Excitatory synapse plasticity at postsynaptic dendritic spines is directly modulated through neurotransmitter signaling. Serotonin (5-HT) is a primary neuromodulator in the brain that activates 14 subtypes of G-protein-coupled receptors, each with distinct expression patterns and downstream signaling mechanisms. Disruptions of serotonergic transmission during development and adulthood induce long-lasting behavioral and neuronal changes, often characterized by profound alterations in the structure and function of dendritic spines, suggesting 5-HT plays a critical role in excitatory synaptic plasticity. This review summarizes research demonstrating the importance of 5-HT receptors in the development and maturation of excitatory postsynaptic synapses. We focus on 5-HT receptor-mediated signaling and how different 5-HT receptors influence the various stages in the lifetime of a dendritic spine from spinogenesis to stabilization, potentiation, and depression. Finally, we highlight recent advances from the past decade in 5-HT research, focusing on how atypical serotonergic signaling alters excitatory synapse plasticity and how serotonergic psychedelics affect dendritic spine structure and function.

Although opioids are effective in treating pain, they cause serious side effects. The use of regional anesthesia, although effective in the perioperative period, may not be suitable if mobility and lack of numbness is desired. Hence, there is a clear need for novel pain therapies. Low-voltage activated (T-type) calcium channels (Ca_V3.2 isoform) could be a promising therapeutic target for the development of novel pain therapies. Indeed, our published findings suggest that novel neuroactive steroid (NAS) analogs that modulate the activity of Ca_V3.2 channels have unique anti-nociceptive properties. However, the concern with current NASs appears to be their hypnotic/sedative properties, thus potentially hindering the future development of NASs for novel pain therapies. Hence, we developed a new line of NASs that are effective blockers of neuronal Ca_V3.2 channels in pain pathways while having more favorable pharmacodynamic properties, i.e., lack of sedative/hypnotic side effects. We present two promising novel analogs of NASs—B372 ((3β,5α,17β)-3-Hydroxyandrostan-17-carbonitrile) and YX23 ((3β,5α,17β)-3-Methoxyestran-17-ol). Using an in vitro approach, we show that B372 and YX23 are effective in blocking Ca_V3.2 channels. Using an in vivo approach, we show that they are effective anti-nociceptives in wild-type but not Ca_V3.2 knock-out mice. Importantly, we show that they lack sedative/hypnotic effects.

Three recent studies claimed that induction of long-term potentiation of synaptic strength requires structural rather than enzymatic functions of the Ca²⁺/calmodulin-dependent protein kinase II (CaMKII). One study used the CaMKII D135N mutation, which was claimed to abolish enzymatic activity without affecting the structural function, that is, binding to GluN2B. We found here that the D135N mutant indeed abolished enzymatic kinase activity and autophosphorylation at T286. However, D135N mutation additionally reduced binding to GluN2B and prevented persistence of co-condensation with GluN2B. Similar to the T286A mutant, GluN2B binding of the D135N mutant could be partially rescued by AS283, an inhibitor that directly enhances GluN2B binding. This partial effect on GluN2B binding has to be taken into account when using D135N to distinguish between enzymatic versus structural functions of CaMKII. Nonetheless, as discussed here, the D135N mutant indeed supports a structural rather than enzymatic CaMKII function in long-term potentiation induction. SIGNIFICANCE STATEMENT: This study characterized the Ca²⁺/calmodulin-dependent protein kinase II D135N mutant and found abolished kinase activity and reduced GluN2B binding. These findings support the nonenzymatic role of Ca²⁺/calmodulin-dependent protein kinase II in long-term potentiation induction but provide important information for the interpretation of results obtained with this tool mutant.