Drug seeking behaviour is substantially influenced by drug-paired conditioned stimuli (CSs). The unexpected presentation of these Pavlovian CSs does not only immediately grab attention, but it also generates arousal and motivational states manifested as approach behaviour, and invigoration of actions aimed at procuring the drug. When they are response produced CSs act as conditioned reinforcers, which invigorate seeking behaviours over extended periods, thereby bridging delays to the procurement of the outcome. The acquisition of cue-controlled drug-seeking depends critically on the basolateral amygdala (BLA) whereas its well-established performance relies on the central nucleus of the amygdala (CeN). However, how, and why this shift from BLA to CeN occurs has not been elucidated. Using behavioural models of drug taking and seeking, I aimed to elucidate the cellular plasticity mechanisms in the amygdala that underlie this functional shift. The General Introduction in Chapter 1 gives an overview of the literature informing the approach I took, the methodology of which is presented in Chapter 2, The General Methods.

In Chapter 3, I present, and justify the choice of, the different behavioural manipulations to which rats in these experiments were exposed, namely, cocaine self-administration and cue-controlled cocaine seeking (CCCS), as measured after short or prolonged exposure to a second order schedule of reinforcement (SOR), respectively.

Using the behavioural models introduced in Chapter 3, in Chapter 4, I demonstrated electrophysiologically that the BLA undergoes significant changes in synaptic and intrinsic plasticity during the development of a well-established CCCS habit (incentive habit): while it is highly active during the initial stages of the acquisition of CCCS, it shows little synaptic activity and low excitability after a prolonged history of SOR. The CeN, however, does not show any changes in synaptic or intrinsic plasticity after performance of these behaviours, indicating that it only takes control over the behaviour as the BLA becomes disengaged during the long-term expression of CCCS.

In order to identify potential molecular mechanisms underlying these changes, I then investigated both pre- and postsynaptic adaptations in the BLA. Therefore, in Chapter 5, I used qPCR and Western Blots to assess changes in expression of postsynaptic receptors and ion channels involved in synaptic and intrinsic plasticity. I show that BLA glutamate and potassium channel expression changes across the acquisition and maintenance of CCCS, in line with my observations from slice electrophysiology.

However, there may also be presynaptic adaptations that could be driving this plasticity. Hence, in Chapter 6, I investigated the extent to which plasticity in the BLA is driven by changes in activity in areas of the prefrontal cortex (PFC) from which it receives inputs, namely the orbitofrontal cortex, the prelimbic cortex, and the infralimbic cortex. Using the immediate early gene cFos as a marker of cellular activity, I performed RNAscope on brain sections encompassing the PFC to explore the behavioural relevance of specific PFC-BLA projections. Taken together the results presented in this work shed light on the mechanisms that may cause the shift in control over behaviour from the BLA to the CeN over the course of the development of an incentive cocaine seeking habit, thereby opening new avenues for the development of new therapeutic strategies for the treatment of substance use disorders.