Trends in Neurosciences
Volume 26, Issue 9, September 2003, Pages 501-506
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Dendritic processing within olfactory bulb circuits

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Abstract

Odors elicit a well-organized pattern of activation in glomeruli across the surface of the olfactory bulb. However, the mechanisms by which this map is transformed into an odor code by the bulb circuitry remain unclear. Recent physiological studies in bulb slices have identified several synaptic processes that could be involved in sharpening odorant signals. Mitral cells within a single odorant receptor-specific network can be synchronized by dendrodendritic excitatory interactions in a glomerulus, whereas mitral cells in different networks engage in long-lasting lateral inhibition mediated by dendrodendritic synapses with interneurons. The emerging picture is one in which groups of mitral cells use a unique set of mechanisms to accomplish computational functions similar to those performed by analogous modular structures in other sensory systems.

Section snippets

Intraglomerular and interglomerular circuits

Olfactory bulb glomeruli are thought to be the functional units of olfactory bulb processing [6]. This designation reflects both the homogeneity of the sensory inputs received by the cells in a glomerular unit and the degree to which the neurons in the same glomerular unit are interconnected. In each glomerulus the primary dendritic tufts of ∼50 mitral and tufted cells receive convergent input from ∼5000 axons of olfactory receptor neurons (ORNs), all of which express the same odorant receptor

Self-excitation and inhibition

The first evidence for self-excitation in mitral cells came from the turtle olfactory bulb, when injection of a depolarizing current into a mitral cell elicited a prominent glutamate receptor-dependent after-depolarization [16]. Based on its insensitivity to blockade of the mitral cell action potential (with tetrodotoxin, TTX), it was suggested that self-excitation originated at dendrodendritic synapses rather than at recurrent axon collaterals. More recent patch-clamp studies in rodent slices

Intraglomerular excitation and inhibition

Recent physiological experiments have shown that activation of mitral cells also causes intraglomerular excitation 1, 10, 18, 29, 30 (Fig. 2a). Paired mitral cell recordings have shown that this excitatory coupling coexists with intraglomerular inhibition (Fig. 3b), but that the excitation is larger than inhibition and that excitation and inhibition have different time scales [30]. Intraglomerular excitation in response to stimulation of a single mitral cell is observed in all pairs that have

Interglomerular circuits

The activity of mitral cells in a particular glomerular unit also is influenced by the activity of mitral cells associated with other glomeruli. The main such interglomerular circuit involves the activation of granule cells by glutamate released from mitral cell dendrites. These cells in turn release GABA onto mitral cell somata and dendrites, inhibiting mitral cell activity. The mechanisms underlying lateral inhibition of mitral cells by granule cells are thought to be similar to

Impact of olfactory bulb circuitry on odor coding

The odor code begins in the olfactory bulb as a map of glomerular activation, with each glomerulus reflecting one activated odorant receptor class 37, 38. It has been widely suggested that the function of the bulb is to sharpen the odor code through lateral inhibition between different odorant receptor-specific networks of mitral cells 35, 39 (see also Ref. [40] for discussion of the insect olfactory system). We favor a modified model of lateral interactions that incorporates excitation as much

Concluding remarks

In many sensory systems, including the olfactory bulb, rodent barrel cortex and primary visual cortex, inputs and local circuitry are organized in modular fashion – with similar inputs being received by groups of cells that are highly interconnected. This modular organization offers several interesting functional possibilities. Strong excitatory connections within a sensory module can serve to reinforce the activity of cells in the module, leading to effective ‘all-or-none’ activation of the

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