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. We have found that hippocampal inhibitory interneurons contain functional somato-dendritic nicotinic receptors, a finding that will have a large impact on the nicotinic receptor field; these interneurons also possess functional HT3Rs, which are also thought to be involved in cognitive processes (and thus similar to nicotinic receptors). Although there are many fewer interneurons than the numbers of principle excitatory cells, a single interneuron can innervate and regulate the activity of hundreds of excitatory cells in the hippocampus. Although the nicotinic and 5-HT3R channels are known to be involved in a variety of physiological processes, the precise nature of these actions are 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 important functional roles in the CNS; e.g., 5-HT3 ligands may be powerful anxiolytic agents, useful for the relief of migraines, and powerful cognition-enhancing agents. Besides our work in 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 release of GABA from these neurons. Cytoplasmic calcium is known to regulate the function of various ligand-gated ion channels. We have recently reported that calcium influx through voltage-gated calcium channels modulates the function of the 5-HT3R channel. Future studies will explore the links between calcium, phosphorylation, and channel activity. Lastly, we have combined patch-clamp recording with single-cell RT-PCR techniques and have found that hippocampal inhibitory interneurons selectively express the 4 and 7 nicotinic receptor subunits, and we have confirmed the GABAergic nature of these inteerneurons by demonstrating their selective expression of GAD (glutamic acid decarboxylase; both GAD65 and GAD67), and VGAT, the vesicular GABA transporter.
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