HBK COMPOUNDS RESHAPE GLUTAMATERGIC SYNAPTIC ARCHITECTURE WITHOUT AFFECTING SYNAPTOGENESIS

HBK compounds are methoxyphenylpiperazine derivatives that primarily target serotonergic receptors (5-HT1A and 5-HT7), dopamine D2 receptors, and sigma-1 receptors in the central nervous system. They exhibit antidepressant-like and procognitive effects in rodent models, making them promising candidates for the treatment of depression associated with cognitive impairments.
Co-funded by an Erasmus+ Cooperation Partnership programme, this study investigated a subset of these compounds (HBK-10, HBK-14, HBK-15, HBK-18, and HBK-19) in primary hippocampal cultures to elucidate the cellular and molecular mechanisms underlying their effects at central synapses. To assess their impact on glutamatergic synapse formation and maturation, compounds were applied at 1 µg/ml at 1, 3, and 7 days in vitro (DIV), corresponding to the peak period of synaptogenesis. Cells were fixed at 10 DIV, and immunocytochemistry was performed to detect pre- and postsynaptic markers (VGlut1 and PSD95). Synapse density was quantified by analyzing the colocalization of these markers using MetaMorph software.
None of the tested compounds altered the overall density of glutamatergic synapses formed during the first week in vitro. However, HBK-10, HBK-15, and HBK-18 significantly reduced PSD95 accumulation at postsynaptic sites. Moreover, HBK-15 and HBK-18 also decreased the intensity and size of VGlut1 clusters in presynaptic boutons.
These findings indicate that, in this in vitro model, HBK compounds do not affect glutamatergic synaptogenesis but that specific derivatives (HBK-10, HBK-15, and HBK-18) induce a downscaling of glutamatergic synapses. This modulation of synaptic structure may contribute to understanding how these multimodal ligands influence hippocampal circuit function and cognitive processes in vivo.