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the junction between two neurons (axon-to-dendrite) or between a neuron and a muscle; "nerve impulses cross a synapse through the action of neurotransmitters"

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シナプス(神経細胞の連接部)

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/10/24 06:12:05」(JST)

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Dendrodendritic synapses are connections between the dendrites of two different neurons. This is in contrast to the more common axodendritic synapse (chemical synapse) where the axon sends signals and the dendrite receives them. Dendrodendritic synapses are activated in a similar fashion to axodendritic synapses in respects to using a chemical synapse. These chemical synapses receive a depolarizing signal from an incoming action potential which results in an influx of calcium ions that permit release of Neurotransmitters to propagate the signal the post synaptic cell. There is also evidence of bi-directionality in signaling at dendrodendritic synapses. Ordinarily, one of the dendrites will display inhibitory effects while the other will display excitatory effects.^[1] The actual signaling mechanism utilizes Na+ and Ca2+ pumps in a similar manner to those found in axodendritic synapses.^[2]

History

In 1966 Wilfrid Rall, Gordon Shepard, Thomas Reese, and Milton Brightman found a novel pathway, dendrites that signaled to dendrites.^[3] While studying the mammalian olfactory bulb, they found that there were active dendrites that couple and send signals to each other. The topic was then only explored sporadically due to difficulties with techniques and technology available to further investigate dendrodendritic synapses. Investigations into this phenomenon of active dendrites has resurfaced with vigor at the start the 21st century.

The study of dendrodendritic synapses in the olfactory bulb provided some early examples of ideas about neuronal organization relating to dendritic spines^[1]

One spine could serve as an input-output unit
One neuron could contain multiple dendritic spines
These spines are widely spaced, indicating some independent function
Synaptic input-output events can occur without axonal stimulation

Location

Dendrodendritic synapses have been found and studied in both the olfactory bulb and the retina. They have also been found though not extensively studied in the following brain regions: thalamus, substantia nigra, locus ceruleus.^[4]

Olfactory bulb

Dendrodendritic synapses have been studied extensively in the olfactory bulb of rats where it is believed they help in the process of differentiating smells. The granule cells of the olfactory bulb communicate exclusively through dendrodendritic synapses because they lack axons. These granule cells form dendrodendritic synapses with mitral cells to convey odor information from the olfactory bulb. Lateral inhibition from the granule cell spines helps to contribute to contrasts between odors and in odor memory.^[4]

Dendrodendritic synapses have also been found to have similar effects on olfactory input from the glomeruli of the antennal lobe of insects.

Retina

The spatial and color contrast systems of the retina operate in a similar manner. Dendrodendritic homologous gap junctions have been found as a way of communication between dendrites in the retinal α-type Ganglion cells to produce a faster method of communication to modulate the color contrast system.^[5] Using bidirectional electrical synapses in the dendrodendrtic syanpses they modulate inhibition of different signals thus allowing for a modulation of the color contrast system. This dendritic function is an alternative modulatory system to that of pre-synaptic inhibition which is presumed to also help differentiate different contrast in the visual sense.^[6]

Neuroplasticity

Dendrodendritic synapses can play a role in neuroplasticity. In a simulated disease state where axons were destroyed, some neurons formed dendrodendritic synapses to compensate.^[7] In experiments where deafferentation or axotomy was performed in the lateral geniculate nucleus (LGN) of cats it was found that pre-synaptic dendrites began to form to compensate for the lost axons.^[7] These pre-synaptic dendrties were revealed to form new dendrodenritic excitatory synapses in the cells that had survived. The development of presynaptic dendrites forming dendrodendritic synapses in the Cerebellar Cortex of mice has also been found following the differentiation of that region.^[7] This type of dendritic reactive synaptogenesis is thought to occur in order to re-saturate the region which has become vacant postsynaptic sites following neurodegeneration caused by deafferentation or axotomy in order to restore partial functionality to the affected region.^[7] Partial recorvery within the LGN has been shown thus supporting the validity of dendrodendritic synapses between neighboring relay neurons functionality.^[7]

References

^ ^a ^b Shepard, G.M. (1996). "The dendritic spine: a multifunctional integrative unit". J. Neurophysiol 75: 2197–2210. PMID 8793734.
^ Masurkar, Arjun; Chen, Wei (Jan 25, 2012). "The influence of single bursts versus single spikes at excitatory dendrodendritic synapses". European Journal of Neuroscience 35: 389–401. doi:10.1111/j.1460-9568.2011.07978.x.
^ Rall, W; Shepard, G.M.; Reese, T.S.; Brightman M.W. (January 1966). "Dendrodendritic synaptic pathway for inhibition in the olfactory bulb". Experimental Neurology 14 (1): 44–56. doi:10.1016/0014-4886(66)90023-9.
^ ^a ^b Shepard, G.M. (July 2009). "Dendrodendritic synapses: past, present and future.". Annals of the New York Academy of Sciences 1170. doi:10.1111/j.1749-6632.2009.03937.x. PMC 3819211.
^ Hidaka, Sid; Akahori, Y.; Yoshikazu, K. (Nov 17, 2004). "Dendrodendritic Electrical Synapses between Mammalian Retinal Ganglion Cells.". The Journal of Neuroscience 24 (46): 10553–10567. doi:10.1523/JNEUROSCI.3319-04.2004. PMID 15548670.
^ Eggers, Arika; McCall, Maureen; Lukasiewicz, Peter (Jul 15, 2007). "Presynaptic inhibition differentially shapes transmission in distinct circuits in the mouse retina". The Journal of Physiology 582: 569–582. doi:10.1113/jphysiol.2007.131763.
^ ^a ^b ^c ^d ^e Hamori, J (2009). "Morphological plasticity of postsynaptic neurons in reactive synaptogenesis.". J Exp Biol 153: 251–260. PMID 2280223.

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English Journal

BDNF over-expression increases olfactory bulb granule cell dendritic spine density in vivo.

McDole B1, Isgor C1, Pare C1, Guthrie K2.
Neuroscience.Neuroscience.2015 Sep 24;304:146-60. doi: 10.1016/j.neuroscience.2015.07.056. Epub 2015 Jul 23.
Olfactory bulb granule cells (GCs) are axon-less, inhibitory interneurons that regulate the activity of the excitatory output neurons, the mitral and tufted cells, through reciprocal dendrodendritic synapses located on GC spines. These contacts are established in the distal apical dendritic compartm
PMID 26211445

Nectin-1 spots as a novel adhesion apparatus that tethers mitral cell lateral dendrites in a dendritic meshwork structure of the developing mouse olfactory bulb.

Inoue T1,2,3, Fujiwara T3,4, Rikitake Y1,3,5, Maruo T1,2,3, Mandai K1,2,3, Kimura K6, Kayahara T7, Wang S3,4, Itoh Y3,4, Sai K4, Mori M3,8, Mori K9,10, Mizoguchi A3,4, Takai Y1,2,3.
The Journal of comparative neurology.J Comp Neurol.2015 Aug 15;523(12):1824-39. doi: 10.1002/cne.23762. Epub 2015 May 13.
Mitral cells project lateral dendrites that contact the lateral and primary dendrites of other mitral cells and granule cell dendrites in the external plexiform layer (EPL) of the olfactory bulb. These dendritic structures are critical for odor information processing, but it remains unknown how they
PMID 25967681

Nectin-1 spots regulate the branching of olfactory mitral cell dendrites.

Fujiwara T1, Inoue T2, Maruo T2, Rikitake Y3, Ieki N4, Mandai K2, Kimura K5, Kayahara T6, Wang S1, Itoh Y1, Sai K7, Mori M8, Mori K4, Takai Y9, Mizoguchi A10.
Molecular and cellular neurosciences.Mol Cell Neurosci.2015 Jul 11;68:143-150. doi: 10.1016/j.mcn.2015.07.003. [Epub ahead of print]
Olfactory mitral cells extend lateral secondary dendrites that contact the lateral secondary and apical primary dendrites of other mitral cells in the external plexiform layer (EPL) of the olfactory bulb. The lateral dendrites further contact granule cell dendrites, forming dendrodendritic reciproca
PMID 26169026

Japanese Journal

Muscarinic receptor type 1 (M1) stimulation, probably through KCNQ/Kv7 channel closure, increases spontaneous GABA release at the dendrodendritic synapse in the mouse accessory olfactory bulb

Takahashi Yoshito,Kaba Hideto
Brain Research 1339, 26-40, 2010-06-21
NAID 120002220975

Synaptic Mechanisms Underlying Pheromonal Memory in Vomeronasal System

Ichikawa Masumi
Zoological science 20(6), 687-695, 2003-06-25
… The vaginocervical stimulation at mating reduces the dendrodendritic feedback inhibition of principal neurons (mitral/tufted (MT) cells) in the AOB and enhances their cell activity. … The enhancement of activity induces on these plastic changes in dendrodendritic synapses, which in turn enhance GABA-mediated inhibition of MT cell activity. …
NAID 110006162743