Prostaglandin E₂ in the midbrain periaqueductal gray produces hyperalgesia and activates pain-modulating circuitry in the rostral ventromedial medulla

Recent years have seen significant advances in our understanding of the peripheral and spinal mechanisms through which prostaglandins contribute to nociceptive sensitization. By contrast, the possibility of a supraspinal contribution of these compounds to facilitated pain states has received relatively little attention. One possible mechanism through which prostaglandins could act supraspinally to facilitate nociception would be by recruitment of descending facilitation from brainstem pain-modulating systems. The rostral ventromedial medulla (RVM) is now known to contribute to enhanced responding in a variety of inflammatory and nerve injury models. Its major supraspinal input, the midbrain periaqueductal gray (PAG), expresses prostanoid receptors and synthetic enzymes. The aim of the present study was to determine whether direct application of prostaglandin E₂ (PGE₂) within the ventrolateral PAG is sufficient to produce hyperalgesia, and whether any hyperalgesia could be mediated by recruiting nociceptive modulating neurons in the RVM. We determined the effects of focal application of PGE₂ in the PAG on paw withdrawal latency and activity of identified nociceptive modulating neurons in the RVM of lightly anesthetized rats. Microinjection of PGE₂ (50fg in 200nl) into the PAG produced a significant decrease in paw withdrawal latency. The PGE₂ microinjection activated on-cells, RVM neurons thought to facilitate nociception, and suppressed the firing of off-cells, RVM neurons believed to have an inhibitory effect on nociception. These data demonstrate a prostaglandin-sensitive descending facilitation from the PAG, and suggest that this is mediated by on- and off-cells in the RVM.