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 critical periods of postnatal life. On the one hand, this susceptibility makes the developing brain particularly vulnerable to toxic insults. On the other hand, the plasticity of the connections between neurons, or synapses, is critical for refining brain circuitry during postnatal development. 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 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. We primarily use the hippocampal slice preparation using neonate and adult rats and mice. The relatively simple laminar structure of the hippocampus, which itself plays an important role in learning and memory, allows electrophysiological studies to be performed easily. To measure synaptic responses, we use techniques that include whole-cell patch clamp recordings and field potential recordings from hippocampal slices either acutely prepared or cultured. To determine how transcription is regulated by neuronal activity, we use molecular and biochemical methods with acutely dissociated hippocampal and cortical neuronal cell cultures, which can be stimulated pharmacologically to mimic long-term potentiation (LTP) and long-term depression (LTD). To understand how synaptic changes persist for up to a lifetime, we study how neuronal activity regulates gene transcription to consolidate synaptic changes. Evidence suggests that the long-term changes in synaptic efficacy require expression of new RNA and toward that end, we have focused on the regulation of gene transcription by neuronal action potentials. Previously, we have shown that action potentials are necessary and sufficient to initiate protein kinase activation and subsequent gene transcription in neurons. These findings have important implications for the interpretation of experiments using NMDA receptor inhibitors to conclude that signals to the nucleus come from the synapse. In an effort to provide a mechanism for how genes might be rapidly transcribed without necessarily involving biochemical signaling from synapses, we have investigated the transcriptional readiness of genes rapidly regulated by neuronal activity. We found that several immediate early genes (IEGs) such as arc/arg3.1 are poised for near-instantaneous transcription by RNA Polymerase II (Pol II) stalled or poised just downstream of the transcription start site in rat neurons. Our data strongly indicate that rapid induction of neuronal IEGs requires poised Pol II and suggest a role for this mechanism in a wide variety of transcription-dependent processes, including learning and memory, development, and drug addiction. These results also support our idea that action potentials can be the critical trigger for transcription of some genes required for the consolidation of synaptic plasticity. Current efforts are aimed at determining the signal transduction events necessary for triggering the Pol II into productive elongation. One approach that we have taken to gain insight into the mechanisms of synaptic plasticity has been by comparing 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 thatoone area of the hippocampus, the CA2, would have a resistance to synaptic plasticity including LTP and LTD, even though we found that synaptic responses in CA2 were very similar to those in the neighboring CA1 and CA3 areas. Recently we found 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. In addition, we have found that a regulator of G-protein signaling (RGS-14), which is highly enriched in CA2, is also a negative regulator of plasticity in CA2. Previous reports have indicated that adenosine A1 receptors (A1Rs) are highly enriched in the CA2. Antagonists of A1Rs can enhance the induction of activity- dependent LTP in the hippocampus, and adenosine, the natural agonist of the receptor, inhibits LTP induction and prevents its stabilization in area CA1. Therefore we tested the role of A1Rs in regulating synapses in CA2 in hippocampal slices in vitro. Caffeine, a naturally-occurring adenosine receptor antagonist, is well known to act both peripherally and centrally for its ability to enhance both attention cognition. Consistent with the enrichment of A1Rs in CA2, we found that a brief application of a physiologically relevant concentration of caffeine or other, more selective A1R antagonists induced a lasting potentiation of synaptic transmission in CA2 but not in areas CA1 or CA3. We also found that caffeine administered orally to rats, at doses comparable to those consumed by humans, potentiated synaptic transmission in area CA2 with no effect on synaptic responses in area CA1. This was the first study to demonstrate a selective enhancement of basal synaptic responses at CA2 synapses in vitro following oral administration of physiologically relevant doses of caffeine in vivo. Most recently, we have found that the social neuropeptides oxytocin and vasopressin act in a way similar to caffeine in that they appear to induce synaptic potentiation in CA2 pyramidal neurons. Unlike caffeine and other A1R antagonists, the peptide-induced potentiation closely resembled typical LTP in that it was NMDA receptor- and calcium-dependent (vs. NMDA receptor and calcium-independent for caffeine). The vasopressin 1b receptor (avpr1b) is highly enriched in CA2 neurons over all other parts of the brain, suggesting that CA2 may play an important role in social behavior; recent work in other laboratories support this view. Using information we learn from CA2, we aim to determine the nature of the developmental down-regulation of synaptic plasticity in the form of critical periods and how plasticity is modulated in different brain areas. Because CA2 and its surrounding regions are anatomically very similar, these findings may therefore lead to identification of critical molecular components in the pathways leading to not only synaptic plasticity, but also those important for neuronal damage and death. Our longer term goal is to determine how neuronal activity leads to synapse elimination (pruning). Toward this goal, we have developed several techniques by which activity-dependent synapse elimination can be studied in live and fixed tissue. We found that electrical or pharmacological stimulation that results in LTD is accompanied by loss of synaptic connections and that smaller synaptic contacts were most likely to be eliminated. These methods will allow us to test our ideas on the molecular mechanisms allowing LTD to lead to synaptic loss. By understanding the molecular and cellular mechanisms of synaptic plasticity during development, we may begin to understand how exposure to environmental toxicants during development can have life-long consequences on cognition and susceptibility to diseases such as autism, schizophrenia, and Alzheimer's disease.
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