Abdallah Hayar, Ph.D.
Sensory coding in the olfactory bulb: Our neurons convey information to each other via chemical and electrical synapses and perform computations that are vital to our survival. My main research interest is to investigate synaptic transmission and function in the olfactory bulb. The olfactory bulb has become an attractive model to study cellular mechanisms underlying the encoding, transfer, processing and decoding of sensory information. Interest in this area was sparked by a series of dramatic breakthroughs over the past decade in our understanding of the organization and function of the peripheral olfactory system, cloning of the olfactory receptors, and identification of the olfactory transduction machinery. These advances have set the stage to unravel the mechanisms of early sensory processing by bulbar circuits. In addition, there has been recently an increase of interest in olfactory dysfunction because the impairment of olfactory bulb seems to be associated with some neurodegenerative diseases. I am interested in investigating the synaptic organization of olfactory bulb glomeruli and the role of glomerular circuitry in olfactory coding in normal and pathological states. We have found that olfactory bulb external tufted cells are endowed with spontaneous rhythmic bursting. Using simultaneous patch-clamp recordings from pairs of neurons, we found that membrane potential oscillations and spontaneous bursting activity are highly correlated in cells associated with the same glomeruli. Synchronous bursting may play an important role in olfactory coding and in regulating the induction of synaptic plasticity at the first input stage of the main olfactory bulb. In summary, the purpose of our research is to unravel the fundamental network mechanisms responsible for encoding and processing odor information. In 2006, Dr. Hayar received the Young Investigator Award for Research in Olfaction.
Schematic model of the intraglomerular circuitry
Glutamatergic external tufted (ET) cells have intrinsic, rhythmic burst firing and are contacted directly by olfactory nerve (ON) terminals. ET cells of the same glomerulus fire in synchrony and directly elicit bursts of EPSPs in other JG interneurons, the periglomerular (PG) and short axon cells (SA), most of which do not receive ON input. PG cell dendrites ramify in a restricted portion of a single glomerulus and thus might serve for local intra-glomerular inhibition via dendrodendritic interactions with ET or mitral cell (MC) dendrites. By contrast, SA cells have dendrites and axons extending throughout several glomeruli and thus might serve for inter-glomerular (i.e. lateral) inhibition. ET cells may represent a rhythm generator of the glomerular network. The synchronously bursting ET cell glomerular ensemble represents an oscillating neuronal circuit that monosynaptically synchronizes the activity of PG and SA neurons within the same glomerulus, and may possibly coordinate the activity of MC projection neurons via glutamate spillover.
Key References
- Hayar A, Shipley MT, and Ennis M (2005) Olfactory bulb external tufted cells are synchronized by multiple intraglomerular mechanisms. J Neuroscience 25:8197-8208(Abstract) (PDF, 934 KB).
- Hayar A,Karnup S, Ennis M, Shipley MT (2004) External tufted cells coordinate intra-glomerular circuit activity. J Neuroscience 24:6676-6685(Abstract) (PDF, 989 KB).
- Hayar A, Karnup S, Shipley MT, Ennis M (2004) Olfactory bulb glomeruli: external tufted cells intrinsically burst at theta frequency and are entrained by patterned olfactory input. J Neuroscience 24:1190-1199 (Abstract) (PDF, 526 KB).
- Aungst JL, Heyward PM., Puche AC, Karnup SV, Hayar A, Szabo G, Shipley MT (2003) Center-surround inhibition among olfactory bulb glomeruli. Nature 426:623-629 (Abstract) (PDF, 798 KB).