Effectiveness of intraoperative use of dexmedetomidine in reducing the incidence of tachyarrhythmia after congenital cardiac surgery in neonates and infants: a doubly robust method estimation analysis.

OBJECTIVES: The antiarrhythmic effects of dexmedetomidine (DEX) have been suggested, but there are controversial reports on the effectiveness of intraoperative use of DEX to reduce the incidence of postoperative tachyarrhythmia (POT). METHODS: From a local European Congenital Heart Surgery Association database, we included patients operated for congenital heart diseases under cardiopulmonary bypass within a 5-year period (2017-2021), during which intraoperative use of high dose of DEX (1-1.4 µg/kg/h) was implemented. A doubly robust matching estimation of the causal effect of DEX on the incidence of POT was conducted. We combined a multimodal estimation model in patients not exposed to DEX (disease risk score) as well as a regression analysis in a matched cohort for patients exposured to DEX. RESULTS: From a cohort of 593 surgeries (514 patients) occurring during the study period, doubly matched analysis consisted of the analysis of 426 surgeries conducted under DEX or not (213 per group). The probability of developing POT in patients exposed to DEX was 6.6% (95% confidence interval 0.032-0.099) vs 14.5% (95% confidence interval 0.098-0.193) in the group of patients not exposed to DEX. The doubly robust matched estimation method showed a mean reduction of 8.8% (95% confidence interval -0.137 to -0.023) of POT when DEX is used for intraoperative anaesthesia. CONCLUSIONS: The use of high doses of DEX during anaesthesia for congenital heart surgery in neonates and infants is associated with a moderate but significant reduction of POT.

Activation-dependent subcellular distribution patterns of CB1 cannabinoid receptors in the rat forebrain.

Chronic cannabinoid exposure results in tolerance due to region-specific desensitization and down-regulation of CB1 cannabinoid receptors (CB1Rs). For most G-protein-coupled receptors, internalization closely follows rapid desensitization as an important component of long-term down-regulation. However, in vivo patterns of CB1R internalization are not known. Here we investigate the subcellular redistribution of CB1Rs in the rat forebrain following activation by agonist CP55 940 or inhibition by antagonist/inverse agonist AM251. At steady state, CB1Rs are mainly localized to the cell membrane of preterminal axon shafts and, to a lesser degree, to synaptic terminals. A high proportion of CB1Rs is also localized to somatodendritic endosomes. Inhibition of basal activation by acute AM251 administration decreases the number of cell bodies containing CB1R-immunoreactive endosomes, suggesting that CB1Rs are permanently activated and internalized at steady state. On the contrary, acute agonist treatment induces rapid and important increase of endosomal CB1R immunolabeling, likely due to internalization and retrograde transport of axonal CB1Rs. Repeated agonist treatment is necessary to significantly reduce initially high levels of axonal CB1R labeling, in addition to increasing somatodendritic endosomal CB1R labeling in cholecystokinin-positive interneurons. This redistribution displays important region-specific differences; it is most pronounced in the neocortex and hippocampus and absent in basal ganglia.

Axonal targeting of the 5-HT1B serotonin receptor relies on structure-specific constitutive activation.

By analogy to other axonal proteins, transcytotic delivery following spontaneous endocytosis from the somatodendritic membrane is expected to be essential for polarized distribution of axonal G-protein coupled receptors (GPCRs). However, possible contribution from constitutive activation, which may also result in constitutive GPCR endocytosis, is poorly known. Using two closely related but differentially distributed serotonin receptors, here we demonstrate higher constitutive activation and spontaneous endocytosis for the axonal 5-HT(1B) R, as compared to the somatodendritic 5-HT(1A) R, both in non-neuronal cells and neurons. Activation-dependent constitutive endocytosis is crucial for axonal targeting, because inverse-agonist treatment, which prevents constitutive activation, leads to atypical accumulation of newly synthesized 5-HT(1B) Rs on the somatodendritic plasma membrane. Using receptor chimeras composed of different domains from 5-HT(1A) R and 5-HT(1B) R, we show that the complete third intracellular loop of 5-HT(1B) R is necessary and sufficient for constitutive activation and efficient axonal targeting, both sensitive to inverse-agonist treatment. These results suggest that activation and targeting of 5-HT(1B) Rs are intimately interconnected in neurons.