Substance P modulation of DAMGO binding in the brain of CXBK and Swiss-Webster mice

The effects of substance P (SP) on the binding of the selective μ opioid agonist [³H]DAMGO to brain membranes of CXBK and Swiss-Webster (SW) mice were compared. We have previously shown that subnanomolar concentrations of SP and N-terminal fragments of SP modulate DAMGO binding in SW brain membranes and hypothesized that modulation occurs via SP interaction with μ₁ sites. In the present study, binding assays using CXBK mice, a strain deficient in μ receptors including μ₁ sites, were performed to assess the effect of μ receptor deficiency on SP-induced modulation of DAMGO binding. Whereas the addition of 0.1 nM SP to the binding mixtures produced up to 30% increase in the values of K_d and maximum binding capacity (R) for the SW strain, SP produced little or no change in the case of CXBK strain. Maximum binding capacity for DAMGO was 43% less in the brain of CXBK mice than in SW mice. No difference was observed in the estimated binding parameters of the spinal cord for the two strains. Whereas pretreatment of brain membranes of SW mice using β-funaltrexamine (β-FNA) increased from 2- to 10-fold the modulatory effect of SP, CXBK brain membranes pretreated with β-FNA remained nearly insensitive to modulation by SP. The effect of SP on the affinity of DAMGO binding in SW mice, but not in CXBK mice, was reversed by the addition of GTP. It is concluded that μ receptor deficiency can markedly influence SP-induced modulation of DAMGO binding.