Ventral 5-HT neurons

A cluster of serotonin-containing neurons has been identified on the subpial surface of ventral medulla, marked by the white asterisk in the figure below.

From raphe neurons

The ventral 5-HT neurons are also referred to as supramedullary 5-HT neurons. This cluster of 5-HT-containing neurons is unique because it is situated more superficially than the brain parenchyma, above the layer of white matter and glia cells. Located merely 200-400 um beneath the pia layer, the supramedullary neurons are nearby to blood vessels, cerebrospinal fluid (CSF), and subarachnoid spaces.

Supramedullary neurons are classified by Sotnikov as primary sensory neurons of the brain, which are notable for the sensing capabilities of their dendrites. The dendritic processes of supramedullary neurons directly abut the surface of the brain, suggesting a sensory role for them in monitoring nutrient and gas levels in the CSF. The raphe and supramedullary neurons are thought to function as a response system to glucose, pH, and CO2 overload.

Ventral 5-HT neurons are an excellent example of Sotnikov's sensory interoceptor neurons because they have a bipolar cell body shape, and a superficial location, so these neurons were presumably born recently from epithelial layers and have not migrated deeply into the body cavity. These neurons may be involved in early sensory processes that are not yet affected by top-down influences.

Using an antibody to 5-HT-binding protein, Gorcs and colleagues stained the ventral 5-HT neurons, in Figure 3 below. Cell bodies are spread along the midline from the caudal tip of the inferior olive (IO) to the level of the oculomotor nucleus. The cell bodies cluster by cranial nerve roots of CN 9, CN 10 (vagus), and CN 12 (hypoglossal).

In summary,

"The presence of serotonergic neurons in contact with CSF in the supraependymal plexus, in the area postrema, and in the subpial location on the ventral surface of the brain apparently projecting to the leptomeninges is consistent with the possibility that 5-HT plays a humoral role in the CSF." (A.L. Kirchgessner, 1988)

Supermedullary neurons of pufferfish have exceptionally high DNA contents and ribosomal gene expression, indicating that these 5-HT-containing neurons are very active in protein synthesis, like raphe neurons in the medulla.


References

Gorcs T. J., Z. Liposits, S. L. Palay and V. Chan-Palay. (1985). Serotonin neurons on the ventral brain surface. Proc. Natl. Acad. Sci. U.S.A. 82, 7449-7452.

Bennett M. V., Y. Nakajima and G. D. Pappas. (1967). Physiology and ultrastructure of electrotonic junctions. I. Supramedullary neurons. J. Neurophysiol. 30, 161-179.

Cuoghi B. and M. Marini. (2001). Ultrastructural and cytochemical features of the supramedullary neurons of the pufferfish Diodon holacanthus (L.) (Osteichthyes). Tissue Cell. 33, 491-499.

Kirchgessner A. L., M. D. Gershon, K. P. Liu and H. Tamir. (1988). Costorage of serotonin binding protein with serotonin in the rat CNS. J.Neurosci. 8, 3879-3890.