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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 axodendritic synapses. 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. The actual signaling mechanism utilizes Na+ and Ca2+ pumps in a similar manner to those found in axodendritic synapses.

History
In 1966 Wilfrid Rall, Gordon Shepard, Thomas Reese, and Milton Brightman found a novel pathway, dendrites that signaled to dendrites. While studying the mammalian olfactory bulb they found that there were active dendrites that coupled 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


 * One spine could serve as an input-output unit
 * One neuron can 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.

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.

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.

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.