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 An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action ...

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. 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. 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 Shepherd, 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 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 * 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 The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...

. They have also been found though not extensively studied in the following brain regions:

thalamus The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all direction ...

, substantia nigra,

locus ceruleus The locus coeruleus () (LC), also spelled locus caeruleus or locus ceruleus, is a nucleus in the pons of the brainstem involved with physiological responses to stress and panic. It is a part of the reticular activating system. The locus coer ...

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 cell A granule is a large particle or grain. It can refer to: * Granule (cell biology), any of several submicroscopic structures, some with explicable origins, others noted only as cell type-specific features of unknown function ** Azurophilic granul ...

s 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 The antennal lobe is the primary (first order) olfactory brain area in insects. The antennal lobe is a sphere-shaped deutocerebral neuropil in the brain that receives input from the olfactory sensory neurons in the antennae and mouthparts. Functi ...

of insects.

Retina

The spatial and color contrast systems of the retina operate in a similar manner. Dendrodendritic homologous

gap junction Gap junctions are specialized intercellular connections between a multitude of animal cell-types. They directly connect the cytoplasm of two cells, which allows various molecules, ions and electrical impulses to directly pass through a regula ...

s 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. Using bidirectional electrical synapses in the dendrodendrtic 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.

Neuroplasticity

Dendrodendritic synapses can play a role in

neuroplasticity Neuroplasticity, also known as neural plasticity, or brain plasticity, is the ability of neural networks in the brain to change through growth and reorganization. It is when the brain is rewired to function in some way that differs from how it ...

. In a simulated disease state where axons were destroyed, some neurons formed dendrodendritic synapses to compensate. In experiments where deafferentation or

axotomy An axotomy is the cutting or otherwise severing of an axon. Derived from axo- (=axon) and -tomy (=surgery). This type of denervation is often used in experimental studies on neuronal physiology and neuronal death or survival as a method to better u ...

was performed in the

lateral geniculate nucleus In neuroanatomy, the lateral geniculate nucleus (LGN; also called the lateral geniculate body or lateral geniculate complex) is a structure in the thalamus and a key component of the mammalian visual pathway. It is a small, ovoid, ventral proj ...

(LGN) of cats it was found that pre-synaptic dendrites began to form to compensate for the lost axons. These pre-synaptic dendrites 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. 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 A neurodegenerative disease is caused by the progressive loss of structure or function of neurons, in the process known as neurodegeneration. Such neuronal damage may ultimately involve cell death. Neurodegenerative diseases include amyotrophic ...

caused by deafferentation or axotomy in order to restore partial functionality to the affected region. Partial recovery within the LGN has been shown thus supporting the validity of dendrodendritic synapses between neighboring relay neurons functionality.

References

{{reflist Neurohistology Neuroplasticity