Parabrachial Internal Lateral Neurons Convey Nociceptive Messages from the Deep Laminas of the Dorsal Horn to the Intralaminar Thalamus

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Abstract
This study investigates the physiological properties of parabrachial internal lateral (PBil) neurons that project to the paracentral thalamic (PC) nucleus using antidromic activation and single-unit recording techniques in anesthetized rat. We reported here that most of these neurons responded exclusively to the nociceptive stimulation of large receptive fields with a sustained firing that often outlasted the stimulus up to several minutes. These responses were depressed by intravenous morphine. Our results demonstrated a novel spino–PBil–PC pathway, which transmits nociceptive messages to the PC nucleus, which in turn projects to the prefrontal cortex. Recent clinical imaging studies showed the important participation of prefrontal cortex in emotional response to pain. This spino–PBil–PC pathway may explain how nociceptive messages reach the prefrontal cortex and thus trigger unbearable aversive aspects of pain.