Introducing the brain: how neurons sense smells (Introduction)

by David Turell , Tuesday, November 28, 2023, 20:12 (359 days ago) @ David Turell

Through the use of two specific proteins:

https://medicalxpress.com/news/2023-11-neurotransmitters-discriminate-odors.html

"A longstanding hypothesis in neurobiology was that a single neuron releases a single type of neurotransmitter, a molecule used by neurons to communicate with one another. In recent decades, several neurons have been found to release more than one neurotransmitter. This phenomenon called co-transmission is increasingly gaining recognition as a powerful and versatile molecular mechanism useful for the dynamic regulation of diverse neural circuits.

"However, precisely how co-transmission affects the firing of target neurons and the overall behavior of an animal remains to be elucidated.

"A recent study in the laboratory of Dr. Benjamin Arenkiel, professor at Baylor College of Medicine...has dissected how co-transmission of two neurotransmitters—gamma-aminobutyric acid (GABA) and dopamine—from a specific group of olfactory bulb neurons modulates the activity of the entire circuit.

"GABA and dopamine are both present and co-transmitted in many brain cells. The Arenkiel lab focused on a type of olfactory inhibitory neurons called the superficial short axon cells (sSACs) that receive inputs from various olfactory sensory neurons.

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"Using a behavioral assay that hinges on the innate ability of mice to detect hidden food, they found that mice that lack both GABA and dopamine were unable to detect odors. Inhibiting either of these neurotransmitters individually did not lead to any impairments in odor detection. However, mice in which either GABA or dopamine release was blocked could not differentiate between pairs of molecularly similar odors that normal mice could easily tell apart.

"'Based on these observations, we conclude that both GABA and dopamine are individually sufficient to detect odors, whereas they likely act cooperatively with one another to discriminate similar odors," added Dr. Lyons-Warren.

"The sSACs receive inputs from various olfactory sensory neurons and send those signals to tufted cells and mitral cells, the primary output neurons that synapse with granule cell neurons present in the deepest layer of the olfactory bulb. Thus, sSACs are a part of the initial circuitry in the olfactory bulb that helps detect, decode, and process olfactory information that the brain receives from the external environment.

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"First, they found GABA and D1 dopamine receptors, which are present throughout the olfactory bulb and importantly, on the known and new targets of sSACs were involved in odor detection.

"Using Channelrhodopsin-assisted circuit mapping (CRACM), they found that although sSACs are connected to external tufted cells via both GABA and dopamine receptors, they only release dopamine to granule cells. Furthermore, they found that impaired release of sSAC GABA or dopamine impacts mitral cell firing frequency, which in turn increases the number of glomeruli that respond to a given odor and leads to reduced odor discrimination.

"'This study provides crucial mechanistic insights into co-transmission specifically in the context of olfaction and shows how this type of neuromodulation sculpts distinct responses to sensory inputs," Dr. Arenkiel said.

"'It also nicely illustrates how co-transmission allows a single cell type to mount varied responses to the same stimuli by differentially modulating different target neurons. Given that co-transmission is now known to occur in various brain cell types, this will serve as the foundation for further explorations on neuromodulatory effects of multiple neurotransmitters in olfaction as well as in other sensory processes.'"

Comment: Of course, infants have to experience each new smell and learn to recognize it. Our sense of smell has declined as we use other senses to help us live. It is a specifically complex system requiring design (Dempski).