Ralph U. Esposito National Institute of Mental Health
Recent advances in neuroanatomy, electrophysiology, and neuropharmacology have contributed enormously to our understanding of the neural structures that support rewarding brain stimulation in animals and of their functions during more naturalistic behaviors. It has been proposed that these so-called "reward pathways" constitute a brain "system" that appears to have evolved to amplify the effects of biologically significant stimuli and related responses in order to modify the organism's adaptive repertoire. This biological function would of necessity be related to the psychological constructs of attention, reward, and memory.
Various biochemical, anatomical, and pharmacological studies have indicated a critical role for the central catecholamines, particularly dopamine, in the mediation of brain stimulation reward. The evidence for dopamine involvement in the mediation of mesotelencephalic self-stimulation is particularly cogent. It is hypothesized herein that these ventral midbrain dopaminergic neurons, in conjunction with opiate-mu receptors, function to bias the sensitivity of certain forebrain structures (i.e. frontal cortex, cingulate cortex, amygdala) involved in the integration of higher order perceptual input with information concerning stimulus significance. This "reward-selective" attentional process serves to activate the organism to orient and respond to motivationally significant stimuli. Sustained activity in dopaminergic pathways, in conjunction with opiate-delta neurons, provides immediate feedback for continuation of behavioral sequences that result in positive hedonic consequences. Thus, opiate-mu receptors may function as filters in "reward- selective" attention for external stimuli whereas opiate-delta receptors may be involved in the concurrent filtering for internal stimuli of significance based on past associations and internal drive states. This latter