combined-vivo-anatomical-functional-tracing-ventral-tegmental-area

Combined In Vivo Anatomical and Functional Tracing of Ventral Tegmental Area Glutamate Terminals in the Hippocampus

Optogenetic modulation of neuron sub-populations in the brain has allowed researchers to dissect neural circuits in vivo and ex vivo. This provides a ...

Louisiana State University School of Veterinary Medicine

N-Methyl-D-Aspartate Receptor 1 (NMDAR)-linked Ca current represents a significant percentage of post-synaptic transient that modulates synaptic strength and is pertinent to dendritic spine plasticity. In the hippocampus, Ca transient produced by glutamatergic ionotropic neurotransmission facilitates Ca-Calmodulin-dependent kinase 2 (CaMKII) Thr286 phosphorylation and promote long-term potentiation (LTP) expression. At CA1 post-synaptic densities, Ca transients equally activate small conductance (SK2) channel which regulates excitability by suppressing Ca movement. Here, we demonstrate that upstream attenuation of GluN1 function in the hippocampus led to a decrease in Thr286 CaMKIIα phosphorylation, and increased SK2 expression. Consistent with the loss of GluN1 function, potentiation of SK channel in wild type hippocampus reduced CaMKIIα expression and abrogate synaptic localization of T286 pCaMKIIα. Our results demonstrate that positive modulation of SK channel at hippocampal synapses likely refine GluN1-linked plasticity by tuning dendritic localization of CaMKIIα.

SK Channel Modulates Synaptic Plasticity by Tuning CaMKIIα/β Dynamics.

Mutation of the  gene underlies a broad range of developmental neuropsychiatric defects, including schizophrenia, depression, and bipolar disorder. The pathophysiological phenotypes linked with  mutation are due to the truncation of the DISC1 primary protein structure. This leads to a defective post-synaptic scaffolding and kinase-GSK3β and Erk1/2-signaling. As a result, synaptic function and maintenance are significantly impaired in the  mutant brain. Among several other pathways, GSK3β and Erk1/2 are involved in insulin-like growth factor 1 receptor (IGF-1Rβ) kinase signaling. Although  mutation alters these kinases, it is unclear if the mutation impacts IGF-1R expression and activity in the brain. Here, we demonstrate that the expression of active IGF-1Rβ (pIGF-1Rβ) is altered in the hippocampus and prefrontal cortex (PFC) of  mutant mice and vary with the dose of the mutation (homozygous and heterozygous). The expression of pIGF-1Rβ decreased significantly in 129S () brains. In contrast, 129S:B6 () brains were characterized by an increase in pIGF-1Rβ when compared with the C57BL/6 () level. The decrease in pIGF-1Rβ level for the 129S brains was accompanied by the loss of Akt activity (S473 pAkt) and decreased Ser9 phosphorylation of GSK3β (increased basal GSK3β). Additionally, hippocampal and cortical pErk1/2 activity increased in the 129S hippocampus and cortex. Although 129S:B6 recorded alterations in pIGF-1Rβ-pAkt-GSK3β (like 129S), there was no observable change in pErk1/2 activity for the heterozygote () mutant. In addition to GSK3β inhibition, we conclude that pIGF-1R, pAkt, and pErk1/2 are potential targets in  mutant brain. On the other hand, pIGF-1R and pAkt can be further explored in  brain.

 Carrier Mice Exhibit Alterations in Neural pIGF-1Rβ and Related Kinase Expression.

N-methyl-D-aspartate receptor (NMDAR) modulates the structural plasticity of dendritic spines by impacting cytoskeletal organization and kinase signaling. In the developing nervous system, activation of NMDAR is pertinent for neuronal migration, neurite differentiation, and cellular organization. Given that small conductance potassium channels (SK2/3) repress NMDAR ionotropic signaling, this study highlights the impact of neonatal SK channel potentiation on adult cortical and hippocampal organization. Neonatal SK channel potentiation was performed by one injection of SK2/3 agonist (CyPPA) into the pallium of mice on postnatal day 2 (P2). When the animals reached adulthood (P55), the hippocampus and cortex were examined to assess neuronal maturation, lamination, and the distribution of synaptic cytoskeletal proteins. Immunodetection of neuronal markers in the brain of P2-treated P55 mice revealed the presence of immature neurons in the upper cortical layers (layers II-IV) and CA1 (hippocampus). Also, layer-dependent cortical-cell density was attenuated due to the ectopic localization of mature (NeuN+) and immature (Doublecortin+ [DCX+]) neurons in cortical layers II-IV. Similarly, the decreased count of NeuN+ neurons in the CA1 is accompanied by an increase in the number of immature DCX+ neurons. Ectopic localization of neurons in the upper cortex and CA1 caused the dramatic expression of neuron-specific cytoskeletal proteins. In line with this, structural deformity of neuronal projections and the loss of postsynaptic densities suggests that postsynaptic integrity is compromised in the SK2/3+ brain. From these results, we deduced that SK channel activity in the developing brain likely impacts neuronal maturation through its effects on cytoskeletal formation.

Positive Modulation of SK Channel Impedes Neuron-Specific Cytoskeletal Organization and Maturation.

Reciprocal connection between the ventral tegmental area (VTA) and the hippocampus forms a loop that controls information entry into long-term memory. Compared with the widely studied VTA dopamine system, VTA glutamate terminals are anatomically dominant in the hippocampus and less understood. The current study employs anterograde and retrograde labeling of VTA dopamine and glutamate neurons to map the distribution of their terminals within the layers of the hippocampus. Also, functional tracing of VTA dopamine and glutamate projections to the hippocampus was performed by photostimulation of VTA cell bodies during CA1 extracellular voltage sampling in vivo. VTA dopamine terminals predominantly innervate the CA1 basal dendrite layer and modulate the firing rate of active putative neurons. In contrast, anatomical dominance of VTA glutamate terminals in the CA1 pyramidal cell and apical dendrite layers suggests the possible involvement of these terminals in excitability regulation. In support of these outcomes, photostimulation of VTA dopamine neurons increased the firing rate but not intrinsic excitability parameters for putative pyramidal units. Conversely, activation of VTA glutamate neurons increased CA1 network firing rate and burst rate. In addition, VTA glutamate inputs reduced the interspike and interburst intervals for putative CA1 neurons. Taken together, we deduced that layer-specific distribution of presynaptic dopamine and glutamate terminals in the hippocampus determinines VTA modulation (dopamine) or regulation (glutamate) of excitability in the CA1 neural network.

Article	Year
SK Channel Modulates Synaptic Plasticity by Tuning CaMKIIα/β Dynamics. Frontiers in synaptic neuroscience\| PubMed ID: 31736736	2019
Carrier Mice Exhibit Alterations in Neural pIGF-1Rβ and Related Kinase Expression. Frontiers in cellular neuroscience\| PubMed ID: 32431597	2020
Positive Modulation of SK Channel Impedes Neuron-Specific Cytoskeletal Organization and Maturation. Developmental neuroscience\| PubMed ID: 32580196	2020
Distribution of VTA Glutamate and Dopamine Terminals, and their Significance in CA1 Neural Network Activity. Neuroscience\| PubMed ID: 32652172	2020

Amita Shrestha

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