Dendrodendritic synapses

It is now well ascertained that dendrites are capable of propagating action potentials not only in distal to proximal direction, but also in the reverse direction by back-propagation after initiation at the cell body (Ludwig and Pittman, 2003). The so-called law of dynamic polarization enunciated by Cajal (see Berlucchi, 1999) was aimed at stating the unidirectional propagation of excitations within the nervous system, and assumed that nerve impulses are conducted from the dendrite or soma to axon terminals. This dogma is now being reconsidered, not only in view of the evidence of dendrodendritic synapses, but also in view of the existence of electrical synapses in which the flow of information can be bidirectional. 

Sassoe-Pognetto, M., and Ottersen, O.P. 2000. Organization of Ionotropic Glutamate Receptors at Dendrodendritic Synapses in the Rat Olfactory tdaVo. J Neurosci 20, 2192-2201. 

The EPL lies beneath or deep to the glomeruli, and it primarily consists of dense neuropil formed by the dendrites of mitral cells and GCs that ascend from the MCL and GCL, respectively. Relative to other MOB layers, the EPL has a low cell density. In Nissl-stained sections, however, it can nevertheless be seen to contain significant numbers of neurons ( Figure 6-3). These include several subtypes of tufted cells and intrinsic interneurons, which are described later. Because tufted cells are in many aspects similar to mitral cells, and as mitral and tufted cell dendrites cannot be distinguished ultrastructurally, the term mitral/tufted cell is often used when generalizing to these two cell populations. The dominant feature of the EPL is nevertheless the extensive dendrodendritic synapses between mitral/tufted cells and GCs. 

The GCL is the deepest neuronal layer in the bulb, and it contains the largest number of cells. Most of the neurons of the GCL are the GCs, but there are also small numbers of Golgi cells, Cajal cells, and Blanes cells. As discussed earlier, the GCs are inhibitory GABAergic cells that form dendrodendritic synapses with mitral/tufted cells in the EPL. 

Panzanelli P, Homanics GE, Ottersen OP, Fritschy J-M, Sassoe-Pognetto M. 2004. Pre- and postsynaptic GABAA receptors at reciprocal dendrodendritic synapses in the olfactory bulb. Eur J Neurosci 20 2945-2952. 

Sassoe-Pognetto M, Ottersen OP. 2000. Organization of ionotropic glutamate receptors at dendrodendritic synapses in the rat olfactory bulb. J Neurosci 20 2192-2201. 

Sassoe-Pognetto M, Utvik JK, Camoletto P, Watanabe M, Stephenson FA, et al. 2003. Organization of postsynaptic density proteins and glutamate receptors in axodendritic dendrodendritic synapses of the rat olfactory bulb. J Comp Neurol 463 237-248. 

Fig. 5. Basic circuitry of the main olfactory bulb. Axons of ORNs form the olfactory nerve (ON). These axons terminate in the glomeruli onto mitral (M) and tufted cells (external tufted cell, ET middle tufted cell, MT) and onto juxtaglomerular neurons including periglomerular cells (PG), ET cells and short axon cells (SA). There are one way and reciprocal synapses between the apical dendritic branches of mitral and tufted cells and the dendrites of juxtaglomerular neurons (upper inset - glomerular synapses). The lateral dendrites of mitral and tufted cells form one way and reciprocal synapses with the apical dendrites of granule cells (lower inset - dendrodendritic synapses).

The transmitter(s) of these olfactory cortical projections to the bulb are not known although glutamate is suspected because that excitatory amino acid is found in many cells in layers II and III of piriform and entorhinal cortex (Kaneko and Mizuno, 1988) (Table 4). These feedback projections from olfactory cortex to the olfactory bulb are believed to primarily excite the GABAergic granule cells in MOB which in turn inhibit firing of mitral cells (Nicoll, 1971) via dendrodendritic synapses between granule and mitral cell dendrites (Halasz and Shepherd, 1983). 

Rail, W. (1972) Dendritic neuron theory and dendrodendritic synapses in a simple cortical system. In F.O. Schmitt (Ed.), The Neurosciences Second Study Program, pp. 552-565. Rockefeller University Press, New York. 

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