Abstract

Environmental stimuli that predict alcohol are powerful triggers for relapse in humans. These stimuli become established as alcohol cues through Pavlovian conditioning and can be grouped into discrete cues (e.g. smell, sight, taste of alcohol) and contexts (e.g. bar). In the current thesis, a Pavlovian conditioning with context alternation procedure was used to measure the capacity for an alcohol-associated context (alcohol context) to influence responding to a discrete alcohol conditioned stimulus (CS). Responding to a discrete alcohol CS was elevated in an alcohol context compared to an equally familiar neutral context. By incorporating other behavioural procedures, the influence of context over responding to a discrete alcohol CS was shown to be persistent and temporally sensitive.

Pharmacology and chemogenetic approaches revealed that dopamine neurotransmission and the activity of ventral tegmental area (VTA) dopamine neurons was necessary for responding to a discrete alcohol CS in a neutral context. Further, chemogenetic inhibition of the dopaminergic projection from the VTA to the nucleus accumbens (NAc) core reduced CS responding in the alcohol and neutral context. Differently, chemogenetic inhibition of the dopaminergic projection from the VTA to the NAc shell selectively reduced CS responding in the alcohol context, and not in the neutral context. Thus, the dopaminergic VTA-to-NAc core projection was necessary for responding to a discrete alcohol CS whereas the dopaminergic VTA-to-NAc shell projection was necessary for the elevation of this behaviour in an alcohol context.

Targeting of tyrosine hydroxylase (TH) positive VTA dopamine neurons and the use of chemogenetics to manipulate neural activity in a circuit-specific manner were validated using immunocytochemistry and electrophysiology, respectively. Chemogenetic designer receptors were selectively (~95%) expressed in TH positive VTA neurons. Chemogenetic inhibition or excitation of VTA dopamine terminals in the NAc modulated the amplitude of electrically-evoked excitatory postsynaptic currents in NAc core medium spiny neurons (MSNs). Thus, a circuit-specific chemogenetic approach could be used to target VTA dopamine neurons and bidirectionally affect the activity of postsynaptic MSNs.

Based on the finding that the VTA-to-NAc shell projection was necessary for the elevation of CS responding in the alcohol context, the sufficiency of activating VTA dopamine neurons and their projection to the NAc shell to drive CS responding was tested. Chemogenetic activation of VTA dopamine neurons failed to impact responding to a discrete alcohol CS in a neutral context, despite affecting feeding behaviour in the same rats. Further, chemogenetic activation of the dopaminergic VTA-to-NAc shell projection failed to affect CS responding in the alcohol and neutral context. Thus, chemogenetic activation of VTA dopamine neurons or their projections to the NAc shell was not sufficient to elevate responding to a discrete alcohol CS.

The lack of an effect of activating dopaminergic substrates on responding to a discrete alcohol CS suggested that non-dopaminergic circuits are recruited to elevate responding to a discrete alcohol CS in an alcohol context. This hypothesis was examined by microinfusing a glutamate-like agonist in the NAc shell during tests for CS responding in the neutral and alcohol context. Responding to a discrete alcohol CS was selectively reduced by microinfusion of a glutamate-like agonist in the alcohol context and was unaffected in the neutral context. This selective effect on CS responding in the alcohol context implicates glutamatergic activity in the NAc shell in the influence of context over responding to a discrete alcohol CS.

In summary, responding to a discrete alcohol CS is elevated in an alcohol context and the influence of context over this behaviour is persistent and temporally sensitive. Responding to a discrete alcohol CS requires the activity of VTA dopamine neurons and their projections to the NAc core. The elevation of responding to a discrete alcohol CS in an alcohol context requires the dopaminergic projection from the VTA to the NAc shell. Attempts to elevate responding to a discrete alcohol CS by activating dopaminergic or glutamatergic substrates either failed to affect or reduced CS responding, respectively. Thus, the circuits sufficient to elevate responding to a discrete alcohol CS may involve the coordinated activation of dopaminergic and glutamatergic inputs to the NAc shell. Ultimately, a detailed description of the circuits that mediate the influence of discrete cues and contexts over responding for alcohol will inform the development of efficacious behavioural and pharmacological therapies for alcohol use disorder.