Segmental and laminar organization of the spinal neurons projecting to the periaqueductal gray (PAG) in the cat suggests the existence of at least five separate clusters of spino-PAG neurons

Abstract

The present retrograde tracing study in the cat describes the spinal cord projections to the periaqueductal gray (PAG), taking into account different regions of the PAG and all spinal segments. Results show that injecting different parts of the PAG leads to different laminar and segmental distributions of labeled spinal neurons. The impression was gained that at least five separate clusters of spinal neurons exist. Cluster I neurons are found in laminae I and V throughout the length of the cord and are probably involved in relaying nociceptive information to the PAG. Cluster II neurons lie in the ventrolateral part of laminae VI-VII of the C1-C4 spinal cord and were labeled by injecting the ventrolateral or lateral part of the rostrocaudal PAG or the deep tectum. Cluster III neurons are located in lamina X of the thoracic and upper lumbar cord and seem to target the PAG and the deep tectum. Cluster IV neurons are located in the medial part of laminae VI-VII of the lumbosacral cord and seem to project predominantly to the lateral and ventrolateral caudal PAG. These neurons may play a role in conveying tactile stimuli to the PAG during mating behavior. Neurons of cluster V are located in the lateral part of lamina I of L6-S2 and in laminae V-VII and X of S1-S3. They are labeled only after injections into the central portion of the lateral and ventrolateral caudal PAG and probably relay information concerning micturition and mating behavior.

Neuropathic pain-induced enhancement of spontaneous and pain-evoked neuronal activity in the periaqueductal gray that is attenuated by gabapentin

“…It has been well-documented that altered discharges of spinal neurons contribute to chemotherapy-induced neuropathy [8; 10; 67]. It is possible that enhanced activity in spinal neurons after neuropathy could drive the excitability changes in the vlPAG, since these neurons receive direct projections from spinal cord neurons [7; 28; 44; 45]. …”

The Role of the Periaqueductal Gray in the Modulation of Pain in Males and Females: Are the Anatomy and Physiology Really that Different?

“…Since then, the anatomical and physiological organization of the PAG and its descending projections to the rostral ventromedial medulla (RVM) and dorsal horn of the spinal cord have been well characterized in a variety of species, including the rat [2–9], cat [10–18], primate [19, 20], and rabbit [21] (see Figure 1). The PAG-RVM-spinal cord pathway comprises an essential neural circuit for opioid-based antinociception [6, 18, 22].…”

Identification of neural circuits involved in female genital responses in the rat: a dual virus and anterograde tracing study

“…The PAG sends dense projections to the nucleus paragigantocellularis (nPGi), which subsequently projects to motoneurons in the thoracolumbar spinal region that innervate the axial musculature, as well as to lumbosacral neurons that innervate the pelvic organs (17, 18, 27, 29,51,61). The PAG also receives direct inputs from the lumbosacral spinal cord (34,36,37,62,96) and may function to integrate ascending and descending information from multiple brain regions related to sexual behavior.Although the MPO, VMN, and PAG have been implicated in some aspects of female sexual behavior, the role of these regions in regulating genital arousal and sexual climactic-like responses are unknown. Vaginal vasocongestion, muscle contractions, and clitoral engorgement occur during genital arousal and sexual climax (5, 24,35,43).…”

“…The PAG sends dense projections to the nucleus paragigantocellularis (nPGi), which subsequently projects to motoneurons in the thoracolumbar spinal region that innervate the axial musculature, as well as to lumbosacral neurons that innervate the pelvic organs (17, 18, 27, 29,51,61). The PAG also receives direct inputs from the lumbosacral spinal cord (34,36,37,62,96) and may function to integrate ascending and descending information from multiple brain regions related to sexual behavior.…”