Session-long modulations of accumbal firing during sucrose-reinforced operant behavior

Dopamine transients follow a striatal gradient of reward time horizons

Animals make predictions to guide their behavior and update those predictions through experience. Transient increases in dopamine (DA) are thought to be critical signals for updating predictions. However, it is unclear how this mechanism handles a wide range of behavioral timescales-from seconds or less (for example, if singing a song) to potentially hours or more (for example, if hunting for food). Here we report that DA transients in distinct rat striatal subregions convey prediction errors based on distinct time horizons. DA dynamics systematically accelerated from ventral to dorsomedial to dorsolateral striatum, in the tempo of spontaneous fluctuations, the temporal integration of prior rewards and the discounting of future rewards. This spectrum of timescales for evaluative computations can help achieve efficient learning and adaptive motivation for a broad range of behaviors.

Nucleus accumbens shell neurons' early sensitivity to cocaine is associated with future increases in drug intake

The striatum, both dorsal and ventral, is strongly implicated in substance use disorder. Chronic consumption of abused substances, such as cocaine, can cause an oversaturation of mesostriatal dopamine, which results in alterations in the firing of striatal neurons. While most preclinical studies of drug self-administration (S-A) are focused on these alterations, individual differences in a subject's early responses to drugs can also account for substantial differences in addiction susceptibility. In this study, we modeled longitudinal pharmacokinetics using data from a previous longitudinal study (Coffey et al., 2015) and aimed to determine if firing in specific dorsal and ventral striatal subregions was subject to changes across chronic cocaine S-A, and if individual animal differences in striatal firing in response to early drug exposure correlated with increases in drug intake. We observed that the firing patterns of nucleus accumbens (NAc) core and shell neurons exhibited increasing sensitivity to cocaine over the first 6 S-A sessions and maintained a strong negative correlation between drug intake and neuronal firing rates across chronic S-A. Moreover, we observed that the early sensitivity of NAc shell neurons to cocaine correlated with future increases in drug intake. Specifically, rats whose NAc shell neurons were most inhibited by increasing levels of cocaine upon first exposure exhibited the strongest increases in cocaine intake over time. If this difference can be linked to a genetic difference, or druggable targets, it may be possible to screen for similar addiction susceptibility in humans or develop novel preemptive pharmacotherapies.

Prelimbic and infralimbic medial prefrontal cortex neuron activity signals cocaine seeking variables across multiple timescales

RATIONALE AND OBJECTIVES

The prefrontal cortex is critical for execution and inhibition of reward seeking. Neural manipulation of rodent medial prefrontal cortex (mPFC) subregions differentially impacts execution and inhibition of cocaine seeking. Dorsal, or prelimbic (PL), and ventral, or infralimbic (IL) mPFC are implicated in cocaine seeking or extinction of cocaine seeking, respectively. This differentiation is not seen across all studies, indicating that further research is needed to understand specific mPFC contributions to drug seeking.

METHODS

We recorded neuronal activity in mPFC subregions during cocaine self-administration, extinction, and cue- and cocaine-induced reinstatement of cocaine seeking.

RESULTS

Both PL and IL neurons were phasically responsive around lever presses during cocaine self-administration, and activity in both areas was reduced during extinction. During both cue- and, to a greater extent, cocaine-induced reinstatement, PL neurons exhibited significantly elevated responses, in line with previous studies demonstrating a role for the region in relapse. The enhanced PL signaling in cocaine-induced reinstatement was driven by strong excitation and inhibition in different groups of neurons. Both of these response types were stronger in PL vs. IL neurons. Finally, we observed tonic changes in activity in all tasks phases, reflecting both session-long contextual modulation as well as minute-to-minute activity changes that were highly correlated with brain cocaine levels and motivation associated with cocaine seeking.

CONCLUSIONS

Although some differences were observed between PL and IL neuron activity across sessions, we found no evidence of a go/stop dichotomy in PL/IL function. Instead, our results demonstrate temporally heterogeneous prefrontal signaling during cocaine seeking and extinction in both PL and IL, revealing novel and complex functions for both regions during these behaviors. This combination of findings argues that mPFC neurons, in both PL and IL, provide multifaceted contributions to the regulation of drug seeking and addiction.

Striatal mechanisms underlying movement, reinforcement, and punishment

Direct and indirect pathway striatal neurons are known to exert opposing control over motor output. In this review, we discuss a hypothetical extension of this framework, in which direct pathway striatal neurons also mediate reinforcement and reward, and indirect pathway neurons mediate punishment and aversion.

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.

Orbitofrontal and insular cortex: neural responses to cocaine-associated cues and cocaine self-administration

Based on neuro-imaging studies in cocaine-addicted humans, it is hypothesized that increases in neural activity within several regions of the prefrontal cortex contribute to cue-induced cocaine seeking and cocaine-induced compulsive drug self-administration. However, electrophysiological tests of these hypotheses are lacking. In the present study, animals were trained to self-administer cocaine (0.75 mg/kg) for 14 days. On the 14th day, we conducted electrophysiological recordings of lateral orbitofrontal (LO) and ventral anterior insula (AIV) neurons. A subset of the combined population of recorded neurons showed a change in firing rate in association with one or more of the following discrete events: (1) presentation of a discriminative stimulus that signaled the onset of the self-administration session, (2) occurrence of the first cocaine-directed operant response, (3) occurrence of a cocaine-reinforced press, and (4) presentation of cues normally paired with delivery of the cocaine reinforcer. The majority of the stimulus- and response-related changes in firing involved a brief increase in firing during the stimulus and response event, respectively. In addition to these event-specific responses, approximately half of the recorded neurons exhibited a sustained change in average firing (i.e., discharges per 30-s bin) during the cocaine self-administration session, relative to average firing during a presession, drug-free period (referred to as session changes). The prevalence of session-increases and decreases were not significantly different. These and other findings are discussed in relation to hypotheses about cue-evoked and cocaine-maintained cocaine-directed behavior.

Background firing rates of orbitofrontal neurons reflect specific characteristics of operant sessions and modulate phasic responses to reward-associated cues and behavior

The orbitofrontal cortex plays an important role in the ability of animals to adjust their behavior in response to behavioral outcomes. Multiple studies have demonstrated that responses of orbitofrontal neurons during operant sessions reflect the outcome of particular behaviors. These studies have focused on rapid neural responses to short-duration events such as instrumental behavior and reward-associated discrete cues. We hypothesize that longer-lasting changes in firing are also important for information processing in the orbitofrontal cortex. In the present study, we recorded the activity of 115 single orbitofrontal neurons during a multiphase operant task in which the relationship between a lever-press response and a sucrose reward was varied between the different phases. Approximately one-half of the orbitofrontal neurons exhibited a change in background firing during the operant phases. These changes were observable across multiple behavioral and stimulus events and thus reflected a general shift in background firing. The majority of changes were selective for one or the other of the operant phases. Selective changes contributed to unique patterns of phasic firing time locked to cues and operant behavior in the two operant phases. These findings are consistent with the interpretation that changes in background firing of orbitofrontal neurons reflect operant session characteristics associated with behavioral outcome, and indicate further that changes in background firing contribute to the outcome selectivity of phasic firing patterns. More generally, we propose that the background firing rates of orbitofrontal neurons reflect contextual information, and facilitate context-appropriate event-related information processing and behavioral responses.