Figure 1.

The Brain Reward Cascade with the major neurotransmitter systems involved; green equal sign indicates stimulation and red pound sign stands for inhibition. Stimuli-induced serotonin release in hypothalamus activates 5HT_2a receptors leading to release of hypothalamic opioid peptides. The latter exerts opposite effects via two distinct opioid receptors: (a) inhibition via µ opioid receptor, e.g., GABA_A neurons in substania nigra and (b) stimulation of cannabinoid neurons (e.g., anandamide and 2-archydonoglcerol) via β–endorphin linked δ receptors inhibiting GABA_A neurons at the substania nigra. GABA_A neurons in the substania nigra may be also indirectly disinhibited by cannabinoids, 0 in hedonic and motivational responses [20].