The Role of the Mediodorsal Nucleus of the Thalamus in Context-dependent Learning

Context has an important influence on voluntary behaviors. For example, drug use is more likely in certain settings due to the associations that can be made between the environment and drug taking behaviors. Our lab’s previous research has shown that the medial prefrontal cortex (mPFC) supports voluntary behavior in the context in which it is first learned. In rats, pharmacological inactivation of the mPFC attenuates responding for a reward only in the context in which the behavior was first learned. Preliminary data from another experiment in our lab suggests that the nucleus reuniens (RE), a thalamic nucleus strongly and reciprocally connected with the mPFC, plays a similar role in modulating context-dependent learning. In the current experiment, we examined the mediodorsal (MD) nucleus of the thalamus, a second thalamic nucleus strongly and reciprocally connected with the mPFC. Rats were conditioned to lever press for a sucrose pellet reward in Context A and exposed to Context B for familiarization. Animals were then tested in both contexts after intra-MD infusion of either muscimol (a GABAA receptor agonist) that temporarily inactivates neurons in the region, or a saline vehicle. Rats were then extinguished in Context B and again tested (using the same design as the first test) in both Context A and B. Inactivation of the MD failed to affect responding in either test. Our results suggest that the MD may not be involved in the contextual modulation of operant learning but further study is needed.