of Work: Neuronal communication between cells in the nervous system occurs at the synapse, where the release of neurotransmitter by the presynaptic terminal diffuses across the synaptic cleft and binds to and activates various ligand-gated ion channels on the postsynaptic membrane. To understand the basic mechanisms and regulation of synaptic transmission in the CNS, the lab is focusing on the function and regulation of the ligand-gated ion channels gated by acetylcholine (Ach; this is referred to as the nicotinic receptor) and serotonin (i.e., the 5-HT3 receptor, or 5-HT3R) in the hippocampus. The hippocampus is implicated in memory formation in humans and animals, and is an important site in relation to a variety of neurodegenerative diseases such as Alzheimer's. Therefore, conditions or agents that may have an effect on the development of the hippocampus can have severe consequences in terms of cognitive deficits. In 1997, we discovered that hippocampal inhibitory interneurons contain functional somato-dendritic nicotinic receptors; since then three other leading laboratories have confirmed and extended our findings. To understand how the nicotinic receptors are regulating hippocampal function, we are currently focusing on the cellular and molecular aspects of these channels. As there are more than ten different nicotinic receptor subunits known to be expressed in the mammalian brain, we have been using patch-clamp electrophysiological studies in conjunction with single-cell RT-PCR studies in order to investigate the cellular expression patterns of these different nicotinic receptor subunits from cells where we can also conduct a functional study of the protein. We have discovered that different populations of neurons within the hippocampus express different nicotinic receptor subunits; as the properties of the responses are also different, we have been successful at correlating the expression patterns with functional properties. We are also quantifying the molecular properties of the channels because the exact molecular makeup will determine different aspects of receptor function. For example we have been studied the single-channel properties of nicotinic receptors and find that there are two different channels present in hippocampal interneurons. The properties of these channels have allowed us to make estimates of molecular structure. Finally we are using focal uncaging of nicotinic ligands in order to map the location of functional receptors on the hippocampal interneurons. All of these approaches in general will allow us to determine better what the molecular makeup of the nicotinic receptors is, but also how functional receptors are distributed. Such information is critical to understand how these receptors control hippocampal activity. Although the nicotinic and 5-HT3R channels are known to be involved in a variety of physiological processes, the precise nature of these actions is not currently known, and our findings may help to understand the role that Ach and 5-HT have in regulating hippocampal neuronal activity and possibly cognitive processes. For example, approximately 25% of pregnant women smoke and exposure to nicotine in utero, both in rats and humans, is known or suspected to cause a variety of developmental abnormalities, including brain damage and cognitive impairment. Nicotine may also have positive physiological effects. For example patients with Alzheimer's disease have fewer nicotinic binding sites in the hippocampus, and nicotine alleviates some symptoms in Alzheimer's patients. Mutations in a nicotinic receptor subtype has also been linked to a rare form of epilepsy. The 5-HT3R is also known to serve an important functional role in the CNS; e.g., 5-HT3 ligands may be powerful anxiolytic agents, useful for the relief of migraines, and powerful cognition-enhancing agents. Based on our work on the hippocampus, we have found that subunits of the 5-HT3 and nicotinic receptors can co-assemble to form a novel type of heteromeric ion channel with an enhanced permeability to calcium; this is the first such evidence of promiscuous co-assembly of receptor subunits from different neurotransmitter receptors. We are currently exploring whether this co-assembly may play a significant role in regulating neuronal activity in the brain. Interestingly, as hippocampal inhibitory interneurons selectively express both functional 5-HT3 and nicotinic receptors, it is possible that 5-HT3 and nicotinic subunits may therefore co-assemble in these neurons, resulting in a channel highly permeable to calcium that in turn regulates neuronal activity and therefore the release of GABA from these neurons.
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