Increased expression of cannabinoid CB2 and serotonin 5-HT1A heteroreceptor complexes in a model of newborn hypoxic-ischemic brain damage.

Preclinical work shows cannabidiol as a promising drug to manage neonatal hypoxic-ischemic brain damage (NHIBD). The molecular mechanism is not well defined but the beneficial effects of this phytocannabinoid are blocked by antagonists of both cannabinoid CB2 (CB2R) and serotonin 5-HT1A (5-HT1AR) receptors that, in addition, may form heteromers in a heterologous expression system. Using bioluminescence energy transfer, we have shown a direct interaction of the two receptors that leads to a particular signaling in a heterologous system. A property attributed to the heteromer, namely cross-antagonism, was found in primary cultures of neurons thus indicating the occurrence of the receptor heteromer in the CNS. Oxygen-glucose deprivation to neurons led to an increase of CB2R-mediated signaling and an upregulation of CB2-5-HT1A heteroreceptor complex expression. In situ proximity ligation assays in brain cortical sections were performed to compare the expression of CB2-5-HT1A complexes in rat E20 fetuses and at different postnatal days. The expression, which is elevated in fetus and shortly after birth, was sharply reduced at later ages (even at P7). The expression of heteromer receptors was more marked in a model of NHIBD and, remarkably, the drop in expression was significantly delayed with respect to controls. These results indicate that CB2-5-HT1A heteroreceptor complex may be considered as a target in the therapy of the NHIBD. This article is part of the Special Issue entitled 'Receptor heteromers and their allosteric receptor-receptor interactions'.