Refinement of odor molecule tuning by dendrodendritic synaptic inhibition in the olfactory bulb.

Abstract

Mitral MATERIALS AND METHODS Animal Preparation. Male adult rabbits (1.8-2.6 kg, Japanese White) were anesthetized with an intravenous injection of 30% urethane (1.2 g/kg, Aldrich) and tracheotomy was performed for double cannulation, one for the animals' spontaneous respiration and the other for artificial inhalation of odor-containing air (Fig. 1). The latter cannula was connected with an artificial respirator for drawing odor-containing air through the nasal cavity periodically. Animals were mounted in a stereotaxic apparatus. Body temperature was maintained at 38.0°C by a homeothermic heat pad system (ATB-1100, Nihon Kohden, Tokyo). The cerebrospinal fluid was drained at the atlantooccipital membrane to minimize brain pulsation. After exposure of the dorsal surface of the left olfactory bulb, a recording micropipette was inserted vertically into the dorsomedial region of the MOB, while a bipolar stimulatory electrode was introduced into the lateral olfactory tract (LOT) at the anterolateral portion of the frontal neocortex. The final position of the stimulating electrode was determined by monitoring LOT-evoked field potentials and the electrode was then anchored to the skull with dental cement.Electrophysiology. Extracellular single unit responses and LOT-evoked field potentials were recorded in the MOB using a glass micropipette (-3 Mfl) filled with 2 M NaCl. Signals were taken into a conventional amplifier and monitored on an oscilloscope. The action potentials were amplified and separated using band path filters (50 Hz-3 kHz). The position of the recorded cell was judged by the profile of the LOT-evoked field potentials. Single unit responses were recorded from a M/T cell in the EPL and the mitral cell layer of the dorsomedial region in the MOB. Mitral cells were identified by their Abbreviations: GABA, y-aminobutyrate; M/T cell, mitral/tufted cell; MOB, main olfactory bulb; LOT, lateral olfactory tract; EPL, external plexiform layer; CNQX, 6-cyano-7-nitroquinoxaline-2,3-dione; D-AP5, D-(-)-2-amino-5-phosphonovalerate; AMPA, a-amino-3-hydroxy-5-methyl-4-isoxazolepropionate.

The fundamental role of memory in olfactory perception

“…Thus, features could vary in their complexity from specific carbon chain lengths and functional groups to, perhaps, specific combinations of these more simple components. Whatever constitutes an odorant feature, however, glomeruli and second-order-neuron mitral cells appear to function as feature detectors, refining the feature information through lateral and feedback inhibition [8,9] and excitation [32]. A second opportunity for feature synthesis appears in the activity of mitral cells, where temporal binding of simultaneously active cells (via synaptic interactions on the expansive lateral dendrites of inhibitory granule cells) can lead to precise temporal synchrony of co-active cells that are potentially activated by different features or mixture components [33,34] (see Refs [35,36] for a discussion of similar processes in invertebrates).…”

“…A spatial representation of odorant features is created through precise receptor projections to olfactory bulb glomeruli [3 -7]. This spatial representation is enhanced by convergence and lateral synaptic interactions within the olfactory bulb [8,9], resulting in olfactory bulb output neurons (mitral cells) with feature-detecting receptive fields [10]. Thus, odorants and odor mixtures appear to be processed analytically by the olfactory system.…”

Organization of Ionotropic Glutamate Receptors at Dendrodendritic Synapses in the Rat Olfactory Bulb

“…These synapses are located in the external plexiform layer (EPL) in which they constitute the large majority of the synaptic junctions and consist of an asymmetric mitral-to-granule synapse directly adjacent to a symmetric granule-to-mitral synapse (Price and Powell, 1970a,b). The dendrodendritic circuit forms the basis for feedback inhibition of the mitral cells and provides a mechanism for lateral inhibition that may be relevant for odor discrimination and olfactory learning (Yokoi et al, 1995;Brennan and Keverne, 1997).…”

Spatio-temporal specification of olfactory bulb interneurons

“…Olfactory discrimination, learning and memory have been attributed to changes at reciprocal dendro-dendritic synapses between mitral/tufted cells and interneurons. The continual addition of interneurons, which modulate the coding of olfactory information through lateral inhibition and synchronization of firing of the mitral/tufted cells, might provide a novel substrate for adapting to complex, changing environments (Laurent 2002 ;Yokoi et al 1995 ). Cell death, which is also prominent in the adult OB (Corotto et al 1994 ;Petreanu and Alvarez-Buylla 2002 ), may also effect rapid changes in neural circuits.…”