Second-order stimuli do not always increase overall response rates in second-order schedules of reinforcement in the rat

Acute effects of nicotine amplify accumbal neural responses during nicotine-taking behavior and nicotine-paired environmental cues

Nicotine self-administration (SA) is maintained by several variables, including the reinforcing properties of nicotine-paired cues and the nicotine-induced amplification of those cue properties. The nucleus accumbens (NAc) is implicated in mediating the influence of these variables, though the underlying neurophysiological mechanisms are not yet understood. In the present study, Long-Evans rats were trained to self-administer nicotine. During SA sessions each press of a lever was followed by an intravenous infusion of nicotine (30 µg/kg) paired with a combined light-tone cue. Extracellular recordings of single-neuron activity showed that 20% of neurons exhibited a phasic change in firing during the nicotine-directed operant, the light-tone cue, or both. The phasic change in firing for 98% of neurons was an increase. Sixty-two percent of NAc neurons additionally or alternatively showed a sustained decrease in average firing during the SA session relative to a presession baseline period. These session decreases in firing were significantly less prevalent in a group of neurons that were activated during either the operant or the cue than in a group of neurons that were nonresponsive during those events (referred to as task-activated and task-nonactivated neurons, respectively). Moreover, the session decrease in firing was dose-dependent for only the task-nonactivated neurons. The data of the present investigation provide supportive correlational evidence for two hypotheses: (1) excitatory neurophysiological mechanisms mediate the NAc role in cue-maintenance of nicotine SA, and (2) a differential nicotine-induced inhibition of task-activated and task-nonactivated neurons mediates the NAc role in nicotine-induced amplification of cue effects on nicotine SA.

Distinct contributions of dopamine receptors in the nucleus accumbens core or shell to established cocaine reinforcement under a second-order schedule

Nucleus accumbens dopamine is implicated in the primary and conditioned reinforcing properties of abused drugs. In the present study, specific impairments in responding for intravenous cocaine (0.3 mg/inf/0.1 ml/5 s) under a fixed-ratio 1 (FR-1) or second-order schedule (FI 15 min (FR10:S)) were investigated following infusion of the dopamine antagonist, alpha-flupenthixol, into either the nucleus accumbens core or shell. Infusion of alpha-flupenthixol into the core decreased cocaine intake under the FR-1 and second-order schedules. By comparison, blockade of nucleus accumbens shell dopamine receptors increased cocaine intake under the FR-1 schedule. Under the second-order schedule, cocaine intake and the number of responses was decreased. Effects on responding were more apparent after self-administered cocaine, when impairments in the latency to receive cocaine infusions were no longer evident. These results are discussed with reference to a role for nucleus accumbens shell dopamine in instrumental responding, and a role of nucleus accumbens core dopamine in incentive motivation, perhaps under the control of contextual stimuli.

Neuropsychopharmacology of drug seeking: Insights from studies with second-order schedules of drug reinforcement

Second-order schedules of reinforcement model complex chains of responding for rewards such as food or drugs. Derived from studies of conditioned reinforcement, an important feature of these schedules is that responding is maintained by the response-dependent presentation of conditioned stimuli. They are thus well-suited to the study of the influences over drug seeking exerted by drug-associated stimuli. In the present review, we summarise studies investigating the neurobiology and neuropsychopharmacology of responding for cocaine under a second-order schedule of reinforcement. We conclude that limbic-striatal circuitries underlie drug seeking measured in this way. Emphasis is placed on potential interactions between structures within these subsystems by reviewing neuropsychopharmacological studies in which antagonists selective for either glutamate or dopamine receptors have been infused directly into limbic, cortical and striatal areas.

Nucleus accumbens neurons in the rat exhibit differential activity to conditioned reinforcers and primary reinforcers within a second-order schedule of saccharin reinforcement

The nucleus accumbens has been associated with processing information related to primary reinforcement and reward. Most neurophysiological studies report that nucleus accumbens neurons are phasically excited in response to the onsets of salient events during the seeking of reinforcement and to the delivery of primary reinforcers. However, a minority of studies report inhibition during primary reinforcement. We recorded from 65 neurons in the nucleus accumbens whilst thirsty rats performed under a second-order schedule of saccharin reinforcement. This allowed us to analyse neural activity and behaviour during reinforcer-seeking in the presence of conditioned reinforcers (second-order stimuli, also called 'conditioned stimuli'), and during primary reinforcer consumption. Specifically, we sought to examine the valence of potential neural responses to primary reinforcement, to compare these responses to second-order stimulus-evoked responses, and to determine whether responses were differential to second-order stimuli presented at different time points within the schedule. Fifty out of 65 neurons we sampled responded to the second-order stimulus and/or consumption of the primary reinforcer. Most neurons in our sample exhibited excitation following the second-order stimulus and inhibition to the primary reinforcer, a pattern also present over the average response of the neural population. However, there was no systematic variation in neural responses evoked by second-order stimuli presented at different temporal proximities to primary reinforcement. Our results provide evidence that partially overlapping mechanisms within the nucleus accumbens differentially process conditioned reinforcers and primary reinforcers.