CNR1 gene deletion affects the density of endomorphin-2 binding sites in the mouse brain in a hemisphere-specific...

Endomorphin-1 (EM-1) and endomorphin-2 (EM-2) are two endogenous tetrapeptides with very high affinities for the μ-opioid receptor. Until recently, the precise neuroanatomical localization of the binding sites for these peptides was unknown. However, the recent synthesis of tritiated forms of these molecules has permitted these binding sites to be analysed with a very high degree of neuroanatomical specificity. Preliminary studies demonstrated a superior binding profile for EM-2, with less non-specific binding than EM-1. As the endogenous cannabinoid and opioid systems interact at several levels, we investigated how deletion of the CNR1 gene, which encodes the cannabinoid receptor 1 (CB(1)R) protein, affects the brain distribution of EM-2 binding sites. Our results revealed no differences in the average density of EM-2 binding sites in CB(1) receptor knockout (CB(1)R KO) and WT mice. However, when both hemispheres were analysed separately, we detected specific alterations in the distribution of EM-2 binding sites in the right hemisphere of CB(1)R KO mice. While, the density of EM-2 binding sites in CB(1)R KO mice was higher in the CA3 hippocampal field and in the pontine tegmental nuclei, it was lower in the superior colliculus and ventral tegmental area than in WT controls. No differences were observed in the left hemisphere for any of the regions analysed. For the first time these findings demonstrate a lateralization effect on cerebral opioid binding sites that may be mediated by the central cannabinoid system.