Mechanisms Of Synaptic Plasticity In The Adult And Developing Nervous System

National Institute Of Environmental Health Sciences

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Abstract

A longstanding issue in environmental health is the need to understand the role the environment plays in human brain development. The brain of the neonate is particularly susceptible to disruption of the sensory environment, which can have profound effects on its physiology and morphology. Such susceptibility of the developing brain to environmental influence by sensory manipulation or to environmental toxicants is particularly pronounced during defined sensitive periods of postnatal life. On the one hand, the plasticity of the connections between neurons, or synapses, is critical for refining brain circuitry during postnatal development. However, on the other hand, it is this very plasticity that makes the developing brain particularly vulnerable to various insults. In addition, similar mechanisms for changing synapses are likely to serve the basis for learning in the adult. Our primary interest, therefore, has been to determine the molecular basis of regulating long-lasting synaptic plasticity. Toward our goal of learning how neuronal activity can induce lasting modifications in neurons, we use a diverse collection of molecular, biochemical, electrophysiological, and imaging techniques. Most of our studies are performed using neonatal and adult mice. To study synaptic plasticity, we use techniques that include whole-cell patch clamp recordings from hippocampal slices maintained in vitro, and we can stimulate them either electrically or pharmacologically to induce long-term potentiation (LTP) or long-term depression (LTD). In addition, we now use molecular and biochemical methods and in vivo recording and tracing techniques to address the role of the different hippocampal subregions in specific behaviors. One approach that we have taken to gain insight into the mechanisms of regulating synaptic plasticity has been to compare highly plastic brain areas, such as the CA1 area of hippocampus, with less plastic areas. From the expression pattern of some genes, we predicted, and found, that one area of the hippocampus, the CA2, would be more resistant to synaptic plasticity such as LTP and LTD, even though we found that synaptic responses in CA2 were very similar to those in the neighboring CA1 and CA3 areas. We later established that dendritic spines in CA2 have very different calcium dynamics from spines in CA1 and CA3 in that both calcium buffering capacity and rates of calcium extrusion were higher in CA2 spines when compared with those in the neighboring regions. RNAseq experiments to measure mRNA from specific cellular compartments of tissue isolated by laser capture microscopy (LCM) revealed that CA2 neurons express an unusually large number of genes related to mitochondrial calcium regulation. In addition, we have found that a regulator of G-protein signaling (RGS-14), and a number of G-protein coupled receptors including adenosine A1 receptors (A1Rs) and receptors for the social neuropeptides oxytocin and vasopressin are highly expressed in CA2 neurons and regulate plasticity there. Consistent with this cell signaling regulated by such social neuropeptides is recent data by our lab and others showing that CA2 may play an important role in social behavior such as aggression. More recently, we had found that a specialized extracellular matrix, called perineuronal nets (PNNs), plays a role in limiting synaptic plasticity in area CA2 and that that they develop prematurely and to a greater degree in a mouse model of Rett Syndrome, a developmental disorder often accompanied by impairments in social cognition, among other abnormalities. In addition, we found that all of the markers we have for CA2 are disrupted in mice lacking the mineralocorticoid receptor, encoded by the NR3C2 gene, which, when impacted in humans, results in a different syndromic autism. To better understand the relationship between CA2 pyramidal neurons and the axons from the dentate gyrus, we explored in detail this pathway in mice with immunofluorescence and viral tracing methods. Here, we sought to characterize the staining patterns of commonly used CA2 markers along the dorsal-ventral hippocampal axis and determine how these markers align along the proximodistal axis. We used a region of CA2 that stained for both Regulator of G-protein Signaling 14 (RGS14) and Purkinje Cell Protein 4 (PCP4; "double-labeled zone" DLZ) as a reference. We found that certain commonly used CA2 molecular markers may be better suited for drawing distinct boundaries between CA2/3 and CA2/1. In addition, these patterns are dependent on position along the dorsal-ventral hippocampal axis. Finally, we found that, consistent with previous findings, mossy fiber axons coming from the dentate gyrus innervating CA2 were both smaller and fewer in number than boutons in CA3. Our results indicate that certain molecular markers may be better suited than others when defining the proximal and distal borders of area CA2 and that the presence or absence of complex spines alone may not be suitable as a distinguishing feature differentiating CA3 from CA2 neurons. This study should help other researchers in their efforts to study CA2 in mice and other species. If CA2 is important in regulating social behaviors such as aggression in mice, we thought it important to determine if we could find evidence of it in other animals, particularly those that show aggressive behavior. Therefore we took an opportunity to study the hippocampi of foxes from the Russian Farm-Fox study, which had bred foxes to be either tame or aggressive. As a first step, we first sought to identify CA2 in foxes (Vulpes vulpes), as no clearly defined area of CA2 had been described in species such as cats, dogs, or pigs. In this study, we stained tissue sections with markers of CA2 pyramidal cells commonly used in tissue from rats and mice. We observed that antibodies against PCP4 best stained the pyramidal cells in the area spanning the end of the mossy fibers and the beginning of the pyramidal cells lacking mossy fibers, resembling the pattern seen in rats and mice. Our findings indicate that foxes do have a "molecularly defined" CA2, and further, they suggest that other carnivores like dogs and cats might as well. With this being the case, these foxes could be useful in future studies looking at CA2 as it relates to aggression. Finally, although our previous work had demonstrated a lack of typical long-term potentiation of stratum radiatum synapses, in CA2, little was known about long-term depression, or weakening of synapses there. CA2 neurons express high levels of several known and potential regulators of metabotropic glutamate receptor (mGluR)-dependent signaling including Striatal-Enriched Tyrosine Phosphatase (STEP) and several Regulator of G-protein Signaling (RGS) proteins, yet the functions of these proteins in regulating mGluR-dependent synaptic plasticity in CA2 are completely unknown. Using whole cell voltage-clamp recordings from mouse pyramidal cells, we found that mGluR agonist-induced long-term depression (mGluR-LTD) is more pronounced in CA2 compared with that observed in CA1. This mGluR-LTD in CA2 was found to rely on protein synthesis, STEP, and RGS14, but not RGS4, in CA2. Supporting a role for CA2 synaptic plasticity in social cognition, we also found that RGS14 KO mice had impaired social recognition memory as assessed in a social discrimination task. These results highlight possible roles for mGluRs, RGS14, and STEP in CA2-dependent behaviors, perhaps by biasing the dominant form of synaptic plasticity away from LTP and toward LTD in CA2. As RGS14 is highly expressed in human CA2, we expect this finding to be relevant to human brain function.