Protein kinases A and C are involved in the mechanisms underlying consolidation of cocaine place conditioning

Abstract

Using a balanced conditioned place preference (CPP) paradigm, we studied the role of protein kinases A (PKA) and C (PKC) on the acquisition, consolidation and expression of cocaine place conditioning. H7, a non-selective inhibitor of protein kinases, was administered intracerebroventricularly at 1 and 10 μg/10 μl. The higher dose significantly reduced the time spent by rats in the cocaine compartment when given immediately after each conditioning session (consolidation), whereas it had no effect when administered before cocaine during the training phase (acquisition) or before testing for place preference in the absence of cocaine (expression). The same effect was found on administering immediately after each training session 3 μg/10 μl chelerythrine, a selective PKC inhibitor, or 10 μg/10 μl H89, a selective PKA inhibitor, suggesting that both kinases contribute to the consolidation of stimulus-reward association which determines rats' behavior in the cocaine CPP. Changes in the activity of PKA and PKC may thus be part of the cascade of events that contribute to enhancing synaptic responses in the consolidation phase of cocaine CPP and determine rats' behavior associated with the memory of the rewarding effect of cocaine during cocaine CPP expression. These findings may have implications for the study of cocaine `craving' and relapse.

Introduction

Protein kinases appear to play an important role in various plastic phenomena such as long-term potentiation (LTP), kindling and learning. LTP, a stable form of synaptic plasticity widely studied as a cellular model of learning and memory, has been amply investigated with the aim of clarifying the neurochemical mechanisms involved in various phases of the phenomenon 26, 30. Evidence is accumulating that protein kinase C (PKC) activation is necessary to convert short-term potentiation to LTP suggesting that the kinase is involved in its induction and consolidation. Protein kinase A (PKA), on the other hand, seems to be involved in the expression of cAMP-inducible genes 2, 30in a late phase of LTP [28].

Addiction to drugs is considered to be the result of plastic changes in multiple neural circuits and progress is being made in understanding the mechanisms involved in sensitization and cue-dependent memories of drug use 4, 8, 9, 19, 20, 44. The conditioned place preference (CPP) model is commonly used for studying the reinforcing properties of drugs 3, 7, 42and the ability of cues associated with drugs of abuse to elicit place preference may have important implications for the study of drug `craving' and relapse 24, 33.

The CPP consists of an acquisition phase during which rats receive the drug in one distinctive environment, and a test or expression phase in which drug-free animals are tested for their preference for the environment previously paired with the drug. A recent study in our laboratory showed that dopaminergic D₁ and glutamatergic NMDA receptors were involved in the establishment of cocaine CPP whereas AMPA/kainate receptors were important only for CPP expression [8]. Cocaine raises the extracellular concentrations of dopamine and glutamate in the nucleus accumbens 23, 35and after repeated treatment it raises the levels of adenylate cyclase and cAMP-dependent protein kinase activity in the nucleus accumbens [40]. Moreover, chronic cocaine causes sensitization to the motor stimulant properties and this effect is blocked by NMDA receptor antagonists 21, 22. An interesting interaction between dopamine D₁ and NMDA receptors is also suggested by the finding that antagonists at these receptors block the induction of c-fos by cocaine in the rat brain 41, 45. Because the dopaminergic D₁ receptors stimulate adenylate cyclase [23]and various glutamate receptors lead to activation of PKC 2, 32, 34, it is likely that PKA and PKC are involved in various phases of cocaine CPP.

While various studies have associated changes in PKA with adaptive phenomena consequent to chronic use of opioids (see Ref. [27]), there is little information on the role of these two kinases in cocaine-induced plastic phenomena. Chronic treatment with cocaine has been reported to activate PKA [40]and a recent study reported that administration of H7, a non-selective protein kinase inhibitor [17], into the ventral tegmental area blocked the development of sensitization to cocaine [37].

In the present study we evaluated the roles of PKA and PKC in the acquisition, consolidation and expression of cocaine CPP using H7, a non-selective inhibitor of protein kinases. Since H7 selectively reduced the consolidation of cocaine CPP, we administered immediately after each conditioning session two compounds having different selectivity in inhibiting PKA or PKC, H89 and chelerythrine 11, 16, to clarify which protein kinase mainly contributed to the effect of cocaine.