Structure of dopamine-releasing neurons relates to the type of circuits they form for smell-processing

(Press-News.org) Closely related subtypes of dopamine-releasing neurons may play entirely separate roles in processing sensory information, depending on their physical structure.  

New research from the Institute of Psychiatry, Psychology & Neuroscience (IoPPN) at King’s College London has found that variations in the physical structure of neurons might have a striking impact in the role that they play when processing sensory information.  

It identified two different sub-types of interneuron in the olfactory bulb which is where the brain first processes information about smell. One of these sub-types was found to communicate in a highly unusual manner, releasing signals from a part of the neuron that has most commonly been associated with receiving signals.   

Published in eLife, the research examined the olfactory bulbs of mice brains to assess the structure of different sub types of neuron that produce and release the chemical dopamine. It is the first study to provide anatomical and physiological evidence that distinct subtypes of dopaminergic neurons in the bulb transmit signals in fundamentally different ways based on the shape and structure of the cell. 

Most neurons send their signals by releasing chemical neurotransmitters via long thin arm-like protrusions of the cell called axons. In the classic view, neurons also receive signals from other neurons via other types of branching tree-like protrusions, known as dendrites. This distinction in role performed by these different structures forms the backbone of how neurons are believed to function. However, this new study provides evidence that these structures in the neuron might not always behave this way.  

Researchers in this study found that neurons within the olfactory bulb can be separated into two distinct subtypes, distinctly characterised by how they transmit and receive signals.  

 One type of dopamine interneuron in the olfactory bulb did not possess an axon at all, rather it released neurotransmitter signals from its dendrites, normally the input not the output part of the cell. These cells are called ‘anaxonic neurons’. 

These unusual anaxonic neurons acted locally within the olfactory bulb and were able to self-inhibit, which means that they can turn down their own activity levels. 

This study was the first to show that a separate sub-type of neurons in the olfactory bulb that have axons, known as “axon-bearing dopaminergic neurons”, do not release signals from their dendrites and cannot self-inhibit.  These neurons followed the classical model of how neurons send signals to each other with the release sites contained almost wholly within the axon. These axons travel long distances across the olfactory bulb rather than influencing their own cell’s electrical activity through self-inhibition, like the anaxonic neurons. 

Dr Ana Dorrego-Rivas, a post-doctoral researcher at King’s IoPPN and the study’s first author said, “Our findings support that the two dopaminergic subclasses play fundamentally different roles in the olfactory bulb. While the neurons without an axon act locally, shaping the processing of odour signals within specific spherical structures in the brain; axon-bearing cells act over long distances, coordinating activity between these spherical structures and potentially enhancing contrast between distinct odours. Despite the fact that they both release dopamine and are located in the olfactory lobe, our findings suggest that these neurons contribute to smell processing in remarkably different ways.” 

Professor Matthew Grubb, Professor in Neuroscience at King’s IoPPN and the study’s senior author said, “The olfactory system is weird and wonderful, so it was a big surprise to find some cells there that behave just like ‘standard’ neurons. It’s going to be fun to try to figure out how these abnormally-normal cells contribute to the perception of smell stimuli.” 

Strikingly different neurotransmitter release strategies in dopaminergic subclasses (doi.org/10.7554/eLife.105271.1) (Dorrego-Rivas, Grubb et al.) was published in eLife. 

This research was funded by the Wellcome Trust, the European Research Council, and UKRI through the Biotechnology and Biological Sciences Research Council and the Medical Research Council. 

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