Sukumar Vijayaraghavan, Ph.D. Associate Professor Department of Physiology & Biophysics
	UCHSC at Fitzsimons RC-1 North Tower, P18-7121 PO Box 6511, Mail Stop F8307 Tel (303) 724-4531 Fax (303) 724-4501	E-mail: sukumar.v@UCHSC.edu Curriculum vitae BNAT program member

RESEARCH

Acetylcholine (ACh) is a transmitter that is involved in most brain functions that involve learning and memory.  Dysfunction of the cholinergic system is thought to underlie neurodegenerative diseases like Alzheimer’s disease and many of the drugs used to treat these diseases are aimed at altering cholinergic signaling in the brain.  However, the development of an effective therapy for these illnesses must be preceded by an understanding of the physiological function of this transmitter system. Our work is aimed at understanding the mechanics of cholinergic signaling and how it modulates brain function.  We use a combination of biochemistry, electrophysiology and live cell imaging to address these questions.
            ACh acts via two receptors, the nicotinic acetylcholine receptors (nAChRs) and the muscarinic acetylcholine receptors (mAChRs).  Part of understanding the cholinergic system involves examining what the physiological actions of these receptors are.  We are studying nAChR and mAChR function in the following projects, while at the same time obtaining information on some fundamental issues involving signaling in the mammalian brain.
AChRs on astrocytes:
            Recent studies have indicated that astrocytes have an unusual form of excitability mediated by calcium signals.  Our studies indicate that nAChR activation on these cells result in intra- and inter-cellular calcium waves (see movie below) by mobilizing calcium from intracellular stores. Our interest is in the purpose of this signaling and its physiological consequences for glia-neuron interactions.

Legend: Intercellular calcium waves in cultured astrocytes triggered local activation of nAChRs in one astrocyte (middle of the frame).  Calcium rise in one astrocyte is propagated to surrounding cells- a form of intercellular communication in these cells.

Presynaptic nAChRS on mossy fiber terminals of the hippocampus
            A dominant effect of nAChR activation appears to be an increase in neurotransmitter release.  This is true in many brain areas and for nearly all transmitter systems.  We are examining, in collaboration with Dr. Geeta Sharma (link), the mechanism underlying nAChR-mediated glutamate release at the mossy fiber-CA3 pyramidal cell synapse.  These are large complex boutons (see figure below) that contain multiple active zones.

Legend:  A diI stained granule cell showing a long axon interrupted by bead-like mossy fiber boutons. Scale bar 10 mm.

Activation of nAChRs results in a store calcium-mediated increase in the frequency of glutamate release at this synapse and, we believe, a calcium/calmodulin kinase II-dependent synchronized release of multiple quanta.  Our studies show that this effect is mediated by a subtype of nAChRs containing the a7 gene product that are located at the mossy fiber terminals.  This is evidenced by the ACh-induced calcium transient that we have measured from individual mossy fiber boutons (see below).

Legend:  Fura-2 loaded mossy fiber boutons.  Application of ACh in the presence of atropine generates a calcium transient in the bouton which is blocked by an antagonist of the a7-nAChR a-bungarotoxin (aBTx).

  The exciting finding from our lab is that this effect of nAChRs on glutamate release is independent of presynaptic action potential and can elicit postsynaptic firing.   One project in the lab is aimed at understanding the role of presynaptic action potential independent form of transmission in synaptic signaling and plasticity.

Cholinergic signaling in the main olfactory bulb (MOB)
            AChRs have been shown to be involved in olfactory perceptual learning.  Disruption of the cholinergic system in the bulb impairs the ability of an animal to learn to discriminate between two closely related odors.  We are examining the cellular mechanisms underlying cholinergic modulation of MOB function. Our studies show that AChRs are widely distributed among cell types in the MOB (see figure below) and modulate the release of the inhibitory transmitter GABA.

Legend: Calcium responses to cholinergic agonists in various cell types of the rat MOB.  Carbachol was used as a mAChR agonist and nicotine as a nAChR agonist.  Left panel shows Oregon Green BAPTA 488 loaded cells in the MOB. The middle panel shows traces from cells challenged with 50 mM carbachol and the right panel shows a compilation of responses from a number of cells showing the fraction that respond to each agonist. The bulb granule cells have mainly mAChRs while the mitral cells predominantly respond to nAChR agonists.  Periglomerular (PG) cells respond to both.

Further, in collaboration with Prof. Diego Restrepo, we are beginning to correlate these mechanisms to changes in olfactory behavior in an attempt to obtain insights into cellular processes that underlie behavioral modifications in the MOB.

Cholinergic transmission in the brain
            In spite of the recognized importance of the cholinergic system, very little is known about the mechanistic bases of cholinergic transmission.  The main reason for this paucity of information is because of the anatomy of the cholinergic system, where  two clusters of cholinergic neurons send diffuse innervation to the entire brain.  In order to facilitate the examination of the mechanisms underlying cholinergic transmission, our lab is currently developing transgenic animals which will have their cholinergic processes labeled with tau-GFP.  We believe the generation of these mice will allow for a detailed examination of how signaling occurs across a cholinergic synapse.

Selected Publications

• Ghatpande, A.S., Sivaraaman, K., and Vijayaraghavan, S. (2006). Store Calcium Mediates Cholinergic effects on mIPSCs in the Rat Main Olfactory Bulb. J. Neurophysiol. 95, 1345-1355. pdf    
• Sharma, G. and Vijayaraghavan, S. (2003). Modulation of presynaptic store calcium induces release of glutamate and postsynaptic firing.  Neuron 38, 929-939.  pdf    
• Sharma, G., and Vijayaraghavan, S. (2001). Nicotinic cholinergic signaling in hippocampal astrocytes involves calcium-induced calcium release from intracellular stores. Proc. Natl. Acad. Sci. 98:4148-4153. pdf      
• Berger, F., Gage, F.H., and Vijayaraghavan, S. (1998). Nicotinic Receptor-Induced Apoptotic Cell Death of Hippocampal Progenitor Cells. J. Neurosci. 18(17):6871-6881. pdf 
• Vijayaraghavan, S., Huang, B., Blumenthal, E.W., and Berg, D.K. (1995). Arachidonic acid as a possible negative feedback regulator of neuronal acetylcholine receptor function. J. Neurosci 15, 3679-3687. pdf 
• Vijayaraghavan, S. (1994). Science for art's sake. Nature 372, 590. Zhang, Z-w., pdf 
• Vijayaraghavan,S., and Berg, D.K. (1994). Neuronal acetylcholine receptors that bind a-bungarotoxin with high affinity function as ligand-gated ion channels. Neuron 12, 167-177. 
• Vijayaraghavan, S., Pugh, P.C., Zhang, Z-w., Rathouz, M.M., and Berg, D.K. (1992). Nicotinic receptors that bind a-bungarotoxin on neurons raise intracellular free Ca++. Neuron 8, 353-362.
• Vijayaraghavan, S., Schmid, H.A., Halvorsen, S.W., and Berg, D.K. (1990). Cyclic AMP dependent phosphorylation of a neuronal nicotinic acetylcholine receptor a-type subunit. J. Neurosci. 10, 3255-3262. pdf 

PubMed search (Vijayaraghavan S)