Role of 5-HT1A receptor-mediated serotonergic transmission in the medial prefrontal cortex in acute restraint stress-induced augmentation of rewarding memory of cocaine in mice.

Stress enhances cocaine craving. We recently reported that acute restraint stress increases cocaine conditioned place preference (CPP) in mice; however, the underlying mechanisms remain unclear. This study aimed to examine the role of serotonergic transmission in the medial prefrontal cortex (mPFC) in cocaine CPP enhancement by acute restraint stress, which increases extracellular serotonin (5-HT) levels in the mPFC. Intra-mPFC infusion of the selective serotonin reuptake inhibitor (S)-citalopram prior to the test session significantly increased the cocaine CPP score under non-stressed conditions. This is indicative of the substantial role of increased mPFC 5-HT levels in cocaine CPP enhancement. Moreover, intra-mPFC and systemic administration of the 5-HT1A receptor antagonist WAY100635 immediately before restraint stress exposure significantly attenuated stress-induced cocaine CPP enhancement. Our findings suggest that enhanced serotonergic transmission via 5-HT1A receptors in the mPFC is involved in acute stress-induced augmentation of rewarding memory of cocaine; moreover, the 5-HT1A receptor could be a therapeutic target for stress-induced cocaine craving.

mTORC1 Activation in Osteoclasts Prevents Bone Loss in a Mouse Model of Osteoporosis.

The mechanistic/mammalian target of rapamycin (mTOR) is widely implicated in the pathogenesis of various diseases, including cancer, obesity, and cardiovascular disease. Bone homeostasis is maintained by the actions of bone-resorbing osteoclasts and bone-forming osteoblasts. An imbalance in the sophisticated regulation of osteoclasts and osteoblasts leads to the pathogenesis as well as etiology of certain metabolic bone diseases, including osteoporosis and osteopetrosis. Here, we identified mTOR complex 1 (mTORC1) as a pivotal mediator in the regulation of bone resorption and bone homeostasis under pathological conditions through its expression in osteoclasts. The activity of mTORC1, which was indicated by the phosphorylation level of its downstream target p70S6 kinase, was reduced during osteoclast differentiation, in accordance with the upregulation of Hamartin (encoded by tuberous sclerosis complex 1 [Tsc1]), a negative regulator of mTORC1. Receptor activator of nuclear factor-κB ligand (RANKL)-dependent osteoclastogenesis was impaired in Tsc1-deficient bone marrow macrophages. By contrast, osteoclastogenesis was markedly enhanced by Raptor deficiency but was unaffected by Rictor deficiency. The deletion of Tsc1 in osteoclast lineage cells in mice prevented bone resorption and bone loss in a RANKL-induced mouse model of osteoporosis, although neither bone volume nor osteoclastic parameter was markedly altered in these knockout mice under physiological conditions. Therefore, these findings suggest that mTORC1 is a key potential target for the treatment of bone diseases.

The L-type amino acid transporter LAT1 inhibits osteoclastogenesis and maintains bone homeostasis through the mTORC1 pathway.

L-type amino acid transporter 1 (LAT1), which is encoded by solute carrier transporter 7a5 (Slc7a5), plays a crucial role in amino acid sensing and signaling in specific cell types, contributing to the pathogenesis of cancer and neurological disorders. Amino acid substrates of LAT1 have a beneficial effect on bone health directly and indirectly, suggesting a potential role for LAT1 in bone homeostasis. Here, we identified LAT1 in osteoclasts as important for bone homeostasis. Slc7a5 expression was substantially reduced in osteoclasts in a mouse model of ovariectomy-induced osteoporosis. The osteoclast-specific deletion of Slc7a5 in mice led to osteoclast activation and bone loss in vivo, and Slc7a5 deficiency increased osteoclastogenesis in vitro. Loss of Slc7a5 impaired activation of the mechanistic target of rapamycin complex 1 (mTORC1) pathway in osteoclasts, whereas genetic activation of mTORC1 corrected the enhanced osteoclastogenesis and bone loss in Slc7a5-deficient mice. Last, Slc7a5 deficiency increased the expression of nuclear factor of activated T cells, cytoplasmic 1 (Nfatc1) and the nuclear accumulation of NFATc1, a master regulator of osteoclast function, possibly through the canonical nuclear factor κB pathway and the Akt-glycogen synthase kinase 3ß signaling axis, respectively. These findings suggest that the LAT1-mTORC1 axis plays a pivotal role in bone resorption and bone homeostasis by modulating NFATc1 in osteoclasts, thereby providing a molecular connection between amino acid intake and skeletal integrity.