Serotonin nerve terminals in adult rat neocortex

Area of Science:

• Neuroscience
• Cell Biology
• Neuroanatomy

Background:

• Serotonin (5-HT) is a key neurotransmitter involved in various brain functions.
• Understanding the precise localization and ultrastructure of serotonin terminals is crucial for elucidating its role.
• Previous methods lacked the resolution to definitively characterize serotonin axonal processes in the cortex.

Purpose of the Study:

• To identify and characterize axonal processes that take up and retain exogenous tritiated serotonin ([3H]5-HT) in the rat fronto-parietal cortex.
• To determine the origin and ultrastructural features of these serotonin-containing axons.
• To investigate the potential roles of specific organelles within serotonin varicosities.

Main Methods:

• High-resolution radioautography using [3H]5-HT on adult rat brain tissue.
• Topical superfusion of cortical slices with [3H]5-HT.
• Electron microscopy and topometric analysis of serial thin sections.
• Comparative analysis with control groups (6-hydroxydopamine pretreatment, midbrain raphe lesions).

Main Results:

• Specific labeling of axonal processes in the upper cortical layers was achieved, originating from serotonin cell bodies in Dahlström and Fuxe groups B7 and B8.
• Serotonin fibers were characterized as tenuous processes with enlarged varicosities containing mitochondria, small agranular vesicles, and large granular vesicles.
• Exogenous [3H]5-HT concentrated in varicosities, with preferential uptake by mitochondria and vesicular organelles.
• Only a small fraction (5%) of serotonin varicosities showed synaptic membrane differentiations, contrasting with unlabeled nerve endings (50%).
• Large granular vesicles, though not always visible, were estimated at ~7 per varicosity, suggesting a role beyond primary storage, possibly in serotonin metabolism.

Conclusions:

• The study provides detailed ultrastructural evidence for cortical serotonin (5-HT) fibers, confirming their origin and specific morphological characteristics.
• The findings suggest that serotonin can be released from varicosities lacking typical synaptic junctions, implying broader functional implications.
• The dynamic properties and potential metabolic roles of serotonin varicosities highlight their significance in cortical function, plasticity, and regrowth.