Abstract

Exploring the molecular genetic cascades responsible for behavioral responses to opioids can improve our understanding of drug use initiation. We generated high-precision time-series data for 105 morphine- and naloxone-related traits across [~]700 young adult BXD mice (64 diverse strains and both sexes) for 3 hours after a single morphine injection. Variations in responses were mapped using high precision sequencing-based genotypes. The initial locomotor responses to morphine map precisely to the {micro} opioid receptor gene (MOR or Oprm1) on chromosome (Chr) 10 with a peak linkage of 12.4 (-log10P). The B allele inherited from C57BL/6J is associated with up to 60% higher activity. This effect climaxes at 75 min but is exhausted by 160 min. A second major modulator of opioid activation emerges after about 100 min and is located on Chr 16 with peak linkages of 10.6 (-log10P) in females, also associated with a high B allele. This locus includes only one compelling candidate--fibroblast growth factor 12 (Fgf12), a 600 Kb gene that controls sodium current kinetics at the axon hillock. A strong and transient epistatic interaction exists between the Oprm1 and Fgf12 loci during a short time window (45-75 min). The combination of a B haplotype at Oprm1 with a D haplotype from DBA/2J at Fgf12 is associated with unusually high activity. In a complementary study in heterogeneous stock rats we demonstrate that Oprm1 and Fgf12 are co-expressed in one specific subtype of Drd1+ medium spiny neuron. A Bayesian network analysis supports an Oprm1-to-Fgf12 network that involves a MAP kinase cascade--Mapk8ip2, Map3k11, and Map3k12--that we hypothesize modulates FGF12 phosphorylation, Nav1.2 sodium channel state, and locomotor activation. OPRM1 and FGF12 networks in human GWAS data highlight enrichment of signals associated with substance use disorder. This is the first demonstration of a time-dependent epistatic interaction modulating drug response in mammals and the first linkage of Fgf12 to opioid sensitivity and potentially to sodium channel activity.