plausibly represents one of the molecular mechanisms underlying the excitatory effect of opiods, contributing to tolerance, drug dependence and opioid-induced hyperalgesia (60–62). Furthermore, similarities in the tissue-specific expression patterns between MOR-1 and MOR-1K, with lower abundance of MOR-1K relative to MOR-1 isoforms, support the possibility of a regulatory function for MOR1-K. To date, no systematic studies involving the genetics, molecular and cell biology of this form of OPRM1 receptor have been reported; and its biological function clearly warrants further research.
