Retrograde synapses are classic axo-dendritic synapses. However, in a retrograde synapse, the postsynaptic cell releases a gaseous neurotransmitter (e.g., carbon monoxide or nitric oxide) that increases the amount of neurotransmitter released by the presynaptic cell. 

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The classic synapses are axo-dendritic synapses (i.e., synapses that forms between a dendrite and an axon) or axo-somatic synapses (i.e., synapses that form between the axon and the cell body or soma). However, there are many other different types of synapses. Synapses can vary by location on the neuron as well as by the mechanism of synaptic transmission (i.e., electrical or chemical transmission). 

Types of Synapses

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

A synapse that forms between two axons and allows regulation of neurotransmitter release by the presynaptic neuron. 

Axo-axonic Synapse

Retrograde Synapse

A synapse that forms between two specialized dendrites that allows for coordination of neural activity. 

Dendro-dendritic Synapse

Non-directed synapses are synapses that involve the axonal swellings in the presynaptic cell releasing neurotransmitters to broadly affect surrounding tissue. The axonal swellings on the presynaptic cell are known as varicosities. 

Non-directed Synapses

An electrical synapse is a specialized connection where two neurons are physically linked by structures called gap junctions. These junctions contain channels that permit electrical current to pass directly from the presynaptic to the postsynaptic cell. This direct physical connection allows for extremely rapid, almost instantaneous, signal transmission between neurons.

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