Netrin-1 receptor DCC is required for the contralateral topography of lamina I anterolateral system neurons.

Anterolateral system (AS) neurons relay nociceptive information from the spinal cord to the brain, protecting the body from harm by evoking a variety of behaviours and autonomic responses. The developmental programs that guide the connectivity of AS neurons remain poorly understood. Spinofugal axons cross the spinal midline in response to Netrin-1 signalling through its receptor deleted in colorectal carcinoma (DCC); however, the relevance of this canonical pathway to AS neuron development has only been demonstrated recently. Here, we disrupted Netrin-1:DCC signalling developmentally in AS neurons and assessed the consequences on the path finding of the different classes of spinofugal neurons. Many lamina I AS neurons normally innervate the lateral parabrachial nucleus and periaqueductal gray on the contralateral side. The loss of DCC in the developing spinal cord resulted in increased frequency of ipsilateral projection of spinoparabrachial and spinoperiaqueductal gray neurons. Given that contralateral spinofugal projections are largely associated with somatotopic representation of the body, changes in the laterality of AS spinofugal projections may contribute to reduced precision in pain localization observed in mice and humans carrying Dcc mutations.

Phox2a Defines a Developmental Origin of the Anterolateral System in Mice and Humans.

Anterolateral system neurons relay pain, itch, and temperature information from the spinal cord to pain-related brain regions, but the differentiation of these neurons and their specific contribution to pain perception remain poorly defined. Here, we show that most mouse spinal neurons that embryonically express the autonomic-system-associated Paired-like homeobox 2A (Phox2a) transcription factor innervate nociceptive brain targets, including the parabrachial nucleus and the thalamus. We define the Phox2a anterolateral system neuron birth order, migration, and differentiation and uncover an essential role for Phox2a in the development of relay of nociceptive signals from the spinal cord to the brain. Finally, we also demonstrate that the molecular identity of Phox2a neurons is conserved in the human fetal spinal cord, arguing that the developmental expression of Phox2a is a prominent feature of anterolateral system neurons.

Recruitment of Spinoparabrachial Neurons by Dorsal Horn Calretinin Neurons.

The dorsal horn of the spinal cord is the first integration site of somatosensory inputs from the periphery. In the superficial layers of the dorsal horn, nociceptive inputs are processed by a complex network of excitatory and inhibitory interneurons whose function and connectivity remain poorly understood. We examined the role of calretinin-expressing interneurons (CR neurons) in such processing and show that they receive direct inputs from nociceptive fibers and polysynaptic inputs from touch-sensitive Aß fibers. Their activation by chemogenetic or optogenetic stimulation produces mechanical allodynia and nocifensive responses. Furthermore, they monosynaptically engage spinoparabrachial (SPb) neurons in lamina I, suggesting CR neurons modulate one of the major ascending pain pathways of the dorsal horn. In conclusion, we propose a neuronal pathway in which CR neurons are positioned at the junction between nociceptive and innocuous circuits and directly control SPb neurons in lamina I.

DCC Is Required for the Development of Nociceptive Topognosis in Mice and Humans.

Avoidance of environmental dangers depends on nociceptive topognosis, or the ability to localize painful stimuli. This is proposed to rely on somatotopic maps arising from topographically organized point-to-point connections between the body surface and the CNS. To determine the role of topographic organization of spinal ascending projections in nociceptive topognosis, we generated a conditional knockout mouse lacking expression of the netrin1 receptor DCC in the spinal cord. These mice have an increased number of ipsilateral spinothalamic connections and exhibit aberrant activation of the somatosensory cortex in response to unilateral stimulation. Furthermore, spinal cord-specific Dcc knockout animals displayed mislocalized licking responses to formalin injection, indicating impaired topognosis. Similarly, humans with DCC mutations experience bilateral sensation evoked by unilateral somatosensory stimulation. Collectively, our results constitute functional evidence of the importance of topographic organization of spinofugal connections for nociceptive topognosis.