Extracellular glutamate levels in prefrontal cortex during the expression of associative responses to cocaine related stimuli

Abstract

We conducted studies to examine the potential role of glutamate in the prefrontal cortex (PFC) during conditioned responses to stimuli (flashing light and metronome) previously associated with cocaine administration. During training, PAIRED subjects received cocaine injections (15 mg/kg) during stimuli sessions while UNPAIRED subjects received saline injections (but received cocaine in the home cage an hour later). We showed previously that PAIRED subjects exhibit conditioned locomotion when tested with the stimuli alone. In this study, we further demonstrated that the expression of behavioral sensitization in response to cocaine challenge is under conditioned control in PAIRED subjects. Then, we used microdialysis to examine extracellular levels of glutamate in the PFC in response to presentation of the conditioned stimuli alone and challenge with cocaine in the presence of the conditioned stimuli. Although PAIRED subjects demonstrated conditioned locomotion and conditioned control of sensitization during the microdialysis experiment, PFC glutamate levels were unaltered during the tests and did not differ between the PAIRED and UNPAIRED subjects.

Introduction

The contribution of associative factors in triggering relapse has long been acknowledged (Wikler, 1973). More recently, several investigators have demonstrated that during cue-elicited cocaine craving in humans, portions of the prefrontal cortex (PFC) as well as other limbic brain regions are activated (Grant et al., 1996, Maas et al., 1998, Childress et al., 1999, Garavan et al., 2000, Kilts et al., 2001, Wexler et al., 2001). This PFC activation is not limited to subjects with a history of psychostimulant abuse; alcoholics show PFC activation in response to alcohol specific cues as compared to social drinkers (George et al., 2001). The magnitude of PFC activation has been correlated with the level of craving reported by the subjects in response to the stimuli, supporting the notion that these activations are relevant to craving (Grant et al., 1996, Maas et al., 1998).

Electrophysiological studies in monkeys have demonstrated that neurons of the PFC show activity related to the expectancy of reward with the pattern of activity depending on the type of reward predicted to occur (Watanabe, 1996). Furthermore, the magnitude of PFC neuronal activity during reward expectation is related to the size of the expected reward—increased firing rates being associated with expectation of larger rewards (Leon and Shadlen, 1999). It has been hypothesized that compulsive drug-seeking behavior results from alterations in the function of circuits involving the PFC, limbic regions such as the basolateral amygdala, and the nucleus accumbens. These alterations are proposed to result in sensitization of stimulus-reward associations coupled with defective functioning of inhibitory control mechanisms that normally govern reward-seeking behavior triggered by stimulus presentation (Jentsch and Taylor, 1999, Everitt and Wolf, 2002). Indeed, studies in monkeys have shown that cocaine administered in a non-contingent manner impairs reversal learning and leads to response perseveration, signs consistent with PFC malfunction (Jentsch et al., 2002). The importance of PFC activity in the expression of conditioned responses has been demonstrated by Franklin and Druhan (2000b) who showed that inactivation of the PFC (via muscimol application) prior to placement in an environment paired with cocaine attenuated the conditioned response.

Despite the known role of the PFC in reward expectation, only one study to date has examined neurochemical changes in the PFC during a conditioned response to stimuli paired with a drug of abuse (Carey and Damianopoulos, 1994). These investigators found that, following expression of conditioned locomotion, the PFC of paired rats had higher tissue levels of 5-HT and 5-HIAA as determined by post mortem analysis. In the present study, we focused on glutamate transmission for several reasons: (1) glutamate is the major excitatory transmitter of the PFC, (2) glutamate transmission is necessary for the expression of conditioned locomotion (Cervo and Samanin, 1996, Tzschentke and Schmidt, 1997, Mead et al., 1999, Hotsenpiller et al., 2001), and (3) the PFC receives excitatory glutamatergic inputs from the basolateral amygdala (Sarter and Markowitsch, 1983, Kita and Kitai, 1990, McDonald, 1991, McDonald, 1992, Shinonaga et al., 1994, Matsuda and Fujimura, 1995, Bacon et al., 1996), a region consistently shown to be important for the expression of conditioned responses to stimuli formerly associated with the self-administration of cocaine (Meil and See, 1997, Grimm and See, 2000, Kruzich and See, 2001).

We have previously characterized conditioned locomotion produced by pairing of discrete stimuli (flashing light and metronome) with cocaine, and demonstrated that the expression of conditioned locomotion during a drug-free test is associated with increased glutamate levels in the nucleus accumbens as measured by microdialysis (Hotsenpiller et al., 2001). In the present study, we conducted additional experiments to show that the expression of behavioral sensitization in response to cocaine challenge is under conditioned control in PAIRED subjects. Then, we used microdialysis to examine extracellular levels of glutamate in the PFC during conditioned locomotion and during the expression of behavioral sensitization to cocaine challenge under the control of conditioned stimuli. Both responses could be studied sequentially during the same test session, because discrete stimuli can be introduced and then removed at specific times during the microdialysis experiment.