Periaqueductal Gray (PAG)

Introduction

Periaqueductal Gray (Pag) is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.

Overview

The periaqueductal gray (PAG), also known as the central gray, is a column of gray matter surrounding the cerebral aqueduct (aqueduct of Sylvius) in the midbrain. The PAG is one of the most functionally diverse structures in the central nervous system, serving as a critical integration center for pain modulation, defensive behavior, autonomic regulation, vocalization, and sleep-wake control [@behbehani1995]. It receives input from descending cortical and limbic pathways and projects to brainstem and spinal cord circuits that control pain perception, autonomic function, and defensive behaviors [@bandler1994].

In neurodegenerative diseases, the PAG shows alpha-synuclein-containing glial cytoplasmic inclusions in multiple system atrophy, and [tau protein](/proteins/tau) neurofibrillary tangles in progressive supranuclear palsy. PAG pathology contributes to the pain syndromes, autonomic dysfunction, sleep disturbances, and cardiovascular dysregulation that are prominent non-motor features of these conditions. The PAG extends caudally, spanning approximately 14 mm in the human brain [@benarroch1994].

Columnar Organization

Introduction

Periaqueductal Gray (Pag) is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.

Overview

The periaqueductal gray (PAG), also known as the central gray, is a column of gray matter surrounding the cerebral aqueduct (aqueduct of Sylvius) in the midbrain. The PAG is one of the most functionally diverse structures in the central nervous system, serving as a critical integration center for pain modulation, defensive behavior, autonomic regulation, vocalization, and sleep-wake control [@behbehani1995]. It receives input from descending cortical and limbic pathways and projects to brainstem and spinal cord circuits that control pain perception, autonomic function, and defensive behaviors [@bandler1994].

In neurodegenerative diseases, the PAG shows alpha-synuclein-containing glial cytoplasmic inclusions in multiple system atrophy, and [tau protein](/proteins/tau) neurofibrillary tangles in progressive supranuclear palsy. PAG pathology contributes to the pain syndromes, autonomic dysfunction, sleep disturbances, and cardiovascular dysregulation that are prominent non-motor features of these conditions. The PAG extends caudally, spanning approximately 14 mm in the human brain [@benarroch1994].

Columnar Organization

The PAG is organized into four longitudinal columns, each with distinct functions, connectivity, and neurochemistry [@bandler1994].

• Lateral column (lPAG): Integrates nociceptive information and produces context-dependent defensive responses. Projects to the rostral ventromedial medulla (RVM), a key relay in descending pain modulation.
• Ventrolateral column (vlPAG): Mediates passive coping behaviors (freezing, immobility), bradycardia, hypotension, and opioid-mediated analgesia. The vlPAG is the origin of the descending analgesic pathway and is the richest source of endogenous opioid peptides in the brain ( modulate PAG function. [@behbehani1995]

Connectivity

Afferent Inputs

The PAG receives convergent input from multiple brain regions:

• prefrontal cortex and cingulate cortex: Top-down modulation of pain perception and emotional regulation
• amygdala: Particularly the central nucleus; conveys fear and threat-related information
• hypothalamus: Integrates homeostatic state (hunger, thirst, thermoregulation) with defensive behavior
• Insular cortex: Interoceptive and visceral information
• Spinal cord: Ascending nociceptive signals via the spinomesencephalic tract [@fields1983]

Efferent Projections

The PAG exerts its functions through projections to:

• Rostral ventromedial medulla (RVM): The primary relay for descending pain modulation; PAG-RVM-spinal cord pathway is the canonical endogenous analgesic system.
• locus coeruleus: Noradrenergic arousal and pain modulation
• Raphe nuclei: Serotonergic modulation of pain and mood
• Nucleus of the solitary tract: Cardiovascular and respiratory control
• hypothalamus: Autonomic and neuroendocrine responses
• thalamus: Midline and intralaminar nuclei; arousal and pain perception
• Motor nuclei: Vocal cord musculature (vocalization), facial nerve (emotional expression) [@ossipov2010]

Connectivity Diagram

Normal Functions

Pain Modulation

The PAG is the master control center for endogenous pain inhibition. Electrical stimulation of the vlPAG produces profound analgesia — sufficient for surgical procedures in experimental animals — through activation of the descending PAG-RVM-spinal cord pathway. This system is also affected in multiple system atrophy [@linnman2012].

Sleep-Wake Regulation

PAG neurons, particularly dopaminergic neurons in the vlPAG, contribute to wakefulness regulation. vlPAG dopaminergic neurons are wake-active, and their lesion increases sleep duration. The PAG is affected by Lewy body pathology (Braak stages 2–3), and its degeneration contributes to several prominent non-motor symptoms [@benarroch2012].

Pain: Chronic pain affects 60–85% of PD patients and is poorly understood. A 2023 study demonstrated that PAG dopaminergic neuron loss is a key contributor to PD-associated pain. Using the 6-OHDA model, researchers found reductions in dopaminergic (DRD5-expressing) neurons in the PAG, and pharmacological activation of D1-like receptors in the PAG alleviated mechanical hypersensitivity. The PAG shows prominent accumulation of alpha-synuclein-containing glial cytoplasmic inclusions (GCIs) across all columns. A systematic neuropathological study found significant loss of VGLUT2-immunoreactive neurons in the ventrolateral, lateral, and dorsomedial columns of MSA patients. [Tau protein](/proteins/tau) neurofibrillary tangles densely infiltrate the PAG, particularly in proximity to the oculomotor and trochlear nuclei, with loss of neurons in specific columns. The functional significance of PAG degeneration in AD is not fully understood but may contribute to the altered pain perception and autonomic changes observed in advanced AD ([Parvizi et al., 2000](https://pubmed.ncbi.nlm.nih.gov/10976641/)).

Therapeutic Targeting

Deep Brain Stimulation

PAG/periventricular gray (PVG) [deep brain stimulation](/therapeutics/deep-brain-stimulation) has been used to treat chronic, intractable pain conditions, particularly nociceptive and central pain syndromes. Stimulation of the vlPAG activates the descending analgesic pathway, producing long-term pain relief in selected patients. This approach has been explored for PD-related pain refractory to dopaminergic therapy.

Pharmacological Approaches

Understanding PAG dopaminergic neuron loss in PD suggests that targeted D1/D5 receptor agonists acting on the PAG could provide analgesic benefits. Currently, levodopa partially ameliorates PD pain, likely through restoration of PAG dopaminergic function among other mechanisms.

See Also

• [Deep Brain Stimulation](/treatments/deep-brain-stimulation)
• [Locus Coeruleus](/brain-regions/locus-coeruleus)
• [Raphe Nuclei](/brain-regions/raphe-nuclei)
• [Substantia Nigra](/brain-regions/substantia-nigra)
• [Parkinson's Disease](/diseases/parkinsons-disease)
• [Multiple System Atrophy](/diseases/multiple-system-atrophy)
• [Progressive Supranuclear Palsy](/diseases/progressive-supranuclear-palsy)

External Links

• [Allen Human Brain Atlas — PAG Expression Data](https://human.brain-map.org/microarray/search/show?search_term=periaqueductal+gray)
• [Periaqueductal Gray — StatPearls](https://www.ncbi.nlm.nih.gov/books/NBK554391/)

Brain Atlas Resources

• [Allen Human Brain Atlas — Periaqueductal Gray expression search](https://human.brain-map.org/microarray/search/show?search_term=Periaqueductal+Gray)
• [Allen Mouse Brain Atlas — Periaqueductal Gray search](https://mouse.brain-map.org/search/index.html?query=Periaqueductal+Gray)
• Allen Cell Type Atlas: [Transcriptomic cell type reference](https://portal.brain-map.org/atlases-and-data/rnaseq)
• BrainSpan Developmental Transcriptome: [Periaqueductal Gray developmental expression](https://www.brainspan.org/rnaseq/search/index.html?search_term=Periaqueductal+Gray)

Background

The study of Periaqueductal Gray (Pag) has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.

Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.

References