Yijun-Cui et al, University of California, Los Angeles :: www.nature.com/neuro :: Nature Neuroscience, Vol. 17, February 2014
Summary and review of the above paper
A sub-population of the striatal direct-pathway neurons, known for being connected to voluntary movement, can be the basis of opiate-reward driven activities. The brain’s opioid system is basic to the reward value of stimuli and consequent behaviours. Opioid receptors bind to the brain’s opioid peptides such as enkephalin, β-endorphin, or dynorphin. μ-opioid receptors act to suppress the neuronal activity of neurons that otherwise inhibit the release of dopamine from ventral tegmentum and substantia nigra neurons.
This study assessed the importance of μ-opioid receptors in the medium spiny neurons of the direct-pathway of the striosome and nucleus accumbens. The existence of such receptors in this direct-pathway is sufficient for opiate related behaviours. However, the study suggested that this brain region was not responsible for the analgesic effects of opiates, nor the symptoms involved in withdrawing from the use of opiates.
The release of dopamine triggered by opiates from the ventral tegmentum and substantia nigra is seen as important for the reward process. Striatal and nucleus accumbens dopamine transmission is connected to opiate reinforcement. However, other populations of μ-opioid receptors outside of the striosome and nucleus accumbens may also have a role in opiate reward. Opiates are known to provide reward through both dopamine and other mechanisms.