Nigrocaudate and nigroputaminal projections in the monkey

Area of Science:

• Neuroscience
• Neuroanatomy
• Histochemistry

Background:

• The substantia nigra pars compacta (SNc) is a key component of the basal ganglia, involved in motor control and reward.
• Acetylcholinesterase (AChE) histochemistry reveals distinct compartmentalization within the SNc, but its functional significance remains debated.
• The striatum, a major target of SNc projections, is also compartmentalized into AChE-poor striosomes and an AChE-rich matrix.

Purpose of the Study:

• To investigate the relationship between retrograde neuronal labeling in the SNc and its AChE compartmentalization.
• To determine if the segregation of nigrocaudate and nigroputaminal projections corresponds to SNc AChE compartments.
• To explore the functional implications of SNc histochemical compartmentalization in relation to striatal organization.

Main Methods:

• Retrograde neuronal tracing by injecting tracers into the caudate nucleus and putamen of squirrel monkeys.
• Analysis of retrograde cell labeling within the nigral complex, specifically the SNc.
• Histochemical staining for acetylcholinesterase (AChE) to identify SNc compartments.

Main Results:

• Precise topography was confirmed in the nigrostriatal projection system, including nigrocaudate and nigroputaminal pathways.
• No double-labeled nigral cells were found when two different tracers were injected, indicating largely independent nigrocaudate and nigroputaminal projecting cell groups.
• Labeled nigral cells were found within the same AChE compartments, regardless of whether they projected to the caudate or putamen, demonstrating that projection segregation does not solely explain SNc AChE compartmentalization.

Conclusions:

• The segregation of nigrocaudate and nigroputaminal projecting cells does not fully account for the AChE compartmentalization of the SNc.
• SNc histochemical compartmentalization is likely related, at least in part, to the subdivision of the striatum into AChE-poor striosomes and AChE-rich extrastriosomal matrix.
• This suggests a functional link between SNc organization and striatal compartmentalization, potentially influencing basal ganglia circuitry and function.