The amygdala has long been associated with emotion and motivation, playing an essential part in processing both fearful and rewarding environmental stimuli. How can a single structure be crucial for such different functions? With recent technological advances that allow for causal investigations of specific neural circuit elements, we can now begin to map the complex anatomical connections of the amygdala onto behavioural function. Understanding how the amygdala contributes to a wide array of behaviours requires the study of distinct amygdala circuits.

Situations where rewards are unexpectedly obtained or withheld represent opportunities for new learning. Often, this learning includes identifying cues that predict reward availability. Unexpected rewards strongly activate midbrain dopamine neurons. This phasic signal is proposed to support learning about antecedent cues by signaling discrepancies between actual and expected outcomes, termed a reward prediction error. However, it is unknown whether dopamine neuron prediction error signaling and cue-reward learning are causally linked. To test this hypothesis, we manipulated dopamine neuron activity in rats in two behavioral procedures, associative blocking and extinction, that illustrate the essential function of prediction errors in learning. We observed that optogenetic activation of dopamine neurons concurrent with reward delivery, mimicking a prediction error, was sufficient to cause long-lasting increases in cue-elicited reward-seeking behavior. Our findings establish a causal role for temporally-precise dopamine neuron signaling in cue-reward learning, bridging a critical gap between experimental evidence and influential theoretical frameworks.

Summary Currently there is no general approach for achieving specific optogenetic control of genetically-defined cell types in rats, which provide a powerful experimental system for numerous established neurophysiological and behavioral paradigms. To overcome this challenge we have generated genetically-restricted recombinase-driver rat lines suitable for driving gene expression in specific cell-types, expressing Cre recombinase under control of large genomic regulatory regions (200–300 Kb). Multiple tyrosine hydroxylase (Th)::Cre and choline acetyltransferase (Chat)::Cre lines were produced that exhibited specific opsin expression in targeted cell-types. We additionally developed methods for utilizing optogenetic tools in freely-moving rats, and leveraged these technologies to clarify the causal relationship between dopamine (DA) neuron firing and positive reinforcement, observing that optical stimulation of DA neurons in the ventral tegmental area (VTA) of Th::Cre rats is sufficient to support vigorous intracranial self-stimulation (ICSS). These studies complement existing targeting approaches by extending generalizability of optogenetics to traditionally non-genetically-tractable but vital animal models.

Background Addictions are defined by a loss of flexible control over behavior. The development of response habits may reflect early changes in behavioral control. The following experiments examined the flexibility of alcohol-seeking following different durations of self-administration training and tested the role of the dorsal striatum in the control of flexible and habitual alcohol self-administration. Methods Rats were trained to lever-press to earn unsweetened ethanol (10%). The sensitivity of the lever-press response to devaluation was assessed by prefeeding the rats either ethanol or sucrose prior to an extinction test following different amounts of training (1,2,4, and 8 weeks). We subsequently tested the role of the dorsomedial striatum (DMS) and dorsolateral striatum (DLS) in controlling alcohol seeking using reversible inactivation techniques (baclofen/muscimol: 1.0/0.1mM, 0.3μl per side). Results We find that operant responding for ethanol early in training is goal-directed and reduced by devaluation, but after 8 weeks of daily operant training, control has shifted to a habit-based system no longer sensitive to devaluation. Further, following relatively limited training, when responding is sensitive to devaluation, inactivation of the DMS greatly attenuates the alcohol-seeking response whereas inactivation of the DLS is without effect. In contrast, responding that is insensitive to devaluation following 8 weeks of training becomes sensitive to devaluation following inactivation of the DLS, but unaffected by inactivation of the DMS. Conclusions These experiments demonstrate that extended alcohol self-administration produces habit-like responding and that response control shifts from the DMS to the DLS across the course of training.

Alcoholism is a devastating disease that manifests as uncontrolled drinking. Consumption of alcohol is regulated by neurochemical systems within specific neural circuits, but endogenous systems that may counteract and thus suppress the behavioral effects of ethanol intake are unknown. Here we demonstrate that BDNF plays a role in reducing the behavioral effects of ethanol, including consumption, in rodents. We found that decreasing the levels of BDNF leads to increased behavioral responses to ethanol, whereas increases in the levels of BDNF, mediated by the scaffolding protein RACK1, attenuate these behaviors. Interestingly, we found that acute exposure of neurons to ethanol leads to increased levels of BDNF mRNA via RACK1. Importantly, acute systemic administration of ethanol and voluntary ethanol consumption lead to increased levels of BDNF expression in the dorsal striatum. Taken together, these findings suggest that RACK1 and BDNF are part of a regulatory pathway that opposes adaptations that lead to the development of alcohol addiction.

Environmental cues, through Pavlovian learning, become conditioned stimuli that guide animals toward the acquisition of rewards (for example, food) that are necessary for survival. We tested the fundamental role of midbrain dopamine neurons in conferring predictive and motivational properties to cues, independent of external rewards. We found that brief phasic optogenetic excitation of dopamine neurons, when presented in temporal association with discrete sensory cues, was sufficient to instantiate those cues as conditioned stimuli that subsequently both evoked dopamine neuron activity on their own and elicited cue-locked conditioned behavior. Notably, we identified highly parcellated functions for dopamine neuron subpopulations projecting to different regions of striatum, revealing dissociable dopamine systems for the generation of incentive value and conditioned movement invigoration. Our results indicate that dopamine neurons orchestrate Pavlovian conditioning via functionally heterogeneous, circuit-specific motivational signals to create, gate, and shape cue-controlled behaviors.

This study compared the contribution of the general activating and specific cueing properties of Pavlovian stimuli to Pavlovian-instrumental transfer (PIT) and the role of the ventral tegmental area (VTA) in mediating these effects. In Experiment 1, hungry rats initially received Pavlovian training, in which three distinct auditory stimuli predicted the delivery of three different food outcomes. Next, the rats were trained to perform two instrumental actions, each earning a unique outcome selected from the three used in Pavlovian conditioning. Finally, the effects of the three stimuli on performance of the two actions were assessed in extinction. Presentation of a stimulus that had been paired with the same outcome as an action increased its performance relative to the other action, demonstrating that PIT effects can be outcome selective. In contrast, presentation of the stimulus that predicted the outcome that was not earned during instrumental training facilitated the performance of both actions indiscriminately. This effect, but not the outcome-selective effect, was abolished by a shift from a hungry to a relatively sated state. Experiment 2 examined the effects of inactivation of the VTA on these two forms of PIT. VTA inactivation was found to attenuate PIT but, unlike satiety, did not appear to differentially affect the general or the outcome-selective forms of PIT. The VTA appears therefore to play an important but general role in the initiation of instrumental actions, enabling cues to influence performance whether they enhance responding by changes in arousal or by retrieving particular actions based on their consequences.

Addiction is characterized by compulsive alcohol or drug taking and seeking, and the dorsal striatum has been implicated in such maladaptive persistent habits. The NMDA receptor (NMDAR), which is a major target of alcohol, is implicated in striatal-based habit learning. We found that, in the dorsal striatum, alcohol (ethanol) exposure produced an increase in the phosphorylation of the NR2B subunit of the NMDAR, and a corresponding increase in the activity of Fyn kinase, which phosphorylates NR2B. We further observed an ethanol-mediated long-term facilitation (LTF) of the activity of NR2B-containing NMDARs (NR2B-NMDARs) in the dorsal striatum. This LTF is Fyn kinase dependent, because it was observed in Fyn wild-type but not in Fyn knock-out mice. Importantly, none of these biochemical and physiological changes was observed in the ventral striatum. Finally, dorsal but not ventral striatum infusion of a Fyn or NR2B-NMDAR inhibitor reduced rat operant self-administration of ethanol. Our results suggest that the Fyn-mediated phosphorylation and LTF of NR2B-NMDAR activity in the dorsal striatum after exposure to ethanol may underlie aberrant plasticity that contributes to mechanisms underlying alcohol drinking behavior.