Decreased G-protein coupling of serotonin 5-HT(1A) receptors in the brain of 5-HT(1B) knockout mouse

The firing of central serotonin (5-hydroxytryptamine, 5-HT) neurons and their capacity to release 5-HT are subjected to a receptor-mediated auto-control via 5-HT(1A) and 5-HT(1B) receptors respectively located on the somata/dendrites (5-HT(1A) autoreceptors) and preterminal axon arborizations (5-HT(1B) autoreceptors) of these neurons. To further characterize mutual adaptations of these two receptor subtypes in the absence of one of them, activation of G-protein coupling by agonist was measured and compared to wild-type (WT) in 5-HT(1A) and 5-HT(1B) homozygous knockout (KO) mice. As expected, in WT, the non-selective 5-HT(1A/1B) receptor agonist 5-carboxyamidotryptamine (5-CT) stimulated guanosine 5'-O-(gamma-[(35)S]thio)triphosphate ([(35)S]GTP(gamma)S) incorporation in many brain regions endowed with one and/or the other receptor. In the respective KOs, no stimulation was measured in regions known to express only or mainly the deleted receptor. In the 5-HT(1A) KOs, the amplitude of G-protein activation in regions endowed with 5-HT(1B) receptors was unchanged by comparison to WT. In the 5-HT(1B) KOs, the magnitude of the 5-CT stimulation was the same as WT in all regions containing 5-HT(1A) receptors, except in the amygdala, where it was significantly lower, even if this region was one of the most strongly activated in the WT. A similar result was obtained in the amygdala of 5-HT(1B) KOs after activation by the selective 5-HT(1A) receptor agonist R-(+)8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT). Under these conditions, however, there was in addition a significant lowering of the stimulated (but not basal) [(35)S]GTP(gamma)S incorporation by comparison to WT in all regions endowed with 5-HT(1A) receptors, including the dorsal raphe nucleus. Thus, eventhough agonist radioligand binding to either 5-HT(1A) or 5-HT(1B) receptors is unchanged in the reciprocal KOs, it appears that a compensatory decrease in the efficiency of G-protein coupling to 5-HT(1A) receptors has developed in the 5-HT(1B) mutant. This could represent the first indication of a cross-talk between these two 5-HT receptor subtypes, at least in brain regions where they are co localized in the same neurons.