPu indicating a region specific role for pERK/pCREB/FosB intracellular signaling in the retrieval of cocaine-context associations. Sex differences in cocaine’s mechanisms of action and behavioral effects have been widely reported. However, little is known about how sex influences intracellular signaling cascades involved with drug-environment associations. We investigated whether ERK/CREB intracellular responses in the mesocorticolimbic circuitry underlying cocaine environmental associations are sexually dimorphic. We used a standard 4 day conditioned place preference (CPP) paradigm using 20mg/kg cocaine—a dose that induced CPP in male and female Fischer rats. In the nucleus accumbens (NAc) following CPP expression, cocaine treated animals showed increased phosphorylated ERK (pERK), phosphorylated CREB (pCREB) and ΔFosB protein levels. In the hippocampus (HIP) and caudate putamen (CPu), pERK and FosB/ΔFosB levels were also increased, respectively. Cocaine females had a larger change in HIP pERK and CPu ΔFosB levels than cocaine males; partly due to lower protein levels in saline female rats when compared to saline males. Prefrontal cortex (PfC) pCREB levels increased in cocaine males, but not females, whereas PfC pERK levels were increased in cocaine females, but not males. CPP scores were positively correlated to NAc pERK, HIP pERK and CPu FosB protein levels, suggesting that similar to males, the ERK/CREB intracellular pathway in mesocorticolimbic regions undergoes cocaine induced neuroplasticity in female rats. However, there seem to be intrinsic (basal) sexual dimorphisms in this pathway that may contribute to responses expressed after cocaineCPP. Taken together, our results suggest that cellular responses associated with the expression of learned drug-environment associations may play an important role in sex differences in cocaine addiction and relapse. Abbreviations: CPP, conditioned place preference; CPu, caudate putamen; CREB, cAMP response element binding protein; DA, dopamine; DAR, dopamine receptor; ERK, extracellular regulated kinase; HIP, hippocampus; MAPK, mitogen activated protein kinases; MEK, mitogen activated extracellular-regulated protein kinase; NAc, nucleus accumbens; pCREB, phosphorylated CREB; pERK, phosphorylated ERK; PfC, prefrontal cortex; VTA, ventral tegmental area