Limbic influence on the periaqueductal gray: a single unit study in the awake squirrel monkey

Area of Science:

• Neuroscience
• Systems Neuroscience

Background:

• The periaqueductal gray (PAG) is a midbrain structure involved in various functions, including pain modulation and defensive behaviors.
• The limbic system, comprising the amygdala and hippocampus, plays a crucial role in emotion and memory.
• Understanding the neural circuitry connecting the limbic system to the PAG is essential for deciphering complex behavioral processes.

Purpose of the Study:

• To investigate the influence of the amygdala and hippocampus on single neurons within the periaqueductal gray (PAG).
• To characterize the response patterns of PAG neurons to electrical stimulation of different amygdala subregions and the hippocampus.

Main Methods:

• Extracellular single-unit recordings were performed in the PAG of rodents.
• Electrical stimulation was applied to the basolateral, corticomedial, and lateral amygdala, as well as the anterior hippocampus.
• Neuronal responses, including firing rates, latencies, and response patterns, were analyzed.

Main Results:

• Fifty-one percent of sampled PAG neurons exhibited a limbic influence.
• PAG neurons responded to basolateral amygdala stimulation with short-duration excitatory responses (mean latency 30 ms).
• Stimulation of different amygdala subregions elicited varied excitatory and inhibitory response patterns in PAG neurons.
• Hippocampal stimulation, particularly anterior regions, evoked excitatory or inhibitory responses in a smaller subset of PAG neurons (15%).
• High-frequency basolateral amygdala stimulation increased neuronal firing probability and reduced response latency and variability in the PAG.

Conclusions:

• A significant portion of PAG neurons are modulated by limbic inputs from the amygdala and hippocampus.
• The PAG displays differential activation patterns in response to specific amygdala subregions, suggesting a complex integration of limbic information.
• These findings elucidate the intricate neural pathways between the amygdala, hippocampus, and PAG, contributing to our understanding of their roles in behavior.