Cortical representations of olfactory input by trans-synaptic tracing

Abstract

In the mouse, each class of olfactory receptor neurons expressing a given odorant receptor converges their axons onto two specific glomeruli in the olfactory bulb (OB), thereby creating an odor map. How is this map represented in the olfactory cortex? Here we combine rabies virusdependent retrograde mono-transsynaptic labeling with genetics to control the location, number and type of 'starter' cortical neurons, from which we trace their presynaptic neurons. We find that individual cortical neurons receive input from multiple mitral cells representing broadly distributed glomeruli. Different cortical areas represent the OB input differently. For example, the cortical amygdala preferentially receives dorsal OB input, whereas the piriform cortex samples the whole OB without obvious bias. These differences likely reflect different functions of these cortical areas in mediating innate odor preference or associative memory. The transsynaptic labeling method described here should be widely applicable to mapping connections throughout the mouse nervous system. The functions of mammalian brains are based on activity patterns of large numbers of interconnected neurons that form information processing circuits. Neural circuits consist of local connections, where pre-and post-synaptic partners reside within the same brain area, and long-distance connections, which link different areas. Local connections can be predicted by axon and dendrite reconstructions1, and confirmed by physiological recording and stimulation methods2. Long-distance connections are more difficult to map, as commonly used methods can only trace bulk projections with a coarse resolution. Most methods cannot distinguish axons in passing from those that form synapses, or pinpoint neuronal types to

Molecular layer perforant path-associated cells contribute to feed-forward inhibition in the adult dentate gyrus

“…We used a genetically modified rabies virus to transsynaptically trace monosynaptic inputs to newborn DGCs (4). This genetically modified rabies virus has recently been used to map neural connectivity in the piriform cortex, amygdala, retinal ganglion cells, and motor neurons (5)(6)(7)(8)(9). Unlike traditional tracers, this genetically modified rabies virus is able to target particular cell types and subsequently retrogradely transport across a single synapse, thereby only labeling direct connections onto the initially infected cells (4).…”

Olfactory projectome in the zebrafish forebrain revealed by genetic single-neuron labelling

“…These findings are consistent with an optical imaging study that found no apparent spatial organization of odour-evoked activity patterns in the piriform cortex 11 . In contrast, the anterior olfactory nucleus and the cortical amygdala receive topographic and biased projections from the OB, respectively 9,10 . Thus, a conceptual organization has been proposed in which the secondary olfactory pathway bifurcates to transform odour information into stereotyped and random representations, features suited for directing innate and learned behaviours, respectively 12,13 .…”

“…The odour map in the OB is transmitted by output neurons, mitral cells, to higher olfactory centres and eventually translated to elicit appropriate behavioural and physiological responses. Recent anatomical studies in mice showed that axons from identified glomeruli project diffusely throughout the piriform cortex [6][7][8][9] , the largest target area of the OB, and that piriform neurons receive convergent inputs from multiple mitral cells distributed throughout the OB 10 . These findings are consistent with an optical imaging study that found no apparent spatial organization of odour-evoked activity patterns in the piriform cortex 11 .…”

Olfaction : quand le cortex redistribue les cartes

“…Il est important de noter que la cellule internalisée est vivante au moment de l'entose et que la cellule hôte joue un rôle passif au cours de ce processus. Ce mécanisme d'internalisation homotypique contrôlé par les cadhérines permet de distinguer l'entose des autres processus de cannibalisme cellulaire [1][2][3]. Par la suite, la cellule internalisée est digérée lentement par les lysosomes de la cellule hôte dans la plupart des cas (Figure 2A).…”

“…Après réplication, la particule virale est transportée jusque dans les dendrites et traverse les espaces synaptiques pour envahir les partenaires présynaptiques de la cellule infectée. Différentes modifications génétiques du virus utilisé permettent : (1) l'expression d'une protéine fluorescente pour visualiser l'ensemble des neurones présynap-tiques ; (2) [8]. En second lieu, il apparaît que deux cellules mitrales d'un même glomérule ont pour cible des territoires corticaux aussi distants que deux cellules mitrales émanant de glomérules distincts (Figure 1).…”