Identification of brain-to-spinal circuits controlling the laterality and duration of mechanical allodynia in mice

Mechanical allodynia (MA) represents one prevalent symptom of chronic pain. Previously we and others have identified spinal and brain circuits that transmit or modulate the initial establishment of MA. However, brain-derived descending pathways that control the laterality and duration of MA are still poorly understood. Here we report that the contralateral brain-to-spinal circuits, from Oprm1 neurons in the lateral parabrachial nucleus (lPBN^Oprm1), via Pdyn neurons in the dorsal medial regions of hypothalamus (dmH^Pdyn), to the spinal dorsal horn (SDH), act to prevent nerve injury from inducing contralateral MA and reduce the duration of bilateral MA induced by capsaicin. Ablating/silencing dmH-projecting lPBN^Oprm1 neurons or SDH-projecting dmH^Pdyn neurons, deleting Dyn peptide from dmH, or blocking spinal κ-opioid receptors all led to long-lasting bilateral MA. Conversely, activation of dmH^Pdyn neurons or their axonal terminals in SDH can suppress sustained bilateral MA induced by lPBN lesion.