The nucleus accumbens, a site within the ventral striatum, is best known for its prominent role in mediating the reinforcing effects of drugs of abuse such as cocaine, alcohol, and nicotine. Indeed, it is generally believed that this structure subserves motivated behaviors, such as feeding, drinking, sexual behavior, and exploratory locomotion, which are elicited by natural rewards or incentive stimuli. A basic rule of positive reinforcement is that motor responses will increase in magnitude and vigor if followed by a rewarding event. It is likely, therefore, that the nucleus accumbens may serve as a substrate for reinforcement learning. However, there is surprisingly little information concerning the neural mechanisms by which appetitive responses are learned. In the present study, we report that treatment of the nucleus accumbens core with the selective competitive N-methyl-D-aspartate (NMDA) antagonist 2-amino-5-phosphonopentanoic acid (A P-5; 5 nmol͞0.5 l bilaterally) impairs responsereinforcement learning in the acquisition of a simple leverpress task to obtain food. Once the rats learned the task, AP-5 had no effect, demonstrating the requirement of NMDA receptor-dependent plasticity in the early stages of learning. Infusion of AP-5 into the accumbens shell produced a much smaller impairment of learning. Additional experiments showed that AP-5 core-treated rats had normal feeding and locomotor responses and were capable of acquiring stimulusreward associations. We hypothesize that stimulation of NMDA receptors within the accumbens core is a key process through which motor responses become established in response to reinforcing stimuli. Further, this mechanism, may also play a critical role in the motivational and addictive properties of drugs of abuse.The basal ganglia constitute a group of structures in the mammalian forebrain that have traditionally been considered as motor control regions. Indeed, one of the most prominent signs of basal ganglia disorders is motor disturbance or impairment (1). However, in more recent years empirical work has implicated the striatum (a major component of the basal ganglia) in cognition, reinforcement mechanisms, and sensorimotor integration (2, 3). Although the outflow of the striatum indeed reaches major extrapyramidal motor centers such as globus pallidus, substantia nigra, and subthalamic nucleus, the input to this structure, arising from the neocortex, limbic system, and midbrain, suggests that it plays a complex integrative role in adaptive motor actions (4, 5).One region of the striatum that has received considerable attention in this regard is the nucleus accumbens. The nucleus accumbens is best known for its role in mediating the reinforcing and rewarding properties of drugs of abuse (6). Drugs such as cocaine, heroin, alcohol, and even nicotine are hypothesized to produce their rewarding effects via activation of accumbens dopamine (7), and it has been recently postulated that chronic neuroadaptations in this system may underlie the addiction process (8...

The nucleus accumbens, a brain structure ideally situated to act as an interface between corticolimbic information-processing regions and motor output systems, is well known to subserve behaviors governed by natural reinforcers. In the accumbens core, glutamatergic input from its corticolimbic afferents and dopaminergic input from the ventral tegmental area converge onto common dendrites of the medium spiny neurons that populate the accumbens. We have previously found that blockade of NMDA receptors in the core with the antagonist 2-amino-5-phosphonopentanoic acid (AP-5; 5 nmol) abolishes acquisition but not performance of an appetitive instrumental learning task (Kelley et al., 1997). Because it is currently hypothesized that concurrent dopamine D 1 and glutamate receptor activation is required for long-term changes associated with plasticity, we wished to examine whether the dopamine system in the accumbens core modulates learning via NMDA receptors. Co-infusion of low doses of the D 1 receptor antagonist SCH-23390 (0.3 nmol) and AP-5 (0.5 nmol) into the accumbens core strongly impaired acquisition of instrumental learning (lever pressing for food), whereas when infused separately, these low doses had no effect. Infusion of the combined low doses had no effect on indices of feeding and motor activity, suggesting a specific effect on learning. We hypothesize that co-activation of NMDA and D 1 receptors in the nucleus accumbens core is a key process for acquisition of appetitive instrumental learning. Such an interaction is likely to promote intracellular events and gene regulation necessary for synaptic plasticity and is supported by a number of cellular models. Key words: glutamate; plasticity; striatum; intracellular signals; rat; reinforcement; rewardThe nucleus accumbens, a forebrain structure known to subserve behaviors governed by natural reinforcers, receives excitatory glutamatergic input from prefrontal cortex, hippocampus, thalamus, and amygdala (McGeer et al., 1977;Walaas and Fonnum, 1979;Young and Bradford, 1986;Fuller et al., 1987;Robinson and Beart, 1988), as well as a major dopaminergic innervation from the ventral tegmental area (Lindvall and Bjorklund, 1978). These innervations converge on the dendritic spines of the medium spiny neurons that populate the nucleus accumbens (Totterdell and Smith, 1989;Sesack and Pickel, 1990;Smith and Bolam, 1990). Therefore, these neurons are in a unique position to recognize context-driven patterns of activation and to transfer this information to planning and motor regions for appropriate behavioral responses (Houk et al., 1995). Recently, there has been much interest in the neuromodulatory effects of dopamine (DA) receptor activation on NMDA receptor state, as well as the intracellular mechanisms that may govern their interaction. For example, DA D 1 receptor activation in striatal slices potentiates responses mediated by NMDA receptor activation (an effect that is blocked by the D 1 receptor antagonist SCH-23390), whereas dopamine D 2 receptors have an atte...

Although corticotropin-releasing hormone (CRH), a regulator of stress responses, acts through two receptors (CRH1 and CRH2), the role of CRH2 in stress responses remains unclear. Knock-out mice without the CRH2 gene exhibit increased stress-like behaviors. This profile could result either directly from the absence of CRH2 receptors or indirectly from developmental adaptations. In the present study, CRH2 receptors were acutely blocked by alpha-helical CRH (alpha(h)CRH, CRH1/CRH2 antagonist; 0, 30, 100, and 300 ng) infusion into the lateral septum (LS), which abundantly expresses CRH2 but not CRH1 receptors. Freezing, locomotor activity, and analgesia were tested after infusion. Intra-LS alpha(h)CRH blocked shock-induced freezing without affecting activity or pain responses; infusions into lateral ventricle or nucleus of the diagonal band had no effects. The same behavioral profile was obtained with d-Phe-CRH((12-41)) (100 ng), another CRH1/CRH2 antagonist. A selective CRH1 antagonist (NBI27914), in doses that reduced freezing on intra-amygdala (central nucleus) infusion (0, 0.2, and 1.0 microg), did not affect freezing when infused into the LS. Ex vivo autoradiography revealed that binding of [125I]sauvagine, a mixed CRH1/CRH2 agonist, was prevented in the LS by previous intra-LS infusion of alpha(h)CRH but not NBI27914. In vitro studies demonstrated that [125I]sauvagine binding in the LS could be inhibited by a CRH1/CRH2 antagonist but not by the selective CRH1 receptor antagonist, confirming that in the LS, alpha(h)CRH antagonized exclusively CRH2 receptors. Acute antagonism of CRH2 receptors in the LS thus produces a behaviorally, anatomically, and pharmacologically specific reduction in stress-induced behavior, in contrast to results of recent knock-out studies, which induced congenital and permanent CRH2 removal. CRH2 receptors may thus represent a potential target for the development of novel CRH system anxiolytics.

Methods for quantifying DNA damage, as well as repair of that damage, in a high-throughput format are lacking. Single cell gel electrophoresis (SCGE; comet assay) is a widely-used method due to its technical simplicity and sensitivity, but the standard comet assay has limitations in reproducibility and throughput. We have advanced the SCGE assay by creating a 96-well hardware platform coupled with dedicated data processing software (CometChip Platform). Based on the original cometchip approach, the CometChip Platform increases capacity ~200 times over the traditional slide-based SCGE protocol, with excellent reproducibility. We tested this platform in several applications, demonstrating a broad range of potential uses including the routine identification of DNA damaging agents, using a 74-compound library provided by the National Toxicology Program. Additionally, we demonstrated how this tool can be used to evaluate human populations by analysis of peripheral blood mononuclear cells to characterize susceptibility to genotoxic exposures, with implications for epidemiological studies. In summary, we demonstrated a high level of reproducibility and quantitative capacity for the CometChip Platform, making it suitable for high-throughput screening to identify and characterize genotoxic agents in large compound libraries, as well as for human epidemiological studies of genetic diversity relating to DNA damage and repair.

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