Targets of horizontal connections in macaque primary visual cortex.

Area of Science:

• Neuroscience
• Cell Biology
• Cortical Circuitry

Background:

• Pyramidal neurons in the cerebral cortex exhibit extensive horizontal axonal collateral systems.
• The function and specificity of these long-range connections remain incompletely understood at the ultrastructural level.

Purpose of the Study:

• To investigate the synaptic characteristics and target specificities of horizontal connections made by superficial layer pyramidal cells in the primate striate cortex.
• To elucidate the ultrastructural basis of information flow within cortical circuits.

Main Methods:

• Intracellular injection of horseradish peroxidase (HRP) in two superficial layer pyramidal cells of the primate striate cortex.
• Electron microscopic (EM) serial reconstruction of labeled axon terminals and their postsynaptic targets.
• Light microscopy to examine the distribution of axon collaterals.

Main Results:

• Labeled axon terminals formed asymmetric synapses with round vesicles, indicative of excitatory transmission.
• Approximately 80% of postsynaptic targets were identified as pyramidal cells (dendritic spines and shafts).
• The remaining 20% of targets were dendritic shafts of smooth stellate cells, likely inhibitory (GABAergic) neurons, potentially small-medium basket cells.

Conclusions:

• Horizontal connections from pyramidal cells exhibit a mixed output, targeting both excitatory pyramidal cells and inhibitory stellate cells.
• This pattern of connectivity suggests these long-range connections mediate diverse postsynaptic effects.
• These horizontal connections may underlie the observed influences between the receptive field center and its surround in the visual cortex.