Hypothalamic Kisspeptin Neurons and the Control of Homeostasis

Hypothalamic kisspeptin (Kiss1) neurons provide indispensable excitatory transmission to GnRH neurons for the coordinated release of gonadotropins, estrous cyclicity and ovulation. But maintaining reproductive functions is metabolically demanding so there must be a coordination with multiple homeostatic functions, and it is apparent that Kiss1 neurons play that role. There are two distinct populations of hypothalamic Kiss1 neurons, namely arcuate nucleus (Kiss1 ARH) neurons and anteroventral periventricular and periventricular nucleus (Kiss1 AVPV/PeN) neurons in rodents, both of which excite GnRH neurons via kisspeptin release but are differentially regulated by ovarian steroids. Estradiol (E2) increases the expression of kisspeptin in Kiss1 AVPV/PeN neurons but decreases its expression in Kiss1 ARH neurons. Also, Kiss1 ARH neurons co-express glutamate and Kiss1 AVPV/PeN neurons co-express GABA, both of which are upregulated by E2 in females. Also, Kiss1 ARH neurons express critical metabolic hormone receptors, and these neurons are excited by insulin and leptin during the fed state. Moreover, Kiss1 ARH neurons project to and excite the anorexigenic proopiomelanocortin (POMC) neurons but inhibit the orexigenic neuropeptide Y/Agouti-related peptide (NPY/AgRP) neurons, highlighting their role in regulating feeding behavior. Kiss1 ARH and Kiss1 AVPV/PeN neurons also project to the pre-autonomic paraventricular nucleus (satiety) neurons and the dorsomedial nucleus (energy expenditure) neurons to differentially regulate their function via glutamate and GABA release, respectively. Therefore, this review will address not only how Kiss1 neurons govern GnRH release, but how they control other homeostatic functions through their peptidergic, glutamatergic and GABAergic synaptic connections, providing further evidence that Kiss1 neurons are the key neurons coordinating energy states with reproduction.

Analyses of the Autism-associated Neuroligin-3 R451C Mutation in Human Neurons Reveals a Gain-of-Function Synaptic Mechanism

Mutations in many synaptic genes are associated with autism spectrum disorders (ASDs), suggesting that synaptic dysfunction is a key driver of ASD pathogenesis. Among these mutations, the R451C-substitution in the NLGN3 gene that encodes the postsynaptic adhesion molecule Neuroligin-3 is noteworthy because it was the first specific mutation linked to ASDs. In mice, the corresponding Nlgn3 R451C-knockin mutation recapitulates social interaction deficits of ASD patients and produces synaptic abnormalities, but the impact of the NLGN3 R451C-mutation on human neurons has not been investigated. Here, we generated human knock-in neurons with the NLGN3 R451C-mutation. Strikingly, analyses of NLGN3 R451C-mutant neurons revealed that the R451C-mutation decreased NLGN3 protein levels but enhanced the strength of excitatory synapses without affecting inhibitory synapses. No significant cell death and endoplasmic reticulum stress were detected. Importantly, the augmentation of excitatory transmission was confirmed in vivo with human neurons transplanted into mouse forebrain. Using single-cell RNA-seq experiments with co-cultured excitatory and inhibitory NLGN3 R451C-mutant neurons, we identified differentially expressed genes in relatively mature human neurons that corresponded to synaptic gene expression networks. Moreover, gene ontology and enrichment analyses revealed convergent gene networks associated with ASDs and other mental disorders. Our findings suggest that the NLGN3 R451C-mutation induces a gain-of-function enhancement in excitatory synaptic transmission that may contribute to the pathophysiology of ASDs.

Expression and Neurotransmitter Association of the Synaptic Calcium Sensor Synaptotagmin in the Avian Auditory Brain Stem

In the avian auditory brain stem, acoustic timing and intensity cues are processed in separate, parallel pathways via the two division of the cochlear nucleus, nucleus angularis (NA) and nucleus magnocellularis (NM). Differences in excitatory and inhibitory synaptic properties, such as release probability and short-term plasticity, contribute to differential processing of the auditory nerve inputs. We investigated the distribution of synaptotagmin, a putative calcium sensor for exocytosis, via immunohistochemistry and double immunofluorescence in the embryonic and hatchling chick brain stem (Gallus gallus). We found that the two major isoforms, synaptotagmin 1 (Syt1) and synaptotagmin 2 (Syt2), showed differential expression. In the NM, anti-Syt2 label was strong and resembled the endbulb terminals of the auditory nerve inputs, while anti-Syt1 label was weaker and more punctate. In NA, both isoforms were intensely expressed throughout the neuropil. A third isoform, synaptotagmin 7 (Syt7), was largely absent from the cochlear nuclei. In nucleus laminaris (NL, the target nucleus of NM), anti-Syt2 and anti-Syt7 strongly labeled the dendritic lamina. These patterns were established by embryonic day 18 and persisted to postnatal day 7. Double labeling immunofluorescence showed Syt1 and Syt2 were associated with Vesicular Glutamate Transporter 2 (VGluT2), but not Vesicular GABA Transporter (VGAT), suggesting these Syt isoforms were localized to excitatory, but not inhibitory, terminals. These results suggest that Syt2 is the major calcium binding protein underlying excitatory neurotransmission in the timing pathway comprising NM and NL, while Syt2 and Syt1 regulate excitatory transmission in the parallel intensity pathway via cochlear nucleus NA.

AMPA Receptors: A Key Piece in the Puzzle of Memory Retrieval

Retrieval constitutes a highly regulated and dynamic phase in memory processing. Its rapid temporal scales require a coordinated molecular chain of events at the synaptic level that support transient memory trace reactivation. AMPA receptors (AMPAR) drive the majority of excitatory transmission in the brain and its dynamic features match the singular fast timescales of memory retrieval. Here we provide a review on AMPAR contribution to memory retrieval regarding its dynamic movements along the synaptic compartments, its changes in receptor number and subunit composition that take place in activity dependent processes associated with retrieval. We highlight on the differential regulations exerted by AMPAR subunits in plasticity processes and its impact on memory recall.

Basal Synaptic Transmission and Long-Term Plasticity at CA3-CA1 Synapses Are Unaffected in Young Adult PINK1-Deficient Rats

Loss of function mutations in PARK6, the gene that encodes the protein PTEN-induced kinase 1 (PINK1), cause autosomal recessive familial Parkinson’s disease (PD). While PD is clinically diagnosed by its motor symptoms, recent studies point to the impact of non-motor symptoms, including cognitive dysfunction in the early pre-motor stages of the disease (Aarsland et al., 2004; Chaudhuri and Schapira, 2009). As the hippocampus is a key structure for learning and memory, this study aimed to determine whether synaptic transmission is affected at CA3-CA1 excitatory synapses in PINK1 knockout rats at an age when we recently reported a gain of function at excitatory synapses onto spiny projection neurons in the dorsal striatum (Creed et al., 2020) and when motor symptoms are beginning to appear (Dave et al., 2014). Using extracellular dendritic field excitatory postsynaptic potential recordings at CA3-CA1 synapses in dorsal hippocampus 4-to 5- month old PINK1 KO rats and wild-type littermate controls, we observed no detectable differences in the strength of basal synaptic transmission, paired-pulse facilitation, or long-term potentiation. Our results suggest that loss of PINK1 protein does not cause a general dysfunction of excitatory transmission throughout the brain at this young adult age when excitatory transmission is abnormal in the striatum.

Extracellular Metalloproteinases in the Plasticity of Excitatory and Inhibitory Synapses

Long-term synaptic plasticity is shaped by the controlled reorganization of the synaptic proteome. A key component of this process is local proteolysis performed by the family of extracellular matrix metalloproteinases (MMPs). In recent years, considerable progress was achieved in identifying extracellular proteases involved in neuroplasticity phenomena and their protein substrates. Perisynaptic metalloproteinases regulate plastic changes at synapses through the processing of extracellular and membrane proteins. MMP9 was found to play a crucial role in excitatory synapses by controlling the NMDA-dependent LTP component. In addition, MMP3 regulates the L-type calcium channel-dependent form of LTP as well as the plasticity of neuronal excitability. Both MMP9 and MMP3 were implicated in memory and learning. Moreover, altered expression or mutations of different MMPs are associated with learning deficits and psychiatric disorders, including schizophrenia, addiction, or stress response. Contrary to excitatory drive, the investigation into the role of extracellular proteolysis in inhibitory synapses is only just beginning. Herein, we review the principal mechanisms of MMP involvement in the plasticity of excitatory transmission and the recently discovered role of proteolysis in inhibitory synapses. We discuss how different matrix metalloproteinases shape dynamics and turnover of synaptic adhesome and signal transduction pathways in neurons. Finally, we discuss future challenges in exploring synapse- and plasticity-specific functions of different metalloproteinases.

Ketamine Produces a Long-Lasting Enhancement of CA1 Neuron Excitability

Ketamine is a clinical anesthetic and antidepressant. Although ketamine is a known NMDA receptor antagonist, the mechanisms contributing to antidepression are unclear. This present study examined the loci and duration of ketamine’s actions, and the involvement of NMDA receptors. Local field potentials were recorded from the CA1 region of mouse hippocampal slices. Ketamine was tested at antidepressant and anesthetic concentrations. Effects of NMDA receptor antagonists APV and MK-801, GABA receptor antagonist bicuculline, and a potassium channel blocker TEA were also studied. Ketamine decreased population spike amplitudes during application, but a long-lasting increase in amplitudes was seen during washout. Bicuculline reversed the acute effects of ketamine, but the washout increase was not altered. This long-term increase was statistically significant, sustained for >2 h, and involved postsynaptic mechanisms. A similar effect was produced by MK-801, but was only partially evident with APV, demonstrating the importance of the NMDA receptor ion channel block. TEA also produced a lasting excitability increase, indicating a possible involvement of potassium channel block. This is this first report of a long-lasting increase in excitability following ketamine exposure. These results support a growing literature that increased GABA inhibition contributes to ketamine anesthesia, while increased excitatory transmission contributes to its antidepressant effects.

Glycine transporter-1 antagonist provides 1 neuroprotection in vivo

Glycine fulfills several roles in biology including protein synthesis, inhibitory transmission via glycine receptor activation and excitatory transmission through glutamate-sensitive N-methyl-D-aspartate receptors (NMDARs). Low glycine doses enhance NMDAR function while high doses trigger glycine-induced NMDAR internalization (GINI) in vitro. The physiological relevance of GINI has been questioned given that the high-affinity glycine transporter type 1 (GlyT1), located on astrocytes and neurons, maintains synaptic glycine concentrations far below the level that would saturate the glycine binding site (GBS) on NMDARs. Here, we report evidence that GINI occurs also in vivo and is neuroprotective following ischemic insult. Mice pre-treated with a GlyT1 antagonist (GlyT1-A), which increased glycine levels, exhibited smaller stroke volume, reduced cell death, and minimized behavioural deficits following stroke induction by either photothrombosis or endothelin-1. We demonstrate that in a modified in vitro ischemic paradigm, glycine is released at levels surpassing what occurs during ischemia alone. Therefore, glycine accumulates in the synaptic cleft, enhances occupancy of GBS and reaches the set point to trigger GINI. We report that GINI is observed during stroke, in vivo, only in the presence of a GlyT1-A. Moreover, we show evidence of a protective effect on the vasculature in the peri-infarct area. Therefore, these data strongly suggest that GlyT1 is a therapeutic target to prevent cell death following an ischemic event.