The amygdalar opioid system.

This chapter reviews the anatomy and physiological effects of the opioid peptides and their receptors in the amygdala, and behavioral responses during genetic or pharmacological manipulations of the amygdalar opioid system in preclinical studies and what is known from human imaging studies. The amygdalar opioid system consists of the neuropeptides enkephalin, dynorphin, and β-endorphin, and their receptors, which are localized throughout the amygdala. Enkephalins activate both mu and delta-opioid receptors (MOR, DOR), while dynorphin activates kappa opioid receptors (KOR). Opioid receptor activation has postsynaptic inhibitory effects on selected neuronal populations in the amygdala, as well as presynaptically inhibiting both GABA and glutamate release. The amygdalar opioid system plays important roles in nociception, stress and anxiety-related responses, associative learning and conditioned fear, ethanol effects, and opiate dependence or withdrawal. Historically studies focused on enkephalin and MOR-mediated effects, although roles for the dynorphin/KOR system and actions of enkephalin via DOR are emerging.