The answer is B. The neuropeptide galanin acts as a modulator of neurotransmission in the CNS and PNS. It is coexpressed with different neurotransmitters and coreleased by the major ascending noradrenergic, serotoninergic, histaminergic, and cholinergic systems. A recent study showed that heteromers between galanin Gal1 receptor and the mu-opioid receptor attenuate the potency of methadone, but not other opioids, in stimulating dopamine release in the ventral tegmental area. Such release may contribute to the euphoric effects of other opioids and fewer “high” feelings on methadone.

Publication reference : Cai N et al. Opioid–galanin receptor heteromers mediate the dopaminergic effects of opioids. J Clin Invest 2019; 129: 2730-44.
Reference URL: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6597217/