Background

Adaptive behaviors shaped by past experiences are essential for animal survival, as they enable flexible responses to changing and potentially threatening environments. Noxious stimuli such as pain are commonly used to induce aversive learning and have been extensively studied as instructive signals. Pavlovian threat conditioning is a well-established behavioral paradigm to explore the neural mechanisms underlying associative aversive learning [1–3]. In Pavlovian threat conditioning, sensory signals with negative valence function as an unconditioned stimulus (US) to induce learning when paired with an emotionally neutral conditioned stimulus (CS). Previous studies have shown that the lateral parabrachial nucleus (PB) in the pons receives nociceptive signals from the dorsal horn and sends direct projections to the central amygdala (CeA) [4–6]. This PB-CeA pathway is vital for transmitting aversive signals that act as the US in aversive learning [7–12].

Long-term potentiation (LTP) of synaptic transmission is widely recognized as a fundamental neuronal mechanism that drives learning and memory. Synaptic transmission in the PB-CeA pathway is potentiated in rodent pain models, as well as following repeated exposure to aversive stimuli [13–23]. However, the specific causal role of the LTP within the US pathway, the PB-CeA, in associative aversive learning has remained unclear, leaving its physiological significance in adaptive behavioral regulation unresolved. Recently, we developed pathway-specific in vivo LTP induction methods and mathematical models. Using this approach, we demonstrated that LTP in the PB-CeA pathway enhances negative value and thereby updates future learning rules [24].

Here, we present an efficient protocol to combine optogenetic in vivo LTP induction and mathematical modeling to investigate value plasticity and learning-rule changes in mice. The pathway-specific in vivo LTP protocol enables causal investigation of the relationship between pathway-specific synaptic plasticity in the PB-CeA pathway and behavioral outputs. To construct the mathematical model, the Rescorla–Wagner model [25], a framework that describes associative learning in classical conditioning, was employed. Furthermore, by integrating behavioral data into mathematical models based on the Rescorla–Wagner model, this approach enables investigations of experience-dependent alterations in emotional value and learning rules driven by pathway-specific synaptic plasticity. Accordingly, this protocol may serve as a framework for understanding the value plasticity of the instructive signal in various organisms and for the development of treatments for psychiatric disorders, such as post-traumatic stress disorder (PTSD).