Dendrodendritic synapsesare connections between thedendritesof two different neurons. This is in contrast to the more commonaxodendritic synapse(chemical synapse) where theaxonsends signals and the dendrite receives them. Dendrodendritic synapses are activated in a similar fashion to axodendritic synapses in respects to using a chemical synapse. An incoming action potential permits the release of neurotransmitters to propagate the signal to the post synaptic cell. There is evidence that these synapses are bi-directional, in that either dendrite can signal at that synapse. Ordinarily, one of the dendrites will display inhibitory effects while the other will display excitatory effects.[1]The actual signaling mechanism utilizesNa+andCa2+pumpsin a similar manner to those found in axodendritic synapses.[2]
History
editIn 1966 Wilfrid Rall, Gordon Shepherd, Thomas Reese, and Milton Brightman found a novel pathway, dendrites that signaled to dendrites.[3]While studying the mammalianolfactory bulb,they found that there were active dendrites that couple and send signals to each other. The topic was then explored only 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 todendritic spines[4]
- 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
editDendrodendritic synapses have been found and studied in both theolfactory bulband theretina.They have also been found though not extensively studied in the following brain regions:thalamus,substantia nigra,locus ceruleus.[5]
Olfactory bulb
editDendrodendritic synapses have been studied extensively in the olfactory bulb of rats where it is believed they help in the process of differentiating smells. Thegranule cellsof the olfactory bulb communicate exclusively through dendrodendritic synapses because they lack axons. These granule cells form dendrodendritic synapses withmitral cellsto 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.[5]
Dendrodendritic synapses have also been found to have similar effects on olfactory input from theglomeruliof theantennal lobeof insects.
Retina
editThe spatial and color contrast systems of the retina operate in a similar manner. Dendrodendritic homologousgap junctionshave 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.[6]Using bidirectional electrical synapses in the dendrodendritic synapses 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.[7]
Neuroplasticity
editDendrodendritic synapses can play a role inneuroplasticity.In a simulated disease state where axons were destroyed, some neurons formed dendrodendritic synapses to compensate.[8]In experiments where deafferentation oraxotomywas performed in thelateral geniculate nucleus(LGN) of cats it was found that pre-synaptic dendrites began to form to compensate for the lost axons.[8]These pre-synaptic dendrites were revealed to form new dendrodendritic 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.[8]This type of dendritic reactivesynaptogenesisis thought to occur in order to re-saturate the region which has become vacant postsynaptic sites followingneurodegenerationcaused by deafferentation or axotomy in order to restore partial functionality to the affected region.[8]Partial recovery within the LGN has been shown thus supporting the validity of dendrodendritic synapses between neighboring relay neurons functionality.[8]
References
edit- ^Shepherd, G.M. (1996). "The dendritic spine: a multifunctional integrative unit".J. Neurophysiol.75(6): 2197–2210.doi:10.1152/jn.1996.75.6.2197.PMID8793734.
- ^Masurkar, Arjun; Chen, Wei (Jan 25, 2012)."The influence of single bursts versus single spikes at excitatory dendrodendritic synapses".European Journal of Neuroscience.35(3): 389–401.doi:10.1111/j.1460-9568.2011.07978.x.PMC4472665.PMID22277089.
- ^Rall, W; Shepherd, 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.PMID5900523.
- ^Shepard, G.M. (1996). "The dendritic spine: a multifunctional integrative unit".J. Neurophysiol.75(6): 2197–2210.doi:10.1152/jn.1996.75.6.2197.PMID8793734.
- ^abShepherd, G.M. (July 2009)."Dendrodendritic synapses: past, present and future".Annals of the New York Academy of Sciences.1170:215–223.doi:10.1111/j.1749-6632.2009.03937.x.PMC3819211.PMID19686140.
- ^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.PMC6730298.PMID15548670.
- ^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(2): 569–582.doi:10.1113/jphysiol.2007.131763.PMC2075342.PMID17463042.
- ^abcdeHamori, J (2009). "Morphological plasticity of postsynaptic neurons in reactive synaptogenesis".J Exp Biol.153:251–260.doi:10.1242/jeb.153.1.251.PMID2280223.