Summary: Animals differ in how they respond to cues that predict rewards. Some animals direct their attention to the predictive stimulus itself (sign trackers), while others go straight to the location where the reward will appear (goal trackers). New research from the University of Pittsburgh shows that sign tracking depends on dopamine signaling in a specific brain circuit, whereas goal tracking relies on a dopamine-independent process. These findings offer insight into why certain individuals may be more susceptible to impulsive behavior and addiction-related vulnerabilities.
Using optogenetics in rats, researchers manipulated dopamine neurons in the ventral tegmental area (VTA) at the exact moment the reward was delivered. Temporarily inhibiting dopamine release at reward time prevented animals from learning to approach the cue, while increasing dopamine did not accelerate sign-tracking learning and produced only transient effects. By contrast, neither inhibiting nor enhancing dopamine affected goal-tracking behavior, indicating two distinct neural mechanisms for cue-reward learning.
Key Facts:
- Sign tracking and goal tracking reflect fundamentally different learning systems in the brain.
- Dopamine signaling in the nucleus accumbens and VTA is necessary for acquiring sign-tracking behavior but not for goal tracking.
- Understanding these pathways clarifies biological risk factors for impulsivity, risk-taking, and relapse in substance use disorders.
Source: University of Pittsburgh
Most students know the classical Pavlov example: dogs learned to associate a bell with food and began to salivate at the sound alone. But animals do not always respond the same way to a conditioned cue. Some will approach and even interact with the cue itself—pawing, sniffing or nibbling it—before attending to the food reward. These animals are called sign trackers. Others, known as goal trackers, move directly to the place where the reward will appear when the cue is presented.
In a study published in the Journal of Neuroscience, Sara Morrison and colleagues demonstrated that sign tracking and goal tracking are produced by separate learning mechanisms. Specifically, sign tracking requires dopamine in the nucleus accumbens (NAc) at the time the reward is received, whereas goal tracking does not depend on this dopamine signal.
The research team used genetically modified TH::Cre rats expressing light-sensitive proteins in dopamine neurons, allowing precise optogenetic control of VTA dopamine activity. By either inhibiting or stimulating those neurons during reward delivery, researchers could observe how changes in dopamine release affected both behavior and neural activity in the NAc.
When dopamine neurons were inhibited at reward time, rats failed to develop sign-tracking behavior even though goal-tracking behavior was unaffected. After inhibition ceased, some animals resumed sign tracking over subsequent days, indicating that the dopamine signal is critical for acquiring the behavior but not for its expression once established. Conversely, artificially boosting dopamine at the time of reward did not speed the acquisition of sign tracking; when the extra stimulation stopped, acquisition was temporarily impaired, suggesting the learning system scales to the expected level of reward-related dopamine.
Electrophysiological recordings revealed that both inhibition and stimulation of VTA dopamine neurons rapidly altered activity in a subset of nucleus accumbens neurons and shifted cue- and reward-related firing across training sessions. These neural changes parallel the behavioral effects and support a causal link among VTA dopamine activity, NAc encoding, and sign-tracking learning.
The distinction between dopamine-dependent sign tracking and dopamine-independent goal tracking has important implications for understanding motivated behavior. Sign tracking is associated with stronger attraction to reward cues, greater persistence of cue-driven responses after reward removal, and links to impulsivity, risk-taking, and susceptibility to relapse. Identifying the circuit dynamics that make cue-driven responses “sticky” could help target interventions for disorders in which maladaptive cue attraction drives harmful behavior.
About this neuroscience research news
Author: Brandie Jefferson
Source: University of Pittsburgh
Contact: Brandie Jefferson – University of Pittsburgh
Image: Image credited to Neuroscience News
Original Research: Closed access. “Modulation of Dopamine Neurons Alters Behavior and Event Encoding in the Nucleus Accumbens during Pavlovian Conditioning” by Sara Morrison et al., Journal of Neuroscience. DOI available in the original publication.
Abstract
Modulation of Dopamine Neurons Alters Behavior and Event Encoding in the Nucleus Accumbens during Pavlovian Conditioning
When a predictive cue is spatially separated from its associated reward, animals can develop different conditioned responses: approaching the cue itself (sign tracking) or approaching the reward location (goal tracking). Previous work suggested that reward-evoked activity in the nucleus accumbens core varies between sign trackers and goal trackers and might reflect distinct dopamine release patterns. However, a causal chain linking dopamine release, NAc activity, and sign tracking had not been established.
Using male and female TH::Cre rats with inhibitory or excitatory opsins expressed in VTA dopamine neurons, the study tested how optical manipulation of dopamine at reward time affects both behavior and NAc neuronal encoding. Inhibiting VTA dopamine neurons at reward delivery suppressed acquisition of sign tracking but left goal tracking intact. Stimulating VTA dopamine neurons did not accelerate sign-tracking acquisition; paradoxically, stopping stimulation temporarily impeded further acquisition. Both inhibition and stimulation caused rapid changes in activity of a subset of NAc neurons and altered cue- and reward-related neural responses across sessions.
These results support the conclusion that sign tracking and goal tracking reflect two distinct learning processes—one dependent on dopamine signaling in the NAc and one independent of it—and that VTA dopamine influences sign tracking in part by modulating NAc core activity.